Molecular Orientation And Strain-Induced Crystallization In Trans-Polypentenamer

Structure evolution during uniaxial deformation of an unfilled 83% trans-polypentenamer sample is investigated as a function of temperature using in situ synchrotron wide-angle X-ray scattering (WAXS) and Raman spectroscopy combined with simultaneous stress and strain measurements. Three deformation regimes were identified: (1) an initial linear elastic regime showing a linear stress-strain response, where only changes in amorphous orientation occurs. (2) At a critical strain, which is an increasing function of temperature, strain-induced crystallization (SIC) starts and develops at a fast rate. In this regime, the amorphous chain alignment accelerates and the nascent crystals orient in the stretching direction very rapidly. (3) At a second critical strain, which also increases with temperature, a sudden slowdown in crystallization and chain alignment is observed. We postulate that the crystallization slowdown results from crystal network saturation, at which point the system behaves as a reinforced elastomer, yielding a strong strain-hardening response. Crystallization measurements by Raman spectroscopy perfectly mirror those obtained by WAXS. Moreover, Raman measurements provide evidence of a preorganization of the chains (enrichment in C-C trans conformations) previous to crystal formation.