Raman spectroscopic study of the structure of water in aqueous solutions of amphoteric polymers

Abstract

Methacrylic acid (MA) and [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC) were polymerized to give amphoteric copolymers with various compositions. The structure and H-bonding of water in an aqueous solution of the copolymer were analyzed using the contours of the O–H stretching in the polarized Raman spectra. For comparison, the H-bonded network structure of aqueous solutions of homopolymers (polyMA and polyMAPTAC) was also examined. From the relative intensity of the collective band (C value) corresponding to a long range coupling of the O–H stretching in the aqueous polymer solutions, the number of H-bonds disrupted due to the presence of one monomer residue of the polymers (N_corr) was determined. The N_corr value for polyMA was largely positive, and with an increase in the content of the MAPTAC residue, the N_corr value became smaller, and after passing a minimum (which was still slightly positive) at a roughly equivalent molar ratio, the N_corr value increased again. This is in significant contrast with the larger positive N_corr values for the homopolymers (both polyMA and polyMAPTAC), and other ordinary polyelectrolytes such as sodium polyethylenesulfonate, poly-L-lysine hydrobromide and sodium polyacrylate. Furthermore, the N_corr value for the copolymer (MA ∶ MAPTAC = 56 ∶ 44) became much smaller by the neutralization of MA residues in the copolymer with sodium hydroxide, and comparable to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone) and zwitterionic polymers such as poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and poly[3-sulfo-N,N-dimethyl-N-(3′-methacryloylaminopropyl)propanaminium inner salt]. The present results clearly indicate that the amphoteric polymers with comparative contents of cationic and anionic groups do not significantly disturb the H-bonded network structure of water, probably due to the counteraction of the electrostatic hydration effect by the proximity between the anionic and cationic side groups.