Viscosity of COC polymer (TOPAS) near the glass transition temperature: Experimental and modeling

TOPAS is a copolymer that is commonly used for making micro fluidic devices for life science applications using the hot embossing technique. In this technique, both micro-scale and nano-scale channels are imprinted onto the copolymer at temperatures near or above its glass transition temperature (Tg). The viscosity of the polymer plays a dominant role in microchannel replication, and the ability to model the polymer rheological flow behavior will enable understanding and prediction of the hot embossing process. Typically in hot embossing, the polymer is deformed at between Tg to Tg + 50 °C and it is difficult to obtain the rheological properties within this temperature range using conventional viscometers. In this paper, the viscosity was measured for four different grades of TOPAS, a cyclic olefin copolymer (COC), namely TOPAS-8007, TOPAS-5013, TOPAS-6015 and TOPAS-6017, in a temperature range from Tg + 20 °C to Tg + 60 °C which is of most relevance to hot embossing. Measurements were made in the dynamic mode and the Cox-Merz rule was applied to obtain the shear viscosity versus shear rate data. The Carreau-Yasuda viscosity model with WLF-type temperature dependence fits the experimental data reasonably well at high shear rates in this temperature range. However, there was significant discrepancy between the model and the experimental data at low shear rates. In contrast, it was found that the Cross model can capture the experimental data reasonable well even at low shear rates. Therefore, the rheological behavior of COC can be represented using the Cross model.