Preparation and processing of porous sulfur foams having low thermal conductivity.

Sulfur-containing polymers prepared via the inverse vulcanization technique have attracted considerable attention due to the feasibility of the method to produce stable polysulfides with up to 50-90 wt% of sulfur and their wide range of applications from Li-S batteries to catalysis, self-healing and optical materials. Despite many applications, the development of new advanced materials using sulfur is still in the initial stage. Herein, we reported the preparation and processing of a porous sulfur foam for low thermal conductivity applications by combining inverse vulcanization and template removal techniques. Initially, water-soluble template-embedded cross-linked polysulfides were prepared and hot-pressed to the required shape and size. Later, pores were generated by dissolving the template in water. The porosity of the foam was altered by varying the particle size of template materials. The effects of the templates on the porosity and morphology were discussed and correlated with thermal conductivity. The sulfur foam with a smaller pore size and high porosity showed significant decrease in the thermal conductivity up to ∼0.032 W m-1 K-1 at 25 °C, which was much lower than that of pristine sulfur (0.205 W m-1 K-1). The present method offers flexibility to modify the foam structure and properties during preparation and processing.

NMR and EPR Structural Analysis and Stability Study of Inverse Vulcanized Sulfur Copolymers.

Sulfur copolymers with high sulfur content find a broad range of applications from Li-S batteries to catalytic processes, self-healing materials, and the synthesis of nanoparticles. Synthesis of sulfur-containing polymers via the inverse vulcanization technique gained a lot of attention due to the feasibility of the reaction to produce copolymers with high sulfur content (up to 90 wt %). However, the interplay between the cross-linker and the structure of the copolymers has not yet been fully explored. In the present work, the effect of the amount of 1,3-diisopropenyl benzene (DIB) cross-linker on the structural stability of the copolymer was thoroughly investigated. Combining X-ray diffraction and differential scanning calorimetry, we demonstrated the partial depolymerization of sulfur in the copolymer containing low amount of cross-linker (<30 wt % DIB). On the other hand, by applying NMR and electron paramagnetic resonance techniques, we have shown that increasing the cross-linker content above 50 wt % leads to the formation of radicals, which may severely degrade the structural stability of the copolymer. Thus, an optimum amount of cross-linker is essential to obtain a stable copolymer. Moreover, we were able to detect the release of H2S gas during the cross-linking reaction as predicted based on the abstraction of hydrogen by the sulfur radicals and therefore we emphasize the need to take appropriate precautions while implementing the inverse vulcanization reaction.