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No / The rheological behavior of polymeric nanocomposites provides major determination for their processability. In this work, three carbon nanotubes (CNTs) with varied geometries were adopted as nanofiller and then were introduced into poly(methyl methacrylate) (PMMA) matrix with different loadings (0.07–1.0 wt %). The different preparation routine led to varied CNTs dispersion states, on which the shear viscosity and the compressibility of their melts were proved to be sensitive. The technology for the preparation of their nanocomposites played a crucial role in controlling their rheological behavior. With melting blended bare CNTs, the dynamic shear viscosity of PMMA/CNTs increased with the increase of CNTs content, accompanied by aggregated CNTs in which no polymer matrix was entrapped. With the help of surface modification and pre‐mixing, well dispersed CNTs were obtained and a rather low aggregation rate ca. 0.029% was revealed. The well dispersed CNTs with an organic layer which was constructed by small molecules and presented lower viscosity. Such CNTs led to no remarkable clusters within polymer host and played the role of lubricant with an increased‐mobility layer, which can be reflected from the weighted relaxation time spectra.

In this paper we present an exponential stability result for a diffusion equation arising from dumbbell models for polymer flow. Using the methods of semigroup theory, we show that the semigroup U(t) associated with the diffusion equation is well defined and that all solutions converge exponentially to an equilibrium solution. Both finitely and infinitely extensible dumbbell models are considered. The main tool in establishing stability is the proof of compactness of the semigroup. / Ph. D.

This work has been concerned with the dimensional stability and the structure-property relationships of thermoplastics reinforced with particulate and fiber fillers. The first part of this study was concerned with ascertaining the main causes of warpage observed for injection-molded thermoplastics reinforced with high aspect ratio fibers. Typically, warpage in injection-molded fiber reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow-induced stresses generated during mold filling do not play a significant role in the warpage. The warpage of PP composites reinforced with TLCP fibers was found to increase with an increase in fiber loading. The shrinkage and the thermal expansion of the TLCP/PP composites and, hence, the thermally induced stresses decreased with an increase in fiber loading while the flow-induced stresses increased. The increase in the flow-induced stresses was attributed to an inhibition of stress relaxation and greater generation of orientation of the polymer chains with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber reinforced thermoplastics, the flow-induced residual stresses must be accounted for. The second part of this work was concerned with minimizing the particle loading of reinforced PC/PBT composites while maintaining the stiffness, i.e. modulus, and the dimensional stability of injection molded flat panels. This was accomplished by using high aspect ratio (≈100-150) nano-clays as opposed to micron-size talc (≈5-10). It was found that by using nano-clays surface modified with a quaternary ammonium salt that contained two hydroxyl groups as opposed to fine talc particles, the level of particle reinforcement could be reduced from 6 to 1 wt% without sacrificing the modulus of the reinforced PC/PBT composites. Further benefits included a 26% increase in flexural strength, 77% increase in the tensile toughness and 3% reduction in the density of the reinforced PC/PBT composites. An increase in the modulus and tensile toughness was observed even though there was evidence of loss in molecular weight of the PC/PBT matrix, which was supported by the rheological behavior of the composites. / Ph. D.

This thesis is concerned with the development of engineering technologies that facilitate melt spinning of carbon fiber precursors in both an environmentally sound and cost effective manner. More specifically, methods were developed to avoid a degradative process in acrylonitrile copolymers (typically used in textiles and as carbon fiber precursors) that occurs as melt spinning temperatures are approached. The following set of analyses was developed to define the rheological properties required for a melt processable acrylic copolymer suitable for use as a carbon fiber precursor, and accordingly facilitated development of a processing window: measurement of steady shear viscosity as a function of both temperature and time, measurement of the magnitude of the complex viscosity (|η*|) as a function of temperature using a temperature sweep, and measurement of the angular frequency dependence of |η*|. Through a systematic screening process, the following properties were identified to afford melt spinnable acrylic precursors suitable for conversion to carbon fibers: emulsion polymerization, 85-88 mole % acrylonitrile, 11-14 mole % methyl acrylate, 1 mole % acryloyl benzophenone, intrinsic viscosity < 0.6 dL/g, steady shear viscosity ≤ 1000-2000 Pa*s at a shear rate (γ) of 0.1 s⁻¹, viscosity increases ≤ 45% over a period of 1800 seconds at 200-220°C and γ=0.1 s⁻¹. Use of the rheological analyses assisted in development of a melt spinnable carbon fiber precursor, which resulted in carbon fibers possessing a tensile strength and modulus of approximately 1.0 and 120 GPa, respectively. A second approach was evaluated using carbon dioxide (CO₂) to plasticize AN copolymers to an extent that facilitates processing at reduced temperatures, below where thermal degradation is significant. A batch saturation method to absorb CO₂ in AN copolymers was developed. Differential scanning calorimetry and thermogravimetric analyses were used to measure the glass transition temperature (T_g) reduction and amount of absorbed CO₂ (respectively). A pressurized rheometer and measurement procedure was designed to obtain viscosity measurements of saturated AN copolymers. Up to 6.7 wt. % CO₂ was found to absorb into a 65 mole % AN copolymer with the saturation method used, resulting in a 31°C glass transition temperature (T_g) reduction, 60% viscosity reduction, and 30°C potential processing temperature reduction. It was found that CO₂ can absorb into copolymers containing up to 90 mole % AN (with the absorption methods used) with the following results (for a 90/10 mole % AN/MA copolymer): 3.0 wt. % uptake, 27°C T_g reduction, 56% viscosity reduction, and potential processing temperature reduction of 9°C. Via estimates of the required pressure, sealing fluid flow rate, and length of a pressure chamber to prevent foaming of the saturated polymer melt during extrusion, melt spinning of saturated AN copolymers appears feasible. / Ph. D.

