Crystallization from the melt

Crystallization from the melt is distinguished by the competitive dynamics between chains or segments of 

In the previous sections it was observed that when polymers are crystallized from dilute solutions, they form lamellar-shaped single crystals. These crystals exhibit a folded-chain habit and are of the order of 100 to 200 A thick. From somewhat more concentrated solutions, various multilayered dendritic structures are observed. 

Spherulites are remarkably easy to grow and observe in the laboratory (36). Simple coohng of a thin section between crossed polarizers is sufficient, although controlled experiments are obviously more demanding. It is observed that each spheruhte exhibits an extinction cross, sometimes called a Maltese cross. This extinction is centered at the origin of the spherulite, and the arms of the cross are oriented parallel to the vibration directions of the microscope polarizer and analyzer. 

Usually the spherulites are really spherical in shape only during the initial stages of crystallization. During the latter stages of crystallization, the 

X-ray microdiffraction (37) and electron diffraction (38) examination of the spherulites indicates that the c-axis of the crystals is normal to the radial (growth) direction of the spherulites. Thus the c-axis is perpendicular to the lamellae flat surfaces, showing the resemblance to single-crystal structures. 

Rgure 6.15 Different types of light-scattering patterns are obtained from sphemlitic polyethylene using (a) V and (b) Hy polarization (54). Note the twofold symmetry of the Vy pattern, and the fourfold symmetry of the Hy pattern. This provided direct experimental evidence that spherulites were anisotropic. 

Whereas crystallization from dilute solutions may result in the formation of single polymer crystals, this perfection is not achieved when deahng with polymers cooled from the melt. The basic characteristic feature is still the lamellar-hke crystallite with amorphous surfaces or interfaces, but the way these are formed may be different, based on the careful investigation of melt-crystallized polymers using neutronscattering techniques. The two models that have been proposed to deseribe the fine structure of these lamellae and their surface characteristics in semicrystalline polymers differ mainly in the way the chains are thought to enter and leave the ordered lamellae regions. These are  

The regular folded array with adjacent reentry of the ehains, but with some loose folding and emergent chain ends or cilia that contribute to the disordered surface 

The switchboard model, where there is some folding of the chains but reentry is now quite random 

Both are represented schematically in Figme 11.5, but the exact nature of the structure has been the subject of considerable controversy. Although the morphology of the single crystals grown from dilute solutions may be more regular and resemble the first model, for polymers that are crystallized from the melt (and this is by far 

FIGURE 11.5 Schematics of possible chain morphology in a single polymer crystal (a) regular folding with adjacent reentry of chains, (b) switchboard model with random reentry of chains. 

When macromolecules possess a certain amount of symmetry, then there is a strong accompanying tendency to form ordered domains, or crystalline regions. CrystaOinUy, however, in polymers differs in nature from that of small molecules. When the small molecules crystallize, each crystal that forms is made up of molecules that totally participate in its makeup. But, when polymers crystallize from a melt, which means that certain elements of the polymeric system or segments of the polymeric chains have attained a form of a three-dimensional order. Complete crystallization, from the melt, however, is seldom if ever achieved The ordered conformations may be fuUy extended or may be in one of the helical forms as shown above. This resembles orderly arrangement of small molecules in crystals. The crystalline domains, however, are much smaUer than the crystals of small molecules and possess many more imperfections. 

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 

Structural regularity of the chains that leads to establishment of identity periods. 

Free rotational and vibrational motions in the chains that allow different conformations to be assumed. 

Presence of structural groups that are capable of producing lateral intermolecular bonds (van der Wall forces) and regular, periodic arrangement of such bonds. 

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Unlike other synthetic polymers, PVDF has a wealth of polymorphs at least four chain conformations are known and a fifth has been suggested (119). The four known distinct forms or phases are alpha (II), beta (I), gamma (III), and delta (IV). The most common a-phase is the trans-gauche (tgtg ) chain conformation placing hydrogen and fluorine atoms alternately on each side of the chain (120,121). It forms during polymerization and crystallizes from the melt at all temperatures (122,123). The other forms have also been well characterized (124—128). The density of the a polymorph crystals is 1.92 g/cm and that of the P polymorph crystals 1.97 g/cm (129) the density of amorphous PVDF is 1.68 g/cm (130). 

