Formation of organic–inorganic nanomatrix structure with nanosilica networks and its effect on properties of rubber

Tunable distribution of silica nanoparticles in water-borne coatings via strawberry supracolloidal dispersions

HYPOTHESIS

Water-borne coatings are rapidly expanding as sustainable alternatives to organic solvent-borne systems. Inorganic colloids are often added to aqueous polymer dispersions to enhance the performance of water-borne coatings. However, these bimodal dispersions have many interfaces which can result in unstable colloids and undesirable phase separation. The covalent bonding between individual colloids, on a polymer-inorganic core-corona supracolloidal assembly, could reduce or suppress instability and phase separation during drying of coatings, advancing its mechanical and optical properties.

METHODS

Aqueous polymer-silica supracolloids with a core-corona strawberry configuration were used to precisely control the silica nanoparticles distribution within the coating. The interaction between polymer and silica particles was fine-tuned to obtain covalently bound or physically adsorbed supracolloids. Coatings were prepared by drying the supracolloidal dispersions at room temperature, and their morphology and mechanical properties were interconnected.

FINDINGS

Covalently bound supracolloids provided transparent coatings with a homogeneous 3D percolating silica nanonetwork. Supracolloids having physical adsorption only, resulted in coatings with a stratified silica layer at interfaces. The well-arranged silica nanonetworks strongly improve the storage moduli and water resistance of the coatings. These supracolloidal dispersions offer a new paradigm for preparing water-borne coatings with enhanced mechanical properties and other functionalities, like structural color.

A polystyrene/silica hybrid nanomatrix formed in natural rubber

Our work aims to investigate the morphology and properties of natural rubber (NR) dispersed in a polystyrene/silica hybrid nanomatrix. The hybrid nanomatrix structure was formed by the graft copolymerization of vinyltriethoxysilane onto NR grafted with styrene. This hybrid nanomatrix structure was composed of nanosilica with a size of less than 100 nm, and the incorporation of polystyrene resulted in outstanding mechanical and viscoelastic properties. The tensile strength of NR with the hybrid nanomatrix structure was 23 MPa, and the storage modulus was 2.54 MPa, which were 2.5 and 15 times higher than those of the silica nanomatrix. The outstanding mechanical and viscoelastic properties of this NR material were attributed to the formation of the hybrid nanomatrix structure. The roles of polystyrene and silica and their synergetic effect were clarified by investigating the morphology and properties of the hybrid nanomatrix after acetone extraction and annealing.

DETERMINATION OF A SUITABLE CONDITION OF GRAFT COPOLYMERIZATION OF VINYLTRIETHOXYSILANE ONTO NR TO FORM NANOMATRIX STRUCTURE

Graft copolymerization of vinyltriethoxysilane (VTES) onto NR particles in the latex stage is a unique reaction, since it occurs together with hydrolysis and condensation of the triethoxysilane group of VTES to form a colloidal silica linking to the rubber particles. These reactions may contribute to the formation of a silica nanomatrix structure that consists of a dispersoid of rubber particles as the major component and a silica matrix as the minor component. Here, the graft copolymerization of VTES followed by hydrolysis and condensation is investigated to determine a suitable condition to prepare NR with a silica nanomatrix structure. The mechanical properties of the resulting graft copolymer are discussed in relation to the morphology, silica content, and gel content of the rubber. Based on morphological observations, NR particles with an average diameter of approximately 1 μm are well dispersed in a nanomatrix consisting of silica nanoparticles. The thickness of the silica nanomatrix increases as the monomer concentration increases, and a long incubation time generates large silica nanoparticles. The tensile strength and viscoelastic properties are significantly improved by forming the silica nanomatrix structure, with its continuous structure that prevents the NR particles from merging.