Spherulitic Growth

For polymers that crystallize from the melt, an important parameter in the characterization of the two-phase systems, is the weight fraction of the crystalline regions. The degree of crystallinity that can be reached is dependent on the temperature at which crystallization takes place. At low temperatures one attains a much lower degree of crystallization than at higher temperatures. This implies that crystallization remains incomplete for kinetic reasons [7]. Normal conditions of cooling of the molten polymer establish the crystalline texture of the polymer and usually result in formation of very tiny crystals. These crystals are part of a closely spaced cluster called spherulite. The formation of a single nucleus in a polymer cooled below its melting point favors the formation of another nucleus in its vicinity due to creation of local stresses. Microscopic examinations with polarized light of many polymeric materials that crystallized from the melt show the specimen packed with spherulites. Often these appear to be symmetrical structures with black crosses in the center [38]. It is believed [39] that these spherulites grow from individual nuclei. Ribbons of crystallites grow from one spherulitic center and fan out in all directions. Initially they are spherical but because of mutual interference irregular shapes develop. The diameters of spherulites range from 0.005 to 0.100 mm. This means that a spherulite consists of many crystalline and noncrystalline regions. The black crosses seen in the spherulites are explained [39] by assuming that the crystallites are arranged so that the chains are preferentially normal to the radii of the spherulites. Spherulitic morphology is not the universal mode of polymer crystallization. Spherulitic morphology occurs usually when nucleation is started in a molten polymer or in a concentrated solution of a polymer. Spherulitic growth is illustrated in Fig. 2.13. The size and number of spherulites in the polymer tends to affects the physical properties. Thus, the impact strength of polymer films or their flex life usually increases as the spherulite size decreases. On the other hand, there does not appear to be any correlation between the yield stress and ultimate elongation and the size of the spherulites. Rhythmic crystal growth is generally encountered in thin films of semicrystalline polymers. This is believed to be due to formation of ring-banded spherulites and attributed to the periodical lamellae twisting along the radial growth direction of the spherulites [42]. Recently, Gu and coworkers [43] reported that by using mild methylamine vapor etching method, the periodical cooperative twisting of lamellar crystals in ring-banded spherulites was clearly observed. When the melt or the solutions are stirred epitaxial crystallinity is usually observed. One crystalline growth occurs right on top of another. This arrangement is often called shish-kebab

Fig. 2.13 Spherulitic growth (from ref [50]) morphology. It contains lamella growth on long fibrils. Drawing of a crystalline polymer forces the spherulites to rearrange into parallel arrangements known as drawn fibrilar morphology. In order for the ordered phase to crystallize from an amorphous melt a nucleation barrier must be overcome. This barrier is a result of interfacial energy between the ordered phase and the melt that causes super cooling. Sirota [44] suggested that in order for the nucleation barrier of the stable phase to be sufficiently high to form out of the melt, another phase with a lower nucleation barrier and a free energy intermediate between that of the stable phase and the melt must form. This, he points out, is implied by Oswald’s rule [45] and evidence presented by Keller [35] that crystallization in semicrys talline polymer systems is mediated by a transient metastable phase [47, 48]. Stroble and coworkers demonstrated that lamellar thickness is determined by a transition between the metastable phase and the stable crystal phase [46–49]. In addition, by relating the crystallization temperature, melting temperature, and crystalline lamellar thickness, he suggested that lamellar growth fronts are thin layers of a mesomorphic phases. He feels that these phases thicken until such thickness is reached that stable crystal phases are favored. The conversion occurs in a block wise fashion but results in granular structures that were observed in many polymers [46–49]. This conver sion is a stabilization process that lowers the free energy of the newly formed crystallites and prevents themfromreturning to the mesomorphic phase upon subsequent elevation of the temperature [46–49]. Such a concept of crystallization, however, is not universally accepted. Sirota [49], pointed out, however, that this picture and the thermodynamic framework are generally correct in many cases. Sirota [49] believes that the origin of granular structures, mentioned above, can be understood in the following way. The initial nucleation and growth take place by stem addition. into mesophases. Lamellae thickness occurs while the chains are in the more mobile mesophase. When the thicknesses grow large enough to allow conversion from mesophases to crystals, the average densities in the lamellae have been set and the crystals break up into blocks. The transitions from mesophases to crystals also involves increases in lateral packing densities. In semicrystalline polymers, the entanglements in the amorphous regions, as well as the folding of the chains and the lamellar spanning the chains, will also have an effect. These effects limit the allowable lateral displacements morphology. It contains lamella growth on long fibrils. Drawing of a crystalline polymer forces the spherulites to rearrange into parallel arrangements known as drawn fibrilar morphology. In order for the ordered phase to crystallize from an amorphous melt a nucleation barrier must be overcome. This barrier is a result of interfacial energy between the ordered phase and the melt that causes super cooling. Sirota [44] suggested that in order for the nucleation barrier of the stable phase to be sufficiently high to form out of the melt, another phase with a lower nucleation barrier and a free energy intermediate between that of the stable phase and the melt must form. This, he points out, is implied by Oswald’s rule [45] and evidence presented by Keller [35] that crystallization in semicrys talline polymer systems is mediated by a transient metastable phase [47, 48]. Stroble and coworkers demonstrated that lamellar thickness is determined by a transition between the metastable phase and the stable crystal phase [46–49]. In addition, by relating the crystallization temperature, melting temperature, and crystalline lamellar thickness, he suggested that lamellar growth fronts are thin layers of a mesomorphic phases. He feels that these phases thicken until such thickness is reached that stable crystal phases are favored. The conversion occurs in a block wise fashion but results in granular structures that were observed in many polymers [46–49]. This conver sion is a stabilization process that lowers the free energy of the newly formed crystallites and prevents themfromreturning to the mesomorphic phase upon subsequent elevation of the temperature [46–49]. Such a concept of crystallization, however, is not universally accepted. Sirota [49], pointed out, however, that this picture and the thermodynamic framework are generally correct in many cases. Sirota [49] believes that the origin of granular structures, mentioned above, can be understood in the following way. The initial nucleation and growth take place by stem addition. into mesophases. Lamellae thickness occurs while the chains are in the more mobile mesophase. When the thicknesses grow large enough to allow conversion from mesophases to crystals, the average densities in the lamellae have been set and the crystals break up into blocks. The transitions from mesophases to crystals also involves increases in lateral packing densities. In semicrystalline polymers, the entanglements in the amorphous regions, as well as the folding of the chains and the lamellar spanning the chains, will also have an effect. These effects limit the allowable lateral displacements.

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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

Crystallization from Solution

Crystallization from the Melt

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity