We are studying the effect of compressed fluid sorption on phase transitions in polymer blends and block copolymers. The locations of upper order-disorder transitions (UODT) in nearly symmetric styrene-isoprene diblock copolymers in the presence of CO2 and ethane were determined in-situ using small angle neutron scattering (SANS) and optical birefringence measurements. Sorption of either fluid depresses UODT by as much as 70 K over the range of conditions studied. The magnitude of the depression can be controlled by manipulating the mass fraction of fluid sorbed through pressure mediated adjustments in solvent density. For CO2, which is a poor, weakly selective solvent for the blocks, the dependence of the transition on the volume fraction of sorbed solvent is determined and compared to the dilution approximation.
In contrast to solvent-induced miscibility observed in
styrene-isoprene diblock copolymers, CO2 sorption induces
phase segregation in systems exhibiting lower critical phase
transitions including blends of polystyrene/poly(vinyl
methyl ether) [PS/PVME], polyisoprene/polybutadiene, and
polystyrene-block-poly(n-butyl methacrylate). We have
studied the behavior of the PS/PVME/CO2 systems in detail
using SANS and high-pressure fluorescence spectroscopy.
Sorption of a few percent of CO2, which is a poor solvent
for both components but is selective for PVME, depresses the
lower critical solution temperature by nearly 100 K.
Stability arguments suggest the primary driver for phase
separation is the rapid and disparate increases in swollen
volumes and compressibilities of the blend components upon
CO2 sorption. In all cases, the influence of hydrostatic
pressure, inherent to the use of compressed solvents, is
small in comparison to the solvent effects over the range of
pressures studied.
[K8.002] Pressure-Induced Nucleation in Binary Polymer Blends
Nitash Balsara (Polytechnic University, Brooklyn, NY 11201)
The initial stages of nucleation during liquid-liquid phase
separation in mixtures of high molecular weight polymers was
studied by time-resolved small angle neutron scattering.
Phase separation was induced by increasing pressure. We
focused on deep quenches in the nucleation regime. For each
quench, we find that the scattering intensity is independent
of time in the high scattering vector regime, q greater than
q* (q* is a critical scattering vector), during the initial
stages of phase separation. This implies the absence of
structures with length scales smaller than R*=1/q* during
nucleation. This aspect of nucleation is consistent with
classical theories, which predict the existence of a
critical nucleus. However, the dependence of R* on pressure
(i.e. quench depth) is not in agreement with any theory of
nucleation that we are aware of.
[K8.003] Self-Assembly in Aqueous Solution at Kilobar Pressures
Michael Paulaitis (Johns Hopkins University)
This abstract not available.
[K8.004] Effect of Pressure and Temperature on the Phase Behavior of Homopolymers and Blockcopolymer Amphiphiles in Supercritical Carbon Dioxide
George D. Wignall (Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831)
Above its critical point, carbon dioxide (CO2) forms a
supercritical fluid (SCF) that has potential to replace many
of the organic solvents currently used in polymer synthesis
and processing. Small-angle neutron scattering (SANS) has
been applied to characterize CO2-soluble molecules (e.g.
fluoropolymers and siloxanes) and as in organic solvents,
the chain dimensions are unperturbed by long range
interactions at the theta temperature. Above this point, the
system exhibits a "good solvent" domain, where the molecules
expand beyond the unperturbed dimensions. However, unlike
organic solvents, this transition can be made to occur at a
critical "theta pressure" in CO2. Below the theta
temperature and pressure, the system enters the "poor
solvent" domain, where diverging concentration fluctuations
prevent the chains from collapsing and allow them to
maintain their unperturbed dimensions. Other polymers such
as polystyrene (PS) are insoluble in CO2, though they may be
solubilized by means of di-block PS-fluoropolymer
surfactants, which aggregate into spherical micelles. SANS
has shown that these consist of a CO2-philic (fluoropolymer)
shell surrounding a CO2-phobic core, which can solubilize
added homopolymers such as PS. A unique attribute of SCFs is
that the solvent quality may be adjusted over a wide range
by varying the density (pressure), and thus control the
solvent-solute interactions of homopolymers. Similarly, the
self-assembly of blockcopolymer amphiphiles can be
reversibly controlled from unimers to core-shell micelles
and this establishes a critical micelle density, a
phenomenon distinctive of highly compressible SCFs.
[K8.005] Effect of Pressure on Phase Behavior of Semi-Crystalline Polymer Blends
Maulik Modi, Ramanan Krishnamoorti (University of Houston)
The pressure dependence of the phase behavior of
semi-crystalline polyolefin blends has been studied using
small angle neutron scattering. Experiments were conducted
in the melt state for seven chemically distinct mixtures at
their critical composition. The upper critical solution
temperature for all these blends increased with increasing
temperature. Further, the crystallization temperature for
the components also increased with increasing pressure with
approximately the same pressure coefficient. The molecular
significance of these results will be dicussed.
[K8.006] The Effect of Pressure on Polyolefin Blend Miscibility
Jane E.G. Lipson (Dartmouth College), K. Willets, J. Luettmer-Strathmann
The issue of miscibility in polyolefin blends has been the
subject of several recent studies. Both upper and lower
critical solution temperatures (UCSTs and LCSTs) may be
observed in blends having limited miscibility, depending on
the choice of components. In previous work two of us (JEGL
and J L-S) have made use of the lattice Born-Green-Yvon
integral equation theory in analyzing and interpreting the
effect of local structure on blend compatibility. In this
work we turn to the subset of polyolefin blends which
exhibit UCST behaviour and start by making use of the
Clapeyron equation in correlating the volume change on
mixing with the effect of changing pressure on miscibility.
We then investigate the correlation between characteristic
properties of the system, such as the molecular weights of
the components and the mixed-component segment interaction
energy, with the pressure dependence of the UCST.
[K8.007] Temperature- and Pressure- dependent Composition Fluctuations in a d-PB/PS Polymer Blend and Diblock Copolymer
Dietmar Schwahn (Institut für Festkörperforschung, Jülich, Germany), Henrich Frielinghaus (Danish Polymer Centre, Riso National Laboratory, Denmark), Basil Abbas, Lutz Willner (Institut für Festkörperforschung, Jülich, Germany)
The study of thermal composition fluctuations in binary polymer blends delivers the spinodal and binodal phase boundaries, the Flory-Huggins interaction parameter, and the Ginzburg number. The Ginzburg number describes the crossover temperature where fluctuations become important and a transition from the range of the Flory-Huggins mean field theory to the 3d-Ising behavior is observed. According to former estimations polymer blends should be well described by the Flory-Huggins theory with noticeable deviations only very near the critical temperature. SANS experiments, however, have shown that the effect of thermal composition fluctuations is much larger in polymer blends and could even appreciably enhance those in low molecular liquid mixtures. By subsequent experiments in pressure fields it became furthermore clear that the packing of polymers has a strong influence on the deviation from the Flory-Huggins theory. This means that more sophisticated theories including the effect of fluctuations are necessary to interpret the phase behavior of polymer blends and the corresponding SANS data. Another topic is related to the corresponding diblock copolymer where the blocks are the same as the components of the blend.