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The tensile properties of polymeric liquids. / Rhodri Brad
Swansea University Author: Rhodri Brad
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The work reported in this Thesis reports on studies of the tensile strength of polymeric liquids by two experimental techniques, namely the Bullet-Piston (B-P) technique and a Capillary Break-up Extensional Rheometer (CaBER). The motivation for this work lies in the fact that although many associati...
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The work reported in this Thesis reports on studies of the tensile strength of polymeric liquids by two experimental techniques, namely the Bullet-Piston (B-P) technique and a Capillary Break-up Extensional Rheometer (CaBER). The motivation for this work lies in the fact that although many associations exist between the cavitation properties of fluids and their extensional flow properties, these associations have never been systematically investigated due to a lack of an appropriate cavitation technique. The work presented m this thesis addresses this, using two custom-built instruments (a filament stretching device and a dynamic stressing technique for cavitation studies). Together, these were used to investigate the appropriate rheological and cavitational characteristics of a range of fluids including model polymer solutions. In experiments in which samples of degassed, deionised water are subjected to dynamic stressing by pulses of tension, the pulse reflection technique allows the rate of development of tension in the liquid, Of, to be varied m a systematic manner, in order to investigate its influence on the resulting measurement of the liquid's 'effective' tensile strength, Fc. Results are reported for a range of stressing rates, Of, ≈ 0.19bar/mus < Of < 0.77bar/mus. These experiments, which are the first of their kind to be reported on water, show an approximately four-fold increase of Fc at the highest stressing rate, this value being 224bar (for Of= 0.77bar/mus) compared to 59bar (for Of= 0.187bar/mus). The present work has resolved a longstanding anomaly concerning the role of polymeric additives in determining the cavitation thresholds of dilute aqueous polymer solutions. For the first time it is shown that with increasing molecular weight there is an increased effective tensile strength of the solution. However, the results reveal that increasing polymer concentration results in a stress saturation level in terms of effective tensile strength. This work is also the first to relate cavitational failure of a fluid and its extensional properties in terms of two appropriately chosen stress parameters, and to report the relationship between these stress parameters on the basis of an experimental study involving two different techniques over a range of stress rates and a wide range of polymer concentration and molecular weight. Despite differences in the magnitudes of the tensile stress parameters, both techniques show that the relevant parameter increases with polymer concentration and molecular weight, but that such stress levels become effectively saturated at essentially the same levels of concentration and molecular weight. This information has never previously been available. As a result of the work reported m this thesis it may now be possible to conduct fluid breakup measurements in extensional flow experiments m order to ascertain the likely levels of cavitation threshold stress for dilute aqueous polymer solutions.
Faculty of Science and Engineering