Condis crystal examples

In the following Sections it will first be discussed that there is a good chance that, particultirly for different condis crystals, a full spectrum of increasingly more cooperative transitions may be possible. Conformational disorder that can be introduced without much cooperation from neighboring groups appears gradually, without first order transition. Such condis states are difficult to identify by thermal means, and microscopic technique must be used for the identification of dynamic disorder. In the major body of the review. Sections 3-6, many condis crystal examples of small molecules and macromolecules have been treated in sufficient detail to allow clear identification of the mesophases, or at least permit an educated guess of the phase-nature. 

As an example of the second case, we may have conformationally disordered chains, but long-range order in the positions of the chain axes (condis crystals [5]). Fiber spectrum features are the occurrence of sharp reflections on the equator only and diffuse reflections on the other layer lines. 

The need ultimately to include conformational disorder in the system of mesophase materials has already been pointed out by Smith (Ref.7), p. 193) and becomes obvious when reading discussions of the behavior of typical condis crystals (see, for example, Ref.108))... 

Only fragmentary details about the structure of the main-chain liquid crystals are known (for a review see Ref.86)). Often condis crystals are confused with liquid crystals, and in many cases lyotropic liquid crystals are not separated from thermotropic materials. The problem is complicated since flexible chains, such as for example poly(gamma-benzyl glutamate)47), can become rigid by a coil-to-helix transformation. Similarly, external stress or quenching can lead to incomplete orientation which may be described as a mesophase. 

No general rules about the entropy of transitions, as were found for liquid and plastic crystal transitions, can be set up for condis crystals. Two typical examples may illustrate this point. Polytetrafluoroethylene has a relatively small room-temperature transition-entropy on its change to the condis state and a larger transition entropy for final melting. Polyethylene has, in contrast, a higher condis crystal transition entropy than melting entropy (see Sect. 5.3.2). 

Small molecules may also form condis crystals, provided they posses suitable conformational isomers, It is of interest to note that several of the organic molecules normally identified as plastic crystals are probably better described as condis crystals. Their motion was, as already shown in Sect. 5.2.2, not the complete reorientation of the presumed rigid molecule, but rather an exchange between a limited number of conformational isomers. The examples treated in Sect. 5.2.2 are 2,3-dimethyl-butane, cyclohexanol and cyclohexane. 

Condis crystals and glasses of macromolecules are a newly recognized type of mesophase. The mobility in this mesophase may lead to chain extension, and as a corollary, it may be possible that mechanical deformation can cause the stabilization of the condis state. Several examples of stable condis crystals are documented, but there seem to be also examples of metastable condis crystals which are produced as intermediates to crystallization. The size of the condis crystal transitions vary depending on the number of conformational isomers involved in the cooperative transitions. 

In this discussion at attempt will be made to describe in greater detail the structure and motion for a larger number of condis crystals. A special effort will be made to point-out the differences between condis crystals on the one hand, and liquid and plastic crystals on the other. It seems reasonable, and has been illustrated on several examples, that molecules with dynamic, conformational disorder in the liquid state show such conformational disorder also in the liquid crystalline and plastic crystalline states The major need in distinguishing condis crystals from other mesophases is thus the identification of translational motion and positional disorder of the molecular centers of gravity in the case of liquid crystals, and of molecular rotation in the case of plastic crystals. 

Values for tte internal variabtes in thetmodynamic, internal equilibriwn are generally uniquely defined by the values for the external variables. For instance, in a simple, thermomechanical system (i.e. one that reacts mechanically solely volume-elastically) the equilibrium concentrations of the conformational isomers are uniquely described by temperature and pressure. In this case the conformational isomerism is not explicitly percqitible, but causes only overall effects, for example in the system s enthalpy or entropy. Elastic macroscopic effects may, however, occur when the relationship between internal and external variables is not single-valued. Then the response-functions of the system diverge or show discontinuities. The Systran undergoes a thermodynamic transformation. The best-known example of sudi a transformation based on conformational isomerism is the helix-coil transition displayed by sonte polymers in solution. An example in the scdid state is the crystal-to-condis crystal transition discussed in this paper. The conditions under which such transformations occur are dealt with in more detail in Sect 2.2. 

