Anomalous melting

Polymer melt rheologists constantly applied the notion of wall slip in describing a variety of polymer melt flow anomalies [ 10,11,14]. However, no explicit knowledge of a molecular mechanism for and microscopic origin of wall slip had been available. Therefore, for 40 years no explanation was given about why wall slip was pertinent and how it produced such melt flow anomalies as flow oscillations and sharkskin-like extrudate distortion. One objective of current research around the world is to explore the molecular nature of wall slip and establish a correlation between wall slip and some of the anomalous melt flow phenomena. 

One leading explanation attributes the anomalous melt flow behavior (i.e., flow discontinuity and oscillation) to constitutive instabilities [65]. In other words, the anomalies would be constitutive in nature and non-interfacial in origin. Such an opinion has not only been expressed phenomenologically by Tordella [9b] and many other rheologists but found support from several theoretical studies [65-67]. However, these theories only attempt to describe inherent bulk flow behavior. Thus, a connection between the anomalous flow phenomena and constitutive instabilities was often explored without any account for possible molecular processes in the melt/wall interfacial region. 

Wu, J. McKenna, G. B., Anomalous Melting behavior of Cyclohexane and Cyclooctane in Poly(dimethylsiloxane) Precursors and Model Networks. J. Polym. Set. PartB Polym. Phys. 2008,46, 2779-2791. 

More than 80 years ago, helium was found to exhibit a liquid-liquid phase transition at very low temperature T < 3K) [19]. However, this phase transition is due to quantum mechanical effects (and it is not a first-order phase transition either). That first-order phase transitions could exist in classical liquids, at much higher temperatures than that characterizing helium s LLPT, was not realized until much more recently. In 1967, Rapoport published an article on the anomalous melting curve maxima observed in systems such as cesium and rubidium [20]. His work was based on statistical mechanics calculations using a two-species model for liquids. In this work, he noticed that for particular parameterizations, the model predicted the existence of an LLPT. However, due to lack of experimental evidence at that time, he did not explore the predictions of the model for polymorphic liquids [20,21]. Similar models to that used in Ref. [20] were studied by Aptekar and Ponyatovsky (see Ref. [22] and references therein). 

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