Ultraviolet-Assisted Direct Ink Write (UV-DIW) is a material extrusion additive manufacturing (AM) technology in which a viscous ink, often at room temperature, is selectively extruded through a translating nozzle to selectively deposit material. The extruded ink is solidified via UV irradiation (photocuring) and three-dimensional parts are created by repeating the process in a layer-by-layer fashion. UV-DIW is an attractive AM technology due to its ability to (1) extrude highly viscous inks (i.e. >10,000 Pa·s if ink exhibits shearthinning behavior) (2) the promise of leveraging the broad photopolymer material library and chemistries established for other AM technologies capable of processing photopolymers and (3) the promise of processing a wide range of inks, which enables the fabrication of metal, ceramic, polymer, bio-based, and multi-material parts. Currently, the technology faces a few shortcomings including (1) limited material selection for UV-DIW due to requirement for inks to be photocurable and limited mechanical properties of photocurable materials (2) lack of feature resolution and topological complexity of printed parts and (3) lack of material screening models providing robust definition of the material requirements (e.g., viscosity, cure time, strength) for successful UV-DIW printing. To address these shortcomings, the goal of this work is to gain a fundamental understanding of the rheological and reactive properties required for successful Ultraviolet-Assisted Direct Ink Write (UV-DIW). The first approach to answering the fundamental research question is establishing the existing rheology experiments used to characterize DIW inks and the relationships between rheology and printability. An in-depth literature review of the techniques and relationships was compiled to better understand ink requirements for successful printing (Chapter 2). This broad survey is not limited to only UV-DIW, but includes all variations of DIW. The first part of the review provides a summary of the rheological experiments that have been used to characterize a wide variety of DIW inks. The second part of this review focuses on the connections between rheology and printability. This survey helps identify the required rheological properties for successful printing that is then used throughout the rest of this work. Additionally, this review identifies shortcomings in current work and proposes areas for future work. From this exhaustive literature review, a systematic roadmap was developed that investigators can follow to quickly characterize the printability of new inks, independent of that ink's specific attributes (Chapter 3). The roadmap simplifies the trends identified in literature into a brief and intuitive guide to the rheology experiments relevant to DIW printing and the relationship between those experiment and printing results. The roadmap was demonstrated by evaluating the printability of two inks: (1) a silicone ink with both yield-stress and reactive curing behavior and (2) urethane acrylate inks with photocuring behavior. Experimental printing studies were used to support the conclusions on printability made in the roadmap. The second main approach focuses on the development of three novel UV-DIW inks to address the current limited material selection for UV-DIW and help better understand the rheological and reactive properties required for successful printing. For the three novel UVDIW inks, the iterative process of ink synthesis, analysis of ink rheology, and printability evaluation was detailed. Data from the development process contributed to gaining a fundamental understanding of how rheology and reactivity affect printability. The three inks each had novel rheological properties that impacted their printing behavior: (1) photocuring (2) yield-stress behavior + photocuring and (3) photocuring + thermal curing. Additionally, each ink had unique properties that expands material selection for UV-DIW including (1) an all-aromatic polyimide possessing a storage modulus above 1 GP a up to 400 °C (Chapter 4), (2) a styrene butadiene rubber (SBR) nanocomposite with elongation at break exceeding 300 % (Chapter 5), and (3) a dual-cure ink enabling the printing of inks containing over 60 vol% highly opaque solids (Chapter 6). The third approach details the development of two UV-DIW process models to better understand the process physics of the UV-DIW process and give insight to the properties of a successful ink. The first process model uses data from photorheology experiments to model how a photocurable ink spreads upon deposition from the nozzle, accounting for transient UV curing behavior (Chapter 7). This model allows for the rapid evaluation of an ink's behavior during the solidification sub-function of UV-DIW solely based on its rheology, without the time-consuming process of trial-and-error printing or complex computer simulations. The second process model combines modeling with a novel experimental method that uses a UV photorheometer to accurately characterize the relationship between cure depth and UV exposure for a wide range of photopolymers (Chapter 8). This model helps understand an inks photocuring behavior and ensure a sufficient cure depth is produced