MESOMORPHIC BEHAVIOR IN POLY(DIPHENYLSILOXANE) .2. AN EXAMINATION OF DIPHENYLSILOXANE OLIGOMERS

Oligomers of diphenylsiloxane with number average molecular weights ra nging from approximately 2200 to 3450 have been prepared and character ized. Samples with an M(n) above a critical value of approximately 240 0 form a mesomorphic phase similar to that formed by poly(diphenylsilo xane), when heated above the melting point of the crystalline phase. A bove this critical molecular weight, the highly birefringent mesomorph ic phase formed by the oligomers transforms completely to the isotropi c state at a temperature that rises sharply with increasing molecular weight. Slow cooling of the molten isotropic phase results in the deve lopment of the mesophase as three-dimensional crystallites in the surr ounding isotropic melt. The experimental evidence suggests that the PD PhS mesophase is a disordered crystalline phase in which the constitue nt oligomers have a degree of dynamic conformational disorder within t he confines of a three-dimensional lattice structure.