Inverse vulcanization
Inverse vulcanization is a solvent-free copolymerization process, firstly developed at the University of Arizona in 2013. Because of the high global production of sulfur as a by-product from the crude oil and natural gas refining processes, new methodologies to exploit this resource are under investigation. The inverse vulcanization allows to synthetize a low cost and chemically stable sulfur-rich material, which has different applications such as lithium-sulfur batteries, mercury capture and infrared transmission.
Synthesis
This process is based on a typical property of sulfur which is the catenation. In chemistry, catenation is the bonding of atoms of the same element into a series, called a chain. Hence, from a chemical point of view, the inverse vulcanization is similar to the sulfur based crosslinking, i.e. vulcanization, of an unsaturated elastomer, such as natural rubber. The product of inverse vulcanization is made by long sulfur linear chains linked together by organic molecules. This is a great difference with the crosslinking networks arising from vulcanization, which are based on short sulfur bridges, even made by one or two sulfur atoms.The polymerization process consists in the heating of elemental sulfur above its melting point, in order to favor the ring-opening polymerization process of the S8 monomer, occurring at 159°C. As a result, the liquid sulfur is constituted by linear polysulfide chains with diradical ends, which can be easily bridged together with a modest amount of small dienes, such as 1,3-diisopropylbenzene, 1,4-diphenylbutadiyne, limonene, divinylbenzene, dicyclopentadiene, styrene, 4-vinylpyridine, cycloalkene and ethylidene norbornene, or longer organic molecules as polybenzoxazines, squalene and triglyceride.
Chemically, the diene carbon-carbon double bond of the substitutional group disappears, forming the carbon-sulfur single bond which binds together the sulfur linear chains.
The huge advantage of such a polymerization is the absence of a solvent : sulphur acts as comonomer and solvent. This makes the process highly scalable at the industrial scale. As evidence, the kilogram-scale synthesis of the poly has been already correctly accomplished.
Products. Characterization and properties
Vibrational spectroscopy was performed to investigate the chemical structure of the copolymers: the presence of the C-S bonds was detected through Infrared or Raman spectroscopies. The high amount of S-S bonds makes the copolymer highly IR inactive in the near and mid-infrared spectrum. As a consequence, sulfur-rich materials made via inverse vulcanization are characterized by a high refractive index, whose value depends again upon the composition and crosslinking species.As shown by the thermogravimetric analysis, the copolymer thermal stability increases with the amount of the added crosslinker; in any case, all the tested compositions degrade above 222°C.
Focusing on the mechanical features, the copolymer behavior, included the glass-transition temperature, depends upon composition and crosslinking species. For given comonomers, the behavior of the copolymers as a function of the temperature depends on the chemical composition, for example, the poly behaves as a plastomer for a diene content between 15-25%wt, and as a viscous resin with the 30-35%wt of DVB. On the other hand, the poly acts as thermoplastic at 15-25%wt of DIB, while it becomes a thermoplastic-thermosetting polymer for a diene concentration of 30-35%wt.
The possibility to break and reform the chemical bonds along the polysulfides chains allows to repair the copolymer by simply heating above 100°C. This feature increses the reforming and recyclability of the high molecular weight copolymer.
The high amount of S-S bonds makes the copolymer highly IR inactive in the near and mid-infrared spectrum. As a consequence, sulfur-rich materials made via inverse vulcanization are characterized by a high refractive index, whose value depends again upon the composition and crosslinking species.
Applications
The sulfur-rich copolymers made via inverse vulcanization can be applied in many technological fields, thanks to the simple synthesis process and their thermoplasticity.Lithium-sulfur batteries
This new way of sulfur processing has been exploited for the cathode preparation of long-cycling lithium-sulfur batteries. Such electrochemical systems are characterized by a greater energy density than commercial Li-ion batteries, but they are not stable for a long service life. Simmonds et al. first demonstrated an improved capacity retention for over 500 cycles with an inverse vulcanization copolymer, suppressing the typical capacity fading of sulfur-polymer composites. Indeed, the poly, briefly defined as poly, showed a higher composition homogeneity compared with other cathodic materials, together with a greater sulfur retention and an enhanced adjustment of the polysulfides volume variations. These advantages made possible to assembly a stable and durable Li-S cell. After that, other copolymers via inverse vulcanization were synthetized and tested inside these electrochemical devices, again providing exceptional stability over cycles.| Cathode | Date | Source | Specific Capacity after cycling |
| Poly | 2014 | University of Arizona | 1005mA⋅h/g after 100 cycles |
| Poly | 2015 | University of Arizona | 800mA⋅h/g after 300 cycles |
| Poly | 2016 | University of the Basque Country | 700mA⋅h/g after 500 cycles |
| Poly | 2016 | University of the Basque Country | 616mA⋅h/g after 200cycles |
| Poly | 2016 | Istanbul Technical University | 400mA⋅h/g after 50 cycles |
| Poly | 2017 | University of Arizona | 485mA⋅h/g after 1000 cycles |
In order to overcome the great disadvantage related to the material low electrical conductivity, researchers started to add special carbon-based particles, to increase the electron transport inside the copolymer. Furthermore, such carboneciuous additives improve the polysulfides retention at the cathode through the polysulfides-capturing effect, increasing the battery performances. Examples of employed nanostructures are long carbon nanotubes, graphene and carbon onions.
Mercury capture
Sulfur element is chemically compatible with many metallic cations, forming sulfides or sulfates species. This feature could be exploited to remove toxic metals from soil or water. However, pure sulfur cannot be employed to manufacture a functional filter, because of its low mechanical properties. Therefore, inverse vulcanization was investigated to produce porous materials, in particular for the mercury capturing process. The liquid metal binds together with the sulfur-rich copolymer, remaining mostly inside the filter. Mercury is dangerous for the environment and highly toxic for humans, making its removal fundamental.Infrared transmission
Polymers are scantily used for IR optical applications because of their low refractive index ; their poor transparency towards the infrared radiation limits their exploitation in this sector. On the other hand, inorganic materials are characterized by high-cost and complex processability, detrimental factors for the large-scale production.Sulfur-rich copolymers, made via inverse vulcanization, represent a great alternative thanks to the simple manufacturing process, low cost reagents and high refractive index. As mentioned before, the latter depends upon the S-S bonds concentration, leading to the possibility of tuning the optical properties of the material by simply modifying the chemical formulation. Such possibility of changing the material refractive index to fullfil the specific application requirements, makes these copolymers applicable in the military, civil or medical fields.