Crystalline polymers positional disorder

Asymmetrical Peaks are rarely found in WAXS from polymers, but they are ubiquitous in the MAXS of liquid crystalline polymers. For asymmetrical peaks in isotropic patterns it is best to determine the peak position from the maximum of the peak, if peak asymmetry is a result of linear or planar disorder. Linear disorder means that the crystals are more or less one-dimensional (a tower of unit cells). Planar disorder means that the crystallites are made from only very few layers of unit cells (cf. Guinier [6] Chap. 7). 

The above conclusion is unfortunate for the case of polymeric solutes, because then-entropies of dissolution are unusually small. The repeat units can not become as disordered as can the corresponding monomer molecules since they are constrained to be part of a chain-like structure. Such disordering is particularly difficult if the chain is stiff. Thus, in this situation dissolution is even less likely. Crystalline polymers are also more difficult to dissolve than are their amorphous counterparts since the enthalpy of dissolution also contains a large, positive contribution from the latent heat of fusion. 

The temperature position for this process for a number of crystalline polymers is given in Table I. The apparent activation energies given were obtained from the slope of a plot of the log of the measuring frequency vs. the reciprocal of the temperature at which the loss maximum occurs. The motion responsible for these phenomena is believed to involve rotation about main-chain bonds in amorphous or disordered parts, leading to translation of chain segments from one position to another. 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

More in general, solid mesophases not only include crystalline forms of polymers containing a large amount of disorder in the conformation of chains and long-range order in the position of chain axes as in condis crystals, but also crystalline polymers characterized by disorder in the lateral packing of conformationally ordered chains [13,14]. 

The normal vibration calculations based on a correct structure and correct potential field permit a good correlation to be made between predicted and observed ateorption bands in the FIR spectra of high-crystalline polymers in spite of the disordered regions existing in polymer crystals. The size and defects of these ciystals influence band shape and position because of finite boundary conditions. It also may give rise to additional al orption bands not predicted by the calculation (because the selection rule cannot be applied in this case). The additional bands are observed, indeed, in tlK FIR spectra at the frequencies corresponding to tlK maxima in tte spectrum density of phonon states in the low-frequency regon [19, 23]. 

Nevertheless, when we carry out x-ray crystallinity measurements on textile fibers, we must consider distortions that always affect crystalline material. Even in a completely crystalline material, the scattered x-ray intensity is not located exclusively in the diffraction peaks. That is because the atoms move away from their ideal positions, owing to thermal motion and distortions. Therefore, some of scattered x-rays are distributed over reciprocal space. Because of this distribution, determinations of crystallinity that separate crystalline peaks and background lead to an underestimation of the crystalline fraction of the polymer. In this paper, we attempt to calculate the real crystallinity for textile fibers from apparent values measured on the x-ray pattern. This is done by taking into account the factor of disorder following Ruland s method (3). 

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