A simple FTIR spectroscopic method for the determination of the lower critical solution temperature of N-isopropylacrylamide copolymers and related hydrogels

Linear and crosslinked polymers based on N-isopropylacrylamide (NIPAAm) exh ibit unusual thermal properties. Aqueous solutions of poly(N-isopropylacryl amide) (PNIPAAm) phase-separate upon heating above a lower critical solutio n temperature (LCST), whereas related hydrogels undergo a swelling-shrinkin g transition at an LCST. A linear copolymer made of NIPAAm/acryloxysuccinim ide (98/2 mol/mol) and two hydrogels with different hydrophilicities were p repared. Fourier transform infrared (FTIR) spectroscopy was employed to det ermine the transition temperature and provide insights into the molecular d etails of the transition via probing of characteristic bands as a function of temperature. The FTIR spectroscopy method described here allowed the det ermination of the transition temperature for both the linear and crosslinke d polymers. The transition temperatures for PNIPAAm and the gel resulting f rom the crosslinking with polylysine or N,N'-methylenebisacrylamide (MBA) w ere in the same range, 30-35 degrees C. For the gels, the transition temper ature increased with the hydrophilicity of the polymer matrix. The spectral changes observed at the LCST were similar for the free chains and the hydr ogels, implying a similar molecular reorganization during the transition. T he C-H stretching region suggests that the N-isopropyl groups and the backb one both underwent conformational changes and became more ordered upon heat ing above the LCST. An analysis of the amide I band suggests that the amide groups of the linear polymer were mainly involved in hydrogen bonding with water molecules below the LCST, the chain being flexible and disordered in a water solution. During the transition, around 20% of these intermolecula r hydrogen bonds between the polymer and water were broken and replaced by intramolecular hydrogen bonds. Similar changes were also observed at the LC ST of a gel crosslinked with MBA. (C) 2000 John Wiley & Sons, Inc.