EXPERIMENTAL-OBSERVATION OF COIL-TO-GLOBULE TYPE TRANSITIONS AT INTERFACES

Coil-to-globule type transitions have been measured by dynamic light scattering for poly(N-isopropylacrylamide) chains attached to the surfaces of polystyrene latex particles dispersed in water. Both sterically and electrosterically stabilized latices were studied. The transitions were induced merely by changing the temperature. That observed on heating the electrosterically stabilized system appeared to conform with theoretical predictions in that the transition was continuous and spanned a broad (20-degrees-C) temperature range compared with that for free chains (1-2-degrees-C); furthermore, most of the reduction in chain dimensions occurred under worse than theta-solvency conditions under which the latices remained stable due to residual electrostatic stabilization. In sharp contrast, the latices that were solely sterically stabilized appeared to display a coil-to-globule type transition under solvency conditions that were better than a theta-solvent. The apparently paradoxical results that involved the simultaneous observation of steric stabilization and a coil-to-globule transition have been interpreted in terms of the theory of n-cluster formation proposed by de Gennes and co-workers. The addition of the anionic surfactant sodium dodecyl sulfate at concentrations below its CMC had little effect on the coil-to-globule type transitions, whereas the addition of the cationic surfactant dodecyl trimethyl ammonium bromide below its CMC had a profound effect. For the sterically stabilized latex, the cationic surfactant eliminated the occurrence of the transition; for the electrosterically stabilized latex, its presence seemingly converted the latex into a purely sterically stabilized one so that the coil-to-globule type transition now took place under better than theta-solvency conditions and flocculation occurred upon passage into the worse than theta-solvent domain. These effects can again be rationalized on the basis of the promotion or inhibition of n-cluster formation. (C) 1994 Academic Press, Inc.