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

Poly(p-phenylene phenylthio-terephthalate) (PPTT) forms nematic melts and i s highly crystalline in the solid state, despite the probable random 2- and 3-disposition of the S-phenyl substituents. The X-ray pattern of melt-spun fibers of PPTT contains 24 Bragg reflections that are indexed by a monocli nic unit cell with dimensions a = 28.6 Angstrom, b = 4.81 Angstrom, c = 12. 57 Angstrom (fiber axis), and gamma = 101.6 degrees. The cell contains mono mer units of four chains that are arranged in pairs with the thiophenyl sid e chains interdigitated; successive pairs of chains are staggered by about c/2. We used molecular mechanics modeling to simulate arrays of chains with random 2- and 3-disposition of the side chains on the terephthalic acid un its and compared the results with those for an idealized structure in which all the substituents were at the 2-position. The refined model for random substitution is more distorted, but the average separations of the monomer units are within the experimental errors of the observed unit cell dimensio ns, and their standard deviations are very similar to those derived from th e line-broadening data. The potential energy of the model with random subst itution is only about 1.9 kcal/mol of monomer higher than that for the mode l with all-2-substitution, indicating random substitution is not a major pr oblem to the formation of an ordered structure. Compared to the structure f ormed by the analogous polyester, poly(p-phenylene phenylterephthalate) (PP T), the additional flexibility due to the thio-ether linkage between the ba ckbone and phenyl side groups in PPTT allows better chain packing both with in and between the layers of stacked chains.