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.