EFFECTS OF CHEMICAL-STRUCTURE AND PRECURSOR ON OPTICAL-PROPERTIES, THERMOMECHANICAL PROPERTIES, AND MOLECULAR-ORIENTATION AND ORDER OF THIN-FILMS OF STIFF AROMATIC POLYIMIDES

Thin films of rigid poly(p-phenylene pyromellitimide) (PMDA-PDA) and s emi-rigid poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepa red by thermal imidization of the respective poly(amic acid) and poly( amic ethyl ester) precursors, were characterized with respect to their optical, thermomechanical and structural properties. Both polyimides exhibit an unusually large anisotropy between the in-plane and out-of- plane refractive indices, with Delta n ranging from 0.198 to 0.216 for PMDA-PDA and from 0.230 to 0.242 for BPDA-PDA, nearly independent of the nature of the initial polyimide precursor, him thickness, and him preparation method. PMDA-PDA films exhibit low coefficients of thermal expansion (CTE's) of 6.5 and 8.2 ppm/degrees C for the acid-derived a nd the ester-derived polyimides, respectively. In comparison, the BPDA -PDA films show CTE values of 4.3 and 18.0 for the acid-derived and es ter-derived samples, respectively, despite the small differences in th eir optical anisotropies. Wide-angle x-ray diffraction patterns obtain ed in reflection and, transmission for the various samples reveal a st rong in-plane chain orientation for both PMDA-PDA and BPDA-PDA polyimi des, with somewhat better intermolecular packing order for the ester-d erived polyimide films. These effects of chemical structure and precur sor on properties and structures of the polyimide films are discussed in light of recent theoretical considerations of semiflexible polymers .