The concentration Auctuations induced in a semidilute polymer solution by shear flow are directly visualized for the first time. The spatial structure of the "butterAy pattern" observed by light scattering is elucidated, being the result of hydrodynamic rippling waves which have a distribution of tilt angles.The minimal tilt angle depends on shear rate in a power law fashion, causing the butterAy to almost close upon itself at high shear rates. The concentration waves have a time scale related to a shear rate j -l sec and periodicity A, -l 0 pm, indicating a link to reptation processes. PACS numbers: 61.2S. Hq, 47.50.+d, 83.50.Ax Shear induced phase separation is one of the striking results of combining hydrodynamics with the special elastic and viscous properties of polymers, and has been the center of a concentrated effort in recent years fl-10]. It originates in the profound effects of entanglement and coil stretching upon the flow of semidilute, high molecular weight polymer solution. While our notion of mixing by shear flow is intuitively consistent with shear induced homogenization in lower molecular weight systems [11], the appearance of systems where shear induces phase separation was quite remarkable. When shear induced separation was first observed experimentally [1,2], it was attributed to an elastic term in the free energy [2). Such a contribution could move the critical curve suSciently to allow phase separation at elevated temperatures. More recently, Helfand and Frederickson (HF) suggested [3) that this separation is not a thermodynamic phase transition, but rather a nonequilibrium result of the coupling between the stress field and the polymer concentration field.A first clue to the hydrodynamic origin of the separation can be found in light scattering experiments, which showed a strong anisotropy of the scattering pattern with respect to the flow direction [6,8]. In these experiments the salient feature is the "butterfly pattern" obtained from small-angle light scattering (SALS). It is characterized by a dark streak in the direction perpendicular to the flow, where there is no enhancement of the scattering. In the direction of the flow a strong enhancement of the scattering occurs that is limited in width to an opening angle 2a which increases with shear rate. Such anisotropy would not occur if the transition were a thermodynamic phase transition. A qualitative confirmation of the HF theory was given in a Couette flow light scattering experiment [7), where the experimental axes of the scattering rays were made to correspond to those in the calculation of HF. In this paper we present the first direct observation and visualization of the actual participating hydrodynamic modes.The butterfly pattern has also been observed in other polymer systems, and is attracting much attention [12][13][14][15][16][17]. It has shown up in stretched rubber, polymer gels, and polymer melts. There it is found at much smaller wavelengths, by use of neutron scattering. It is worth noting that the butterfly pattern w...
We investigated time evolution of structures induced by imposing shear flow on semidilute solutions of high molecular weight polystyrene (PS) by means of both flow light scattering and rheology. PS with weight-average molecular weights of 5.48 × 10 6 and 2.89 × 10 6 were dissolved in dioctyl phthalate (DOP). After imposing shear flow with a shear rate γ > γ c (the critical shear rate for shear-induced concentration fluctuations), the unique butterfly-type scattering pattern appeared. The pattern had a scattering maximum along the flow direction. The wave number qm and the intensity Im at the scattering maximum decreased and increased, respectively, with time and eventually reached steady state values. The rheological experiments that were carried out on the same solution under the same shear flow revealed two stress overshoots. The second overshoot was found to be related to the development of butterfly pattern and its time change, i.e., to the formation and growth of shear-induced structures.
Shear-induced structure formation in semidilute solutions of high molecular weight polystyrene (PS) was investigated by rheology, shear-dichroism, shear-birefringence, shear-small-angle light-scattering (SALS) and shear-optical-microscopy methods. At the shear rate γ ˘c < γ ˘< γ ˘a, where γ ˘c and γ ˘a are the critical shear rates for the onset of the shear-enhanced concentration fluctuations and the onset of the anomalies in the rheological and scattering behaviors respectively, we observed negative dichroism, almost zero birefringence, and butterfly type LS patterns. All these results are consistent with the optical images captured by shear-microscopy and show the shear-induced concentration fluctuations with the following characteristics: the amplitude of the fluctuations is enhanced along the flow direction, and the domain structure (regions rich in polymer concentration) formed is extended normal to flow. However, at the shear rate γ ˘> γ ˘a, the dichroism became positive and the absolute values of both the dichroism and the birefringence increased drastically. The changes in the dichroism and the birefringence are believed to be related to formation of the stringlike structures and molecular orientation both parallel to flow.
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