Structure–Property Relationships and Mechanical Performance of Inverse Vulcanized Sulfur-Soybean Oil Polymer Films and Their Biochar-Reinforced Biocomposites

In this work, a series of inverse vulcanized biopolymers (BP) based on elemental sulfur and soybean oil were synthesized with varying sulfur content (20–80 wt %). Also, using reactive molding films of the polymers was obtained (F-BP). The effects of the sulfur content on the mechanical and physicochemical properties of F-BP materials were systematically evaluated. Mechanical tests revealed that increasing sulfur content significantly enhanced stiffness and tensile strength, attributed to the formation of denser S–S cross-linked networks. In addition, ecofriendly biocomposite films (F-BP-C) were developed by reinforcing the polymer matrix with biochar derived from carbonized barley biomass. The incorporation of biochar improved the mechanical response, increasing both the stiffness and flexural strength. In particular, a 25% weight loading of biochar (F-BP-C25) led to improvements of up to ∼2.8 times in Young modulus and ∼4 times in flexural strength, demonstrating its function as a reinforcing material within the polymer network. Furthermore, we found that the porosity and surface friction of the materials can be adjusted based on the formulation, allowing for tailored properties depending on the application. F-BP60-C25 showed the best combination of mechanical, thermal, and surface properties among the formulations studied, positioning it as a promising candidate for use as a functional coating material with thermal insulation capabilities. This integrated mechanical–thermal response of polymeric materials highlights their strong potential for thermal insulation and surface-related applications where mechanical robustness is essential. Moreover, the development of such materials through environmentally friendly approaches, particularly by utilizing renewable and biobased resources, further enhances their value, aligning with global efforts toward sustainable materials science and reducing environmental impact.