Overcooled liquid

The Gibbs specific function notion for nonequilibrium phase transition overcooled liquid —> solid body is connected closely to local order notion (and, hence, fractality notion, see chapter one), since within the ffamewoiks of the cluster model the indicated transition is equivalent to cluster formation start. In Fig. 1.1, the dependence of clusters relative fraction (p, on the... 

V.P. Skripov, V.P.Koverda. Spontannaja Kristallizacija Pereokhlazhdennykh Zhidkostejj (Spontaneous Crystallization of Overcooled Liquids). - M. Nauka, 1984. (in Russian). 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

Besides the overcooling phenomenon due to the liquid nature of the system s components, the above investigation demonstrated that the free water fi-action of the sample maintains its liquid state down to temperatures near the homogeneous nucleation temperature of water (=233 K) without requiring a quenching procedure, even with applied thermal rates as low as 0.1 K/min (Fig. 22). 

For the same reason of higher amplitude of near-surface atomic vibrations, crystallization cannot start from the liquid-phase surface. Because of this, unlike the melting ptrooess, the crystallization process is homogeneous and needs overcooling of the system by dozens or even hundreds degrees relative to the equilibrium temperature of the phase transition. 

Under conditions of overcooling or supersaturation, under which the crystal grows, a new material continuously transfers to the liquid phase from the mother phase (gaseous phase, melt, or solution that differ from the liquid phase on the crystal surface in the composition, degree of ordering, or concentration) and is continuously ordered inside this liquid phase according to the distribution of the potential field of the crystal. As a result, the thickness of the liquid layer on the crystal surface increases and becomes larger than the equilibrium thickness (Fig. 22 (b)). 

Crystallization of cellulose acetates (d.s. 2.2—3.0) has been achieved by annealation in the presence of either nitromethane or nitromethane-n-butanol in overcooled dilute solutions. Cellulose acetate I crystallized over the whole range of compositions when annealed above the glass temperature thermal treatment of cellulose acetate II brought about crystallization of samples of d.s. 2.6, whereas heating in the presence of polar liquids resulted in crystallization over the whole range of compositions. Cellulose acetate I was converted into cellulose acetate II on heating in ethanol, and cellulose acetate II crystallized spontaneously from acetone at room temperature. A comparison of the crystalline forms obtained from cellulose acetates of different d.s. was also made. 

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