Superdendrites in Directional Solidification of Polymer-Solvent Mixtures
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ABSTRACT The directional solidification of the transparent binary alloy succinonitrile-poly(ethylene oxide) was studied in an experiment in which solidification speeds of about 2 mm/sec could be reached without loss of the linear temperature gradient. The low diffusivity of the polymer solute allowed the study of the dynamics of rapid solidification using an optical microscope. For both normal and doublonic dendrites we observed a transition to large triangular "superdendrites" above a certain solidification speed and we report measurements of the primary and secondary spacing as a function of the pulling speed. Our measurements suggest that the observed triangular shape is due to a decoupling of primary and secondary growth at large undercooling.
INTRODUCTION Rapid solidification of metallic alloys at solidification speeds v > 1 m/sec results in a variety of morphological microstructures that have been the subject of extensive studies [1]. The high solidification rates and opacity of metallic alloys make in situ experiments with direct visualization of the growing microstructures difficult [2, 3]. At modest solidification rates, model systems using transparent alloys such as succinonitrile-acetone have given much insight into the spatio temporal dynamics of dendritic growth [4]. However, rapid solidification rates with strongly nonequilibrium growth conditions are difficult to realize experimentally. In addition, time-resolved observation of the rapid spatio-temporal dynamics is impossible with current imaging techniques. We have attempted to overcome these difficulties by using a slowly diffusing polymeric solute, poly(ethylene oxide) (PEO), which has a diffusivity typically three orders of magnitude lower than the diffusivity of its low molecular weight counterparts (e.g. acetone). The time scale for redistribution of solute ("impurity") is increased which results in a drastic reduction of the solidification speeds for a given undercooling. This makes possible the study of the dynamics of the solidification front using standard video microscopy. The disadvantages of this approach are a low solubility of polymer in the solvent and a relative lack of knowledge of the physical properties of the model system. The solidification of binary polymer-solvent alloys is of interest not only as a model system for metallic solidification, but also in its own right. An understanding of the solidification process may allow the development of methods for the controlled growth of porous polymeric materials with crystalline additives. This was recognized by Smith and Pennings who investigated a number of eutectic polymer-solvent alloys [5, 6], but did not study the dynamics of the evolving liquid-solid interface.
65 Mat. Rea. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
EXPERIMENT Our solidification experiments were carried out using two different temperature stages. In the first design, sample cells of dimensions 50 pm by 15 mm by 75 mm were constructed from polished glass plates that were epoxied together to f
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