3D Printable Modular Soft Elastomers from Physically Cross-linked Homogeneous Associative Polymers.

Three-dimensional (3D) printing of elastomers enables the fabrication of many technologically important structures and devices. However, there remains a critical need for the development of reprocessable, solvent-free, soft elastomers that can be printed without the need for post-treatment. Herein, we report modular soft elastomers suitable for direct ink writing (DIW) printing by physically cross-linking associative polymers with a high fraction of reversible bonds. We designed and synthesized linear-associative-linear (LAL) triblock copolymers; the middle block is an associative polymer carrying amide groups that form double hydrogen bonding, and the end blocks aggregate to hard glassy domains that effectively act as physical cross-links. The amide groups do not aggregate to nanoscale clusters and only slow down polymer dynamics without changing the shape of the linear viscoelastic spectra; this enables molecular control over energy dissipation by varying the fraction of the associative groups. Increasing the volume fraction of the end linear blocks increases the network stiffness by more than 100 times without significantly compromising the extensibility. We created elastomers with Young's moduli ranging from 8 kPa to 8 MPa while maintaining the tensile breaking strain around 150%. Using a high-temperature DIW printing platform, we transformed our elastomers to complex, highly deformable 3D structures without involving any solvent or post-print processing. Our elastomers represent the softest melt reprocessable materials for DIW printing. The developed LAL polymers synergize emerging homogeneous associative polymers with a high fraction of reversible bonds and classical block copolymer self-assembly to form a dual-cross-linked network, providing a versatile platform for the modular design and development of soft melt reprocessable elastomeric materials for practical applications.

Additive Manufacturing of Poly(phenylene Sulfide) Aerogels via Simultaneous Material Extrusion and Thermally Induced Phase Separation.

Additive manufacturing (AM) of aerogels increases the achievable geometric complexity, and affords fabrication of hierarchically porous structures. In this work, a custom heated material extrusion (MEX) device prints aerogels of poly(phenylene sulfide) (PPS), an engineering thermoplastic, via in situ thermally induced phase separation (TIPS). First, pre-prepared solid gel inks are dissolved at high temperatures in the heated extruder barrel to form a homogeneous polymer solution. Solutions are then extruded onto a room-temperature substrate, where printed roads maintain their bead shape and rapidly solidify via TIPS, thus enabling layer-wise MEX AM. Printed gels are converted to aerogels via postprocessing solvent exchange and freeze-drying. This work explores the effect of ink composition on printed aerogel morphology and thermomechanical properties. Scanning electron microscopy micrographs reveal complex hierarchical microstructures that are compositionally dependent. Printed aerogels demonstrate tailorable porosities (50.0-74.8%) and densities (0.345-0.684 g cm-3 ), which align well with cast aerogel analogs. Differential scanning calorimetry thermograms indicate printed aerogels are highly crystalline (≈43%), suggesting that printing does not inhibit the solidification process occurring during TIPS (polymer crystallization). Uniaxial compression testing reveals that compositionally dependent microstructure governs aerogel mechanical behavior, with compressive moduli ranging from 33.0 to 106.5 MPa.

3D Printing Carbonaceous Objects from Polyimide Pyrolysis.

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.

Dissociative Carbamate Exchange Anneals 3D Printed Acrylates.

Relative to other additive manufacturing modalities, vat photopolymerization (VP) offers designers superior surface finish, feature resolution, and throughput. However, poor interlayer network formation can limit a VP-printed part's tensile strength along the build axis. We demonstrate that the incorporation of carbamate bonds capable of undergoing dissociative exchange reactions provides improved interlayer network formation in VP-printed urethane acrylate polymers. In the presence of dibutyltin dilaurate catalyst, the exchange of these carbamate bonds enables rapid stress relaxation with an activation energy of 133 kJ/mol, consistent with a dissociative bond exchange process. Annealed XY tensile samples containing a catalyst demonstrate a 25% decrease in Young's modulus, attributed to statistical changes in network topology, while samples without a catalyst show no observable effect. Annealed ZX tensile samples printed with layers perpendicular to tensile load demonstrate an increase in elongation at break, indicative of self-healing. The strain at break for samples containing a catalyst increases from 33.9 to 56.0% after annealing but decreases from 48.1 to 32.1% after annealing in samples without a catalyst. This thermally activated bond exchange process improves the performance of VP-printed materials via self-healing across layers and provides a means to change Young's modulus after printing. Thus, the incorporation of carbamate bonds and appropriate catalysts in the VP-printing process provides a robust platform for enhancing material properties and performance.

Ultraviolet-Assisted Direct Ink Write to Additively Manufacture All-Aromatic Polyimides.

All-aromatic polyimides have degradation temperatures above 500 °C, excellent mechanical strength, and chemical resistance, and are thus ideal polymers for high-temperature applications. However, their all-aromatic structure impedes additive manufacturing (AM) because of the lack of melt processability and insolubility in organic solvents. Recently, our group demonstrated the design of UV-curable polyamic acids (PAA), the precursor of polyimides, to enable their processing using vat photopolymerization AM. This work leverages our previous synthetic strategy and combines it with the high solution viscosity of nonisolated PAA to yield suitable UV-curable inks for UV-assisted direct ink write (UV-DIW). UV-DIW enabled the design of complex three-dimensional structures comprising of thin features, such as truss structures. Dynamic mechanical analysis of printed and imidized specimens confirmed the thermomechanical properties typical of all-aromatic polyimides, showing a storage modulus above 1 GPa up to 400 °C. Processing polyimide precursors via DIW presents opportunity for multimaterial printing of multifunctional components, such as three-dimensional integrated electronics.