COIL GLOBULE TRANSITION OF POLY(N-ISOPROPYLACRYLAMIDE) - A STUDY OF SEGMENT MOBILITY BY FLUORESCENCE DEPOLARIZATION

Time-resolved measurements of fluorescence polarization anisotropy were used to monitor the "freezing" of local motions due to the temperature-induced conformational transition of poly(N-isopropyl-acrylamide) in aqueous solutions. The label dye dansyl was covalently bound to polymers containing aminostyrene which were synthesized by radical polymerization. The reorientational relaxation of the labels is anisotropic and can be resolved into two components. These two relaxation modes are interpreted in terms of a simple model as (i) rotations of the label around a free axis (this mode is sensitive to microviscosity) and (ii) polymer-coupled reorientations reflecting the chain flexibility. Both relaxation times diverge above a transition temperature of 31-degrees-C. Our findings confirm the picture of a conformational transition from extended coils to a compact globular state of the polymer.