(317b) Diffusion-Softening of a Mechanically Adaptive Cellulose Nanocrystal Polyurethane Composite

Authors: 
Pritchard, C., Virginia Polytechnic Institute and State University
Fallon, J., Virginia Polytechnic Institute and State University
Bortner, M. J., Virginia Polytechnic Institute and State University
Cellulose nanocrystals (CNCs) are an exciting filler for emerging bio-renewable nanocomposite systems. CNCs provide mechanical reinforcement through the development of a strong hydrogen bonding network within the matrix, when loaded above a critical percolation threshold, greatly enhancing the storage modulus. This effect is reversible upon the introduction of water, where competing interactions consume hydrogen bonding sites thereby reducing the mechanical reinforcement provided by CNCs.

We explore the dynamics of the mechanical softening upon exposure to water and develop a relationship between storage modulus and water diffusion as functions of filler loading. Dynamic mechanical analysis (DMA) coupled with a relative humidity accessory enabled direct evaluation of the mechanical adaptivity in response to the water stimulus and thermogravimetric analysis – sorption analysis (TGA-SA) was implemented to evaluate the diffusivity of water in a high relative humidity environment. Following a hindered diffusion model, the ratio of bound-unbound water was characterized for its impact on mechanical softening providing new insight into the softening kinetics. Finally, an empirical relationship between water and filler concentration on storage modulus was developed and compared to existing theories such as Halpin-Kardos, and percolation theory.

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