Conductive Cellulose Composites with Low Poisson's Ratio via Thermoresponsive Shrinkage

Controlling deformation behavior in porous materials is critical for the development of mechanically adaptive systems. However, conventional porous elastomers often undergo undesired lateral contraction under tension. Here, a cellulose-based composite with a low Poisson’s ratio was developed through directional pore engineering and thermoresponsive shrinkage. Anisotropic hydroxypropyl cellulose (HPC) foams were prepared via directional melt crystallization (DMC). A butanetetracarboxylic acid (BTCA)-mediated two-step crosslinking strategy enabled controlled shrinkage above the lower critical solution temperature (LCST) of HPC and stabilized the resulting re-entrant architecture. Subsequent Ecoflex infiltration produced a flexible composite. The shrinkage strain reached 30% perpendicular to the crystallization direction and 20% parallel to it. Tensile testing revealed Poisson’s ratio as low as 0.02 under optimized first crosslinking conditions, approaching near-zero values when shrinkage was conducted in aqueous polymer solution. An inverse correlation between shrinkage strain and Poisson’s ratio was observed, indicating enhanced auxetic behavior with increasing shrinkage. Electrical conductivity was introduced by incorporating acetylene black (AB) and carbon nanotubes (CNTs). The resulting conductive and stretchable composite demonstrates potential for strain-sensing and soft robotic applications.