Shear-Induced “Homogenization” of a Diluted Polymer Blend

Abstract

The miscibility of a diluted polymer blend under steady shear has been investigated in the twophase region using fluorescence and phase-contrast microscopy. Critical exponents describing the shape of the coexistence curve and the shift of the critical temperature DT c ͑ ᠨ g͒ were compatible with expectations based on renormalization group and mode-coupling theories, but the introduction of a reduced variable description of DT c suggested a tendency for T c to saturate at high shear rates.[S0031-9007 (97)02774-9] PACS numbers: 83.80.Es, 64.75. + gThe effects of shear on the critical temperature T c has been studied in small molecule binary mixtures as well as polymeric blends [1][2][3][4]. Because of the relatively large influence of shear and the slow dynamics of polymeric systems, it has been possible to perform more comprehensive measurements on bulk [3] and diluted [4] polymer blends using a combination of small angle neutron scattering and dynamic light scattering techniques. A comparison [5] of experimental results obtained in the one phase region with the theory of Onuki and Kawasaki [6] indicates a remarkable consistency. However, many questions remain about the influence of shear on the phase stability of mixtures. For example, the combined influence of additives (e.g., solvent for the blend) and shear on the phase stability of mixtures and the nature of mode-coupling effects in the unstable regime have received limited attention. The present paper considers these problems based on the existing Onuki-Kawasaki framework.Previous measurements [2] on diluted polymer blends in the unstable regime under conditions of steady shear showed a marked decrease of scattering intensity at high rates of shear. This shear-induced "homogenization" effect was interpreted in terms of a shift of the phase boundary with shear, as in earlier shear-scattering measurements on small molecule mixtures in the one-phase region [1]. The effect of droplet distortion on the shear-scattering measurements was minimized by considering the projection of the scattering data along the vorticity direction. Estimates of the integrated intensity along this direction from shear light scattering measurements showed a tendency to strongly decrease and then to saturate at high shear rates. The characteristic shear rate ᠨ g c , corresponding to this saturation point, was then employed along with theoretical arguments to estimate the shift of the phase boundary with shear. While this procedure led to estimates of the critical temperature shifts with shear that were apparently consistent with the Onuki-Kawasaki (OK) theory [6], it seems unclear to us whether the diminished scattering intensity ("homogenization") reflected a true shift of the phase boundary or merely the breakup of droplets under shear to a scale undetectable by light scattering measure-ments. Since this kind of scattering intensity measurement depends on the history of the shearing measurement, the procedure of extrapolation, and the sensitivity of the detection system, it seems nece...

Phase transitions of fluids in shear flow

“…We believe that the observed downward shift should be due to the hydrodynamic interaction, which is consistent with (2.26). Very recently Yu et al [68] have used fluorescence and phasecontrast microscopy on a similar ternary mixture of PS + PB in DOP, and have reported that the shift tends to saturate at very high shear.…”

Shear-induced structure in polymer blends with viscoelastic asymmetry

“…When subjected to stresses induced by shear flow, the response of such mixtures depends strongly on a number of factors, including the viscoelasticity of the components, the interfacial tension between the phases, the proximity to a critical point, and the presence of a surfactant. Semidilute entangled polymer solutions, for example, exhibit shearinduced phase separation, 21,28 -30 while the shear typically has a homogenizing effect on phase-separating polymer blends, 31 which can form stringlike patterns in the presence of the flow field. [10][11][12]32,33 The addition of modest amounts of block-copolymer surfactant, however, can lead to shearinduced order 34 and shear-induced turbidity 35 in such blends, and a universal understanding of the shear response of these fluids, although highly desirable from both a pure and applied perspective, is clearly a daunting and challenging task.…”

Phase‐separated polymer blends: Complementary studies between scanning electron microscopy, epifluorescence microscopy, and fluorescence microspectroscopy

“…In addition, fluorescence techniques are useful in studies of polymer compatibility either by time‐resolved or by steady‐state methods 11–16. Some of these methods involve comparison among the temperature dependence of the fluorescence spectra of fluorophores dissolved in the blends and in the isolated homopolymer 17–19. Other methods are based on the measurement of the nonradiative energy transfer efficiency that depends on the distance between both fluorophores and, thus, on the interpenetration between the phases 15, 20–24…”

“…Although the resolution and/or the contrast of the image of the domain structure is worse with fluorescence than with phase contrast microscopes, epifluorescence microscopy was revealed to be quite a useful technique for analyzing the distribution of the fluorescent component of the blend. The reason is the high sensitivity of the fluorescence intensity to small composition changes undetectable by the light‐scattering technique 19…”