A comparison between the 'coil-to-globule' transition of linear chains and the "volume phase transition" of spherical microgels

The coil-to-globule transition of individual poly(N-isopropylacrylamide) (PNIPAM) linear chains and the volume phase transition of spherical PNIPAM microgels were studied by a combination of static and dynamic laser light scattering. The thermodynamically stable collapsed single-chain globule was observed for the first time. The ratio of R-g/R-h lower than 0.774, predicted for a uniform sphere, indicates that the coil-to-globule transition is not an 'all-or-nothing' process, where R-g and R-h are the gyration and hydrodynamic radii, respectively. The time scale less than 100 s observed in the coil-to-globule transition is too short to support a suggested high chain-knotting density inside the globule. At the collapsing limit, the single-chain globule and microgel particle still contain 80% and 70% of water respectively in their hydrodynamic volumes. As for the volume phase transition, our results indicated that the volume change of the microgels is practically continuous, in contrast to the discontinuous volume phase transition observed in bulk PNIPAM gels. The discrepancy between a microgel and a bulk gel can be attributed to shear modulus. (C) 1998 Elsevier Science Ltd. All rights reserved.