Strain Induced Crystallization in Stereoregular Poly[Methyl(3,3,3-Trifluoropropyl) Siloxane] Networks

Abstract

Elastomeric networks based on poly[methyl(3,3,3-trifluoropropyl)siloxane] having a minimum level of stereo-regularity crystallize and self-reinforce when stretched at room temperature. The effect is especially pronounced in networks where the cross-links are uniformly spaced. The polymers are made by modification of a recently described anionic ring opening copolymerization of prescribed mixtures of cis- and trans-1,3,5-trimethyl-1,3,5-tris (3′, 3′, 3′-trifluoropropyl) cyclotrisiloxane. This provides polymers enriched in stereoregularity with nearly monodisperse molecular weight distributions and reactive groups at the chain ends. The end-linked networks show markedly enhanced mechanical properties due to self-reinforcement from strain-induced crystallization. In addition to applied strain, self-reinforcement depends on temperature and the amount of stereoregularity introduced to the polymer. Strained specimens having high levels of stereoregularity and pre-existing crystallinity exhibit plastic flow and, upon release of the applied strain, stress-hardening. Wide angle X-ray scattering (WAXS) on strained specimens establishes that, although the relative degree of crystallinity varies, the basic crystal structure is unaltered by cross-linking, amount of strain, or amount of stereoregularity. Unit cell parameters from WAXS indicate a monoclinic crystal and suggest a 4*11 chain conformation.