Sangwoo
Park‡§
a,
Minju
Chung‡¶
a,
Alexandros
Lamprou
b,
Karsten
Seidel
c,
Sanghoon
Song
d,
Christian
Schade
e,
Jeewoo
Lim
*d and
Kookheon
Char
*a
aSchool of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea. E-mail: khchar@snu.ac.kr
bFunctional Polymers Global Research, Innovation Campus Asia Pacific, BASF, 200137 Shanghai, China
cMaterial Physics, Analytics & Formulation Research, BASF SE, 67056 Ludwigshafen, Germany
dDepartment of Chemistry, Kyung Hee University, Seoul 02447, Republic of Korea. E-mail: jeewoo@khu.ac.kr
eFunctional Polymers Global Research, BASF SE, 67056 Ludwigshafen, Germany
First published on 15th December 2021
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.
While numerous compounding methods for achieving physical blends of sulfur with carbon-based materials,1,2 inorganic nanoparticles,3 and construction materials (e.g. sulfur cement/concrete)4,5 have been reported, chemical methods of sulfur utilization, such as the direct use of elemental sulfur in polymerization reactions, had been scarce, largely due to the low stability of high sulfur content polymers. Recently, bulk free-radical copolymerization of molten sulfur with 1,3-diisopropenylbenzene (DIB) has been reported.6 The method, dubbed “inverse vulcanization” allowed for the formation of polysulfide polymers with unprecedentedly high sulfur contents (50–90 wt%) which were much less prone to depolymerization through cyclooctasulfur elimination.7,8 Since the first report of inverse vulcanization in 2013, a large number of multifunctional comonomers, such as divinylbenzene,9,10 diethynyl benzene,11 myrcene,12 limonene,13 dicyclopentadiene,12 and diallyl disulfide14 have been used in inverse vulcanization.
The polymers from inverse vulcanization display attractive properties such as high (n > 1.80) refractive index,7,15,16 high energy density in Li–S batteries,6,17–19 and self-healing.8,20 Despite this, their relatively poor mechanical properties have limited further expansion of the scope of applications. Recent studies show that the strength of inverse vulcanization polymers are much lower than those of conventional polymers, with an upper limit of tensile strength at break (σmax) of lower than 10 MPa.8,20–22 Employing a mixture of crosslinkers21,23 have allowed for some control over the shear modulus of inverse vulcanization products, but the approach has yet to provide higher strength materials. The crosslinkers used in inverse vulcanizations thus far were limited to homobifunctional molecules with unsaturated carbon–carbon bonds, where addition reaction of sulfur radicals (R–S·) is the only available chemistry (Scheme 1),6,10,11,14,21,23 and introducing new types of chemistry for crosslink formation has recently emerged as a compelling strategy for the enhancement of mechanical properties of inverse vulcanized polymers. In 2020, Hasell and coworkers reported a two-step process involving a reaction of sulfur with sorbitan oleate (Span 80), a non-ionic surfactant composed of a polar headgroup containing three hydroxyl groups and a hydrophobic tail composed of an oleyl group.24 The reaction of sulfur and Span 80 gave a prepolymer, which was subsequently treated with a diisocyanate to form urethane crosslinks. The resulting polymers showed shape-memory behavior with σmax as high as 20.56 MPa for the products with the highest degree of crosslinking, representing a significant improvement in strength compared to previously reported inverse vulcanization products. Further work by the group introduced similar polymers with improved tensile elongation and toughness.25 Recently, Pyun and coworkers synthesized segmented block copolymer sulfur polyurethanes showing both high tensile strength (13–24 MPa) and ductility (348% strain at break), using inverse vulcanization products as prepolymers.26
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Scheme 1 Overall scheme for inverse vulcanization process illustrating comparison of adopted reaction chemistry and suitable monomers of previous reports and this work. |
Based on our recent finding that inducing radical propagation of divinylbenzene (DVB) during inverse vulcanization leads to products with enhanced thermal properties,9 we envisioned that heterobifunctional comonomers, which could simultaneously undergo addition reactions with sulfur radicals and cross-linking involving a different mechanism, may lead to inverse vulcanization polymers with unprecedented strength. We report herein an inverse vulcanization using a single comonomer, allyl glycidyl ether (AGE), to give high sulfur content (>50 wt% S) polymers having σmax of over 60 MPa in a single step (Scheme 1). Although AGE is immiscible with sulfur even at elevated temperatures, the bulk polymerization of sulfur and AGE leads to reactive compatibilization, yielding a homogeneous polymer over a wide range of sulfur contents. Isothermal kinetic studies showed two different types of reactions occurring simultaneously, an observation attributed to the concurrent radical polymerization of sulfur and epoxide crosslinking. The resulting polymer exhibited stable macroscopic shape memory properties with excellent shape recovery and reprogrammability, properties which are attributed to the presence of both strong and weak linkages within the polymer network. High contents of polysulfide groups within the polymer matrix allowed for facile reprogramming and reprocessing of the shape memory polymer.
