Molecular simulation on structure–property relationship of polyimides with methylene spacing groups in biphenyl side chain

Abstract

In our early research, the experiment of three synthesized polyimides (PP0DA-PI, PP2DA-PI and PP6DA-PI) with different methylene spacing groups in biphenyl side chain but with the same backbone reveals that the PP6DA-PI with six methylene flexible spacing groups in biphenyl side chain has lower intrinsic viscosity, density and glass transition temperature than those of the PP0DA-PI without methylene spacing group and PP2DA-PI with two methylene spacing groups in biphenyl side chains but keeps the mechanical property to that of the PP0DA-PI and PP2DA-PI. The aim of this paper is to explain the addition of the methylene spacing groups leading to the diversity of polyimide’s properties; molecular mechanics and molecular dynamics techniques were adapted to generate reasonable molecular models to establish the structure–property relationship of polyimide. Lots of interesting results have been obtained as follows. The modeling of chain conformation shows that the biphenyl side groups in PP0DA-PI are almost vertical to the backbone, whereas the biphenyl side groups in PP6DA-PI are almost parallel to the backbone indicating that PP6DA-PI exhibits a strong intra-chain interaction leading to in situ reinforcement interaction between the main chain and the side chain validated by the charge density, dipole moment and force current property of the two phenyls in biphenyl side chains. For this reinforcement in PP6DA-PI, the original modulus has been enhanced to some extent. Meanwhile, though the PP6DA would be more reactive due to higher charge density on N atom of the diamine monomer, the bulk structure of the side chain hindering the polymerization approach of the monomers leads to lower intrinsic viscosity of PP6DA-PAA. The density of PP6DA-PI does not decrease excessively with respect to the increase in chain length because of the bended side chain in this system. Furthermore, using conformational grid scan, potential energy contour map illuminates that the addition of the methylene spacing group in biphenyl side chain decreases the energy barrier in synchronous rotational bonds of the backbone resulting in lower glass transition temperature of PP6DA-PI.