Crystallization from Solution

For many polymers, crystal growth can also take place from dilute solutions and from such solutions they yield single crystals. Crystal formations in polymers were studied intensively almost from the time of recognition of their existence in macromolecules. Single crystals of organic polymers were recognized as early as 1927 [26]. It became the subject of intensive investigations after observations that linear polyethylene can crystallize into single crystals. The observations made on polyethylene were followed by observations that it is possible to also grow single crystals of other polymers. Some of them are polyoxymethylene [30], polyamides [31], polypropylene [32], polyoxyethylene [33],

Fig. 2.11 Electron diffraction pattern and single crystal of polyacrylonitrile (from ref [50])

Fig. 2.12 Models of chain folding. (a) adjacent reentry model, and (b) nonadjacent reentry model cellulose [34], and others [35]. The electron diffraction pattern and a single crystal of polyacryloni trile are shown in Fig. 2.11. There is good correlation between the structural regularity of a specific polymer and the appear ance of its single crystals. Relatively larger single crystals with smooth, sharp edges, and little random growth are formed only from polymers that are known to have high regularity in their structures. To explain the arrangement of the chains in the single crystals a theory of folded chain lamella was proposed in 1947 [36]. It states that the basic units in single crystals are lamellae about 100 A ˚ thick. The evidence for the existence of lamellae-like crystallites comes from microscopic observations. Direct microscopic observations, however, do not yield information about chain structure on the molecular level. The thickness of the lamellae relative to the chain length led some to postulate that the molecules are arranged perpendicular or nearly so to the layers. Also that sharp, folded configurations form with the fold length corresponding to the layer thickness. In this view, one polymer molecule is essentially constrained to one lamellae and the interface is smooth and regularly structured with the chain folding back and forth on itself like an accordion during crystallization. Two models of chain folding are visualized. In one model, the chain folding is regular and sharp with a uniform fold period. This is called the adjacent-reentry model. In the other one, the molecules wander through the irregular surface of a lamella before reentering it or a neighboring lamella. This mode is called switchboard or nonadjacent-reentry model. The two are illustrated in Fig. 2.12.Some early experimental evidence failed to support the lamellae concept [37]. Since, much evidence was presented to support it, today it is accepted as an established fact [5]. Recently, Hosoda and coworkers [37] reported studies of the morphology of specially synthesized polyethylene with n-butyl branches precisely spaced on every 39th carbon. This was compared with a commercial ethylene/1-hexene copolymer possessing the same branching probability. The goal of their work was to elucidate the effect of the intramolecular sequence length heterogeneity on the lamella crystal thickness and its distribution. The commercial copolymer was found to have an orthorhombic crystalline polymorphism, which is normal for such polyethylenes and different from that of the specially synthesized material. The synthesized one exhibits a narrow lamella thickness distribution with the average thickness corresponding exactly to the space length between two consecutive branches. This suggests to them. complete exclusion of n-butyl branches from the crystal stem. On the other hand, the 1-hexene copolymer forms much thicker lamellae and a broader thickness distribution than the synthesized polyethylene with butyl branches. Here, the average thickness is 1.5 times larger than that calculated from the most probable ethylene sequence length obtained from NMR, or for a theoretical ethylene sequence Length distribution, indicating that the lamellae are composed predominantly of the sparsely branched longer ethylene sequences. Linear polyethylene single crystals often exhibit secondary structural features that include corrugations and pleats. It was suggested that the crystals actually grow in form of pyramids, but that these pyramidal structures collapse when the solvent is removed during preparation for micros copy [42]. Various investigators described other complex structures besides pyramidal. Typical among these descriptions are sheaf-like arrays that would correspond to nuclei. Also there were observations of dendridic growths, of clusters of hollow pyramids, of spiral growths, of epitaxial growths, of moire´ patterns, etc. [42, 43].

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مواضيع ذات صلة

Autoacceleration

Ceiling Temperature

Effect of Reaction Medium

Steric, Polar, and Resonance Effects in the Propagation Reaction

Propagation

Capture of Free Radicals by Monomers

Optical Activity in Polymers

Spherulitic Growth

Crystallization from the Melt

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity