Structure of poly(N-isopropylacrylamide) brushes and steric stability of their grafted cellulose nanocrystal dispersions
Thermo-responsive poly(N-isopropylacrylamide) (poly(NIPAAm)) brushes were grafted from the surface of cellulose nanocrystals (CNC) via living radical polymerization (LRP) using different initiator and monomer concentrations. The dry film thickness of the poly(NIPAAm) layer around CNC was calculated based on Scanning Electron Microscopy (SEM) and dynamic light scattering (DLS) measurements. The wet film thicknesses of grafted poly(NIPAAm) brushes in water were calculated to be 15 and 9nm for NIPAAm-CNC-1 and NIPAAm-CNC-2, respectively. Grafted chain densities and wet film thicknesses at below and above the critical temperature (T=34°C) of polyNIPAAm were calculated by applying mean-field analytical theory. The non-ionic poly(NIPAAm) brushes screened the surface charges of CNC particles, leading to a significant decrease in the absolute zeta potential values for the poly(NIPAAm) grafted CNCs compared to the unmodified and initiator modified CNC samples. Nevertheless, the colloidal stability of poly(NIPAAm) grafted CNC particles were still maintained by steric stabilization below the critical temperature On the other side, hydrophobic attractions among poly(NIPAAm) grafted CNC rods above 34°C lead to coagulation and phase separation. While both poly(NIPAAm) grafted CNC samples showed thermo-responsive behavior, the reversibility of this temperature triggered property was dependent on grafting density.