ISTANBUL TECHNICAL UNIVERSITY  GRADUATE SCHOOL M.Sc. THESIS JANUARY, 2022 INVESTIGATION OF THE THERMAL AND RHEOLOGICAL PROPERTIES OF PET/PBT BLENDS Emine Büşra BENLİ Department of Polymer Science and Technology Polymer Science and Technology Programme Department of Polymer Science and Technology Polymer Science and Technology Programme JANUARY, 2022 ISTANBUL TECHNICAL UNIVERSITY  GRADUATE SCHOOL INVESTIGATION OF THE THERMAL AND RHEOLOGICAL PROPERTIES OF PET/PBT BLENDS M.Sc. THESIS Emine Büşra BENLİ (515171014) Thesis Advisor: Assoc. Prof. Dr. M. Reza NOFAR Polimer Bilimi ve Teknolojisi Anabilim Dalı Polimer Bilimi ve Teknolojisi Programı OCAK, 2022 ISTANBUL TEKNİK ÜNİVERSİTESİ  LİSANSÜTÜ EĞİTİM ENSTİTÜSÜ PET/PBT HARMANLARININ TERMAL VE REOLOJİK ÖZELLİKLERİNİN İNCELENMESİ YÜKSEK LİSANS TEZİ Emine Büşra BENLİ (515171014) Tez Danışmanı: Doç. Dr. M. Reza NOFAR v Thesis Advisor : Assist. Prof. Dr. M. Reza NOFAR .............................. İstanbul Technical University Jury Members : Prof. Dr. Sevim İŞÇİ TURUTOĞLU ………................ Istanbul Technical University Prof. Dr. Ali DURMUŞ ..……………....... Istanbul University-Cerrahpasa Emine Büşra BENLİ, a M.Sc. student of ITU Graduate School of Science Engineering and Technology student ID 515171014, successfully defended the thesis entitled “INVESTIGATION OF THE THERMAL AND RHEOLOGICAL BEHAVIOR OF PET/PBT BLENDS”, which she prepared after fulfilling the requirements specified in the associated legislations, before the jury whose signatures are below. Date of Submission : 14 January 2022 Date of Defense : 28 January 2022 vi vii To my dearest familiy, viii ix FOREWORD I would like to express my deepest appreciation to my supervisor, Assoc. Prof. M. Reza NOFAR, for his precious guidance, encouragement, support, and patience. I would like to thank Prof. F. Seniha GÜNER for their support in running experiments in their laboratories, specifically for providing a rheometer. I also express my gratitude to Res. Assist. Yonca ALKAN GÖKSU in running FTIR experiments. I wish to thank Res. Assis. Burcu ÖZDEMIR for her usual smile, help, and support. I also acknowledge the support from The Scientific and Technological Research Council of Turkey-TUBITAK project (Project No: 220N342). I also would like to thank my colleagues from Korozo Packaging-Innovation Team for their support and kind cooperation during this work. I would also like to especially thank my dear friends and my beloved, who helped and motivated me with their words and presence in my difficult times. Finally, I would like to dedicate my thesis to my lovely family. I always feel their love and support in my life. January 2022 Emine Büşra BENLİ (Chemical Engineer) x xi TABLE OF CONTENTS Page FOREWORD ............................................................................................................. ix TABLE OF CONTENTS .......................................................................................... xi ABBREVIATIONS ................................................................................................. xiii SYMBOLS ................................................................................................................ xv LIST OF TABLES ................................................................................................. xvii LIST OF FIGURES ................................................................................................ xix SUMMARY ............................................................................................................. xxi ÖZET ...................................................................................................................... xxiii 1. INTRODUCTION .................................................................................................. 1 2. LITERATURE REVIEW ...................................................................................... 3 Poly (ethylene terephthalate) (PET) ................................................................... 3 2.1.1 Synthesis of PET ......................................................................................... 3 2.1.2 Thermal properties of PET .......................................................................... 4 2.1.3 Mechanical properties of PET ..................................................................... 5 2.1.4 Rheological properties of PET .................................................................... 6 2.1.5 Applications of PET .................................................................................... 8 Poly (butylene terephthalate) (PBT) .................................................................. 8 2.2.1 Synthesis of PBT ......................................................................................... 9 2.2.2 Thermal properties of PBT ....................................................................... 11 2.2.3 Mechanical properties of PBT .................................................................. 12 2.2.4 Rheological properties of PBT .................................................................. 12 2.2.5 Applications of PBT .................................................................................. 13 PET/PBT Binary Blends .................................................................................. 13 2.3.1 Miscibility and blend morphology ............................................................ 14 2.3.2 Thermal properties of PET/PBT blends .................................................... 16 2.3.3 Mechanical properties of PET/PBT blends ............................................... 18 2.3.4 Rheological properties of PET/PBT blends .............................................. 19 2.3.5 Compatibilization of PET/PBT blends...................................................... 20 Novelty of the Current Research ...................................................................... 21 3. EXPERIMENTAL ............................................................................................... 23 Materials ........................................................................................................... 23 Experimental Design ........................................................................................ 23 Characterization ............................................................................................... 24 3.3.1 Differential scanning calorimetry (DSC) .................................................. 24 3.3.2 Rheological analysis ................................................................................. 25 3.3.3 Fourier transform infrared (FTIR) spectroscopy analysis ......................... 26 4. RESULTS and DISCUSSION ............................................................................. 27 Thermal and Crystallization Behavior ............................................................. 27 4.1.1 Crystallization behavior of PET and PBT ................................................. 27 4.1.2 Crystallization behavior of PET/PBT blends ............................................ 31 Rheological Behavior ....................................................................................... 36 4.2.1 Time sweep test ......................................................................................... 36 xii 4.2.2 Frequency sweep test ................................................................................ 39 Structural Properties ......................................................................................... 41 4.3.1 FTIR analysis ............................................................................................ 41 5. CONCLUSION ..................................................................................................... 43 REFERENCES ......................................................................................................... 45 CURRICULUM VITAE .......................................................................................... 51 xiii ABBREVIATIONS BDO : 1,4-butanediol BHBT : Bis (4-hydroxybutyl-terephthalate) BHET : Bis-(2-hydroxyethyl)-terephthalate CE : Chain extender C-NMR : Carbon nuclear magnetic resonance DMT : Dimethyl terephthalate DSC : Differential scanning calorimetry EG : Ethylene glycol FTIR : Fourier transform infrared spectroscopy HDT : Heat deflection temperature H-PET : High molas mass PET L-PET : Low molas mass PET l-PET : Lower molas mass PET MFR : Melt flow rate NMR : Nuclear magnetic resonance PBT : Poly (butylene terephthalate) PET : Poly (ethylene terephthalate) PMDA : Pyromellitic di-anhydride SEM : Scanning electron microscopy THF : Tetrahydrofuran TPA : Terephthalic acid xiv xv SYMBOLS ƞ : Intrinsic viscosity ƞ* : Complex viscosity Tcr : Crystallization temperature Tg : Glass transition temperature Tm : Melting temperature w : Weight fraction γ : Shear rate ΔHcr : Crystallization enthalpy ΔHm : Melting enthalpy ΔHmPBT : PBT crystal melting temperature ΔHm-PET : PET crystal melting temperature xvi xvii LIST OF TABLES Page Table 2.1: Typical mechanical properties of PET .................................................... 6 Table 2.2: Intrinsic viscosities for different PET processing and applications. ....... 6 Table 2.3: MFI results of the samples. ..................................................................... 7 Table 2.4: The typical mechanical properties of PBT. .......................................... 12 Table 2.5: Thermal Crystallization Data for PET/PBT Block Copolymers. ......... 16 Table 3.1: The typical properties of Joncry ADR 4368 ®. .................................... 23 Table 3.2: Codifications of PET and PBT flakes with/without Joncryl. ................ 23 Table 3.3: Compounding ratios and codifications of PBT/ PET blends. ............... 24 Table 4.1: A summary of DSC conditions. ............................................................ 27 Table 4.2: Thermal properties of processed PET and PBT. ................................... 28 Table 4.3: Thermal properties of chain extended PET and PBT samples. ............ 29 Table 4.4: Thermal properties of PET and PBT samples w/wo CE at different cooling rates. ......................................................................................... 31 Table 4.5: Thermal properties of PET/PBT blends. ............................................... 32 Table 4.6: Thermal properties of chain extended PET/PBT blends. ..................... 34 Table 4.7: Thermal properties of blends against varying cooling rates. ................ 36 Table 4.8: Complex viscosity changes of PET and PBT samples during time sweep test after 20 min. ........................................................................ 38 Table 4.9: Complex viscosity changes of PET/PBT blend samples during time sweep test after 20 min. ........................................................................ 