Sustainable inverse-vulcanised sulfur polymers

Abstract

We demonstrate two renewable crosslinkers that can stabilise sustainable high sulfur content polymers, via inverse-vulcanisation. With increasing levels of sulfur produced as a waste byproduct from hydrodesulfurisation of crude oil and gas, the need to find a method to utilise this abundant feedstock is pressing. The resulting sulfur copolymers can be synthesised relatively quickly, using a one-pot solvent free method, producing polymeric materials that are shape-persistent solids at room temperature and compare well to other inverse vulcanised polymers. The physical properties of these high sulfur polymeric materials, coupled with the ability to produce them sustainably, allow broad potential utility.

Fig. 1. (a) General scheme outlining the synthesis of sustainable inverse vulcanised polymers (b) structures of the crosslinkers used, squalene (SQ) and perillyl alcohol (PER). (c) Photographs of (i) sulfur, squalene, and perillyl alcohol (L to R) and the resultant inverse-vulcanised polymers cast as pegged bricks: (ii) S-squalene copolymer, black solid, graduations show mm; (iii) S-perillyl alcohol copolymer, semi-transparent ruby red solid.

Fig. 2. ¹H NMR for both the sulfur–perillyl alcohol 50 : 50 copolymer (a) and the perillyl alcohol monomer (b). Loss of vinylic proton resonances indicate a successful crosslinking by addition across the double bonds, though some aromatic H environments are detected, suggesting some possible hydrogen abstraction. The formation of new peaks in the 3.5–4 ppm region is consistent with the formation of C–S bonds. * = chloroform.

Fig. 3. (a) Stacked p-XRD patterns for different sulfur : squalene copolymer ratios and polymorphs of elemental sulfur and (b) stacked p-XRD patterns for different sulfur : perillyl alcohol copolymer ratios and polymorphs of elemental sulfur.

Fig. 4. (a) Stacked DSC curves for different ratios of sulfur–squalene copolymers. The T_g of the polymers can be seen at 22 °C (50 : 50 wt% sulfur : squalene), 35 °C (50 : 50 wt% sulfur : squalene), and 14 °C (50 : 50 wt% sulfur : squalene). The 80 : 20 and 90 : 10 wt% sulfur : squalene products both show melting transitions for crystalline S₈ at ∼120 °C. (b) Stacked DSC curves of different ratios of sulfur–perillyl alcohol copolymers. The T_g of the polymers can be seen at 20 °C (50 : 50 wt% sulfur : perillyl alcohol), 31 °C (50 : 50 wt% sulfur : perillyl alcohol), and 13 °C (50 : 50 wt% sulfur : perillyl alcohol). The 80 : 20 sulfur : perillyl alcohol product shows melting of crystalline S₈ at ∼120 °C. (c) Stacked GPC comparison of perillyl alcohol monomer and sulfur copolymer.

Fig. 5. (a) Stacked pXRD patterns for 70 : 30 wt% sulfur–perillyl alcohol copolymer at room temperature, after heating to 140 °C to ‘cure’ the detected trace S₈ crystals, and after 24 hours back at room temperature. (b) Stacked pXRD patterns for a slurry of sulfur and perillyl alcohol monomer, after heating to 185 °C to induce reaction, and after 24 hours at room temperature.

Fig. 6. Sulfur polymer samples on, after breaking into powder, centre, and then after being reformed into a monolith again, right: (a) sulfur–perillyl alcohol copolymer, and (b) sulfur–squalene copolymer. Both samples were made with 50 wt% sulfur.

Fig. 7. Mercury uptake results for mercury chloride and methylmercury chloride from a 2.5 ppm aqueous solution after 1 hour.