Rheo-optical investigation of immiscible polymer blends
Introduction
Properties of immiscible polymer blends are known to depend strongly on the detailed morphology (see e.g. Ref. [1]). During processing, flow can cause drastic changes in the microstructure in two-phase fluids. The resulting morphology will depend on process parameters and on material parameters such as viscosity η and elasticity of the components and their interfacial tension Γ. The evolution of the microstructure in fluid–fluid systems has been the subject of many theoretical and experimental studies since the early work by Taylor2, 3. The majority of the direct experimental evidence has been obtained by microscopic observation of single droplets. For real polymer blends, which are processed at elevated temperatures, most of the structural information has been derived from the analysis of specimens which have been cooled-down from the melt after processing (see e.g. Refs 4, 5). Scattering techniques have been suggested to study morphology in real time during flow. This topic has been reviewed recently[6], and it turns out that collecting scattering patterns is not always the most optimal manner with which to probe structural changes.
Here we will investigate the possibility of using rheo-optical techniques, in particular linear conservative dichroism Δn″, to follow in situ the time evolution of the microstructure. Dilute mixtures of immiscible polymers subjected to simple shear flow are used for this purpose. Conservative dichroism measures polarisation-dependent attenuation of light. In a non-absorbing material the transmitted light intensity in perpendicular polarisation directions will be different if the scattering is anisotropic. In this manner dichroism probes changes in the microstructure. Dichroism measurements during flow have been used successfully to detect flow-induced structural changes in particulate7, 8, 9, 10and phase-separating systems[11]. In immiscible polymer blends, the flow will affect size, shape, orientation and relative position of the droplets. This will normally generate a change in the global anisotropy and hence in dichroism. Therefore it should be possible to follow the various structural changes in such a system by the said technique. The dichroism measurements will be supplemented here by flow-SALS to support structural interpretation of the data and to provide additional information.
Section snippets
Experimental
The measurements of the conservative linear dichroism have been performed on an Optical Analyzer (ROA) from Rheometric Scientific Inc. This instrument uses a polarisation modulation technique to measure linear birefringence and dichroism[12]. An He–Ne laser serves as a light source with a wavelength of 633 nm. Before the light reaches the sample it is modulated by a halfwave plate rotating at 2000 revolutions per second. Only the light that leaves the sample at zero angle is detected.
Behaviour during steady-state shear flow
Fig. 3 shows the dichroism as a function of shear rate for the two 1.0% blends at 15°C. The data have been obtained by the procedure described in the previous paragraph. At 15°C the viscosity ratio is unity. The accessible range of shear rates is limited by shear fracture, in particular when the more elastic PDMS constitutes the continuous phase. The two samples produce similar values for the dichroism during flow. As the viscosity ratio is unity, this result could be expected for blends with
Conclusions
The flow-induced changes of the microstructure in immiscible polymer blends can be investigated by means of rheo-optical techniques. Linear conservative dichroism and flow small-angle light scattering are particularly useful for that purpose. It has been demonstrated that these techniques can detect detailed changes in microstructure in real time during flow. The various mechanisms for structural change could be distinguished: droplet deformation, fibril formation, droplet break-up and
Acknowledgements
Partial support from the European Union through a Brite/Euram grant (No. CT 92 0213), from the Nationaal Fonds voor Wetenschappelijk Onderzoek (Belgium) through a FKFO grant and from the Research Council of the K.U. Leuven are gratefully acknowledged.
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Present address: Department of Chemical Engineering, ZheJiang University, Hangzhou 310027, Peoples Republic of China.