Rheology is the study of flow and/or deformation of materials under applied forces. Using a plate-plate geometry, where the top and bottom features are flat plates, the measurement of rheological properties such as shear viscosity, normal stress, dynamic modulus and phase angle are enabled. Although this rotational rheology technique is well established, there is continuously growing interest in the combination of rotational rheology with other characterisation techniques to allow simultaneous measurements of viscoelasticity and additional physical properties.
Oleksandr Mykhaylyk et al. recently reported a new technique, shear-induced polarised light imaging (SIPLI), which combines rotational rheology with a reflection polariscope in order to study the behaviour of macromolecules and nanoparticles through changes in the birefringence properties of a material. Birefringence occurs when there is shear-induced orientation of particles which creates optical anisotropy. Birefringent materials change the orientation of the polarised light used to illuminate the sample, allowing it to pass through an analyser which is orthogonal to the axis of polarised light. The result of shear-induced orientation is a characteristic Maltese cross pattern. Conversely, non-birefringent samples do not change the orientation of plane-polarised light and thus the resulting polarised light image appears dark.
One application for SPILI presented by Mykhaylyk et al., is for the study of thermo-responsive block copolymer micelles. A number of particle morphologies can be produced from the self-assembly of amphiphilic block copolymers in a solvent selective for one of the blocks. These include spheres and vesicles, which form free-flowing liquids, and worms, which form free-standing physical hydrogels. Unlike the non-birefringent nature of the sphere and vesicle morphologies, the uniaxial anisotropy of the worm morphology results in birefringence. Consequently, the thermo-reversible worm-to-sphere transition, and associated reversible de-gelation, of a poly(glycerol monomethacrylate)-block-poly(2-hydroxypropyl methacrylate) hydrogel is an ideal system for SIPLI characterisation.
The rheology data presented in the study confirms the expected reduced viscosity on cooling to 5 °C associated with degelation during the worm-to-sphere morphology transition. Simultaneously, a loss of the Maltese cross pattern is observed due the formation of isotropic spherical particles. On heating, the opposite sphere-to-worm transition occurs resulting in the reappearance of the Maltese cross pattern and an increase in viscosity due to regelation. Here, SIPLI has allowed the study of structural-rheological property relationships of thermo-responsive block copolymer micelles. This demonstrates just one application of SIPLI, with many other advantages and applications of SIPLI discussed in the original article.
Original Article: Applications of Shear-Induced Polarized Light Imaging (SIPLI) Technique for Mechano-Optical Rheology of Polymers and Soft Matter Materials O. O. Mykhaylyk, N. J. Warren, A. J. Parnell, G. Pfeifer and J. Laeuger, J. Polym. Sci. Part B Polym. Phys., 2016, 1–20.
Article by Sarah Byard; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.