In this dissertation, new ways for controlling the internal structures of a system of polymer composites, polymer blends, and hydrogel composites by means of external electric or magnetic fields are presented. The first part of this study addresses the development of an anisotropic phase-separated morphology in polymer blends by using electrically pre-oriented clay particles. It was observed that electrically pre-oriented montmorillonite clay particles in a homogenous single-phase blend lead to anisotropic phase-separated morphology of the blends, undergoing demixing upon temperature shift to a two-phase regime. The initial co-continuous microstructure developed into a coarsened and directionally organized phase-separated morphology parallel to the direction of oriented clay particles (applied AC electric field direction) over the annealing time. It was also found that the degree of clay orientation under AC electric field was linearly proportional to the degree of polymer-phase orientation. The temporal morphological evolution was thoroughly analyzed by electron microscopy and X-ray diffraction studies. The second part of the study covers anisotropic hydrogel nanocomposites developed by orienting magnetically sensitive nontronite clay minerals under the strong magnetic fields. Anisotropic hydrogel nanocomposites were formed by magnetic-field assisted orientation of nontronite clays suspended in a hydrogel precursor solution followed by a gelation process. The degree of orientation of nontronite minerals was quantitively characterized by birefringence and small-angle X-ray scattering. The resultant hydrogels exhibited anisotropic optical, mechanical, and swelling properties along the direction of oriented clay minerals. Anisotropic water swelling behaviors can be particularly applied in medical dressing materials, where vertical wicking of fluid into the wound dressing is sought after for minimizing periwound maceration damage.

This dissertation presents the fabrication of a silicate-based nanocomposite hydrogel with outstanding shear-thinning properties, viscoelastic behaviour, and water retention capacity. Due to their adaptable mechanical properties, bioavailability, and water retention capacity, these nanocomposite hydrogels have been extensively used for biomedical applications. Laponite nanoparticles are among the most utilized silicate-based minerals. These clay nanoparticles are composed of platelets that are positively charged on the edges and negatively charged on the surface. The high aspect ratio of the polyanionic surface of the Laponite nanoparticles can absorb and trap ionic functional groups with non-covalent interactions. These silicate-based nanocomposite hydrogels are produced by dispersing Laponite nanoparticles in deionized water, forming a homogenous colloid. The uniform dispersion of these nanoparticles in aqueous solutions forms a “house of cards” structure, which eliminates particle aggregation and improves their surface interaction with ionic compounds. The fabrication process is followed by the addition of the stable colloid to various organic and inorganic mixtures including, chitosan, alginate, graphene oxide, and gelatin. The chemical, physical, and mechanical properties of these nanocomposites are experimentally evaluated. Silicate-based nanocomposite hydrogels offer unique rheological characteristics, which facilitate the injection process while preserving the mechanical integrity of the construct following extrusion. The injectability of these nanocomposites was assessed by evaluating their shear-thinning properties through multiple rheological analyses. As per the definition of shear-thinning, the viscosity of nanocomposites is directly affected by the applied shear stress; the viscosity of these compositions decreases under shear stress and reverts to the original viscosity after removal of the force. Accordingly, nanocomposite hydrogels with shear-thinning properties can be utilized for extrusion-based 3D printing and for depositing drugs in localized tissue without the jeopardy of being washed away by circulating blood. In addition, the large number of surface interactions and cationic exchange capacity of Laponite nanoparticles improve electrostatic interactions between the nanocomposite components and a wide range of ionic compounds. Accordingly, these chemical properties facilitate the incorporation of stimuli-responsive materials into the polymeric structure of the nanocomposite, allowing for the utilization of these hydrogels in on-demand drug delivery applications. These properties of the silicate-based nanocomposite hydrogels are investigated through swelling and release studies, Fourier transforms infrared spectroscopy (FTIR), and zeta potential measurements. The results of these experiments indicate that the non-covalent electrostatic interactions and chemical properties of these hydrogels improve the solubility and loading efficiency of therapeutic agents. Silicate-based nanocomposite hydrogels may also be utilized for developing electrical conductive bioinks for extrusion-based three-dimensional (3D) printing. Adjusting the viscosity and shear-thinning properties of the hydrogel plays a significant role in the printability of a bioink. For instance, a highly viscous bioink disrupts extrusion, while a bioink with a low viscosity results in the formation of droplets instead of the desired cylindrical filaments. Optimized formulations of the nanocomposite hydrogels are investigated by conducting various mechanical property measurements. Consequently, the unique chemical and rheological properties of the proposed hydrogels make them superior candidates for drug delivery and tissue engineering applications. / Graduate / 2022-03-30

