  Nanoscale polymeric thin films are widely used in diverse applications such
as energy devices, flexible electronics and biosensors, where a satisfactory
mechanical performance is of vital importance to realize their full
functionality. It has been evidenced that the elastic properties of polymer
films are often strongly affected by their thickness; however, the underlying
mechanism of this phenomenon, especially a thorough understanding at the
microscopic level, has yet to be achieved. Here we established a coarse-grained
molecular dynamics (CGMD) based computational framework, combining with
experimental verifications, aiming to reveal the conformational origin of the
stiffening behavior of crosslinked polymeric thin films. By imposing systematic
controls over the polymer network structures, we found that the bi-axial
modulus changes are essentially consequent of the alteration of polymer
conformations. A unified theory was then proposed, to quantitatively clarify
the correlation between the elastic properties of the system and the
distributional variations of the chain end-to-end distances, with predicting a
significant hardening effect on top of the conventional entropic elasticity
with largely uncoiled chains. Adopting processing protocols inspired by the
modeling, our experiments showed that PDMS films at approximately the same
thickness may exhibit a two order of magnitude difference in their moduli. The
good agreement between experiments and simulations illustrated our findings as
an effective guideline for tailoring the elastic properties of polymer films at
nanoscale.
