Physical Chemistry Chemical Physics 2018-04-10

Stereodynamic insight of the thermal history effect on poly(vinyl chloride) calorimetric sub-glass and glass transitions as fragile glass model

Jean-Mathieu Pin, Ehsan Behazin, Manjusri Misra, Amar K. Mohanty

Index: 10.1039/C8CP00755A

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Abstract

The dynamic thermal history impact of poly(vinyl chloride) (PVC) has been explored in a wide range of pre-cooling rates, from 1 to 30 oC.min-1. A first macroscopic insight of the dynamic thermal history influence has been highlighted through a decrease of the apparent activation energy (Eapp) in the first stage of the glass transition. The overall glass transition Eapp surface was successfully model in a polynomial fashion regarding the pre-cooling range. In order to associate Eapp variations along the glass transition conversion to the stereochemistry evolution during the polymeric relaxation, Raman scattering has been used. Herein, the selection of atactic PVC as polymer model permits to monitor the glassy polymer segment stereodynamic during the heating ramp through the C-Cl stretching. The intermolecular H-Cl dipole interactions as well as intramolecular conformational reorganizations between syndiotactic, isotactic and heterotactic polymer sequences have been associated to non-cooperative and cooperative motions, i.e. β- and α-process respectively. The fruitful comparison of the two extreme values of the pre-cooling rates permits to propose a thermokinetic scenario that explains the occurrence, intensity, and inter-dependence of β- and α-processes in the glassy state and during the glass transition. This scenario could potentially be generalized to all the other polymeric glass former.