All-aromatic liquid crystalline thermosets with high glass transition temperatures

Fast-Curing of Liquid Crystal Thermosets Enabled by End-Groups Regulation and In Situ Monitoring by Triboelectric Spectroscopy

The development of high-performance polymer is crucial for the fabrication of triboelectric nanogenerators (TENGs) used in extreme conditions. Liquid crystal polyarylate thermosets (LCTs) demonstrate great potential as triboelectric material by virtue of exceptional comprehensive properties. However, there are only a few specific end-groups like phenylethynyl matching the LCT polycondensation temperature (above 300 °C). Moreover, the excellent properties of LCTs rely on the crosslinked network formed with long curing time at high temperature, restricting their further application in triboelectric material. Herein, a fast-curing LCT is designed by terminating with 4-maleimidophenol possessing appropriate reactivity. The resultant LCT (MA-LC-MA) exhibits much lower polycondensation temperature (250-270 °C) and curing temperature of 300 °C within only 1 min compared to typical LCTs (cured at 370 °C for 1 h). Furthermore, the cured MA-LC-MA retains a high glass transition temperature of 135 °C, storage modulus of 6 MPa even at 350 °C, and great electrical output performance. Additionally, triboelectric measurement related to the dielectric properties that vary with crosslinked network is innovatively utilized as an analysis technique of curing progress. This work provides a new strategy to design high-performance TENGs and promotes the development of next generation thermosets in extreme conditions.

Oxidative NHC-Catalysis as Organocatalytic Platform for the Synthesis of Polyester Oligomers by Step-Growth Polymerization

The application of N-heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil-based (ethylene glycol, phthalaldehydes) and bio-based (furan derivatives, glycerol, isosorbide) monomers. The catalytic dimethyl triazolium/1,8-diazabicyclo[5.4.0]undec-7-ene couple and stoichiometric quinone oxidant afforded polyester oligomers with a number-average molecular weight (M_n ) in the range of 1.5-7.8 kg mol^-1 as determined by NMR analysis. The synthesis of a higher molecular weight polyester (polyethylene terephthalate, PET) by an NHC-promoted two-step procedure via oligoester intermediates is also illustrated together with the catalyst-controlled preparation of cross-linked or linear polyesters derived from the trifunctional glycerol. The thermal properties (TGA and DSC analyses) of the synthesized oligoesters are also reported.

High-Temperature Shape Memory Behavior of Semicrystalline Polyamide Thermosets

We have explored semicrystalline poly(decamethylene terephthalamide) (PA 10T) based thermosets as single-component high-temperature (>200 °C) shape memory polymers (SMPs). The PA 10T thermosets were prepared from reactive thermoplastic precursors. Reactive phenylethynyl (PE) functionalities were either attached at the chain termini or placed as side groups along the polymer main chain. The shape fixation and recovery performance of the thermoset films were investigated using a rheometer in torsion mode. By controlling the M_n of the reactive oligomers, or the PE concentration of the PE side-group functionalized copolyamides, we were able to design dual-shape memory PA 10T thermosets with a broad recovery temperature range of 227-285 °C. The thermosets based on the 1000 g mol^-1 reactive PE precursor and the copolyamide with 15 mol % PE side groups show the highest fixation rate (99%) and recovery rate (≥90%). High temperature triple-shape memory behavior can be achieved as well when we use the melt transition ( T_m ≥ 200 °C) and the glass transition ( T_g = ∼125 °C) as the two switches. The recovery rate of the two recovery steps are highly dependent on the crystallinity of the thermosets and vary within a wide range of 74%-139% and 40-82% for the two steps, respectively. Reversible shape memory events could also be demonstrated when we perform a forward and backward deformation in a triple shape memory cycle. We also studied the angular recovery velocity as a function of temperature, which provides a thermokinematic picture of the shape recovery process and helps to program for desired shape memory behavior.

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

We have synthesized and characterized a new family of nematic all-aromatic polyesteramide thermosets based on 6-hydroxy-2-naphthoic acid (HNA), terephthalic acid (TA), and 4-acetamidophenol (AAP). In order to incorporate a high concentration of the amide-based monomer (AAP), the melt transition ( T K-N) and melt viscosity had to be lowered in order to maintain melt processable intermediates. Precursor thermoplastic reactive oligomers, end-capped with phenylethynyl functionalities, were prepared using standard melt condensation techniques with a target M n of 1000–9000 g mol−1. The reactive oligomers with 20–30 mol% AAP could easily be processed into films, and the films exhibit good tensile properties in terms of tensile strength (70–80 MPa) and elongation at break (7–10%). A glass transition of 191°C could be obtained when a 1000 g mol−1 oligomer (HNA/TA/AAP(20)–1 K) was thermally cross-linked. When the AAP concentration reaches 35 mol%, the rigidity of the backbone and the hydrogen bonding interactions are enhanced, which make HNA/TA/AAP(35) polymers difficult to process.

