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Title: Floresans Ve Dilatometrik Teknik Kullanılarak Stiren (s) Ve Divinilbenzen (dvb)'in Serbest Radikal Zincir Kopolimerizasyonu İle Ağ Yapı Oluşumunun İncelenmesi
Other Titles: Free-radical Crosslinking Copolymerization Of Styrene And Divinylbenzene Using Both Fluorescence And Dilatometric Technique
Authors: Pekcan, Önder
Kaya, Demet
Fizik Mühendisliği
Physics Engineering
Keywords: Ağ yapısı
Serbest radikaller
Network architecture
Free radicals
Issue Date: 1998
Publisher: Fen Bilimleri Enstitüsü
Institute of Science and Technology
Abstract: Bu çalışmada, kararlı durum floresans tekniği ve hacim azalması esasına dayanan dilatometrik yöntem kullanılarak, değişik sıcaklık ve çapraz bağlayıcı oranlarında, bulk olarak stiren (S) ve teknik divinilbenzen (D VB) 'nin serbest radikal zincir kopolimerizasyonu ile ağ yapı oluşumu incelenmiştir. S-DVB kopolimerizasyonunda floresans teknikle uyarılmış aromatik molekülün emisyon şiddeti takip edilerek jel etkisinin başlangıç noktası ve dilatometrik yöntem ile de hacim küçülmesi izlenerek jel noktası ve bu noktaya kadar olan monomer dönüşümü sıcaklık ve çapraz bağlayıcının bir fonksiyonu olarak incelenmiştir. Ayrıca floresans teknikle yüksek çapraz bağlayıcı konsantrasyonları ve düşük sıcaklıklar için, ortamda iki farklı konsantrasyona sahip iki farklı jel fazının oluşması ile floresans ışığın saçılarak oluşturduğu kritik opalescence gözlenmiştir. Tez, giriş niteliğindeki ilk bölüm dahil olmak üzere toplam beş bölümden oluşmaktadır. İkinci bölümde j ellerin sentez yöntemlerinden serbest radikal zincir kopolimerizasyonu ve kinetiği ana hatları ile incelenmiştir. Üçüncü bölümde kullandığımız floresans teknik, dördüncü bölümde floresans ve dilatometrik tekniği kullanarak yaptığımız deneyler ve elde edilen sonuçlar anlatılmakta, beşinci ve son bölümde ise elde edilen sonuçlar hakkında kısa bir yorum yapılmaktadır
A gel is a form of matter intermediate between a solid and a liquid. It consists of polymers, or long-chain molecules, crosslinked to create a tangled network and immersed in a liquid medium. The properties of the gel depend strongly on the interaction of these two components. The liquid prevents the polymer network from collapsing into a compact mass; the network prevents the liquid from flowing away. Depending on chemical composition and the other factors, gels vary in consistency from viscous fluids to fairly rigid solids, but typically they are soft and resilient or in a word, jellylike. Gels are classified by the strength of the cross-linkages. Some gels are cross-linked chemically by covalent bonds, whereas other gels are cross-linked physically by weak forces such as hydrogen bonds, van der Waals forces, or ionic interactions. An example of a physically crosslinked gel is jello, whereas examples of chemically crosslinked gels are the polystyrene and polyacrylamide gels. It is convenient to clasify the chemical gels in to two main categories depending on their synthesis mechanism as first suggested by Carothers in 1929. One is step-growth polymerization which is often called condensation polymerization since it is almost exclusively concerned with condensation reactions taking place between multifunctional monomer molecules. The other category which is concerned with our present work is additional polymerization where the monomer molecules add on to a growing chain one at a time. Monomers for additional polymerization normally contain double bonds. The double bond is susceptible to attack by either free radikal orionic initiators to form a species known as an active centre. If the active centre is free radical, it is called "free radical additional (chain) polymerization". Free radicals are species contain unpaired electrons. They are extremely reactive and will react with monomers containing double bond to form an active centre which is capable of reacting with further monomer molecules to give a macromoleculer chain. Free radical chain polymerization reaction takes place in three steps; initiation, propagation and termination. The initiation reactions takes place in two stages. First of all, the initiator molecules decompose to form free radicals and then the radicals react with monomer molecules to form active centre. Chain propagation takes place by the rapid addition of monomer molecules to the growing chain. The average life times of the growing chain are extremely short and several thousand additions can take place within a few seconds. This means that there may be a monomer addition every few miliseconds to each growing chain. The most important mechanisms of termination are when two growing chains interact with each other and become mutually terminated