Poli(stiren-b-etilen oksit) blok kopolimerlerinin sentezi ve karakterizasyonu

Abstract

Termoplas tik işieme tekniklerine uygun, esnek özellik teki blok kopolimerler, termoplas tik elastomerler olarak ad landırılır. Bunlar camsı ve kauçuksu blokların birleşmesinden oluşan kopol imer lerdir. Bu çalışmada termoplastik elastomer nitelikleri taşı yan poli(stiren-b-etilen oksit) blok kopolimerler i hazırlan dı ve bu kopolimerlerin, kimyasal, fiziksel ve mekanik karak- terizasyonu yapıldı. Bu amaçla, kopolimerizasyon deneyleri aşamalı olarak yürütüldü. İlk aşamada, dihidroksil son grup ları bulunan 4000 ve 20000 molekül ağırlıklı polieterler (PEG-4000 ve PEG-20000) bir alifatik diizosiyanat (izoforon diizosiyanat) ile 60 C'de kaplama (capping) reaksiyonuna so kuldu. Bu reaksiyonun sonunda, zincirin iki ucunda izosiyanat bulunan Uretanlar elde edildi. 2, Aşamada, bu Uretanlar t-btl- bil hidroperoksit ile udu mcukl ığıııda, kuLalizür ugllgiııdu ve karanlıkta reaksiyona sokularak prepolimer dikarbamatlar elde edildi. 3. Aşama kopolimerizasyon aşamasıdır. Bu aşama da, prepolimer diperkarbamat peroksidik bir baslatıcı olmak Üzere, stirenin polimerizasyonunda kullanıldı. Peroksikarba- matların baslatıcı yüzdeleri ayarlanarak hazırlanan kopolimer örneklerinin değişik oranlardaki bloklardan oluşması sağlandı. Kopolimerizasyon vakumda blok veya çözelti polimerizasyonu ile sabit sıcaklıkta veya sıcaklık programlaması ile gerçek leştirildi. Bu yöntemle gerçekleştirilen polimerizasyon aşamaları, kimyasal ve spektroskopik deneylerle izlendi ve blok kopoli merlerin molekül yapıları belirlendi. İR spektrumlarmdan da yararlanılan örneklerin blok kopolimer oldukları kanıtlandı. Hazırlanan 12 örneğin stiren ağırlık yüzdelerinin % 96.0 ile % 63.5 arasında değiştiği, UV-analizi ile belirlendi. Bu ör- - II - neklerin viskozite- ve sayı-ortalama molekül ağırlıkları bu lundu. Stiren yüzdesi azaldıkça, molekül ağırlığının azalmak ta olduğu gözlendi. Osraometre ile belirlenen sayı-ortalama molekül ağırlıkları, viskozite-ortalama molekül ağırlıkları ile karşılaştırılarak heterojenlik indisi hakkında bir fikir vermek üzere M /M oranı hesaplandı, ölçülen değerler için bu oranın 1.4 ile 2.3 arasında değiştiği bulundu. Tensilonda yapılan çekme deneyi ile elde edilen geril- me-şekil değiştirme eğrilerinden, örneklerin kopma uzaması, çekme ve kopma mukavemetleri ile Young modülleri hesaplandı. Uzamada artışın % 21 ile % 106 arasında değiştiği belirlendi. Polistiren ile karşılaştırıldığında, stiren yüzdeleri azal dıkça, çekme ve kopma mukavemetleri ile Young modülü değerle rinde düşme olduğu görüldü.

