Shear-induced Phase Separation Research Articles

Shear flow and the phase behaviour of polymer blends

The effect of simple shear flow on the phase diagram of partially miscible polymer blends has been analysed through the use of a generalised Gibbs energy of mixing, modified by a stored energy term. The latter is defined as the energy stored by the polymeric molecules during flow, and is seen to have a significant influence on the magnitude and direction of the change in the phase boundary caused by shear. Computer simulations indicate that flow induces mixing when the excess stored energy is negative and phase separation when the excess stored energy is positive. The dependence of the stored energy on the viscosity of the polymer blend was investigated for seven model blends exhibiting different viscosity-composition behaviours. A negative deviation of the viscosity-composition curve from the linear mixing rule was found to be a sufficient condition for a negative excess stored energy, resulting in shear-induced mixing. This prediction is in qualitative agreement with the experimental results on the effect of shear on the phase diagram of poly(styrene-co- acrylonitrile) poly(methymethacrylate) blends. In contrast, a positive deviation of the viscosity-composition curve from additivity was found to be a necessary but not sufficient condition for a positive excess stored energy which would cause shear-induced phase separation. This analysis seems to indicate that the viscosity-composition behaviour of a polymer blend could be used to qualitatively anticipate the change in the phase diagram of a polymer blend upon flow.

Structure Evolution of a Polymer Solution at High Shear Rates

We utilized large angle light scattering to study the structure of a sheared semidilute polymer solution. Below a critical shear rate γ̇c we observe an enhancement of concentration fluctuations in accordance with previous experimental and theoretical work. Above γ̇c we find dramatic changes in the transient and steady state scattering behavior which indicate a shear-induced phase separation. A new four-peaked structure which coarsens in time is observed which is superimposed upon the scattering characteristic of low shear. The four-peaked scattering structure evolves into two peaks along the gradient direction, implying the formation of phase-separated domains elongated along the shear direction.

Shear-induced phase separation of complex fluids: The role of flow-concentration coupling.

We propose a classification scheme for the instabilities that might arise in sheared complex fluids. A central role is played by the coupling between the flow and the solute concentration, which is a combination of the parameters describing (a) the tendency of solute molecules to migrate to regions of small or high shear rates and (b) the variation of the viscosity with concentration. Using a mean field approach, we show that the nature and the geometry of the instability can be predicted from the knowledge of the coupling parameter, the diffuson coefficient, and the derivative of the stress with respect to the shear rate. We also successfully compare a description of the variation of the stress at and just beyond the instability threshold, with experimental results from a wormlike surfactant solution of ${\mathrm{CPClO}}_{3}$.

Rheo-optical studies of stress-induced phase changes in blends exhibiting UCST: Polystyrene and polyisobutylene

Phase transition for blends of polystyrene (PS) (Mw=800, Mw/Mn=1.3)/polyisobutylene (PIB) (Mw=500) were studied in the quiescent state and under shear flow. The cloud points of blends were determined by turbidity measurements at a fixed cooling rate of 0.5 °C/min. The phase diagram of PS/PIB blends shows a UCST behavior with a bimodal miscibility gap at low temperature. The shear effects on the phase transition behavior of PS/PIB blend was investigated by a flow-turbidity apparatus which was constructed by modifying a Rheometrics Mechanical Spectrometer (RMS). No shear-induced phase separation was observed in the one-phase region. For temperatures within the two-phase region, the transmitted light intensity increased abruptly when the shear was applied. The intensities decreased and approached to the initial values when the shear was removed. Several possible explanations will be discussed to explain these results. The one that appears to be most valid is that of shear-induced phase mixing.

Rheo-Optiics for Multicomponent Liquids

AbstractThe application of techniques in optical rheometry for the study of multicomponent systems is reviewed. Small angle light scattering (SALS) patterns are related to the structure of concentration fluctuations with length scales of the order of the wavelength of light. Scattering techniques such as SALS and scattering dichroism have been applied to monitor the transient evolution of anisotropic concentration fluctuation enhancement during simple shear induced phase separation in a semi-dilute solution of polystyrene (PS) in dioctyl phthalate(DOP). Furthermore, the Onuki- Doi theory relating scattering dichroism and structure factor has been used to verify the consistency between scattering dichroism and anisotropy in structure factor. Infrared polarimetry is a useful technique in probing the transient microstructural orientation of individual chemical species in multicomponent systems. The simultaneous measurement of intrinsic infrared dichroism and birefringence is particularly effective and has been employed to monitor component relaxation dynamics in miscible blends of poly(ethylene oxide) and poly(methyl methacrylate). Polarization Modulated Laser Raman Scattering (PMLRS) has been successfully employed to study the orientation dynamics of a polymer melt subjected to transient uniaxial extension. PMLRS provides quantitative information about the time evolution of both the second and fourth moments of the orientation distribution function of molecular segments.

Shear-Induced Phase Separation in Polymer Solutions

We examine spinodal decomposition induced in sheared semidilute polymer solutions which were stable in quiescent states. We find a transverse elastic force acting on polymers. It causes a diffusive flux of polymers from lower to higher concentration regions in mechanical equilibrium. This mechanism can also explain formation of a slipping layer in capillary flows. The scattering amplidue is also calculated in weak shear.