All‐aromatic liquid crystalline thermosets with high glass transition temperatures

Abstract

We have synthesized and characterized a new family of low melting allaromatic ester-based liquid crystal oligomers end-capped with reactive phenylethynyl end groups. In a consecutive, high-temperature step, the reactive end groups were thermally activated and polymerization was initiated. This reactive oligomer approach allows us to synthesize liquid crystal thermosets with outstanding mechanical and thermal properties, which are superior to well-known high-performance polymers such as PPS and PEEK. We have modified an intractable LC formulation based on hydroquinone and terephthalic acid, with M n ¼ 1000, 5000, and 9000 g mol À1 , and varied the backbone composition using isophthalic acid, resorcinol, 4hydroxy-benzoic acid, 6-hydroxy-2-naphthoic acid, and chlorohydroquinone. All fully cured polymers showed glass transition temperatures in the range of 164-275 C, and high storage moduli at room temperature ($ 5 GPa) and elevated temperature ($ 2 GPa at 200 C). All oligomers display nematic mesophases and in most cases, the nematic order is maintained after cure. Rheology experiments showed that the phenylethynyl end group undergoes predominantly chain extension below 340 C and crosslinking above this temperature. Highly aligned fibers could be spun from the nematic melt, and we found that the order parameter hP 2 i was not affected by the chain extension and crosslink chemistry.Scheme 2. Synthesis of the terephthalic acid (TA)-and hydroquinone (HQ)-based liquid crystalline oligomers with phenylethynyl end groups. The oligomers in series 1 are terminated with the more linear para-substituted reactive end groups, whereas the series 2 are end capped with the meta-substituted phenylethynyl functionalities. ALL-AROMATIC LIQUID CRYSTALLINE THERMOSETS

Flexible all‐aromatic polyesterimide films with high glass transition temperatures

“…For example, introducing non‐aromatic bulky side‐chain substitutions will increase the T g , but at the same time the thermal stability is compromised. Introducing aromatic ketone‐, ether‐, amide‐ and imide‐based monomers (Figure ) lowers the backbone symmetry, disrupts crystallization and suppresses the T m while retaining the favorable thermomechanical properties . However, little has been reported on the room temperature stress–strain behavior of melt processed films or fibers.…”

High-Temperature Shape Memory Behavior of Semicrystalline Polyamide Thermosets

“…We attempted to investigate the cure chemistry using Raman and FTIR spectroscopy. 33 , 34 However, Raman spectroscopy failed due to a strong fluorescence background, and FTIR cannot detect the acetylene bond of PE because of IR insensitivity. Hence, direct evidence is not available to confirm the chemical change of the PE functionalities.…”

High-performance all-aromatic liquid crystalline esteramide-based thermosets

“…Introducing nonlinear aromatic monomers, such as substituted naphthalenes; 2,5-thiophenes; 1,3-substituted benzenes; and ketone-, ether-, amide-, and imide-based monomers, lowers the backbone symmetry, disrupts crystallization, and suppresses the T K-N , whereas the liquid crystalline (LC) phase might be lost. [6][7][8][9][10][11][12][13][14] Another approach is to lower the molecular weight of the polymer backbone, that is, prepare oligomers and introduce reactive functionalities such as phenylethynyl or maleimide end groups at the chain ends (typically M n of reactive oligomers ¼ 1000-9000 gÁmol À1 ). This lowers the T K-N and melt viscosity and makes it possible to process all-aromatic reactive oligomers, which are allowed to chain extend and/or cross-link in a successive high-temperature posttreatment step.…”