Poli(2-etil-2-oksazolin)sulu çözeltilerinin fiziko-kimyasal karakterizasyonu ve polimerin kendiliğinden yapılanması

Poly(2-ethyl-2-oxazoline) (PEOX) is a non-toxic, biocompatible and thermoresponsive polymer which is soluble in water at room temperature. Aqueous solutions of PEOX exhibit lower critical solution behavior and phase separate macroscopically above a cloud point temperature (Tc) which depends on several factors including polymer concentration, polymer molecular weight, ionic strength and the presence of organic additives. Despite the common use of PEOX in biomedical applications, the physico-chemical factors affecting its thermoresponsive behavior and self-assembly in aqueous solutions are yet to be understood. In this thesis, we report on the effect of a series of sodium salts on Tc of aqueous PEOX solutions and the self-assembly of PEOX into crystalline nanofibers both above and below Tc. For 1 mg/mL aqueous PEOX-500K (molecular weight of 500,000 g/mol) solutions Tc was ~61 °C in water. The effects of 11 anions were investigated and Tc was controlled between 20 °C and 70 °C by salt addition. Kosmotropic anions decreased Tc linearly while chaotropic anions increased Tc non-linearly with salt concentration. Our results indicate that the dominant mechanism was the dehydration of PEOX for divalent kosmotropic anions (CO32?, SO42?, S2O32?) and direct binding for chaotropic anions (NO3?, I?, ClO4?, SCN?). For the remaining monovalent kosmotropic anions (H2PO4?, F?, Cl?, Br?), a combination of dehydration and surface tension mechanisms was in effect. When aqueous PEOX solutions were kept above Tc for long times (of the order of days to weeks), PEOX self-assembled into crystalline nanofibers which remained stable even below Tc. The addition of salts enhanced the self-assembly process. The self-assembled nanofibers were characterized by microscopic techniques, differential scanning calorimetry and X-ray diffraction. The melting temperature and the lattice constants of the crystalline PEOX nanofibers were consistent with those of other poly(2-alkyl-2-oxazoline)s (PAOX) which confirmed the successful crystallization of PEOX for the first time. A model was proposed for the crystal structure and the role of inter-molecular interactions in the self-assembly process was discussed. The self-assembly of PEOX into crystalline nanofibers was also observed below Tc, but the rate of self-assembly was even slower. The growth of aggregates as determined by dynamic light scattering indicated a nucleation and growth mechanism. The thermoresponsive behavior of thin PEOX coatings on solid substrates was also demonstrated. Thermoresponsive PEOX coatings were given antimicrobial functionality by addition of silver nano-particles. The durability of the PEOX coatings against water adsorption was improved by sol-gel technique using tetra ethyl ortho silicate (TEOS).