Discontinuous Enhancement of Crystal Growth below Glass Transition(Poster session 1, New Frontiers in Colloidal Physics : A Bridge between Micro- and Macroscopic Concepts in Soft Matter)

Abstract

結晶成長速度はガラス転移温度に近づくにつれて結晶化速度は急激に遅くなり、通常はガラス転移温度(T_g)以下では結晶化が進行しないと考えられている。しかし、T_gを境に結晶化が不連続に促進されるという不思議な現象が幾つかの物質について報告されている。この原因について、我々はガラスから結晶へ凝集した際、その界面に過剰な自由体積が与えられるために、その周りのガラスが液体状態として振舞ったと考え、このモデルの妥当性を実験により確かめた。

The growth rate G of crystal can be described in terms of the difference in the free-energy between liquid and crystal, ΔF, and the translational diffusion constant, D, as G∝ΔFD. Thus, G becomes extremely small when T is either near melting temperature Tm or near glass transition temperature T_g. Contrary to this common sense, Oguni and coworkers found discontinuous enhancement of crystal growth (DECG) behavior just below T_g for some molecular liquids by observing the growth of a crystal from the surface of a crystal previously formed above T_g. The increase of the growth rate across T_g is far more than one order of magnitude. They explained this phenomenon by their homogeneous nucleation based (HNB) crystallization scenario. It was also suggested that this phenomenon occurs only in molecules having phenyl rings. However, why the growth rate is suddenly accelerated by many orders of magnitude across T_g and what are the roles of phenyl rings remain open questions.