Inverse Vulcanized Polymers with Shape Memory, Enhanced Mechanical Properties, and Vitrimer Behavior

Abstract The invention of inverse vulcanization provides great opportunities for generating functional polymers directly from elemental sulfur, an industrial by‐product. However, unsatisfactory mechanical properties have limited the scope for wider applications of these exciting materials. Here, we report an effective synthesis method that significantly improves mechanical properties of sulfur‐polymers and allows control of performance. A linear pre‐polymer containing hydroxyl functional group was produced, which could be stored at room temperature for long periods of time. This pre‐polymer was then further crosslinked by difunctional isocyanate secondary crosslinker. By adjusting the molar ratio of crosslinking functional groups, the tensile strength was controlled, ranging from 0.14±0.01 MPa to 20.17±2.18 MPa, and strain was varied from 11.85±0.88 % to 51.20±5.75 %. Control of hardness, flexibility, solubility and function of the material were also demonstrated. We were able to produce materials with suitable combination of flexibility and strength, with excellent shape memory function. Combined with the unique dynamic property of S−S bonds, these polymer networks have an attractive, vitrimer‐like ability for being reshaped and recycled, despite their crosslinked structures. This new synthesis method could open the door for wider applications of sustainable sulfur‐polymers.


Introduction
Sulfur, as aby-product of the purification of crude oil and gas reserves,i sw idely available and low cost. [1] The" excess sulfur problem" has drawn the attention of material researchers looking to exploit this underused resource. [2,3] On the basis of ring-opening polymerization (ROP) of molecular sulfur rings,r esearchers have created new materials directly from elemental sulfur to alleviate this problem. To date,m ultiple routes for producing sulfur-polymer materials directly from waste sulfur have been proposed, including the reaction of thiols with elemental sulfur, [4,5] the reaction of element sulfur with p-diiodobenzene, [6,7] multicomponent polymerizations (MCPs) of sulfur with other molecules, [8,9] sulfur radical transfer and coupling (SRTC) reaction with benzoxazine compounds, [10] and inverse vulcanization of sulfur with vinyl groups. [11][12][13][14][15][16] Among those methods," inverse vulcanisation", coined by Pyun et al. in 2013, [11] has gained much attention for its outstanding benefits:s imple,s olvent-free,a nd high utilization of sulfur.I ti sn otable that inverse vulcanised polymers show various advantageous functions,like mercury capture, [12,13] self-healing capability, [17,18] optical application, [19,20] electrochemical properties, [21,22] and antimicrobial properties. [23] However,p oor mechanical properties of these exciting new materials currently limit their wider application and scale of use.A lso,t here is still little literature on the mechanical properties of inverse vulcanized polymers.A ccording to the available reports, [11,17,18,[24][25][26][27][28][29][30] most materials show aq uite low strength compared to conventional polymers.T he reported highest stress of this material is 8.69 MPa of copolymer poly(S-DIB), which means that not much force is required to break the polymers.The change of crosslinkers seem to be the mostly reported method used for improving the related mechanical properties.E ither using an ew crosslinker or blending two different crosslinkers,t he rigidity modulus of sulfur-polymers could be modified from high to low,b ut the strength is still always low.T his means that polymers can be made either stiff or flexible,but not strong. From asynthetic perspective,c onstraining ourselves to ao ne-step polymerization routes limits the opportunities for realizing multiple performance adjustments.
