Structure–property study of polyimides derived from PMDA and BPDA dianhydrides with structurally different diamines

A series of aromatic diamines were polymerized with two aromatic dianhydrides, pyromellitic dianhydride and 3,3′,4,4′-biphenyltetracarboxylic dianhydride, and the resulting poly(amic acid)s were thermally cyclodehydrated to aromatic polyimides. The polyimides were characterized by determining the glass transition temperatures (Tg), thermal stability, coefficients of thermal expansion, and wide-angle X-ray diffraction. Structure–property relationships are elucidated and discussed in terms of the structural fragments in the polymer chain. The PMDA-based polyimides generally revealed a higher Tg than the corresponding BPDA-based analogues. Generally, the dilution of the imide content by the insertion of oxyphenylene segments into the diamines significantly reduced the Tg. The introduction of m- or o-phenylene units into the polymer backbone usually resulted in a decrease in Tg. The attachment of pendant groups on the backbone may lead to decreased or increased Tgs, depending on the structure of pendant groups. As evidenced by X-ray diffraction, the polyimides derived from rigid, rod-like diamines or the diamines having two or three p-oxyphenylene showed a higher crystalline tendency. The presence of aliphatic pendant groups slightly reduced the thermal stability of the polyimides. The other structural changes did not show a dramatic influence on the thermal stability. Some polyimides obtained from p- or m-phenylenediamine had low thermal expansion coefficients below 2×10−5 °C−1.