Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films

Abstract

The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10R g to 107R g . For 15k four-arm star polystyrene (SPS), confinement appears at a thickness between 112 nm (40R g ) and 72 nm (26R g ), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7R g ) and 6k and 14k cyclic (24 and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6R g ) thick highly viscous layer at the substrate, which is significantly thicker than the 1R g thick "irreversibly adsorbed" layer. For a 29 nm (10R g ) thick film, more striking confinement occurs due to the combined influence of both interfaces. These results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.

Unexpected Molecular Weight Dependence to the Physical Aging of Thin Polystyrene Films Present at Ultra‐High Molecular Weights

“…Based on our experimental investigations, we attribute the ~45% faster physical aging response of ultra‐high MWs ≥ 6500 kg mol −1 thin PS films, relative to equivalent films of (merely) high MWs ≤ 3500 kg mol −1 , to a diminished gradient in dynamics near the free surface of these films. These results contribute to the growing reports in the literature that chain connectivity and entropy are additional factors influencing interfacial effects and dynamical gradients in thin films beyond the well‐characterized enthalpic interactions.…”

“…However, several recent studies have reported differences in confinement behavior associated with chain length, stiffness, entanglements, and chain architecture that are unexpected given the primarily enthalpic understanding developed over the years. This suggests that entropic effects may also play a modifying role in understanding confinement effects.…”

“…This suggests that entropic effects may also play a modifying role in understanding confinement effects. For example, changes in dynamics due to tethering and adsorption of chains to substrates have been reported, as well as variations for different chain architectures such as stars and cyclic chains that otherwise have identical chemical composition, suggesting that the presence of a boundary or interface alters the available entropic conformations and influences the local dynamics, altering the nature of the dynamical gradient. Recent studies have also reported unexpected MW dependences and chain stiffness effects to the glass transition temperature ( T g ) and surface dynamics that contradict the general understanding that had historically been formed.…”

Effect of Molecular Architecture on Conformational Relaxation of Polymer Chains at Interfaces

“…Thus far, it has been reported that the dynamics of branched polymers in thin films is different from those of corresponding linear polymers. [ 33 − 36 ] This can be explained in terms of the entropic interaction between star‐shaped polystyrene (sPS) chains and the solid surface. [ 34 ] Also some studies showed that the suppressed relaxation dynamics of interfacial sPS chains could extend away from the Si/SiO x substrate over a length scale of several times larger than that for linear PS (lPS) chains.…”

“…[ 34 ] Also some studies showed that the suppressed relaxation dynamics of interfacial sPS chains could extend away from the Si/SiO x substrate over a length scale of several times larger than that for linear PS (lPS) chains. [ 35,36 ] Despite such growing evidence of the fundamental role of molecular architecture in controlling the relaxation dynamics in a thin film, it seems that direct observations of the relaxation processes of interfacial chains have yet to be addressed.…”