39 Table 4.10: Complex viscosity changes of PET and PBT samples during frequency sweep test. ............................................................................................. 40 Table 4.11: Complex viscosity changes of PET/PBT blend samples during frequency sweep test. ............................................................................ 41 xviii xix LIST OF FIGURES Page Figure 2.1: Chemical Structure of PET. .................................................................. 3 Figure 2.2: PET Synthesis Route. ........................................................................... 4 Figure 2.3: DSC diagram in both first heating and cooling processes for PET. ..... 5 Figure 2.4: In-line steady shear measurements of PMDA-modified PET: (O) PET control, (▲) PET-A and (■) PET-B. .................................................... 8 Figure 2.5: The chemical structure of poly(butylene terephthalate). ...................... 9 Figure 2.6: Transesterification of DMT and BDO. ................................................. 9 Figure 2.7: (1) Formation of bis-HBT and other hydroxybutyl-terminated terephthalate oligomers by transesterification of DMT with BD and (2) polycondensation of bis-HBT and hydroxybutyl-terminated oligomers resulting in PBT. ................................................................ 10 Figure 2.8: Synthesis of PBT from TPA as starting material. .............................. 11 Figure 2.9: Time sweep experiments of PBT at various temperatures. ................ 13 Figure 2.10: Scanning electron microscopy photomicrographs of PET/PBT blends (original magnification x 1000): (a) 20:80, (b) 40:60, (c) 50:50, (d) 60:40, (e) 70:30, (f) 80:20, and (g) 80:20 A. ...................................... 15 Figure 2.11: PET/PBT blends glass transition temperature versus PET concentration as determined by DSC. ................................................ 16 Figure 2.12: Melting temperatures of PBT and PET versus PET concentration in the blend as determined by DSC. ....................................................... 17 Figure 2.13: A plot of impact strength against the PET content in PET/PBT blends . ............................................................................................... 18 Figure 2.14: Chemical structure of Joncryl ADR 4368 ®. ..................................... 20 Figure 3.1: Schematics of all together processing methods through blending. .... 24 Figure 3.2: Perkin Elmer DSC 4000 system device.............................................. 25 Figure 3.3: MCR-301 rotational rheometer (Anton Paar, Austria). ...................... 26 Figure 3.4: An example of an FTIR device. ......................................................... 26 Figure 4.1: First heating, cooling, and second heating thermograms of processed PET (a) and PBT (b). .......................................................................... 28 Figure 4.2: First heating, cooling and second heating thermograms of chain extended PET (a) and PBT (b). .......................................................... 29 Figure 4.3: DSC curves of cooling (a, c, e, g) and second heating (b, d, f, h) cycles of PET and PBT samples. ................................................................... 30 Figure 4.4: First heating, cooling and second heating thermograms of PET/PBT blends. ................................................................................................. 32 Figure 4.5: First heating, cooling and second heating thermograms of chain extended PET/PBT blends .................................................................. 33 Figure 4.6: DSC curves of cooling (a, c, e, g) and second heating (b, d, f, h) cycles of blends. ............................................................................................ 35 Figure 4.7: Time sweep test results of PET (a) and PBT (b) samples at 260℃. ... 37 Figure 4.8: Chemical representation of chain extension reaction of polyterephthalates with a generic epoxide. ........................................ 38 xx Figure 4.9: Time sweep test results of PET/PBT blends at 260℃. ....................... 39 Figure 4.10: Frequency sweep test results of PET (a) and PBT (b) samples at 260℃. .................................................................................................. 40 Figure 4.11: Frequency sweep test results of PET/PBT blend samples at 260℃. .. 41 Figure 4.12: FTIR results of processed and chain extended PET and PBT samples. . 42 Figure 4.13: FTIR results of PET/PBT blend samples. .......................................... 42 xxi INVESTIGATION OF THE THERMAL AND RHEOLOGICAL PROPERTIES OF PET/PBT BLENDS SUMMARY The use of polymer materials, due to their easy processing and low cost, is becoming increasingly common today and is frequently used in the industry. Polymer blending is a simple, effective, and cost-effective approach to obtaining a new composite with desired combinations of properties without overtly compromising its advantages. Polymer blends are formed by the physical mixing of two or more polymers. Poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) are two of the most important aromatic-aliphatic polyester resins in the industry. PET and PBT have similar chemical structures and can undergo transesterification reactions at high temperatures. Therefore, they form a miscible blend in the amorphous state without the need for compatibilizers to be used. PET resins are difficult to process due to their slow crystallization rate, high heat deflection temperature, low melt strength, and hardness. Thanks to the flexible butylene groups in the PBT structure, it is easier to process because it has a higher crystallization rate and better melt strength. Blending PBT with PET provides a product with good mechanical and high electrical insulating properties and improves machinability, surface appearance, heat deflection temperature, impact strength, and dimensional stability. Through these features, it is used in automotive parts, household and kitchen appliances that require mechanical, high heat, and chemical resistance. These blends are also used to produce visible parts of devices that require a smooth and glossy surface appeal. While blending PET and PBT provides new structures with superior properties, these structures may also have disadvantages such as brittleness and relatively low mechanical properties. Using chain extenders when forming blends can result in improvements in the shear thinning, shear and elongation viscosity, melt elasticity and melt strength behavior of polymers, and thus in the performance of the final product. In this thesis, PET/PBT blends were produced using a twin-screw extruder. The pellets produced with the extruder were then formed into test specimens using injection molding. Multi-functional styrene-acrylic additive with epoxy reactive groups, whose trade name is Joncryl ADR 4368 ®, was used as the chain extender agent. First, PET and PBT samples were extruded. Afterward, 0.75% by weight Joncryl additive was physically mixed with the obtained samples, and pellets were produced in the extruder. While preparing the blends, PET was added to PBT at rates of 25, 50, and 75% by weight. While Joncryl was added to the PET/PBT blends, pellets were produced in the extruder by physically mixing the PBT, PET, and Joncryl additive material. xxii In this thesis, the effects of using different ratios of PET and adding chain extender on PET/PBT blends were investigated. The thermal properties and crystallization behaviors of the developed blend materials were investigated using the differential scanning calorimetry (DSC) method. The rheological properties were investigated using a rotational rheometer. Fourier transform infrared (FTIR) spectroscopy was used to observe the structural changes in the samples. xxiii PET/PBT BLENDLERİN TERMAL VE REOLOJİK ÖZELLİKLERİNİN İNCELENMESİ ÖZET Polimer malzemelerin kullanımı, kolay işlenebilmeleri ve düşük maliyetleri sebebiyle günümüzde giderek yaygınlaşmaktadır ve endüstride sıklıkla kullanılmaktadır. Polimer harmanlama, avantajlarından açıkça ödün vermeden istenen özellik kombinasyonlarına sahip yeni bir kompozit elde etmek için basit, etkili ve düşük maliyetli bir yaklaşımdır. Polimer harmanları, iki ya da daha fazla polimerin fiziksel karışımı ile meydana gelmektedir. Poli(bütilen tereftalat) (PBT) ve poli(etilen tereftalat) (PET), endüstrideki en önemli aromatik-alifatik polyester sınıfına ait reçinelerinden ikisidir. PET ve PBT’nin kimyasal yapıları birbirine benzerdir ve yüksek sıcaklıklarda transesterifikasyon reaksiyonuna girebilirler. Bu nedenle, uyumlaştırıcıların kullanılmasına gerek kalmadan amorf halde birbirine karışmış bir harman oluştururlar. PET reçinelerinin işlenmesi yavaş kristallenme hızı, yüksek ısı sapma sıcaklığı, düşük erime mukavemeti ve sertliği sebebiyle zordur. PBT yapısındaki esnek butilen grupları sayesinde daha yüksek kristalizasyon hızına ve daha iyi eriyik mukavemetine sahip olduğu için proses edilmesi daha kolaydır. PBT'yi PET ile karıştırmak, iyi mekanik ve yüksek elektriksel yalıtım özelliklerine sahip bir ürün sağlar ve işlenebilirliği, yüzey görünümünü, ısı sapma sıcaklığını, darbe dayanımını ve boyutsal kararlılığı geliştirir. Bu özellikler sayesinde mekanik, yüksek ısı ve kimyasal dayanım gerektiren otomotiv parçalarında, ev ve mutfak aletlerinde kullanılmaktadır. Bu karışımlar, pürüzsüz ve parlak yüzey çekiciliği gerektiren cihazların görünür parçalarının üretilmesi için de kullanılır. PET ve PBT'yi karıştırmak üstün özelliklere sahip yeni yapıların elde edilmesini sağlarken bu yapılar kırılganlık, nispeten düşük mekanik özellikler gibi dezavantajlara da sahip olabilir. Harmanlar oluşturulurken zincir uzatıcı kullanmak polimerlerin kesme incelmesi, kesme ve uzama viskozitesi, eriyik elastikiyeti ve eriyik mukavemeti davranışında ve dolayısıyla son ürünün performansında iyileşmelere neden olabilir. Bu tez çalışmasında, PET/PBT harmanları çift vidalı ekstrüder kullanılarak üretilmiştir. Ekstrüder ile üretilen pelletler daha sonra enjeksiyon kalıplama kullanılarak test numuneleri haline getirilmiştir. Zincir bağlayıcı ajan olarak ticari adı Joncryl ADR 4368 ® olan epoksi reaktif gruplara sahip multi-fonksiyonel stiren akrilik katkı malzemesi kullanılmıştır. Öncelikle, PET ve PBT numuneleri ekstrüde edilmiştir. Daha sonra elde edilen numunelere ağırlıkça % 0.75 oranında Joncryl katkı malzemesi fiziksel olarak karıştırılarak ekstrüderde pelletler üretilmiştir. xxiv Harmanlar hazırlanırken PET, PBT içerisine ağırlıkça % 25, 50 ve 75 oranlarında eklenmiştir. Joncryl, PBT/ PET harmanlarına eklenirken PBT, PET ve Joncryl katkı malzemesi fiziksel olarak karıştırılarak ekstrüderde pelletler üretilmiştir. Bu tez çalışmasında, farklı oranlarda PET kullanmanın ve zincir uzatıcı ilavesinin PET/ PBT harmanları üzerine etkisi incelenmiştir. Geliştirilen harman malzemelerin termal özellikleri ve