Layer Structures. 3. Poly(p-phenylene−terephthalate)s with One, Two, or Four Alkyl Substituents:  Thermotropic and Isotropic Rigid Rods

Abstract

Nine polyesters were synthesized by polycondensation of silylated hydroquinone with substituted terephthaloyl chlorides. Three mono(alkylthio), three bis(alkylthio), and three tetra(alkylthio)terephthaloyl chlorides were used as reaction partners. The length of the alkyl side chains was varied among 8, 12, and 16 carbons. WAXS powder patterns indicate that these polyesters form three different types of sanidic layer structures in the solid state depending on the number of side chains per repeating unit. All three layer structures have an interdigitating of the alkyl side chains in common. As evidenced by 13C NMR CP/MAS spectroscopy and DSC measurements, the longer side chains form crystalline paraffin domains between the stacks of the main chains. The monosubstituted polyesters form two different LC phases, a biaxial nematic phase with layer structure and at higher temperatures a normal nematic phase. The disubstituted polyesters exclusively form a biaxial nematic melt. Increasing length and number of the side chains reduce the isotropization temperature of all polyesters, and the tetrasubstituted polyesters form an isotropic melt directly upon melting of the crystallized side chains. This result demonstrates that rigid rod polymers are not necessarily mesogenic. The failure to form a LC phase and the good solubility in many common solvents is attributed to poor attractive electronic interactions between the main chains. A more detailed interpretation was achieved by computer modeling.

Molecular Dynamics Simulations of Polymers of Unsubstituted and Substituted Poly(p-phenylene terephthalate)s in the Bulk State

“…The isotropization temperature (T i ) was 643 K as revealed by optical microscopy at a heating rate of 10 K/min. [7] Thus, the simulations at 300 K, 375 K, and 450 K should yield the structure in the crystalline state and the simulations at 575 K, 600 K, and 625 K, the structure in the liquid crystalline state, respectively. The whole simulation time varied for different polymers and temperatures from 300 ps to 2 700 ps.…”

“…The starting structure for the simulation of PTA8 was obtained from the experimental X-ray diffraction pattern of a polymer very similar to PTA8. [7] It had sulfur as the linking atom between the main chain and the side chains instead of oxygen, but this detail is not assumed to be essential when the average orientation is concerned. There are several other results indicating that rigid rod polymers based on monosubstituted building blocks tend to form "double stacks" with a back-to-back array of the main chains [7,34] , see Figure 2.…”

Synthesis and light‐emitting properties of a novel π‐conjugated poly[di(p‐phenyleneethynylene)‐alt‐ (p‐phenylenecyanovinylene)] containing n‐octyloxy side branches

“…This peak was characteristic of a typical layer structure, 29 in which the main chains form rigid backbone layers and the flexible side chains form a separate crystal region.…”

Layer structures, 13. Chiral sanidic polyesters derived from 2,5-bis(alkylthio)terephthalic acids

“…Polyesters derived from 2,5-bis(alkoxy)-or 2,5-bis(alkylthio)-substituted terephthalic acids and hydroquinone [1][2][3][4][5] or 4,49-dihydroxybiphenyl 6) form so-called sanidic (board-like 1,7) ) layer structures in the solid state. In contrast to smectic layers the sanidic layers consist of stacks of main chains with the side chains pointing sideward, so that the layer planes parallel the main chains.…”