The case for PE is less straightforward and also noted for the change in dehydration free energy being negative. PE lacks any atom like chlorine that would strongly attract water, suggesting that changes in hydrophobicity metrics are based on changes in polymer mechanics. To test this idea, an artificial comparison is made between PE and an amorphous but "frozen" PE, denoted PE • , for which polymer atoms are fixed in place; the polymer dynamics of PE • are thus much more similar to PE * than to PE. Fig. shows that the change in hydrophobicity metrics for PE • and PE * are virtually identical, confirming that the changes are not tied to specific elements of the crystal structure itself. This result also provides some insight regarding why responses in hydrophobicity metrics differ. In particular, amorphous PE is soft, typified by a low glass-transition temperature, which leads to polymer deformation around the water droplet and a smaller observed nanoscale contact angle (see Supporting Information, Section S10). This highlights a need to carefully consider the meaning and extraction of nanoscale contact angles formed at soft surfaces. The origin of -∆(β∆f σ ) being negative for PE is unclear. We speculate that increased surface roughness of the dynamic PE slab (Figure ) could manifest in a slightly more hydrophobic environment for interfacial water based on analogy to superhydrophobic surfaces, for which microstructuring can enhance hydrophobic behavior. Meanwhile, characterization of the water dynamics (Fig. ) do not offer a clear distinction between PE and PE * . In practical terms, PE and PE * present similar hydrophobic character relative to other polymers.