Philip J. Scott’s research while affiliated with Virginia Tech and other places

Additive manufacturing (AM), also known as 3D printing, is a promising technology to produce complex shapes with little waste material in a distributed fashion. AM can be implemented with various materials, but metal and polymer are dominant feedstocks. During the deposition process for polymeric AM, the polymer may encounter repetitive softening or melting, applied shear flow including multiple constrictions, cooling and solidification, solvent evaporation, polymerization and crosslinking, or coalescence across interfaces. This leads to the need to invoke polymer science principles to rationally predict the response of the polymeric feedstock to the shear fields, thermal gradients, and reaction fronts that they will encounter in the complex 3D printing process. Elucidating the fate of the macromolecules as they experience the 3D printing process provides a foundation to design new materials that are formulated to bias the formation of robust structures and interfaces. As such, polymers in AM offer a unique opportunity to apply polymer science principles to address shortcomings, offer potential solutions, and advance the technology. This article provides a focused discussion of how polymer science principles drive the growth of three common polymeric AM techniques, fused filament fabrication, direct‐ink write, and vat polymerization.

Inorganic-polymer hybrid colloids present a modular and tunable route to fabricate polymer nanocomposites from low viscosity precursors; however, their use in additive manufacturing remains limited. This manuscript describes photocurable “hybrid colloids” to enable layered fabrication of elastomeric nanocomposites, i.e., combination of continuous-phase photocrosslinking chemistry with hybrid colloids of water-dispersible silica nanoparticles and styrene-butadiene rubber (SBR) latex particles. Varying the relative concentrations of polymeric and inorganic particles afforded precise tuning of filler loading in the final nanocomposite and introduced a bimodal particle size distribution with desirable rheological behavior for extrusion-based additive manufacturing. Specifically, ultraviolet-assisted direct ink write (UV-DIW) processing of the photocurable hybrid colloid pastes generated free-standing green bodies, which contained a combination of SBR and silica nanoparticles. Subsequent drying of the green bodies allowed SBR particle coalescence and penetration through the scaffold and surrounding the silica nanoparticles, which yielded a semi-interpenetrating network (sIPN) nanocomposite. Facile tuning of silica concentrations in the hybrid colloid enabled tuning of both the colloidal ink rheology and mechanical properties of the final sIPN nanocomposites to achieve additive manufacturing of silica-SBR nanocomposites with ultimate tensile strains exceeding 300% and ultimate tensile strengths above 10 MPa.

Unparalleled temporal and spatial control of colloidal chemical processes introduces immense potential for the manufacturing, modification, and manipulation of latex particles. This review highlights major advances in photochemistry, both as stimulus and response, to generate unprecedented functionality in polymer colloids. Light-based chemical modification generates polymer particles with unique structural complexity, and the incorporation of photoactive functionalities transforms inert particles into photoactive nanodevices. Latex photo-functionality, which is reflected in both the colloidal and coalesced states, enables photochromism, photoswitchable aggregation, tunable fluorescence, photoactivated crosslinking and solidification, and photomechanical actuation. Previous literature explores the capacity of photochemistry, which complements the rheological and processing advantages of latex, to expand beyond traditional coatings applications and enable disruptive technologies in critical areas including nanomedicine, data security, and additive manufacturing.

Vat photopolymerization (VP) additive manufacturing fabricates intricate geometries with excellent resolution; however, high molecular weight polymers are not amenable to VP due to concomitant high solution and melt viscosities. Thus, a challenging paradox arises between printability and mechanical performance. This report describes concurrent photopolymer and VP system design to navigate this paradox with the unprecedented use of polymeric colloids (latexes) that effectively decouple the dependency of viscosity on molecular weight. Photocrosslinking of a continuous-phase scaffold, which surrounds the latex particles, combined with in-situ computer-vision print parameter optimization, which compensates for light scattering, enables high-resolution VP of high molecular weight polymer latexes as particle-embedded green bodies. Thermal post-processing promotes coalescence of the dispersed particles throughout the scaffold, forming a semi-interpenetrating polymer network (sIPN) without loss in part resolution. Printing a styrene-butadiene rubber (SBR) latex, a previously inaccessible elastomer composition for VP, exemplified this approach and yielded printed elastomers with precise geometry and tensile extensibilities exceeding 500%.

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.

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.

The synthesis of 1,4-bis[4-(1-phenylethenyl)benzyl] piperazine and subsequent reaction with sec-butyllithium enables a novel piperazine-containing difunctional organolithium initiator for the living anionic polymerization of isoprene. Piperazine provides a polar unit within the difunctional initiator, promoting ion dissociation and miscibility with hydrocarbon solvents, and enabling the formation of well-defined polyisoprene homopolymers with predictable molecular weights and controlled microstructure. In situ Fourier transform infrared spectroscopy monitors the dilithium initiator formation and the anionic polymerization of isoprene, revealing kinetic insight into this synthetic method. Furthermore, sequential monomer addition with styrene affords poly(styrene-block-isoprene-block-styrene) triblock copolymers with controlled molecular weights and narrow polydispersities. This novel initiator facilitates the synthesis of thermoplastic elastomers with desired cis-1,4 microstructure in polydienes.