Isotropic-mesophase transition supercooling

In contrast to the isotropic mesophase transition, complete supercooling to the glassy mesophase state is possible. The crystallization from the mesophase may be irreversible. 

This chapter contains the following sections Brief overview of rigid-flexible (RF) polymers Supercooling at the isotropic-mesophase transition Memory of thermal history in the isotropic phase and Memory of thermal history in the nematic-isotropic biphase and aging of the mesophase. 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

The polymers were formed by condensation of 4,4dihydroxy-benzene and 2,2 -dimethyl-4,4 dihydroxyazoxybenzene with various diacid chlorides acting as flexible spacer groups Polydisperse homopolymers and copolymers, sharp fractions of homopolymers and mixtures of polydisperse polymers with a low mass mesogen were investigated. Supercooling at the mesophase-isotropic and solid-mesophase transitions, sharpness of the nematic-isotropic transition (range of N+I biphase), polymer crystallization from the mesophase melt, and enhancement of crystallinity upon addition of a low mass nematic, were studied. 

The way, or sequence, in which thermotropic transitions occur is defined in the following ways. The liquid crystal to isotropic liquid transition is called the clearing or isotropization point, and this transition, like those between liquid crystal phases, is essentially reversible and occurs with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Mesophases formed on the first heating cycle of a material are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed... 

The distinction between macromolecular liquid crystals and condis crystals may be helped by experience, although the differences are usually only a matter of degree. Liquid crystals seem always to convert fully from the liquid state to the liquid crystalline state, and the conversion to the mesophase on cooling shows rdatively little supercooling (few kelvins). Condis crystals, in contrast, show more often than not partial conversion from the melt and a relatively high supercoolir (10 or more kelvins), i.e. they are much closer in their behavior to fully-ordered, macromolecular crystals. The crystallization kinetics of liquid crystals has foimd little attention in the past. It seems describable by Avrami expressions with exponents of 2 to 4 >. Condis crystals, in contrast, have in the few analyzed cases shown exponents of less than 2 2 W) tjie transition entropy of the liquid crystal to the isotropic melt is... 

The mesomorphic state is a mesophase between a crystalline solid and an isotropic liquid where the molecules are separated in parallel layers of quasicrystalUne order. Here, the molecules show optical activity. In this state, transition and supercooling are possible. 

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