Molecular orientation crystals

Nylons are semicrystalline polymers whose properties are controlled primarily by their amide concentration, molecular orientation, crystallization conditions, and the level of absorbed water. As discussed earlier, the level of crystallinity and hence product stiffness, is maximized by high concentrations of amide groups, high orientation, slow cooling, and the absence of absorbed water. 

The higher-order structures (morphologies) of injection-molded polypropylene (PP), which should be considered, are crystalline form, lamellar thickness, spherulite size, crystallinity, molecular orientation, crystal orientation, dispersion state of a blended polymer, length and orientation of reinforcing fibers, crystalline texture (skin-core structure), etc. 

Silicone rubber (SM) comprises a versatile family of semiorganic synthetics that look and feel like NR, yet have a completely different type of structure and performance from other rubbers. They have no molecular orientation, crystallization, or propensity for stretching and must be strengthened by reinforcements. Although they are on the high end of the cost scale, they can be made to withstand temperatures as high as 315 C (600°F) without deteriorating, and they retain their flexibility down to — 129 C ( — 200 F). 

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

Orientation. Most articles made of HDPE, including film, fiber, pipes, and injection-molded articles, exhibit some degree of molecular and crystal orientation (21). In some cases, orientation develops spontaneously for example, during melt flow into a mold and its subsequent crystallisation. When blown HDPE film and fiber are manufactured, orientation can be introduced dehberately by stretching. 

Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of the respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of up to six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes. 

The substituted five-ring OPVs have been processed into poly crystal line thin films by vacuum deposition onto a substrate from the vapor phase. Optical absorption and photolumincscence of the films are significantly different from dilute solution spectra, which indicates that intermolecular interactions play an important role in the solid-state spectra. The molecular orientation and crystal domain size can be increased by thermal annealing of the films. This control of the microstruc-ture is essential for the use of such films in photonic devices. 

Thermodynamics of Crystallization of Flexible-Chain Polymers Under Conditions of Molecular Orientation. 217... 

A characteristic feature of the structure of samples obtained under the conditions of molecular orientation is the presence of folded-chain crystals in addition to ECC. Kawai22 has emphasized that the process of crystallization from the melt under the conditions of molecular orientation can be regarded as a bicomponent crystallization in which, just as in the case of fibrous structures in the crystallization from solutions, the formation of crystals of the packet type (ECC) occurs in the initial stage followed by the crystallization with folding . 

In principle, it is possible to obtain ECC in the absence of molecular orientation if the crystallization is carried out very slowly at high temperatures close to the melting temperature. Thus, Mandelkern obtained polyethylene crystals similar to ECC in their thermodynamic characteristics by a 40 days crystallization of an isotropic melt28. These experiments also characterize one of the possible paths of the generation of order in polymers order through fluctuations 29 (see below). 

In spite of the presence of ECC, the sample exhibiting a domain structure remains unoriented on the macroscopic level. Figure 3 c shows a great difference in the structures obtained, if molecular orientation exists and if hydrostatic compression is applied. Although the method of hydrostatic compression of the melt is of paramount importance from the scientific view point just for samples crystallized under pressure it was possible to prove unequivocally the existence of ECC), it does not allow a direct preparation of oriented samples of high strength (they are brittle and readily crumble to powder under minimum strain). However, the material obtained in this way can probably serve as a semi-finished product for further technological treatment that would improve its mechanical properties. 

Effect of Molecular Orientation on Crystallization of Flexible-Chain... 

We carried out thermodynamic studies on the crystallization from melts of flexible-chain polymers uniaxially stretched at various degrees of molecular orientation in the melt and studied the effect of the stretching stress on thermodynamic parameters such as degree of... 

Fig. 4) for relatively long chains37. However, under conditions of molecular orientation, the distribution function is usually displaced towards higher fi and the analysis of the crystallization process should be carried out over a wide range of fi values. 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

This method of crystallization leading to die formation of ECC was called orientational or directed orientational crystallization16. To carry out this crystallization it is necessary to achieve in the melt or in solution a molecular orientation that causes chain extension, i.e. an increase of parameter /8. 

Figure 11 shows that the molecular weight distribution in the melt (presence of short chains) can account for the coexistence of two types of crystals in the absence of molecular orientation or at a slight stretching of the melt. However, there is a purely thermodynamic reason for the appearance of this main structural feature of samples crystallized under conditions of molecular orientation, even at high degrees of orientation, when virtually the whole distribution function is displaced into the region of /S > /3cr. 

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