Tez özetleri Astronomi ve Uzay Bilimleri Anabilim Dalı

Synthesis and Characterization of N-vinylcaprolactam Based Gels

In this study, poly(N-vinylcaprolactam-co-2-diethylaminoethyl methacrylate) [P(VC-co-DEAEMA)] and poly(N-vinylcaprolactam-co-itaconic acid) [P(VC-co-IA)] copolymer gels were synthesized in ethanol by free radical crosslinking polymerization method at 60 ^oC for 24 h in the presence of azobisisobutyronitrile (AIBN) initiator. Total initial monomer concentration was kept constant at 2M and AIBN concentration was 0.5 mole % of the total monomer concentration. Homopolymer gels of N-vinylcaprolactam and diethylaminoethyl methacrylate (PVK and PDEAEMA) were also synthesized at the same conditions and used for comparison purposes. For PVK and some P (VK-ko-IA) gels as media for the synthesis of ethanol-water mixture (80-20, V / V) is also used.PVK and some P(VC-co-IA) gels, ethanol-water mixture (80/20, V/V) was used as polymerization medium. N-vinylcaprolactam monomer (VC) was preferred in this study, because of the limited number of studies in the literature about the gels based on VC. Lower Critical Solution Temperature (LCST) of its homopolymer (PVC) was found to be around of physiological temperature (32 ^oC). In this study, P(VK-co-DEAEMA) and P(VK-co-IA) gels were synthesized and characterized for the first time.

PVK, PDEAEMA, P(VK-co-DEAEMA) and P(VC-co-IA) gels were prepared using ethylene glycol dimethacrylate (EGDMA) or allyl methacrylate (AMA) crosslinking agents in order to determine the effect of cross-linker type on equilibrium swelling values (ESV) and percentage of gelation.

All the P(VK-co-IA) gels were prepared in the presence of AMA and the effect of presence of water in the synthesis was investigated on the percentage of gelation and swelling behavior of the gels. Swelling behaviors of the prepared homopolymer and copolymer gels were investigated in buffer solutions with different pH values (2.2, 5.3, 7.4, 10.0 ve 12.40, constant ionic strength, I=0.08 M), in distilled water at various temperatures (between 8 and 50 ^oC) and in solvent/solvent mixtures. Equilibrium Swelling Values (ESV) of the gels were determined by gravimetric method. Structural characterization, morphological investigations and thermal caharacterization of the synthesized gels were carried out using Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM) and Thermal Gravimetric Analysis (TGA), respectively. In addition to gravimetric method, Differential Scanning Calorimetry (DSC) was used to determine the temperature effect on swelling behavior of the gels in distilled water.

For the homopolymer (PVK, PDEAEMA) and copolymer [P(VK-co-DEAEMA)] gels prepared in the presence of EGDMA and AMA, it was observed that PVK gel had the lowest percentage of gelation and the highest value of equilibrium swelling value (ESV). Compared to EGDMA, crosslinking agent AMA increased the percentage of gelation, decrease the ESV of the gels. In addition, ESV of the gels reduced with the increase of DEAEMA content in the gel compositon. ESV of the gels prepared with EGDMA decreased from 8 to 30 ^oC, and then increased slightly. ESV of the gels prepared with AMA continuously decreased depending on the temperature increase.

A maximum point was observed at pH 7.4 which was the highest value of the ESV for the gels prepared with EGDMA and contain % 5, 10, 15 mole DEAEMA. For these gels the minimum value of the ESV was achieved in the most acidic and basic enviroments. The gels prepared with AMA showed the same behavior at pH 7.4, but lower ESVs were obtained compared to gels prepared with EGDMA.

Swelling behavior of the gels prepared in the presence of different type of cross-linkers (EGDMA or AMA) was investigated in different solvents (water, ethanol, methanol, iso-propyl alcohol, chloroform, toluene, acetone) and the highest value of ESV was obtained for chloroform which was confirmed by calculated solubility parameters. When swelling kinetics was investigated at pH 7.4 for the gel contained highest mole percentage of (%95) VK, it was determined that gels prepared with EGDMA and AMA showed Non-Fickian and Fickian swelling behavior, respectively.

PVK and P(VK-co-IA) gels were prepared in ethanol and ethanol-water (80/20, V/V) mixture using AMA crosslinking agent. For the gels prepared in ethanol, the lowest percentage of gelation was obtained for the gel with highest mole percentage of % 10 IA and the highest equilibrium swelling value (ESV) was obtained for PVK. In the case of using of water in polymerization medium, the highest percentage of gelation and ESV was reached for PVK and the gel containing 5 mole % IA, respectively. Obtained results for the swelling equilibrium in ethanol were verified by SEM images.
ESV of the gels in distilled water increased with decrease in temperature and LCST was obtained at 30-35 ^oC. In buffer solutions with various pH values, for all P(VK-co-IA) gels demonstrated the lowest ESV in the most acidic enviroment and ESV reached its highest value at pH 10. Swelling behavior of the gels was investigated in different solvents (water, ethanol, methanol, iso-propyl alcohol, chloroform, toluene, acetone) and the highest value of ESV was obtained for chloroform which was confirmed by calculated solubility parameters. When swelling kinetics was investigated at pH 10.0 for the copolymer gels [P(VK-co-IA)]contained 5 mole ve 10 mole % IA, it was determined that gel containing 5 mole % and 10 mole % of IA indicated Non-Fickian and approximate Case II swelling behavior, respectively.
Key words: N-vinylcaprolactam, 2-diethylaminoethyl methacrylate, itaconic acid, solvent, swelling kinetics, pH-sensitive, temperature-sensitive