Jingling Yan

Thermosetting polyimides and composites based on highly soluble phenylethynyl-terminated isoimide oligomers

Highly soluble phenylethynyl-endcapped isoimide oligomers were synthesized using 2,3,3′,4′-biphenyltetracarboxylic dianhydride (3,4′-BPDA) and aromatic diamines as the monomers, 4-phenylethynyl phthalic anhydride (4-PEPA) as the end-capping reagent, and trifluoroacetic anhydride as the dehydrating agent; then high performance thermosetting polyimides and composites were produced from these oligomers via the thermal crosslinking reaction of the phenylethylnyl group and the material properties were fully investigated. A series of isoimide oligomers with different molecular weights and a variety of chemical architectures were prepared by polycondensation of 3,4′-BPDA, 4-PEPA, and aromatic diamines including m-phenylenediamine (m-PDA), 2,2′-bis(trifluoromethyl)benzidine (TFMB), and 3,4′-oxydianiline (3,4′-ODA), followed by cyclization with trifluoroacetic anhydride. These isoimide oligomers were characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), rheological measurements, intrinsic viscosity measurement, and solubility tests. Compared to their imide analogues, isoimide oligomers showed much higher solubility in low boiling point solvents, and slightly lower melt viscosity, which can be attributed to their unique asymmetric architecture. These resins were formulated into thermosetting polyimides and composites by thermal crosslinking of the phenylethynyl group and conversion from isoimide to imide at elevated temperatures. The properties of the thermosets and composites were studied using mechanical property measurements, dynamic mechanical thermal analysis (DMTA), and thermogravimetric analysis (TGA). The cured polyimides exhibited extremely high glass transition temperatures (Tg) up to 467 °C, and 5% weight loss temperatures (T5%) up to 584 °C in a nitrogen atmosphere. The polyimide/quartz fiber composites possessed excellent high temperature mechanical properties due to the high glass transition temperatures of matrix resins.

Anion exchange membranes by bromination of benzylmethyl-containing poly(fluorene ether sulfone)s

Fluorene-containing anion exchange membranes were synthesized via the bromination reaction of poly(sulfone)s derived from 9,9-bis(3,5-dimethyl-4-hydroxyphenyl)fluorene (DMHPF), 4-fluorophenyl sulfone and 4,4′-biphenol, quaternization reaction using trimethylamine, and ion exchange; then the properties of these AEMs were fully characterized, and the structure–property relationship regarding this series of AEMs was elucidated. Brominated poly(fluorene ether sulfone)s (BrPFES) were characterized using proton nuclear magnetic resonance (1H NMR), and the bromination conversion, degree of functionalization (DF), and conversion of benzylmethyl groups were calculated. BrPFES was then quaternized by heterogeneous amination using trimethylamine, and then converted to quaternary ammonium bicarbonate by ion exchange. The properties of these AEMs were studied in terms of water uptakes, conductivities, swelling ratios, and mechanical properties. Compared to their homopolymer counterparts with similar ion exchange capacities (IEC), AEMs based on copolymers showed slightly lower conductivities but much lower water uptakes and swelling ratios, which can be explained by the fact that the continuation of hydrophilic domains in copolymers was interrupted by the incorporation of hydrophobic 4,4′-biphenol segments.

Bio-based epoxy-anhydride thermosets from six-armed linoleic acid-derived epoxy resin

