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-isopropylacrylamide) chains at latex interfaces in mixed dispersion media were studied. A combined collapse-swelling behavior was observed on the addition of lower alcohols (methanol, ethanol, or 2-propanol) to an aqueous dispersion medium. The PNIPAM chains collapsed to a minimum size in the water-rich region and then reswelled on further addition of the alcohol. Both collapse and reswelling were reversible. The collapse of the higher molecular weight PNIPAM chains with narrower polydispersity was found to proceed almost to completion and to be relatively sharp prior to the attainment of theta conditions. It was found that the minimum size of the collapsed chains was dependent upon both the number of carbon atoms in the alcohol and the temperature studied. The volume fraction of alcohol corresponding to this minimum size was also dependent upon the number of carbon atoms in the alcohol but was essentially independent of the temperature of the dispersion medium. The experimental observations have been explained by the variations in the hydrophobic interactions and the hydration of the polymer chains. The results suggest that the collapse of interfacial PNIPAM chains in the alcohol-water mixed media occurs as a consequence of the effects of alcohols on the environment of the PNIPAM chains, whereas the chain swelling results from the direct interaction of the alcohol molecules with the hydrophobic groups in the PNIPAM macromolecules. The attractive hydrophobic interactions in PNIPAM layers are believed to play an important role in promoting the chain collapse after the interactions between the PNIPAM chains and the alcohols are diminished. The validity of this interpretation was supported by similar studies on poly(acrylamide). (C) 1996 Academic Press, Inc.