Structure-Process-Property Relationships of Cellulose Nanocrystal Thermoplastic Urethane Composites

Nanomaterials are becoming increasingly prevalent in final use products as we continue to improve our understanding of their structure and properties and optimize their processing. The useful applications for these materials extend from new drug delivery systems to improved materials for various transport industries and many more. Nanoscale materials which are commonly used include but are not limited to carbon nanotubes, graphene, silica, nanoclays, and cellulose nanocrystals. The literature presented herein aims to investigate structure-process-property relationships of cellulose nanocrystal (CNC) polymer composites. These CNC nanocomposites are unique in that they provide a dynamic mechanical response when exposed to H2O. Currently, these nanocomposite systems are most commonly solvent cast into their final geometry. In order to enable the use of these materials in more commercial processing methods such as extrusion, we must understand their inherent structure-process-property relationships. To do this, we first characterize the influence of temperature and shear orientation on the unique mechanical adaptive response. Next, the melt processability of the nanocomposite was characterized using material extrusion (MatEx) additive manufacturing (AM). Additionally, the diffusion behavior of water within the film, which controls the dynamic mechanical response, was probed to better predict the concentration dependent behavior. Finally, a literature review is presented which outlines the state of the art for melt extrusion AM of fiber filled polymer composite materials and provides insight into how we can further improve mechanical properties through further addition of composite filler materials.

The initial focus of the dissertation is on the influence of melt processing CNC thermoplastic urethane (TPU) composites and the resulting impact on the mechanical adaptive response. Dynamic mechanical analysis (DMA) fitted with a submersion clamp was used to measure the mechanical softening of the composite while submerged in water. Small angle x-ray scattering (SAXS) and polarized raman spectroscopy were used to qualify the orientation of the various CNC/TPU composite samples. The results of the orientation measurements show that solvent casting the films orient CNCs into a mostly random state and melt extrusion induces some degree of uniaxial orientation. The DMA results indicate that at the processing conditions tested, the uniaxial orientation and thermal exposure from the melt processing do not significantly impact the mechanical responsiveness of the material.

The next objective of this work was to expand upon the aforementioned learnings and determine the CNC composite material processability using MatEx. The ability to process mechanically dynamic CNC/TPU composites with a selective deposition process capable of generating complex geometries may enable new functionality and design freedom. To realize this potential, a two factor (extrusion temperature and extrusion speed) three level (240, 250 and 260 ℃/ 600, 1100 and 1600 mm/min) design of experiments (DOE) was utilized. The resulting printed parts were characterized by DMA to determine their respective mechanical adaptivity. Processing conditions did prove to have a significant impact on the mechanical adaptivity of the printed part. A correlation between applied energy and mechanical adaptivity demonstrates how increasing residence time and temperature can reduce mechanical performance. The shape fixity of the printed parts was calculated to be 80.4% and shape recovery was 44.2%. A 3D prototype part was also produced to demonstrate the unique properties of this material.

Although the understanding of the melt processing behavior of these CNC composites had been improved, a stronger understanding of the moisture diffusion behavior within the composite is required to fully realize and control their potential. Therefore, a study was undertaken to capture the diffusion behavior and correlate it to the mechanical responsive mechanism. To do this, a thermogravimetric sorption analysis (TGA-SA) instrument was used to monitor the mass uptake as a function of time exposed to a humid environment. These data were then compared to DMA data collected for the same samples exposed to a similar degree of humidity. All studies were conducted as a function of concentration in order to better elucidate the influence that percolating network structures may have on the resultant properties. Interestingly, the results show how increasing addition of CNCs results in a decrease in the rate of diffusivity, which is counter to what has been commonly hypothesized. It is hypothesized that increasing CNC content restricts the mobility of surrounding amorphous matrix material, thus increasing the resistance for diffusion of a water molecule. However, the rate of mechanical adaptation was found to increase with increasing CNC content, which is believed to be a result of the increased connectivity, enabling further transport of water molecules.