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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.

Loblolly pine (<u>Pinus taeda</u>) occurs in mixed stands with hardwood species including red maple (<u>Acer rubrum</u>) and sweetgum (<u>Liguidambar styraciflua</u>) across much of the southern United States. Rising atmospheric CO₂ concentrations over the next several decades could influence competitive interactions between these species. This research examined the effects of increased atmospheric CO₂ concentration in combination with the availability of other resources on growth and gas exchange characteristics of these species grown in direct competition in miniature stands and compared characteristics of miniature stands with field-grown stands of similar composition. In a greenhouse study, loblolly pine and red maple were grown in monoculture and 50:50 replacement mixtures at a 2.54 x 2.54 cm spacing for two growing cycles under ambient (408 ppm) or elevated (806 ppm) CO₂ concentrations and well-watered or droughted (0.5x well-watered) conditions. Loblolly pine dominated all mixed stands and was proportionally larger while red maple was smaller in mixtures versus monocultures. This pattern was also observed in weed-free f ld-grown stands after five growing seasons. Biomass data from loblolly pine and red maple grown in monocultures and 50:50 replacement mixtures at 2.54, 5.08 and 7.62 cm2 initial spacing suggest that relative dominance of species in miniature stands is not dependent on initial spacing. Dominance of red maple over loblolly pine in all mixed stands in this experiment suggests that soil medium is important in determining relative growth of these species. Photosynthesis rates and response to elevated CO₂ differed between whole stands and individual seedlings from these stands. Individual seedlings tended to be more responsive to elevated CO₂ compared to whole stands suggesting reduced responsiveness of lower canopy leaves may dampen the effect of elevated CO₂ on canopy photosynthesis rates. Growth under elevated CO₂, alone or in combination with water did not affect relative sizes of these species. These results suggest that competitive interactions will not change between these species in response to growth in high CO₂ environments. / Ph. D.

Polymer nanocomposites have been proposed as lightweight replacements for traditional composite materials in various applications. Montmorillonite (MMT) and carbon nanotubes (CNTs) are two nanofillers which have accrued significant interest in the past 20 years due to their superior mechanical and electrical properties, respectively. However, efficient dispersion of the nanofiller and damage to CNTs prevent widespread utilization of these materials in polymer nanocomposites. Novel methods of nanocomposite generation combining the use of supercritical carbon dioxide (scCO2) with melt compounding have been investigated to overcome these issues. The focus of this work is on developing the scCO2 treatment of nanomaterial for thermoplastic nanocomposite generation. First, the supercritical carbon dioxide aided melt blending method was applied to nanoclay nanocomposites of Nylon 6/ and organoclay where the polymer may interact with the nanoclay surface. Second, the effect of scCO2 processing of CNTs was investigated with special consideration to the processing variables. Finally, a study was carried out to analyze the electrical conductivity of polycarbonate nanocomposites generated using CNTs deagglomerated by scCO2 processing. The initial focus of this dissertation is the use of supercritical carbon dioxide (scCO2) as a processing aid in the generation of nylon 6 nanocomposites in which the nylon 6 may interact with the nanoclay surface. Wide-angle X-ray diffraction, transmission electron microscopy, rheology, and tensile tests were carried out to investigate the effect of processing with scCO2 on the final composite morphology and properties. It was observed that mechanical properties of composites prepared with the scCO2 aided melt blending method were similar to or higher than those reported in the literature for samples prepared with twin screw compounding. At 7.6 wt% nanoclay the modulus value reaches 4.75 +/- 0.194 GPa which is one of the highest increases (1.7 GPa) reported for these materials processed at intermediate concentrations. Beyond 7.6 wt% the improvement due to scCO2 processing matched that of direct blending. The next objective of this work is to develop a method for the deagglomeration of commercially available multi-walled carbon nanotubes (MWCNTs) by manipulating processing variables and observing carbon nanotube aspect ratios after deagglomeration. High levels of deagglomeration of Baytubes C 150 P and Nanocyl NC-7000 MWCNT agglomerates were observed, resulting in 30 fold and 50 fold decreases in bulk density, respectively, with median agglomerate sizes < 8 um in diameter. These results were obtained while retaining the aspect ratio of the as-received nanomaterial, irrespective of the MWCNT agglomerate morphology. It was found that the supercritical temperature and pressure of 40 deg C and 7.86 MPa were the optimal temperature and pressure for maximum deagglomeration without damaging the MWCNTs. The final goal of this work is to apply the scCO2 aided melt blending process to generate polycarbonate/ carbon nanotube (CNT) nanocomposites with enhanced electrical conductivity and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Different degrees of scCO2 processed Baytubes C 150 P CNT were benignly deagglomerated with scCO2 resulting in 5 fold (5X), 10X, and 15X decreases in bulk density from the as-received CNTs. The CNT were melt compounded with polycarbonate using single screw melt extrusion and compression molded into plaques. A surface conductivity of 4.8 x 10-8 +/- 2.0 x 10-9 S was observed for samples prepared with the scCO2 aided melt blending at 15X scCO2 processing. Electrical percolation thresholds as low as 0.83 wt% were observed for composites prepared with 15X CNTs using the scCO2 aided melt blending method, while concentrations as high as 1.5% are required without scCO2 processing. The percolation concentration in nanocomposites prepared with 15X scCO2 processing and single screw extrusion is competitive with values reported for similar nanocomposites generated using twin screw melt compounding in the literature. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO2 processing compared to direct melt blending but was found to be significantly worse than that of twin screw melt compounded nanocomposites from the literature. Because the percolation thresholds are similar with substantially different extents of dispersion, the importance of maintaining longer CNTs during nanocomposite generation is emphasized. / Ph. D.

Ecological, health and environmental concerns are driving the need for bio-resourced foams for the building industry and for other applications. This is because insulation is one of the most important aspects of the building envelope. Global building insulation is expected to reach USD 27.74 billion in 2022. Conventional insulation materials currently used in buildings are made from nonrenewable products (petroleum, fiber glass). However, they yield increasing unrecyclable eco-unfriendly waste at the end of their lives; styrene and polyurethane generates over 100,000 kg of waste insulation in US alone yearly. This is because they are non-biodegradable and can remain as microplastics in the environment for 1000 years. Polyurethane contains the same amount of energy as coal. Additionally, most of the processing techniques and blowing agents used in this manufacturing of these foams are cancerous and injurious to health when inhaled. Because buildings and their construction together account for 36% of global energy use and 39% of energy-related carbon dioxide emissions annually, there is a need to develop eco-friendly foams that will serve as possible substitutes to the currently used petroleum-based foams. This dissertation examined the development and characterization of eco-friendly foams that were developed using the melt mixing technique of bio-resourced polymers with the use of environmentally benign carbon dioxide as blowing agent. This study was conducted and financially supported by the National Science Foundation. A collaborative research: Engineering Fully Bio-based Foams for the Building Industry. Award NSF-CMMI: 1728096.