Spontaneous molecular orientation of polyimides induced by thermal imidization (5). Effect of ordered structure formation in polyimide precursors on CTE

Six poly(amic acid) (PAA) systems based on pyromellitic dianhydride (PMDA) formed some ordered structures with optical anisotropies clearly detectable on an optical polarizing microscope (POM) in N-methyl-2-pyrrolidone (NMP) at room temperature at high solute concentrations (15–25 wt.%) with complete sol–gel transition reversibility, whereas PAA systems based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) with a variety of diamine components showed no optical anisotropy in solution. However, a fluorescence probe technique combined with solution viscosity measurements suggested that a PAA derived s-BPDA with 1,4-phenylenediamine (PDA), i.e., PAA(s-BPDA/PDA) forms some ordered structure with a POM-undetectable very local scale during prolonged storage in NMP at room temperature. The introduction of the biphenyldiimide (BPDI) units at 33% into the PAA(s-BPDA/PDA) main chains by copolymerization allowed the formation of optically anisotropic gels with a smectic liquid crystal-like ordered structure by cooling the NMP solution at −20 °C. PI films derived from s-BPDA with PDA, i.e., PI(s-BPDA/PDA) were prepared upon thermal imidization of the BPDI-containing PAA films dried at 40 °C for 2.5 h. An increase in the BPDI content caused a gradual decrease in the linear coefficient of thermal expansion (CTE) of the PI films. This can be interpreted as a result of an intensified pre-orientation at the stage of the PAA cast films by incorporation of the BPDI units. When the BPDI-containing PAA solutions were heated at 70 °C for 4 min prior to the drying process at 40 °C, the ordered structures can be cancelled without imidization, and the CTE values of the resulting PI films appreciably increased compared to the case without heating at 70 °C. A similar effect was observed even in the BPDI-free original s-BPDA/PDA system. The results suggest the presence of a POM-undetectable very locally ordered structure in the PAA cast films, which promotes the pre-orientation of the PAA chains in the cast films and consequently can contribute to a further decrease in the CTE of the PI(s-BPDA/PDA) films.