COIL GLOBULE TRANSITION OF POLY(N-ISOPROPYLACRYLAMIDE) - A STUDY OF SURFACTANT EFFECTS BY LIGHT-SCATTERING

The phase behavior of a fractionated high molecular weight sample of poly(N-isopropylacrylamide) in dilute aqueous solution containing a surfactant, sodium dodecyl sulfate (SDS), is studied with temperature and surfactant concentration as independent variables. Static and dynamic light scattering are used as methods of investigation. Without surfactant, the polymer exhibits a lower critical solution temperature (LCST) of 34-degrees-C, above which it precipitates. At SDS concentrations of only 250 mg/L, aggregation is completely prevented (intermolecular solubilization) and the behavior of isolated polymer molecules can be studied in the whole temperature range: Upon heating, the polymer undergoes a coil-to-globule phase transition with a volume reduction by a factor of more than 300. The transition temperature depends on the surfactant concentration. It is first constant and equal to the LCST, but begins to increase above 300 mg/L surfactant. Therefore when increasing the surfactant concentration at constant temperatures above the LCST, we cross the phase boundary and observe intramolecular solubilization, i.e., a surfactant-induced globule-to-coil transition. Below the LCST, the surfactant causes an expansion of the polymer coils also setting on at 300 mg/L.