An editable shape-memory transparent wood for smart building


 Transparent wood (TW) with excellent optical and thermal management performance has been developed recently as a promising energy-efficient building material. Here, an editable shape-memory TW (ESMTW) is developed through in situ polymerization of epoxy vitrimers into a delignified wood scaffold. The ESMTW possesses high strength at low temperature and flexibility at high temperature, while it exhibits excellent shape-manipulation capability under thermal-stimulus. Meanwhile, the ESMTW shows unique light guiding and directional scattering effects. The light illuminance difference observed in our house model with a common glass ceiling is 81 times, whereas it is only 16 times with a ceiling made of the ESMTW. Most importantly, the transmitted light intensity distribution is tunable owing to its shape-management capability. Additionally, the resultant TW possesses great thermal insulation properties, mechanical strength, and high impact absorption ability. The combination of characteristics enables TW to exhibit great promise as an advanced functional and intelligent building material.


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It has been forecast that from 2012 to 2040, global energy consumption will increase 20 by 48% and carbon dioxide (CO2) emissions by 34%. [1][2][3] Residential buildings account 21 for 30-40% of total energy consumption and CO2 emissions, and their windows contribute approximately 10-25% of total thermal loss due to poor thermal 23 management ability. 4-6 Glass is extensively used in building windows and rooftops 24 owing to its high optical transmittance and low thermal expansion coefficient (7-10 25 ppm K -1 ). 7 However, common glass windows suffer from some weaknesses. First, glass 26 has a relatively high thermal conductivity (approximately 1 W m -1 K -1 ), resulting in 27 high thermal loss and low energy efficiency. 8 Second, glass is brittle and easy to break 28 under sudden impact, causing potential safety risks. Moreover, massive amounts of CO2 29 are emitted during glass production, accelerating the environmental greenhouse effect. 9

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In addition, it may have problems with glaring and shadowing effects. 10 Therefore, it is 31 urgent to develop alternative transparent materials with great thermal insulation, optical 32 properties, security, and sustainability to be used as a replacement for traditional glass. 33 Wood has been considered one of the most promising alternatives to green and 34 energy-efficient building materials. Recently, transparent wood (TW) has attracted 35 significant attention as an energy-saving building material because of its many merits, 36 including high anisotropy, high optical transmittance, adjustable optical haze, efficient 37 thermal insulation, great mechanical robustness with shatterproof features, high impact 38 energy absorption, renewability, and great potential for multi-functionalization. 11, 12 TW Although some progress has been made on functionalized TW for smart building 56 materials, the scope has focused on two-dimensional (2D) applications, such as 57 windows, ceilings, and rooftops. If TW were endowed with shape-manipulation ability, 58 such as shape transformation, shape editing, or even shape memory, its application 59 could extend from 2D to 3D, for example, in curved or irregularly shaped windows, 60 screens, ceilings, rooftops, and transparent decorations. This would be great progress 61 in advanced functional TW fields for smart building materials. However, to date, an 62 editable shape-memory TW has not been reported.  Benefiting from the unique property of vitrimers, the TW exhibited stiffness at low 92 temperature (less than Tg) and flexibility at elevated temperature (greater than Tg), and 93 it showed shape-memory behavior by Tg-induced phase change and editable shape-

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Microstructures and chemical components. 112 The reaction and transesterification mechanism of the vitrimers and the fabrication 113 procedure of the ESMTW are shown in Fig. 1. The colorless and transparent vitrimers 114 were synthesized through a mild and rapid click reaction between E51 and Tetra-SH      shape of the ESMTW was a rectangular strip. It was deformed into an s-shape under an 195 external force at 60 °C and rapidly fixed at 0 °C to obtain a temporary shape, which 196 was stable below Tg. The temporary s-shape returned to the original rectangular shape 197 when reheated to 60 °C. Next, a new temporary u-shape was formed, which also 198 returned to the original rectangular shape at 60 °C. To further investigate the editable 199 shape-memory behavior during transesterification, according to the result of the 200 dilatometry experiment ( Supplementary Fig. 3), the re-editing temperature was set to 201 180 °C (above Tv). Notably, decomposition of the wood, DW, and ESMTW did not 202 occur until the temperature was above 200 °C, as shown by Thermogravimetric analysis 203 (TGA) in Supplementary Fig. 7. The re-editing permanent u-shape was formed through 204 an external force applied at 180 °C for 10 min, gradually cooled to room temperature, 205 and then rapidly fixed at 0 °C. The newly permanent u-shape was further deformed at 206 60 °C and fixed at 0 °C to get the temporary n-shape and s-shape. When reheated to 207 60 °C, these temporary shapes returned to the permanent u-shape.

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The shape-memory recovery ratio of the ESMTW samples at different temperatures  Supplementary Fig. 8 and 9, and the 212 corresponding recovery ratios are presented in Fig. 3c and 3e. The bent samples all 213 recovered to their original rectangular shape over the heating duration and exhibited a 214 faster recovery speed with increasing heating temperature. When the heating 215 temperature was above Tg, the recovery ratio of the T-ESMTW was above 86.7%, and 216 that of the L-ESMTW was above 94.4% within 30 min. Additionally, multicycle 217 recovery performance at 60 °C within 30 min was conducted, as shown in Fig. 3d and   218 3f. After the multicycle tests, the recovery ratios of the T-ESMTW and L-ESMTW 219 showed almost no change, indicating the great shape-memory and recovery capability 220 of the ESMTW.

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A possible mechanism for the editable shape-memory behavior of the ESMTW was 222 proposed, and the potential molecular structure changes during the shape-memory and 223 shape re-editing processes are illustrated in Fig. 3g. The shape-manipulation capability       The stress-strain curves of the balsa wood, DW, and ESMTW along the transverse and 353 longitudinal directions are shown in Fig. 7a and 7b. The fracture strength, elastic 354 modulus, and toughness are listed in Table 1. After delignification, the fracture strength,   conductivities of the T-ESMTW and L-ESMTW were 0.3002 W m -1 K -1 and 0.2898 W 420 m -1 K -1 , respectively, which were at least one-third that of common glass. All these 421 characteristics enable the ESMTW to be a promising candidate for green, energy-422 efficient, and advanced and intelligent building materials in 3D applications.