kristalizasyon davranışları, diferansiyel taramalı kalorimetri (DSC) yöntemi kullanılarak incelenmiştir. Reolojik özellikleri rotasyonel reometre kullanılarak incelenmiştir. Numunelerdeki yapısal değişiklikleri gözlemlemek için Fourier Dönüşümlü Kızılötesi (FTIR) Spektroskopisi kullanılmıştır. https://www.google.com/search?rlz=1C1GCEU_en-GBTR923TR923&sxsrf=AOaemvItre3WHv6c626SWFLZ1TnJrnEQtA:1641755870390&q=Fourier+D%C3%B6n%C3%BC%C5%9F%C3%BCml%C3%BC+K%C4%B1z%C4%B1l%C3%B6tesi+(FTIR)+Spektroskopisi&spell=1&sa=X&ved=2ahUKEwjHgtmbsaX1AhXdR_EDHQlFAPkQBSgAegQIAhA3 1 1. INTRODUCTION The use of polymer materials is becoming increasingly common and frequently used in industry due to their easy processing and low cost. Most of these are petroleum- based, but polymers from renewable resources have recently been developed and widely used. Polymer blending is a simple, effective, and cost-effective approach to obtaining a new composite with desired combinations of properties without overtly compromising its benefits. PET resins are difficult to process due to their slow crystallization rate, and flexible butylene groups in the structure of PBT allow it to have a higher crystallization rate and thus easier processing. Blending PBT with PET provides a product with good mechanical properties and improves machinability, surface appearance, and dimensional stability. The use of chain extenders when forming blends can result in improvements in the shear thinning, shear viscosity, and melt strength behavior of polymers and therefore in the performance of the final product. In this master thesis, PET blends with PBT were developed with different compositions while reactive twin-screw extrusion (TSE) was also used in presence of a chain extender to further improve the deteriorated properties of the noted blends. Injection molding was also employed subsequent to TSE to prepare the testing samples. The thermal properties and crystallization behaviors, rheological properties structural changes in the samples were investigated. 2 3 2. LITERATURE REVIEW Poly (ethylene terephthalate) (PET) Polyethylene terephthalate (PET) is a semi-aromatic thermoplastic polyester (Figure 2.1) which is widely used in many industrial applications [1-3]. While the majority of these applications include PET as an effective packaging material for food and drink products [4], it has also remarkable importance in textile fibers, automotive components, and biomedical purposes [5]. Such extensive use is mainly due to its superior mechanical properties, high chemical resistance, good barrier properties against oxygen and water vapor, and thermal stability along with its low production cost, processability, and recyclability [6-9]. As a result of such broad PET usage, mainly in the containers of soft drinks, its global production was over 56 million tons in 2016 and over 70 million tons in 2020 [5]. Figure 2.1: Chemical structure of PET [1]. 2.1.1 Synthesis of PET The production of PET consists of two steps, pre-polymerization and melt polycondensation (Figure 2.2). Pre-polymerization proceeds via two reactions; direct esterification of terephthalic acid (TPA) with ethylene glycol (EG) or transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG). During ester interchange reactions water (when TPA is used) or methanol (when DMT is used) is removed and an oligomeric product (n=1-4) bis-(2-hydroxyethyl)- terephthalate (BHET) is obtained. In the second step, ethylene glycol is distilled during the reaction, BHET monomer is further polymerized and high molecular weight PET is obtained with reduced pressure and increased temperature. 4 The final molecular weight is influenced by factors such as time, temperature, and vacuum. During polymerization, several factors can affect the final molecular weight such as time, temperature, and vacuum. While the first step reaction is carried out at temperatures ranging from 200 to 260°C and is catalyzed by metal salts, during the second step temperature increases around 280°C in the presence of the catalyst under a high vacuum (<1 kPa). The melt polymerization time can be in the range of 2–5 h. [10-12] Figure 2.2: PET synthesis route [11]. 2.1.2 Thermal properties of PET The processability of PET largely depends on its thermal properties, while its thermal properties depend on the degree and quality of crystallinity. Due to the short ethylene chain between the aromatic rings, PET has a low crystallization rate and PET samples constantly show multiple melting endotherms, which depend on crystallization occurs wide temperature range and this behavior causes different sizes and perfection crystals. [13, 14] PET samples could be in an amorphous state, semicrystalline state, or fully oriented state depending on PET processing and thermal history [15]. Amorphous, transparent PET could be obtained by cooling rapidly from the melt to a temperature below Tg and PET has a Tg of 67 ⁰C in the amorphous state. Heating the solid amorphous PET to a temperature above Tg could produce semi-crystalline PET which has a Tg of 81 5 ⁰C in the semi-crystalline state. The melting temperature of PET is a range between 250-265 ⁰C, while fully annealed PET has a melting point of 280 ⁰C. DSC diagram of PET is given in Figure 2.3 [16]. The degree of crystallinity and morphology have a significant effect on the thermal properties and hence processability of the final polymer [16]. The crystallinity in PET could also be strongly influenced by the molecular weight of the polymer. Higher degree of crystallinity could be obtained with low-molecular-weight grades since low molecular weight allows the chain to align easily compared to higher molecular weights. The final mechanical properties can be affected by a combination of molecular weight and crystallinity [11, 14, 17-19]. Figure 2.3: DSC diagram in both first heating and cooling processes for PET [16]. 2.1.3 Mechanical properties of PET Due to its chemical nature, PET exhibits good mechanical properties that are summarized in Table 2.1. It has a tensile strength of around 55 – 75 MPa with a high flexural modulus (2412-3102 MPa). Moreover, it is almost impermeable to most gases and liquids that could be further improved with increasing crystallinity. It also exhibits resistance to dissolution by common solvents. However, the processing of PET is difficult because of its low melt strength and slow rate of crystallization [17,18, 20-22] that is mainly caused by ethylene units in its structure. 6 Table 2.1: Typical mechanical properties of PET [11, 23]. Properties Units Average Values Tensile Strength MPa 55-75 Tensile Elongation % 50-165 Tensile Modulus MPa 2000-4000 Flexural Strength MPa 96,5-124,1 Flexural Modulus MPa 2413-3102 Izod Notched Impact Strength (at room temperature) J/m 13-35 2.1.4 Rheological properties of PET The most available PET has an inherently poor melt strength and viscosity, relatively low molecular weight, and narrow molecular weight distribution. Regarding these features, they are not suitable for applications such as foaming or extrusion blow molding. Improving the properties of PET such as die swell, melt strength, and melt flow index will be possible by increasing the molecular weight and broadening the molecular weight distribution [24-25]. PET can be manufactured through modification with a chain extension, grafting, branching, blending, controlled cross-linking, or controlled degradation since different applications require different properties [24]. Intrinsic viscosities of PET with respect to the required processing and application are shown in Table 2.2 [11]. Table 2.2: Intrinsic viscosities for different PET processing and applications [11]. Intrinsic viscosity [η] (dL/g) Processing Applications 0.68 Injection molding Cups, connectors 0.76 Blow molding, film extrusion, electrospinning Bottles for mineral water, containers, cas film, fiber 0.80 Blow molding General bottle applications 0.84 Blow molding, thermoforming Bottles for carbonated drinks, oils, cosmetics, and food 0.90 Blow molding Detergent bottles, jars, dreep-draw cups Kruse M. [26] investigated the three grades of PET produced by different synthesis routes, distinguishable by molar mass and concentration of functional end groups, in terms of thermal and thermo-oxidative degradation and preparation. These grades are high molar mass PET (H-PET), low molar mass PET (L-PET), and lower molar mass PET (l-PET). In thermal stability studies, a degradation mechanism and a decrease in storage modulus in the air atmosphere, and an increase in modulus in the nitrogen 7 atmosphere was observed due to polycondensation. Over time, the degradation reaction in the air atmosphere becomes predominant with an increase in modulus due to a crosslinking induced by oxygen. The development reaction in a nitrogen atmosphere is deactivated by thermal decomposition. High molar mass PET was found to be more stable in a nitrogen atmosphere and less stable in air than low molar mass PET, and vice versa. Forsythe et al. [25] investigated the enhancement of the melt strength of PET with the addition of pyromellitic di-anhydride (PMDA) as a chain extender additive at various concentrations. For a plastic melt, the ratio between shear stress and shear rate in a narrow range is defined as the apparent viscosity and is measured by observing the melt flow through a capillary tube. The apparent viscosity provides information about melt flow during various processing conditions and is constant for Newtonian materials and variable for non-Newtonian plastic materials. Therefore, the adjective 'apparent' is used to describe viscosity at a given shear rate [27-30]. It can be seen in Figure 2.4, while the apparent viscosity of the PMDA modified PET samples increased at lower shear rates which is an indication of the branching of the PET, shear sensitivity increased especially at the higher shear rates. It was shown that in Table 2.3 results melt flow index (MFI) decreased from 22 g/10 min to 15 g/10 min and 11 g/10 min after the addition of 0.10 wt% (PET-A) and 0.20 wt% (PET-B) of PMDA, respectively. Table 2.3: MFI results of the samples [25]. Materials MFI (g/10 min) PET Control 22 PET-A 15 PET-B 11 8 Figure 2.4: In-line steady shear measurements of PMDA-modified PET: (O) PET control, (▲) PET-A and (■) PET-B [25]. 