Macromolecules, Vol.29, No.23, 7402-7411, 1996
Thermoreversible Gelation of a Rodlike Polymer
Thermoreversible gelation of poly(gamma-benzyl-alpha-L-glutamate) in toluene has been studied by differential scanning calorimetry and by static and dynamic light scattering. For high molecular weights, this system tends to form clear gels when rapidly cooled to low temperatures and more cloudy gels for quenches to temperatures near or slightly above ambient. Differential scanning calorimetry measurements on low-mass polymers show that the melting temperature and also the width of the melting transition depends upon both concentration and type of quench. Gels quenched rapidly to -10 degrees C melt at lower temperatures, and more sharply, than gels formed slowly at 25 degrees C. A series of autocorrelation functions, intensity measurements, and visual observations during slow gelation at 30 degrees C showed that a clear gel formed first, followed by a slightly cloudy gel and partial heterodyning. For visible light at commonly used scattering angles, the apparent fractal dimension of clear and cloudy gels was not dramatically different, a rather extended structure being indicated in either case. Dynamic light scattering measurement during stepwise cooling was used to follow the decrease in apparent diffusion and increase in the gel fraction, as defined on the dynamic light scattering time and distance scales, On melting either clear or cloudy gels, an estimate of the melting temperature could be identified from the power law behavior of the autocorrelation function. The power law regime spanned 3-5 decades of time. Parameters associated with the line shape of the correlation function echoed the results of differential scanning calorimetry on low-mass polymer : a sharp melting transition was observed for the rapidly quenched gel, while the slowly cooled gel melted in a complex fashion over a broader temperature range. These observations reflect the competition between connectivity and phase transitions.
Keywords:DYNAMIC LIGHT-SCATTERING;BENZYL ALCOHOL SYSTEM;SOL-GEL TRANSITION;ROD-LIKE POLYMERS;ISOTROPIC SOLUTIONS;STIFF POLYMERS;POLY(GAMMA-BENZYL L-GLUTAMATE);CONCENTRATION FLUCTUATION;SPINODAL DECOMPOSITION;MOLECULAR AGGREGATION