Capturing the Dynamic Behavior of Adsorbed Polymers

Anna C. Balazs

The author is in the Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA. E-mail: balazs{at}vms.cis.pitt.edun

The ability of the chains to display such interfacial activity is dependent not only on their static structure but also on their dynamic behavior. Until now, our knowledge of the adsorbed chains was confined to their static conformations. On page 2041 of this issue, however, Fytas et al. report on a technique that allows measurement of the dynamic fluctuations within the layer (3). This method now makes it possible to investigate how the collective microscopic motion of polymers at interfaces affects such macroscopic phenomena as adhesion, lubrication, and wetting.

Fytas et al. focus on the properties of terminally anchored polymers (3). The polymers are composed of two distinct blocks: a long polystyrene block and a shorter polyethyleneoxide fragment. The chains and substrate are immersed in a solvent that is compatible with the polystyrene but incompatible with the polyethyleneoxide fragment. The solvent incompatibility drives the polyethyleneoxide to bind to the surface and thereby anchor the chains to the substrate. The polystyrene blocks stretch into the favorable solvent, forming "bristles" (figure, top); the layer is referred to as a polymer brush (4).

The height of the brush is dependent on the molecular weight of the solvophilic block, and thus, the chains can form a relatively thick layer. Thick films play an important role in a number of technologically relevant applications. When long chains are terminally anchored to the surface of colloidal particles, the steric repulsion between the tethered layers prevents the particles from aggregating and precipitating out of solution (5). It is through this mechanism, for example, that dye and pigment particles stay suspended in solution. The thick layers also form effective lubricants, reducing the friction and wear between mechanical parts. Finally, these tethered and extended macromolecules act as model systems for the chains within lipid bilayers; thus, investigating the interactions between the layer and other chains or the surrounding solvent provides insight into the properties of the bilayer.

Because of the potential utility of the these thick, end-grafted layers, considerable attention has been focused on determining their properties. The majority of the earlier studies have focused on characterizing the static equilibrium properties of the layer. For example, both experimental measurements (6) and theoretical calculations (4, 7) have been used to probe the density profiles of the layer. Little, however, is known about the dynamic behavior of the brush at thermal equilibrium. Here, thermal fluctuations (thermally activated collective motions of the chains) cause part of the chains to be more extended and other regions to be more compressed. One could view these motions as undulations in the surface of the brush (figure, bottom). Theoretically, these undulations or deformations are predicted to have a characteristic size, which is comparable to the thickness of the adsorbed layer, L₀ (8). Experiments, however, have lagged behind theory in the ability to probe the dynamic behavior of the anchored layer.

Fytas et al. used dynamic light scattering to determine the behavior of end-grafted chains (3). The advantage of dynamic light scattering is that one can obtain correlations between deformations at times t₁ and t₂. Thus, one can probe not only the characteristic size but also the lifetime of the undulations. In their elegant experiment, a laser beam was aimed at a glass prism that was coated on the opposite side with a layer of end-anchored polymers. The light was totally reflected from the surface of the glass, yet a damped, oscillating electric field from the laser beam penetrated the layer and scattered from the grafted chains. The scattered field characterizes density fluctuations of a size 2 /q, where q = |q|, q = k_s k_i, and k_s and k_i are the scattered and incident light wave vectors, respectively. Using this method, the researchers indeed found that the long-lived deformations have a specific size and that fluctuations at other length scales rapidly die away. The results also reveal that the scattering intensity is proportional to (qL₀)² for small q. Because q is inversely proportional to wavelength, this relation indicates that the long-wavelength disturbances are suppressed.

The findings are significant in that they provide insight into the collective motions of the chains at thermal equilibrium. The technique can now be used to determine how characteristics, such as chain rigidity and solvent quality, affect the deformations of the layer. Thus, the technique can be used as a probe of the local environment. With small modifications to the experimental setup, the method can also be used to determine how the fluctuations are affected by the presence of a second end-grafted layer where the bristles of both brushes are facing each other.If the separation between the brushes is varied, the results will reveal how the dynamic behavior of lubricating layers is affected as the layers come into contact. The results can also show how the chain deformations can promote or hinder adhesion between the layers. Performing such experiments on biopolymers or lipids can provide information on the fluctuations of the chains within cell membranes

References

1. R. S. Ward, IEEE Eng. Med. Biol. Mag. 6, 22 (1989).
2. G. B. Sigal, M. Mammen, G. Dahman, G. M. Whitesides, J. Am. Chem. Soc. 118, 3789 (1996).
3. G. Fytas et al., Science 274, 2041 (1996).
4. S. T. Milner, ibid. 251, 905 (1991).
5. D. H. Napper, Polymer Stabilization of Colloidal Dispersion (Academic Press, London,1983).
6. See reference (7) in Fytas et al. (3).
7. A. M. Skvortsov et al., Polym. Sci. USSR 30, 1706 (1988); C. Yeung, A. C. Balazs, D. Jasnow, Macromolecules 26, 1914 (1993); K. Huang and A. C. Balazs, ibid., p. 4736.
8. F. J. Solis and G. Pickett, Macromolecules 28, 4307 (1995).