RAMAN SPECTROSCOPY AS A TECHNIQUE FOR STUDYING THE STRUCTURE AND MECHANISM OF THE VOLUME PHASE TRANSITION OF POLY(N-­‐ISOPROPYLACRYLAMIDE

摘要

The volume phase transition, VPT, of poly(N-­‐isopropylacrylamide), PNIPAM, was examined using Raman Spectroscopy. The visible Raman spectra of a solution of PNIPAM microsphere particles were examined at different temperatures above and below the lower critical solution temperature, LCST, of the particles. The frequency shifts of the C-­‐H stretch vibrations of the isopropyl moiety of the NIPAM monomer unit indicated changes in hydration of the PNIPAM. The highest frequency C-­‐H stretch vibrational band derives from the C-­‐H stretch of the methyl groups and downshifts as the solution temperature increases and the polymer collapses indicating dehydration. The C-­‐H stretch of the C-­‐H in the isopropyl moiety shows a downshift at a lower temperature than that of the methyl group, indicating that the disruption of the amide hydration may be the trigger of the cooperative dehydration that the NIPAM monomers experience in the VPT. Raman spectroscopy was also used to examine the effects of the presence of perchlorate salt on the VPT of PNIAPM and particle aggregation. Using UV Resonance Raman spectroscopy, the hydration of the amide moiety of the NIPAM monomer unit can be closely monitored. The spectra were collected at temperatures above and below the LCST, in the presence and absence of 0.5 M NaClO4. The amide I vibration frequency shift indicated a large increase in the number of amides that are completely dehydrated in the salt solution as compared to the particles in DI water above the LCST. The salt causes aggregation in the particles as it screens the repulsion of the charges in the ion-­‐comonomers that normally help to support the expanded and dispersed particles.