How elastic moduli affect ambient pressure drying of poly(methylsilsesquioxane) gels

Abstract

To understand which properties of wet gels decide whether they can be dried under ambient pressure or not, the author prepared fifty-six gels from solutions of different mass ratios of tetra-functional tetramethoxysilane (TMOS), tri-functional methyltrimethoxysilane (MTMS) and difunctional dimethyldimethoxysilane (DMDMS). These gels were dried under ambient pressure, and the mechanical properties of the wet gels were investigated by uniaxial compression test. The stress-strain curve of the wet gels was composed of two parts (the 1st and 2nd parts) with different slopes. When the ratio of the 1st and 2nd parts (E_2nd/E_1st, EM ratio) exceeded 2.3, the gel was dried without crack accompanying spring-back to obtain a dried gel with bulk density lower than 0.2 g/cm². This finding means that the elastic modulus of a wet gel is a good criterion to predict whether or not they can be dried under ambient pressure without cracking to obtain a xerogel whose properties are close to the aerogel counterpart. Choosing a starting composition from fifty-six formulas, which gives the highest EM ratio, the author obtained a crack-free and transparent aerogel monolith with the dimension of 300 × 300 × 8 mm³ through ambient pressure drying.

Highlights

• Aerogels were obtained from three-component system; TMOS, MTMS and DMDMS.

• While MTMS formed the main network, TMOS improved transparency and DMDMS affected elastic moduli of the gel.

• A 300 × 300 × 8 mm³ crack-free aerogel monolith was obtained under ambient pressure drying.

• Elastic modulus of the wet gel was an important factor for successful ambient pressure drying.