Synthesis and Properties of Thermotropic Copolyesters Based on Poly(ethylene terephthalate) and 4′-Acetoxy-4-biphenyl-carboxylic Acid

Abstract

A series of novel copolyesters based on polyethylene terephthalate (PET) and 4′-hydroxy-biphenyl-4-carboxylic acid (HBCA) was obtained by melt polycondensation of bis(2-hydroxyethyl) terephthalate and 4’-acetoxybiphenyl-4-carboxylic acid (ABCA) as co-monomers with Sb2O3 as a catalyst. Using this synthetic procedure, a set of copolymers containing 20–80 mol% of HBCA units was prepared. According to NMR spectroscopy, the copolymers were of random composition. Copolyesters comprising 60–80 mol% of HBCA possessed increased heat resistance and formed nematic melts at 270 °C and higher. The liquid crystal (LC) phase formation was accompanied by transition to non-Newtonian characteristics of the melt flow, as well as an equalization of storage and loss moduli values. According to XRD and polarizing microscopy, the LC glassy phase of the copolyesters coexists with crystalline regions of poly-(4’-hydroxy-4-biphenylcarboxylate), non-melting up to 400 °C and above. The mechanical characteristics of these LC copolyesters showed similar or better values than those of well-known LC polymers. These novel copolyesters can be useful in obtaining heat-resistant materials with an ordered structure and, as a consequence, improved performance.

Fully Aromatic Thermotropic Copolyesters Based on Vanillic, Hydroxybenzoic, and Hydroxybiphenylcarboxylic Acids

“…Generally, TMCPs exhibit a schlieren texture typical of nematic melts. Similar results were obtained for copolyesters based on hydroxy acids and aromatic diols/diacids of various structures [19,43,49,50].…”

“…Generally, TMCPs exhibit a schlieren texture typical of nematic melts. Similar results were obtained for copolyesters based on hydroxy acids and aromatic diols/diacids of various structures [19,43,49,50]. The ternary copolyesters did not demonstrate isotropization up to 375 °C, which was the apparatus limit and higher than the decomposition temperature.…”

“…At the same time, they have noticeably lower glass transition temperatures (~117 • C) even compared to the binary VA-HBCA copolyesters (~135 • C) and especially compared to the 3HBA-3HBCA copolyesters (4 ′ -hydroxybiphenyl-3-carboxylic), for which T g can reach 190 • C [44]. As has been repeatedly shown, including in our early works [19,38], in polyesters based on aromatic hydroxy acids, a comonomer with biphenyl units-in this case, HBCA-makes a large contribution to heat resistance. The ratio of biphenyl and monophenyl comonomer units in the binary VA-HBCA copolymers is higher than in the ternary VA-HBA-HBCA copolymers, which leads to a decrease in T g values.…”

“…The average values of their mechanical characteristics were as follows: tensile strength of 39 MPa, elastic modulus of 125 MPa, and relative elongation at break of 4%. These characteristics are close to those for semi-aromatic copolyesters of polyethylene terephthalate and HBCA [19], but they are much lower than the typical values for fully aromatic thermotropic polymers. We assume that this problem is associated with the fairly The ternary copolyesters did not demonstrate isotropization up to 375 • C, which was the apparatus limit and higher than the decomposition temperature.…”

“…In short, fully aromatic TMCPs are produced from conventional oil-based monomers: hydroquinone; 4,4′-dihydroxybiphenol; 4-hydroxybenzoic, 6-hydroxy-2-naphthoic, terephthalic, isophthalic, and 2,6-naphthalenedicarboxylic acids; etc. Aliphatic–aromatic TMCPs are prepared through chemical modification of poly(ethylene terephthalate) by 4-acetoxybenzoic acid via ester–ester exchange (X7G Eastman Kodak, Rodrun Unitika), resulting in a copolymer of 4-hydroxybenzoic acid with flexible spacer units of PET [ 19 ].…”