This work studies the effect of film fabrication parameters on the structure of highly-responsive multilayer films of poly(4-vinylpyridine), used to form neutral single-component nanothick hydrogels. The effect of crosslinking degree and chain deposition on hydration and thickness are probed with neutron reflectometry (NR). The benefit of NR in hydrogel experiments is that it allows the elucidation of internal structure in a way not available using photon-based measurements. Neutrons interact directly with the nuclei of constituent atoms and, using isotopic substitution, can be made sensitive to specific polymer layers and to the presence of water. By using polymers or solvents containing deuterium, resolution of internal structure at the nanometer scale is possible. This capability allows us to understand well-stratified films and hydrogels as unified structures, enabling us to resolve polymer layers with distinct chemistries, or to see the way stimuli-responsive polymers absorb water. This view, coupled with more conventional techniques such as ellipsometry, deepens our understanding of these materials. Specifically, NR has been used to track internal changes as pH passes through the pKa, driving the film between its hydrophobic basic and hydrophilic acid states.  <br/>Films of PVP can undergo an over 10-fold increase in volume at low pH if deposited through the spin-assisted layer-by-layer (LbL) technique, which results in high-entropy stratified layers. The degree of swelling and collapse are closely related to the method used to deposit the layers (either spinning or dipping) and the degree of crosslinking (4% of crosslinked backbone groups versus 7%). The least-crosslinked hydrogel with high stratification can increase volume 11.5 fold versus the dry film, corresponding to a 0.91 ± 0.05 D2O volume fraction. Above the pKa, the volume increase is 3 fold, consisting of 0.60 ± 0.01 water. In addition to gross swelling, proton/deuteron contrast highlights the distribution of water throughout the film.

Stimuli-responsive poly(4-vinylpyridine) hydrogel structure and hydration via neutron reflectometry

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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