Thermal and structural analysis of BPDA based hairy-rod polyimides.

摘要

Aromatic main chain polymers are desirable for use in applications such as high performance thermoplastics and high modulus fibers. A drawback to the use of these polymers is their limited solubility. The addition of flexible side chains to the rigid main chain increases the solubility of the polymer and decreases the transition temperatures to values at which the materials may be easily processed.; A series of soluble hairy-rod polyimides has recently been synthesized from 3,3{dollar}spprime,4,4spprime{dollar}-biphenyltetracarboxylic dianhydride (BPDA) and 4,4{dollar}spprime{dollar}-diaminobiphenyl substituted at the 2,2{dollar}spprime{dollar} positions with bromine and with ester side chains of varying length at the 6,6{dollar}spprime{dollar} positions. These fully imidized materials yield excellent solubility in common solvents such as tetrahydrofuran and chloroform.; The objective of this research has been the determinations of the structure/property relationships of this series of polyimides as a function of side chain length. By understanding the effect of side chain length on a variety of material parameters, a polymer can be tailored to have properties desired for specific applications. The goal of the analysis of this set of materials was to determine the suitability of these polyimides for use as alignment layers in liquid crystal displays (LCDs).; Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) revealed two cooperative relaxations, both of which varied in temperature with corresponding changes in the side chain length. Variable temperature solid-state nuclear magnetic resonance (NMR) experiments were carried out in order to determine the molecular contributions to each of the observed DMA and DSC relaxations. It was determined that the low temperature relaxation is associated with the onset of motion in the side-chain and the high temperature relaxation is associated with motion in the main chain.; Wide angle X-ray diffraction was used to analyze the structural packing behavior as a function of side chain length. Long side chain (12-18 carbon atoms) samples have been shown to adopt a monoclinic unit cell while short side chain (8-10 carbon atoms) samples possess a hexagonal unit cell.; Pretilt measurements revealed a tendency for increasing pretilt angle with increasing side chain length. Long term measurements suggested instability of the alignment layer with time, perhaps indicating that this series of materials is too soluble for application in LCDs.