Justin M. Sirrine’s research while affiliated with Virginia Tech and other places

Elastomers are a unique and important class of polymers that enjoy applications in virtually every industry, including healthcare, aerospace, automotive, and apparel. However, due to inherent physical, thermal, and mechanical properties of elastomers, the additive manufacturing (AM) of elastomers remains challenging. These challenges are discussed in the context of various AM processes, including powder bed fusion, material extrusion, and vat photopolymerization. This review provides an in-depth discussion of the current state of silicone and polyurethane polymers for AM, and also discusses polyesters/polycarbonates, liquid crystalline elastomers, and monomer compositions that provide elastomeric properties upon photocuring. Finally, the current state of common, commercially-available elastomers for AM is provided, as well as an outlook for this rapidly progressing field. We expect this review to be useful for any research involved with the additive manufacturing of elastomers.

This work describes the first example of a hydrogenated polybutadiene elastomer photopolymer that addresses the process constraints of vat photopolymerization (VP) additive manufacturing. A synthetic method, which involves simultaneous thiol-ene step growth chain extension and acrylate crosslinking, addresses traditional challenges associated with this leading 3D printing platform. This facile, one-pot strategy combines the processing advantages of low molecular weight oligomers with the tunable thermomechanical and mechanical performance of higher molecular weight polymeric networks directly during printing, without requiring a post-processing step. The addition of photo-initiator to mixtures of liquid polybutadiene oligomer and miscible dithiols enabled selective photocuring under UV exposure to form high-strain, elastic parts in comparison to neat diacrylate systems. Photolithographic printing of these photopolymers enabled the fabrication of three-dimensional, hydrocarbon elastomer objects. Photorheology elucidated curing behavior as a function of composition and UV intensity, while optical imaging and SEM revealed quality and resolution.

Melt polymerization enabled the synthesis of semi-aromatic (co)polyesters containing 1,4-cyclohexanedimethanol (CHDM), 4,4′-bibenzoate (4,4′BB), and 3,4′-bibenzoate (3,4′BB). Proton nuclear magnetic resonance ( ¹ H NMR) spectroscopy confirmed monomer incorporation, and size exclusion chromatography (SEC) revealed molecular weights and polydispersity indices (PDIs) consistent with high conversion melt phase synthesized polyesters. All bibenzoate-based polyesters exhibited a high onset of 5 wt % loss temperature according to thermogravimetric analysis (TGA) (>350 °C), and differential scanning calorimetry (DSC) provided compositionally dependent glass transition temperatures (T g s) approaching 135 °C and crystalline melting temperatures where applicable. Dynamic mechanical analysis (DMA) probed sub-T g β-relaxations with minimal changes in intensity, suggesting that cyclohexyl ring relaxations dominated the low temperature energy absorption for all (co)polyester compositions. Time-temperature superposition (TTS) analysis from melt rheology revealed increasing characteristic relaxation times with increasing 4,4′BB content, which was attributed to the linear 4,4′BB stiffening the polymer chain. Increased kinked 3,4′BB content promoted chain entanglement, resulting in a lower entanglement molecular weight and a higher number of entanglements per chain (N/N e ). Similarly, increases in 3,4′BB content improved tensile yield strength and Young's modulus due to a higher polymer density and potentially due to an increase in entanglement density. Finally, scanning electron microscopy (SEM) suggested mostly brittle failure after necking and strain hardening in tensile specimens. As a result, structure-property relationships afforded insight into regioisomer impacts on thermal, rheological, and mechanical performance for bibenzoate-based (co)polyester regioisomers.

Photocuring and vat photopolymerization (VP) additive manufacturing (AM) is reported for two families of fully amorphous poly(dimethyl siloxane) (PDMS) terpolymers containing either diphenylsiloxy (DiPhS) or diethylsiloxy (DiEtS) repeating units. A thiol‐functionalized PDMS crosslinker enables rapid crosslinking in air using efficient thiol–ene addition. Differential scanning calorimetry and dynamic mechanical analysis (DMA) confirm the absence of crystallinity for the DiPhS‐containing systems, while DMA shows a rubbery plateau extending to greater than 200 °C for the DiEtS‐containing system. VP‐AM of both photopolymer systems afford well‐defined 3D geometries, including high aspect ratio structures, which demonstrate feasibility of these photopolymers for the 3D printing of unique geometric objects that require elastomeric performance to temperatures as low as −120 °C. Vat photopolymerization additive manufacturing with vinyl‐functional siloxane terpolymers and a thiol‐functional siloxane, enable thiol–ene addition for rapid crosslinking in the presence of air. Thermal analysis confirms the absence of crystallinity, and dynamic mechanical analysis displays a rubbery plateau width of more than 200 °C. These photopolymer compositions hold promise for 3D printed elastomers that offer temperature‐independent moduli from room temperature to −120 °C.

Poly(dimethyl siloxane)-containing (PDMS), segmented polyureas represent an important class of high-performance elastomers that leverage the low glass transition temperature (Tg) of PDMS and superior mechanical reinforcement from bidentate hydrogen bonding. Current synthetic methods exclusively employ highly reactive/toxic isocyanate reagents and volatile organic solvents; the latter must be quantitatively removed prior to use. This report details an isocyanate-, solvent-, and catalyst-free synthetic method towards PDMS polyureas using urea and a disiloxane diamine chain extender in the melt phase. Melt polymerization afforded segmented PDMS polyureas, which formed optically clear, mechanically ductile, freestanding films. Observation of distinct thermal transitions with differential scanning calorimetry and dynamic mechanical analysis, corresponding to the respective segments, suggested microphase separation. Tensile and hysteresis measurements corroborated similarities between these PDMS polyureas and their isocyanate-containing analogues with strain at break ranging from 495 to 1180%. This facile, isocyanate-free approach provides a commercially viable alternative to the current industrial process for high performance elastomers.

