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. / 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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Research efforts have focused on synthesis of linear, long-chain branched, and novel crosslinked polyesters for applications spanning from pressure sensitive adhesives to biomedical applications. Altering polymer topology and functionality using different synthetic strategies was enabled tailoring the thermomechanical, rheological, and adhesive properties of polyesters. The synthesis and characterization of linear, long-chain branched, and crosslinked networks are described focusing on the structure-property relationships. Aliphatic low-Tg polyesters with linear and long-chain branched topology were synthesized using melt polycondensation for pressure sensitive adhesive applications. Relationships between molecular weight, polymer composition, and adhesive performance were investigated. Melt rheological studies and the characterization of adhesive properties indicated that adhesive performance was enhanced with increasing molecular weight. Moreover, a series of long-chain branched low-Tg polyester were investigated to determine the influence of branching and molecular weight. Tailoring the degree of branching enabled the control of rheological and adhesive properties. Characterization of adhesive properties revealed that long-chain branched polymers displayed an enhanced cohesive strength. In addition, utilization of different comonomer compositions allowed tailoring thermal and adhesive properties of low-Tg polyesters over a wide range. Biodegradable networks were synthesized for the first time using base-catalyzed Michael addition of acetoacetate functionalized polyesters with acrylates. Linear and star-shaped poly(caprolactone) (PCL) oligomers with different molecular weights were functionalized and crosslinked. Thermomechanical properties were evaluated as a function of precursor molecular weight and crosslink density. The glass transition temperature and the extent of crystallinity of the networks were dependent on the molecular weight of the PCL segment. Moreover, dynamic mechanical analysis (DMA) indicated that molecular weight of the oligomeric precursors influenced the plateau modulus of the networks as a result of the differences in crosslink density of the networks. In addition, covalently crosslinked networks were synthesized from Michael addition reaction of acetoacetate-functional oligomeric poly(trimethylene succinate)s and poly(trimethylene adipate)s with neopentylglycol diacrylate. The oligomeric polyesters with telechelic hydroxyl functionality were synthesized from renewable monomers, adipic acid, succinic acid, and 1,3-propanediol using melt polycondensation. The molecular weights of the precursors were varied systematically to probe the influence of molecular weight on thermomechanical properties of the networks. The extent of crystallinity and mechanical properties were dependent on the molecular weight of the oligomeric polyester precursors which also controlled crosslink density. Moreover, Michael addition chemistry was utilized to crosslink low-Tg polyesters to improve cohesive strength for PSA applications. In order to determine the influence of temperature and catalyst levels, crosslinking reactions were monitoring using measurement of loss and storage moduli during the reaction. Networks having different levels of gel fractions were investigated to elucidate the influence of degree of crosslinking on thermomechanical and adhesive properties of low-Tg polyesters. / Ph. D.

Les composites élastomères sont actuellement utilisés dans de nombreuses applications commerciales et ont montré de grandes promesses pour l'utilisation dans les nouvelles technologies. Cela soulève la pratique, ainsi que théorique nécessaire pour comprendre le lien entre la microstructure sous-jacente de composites en élastomère et de leurs propriétés mécaniques et physiques, et comment celui-ci peut être améliorée avec des changements dans l'ancienne. Dans ce contexte, l'objectif principal de cette thèse est le développement d'une analyse, le cadre homogénéisation non linéaire pour déterminer la réponse globale des composites élastomères soumis à des déformations finies. Les comptes-cadre pour l'évolution de la microstructure sous-jacente, ce qui entraîne des changements dans la géométrie finie induite par la charge appliquée. Ce point est essentiel que l'évolution de la microstructure peut avoir un assouplissement significatif géométrique (ou raidissage) effet sur la réponse globale du matériau, qui, à son tour, peut conduire à l'élaboration éventuelle d'instabilités macroscopiques. Le concept principal derrière la méthode d'homogénéisation non linéaire proposé est la construction de principes variationnels appropriés en utilisant l'idée d'un "portail composite linéaire», qui a finalement permettre la conversion des estimations disponibles homogénéisation linéaire dans les estimations analytiques pour la grande déformation de réponse global de l' non linéaire des composites en élastomère. Cette thèse comprend des applications de la théorie proposée pour les différentes classes des élastomères renforcés et poreux aléatoire et des microstructures périodiques. Une analyse complète du comportement efficace, l'évolution de la microstructure et le développement d'instabilités macroscopiques est prévu pour toutes ces applications.

