4D Printing of Moisture Responsive Cellulose Nanocrystal Polymer Composites

Improving material selection for additive manufacturing (AM) has gained significant attention in recent years. Several researchers and companies have been pushed by consumers and industry to stretch the capabilities of the most common form of AM, known as fused filament fabrication (FFF), which involves extrusion of a polymer filament in a layer-by-layer fashion.

We have built on the development of a smart mechanically dynamic thermoplastic urethane/cellulose nanocrystal (TPU/CNC) composite, one which can respond to changes in environment simply by moisture exposure, to potentially increase design freedom and realize opportunities for expansion of FFF into new functional products. TPU/CNC composites have been documented to change modulus significantly upon exposure to moisture through decoupling of CNC hydrogen bonding and disruption of the mechanically percolating network. However, the effects of processing on the mechanical response of the composites is not well documented or understood.

Here we investigate the interplay between processing and structure-property relationships for 3-D printed TPU/CNC composites. We evaluate the impact of melt processing parameters on moisture induced mechanical modulus change following 1) TPU/CNC filament production from a masterbatched, solvent cast composite and 2) subsequent FFF of TPU/CNC filament into functional parts. The interplay between thermal history and shear induced particle orientation are investigated to determine effects on the dynamic modulus change upon moisture exposure. Further, we analyze the impact of geometry on moisture diffusion kinetics, and reversibility of the mechanical modulus change. Finally, we present the impact of FFF process parameters on the mechanical response of 3D printed parts, including the ability for the printed composite parts to change and/or hold their shape upon exposure to moisture and/or drying. These analyses help us understand how processing influences the structure and behavior of the composite, notably on its ability to mechanically adapt upon moisture exposure. This information is critical to successfully realize the ability to design and 3D print complex, mechanically moisture responsive composite structures.