J. Phys. II France
Volume 2, Numéro 5, May 1992
Page(s) 1053 - 1063
DOI: 10.1051/jp2:1992185
J. Phys. II France 2 (1992) 1053-1063

Critical behavior of trifunctional randomly branched polycyanurates

Jörg Bauer and Walther Burchard

Institute of Macromolecular Chemistry, Universitry, of Freiburg, W-7800 Freiburg, Germany

(Received 9 January 1992, accepted 31 January 1992)

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 $(M_{\rm w})$ and the characteristic (M*) molecular weights and the radius of gyration $(R_{\rm g})$ respectively. All these independently measured exponents together with $\tau$, describing 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. Furthermore, it was 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 gel-point into the gel state was analyzed quantitatively by a hybride of a stretched exponential and a power law function. I was shown that the basic relations between $\gamma,$ $\sigma$, $\nu$ and $\tau$ are fulfilled and should be universally valid. On the other side, the relations, which link the characteristic exponents of the time correlation function at the gelpoint to the basic exponents, gave incoherent predictions. In that way, one can assume that their universal validity is limited, since some additional assumptions on the local dynamics are needed.

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