Microgel Heterogeneous Morphology Reflected in Temperature-Induced Volume Transition and¹H High-Resolution Transverse Relaxation NMR. The Case of Poly(<i>N</i>-vinylcaprolactam) Microgel

Abstract

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 1H 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 NMR 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 T2 of each decay components. The quantitative characterization of PVCL microgels by size−temperature data and 1H transverse relaxation NMR shows the consistency of the assumptions made in the generalization of Flory theory of microgels for heterogeneous morphology.