Cohesive and adhesive properties of polycaprolactone/silica hybrid coatings on poly(methyl methacrylate) substrates

Organic-inorganic hybrid materials were prepared using the sol-gel process in order to improve the scratch resistance of poly(methyl methacrylate) (PMMA) substrates. Sol-gel solutions based on tetraethoxysilane and alpha,omega-triethoxysilane terminated polycaprolactone with two different organic/inorganic weight ratios (5/5 and 7/3) were applied and cured on the substrate. The final coatings consisted of interconnected nanodomains of silica and polycaprolactone, which increased in size and resulted into microdomains when the content of organic phase increased. The surface energies of coated and uncoated PMMA, and the cohesive and adhesive properties of the hybrid coatings were examined using contact angle measurements, and fragmentation tests, respectively. The surface energy of the hybrid coatings and their work of adhesion to the PMMA substrate were found to be similar, with a higher polar contribution at increasing silica content. In contrast, the strain to failure, cohesive strength, toughness and adhesion to PMMA of the coating with a 5/5 organic/inorganic ratio were considerably higher, compared to the coating with a 7/3 organic/inorganic ratio. These results demonstrate that the adhesion strength is the result of a series of complex phenomena that probably involves diffusion of reactants in PMMA and their crosslinking, as suggested by electron energy loss spectroscopy analyses, other than effects deriving from residual stresses and from the morphology produced during the formation of the coating.