Failure and mechanical properties of block copolymer thin films*

The physical properties of glassy polymer films can change drastically under
nanoscale confinement. These changes are often attributed to increased
average molecular mobility and reduction in entanglement density, and both
are known to alter mechanical behavior. While most previous work has focused
on the properties of homopolymers under confinement, and the mechanical
response of block copolymer thin films is still comparatively unexplored. We
have used molecular dynamics simulations to investigate the mechanical
response of free standing, lamellar-forming block copolymer thin films made
from polymer chains that span from slightly entangled to highly entangled at
different film thicknesses. We noticed that the failure and mechanical
properties become substantially distinct from bulk as the thickness is
decreased. The dynamic response of the thinnest films studied show a
different local response compared to thicker films. For all the block
copolymer films, we find that the plastic rearrangements initially
concentrate at the boundary between the two phases of the lamellae until
close to failure, when the plasticity moves to the center of a domain.