Six-armed linoleic acid-derived epoxy resin with a rigid triazine core (EHL) was prepared through the esterification reaction between linoleic acid and hexamethylol melamine, followed by epoxidation of unsaturated fatty acid chains by using hydrogen peroxide. Bio-based epoxy-anhydrides thermosets were then produced from this resin by using 4-methyl hexahydrophthalic anhydride as the hardener and 1,8-diazabicyclo[5.4.0]undec-7-ene as the catalyst; the properties of these thermosets were then systematically investigated. The epoxy oligomer was fully characterized and confirmed through Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and matrix-assisted laser desorption and ionization time-of-flight mass spectrometry. The physical properties of this oligomer were also studied according to its bulk viscosity, epoxy equivalent weight, and density. The curing extent of the bio-based epoxy-anhydride thermosets were measured through differential scanning calorimetry and gel content testing. The properties of these thermosets were characterized through tensile testing, dynamic mechanical thermal analysis, and thermogravimetric analysis. Compared with thermosets based on benchmark bio-based epoxy resins, such as epoxidized soybean oil and epoxidized sucrose soyate, EHL-based thermosets showed higher glass transition temperatures, and enhanced tensile strength and modulus for a given cross-link density. These enhancements can be rationalized according to the rigidity of the triazine core, and the cohesive energy stemmed from the inter-molecular interaction of highly polarized heterocycle.

Synthesis and properties of polyimides derived from bis(4-aminophenyl)isohexides:

Several partially bio-based polyimides have been successfully synthesized by polycondensation between bis(4-aminophenyl)isohexides with various commercial dianhydrides. Flexible and free-standing films were readily obtained from their poly(amic acid) or polyimide solutions. A systematic investigation of the structure–property relationship of polyimides highlights the significant impact of the isohexides moieties on their physical and mechanical properties (glass transition temperature, inherent viscosity, thermal stability, solubility, and mechanical properties). The results revealed that these polyimides exhibited comparable thermal stability and mechanical properties to those of petrochemical-based ones.

Colorless polyimides derived from 2R,5R,7S,10S-naphthanetetracarboxylic dianhydride

A novel alicyclic dianhydride, 2R,5R,7S,10S-naphthanetetracarboxylic dianhydride (HNTDA), was prepared through the hydrogenation of tetramethyl 1,4,5,8-naphthalenetetracarboxylate, followed by deprotection and dehydration reactions. Single crystal X-ray diffraction results revealed that HNTDA possesses an irregular L-shaped architecture with one anhydride ring equatorial, and the other axial on the naphthane ring. Conventional one-step solution polycondensation of HNTDA with commercial diamines enabled the preparation of fully or semi-alicyclic polyimides with sufficient molecular weights. The properties of HNTDA-based polyimides were systematically characterized and compared with their counterparts based on 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride (HPMDA). Dynamic mechanical thermal analysis, thermogravimetric analysis, thermal mechanical analysis, and tensile testing indicated that HNTDA-based polymers exhibit markedly higher thermal and mechanical properties compared with HPMDA-based ones. These improvements can be rationalized according to the rigid, bulky, fused naphthanetetracarboxydiimide moiety, and the pronounced inter- and intra-molecular interactions. Furthermore, HNTDA-based polyimides were colorless or pale-yellow, and displayed good optical transparency, although their cutoff wavelength values were higher than those of HPMDA-based ones.

Bio-Based Poly(Ether Imide)s from Isohexide-Derived Isomeric Dianhydrides

In this work, four isohexide-derived isomeric dianhydrides were synthesized through a four-step procedure using isohexide and chloro-N-phenylphthalimides as the starting materials. The one-step solution polymerization of these dianhydrides with petroleum- or bio-based diamines enabled the synthesis of poly(ether imide)s (PEIs), which had viscosities of 0.41 to 2.40 dL∙g−1. The isohexide-derived PEIs were characterized based upon their solubility and their thermal, mechanical, and optical properties. The results showed that most of the isohexide-derived PEIs possessed comparable glass transition temperatures (Tg), tensile strengths, and moduli to petroleum-based PEIs. However, the thermo-oxidative stability of the PEIs was found to be lower than that of the common petroleum-based PEIs. Moreover, the PEIs displayed good optical activity, which originated from their unique chiral isohexide moieties. The isomeric effects of dianhydride monomers on the properties of the resulting PEIs were comparatively studied. The results suggested that the corresponding 4,4′-linked PEIs possessed lower Tg, higher mechanical properties, and higher specific rotations compared to 3,3′-linked polymers. Meanwhile, the polyimides with isomannide residue displayed higher Tg and more specific rotations than the corresponding polymers with isosorbide residue. These results contributed to more restricted rotations of phthalimide segments in 3,3′-linked or isomannide containing polyimides.

