Water Uptake of Poly(2-<i>N</i>-Alkyl-2-Oxazoline)s: Temperature-Dependent Fourier Transform Infrared (FT-IR) Spectroscopy and Two-Dimensional Correlation Analysis (2DCOS)

A library of poly(2-oxazoline)s with varying length of the alkyl side-chain has been investigated by variable-temperature Fourier transform infrared (FT-IR) spectroscopy. These polymers are suitable for studies of structure-property relationships as their cationic ring-opening polymerization and the relatively facile monomer synthesis enable a control of the molecular structure. In this contribution, the number of carbon atoms in the linear side-chain is systematically varied from a short methyl to a long nonyl group. Previous studies showed that the sample library can be split in two groups: poly(2-oxazoline)s with a short side-chain (methyl-, ethyl-, and isopropyl-) exhibit hygroscopic behavior, while those with longer side-groups (butyl- and longer) were found to be semi-crystalline. To gain further insight into the mechanisms of hydrogen bonding and crystallization, temperature-dependent infrared (IR) spectroscopy has been applied in the current study. The processes involved have been monitored by generalized two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window two-dimensional correlation spectroscopy (PCMW2D) in the C=O stretching region around 1645 cm(-1). These advanced analysis techniques provided valuable additional information on the material behavior during heating. As water is removed from the samples in the course of the heating process, it was possible to clearly distinguish between "loosely associated" and hydrogen-bonded water. Furthermore, the melting process of the semi-crystalline samples could be depicted. For the poly(2-isopropyl-2-oxazoline) even a crystallization process could be monitored in the temperature range between glass transition and melting.