Abstract
Temperature-responsive oxazoline-based polymer brushes have gained increased attention as biocompatible surfaces. In aqueous environment, they can be tuned between hydrophilic and hydrophobic behavior triggered by a temperature stimulus. This transition is connected with changes in molecule-solvent interactions and results in a switching of the brushes between swollen and collapsed states. This work studies the temperature-dependent interactions between poly(2-oxazoline) brushes and water. In detail, thermoresponsive poly(2-cyclopropyl-2-oxazoline), nonresponsive hydrophilic poly(2-methyl-2-oxazoline), as well as a copolymer of the two were investigated with in situ infrared ellipsometry. Focus was put on interactions of the brushes' carbonyl groups with water molecules. Different polymer-water interactions could be observed and assigned to hydrogen bonding between C=O groups and water molecules. The switching behavior of the brushes in the range of 20-45 °C was identified by frequency shifts and intensity changes of the amide I band.
Highlights
Interactions of POx with water depend on the specific sidechain chemistry
The focus was put on the amide I carbonyl infrared signature18 originating from the POx N–C1⁄4O groups.32 (C1⁄4O) groups, as well as on the influence of the PGMA anchoring layers on the POx amide I bands
It does not overlap with (C1⁄4O) of the brushes, which are located at much lower wavenumbers because of mesomeric effects within the N–C1⁄4O groups.32 (C1⁄4O)POx is positioned at 1659 cmÀ1, 1651 cmÀ1, and 1654 cmÀ1 for PMeOx, PcPrOx, and copolymer, respectively
Summary
Interactions of POx with water depend on the specific sidechain chemistry. Different alkyl groups can lead to more hydrophilic or more hydrophobic POx, or to an LCST behavior often tunable via parameters like molecular weight. Other temperature-responsive POx have quite a low glass transition temperature (Tg < 45 C), which is a severe disadvantage in sample handling and storage.16,17 For these reasons, Bloksma et al have introduced poly(2-cyclopropyl-2-oxazoline) (PcPrOx), an amorphous polymer with a reversible phase transition in solution as well as a Tg of about 80 C, which is sufficiently high for biomedical applications.. A few publications exist about the switching behavior of thin POx films, and these involve only noncyclic poly(2-oxazoline)s like PiPrOx. In the present study, in situ infrared spectroscopic ellipsometry (IRSE) was applied to characterize the thermoresponsive behavior of POx brush films consisting of either PcPrOx or a statistical copolymer containing 75% cPrOx and 25% MeOx. The temperature sensitivity of the brushes was monitored between 20 and 45 C via the characteristic amide I carbonyl vibrational band.. The temperature sensitivity of the brushes was monitored between 20 and 45 C via the characteristic amide I carbonyl vibrational band. This band is a direct measure for changes in polymer–water interactions since its frequency shifts when the polymer’s N–C1⁄4O groups are involved in hydrogen bonding
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