Infrared spectroscopy of aqueous poly(ethylene glycol)-b-poly(n-isopropylacrylamide) through the lower critical solution temperature window

Abstract

The effect of the poly(ethylene glycol) subunit on the lower critical solution temperature phase transition of poly(ethylene glycol)-b-poly(N-isopropylacrylamide) in D₂O was examined using Fourier-transform infrared spectroscopy. Comparison of the transmission spectra of dry PEG, PNIPAM, and PEG-b-PNIPAM revealed no discernable differences in the spectra of PNIPAM and the block copolymer at the current ratio of 1 unit of PEG to 5 units of PNIPAM. The lack of major variations in the spectra of aqueous PEG with temperature indicated that any differences in spectroscopic behavior between PEG-b-PNIPAM and PNIPAM were likely due to the solubility properties inherent to PEG itself. The DFT normal mode analysis of a NIPAM dimer model yielded the peak assignments used to interpret the spectra of PNIPAM and PEG-b-PNIPAM. When heated the spectra of both polymers showed a clear red shift of bands in the C-H bending and stretching regions, which was related to the destabilization of proximal D₂O molecules around the hydrophobic backbone and isopropyl group. An apparent blue shift was observed in the amide I region, wherein peaks associated with C=O⋯D-O-D intermolecular hydrogen bonding decreased in intensity and peaks associated with C=O⋯D-N intramolecular hydrogen bonding increased in intensity. While cooling, each spectral region exhibited behavior opposite to that observed while heating. The peaks in the C-H bending and stretching regions showed a clear red shift and decrease in intensity, possibly indicating the disruption and reformation of weak hydrogen bonds between C-H and O-D₂ while heating and cooling, respectively. The contraction of C-H bonds and associated blue shift during rehydration occurred when electron density was transferred from D₂O to a remote part of the C-H group, leading to structural relaxation. An apparent red shift was observed in the amide I region, wherein peaks associated with intermolecular hydrogen bonding increased in intensity and those associated with intramolecular hydrogen bonding decreased in intensity. The spectral changes observed in the amide I and C-H stretching regions provided insights into the microdynamics of the coil-to-globule process. Analysis of the absorbance and wavenumber as a function of temperature yielded distinct transition temperatures associated with each functional group. The sequence of events during the coil-to-globule transition for PNIPAM was the dehydration of hydrophobic regions, then disruption of intermolecular amide-D₂O hydrogen bonds, then formation of intramolecular hydrogen bonds between adjacent units of NIPAM - all within a ~1 ℃ window. This differed for the copolymer in that stabilization of the hydrophobic backbone by PEG resulted in a different sequence: formation of intramolecular hydrogen bonds, followed by loss of intermolecular hydrogen bonds, then dehydration of the isopropyl group, followed by dehydration of the backbone - within a ~0.5 ℃ window. The high solubility of PEG appeared to partially mitigate the hydrophobic collapse, resulting in a higher transition temperature for the backbone and isopropyl groups.