Local glass transition of polyisoprene induced by graphene planes observed in silico through atomistic molecular dynamics simulations

Abstract

A study on the molecular mechanisms of natural rubber reinforcement due to the addition graphene planes was carried out by molecular dynamics simulations. Simulations of pure natural rubber (Polyisoprene; PI) and Polyisoprene-graphene composite (PI-GRA) were performed and analyzed for their glass transition temperatures (Tg). In the simulations, simulated annealing technique was used to change the temperature from 100 K to 300 K in order to observe the glass transition from the point where the sudden increase in volume and sudden decrease in density were observed. Glass transition temperature (Tg) from the molecular dynamics trajectory of pure PI was 207.9 K, close to the value Tg = 211 K obtained from the experiment. Also, we found that glass transition temperature (Tg) increased to 220.1 K when adding graphene planes into the PI matrix. In addition, further analysis of MD trajectories showed that the overall density of pure 16-mer cis-1, 4-polyisoprene at low temperature (100 K) is higher than the density at high temperature (300 K) and the threshold density for glass transition was 929.4 kg/m3. For PI-GRA at 100 K, density of the whole system was higher than the threshold density, which showed that the whole polymer was in glass phase. However, at 300 K, the network of PI molecules nearby the graphene planes were orderly organized and possessed higher density than the threshold density, which showing the properties of glass. In the middle area, PI molecules were arranged disorderly and has a lower density than the threshold, showing the properties of rubber.