Condis crystal motion

In a condis crystal cooperative motion between various conformational isomers is permitted. In the CD-glass this motion is frozen, but the conformationally disordered structure remains. For a condis crystal it is not necessarily expected that all possible conformations can be reached, but all conformations of the same type are involved in the condis crystal motion. If conformational isomers of low energy exist which leave the macromolecules largely in a parallel, extended, low energy conformation, conditions for the formation of condis crystals are given. The conformational changes involve more or less hindered rotations about backbone bonds or side chain bonds and are thus the some degree related to the orientational motion in plastic crystals. 

The condis crystal motion results in a similar rotation as in the plastic crystal, but the molecules do not rotate as a whole, rather they undergo segmental motion and... 

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. 

Comparison between the various condis crystals shows that large variations in the amount of conformational 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 the tetracosane of Fig. 23, one estimates about 16 gauche conformations per 100 carbon atoms, and for the high pressure phase of polyethylene (see Sect. 5.3.2), one expects 37 gauche conformations per 100 carbon atoms 171). 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. 

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. 

The series of p-oligophenyls illustrates thus, as shown in Fig. 5.1, that transition from the rigid crystal to the condis crystal can go with a displacive or with an order-disorder transition. Larger motion can occur with little correlation and thus... 

The problem of identifying phases in the area between highly-ordered, smetic liquid crystals and condis crystals is well recognized and often only solvable when a complete motional, structural, and entropic analysis is available. 

In the process of identification of condis crystals it was observed that conformational mobility alone is not sufficient to prove the presence of a condis phase. Large amplitude molecular jump motion may be possible already in crystals without disorder if the symmetry is identical before and after the jump. The frequencies of these jumps can be surprisingly large and the moving parts of the molecules substantial. In the condis phase quick reptation can lead to extension of folded chain crystals, and is possibly also involved in rearrangements on mechanical deformation and membrane functions. 

In a condis crystal cooperative motion between various con- formational isomers is permitted. In the CD- glass this motion is frozen, but the conformationally disordered structure remains. 

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. 

In Sect. 2.5 a similar two-step melting was discussed for the condis state of trans-1,4-polybutadiene. The c/ -isomer shows in Fig. 2.113 complete gain of the entropy of fusion at a single melting temperature, while the trans isomer loses about 2/3 of its entropy of transition at the disordering transition. The structure of the trans isomer is close to linear, so that conformational motion about its backbone bonds can support a condis crystal sttucture with little increase in volume of the unit cell. 

B Defect motion in the condis crystal after a first order transition... 

In order to substantiate these assumptions about the phase structures of MBPE-9, the molecular motion was studied by solid state NMR, as shown in Fig. 5.152. The three chosen temperatures are just before initial ordering in the melt, between the two exotherms of Fig. 5.151, and below the second exotherm after completed cooling. Two spectra are shown at each temperature. One is sensitive to mobile atoms, as found in the melt, the other is sensitive to the less mobile atoms as in condis crystals. 

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