Crystal condis

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. 

Microhardness (MH), has been shown to be a convenient additional technique to detect accurately the ferro to paraelectric phase changes in these copolymers. The increase of MH as a function of VF2 polar sequences observed at room temperature is correlated with the contraction of the p-all-trans unit cell On the other hand, the fast exponential decrease of MH with increasing temperature, observed above Tc, is similar to that obtained for glassy polymers above Tg and suggests the existence of a liquid crystalline state in the high temperature paraelectric phase. This phase is characterized by a disordered sequence of conformational isomers (tg-, tg+, tt) as discussed for Condis crystals [109]. 

The WAXS pattern can be calculated in a quantitative way when we arrange 10 x 12 PTFE stems of length 40 A (two monomers) on a two-dimensional hexagonal lattice with a = 5.66 A (Fig. 24). Therefore, the lateral size of these Condis-crystals is at least 60 A. 

The term condis crystal , which is a contraction of the term conformationally disordered crystal , was coined to designate the most important mesophase for flexible, linear macromolecules. We are not aware of prior naming of this class of mesophases4. 

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))... 

The third group of mesophase materials represents the conformationally disordered crystals, called condis crystals. The physical properties of condis crystals, which largely maintain positional and orientational order, change in much too subtle a way from the fully ordered crystals so that a common property could be attached to their name. 

Because all glasses are clearly solids, the names liquid crystal glass or plastic crystal glass are awkward. The terms LC-, PC- and CD-glass thus stand for glass obtained by quenching a liquid crystal, plastic crystal, or condis crystal, respectively. 

The formation of macromolecular liquid crystals usually seems to be virtually complete. Also, when the mesogenic group is sufficiently mobile, there is only little supercooling of the transition to the liquid crystalline state. Plastic and condis crystals, on the other hand, are expected to behave more like the fully ordered, macromolecular crystals, i.e., partial crystallinity is expected in these cases. 

The possible transitions of plastic and condis crystal-forming materials are shown in Fig. 4. For plastic crystals, this diagram is fully based on information on low molecular weight materials. No flexible, linear macromolecules which resemble plastic crystalline behavior have been reported (see Sect. 5.2.3). Similarly, little attention has been paid in the past to conformationally disordered mesophases in small molecules. In fact, some of the plastic crystals of larger organic molecules may actually be condis crystals (see Sects. 5.2,2 and 5.3.3). Since the positional order is preserved in both plastic and condis crystals, the possible phase relations are similar. The major difference from the liquid crystals is the possibility of partial mesophase formation. 

Fig. 4. Schematic diagram of the possible states of plastic or condis crystal forming materials. Only the left side of the diagram corresponds to equilibrium. The states towards the right are increasingly metastable or unstable, but are often reached for flexible, linear macromolecules...

The crystal-mesophase transitions at Td are first order transitions. In the case of plastic crystals, they show usually the major heat of transition. In the condis crystals the magnitude of the transitions depends on the number of bonds which gain con-... 

Little information is available for the crystallization of condis crystals from the isotropic melt, but even less difference from the crystallization of fully ordered crystals is expected. It will be shown in Sect. 5.3 that one possible special effect of crystallization of macromolecular condis crystals is the ability to chain extend after initial crystallization. This process has been analyzed to some degree and occasional low Avrami exponents have been reported39). No information is available for the kinetics of... 

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. 

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. 

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). 

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

Polyoxybenzoate is a stiff chain, lyotropic liquid crystalline material, as was discussed on the basis of its copolymers with ethylene terephthalate (see Sect. 5.1.4). The crystal structure of the homopolymer polyoxybenzoate was shown by Lieser 157) to have a high temperature phase III, described as liquid crystalline. X-ray and electron diffraction data on single crystals suggested that reversible conformational disorder is introduced, i.e. a condis crystal exists. Phase III, which is stable above about 560 K, has hexagonal symmetry and shows an 11 % lower density than the low temperature phases I and II. It is also possible to find sometimes the rotational disorder at low temperature in crystals grown during polymerization (CD-glass). 

Poly (ethylene terephthalate) can be grown to extended chain crystals similar to the nylons 164). No special high pressure polymorph has been suggested. But there may be, as in the nylons, the possibility that a continuous increase in mobility exists in the crystal phase at higher pressure and temperature. This mobility would have to be based on conformational changes, i.e. a condis crystal phase. 

Table 8. Transition Parameters of Macromolecular Condis Crystals...

The thermal data on possible condis crystals are collected in Table 8. Substantial entropy gains are observed at the disordering transition, but variations are large, depending on the amount of conformational mobility gained. When compared to the total entropy of fusion, the listed entropies of disordering vary from 30 %, for polytetrafluoroethylene, to close to 100% for the polyphosphazenes. 

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. 

A series of other condis crystals are the larger cyclo-alkanes, analyzed by Gross-mann 171). Figure 23 illustrates the transition behavior of cyclotetracosane and cyclo-hexanonacontane. The melting transition of cyclotetracosane has only about 1/4 the heat of transition at the disordering to the condis phase. The condis phase has a much higher symmetry and fewer X-ray diffraction lines. Infrared and Raman spectroscopy indicate that practically no additional conformational isomers are introduced on... 

Fig. 23. Heat capacities of two cycloalkanes in the transition regions. The low temperature polymorphs are fully ordered. The high temperature transitions lead to the isotropic melts. The condis crystals exist between the two endotherms (Scanning calorimetry, heating rate 10 K/min, drawn after data of Ref.171>)...

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