Pressure Effects on the Thermodynamics of Polymer Blends

Abstract

Simple thermodynamic arguments are evoked to show that the pressure dependence of the Flory interaction parameter of miscible polymer blends, as derived from small-angle neutron scattering experiments, is directly related to the volume changes on mixing for these systems. This approach is validated by comparison to available experimental data. We make contact with existing theoretical approaches and show that it is thermodynamically inconsistent to model a blend with nonzero as having no volume change on mixing. Since the entropic contributions that arise from these volume changes on mixing account for more than 50% of the pressure dependence of , the use of regular solution theory, where the pressure dependence is purely enthalpic in origin, gives an imperfect understanding of these situations. We finally show that, since the volume change on mixing is directly proportional to the parameter under ambient pressures, the critical temperature for any polymer blend will vary in an apparently "universal" fashion on pressurization. This conclusion is in agreement with experimental data for a large number of polymer blends. IntroductionThe role of pressure on the thermodynamics of polymer mixtures has been intensely studied in the past few years since it has direct applications to processing and to novel synthesis schemes involving environmentally benign supercritical fluids. Pressure effects are also of interest from a fundamental standpoint since the thermodynamics of typical polymer blends are understood in the framework of the incompressible random phase approximation. 1-6 A rigorously incompressible system should be unaffected by pressure. However, since experimental results show that the critical temperature for polymer-polymer demixing, T c , is strongly affected by pressure, typically dT c /dP ≈ 25-50 K/kbar, it is clear that polymer blends show significant departures from this ideal limit. 5,7 Most successful approaches in this area incorporate free volume effects, either into the free energy of the system or into an equation of state. The pressure dependence of T c and , the Flory-Huggins interaction parameter, is then numerically derived. 7-14 While these methods have been quite successful at explaining the measured dT c /dP, they provide little physical insight into the effect of pressure.A line of thinking that helps build a picture at the molecular level for the effect of pressure on polymer blend thermodynamics is presented by Rabeony et al. 6 Here it is postulated that the central quantity is the interaction energy density, X ≡ ( /v 0 )RT. is the Flory interaction parameter, 15 v 0 is an arbitrary reference volume, R is the gas constant, and T is the temperature. The X values obtained from some of the blends considered appear to be unique functions of system density, and data from several pressures and temperatures for each of these systems can be collapsed onto a master curve. Since this approach explicitly assumes that volume changes on mixing are negligible, it suggests that the only role of pressu...

Pressure Effects on Thermodynamics of Polymer Containing Systems

“…In past work, a wide range of phase behavior at chosen composition or near critical point of polymer solutions and polymer blends was found (Beiner et al 1998(Beiner et al , 2002Hammouda et al 1997;Janssen et al 1993;Lefebvre etal 1999;Schwahn et al 2001;Wolf & Blaum, 1977;Wolf & Jend, 1977;Zeman &Patterson, 1972;Zeman et al 1972). There are also many works on the theories about the pressure effects on the thermodynamics of polymer liquid and blends An & wolf, 1998;Dudowicz & Freed, 1995, 2006Kumar, 2000;Patterson & Robard, 1978;Walsh & Rostami, 1985). As several outstanding problems remain unexplained in these blends, we decided to investigate the dependence on pressure, an independent thermodynamic variable.…”

Effect of Pressure on the Phase Behavior and Segmental Dynamics in Blends of Polystyrene with Poly(methylphenyl siloxane)

“…[11][12][13][14][15][16] These observations have motivated numerous studies of the thermodynamics and dynamics of polymer blends. Although the effect of pressure, with its relevance in polymer blend processing, is much less investigated, there are some recent experimental [17][18][19][20][21][22][23][24][25][26] and theoretical 27,28 efforts that focus on the effects of pressure on the dynamic heterogeneity and phase behavior. With respect to the issue of dynamic heterogeneity, there have been two studies in athermal polymer blends/copolymers: polyisoprene-b-poly(vinyl ethylene) (PI-b-PVE) (ΔT g ≈ 60 K) 17 and poly(methyl methacrylate)/poly(ethylene oxide) (PM-MA/PEO) (ΔT g ≈ 180 K).…”

Effect of Pressure on the Miscibility of Polyethylene/Poly(ethylene‐alt‐propylene) Blends

“…The phase behavior of upper critical solution temperature (UCST) blends can be altered significantly by pressure ( p ) 1–7. This observation is particularly relevant to injection molding processes, where p can reach 100 MPa (1 000 atm).…”