Chains end-grafted in a theta-solvent and polymer melts: Comparison of force-distance profiles

We find an unexpected analogy between polymer melts compressed between strongly-attractive solid surfaces, and end-attached polymers in near-theta-solvent. End-grafted polystyrene (PS) chains with various graft density were produced by immersing mica, coated with adsorbed diblock copolymers of PS/PVP, PS/polyvinylpyiridine, into trans-decalin at 24°C (this solvent is a near-theta-solvent for PS but a non-solvent for PVP), and the force-distance relations were measured using a surface forces apparatus. The end-grafted PS chains repelled one another in spite of the theta-solvent situation. Repulsive forces began at a thickness, per adsorbed layer, equivalent to 4-5 times the unperturbed radius of gyration of the PS chain. These cases of symmetrically-opposed PS layers are compared to the asymmetric case (PS on one mica surface, the other mica surface bare) and to the case of adsorbed PS homopolymer. Force-distance profiles in the presence of unattached PS homopolymer in solution at concentration c* (the overlap concentration) displayed repulsion beginning at considerably larger separation than in the presence of pure solvent. Finally, we compare to confined melts of linear polydimethylsiloxane (PDMS), cyclic PDMS and a linear perfluoropolyether (Demnum). For these cases of polymer melts confined between adsorbing surfaces, force-distance relations could be described by the same functional relations as for end-attached chains in near-theta-solvent.