THERMOREVERSIBLE GELATION OF A RODLIKE POLYMER

Thermoreversible gelation of poly(gamma-benzyl-alpha-L-glutamate) in t oluene has been studied by differential scanning calorimetry and by st atic and dynamic light scattering. For high molecular weights, this sy stem tends to form clear gels when rapidly cooled to low temperatures and more cloudy gels for quenches to temperatures near or slightly abo ve 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, an d more sharply, than gels formed slowly at 25 degrees C. A series of a utocorrelation functions, intensity measurements, and visual observati ons during slow gelation at 30 degrees C showed that a clear gel forme d first, followed by a slightly cloudy gel and partial heterodyning. F or visible light at commonly used scattering angles, the apparent frac tal dimension of clear and cloudy gels was not dramatically different, a rather extended structure being indicated in either case. Dynamic l ight 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, O n melting either clear or cloudy gels, an estimate of the melting temp erature could be identified from the power law behavior of the autocor relation function. The power law regime spanned 3-5 decades of time. P arameters associated with the line shape of the correlation function e choed the results of differential scanning calorimetry on low-mass pol ymer: a sharp melting transition was observed for the rapidly quenched gel, while the slowly cooled gel melted in a complex fashion over a b roader temperature range. These observations reflect the competition b etween connectivity and phase transitions.