Reinforcement mechanism of silica surface hydroxyl: The opposite effect

Silica is one of the best reinforcing fillers for silicone rubber due to its similar structure to the main chain of silicone rubber and abundant surface hydroxyl groups. However, for the role of surface hydroxyl groups in reinforcement, there are many conflicting conclusions. This paper combines experiments and molecular dynamics calculations to illustrate that the reinforcing mechanism of the hydroxyl group is two-sided. Firstly, at 30 phr, the tensile strength increases from 6.34 ± 0.47 MPa to 8.40 ± 0.37 MPa as the number of hydroxyl groups decreases; the Payne effect decreases, and the bound rubber content increases from 25.7% to 34.0%, and the filler-rubber interfacial interaction increases; molecular dynamics calculations show that as the number of hydroxyl groups increases, (I) the interaction forces between the silica particles increase, inhibiting their dispersion in the silicone rubber; (II) physical-adsorption water increases and the strength of the rubber-filler interfacial interaction decreases, reducing the reinforcing effect. Secondly, as the amount of silica added gradually reduces, the difference in tensile strength between Silica (13.1 OH/nm2) /SR composites and A200 (4.5 OH/nm2) /SR composites decreases. At 5 phr, the tensile strength of Silica-5 phr is 0.95 ± 0.07 MPa, which is higher than that of A200-5 phr, 0.87 ± 0.19 MPa; the bound rubber content and the filler-rubber interfacial interaction of Silica-5 phr are higher than A200-5 phr; molecular dynamics calculations show that the strength of the filler-rubber interfacial interaction increases gradually with increasing hydroxyl groups for similar dispersion states. This paper would help design more desirable silica structures with a highly branched structure and multiple hydroxyl groups as an ideal reinforcing agent.