Europhys. Lett., 71 (5), pp. 763–769 (2005) DOI: 10.1209/epl/i2005-10162-7 EUROPHYSICS LETTERS 1 September 2005 The effects of long-ranged and short-ranged forces in confined near-critical polymeric liquids C. Carelli 1 ( ∗ ), R. A. L. Jones 2 , R. N. Young 2 , R. Cubitt 3 , R. Krastev 4 , T. Gutberlet 5 , R. Dalgliesh 6 , F. Schmid 7 and M. Sferrazza 8 ( ∗∗ ) 1 Department of Physics, University of Surrey - Guildford, UK 2 Department of Physics and Astronomy, University of Sheffield - Sheffield, UK 3 ILL, Institute Max Von Laue-Paul Langevin - Grenoble, France 4 Hahn-Meitner Institut - Berlin, Germany 5 SINQ, Paul Scherrer Institut - Villigen, Switzerland 6 ISIS, Rutherford Appleton Laboratory - Chilton, UK 7 Fakult¨ at f¨ ur Physik, Universit¨ at Bielefeld - Bielefeld, Germany 8 D´ epartement de Physique, Universit´ e Libre de Bruxelles Boulevard du Triomphe, CP 223, 1050 Bruxelles, Belgium received 19 April 2005; accepted in final form 8 July 2005 published online 3 August 2005 PACS. 61.41.+e – Polymers, elastomers, and plastics. PACS. 61.12.Ha – Neutron reflectometry. PACS. 68.35.Fx – Diffusion; interface formation. Abstract. – The relative importance of long- and short-ranged forces on the thermal fluctu- ations at polymer interfaces was investigated with neutron reflection. Polyolefin blends were synthesised to create polymer pairs with a tuned interaction parameter, allowing the explo- ration of situations from near criticality to strongly immiscible cases in thin-film systems. We have observed for the polymer interfacial width, a transition from a region where long-ranged forces dominate, at higher degree of incompatibility, to a region approaching criticality where short-ranged truncation forces are more relevant. The problem of the structure of the interface between two coexisting fluid phases has a long history, dating back to van der Waals [1], but there has been a recent burgeoning of theoretical work in this area, particularly in relation to the effects of confinement and reduced dimensionality on the fluid-phase behaviour [2]. The experimental effort has been a lot smaller than the theoretical one, largely because the study of the structure of interfaces between two coexisting fluids and the control of the strength of surface fields in confining geometries present difficult experimental problems. However, one subset of fluid mixtures has proved very fruitful for experiments —coexisting polymeric liquids, where the technique of neutron reflection has proved invaluable for probing the structure of interfaces with sub-nanometer resolution [3]. For confined thin polymer systems, the following questions are still open to debate: How is ( * ) Present address: Ecole Sup´ erieure de Physique et Chimie Industrielles (ESPCI) - Paris, France. ( ** ) E-mail: msferraz@ulb.ac.be (corresponding author) c  EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2005-10162-7