Molecular-weight-dependent thermal resistivity response of CNT/PNIPAm hydrogels

Thermoresponsive CNT/PNIPAm hydrogels exhibit resistivity changes during deswelling through two competing effects: water loss reduces ionic conduction and increases resistivity, whereas volume contraction densifies the CNT network and decreases resistivity. However, what determines the response direction remains unclear. Here, we investigate the effect of poly(vinylpyrrolidone) (PVP) molecular weight across twelve dispersion conditions. Heating from 25 to 50 °C produced predominantly opposite responses: resistivity decreased with low-molecular-weight PVP (K-15) and increased with high-molecular-weight PVP ( K −30), reaching Δ ρ values of −1.82 and + 1.89 MΩ·mm, respectively, in the most responsive conditions. Raman spectra indicated comparable non-covalent PVP–CNT interactions for both molecular weights, suggesting that CNT surface interactions are unlikely to govern the response direction. Water loss and volume shrinkage correlated with response magnitude but did not explain its direction. Water loss consistently exceeded shrinkage, accompanied by a decrease in apparent density from 0.83 to 1.13 to 0.73–0.89 g cm −3 . These results suggest that thermal resistivity response reflects competition between ionic conduction loss and CNT-network densification, with balance depending on PVP molecular weight. Thus, PVP molecular weight provides a practical design parameter for tuning thermal electrical response of CNT/PNIPAm hydrogels.