Polymeric hollow microcapsules (PHM) via cellulose nanocrystal stabilized Pickering emulsion polymerization

A practical and sustainable method to prepare polymeric hollow microcapsules (PHMs) using cellulose nanocrystal (CNC) stabilized Pickering emulsion polymerization was developed. Pristine CNCs hydrolyzed from wood pulp are hydrophilic and could be employed as emulsifiers to prepare oil-in-water (O/W) Pickering emulsions. The O/W Pickering emulsions were used as templates for the Pickering emulsion polymerization of hydrophobic monomers inside the emulsion droplets. The crosslinked hydrophobic polymers phase separated and partitioned to the interface of the Pickering emulsion, leading to the formation of hydrophobic PHMs. Correspondingly, cinnamate modified CNCs with less surface hydrophilicity were employed as emulsifiers to obtain water-in-oil (W/O) inverse Pickering emulsions, which were then used as templates for inverse Pickering emulsion polymerization of hydrophilic monomers to prepare hydrophilic PHMs. Therefore, both hydrophobic and hydrophilic PHMs could be obtained via this approach. Herein, polystyrene, poly(4-vinylpyridine), and poly(N-isopropyl acrylamide) hollow microcapsules were prepared as models, where the size, crosslinking density, shell structure and stimuli-responsive properties of PHMs could be tuned by varying the synthesis parameters.