There are several distinct molecular origins that underlie observed trends. To first order, the propensity for hydrogen bonding is a major factor that has already been discussed. For example, hydrogen bonding between N66 * and water is scarce by comparison to N66, and PVA forms more extensive hydrogen bonding with water than PVA † . However, the role of hydrogen bonding is not so relevant for PVC and PE. In addition, water orientation is relatively unaffected by crystallinity (Figs. ) and cannot explain the disparity in hydrophobic behavior. Instead, the increased rigidity of the polymer surface, which inhibits local deformation and rearrangement, governs the observed behavior. In the case of PVC * , as simulated, approximately half of all chlorine atoms are oriented towards the "bulk" of the polymer, which decreases overall water attraction and thus increases hydrophobicity.