Crystallization from the molten state

Polymers crystallize from the molten state by the two-step process of nucleation and crystal growth. Nucleation initiates crystallization, followed by the addition of linear chain segments to the crystal nucleus. 

Random copolymers of VF2/F3E when crystallized from the molten state above the Curie temperature show a microstructure in the form of very thin needle-like morphological units which are probably semicrystalline. Figure 5a illustrates the needle-like microstructure of the copolymer 80/20 melt crystallized in the paraelectric phase observed at 140 °C. After codling at room temperature the microstructure of the ferroelectric crystals is such that what appear in the optical microscope as radial fibers are, in fact, stacks of thin platelet-like morphological units (see Fig. 5b). 

Dibenzobarrelene diisopropylester 12 is an achiral molecule, but it forms dimorphic crystals. A chiral crystal, P212121, can be obtained by crystallization from the molten state. Irradiation of such crystals gives 13 in over 95% enantiomeric excess (ee) (Scheme 4.6) [8]. 

However, in most cases the structural sizes inferred from this treatment do not agree with the stmctures observed by means of electron microscopy [18]. For instance, polymers crystallized from the molten state rarely have orders of multiple dispersion (as required by diffraction) and, moreover, they present a wide first-order peak as shown in Figure 19.7 [19]. Then, when applying the Bragg s law, the obtained period will be distorted considerably from the average period of the structure [14], This discrepancy of values can decrease, but not vanish, when applying the Lorentz factor to the curve of observed intensity. 

Most metals crystallize from the molten state in this manner, but because of the filling-in process the final crystalline mass may show little outward appear-... 

Polypropylene (PP) has two crystalline forms. A monoclinic crystal (a-type) is obtained by slow crystallization from the molten state and a hexagonal crystal ( -type) is formed by annealing while a temperature gradient is maintained across the sample. Figure 5.25 A presents DSC heating curves of an isotactic PP film prepared by temperature gradient annealing [23]. The film was pressed at 473 K and cooled to 443 K, where the pressure was released. The... 

Fig. 19.16. Common logarithm of I as a function of crystallization temperature (Tc) for i-PS [61]. Circle and triangle symbols indicate as previous temperature of melting at 230°C and 250°C, respectively. Open and solid syndxds indicate as crystallization from the molten state and from the glassy state, respectively...

Figure 5.29. Representation of the directions of the growth of the lamellae starting from a microcrystalline nucleus (crystallization from the molten state).

Crystallization from the molten state is an important phenomenon whose mechanism, thermodynamic aspects and kinetics will be described in Chapter 12. 

Polyethylene crystallizes from the molten state or solution when prevailing conditions make the crystalline state more stable than the disordered one. The processes by which polyethylene crystallizes reflect the properties of the disordered state from which the ordered phase condenses. Thus, for instance, levels of chain entanglement, molecular dimensions, and viscosity all play important roles. The factors affecting the structure of the disordered state are both intrinsic to the molecules and extrinsic to the surrounding conditions. The principal molecular factors are the molecular weight, molecular weight distribution, and concentration, type, and distribution of branches. External factors include temperature, pressure, shear, concentration of solution, and polymer-solvent interactions. 

There are two basic crystallisation forms for sulphur - monoclinic and rhombic. Rhombic is the most stable form, at least up to 96 °C the other types revert to this stable form at a rate dependant upon temperature. When sulphur solidifies from the molten state (melting point 114 °C) the crystalline form which occurs is monoclinic (needle-like crystal structure). Below 96 °C, the monoclinic form becomes metastable and changes into the rhombic form. 

Demonstration 4.1 shows that the formation rate makes the difference. Glass, as it cools, solidifies quickly from the molten state. There is no time for the atoms and ions to line up in an orderly structure. They are frozen in place like pebbles in ice. Silver crystals, however, form slowly as silver atoms line up in a face-centered cubic arrangement. The atoms are as uniform as soldiers waiting for inspection. 

Crystallization of polymers can be carried out from the molten state or from solutions. These procedures lead to different morphologies. Starting from solution, single crystals are obtained with the shape of lamella (plates). Starting form the melt, small crystalline lamellae or crystallites are formed that are organized into complex structures known as spherulites. 

Crystals can be grown from the molten state just as water is frozen into ice, but it is not easy to remove impurities from crystals made in this way. Thus most purifications in the laboratory involve dissolving the material to be purified in the appropriate hot solvent. As the solvent cools, the solution becomes saturated with respect to the substance, which then crystallizes. As the perfectly regular array of a crystal is formed, foreign molecules are excluded and thus the crystal is one pure substance. Soluble impurities stay in solution because they are not concentrated enough to saturate the solution. The crystals are collected by filtration, the surface of the crystals is washed with cold solvent to remove the adhering impurities, and then the crystals are dried. This process is carried out on an enormous scale in the commercial purification of sugar. 

Owing to their polymeric nature, many silicate compounds and systems tend to form glasses. When cooling rapidly from the molten state, a part of the sample crystallizes, while the other part remains glassy. This is the main disadvantage while measuring their heat capacity, heat content, enthalpy of fusion, and mixing. 

FIGURE 10.20 Dynamic mechanical thermal analysis of polystyrene that has been fast-cooled from the molten state, illustrating two relaxation events that may be attributed to crystal disordering. (Reprinted from Wetton, R.E. et al., Thermochim. Acta 175, 1, 1991. With permission.)... 

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