Floresans tekniği kullanılarak Sol-Gel faz geçişlerinin incelenmesi

Abstract

Bu çalışmada, kararlı durum floresans tekniği kullanarak, 75°C de toluenli ve toluensiz durumda metil metakrilat (MMA) ve etilen glikol dimetakrilat'ın (EGDM) serbest-radikal çapraz bağlanma kopolimerizasyonu incelenmiştir. Yukarıdaki sistemin jelleşmesi süresince uyarılmış aromatik molekülün emisyon şiddeti takip edilerek sol-gel faz geçişi gözlendi. Sonuçlar "bond percolation" (sızma) teorisi kullanılarak yorumlandı. Kritik üs J3 ve jel noktası pc hem kah hem de çözelti polimerizasyonu için sırasıyla 0.45 ve 0.36 olarak bulundu. Tez, giriş niteliğindeki ilk bölüm dahil beş bölümden oluşmaktadır. İkinci bölümde jelleşme olayını inceleyen Flory - Stockmayer Teorisi (Klasik Teori) ve Sızma Teorisi anahatlarıyla özetlenmiştir. Üçüncü bölümde bu çalışmada kullandığımız floresans teknik, dördüncü bölümde yaptığımız deneyler ve elde edilen sonuçlar anlatılmakta, beşinci ve son bölümde elde edilen sonuçlar hakkında kısa bir yorum yapılmaktadır.

