Comparison\nof the Swelling Behavior of Poly(<i>N</i>‑Isopropylacrylamide)\nand Poly(<i>N</i>‑Vinylisobutyramide) Thin Films\nunder Water Vapor Exposure

Abstract

Poly(<i>N</i>-isopropylacrylamide) (PNIPAM)\nis known\nfor exhibiting lower critical solution temperature behavior in water.\nA structural isomer of PNIPAM, the likewise LCST-type polymer poly(<i>N</i>-vinylisobutyramide) (PNVIBAM), is compared to PNIPAM in\na thin film with respect to their swelling behaviors and water uptake\nkinetics in a humid atmosphere. Based on spectral reflectance, Fourier-transform\ninfrared (FT-IR) spectroscopy, and time-of-flight neutron reflectometry,\nthe amount and kinetics of uptaken water and its distribution inside\nthe films correlate with molecular changes. It is observed that PNVIBAM\nswells less than PNIPAM. The FT-IR signals reveal a lower water affinity\nfor PNVIBAM than for PNIPAM and larger hydrophobic clathrates in PNVIBAM,\nwhich hinders the introduction of water. Additionally, N₂-dried PNIPAM films still contain primary water, whereas PNVIBAM\ncan be fully dried. The first step of water uptake of the main layer\ndescribes a filling of the free volume, reaching a water content of\n3.8% in PNVIBAM and 6% in PNIPAM.