2.1.5 Applications of PET PET is the most widely used material due to its excellent mechanical, thermal properties and lowest cost compared to other engineering thermoplastic polyesters. PET has long been used in fiber applications including clothing, home furnishings, and tire cord. The fiber applications of PET depend on exceptional crease-resistance, work recovery, and low moisture absorption. As a plastic, PET has been used in the manufacture of films and more recently as blow-molded bottles for carbonated soft drinks. Biaxially oriented PET film is used industrially in magnetic tape, X-ray, and photographic films. Both oriented and unoriented PET films are also used in food packaging applications such as boil-in-bag food pouches. PET is inherently an electrical insulator, which allows for applications such as electrical connectors [24, 31-33]. Poly (butylene terephthalate) (PBT) PBT is a semicrystalline engineering thermoplastic (Figure 2.5) representing the second-leading commercial polyester in terms of significance. The global PBT market is expected to reach 1,5 million metric tons by 2026 [34]. PBT is a fast-crystallizing polymer and, therefore, well suited for extrusion and injection-molding applications with high cycle times [35]. When a material with high strength, good dimensional stability, resistance to various chemicals, and good insulation is required, PBT is the preferred choice given its excellent properties. The 9 same is true when bearing and wear characteristics are decisive factors in material selection. That's why valves, food processing machinery parts, wheels, and gears are also can be made from PBT. PBT exhibits good electrical insulating properties (contact and surface resistance) along with high dielectric strength and good tracking current resistance, which are stable over a wide temperature and humidity ranges. This makes PBT a reliable and excellent building material for electrical and electronic equipment. [35] Figure 2.5: The chemical structure of poly(butylene terephthalate) [32]. 2.2.1 Synthesis of PBT Polybutylene terephthalate (PBT) is a very important semicrystalline commercial thermoplastic that is synthesized from 1,4-butanediol (BDO) with either terephthalic acid (TPA) or dimethyl terephthalate (DMT) with the presence of a polyesterification catalyst by condensation polymerization (Figure 2.6) [20, 23, 35, 36]. The most commonly used catalysts are tetraalkyl titanates for PBT polymerization. [35, 36] Traditionally the polymerization of PBT starts with forming of the diester, bis (4- hydroxybutyl-terephthalate) (BHBT), or ester oligomers with hydroxybutyl ester end groups by a transesterification reaction of BDO with DMT as a first stage [11, 36]. Figure 2.6 shows the diester structure produced from transesterification reaction in BDO and DMT [11]. Figure 2.6: Transesterification of DMT and BDO [11]. Transesterifications also termed as ester exchange or ester interchange reactions, include hydroxy-ester, carboxy-ester, and ester-ester reactions. Hydroxy-ester interchange (called alcoholysis) plays a predominant role in most industrial preparations of aliphatic-aromatic polyesters such as PBT [35]. 10 During the first step, the temperature increases from 150 °C to approximately 210 °C. Methanol is the major byproduct when DMT is used in the former route and the first stage is completed with the removal of methanol during the reaction [11, 35, 36]. In the second stage, bis-HBT, and other low molecular weight oligomers, occurred during the transesterification step, further react by polycondensation between two hydroxybutyl end groups resulting in PBT homopolymer, whereas BD is the elimination product. The catalyst is usually applied during the reaction. The most commonly used catalysts are tetraalkyl titanates for PBT polymerization [35, 37]. As the polycondensation reaction progresses, PBT molecular weight builds up and the melt viscosity increases significantly. The polycondensation takes place at temperatures close to 260°C (well above the melting temperature of PBT) and a reduced pressure below 100 Pa. Owing to the high temperatures during the second stage, oxidative thermal decomposition and yellowing occur. These decomposition reactions place the upper limit on the molecular weight that can be realized by the normal melt condensation reaction. The second step is completed when the desired molecular weight of PBT is achieved (Figure 2.7) [11, 35, 37]. Figure 2.7: (1) Formation of bis-HBT and other hydroxybutyl-terminated terephthalate oligomers by transesterification of DMT with BD and (2) polycondensation of bis-HBT and hydroxybutyl-terminated oligomers resulting in PBT [37]. The other synthesis process of PBT is based on a direct esterification reaction of TPA with an excess amount of BDO and is very similar to the DMT-based route, described before. In a TPA-based reaction, there is a large amount of water generated 11 in the first-stage reaction of an acid with a diol. During the reaction, water is removed from the reaction vessel because the water generation during PBT polymerization is undesirable. The second step of the process is to produce PBT from bis(4-hydroxybutyl-terephthalate). The ester oligomer formed is very similar for both routes, with either TPA or DMT as the starting raw material. The major side reaction in the production of PBT is the generation of tetrahydrofuran (THF). The TPA-based process produces as much as twice the amount of THF by- products as that generated by the DMT-based process (Figure 2.8) [11, 35, 37]. Figure 2.8: Synthesis of PBT from TPA as starting material [35]. Synthesis of PBT from DMT monomer (i.e., the first method) is more advantageous from the synthesis from TPA (i.e., the second method) because the environmentally aggressive chemicals are not used (such as bromides or acetic acid) in the first method. Thus, this eliminates the need for expensive, highly corrosion-resistant reaction vessels. Furthermore, at the beginning of the production of terephthalate- based polyesters, DMT was predominantly used for their synthesis because it was relatively easily purified in comparison with TPA [35, 37]. 2.2.2 Thermal properties of PBT PBT has a very fast crystallization rate due to the existence of flexible butylene groups within its molecular structure and the thermal history of the material influences its crystallization and melting behavior. Completely crystalline PBT homopolymer has a glass-transition (Tg) range of 30-50°C and the melting temperature (Tm) is usually between 222 and 232 °C. The glass transition 12 temperature and the melting temperature can shift to higher values due to the perfection of the crystals by annealing PBT for a long time. Also, while the typical crystallinity degree of PBT is between 35-40 %, the crystalline portion of the PBT can be reached 60% by annealing. PBT usually contains different sizes of crystals, and therefore, multiple melting peaks can occur on the DSC curve. [11, 35, 37] 2.2.3 Mechanical properties of PBT PBT is characterized by good tensile strength, high rigidity, and toughness due to the existence of its crystalline phase; low creep at elevated temperatures; excellent dimensional stability; low coefficient of friction; good chemical, grease, oil, and solvent resistance; minimal moisture absorption; and excellent electrical properties [11, 18, 23, 36, 38, 39]. The degree of crystallinity strongly influences the mechanical properties of PBT. The crystalline PBT regions give the resin its resistance to solvents and mechanical strength. The amorphous region gives the material much of its elongation properties Fast crystallization and low melt strength properties of PBT allow for easy processing [35, 37]. The typical mechanical properties of PBT are listed in Table 2.4 [11, 37, 40]. Table 2.4: The typical mechanical properties of PBT [11, 37, 40]. Properties Units Average Values Tensile Strength MPa 52 Tensile Elongation % 150 Tensile Modulus MPa 2300 Flexural Strength MPa 80 Flexural Modulus MPa 2000 Izod Notched Impact Strength (at room temperature) J/m 35 2.2.4 Rheological properties of PBT PBT is one of the viscoelastic polymers and the melt of PBT shows a low viscosity and very fast crystallization, hence allowing for easy processing in comparison with PET. PBT resins can be processed by using conventional processes such as spinning, extrusion, injection molding, and gas-assisted injection molding [20, 35]. Guclu M. et al. [41] investigated the thermal stability behavior of PBT samples through time sweep rheological experiments at various temperatures for 20 min. As 13 seen in Figure 2.9, the continuous decrease of the complex viscosity of PBT was observed, e.g. the complex viscosity of the PBT decreased by approximately 31% at 245 °C and 40% at 285 °C after 20 minutes. Thus, it can be said that thermal degradation predominates in PBT and becomes more pronounced at high test temperatures. Figure 2.9: Time sweep experiments of PBT at various temperatures [41]. 2.2.5 Applications of PBT PBT exhibits good electrical insulating properties (contact and surface resistance) combined with high dielectric strength and good tracking current resistance, which are stable over wide temperature and humidity ranges. This makes PBT a reliable and excellent building material for electrical and electronic equipment. It is used in insulating parts such as plugboards, contact strips, and plug connections.[35] Polybutylene terephthalate (PBT) is also a commercially important polymer with applications in various industries such as automotive parts, electrical, electronic, and textile. It has similar thermal and chemical resistance and mechanical properties compared to that PET but slightly better impact strength and PBT has a faster rate of crystallization than PET. [35, 36] PET/PBT Binary Blends Poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) are two of the most important aromatic–aliphatic polyester resins in industry. PBT has advantages in crystallization rate, processing, and dimensional stability compared to 14 PET, while PET resins have a higher heat deflection temperature, stiffness, and, because of its low melt strength and slow rate of crystallization the processing of PET is difficult. Polymer blending has been shown as a simple, effective, and low- cost approach to obtain a novel composite with desirable property combinations without clearly sacrificing its advantages. Blending PBT with PET ensures a product with good mechanical and high electrical insulation properties and improves processability, surface appearance, heat deflection temperature (HDT), impact strength, and dimensional stability. [42] 2.3.1 Miscibility and blend morphology PET and PBT form stable blends without the demand for compatibilizing agents in the amorphous phase in spite of their large difference in the crystallization rates. There are several studies showing this behavior, but Escala and Stain's [32] work is the first example. They specify that, based on DSC analysis, infrared studies, and X- ray scattering, the compatibility of the PET/PBT mixture in the amorphous state is demonstrated by the presence of a single glass transition temperature between those of the individual components. However, in reference to the DSC results separate melting points for the two crystalline species were occurred with no evidence of co- crystallization, and, put differently, the blend is immiscible in the crystalline state. [43] Similarly, Avramova [44] reported the miscibility of polymers in the amorphous phase and having a single glass transition temperature for all PET/PBT blend compositions. Also, the blends showed a single crystallization peak and multiple melting peaks with some depression in melting temperatures. Escala and Stain [43] also put forward that the compatibility in PET/PBT blends might be based on the formation of copolymers by transesterification (ester interchange) reactions between both polymers in the blend during melt processing. The transesterification reaction between PBT and PET has been studied by researchers [32, 43, 45-47]. This reaction related to the initial miscibility of the homopolymers in the molten state and on the processing conditions leading to the formation of block copolymers in the initial stage and random copolymers finally. Therefore, the transesterification reaction can affect the morphology hence the properties of the blends.