Using Group Contribution Method and Molecular Dynamics to Predict the Glass Transition Temperature of Poly (p-phenylene isophthalamide)

Aramid fibers mainly include wholly aromatic polyamide and heterocyclic aromatic polyamide , while the wholly aromatic polyamides ( aramids ) are considered to be high-performance organic materials due to their outstanding thermal and mechanical properties. Their high-performances arise from their aromatic structure and amide linkages. The better known commercial aramids , poly(p-phenylene terephthalamide ) ( PPTA) and poly ( m-phenylene isophthalamide) ( MPDI) , are used in advanced technologies and have been transformed into high-strength and flame-retardant fibers and coatings , with applications in the aerospace and armament industry. Poly ( p-phenylene isophthalamide) ( PPIA ) , a new aromatic polyamide, has not yet been commercialized and there are few reports about its comprehensive performance until nowadays. In this paper, Group Contribution( GC) method and molecular dynamics ( MD) simulation were used to simulate the glass transition temperatures ( T-g ) of MPDI and PPTA. Then analysis and comparisons of the glass transition temperatures by GC method and MD simulation with their experimental values are presented. The results show that the glass transition temperature measured by GC method and MD simulation is very close to the experimental value , and that the change of the density , specific volume , radius of gyration and energy along with temperature can characterize the glass transition temperature. Then these two methods were exploited to simulate the T-g of PPIA. The change of the density, specific volume, radius of gyration and energy interactions along with temperature were analyzed in the MD simulation. The results show that the free volume theory can explain the glass transition phenomenon of PPIA, and the change of the non-bond energy interactions with temperature is the essential reason. These results indicates that PPIA has the potential to become another high performance polyamide with its T-g lying between those of both MPDI and PPTA. It is of great significance to emphasize on the synthesis of PPIA with sufficiently high molecular weight. In general, the group contribution method and molecular dynamics simulation can predict the T-g of aromatic polyamide sucessfully , and they can contribute to a deeper understanding on the glass transition phenomenon of aromatic polyamides and the molecular motion behind.