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Unformatted text preview: Osmotic Second Virial Coefficient, Intrinsic Viscosity and Molecular Simulation for Star and Linear Polystyrenes Alberto Striolo, ²,‡ John M. Prausnitz,* ,² and Alberto Bertucco ‡ Chemical Engineering Department, University of California, Berkeley, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720; and Istituto di Impianti Chimici, Universita ´ di Padova, Via Marzolo 9, I-35131 Padova, Italy Received February 28, 2000; Revised Manuscript Received September 9, 2000 ABSTRACT: Experimental osmotic second virial coefficients are reported for polystyrene in toluene (good solvent), cyclohexane ( Θ solvent) and methylcyclohexane (poor solvent) in the temperature range 10- 60 ° C. The Θ temperature for eight-arm star polystyrene in methylcyclohexane is 29 ( 3 ° C. Intrinsic viscosity for polystyrene in cyclohexane and methylcyclohexane has been measured over a wide temperature range. A coil-globule transition has been observed for eight-arm star polystyrene in methylcyclohexane at temperatures close to the Θ temperature. The Θ point for star polymers with three, four, five, and six arms have been determined by standard Monte Carlo simulation calculations. For six-arm star polymers at the Θ point, defined as the well depth at which the radius of gyration squared scales linearly with the number of segments, the osmotic second virial coefficient is zero. As shown by others, for a branched polymer, the osmotic second virial coefficient at good solvent conditions and the Θ temperature are lower than those for a corresponding linear homologue in the same solvent. Introduction Regular star polymers are branched structures where equi-sized linear arms emanate from a central core. Star polymers with up to 450 arms with desired chemical and molecular weight asymmetries can be synthesized. 1- 3 Possible industrial applications of star polymers have been discussed. 4,5 For concentrated solutions, the effect of polymer structure upon solvent sorption is small. 6,7 Numerous experimental and computer simulation studies have been reported for dilute solutions of linear and star polymers in good and Θ solvents. Universal ratios express differences in thermodynamic properties, such as radius of gyration, osmotic second virial coef- ficient, and intrinsic viscosity between star and linear polymers in good or Θ solvents. 2,8 Osmotic second virial coefficients, B 22 , and mean-square radii of gyration in dilute solutions have been measured. 9- 13 In good sol- vents, B 22 is smaller for branched polymers than that for the homologue linear polymers. Several studies 3,8 show that branching lowers the Θ temperature of a solvent- polymer system. Deviations of Θ from that for a linear polymer with the same molecular weight increase as the number of arms increases....
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