[46] 15 Backson et al. [24], Matsuda et al. [19], and Kim et al. [48] applied 13C Nuclear Magnetic Resonance (NMR) on PET/PBT blends to examine the transesterification extent and sequence structure in blends. Backson et al. [24] first heated the PET/PBT blends at 573 K for 30 min and 476 K for 6 h and then used 13C NMR. They focused on the number average sequence length of the minority component in the mixture and this value being about 2 indicates that the minority component is randomly distributed among all the molecules in the mixture. All data are consistent with the formation of random copolymers when the mixtures are heated at 573 K. For example for the 25 PET/75 PBT mixture, the number average sequence length of the PET is nearly 2, indicating that transesterification of the PET is complete and the PET units are randomly distributed in the resultant copolymer molecules. Processing conditions and the properties of each component affect the morphology of polymer blends. Although miscibility has been demonstrated in DSC thermograms in previous studies, Aravinthan and Kale [20] reported that scanning electron micrographs (SEM) (Figure 2.10) of PET/PBT mixtures at close mixing ratios produced co-continuous and fibrillar-like morphologies indicating their immiscibility. Figure 2.10: Scanning electron microscopy photomicrographs of PET/PBT blends (original magnification x 1000): (a) 20:80, (b) 40:60, (c) 50:50, (d) 60:40, (e) 70:30, (f) 80:20, and (g) 80:20 A. 16 2.3.2 Thermal properties of PET/PBT blends It is common knowledge that PET/PBT blends show an average glass transition temperature, one crystallization peak without the presence of co-crystals, and two melting peaks with a slight mutual depression. Many researchers investigated the effects of concentrations of each component, degree of transesterification between PBT and PET, degree of branching and addition of fillers, reinforcements effects on crystallization rate of PET/PBT blends. Glass transition temperatures versus concentrations of each component in PET/PBT blends are reported in the study of Escala and Stein [32], as shown in Figure 2.11, and Misra and Garg [49], as shown in Table 2.4. In both studies, it was observed that there was only one glass transition temperature for all mixtures and the change in glass transition temperature was linearly proportional to the change in composition. Figure 2.11: PET/PBT blends glass transition temperature versus PET concentration as determined by DSC [32]. Table 2.5: Thermal crystallization data for PET/PBT block copolymers. [49] PBT content (%) Tg (°C) 0 69 1.0 68 2.0 68 3.5 66 5.0 64 7.5 61 10.0 58 15.0. 56 20.0 55 17 The melting temperatures of both components were present in the mixtures with a slight decrease depending on the composition. The melting temperatures of PBT and PET versus PET concentration in the blend were reported in the study of Escala and Stein, as shown in Figure 2.12. [32]. Figure 2.12: Melting temperatures of PBT and PET versus PET concentration in the blend as determined by DSC [32]. Due to the higher crystallization behavior of PBT, blending PET with PBT compensates for the processing difficulty caused by the slow crystallization behavior of PET. [22, 33, 50, 51]. The effects of the concentration of each component, the degree of transesterification between PBT and PET, the degree of branching and fillers, the effects of reinforcements on the crystallization rate of PET/PBT blends have been studied in many studies. Aravinthan and Kale [20] investigated the effect of different compositions of PET/PBT blends and reported that introducing PBT weakens the interaction between molecular chains of PET and enhances the movement of polymer molecules, thus decreasing the crystallization temperature. Also, Szostak [22] investigated the effect of different compositions of PBT and reported that introducing the PBT material into PET increases the crystallization speed which results in decreasing the crystallization temperature and increasing the crystallinity. Those features improve and make much easier the processing of PET/PBT blends. Ito et al. [33] investigated how the crystallization behaviors of PET and PBT affected the blend under the influence of different degrees of reactive transesterification during different processing methods. Solution casting processes and reducing 18 extrusion temperature lead to a lower degree of transesterification. They reported that crystallization increased with partial transesterification copolymerization. They confirmed that PBT crystallization was accelerated by the presence of PET crystals in the mixture, which presents moderate transesterification under the isothermal condition at temperatures above 200 °C. Ito et. al., transesterification copolymers enhanced the crystallization of one of the components by the presence of crystals of another component. 2.3.3 Mechanical properties of PET/PBT blends Despite PBT having better processability due to its faster crystallization rate than PET, there are slight differences between their mechanical properties. There are several studies examining the changes in mechanical properties of PET/PBT blends. It is reported by Szostak [22] that increasing PBT content in the blends improved the impact strength due to the long and flexible butylene chain within PBT which enables it to absorb the impact energy. At the same time, Mikiatev and Borukaev [40] stated that increasing PET content in the blend over the value of 20 wt% caused a significant reduction in impact strength of the blend due to the phase separation phenomenon as shown in Figure 2.13. Figure 2.13: A plot of impact strength against the PET content in PET/PBT blends [40]. Both Aravinthan and Kale [20] and Szostak [22] stated that PET has comparatively higher tensile strength than PBT. When the amount of PET increased in PET/PBT blends the tensile and flexural strength slightly increased [20, 22]. Aravinthan and Kale [20], in their study of PET/PBT, blend at different concentrations, reported that 19 the highest tensile and impact strength was obtained in the 50:50 PET/PBT blend. They specified that this synergistic improvement in mechanical properties was due to the co-continuous morphology observed in the 50:50 PET/PBT mixture. Baxi et al. [52] investigated the effect of different concentrations of different modifiers on the performance increase of 60:40 ratio PET/PBT blends. In the SEM analysis, reinforcement and network formation in the matrix of impact modifiers (Elvaloy AC and polypropylene copolymer B220 MN) were observed. These properties resulted in the highest impact strength with an increase of approximately 163% compared to the unmodified PET/PBT blend. 2.3.4 Rheological properties of PET/PBT blends Firstly, the rheological behaviors of PET/PBT blends, which contain PBT up to 10%, were investigated by Mishra and Deopura [53] by using a capillary rheometer. They reported that adding a small amount of PBT (3% by weight) to PET caused an increase in viscosity due to increased molecular entanglement. They observed a reduction in viscosity when more than 4% by weight of PBT in PET and interpreted the results as the appearance of phase separation. Mikitaev and Borukaev [40] reported that melt flow rate (MFR) values at different PET concentrations in the PET/PBT blends. While the addition of up to 20% by weight PET to PBT did not significantly alter the MFR, further increase in PET content resulted in an increase in MFR values and a reduction in melt strength. They reported that PET has a specific character, acting as a relaxing agent for the polymer matrix expanding the interface boundary. Therefore, the addition of more than 20% by weight of PET may result in a partial conversion of compatibility between PBT and PET [38]. Aravithan and Kale [20] stated that the melt viscosity of PET, PBT, and different PET/PBT blends varies over a wide range of shear rates. It is clearly seen that the viscosity of PET is higher than PBT and all blends show Newtonian behavior at the low shear rate and non-Newton at a high shear rate. The viscosity of all blends varies systematically with composition and this can indicate compatibility between PET and PBT. 20 2.3.5 Compatibilization of PET/PBT blends Blending PET and PBT is a useful way to achieve the enhanced combination of the superior properties of these polymers. On the other hand, unfilled and unreinforced PET/PBT blends have disadvantages such as brittleness, relatively low mechanical properties, and electrical/thermal resistance. Various coupling agents and chain extenders are widely used to overcome these deficiencies and to give new uses to polymer blends. The most commonly used functional groups are silane, maleic anhydride, methacrylates, and epoxy-containing compounds. These groups can react with the carboxyl or hydroxyl end group of polyesters and form branched structures during reactive melt processing, thereby significantly increasing the process-ability of a wide variety of polyesters. [54-56]. It is an established method to use multifunctional epoxide chain extenders as chemical modifications to prevent deterioration in reactive processes and improve the processing capacity as well as the final properties of thermoplastics. Joncryl ADR 4368 (Figure 2.14) is a multi-functional epoxy-based styrene-acrylic chain extender, and it is expected to react with both hydroxyl and carboxyl groups of polyesters. In their review, Standau and Nofar et al. [54] focused on the reactive behavior of Joncryl ADR and the induced changes in the rheological behavior of polymers caused by this modification. It has been observed that the addition of Joncryl ADR can lead to significant improvements in the shear thinning, shear and elongation viscosity, melt elasticity, storage modulus, melt strength, and strain- hardening behavior of polymers. Figure 2.14: Chemical structure of Joncryl ADR 4368 ® [54]. 