Super‑adsorbent polyacrylate under swelling in water for passive solar control of building envelope

摘要

Super-adsorbent polymers have the capacity to immobilize huge quantities of water in the form of hydrogel, thanks totheir configuration. A commercial sodium polyacrylate (PA) was analysed as such and at different water uptakes, indicatedthrough the weight ratios PA:H_2O. The hydrogels were prepared using different type of water (tap, distilled anddeuterated) and characterized by Infrared and Raman spectroscopic analyses, nuclear magnetic resonance experiments,CHN elemental analysis, measurements of thermal conductivity and diffusivity. All the measurements were done inorder to assess applications of PA:H_2O gels as Thermal Energy Storage systems for improving thermal performances ofbuilding envelope through passive solar walls. It has been observed that the behaviour of the hydrogels depends bothon temperature and water content. In certain conditions such as low weight ratios, a spontaneous and quick cooling ofthe hydrogel could be observed. The curves of heat flow and average specific heat (c_p) were determined as a functionof temperature in order to investigate the states of water in PA hydrogels. When a few water molecules are present, theyare mainly and strongly bonded with carboxylate groups. Increasing the amount of water, greater shells of solvationaround ionic groups form and water molecules can even interact with neighbouring non-polar hydrocarbon groups.At very high amount of water molecules, they are much more involved into H-bonds among themselves, rather thanwith PA, so that water pools form into the links of polymeric network. Bulk-like water can freeze and melt. Whatever theamount of water in the hydrogel, its thermal capacity is higher than dry polymer, because the heat can be absorbedby the continuous desorption of water from polymer to bulk-like water (water_(gel) → water_(liquid)), which can evaporate astemperature approaches 100 ℃ (water_(gel) → water_(liquid) → water_(vapour)).