(777c) Influence of Processing on Additively Manufactured Mechanically Adaptive Cellulose Nanocrystal Polymer Composites

Cellulose nanocrystal (CNC) polymer composites have the potential to provide enhanced strength and impart stimuli responsive, mechanically adaptive behavior to a number of commercially available polymeric materials. We are currently investigating the potential to transition these unique characteristics from solvent cast polymer films to functional parts generated using extrusion based additive manufacturing (AM). The relationship between structure, process and properties of functional CNC polymer composites has not been explored when utilizing material extrusion based AM technologies. We are establishing relationships between key material properties such as mechanical strength, thermal transitions and morphology to aid in describing this complex relationship. Developing this understanding of the processing dependent material behavior will aid in developing novel CNC based materials for AM technologies.

In this work, we study a CNC / thermoplastic polyurethane (TPU) nanocomposite. Dynamic mechanical analysis (DMA) experiments were conducted with a submersion chamber attachment, which allows for analysis of mechanical properties in both dry and water saturated environments. Preliminary DMA data indicate processing has a significant impact on dry mechanical properties of the cast films (385 MPa), compression molded films (92 MPa) and extruded fibers (63 MPa). The wet mechanical properties of the cast film, compression molded film and extruded fiber are all very similar (25 MPa ± 5 MPa) indicating that thermal history and orientation has a significant impact on the dry mechanical properties. Geometry also has a substantial impact on rate of diffusion and corresponding rate of mechanical switching, with thin film samples adapting in 350 seconds compared to 15 hours for a 6 mm diameter fiber. Small angle X-ray scattering is used to qualitatively analyze the degree of orientation induced by processing the cast film and fiber samples; despite machine direction orientation in an extruded fiber, we observe reduced dry state storage modulus compared to the unoriented solvent cast film. These results suggest the potential impact of thermal history on surface functionality of the CNCs. We deconvolute the separate, competing contributions of CNC orientation and thermal decomposition; the former is expected to substantially increase modulus, while the latter is expected to reduce coupling between CNC’s.