Abstract
The FTIR spectroscopic examination of aromatic polyimide (PI), poly (p-phenylene biphenyltetracarboxydiimide) (BPDA-PDA) prepared under different (thermal and chemical) imidization conditions, revealed a number of IR active vibrational modes sensitive to molecular (conformational and packing) order. The spectral criteria obtained were employed to characterize the structure organization in the BPDA-PDA films cured in a free-standing state as a function of their thermal histories : temperatures of imidization (Ti = 150-400°C) and annealing (Ta = 300-400°C); cure protocol (one-step/stepwise), and film thickness (t = 1-100μm). The investigation revealed that the conformational changes toward the crystalline preferred chain conformation could be observed even at early stages of imidization - at Ti's as low as 150-170°C, whereas the spectral features of the crystal-like order appeared for the samples cured at Ti's close to or above the Tg. It was found that annealings at Ta's as high as 350-400°C do not completely erase the effects of former processing history, suggesting that the thermally induced molecular mobility is not the only factor governing the structure organization processes. For samples obtained under the same processing conditions, thicker films always displayed much higher order, indicating the plasticizing action of the volatiles retained in the film as another important factor. The results are also presented and discussed on the relative contributions to the molecular mobility of the traces of the casting solvent [N, N-dimethylacetamide (DMAc) / N-methyl-2-pyrrolidone (NMP)] as well as that of the by-products of imidization [H2O - for polyamic acid (PAA) / CH3OH - for polyamic ester (PAE)].
