The dynamics of materials are generally modified near their boundaries and there has been much interest recently in quantifying the scale over which interfacial dynamics is perturbed, and identifying factors influencing the dynamics within these regions. Interfacial layers are thought to be especially important in applications of thin glass-forming polymer films and polymer nanocomposites and we performed coarse-grained simulations of both model NPs in an unentangled polymer matrix in the limit of low NP concentration and polymer films on solid substrates to understand how the presence of boundaries alters both the segmental mobility and segmental density in the interfacial regions. We first quantified the interfacial mobility on a layer by layer basis based on a phenomenology similar to bulk materials. The length scale of perturbed mobility was found to be on the order of a few nm, regardless of the polymer-surface interaction strength and boundary type. The NP interfacial mobility scale for moderate cooling is maximized for the planar interface so that boundary curvature can significantly influence this scale. The strength of the polymer-surface interaction, on the other hand, has a remarkably weak effect on this length scale. Brief comparisons with experiment were considered as reality checks. Simulations over a wide tempearture range, encompassing the glass transition, indicated that the thickness of the mobile interfacial region at the 'free’ interface of supported polymer films exhibits a maximum near the glass transition temperature, <i>T<sub>g</sub>.</i> This finding is appears to explain contradictory trends in the tempearture dependence of the mobility interfacial zone reported in some experiments and simulations. We also compared our findings to the interfacial zones of the crystalline and glass-forming metallic materials. Condensed materials generally exhibit greatly altered dynamical and thermodynamic properties in their interfacial zones, explaining the significantly altered properties of thin film and nanocomposites where the relative amount of the material in such an 'interfacial state' is relatively large.

Explorations of the nature of the interfacial zone by simulation

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.