In ao ne-step inverse vulcanisation, it is impossible to control the material properties by adjusting the amount of ac rosslinker if the percentage of sulfur is set at ac ertain desired value.Additionally,once the crosslinking reaction has begun, the crosslinking degree of the polymer is difficult to control. These disadvantages result in few possibilities for significant alteration in physical performances of the polymers through simple replacement of the crosslinker.I nt his study,w ee xplore an ovel synthetic approach that greatly improves the mechanical properties of sulfur-polymers and could increase opportunities for practical applications.I f al inear sulfur-polymer containing reactive chemical groups could be prepared from inverse vulcanization, then an ew network polymer could be obtained through athird monomer crosslinking this pre-polymer.I nt his Scheme,a na lternative kind of chemical bond can be introduced into the sulfurpolymer,w ith the potential to improve its mechanical properties or endow additional functions.H ence,at wo-step polymerization was considered as am ethod to achieve controllable mechanical properties of sulfur-polymers by providing the pre-polymer scope for further modifications. Tsutsumi et al. investigated as imilar strategy for the modification of sulfur-polymers,but they were focused on improving the electrochemical properties of polymers in Li-S batteries application, without characterisation of the physical properties of the materials. [31] Recently,w es howed that at ernary co-polymer system allows delayed curing to be used, [32] which could aid practical production, by allowing al iquid pre-polymer to be transported, stored, and injected into asuitable mould before final setting.F rom the perspective of practical applications,t his one-step synthesis method was replaced with at wo-step method to generate sulfur-polymers,a nticipating that more promising materials could be obtained. Unlike previous work that has relied solely on crosslinking by reaction of sulfur with C=Cb ond positions,h ere we employ ac ombination of two distinct chemistries:s ulfur addition to alkene groups,a nd reaction of isocyanates with alcohols to form urethane linkages.H ence,w ed emonstrate that the designed linear polymer formed from sulfur is chemically stable and could be stored at room temperature for long periods of time,and then it could be further modified by the second urethane forming step into ac rosslinked polymer.B ya djustment of the isothiocyanate crosslink density we demonstrate dramatic increases in the mechanical properties,h ardness and solubility of the resultant material. Owing to the unique chemical nature of the produced polymer,s ome of the samples show asignificant shape memory ability.Inaddition, SÀSdynamic bonds give the crosslinked sulfur-polymer multi dynamic functions,s uch as reprocess-ability of polymer network, comparable to vitrimers. [33][34][35][36][37]

Results and Discussion
Element sulfur exists primarily in the form of an eightmembered ring (S 8 ), which melts on heating and forms polymeric sulfur chains above its floor temperature (159 8 8C) through ar ing opening polymerization process. [9] However, polymers made purely from sulfur are not stable,a nd depolymerize back to S 8 ,e ven at room temperature through ab ack biting mechanism. Based on this principal, ah ighly crosslinked polymer could be obtained after the addition of small vinylic monomers into liquid sulfur, where they react with the growing sulfur-polymers,a nd act to stabilize the material against de-polymerization. That is the foundation of "inverse vulcanization". As mentioned above,our aim was to explore more potential properties and wider applications of sulfur-polymers using two-step polymerization instead of onestep method. In this work, span 80 (Span), at rifunctional monomer containing ac arbon-carbon double bond, was selected to stabilize element sulfur to first form al inear prepolymer (S-Span) in the presence of catalyst zinc diethyldithiocarbamate (Zn (DTC) 2 )( Figure 1). As shown in Figure 1, this pre-polymer with hydroxyl groups on the side chains was further crosslinked by ad ifunctional monomer diphenylmethane 4, 4'-diisocyanate (MDI) to generate three-dimensional sulfur-polymers (S-Span-MDI-X). Here,Xwas defined by the molar ratio of -OH and -NCO,which can be found in Table 1. Detailed illustration and experimental procedures for synthesis of the polymers can be found in the Supporting Information (SI).
Considering the polymerization activity of two monomers, S 8 and Span, we need to prove the viability of this polymerization reaction first, and that both monomers are incorporated to form ac o-polymer. Nuclear magnetic resonance spectroscopy (NMR) was performed to monitor this inverse vulcanization reaction of S 8 with Span ( Figure 2). From the reference NMR spectrum of pure Span (Figure 2a), we can see that the peak (a) at d % 1.3 ppm belongs to -CH 2 groups in ten distinct environments,a nd the peak (b) at d % 5.3 ppm corresponds to the hydrogens adjacent to the C=C bond. Thanks to unreactive property of the former hydrogen in this reaction, the degree of reaction could be analyzed from comparing the change of integral ratio of these two peaks varying with reacting time.The integral ratio of the two peaks in the spectra (Figure 2b)v aried from 0.1:1, 0.05:1, 0.03:1 to 0:1. This result suggests that C=Cb onds were opened as the reaction progressed and finally could be fully consumed in the presence of catalyst Zn (DTC) 2 ,ar eported effective inverse vulcanization catalyst. [38] Before curing, when about 80 %o f double carbon bonds were consumed (after reacting for 1h with catalyst), the pre-polymer S-Span showed ar eversible gel-liquid transformation property and excellent chemical stability at room temperature.
