Shear-Induced Structural and Functional Transformations of Poly(N-vinylcaprolactam) Microgels

Abstract

We here performed an in-depth investigation of the behavior of microgels (μgels) and their associated physicochemical transformations under shear force. Thermo- and mechanoresponsive poly(N-vinylcaprolactam) (PVCL) μgels (d ∼400 nm) cross-linked with a force-responsive mechanofluorophore in different cross-linking degrees were synthesized and examined. Fluorescence spectroscopy (FS), confocal laser scanning microscopy (CLSM), dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryoTEM), high-resolution magic-angle sample spinning (HRMAS) nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) are used to characterize the μgels before, during, and after shearing with different shear rates and intensities. The obtained results suggest nonuniform structural features consisting of a softer outer "corona"and a harder particle "core"(cross-linker-rich). Upon shearing, the μgels rapidly lose their corona and the cores agglomerate altering μgel functionality. Surprisingly, μgels degrade promptly, even when subjected to low shear forces, such as the extrusion through a needle. This has potential implications for all applications in which shear forces in solution are expected, including extrusion, injection, and filtration processes involving colloidal μgel solutions as well as circulation within the bloodstream of living organisms.