Clay B. Arrington’s research while affiliated with Arizona State University and other places

Vat photopolymerization (VP) Additive Manufacturing (AM), in which UV light is selectively applied to cure photo‐active polymers into complex geometries with micron‐scale resolution, has a limited selection of aliphatic thermoset materials that exhibit relatively poor thermal performance. Ring‐opening dianhydrides with acrylate‐containing nucleophiles yielded diacrylate ester‐dicarboxylic acids that enabled photo‐active polyimide (PI) precursors, termed polysalts, upon neutralization with an aromatic diamine in solution. In situ FTIR spectroscopy coupled with a solution and photo‐rheological measurements revealed a previously unknown time‐dependent instability of 4,4′‐oxydianiline (ODA) polysalts due to an aza‐Michael addition. Replacement of the electron‐donating ether‐containing diamine with an electron withdrawing sulfone‐containing monomer, e.g., 4,4′‐diaminodiphenyl sulfone (DDS), prohibited the aza‐Michael addition of the aromatic amine to the activated acrylate double bond. Novel DDS polysalt photocurable solutions are similarly analyzed and validated long‐term stability, which enabled reproducible printing of polyimide organogel intermediates. Subsequent VP AM afforded 3‐dimensional (3D) structures of intricate complexity and excellent surface finish, as demonstrated with scanning electron microscopy. In addition, the novel PMDA‐HEA/DDS solution enabled the production of the first beam latticed architecture comprised of all‐aromatic polyimide. The versatility of a polysalt platform for multi‐material printing is further demonstrated by printing parts with alternating polysalt compositions.

Vat photopolymerization (VP) is an advanced additive manufacturing (AM) platform that enables production of intricate three‐dimensional (3D) monoliths that are unattainable with conventional manufacturing methods. In this work, modification of amorphous poly(arylene ether sulfone)s (PSU) allowed for VP printing. Post‐polymerization telechelic functionalization with acrylate functionality yielded photo‐crosslinkable PSUs across a molecular weight range. 1H NMR spectroscopy confirmed chemical composition and quantitative acrylate functionalization. Addition of diphenyl‐(2,4,6‐trimethylbenzoyl)phosphine oxide (TPO) photo‐initiator to 30 wt. % PSU solutions in NMP provided a photo‐curable composition. However, subsequent photo‐rheological studies elucidated rapid photo‐degradation of the polysulfone main chain, which was especially apparent in high Mn (15 kg·mol−1) PSU formulations. UV‐light intensity and wavelength range were altered to reduce degradation while allowing for efficient crosslinking. The addition of 0.5 wt. % of avobenzone photo‐blocker produced an ill‐defined structure with 6 kg·mol−1 PSU. For higher molecular weights (>12 kg·mol−1), solutions with a low molar mass reactive diluent, i.e., trimethylolpropane triacrylate, enabled the printing of an organogel with a storage modulus (>105 Pa) sufficient for vat photopolymerization. Employing multicomponent solutions provided well‐defined parts with complex geometries through vat photopolymerization. This article is protected by copyright. All rights reserved

