  Polymeric films with greater impact and ballistic resistance are highly
desired for numerous applications, but molecular configurations that best
address this need remain subject to debate. We study the resistance to
ballistic impact of thin polymer films using coarse-grained molecular dynamics
simulations, investigating melts of linear polymer chains and star polymers
with varying number 2<=f<=16 and degree of polymerization 10<=M<=50 of the
arms. We show that increasing the number of arms f or the length of the arms M
both result in greater specific penetration energy within the parameter ranges
studied. Greater interpenetration of chains in stars with larger f allows
energy to be dissipated predominantly through rearrangement of the stars
internally, rather than chain sliding. During film deformation, stars with
large f show higher energy absorption rates soon after contact with the
projectile, whereas stars with larger M have a delayed response where
dissipation arises primarily from chain sliding, which results in significant
back face deformation. Our results suggest that stars may be advantageous for
tuning energy dissipation mechanisms of ultra-thin films. These findings set
the stage for a topology-based strategy for the design of impact-resistant
polymer films.
