X-ray structure analysis of a thermoplastic polyimide

The molecular and crystal structure of a new thermoplastic polyimide, poly\(5,7-dihydro-1,3,5,7-tetraoxobenzo\1,2-c:4,5-c'\ dipyrrole-2,6(H,3H)-diyl)-1,3-phenyleneoxy\1,1'-biphenyl\-4,4'-diy loxy-1,3-phenylene\ (TPI), was analyzed by means of an X-ray diffraction method and a linked-atom least-squares method. From diffraction patterns of oriented films of this polyimide, unit cell dimensions were determined to be a = 7.89 A, b = 6.29 A, c(fiber axis) = 25.11 A, and beta = 90.0 deg C. The space group was P2 sub 1 . The unit cell contains two polymer chains with a 1 /1-helical symmetry. One chain is located at the center of the four equivalent polymer chains. There are no interactions between neighboring molecules but van der Waals interactions. Aromatic ring planes tilt approx 30-40 deg C to the neighboring planes due to the short atomic interactions between directly-linked aromatic moieties. Because of the mutual compensation of two bending parts of the polymer chain, the overall molecular conformation is quite straight, which gives TPI mechanical strength and stability. On the other hand, the four degrees of freedom of TPI decrease the melting temperature to 382 deg C, which enables the melt processing of this polyimide.