Melt polycondensation incorporating meta-substituted hydroxyethylresorcinol (HER) or para-substituted hydroxyethylhydroquinone (HEH) regioisomers enabled the synthesis of poly(ethylene terephthalate) (PET)-based (co)polyesters with targeted comonomer mol %. Incorporation of either HER or HEH lowered the glass transition temperature (Tg) relative to PET, but the 5% weight loss temperature (Td,5%) remained unchanged. Similarly, the semi-crystalline morphology varied as a function of comonomer selection and mol % comonomer. Dynamic mechanical analysis confirmed decreasing β-relaxation intensities upon HER and HEH incorporation, while HER impacted β-relaxation intensity more substantially at similar mol %. HER incorporation also shifted the β-relaxation temperature (Tβ) to lower temperatures considerably more than HEH. Time-temperature-superposition (TTS) and Williams-Landel-Ferry (WLF) analysis of melt rheology probed various physical polymer properties including melt flow activation energy (Ea), fractional free volume at Tg (fg), and molecular weight of entanglement (Me). HER-based (co)polyesters exhibited lower Ea compared to HEH. Likewise, HER incorporation lowered fg while HEH incorporation increased fg. HER-based (co)polyesters possessed a higher Me compared to HEH-based (co)polyesters, although both copolymers resulted in higher Me compared to PET. Tensile testing revealed HER/HEH-based (co)polyesters ultimate mechanical performance compared to amorphous PET. Both HER and HEH incorporation increased Young's moduli and yield strengths, although comonomer incorporation above 25–30 mol % yielded minimal change. Establishing structure-property relationships provided insight into the role of regioisomeric variation at various comonomer levels on thermal, rheological, and mechanical performance for PET-based copolymers.

Fully aromatic polyimides are amenable to efficient carbonization in thin two-dimensional (2D) films due to a complement of aromaticity and planarity of backbone repeating units. However, repeating unit rigidity traditionally imposes processing limitations, restricting many fully aromatic polyimides, e.g., pyromellitic dianhydride with 4,4′-oxidianiline (PMDA-ODA) polyimides, to a 2D form factor. Recently, research efforts in our laboratories enabled additive manufacturing of micron-scale resolution PMDA-ODA polyimide objects using vat photopolymerization (VP) and ultraviolet-assisted direct ink write (UV-DIW) following careful thermal postprocessing of the three-dimensional (3D) organogel precursors to 400 °C. Further thermal postprocessing of printed objects to 1000 °C induced pyrolysis of the PMDA-ODA objects to disordered carbon. The pyrolyzed objects retained excellent geometric resolution, and Raman spectroscopy displayed characteristic disordered (D) and graphitic (G) carbon bands. Scanning electron microscopy probed the cross-sectional homogeneity of the carbonized samples, revealing an absence of pore formation during carbonization. Likewise, impedance analysis of carbonized specimens indicated only a moderate decrease in conductivity compared to thin films that were pyrolyzed using an identical carbonization process. Facile pyrolysis of PMDA-ODA objects now enables the production of carbonaceous monoliths with complex and predictable three-dimensional geometries using commercially available starting materials.

High performance engineering thermoplastics exhibit excellent thermomechanical performance and chemical resistance, however, the rigid polymeric main chains that are derived from highly or fully aromatic repeating units impose processing challenges. The design of photo-reactive precursors now enables additive manufacturing of poly(amide imide) (PAI) precursor organo-gels via UV-assisted direct ink write (DIW) printing. Solution rheology revealed prerequisite high solution viscosities and shear thinning behavior of organic solutions of the PAI precursors. Additionally, photorheology of PAI precursors exhibited rapid gelation and high gel strengths, enabling production of self-supporting organo-gels that are necessary for successful DIW. Thermal post-treatment of the printed organo-gels revealed isotropic linear shrinkage as low as 26% during drying and imidization. The reduced chain rigidity of the poly(amide imide), relative to fully-aromatic polyimides, prevented quantitative thermal removal of the poly (hydroxyethyl acrylate) (poly(HEA)) scaffold, resulting in a decreased glass transition temperature (263 °C) relative to commercial PAI (290 °C). Moreover, crosslinking of the PHEA during thermal imidization limited scaffold removal upon solvent extraction, however, suggested the formation of novel multiphase polymers. Selective dissolution of the PAI from the crosslinked PHEA enabled assessment of the role of the scaffold on thermomechanical properties and characterization of printed PAI. Spectroscopic and thermal analysis confirmed the formation of the desired PAI composition with this photochemical synthetic method.

