Conformational, disorder motion

The positive current which increases rapidly on heating above 40 °C may be caused by the increase in the conformational disorder and thermal motion of the hydrocarbon chains. 

Burling, F. T. and Brunger, A. T. (1994) Thermal motion and conformational disorder in protein crystal structures. Isr. J. Chem. 34,165-175. 

Using a realistic model for PE, the molecular dynamics technique is used to simulate atomic motion in a crystal. The calculations reveal conformational disorder above a critical temperature. The customarily assumed RIS model is found to be a poor description of the crystal at elevated temperature. 

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. 

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. 

These data may reflect that PDES retains both ordered and disordered phases in the range of -60 to -10°C. Above 25°C, PDES takes only a disordered phase and the molecular motion is in the fast-motion region for the single correlation-time model based on BPP theory [22], because the Si Ti values increase as the temperature is increased from 25 to 125°C. That is to say, the disordered phase (I) is conformationally disordered but shows rudimentary intermolecular packing and reflect a single motional state. 

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. 

More direct studies of motion in this group of molecules by various types of NMR were often limited to the liquid crystalline state A largely motionally averaged spectrum for rotation about the molecular director and conformational disorder including in many cases rotation of the phenylenes about their para-axis is observed... 

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. 

The Advanced THermal Analysis System was developed in the 1980 s to be able to interpret the heat capacities of linear macromolecules more precisely. In the solid state, the heat capacity is described by contributions from the vibrations of an approximate spectrum. Any deviation is a sign of additional processes, usually conformational disordering or motion. In the liquid state extensive addition schemes based on group contributions have been developed to judge heat of fusion baselines and increases in heat capacity on devitrification at Tg. 

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. 

Thermodynamics and motion can be used as a base for an operational definition of the solid state. A solid is a phase below its glass- or melting-transition temperature where the molecular motion is almost completely restricted to small-amplitude vibrations. Both transitions are easily determined by thermal analysis (the operation). Recently it has become possible by simulation on supercomputers to establish the link from the microscopic thermal motion of macromolecules to the macroscopic thermal analysis. By solving the equation of motion, one can produce a detailed movie of molecular motion (see Sect 1.3.4, Fig. 1.47). At high temperature, conformational disorder is seen, i.e., the crystal can change to a condis state. Note that even the conformational motion occurs in a picosecond time scale (see Sect. 5.3.4). 

Mesophases are intermediate phases between rigid, fully ordered crystals and the mobile melt, as explained in the introductory discussion of phases in Sect. 2.5, and summarized in Figs. 2.103 and 2.107. The quantitative analysis of melting in Sect. 5.4 shows that with a suitable molecular stracture, three types of disorder and motion can be introduced on fusion (1) positional disorder and translational motion, (2) orientational disorder and motion, and (3) conformational disorder and motion [43]. In case not all the possible disorders and motions for a given molecule are achieved, an intermediate phase, a mesophase results. These mesophases are the topic of this section. Both structure and motion must be characterized for a full description of mesophases. 

Fig. 9. Conformational disorder in a seven-chiun simulation at the given temperatures and times. At low temperature, sceletal vibrations are obvious at high temperature, the motion is more chaotic and shows occasional conformational defects...

Many connections have been established between microscopic MD simulations and macroscopic experimental properties of polymers, such as seen in conformational disorder and heat capacities [1], molecular motion and the vibrational spectrum- [2], stress-induced frequency shifts and conformational changes [3], twist motion and the dielectric a-relaxation [4], molecular diffusion... 

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