Analytical calculus and Monte Carlo simulation of crosslinked polymer formation in the copolymerization of tetraethoxysilane and poly(dimethylsiloxane)

To understand the fundamental aspects of the polycondensation reaction of h ydrolyzed tetraethoxysilane (TEOS) and silanol-terminated poly(dimethylsilo xane) (PDMS), we modeled the reaction system as a step-growth polymerizatio n of A(4) and polydisperse A(2), assuming the reactivities of all functiona l groups are equal. The analytical solution for the weight-average molecula r weight is developed, and in addition, a Monte Carlo simulation is conduct ed to investigate the detailed structural development. It was found that as long as the molecular weight of PDMS is much larger than TEOS, the apparen t a behavior is significantly different from usual gelling systems. The gel point is relatively insensitive to th weight fraction of crosslinker (TEOS ), the polydispersity index may decrease during polymerization before the r apid increase to infinity, and the molecular weight distribution profile ma y not show a significant broadening toward gelation. Even though the presen t model assumes a complete random reaction process among functional groups, formation of a heterogeneous structure in which a tight core consisting of TEOS-based molecules is surrounded by soft PDMS chains was observed in the Monte Carlo simulation.