New All-aromatic Liquid Crystalline Polyester Thermosets with High Glass-Transition Temperatures for Composite Applications

Wholly aromatic thermotropic liquid crystalline polymers (TLCPs) are an important class of high-performance materials, which offer a combination of excellent thermal and mechanical properties. TLCPs can be processed from the nematic melt into high-modulus fibers, films and molded articles. However, despite their all-aromatic composition, main-chain TLCPs exhibit low glass transition temperatures (100-120 o C) and low moduli at elevated temperatures, which significantly limits their final use temperature. Recently we demonstrated an alternate route towards all-aromatic TLCPs.[1] We synthesized oligomers end-capped with reactive phenylethynyl functionalities. This approach allows us to take advantage of the low melt viscosity inherent to oligomeric species. In a latter, high temperature step, the reactive end-groups are activated and chain extension or crosslinking is initiated. In this paper we will present our current work, where we are utilizing this reactive oligomer approach towards the synthesis of melt processable all-aromatic esterbased TLCPs with Tg’s in excess of 200 o C. We have modified a well-known LCP formulation based on Hydroquinone (HQ) and Terephthalic acid (TA), an LCP with a Tg of 267 0 C and a Tm of approximately 470 0 C, which is far above the decomposition temperature of this polymer.[2] We synthesized reactive oligomers, based on TA and HQ, with Mn = 1000, 5000, and 9000 g/mol, and varied the backbone chemical composition using Isophthalic acid (IA), 6-Hydroxy-2-naphthoic acid (HNA), 4-Hydroxy-benzoic acid (HBA) or Chloro hydroquinone (Cl-HQ). All fully cured polymers showed high glass-transition temperatures in the range of 164-275 0 C, and exhibit high storage moduli at elevated temperatures (>1 GPa at 200 0 C). Some oligomers exhibited low melt viscosities and and are therefore easy to process in complex articles such as fiber-reinforced composites. The cured polymers have excellent flexural and adhesive properties. All oligomers display nematic mesophases and in most cases, the nematic order is maintained after cure. Based on these preliminary results, we believe that our polymers offer similar or even improved properties over current state-of-the-art high-performance polymers such as PPS, PEKK and PEEK.