Crystallization from the Melt

Certain basic information was established about the crystallization from the melt [5]: The process is a first-order phase transition and follows the general mathematical formulation for the kinetics of a

Fig. 2.9 Illustration of the helical conformation of isotactic vinyl polymers phase change. Equilibrium conditions, however, are seldom if ever attained and as a result complete crystallinity, as stated above, is not reached. The tendency to crystallize depends upon the following: 1. Structural regularity of the chains that leads to establishment of identity periods. 2. Free rotational and vibrational motions in the chains that allow different conformations to be assumed. 3. Presence of structural groups that are capable of producing lateral intermolecular bonds (van der Wall forces) and regular, periodic arrangement of such bonds. 4. Absence of bulky substituents or space irregularity, which would prevent segments of the chains from fitting into crystal lattices or prevent laterally bonding groups from approaching each other close enough for best interaction. Natta and Carradini [30] postulated three principles for determining the crystal structures of polymers These are: 1. It is possible to assume that all mer units in a crystal occupy geometrically equivalent positions with respect to the chain axis. This is known as the Equivalence Postulate. 2. The conformation of the chain in a crystal is assumed to approach the conformation of minimum potential energy for an isolated chain oriented along an axis. This is the Minimum Energy Postulate. 3. As many elements of symmetry of isolated chain as possible are maintained in the lattice, so equivalent atoms of different mer units along an axis tend to assume equivalent positions with respect to atoms of neighboring chains. This is the Packing Postulate. X-ray diffraction studies of polymers crystallized from the melt show recognizable features in some of them. The Bragg reflections, however, appear more broad and diffuse than those obtained from well-developed single crystals. Such broadening could be the result of the crystals being small. It could also be the result lattice defects. Because diffraction patterns are too weak, it is impossible to reach a conclusion. The majority opinion, however, leans toward the small crystal size as being the

Fig. 2.10 Illustration of a fringed micelle cause of the broadening [5]. The crystals from the melt are approximately 100 200 200 A ˚ in size. Rough estimates from these diffraction studies indicate that the size of crystals, or crystallites, rarely exceeds a few hundred angstroms. The fact that there is a substantial background of diffuse scattering suggests that considerable amorphous areas are also present. Because the chains are very long, it has often been suggested that individual chains contribute to several different crystalline and amorphous domains. This resulted in a proposal of a composite single-phase structure [5], a fringed micelle or a fringed crystallite model. This is illustrated in Fig. 2.10. The fringes are transition phases between the crystalline and the amorphous regions. Some analytical studies, however, failed to support this concept. The proportion of crystalline matter in a polymer is defined as the degree of crystallinity. It can be expressed in volume or in mass. By expressing it in volume, it would be:

where, Vc, Va, and V represent the respective volumes of the crystalline and amorphous regions and the total volume. When expressed in terms of mass, the equation would be:

where, mc, ma, and m are the masses of crystalline, amorphous phases, and the total mass.

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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 Solution

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity