A series of poly(ester imide)s (PEIMs) has been synthesized from N-[4-(chloroformyl)phenyl] -4-(chloroformyl)phthalimide and different diols with methylene units of 4 to 12 (n). PEIMs with both even and odd methylene units exhibit a monotropic mesophase behavior during cooling due to the supercooling necessary for crystallization. Identification of this mesophase as a monotropic liquid crystal with smectic A order has been carried out via differential scanning calorimetry, wide angle X-ray diffraction (WAXD), small angle X-ray scattering, polarized light microscopy, and transmission electron microscopy. It is found that the methylene units in these chain molecules are also largely responsible for the formation of this liquid crystal phase. After investigating different types of main chain mesogen-nonmesogen liquid crystal polymers, a general concept considering the contributions to enthalpy aind entropy changes during the liquid crystal transitions may be associated with the relative rigidity, linearity, and regularity of the mesogenic groups compared to the methylene units. Detailed WAXD study of fiber and powder patterns indicates that the degree of orientation and the order correlation lengths along and perpendicular to the direction of chain molecules (chain lateral packing and layer structure) not only increase with the number of methylene units but also show an even-odd alternation. A possible chain packing model is suggested. The morphology and defects of this smectic A liquid crystal phtwe are also discussed.

A new high molecular weight aromatic polyimide has been synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-dimethyl-4,4'-diaminobiphenyl (DMB) in p-chlorophenol at elevated temperature. BPDA-DMB fibers have been spun by a dry-jet wet spinning method. The fibers were elongated and annealed at elevated temperatures above 400 "C to achieve excellent mechanical properties. In seven times drawn fibers, the BPDA-DMB molecule packs into a triclinic unit cell with dimensions of u = 2,10(2) nm, b = 1,523(8) nm, c = 4,12(7) nm, a = 61,2(6) ", / 3 = 50,7(7) O, and y = 78,9(6) with the number of chain repeating units per unit cell (2) is sixteen. After annealing at elevated temperatures, the fibers produce a small modification of the unit cell [a = 2,048(6) nm, b = 1,529(5) nm, c = 4,00(2) nm, a = 62,1(3) ' , /3 = 52,2(3) O and y = 79,6(3) "I. By increasing the draw ratio, both the crystallinity and crystal orientation increase. The BPDA-DMB fibers possess a decomposition temperature of 530 "C in nitrogen and 500°C in air at a 5% weight loss when the heating rate is 10"C/min. After extensive drawing, BPDA-DMB fibers exhibit a tensile strength of 3,3 GPa and a tensile modulus of over 130 GPa.Dynamic mechanical behavior of the fibers show both a (glass transition) and p (sub-glass transition) relaxations above room temperature. The nature of the sub-glass transition behavior is described as a noncooperative motion attributed to the diamine portion of the molecule. The activation energy for this relaxation in as-spun fibers is 109 kJ/mol and increases to 144 kJ/mol by increasing the draw ratio. This p relaxation is found to be crystallinity dependent. The a transition is also suppressed by crystallinity which increases with draw ratio.

Most aromatic polyimides and copolyimides show a β relaxation process below their α relaxation processes in dynamic mechanical and dielectric measurements. The origin of this β relaxation has been under active discussion. In order to gain a better understanding of the origin of the β relaxation process, the dynamic mechanical behavior of a series of segmented rigid‐rod polyimide and copolyimide films with different dianhydrides and diamines has been systematically studied. For some of the polymers, crystallinity and orientation effects on the β relaxation process were also investigated. It is found that the β relaxation process in the unoriented films can mainly be attributed to a relatively noncooperative motion in uncrystallized diamines. However, this noncooperative nature may be gradually lost by increasing the crystallinity and orientation. Molecular modeling and computation have led to a discussion of possible diamine motion in the β relaxation process.

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