Effect of process variables on optimisation of mechanical properties of silica/poly(tetrahydrofuran) sol-gel hybrids

Abstract Inorganic/organic sol-gel hybrid materials consist of inorganic and organic co-networks which interact at the molecular level and behave as a single material. The aim is to gain synergy of properties between the two components, with the inorganic providing mechanical strength and organic providing elasticity. This becomes more challenging as the thickness of a sol-gel material increases, due to risk of cracking. Here, we investigate the effect of sol-gel processing variables for large silica/poly(tetrahydrofuran) (SiO 2 -PolyTHF) monolithic hybrids (a size range of several centimetres) to optimise their mechanical properties. Cracking on drying can occur due to combinations of shrinkage and capillary stresses inside the monoliths, but can be mitigated by optimisation of process variables and hybrid composition. The effect of these variables on success rate of producing crack-free monoliths was assessed, then full characterisation carried out on samples made with successful protocols. Computational scribe calculations on X-ray micro-computed tomography images determined the shrinkage during different drying methods. Chemical, thermal, mechanical, and structural analyses were applied to devise a reproducible synthesis method for hybrid monoliths. The monolithic hybrids, prepared using both oven-dried (OD) and freeze-dried (FD) techniques, demonstrated compressive strengths of 44.1 MPa and 30.5 MPa, respectively, when made with an inorganic content of ~31 wt.%. These materials can withstand strains of up to 54.5%. Mechanical testing in wet and dry conditions, i.e. after 60 days immersion in phosphate buffered saline solution (PBS), enabled observation of the viscoelastic response after 1000 cycle compression loading, demonstrating that the materials can work under wet conditions. Graphical Abstract