Fluorescence and phosphorescence intensities of aromatic molecules are effected by both radiative and non-radiative processes. If the possibility of perturbation due to oxygen is excluded, the radiative probabilities are found to be relatively independent of environment and even of molecular species. Environmental effects on non-radiative transitions which are primarily intramolecular in nature are believed to arise from a break down of the Bom - Oppenheimer approximation. The role of the solvent in such a picture is to add the quasi-continuum of states needed to satisfy energy resonance conditions. The solvent acts as an energy sink for rapid vibrational relaxation which occurs after the rate limiting transition from the initial state. Birks et al studied the influence of solvent viscosity on fluorescence charesteristics of pyrene solutions in various solvents and observed that the rate of monomer internal quenching is effected by solvent quality. Weber et al reported the solvent dependence of energy trapping in phenanthrene block polymers and explained the decrease in fluorescence yield with the static quenching, caused by the solvent induced trapping states. As the temperature of liquid solution is varied, the environment about the molecule changes and much of the change in absorption spectra and fluorescence yields in solution can be related to the changes in solvent viscosity. A matrix that changes little with temperature will enable one to study molecular properties themselves without changing environmental influence. Poly (methyl methacrylate) (PMMA) has been used such a matrix in many studies. Recently it has been reported viscosity effect on low frequency, intramolecular vibrational energies of excited naphthalene in swollen PMMA latex particles. In this thesis we aimed to use these properties of aromatic molecules to monitor the sol-gel phase transition in free-radical crosslinking copolymerization. We employed lattice percolation model, where monomers are thought to occupy the sites of a periodic lattice. Between two nearest neighbors of this lattice sites a bond is formed randomly with the probability p. Thus, for p = 0, no bonds have been formed and all monomers remain isolated clusters. However, in the other extreme, i.e. for p = 1 all monomers in the lattice have clustered into one infinite network. This network is called a gel and a collection of finite clusters is called a sol. Usually, there is a sharp phase transition at some critical point p = pc, where an infinite cluster starts to appear. This point is called the gel point, for p below pc only a sol exists but for p above pc both sol and gel coexist together. Thus, gelation is a phase transition from a state without gel to a state with gel. The sol-gel transition happens in "asymptopia" which is in general given by the following relation G = B ( p - pc)P ( 1 ) with a suitable constant p, called critical exponent. Here G is the gel fraction which is non zero for p above pc. The asymtotic proportionality factor B is referred as the critical amplitude. In this work we plan to probe the sol-gel transition in free-radical crosslinking copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM) by using steady-state fluorescence technique. The radical copolymerization of MMA and EGDM was performed in bulk or in toluene solutions at 75°C in the presence of 2,2' - azobisisobutyronitrile (AIBN) as an initiator. Pyrene methyl pivalate (PMP) was used as a fluorescence probe to detect the gelation process, where below pc, MMA, linear and branched PMMA chains act as an energy sink for the excited PMP but above pc, PMMA network provides an ideal, unchanged environment for the excited PMP molecules. Naturally, from these experiments one may expect a drastic increase in fluorescence intensity, I, of PMP around the gel point. EGDM has been commonly used as crosslinker in the synthesis of polymeric networks. Here, for our use, the monomers MMA (Merck) and EGDM (Merck) were freed from the inhibitor by shaking with a 10% aqueous KOH solution, washing with water and drying over sodium sulfate. They were then distilled under reduced pressure over copper chloride. The initiator, AIBN (Merck) was recrystallized twice from methanol. The polymerization solvent, toluene (Merck), was distilled twice over sodium. Steady-state fluorescence measurements were carried out using Model LS-50 Spectrometer of Perkin Elmer, equipped with temperature controller. All measurements were made at 90° position and slit widths were kept at 2.5 nm. VI Three different sets of experiments were carried out; in the first set, AIBN (0.26 wt %) was dissolved in MMA and this stock solution was divided and transferred into round glass tubes of 15 mm internal diameter for fluorescence measurements. Four different samples were prepared with various EGDM contents for bulk polymerization Details of the samples are listed in Table 1. All samples were deoxygenated by bubbling nitrogen for 10 minutes and then radical copolymerization of MMA and EGDM was performed at 75 + 2°C in the fluorescence accessory of spectrometer. PMP molecule was excited at 345 nm and variation in fluorescence emission intensity I was monitored with the time-drive mode of the spectrometer, by staying at the 395 nm peak of the PMP spectrum. No shift was observed in the wave length of the maximum intensity of PMP and all samples kept their transparency during the polymerization process. Scattering light from the samples were also monitored during gelation experiments and no serious variation was detected at 345 nm intensity. Normalized PMP intensities versus reaction time are plotted in Fig.1 for samples with various crosslinker (EGDM) contents. Gelation curves in Fig.1 represent asymptotic behaviours, which give evidence to typical critical phenomenon. tOCO 2300 3000 400 : t (time) (sec) Rg.1. Variation in PMP fluorescence intensity, I against reaction time, t during the crosslinking copolymerization of MMA and EGDM in bulk for various EGDM contents. Time drive mode of the spectrometer was employed for the data collection. Numbers 2,3 and 4 corresponds to samples in experimental set I, in Table 1. In order to quantify the above results, we assumed that the reaction time, t for the polymerization is proportional to the probability p and the fluorescence intensity monitors the growing gel fraction G, then Eq. 1 can be written as Vll I = A ( t - tc)' (2) Here, the critical time, tc corresponds to the gel point, pc and A is the new critical amplitude. Below tc, since PMP molecules are free, they can interact and quenched by sol molecules, as a result I presents small values. However, above tc, since most of the PMP molecules are frozen in the EGDM network, I intensity gives very large values. The plot of Log I = Log A + J3 Log ( t - tc) for the data shown in Fig.1 are presented in Fig. 2, where tc is chosen for the best linear fit, immidiatly after gelation started. The critical exponents, p were determined and listed in Table 1 together with tc and pc values for the corresponding samples in bulk polymerization. Gelation times, tc are shifted to smaller values as EGDM content increased. However p values varied around 0.45, independent of the EGDM content, as expected from the bond percolation theory. However, pc values are found to be slightly larger than the predicted value ( 0.248 ) in the corresponding theory. 2.30- UO 1.30 UO ISO 180 170 180 10 log (l-tc ) Fig.2. Log - log plot of Eq.(2) for the data given in Fig. 1. 10"1 < region was chosen for the best fit to obtain p values. 1- U < 10' i-2 In the second set of experiments, three different samples were prepared with various toluene contents using the stock solution of the first experimental set. The amounts of toluene in the samples are shown in Table 1. Fluorescence measurements were carried out with these samples at 75 + 2°C during solution polymerization in toluene, where EGDM content was kept as 0.01 Vol.% PMP concentration was via taken as 4x1 0"4 M and excitation wavelength chosen, again as 345 nm. Fluorescence intensity, I versus reaction time t was monitored for all samples. Results are shown in Fig 3. Asymtotic behaviour was abserved only in the sample with 0.25 Vol.% toluene content. The other two samples were not be able to form gel, presumable toluene molecules prevent the formation of EGDM network in these high solvent content samples (2 and 3 in Table 1 and Fig 3). Critical values, p, tc and pc are obtained by fitting the data to Eq.( 2) and results are listed in Table 1. In the final and third experimental set, the amount of toluene was fixed at 0.25 % and the samples were prepared with different EGDM contents. Gelation was monitored in four different samples by detecting PMP fluorescence intensity vs. reaction time at 75°C. As the EGDM content is increased, tc decreases indicating that gelation occurs earlier at high crosslinker contents. Furthermore, comparison of the experiments show that, at a given crosslinker content, the gel formation is retarded with rising dilution. All of these results are in accordance with the theory of gelation, p and tc values were obtained from Eq.( 2 ) and the results are listed in Table 1 together with EGDM content. Here, p values are also found to be very close to universal exponent (0.45) in bond percolation theory, woo looo 2000 3000 t (time) (sec) 4000 Fig.3. Variation in PMP intensity, I versus reaction time, t during the crosslinking copolymerization of MMA and EGDM in toluene. Data were collected with the time drive mode of the spectrometer. Numbers 1, 2 and 3 correspond to samples in experimental set n.Sample 4 of m experiment is given for comparison. however tc values were measured much larger than the theory predicts. IX In these experiments, results produced quite reasonable values for the critical exponent p during soi-gel phase transition in three dimension. In summary, this thesis introduced a novel technique to study gelation phenomenon in which experiments are quite simple to perform and the fluorescence spectrometer is inexpensive to obtain or already accessible in any laboratory. Table 1. Experimentally obtained p and tc (or pc ) values using Eq.2 (or Eq.1) during bulk or toluene polymerization 4 0.024 0.49 890 | 0.44