Existing highly porous polymers are not suitable for applications requiring optical transparency and low haze due to significant visible light scattering. However, materials that are both porous and transparent are desirable for applications including energy-efficient, thermally insulating window glazings. Here, we demonstrate design rules for the synthesis of visibly transparent polymer aerogels using controlled radical polymerization to reduce the size, and therefore the scattering cross section, of pores and pore walls in the aerogel. In this case, poly(divinylbenzene) aerogel monoliths synthesized using stable free radical polymerization (SFRP) exhibit a porosity of 43% and a transmittance greater than 60%; this performance corresponds to a greater-than-sevenfold improvement to visible transmittance and a modest decrease in porosity (by approximately one third) when compared to samples prepared via uncontrolled radical polymerization, which are effectively opaque. The scalable, one-pot synthesis uses inexpensive starting materials and ambient drying. We demonstrate through nitrogen adsorption porosimetry and synchrotron small-angle X-ray scattering (SAXS) that controlled radical polymerization improves polymer optical properties by reducing the diameter of pores (~&lt; 10 nm) and of the polymer aggregates that make up the pore walls (36 nm). We further demonstrate how changes to the SFRP formulation that modulate the ratio of active chain ends to free radicals can be used to rationally tune aerogel properties. As the process presented here can be extended to different classes of monomers as well as different types of controlled radical polymerization, our results provide a general set of design rules for the radical polymerization of transparent porous polymers.

Transparent mesoporous polymer aerogels via stable free radical polymerization

ACS Fall 2020 Virtual Meeting

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