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The development of future electronics depends on the availability of suitable functional materials. Printed electronics, for example, relies on access to highly conductive, inexpensive and printable materials, while strong light absorption and low carrier recombination rates are demanded in photocatalysis industry. Despite all efforts to develop new materials, it still remains a challenge to have all the desirable aspects in a single material. One possible route towards novel functional materials, with improved and unprecedented physical properties, is to form composites of different selected materials. In this work, we report on hydrothermal growth and characterization of graphene/zinc oxide (GR/ZnO) nanocomposites, suited for electronics and photocatalysis application. For conductive purposes, highly Al-doped ZnO nanorods grown on graphene nanoplates (GNPs) prevent the GNPs from agglomerating and promote conductive paths between the GNPs. The effect of the ZnO nanorod morphology and GR dispersity on the nanocomposite conductivity and GR/ZnO nanorod bonding strength were investigated by conductivity measurements and optical spectroscopy. The inspected samples show that growth in high pH solutions promotes a better graphene dispersity, higher doping and enhanced bonding between the GNPs and the ZnO nanorods. Growth in low pH solutions yield samples characterized by a higher conductivity and a reduced number of surface defects. In addition, different GR/ZnO nanocomposites, decorated with plasmonic silver iodide (AgI) nanoparticles, were synthesized and analyzed for solar-driven photocatalysis. The addition of Ag/AgI generates a strong surface plasmon resonance effect involving metallic Ag0, which redshifts the optical absorption maximum into the visible light region enhancing the photocatalytic performance under solar irradiation. A wide range of characterization techniques including, electron microscopy, photoelectron spectroscopy and x-ray diffraction confirm a successful formation of photocatalysts. Our findings show that the novel proposed GR-based nanocomposites can lead to further development of efficient photocatalyst materials with applications in removal of organic pollutants, or for fabrication of large volumes of inexpensive porous conjugated GR-semiconductor composites.

<p>Nanomaterials have been heavily studied in the past two decades. Previous findings have demonstrated that the characteristics of nanocomposites and the performance of nanomaterial-based devices are both determined by the interfacial characteristics of the nanomaterials. However, there are still some remaining challenges from interfacial characteristics to device applications, which are specified as follows: the difficulty in identifying the interfacial contacts of nanostructured surfaces, the instability of nanocomposite surfaces, and the under-researched mechanism of the correlation between interfacial characteristics and the performance of devices.</p> <p>Therefore, the main theme of this thesis is to investigate the interfacial contacts of nanostructured solid-liquid interfaces by direct observation, and to develop a stable nanocomposite based on which the direct observation of the interfacial contact can be better conducted, and to eventually investigate the effect of interfacial contacts on the performance of organic solar cells.</p> <p>As the previous identification of the solid-liquid interface is limited to a microscale range, a direct method of tracing the different wetting states of water was developed, on nanostructured surfaces. This method provided an answer to a long standing question of, whether there is a transition from Wenzel to Cassie states in the sliding angle drop on nanocomposite thin films. In order to complete the observation of the wetting states of water, a stable superhydrophobic nanocomposite thin film with hierarchical structure was developed.</p> <p>Furthermore, with the knowledge of identifying the wetting states and the preparing procedures of the nanocomposites, a surfactant-free small-molecule nanoparticle organic solar cell with a much improved fill factor was developed by spin coating. The inverse correlation of series resistance and parallel resistance was discovered, due to the morphology change and the variation of the charge carrier concentration near the donor-acceptor interface in small-molecule organic solar cells.</p> / Doctor of Philosophy (PhD)

One of the major classes of polymer matrix resins under consideration for structural composite applications in the infrastructure and construction industries is vinyl ester resin. Vinyl ester resin is comprised of low molecular weight poly(hydroxyether) oligomers with methacrylate endgroups diluted with styrene monomer. The methacrylate endgroups cure with styrene via free radical copolymerization to yield thermoset networks. The copolymerization behavior of these networks was monitored by Fourier Transform Infrared Spectroscopy (FTIR) at various cure conditions. Reactions of the carbon-carbon double bonds of the methacrylate (943 cm-1) and styrene (910 cm-1) were followed independently. Oligomers possessing number average molecular weights of 700 g/mole were studied with systematically increasing levels of styrene. The Mortimer-Tidwell reactivity ratios indicated that at low conversion more styrene was incorporated into the network at lower cure temperatures. The experimental vinyl ester-styrene network compositions deviated significantly from those predicted by the Meyer-Lowry integrated copolymer equation at higher conversion, implying that the reactivity ratios for these networks may change with conversion. The kinetic data were used to provide additional insight into the physical and mechanical properties of these materials. In addition to establishing the copolymerization kinetics of these materials, the development of halogen free fiber reinforced vinyl ester composites exhibiting good flame properties was of interest. Flame retardant vinyl ester resins are used by many industries for applications requiring good thermal resistance. The current flame-retardant technology is dependent on brominated vinyl esters, which generate high levels of smoke and carbon monoxide. A series of halogen free binder systems has been developed and dispersed in the vinyl ester to improve flame retardance. The binder approach enables the vinyl ester resin to maintain its low temperature viscosity so that fabrication of composites via Vacuum Assisted Resin Transfer Molding (VARTM) is possible. The first binder system investigated was a polycaprolactone layered silicate nanocomposite, which was prepared via intercalative polymerization. Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) data indicated a mixed morphology of exfoliated and intercalated structures. The mechanical properties and the normalized peak heat release rates were comparable to the neat vinyl ester resin. Alternative binder systems possessing inherent flame retardance were also investigated. A series of binders comprised of novolac, bisphenol A diphosphate, and montmorillonite clay were developed and dispersed into the vinyl ester matrix. Cone calorimetry showed reductions in the peak heat release rate comparable to the brominated resin. / Ph. D.

