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.