PBLG/tolüen organojellerinin jelleşme kinetiği ve mekanizması

Poly (?-benzyl-L-glutamate) (PBLG) is a synthetic polypeptide which can form thermoreversible organogels with common organic solvents such as toluene. In this thesis, thermoreversible gelation of PBLG has been studied by dynamic light scattering (DLS), micro-viscometer, differential scanning calorimetry (DSC), and ultraviolet-visible spectrometer. In this model system, to understand how rodlike polypeptides form a gel, we have investigated the effect of time, temperature, concentration and addition of other competing molecules. PBLG can form thermoreversible gels in toluene above the critical concentration which was determined to be 4-5 mg/ml close to critical gelation temperature which was found to be between 23°-26°C. The transition temperatures for PBLG/toluene solutions were determined as 51.04°C and 19.65°C by DSC. The time required for gelation of PBLG at different temperatures, decreased with concentration. With increasing PBLG concentration, aggregate size, viscosity and scattered light intensity increased due to growth of aggregates. Turbidity measurements by means of UV-visible spectrometer showed that macroscopic gel was formed only when aggregates fused to each other. From the comparison of the data obtained by different techniques, it was found that each technique was sensitive to different stages of gelation in time. Though onset of PBLG gelation could be detected via DLS, growth of aggregates could be followed by viscometer. Last stage in which the system formed a macroscopic gel could only be observed by turbidity measurements using UV-visible Spectrometer. We propose a model for the gelation of PBLG molecules in toluene in time. Turbidity measurements as a function of time for different concentrations of PBLG/toluene solutions were analyzed on the basis of Avrami equation. The Avrami exponent n was found to be between 2-5 depending on PBLG/toluene concentration indicating different growth mechanisms.It has been achieved to incorporate polystyrene (PS) into PBLG/toluene solutions with high loading percentages. The critical concentration above which PBLG does not form macroscopic gel was determined as 82% by weight. Incorporation of PS did not affect the thermoreversibility of PBLG/toluene solutions. DLS results of PS-PBLG/toluene solutions were similar with PBLG/toluene solutions. This indicates that PS has no significant effect on the onset of structure formation which is the first stage of gelation. On the other hand, PS plays a key role in growth of aggregates and their fusion to each other at the later stages of gelation. Network formation of PBLG in PS matrix was confirmed by microscopic measurements.