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Abstract

In order to improve the melt processability of all-aromatic reactive liquid crystal oligomers and to allow solution processing, a series of branched liquid crystalline oligomers end-capped with reactive phenylethynyl end-groups were explored. 5-hydroxyisophthalic acid (HIA) was used as a trifunctional, A₂B-type monomer, and the concentration of HIA in the polymer backbone was varied from 2.5 to 50 mol.%. The incorporation of a small concentration of HIA, i.e. 2.5 mol.%, has the tendency to reduce the crystal-to-nematic (K—N) transition, whereas an HIA concentration of 50 mol.% resulted in an isotropic polymer. A fully cured liquid crystal thermoset based on terephthalic acid (TA), hydroquinone (HQ), isophthalic acid (IA) and 10 mol.% HIA, labeled TA/HQ/IA/HIA(10)-5K _p , showed a high glass-transition temperature (T _g) of 300 ^°C, which is an increase of ∼80 ^°C over its linear counterpart. The thermal stability of all cured polymers was excellent both in air and nitrogen atmosphere ( T _d(5%) > 450 ^°C). Room temperature storage moduli varied from 2.3—5.1 GPa and most thermosets display moduli of 1 GPa or higher at 200 ^°C. The branched oligomers showed stable nematic mesophases, and after a 1 h cure at 370 ^°C a nematic thermoset was obtained exhibiting a fixed nematic texture. Although branching had a positive effect on lowering the T _m (K—N) and increasing the after cure T _g, the reactive oligomers remained insoluble. In addition, branching has a negative effect on the stress—strain behavior. Room temperature tensile strengths are in order of 38 MPa and the films fail at 3% elongation.

Keywords

Liquid crystal branched reactive oligomers thermosets

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Synthesis,Characterization and Properties of Branched All-aromatic Liquid Crystal Thermosets

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