Synthesis, characterization, and properties of thermoplastic polyimides derived from 4,4’-(hexafluoroisopropylidene)diphthalic anhydride in diethylene glycol dimethyl ether

By regulating the order of monomer addition, four kinds of copolymer polyimides (PIs) were prepared using diethylene glycol dimethyl ether (DEGDE) and N, N-dimethylacetamide (DMAc) as the solvents. The molecular weights of polyamide acids (PAAs) ranged from 3.8 × 105 to 9.2 × 105. All of the films displayed high glass transition temperatures ( Tgs) ranging from 313°C to 346°C. The polymer films show excellent thermal stabilities with 5% weight loss at temperatures of 505–524°C and char yields at 800°C were as high as 55% under nitrogen. The peel strengths of flexible copper (Cu) clads were in the range from 0.337 N cm−1 to 0.598 N cm−1. Compared to the molecular weight and peel strength of fresh PAA, those of PAAs prepared using DMAc significantly decreased after storage for 3 months at 0°C. However, when DEGDE was used as the solvent, the molecular weights of the PAAs and the thermal properties of the PIs were maintained after long storage time.

Non-isothermal curing kinetics, chemorheological behaviour, and IR spectral study of two trifunctional phenylethynyl-terminated imide oligomers compared with the corresponding bifunctional structure

Two trifunctional phenylethynyl-terminated imide oligomers, m-TPEPA and p-TPEPA, were systematically compared with the corresponding bifunctional phenylethynyl-terminated imide oligomer, BPEPA. The non-isothermal curing kinetics, Master Plots method, and rheological behaviour of the three high performance oligomers were systematically studied by dynamic DSC, small amplitude oscillatory shear rheometry, and infrared spectroscopy. The results show that the activation energy ( Ea) depended on the extent of conversion evaluated with three different methods, and the lower Ea values of the trifunctional oligomers, which may lead to different curing reactions, compared with the sustained growth of Ea values of BPEPA. The chemorheological properties of the oligomers were measured and fit numerically with the dual Arrhenius model and gel model. Moreover, the degree of cure (agel ) at the gel time (tgel ) was calculated by the value of glass transform temperature at different curing temperature combined with rheological and isothermal DSC results. According to structural changes during the curing reaction characterised using FTIR spectra, it was inferred that the structures of the cured trifunctional imide resins have cis-configurations, which may change the part of the configuration from cis to trimerisation with the lower activation energies found in the later part of the conversion, while the difunctional imide structure is trans.

Rheological behaviour and cure kinetic studies of a trifunctional phenylethynyl-terminated imide oligomer

A phenylethynyl-terminated imide oligomer (PETI)was prepared from a triamine and 4-phenylethynylphthalic anhydride. The oligomer can be used to prepare high performance resin-based composite material via resin transfer moulding due to its low melt viscosity. The cured resin also exhibited excellent thermal stability with 5% weight loss at temperature above 531°C and higher glass transition temperature Tg (410°C) than PETI series as a result of the introduction of star-branched units. The rheological properties of the oligomer were measured and numerically fit with the dual Arrhenius model. The curing of the resin was also studied by dynamic and isothermal differential scanning calorimetry measurements. Based on the experimental data, both the curing reaction rate and the curing degree were calculated in the dynamic mode and tested under isothermal conditions.

Synthesis and properties of aligned all-aromatic liquid crystal networks

A series all-aromatic main-chain liquid crystalline (LC) polymers based on 6-hydroxy-2-naphtoic acid (HNA), biphenyl-4,4′-dicarboxylic acid (BA), resorcinol (RS), and a cross-linkable ethynyl functionality, 4,4′-(1,2-ethynediyl)bisbenzoic acid (EBA) were synthesized using a one-pot melt condensation method. In contrast with reactive LC oligomers where reactive functionalities were placed at the chain terminus, the reactive ethynyl group is now used as a cross-linking functionality placed in the polymer main-chain. Differential scanning calorimetric analysis reveals that these polymers can be cured at temperatures above 350°C in the nematic melt and exhibit excellent thermal properties, that is, temperature at 5% weight loss > 465°C, and reaches a maximum after cure glass transition temperature ( Tg) of 167°C. When only 5 mol% of BA was replaced with EBA, a slightly cross-linked nematic network, HNA/BA/RS/EBA-5, was obtained and thin films thereof could be stretched by 400% strain above Tg in four consecutive steps. A maximum order parameter [Formula: see text] of 0.54 could be achieved, and this moderately aligned LC network displays a room temperature storage modulus ( E′) of 29 GPa, a tensile strength of 330 MPa, a tensile modulus of 7 GPa, and an elongation at break of 7%. Cross-linking also improved the transverse properties of the nematic films. E′ of HNA/BA/RS/EBA-5 is 1.0 GPa at 150°C after being stretched by 400% versus 0.16 GPa for the non-cross-linked reference polymer. An affine deformation model was used to calculate E′ using [Formula: see text], and the predicted values are in close agreement with the experimental results.

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

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 ( Tg) 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 ( Td(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 Tm (K—N) and increasing the after cure Tg, 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.