The vitrified product of the reaction of sulfur and AGE (poly(S-r-AGE), S–AGE) was insoluble in organic solvents, similarly to previously reported thermosets from the copolymerization of molten sulfur, limiting the number of available analytical methods for its characterisation.12,16 The composition of S–AGE was probed using scanning electron microscopy (SEM). SEM-EDS analysis of free-standing films of S–AGE indicated a uniform distribution of carbon, oxygen, and sulfur atoms, suggesting that S–AGE is a homogeneous polymeric material (Fig. 1b). Thermogravimetric analysis (TGA) of S–AGE showed typical one-step mass loss of random copolymers with higher thermal onset temperature of decomposition compared to that of elemental sulfur (Fig. 1c). A simple mixture of sulfur and AGE displayed a much lower onset temperature, attributed to the evaporation of AGE, followed by the loss of elemental sulfur. Powder X-ray diffraction (PXRD) profile of S–AGE prepared from 50 wt% sulfur did not feature peaks associated with elemental sulfur, suggesting the absence of unreacted sulfur (Fig. S1†). Elemental analysis results showed carbon, hydrogen, and sulfur composition nearly identical to theoretical values (Table S1†). The formation of a homogeneous polymeric product from an initially biphasic mixture suggested that the polymerization proceeded via reactive compatibilization with initially formed polymers possibly acting as compatibilizers.
Differential scanning calorimetry (DSC) of S–AGE revealed a gradual increase in the glass transition temperatures (Tg) with increasing AGE content up to 50 wt% (Fig. 1d). This ascending trend of Tg along with increasing comonomer content is similar to the trends observed in the bulk copolymerization of elemental sulfur with homobifunctional comonomers such as divinylbenzene10 and diisopropenylbenzene,6 suggesting that AGE is serving as a cross-linker for sulfur polymerization despite having only one double bond per molecule. Increase in AGE content beyond 50 wt% resulted in decreasing Tg, presumably due to the plasticizing effect of dangling or unreacted AGE molecules present in the polymer matrix.27 When AGE content was increased to 80 wt%, the resulting reaction mixture did not vitrify and remained a non-viscous liquid, and Tg could not be measured.
Inverse vulcanization involves the reactions of sulfur radicals generated through thermal homolytic ring-opening of cyclooctasulfur.6 While the radicals from the ring-opening of sulfur readily add to olefins, hydrogen atom abstraction that results in the formation of nucleophilic radicals is also a viable reaction pathway, given that sulfur radicals are electrophilic. Since allyl radicals are nucleophilic,28 abstraction of allylic hydrogen of AGE by sulfur is favorable. The reaction would produce polysulfanes in situ, which, like thiol, is able to attack the epoxide ring of the glycidyl group. A similar sequence of reactions had been proposed in a mixture of 2-allylphenol and bisphenol-A diglycidyl ether, which was cured at 190–210 °C with a small (∼7 wt%) amount of elemental sulfur.29 Based on the results and the reactivity of sulfur radicals, a possible mechanism of the polymerization of sulfur and AGE is proposed, where sulfur radicals participate both in addition reactions and hydrogen abstraction reactions with the allyl group of AGE. Compatibilization of sulfur and AGE is attributed to the formation of a polymer from the radical addition reaction of sulfur radicals to the allyl double bonds. The polysulfane resulting from the hydrogen abstraction reaction subsequently reacts with the epoxide, resulting in a vitrified network polymer (Scheme 2).
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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. |
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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. |
The 1H–13C cross-polarization (CP) spectrum (Fig. 3b) exhibits rather broad signals, which suggests a lack of crystallinity. Signal buildup under CP (Fig. S2†), as well as additional direct excitation experiments with short recycle delays (Fig. S3†), point to a very low local molecular mobility and a rather uniform proton density. This suggests a high degree of cross-linking. Also, the absence of significant differences between the CP and the fully relaxed direct polarization spectra confirms that the CP spectrum can be considered quantitative to a good approximation (Fig. S4†). The absence of resonances around 130 ppm also indicates the nearly complete consumption of vinyl double bonds. The resonances at 48.1 ppm and 41.6 ppm could be attributed to C–S connectivity, i.e. R2C–S and RC–S, respectively. The intense resonances at 70 ppm and 73.8 ppm, within the same broad peak, could be attributed to ring-opened epoxides, i.e. R2C–OH and R(OH)C–O, respectively, consistent with Scheme 2a. A spinning sideband from that resonance is clearly visible around 138 ppm. Because of the relatively broad resonances, the possibility of remaining epoxide rings cannot be fully excluded. The resonance around 18 ppm could be due to terminal methyl groups formed according to the substructure 3 (Scheme 2b).