Thep re-polymer S-Span could easily flow with low viscosity at high temperature (above 100 8 8C), but changed into ag el state with high viscosity and vitrified after cooling down to room temperature (about 20 8 8C) ( Figure S1). Figure 3a shows that the viscosity of pre-polymer S-Span decreases sharply with the increase of temperature under the shear rate control mode of the rheometer. In addition, this pre-polymer showed at ypical shear-thinning property corresponding to traditional linear polymer behavior,a nd as ah igher temperature was applied less shear force was required to reduce the viscosity of the pre-polymer (Figures 3b,c and d). Moreover,t his pre-polymer can be stored at room temperature for long periods,w ithout separation or gelation. From observation using NMR, Differential scanning calorimetry (DSC), Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR) and Gel permeation chromatography (GPC) ( Figures S2, S3, S4, S5 and Figure 4) during storage periods at room temperature,t his pre-polymer exhibited excellent chemical stability,w ithout any structural changes or molecular weight decrease during the observation periods (maximum of 30 days). Therefore, this pre-polymer could be stored, transported, and be ready for further modification as required.
It was further discovered that before the introduction of the crosslinker MDI into the polymerization system, prepolymer S-Span could be cured into alinear polymer poly (S-   Span) in the solid-state but lacking in shape persistence ability ( Figure S6). As shown by PXRD ( Figure S7), no detectable unreacted crystalline S 8 remained in the polymer after curing for above 3h.A ccording to FT-IR ( Figure S8), the peak belonging to stretching vibration of C=CÀHand C=CinSpan at 3080 cm À1 and 1600 cm À1 disappeared after curing, and there are new peaks at 465 cm À1 ,5 50 cm À1 ,a nd 660 cm À1 , suggesting that double carbon bonds were fully consumed and new C-S bonds had been formed. Additionally,a ss hown in Figure 4, there is an increase in weight average molecular weight (M w )w ith curing time.F or example,t he M w significantly increased from about 4000 gmol À1 before curing, to 11 151 gmol À1 after curing for 3h,a nd reached at 35 822 gmol À1 after curing for 6h with less change of molecular weight dispersity (M w /M n was in the range of 2.5-3) (Figure 3b). After about 20 hc uring,asolid-state linear polymer poly (S-Span) with good thermal stability formed ( Figure S9) due to the chain entanglement, as confirmed by thermogravimetric analysis (TGA). However,t his polymer showed poor shape-persistence ability at room temperature as its glass transition temperature (T g )ofÀ26.2 8 8Cistoo low to freeze the polymer chains at room temperature,observing from DSC curve ( Figure S10), and comparatively low molecular weight allows the solid to be deformed easily.
Motivated by the evidence that long sulfur-based polymer chains could be formed through S 8 reaction with Span, and that the pre-polymer can be stored at room temperature for along period, the next step was to use adesigned method of further modification of the pre-polymer.T hus,the polymers performance,like shape retention ability,physical properties, and other potential applications,c ould be explored and potentially controlled. So,adifunctional isothiocyanate,MDI was selected as the crosslinker to produce polymer networks. Thec rosslinked polymers S-Span-MDI-X, where Xi s1 ,2 ,3 and 4r esponding to the theoretical crosslinking degree of 100 %, 50 %, 25 %and, 12.5 %, respectively,were synthesized and characterized, which are discussed below.