21 Novelty of the Current Research Investigations on thermal, mechanical, viscoelastic, and rheological properties of either PET/PBT binary blends or PET/PBT composites reinforced with nano- additives or glass fibers exist in the literature. Various chain extenders are widely used to improve the properties of PET/PBT blends. The most commonly used functional groups are silane, maleic anhydride, methacrylates, and epoxy-containing compounds. It is an established method to use multifunctional epoxide chain extenders as chemical modifications to prevent degradation in reactive processes and to improve the processing capacity as well as the final properties of thermoplastics. Differing from the literature, in this thesis study, a multi-functional epoxy-based styrene-acrylic chain extender was used to improve the properties of PET/PBT blends. A commercial chain extender with the trade name Joncryl ADR 4368 ® was used in the blends. The effect of various PET concentrations on PET/PBT binary blends was investigated. In addition, Joncryl was added to the blends by the all- together method. In this method, PBT, PET, and Joncryl were physically mixed and extruded all together from a hopper to the pellets. Extensive studies on crystallization behavior, rheologic properties, and structural properties of the blends were examined. 22 23 3. EXPERIMENTAL Materials A commercial PBT with a 45 cm3/10 min melt volume rate was supplied from Lanxess Deutschland GmbH. PET supplied from Neo Group with an intrinsic viscosity of 0.76–0.02 dL.g−1. Joncryl ADR 4368 ® is a multi-functional epoxy- based styrene-acrylic chain extender was supplied from BASF. The chemical structure and typical properties of Joncryl are given in Figure 2.14; Table 3.1, respectively. Table 3.1: The typical properties of Joncry ADR 4368 ®. Properties Unit Values Glass Transition Temperature oC 54 Molecular Weight - 6800 Epoxy Equivalent Weight g/mol 285 Experimental Design In this thesis study, blending PET and PBT with different ratios (i.e. 75/25, 50/50, and 25/75) and the effect of the addition of Joncryl on neat polymers and blends were investigated. First of all, PBT and PET samples were prepared through extruded without and with 0.75 wt% Joncryl. Codifications and compounding ratios of samples are summarized in Table 3.2. Table 3.2: Codifications of PET and PBT flakes with/without Joncryl. Codification of the samples PET or PBT (wt%) Joncryl (wt%) PET_as recieved 100 0 PET_processed 100 0 PET_CE0.75 99.25 0.75 PBT_as recieved 100 0 PBT_processed 100 0 PBT_CE0.75 99.25 0.75 24 Then, the blends were prepared through all together method. During blending 25, 50, and 75 wt% of PET was mixed with PBT, and chain extender were physically added and extruded all together from a hopper to pellets. An illustration of this processing method was given in Figure 3.1 and codifications in Table 3.3. Figure 3.1: Schematics of all together processing methods through blending. Table 3.3: Compounding ratios and codifications of PBT/ PET blends. Codification of the samples PBT (wt%) PET (wt%) Chain Extender (wt%) 2575 25 75 0 5050 50 50 0 7525 75 25 0 2575_0.75CE 25 75 0.75 5050_0.75CE 50 50 0.75 7525_0.75CE 75 25 0.75 The disk shape rheological samples (25mm diameter and 1.5mm thickness) were prepared through compression molding at 260 °C for 5 minutes by gradually increasing the pressure up to 1.5 tons. After 5 minutes, the samples were fast cooled with water before being taken out from the mold. Before molding, pellets were dried under vacuum overnight at 60℃. When preparing the samples, 260°C was preferred which PBT and PET can work without degradation. Since the rapid degradation occurs for polyesters in the air atmosphere, preparation of samples and rheological measurements were carried out under a nitrogen atmosphere. Characterization 3.3.1 Differential scanning calorimetry (DSC) Perkin Elmer DSC 4000 system (see in Figure 3.2) was used to thermal studies of the samples that related to the melting and cooling and analysis were occurred under 50 ml/min flow rate of inert nitrogen gas. Thermal analysis of the samples was carried out by heating the samples from 30 to 300 °C, then cooling them to 30 °C, and then 25 heating them back to 300 °C during a second scan. Further studies were carried out on the crystallization behavior of the samples by performing DSC analyses at cooling rates of 2, 5, 10, and 20 °C/min, keeping the first and second heating rates constant at 5 °C/min. Figure 3.2: Perkin Elmer DSC 4000 system device. 3.3.2 Rheological analysis Rheological measurements were performed at 260°C with the MCR-301 rotational rheometer equipped with a parallel-plate geometry (plate diameter 25mm) with a gap of 1 mm. (Anton Paar, Austria) shown in Figure 3.3. The behavior of the material at a fixed frequency, amplitude, and temperature is determined by the time sweep test to observe the material's thermal stability over time. Due to the sensitivity of PET and PBT to thermal degradation, the time sweep test was carried out at 260℃ using strain amplitude of 0.05 and frequency 1 rad/s to determine thermal stability. The frequency scanning test defines the deformation interval of the material that occurs over time. At high frequencies, the material is given a short time period, while at low frequencies, deformations are determined for much longer periods. In this way, the behavior of the polymers and their stability are determined in the long or short term. The frequency sweep test was applied at 260 ℃ starting from ω = 628 rad / s and decreasing to 0.1 rad/s. 26 Figure 3.3: MCR-301 rotational rheometer (Anton Paar, Austria). 3.3.3 Fourier transform infrared (FTIR) spectroscopy analysis FTIR spectra of the PET and PBT samples and PET/PBT blends with and without CE were recorded at room temperature in the mid-IR range (400–4000 cm−1) using a Bruker FTIR spectrometer equipped with a Bruker Platinum ATR accessory. Sample measurements were conducted on dried powdered samples. Each spectrum was taken over 12 scans with a resolution of 2 cm−1. The results were analyzed using OPUS software (Bruker Optics). The FTIR device is shown in Figure 3.4. Figure 3.4: An example of an FTIR device. 27 4. RESULTS and DISCUSSION Thermal and Crystallization Behavior The thermal behavior of the materials was accomplished by applying heating, cooling, and heating cycles by using DSC analysis. During the DSC analysis, the first and second heating rates were fixed at 5°C/min and four different cooling rates were applied. A summary of DSC conditions is given in Table 4.1. Table 4.1: A summary of DSC conditions. First Heating Cooling Second Heating (°C/min) (°C/min) (°C/min) 1st Condition 5 2 5 2nd Condition 5 5 5 3rd Condition 5 10 5 4th Condition 5 20 5 Based on the information obtained from the DSC results, the crystallinity degree (Xc) for the samples was calculated according to Equation (1) below. 𝑋𝑐 (%) = (ΔH𝑃𝐵𝑇 − ΔH𝑐𝑐𝑃𝐵𝑇) + (ΔH𝑃𝐸𝑇 − ΔH𝑐𝑐𝑃𝐸𝑇) (𝑤𝑃𝐵𝑇 ∗ ΔH𝑃𝐵𝑇 ° ) + (w𝑃𝐸𝑇 ∗ ΔH𝑃𝐸𝑇 ° ) 𝑥100 4.1.1 Crystallization behavior of PET and PBT First heating, cooling, and second heating curves with 5°C/min of as received and processed both of PBT and PET materials are shown in Figure 4.1. The transition temperatures and calculated crystallinity are given in Table 4.2. The glass transition temperatures of samples did not affect by processing. At the first heating, the PBT samples showed a single melting peak in both cases. While the as received PET sample showed two shoulders, the processed PET sample showed a single melting peak. In addition, it was observed that PET showed cold crystallization behavior after processing. The cooling curves revealed that the crystallization temperatures of both PET and PBT increased after processing. The molecular weight of the polymers could have decreased after processing due to thermal degradation which at the end 28 increased the crystallization temperature since the smaller molecules have higher mobility. In the second heating thermograms of PETs, no cold crystallization peaks were observed due to the slow cooling rate. The melting temperature and the crystallinity of PET slightly increased after processing which could be resulted from the decreased molecular weight after processing which again increased the perfection of crystals due to increased chain mobility. PBT showed a double melting peak after processing which could be attributed to the presence of crystals with different perfection degrees. Figure 4.1: First heating, cooling, and second heating thermograms of processed PET (a) and PBT (b). Table 4.2: Thermal properties of processed PET and PBT. Materials Tg Tccr ΔHccr Tmlow Tmhigh ΔHm Tcr ΔHcr Tmlow Tmhigh ΔHm Xc (oC) (oC) (J/g) (oC) (oC) (J/g) (oC) (J/g) (oC) (oC) (J/g) (%) PET_as received 79 - - 232 249 52 170 21,5 - 246,8 29 20,7 PET_processed 77 116 27,5 - 251 49 192 39,8 - 248,5 31 22,1 PBT_as received 53 - - - 225 49 186 46,5 - 224,5 43 29,6 PBT_processed 52 - - - 225 37 195,5 39,7 215 225 32 22,1 As stated in Figure 4.2 and Table 4.3, the addition of chain extender into PET and PBT did not affect glass transition temperatures distinctly. The chain extended PET showed a cold crystallization peak similar to that of neat PET. Based on the first heating cycles it can be said that the addition of CE to PBT did not affect melting behavior distinctly. On the other hand, the addition of CE to PET lowered the melting temperature slightly but decreased the cold crystallinity. The chain extended cooling curves showed that the addition of chain extender slightly increased the crystallization temperature of PBT, but did not change the crystallization temperature of PET. This may be due to inhibition of crystallinity due to the lower mobility of the branched molecules. However, when the second heating cycles were examined, it was seen in the second heating graphs that the 29 addition of CE did not affect the melting temperatures much, but increased the crystallinity of PET somewhat. The melting temperature probably did not increase due to the presence of branched molecules preventing perfect crystals from being obtained. However, the increased crystallinity indicates that the crosslinking points can act as crystal nucleation points and increase the overall crystallinity. Figure 4.2: First heating, cooling and second heating thermograms of chain extended PET (a) and PBT (b). Table 4.3: Thermal properties of chain extended PET and PBT samples. Materials Tg Tccr ΔHccr Tmhigh ΔHm Tcr ΔHcr Tmlow Tmhigh ΔHm Xc (oC) (oC) (J/g) (oC) (J/g) (oC) (J/g) (oC) (oC) (J/g) (%) PET_processed 77 116 27,5 251 49 192 39,8 - 248,5 31 22,1 PET_0.75CE 78 115 21,6 249,6 36 192 29,7 - 248,1 34 24,3 PBT_processed 52 - - 225 37 195,5 39,7 215 225 32 22,1 PBT_0.75CE 50 - - 224,5 33 198,4 35,5 215,5 223 26 17,9 Different cooling rates of 2, 5, 10, and 20 °C/min were applied to all materials with a constant 5°C/min heating rate to investigate the crystallization behaviors in cooling cycles and the effect of cooling rate on thermal properties in second heating cycles. Faster cooling rate resulted in a decrease in crystallization temperature of PET_processed, PBT_processed, and their chain extended versions (Figure 4.3 and Table 4.4). This result could be attributed to the fact that the molecules couldn’t have enough time to align and crystallize when cooled rapidly. Therewithal, it is important to attention that the addition of a chain extender enables PET to crystallize even at a very high cooling rate. For example, with the highest cooling rate as received PET couldn’t crystallize at all and the processed PET revealed a crystallization at 164oC while the chain extended PET showed a crystallization peak at 171oC. In the case of PBT, however, chain extender addition did not show the same effect, since PBT already has very high crystallization kinetics. The addition of a chain extender could prevent chain 30 movement. In second heating cycles, double crystal melting peaks turned into a single peak with increased cooling rates. Possessing a lower time for crystal growth at higher cooling rates could cause a more homogeneous crystallization with a single-crystal melting peak. With the increased cooling rate, the Tm of the PET samples increased. Figure 4.3: DSC curves of cooling (a, c, e, g) and second heating (b, d, f, h) cycles of PET and PBT samples. 