Influence of Cohesive Energy and Chain Stiffness on Polymer Glass Formation

摘要

The generalized entropy theory is applied to assess the joint influence of the microscopic cohesive energy and chain stiffness on glass formation in polymer melts using a minimal model containing a single bending energy and a single(monomer averaged) nearest neighbor van der Waals energy. The analysis focuses on the combined impact of the microscopic cohesive energy and chain stiffness on the magnitudes of the isobaric fragility parameter m_P and the glass transition temperature T_g. The computations imply that polymers with rigid structures and weak nearest neighbor interactions are the most fragile, while T_g becomes larger when the chains are stiffer and/or nearest neighbor interactions are stronger. Two simple fitting formulas summarize the computations describing the dependence of m_P and T_g on the microscopic cohesive and bending energies. The consideration of the combined influence of the microscopic cohesive and bending energies leads to the identification of some important design concepts, such as iso-fragility and iso-T_g lines, where, for instance, iso-fragility lines are contours with constant m_P but variable T_g. Several thermodynamic properties are found to remain invariant along the iso-fragility lines, while no special characteristics are detected along the iso-T_g lines. Our analysis supports the widely held view that fragility provides more fundamental insight for the description of glass formation than T_g.