Cellulose nanocrystal/polymer nanocomposite latex via surfactant-free emulsion polymerization

Abstract

Polar cellulose nanocrystals (CNCs) are difficult to disperse well in hydrophobic polymer matrices, and usually surfactants or expensive and time-consuming functionalization of the reinforcement or matrix have been attempted to improve dispersion. Here, we report a new, facile method to prepare CNC/polymer nanocomposite latices directly via surfactant-free emulsion polymerization of vinyl monomers and CNCs initiated by a cationic, water-soluble, free-radical initiator. The distribution of CNCs in the latex particles can be tailored through controlling the reaction procedures, namely, whether the monomers are added before or after preheating the CNCs with the initiator and has been confirmed by scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM) studies. CNCs distribute inside the latex particles if the monomers are added after the preheating step, and owing to the more even distribution of CNCs, such nanocomposite latices exhibit better mechanical properties than those prepared without preheating or via physically mixing CNCs with emulsions polymerized using conventional surfactant system. A latex particle formation mechanism is proposed based on these phenomena and polymerization kinetics studied in this work. Unlike typical CNC-based Pickering emulsion polymerization, our process can be operated without the need for intense mechanical shearing and provide an industrially scalable approach to prepare stable CNC/polymer nanocomposite latices that can be melt processed or used directly in a variety of applications, such as paints and coatings, adhesives, or cosmetics.