Langmuir–Blodgett films of physical and chemical polyimide mixtures from linear and network precursors

Physical and chemical mixture Langmuir–Blodgett polyimide films were obtained by reacting mellitic acid hexamethyl ester with pyromellitic anhydride and 4,4′-diaminodiphenyl ether followed by formation of polyamic acid salts with octadecyl amine. The additions of mellitic acid hexamethyl ester provided more reinforcement to the copolymers than the blends because of the more evenly distributed linear and network segments as observed from the Π-A isotherms. The alkyl chains randomly tilted to the normal of the substrate at an angle about 20° while the tilt angels increased with the increase of the amount of mellitic acid hexamethyl ester. The ordered chain packing pattern along the dipping direction was disrupted and even disappeared if more mellitic acid hexamethyl ester was added as shown from the atomic force microscopy images. Accordingly, the X-ray diffraction results showed that the crystallinity decreased with the increase of the amount of mellitic acid hexamethyl ester. The branched component, the mellitic acid hexamethyl ester, had much more effects both in structure and morphology on the copolymers than on the blends.