Temperature-responsive hydroxypropylcellulose based thermochromic material and its smart window application

Abstract

Thermochromic materials are the most cost effective smart window materials and the organic hydrogel material has large solar modulating ability (∆T sol ) and the luminous transmittance (T lum ) compared with inorganic VO 2 based materials. Here we report a green and new organic thermochroic material based on hydroxypropylcellulose. With increasing addition of NaCl from 0.5% (wt. %) to 5% (wt. %), the LCST (lower critical solution temperature) could reduce from 42 o C to 30 o C. The morphology change of freeze dried samples below and above LCST proves that the phase change of this hydroxypropylcellulose based hydrogel is due to the solubility change of polymer in water. The fine-tuned recipe can give an outstanding solar modulating ability (∆T sol ) of 25.7% and high averaged T lum of 67.4% with LCST of 38 o C.As shown in Fig 3b, DSC suggests that LCST of pure HPC is 46 o C, with dramatically reduced LCST to 38 o C when adding 0.5% NaCl. When the concentration of NaCl increases to 5%, the τ c of HPC could reduce to 31 o C. In pure HPC, at low temperatures more hydrogen bonds exist between water and the HPC polymer fibers, as the temperature increases, these hydrogen bonds are gradually broken, indicating a continuous phase separation. By adding NaCl, the hydrogen bonding between HPC and water weakens, facilitating the formation of finer polymeric web structure, which acts as a scattering center to block the visible light, resulting in opaque status above LCST. 43 The solar light transmittance (250-2500 nm) profiles at 20 °C and 45 °C are shown in Fig. 3c with fixed thickness of 0.35 mm for all samples. For pure HPC film, it has the highest solar transmittance for both temperatures. T lum(20 °C) for all the samples are larger than 80% remains constant (Table 1). Both T lum(45 °C) and T sol(45 °C) decrease with increasing NaCl concentration from 0.5% to 5%, which is due to the reduced LCST (Fig. 3a). As can be seen from Fig. 3d, the ∆T sol , ∆T IR, ∆T lum were all increased with increasing NaCl concentration. For HPC/NaCl (0.5%) sample, the ∆T sol is 25.7% with high averaged T lum at 67.4 % (Table 1), which indicates it is a suitable candidate for smart window application. Fig. 3 (a). Hysteresis loop for the temperature-dependent transmittance of the HPC/NaCl (0-5%) samples with 0.7 mm thickness at a wavelength 580 nm and the curve of LCST vs NaCl

Infrared Regulating Smart Window Based on Organic Materials

“…The hydrogel film of a specific thickness is transparent at room temperature but as the temperature rises above the lower critical solution temperature (LCST), the film starts scattering, resulting in less light entering the interior spaces ( Figure 16 ). Similar behavior has also been demonstrated using hydroxypropylcellulose based hydrogel . Photo‐thermotropic hydrogels can alter their transparency and reflective properties when illuminated by sunlight .…”

“…Similar behavior has also been demonstrated using hydroxypropylcellulose based hydrogel. [103] Photo-thermotropic hydrogels can alter their transparency and reflective properties when illuminated by sunlight. [104] In these systems, a material such as graphene is dispersed in a hydrogel network.…”

Design Principles for Aqueous Interactive Materials: Lessons from Small Molecules and Stimuli‐Responsive Systems

“…Changing the hydrophilic/hydrophobic balance allowed for the UCST of polyampholyte hydrogels to be tuned between 25 and 55 °C 13. Similarly, the LCST of hydroxypropyl cellulose was tuned between 30 and 50 °C through addition of glycerol, which at the same time served as an antifreezing agent 14,15. Poly(ethyleneglycol) (PEG) usually exhibits a LCST of 52 °C and can mediate thermoresponsive behavior in materials.…”

Humidity-gated, temperature-responsive photonic infrared reflective broadband coatings

“…14 To avoid interference with visible light, the reection band should be able to shi to longer wavelengths upon cooling allowing the highest intensity IR light from the sun (from around 800-1200 nm) to transmit though the window and contribute to passive heating. Temperature responsive materials switching between transparent and scattering or absorbing states, such as hydrogels, 15,16 phase changing materials 12,[17][18][19] or polymer composite coatings 20 have been developed for temperature responsive smart window applications. Also static IR reecting coatings have been studied intensively.…”