Article pubs.acs.org/Langmuir
Spreading Dynamics of Molten Polymer Drops on Glass Substrates Yichuan Zhang,*,†,‡ Carlos A. Fuentes,† Robin Koekoekx,§ Christian Clasen,§ Aart W. Van Vuure,† Joel̈ De Coninck,‡ and David Seveno† †
Department of Materials Engineering, KU Leuven, 3001 Leuven, Belgium Laboratory of Surface and Interfacial Physics, Université de Mons, 7000 Mons, Belgium § Department of Chemical Engineering, KU Leuven, 3001 Leuven, Belgium ‡
S Supporting Information *
ABSTRACT: Wetting dynamics drive numerous processes involving liquids in contact with solid substrates with a wide range of geometries. The spreading dynamics of organic liquids and liquid metals at, respectively, room temperature and >1000 °C have been studied extensively, both experimentally and numerically; however, almost no attention has been paid to the wetting behavior of molten drops of thermoplastic polymers, despite its importance, for example, in the processing of fiber-reinforced polymer composites. Indeed, the ability of classical theories of dynamic wetting, that is, the hydrodynamic and the molecular-kinetic theories, to model these complex liquids is unknown. We have therefore investigated the spreading dynamics on glass, over temperatures between 200 and 260 °C, of two thermoplastics: polypropylene (PP) and poly(vinylidene fluoride) (PVDF). PP and PVDF showed, respectively, the highest and lowest slip lengths due to their different interactions with the glass substrate. The jump lengths of PP and PVDF are comparable to their Kuhn segment lengths, suggesting that the wetting process of these polymers is mediated by segmental displacements. The present work not only provides evidence of the suitability of the classical models to model dynamic wetting of molten polymers but also advances our understanding of the wetting dynamics of molten thermoplastics at the liquid/solid interface.
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INTRODUCTION Numerous modern technologies are dependent on the precise control of liquid spreading, such as the application of adhesives, painting, and oil recovery as the movement of a liquid front often controls their efficiency and stability.1−4 As a consequence, spreading phenomena have been studied extensively, from both theoretical and experimental aspects.5−8 To date, the spreading dynamics of both classical liquids (water and organic liquids) and liquid metals at, respectively, room temperature and temperature above 1000 °C are well documented.9−11 However, compared with classical liquids and liquid metals, little attention has been paid to the wetting behaviors of molten thermoplastic polymers despite their importance, for example, in the processing of fiber-reinforced polymer composites.12 The root of this issue might be due to the experimental difficulties associated with the manipulation of high viscous molten polymer liquids, to their sensitivity to temperature, and to the precise control of the environment. To describe the dynamics of wetting, two main different theoretical approaches have been proposed: the hydrodynamic approach (HD) and the molecular-kinetic theory (MKT), which differ from each other mostly in the consideration of the dominant channel of dissipation.2,3,5,6 The first approach emphasizes the dissipation due to viscous flows generated in the core of the spreading drop and has been scrutinized by many researchers during the last several decades.13−16 In © 2017 American Chemical Society
contrast with the HD approach, the MKT proposed by Blake and Hayes17 concentrates on the dissipative processes occurring in the vicinity of the advancing contact line. This prediction has also been successfully tested against numerous experimental and numerical liquid/solid systems,3,5,10,18 yet, to our knowledge, there is no knowledge about the relevancy of the HD approach and the MKT to model the behavior of molten thermoplastic polymers. Actually, the slip length on the microscale and the contact line friction on the molecular scale, obtained from the HD approach and MKT, respectively, can provide a comprehensive basis for understanding and elucidating the flow behaviors of polymer melts during processing. This is important for controlling the structural evolution and suppressing unstable surface defects (such as sharkskin commonly observed in polymer extrudates) of polymer products.19−21 This work therefore investigates the spreading dynamics of two typical thermoplastics, polypropylene (PP) and polyvinylidene fluoride (PVDF), on clean glass substrates. These polymers were chosen based on the fact that they should have different physical interactions with this substrate and are commonly used to process glass fiber-reinforced polymers. The Received: May 3, 2017 Revised: July 13, 2017 Published: August 2, 2017 8447
DOI: 10.1021/acs.langmuir.7b01500 Langmuir 2017, 33, 8447−8454
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Figure 1. Schematic illustration of the testing procedure. (a) Pendant drop for surface tension measurements and (b,c) spreading dynamics. melting of Sn (melting point: 231.9 °C) at different positions away from the bottom substrate. (The distance between the needle tip and the substrate is ∼15 mm.) The melting of Sn was observed at 231.8, 232.6, 234.2, 233.6, and 232.2 °C at, respectively, positions of 0, 4, 8, 12, and 15 mm away from the substrate. Therefore, the temperature deviation is