This reaction can be violent partiy because of the heat Hberated in the solvation of the hydrogen chloride. The hydrolysis can be moderated by adding PCl to a saturated solution of HCI Subsequentiy, the water and hydrogen chloride are boiled until the temperature reaches 180°C. On cooling, phosphonic acid crystallizes from the melt. 

Joseph D Henry/ Jr./ Ph.D./ P.E./ Senior Fellow, Deportment of Engineering and Public Policy, Carnegie Mellon University Member, American Institute of Chemical Engineers, American Society for Engineering Education. (Section Editor, Alternative Solid/Liquid Separations, Crystallization from the Melt)... 

There is no unanimity in regard to the exact mechanism of ECC formation under high pressure. Wunderlich et al. [11-18] suggested that when a flexible polymer molecule crystallizes from the melt under high pressure, it does not grow in the form of a stable extended chain, rather it deposits as a metastable folded chain. 

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 . 

Also the polymorphic behavior of s-PS can be altered by blending, in particular with poly-2,6-dimethyl-l,4-phenylene oxide (PPO), both for the case of crystallization from the melt [104] and for the case of crystallization from the quenched amorphous phase [105]. 

As a first example of applying the techniques described in section 2 let us look at the chain motion of linear polyethylene (LPE). A detailed study of a perdeuterated sample, isothermally crystallized from the melt, has been carried out in our laboratory24,25,44). Since all of this work is published and, in fact, has been reviewed extensively17 we can restrict ourselves to stating the main conclusions here ... 

Fig. 14.2H NMR spectra of LPE, isothermally crystallized from the melt at 396 K (Mw as 100000, Mw/Mn as 10, Merck, Darmstadt) at 55 MHz obtained from a complex FT of the solid echo for various temperatures...

Fig. 5. Log-iog plot illustrating the hardness dependence of density for PE samples crystallized from the melt. The plot yields two straight sections which can be ascribed to two preferential deformation modes a crystal destruction and b compression of amorphous domains...

Figure 4 Experimental stress-strain curves for UHMW polyethylene (Mw = 1.5 X 10, Mn = 2 X 10 ) crystallized from the melt and from solutions of various initial polymer concentrations 0. T = 120°C and e = 500%/min.

In crystallization from the melt, as in the freezing of water or the solidification of molten sugar, the liquid phase is one component and temperature alone is the determining factor in whether or not crystallization will take place. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or crystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binary mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirectly cooled, and/or solvent is evaporated to effect crystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt crystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining section of the apparatus. 

TABLE 20-1 Comparison of Processes Involving Crystallization from the Melt... 

We can nucleate crystallization from the melt by incorporating finely ground inorganic crystalline compounds such as silica. Nucleation of injection molded nylons has three primary effects it raises the crystallization temperature, increases the crystallization rate, and reduces the average spherulite size. The net effect on morphology is increased crystallinity. This translates into improved abrasion resistance and hardness, at the expense of lower impact resistance and reduced elongation at break,... 

Finally, we were led to the last stage of research where we treated the crystallization from the melt in multiple chain systems [22-24]. In most cases, we considered relatively short chains made of 100 beads they were designed to be mobile and slightly stiff to accelerate crystallization. We could then observe the steady-state growth of chain-folded lamellae, and we discussed the growth rate vs. crystallization temperature. We also examined the molecular trajectories at the growth front. In addition, we also studied the spontaneous formation of fiber structures from an oriented amorphous state [25]. In this chapter of the book, we review our researches, which have been performed over the last seven years. We want to emphasize the potential power of the molecular simulation in the studies of polymer crystallization. 

The molecule is either fully flexible or semi-flexible. The fully flexible chains are generally harder to crystallize than semi-flexible chains [35]. In the latter part of the paper (Sect. 5), where we discuss crystallization from the melt, we consider a semi-flexible chain, the flexibility of which is adjusted to reproduce the characteristic ratio of real polyethylene. We there make the... 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

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