Abstract:

We measured the non-coincidence effect (NCE) in the Raman spectrum of poly(N-isopropylacrylamide) (PNIPAM) in aqueous methanol at -13 ℃ (below the lower critical solution temperature, LCST). This was done to investigate the solubility of PINPAM in aqueous methanol by considering molecular interactions. Changes of NCE when adding PNIPAM to an aqueous methanol solution show that at the molar fraction of methanol x=1.0-0.90, PNIPAMpreferentially adsorbs methanol and at x=0.80-0.50, PNIPAMpreferentially adsorbs water. At x=0.50-0.20, PNIPAM breaks down the ring structure composed of two methanol molecules and one water molecule. Further comparison between the changes of NCE when adding PNIPAM and its monomer NIPPA indicates that PNIPAM adsorbs methanol cooperatively through hydrophobic interactions. This cooperative interaction breaks down the methanol-water ring structure at low methanol concentrations. The cononsolvency phenomenon of PNIPAM in a methanol solution when heating above the LCST temperature is attributed to a reformation of the methanol-water ring structure.

Key words: Raman spectroscopy, PNIPAMpolymer, Aqueous methanol, Noncoincidence effect, Solubility, Cooperative