| Summary: | 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 Tg'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 Tg 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 Tg 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. === Doctor of Philosophy === Additive manufacturing (AM) revolutionizes the fabrication of complex geometries,
however the utility of these 3D objects for real world applications remains hindered by
characteristically poor mechanical properties. As a primary example, many AM process restrict
the maximum viscosity of suitable materials which limits their molecular weight and mechanical
properties. This dissertation encompasses the design of new photopolymers to circumvent this
restriction and enhance the mechanical performance of printed materials, with an emphasis on
elastomers. Primarily, my work investigated the use of latex polymer colloids, polymer particles
dispersed in water, as a novel route to provide high molecular weight polymers necessary for
elastic properties in a low viscosity, liquid form. The addition of photoreactive molecules into the
aqueous phase of latex introduces the necessary photocurability for vat photopolymerization (VP)
AM. Photocuring in the printer fabricates a three-dimensional object which comprises a hydrogel
embedded with polymer particles. Upon drying, these particles coalesce by penetrating through
the hydrogel scaffold without disrupting the printed shape and provide mechanical properties
comparable with the high molecular weight latex polymer. As a result, this work introduces high
molecular weight, high performance polymers to VP and reimagines latex applications beyond 2D
coatings. Further investigations demonstrate the versatility of this approach beyond elastomers
with successful implementations with glassy polymers and inorganic (silica) particles which yield
nanocomposites.
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