Microgel Heterogeneous Morphology Reflected in Temperature-Induced Volume Transition and 1H High-Resolution Transverse Relaxation NMR. The Case of Poly(N-vinylcaprolactam) Microgel

The Flory temperature-induced volume transition theory for homopolymer microgels was generalized for the case of bimodal heterogeneous morphology. The most probable morphological parameters were selected from the microscopic and thermodynamic constraints imposed by H-1 transverse relaxation NMR and Flory equation of state in the approximation of a homogeneous morphology. Proton transverse magnetization relaxation NMR proved directly the existence of a bimodal heterogeneous morphology of the PVCL microgel particle. The volume polymer fractions in the deswollen state and the number of subchains for the core and corona were obtained from size-temperature data for a series of differently cross-linked microgels made of poly(N-vinylcaprolactam) (PVCL) in water. The cross-link density effect given by the different amounts of cross-linker used was investigated using dynamic light scattering. From the combination of Flory heterogeneous theory and MAR transverse relaxation, the number of polymer subchains in core and corona was shown to increase with the amount of cross-linker. Moreover, the ratio of the cross-link density in core and corona could be evaluated from the transverse relaxation times T-2 of each decay components. The quantitative characterization of PVCL microgels by size-temperature data and H-1 transverse relaxation NMR shows the consistency of the assumptions made. in the generalization of Flory theory of microgels for heterogeneous morphology.