by_x000B_one of two specific reactions. One of these reactions is combination where the two growing chains join together to form a single polymer molecule. Alternatively a hydrogen atom can be transferred from one chain to the other in a reaction known as disproportional. The gel effect also called the Trommsdorf effect in free-radical polymerization of vinyl monomers is a well-known phenomenon that is accompanied by an increase in both rate and degree of polymerization. Analysis of the gel effect has been the subject of continued investigations for many years. Methyl methacrylate (MMA) polymerization in bulk also shows a very pronounced gel effect caused by the diffusion control of the termination reaction. This is a result of increased viscosity of the reaction solution of poly(methyl methacrylate) (PMMA) in MMA monomer. Thus, during the free radical polymerization of MMA, the monomer conversion first increases only slightly but then it accelerates due to the gel effect. Compared to MMA, the diffusion control of the termination reactions and the resulting gel effect is less obvious in styrene polymerization. In the present work, we attempt to study the free-radical crosslinking copolymerization (FCC) of styrene (S) and commercial divinylbenzene (DVB) using both the steady state fluorescence and dilatometric techniques. Our aim was primarily to answer the question, which critical time - the time required for the onset of gelation or that for the onset of the gel effect - can be monitored using the steady-state fluorescence technique in S - DVB copolymerization. The copolymerization reactions were carried out in bulk using the monomers S and commercial DVB, a mixture of para- and meta- isomers of DVB and ethylstyrene, at various temperatures and with various amounts of DVB as the crosslinker. Benzoyl peroxide and pyrene (Py) were used as the initiator and the fluorescence probe for the in situ polymerization experiments, respectively. The monomer conversions up to the onset of gelation and the gel points were recorded by dilatometry, whereas the critical times required for a drastic increase in the fluorescence intensity of Py were monitored by the in situ fluorescence experiments. In this work, mainly two sets of FCC experiments were performed; in the first set, different DVB content in the range from 0.4 to 12 mole % was used for each FCC experiment at a constant temperature (70°C). In the second experimental set, FCC reactions were performed separately at various temperatures between 60 and 90°C for constant DVB content (12 mol %). In both sets of experiments gelation was monitored against the reaction time /. Pyrene (Py) was used as a fluorescence probe for the in situ steady-state polymerization experiments, where styrene and mobile polymer chains act as an energy sink for the excited Py during polymerization. Later, the formation of bulk polystyrene provides an ideal, unchanged environment for the excited Py molecules. Naturally, from these experiments one may expect a substantial increase in fluorescence intensity, I, of Py at a certain time interval. For the fluorescence measurements, reaction mixtures were transferred into round glass tubes of 15 mm internal diameter and they were deoxygenated by bubbling nitrogen for 10 min. The copolymerization of S and DVB was performed in the fluorescence accessory of spectrometer. The Py molecule was excited at 363 nm during the in situ experiments, and the variation in the fluorescence emission intensity, I, was monitored with the time-drive mode of the spectrometer, by staying at the 393 nm peak of the Py spectra. In situ steady-state fluorescence_x000B_measurements were carried out using the Model LS-50 spectrometer of Perkin Elmer, equipped with temperature controller. All measurements were made at 90° position and slit widths were kept at 7 nm. In the floresans experiments, we always observed a sudden increase in the fluorescence intensity of Py after crossing a critical time. Let tr be the time needed for an abrupt increase in the Py intensity. Below tr, since / presents small values, Py molecules are relatively free, they can interact and be quenched by other molecules. However, above tr, I gives large values indicating that the reaction mixture is highly viscous and Py molecules are trapped in a polystyrene network. Normalized and smoothed Py intensities, / versus reaction times are plotted in Figures 1 A and B for various DVB content and temperature, respectively. It is seen that all curves present sudden increase at a given reaction time, and then reach an equilibrium at later times. According to Figures 1 A and B, at low DVB content or at low temperature, the increase in / takes place at longer times, indicating