Bio-based core–shell casein-based silica nano-composite latex by double-in situ polymerization: Synthesis, characterization and mechanism

Abstract

Casein-based silica nano-composite latex as bio-based film-forming material was synthesized from casein, caprolactam, acrylate, tetraethoxysilane and silane coupling agent via double-in situ emulsion polymerization. The objective of this study was to solve the problem of casein film deficiencies, such as hard, brittle and poorer water resistance. To investigate the influences of silica on properties of the latexes and films, the resultant nano-composite and the control system (without silica) were both characterized by flourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), thermogravimetry (TG), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), separately. FT-IR spectra suggested the successful incorporation of nano-silica particles with the matrix as well as the expected grafting of polyacrylate onto casein chains. Morphology of resultant composite latex exhibited evident core–shell structure with the average size of around 80nm, and the improved shell thickness indicated the encapsulation of silica on the outer layer of the particle, which was verified by TEM results. TG analysis demonstrated the enhanced thermostability of casein-based silica nano-composite, in comparison with the control system (without nano-silica). SEM, AFM and XPS results confirmed that the substantial silica dispersed uniformly near the outer layer of the films, and showed good compatibility between the inorganic and organic phases in the casein-based silica composite. Additionally, compared with the latex film without silica, the hybrid latex film containing silica showed higher hydrophobicity, lower water absorption, enhanced tensile strength and decreased flexibility.