CRITICAL-BEHAVIOR OF TRIFUNCTIONAL RANDOMLY BRANCHED POLYCYANURATES

The behavior near the gelation threshold of trifunctional randomly branched polycyanurates is studied by static and dynamic light scattering. By static measurements the critical exponents gamma, sigma and nu were obtained, which describe the divergence of the weight average (M(w)) and the cutoff (M*) molecular weights and the radius of gyration (R(g)) respectively. All these independently measured exponents together with tau, characterizing the power law behavior of the molecular weight distribution and measured by size exclusion chromatography coupled with light scattering, confirm the predictions of the three-dimensional percolation theory. With the help of size exclusion chromatography coupled with a light scattering and a viscosity detector, a fractal dimension D = 2.24 is obtained. On the other side, from the corresponding exponent for the whole unfractionated samples a fractal dimension D = 2.21 results, using a theory of Daoud. This suggests that the fractal dimension of the polycyanurates in dilute solution lies between the theoretical predictions D = 2.5 for the unswollen and D = 2.0 for the completely swollen state. Furthermore, it is shown by dynamic light scattering that the power law behavior over some decades in time of the time autocorrelation function and the divergence of the mean relaxation time are characteristics of the gelpoint. The development with increasing reaction time of the time correlation function of the gelling system from the pregel through the gelpoint into the gel state is analyzed quantitatively by a hybrid of a stretched exponential and a power law function.