Nano-scale Creep Compliance of Hybrid Aerogels
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Nano-scale Creep Compliance of Hybrid Aerogels N. de la Rosa-Fox1, V. Morales-Flórez2, M. Piñero3, L. Esquivias2,4 1 Física de la Materia Condensada, Facultad de Ciencias-UCA, 11580 Pto Real, Cádiz 2 ICMSE (CSIC-US) Av. Américo Vespucio 49, 41092, Sevilla 3 Dpto. Física Aplicada, CASEM-UCA, 11580 Pto Real, Cádiz 4 Dpto. Física de la Materia Condensada, Facultad de Física US.41012 Sevilla
ABSTRACT The copolymerization between TEOS (tetraethoxysilane) and silane derivatives was promoted by the application of high power ultrasound to the precursor liquid mixtures in the same way as in the classical sol-gel method. The specific organic precursor was selected from the silanol-terminated polymer family with different molecular functionality and the inorganic precursor was from the silicon alkoxide family. Ultrasound, through the acoustic cavitation process, influences the formation of a very fine distribution of silica particles and avoids cyclidation of the polymer, thus favoring copolymerization with the inorganic particles and leading to the formation of a highly porous and rubber-like solid aerogel. Creep compliance curves, corresponding to the time-dependent depth response to a step load, are imprint site dependent, with pore, soft and stiff sites discerned. In all cases, an instantaneous elastic deformation is apparent. For longer test times, depending on the imprint sites, elastic deformation and newtonian flow produce the rise and fall of the creep curve. Linear parts of the curve on a log-log scale indicate potential growth with small exponents for the creep compliance level. Isochrones stress-strain diagrams show a superlinear trend and an increase with time, which reveals the nonlinear viscoelasticity of these hybrid aerogels. The elasto-plastic response to creep can be tuned by the molecular functionality of the different silane derivatives studied. INTRODUCTION Hybrid organic/inorganic aerogels are nanostructured materials composed of an entangled network of silica particles surrounded by the polymeric organic phase and covalently bonded as a true copolymerization (hybrid Type C from Mackenzie’s classification [1]). The mechanical behavior of the system changes drastically depending on the organic content. This property can be tuned to give materials ranging from a brittle solid (pure SiO2 aerogel) to an elastomeric solid (Ormosil aerogel). These hybrid aerogels are of increasing interesting because of the improved mechanical properties [2]. In one sense, knowledge of creep behavior is one of the most important mechanical property for the structural design of aerogels controlled by the dimensional stability of the material. Furthermore, creep experiments are of fundamental interest in any application where the aerogel must sustain loads in different fluids [3]. In assessing this parameter, nanomechanical creep testing has a significant capability to treat the mechanical response of hybrid aerogels such ε (t) ) is calculated as the as time-dependent deformation. Thus, the creep compliance ( J (t) =
σ0
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