Surface Dynamics of “Dry” Homopolymer Brushes

Abstract

The dynamics of the surface height fluctuations on layers of covalently tethered, nearly monodisperse polymer chains synthesized by atom transfer radical polymerization were studied using X-ray photon correlation spectroscopy. The data reveal that both polystyrene and poly(n-butyl acrylate) "brushes" have structure at the surface with length scales in the region of 620-3100 nm, but the surface features show no relaxation in a time window of 0.1-1000 s, even at temperatures more than 130 °C above the glass transition temperature of the corresponding untethered chains. This remarkable alteration of the dynamics is compared with the suppression of fluctuations on this length scale anticipated by thermodynamic theories. The alteration of surface dynamics by tethering has implications for wetting, friction, and adhesion.

X‐ray Photon Correlation Spectroscopy Studies of Surfaces and Thin Films

“…Polymer conformations and molecular architectures also play important roles in surface capillary waves. Polystyrene brushes densely covalently bonded to supporting substrates have exhibited completely frozen surface fluctuations on time scales as long as 1000 s even at temperatures 130 °C above T g , while untethered films of identical thickness and polystyrene molecular weight show very fast surface fluctuations . This is qualitatively consistent with thermodynamic theories and can be ascribed to the huge entropic cost required for the collective lateral chain displacement.…”

“…Later, Jiang et al reported viscoelasticity in ultra‐thin polymer films of thickness in a range of 1–4 times of the radius of gyration. This viscoelasticity is only apparent in thin films, not in the thicker films in the study of Kim et al,[37a] and it arises from pinning of the chains to the solid substrate and entanglement effects giving a brush‐like behavior and suppressing thermal fluctuations . This brush‐like behavior gives rise to a shear modulus or a complex component to the viscosity.…”

Lubrication with Oil-Compatible Polymer Brushes

“…It should also be borne in mind that the glass temperatures of surface-tethered polymers that have constrained freedom may in fact be higher than those reported for free polymers [47]. An interesting X-ray photon-correlation-spectroscopy study by Akgun et al [48] of end-grafted and free polystyrene films showed a significant reduction in surface-height fluctuation for the grafted system. Based on the above, for the polymer brushes in their collapsed state (e.g., immersed in ethanol), the difference between the glass-transition temperature for a specific polymer and the temperature at which the friction measurements were carried out will influence the contact properties, which are determined by the mobility of the polymeric chains at the temperature of the measurements.…”

Surface and Interface Analyses of Polymer Brushes by Synchrotron Radiation

“…There is pioneering work about the dynamics of polymer brushes by XPCS conducted by Akgun et al 65) They studied the dynamics of polystyrene and poly(n-butyl acrylate) brushes grafted on the silicon substrates by XPCS in GI geometry with length scales in the region of 620-3100 nm, but the surface features show no relaxation in a time window of 0.1-1000 s, even at temperatures more than 130 C above the glass transition temperature of the corresponding untethered chains. As a different approach to the dynamics of polymer brushes, the study of dynamical behavior of polymer grafted nanoparticles is significant.…”