Advancing Elastomers to Additive Manufacturing Through Tailored Photochemistry and Latex Design

Additive manufacturing (AM) fabricates complex geometries inaccessible through other manufacturing techniques. However, each AM platform imposes unique process-induced constraints which are not addressed by traditional polymeric materials. Vat photopolymerization (VP) represents a leading AM platform which yields high geometric resolution, surface finish, and isotropic mechanical properties. However, this process requires low viscosity (<20 Pa·s) photocurable liquids, which generally restricts the molecular weight of suitable VP precursors. This obstacle, in concert with the inability to polymerize high molecular weight polymers in the printer vat, effectively limits the molecular weight of linear network strands between crosslink points (Mc) and diminishes the mechanical and elastic performance of VP printed objects.

Polymer colloids (latex) effectively decouple the relationship between viscosity and molecular weight by sequestering large polymer chains within discrete, non-continuous particles dispersed in water, thereby mitigating long-range entanglements throughout the colloid. Incorporation of photocrosslinking chemistry into the continuous, aqueous phase of latex combined photocurability with the rheological advantages of latex and yielded a high molecular weight precursor suitable for VP. Continuous-phase photocrosslinking generated a hydrogel scaffold network which surrounded the particles and yielded a solid "green body" structure. Photorheology elucidated rapid photocuring behavior and tunable green body storage moduli based on scaffold composition. Subsequent water removal and annealing promoted particle coalescence by penetration through the scaffold, demonstrating a novel approach to semiinterpenetrating network (sIPN) formation. The sIPN's retained the geometric shape of the photocured green body yet exhibited mechanical properties dominated by the high molecular weight latex polymer. Dynamic mechanical analysis (DMA) revealed shifting of the latex polymer and photocrosslinked scaffold T_g's to a common value, a well-established phenomenon due phasemixing in (s)IPN's. Tensile analysis confirmed elastic behavior and ultimate strains above 500% for printed styrene-butadiene rubber (SBR) latexes which confirmed the efficacy of this approach to print high performance elastomers.

Further investigations probed the versatility of this approach to other polymer compositions and a broader range of latex thermal properties. Semibatch emulsion polymerization generated a systematic series of random copolymer latexes with varied compositional ratios of hexyl methacrylate (HMA) and methyl methacrylate (MMA), and thus established a platform for investigating the effect of latex particle thermal properties on this newly discovered latex photoprocessing approach. Incorporation of scaffold monomer, N-vinyl pyrrolidone (NVP), and crosslinker, N,N'-methylene bisacrylamide (MBAm), into the continuous, aqueous phase of each latex afforded tunable photocurability. Photorheology revealed higher storage moduli for green bodies embedded with glassy latex particles, suggesting a reinforcing effect. Post-cure processing elucidated the necessity to anneal the green bodies above the T_g of the polymer particles to promote flow and particle coalescence, which was evidenced by an optical transition from opaque to transparent upon loss of the light-scattering particle domains. Differential scanning calorimetry (DSC) provided a comparison of the thermal properties of each neat latex polymer with the corresponding sIPN.

Another direction investigated the modularity of this approach to 3D print mixtures of dissimilar particles (hybrid colloids). Polymer-inorganic hybrid colloids containing SBR and silica nanoparticles provided a highly tunable route to printing elastomeric nanocomposite sIPN's. The bimodal particle size distribution introduced by the mixture of SBR (150 nm) and silica (12 nm) nanoparticles enabled tuning of colloid behavior to introduce yield-stress behavior at high particle concentrations. High-silica hybrid colloids therefore exhibited both a shear-induced reversible liquid-solid transition (indicated by a modulus crossover) and irreversible photocrosslinking, which established a unique processing window for UV-assisted direct ink write (UV-DIW) AM. Concentric cylinder rheology probed the yield-stress behavior of hybrid colloids at high particle concentrations which facilitated both the extrusion of these materials through the UV-DIW nozzle and the retention of their as-deposited shaped during printing. Photorheology confirmed rapid photocuring of all hybrid colloids to yield increased moduli capable of supporting subsequent layers. Scanning electron microscopy (SEM) confirmed well-dispersed silica aggregates in the nanocomposite sIPN's. DMA and tensile confirmed significant reinforcement of (thermo)mechanical properties as a result of silica incorporation. sIPN's with relative weight ratio of 30:70 silica:SBR achieved maximum strains above 300% and maximum strengths over 10 MPa.

In a different approach to enhancing VP part mechanical properties, thiol-ene chemistry provided simultaneous linear chain extension and crosslinking in oligomeric diacrylate systems, providing tunable increases to Mc of the photocured networks. Hydrogenated polybutadiene diacrylate (HPBDA) oligomers provided the first example of hydrocarbon elastomer photopolymers for VP. 1,6-hexanedithiol provided a miscible dithiol chain extender which introduced linear thiol-ene chain extension to compete with acrylate crosslinking. DMA and tensile confirmed a decrease in T_g and increased strain-at-break with decreased crosslink density.

Other work investigated the synthesis and characterization of first-ever phosphonium polyzwitterions. Free radical polymerization synthesized air-stable triarylphosphine-containing polymers and random copolymers from the monomer 4-(diphenylphosphino) styrene (DPPS). ³¹P NMR spectroscopy confirmed quantitative post-polymerization alkylation of pendant triarylphosphines to yield phosphonium ionomers and polyzwitterions. Systematic comparison of neutral, ionomer, and polyzwitterions elucidated significant (thermo)mechanical reinforcement by interactions between large phosphonium sulfobetaine dipoles. Broadband dielectric spectroscopy (BDS) confirmed the presence of these dipoles through significant increases in static dielectric content. Small-angle X-ray scattering (SAX) illustrated ionic domain formation for all charged polymers.