X-RAY-ANALYSIS OF THE STRUCTURE OF A WHOLLY AROMATIC POLYESTER WITH ALKOXY SIDE-CHAINS

X-ray diffraction has been used to determine the structure of the poly(p-phenylene 2,5-di-hexadecyloxy-terephthalate), which is one of a series of polyesters with stiff aromatic mainchains and flexible aliphatic sidechains currently being studied by this laboratory. The X-ray data for oriented melt quenched films show that this polymer adopts an interesting and novel structure, in which the backbones and sidechains form separate incommensurate lattices. The backbones are fully extended, and adopt a monoclinic structure with unit cell dimensions: a =26.2angstrom, b=7.3angstrom, c = 12.6angstrom, and alpha = 130-degrees-C. They are in contact along the b axis, thereby forming stable sheets that are separated by intercalated, highly extended sidechains, which point along the a axis. The sidechains are packed in a columnar, approximately hexagonal structure that is similar to the hexagonal form of linear alkanes. The repeat for the sidechain lattice along the backbone direction is c = 13.2angstrom, and thus the two lattices are incommensurate. We envisage a small disordered boundary layer between the backbone and sidechain lattices, containing some of the initial and final carbons of the sidechain, and accommodating any conformational isomerism. This allows an independent lattice to be formed by the central sections of the sidechains, but this cannot propagate indefinitely because of the different c dimension and the restrictions imposed by covalent linkage to the backbones. As a result, the sidechain crystallites are only approximately 50angstrom wide, compared to approximately 200angstrom for those formed by the backbones. Simulated X-ray patterns for the above incommensurate structure show good qualitative agreement with the observed data.