ADHESION OF THIN, DRY BLOCK-COPOLYMER LAYERS ADSORBED ON MICA

Adhesion was examined for dry, thin layers of poly(2-vinylpyridine) (PVP)-polyisoprene (PI) diblock and graft-block copolymers, the latter having PVP blocks as the trunk and PI blocks as the branch. Those copolymers were selectively adsorbed at the PVP blocks on mica from toluene solutions. In toluene the diblock and graft-block copolymers appeared to obey the same thermodynamics of adsorption that was discussed by Parsonage et al. In dry states the adsorbed copolymers formed very thin layers having the PVP blocks at the bottom and the PI blocks at the top, and adhesion of those layers was related to interaction of the PI blocks having entropically unfavorable, flattened conformations. Adhesion energies per unit area (surface free energy decrease on contact), gamma, were measured for two cases, symmetric or asymmetric contact of identical or different PI block layers. For both cases gamma was larger than the surface energy per unit area, gamma(homo-PI), for homo-PI films of macroscopic size. gamma(homo-PI) represented the van der Waals contribution to adhesion, and the extra adhesion energy for the PI blocks, gamma-gamma(homo-PI), was related to their flattened conformation: On contact of two adsorbed layers, the PI blocks belonging to each layer could interpenetrate each other and increase their conformational entropy. This entropy gain was considered to be the origin of the extra adhesion. For both symmetric and asymmetric contacts, the extra adhesion energy reduced per PI block, Delta gamma*, was found to be proportional to N*kT, with N* and kT being the PI block length normalized by the PI layer thickness and the thermal energy, respectively. This proportionality as well as the Delta gamma* values themselves were satisfactorily explained from a calculation of entropy gain for confined tethered chains brought into contact, supporting the validity of the above mechanism of the extra adhesion. The adhesion energies of the adsorbed copolymer layers were also discussed in relation to a stretched-chain pullout mechanism experimentally observed by Creton et al.