Ultrahigh–molecular weight polyethylene [0.5 weight % (wt %); 429015, Sigma-Aldrich; polymer] was mixed with 99.3 wt % of paraffin oil (76235, Sigma-Aldrich; solvent) and 0.2 wt % of butylated hydroxytoluene (W218405, Sigma-Aldrich; antioxidant) in a sealed beaker at room temperature. The solution was then heated in a silicone oil bath at 150°C and mixed using a magnetic stirrer. After complete dissolution of the polymer in the solvent (~30 min), the homogeneous solution was poured in a preheated circular aluminum mold (13.5-cm diameter and 10-mm depth). The mold was then inserted in a water bath (5°C), initiating the TIPS and resulting in a polymer gel. Next, the paraffin oil was removed using a three-step solvent exchange in hexane. Another three-step solvent exchange in ethanol was performed to remove the hexane, a necessary step to ensure chemical compatibility with our critical point dryer. Last, the gel was dried using a critical point dryer (Automegasamdri-938, Tousimis), which replaced the ethanol from the PEA with air while preventing collapse of the porous structure. The initial polymer concentration was chosen to maximize solar reflectivity, IR transmittance, and structural integrity of the gel during fabrication, while the sample thickness was chosen as a compromise between number of samples needed for the experiments to achieve the desired thicknesses and fabrication time (i.e., solvent exchange and critical point drying are diffusion-limited processes).

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