Surface modification of high-performance polyimide fibers by oxygen plasma treatment

In order to improve the interfacial compatibility between the fibers and epoxy resins, polyimide (PI) fibers were modified using oxygen plasma with various treatment powers. The properties of PI fibers before and after surface modification were comparatively characterized according to their chemical composition, surface morphology, surface free energy, single filament tensile strength, and interfacial shear strength (IFSS). Most of the fiber properties, including the ratio of oxygen to nitrogen, oxygen concentration, surface free energy, and IFSS firstly increased with the plasma power, and then decreased when the plasma power was higher than 120 W. With a plasma power of 120 W, the oxygen concentration and the ratio of oxygen to carbon atoms (O/C) was 26% (22% for untreated fiber) and 0.38 (0.28 for untreated fiber), respectively. Meanwhile, the values of surface free energy and IFSS were 89% and 30% higher than those for untreated fiber, respectively. Furthermore, the values of single filament tensile strength for modified fibers were only 2.3% lower than those for pristine one. These results indicated that the compatibility between PI fibers and epoxy resins was greatly improved without compromising mechanical properties.

Synthesis and characterization of poly(N-vinyl-1,2,3-triazole)s derived from monomers obtained by highly efficient Wolff's cyclocondensation

Four N-vinyl-1,2,3-triazole monomers were prepared in high yields from 2-aminoethanol and α-diazo-β-oxoamides via modified Wolffs cyclocondensation reactions. Then a series of poly(N-vinyl-1,2,3-triazole)s were synthesized through conventional free radical polymerization of these monomers. The chemical structures of these monomers and polymers were confirmed by proton and carbon nuclear magnetic resonance (1H and 13C NMR). The properties of poly(N-vinyl-1,2,3-triazole)s were systematically characterized by a variety of analytical techniques including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and solubility testing. The results indicated that these polymers exhibited higher glass transition temperatures (196–212 °C) than most of the commodity vinyl polymers such as polystyrene (100 °C) and poly(methyl methacrylate) (110 °C). Moreover, these polymers can be further functionalized through hydrolysis of amide groups. These results revealed that this methodology has great potential for constructing 1,2,3-triazole-containing monomers and polymers.

2,3,3′,4′-Oxydiphthalic dianhydride-based phenylethynyl-terminated imide oligomers for low-temperature resin transfer molding applications

A series of imide resins were prepared using 2,3,3′,4′-oxydiphthalic dianhydride, 4-phenylethynylphthalic anhydride, and aromatic diamines (1, 3-bis(3-aminophenoxy), 3,4′-oxyaniline, 2,2′-bis (trifluoromethyl) benzidine, and m-phenylenediamine) as starting materials. These imide oligomers were characterized by means of Fourier transform infrared spectroscopy, differential scanning calorimetry, intrinsic viscosity measurements, and rheological measurements. Some of these oligomers exhibited low (<1 Pa·s at 250°C) and stable (>2 h at 250°C) melt viscosity, which was highly desirable for resin transfer molding process. Thermosetting polyimides (PIs) were then produced from these oligomers via thermal cross-linking reaction of phenylethynyl group. The properties of the thermosets were studied using tensile and flexural testing, dynamic mechanical thermal analysis, and thermogravimetric analysis. The cured PIs exhibited a good combination of thermal and mechanical properties, with a tensile strength of 31–76 MPa, flexural strength of 38–142 MPa, glass transition temperatures of 233–358°C, and 5% weight loss temperatures of 540–545°C under nitrogen atmosphere.