Photo-crosslinkable, self-healable and reprocessable rubbers
Graphical abstract
Introduction
Natural and synthetic rubbers are polymers widely used in various fields such as tires, shoes, gloves and seals, due to their unique elasticity and mechanical strength. For these vital applications, rubbers need to be vulcanized or crosslinked with the formation of irreversible three-dimensional networks. However, the vulcanization always has been considered to be a serious energy-intensive and time-consuming process [1], [2]. Compared with thermally-induced crosslinking, the photo-crosslinking process is expanding rapidly on an industrial scale in coating, adhesive, photoresist, microelectronic and 3D printing, owing to its unique advantages like low energy consumption, fast curing rate, high efficiency and broad adaptability [3]. Therefore, if the photocrosslinking technology could be applied to rubber industry, the energy consumption and low efficiency would be effectively resolved.
To date, the principal approach to develop photocrosslinkable rubber or elastomer lies in the introduction of UV-curable or photosensitive groups into rubber chains. As the well-known photosensitive group, acrylates are grafted onto natural rubber by acrylating epoxidized natural rubber with acrylic acid to produce acrylated natural rubber [4], [5]. Besides, the inactive double bonds in rubber backbones are also photopolymerized to form tridimensional networks when acrylate monomers are presented [4], [5], [6], [7]. As an alternative measure, cinnamoyl modified natural rubber and butyl rubber likewise can be fleetly cured upon exposure to UV light through [2 + 2] cycloaddition mechanism [8], [9], [10].
In addition, UV-induced thiol-ene photopolymerization is also used as crosslinking agents for photocrosslinking between double bonds in natural rubber, isoprene rubber [11], carboxylated nitrile butadiene rubber [12], polybutadiene rubber [13], styrene/butadiene/styrene elastomer [14], [15], [16], butadiene-acrylonitrile copolymers [17] or norbornene-functionalized poly(glycerol sebacate) elastomer [18] and multi-thiols. Comparatively, the thiol-ene photopolymerization is a much more efficient way to crosslink elastomer than the copolymerization of double bonds with acrylate monomers [16]. The aforesaid studies have clearly indicated that mechanical properties of the photocured rubbers relied mainly on the dosages of crosslinking agent, however, the thiol-ene photopolymerization kinetics has not been explicitly revealed in site.
The irreversible crosslinked networks endow rubber products with excellent properties, but in the meanwhile they also render the rubbers insoluble and infusible. Thus, the crosslinked rubbers are hard to be reprocessed or recycled as thermoplastics. Postconsumer rubbers had to be mainly dumped as landfills, incinerated as a fuel or reclaimed as fillers or additives to produce menial items [19]. Nevertheless, these dispositions maybe still result in serious environmental burden and significant loss of value [20]. A sustainable solution is therefore urgently required for recycling waste rubber products.
Constitutional dynamic chemistry involves molecular structures of constructing dynamic crosslinked networks based on reversible physical interaction or chemical reaction, in which the crosslinkages are capable of reversibly breaking and reforming, usually under equilibrium control [21], [22]. Taking advantage of these special features, self-repairing or healing, controllable stress reduction or shape change, and recycling or reprocessing of conventional thermosets are realized [23], [24], [25]. A variety of constitutional dynamic chemistries, such as Diels-Alder (DA) reaction [26], [27], [28], [29], alkoxyamine moieties [30], [31], disulfide metathesis [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], transesterification reaction [42], [43], [44], olefin metathesis [45], [46], ionic association [47], [48], [49], [50], [51], supramolecular chemistry [52], [53], [54], [55] and metal-ligand interaction [56], [57], [58], [59], [60] have been incorporated into rubber or elastomer networks, imparting them with intrinsic self-healing and reprocessing properties via rearranging the crosslinked networks between contacted interfaces. Nevertheless, either the synthesis of the self-healable/recyclable rubbers is specially designed and quite complicated, or the self-healing/reprocessing process is mostly activated by heat that is less economical than light. If the self-healable and reprocessable rubbers are prepared directly by commercially available materials, and meanwhile the crosslinking, healing and recycling are driven by light, it would be a far more facile and sustainable strategy. However, limited attention has been paid to this aspect to date.
In the present work, a facile method is designed to develop rubbers that can be vulcanized, self-healed and reprocessed by exposure to UV irradiation. The rubbers are produced from commonly available polybutadiene rubber (BR) and liquid polysulfide (containing both disulfide bonds and thiol end groups) via thiol-ene photopolymerization. The photopolymerization kinetics are in situ characterized by real-time infrared spectrum (RT-IR), photo-differential scanning calorimeter (photo-DSC) and photodynamic rheology. Moreover, the dynamic reversibility and kinetics of photo-activated disulfide metathesis are also carefully studied. Meanwhile, due to the UV-induced disulfide metathesis on the interface, the resultant rubbers can rearrange the crosskinked networks, being endowed with eminent capacities of self-repairing, reprocessing and recycling. The influence of the content of disulfide linkages, irradiation intensity and time on the self-healing, reprocessing and recycling abilities is analyzed. It is believed that the photo-crosslinkable, self-healable and reprocessable rubbers would be conducive to energy saving, lifetime extension and reduction of waste of rubber products.
Section snippets
Materials
Commercially available cis-1,4-polybutadiene rubber (trade name: BR 9000, Mn = 72402, Mw = 205957 and PDI = 2.84 detected by GPC, the molar fraction of vinyl = 1.61 mol/100 g rubber as determined by 1H NMR) was purchased from Beijing Yanshan Petrochemical Co. Ltd., Sinopec, China. Liquid polysulfide polymer (trade name: G4, brown liquid, viscosity (25 °C) = 1.1 Pa·s, average molecular weight <1100 g/mol, SH content = 5.0–7.0%, sulfur content = 37–38%) was kindly provided by Akzo Nobel
Results and discussion
It has been known that free radicals are generated in the process of UV-induced disulfide metathesis [39], [62], [63]. Zhu et al. also certified that the exchange reaction of diselenide belongs to the same family (VI A) with disulfide following [2 + 1] radical-mediated reaction mechanism [64]. Matyjaszewski et al. further testified that the reshuffling of thiuram disulfide under the stimulation of visible light follows degenerative radical transfer and radical crossover reaction mechanisms [65]
Conclusions
Commercially available polybutadiene rubber can be photo-crosslinked with liquid polysulfide polymers as crosslinker and Darocur 1173 as photoinitiator in virtue of thiol-ene photoclick reaction. The photocuring kinetics exhibits that the photo-crosslinking can be realized within a few seconds and shows excellent spatiotemporal control. According to model disulfide exchange reaction of small molecule, disulfide metathesis quickly reaches its reaction equilibrium within minutes at UV intensity
Acknowledgment
The authors thank the support from the National Natural Science Foundation of China (Grants: 51333008, 51773229 and 21604014).
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