Understanding the response of fillers in the epoxy resin crosslinking process and characterizing polymer-filler dynamics are key features that guide the engineering of new thermosetting resin composites. In this work, we investigate X-ray photon correlation spectroscopy (XPCS) as a thermal analysis tool to track the curing reaction of a thermoset composite based on surface-functionalized cellulose nanocrystals (mCNC). Of interest is the influence of the mCNC on the curing kinetics and macroscopic properties of a commonly used epoxy resin. Here, scattering techniques (XPCS and small-angle X-ray scattering) are used to reveal changes on the nanoscale. In contrast, traditional methods such as differential scanning calorimetry (DSC) and curing rheology are used to understand the kinetics and properties on macroscopic length scales. Three levels of modification (increasing hydrophobicity) were chosen to observe the influence of surface modification. Before cure (T₀), the highly functionalized CNC shows a 44% increase in complex viscosity (η*), while the less functionalized CNC shows a similar η* value to that of the neat resin. As the cure cycle progresses (T_cure>T>T₀), results from DSC, rheology, and XPCS further show the enhancement in dispersion for the highly functionalized CNC. The results show a clear difference in the maximum drift velocity (v_d), maximum heat flow (Q), and complex viscosity (η*) during the ramp to T_cure. Such results suggest that the highly functionalized CNC contains a well-dispersed network of particles due to the higher level of surface modification. During the isothermal cure (T_cure), there is a transition in elastic modulus (G') that occurs only for the system containing the highly functionalized CNC particle. Additionally, the neat and surface modified CNCs show a final G' value identical to that of the neat resin. We believe that heat-induced aggregation occurs, and the crosslinked resin ultimately dominates the macroscopic properties of the final cured system for all samples. The results from the three techniques (DSC, rheology, XPCS) are in good agreement and showcases XPCS as a beneficial experimental tool for characterizing the microscopic dynamics of particulate-filled thermosetting resins. Hence, we envision this to be a fundamental study for the design of many different solid-based fillers for curing thermosetting resin composites.

Roneisha Haney

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