Conformational origin of glassy-state relaxation and ductility in aromatic polycarbonates

摘要

A new two-state conformational transition is proposed to explain the large, low-temperature mechanical loss peak seen in glassy polycarbonates. Restricted Hartree Fock ab initio calculations at the 6-31G~(**) level for diphenyl carbonate (DPC), a key model compound of bisphenol-A polycarbonate, reveal two inequivalent trans-trans carbonate-ring conformations both of which will exist in solution, melt or glassy states. These calculations appear to be the first high level ones (with full geometry optimization) reported for DPC, and the findings are consistent with earlier ab initio results for phenyl formate and other smaller model compounds and also with single-crystal X-ray data for DPC and oligomers. In addition to a trans-trans conformer DPC with both phenyl rings on the same side of the carbonate unit (called the 'syn' conformer) which is seen in the crystalline state of DPC, an 'anti' conformer f lower energy is found, which has its two phenyl rings located on opposite sides of the plane of the carbonate unit. Analysis of these calculated ground state geometries and energies as well as experimental single crystal X-ray results indicates that the 'anti' conformer has the lowest energy in the gas phase and solution, while the 'syn' conformation is stabilized relative to the 'anti' in the bulk, probably because of aromatic ring interactions between neighbour chain segments. In the glassy state of either DPC or polycarbonate, one expects a nearly random mixture of 'syn/anti' conformers, and the prominent low-temperature mechanical loss peak observed in many polycarbonates is consistent with a molecular level two-state process consisting of 'syn/anti' carbonate conformer conversions. These conformational transitions must involve rotation and translation of both the carbonate units and, most importantly, the neighbouring phenyl groups. The possible influence of these conformational changes and the accompanying correlated molecular motions on polymer ductility and ageing is briefly discussed.