Cellulose nanocrystals (CNCs) are attractive nanomaterials due to their superior mechanical properties, renewability, and natural abundance. Their surface hydroxyl groups, along with surface charges induced during their production, allow CNCs to be easily dispersed in an aqueous medium, especially with sustainable water-based production methods such as emulsion polymerization. Moreover, their surface functionality makes them highly suitable for modification, thereby making them even more versatile. Emulsion polymer latexes are heterogeneous mixtures, having a continuous aqueous phase along with a dispersed organic phase. Latex polymers are used in a wide range of applications such as in coating and adhesive films. Because of the bi-phasic nature of emulsion polymerizations, the surface properties of CNCs play a crucial role in their location relative to the organic phase, and how well-dispersed they are in the cast films. In this thesis, three grades of CNCs (Celluforce Inc.) with either hydrophilic, partially-hydrophobic, or hydrophobic surface properties, were combined with conventional emulsion and miniemulsion polymer formulations to investigate their effect on the properties of pressure sensitive adhesive (PSA) films. In the first instance, hydrophilic CNCs were tested in a seeded semi-batch emulsion polymerization. Using a sequential experimental design, the effects of polar comonomer, surfactant, chain transfer agent, and CNC loading on latex stability and PSA properties were studied. By increasing polymer chain entanglements and the work of adhesion, the hydrophilic CNCs were observed to simultaneously improve the three key properties of acrylic-based PSA films, i.e., tack, peel strength and shear strength. In the second part of this project, we compared the role of hydrophilic and partially-hydrophobic CNCs in PSA property modification. Viscosity measurements and atomic force microscopy revealed differences in the degree of association between the two types of CNCs and the latex particles. Dynamic strain-sweep tests showed that hydrophilic CNC nanocomposites softened at lower strains than their partially-hydrophobic counterparts. This behaviour was confirmed via dynamic frequency tests and modelling of the nanocomposites’ storage moduli, which suggested the formation of CNC aggregates of, on average, 3.8 and 1.3 times the length of CNCs. These results confirmed that the partially-hydrophobic CNCs led to improved CNC dispersion in the PSA films and ultimately, enhanced PSA properties. In the third part of the project, mini-emulsion polymerization (MEP) was used to embed the hydrophobic CNCs within the polymer particles in contrast to the hydrophilic and partially-hydrophobic CNCs which resided mainly in the aqueous phase or near the water-particle interface. Higher CNC loadings led to increased particle size, decreased polymerization rate and number of particles, while only slightly increased the viscosity and the work of adhesion. PSA film properties decreased upon the incorporation of hydrophobic CNCs. Transmission electron microscopy showed that CNCs were expelled from the latex particles at higher loadings, suggesting the incompatibility of the acrylic polymer and the CNCs’ modifying agents. The ability to modify CNCs enables one to achieve a range of hydrophilicity/hydrophobicity. This makes them extremely versatile in a heterogeneous mixture such as in an emulsion polymerization. Because emulsion polymers are used in a wide range of applications with a broad spectrum of properties (i.e., not only as adhesives but as non-tacky coatings), our ability to control CNC location relative to the polymer particles in the latex opens the door to a world of high value-added sustainable polymer products.

