The structure of wholly aromatic polyesters with bulky side chains: Poly(phenylene phenyl-terephthalate)

Abstract

The thermotropic polyester prepared from phenyl-terephthalic acid and hydro-quinone is highly crystalline, despite the probable random 2-and 3-disposition of the phenyl substituents. The x-ray pattern of melt-spun fibers contains 18 Bragg reflections that are indexed by a monoclinic unit cell with dimensions a = 28.0 Å. b = 4.89 Å, c = 12.48 Å (fiber axis), and γ = 114.8°, containing monomer units of four chains. In the ac-plane, the chains are arranged in pairs with the phenyl side chains interdigitated; successive pairs of chains are staggered by about c/2. We have used molecular mechanics modeling to simulate arrays of chains with random 2-and 3-disposition of the side chains on the terephthalic acid units and have compared the results with those for a similar structure in which all the substituents were at the 2-position. The refined model for random substitution is distorted, but the average separations of the monomer units are within experimental error of the observed unit cell dimensions, and their standard deviations are very similar to those derived from the line-broadening data. The potential energy of the random substitution model is only about 1 kcal/mol of monomer higher than that for the model with all 2-substitution, indicating the random substitution is not a problem for the formation of an ordered structure.