Oriented crystallization

Oriented Crystallization.—Hoffman has developed a theory for the growth of fibrous crystals with extended-chain morphology i.e., the core fibril or shish which develops on flow-induced crystallization. An embryonic fibril connected by bundle nuclei is produced. End surfaces resulting from the repulsion of amorphous chains in the regions between the nuclei build up commutatively as the nuclei mature. Volume strain in each nuclei limit the diameter of the core fibril to 15—50 nm. The model leads to a set of extended chains crystallites of stable diameters interrupted by short and highly strained amorphous regions. 

Posthuma de Boer and Pennings have investigated the stretching of polyethylene networks swollen to an equilibrium extent and followed the retroactive forces with crystallization. The product has a shish-kebab morphology identical to that normally prepared by flow-induced crystallization. 

In a thermodynamic analysis of oriented crystallization, Janecki and Ziabicki have considered the effect of partial crystallization on the subsequent development. The free-energy change is orientation dependent and this in turn contributes to crystal orientation. In a simple tetrahedral model of the network the change is conflgurational entropy due to partial crystallization is calculated and its effect on the distribution of crystal orientation derived. Calculations were carried out on uniaxially stretched polyethylene. 

Macro Crystals.—Nearly defect-free crystals of polymers have been produced by direct polymerization of monomer at gas-solid or liquid-solid interface. Poly-(diacetylene) single crystals have been produced from irradiation of the crystalline monomer and involves the instantaneous polymerization and crystallization of 

123 Single crystals of poly(oxymethylene) - are also produced by polymerization, involving a cation-initiated insertion of the monomer unit directly into a chain fold of the single crystal. The molecular chains are perpendicular to the base of the crystal and it thickens progressively by the mechanism of insertion into the folds on the surface. Clearly these crystals offer a unique opportunity for study of the structure and properties of the polymeric crystalline phase in the absence of amorphous regions. 

The mechanism of fibrillar growth has been interpreted in terms of formation of bundle-like nuclei which grow longitudinally. 

The structure of shishkebabs has been examined in detail by A -ray diffraction and electron microscopy and considered to be made up of a central core of extended chain fibrils with a helical arrangement of lamellae. The periodicity of the helix and the lamellar thickness were both uniform and regular.  

---

To answer questions regarding dislocation multiplication in Mg-doped LiF single crystals, Vorthman and Duvall [19] describe soft-recovery experiments on <100)-oriented crystals shock loaded above the critical shear stress necessary for rapid precursor decay. Postshock analysis of the samples indicate that the dislocation density in recovered samples is not significantly greater than the preshock value. The predicted dislocation density (using precursor-decay analysis) is not observed. It is found, however, that the critical shear stress, above which the precursor amplitude decays rapidly, corresponds to the shear stress required to disturb grown-in dislocations which make up subgrain boundaries. 

MW and MWD are very significant parameters in determining the end use performance of polymers. However, difficulty arises in ascertaining the structural properties relationship, especially for the crystalline polymers, due to the interdependent variables, i.e., crystallinity, orientation, crystal structure, processing conditions, etc., which are influenced by MW and MWD of the material. The presence of chain branches and their distribution in PE cause further complications in establishing this correlation. 

For [001] and [IlO] orientations where no stress is resolved onto <110] ordinary slip, <101] superlattice slip is observed up to the peak temperature. <101] dislocatiom predominantly lie along their screw orientation up to the peak temperature. This is consistent with the recent results of TEM observations on [001] si"gle crystals by Stucke et al. [9]. At temperatures below the peak, CRSS is much higher for the [001] orientation than for the [IlO] orientation. However, both the peak temperature and peak stress are lower for the former orientation than the latter. TTie lower peak temperature for the [001] orientation is associated with the occurrence of twiiming of the lll <112]-type above the peak temperature. Such twinning can not occur for the [IlO] orientation in compression. Deformation of [Il0]-oriented crystals above the peak is carried by slip on 111 <112]. 

At room temperature, NiAl deforms almost exclusively by (100) dislocations [4, 9, 10] and the availability of only 3 independent slip systems is thought to be responsible for the limited ductility of polycrystalline NiAl. Only when single crystals are compressed along the (100) direction ( hard orientation), secondary (111) dislocations can be activated [3, 5]. Their mobility appears to be limited by the screw orientation [5] and yield stresses as high as 2 GPa are reported below 50K [5]. However, (110) dislocations are responsible for the increased plasticity in hard oriented crystals above 600K [3, 7]. The competition between (111) and (110) dislocations as secondary slip systems therefore appears to be one of the key issues to explain the observed deformation behaviour of NiAl. 

Since some earlier work based on anisotropic elasticity theory had not been successful in describing the observed mechanical behaviour of NiAl (for an overview see [11]), several studies have addressed dislocation processes on the atomic length scale [6, 7, 8]. Their findings are encouraging for the use of atomistic methods, since they could explain several of the experimental observations. Nevertheless, most of the quantitative data they obtained are somewhat suspicious. For example, the Peierls stresses of the (100) and (111) dislocations are rather similar [6] and far too low to explain the measured yield stresses in hard oriented crystals. 

Elyashevich, G. K. Thermodynamics and Kinetics of Orientational Crystallization of Flexible-Chain Polymers. Vol. 43, pp. 207 — 246. 

Thermodynamics and Kinetics of Orientational Crystallization of Flexible-Chain Polymers... 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Fig. 13. Phase topogram. Curves 1 and 2-melting temperatures of FCC and ECC as a function of the degree of extension / curve 3 - dependence /4, on crystallization temperature. Arrows show the way of orientational crystallization ...

The generation of an intermediate phase during melting under isometric conditions of orientationally crystallized polyethylene has also been observed56 at temperatures exceeding the melting temperature of ECC. The authors suppose that the mesophase... 

If this sample contains also folded-chain crystals (reasons for their appearance during orientational crystallization were stated before), under isometric conditions they undergo melting at a higher temperature (at point 1 with respect to the oriented melt with transition to line A2) than under the conditions of free heating (point 1 with transition in the isotropic melt to line At). 

This model of the structure of orientationally crystallized samples based on experimental data is in good agreement with the results of the foregoing thermodynamic analysis which resulted in relationships describing the formation of two structures, FCC and ECC, during the crystallization of strongly oriented melts of flexible-chain polymers. 

Fig. 21 a-c. Schematic representation supramolecular structure of a crystalline rigid-chain polymer (a), an idealized ECC of a flexible-chain polymer (b) and an orientationally crystallized sample with a spatial ECC framework (c)... 

It should be noted that the fraction of ECC in samples obtained by other methods described in Sect. 2 is approximately as small as that of the framework in the orientation-ally crystallized samples. These methods differ in details but depend on the mechanical treatment of the crystallizing system and are therefore given the common name stress-induced crystallization . Although the structure of the samples obtained by these methods has some features in common with that of orientationally crystallized samples, the thermodynamics and kinetics of orientational crystallization are fundamentally different from the mechanism of stress-induced crystallization. 

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

---