All‐aromatic ester‐based thermosets: The effect of crosslinking in the nematic or isotropic state

Abstract

We'll report on two series all‐aromatic main‐chain reactive oligomers that can be crosslinked in either the nematic phase or in the isotropic phase. This series is unique in that both model systems have an identical backbone geometry, comprised of hydroquinone with or without a phenyl substituent and phenyl substituted terephthalic acid. Crosslinking the oligomers (Mn of 1–9 kg/mol) via maleimide end‐groups in the nematic or isotropic phase yields networks with similar crosslink densities (Mc) and similar thermal properties. Crosslinked films exhibit high decomposition temperatures (>395°C) and amorphous thermoset films exhibit Tg's in the 141–190°C range whereas nematic thermoset films give Tg's that range from 143 to 176°C, as measured by DMTA. However, the phase type appears to have a major effect on the stress–strain behavior of the films. All films prepared by crosslinking un‐aligned nematic oligomers show poor stress–strain behavior (σ = 20–63 MPa, ε = 0.5%–5.4%), whereas crosslinking the amorphous oligomers results in films with excellent stress–strain properties (σ = 94–97 MPa, ε = 7.1%–13.3%). The superior toughness of the cured amorphous films can be attributed to the larger free volume induced by steric crowding of the phenyl substituents in the polymer repeat unit.

Chemistry and Rheological Behavior of Cross-Linked Liquid Crystal Polyarylate with a Mixed End-Capping Strategy

“…The phenylethynyl group (PEPA) could be cured at 370 °C but is kept latent during polycondensation at ∼300 °C, which had successfully terminated various aromatic thermoplastic polymers (i.e., LCP and polyesterimide) in the past 2 decades. − This strategy not only effectively reduced melting point ( T m ) and improved processability of thermoplastics but also significantly enhanced their thermomechanical properties with the formation of cross-linked networks through the cure reaction of PEPA. For instance, PPTA exhibited a glass transition temperature ( T g ) of 267 °C and a T m of 610 °C, which made it rather intractable that the polymer in fact decomposed before it melted.…”