The design of linear high performance polyetherimides (PEIs) comprising 3,3′- and 4,4′-bisphenol-A dianhydride (bis-DA) and m-phenylene diamine (mPD) provided an opportunity to elucidate the influence of dianhydride regiochemistry on thermomechanical and rheological properties. This unique pair of regioisomers allowed the tuning of the thermal and rheological properties for high glass transition temperature polyimides for emerging engineering applications. Step-growth polycondensation enabled the production of high molecular weight PEIs for subsequent thermal and mechanical analysis. Monofunctional phthalic anhydride as an endcapper and monomeric stoichiometric imbalances enabled predictable number-average molecular weights ranging from 3,600 to 35,400 g mol⁻¹, as confirmed with size exclusion chromatography coupled with light scattering detection for absolute molecular weight determination. The selection of the dianhydride regioisomer influenced the weight loss profile as a function of temperature, entanglement molecular weight (Me), glass transition temperature (Tg), tensile strain-at-break, zero-shear melt viscosity, average hole-size free volume (Vh), and the plateau modulus prior to viscous flow during dynamic mechanical analysis. The 3,3′- PEI composition interestingly exhibited a ~20 °C higher glass transition temperature than the corresponding 4,4′- PEI analog. Moreover, melt rheological analysis revealed a two-fold increase in Me for 3,3′- PEI, which pointed to the origin of the differences in mechanical and rheological properties as a function of PEI backbone geometry.

Blends of poly(ethylene terephthalate) (PET) and poly(m-xylylene adipamide) (MXD6) were successfully compatibilized by using a phosphonated PET ionomer in the Na⁺ form (Na⁺-PPET) as a compatibilizer. The compatibilized 90:10 (by weight) polyester/polyamide blends containing various amounts of the phosphonated polyester were prepared by melt mixing in a twin-screw extruder. Scanning electron microscopy (SEM) revealed that the extruded blends contained spherical MXD6 domains dispersed in the PET matrix. The phase-separated MXD6 domain size decreased with increasing ionic monomer concentration, and the efficient compatibilization was attributed to specific interactions between the ionic phosphonate groups on the Na⁺-PPET ionomer and the amide linkages of MXD6. Thermal analysis suggested that the small MXD6 domains nucleated the crystallization of PET upon cooling as indicated by a shift of crystallization temperature of PET to higher temperatures. Furthermore, the specific interactions appeared to inhibit crystallization of MXD6. Biaxial orientation of melt processed films of the blends with a draw ratio of 3 × 3 transformed the spherical MXD6 domains into high aspect ratio platelets that were oriented in the plane of the film. Compared to the uncompatibilized base blends, optical property characterization on the oriented films revealed a decreased haziness by 56% and an improved transparency by 35% with 0.22 mol% of phosphonated ionic monomers incorporated in the polyester component. Moreover, a 33% decrease in oxygen permeability was found for the oriented Na⁺-PPET/PET/MXD6 blend composition containing 0.11 mol% of phosphonated monomers.

Halogen-free poly(ether imide)s (PEIs) are important lightweight engineering materials. Herein we report two facile strategies of “mixed polymers” and “mixed end-cappers” for preparing phosphonium-containing but halogen-free PEIs. As confirmed by X-ray photoelectron spectroscopy, the PEIs from both strategies were free of bromine but contained phosphonium and sulfonate groups and thus termed PEI-SAA-P. The phosphonium in PEI-SAA-P ensured high char yields of 56–57%. PEI-SAA-P showed thermal stability higher than the prepolymers of phosphonium-bromide-terminated PEI and lithium-sulfonate-terminated PEI. Additionally, PEI-SAA-P retained excellent processability as evidenced by the low viscosities at high frequencies. With a molecular weight of 12 kDa, PEI-SAA-P exhibited tensile strengths of 108–111 MPa and Young’s moduli of 3.24–3.29 GPa, comparable to state-of-the-art high-molecular-weight PEIs. The simple strategies of “mixed polymers” and “mixed end-cappers” are effective in preparing halogen-free but phosphonium- and sulfonate-containing PEIs and potentially other high-performance polymers.