HCl gas adsorption/desorption properties of poly(N-isopropylacrylamide) brushes grafted onto quartz resonator for gas-sensing applications

Poly(N-isopropylacrylamide) (PNIPAM) chains were grafted onto a quartz resonator using surface-initiated atom transfer radical polymerization (SI-ATRP). The PNIPAM loading was varied by changing the grafting time and the monomer concentration. AFM analysis revealed that the increase in PNIPAM loading was attributable to increases in the chain length and in the corresponding brush height. The HCl gas adsorption/desorption properties of the resulting PNIPAM brushes were measured by a quartz crystal microbalance (QCM) technique. The PNIPAM brushes adsorbed large amounts of HCl gas, and the adsorption capacity was linearly related to the PNIPAM loading, indicating the homogeneous adsorption of HCl gas inside the PNIPAM brushes. On the other hand, the rate of desorption of the HCl molecules was lower than that of the adsorption process, and slowed with increases in chain length. Reversibility was insufficient, especially at longer chain lengths due to the difficulty of the diffusion of gas molecules from inside the brushes. The reversibility was improved by raising the measuring temperature from 30 °C to 50 °C, but it was still up to 80%, suggesting the irreversibility of the adsorption.