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

Oligomers of diphenylsiloxane with number average molecular weights ranging from approximately 2200 to 3450 have been prepared and characterized. Samples with an M(n) above a critical value of approximately 2400 form a mesomorphic phase similar to that formed by poly(diphenylsiloxane), when heated above the melting point of the crystalline phase. Above this critical molecular weight, the highly birefringent mesomorphic phase formed by the oligomers transforms completely to the isotropic state at a temperature that rises sharply with increasing molecular weight. Slow cooling of the molten isotropic phase results in the development of the mesophase as three-dimensional crystallites in the surrounding isotropic melt. The experimental evidence suggests that the PDPhS mesophase is a disordered crystalline phase in which the constituent oligomers have a degree of dynamic conformational disorder within the confines of a three-dimensional lattice structure.