Investigating Biomimetic Chemoresponsive Cation–π Interactions Using Raman Spectroscopy

ABSTRACT Biomimetic designs are inspired by the complex and unique behavior of naturally occurring materials, and can be applied to many systems, including polymers. ZIPer polymers (Zwitter arene‐ion like polymer) are inspired by byssal threads found on mussels, and their physical state is highly sensitive to various environmental conditions. Specifically, the ZIPer polymer undergoes chemospecific phase transitions, exhibiting potential for its use as an ionic responsive technology. Though this phenomenon has been observed with Raman spectroscopy, little is known about how salt identity or concentration affect polymer inter‐ and intra‐chain interactions. Previous studies have used Raman spectroscopy to analyze ZIPer polymer behavior in the presence of salt; however, the effect is typically only observed with sodium chloride and often only compares spectra at two concentrations. Additionally, studies have mainly focused on the spectral evidence of cation–π interactions, significantly narrowing their spectral range. In order to develop a more predictive framework for ZIPer polymer behavior, a range of salt identities and concentrations need to be tested. This study uses Raman spectroscopy to investigate ZIPer polymer behavior in the presence of a series of salts, namely NaCl, NaOTFA, NaBr, NaBF 4 , and NaPF 6 , each at 0.1 M, 0.5 M, 1.0 M, and 1.5 M concentrations. Moreover, we observe spectral changes in a range from 550 to 2000 cm −1 . Spectral evidence suggests that the cation–π interactions previously hypothesized to be the driver of ZIPer polymer behavior are not the only mechanism determining the chemoresponsive phase transitions. We hypothesize that cation–π interactions and dispersion forces are competing mechanisms controlling ZIPer polymer behavior. Furthermore, we suggest that at certain concentrations the dominating mechanism transitions, and this inflection point is salt identity dependent.