High strength, epoxy cross-linked high sulfur content polymers from one-step reactive compatibilization inverse vulcanization

Abstract

Inverse vulcanization provides a simple, solvent-free method for the preparation of high sulfur content polymers using elemental sulfur, a byproduct of refining processes, as feedstock. Despite the successful demonstration of sulfur polymers from inverse vulcanization in optical, electrochemical, and self-healing applications, the mechanical properties of these materials have remained limited. We herein report a one-step inverse vulcanization using allyl glycidyl ether, a heterobifunctional comonomer. The copolymerization, which proceeds via reactive compatibilization, gives an epoxy cross-linked sulfur polymer in a single step, as demonstrated through isothermal kinetic experiments and solid-state ¹³C NMR spectroscopy. The resulting high sulfur content (≥50 wt%) polymers exhibited tensile strength at break in the range of 10-60 MPa (70-50 wt% sulfur), which represents an unprecedentedly high strength for high sulfur content polymers from vulcanization. The resulting high sulfur content copolymer also exhibited extraordinary shape memory behavior along with shape reprogrammability attributed to facile polysulfide bond rearrangement.

Scheme 1. Overall scheme for inverse vulcanization process illustrating comparison of adopted reaction chemistry and suitable monomers of previous reports and this work.

Fig. 1. Inverse vulcanization of elemental sulfur and AGE. (a) Photographs of reaction between equal masses of elemental sulfur and allyl glycidyl ether (AGE), butyl glicidyl ether (BGE), allyl butyl ether (ABE), and 1,1-dimethyl allyl glycidyl ether (DAGE). Casted free-standing film of poly(S-r-AGE) and the corresponding SEM-EDS elemental mapping (carbon, oxygen and sulfur) images are shown in (b). (c) TGA thermogram of the poly(S-r-AGE), elemental sulfur, and a mixture of sulfur and AGE, and (d) glass transition temperatures, obtained from differential scanning calorimetry, of poly(S-r-AGE) with varying sulfur contents.

Fig. 2. Isothermal DSC and time-dependent NMR studies. Isothermal DSC thermogram of (a) elemental sulfur–AGE mixture (1 : 1 weight ratio) under various temperatures and (b) mixtures of elemental sulfur with AGE, ABE, and BGE conducted at 150 °C. The ¹H NMR spectrum of sulfur–AGE mixtures upon reaction at 140 °C is shown in (c).

Scheme 2. Proposed mechanism of sulfur–AGE copolymerization showing (a) addition of elemental sulfur to the allyl group followed by (b) polysulfane-initiated epoxide ring-opening.

Fig. 3. Probing the bond connectivity of S–AGE. (a) FTIR spectra and (b) semi-quantitative ¹³C solid-state NMR spectrum from ¹H–¹³C cross-polarisation of S–AGE.

Fig. 4. Tensile strength measurement. (a) Digital photograph of the dogbone sample of S–AGE with 50 wt% sulfur and (b) stress–strain curve of S–AGE with sulfur contents of 50 and 70 wt%.

Fig. 5. Evalution and demonstration of shape-memory behavior. (a) Quantitative analysis of dual-shape memory cycle process, (b) shape-memory performances under repetitive bending cycles, and (c) demonstration of dual-shape memory behavior and polysulfide bonds rearrangement-induced shape reconfigurability of S–AGE. Polymerization allows for the formation of highly cross-linked polymer, which, despite being a thermoset, can be thermally reprocessed due to the presence of large quantities of dynamic S–S bonds. The S–AGE displayed excellent shape-memory properties, and we believe that our studies would open a new class of polysulfide materials that could be applied in various fields requiring simple and well-characterized stimuli-responsive material which could be readily prepared in large scales (Video S2. Spontaneous folding of S–AGE (50 wt% sulfur) film into a cube).