Polymer Chain Dynamics of Core−Shell Thermosensitive Microgels

Abstract

Microgels of poly(N-isopropylacrylamide) (PNIPAM) have the ability to change size in response to temperature. PNIPAM microgels with 0.25 wt % cross-linker content were used to investigate the molecular dynamics of the polymer chains in the swollen and collapsed states. The study was performed using incoherent elastic (IES) and quasielastic neutron scattering (IQNS), and pulsed field gradient NMR spectroscopy (PFG−NMR). From IES the volume transition is characterized by a sharp increase of the elastic intensity at the transition temperature. Using IQNS, a diffusive motion of the polymer chains was identified with self-diffusion coefficient D = 1.1 ± 0.2 × 10−11 m2/s at 290 K which decreases down to D = 8.6 ± 0.1 × 10−13 m2/s when the microgel deswells at 327 K. With PFG−NMR spectroscopy we measured two diffusion coefficients in the swollen state that are associated with regions of high and low cross-linking content in the microgel. IQNS and PFG−NMR spectroscopy measurements demonstrate that in the swollen state the polymer is behaving as if it is in solution whereas in the collapsed state it resembles as a solid material.