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Sulfur-based polymers are unique renewable materials that are receiving a growing attention. The utilization of elemental sulfur with a variety of monomers in their preparation in the absence of solvents using the inverse vulcanization are granting them green nature and unique properties. Several characterization techniques have been used to evaluate the properties of sulfur-based polymers. However, the complex structure and lack of solubility undermine the applicability of some standard characterization techniques in the usual manners. This article reviews the characterization methods used for the evaluation of various properties of sulfur-based polymers such as chemical, morphological, structural, thermal, rheological and mechanical properties, all of which vary depending on the type of comonomer involved in the reaction and heat treatment conditions. The successful applications of different characterization techniques including Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, nuclear magnetic resonance (NMR), scanning electron microscopy/X-ray energy dispersion (SEM-EDX), X-ray diffraction (XRD), mechanical tester, rheometer, thermal gravimetric analyzer (TGA) and differential scanning calorimetry (DSC) are discussed. The challenges to the evaluation of the properties of sulfur-based polymers and the innovative applications of the conventional techniques to overcome them are also deliberated.

Vegetable oils are a promising class of bioresources for producing green polymeric materials to reduce the dependence on petro-based polymers. In this study, a green copolymer prepared by thermal copolymerization corn oil with sulphur at its molten state is reported for the first time. The proportions of sulphur to corn oil (w/w%) in the reaction mixture were varied in the range of 50/50 to 80/20 and the reactions were carried out at 170°C for 1 h. The obtained copolymers were characterized using Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and powder X-ray diffraction (PXRD). The percentage of the unsaturated fatty acid portion was found to act as a multifunctional monomer stabilizing polysulphide forming crosslinked structures that vary depending on reactant sulphur content. The obtained copolymers were found to be amorphous thermosets with heavily crosslinked structures and composite morphologies. The copolymers also showed high thermal stability under nitrogen atmosphere. The new copolymers are environmentally friendly hybrid material promoting green chemistry with a potential added value to abundantly available sulphur and corn oil.

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Inverse vulcanization is a facile solvent‐free process, which offers interesting sustainable copolymers from the reaction of sulfur with petro‐based monomers or edible vegetable oils. However, sulfur reaction with the former contradicts green chemistry, whereas the latter reduces the viability of the product and can contribute to the food crisis. Herein, we report the preparation of sulfur‐based polymer (SBP) by the reaction of rubber seed oil, RSO (a non‐edible oil), to produce a sustainable sulfur‐based copolymer for the first time. The properties of the new polymer were evaluated using different techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy dispersive X‐rays (SEM‐EDX‐mapping), powdered X‐ray diffractometer (p‐XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The polymer was found to be soluble in tetrahydrofuran, thermally stable to 200 °C, and a low glass transition temperature (−6.41 to −7.85 °C for a polymer with 50 to 70 wt % S). The polymer morphological and DSC analysis demonstrated a uniform surface possessing a small amount of unreacted microscale sulfur particles that is lesser than similar polymers from other oils, which was confirmed by DSC. The P‐XRD analysis revealed the amorphous nature of the copolymer caused by a heavily crosslinked structure. The effect of the post‐polymerization treatment on the properties of the copolymers was also investigated which revealed that increasing the curing temperature or quenching medium temperature increases the glass transition temperature of the copolymer. The polymer properties were dramatically improved by reducing the amount of the unreacted sulfur by the addition of a small amount of 1,3‐diisopropeynyl benzene (crosslinker), leading to 99.75 % sulfur conversion, the highest ever value achieved in such SBPs. It can be concluded that the use of RSO with sulfur enhances the sustainability of SBP and promotes their adding products

Global enhancement of crop yield is achieved using chemical fertilizers; however, agro-economy is affected due to poor nutrient uptake efficacy (NUE), which also causes environmental pollution. Encapsulating urea granules with hydrophobic material can be one solution. Additionally, the inverse vulcanized copolymer obtained from vegetable oils are a new class of green sulfur-enriched polymer with good biodegradation and better sulfur oxidation potential, but they possess unreacted sulfur, which leads to void generations. In this study, inverse vulcanization reaction conditions to minimize the amount of unreacted sulfur through response surface methodology (RSM) is optimized. The copolymer obtained was then characterized using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). FTIR confirmed the formation of the copolymer, TGA demonstrated that copolymer is thermally stable up to 200 °C temperature, and DSC revealed the sulfur conversion of 82.2% (predicted conversion of 82.37%), which shows the goodness of the model developed to predict the sulfur conversion. To further maximize the sulfur conversion, 5 wt% diisopropenyl benzene (DIB) as a crosslinker is added during synthesis to produce terpolymer. The urea granule is then coated using terpolymer, and the nutrient release longevity of the coated urea is tested in distilled water, which revealed that only 65% of its total nutrient is released after 40 days of incubation. The soil burial of the terpolymer demonstrated its biodegradability, as 26% weight loss happens in 52 days of incubation. Thus, inverse vulcanized terpolymer as a coating material for urea demonstrated far better nutrient release longevity compared with other biopolymers with improved biodegradation; moreover, these copolymers also have potential to improve sulfur oxidation.

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The world is witnessing a growing interest in the utilization of waste materials to reduce environmental burden, add product value and promote sustainability. Such noble goals are being supported by the implementation of a number of approaches, including green chemistry. Currently, accumulation of large amounts of waste elemental sulfur, a by-product produced in gas and petroleum refineries, is posing an environmental concern and requires immediate intervention to search for new routes to make high-value materials from it. Inverse vulcanization is a new copolymerization technique allowing formation of stable sulfur-based polymers (sulfur copolymers) against the depolymerization of sulfur chains. Such a new class of polymers has demonstrated great properties for several environmental and renewable-energy applications. This paper reviews the advances in the evolution of sulfur copolymers prepared by inverse vulcanization with various monomers, including not only petro-based monomers but also renewable monomers as well as vegetable oils. Several applications of the obtained sulfur-based polymers are discussed. The challenges and future research directions to develop new polymers further and extend their applications are elaborated.