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Yayın Synthesis and liquid crystalline behavior of random copolymer of poly(ethylene oxide) macromonomer and liquid crystalline monomer by the photon transmission technique(Taylor & Francis Inc, 2005-12) Yıldız, Sevtap; Yılmaz, Faruk; Özbek, Haluk; Pekcan, Mehmet Önder; Ito, Koichi; Yağcı, YusufRandom copolymers of poly(ethylene oxide) macromonomer with p-vinylbenzyl end-functional group (PEOVB) and liquid crystalline monomer, namely 6-(4-cyanobiphenyl-4'-oxy)hexyl acrylate (COA), were prepared by conventional free radical polymerization. A living anionic polymerization technique was employed for the synthesis of PEO macromonomers bearing p-vinylbenzyl moiety at one end. The photon transmission method was also applied to study the phase transitions of COA monomer and its random copolymer with PEO. It was found that, for both samples, the nematic-smectic A transition is continuous, but the critical fluctuation regions do not allow to obtain 3D XY values. Instead, we have obtained the values close to mean field regime. Scaling of thermal hystersis for random copolymer sample near the nematic-isotropic transition was studied as well. Thermal hysteresis loops were produced under linearly varying temperature. It was shown that the areas of the hysteresis loops scale with the temperature scanning rate with an exponent being equal to 0.614 which is in good agreement with the field-theoretical value.Yayın Small molecule diffusion into swelling Iota-Carrageenan gels: A fluorescence study(Taylor & Francis Group, 2007-04) Ataman, Evren; Pekcan, Mehmet ÖnderSmall molecule diffusion into Iota-Carrageenan gel was studied by using steady-state fluorescence (SSF) technique. Pyranine, dissolved in water was used as fluorescence probe. Fluorescence emission intensity, I-p, and scattered light intensity, I-sc, were monitored to study diffusion and swelling processes at various temperatures respectively. Fickian and Li-Tanaka models were elaborated to produce diffusion, D, and collective diffusion, D-0, coefficients. Diffusion and swelling activation energies were also obtained and found to be 20.5 kj mol(-1) and 28.2 kj mol(-1). respectively.Yayın Synergistic effect of the locust bean gum on the thermal phase transitions of kappa-carrageenan gels(Elsevier Sci Ltd, 2009-03) Arda, Ertan; Kara, Selim; Pekcan, Mehmet ÖnderSynergism between K-carrageenan and locust bean gum (LBG) was studied using the photon transmission technique. Synergistic effects in these polymeric mixtures strongly affected the physical properties of the gel structure. The transmitted light intensities, I-tr, versus temperature variations were investigated during the gelation and liquefaction processes. Slight synergistic peaks were detected in gel-sol and sol-gel transition temperatures for high kappa-carrageenan/LBG ratios (approx. 80/2). Moreover, apparent synergistic peaks were observed in gel-sol and sol-gel transition activation energies for the mixtures approximately with the ratios of 80/10.Yayın Phase transitions of kappa-carrageenan gels in various types of salts(John Wiley & Sons Inc, 2006-11-05) Kara, Selim; Arda, Ertan; Kavzak, Burhan; Pekcan, Mehmet ÖnderSol-gel and gel-sol phase transitions of K-carrageenan in various types of salts were studied using photon transmission technique. Here, LiCl, NaCl, KCl, MgCl2, CaCl2, and SrCl2 were chosen as the proper salts for the K-carrageenan-water system. Photon transmission intensity, I-tr was monitored against temperature to determine the sol-gel and gel-sol temperatures (T-sg and T-gs) and activation energies (Delta E-sg and Delta E-gs). It was observed that T-gs temperatures were notably higher than T-sg due to the hysteresis on the phase transition loops. It was observed that stronger carrageenan gels are formed in the presence of KCl compared to the others.Yayın Study of drying of kappa-carrageenan gel at various temperatures using a fluorescence technique(Taylor & Francis Inc, 2008-01) Tarı İlgin, Özlem; Pekcan, Mehmet ÖnderThe steady-state fluorescence technique was used to study drying of kappa-carrageenan gel at various temperatures. Pyranine was doped as a fluorescence probe, and scattered light, I-sc, and fluorescence intensities, I, were monitored during drying of these gels. It was observed that I decreased linearly as drying time is increased. The moving boundary model based on Case II diffusion was applied to produce the packing constants, k(0), at higher temperatures. Activation energy for drying was found to be 53.9 kJ mol(-1). Supporting gravimetric and volumetric measurements were also carried out during drying of gels and the corresponding activation energies were determined to be 46.4 and 58.9 kJ mol(-1), respectively.Yayın Universality of sol-gel phase transition of kappa-carrageenan in various salts: a steady state fluorescence study(Taylor & Francis Ltd, 2007-08) Pekcan, Mehmet Önder; Tarı İlgin, ÖzlemThermal phase transitions of kappa-carrageenan in NaCl, KCl and CaCl2 solutions were studied using steady state fluorescence (SSF) technique. Pyranine was introduced as a fluorescence probe for studying sol-gel phase transitions. Scattered light, I-sc and fluorescence intensity, I was monitored against temperature to determine the sol-gel (T-sg) phase transition temperatures. It was observed that T-sg values are strongly correlated to NaCl, KCl and CaCl2 contents. The weight average degree of polymerization, DPw, and gel fraction G, exponents (gamma and beta) were measured and found to be in accord with the classical Flory-Stockmayer model, i.e., gamma and beta were found to be close to 1.0, independent of salt content.