Crosslinking density was controlled by adjusting the molar ratio of -NCO group,from MDI, and -OH group,from Span (Table 1a nd SI). Solid 13 CNMR spectra and FT-IR were performed to demonstrate that the expected structure of the polymer network had been obtained. Compared to the solution 13 CNMR spectrum of pure Span in Figure S11, the peak at % 130 ppm belonging to the carbon in the C=Cbond completely disappeared in the solid 13 CNMR spectrum of poly (S-Span) ( Figure S12). Meanwhile,anew peak at % 57 ppm was formed, which was attributed to the chemical shift of aC À Sb ond. That further supports the formation of sulfur-based polymer from the reaction of Span with S 8 . Afterward, the solid 13 CNMR spectra of crosslinked polymers were carried out. Taking the solid 13 CNMR spectrum of crosslinked polymer S-Span-MDI-4 as an example (Figure S13) to analyze,n ew peaks at % 57 ppm and % 154 ppm were attributed to the chemical shifts of C À Sb ond and -NHCOO-bond, respectively.C omparable 13 CNMR results with that of S-Span-MDI-4 were obtained for the other three crosslinked polymers ( Figure S14). Moreover,i nt he FT-IR spectra shown in Figure 5a,a ll the polymers show no unreacted C=Cb onds remaining.A lso,anew peak was observed at 1530 cm À1 corresponding to the amide vibration of -NHCOO-groups.T hese results confirm that double carbon bonds were oxidized and consumed, -NCO groups reacted with -OH groups and carbamate bonds were successfully introduced into the inverse vulcanized sulfur-polymer. PXRD (Figure 5c)f urther proved that no unreacted crystalline sulfur remained in the polymer networks.T herefore,i t can be concluded that the sulfur-polymer networks were successfully synthesized via at wo-step polymerization method, resulting in both SÀSand urethane crosslinks.
Solubility experiments were carried out to further characterize the formation of synthesized polymer network structures (SI). Theresults shown in Figure S15 indicate that as the theoretical crosslinking degree increases,the solubility of the polymers generally decreases.After processing the five polymers by the same procedure,t he linear polymer completely dissolved into tetrahydrofuran (THF), dimethylformamide (DMF) and chloroform forming at ransparent solution, but the crosslinked polymers showed at endencyf or insolubility varying from only partially soluble suspension to an insoluble swelled solid as the degree of theoretical crosslinking increased. Moreover,D SC (Figure 5b)s hows that the T g of the polymer increased to ahigher temperature, but decreased in intensity,w ith the increase of theoretical crosslinking degree (Table 1). This is explained by the higher degree of crosslinking constraining the polymer chains and requiring higher temperatures for free movement. As the glass transition is afeature of regions of linear polymer chains, increased crosslinking reduces the intensity of this transition. In addition, with the increase of the crosslinking degree,t he water contact angle of the polymer also increased. This increase in hydrophobicity is because the concentration of the hydrophilic group,-OH group,i nt he polymers decreases as the crosslinking degree increases.L oss of -OH groups is further demonstrated from the decrease of the peak at % 3400 cm À1 in FT-IR spectra as crosslinking increases (Figure 5a). TGA in Figure 5dillustrates that for all four polymer networks,asimilar T deg,5 % around 200 8 8Cw as obtained (Table 1). So,p olymer networks with designed crosslinking degrees were successfully obtained.
Generally,chemical crosslinking agents are considered to give enhanced mechanical properties to polymers.T ensile strength measurements were performed on the synthesized polymers,and strong theoretical crosslinking degree dependence was observed. Figure 6a shows typical strain-stress curves of sulfur-polymers,i ndicating that compared to the linear polymer,t ensile stress and Youngsm odulus of crosslinked polymers both have been significantly improved by the chemical crosslinking process.T he physical properties of the polymers were controlled, changing from flexible to stiff.The stress and strain at break were analyzed as af unction of theoretical crosslinking degree shown in Figures 6b and c. Stress increased from 0.14 AE 0.01 MPa to am aximum of 20.17 AE 2.18 MPa (a nearly 135-fold increase). This is attributed to the increase of crosslinking density,ascomplex threedimensional networks with ah igher crosslinking degree are not easily destroyed under an external applied force.H owever,after an increase of breaking strain from linear polymer (35.28 AE 0.98) to slightly crosslinked polymer (51.20 AE 5.75), the breaking strain of crosslinked polymer began adecreasing tendency with the increase of theoretical crosslinking degree from 51.2 %t o11.8 %. It was attributed that crosslinking structure restricts polymer chains from free movement and shape change during tensile deformation. Additionally,due to the difference in structure between the linear polymer and crosslinked variant, different fracture morphology was observed by scanning electron microscopy (SEM). It can be seen in Figure S17 that ac ross-section of the linear polymer is noticeably rougher than that of crosslinked polymers.M eanwhile,t he changing chemical structure endows different hardness to the polymers ( Table 1). As shown in Figure 6d, the hardness of the polymers increases from 17.7 AE 0.8 HD of linear polymer to 77.0 AE 1.5 HD of fully crosslinked polymer. It can be seen that the polymers show an obvious trend from soft to hard, as shown in photographs in Figure S18.
