Mechanical Size Effect of Freestanding Nanoconfined Polymer Films

GR Wang and F Najafi and K Ho and M Hamidinejad and T Cui and GC Walker and CV Singh and T Filleter, MACROMOLECULES, 55, 1248-1259 (2022).

DOI: 10.1021/acs.macromol.1c02270

While elastic properties of nanoconfined polymer films have been recognized to show departures from bulk behavior, a careful understanding of the origins of mechanical size effects remains weak. Here, we report a significant mechanical stiffening of freestanding ultrathin poly(methyl methacrylate) films of varying thicknesses (6-200 nm) through atomic force microscopy deflection measurements at ambient conditions. After excluding the substrate influence, the stiffening mechanism is linked to extended chain conformations based on small-angle X-ray scattering and infrared nanoscopic characterization. We advocate that the entropic elasticity of individual chains plays a significant role in polymer mechanics in nanoscale thickness films, where the entanglement density is apparently low, with chains oriented in the plane of the film, unlike a bulk polymer. Molecular dynamics simulations further unveil the dominance of entropic contributions over enthalpic contributions to the chain stiffness that endows polymer films with higher load-bearing capacity and accounts for the stiffening at the nanoscale. The results presented herein provide a mechanistic understanding of molecular origins of the size effect, serving as a potent design strategy for accessing high-performance polymer-based devices.

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