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.

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.

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.

High-strength biodegradable polymer composites have potential applications in a variety of biomedical applications. This study investigates the influence of silk fiber on the properties of the commonly used biodegradable polylactic acid-based composites, focusing on mechanical, rheological, morphological, and degradation characteristics. Mechanical tests revealed that the addition of 2.5 wt% silk fibers enhanced the ductility of PLA composites, increasing tensile strain at break from 7.39% for pure PLA to 11.51% for the composite. However, higher silk contents (≥10 wt%) resulted in lower elongation at breaks but higher moduli, indicating a trade-off between flexibility and the structural rigidity of the composite. Rheological tests demonstrated that the presence of silk fibers up to 7.5% improved the storage modulus, reflecting better network formation within the PLA matrix. Scanning Electron Microscopy (SEM) photomicrographs illustrated improved fiber dispersion, while higher contents introduced voids and stress concentrations, adversely affecting mechanical properties. Degradation tests in phosphate-buffered saline at 37 °C showed that silk additions slowed PLA degradation, suggesting controlled degradation suitable for biomedical applications. The optimal silk fiber content for balancing mechanical integrity and flexibility was identified to be ca 7.5 wt%, providing insights into the design of PLA/silk composites for enhanced performance in practical applications.