Phantom chain simulations for fracture of star polymer networks on the effect of arm molecular weight

Abstract: This study investigated the fracture of star polymer networks made from prepolymers with various arm molecular weights in the range $2 \leq N_a \leq 0$, for node functionalities $3 \leq f \leq 8$ and conversion ratios $0.6\leq\phi_c\leq0.95$ by phantom chain simulations. The networks were created via end-linking reactions of star polymers dispersed in a simulation box with a fixed monomer density $\rho=8$. The resultant networks were alternatively subjected to energy minimization and uniaxial stretch until the break. The stretch at the break, $\lambda_b$, depended on the strand molecular weight $N_s=2N_a+1$ with a power-law manner described as $\lambda_b \sim N_s^0.67$, consistent with the experiment. However, the strand length before stretch is proportional to $N_s^0.5$, which does not explain the observed N_s-dependence of $\lambda_b$. The analysis based on the non-affine deformation theory does not interpret the phenomenon either. Instead, the increase of normalized prepolymer concentration concerning the overlapping concentration with increasing $N_s$ explains the result through a rise in the fraction of broken strands.