Silver-polymer composite material containing highly dispersed silver nanoparticles (AgNPs) of 20-100. nm diameter can be obtained from bare nanosilver. The synthesis consists of three steps. The first step is modification of AgNPs by 2-aminoethanethiol. Second, polyacrylic acid is bonded to the silver 2-aminoethanethiolate by the carbodiimide method. Then esterification of the remaining carboxyl groups of the product by methanol results in formation of a stable colloidal dispersion of AgNPs in the polymer matrix. The method allows obtaining of nanocomposites with silver contents up to 1.4. wt%.

The fractional–order capacitors add an additional degree of freedom over conventional capacitors in circuit design and facilitate circuit configurations that would be impractical or impossible to implement with conventional capacitors. We propose a generic strategy for fractional-order capacitor fabrication that integrates layers of conductive, semiconductor and ferroelectric polymer materials to create a composite with significantly improved constant phase angle, constant phase zone, and phase angle variation performance. Our approach involves a combination of dissolving the polymer powders, mixing distinct phases and making a film and capacitor of it. The resulting stack consisting of ferroelectric polymer-based composites shows constant phase angle over a broad range of frequencies. To prove the viability of this method, we have successfully fabricated fractional-order capacitors with the following: nanoparticles such as multiwall carbon nanotube (MWCNT), Molybdenum sulfide (MoS2) inserted ferroelectric polymers and PVDF based ferroelectric polymer blends. They show better performance in terms of fabrication cost and dynamic range of constant phase angle compared to fractional order capacitor from graphene percolated polymer composites. These results can be explained by a universal percolation model, where the combination of electron transport in fillers and the dielectric relaxation time distribution of the permanent dipoles of ferroelectric polymers increase the constant phase angle level and constant phase zone of fractional-order capacitors. This approach opens up a new avenue in fabricating fractional capacitors involving a variety of heterostructures combining the different fillers and different matrixes.

Access to clean water is one of the world's greatest concerns. Because 97% of global water resources are seawater, desalination via reverse osmosis (RO) membrane process has become a vital technology to obtain drinkable water. At the same time, the discharge of industrial waste effluents containing heavy metal ions to the available water resources (seawater and brackish water) without adequate pre-treatment is a major cause of water pollution. Heavy metal rejection using nanofiltration (NF) membrane process is a recognized water treatment methodology. Thin-film nanocomposite (TFN) membranes have shown vast performance enhancement using both RO and NF processes. However, TFN membrane fabrication has been limited due to poor dispersion of the nanoparticles in the polyamide (PA) layer of the membrane, and the leaching of the often-hazardous nanoparticles from the TFN membranes. For various reasons such as their dispersibility in aqueous media, safety, high aspect ratio, and functionality, cellulose nanocrystals (CNCs) are an ideal nanoparticle for inclusion in TFN membranes. Because of their hydrophilicity, CNCs have more commonly been dispersed in the aqueous monomer solution during PA interfacial polymerization. In this thesis, we investigated two different CNC modification routes to improve CNC dispersion within the trimesoyl chloride (TMC)/n-hexane (non-aqueous) monomer solution. In one case, we acetylated the CNCs (ACNCs) using a straightforward, efficient, solvent-free method to achieve a more uniform CNC dispersion in the PA layer. The resulting ACNCs were less hydrophilic, which allowed increased nanoparticle loading and improved dispersion in the PA layer. In an RO desalination process, compared to unmodified CNC-TFN membranes, the NaCl rejection of the ACNC-TFN membranes remained stable (at 98-99%) up to a 0.4 wt% loading, while water permeability increased by up to 40%. For the second case, we synthesized L-cysteine functionalized CNCs (CysCNCs) and incorporated them into the PA layer for testing in an NF wastewater treatment process. The amine functional groups of L-cysteine covalently bonded with the acyl chloride groups of the TMC monomer. This resulted in improved nanoparticle dispersion but could also have prevented nanoparticle leaching. Moreover, because L-cysteine contains strong chelating groups, their inclusion in the PA layer led to improved heavy metal rejection. A loading of 0.1 wt% CysCNCs in the TFN membranes provided high rejection of both copper and lead ions, 98.1 and 95.2%, respectively. The CysCNCs were also evaluated in an NF desalination process resulting in a 40% increase in water permeability with almost no decline in Na₂SO₄ (97-98%), MgCl₂ and NaCl rejection. The modified CNCs enabled us to overcome the water permeability/selectivity trade-off in CNC-TFN membranes for both RO and NF membrane desalination. Finally, we developed an experimental protocol to investigate the effect of the adsorption of heavy metal ions (if any) on the performance of thin film composite (TFC) and TFN membranes in NF. We confirmed that adsorption occurred, and the equilibrium capacity of the membranes was reached after 8 - 12 h of the experiment. Despite reaching the equilibrium capacity, the water permeability and heavy metal rejection remained at their highest values. This led to the conclusion that the adsorbed heavy metals altered the membrane surface, thereby improving the performance of both TFC and TFN membranes. The ability to modify CNCs enables one to achieve a controlled range of hydrophilicity/ hydrophobicity. This allows one to fine-tune CNC compatibility with the TMC/n-hexane non-aqueous monomer solution and enable improved dispersion in the PA layer, eventually leading to improved TFN membrane performance for both RO and NF processes.

