March 21, 2014 - 3:00pm - 4:00pm
Ricardo Andrado Abstract:
Usually, when studies on uniaxial extension of polymer melts are performed the main concern is the achievement of the largest possible deformation with a homogeneous stretching of the sample at any given deformation rate. Therefore, it is necessary to ensure that the correct experimental conditions are achieved, which, more often than not, is not the case. In fact, despite a wealth of work available in the open literature that highlights the importance of correctness of the experimental method, many researchers still disregard basic experimental phenomena such as, for example, the almost certain discrepancy between the nominal and the true strain rates being imposed on the sample, poor sample preparation, inadequate sample dimensions and the effects of sample sagging.
In this work we build on previous works by our group and use our new MERA, Meissner Extensional Rheometer Accessory, based on the Meissner principle but small enough to fit in the oven of a standard rotational rheometer, to highlight the importance of truly controlling the strain rate in rate-controlled experiments in order to achieve high deformations in uniaxial extension of polymer melts. This can only be achieved by coupling the ability of the MERA to achieve very high strains while visually accessing the sample during deformation and using a feedback control loop to keep the desired rate constant. By using the control loop to compensate for the deviations in the expected cross-section evolution over time it is possible to minimize or even eliminate both slip at the rollers and the planar component of the deformation known to be present when relatively wide samples are used, e.g., to prevent sagging in SER-type devices. With proper experimental care, we have been able to perform true constant rate experiments in various polymers up to Hencky strains of more than 8, depending on the polymers. The latter case corresponds to a linear deformation of almost 4,000.