Fluid Phase Equilibria 222–223 (2004) 247–254 Conformations of homopolymer chains and their phase behavior in a simple supercritical solvent  Martin L´ ısal a,b , Ivo Nezbeda a,b,∗ a E. Hála Laboratory of Thermodynamics, Institute of Chemical Process Fundamentals, Acad. Sci., 165 02 Prague 6-Suchdol, Czech Republic b Department of Physics, J.E. Purkynˇ e University, 400 96 Úst´ ı n. Lab., Czech Republic Available online 15 July 2004 Abstract Using a simple molecular model and the configurational-bias Monte Carlo method combined with the parallel tempering technique, solvent driven changes in conformations of a homopolymer chain in a simple supercritical solvent are systematically investigated. The solvent is modelled as a square-well fluid, and two types of chain are considered: the flexible chain of tangentially touching (i) hard spheres (purely repulsive chain) and (ii) square-well spheres (purely attractive chain). The mean square end-to-end distance and radius of gyration are the main quantities computed and used to characterize the changes in conformations in dependence on the temperature and density of the solvent. It is found that the attractive chain exhibits both the upper and lower critical solution temperatures, whereas the repulsive chain exhibits only the upper critical solution temperature. © 2004 Elsevier B.V. All rights reserved. Keywords: Attractive chains; Conformations of polymer chain; Critical solution temperatures; Repulsive chains; Supercritical square-well solvent 1. Introduction Because of its great practical importance, the confor- mation behavior of polymer chains in supercritical fluids has been attracting a good deal of attention and despite the effort so far invested yet it remains an open problem [1]. Overwhelming majority of previous simulation studies of solvent driven changes in polymer conformations have con- sidered chains on a lattice. Few simulation studies of contin- uous molecular models focused then on chains in athermal solvents (i.e. in the fluid of hard spheres) [2–4], and on attrac- tive chains in dilute mixtures [5,6] and in simple supercriti- cal solvents [7,8]. The primary goal of our current project is the systematic investigation of the conformation behavior of amphiphilic chains in a supercritical solvent in dependence on (i) the type of solvent and its densities and temperatures, and on (ii) the structure of the chain, i.e., in the case of di-  Paper presented at the 15th Symposium on Thermophysical Proper- ties, Boulder, CO, USA, June 22–27, 2003. ∗ Corresponding author. Fax: +420 2 20920661. E-mail address: ivonez@icpf.cas.cz (I. Nezbeda). block polymers, on the ratio of the lengths of the attractive and repulsive segments. Simultaneously, it is desirable to link the changes in the conformations to the observed phase behavior. The chain conformations are a result of two compet- ing effects: the enthalpic effect, which forces the polymer chain to adopt a compact conformation in order to mini- mize its potential energy, and the entropic effect, which fa- vors expanded conformations [9,10]. These two competing effects give rise to rather a complex behavior of amphiphilic chains made up of repulsive and attractive segments and this behavior may be intuitively expected to be a combi- nation of that of its constituent parts. It seems thus natu- ral to begin with the two limiting cases, purely repulsive chains and purely attractive chains; similarly, we begin with a simple solvent (i.e. simple fluid without any specific struc- tural and thermodynamic features). The aim of this work is thus to investigate the conformation and phase behavior of purely attractive and repulsive chains immersed in a su- percritical solvent over a broad range of solvent densities and temperatures using the configurational-bias Monte Carlo method [11] combined with the parallel tempering technique [12]. 0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2004.06.015