Another example of the existence of condis crystalline polymorphs between liquid crystal and crystal phase is OOBPD. Table 5.4 contains a listing of the observed transitions Again, the combined entropies of transition from the condis crystal K1 to the melt [41.6 J/(K mol)] are what is expected for fusion of a single, rigid, non-spherical motif (see Sect. 1). 

The concept of a condis crystal was suggested in 1975 [45] and their existence was documented by many examples in 1984 [39]. Condis crystals have in general an hexagonal packing and some authors have stressed the... 

In this section, we review three representative cases of flow-induced mesophases. PET serves as an example of a semi-rigid polymer with intrinsic rigid building blocks. The shear-induced smectic ordering in the flexible-chain polymer iPP provides a special case of induced rigidity, which is still far from fully understood. PDES has been chosen as an example of a stretch-induced condis crystal. Various experimental techniques have been employed to characterize flow-induced mesophases. While scattering methods probe the ordering, a direct technique to measure the molecular mobility is nuclear... 

The examples document that condis crystals as defined above do, indeed exist. A comparison between the various condis crystals shows that large variations in the amount of conformar-tional disorder and motion is. possible even in similar molecules. The tritriacontane in the condis state possesses about 3 gauche conformations per 100 carbon atoms. For cyclodocosane which is in its transition behavior similar to tetracosane, one estimates about 16 gauche conformations per 100 carbon atoms. The concentration of gauche conformations in cyclodocosane and polyethylene condis crystals are close to the equilibrium concentration in the melt, while the linear short chain paraffin condis crystals are still far from the conformational equilibrium of the melt. 

Conformationally disordered crystals (condis crystals) were discovered in the 1980 s. They show positional and orientational order, but are partially or fully conformationally mobile. The condis crystals complete the comparison of mesophases in Figs. 2.103 and 2.107. Linear, flexible molecules can show chain mobility that leaves the position and orientation of the molecule unchanged, but introduces large-amplitude conformational motion about the chain axis. Again, the symmetry of the molecule is in this case increased. Condis crystals have often a hexagonal, columnar crystal structure. Typical examples of condis crystals are the high-temperature phase of polyethylene, polytetrafluoroethylene, frawj-1,4-polybutadiene, and the low-temperature phases of soaps, lipids and other liquid-crystal forming, flexible molecules. 

Polytetrafluoroethylene and fran -1,4-polybutadiene are two examples of macro-molecular condis crystals. The heat capacity of polytetrafluoroethylene is shown in Fig. 2.63, that of fran -l,4-polybutadiene is illustrated in Fig. 2.112. Both polymers... 

The existence-range of the condis crystal of poly(tetrafluoroethylene), PTFE, can be seen from the phase diagram of Fig. 5.130. The calorimetric heat capacity analysis of PTFE is described as an example of the ATHAS applications in Fig. 2.63, and the entropies of transition, which lead to the high isotropization temperature, are discussed in Sect. 5.4.3. 

Rapidly quenched polypropylene, PP, is an example of metastable condis crystals. The fast crystallization prohibits the development of proper handedness of the helices of Fig. 5.26. The X-ray diffraction-scan in Fig. 5.146 shows the disorder caused by random assembly of left- and right-handed helices. The two broad diffraction maxima at room temperature correspond to the spacings along and between the chains. The state is a glass, since no conformational mobihty is detectable. As soon as the sample is heated to about 360 K, conformational mobility permits perfection of the helices. 

Soap-like molecules are examples of condis crystals of small molecules. This is illusU ated on the case of the homologous series of thallium carboxylates, TK, in Fig. 5.147. These soaps are amphiphilic molecules and form lamellar, smectic liquid crystals without the help of a mesogen by ordering on both sides of the interface between the polar and nonpolar parts of the nanophase-separated molecules. The only regular increases of entropy with chain-length n occur, when adding all transition... 

Figure 4.70 addresses the question of disordering of mesophase polymers [76]. An example of MTDSC of a liquid crystal and a condis crystal is shown. The analysis of low molar mass liquid crystals was discussed in Figures 4.42-4.44 as an example of a sharp, reversible transition and of low latent heat of transition of a sample of low thermal conductivity (Section 4.1). The liquid crystal-forming polymer has a much broader isotropisation... 

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