The mechanical properties of natural rubber (NR) composites depend on many factors, including the filler loading, filler size, filler dispersion, and filler-rubber interfacial interactions. Thus, NR composites with nano-sized fillers have attracted a great deal of attention for improving properties such as stiffness, chemical resistance, and high wear resistance. Here, a coarse-grained (CG) model based on the MARTINI force field version 2.1 has been developed and deployed for simulations of cis-1,4-polyisoprene (cis-PI). The model shows qualitative and quantitative agreement with the experiments and atomistic simulations. Interestingly, only a 0.5% difference with respect to the experimental result of the glass transition temperature (Tg) of the cis-PI in the melts was observed. In addition, the mechanical and thermodynamical properties of the cis-PI-fullerene(C60) composites were investigated. Coarse-grained molecular dynamics (MD) simulations of cis-PI-C60 composites with varying fullerene concentrations (0–32 parts per hundred of rubber; phr) were performed over 200 microseconds. The structural, mechanical, and thermal properties of the composites were determined. The density, bulk modulus, thermal expansion, heat capacity, and Tg of the NR composites were found to increase with increasing C60 concentration. The presence of C60 resulted in a slight increasing of the end-to-end distance and radius of the gyration of the cis-PI chains. The contribution of C60 and cis-PI interfacial interactions led to an enhancement of the bulk moduli of the composites. This model should be helpful in the investigations and design of effective fillers of NR-C60 composites for improving their properties.

Abstract. In this study, we have successfully parameterized the coarse-grained (CG) model of cis-1,4-polyisoprene (main component of natural rubber) based on the MARTINI force field. An isoprene monomer is mapped into one bead of CG model. The structure, bulk and thermodynamics properties of cis-1,4-polyisoprene with new CG model are well comparable to the atomistic simulation model and experiment. Our CG model of cis-1,4-polyisoprene will be helpful to study in the advanced rubber nanocomposite materials. IntroductionRubber is one of the most important natural resources and has a very high impact on Thailand's economy. It is composed mainly of high molecular weight polymer cis-1,4-polyisoprene (cis-PI). Computer simulations can play role in the advanced rubber technologies as the virtual experiments, carried out in silico, to observe and fine-tune the chemical details of both rubber and rubber composites. These numerical experiments can greatly reduce time and cost from trial and error processes in the laboratories. However, there are still rooms for improvement as the traditional detail-riched atomistic model of polymers are still consuming a large amount of computer time to produce the well-equilibrated conformation of polymer networks. Therefore, some efforts were put to provide the more simplified representations of polymer molecules, namely the MARTINI force field [1]. The MARTINI models were built for amino acids, water, phospholipid cell membranes, fullerenes and some other polymers [2][3][4][5], but there is no parameter for rubber molecules yet. In this study, CG model of cis-PI chains were parameterized based on MARTINI scheme [1]. Solvation free energy and chain properties of cis-PI were calculated in water, cyclohexane and in own melt-state. A series of molecular dynamics (MD) simulations of CG model were performed in comparison with the united-atom(UA) model to verify the transferability between two length scales.