Tannaz Alamfard

Cis-1,4-polyisoprene is a widely used elastomer that demonstrates particular thermal and mechanical characteristics, in which the latter is influenced by temperature and strain rate. Molecular dynamic simulations were used to obtain thermal conductivities, glass transition temperatures (T_g), and tensile deformation. Thermal conductivities were calculated by applying the Green–Kubo method, and a decrease in thermal conductivity was observed with increasing temperature. Density–temperature relations were used to calculate T_g, which indicates the transition from the glassy to the rubbery state of the material, and this temperature influences mechanical properties. Investigation of the mechanical properties under uniaxial tensile deformation for constant strain rates indicates an increase in the stiffness and strength of the material at lower temperatures, while increasing molecular mobility at higher temperatures results in reducing these properties. The influence of strain rates at constant temperature highlighted the viscoelastic nature of the structure; increasing strain rates resulted in increases in stiffness, strength, elongation at maximum strength, and elongation at break because of restricted molecular relaxation time. The united-atom force field contributes to higher computational efficiency, which is suitable for large-scale simulations. These results provide important information on the thermo-mechanical properties and tunability of cis-1,4-polyisoprene, which supports applications in the production of interactive fiber rubber composites.

The thermal conductivities and glass transition temperatures of polybutadiene crosslinked with randomly distributed sulfur chains having different lengths from mono-sulfur (S1) to octa-sulfur (S8) were investigated. The thermal conductivities of the related models as a function of the heat flux autocorrelation function, applying an equilibrium molecular dynamic (EMD) simulation and the Green–Kubo method, were studied for a wide range of temperatures. The influence of the length of sulfur chains, degree of crosslinking, and molar mass of the crosslinker on the glass transition temperature and final values of thermal conductivities were studied. First, the degree of crosslinking is considered constant for the eight simulation models, from mono-sulfur (S1) to octa-sulfur (S8), while the molar mass of the sulfur is increases. The results show that the thermal conductivities of the crosslinked structure decrease with increasing temperature for each model. Moreover, by increasing the lengths of the sulfur chains and the molar weight of the crosslinker, thermal conductivity increases at a constant temperature. The MD simulation demonstrates that the glass transition temperature and density of the crosslinked structure enhance as the length of the sulfur chains and molar mass of the sulfur increase. Second, the molar weight of sulfur is considered constant in these eight models; therefore, the degree of crosslinking decreases with the increase in the lengths of the sulfur chains. The results show that the thermal conductivities of the crosslinked structure decrease with the increase in the temperature for each model. Moreover, by increasing the lengths of sulfur chains and thus decreasing the degree of crosslinking, the trend in changes in thermal conductivities are almost the same for all of these models, so thermal conductivity is constant for a specific temperature. In addition, the glass transition temperature and density of the crosslinked structure decrease.

Thermal conductivities of polybutadiene crosslinked with sulfur as a function of the heat flux autocorrelation function by using an equilibrium molecular dynamic (EMD) simulation were investigated. The Green–Kubo method was used to calculate thermal conductivities. All simulations were performed by applying the LAMMPS software (version 3 Mar 2020) package. The united-atom force field (OPLS-UA) from the Moltemplate software (version 2.20.3) was applied in the simulations. The influence of uniform and random distributions of sulfur in polybutadiene on the final value of thermal conductivities was studied by polymeric model structures with similar and variable degrees of crosslinking. The results showed that for identical degrees of crosslinking, the distribution of crosslinkers in the polymeric model structures significantly influenced the final value of thermal conductivity. Moreover, the influence of the crosslinking degree on the final value of thermal conductivity was studied by considering polymeric model structures with different degrees of crosslinking. The results demonstrate that by having a random distribution of sulfur, the thermal conductivity will be enhanced. However, by increasing the degree of crosslinking to the higher percentage in random crosslinked model structures, the value of thermal conductivity drops significantly due to possible higher crystallization of the model structures, which decrease the degree of freedom for phonon contributions.