Short Chains Enhance Slip of Highly Entangled Polystyrenes during Thin Film Dewetting
We investigate the effect of short chains on slip of highly entangled polystyrenes (PS) during thin film dewetting from non-wetting fluorinated surfaces. Binary and ternary mixtures were prepared from monodisperse PS with weight average molecular weights $5 < M_\textrm{w} < 490$ kg/mol. Flow dynamics and rim morphology of dewetting holes were captured using optical and atomic force microscopy. Slip properties are assessed in the framework of hydrodynamic models describing the rim height profile of dewetting holes. We show that short chains with $M_\textrm{w}$ below the polymer critical molecular weight for entanglements, $M_\textrm{c}$, can play an important role in slip of highly entangled polymers. Among mixtures of the same $M_\textrm{w}$, those containing chains with $M<M_\textrm{c}$ exhibit larger slip lengths as the number average molecular weight, $M_\textrm{n}$, decreases. The slip enhancement effect is only applicable when chains with $M<M_\textrm{c}$ are mixed with highly entangled chains such that the content of the long chain component, $\phi_\textrm{L}$, is dominant ($\phi_\textrm{L}<0.5$). These results suggest that short chains affect slip of highly entangled polymers on non-wetting surfaces due to the physical or chemical disparities of end groups, and any associated dynamical effect their presence may have, as compared to the backbone units. The enhanced slip in this regard is attributed to the impact of chain end groups or short chain enrichment on the effective interfacial friction coefficient. Accordingly, for entangled PS, a higher concentration of end groups or short chains at the interface results in a lower effective friction coefficient which consequently enhances the slip length.