Mesoscale modeling of random scission in polymer melts

Understanding thermal degradation of polyolefins on the mesoscale can result in development of more efficient strategies for thermoplastics recycling. Herein we develop an initial framework to model random scission at high temperatures in polyethylene melts on the mesoscale. We use the energy conserving dissipative particle dynamics (eDPD) approach1, in which the temperature is introduced as an additional degree of freedom with respect to the original DPD to ensure energy conservation. To prevent unphysical topological crossings of bonded polymer chains in standard DPD, we utilize mSRP (modified Segmental Repulsive Potential) formulation, which captures the effects of entanglements in polymers melts. We use the LAMMPS open-source simulation package3 along with the corresponding code for mSRP2 and eDPD1 to integrate the momentum and energy conservation equations. In addition, we use our recently implemented pair style srp/react command in LAMMPS; with this approach the bond breaking can be modeled along with the mSRP formulation. We model bond breaking as a stochastic process by setting a probability of bond breaking at each reaction time step dependent of local temperature. Using this approach, we characterize the fragmentation process. Specifically, we track the weight fraction , 𝑤𝑖(𝑥), and the number fraction, 𝑛𝑖(𝑥) , distributions of chain fragments during the random scission, where 𝑥 denotes a fraction of bonds broken. Our results demonstrate that 𝑛𝑖(𝑥) and 𝑤𝑖(𝑥) can be reasonably well approximated by the most probable distribution, also known as Flory-Schulz distribution.