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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%. |
SMPs constitute a class of stimuli-responsive polymers that revert to a certain shape (permanent shape) from a temporarily deformed shape upon exposure to appropriate stimulus such as heat.30,31 Interestingly, we found that free-standing films of S–AGE exhibit thermally induced dual-shape memory behavior with Tg as the transition temperature (Ttrans). The dual-shape memory effect was quantitatively studied through dynamic mechanical analysis (DMA) under single-cantilever bending mode, using S–AGE with 50 wt% sulfur (Fig. 5a).
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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). |
The dual-shape memory cycle consisted of (1) gradual deformation of the film to its maximum displacement (εm) under 55 °C (∼20 °C higher than Ttrans) followed by cooling the film to 5 °C to fix the temporary shape, followed by (2) removal of the deforming force and subsequent heating of the sample to 60 °C to induce shape recovery. The displacement, stress, and temperature profiles were recorded with respect to time. The shape fixity (Rf), which is an important performance factor of SMPs, was calculated as the ratio of εu (unloading strain) to εm (maximum strain) (εu/εm). S–AGE film was found to exhibit excellent shape fixity of 95%. Also, the strain at the end of recovery (εr) approached zero, which means the copolymer showed a nearly complete (100%) shape recovery ratio (Rr = (εm − εr)/εm) as seen in the displacement curve. In addition to quantitative DMA, a manual bending cycle test was also conducted using a film having dimensions of 90 mm × 12 mm and 1 mm thickness to test the durability of S–AGE as an SMP (Fig. 5b and Video S1.† Spontaneous bending of 90 mm × 12 mm and 1 mm thick S–AGE (50 wt% sulfur)). The S–AGE film maintained excellent shape fixity (Rf > 95%) and recovery ratio (Rr ∼ 100%) over 200 cycles of repetitive bending to 180° followed by full recovery, demonstrating a reliable shape memory performance of poly(S-r-AGE).
Since sulfur–sulfur bonds are often utilized in dynamic covalent bond chemistry32,33 and applied in self-healing and other thermoresponsive polymers,34–36 we envisioned that the sulfur-rich S–AGE could readily undergo reprogramming into a new permanent shape through facile S–S bond rearrangement. To demonstrate the reprogrammability of S–AGE SMP, S–AGE film was fashioned into a cube layout (Fig. 5c, flat). Then, the film was folded into a cube and annealed in a convection oven held at 90 °C for 30 minutes to induce thermal S–S bond rearrangements to fix a new permanent shape (Fig. 5c, cube). It should be noted that aluminum foil was used as a substrate to prevent unintended shape recovery during annealing. The aluminum foil was then removed by submerging the cube in aqueous HCl. High chemical resistance, typical of high sulfur-content polymers, allowed for a clean removal of the substrate without affecting physicochemical properties of S–AGE. Shape memory cycle test using the cube gave excellent shape memory behavior, with the new permanent shape being recovered from the flat layout within 8 seconds (Fig. 5c). These results demonstrate the merit of using S–AGE as SMP, since the permanent shape can be easily tuned with simple heat treatment.
Another advantage of the high sulfur content in S–AGE SMP manifested in the reprocessability of the material (Fig. S5†). Hot-pressing ground powder of S–AGE at 150 °C gave a monolithic specimen which exhibited nearly identical shape memory performance to cast films. Despite the high crosslink density of S–AGE with 50 wt% AGE which renders the material with high tensile strength, the presence of dynamic S–S bonds allows the material to be readily recycled.
Lastly, it should be noted that, although many different classes of shape memory polymers (SMP) have been carefully designed and developed, preparation of such SMPs are often complicated and involve multiple components polymerized via multistep procedures. Many SMPs are made of phase-segregated multi-block copolymer structures consisting of hard segments as physical cross-links and soft segments as thermoresponsive domains for shape-memory actuation.37–39 The S–AGE described herein is prepared from a one-step, solvent-free bulk polymerization, which, together with its unprecedentedly high strength, excellent performance as a SMP, reprogrammability, and reprocessability, render the new material worthy of further investigations and applications.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental procedures, supplementary figures and videos. See DOI: 10.1039/d1sc05896g |
‡ These authors contributed equally. |
§ Current address: Battery R&D Center, Samsung SDI, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea. |
¶ Current address: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. |
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