Strong kinetic effect of polyethylene glycol 6000 under directional solidification condition

Abstract

Interface characterizes describes how the atoms/molecules attach themselves to the solid/liquid interface from the liquid when the crystallization takes place, which plays a key role in revealing the kinetic mechanism during the crystal growth. For common non-facet/non-facet metallic systems, the kinetic undercooling is usually small and it becomes only significant when the growth velocity is high. However, high growth velocity can be usually realized under large undercooling condition. In this case, the interface temperature cannot be measured, thus the kinetic undercooling cannot be determined quantitatively either. Compared with the atom and small molecule materials, the polymer has its distinctive characteristic of different long chains, which are entangled together in a liquid state. Thus the crystallization of the polymer system usually proceeds in the two-dimensional manner, which provides an ideal way to obtain large kinetic undercooling under the small growth velocity condition. The directional crystallization technique has been widely adopted to study the scaling law of undercooling and growth velocity due to its accurate controlling of growth velocity and temperature gradient. Therefore, it offers an appropriate way to make a quantitative investigation. In this paper, the in-situ observations of the solidification of polyethylene glycol 6000 at different pulling velocities are performed and the interface temperature is examined as well by using the directional crystallization technique. The effect of the pulling rate on the growth kinetics is examined. The results reveal that the interface temperature decreases and the undercooling increases gradually with the pulling velocity increasing. A change in the growth regime is observed at T=13.5 K, where regime Ⅱ-regime Ⅲ transition occurs according to Hoffman's kinetic theory of polymer crystallization. The comparison of undercooling between the present work and DSC isothermal crystallization is made, and it shows that the data obtained in the directional growth and the isothermal growth follow the same trends but the undercooling in isothermal growth is larger than in directional growth under the same growth velocity. This indicates that the undercooling in the latter case is over-estimated since it contains the thermal undercooling. Undercooling is the driving force for crystallization, which usually includes solute undercooling, curvature undercooling, thermal undercooling, and kinetic undercooling. Because of the flat interface and the pure material, there is no solute undercooling nor curvature cooling in the present case. The thermal undercooling is also zero in the unidirectional crystallization process. Thus the total undercooling in the present work is the kinetic undercooling. The maximum kinetic undercooling reaches 20 K, indicating that the interface kinetic controlling growth takes place due to the two-dimensional nucleation in polymer.