Llj. Cai et S. Granick, Chains end-grafted in a theta-solvent and polymer melts: comparison of force-distance profiles, ADV COLL IN, 94(1-3), 2001, pp. 135-150
We find an unexpected analogy between polymer melts compressed between stro
ngly-attractive solid surfaces, and end-attached polymers in near-theta-sol
vent. End-grafted polystyrene (PS) chains with various graft density were p
roduced by immersing mica, coated with adsorbed diblock copolymers of PS/PV
P, PS/polyvinylpyiridine, into trans-decalin at 24 degreesC (this solvent i
s a near-theta-solvent for PS but a non-solvent for PVP), and the force-dis
tance relations were measured using a surface forces apparatus. The end-gra
fted 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 o
f symmetrically-opposed PS layers are compared to the asymmetric case (PS o
n one mica surface, the other mica surface bare) and to the case of adsorbe
d PS homopolymer. Force-distance profiles in the presence of unattached PS
homopolymer in solution at concentration c* (the overlap concentration) dis
played repulsion beginning at considerably larger separation than in the pr
esence of pure solvent. Finally, we compare to confined melts of linear pol
ydimethylsiloxane (PDMS), cyclic PDMS and a linear perfluoropolyether (Demn
um). For these cases of polymer melts confined between adsorbing surfaces,
force-distance relations, could be described by the same functional relatio
ns as for end-attached chains in near-theta-solvent. (C) 2001 Elsevier Scie
nce B.V. All rights reserved.