Structure, chain orientation, and properties in thin films of aromatic polyimides with various chain rigidities

Several high temperature polyimides, as model polymers with various chain rigidities and chain orders, were synthesized through the polycondensation of p-phenylene diamine (PDA) with five different aromatic dianhydrides [i.e., pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA), 4,4^′-oxydiphthalic anhydride (ODPA), benzophenonetetracarboxylic dianhydride (BTDA), and 4,4^′-hexafluoroisopropylidenediphthalic anhydride (6F)], and investigated in terms of structure and properties. Both fully rodlike PMDA-PDA and ether-linked ODPA-PDA polyimides showed excellent orientation and poor ordering of chain in x-ray diffraction patterns, whereas both biphenyl-linked (BPDA-PDA) and carbonyl-linked (BTDA-PDA) polyimides exhibited excellent orientation and high ordering of chain. However, hexafluoroisopropylidene-linked 6F-PDA polyimide is structureless. The coherence length along the chain axis, which is a measure of chain rigidity and ordering, was estimated from $\text{(00l)}$ peaks of x-ray diffraction patters: 6F-PDA (could not be measured) <ODPA-PDA (95 Å)<BTDA-PDA (98 Å)<BPDA-PDA (104 Å)<PMDA-PDA (130 Å). In films prepared on substrates, all polyimides were preferentially oriented in the film plane. However, the degree of molecular in-plane orientation strongly depended on the coherence length. Higher coherence length, i.e., higher chain rigidity and ordering, caused higher in-plane orientation of chains. However, the chain rigidity is more critically responsible for molecular in-plane orientation, in comparison to the chain ordering. Higher in-plane orientation of chains led to larger anisotropies of refractive indices and dielectric constants, higher in-plane modulus, lower in-plane thermal expansion coefficient, and lower residual stress.