Direct observation of a repulsion between polymer chains in dilute solutions

3 Direct observation of a repulsion between polymer chains in dilute solutions 

Long polymer chains are soluble in low molecular mass liquid solvent. Thus, in solution, polymer chains repel one another. This is remarkable, if we consider colloidal systems in general. Namely, (neutral) colloidal particles, dispersed in a liquid tend to aggregate as a result of the van der Waals forces. Such van der Waals forces exist also between polymer chains in solution, but they are weaker and counterbalanced by entropy contributions. 

The fact that polymer chains in solution repel one another can now be observed directly, by mechanical measurement of the forces. The apparatus, developed by Israelachvili and Tabor,3 is made of two half-cylindrical mica shells, whose axes are perpendicular to each other (see Fig. 1.9). 

The distance d between shells can be monitored with great accuracy (down to 0.1 nm) and the force jF exerted between shells is measured as a function of d(d 0.1 nm). When the apparatus is immersed in pure toluene, the observed4 force-distance profile indicates the existence of attractive forces for d 10 nm. However, when polystyrene chains (A/w = 1.4 x 10s) are terminally anchored on the half-shells, with a surface coverage (3 0.5) x 10 3 g/m2, the result is very different. The force-distance profile indicates a strong repulsion (see Fig. 1.9). The onset of repulsion is at d = 125 7 nm, which is about twice the average end-to-end distance of the corresponding free polystyrene chain in toluene. The experiment thus shows very clearly the existence of repulsive forces, the study of which will be pursued throughout this book. 

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