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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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