Architectural Effects on the Solution Behavior of Linear and Star Polymers in Propane at High Pressures Yue Wu,* ,† Matthew S. Newkirk, † Sean T. Dudek, † Kara Williams, ‡ Val Krukonis, ‡ and Mark A. McHugh † † Department of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond, Virginia 23284, United States ‡ Phasex Corporation, Lawrence, Massachusetts 01843, United States * S Supporting Information ABSTRACT: A star polymer with a divinylbenzene core and statistically random methacrylate copolymer arms is synthesized with reversible addition-fragmentation-transfer method and fractionated with supercritical carbon dioxide and propane to obtain fractions with low molecular weight polydispersity. The phase behavior and density behavior are experimentally determined in supercritical propane for fractionated star polymers and the corresponding linear copolymer arms at temperatures to 423 K and pressures to 210 MPa. Experimental data are presented on the impact of the number of arms, the backbone composition of the lauryl and methyl methacrylate repeat units in the copolymer arms, and the divinylbenzene core on the polymer-propane solution behavior. The star polymer is significantly more soluble because of its unique structure compared with the solubility of the linear copolymer arms in propane. The resultant phase behavior for the two homopolymers and the copolymer arms in propane are modeled using the perturbed chain statistical associating fluid theory (PC-SAFT). Model calculations are not presented for the phase behavior of the star polymers in propane because the PC-SAFT approach is not applicable for star polymer structures. ■ INTRODUCTION Over the past few decades the advent of new chemistries has led to the creation of polymers with unique, well-defined architectures, such as star polymers with a fixed number of branches or, in other words, arms. Star polymers, with low to moderate molecular weight arms, have a globular structure that does not promote chain entanglements. Star polymers can be synthesized from a large range of homopolymer, block, and copolymer arms that can also contain functionilized groups. 1 Once the star polymer is synthesized the functional groups can be readily modified to adjust their physical properties for specific applications in the areas of catalysis, 1 coatings, 2 lubrication, 3 membrane formation, 4 and drug delivery. 5,6 Despite their increasingly mature applications, some funda- mental physical properties, such as phase behavior and solution densities, are still lacking, and an accurate prediction of these properties over wide ranges of temperatures and pressures remains a challenge. Previously reported studies on star polymer-solvent solution behavior are typically performed with incompressible liquids at ambient pressure, not at high pressures with supercritical fluids. Star (sPS) and linear polystyrene (lPS) are primarily used in these studies to investigate the impact of multibranching architectural effects on solution behavior. Liquid solvents used for dissolving sPS and lPS include toluene, cyclohexane (CH), and methylcyclohexane (MCH), which, for these polymers, are characterized as a good, theta (Θ), and poor quality solvents, respectively. 7 Studies have been performed to determine the upper critical solution temperature (UCST), radius of gyration ⟨R g 2 ⟩, and second osmotic virial coefficient B 22 for sPS-solvent and lPS-solvent solutions at the same molecular weight M w , at different compositions, and at atmospheric pressure. Given that star and linear polymers can be readily dissolved in a good solvent such as toluene, the UCST was reported only for each PS in CH and MCH. A 2-4 K decrease in UCST was observed for sPS-CH mixtures relative to that observed for lPS-CH mixtures. 8-11 Furthermore, the critical volume fraction of the sPS-CH system is slightly greater than that for the lPS-CH system, indicating that the star morphology suppresses the impact of molecular weight on solubility relative to the effect found with the linear polymer analogue. 9 Alessi et al. 12 observed a 5-15 K decrease in the UCST for an eight-arm sPS-MCH mixture relative to lPS-MCH mixture with PS of the same molecular weight (M w = 77 000 or 268 000), although these researchers observed no noticeable difference for the critical volume fraction of these mixtures. Literature studies report that ⟨R g 2 ⟩ for the star polymer is always smaller than its linear analogue with the same molecular weight. 7,13 The compact star structure reduces chain-chain penetration, which is one of the reasons for the greater solubility of star polymers in a poor solvent. ⟨R g 2 ⟩ decreases with an increase in the number of arms while it increases with an increase in the arm length. 7 Molecular simulation studies show that arms are stretched in a star polymer to best accommodate both the intra- and interarm repulsions. 14,15 Recently, Wever et al. 16 reported experimental evidence that the hydrodynamic radius, R h , of star polyacrylamide (PAM) increases with an increase in the number of arms, although the ⟨R g 2 ⟩ decreases simultaneously. The increase in R h confirms the Received: March 18, 2014 Revised: May 27, 2014 Accepted: May 30, 2014 Published: May 30, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 10133 dx.doi.org/10.1021/ie5011417 | Ind. Eng. Chem. Res. 2014, 53, 10133-10143