Phantom Chain Simulations for the Fracture of Star Polymer Networks on the Effect of Arm Molecular Weight

This study investigated the fracture of star polymer networks made from pre-polymers with various arm molecular weights in the range, 2 ≤ Na ≤ 20, for node functionalities 3 ≤ f ≤ 8 and conversion ratios 0.6 ≤ φc ≤ 0.95 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 ρ = 8. The resultant networks were alternatively subjected to energy minimization and uniaxial stretch until the break. The stretch at the break, λb, depended on the strand molecular weight Ns = 2Na + 1 with a power-law manner described as λb ∼ Ns0.67, consistent with the experiment. However, the strand length before stretch is proportional to Ns0.5, which does not explain the observed Ns-dependence of λb. The analysis based on the non-affine deformation theory does not interpret the phenomenon either. Instead, the increase of normalized pre-polymer concentration concerning the overlapping concentration with increasing Ns explains the result through a rise in the fraction of broken strands.