that trapping of Py molecules in the reaction system is delayed. The critical time tr can be determined by taking the first derivative of the experimentally obtained / curve with respect to t. The maximum in dl/dt curve corresponds to d2lldt = 0, i.e., to the inflection point in curve /, which gives the tr, on the time axis. At high crosslinker contents and low temperatures, a spike in the flourescence intensity of Py was observed in the close vicinity of the critical time tr. The spikes were attributed to the appearence of heterogeneities in the reaction system, i.e., to the existence of critical opalescence during gelation curves where two different gel phases having two different concentrations occur. The spike in the Py intensity versus time plots, that is, the critical opalescence dissapered at low DVB content and at high temperature regime during in-situ gelation experiments. Below a critical content of DVB which is found to be 3.2 % at 70° C either no phase seperation occurs or domain sizes of the separated phase are much larger than the wavelenght of the fluorescence light. As a result no spikes Ip were observed in the I versus t curves. On the other hand, above the critical temperature, termal fluctuations prevent the gel, forming two different phases having two different concentrations as a result no spikes on Ip were observed during gelation proces at the critical time tr. Here it is shown that in situ fluorescence method can be used to study the critical opalescence both as function of crosslinker density and temperature._x000B_O 2000 4000 6000 8000 10000 12000 14000 Time / sec Fig.l.a. Normalized and Smoothed Py intensities, I versus reaction times t for various DVB contents_x000B_O 1000 2000 3ÖD0 4000 5000 6000 7000 Time / sec Fig.l.b. Normalized and Smoothed Py intensities, I versus reaction times t for various temperatures_x000B_Dilatometric method were also used to monitor S - DVB copolymerization. The conversion of the monomers up to the onset of gelation i.e the gel points were followed by dilatometry. The dilatometers consisted of a blown glass bulb, approximately 25 mL in volume connected to a 30 cm length of 1.5 mm precision-bore capillary tubing with a ground-glass joint. The meniscus of the polymerizing solution was measured throughout the experiment with a millimetric paper to 0.2 mm. The deviation in the data points between two runs was always less than 3%. For the gel point measurements, dilatometers containing a steel sphere of 4.8 mm diameter were used. The midpoint between the last time at which the sphere moves magnetically and that at which it stops moving is taken as the gel point tc. Figures 2 and 3 shows typical plots of fractional monomer conversion x - versus - time / up to the onset of gelation in S - DVB copolymerization. The experimental data from dilatometry were for various polymerization temperatures and DVB contents. Experimental data also show the drastic dependence of the gel point on the polymerization temperature; at a given DVB content, gelation occurs at a lower conversion but, it requires a longer reaction time as the temperature of experiment is decreased. The tr values are plotted versus DVB content and temperature in Figure 4 and Figure 5, respectively. For comparision, the experimental gel point data,tc obtained by dilatometry are also included in the same figure. One can see that tr does not corresponds to tc. Py molecules are starting to be trapped in the rigid polymer environment much later than at the time of the sol-gel phase transitions. The reaction time at which the Py intensity in the fluorescence spectra exhibits a sudden increase does not corresponds to the gel point but it corresponds to the time for the onset of the gel effect. It was shown that the time at the inflection point in the Py intensity curve matches to the reaction time at which the rate of polymerization becomes maximum due to the gel effect._x000B_.20 40 60 - 80 Time / mîn 100 Fig.2. Monomer conversion X versus time t up to the onset of gelation for various DVB contents._x000B_0.30 0.25 - 0.20 X 0.15 0.10 0.05 - 0.00 i ı i r r 20 40 60 80 100 120 140 160 time / min Fig.3.. Monomer conversion X versus time t up to the onset of gelation for various temperatures._x000B_O 50 I T 100 150 time / min 200 250 300 Fig.4. The tt and tc values are plotted versus DVB contents_x000B_100 200 300 time / min 400 500 Fig.5. The tt and tc values are plotted versus temperatures
Description: Tez (Yüksek Lisans ) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1998
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1998
Appears in Collections:Fizik Mühendisliği Lisansüstü Programı - Yüksek Lisans

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