(565c) Xpcs Studies of a Cellulose Nanocrystal Thermoset Ink during Extrusion Printing

Cellulose nanocrystal (CNC)-epoxy composites have received a great deal of attention as light-weight, high strength materials due to the lower cost, biodegradability, and high aspect ratio of the filler particles. The particles have low density (~1.6 g/cm³) and are quickly processed; however, the utilization of CNCs in more common non-polar polymers is still limited to their hydrophilicity and poor dispersibility. Therefore, to utilize such particles as mechanical reinforcements in, for example, epoxy resins, the hydrophilicity of the particle would need to be adjusted. The bulk properties of such composites depend on (i) the properties and shape of the nanofiller, (ii) the dispersion of the nanofiller in the matrix (microstructure), (iii) the orientation of the nanofillers inside the matrix, and (iv) the manufacturing process of the composite. Higher alignment has resulted in enhanced strength and conductivity in several instances. However, a key component that is required and is currently missing in the literature is a clear understanding of the underlying structure-dynamics of the particles, its recovery after processing, and how this influences macroscopic properties, such as tensile strength. This would help in developing novel design rules for processing.

In this work, using state of the art scattering techniques at the National Synchrotron Light Source at the Brookhaven National Lab (BNL), we address the dispersion of surface-modified CNC in a commonly used epoxy resin (EPON 828) and explore processing such composites through direct-ink write (DIW) printing with a DIW printer installed on the beamline. X-ray photon correlation spectroscopy (XPCS), coupled with shear rheology studies, is utilized to study the morphology and dynamics before, during, and after printing of a naturally hydrophilic CNC particle in a mostly hydrophobic thermosetting epoxy resin. The hydrophobicity of the particles was varied over three different concentrations, allowing for a systematic increase of grafting on the surface of the particles. Each level of surface modification was dispersed into the EPON resin at loadings up to 40 wt%. The rheological measurements revealed a clear distinction in macroscopic properties (viscosity, viscoelasticity, recovery) due to changes in the particle-particle and particle-matrix interactions amongst the different levels of modification. Additionally, results from the XPCS studies show drastic changes in the dynamics of the particles as a function of surface modification during the recovery of the material after printing. Results from the scattering studies are then correlated with the macroscopic properties (tensile strength) after printing to delineate the underlying structure-function relationships. Our in-operando results provide insight into the importance of understanding the microstructure properties of such materials to allow for the development of enhanced polymeric nanocomposites.