SÀSb onds have been demonstrated to show at hermally induced dynamic exchange reaction, and dynamic S À Sbonds have been widely reported to be applied in vitrimers to achieve recyclability of traditional thermoset polymers. [39][40][41] Additionally,a ni nverse vulcanized sulfur-polymer was recently reported as sfunctional crosslinker for epoxy thermosets to endow the epoxy material potential self-healing ability due to the particular property of dynamic SÀSb onds. [42] Hence,i ti sp lausible that our sulfur-polymers,t he backbone of which is formed of sulfur chains,s hould possess dynamic properties associated with disulfide vitrimers.T he dynamic property of crosslinked polymers induced by SÀSb onds was demonstrated by using dynamic mechanical analysis (DMA). Fully crosslinked polymers are not usually able to be reprocessed or recycled, as this would require the irreversible breakage of C À Cb onds and degrade the network. Whereas when dynamic chemical bonds are introduced into apolymer, it becomes reprocessable due to the potential for topological rearrangement caused by dynamic covalent chemistry.S o, here fully crosslinked polymer S-Span-MDI-1 was selected to characterize the stress relaxation behavior at varying temperatures.Ifpolymer S-Span-MDI-1 could be proved to show an obvious stress relaxation property,the other three crosslinked polymers with al ower crosslinking degree definitely should have the same ability.
Thestress relaxation characterization of polymer S-Span-MDI-1 was carried out at 120 8 8C, 130 8 8C, 140 8 8C, and 160 8 8C under control of strain at 1%. Figure 7a shows that stress was able to relax to zero at ahigh temperature within 5min. It was indicated that quicker relaxation happened at ah igher temperature.W hen the results were plotted in an Arrhenius plot, al inear correlation was obtained and the activation energy of 40.3 kJ mol À1 was calculated from the slope [SI Eq. (1)].T he other three crosslinked polymers show similar  dynamic properties to each other,b ya ppropriately controlling the temperature (Figure 7c). With the decrease of crosslinking degree,t he temperature required for similar stress relaxation time with S-Span-MDI-1 at 160 8 8Cdecreases. These results prompted further investigation into the recycling of crosslinked sulfur-polymers.R eprocessing experiments were carried out by cutting up the sulfur-polymers, before reforming them using ahot press (Figure 7d).
To be clear, the original samples were also formed by use of ahot press after oven molding, but are marked as pristine in Figure 8, as they had not been intentionally recycled. The exact values of breaking stress and strain of the crosslinked polymers before and after reprocessing are shown in Figures 8a and b. Thes tress decreased after every recycling step, with the highest recovery after the first cycle being 91.3 % recovery for S-Span-MDI-3 and highest after the second cycle 78.0 %r ecovery of S-Span-MDI-2 (Figure 8c). From the FT-IR spectra ( Figure S19) and PXRD curves ( Figure S2) of pristine and reprocessed samples,n os tructure change is observed after reprocessing experiments.T oe xplain the incomplete recovery of stress,wewould attribute some minor chemical degradation resulting from thermal stress exerted via the hot press.However,except for S-Span-MDI-1 after the second cycle,w hich showed ad ecrease in strain, all crosslinked polymers show asignificant strain increase after every time they are recycled (Figure 8d). That was considered to be caused by homogenization of the sulfur chain lengths during rearrangement in the thermal reprocessing process,a nd/or some breakage of the polyurethane crosslinks.B oth of these hypotheses are consistent with the slight decrease of T g after reprocessing evidenced from DSC ( Figure S21). Under the same testing temperature,t he polymer with lower T g can be further deformed without break as the polymer chains are more mobile under stretching force.S o, it is clear that the dynamic S À Sb onds endow recyclability to the crosslinked polymers whilst simultaneously preserving the crosslinked structure.