This thesis is concerned with fused filament fabrication (FFF) of cellulose nanocrystal (CNC) and thermoplastic polyurethane (TPU) nanocomposites, focusing on preliminary optimization of a processing window for 3D printing of mechanically responsive composites and the influence of temperature on mechanical adaptivity, thermal stability, and rheology. CNC thermoplastic nanocomposites are a water responsive, mechanically adaptive material that has been gaining interest in additive manufacturing for 4D-printing applications. Using a desktop FlashForge Pro 3D printer, we first established a viable processing window for a nanocomposite comprising 10 wt% CNCs in a thermoplastic urethane (TPU) matrix, formed into a filament through the combination of masterbatch solvent casting and single screw extrusion. Printing temperatures of 240, 250, and 260°C and printing speeds of 600, 1100, and 1600 mm/min instituted a consistent 3D-printing process that produced characterizable CNC/TPU nanocomposite samples. To distinguish the effects of these parameters on the mechanical properties of the printed CNC/TPU samples, a design of experiments (DOE) with two factors and three levels was implemented for each combination of printing temperature and speed. Dynamic mechanical analysis (DMA) highlighted 43 and 66% increases in dry-state storage moduli values as printing speed increases for 250 and 260°C, respectively. 64 and 23% increases in dry-state storage moduli were also observed for 600 and 1100 mm/min, respectively, as temperature decreased from 260 to 250°C. For samples printed at 240°C and 1600 mm/min, it was determined that that parameter set may have fallen out of the processing window due to inconsistent deposition and lower dry-state storage moduli than what the slower speeds exhibited. As a result, the samples printed at 240°C did not follow the same trends as 250 and 260°C. Further analysis helped determine that the thermal energy experienced at the higher end printing temperatures coupled with the slower speeds decreased the dry-state storage moduli by nearly 50% and lead to darker colored samples, suggesting CNC degradation. Isothermal thermogravimetric analyses (TGA) demonstrated that the CNC/TPU filament would degrade at relative residence times in the nozzle for all the chosen printing temperatures. However, degradation did not eliminate the samples' ability to mechanically adapt to a moisture-rich environment. DMA results verified that mechanical adaptivity was persistent for all temperature and speed combinations as samples were immersed in water. However, for the higher temperatures and slower speeds, there was about a 15% decrease in adaptability. Optimal parameters of 250°C and 1600 mm/min provided the highest dry-state storage modulus of 49.7 +/- 0.5 MPa and the highest degree of mechanical adaptivity of 51.9%. To establish the CNC/TPU nanocomposite's use in 4D printing applications, shape memory analysis was conducted on a sample printed at the optimal parameters. Multiple wetting, straining, and drying steps were conducted to highlight 76% and 42% values for shape fixity and shape recovery, respectively. Furthermore, a foldable box was printed to serve as an example of a self-deployable structure application. The box displayed shape fixity and recovery values of 67% and 26%, respectively, further illustrating significant promise and progress for CNC/TPU nanocomposites in 4D-printed, shape adaptable structures. Further analysis of the effect of degradation during FFF of the CNC/TPU nanocomposite was conducted using rotational rheometry, Fourier-Transform Infrared Spectroscopy (FTIR), and polymer swelling experiments. A temperature ramp from 180 to 270°C showed a significant increase in complex viscosity (h*) at the chosen printing temperatures (240, 250, and 260°C). Moreover, h* of neat TPU suddenly increases at 230°C, indicating a potential chemical crosslinking reaction taking place. 