Structure and Shear-Induced Order in Blends of a Diblock Copolymer with the Corresponding Homopolyme

Abstract

The effect of shear on soft materials is relevant to their processing, for example mixtures of polymers are subjected to shear during flow and this can have a profound influence on their miscibility and rheology. Shear-induced mixing or demixing of polymer blends have both been observed. Shear-induced mixing is the typical behaviour of blends of low molar mass polymers and polymer solu- tions. However, shear-induced demixing can be observed for solutions of high molar mass polymers or polymer blends at high shear rates. Block copoly- mers are widely used to compatibilize otherwise immiscible polymers [1-4]. Approximately ten years ago, observations on polymeric microemulsions were first reported, these being formed at low copolymer concentrations in a blend of two homopolymers with a small amount of the corresponding diblock [5-8] These systems have immense potential due to the intimate mixing of the two homopolymers that results when the interfacial tension is near zero, as it is in a microemulsion phase. The size of the phase separated domains in the microemulsion can be tuned through application of shear, and the focus of this project was to understand how shear changes the structure, in terms of enhanced mixing or demixing and also the degree of alignment. Because of the focus on microemulsions, the work is also relevant to conventional am- phiphilic microemulsions, where the effect of shear has been the focus of a few studies [9, 10]. Light scattering has been used by a number of groups internationally to investigate shear-induced structure formation in polymer blends and solu- tions [11-16]. In the case of shear-induced mixing, shear flow leads to highly oriented stringlike domains which produce a correspondingly anisotropic light scattering pattern perpendicular to the shear direction [11-13]. For the case of shear-induced demixing in semidilute polymer solutions, so-called butterfly patterns have been observed; [14-16] also reported for critical polymer blends in the presence of solvent at high shear rates [12]. Light scattering is the most suitable method for investigating phase separation since this typically leads to turbidity in polymer blends, which is the result of density inhomogeneities with a size of the order of that of the wavelength of light. In addition it has been used to probe the shear-induced ordering of surfactant microemulsions (shear leads to the breakup of the microemulsion or sponge and formation of a lamellar phase [9, 10]). However, we are only aware of the work of one group on the effect of shear on polymeric microemulsions, [17, 18] although it has been used to probe shear-induced order in an A/B/AB (A and B indicate distinct polymers) blends [19]. The discovery of polymer microemulsions creates opportunities to study the intricate ordering of micro- and nano-structured soft materials and is the focus of intense research activity internationally, in particular in the US and Japan. It is also of considerable commercial relevance, extending the compatibilization capabilities of present block copolymer-based systems.