Thes hape memory function of traditional polymers has been studied for many years,i ncluding principles and applications. [43][44][45] However,t he shape memory of inverse vulcanized sulfur-polymers has not been reported to date.The limit of weak or stiff mechanical properties of such sulfurpolymers may be considered as the main challenge for not achieving shape memory function. We have discussed above that sulfur-polymers with controlled physical properties can be obtained. Among them, crosslinked polymers S-Span-MDI-1 and S-Span-MDI-2 show an excellent temporary shape maintenance effect at room temperature.A sp olymer S-Span-MDI-2 has asuitable T g of 28.2 8 8Cand is more flexible than S-Span-MDI-1 at room temperature,i tw as selected as an example to discuss that property.While studying the shape memory property of polymer S-Span-MDI-X, two related sulfur-polymer materials,m ade through different methods, were reported. [5,7] Tr aditionally,chemical crosslinking leads to thermoset shape memory polymers with robust shape memory but low recycling ability.H owever, the introduction of dynamic bonds provides the crosslinked shape memory polymers with the ability to also be recycled. Despite the reversible shape-memory transitions,t he polymer could also obtain an ew shape caused by topological rearrangement from the dynamic reaction of the reversible SÀSbonds.Here, the distinct elastic shape memory and plastic permanent reshape property of ac rosslinked sulfur-polymer was investigated.
As shown in Figure 9a,t he shape (a), ar ectangular film, was able to be reshaped, by heating above the T g and then  cooling, into at emporary shape (a1) that can recover when reheated. Ther ecovered shape (a) can be further deformed into apermanent shape (b) by heating to higher temperature (solid-solid transition temperature (T v )) allowing dynamic S À Sb ond exchange.T his second permanent shape (b), is then still capable of further temporary shape deformation and recovery,a sb efore (b1). After that, the recovered shape (b) can still be further reshaped to at hird distinct form (c) following again by areversible shape memory behavior (c1). Tw ov ideos of this process are available in the supporting information, corresponding to the reversible shape memory behaviors between shape (a) and shape (a1) and between shape (c) and shape (c1). Thef undamental principle of this distinct elasticity and plasticity property is the perfect combination of phase change and topological rearrangement induced by temperature.Asillustrated by Figure 9b,the film can be heated to its glass transition temperature first and then reshaped by an external force.T his temporary shape can be maintained by freezing its polymer chains after cooling down and can recover to the original shape by heating again. The dynamic SÀSbond exchange reaction, which can be triggered by heating up to the T v ,r esults in the topological rearrangement of the polymer network. So,apermanent shape of the film can be obtained through the thermally induced dynamic exchange reaction. And this behavior can be repeatedly carried out without loss of the temporary shape memory effects of the pristine material, suggesting that ac umulative reshaping of this sulfur-polymer network can be obtained as discussed above.

Conclusion
In summary,w ed esigned and synthesized pre-polymer and polymer networks,w ith variable degrees of crosslinking, directly using by-product elemental sulfur through atwo-step polymerization method. Thepre-polymer is chemically stable, is able to be stored at room temperature for long periods of time,a nd is ready for further modification when needed. By adjusting the degree of crosslinking during the second, chemically distinct, reaction step,the physical properties such as glass transition temperature,s olvent resistance,h ardness, contact angle,a nd mechanical properties of the polymers were effectively controlled. Thep olymers show ac lear tendency,varying from weak and soft to strong but hard with the increase of crosslinking degree.I na ddition, the polymer network with as uitable degree of crosslinking shows an excellent shape memory effect. Theunique dynamic property of S À Sb onds provides the synthesized sulfur-polymer networks with reprocessing and plasticity reshaping abilities.W e have realized enhancing the strength of sulfur-polymers (e.g. > 20 MPa tensile strength, an increase of % 135 times), but combining such high strength with high flexibility for sulfurpolymers is still challenging. There is still great potential for awider range of crosslinking degrees and crosslinking agents to be exploring as aw ay of further tuning the properties of sulfur-polymers to meet various practical needs.W eb elieve that the basic principles of this work can be expanded into arange of applications and other research areas.