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

This work deals with the crystallization of polymer chains within silica-poly(dimethylsiloxane) mixtures, irrespective of the type of crystalline structure that is actually present in the system. An analysis of the transverse magnetization relaxation function is proposed as a method for melting point determination as well as for determination of the 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 of 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 does not affect the dimensionality of the crystalline growth. However, it reduces the extent of crystallization and has two antagonistic effects on the overall rate of crystallization. The first is a nucleation effect 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 crystallization goes through a maximum with increasing silica volume fraction.