Mechano-optical rheology of semi-crystalline polymers

O. Mykhaylyk

Soft Matter Analytical Laboratory, Department of Chemistry, The University of Sheffield, Sheffield, S3 7HF, UK

Since polymeric liquids subjected to an external field (flow) often respond with a related anisotropy of their refractive index and stress, flow birefringence is commonly used for structural characterization of these materials. In this respect, rotational geometries are well suited for visual observations of the flow and, consequently, mechano-optical rheology of polymeric liquids. New applications of a rheo-optical method based on a combination of rotational rheology and a recently developed optical technique – shear-induced polarized light imaging (SIPLI) are presented [1]. Simultaneous rheo-optical studies using rheo-SIPLI have already been successfully used for characterization of self-assembled copolymers and liquid crystals [1, 2]. The proposed rheo-optical method is also effective for studying flow-induced crystallization of semi-crystalline polymers (FIC) [3]. Simultaneous optical measurements and mechanical rheology are performed during FIC. These experiments enable a relationship between the shish formation, detected by SIPLI, and the viscosity upturn, measured by the rheometer at the same time, to be established. The results are compared with small-angle x-ray scattering. It is also shown that SIPLI setup can be used for birefringence measurements. The normal stress difference calculated from the birefringence of a sheared polymer melt (polyethylene) correlate well with both the total normal force measured by the rheometer transducer during the same experiment and the first normal stress differences measured independently on the same polymer using cone-and-plate geometry.

[1] O. O. Mykhaylyk, N. J. Warren, A. J. Parnell, G. Pfeifer, and J. Laeuger, J. Polym. Sci., Part B: Polym. Phys. 54, 2151 (2016). (link)
[2] O. O. Mykhaylyk et al., Macromolecules 45, 5260 (2012). (link)
[3] O. O. Mykhaylyk, Soft Matter 6, 4430 (2010). (link)