SILICA POLY(DIMETHYLSILOXANE) MIXTURES - NMR APPROACH TO THE CRYSTALLIZATION OF ADSORBED CHAINS

This work deals with the crystallization of polymer chains within sili ca-poly(dimethylsiloxane) mixtures, irrespective of the type of crysta lline structure that is actually present in the system. An analysis of the transverse magnetization relaxation function is proposed as a met hod for melting point determination as well as for determination of th e extent of crystallization as a function of time during an isothermal crystallization. We first pay attention to pure poly(dimethylsiloxane ) (PDMS), for which we have measured the enthalpy of fusion per mole o f structural units and defined the isothermal crystallization kinetics . The results obtained for the mixtures are compared to those obtained for pure PDMS. It turns out that the presence of silica particles doe s not affect the dimensionality of the crystalline growth. However, it reduces the extent of crystallization and has two antagonistic effect s on the overall rate of crystallization. The first is a nucleation ef fect which is dominant at low silica volume fractions, and the second effect is a topological constraint which becomes dominant and hinders the crystalline growth at high silica volume fractions. As a result of the competition between these two effects, the overall rate of crysta llization goes through a maximum with increasing silica volume fractio n.