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

摘要

Results are presented of the equilibrium forces between two molecularly smooth surfaces immersed in pure monodisperse polymer liquids or lsquo;lsquo;meltsrsquo;rsquo;: liquid polybutadienes of molecular weights sim;1000 and sim;3500, corresponding tonap;20 andnap;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 lsquo;lsquo;a polymer,rsquo;rsquo; (ii) to investigate the nature of the transition from simple liquidhyphen;like behavior (as occurs with shorthyphen;chained alkanes) to that characteristic of longhyphen;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 withn20 already exhibit lsquo;lsquo;polymerhyphen;likersquo;rsquo; 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 liquidhyphen;like ordering of spherical or shorthyphen;chained molecules at surfaces to that characteristic of longhyphen;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.