Pressure-Induced Compatibility in a Model Polymer Blend

Abstract

Photon correlation spectroscopy measurements on mixtures of chemically similar polymers close to their phase boundary show that pressure enhances their miscibility. This unexpected result, which is shown to be caused by negative excess volume changes on mixing for this partially miscible blend, stresses that pressure can play a complex role in determining the miscibility and hence the processing of polymeric mixtures. [S0031-9007 (98)06644-7] PACS numbers: 61.41. + e, 64.60.Ht, 64.75. + g The phase separation of polymer blends is primarily driven by the reduced entropy of mixing as compared to small molecule analogs [1]. This basic fact is captured by incompressible Flory-Huggins (FH) theory [2], which predicts that the free energy of mixing per lattice site, DG mix , isN A and N B are the degrees of polymerization for the two polymers, and f is the volume fraction of species A [1]. x~͑2´1 2 2´1 1 2´2 2 ͒ is the interaction parameter which has a purely enthalpic origin in the theory, and´i j is the energy of interaction between a nearest neighbor i-j pair. In contrast to these ideas experimental work has established that additional factors, not incorporated in FH theory, can play an important role. x has therefore been assumed to be of the general form,x ϵ x h ͞T 2 x s , where x h ͞T and x s are usually interpreted as the enthalpic and entropic contribution, respectively [3,4]. An important factor determining blend behavior is their finite compressibility, an issue which is important, for example, when blend phase behavior is studied as a function of pressure [5]. Note that the FH theory, being incompressible, would suggest that pressure is an irrelevant variable. Relative standard thermodynamics [6] shows that the pressure dependence of the critical temperature is ͑≠T c ͞≠P͒ f TDV mix ͞DH mix . Since DH mix , the enthalpy change on mixing, is positive at the critical point, the sign of this derivative is controlled by DV mix , the volume change on mixing. In all polymer blends investigated to date ͑≠T c ͞≠P͒ f . 0 implying that DV mix . 0. A simple equation of state, such as the lattice fluid model (which extends FH theory through the addition of free volume [3]), shows that to leading order DV mix f͑1 2 f͒ ͕4x 2 ͓͑´1 1 2´2 2 ͒͞RT͔ 2 ͖. For many common blends, such as those studied in past work, the x parameter is large, and therefore the lattice model predicts DV mix . 0. In contrast, for a carefully selected system with chemically similar monomers, where x is positive but small, there exists a possibility for DV mix , 0. These predictions are consistent with the recent work of Foreman and Freed [7] using the lattice cluster model. Such situations, which would result in a previously unobserved pressure induced compatibilization, are the focus of this paper. We conducted free space Monte Carlo simulations on mixtures of chains, where both components are of length N 25. These simulations build on our past work [8] where we considered that the monomers on the different chains interacted through standard Lennard-Jo...

Abnormal Pressure Dependence of the Phase Boundaries in PEE−PDMS and PEP−PDMS Binary Homopolymer Blends and Diblock Copolymers

“…A similar behavior was found in the binary blend PEMS/PDMS 22 and in the diblock copolymers PEP-PEE, 18 PS-PI, 20 and PS-PMPS. 20 To better understand the origin of this behavior, we have to further analyze the FH parameter and the Ginzburg number.…”

“…19,20 In ref 19 the abnormal behavior was indirectly concluded from the ClausiusClapeyron equation after measuring the density discontinuity at the phase transition and the enthalpy of mixing; this prediction was later confirmed in a scattering experiment. 20 In the meantime such an abnormal behavior of the phase boundary was also found for a sample of PEMS/PDMS blend, 22 and experiments on samples of corresponding PB/PS blend and diblock copolymers both show a normal pressure behavior with respect to pressure. 15 The studies presented here were performed on a binary polymer blend and three diblock copolymers with three different monomers.…”

Thermodynamics and Kinetic Processes of Polymer Blends and Block Copolymers in the Presence of Pressurized Carbon Dioxide

“…35 MPa, the maximum pressure examined, the cloud point temperatures of the blends have nearly returned to their initial values at ambient pressure. This trend has not been observed for the This result complements the studies of Beiner et al, [94,100] who have examined the effect of hydrostatic pressure on PDMS/ PEMS blends, and report that this system exhibits atypical behavior for pressurized UCST blends, i.e., the phase miscibility improves (the phase boundary is lowered) upon pressurization. They have attributed [94,100] this unexpected observation to a negative change in DV mix .…”

“…This trend has not been observed for the This result complements the studies of Beiner et al, [94,100] who have examined the effect of hydrostatic pressure on PDMS/ PEMS blends, and report that this system exhibits atypical behavior for pressurized UCST blends, i.e., the phase miscibility improves (the phase boundary is lowered) upon pressurization. They have attributed [94,100] this unexpected observation to a negative change in DV mix . Since the phase boundary of UCST blends depends on both DH mix (which is always positive as a consequence of endothermic mixing) and DV mix through Equation 9, the direction in which the boundary shifts upon hydrostatic pressurization is dictated by the sign of DV mix .…”

Calculation of the effect of macromolecular architecture on structure and thermodynamic properties of linear–tri-arm polyethylene blends from Monte Carlo simulation

“…For all compositions, blend density is higher compared to either of the two pure polymers indicating that the blends examined assume negative excess volume values. Such behavior agrees with lattice model predictions for poly(ethylene-alt-propylene) e head-to-head polypropylene blends [27] and with Monte Carlo simulations on model polyethylene blends [64]. As a result, one may conclude that pressure is expected to have a substantial effect on the mixing properties of this blend.…”