Reactive molecular dynamics simulation and experimental validation of pyrolysis in Cis-1,4-polyisoprene nanocomposite

This study investigates the thermal degradation behavior of cis-1,4-polyisoprene and its nanocomposites using a combined approach of reactive molecular dynamics simulations and experimental techniques. Pyrolysis was experimentally conducted up to 500 °C using a custom-built lab-scale-tubular reactor (20 mm diameter, 300 mm length). Samples were prepared by melt mixing, followed by compression molding into 1 mm thick sheets. Simulations were performed up to 2500 K using the ReaxFF reactive force field, selected for its ability to dynamically model bond breaking and formation in reactive systems. We constructed a simulation system containing ten polymer chains and up to two nano-silica units (576 atoms each) within a 150 Å periodic box, using the NVT ensemble with a Nosé-Hoover thermostat and a time step of 0.25 fs over a total duration of 42 ps. Experimental analyses via TGA, FTIR, and GC-MS confirmed the formation of key pyrolysis products such as isoprene (C₅H₈), ethylene (C₂H₄), and methane (CH₄). The addition of 60 wt% nano-silica extended the degradation time by approximately 100% and increased the activation energy from 121.9 to 133.8 kJ/mol-a 9.77% rise-suggesting a stabilizing role in the thermal degradation process. Mechanistic insights revealed that degradation proceeds via radical-driven scission near double bonds, with nano-silica modulating both the rate and pathway of decomposition. Overall, the results demonstrate a concentration-dependent dual role of nano-silica in thermal degradation and provide a predictive framework for designing heat-resistant rubber nanocomposites and advancing sustainable pyrolysis-based recycling technologies.

Cis-1,4-polyisoprene, a naturally derived elastomer with high flexibility and unsaturation, was used as the base polymer (Mw ≈ 38,000 g/mol, Sigma-Aldrich). Nanocomposites containing 30 wt% and 60 wt% nano-silica were prepared via magnetic stirring and compression molding. Pyrolysis experiments were conducted in a lab-scale tubular reactor under nitrogen flow, and thermal behavior was analyzed using thermogravimetric analysis (STA 504, Bahr, Germany). Volatile products and functional groups were identified via FTIR and GC-MS. Reactive molecular dynamics simulations were performed using LAMMPS with a ReaxFF force field parameterized for C/H/O/Si systems. Simulations employed the NVT ensemble with a Nosé-Hoover thermostat to model bond dissociation and reaction pathways at elevated temperatures (1500-2500 K), enabling direct comparison with experimental results.