This dissertation presents a numerical study using molecular dynamic simulations that interrogates the polymer structure as it is strained continuously in time and correlates it with the stress developed in it. We investigate the role of external control variables such as temperature, strain-rate, chain length, and density. At temperatures higher than glass transition, stress anisotropy is reduced even though bond stretch is greater at higher temperatures. There is a significant increase in stress level with increasing density. At faster rates of loading stress anisotropy increases. Deformation is mostly due to bond stretch and bond bending rather than overall shape and size. Stress levels increase with longer chain length. Cross-linkers beyond a critical value of functionality cause increased constraint on the motion of monomers and uniaxial stress developed increases. Stacking of chains also plays a dominant role in terms of excluded volume interactions. Low density, high temperature, low values of functionality of cross-linkers, and short chain length, facilitate chain uncoiling and chain slipping in crosslinked polymers. Uniaxial stress in linear polymers, on the other hand, is only mildly in uenced by temperature. Sinusoidal strain loading clearly reveals the viscoelastic nature of polymers. Internal structural parameters of the chains such as bond length, bond angle show hysteresis during loading and unloading. However, parameters representing overall size and shape of chains do not display any hysteresis. Small size magnetic particles and their small volume fractions in polymers show no signi cant e ect of applied external magnetic eld on anisotropic stress. Stress increases with lowering temperature, increasing density, decreasing volume fraction of magnetic particles, and increasing chain length

Materials which exhibit non-linear mechanical behaviors under large deformations are generally classified as “soft matter”. Elastomers represent an important class of soft materials which have wide commercial applications and isotropic non-linear behavior. In contrast, biological materials have anisotropic responses due to their heterogeneous and composite architectures. The underlying microstructure determines the arterial macroscopic behavior and is represented through constitutive models to describe the stress-strain relationships. Mechanical characterization and development of constitutive models that describe these non-linear and anisotropic properties are essential to our understanding of the structure-property relationships in these materials. In this study, we use two model systems to link the local microstructure to the overall macroscopic behaviors of soft matter. First, we delineate the roles of individual network topological factors in determining the overall macroscopic behavior of isotropic silicone elastomers using specimens fabricated with differential amounts of crosslinking. We performed mechanical experiments, within a theoretically motivated continuum mechanical framework, using a custom made planar biaxial testing instrument. These experiments demonstrate the contributions of physical entanglements and chemical crosslinks to the overall mechanical properties of silicone elastomers. Further, we show that the slip-link form of strain energy function is better suited to describe the material properties in the low to moderate regions of the stress-strain behavior. However, this model does not predict the stiffening response of elastomers at higher deformations, which is better captured using the Arruda-Boyce form of strain energy function. To explore the effects of individual topological factors on the overall network properties, we performed swelling experiments of silicone specimens in xylene and quantified variations in the polymer-solvent interaction parameter, χ, given by the Frenkel-Flory-Rehner (FFR) model. Further, we characterized the viscoelastic properties using dynamic mechanical analysis. Our results show that χ is not a constant, as assumed in the FFR model, but bears a linear relation to the equilibrium polymer volume fraction. To characterize the contribution of trapped entanglements to the overall mechanical behaviors, we use scaling laws in polymer physics and investigate the dependence of equilibrium volume fraction and experimentally obtained elastic moduli. Further, dynamic mechanical analysis demonstrated an increase in complex modulus with increase in the cross linking density. Finally, we examined variations in the uniaxial and the dynamical mechanical properties of silicone elastomers with storage time. Our results show that the time dependent increase in the modulus correlated with the formation of slip-links in the samples aged for a significantly long time in air. Together, these comprehensive studies show the importance of individual network features which affect the overall macroscopic properties of elastomers. Second, we use a multilayered and composite arterial model system to explore the passive material properties of arteries due to anisotropic layouts of extracellular matrix proteins, collagen and elastin. We characterized the mechanical properties of diseased human ascending thoracic aortic dissected (TAD) tissues, obtained from consenting patients undergoing emergency surgical repair to replace the diseased region, using multiple biaxial tests. We fit these results to micro structurally motivated Holzapfel-Gasser-Ogden model which is frequently used in the arterial mechanics literature. Our results show a higher stiffness for TAD tissues as compared to control aorta, without the presence of atherosclerotic plaques or other arterial disease. To study the directional variation in the mechanical properties of TAD tissues, we compared the stiffness in circumferential longitudinal directions at high and low stress region of equibiaxial experimental data. We observed no differences in the stiffness of TAD tissues in the circumferential and longitudinal directions. Further, we do not see any directional variations in the ultimate tensile stress, maximum extensibility, and modulus calculated in the low stretch region of uniaxial stress-strain response in TAD tissues. Histological analysis of TAD tissues showed a decrease in elastin content and an increase in collagen content as compared to control tissues. Higher TAD tissue stiffness also correlated with reduced elastin content in the arterial walls. To investigate the strain rate dependence of measured mechanical properties we use high testing rates of 1mm/sec to show that the TAD tissues have higher stiffness in the circumferential direction as compared to longitudinal direction. Finally, we used peel experiments to quantify the rupture potential of aortic dissected tissues. Our results show that TAD tissues have reduced delamination strength between layers as compared to control aortic tissues. To the best of our knowledge, no previous study has reported the mechanical property of human TAD tissues and these are the only biomechanical results on TAD tissues reported in specimens from South Asian patients. We hope that such studies will be useful for researchers who rely on microstructure based constitutive models to accurately describe the mechanical environment of cells which are important in the remodeling of tissues and in numerical models to assess mechanical criteria which may lead to the growth or dissection of arterial tissues.

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