Controlled coagulation and redispersion of thermoresponsive poly di(ethylene oxide) methyl ether methacrylate grafted cellulose nanocrystals

Abstract

Hypothesis: Cellulose nanocrystals (CNCs) undergo precipitation in the presence of high concentrations of cationic surfactants in aqueous solutions. To avoid such behavior and/or to promote redispersion of CNC/surfactant mixtures, the CNC surface was grafted with poly di(ethylene oxide) methyl ether methacrylate, P(MEO₂MA).

Experiments: CNC-g-P(MEO₂MA) was characterized using the following techniques ¹³C solid-state nuclear magnetic resonance (¹³C SSNMR), Fourier-transform infrared spectroscopy - attenuated total reflection spectroscopy (FTIR-ATR) and thermal gravimetric analysis (TGA). Isothermal titration calorimetry (ITC), electrophoretic mobility, light scattering and high sensitivity differential scanning calorimetry (HSDSC) were used to study the interaction between CNC-g-P(MEO₂MA) and ionic surfactants, dodecyltrimethylammonium bromide (C₁₂TAB, cationic) and sodium dodecylsulfate (SDS, anionic) at temperatures below and above the LCST.

Findings: CNC-g-P(MEO₂MA) underwent phase separation above its lower critical solution temperature (LCST ∼ 25 °C) and precipitated from solution as seen by HSDSC and transmittance experiments. When C₁₂TAB was added to CNC-g-P(MEO₂MA) it induced the precipitation that prevented the redispersion due to strong electrostatic interactions with the negative charges on the CNC surface. With increasing concentrations of SDS, the polymer phase transition temperature was increased, which can be used to redisperse the CNC complexes. By removing SDS from the mixture via dialysis, the CNC-g-P(MEO₂MA) underwent subsequent phase transition.

Keywords: Anionic surfactant; Cellulose nanocrystals; PEO-based polymer grafting; Temperature-induced phase transition.