The effects of long-ranged and short-ranged forces in confined near-critical polymeric liquids

– The relative importance of longand short-ranged forces on the thermal fluctuations at polymer interfaces was investigated with neutron reflection. Polyolefin blends were synthesised to create polymer pairs with a tuned interaction parameter, allowing the exploration 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 Supérieure de Physique et Chimie Industrielles (ESPCI) Paris, France. (∗∗) E-mail: [email protected] (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/i2005-10162-7 764 EUROPHYSICS LETTERS the nature of the interface modified by interfacial fluctuations, and how is this modified by confinement effects in thin films? It is now clear that the equilibrium interface width in polymer systems is substantially broader than the mean-field prediction, and that the origin of this broadening is related to thermally excited capillary waves. Moreover, in confined systems the spectrum of excited capillary waves is modified, and this has a substantial effect on their contribution to the interfacial width in thin films. However, there is a controversy in the literature about the relative importance of long-ranged van der Waals forces [4–6] and short-ranged “truncation forces” [7, 8] in influencing the capillary wave spectrum. While the long-ranged dispersion forces lead to a logarithmic dependence of the interfacial width on the thickness, the shortranged interactions involve a square-root dependence on the thickness. Recent computer simulations make clear that in principle both forces should be taken into account [9]. To probe this issue, we have performed systematic experiments to investigate the confinement effect on the interfacial width of polymer/polymer systems with a tuneable interaction parameter. In fact, in order to realise the full advantages of polymers as model systems for studying fluid interfaces and critical phenomena in confined geometries, it is necessary to be able to synthesise highly controlled polymers with well-defined interactions. A remarkable class of materials that is ideally fitted for this purpose are random copolymers of ethylene and ethyl-ethylene that are obtained by hydrogenating anionically polymerised polybutadiene with different chain microstructures [10, 11]. By varying the solvent in which the polymerisation takes place, random copolymers of microstructure (C4H8)1−x(C2H3(C2H5))x with a controlled copolymer ratio have been produced. Pairs of these copolymers have an unfavourable interaction, expressed as the Flory-Huggins interaction parameter χ, that depends in a relatively simple way on the copolymers ratios of the components [7, 10]. Different conditions of miscibility were then probed by using a wide range of copolymer ratios that were varied from 0.5 to 0.86, while the molecular weight was about 150000 g/mol. The χ parameter is calculated from the copolymer ratios x1 and x2 of the two components using the relation [10] χ = (a0+a1x̄+a2x̄)(x1−x2). In the previous expression x̄ is the mean copolymer ratio and the coefficients a0, a1 and a2 are linear combinations of the homopolymers interaction parameter χA/B and of the interactions between homopolymer and copolymer χA/A−B and χB/A−B. The values of these coefficients at 83 ◦C are, respectively, 0.062, −0.114 and 0.220 [10]. The degree of immiscibility χN , where N is the degree of polymerisation of the polymers, was then varied from 4.1 to 31. To investigate how longand short-ranged forces interplay at the interface in confined systems approaching criticality, neutron reflectivity experiments were performed on bilayers formed by a film of 50%-deuterated (D) polyolefin and a layer of hydrogenated (H) polyolefin. For each polymer pair, a series of samples was prepared where the thickness of the bottom D layer was varied, while the thickness of the top H film was fixed at about 4000 Å. The bottom D polymer layer was spun cast onto silicon substrate from toluene solutions of different concentrations obtaining layers with thickness between 700 and 9000 Å. The polymer top H layer was spun onto a glass slide and the resulting film was floated in water and then deposited onto the D layer. The samples were annealed in a vacuum oven for 5 days at approximately 356K, well above the glass transition of the polymers [10,11]. Reflectivity profiles were measured on reflectometers at different facilities: on SURF/ CRISP, Rutherford Appleton Laboratory (UK), on D17, Institut Max Von Lau-Paul Langevin (France), on V6, Hahn-Meitner Institut Berlin (Germany) and on AMOR, Paul Scherrer Institut (Switzerland). The resolution used varied between 3% and 6%. Figure 1 shows, as an example, reflectivity curves obtained for a polymer pair with degree of immiscibility of χN = 5.9 for two different thicknesses of the bottom layer: 785 Å and 4225 Å. C. Carelli et al.: The effects of long-ranged etc. 765