31 Table 4.4: Thermal properties of PET and PBT samples w/wo CE at different cooling rates. Materials Tcr ΔHcr Tccr ΔHccr Tmlow Tmhigh ΔHm Xc (oC) (J/g) (oC) (J/g) (oC) (oC) (J/g) (%) PET_as recieved_2 184 27,2 245,4 29,5 21,1 PET_as recieved_5 170 21,3 247 28,7 20,5 PET_as recieved_10 155 27,8 150 15,8 247 26,8 19,1 PET_as recieved_20 - - 154 18,7 246 24,1 17,2 PET_processed_2 202 29 241 30,3 21,6 PET_processed_5 192 39,8 248,5 30,5 21,8 PET_processed_10 183,5 48,9 249 35,3 25,2 PET_processed_20 164 18,8 130,6 2,5 249 29,5 19,3 PET_CE0.75_2 201 34,3 240,4 27,5 19,6 PET_CE0.75_5 192 29,7 238,5 248 34,1 24,4 PET_CE0.75_10 184 29,8 248 36,0 25,7 PET_CE0.75_20 171 21,6 248 30,1 21,5 PBT_as recieved_2 194 35,8 225 30,0 20,7 PBT_as recieved_5 186 46,5 224,5 42,8 29,5 PBT_as recieved_10 179,5 34,5 224 40,0 27,5 PBT_as recieved_20 171,5 38,2 224 33,7 23,3 PBT_processed_2 201 50,5 215 225 47,0 32,4 PBT_processed_5 195,5 39,7 215 225 31,9 22,0 PBT_processed_10 191 38,7 213 225 28,9 20,0 PBT_processed_20 186 44,2 225 38,3 26,4 PBT_CE0.75_2 199 43,6 216 223,5 36,5 25,2 PBT_CE0.75_5 198,5 35,5 216 223 26,5 18,3 PBT_CE0.75_10 190,5 38 213,5 223,5 28,8 19,8 PBT_CE0.75_20 186 38,8 223,5 33,5 23,1 4.1.2 Crystallization behavior of PET/PBT blends Figure 4.4 shows the heat/cool/heat cycles of PET/PBT blends under heating and cooling rates of 5oC/min. As shown in Figure 4.4 and the values in Table 4.5 all blends at different ratios either with or without chain extender showed a composition-dependent single glass transition temperature in both first and second heating cycles. This result means that PET/PBT blends show miscibility in the amorphous state, as stated in the literature. As seen in Figure 4.4, the first heating cycle of blends showed individual melting temperatures nearly the same as the original components. In the case of 75/25, due to the low PBT content, only one Tm peak was observed that correspond to PET. Increasing PET concentration decreased crystallinity in cooling graphs due to lower crystallization of PET. When the samples were cooled slowly, crystallization of the entire blend 32 occurred more slowly and at lower temperatures than in the neat samples. During the second heating, PET/PBT blends with weight ratios of 25/75 and 75/25 revealed a single but much lower melting peak compared to that of PET and PBT. This could indicate the miscibility of crystalline regions of PET and PBT when slowly cooled and the formation of less perfect crystals due to co-crystallization. Moreover, the melting point of the blend systems decreases continuously with increasing PBT content since PBT has a long and flexible butylene chain and could increase chain mobility. The PET/PBT blend with the weight ratio of 50/50 revealed double melting peaks where the melting temperatures of PET and PBT were closed to each other. Moreover, the amount of crystallinity of this blend was found to be higher compared to that of neat polymers and other blends. Figure 4.4: First heating, cooling and second heating thermograms of PET/PBT blends. Table 4.5: Thermal properties of PET/PBT blends. Materials Tg Tcr ΔHcr Tccr ΔHccr Tmlow Tmhigh ΔHm Xc (oC) (oC) (J/g) (oC) (J/g) (oC) (oC) (J/g) (%) PBT_processed 52 195,5 39,7 - - 215 225 31,8 22,06 PET_processed 77 192 39,8 - - - 248,5 30,5 22,14 2575 49 170,5 35,7 - - - 206 27,35 19,06 5050 182 53,97 - - 210,7 228,3 42,98 30,17 7525 64 162,64 20,16 130,89 1,98 - 230,82 24,79 16,15 33 At a constant chain extender concentration of 0.75 wt%, materials showed higher crystallinity than those of blends without chain extender. Branched molecules could act as nucleating points and promote crystallization. The crystallization temperature of the 5050_0.75CE shifted to low temperature indicating that crystallization was affected by transesterification reaction. As a result of transesterification, the closepackness of the crystals decreased due to decreased mobility and affinity thus the Tm of the blend decreased. In addition, in contrast to the 50/50 case, 50/50_0.75CE showed a single melting peak in the second heating cycle, indicating that CE increased miscibility in the crystalline region. The addition of a chain extender generally reduced the melting temperatures of the blends. The first heating, cooling, and second heating curves of the blends are shown in Figure 4.5. The thermal properties of overall blends are listed in Table 4.6. Figure 4.5: First heating, cooling and second heating thermograms of chain extended PET/PBT blends 34 Table 4.6: Thermal properties of chain extended PET/PBT blends. Materials Tg Tcr ΔHcr Tccr ΔHccr Tmlow Tmhigh ΔHm Xc (oC) (oC) (J/g) (oC) (J/g) (oC) (oC) (J/g) (%) 2575 49 170,5 35,7 - - - 206 27,4 19,1 5050 182 53,9 - - 210,7 228,3 42,9 30,2 7525 64 162,6 20,2 130,9 1,9 - 230,8 24,8 16,2 2575_0.75CE 48 162,5 38,5 196,6 23,3 16,2 5050_0.75CE 54 129,2 17,9 100,1 1,5 182,7 18,7 12,1 7525_0.75CE 64 163 35,6 226 27,5 19,1 Different cooling rates of 2, 5, 10, and 20 °C/min were applied to all materials to investigate the crystallization behaviors in cooling cycles and the effect of cooling rate on thermal properties in second heating cycles. Thermal properties of blends against varying cooling rates are in Table 4.7 and shown in Figure 4.6. A faster cooling rate resulted in a decrease in crystallization temperature for all blends. Despite the increased cooling rate for the 75/25, a clear crystallization peak could not be obtained because the crystals did not have enough time to align. Unlike other blends, for 50/50, the crystallization temperature increased with increasing cooling rate from 2°C to 5°C. This increase may be due to the more closed-packed structure and the increased chance of separate crystallization of PBT and PET at earlier temperatures. In other words, the chances of crystalline phase immiscibility may increase with cooling rate, similar to what was shown previously. This separate crystallization of PBT and PET was more pronounced in mixtures without CE. The addition of a chain extender increased the melting temperatures of the blends for all cooling rates. However, with the addition of a chain extender enabled the crystallization of blends with higher PET even for high cooling rates. In the presence of CE, during slow cooling, more vigorous co-crystallization may occur with a more combined structure, as CE can juxtapose both PBT and PET molecules to form an ordered structure. 35 Figure 4.6: DSC curves of cooling (a, c, e, g) and second heating (b, d, f, h) cycles of blends. 36 Table 4.7: Thermal properties of blends against varying cooling rates. Materials Tcr ΔHcr Tccr ΔHccr Tmlow Tmhigh ΔHm Xc (oC) (J/g) (oC) (J/g) (oC) (oC) (J/g) (%) 2575_2 173,5 43,0 203,11 31,8 22,1 2575_5 170,5 35,7 206,2 27,4 19,1 2575_10 166,0 36,0 207,38 25,7 17,8 2575_20 156,6 27,0 205,15 28,3 19,7 5050_2 168,5 51,0 197,62 213,17 41,3 30,0 5050_5 181,9 54,0 210,74 228,95 43,0 30,2 5050_10 178,4 60,4 213,28 232,87 30,4 24,6 5050_20 174,9 66,8 216,14 237 35,1 24,5 7525_2 166,6 20,7 224,49 19,0 13,5 7525_5 162,6 20,2 130,89 1,98 230,82 24,8 16,2 7525_10 146,8 26,5 125,48 4,71 220,33 231 26,2 15,2 7525_20 125,65 14 232,14 25,6 8,2 2575_CE0.75_2 168,7 97,4 194,89 21,6 15,0 2575_CE0.75_5 162,5 38,5 196,62 23,3 16,2 2575_CE0.75_10 158,3 30,2 199,55 23,9 16,6 2575_CE0.75_20 154,3 40,6 201,2 39,8 27,7 5050_CE0.75_2 125,2 26,0 174,8 20,1 14,1 5050_CE0.75_5 129,2 18,0 182,73 18,7 12,1 5050_CE0.75_10 128,0 22,9 189,97 26,4 16,8 5050_CE0.75_20 117,3 18,1 201,71 31,5 16,2 7525_CE0.75_2 161,8 22,1 215,56 18,6 13,2 7525_CE0.75_5 163,0 35,6 226 27,5 19,5 7525_CE0.75_10 148,3 16,4 125,73 4,17 225,56 17,3 9,3 7525_CE0.75_20 121,0 12,8 228,88 30,6 21,6 Rheological Behavior 4.2.1 Time sweep test The time sweep test was carried out for processed and chain extended PET and PBT samples at 260℃ using the constant frequency 1 rad/s to observe the time effect on thermal stability. The complex viscosity (η*) versus time (t) values obtained from the time sweep measurements of the samples are in Figure 4.7 and complex viscosity changes during the time sweep test are reported in Table 4.8. 37 As shown in Figure 4.7 (a), the complex viscosity of PET decreased after processing. The complex viscosity of as received and processed PET increased around 15,7% and 18% after 20 min time sweep experiments, respectively. This increase in complex viscosity may result from polycondensation reactions between the internal and terminal functional groups of PET molecules [43, 53]. As seen in the figure, the complex viscosity of PET increased with the addition of 0.75 wt% CE due to the induced branched structure. Also, the complex viscosity of PET_0.75CE continued to increase during the experiment. There may be two different reasons for the increased viscosity of PET; polycondensation PET and resumption of reaction between epoxy groups with hydroxyl and carboxyl end groups of PET during rheological tests. On the other hand, as shown in Figure 4.7 (b), the continuous decrease of the complex viscosity of as received and processed PBT with time can clearly indicate a severe thermal degradation during time sweep experiments. As seen, the complex viscosity of PBT decreased around 15,2% and 10,2% after 20 min. Such different thermal stability behaviors in PET and PBT samples may indicate that although polycondensation and thermal degradation are present simultaneously in both polyterephthalates, the polycondensation reaction is more pronounced among PET molecules than their thermal degradation. The addition of 0.75wt% of CE increased the complex viscosity of the PBT. However, after 20min of time sweep experiment at 260C, a decrease of 62,2% in complex viscosity was observed. While the complex viscosity after 20min was still higher than that of as received PBT, the percent decrease in complex viscosity of chain extended sample was higher when compared to the percent decrease in complex viscosity of processed PBT. Figure 4.7: Time sweep test results of PET (a) and PBT (b) samples at 260℃. 