Salt-Mediated Tuning of the Cononsolvency Response Behavior of PNIPMAM Thin Films

The development of tuning parameters to influence the response behavior of polymer-based nanodevices is investigated by the addition of NaClO₄ or Mg(ClO₄)₂ to adjust the swelling degree of poly(N-isopropylmethacrylamide) thin films under different vapor atmospheres. By leveraging the cononsolvency effect of the polymer in a mixed vapor of water and acetone, a contraction of the preceding water-swollen films is induced. The relation between the macroscopic and the molecular processes is elucidated by static and time-resolved time-of-flight neutron reflectometry, as well as by in situ Fourier transform infrared spectroscopy. It is found that the addition of NaClO₄ strongly enhances the film thickness response for swelling and contraction, which, therefore, allows the tuning of the film toward stronger responses. Mechanistically, D₂O-amide bonds are formed during swelling and become perturbed upon vapor exchange. Thereby, the D₂O-amide interactions are reduced continuously, while acetone-amide interactions develop, accompanied by increasing amide-amide interactions during the film contraction.