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

Super-adsorbent polymers have the capacity to immobilize huge quantities of water in the form of hydrogel, thanks to their confguration. A commercial sodium polyacrylate (PA) was analysed as such and at diferent water uptakes, indicated through the weight ratios PA:H2O. The hydrogels were prepared using diferent type of water (tap, distilled and deuterated) and characterized by Infrared and Raman spectroscopic analyses, nuclear magnetic resonance experiments, CHN elemental analysis, measurements of thermal conductivity and difusivity. All the measurements were done in order to assess applications of PA:H2O gels as Thermal Energy Storage systems for improving thermal performances of building envelope through passive solar walls. It has been observed that the behaviour of the hydrogels depends both on temperature and water content. In certain conditions such as low weight ratios, a spontaneous and quick cooling of the hydrogel could be observed. The curves of heat fow and average specifc heat (cp) were determined as a function of temperature in order to investigate the states of water in PA hydrogels. When a few water molecules are present, they are mainly and strongly bonded with carboxylate groups. Increasing the amount of water, greater shells of solvation around 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 than with PA, so that water pools form into the links of polymeric network. Bulk-like water can freeze and melt. Whatever the amount of water in the hydrogel, its thermal capacity is higher than dry polymer, because the heat can be absorbed by the continuous desorption of water from polymer to bulk-like water (watergel→waterliquid), which can evaporate as temperature approaches 100 °C (watergel→waterliquid→watervapour).