Liquid structuring at solid interfaces as probed by direct force measurements: The transition from simple to complex liquids and polymer fluids

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Abstract
Results are presented of the equilibrium forces between two molecularly smooth surfaces immersed in pure monodisperse polymer liquids or ‘‘melts’’: liquid polybutadienes of molecular weights ∼1000 and ∼3500, corresponding to n≈20 and n≈65 segments (monomer units) per molecule, respectively. The aims were (i) to establish the minimum segment number when a flexible chain molecule begins to interact like ‘‘a polymer,’’ (ii) to investigate the nature of the transition from simple liquid‐like behavior (as occurs with short‐chained alkanes) to that characteristic of long‐chained polymers, and (iii) to compare the forces in polymer melts with those where dissolved polymers are adsorbed from solution, and thereby ascertain whether there are any fundamental differences between these two types of interactions. The results show that, unlike liquid alkanes (up to hexadecane, n=16), liquid polybutadienes with n>20 already exhibit ‘‘polymer‐like’’ interactions. There is also a striking similarity between the forces measured in pure polymer melts with those previously measured when polymers are adsorbed to saturation onto surfaces from good solvents; viz., a purely monotonic repulsive force law extending out to a few radii of gyration between the surfaces. It is concluded that the transition from the simple liquid‐like ordering of spherical or short‐chained molecules at surfaces to that characteristic of long‐chained polymers is essentially a transition from a quasiordered layering of segments parallel to surfaces, extending a few segment diameters, to a more random disordered state farther out describable by mean field and scaling theories.