to adhere to the previous layer in UV-DIW printing. Lastly, two UV-DIW process modifications are introduced to address research gaps of printing high resolution features and limited material selection. A hybrid DIW + Vat Photopolymerization system is presented to improve the feature size and topographical complexities of parts, while still retaining UV-DIW's ability to print with very high viscosity photoresins (Chapter 9). A high temperature Heated-DIW system is presented to heat inks to over 300 °C and ultimately enable printing of poly(phenylene sulfide) aerogels (Chapter 10). In enabling the DIW of poly(phenylene sulfide) aerogels, the production of ultra-lightweight thermally insulating components for applications in harsh environments is enabled. With the use of additive manufacturing, hierarchical porosity on the macroscale is enabled in addition to the meso-scale porosity inherent to the aerogels. / Doctor of Philosophy / Direct Ink Write (DIW) is a type of three-dimensional (3D) printing that is used to automatically produce a range of 3D geometries. Specifically, the DIW process selectively extrudes a viscous ink, similar in consistency to peanut butter or toothpaste, through a small moving nozzle to create the features of each layer. This process is like using a frosting bag to decorate a cake with icing. Three-dimensional parts are created by repeating this process and depositing layer on top of layer. While seemingly a straightforward process, it remains relatively unclear what properties an ink needs to produce quality parts. To produce quality parts, the ink first needs to be extruded from the nozzle to form homogenous beads with a constant width and free from breaks. Second, the extruded ink needs to retain the shape that it was deposited in. If the ink spreads excessively, the as-deposited features will be lost and a part resembling a blob will be produced. Lastly, the ink deposited on the first layers needs to have enough strength to support the weight of the part. Otherwise, the part will collapse akin to the Leaning Tower of Pisa. To achieve all three steps and produce a quality part, a successful ink needs to be able to flow through the nozzle and then solidify upon deposition. This work focuses on a specific process called Ultraviolet-Assisted Direct Ink Write (UV-DIW) where materials that solidify when exposed to UV light, called photopolymers, are printed. Currently, the properties of the inks, especially how they cure when exposed to UV light, that produce successful printing remains unclear. This work focuses on understanding how the properties of the photopolymer inks affect the printing behavior of the ink. The ultimate goal of this work is to develop guidelines for the properties of successful inks which will help others develop the next generation of materials printed via UV-DIW. Specifically, experiments are used study how inks behave when they flow through the nozzle (rheology) and then solidify when exposed to UV light (reactivity). This behavior is then connected to the inks printing behavior (printability). In working to better understand the connection between rheology, reactivity, and printability multiple approaches were used. These approaches include the development of new materials for printing via UV-DIW, development of a modified UV-DIW printing process that reduces the size of the printed features, and development of models to predict how inks will behave during printing. The new plastic materials that were developed and successfully printed via UV-DIW have outstanding properties including remaining strong up to 400 °C, being extremely flexible, and a plastic containing a large fraction of a solid filler. With each new material, the formulation was varied to change the inks rheological and reactive properties until successful UV-DIW was enabled. Each new formulation introduced material capabilities not previously available to DIW 3D printing. Then, A modified UV-DIW process was developed that takes advantage of the reactivity of the photopolymers to enable the printing of high-resolution features and shapes not previously possible via DIW 3D printing. In this novel process, a projector is used to project patterned UV light at the material and selectively cure small portions of the deposited material, instead of curing all the deposited material. After printing, the uncured ink is washed away resulting in features much smaller than what can be produced when directly extruding them. Finally, the developed process models use the relatively simple rheology and reactivity experiments, to predict how an ink behaves during the UV-DIW process. Using the results of these experiments and the developed models, the inks behavior during the printing process is predicted. These models allow for the properties of new inks to be quickly measured and their printing behavior predicted. New ink formulations can be quickly screened, and optimal process parameters predicted. Overall, this work produces guidelines for the rheological and reactive properties required of a photopolymer ink to produce successful UV-DIW printing. Future researchers can use these guidelines to develop the next generation of materials printed via UV-DIW more easily.