20-minute time sweeps further verified that h* increases along with steady increases in storage (G') and loss (G'') moduli. From these results, it was hypothesized that crosslinking is occurring between CNCs and TPU. Preliminary characterization with FTIR was used to probe the molecular structure of thermally crosslinked samples. At 1060 and 1703 cm-1, there are significant differences in intensities (molecular vibrations) as the temperature increases from 180 to 260°C related to primary alcohol formation and hydrogen bonded carbonyl groups, respectively. The hypothesis is the disassociation of TPU carbamate bonds into soft segments with primary alcohols and hard segments with isocyanate groups. The subsequent increasing peaks at 1060 and 1703 cm-1 may indicate crosslinking of CNCs with these disassociated TPU segments. To quantify potential crosslinking, polymer swelling experiments were implemented. After being submerged in dimethylformamide (DMF) for 24 hours, CNC/TPU samples thermally aged for 15 minutes at 240, 250, and 260°C retained their filament shape and did not dissolve. The 240 and 250°C aged samples had relatively similar crosslink densities close to 900 mole/cm3. However, from 250 to 260°C, there was about a 36% increase in crosslink density. These results suggest that crosslinking is occurring at these printing temperatures because both CNCs and TPU are thermally degrading into reactive components that will lead to covalent crosslinks degradation. Additional characterization is needed to further verify the chemical structure of these CNC/TPU nanocomposites which would provide significant insight for CNC/TPU processing and 3D printing into tunable printed parts with varying degrees of crosslinking. / Master of Science / This thesis is concerned with the development of a processing window for mechanically adaptive cellulose nanocrystal (CNC) and thermoplastic polyurethane (TPU) nanocomposites with fused filament fabrication (FFF) and, evaluating the influence of elevated temperatures on the mechanical, thermal, and rheological properties of said nanocomposite. CNC thermoplastic nanocomposites are a water responsive, mechanically adaptive material that has been gaining interest in additive manufacturing for 4D-printing. Using a desktop 3D-printer, an initial processing window for a 10 wt% CNC in TPU was established with printing temperatures of 240, 250, and 260°C and printing speeds of 600, 1100, and 1600 mm/min. A design of experiments (DOE) was implemented to determine the effects of these parameters on the mechanical properties and mechanical adaptability of printed CNC/TPU parts. Dynamic mechanical analysis (DMA) suggests that combinations of higher temperatures and lower speeds result in reduced storage moduli values for printed CNC/TPU parts. However, mechanical adaptation, or the ability to soften upon water exposure, persists for all the printed samples. Additionally, there was significant discolorations of the printed samples at the higher temperature and slower speed combinations, suggesting thermal degradation is occurring during the printing process. The decrease in storage moduli and discoloration is attributed to thermal energy input, as thermogravimetric analysis indicated thermal degradation was indeed occurring during the printing process regardless of printing temperature. Using the parameters (250°C and 1600 mm/min) that displayed the superior mechanical properties, as well as mechanical adaptivity, shape memory analysis was conducted. The optimal printed part was able to hold 76% of the shape it was strained to, while recovering 42% of the original unstrained shape once immersed in water, indicating potential for shape memory and 4D-printing applications. Furthermore, a foldable box was printed with the optimal parameters and it displayed similar shape memory behavior, illustrating promise for CNC/TPU self-deployable shape adaptable structures. To further study the effect of degradation on the CNC/TPU system, melt flow properties, molecular structure, and polymer swelling were investigated. At the printing temperatures (240, 250, and 260°C), the complex viscosity of the CNC/TPU filament experienced an exponential increase, indicating potential network formation between the CNCs and TPU. Fourier-Transform Infrared Spectroscopy (FTIR) highlighted changes in the molecular structure for the CNC/TPU filament as temperature increased from 240 to 260°C, which suggests that chemical structure changes are occurring because of degradation. The hypothesis is TPU is disassociated into free soft and hard segments that the CNCs can covalently crosslink with, which can potentially be explained by the increases in the FTIR intensities relating to TPU and CNC's chemical structure. To further quantify potential crosslinking between CNCs and TPU, polymer swelling experiments were implemented. The results from these experiments suggest that increasing printing temperatures from 240 to 260°C will lead to higher degrees of crosslinking. Further investigation could yield the validity of this crosslinking and additional optimization of FFF printing with CNC/TPU nanocomposites.