Superdendrites in Directional Solidification of Polymer-Solvent Mixtures

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.

Modeling of Macrosegregation and Solidification Grain Structures with a Coupled Cellular Automaton-Finite Element Model

“…Instability is predicted by the CAFE model and observed in the segregation maps computed for single grains. It develops in conditions very close to those known by several authors [44][45][46] to promote the formation of "superdendrites", i.e. limited temperature gradient in the melt and a decreasing growth front velocity.…”

Preparation of poly(L-lactic acid) honeycomb monolith structure by unidirectional freezing and freeze-drying

“…5c and f and 6c and f). The structure resembles the superdendrite structure observed by Ragnarsson et al (1998). PLLA as well as 1,4-dioxane is likely to contain moisture.…”

“…The solvent was crystallized and grown in the freezing direction due to the constitutional supercooling. When the degree of supercooling was large during the crystallization process, Mullins--Sekerka instability occurred and created crystals with cellular (microtube like) or dendrite (ladder like) structures (Mullins and Sekerka, 1964; Ragnarsson et al, 1998). In the course of directional freezing, PLLA was expelled from solvent crystals to the liquid phase and the constitutional supercooling condition was established at the interface.…”

Preparation of Porous Poly(l-lactic acid) Honeycomb Monolith Structure by Phase Separation and Unidirectional Freezing

“…The solvent was crystallized and grown in the freezing direction by constitutional supercooling. When the degree of supercooling was large, the crystal structure formed dendrite (ladderlike) structures, following the Mullins−Sekerka instability theory. , In our previous paper, moisture in the polymer acted as an impurity, increasing the instability and enhancing the transition from a cellular to a dendritic structure. In the polymer blends studied in the present work, the minor polymer did not hinder the creation of smooth-walled (cellular type) microtubes when the compatibility of both polymers was high.…”

“…The solvent was crystallized and grown in the freezing direction due to the constitutional supercooling mechanism. When the degree of supercooling was large during the crystallization process, Mullins−Sekerka instability occurred and created crystals with cellular (microtube-like) or dendrite (ladderlike) structures. , While the crystals grow, the polymers are expelled from the solvent crystal. In the course of unidirectional freezing, the concentration of PLLA and PEG in the liquid phase surrounded by 1,4-dioxane crystals increased, and the 1,4-dioxane content decreased (Figure ).…”