The mechanical stability of Langmuir-Blodgett multilayers

The properties and stability of confined Langmuir-Blodgett multilayers made of arachidic acid were investigated using the surface forces apparatus. Tw o different types of confined films were studied: one formed by bringing tw o separate LB films into molecular contact, the other formed by bringing on e LB film into direct contact with mica. The measured thickness of confined material depends solely on the total number of trapped molecular layers (2 8 Angstrom/layer). The force required to separate the substrates, however, depends on the type of contact made. More force (equivalent to a work of ad hesion of 36 mJ/m(2)) is required to separate an LB film from mica than fro m another LB film (28 mJ/m(2)). Applied stress impacts the integrity of the LB multilayers; the films are particularly vulnerable to the combination o f shearing and tensile stresses. Microscopic changes in film structure are sufficient to generate anomalous behavior at the macroscopic level, namely pinning of the contact region despite changes in load exerted on the deform able interacting bodies. (C) 2000 Elsevier Science S.A. All rights reserved .