Non‐isocyanate polyurethanes (NIPU) have rapidly emerged as a sustainable, less toxic, and environmentally friendly alternative to traditional isocyanate‐based thermoplastic polyurethane (TPU) synthesis. TPU is widely used in the medical industry due to its excellent mechanical properties and elasticity. However, little work has been done to synthesize and electrospin NIPU into fibrous mats for biomedical applications. In this work, melt polymerization of a plant oil‐based cyclic carbonate monomer with polyether soft segments and various diamines yielded isocyanate‐free, segmented poly(amide hydroxyurethane)s (PAHUs). Electrospinning of segmented PAHUs afforded ductile, free‐standing fibrous mats with Young's modulus values between 7 and 8 MPa, suitable for tissue scaffold applications. PAHU fiber mats exhibited 3–4 times greater water uptake than the electrospun TPU control, demonstrating potential utility in drug delivery. Fibroblasts adhered to electrospun PAHU fibrous mats with viability values over 90% after 72‐h, validating its biocompatibility. The results highlight the high performance and potential of electrospun isocyanate‐free polyurethanes mats for biomedical application.

A novel, poly(dimethyl siloxane)-based photopolymer that exhibits simultaneous linear chain extension and crosslinking was suitable for vat photopolymerization additive manufacturing. Photopolymer compositions consisted of dithiol and diacrylate functional poly(dimethyl siloxane) oligomers, where simultaneous thiol-ene coupling and free radical polymerization provided for linear chain extension and crosslinking, respectively. Compositions possessed low viscosity before printing and the modulus and tensile strain at break of a photocured, higher molecular weight precursor after printing. Photorheology and soxhlet extraction demonstrated highly efficient photocuring, revealing a calculated molecular weight between crosslinks of 12,600 g/mol and gel fractions in excess of 90% while employing significantly lower molecular weight precursors (i.e. < 5300 g/mol). These photocured objects demonstrated a 2× increase in tensile strain at break as compared to a photocured 5300 g/mol PDMS diacrylamide alone. These results are broadly applicable to the advanced manufacturing of objects requiring high elongation at break.

Chemical warfare agent (CWA) sorption into polymeric coatings is an important consideration for preventing human exposure to CWAs. The specific nature of CWA–polymer interactions greatly influences the degree of gas uptake into a polymeric coating. By uncovering the role the various functional groups in CWAs play during gas sorption, better films and coatings can be engineered. This study utilized a quartz crystal microbalance to measure sulfur mustard (HD) analog (simulant) and hexane vapor uptake into polyurethane films. Analysis of the uptake data with Flory–Huggins theory yielded interaction parameter values (χ) between 0.24 and 0.40 for each HD simulant-polyurethane system compared to 0.97 for hexane, revealing the importance of the chlorine and sulfur groups in gas uptake at the thin film interface. We predict similar uptake behavior for HD in polyurethane coatings due to structural similarities between HD and the simulants.

Ultraviolet curable polyureas and polyurethanes enjoy a wide range of applications in the coatings industry. They demand less energy for curing compared to thermal processes that offer relatively low viscosities before photocuring, and provide mechanical property improvements due to hydrogen bonding physical crosslinks. Mechanical properties of the coatings are dependent on a variety of factors, including chemical composition, molecular weight, and molecular weight distribution. Formulations are designed for a wide variety of end use applications, ranging from soft, elastomeric coatings to harder, nonflexible sealants. This report demonstrates that the thermomechanical behavior of photocured coatings is a function of molecular weight distribution. Step-growth polymerization and endcapping afforded a variety of acrylate-terminated, urea-/urethane-containing photocurable oligomers from amine-terminated poly(propylene glycol), dicyclohexylmethane-4,4′-diisocyanate (HMDI), and 2-hydroxyethyl acrylate (HEA) at various stoichiometric ratios. The state-of-the-art supercritical fluid chromatography coupled with evaporative light-scattering detection (SFC-ELSD) enabled the elucidation of oligomeric molecular weight distributions as a function of reaction stoichiometry. SFC-ELSD demonstrated the efficient separation of oligomeric species with single repeat unit resolution (i.e., n = 2 vs. n = 3). Dynamic mechanical analysis probed thermomechanical response of photocured films as a function of molecular weight distribution and demonstrated that the presence of a hydrogen-bonding, small molecule photoactive reaction byproduct, i.e., HEA doubly-endcapped HMDI, had a much more profound effect on thermomechanical response as compared to changes in oligomer molecular weight in the molecular weight range investigated. This combination of chromatographic technique and thermomechanical analysis afforded an in-depth investigation of the structure–property relationships of urea-/urethane-containing photocurable oligomers.

Additive manufacturing, or three-dimensional (3D) printing, has emerged as a viable technique for the production of vascularized tissue engineering scaffolds. In this report, a biocompatible and biodegradable poly(tri(ethylene glycol) adipate) dimethacrylate was synthesized and characterized for suitability in soft-tissue scaffolding applications. The polyester dimethacrylate exhibited highly efficient photocuring, hydrolyzability, and 3D printability in a custom microstereolithography system. The photocured polyester film demonstrated significantly improved cell attachment and viability as compared with controls. These results indicate promise of novel, printable polyesters for 3D patterned, vascularized soft-tissue engineering scaffolds.