Blends, block and graft copolymers, or "interpenetrat ing" networks have received widespread attention in recent years. All of these materials are usually characterized by the presence of two or more polymeric phases in the solid state. Most homopolymer pairs are immiscible with one another and give rise to low strength materials due to the lack of interfacial adhesion between the separate phases. Block and graft copolymers do offer advantages over blends, because the different segments are covalently bonded together. Block copolymers may vary considerably in molecular structure: diblock, triblock, multiblock, starblock and over lap. Oiblocks are of less interest, but have found applica tions as "emulsif iers" for incompatible polymer blends. The physical states of the blocks in the triblock or multiblock systems are usually either glass-rubber or crystalline rubber because of the utility of thermoplastic elastomers. If ABA type triblock copolymers in which A represents an amorphous hard block and B a soft block have thermoplastic and elastomeric properties in their structure, it has been known as thermoplastic elastomers. - IV - At the beginning the preferred block copolymer synthesis is based on living polymerization technique, which was developed by Szwarc. This type of block copolymers are produced industrially by Shell Chemical Company in 1965. These new products were useful for modern thermoplastics processing techniques, have high resilians, high tensile strength, highly reversible elongation and abrasion resistance. The synthesis by anionic methods have a number of advantages and disadvantages in their use. Tobolsky proposed a new method to synthesize block copolymers via functionally terminated oligomers. The oligo mers containing one or several isocyanate group per chain with a hydroperoxide yielded polymeric peroxycarbamates. These are able to initiate the polymerization of a wide variety of monomers as well as dienes. In this work, the stepwise polymerization technique was followed to synthesize block copolymers. The hydroxyl terminated polyethers (PEG-AOOO and PEG-20000) were capped in excess of an aliphatic diisocyanate (IPDI) to avoid chain extension. HO-(CH2CH20)x-H + n OCN-(C1()H18)-NCO prepolymer diisocyanate 0=C=N-(C.nH1Q)-N C-0-(CH^CH,0)x-C-N-(CinHa)-N=C=0 HO OH urethan V - where, x shows the degree of polymerization of polyether. Subsequently the capping reaction product was reacted with an aliphatic hydroperoxide (t-BHP) in the presence of a catalyst (T-12, dibutyl tin dilaurate) under nitrogen atmosphere in dark at room temperature. In this way 6 poly meric peroxycarbamates were prepared according to the following reaction: OCN-(C10H18)-N-C-O-(CH2CH20)x/2 H 0 + m HOO-C-CH, I CH, urethan t-biltil hydroperoxide CH, H.,C-C~0--0-C-N-(CiriHia)-N-C-0-(CH9CH,,0), J i hi 1 U 1 o i il I I X / I CH, 0 H H 0 ?±2 (Prepolymer dipercarbamate) Finally, the peroxycarbamate was used to initiate the free radical polimer izat ion of styrene. The polymerization reaction were carried out in vacuum with temperature program ming, (in some samples, constant temperature polymerization was used.) By this method, 12 poly (styrene-b-ethylene oxide) copolymers were synthesized. A temperature-programmed polymerization starting at relatively low temperature and ending up at high temperatures would give a wide chain length distribution for vinyl blocks, with the absence of unreacted peroxidic prepolymer. The temperature programming process was based on finding a sui- VI Cable temperature at which the rate of decomposition of peroxide would be approximately equal to the rate of consump tion of the vinyl monomer. The reaction steps were followed by Infrared Spectros copy. The characterization of the first step capping reaction product were also made by isocyanate analysis. The peroxygen contents of peroxycarbamates in the second step were deter mined by iodometric titrations. The agreement between theore tical and experimental values of the peroxygen contents of the peroxycarbamates was also satisfactory. In the characterization of copolymers by their IR spectra, the aromatic absorption pattern at 1495 cm and 1605 cm confirm the presence of polystyrene, while the peaks at 3440 cm and 1730 cm result from the urethan linkage in polyethylene oxide segments. The styrene contents of the copolymers were determined by ultraviolet analysis. The molecular weight determinations of the samples were carried out by viscometric and osmometric techniques. M /M_ ' v n ratios were calculated in order to get an idea about the heterogeneity indices. It was observed that, as the styrene content decreases, percent conversion, viscosity and number average molecular weights also decrease. Mechanical characterization of the copolymers was made on the basis of the stress-strain data obtained from the Tensilon. Tensile strength, ultimate tensile strength, percent elongation and Young's modulus of the copolymers were calculated from their stress-strain curves. While styrene content VII changes the percent elongation varies between 21% and 106%. Due to the decrease in styrene content tensile strength, ultimate tensile strength and Young's modulus also decreases,