The Role of Backbone Hydration of Poly(N-isopropyl acrylamide) Across the Volume Phase Transition Compared to its Monomer

Peter Müller-Buschbaum,Martine Philipp,Alfons Schulte,Moritz H Futscher

doi:10.1038/s41598-017-17272-7

Peter Müller-Buschbaum, Martine Philipp + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-17272-7

Copy DOI

Abstract

Thermo-responsive polymers undergo a reversible coil-to-globule transition in water after which the chains collapse and aggregate into bigger globules when passing to above its lower critical solution temperature (LCST). The hydrogen bonding with the amide groups in the side chains has to be contrasted with the hydration interaction of the hydrophobic main-chain hydrocarbons. In the present investigation we study molecular changes in the polymer poly(N-isopropyl acrylamide) (PNIPAM) and in its monomer N-isopropyl acrylamide (NIPAM) in solution across the LCST transition. Employing Fourier-transform infrared spectroscopy we probe changes in conformation and hydrogen bonding. We observe a nearly discontinuous shift of the peak frequencies and areas of vibrational bands across the LCST transition for PNIPAM whereas NIPAM exhibits a continuous linear change with temperature. This supports the crucial role of the polymer backbone with respect to hydration changes in the amide group in combination with cooperative interactions of bound water along the backbone chain.

Highlights

In a poly(N-isopropyl acrylamide) (PNIPAM) aqueous solution there is polymer-bound water in addition to the bulk water acting as the solvent
In the present work we investigate the role of the polymer backbone in hydration changes that are crucial for the lower critical solution temperature (LCST) transition
In order to study the difference between PNIPAM and its monomer NIPAM, the temperature dependence of their characteristic spectral bands is investigated with Fourier-transform infrared (FTIR) spectroscopy

Summary

Introduction

In a PNIPAM aqueous solution there is polymer-bound water in addition to the bulk water acting as the solvent. Katsumoto et al observed large changes in the amide bands of aqueous PNIPAM solutions with temperature-dependent infrared spectroscopy measurements, and attributed these to the formation of intermolecular hydrogen bonds between C=O and N-H of neighboring amide groups[44]. Maeda et al found that the C-H stretching IR bands shift to lower wavenumbers during the heating process due to the dehydration of the isopropyl side chain and the main chain[25] These authors suggested that part of the amide groups of PNIPAM do not form intra- or intermolecular hydrogen bonds between C=O and N-D, but instead form hydrogen bonds with water in the demixed state of the solution. We discuss detailed changes in the side chains (Amide I and Amide II bonds) versus the backbone (C-H groups) and their role in the transition

Methods

Results

Conclusion