In this paper the synthesis and characterization of two series wholly aromatic, main-chain, reactive liquid crystalline oligomers is reported. Phenylethynyl end-capped oligomers, based on 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA), were successfully synthesized using standard high-temperature melt-condensation techniques. Melt processable oligomers, with M n ) 1000, 5000, 9000, and 13 000 g mol -1 , and oligomers with HBA or HNA concentrations as high as 95 mol % were prepared. All oligomers showed well-defined, homogeneous, nematic melt behavior over a broad temperature range. The phenylethynyl endcapped oligomers could be cured at 370 °C and formed films with excellent mechanical and thermal properties, and without losing the nematic order. Low molecular weight oligomers, i.e. M n < 5000 g mol -1 , tend to form nematic thermosets, while oligomers with M n > 5000 g mol -1 , polymerize predominantly via chain extension chemistry. The fully cured nematic polymers exhibit glass-transition temperatures (T g ) up to 203 °C, as determined by DMTA measurements, and thermal stabilities (5 wt % loss) up to 500 °C in both air and nitrogen atmosphere. The cured 1000 and 5000 g mol -1 oligomers, with 95 mol % HBA or HNA compositions, show significant improvements in storage modulus at elevated temperatures as compared to their high-molecular weight counterparts. Rheology experiments showed that these reactive nematic oligomers are melt-stable for at least 30 min at 300 °C and exhibit complex melt viscosities |η*| as low as 1 Pa‚s at 100 rad‚s -1 . In all examples no loss of mesophase could be observed during chain extension or cross-linking, which indicates that phenylethynyl cross-link chemistry is completely compatible with mesophase formation.

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Over the past two decades, there has been an increasing interest in all-aromatic crosslinked liquid crystal (LC) systems for structural, electronics and coatings applications. In this paper we will give a brief overview of our work on oligomeric liquid crystals end-capped with phenylethynyl reactive end-groups. All reactive oligomers were synthesized using standard melt-condensation techniques and the final products form homogeneous nematic melts over a wide temperature range (220-400 uC). The reactive LC oligomers could be cured at elevated temperatures (310-400 uC) to form true rigid-rod nematic networks. As will be demonstrated, this oligomer approach allows us to design all-aromatic nematic networks with a variety of favorable physical, mechanical and processing characteristics. Depending on the backbone chemistry, and end-group concentration, we can access liquid crystal thermosets with Tg's between 110-280 uC and storage moduli (E') of 2-5 GPa. The oligomeric nature of our reactive LCs results in excellent melt processing characteristics (|g*| > 1 Pa.s at 100 rad.s 21 ) and allows for the processing of complex structures such as fiber-reinforced composites. Based on our current results we are convinced that reactive all-aromatic liquid crystals can be interesting alternatives over existing high-performance polymers such as polyarylether ketones (PEEK and PEKK) polyphenylenesulfide (PPS) and polyetherimides (PEIs).

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