COIL-TO-GLOBULE TYPE TRANSITIONS AND SWELLING OF POLY(N-ISOPROPYLACRYLAMIDE) AND POLY(ACRYLAMIDE) AT LATEX INTERFACES IN ALCOHOL-WATER MIXTURES

The coil-to-globule type transition and swelling of poly(N-isopropylac rylamide) chains at latex interfaces in mixed dispersion media were st udied. A combined collapse-swelling behavior was observed on the addit ion of lower alcohols (methanol, ethanol, or 2-propanol) to an aqueous dispersion medium. The PNIPAM chains collapsed to a minimum size in t he water-rich region and then reswelled on further addition of the alc ohol. Both collapse and reswelling were reversible. The collapse of th e higher molecular weight PNIPAM chains with narrower polydispersity w as found to proceed almost to completion and to be relatively sharp pr ior to the attainment of theta conditions. It was found that the minim um size of the collapsed chains was dependent upon both the number of carbon atoms in the alcohol and the temperature studied. The volume fr action of alcohol corresponding to this minimum size was also dependen t upon the number of carbon atoms in the alcohol but was essentially i ndependent of the temperature of the dispersion medium. The experiment al observations have been explained by the variations in the hydrophob ic interactions and the hydration of the polymer chains. The results s uggest that the collapse of interfacial PNIPAM chains in the alcohol-w ater mixed media occurs as a consequence of the effects of alcohols on the environment of the PNIPAM chains, whereas the chain swelling resu lts from the direct interaction of the alcohol molecules with the hydr ophobic groups in the PNIPAM macromolecules. The attractive hydrophobi c interactions in PNIPAM layers are believed to play an important role in promoting the chain collapse after the interactions between the PN IPAM chains and the alcohols are diminished. The validity of this inte rpretation was supported by similar studies on poly(acrylamide). (C) 1 996 Academic Press, Inc.