Highlights
- Thermal resistivity response direction depends strongly on PVP molecular weight.
- Resistivity decreases with K-15 PVP but increases with K-30 PVP upon heating.
- Raman spectra show comparable non-covalent PVP-CNT interactions across molecular weights.
- Water loss-shrinkage mismatch supports competing ionic and CNT-network effects.
Abstract
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 degrees C produced predominantly opposite responses: resistivity decreased with low-molecular-weight PVP (K-15) and increased with high-molecular-weight PVP (K-30), reaching delta-rho values of -1.82 and +1.89 megaohm millimeters, 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 0.83-1.13 to 0.73-0.89 grams per cubic centimeter. These results suggest that thermal resistivity response reflects competition between ionic conduction loss and CNT-network densification, with the balance depending on PVP molecular weight. Thus, PVP molecular weight provides a practical design parameter for tuning the thermal electrical response of CNT/PNIPAm hydrogels.
Research summary
Thermoresponsive PNIPAm hydrogels shrink when heated above their transition temperature. Adding carbon nanotubes (CNTs) turns that volume change into an electrical signal, but two mechanisms act in opposite directions: losing water reduces ionic conduction and raises resistivity, while contraction can densify the CNT network and lower resistivity. This study tests whether the molecular weight of poly(vinylpyrrolidone) (PVP), used to disperse the CNTs, determines which mechanism dominates.
What the study examined
- Twelve CNT dispersion conditions combining low-molecular-weight PVP K-15 or high-molecular-weight PVP K-30 with three CNT:PVP ratios and two sonication amplitudes
- Resistivity, water loss, and volume shrinkage before and after heating the hydrogels from 25 to 50 degrees C
- Raman spectra to compare PVP-CNT interactions and test whether molecular-weight-dependent surface chemistry explains the electrical response
- DLP 3D-printing performance across the same twelve formulations
Main findings
The response direction separated cleanly by PVP molecular weight. Resistivity decreased for every K-15 formulation and increased for every K-30 formulation. The strongest responses occurred at a CNT:PVP ratio of 1:5 under weak sonication: -1.82 megaohm millimeters for K-15 and +1.89 megaohm millimeters for K-30.
Raman spectra showed comparable, predominantly non-covalent PVP-CNT interactions for the two molecular weights. Water loss and shrinkage tracked response magnitude within each group, but similarly shrinking samples could still respond in opposite directions. Water loss exceeded macroscopic shrinkage in every condition, and apparent density decreased after heating, supporting a competition between ionic-carrier loss and CNT-network densification rather than a single controlling effect.
K-30 also performed better in exploratory DLP printing tests: its six formulations produced three successful and three marginal prints with no failures, whereas K-15 produced one successful, two marginal, and three failed prints.
Why it matters
PVP is not merely a processing aid in these conductive hydrogels. Its molecular weight provides a practical way to choose whether heating should increase or decrease electrical resistivity while also influencing printability. That makes PVP molecular weight a useful formulation parameter for designing thermally responsive, 3D-printable hydrogel devices, even though direct separation of ionic and electronic conduction and measurement of internal pore structure remain important next steps.
Citation
Wonseok Tae, Jong Suk Lee, Prabakaran Shankar, Wonsuk Jung. Molecular-weight-dependent thermal resistivity response of CNT/PNIPAm hydrogels. Materials Letters 428 (2027) 141867.