The solution rheological and electrospinning behavior of a series of charge-containing polymers, surface-active agents, and carbon nanotube composites was studied to investigate the effect of intermolecular interactions, including electrostatic interactions, hydrogen bonding, surface activity, and surface functionalization of carbon nanotubes. The synthesis of novel polyelectrolytes with varied topologies, charge content, and counterions tailored the charged macromolecules to elucidate structure-rheology and structure-processing relationships. In addition, the use of additives for electrospinning, including surfactants and nanofillers, allows us to tailor the functionality of electrospun nanofibers for high-performance applications. Novel polyelectrolytes based on poly(2-(N,N-dimethyl)aminoethyl methacrylate) (DMAEMA) were synthesized with the counteranions Cl-, NO3-, (CN)2N-, BF4-, PF6-, triflate (TfO-), and bis(trifluoromethanesulfonyl)imide (Tf2N-). The counteranion selection controlled the thermal transitions and degradation; the larger and more charge-delocalized anions typically resulted in lower Tg and higher decomposition temperature. The polyelectrolyte behavior in solution was nearly independent of anion choice, though solution conductivity depended on the electrophoretic mobility of the counterion. Charge containing copolymers of DMAEMA and di(ethylene glycol) methyl ether methacrylate (MEO2MA) were synthesized and demonstrated that polyelectrolyte behavior in solution was also nearly independent of charge content. Low ionic contents resulted in extended solution conformations and high conductivities. Controlled atom-transfer radical polymerization allowed the synthesis of star-shaped polyelectrolytes with varying arm numbers and lengths. The solution behavior of the stars deviated slightly from the linear polyelectrolytes due to significant counterion condensation within the star core and constrained polymer conformations. The linear and star-shaped polyelectrolytes were electrospun to understand the interplay between polyelectrolyte structure and electrospinnability. Similar to other strong polyelectrolytes described in the literature, PDMAEMA-based polyelectrolytes with polar anions (e.g. Cl-) experienced significant instabilities during electrospinning, requiring high concentrations and viscosities to stabilize the electrospinning jet. The use of large, more hydrophobic anions (BF4-, TfO-) led to increased electrospinnability. Unlike neutral branched polymers, which electrospin nearly identically to linear polymers of similar molecular weight, the star-shaped PDMAEMA-based polyelectrolytes required even higher viscosities than linear polyelectrolytes for stable electrospinning. The correlations between electrospinnability and solution rheological analysis are detailed. The use of surfactants facilitates the electrospinning of neutral polymers at lower concentrations. However, we have demonstrated that specific cylindrical aggregates of surfactants (wormlike micelles) can be electrospun into microfibers under the proper conditions. Ammonium and phospholipids surfactants as well as organogelators were studied using solution rheology and DLS to determine the effects of micellar structure and solution viscosity on the electrospinnability of low molar mass surfactants. In addition, the effects of charged and uncharged surfactants on the electrospinning behavior of poly(methyl methacrylate) were determined. Added surfactant facilitated uniform fiber formation at lower PMMA concentrations. XPS analysis demonstrated the formation of core-shell fibrous structures resulting from the self-migration of surfactants to the fiber surface. Hydrogen bonding also influences fiber formation through electrospinning. Star-shaped poly(D,L-lactide)s (PDLLAs) were end-functionalized with adenine (A) or thymine (T) units. The complementary hydrogen bonding between the adenine and thymine lead to thermoresponsive rheological behavior for mixtures of PDLLA-A and PDLLA-T. The mixtures could be electrospun above the hydrogen bond dissociation temperature and resulted in thicker fibers compared to unfunctionalized PDLLA stars. The hydrogen bonding allows the preparation of polymers with a combination desirable solid-state properties and very low processing viscosities. The effects of carbon nanotube incorporation on electrospinning behavior and fiber morphology were also investigated. Nonfuntionalized and carboxylic-acid functionalized carbon nanotubes were electrospun into polyurethane nanofibers. The nonfunctionalized nanotubes required high-shear melt mixing to disperse within the polyurethane, but remained well dispersed through electrospinning. The surface functionalization with acid groups produced nanotubes which dispersed more readily into the polyurethane solutions. TEM analysis revealed that nanotube dispersion and alignment within the nanofibers was similar for both nonfunctionalized and acid-functionalized nanotubes. / Ph. D.