38 Table 4.8: Complex viscosity changes of PET and PBT samples during time sweep test after 20 min. Sample Name Complex Viscosity Changes (%) PET_as received 15,7 PET_processed 18 PET_0.75CE 41,8 PBT_as recieved -15,2 PBT_processed -10,2 PBT_0.75CE -62,2 Figure 4.8 shows a possible chemical reaction between polyterephthalates and a general epoxide. Figure 4.8: Chemical representation of chain extension reaction of polyterephthalates with a generic epoxide [40]. To observe how the thermal stability of PET/PBT blends would change, a time sweep test was performed at 260℃, which was chosen as the optimum temperature for all materials. The complex viscosity graphs of PET/PBT polymer blends are shown in Figure 4.9. The viscosity of all blends changed depending on the PBT ratio of the blends, and the complex viscosity value increased as the PBT ratio increased due to the higher viscosity of PBT (Table 4.9). In addition, the complex viscosity of the blends with a high PBT ratio decreased during the test, while the complex viscosity of the blends with a high PET ratio increased with the addition of a chain extender. Complex viscosities were found to increase when a chain extender was added, due to increased thermal stability. Moreover, the complex viscosity of the chain extended blends continued to increase during the test. Again, this could be due resumption of chain extender reactions, polycondensation, and transesterification reactions. [41, 57] The curvy behavior of the chain extended blends and the 75/25 39 PET/PBT blend without chain extender shows that there is a competition between the thermal degradation and the previously mentioned reactions. Figure 4.9: Time sweep test results of PET/PBT blends at 260℃. Table 4.9: Complex viscosity changes of PET/PBT blend samples during time sweep test after 20 min. Sample Name Compex Viscosity Changes (%) 2575 -11,7 5050 -23,6 7525 25,5 2575_CE -7,1 5050_CE 16,9 7525_CE 39,3 4.2.2 Frequency sweep test Frequency sweep test was applied to start from ω = 628 rad / s and decrease to 0.1 rad / s. A frequency sweep test was carried out at 260 ℃ for all samples. The complex viscosity (η*) values obtained from the frequency sweep measurements of the PET and PBT samples are reported in Figure 4.10 and complex viscosity changes are indicated in Table 4.10. According to test results, processing of PET reduced the complex viscosity and the addition of CE couldn’t compensate for this decrease much. During the test, the viscosity of as received and chain extended samples increased, while the viscosity of processed sample decreased. Additionally, all PET samples nearly showed Newtonian behavior. 40 As seen in Figure 4.10 (b), a small decrease was observed in the viscosity of the sample processed at low frequencies, while the addition of a chain extender increased the viscosity. The as received and processed PBT samples showed an almost Newtonian behavior throughout the test, while the CE-added sample showed significant Non-Newtonian behavior. This shows that the addition of CE improves the rheological properties of PBT and facilitates its processing by increasing the shear thinning property. Figure 4.10: Frequency sweep test results of PET (a) and PBT (b) samples at 260℃. Table 4.10: Complex viscosity changes of PET and PBT samples during frequency sweep test. Sample Name Compex Viscosity Changes (%) PET_as recieved 20,0 PET_processed 25,7 PET_0.75CE 84,3 PBT_as recieved -17,5 PBT_processed 6,5 PBT_0.75CE -0,8 Frequency sweep test results of PET / PBT blends are given in Figure 4.11. In the samples without CE, as the PBT content increased the complex viscosity of the blends increased over the entire frequency range. Also, in blending with 0.75% by weight PET, the possible polycondensation in PET and thermal degradation in PBT could have offset each other's influence as complex viscosity towards low frequency seemed almost stable. As Figure 4.11 shows, the addition of 0.75 wt% CE improved the complex viscosity of all blends it enhanced the complex viscosity and the thermal stability of the blends with higher PBT content more effectively (Table 4.11). While the blends showed Newtonian behavior without the addition of a chain extender, they showed non- 41 Newtonian behavior with the addition of a chain extender. In addition, the highest viscosity increase was observed in the 7525_0.75CE. The increased shear-thinning behavior and complex viscosity upturn at low frequency is an indication of chain branching. However, it is also important to note that during the frequency sweep tests, the increase of complex viscosity also results from the polycondensation, transesterification, and resumption of chain extender reactions consistent with the time sweep tests. Figure 4.11: Frequency sweep test results of PET/PBT blend samples at 260℃. Table 4.11: Complex viscosity changes of PET/PBT blend samples during frequency sweep test. Sample Name Compex Viscosity Changes (%) 2575 8,0 5050 -6,0 7525 -15,3 2575_CE 64,3 5050_CE 49,4 7525_CE 28,4 Structural Properties 4.3.1 FTIR analysis FTIR analysis was performed to observe the changes in the samples. The main chain extension reactions proceed between the -OH functional groups of polyesters and the epoxide functional groups of CE. Therefore, the intensities of the functional primary alcohol -OH bending and -C-O- stretching signals were compared with the normalized C=O stretch band at 1712 cm-1. As seen in the Figure 4.12, the signal 42 intensities of both the -OH bending band at 1344 cm-1 and the -C-O- stretching band at 978 cm-1 decreased as a result of the chain extension reaction occurring during processing in PET samples containing 0.75% CE by weight. Figure 4.12: FTIR results of processed and chain extended PET and PBT samples. As seen from Figure 4.13, at 1344 cm-1, the -OH bending signal was almost absent in the PET/PBT (25w/75w) mixture, whereas it appeared more clearly in the PET/PBT (75w/25w) sample. This can be attributed to the larger internal reactive sites present in blends with higher PET content. The addition of 0.75% CE by weight to the PET/PBT (50w/50w) mixture also shows a structural change from a wide to a sharper small peak at 978 cm-1, which is an indication of chain extension reactions. Figure 4.13: FTIR results of PET/PBT blend samples. 43 5. CONCLUSION In this thesis study, PET/PBT binary blends at different ratios were investigated. A chain extender with a trade name of Joncryl ADR 4368® was used to improve the blend properties. The addition of chain extender was carried out with all together mixing method. Crystallization properties, rheological behavior, and structures of the materials were investigated. The addition of a chain extender slightly increased the crystallization rate of PBT, but did not change the crystallization rate of PET and did not affect the melting temperatures much, but increased the crystallinity of PET somewhat. Faster cooling rate resulted in a decrease in crystallization temperature of both processed PET and PBT, and their chain extended versions. This result could be attributed to the fact that the molecules couldn’t have enough time to align and crystallize when cooled rapidly. In second heating cycles, double crystal melting peaks turned into a single peak with increased cooling rates. Possessing a lower time for crystal growth at higher cooling rates could cause a more homogeneous crystallization with single-crystal melting peak. All blends at different ratios either with or without chain extender showed a composition-dependent single glass transition temperature in both first and second heating cycles. This result means that PET/PBT blends show miscibility in the amorphous state, as stated in the literature. In the case of 7525, due to the low PBT content, only one Tm peak was observed that correspond to PET. In the 50/50 PET/PBT blend, during the second heating, the melting temperatures of PBT and PET got closer after slow cooling. In the case of 25/75 and 75/25 PET/PBT blends, during the second heating single melting peak was observed which is due to simultaneous crystallization of PET and PBT during slow cooling. In agreement with the literature, this indicates that the crystalline regions of PET and PBT are miscible when slowly cooled. At a constant chain extender concentration of 0.75 wt%, materials showed higher crystallinity than those of blends without chain extender. Branched molecules could act as nucleating points and promote crystallization. The crystallization temperature of 44 the chain extended 5050 shifted to low temperature indicating that crystallization was affected by transesterification reaction. In addition, in contrast to the 50/50 case, 50/50_0.75CE showed a single melting peak in the second heating cycle, indicating that CE increased miscibility in the crystalline region. The addition of a chain extender generally reduced the melting temperatures of the blends. A faster cooling rate resulted in a decrease in crystallization temperature for all blends. Unlike other blends, for 50/50, the crystallization temperature increased with increasing cooling rate from 2°C to 5°C. This increase may be due to the more closed- packed structure and the increased chance of separate crystallization of PBT and PET at earlier temperatures. The complex viscosity of PET increased with the addition of 0.75 wt% CE due to the induced branched structure. Also, the complex viscosity of chain extended PET continued to increase during the experiment. There may be two different reasons for the increased viscosity of PET; the occure polycondensation reactions, and residence time in the extruder is probably too short for a complete reaction between Joncryl and PET, and epoxy groups continue to react with hydroxyl and carboxyl end groups of PET during rheological tests. The continuous decrease of the complex viscosity of PBT samples with time can clearly indicate a severe thermal degradation during time sweep experiments. Complex viscosities were found to increase when chain extender was added, and this increase was more pronounced in blends with higher PBT ratios. In addition, the complex viscosity of the blends with a high PBT ratio decreased during the test, while the complex viscosity of the blends with a high PET ratio increased with the addition of a chain extender. 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