LASER-LIGHT SCATTERING STUDY OF THE PHASE-TRANSITION OF POLY(N-ISOPROPYLACRYLAMIDE) IN WATER .1. SINGLE-CHAIN

The coil-to-globule transition of a single chain of narrowly distributed (M(z)/M(w) < 1.05) high molecular weight (M(w) > 10(7) g/mol) poly(N-isopropylacrylamide) in extremely dilute solutions (similar to 5 mu g/mL) was studied by both static and dynamic laser light scattering. The thermodynamically stable collapsed globule (R(g)/R(h) = 0.97) was observed for the first time. However, when R(g)/R(h) < 0.97, the single-chain globules were ''metastable''; i.e., the globules in the solution were stable only for a finite time. Two-stage chain collapsing kinetics in the thermodynamically stable region was observed. The time scales (similar to 10(2) s) observed in both the chain collapsing and dissolving processes are too short to support the previously suggested high chain-knotting density inside the globule. The R(g)/R(h) can be as low as similar to 0.62, lower than 0.774 predicted for a uniform density sphere. This paradox suggests that the chain density is higher at the center of the globule than that near the surface. The highly collapsed globule still contains similar to 80% water in its hydrodynamic volume.