No description available.

No description available.

Biological tissues routinely experience large strains and undergo large deformations during normal physiologic activity. Biological tissue deformation is well beyond the range of standard strain gauges, and hence must often be captured using expensive and non-portable options such as optical marker tracking methods that may rely upon significant post-processing. This study develops portable gauges that operate in real time and are compatible with the large strains seen by biological materials. The new gauges are based on a relatively new technique for quantifying large strain in real-time (up to 40 %) by use of a piezoresistive nano-composite strain gauge. The nano-composite strain gauges (NCSGs) are manufactured by suspending nickel nanostrands within a biocompatible silicone matrix. The conductive nickel filaments come into progressively stronger electrical contact with each other as the NCSG is strained, thus reducing the electrical resistance that is then measured using a four-probe method. This thesis summarizes progress in the understanding, design and application of NCSGs for biomechanical applications. The advanced understanding arises from a nano-junction-level finite element analysis of gap evolution that models how the geometry varies with strain in the critical regions between nickel particles. Future work will incorporate this new analysis into global models of the overall piezoresistive phenomenon. The improvements in design focused on the manufacturing route to obtain a reliable thin and flexible gauge, along with a modified connection and data extraction system to reduce drift issues that were present in all previous tests. Furthermore, a pottable data logging system was developed for mobile applications. Finally, a method of analyzing the resultant data was formulated, based upon cross-correlation techniques, in order to distinguish between characteristic wave-forms for distinct physical activities. All of these improvements were successfully demonstrated via a gait-tracking system applied to the insole of standard running shoes.

Ceramics have a number of applications as coating material due to their high hardness, wear and corrosion resistance, and the ability to withstand high temperatures. Critical to the success of these materials is the effective heat transfer through a material to allow for heat diffusion or effective cooling, which is often limited by the low thermal conductivity of many ceramic materials. To meet the challenge of improving the thermal conductivity of ceramics without lowering their performance envelope, carbon nanotubes were selected to improve the mechanical properties and thermal dispersion ability due to its excellent mechanical properties and high thermal conductivity in axial direction. However, the enhancements are far lower than expectation resulting from limited carbon nanotube content in ceramic matrix composites and the lack of alignment. These problems can be overcome if ceramic coatings are reinforced by carbon nanotubes with good dispersion and alignment. In this study, the well-dispersed and aligned carbon nanotubes preforms were achieved in the form of vertically aligned carbon nanotubes (VACNTs) and Buckypaper. Polymer derived ceramic (PDC) was selected as the matrix to fabricate carbon nanotube reinforced ceramic nanocomposites through resin curing and pyrolysis. The SEM images indicates the alignment of carbon nanotubes in the PDC nanocomposites. The mechanical and thermal properties of the PDC nanocomposites were characterized through Vickers hardness measurement and Thermogravimetric Analysis. The ideal anisotropic properties of nanocomposites were confirmed by estimating the electrical conductivity in two orthogonal directions.