Three series of randomly branched copolyesters were prepared from mixtures of acetylated trimethylsilylesters of non‐toxic natural hydroxy acids. The first series is based on 1 : 1 molar ratio of β‐(4‐hydroxyphenyl)propionic acid (HPPA, phloretic acid) and 4‐hydroxybenzoic acid (4‐HBA). The second series was prepared from HPPA, 4‐HBA and vanillic acid (VA, molar ratio 1 : 1 : 1), and in the third series VA was replaced by 4‐hydroxycinnamic acid (HCA). The branching density was varied via the feed ratio of triacetylgallic acid. The stability of the nematic phase increases with decreasing branching density, but a partial LC‐character was found even at relatively short length of the linear chain segments. Finally, two star‐shaped and hyperbranched copolyesters were prepared. The incorporation of a star center type molecule allowed the control of the molecular weight, but eliminated the liquid‐crystalline character almost completely. The ternary copolyesters and the quaternary copolyesters of VA were characterized by dynamic rheological measurements, and in the latter case also a few stress‐strain measurements were conducted.

In addition to the homopolyesters, poly(4-hydroxybenzoate) and poly(4-phenylene terephthalate), two series of copolyesters have been prepared with a systematic variation of the molar ratio. The first series was prepared by polycondensation of 4-acetoxybenzoic acid, terephthalic acid and acetylated hydroquinone in a liquid inert reaction medium at 400 °C. The second series was prepared under the same reaction conditions but from silylated 4-acetoxybenzoic acid and silylated terephthalic acid and showed a considerably higher 4-hydroxybenzoate/terephthalate ratio than expected from the feed ratio. The composition of these copolyesters was checked by 1H NMR spectroscopy after hydrolysis. All copolyesters proved to be highly crystalline regardless of the molar composition. WAXS measurements were conducted with synchrotron radiation up to 450 °C. These WAXS patterns indicate a gradual phase transition between 150 and 300 °C which is not detectable in the DSC curves. For the copolyesters containing > 50 mol% or 4-Hybe (4-hydrobenzoic acid) units one or two reversible first-order phase transitions were found in the DSC traces. Electron microscopy revealed that all these copolyesters are crystal powders consisting of particles of irregular shape. However, copolycondensations of silylated 4-acetoxybenzoic acid with silylated terephthalic acid and acetylated hydroquinone yielded copolyesters with a ‘potato-chip morphology’.

SUMMARY: Three series of randomly branched copolyesters were prepared from mixtures of acetylated trimethylsilylesters of non-toxic natural hydroxy acids. The first series is based on 1 : 1 molar ratio of b-(4-hydroxyphenyl)propionic acid (HPPA, phloretic acid) and 4-hydroxybenzoic acid (4-HBA). The second series was prepared from HPPA, 4-HBA and vanillic acid (VA, molar ratio 1 : 1 : 1), and in the third series VA was replaced by 4-hydroxycinnamic acid (HCA). The branching density was varied via the feed ratio of triacetylgallic acid. The stability of the nematic phase increases with decreasing branching density, but a partial LC-character was found even at relatively short length of the linear chain segments. Finally, two star-shaped and hyperbranched copolyesters were prepared. The incorporation of a star center type molecule allowed the control of the molecular weight, but eliminated the liquid-crystalline character almost completely. The ternary copolyesters and the quaternary copolyesters of VA were characterized by dynamic rheological measurements, and in the latter case also a few stress-strain measurements were conducted.