Assessment of the Relaxation Time Under Controlled Shear Flow by Rheo-Optical Characterization

Abstract

An attempt to study the relaxation characteristics of some polymers during a controlled shear flow is described in this paper. In polymers, optical properties can be used to estimate morphological changes under flow and, for this reason, are a trustful tool to evaluate relaxation time. This experiment consists of varying the shear rate imposed on polymeric systems of polystyrene (PS), polypropylene (PP), and high-density polyethylene (HDPE) while collecting the light signal isothermally at distinct temperatures. Then, birefringence is evaluated via an own built optical sensor by measuring the transmitted light intensity with crossed polarizers. Temperatures from 180°C to 220°C are performed and shear rates between 3 and 180 s⁻¹ are imposed on the samples in agreement with the rates implemented in a previous rheological characterization. Each polymer shows characteristic values of birefringence that intensify with the increase in shear imposed by the system. The relaxation time is directly influenced by temperature and shear, decreasing for higher shear rates and temperatures. The relaxation time values found with our optical apparatus are validated by rheometric measurements. The values in both techniques are very similar in the proposed range of temperature, showing the effectiveness of the technique detailed in this article.