Crystallisation crystallites

Crystallisation Crystallites 37,104 37 Donnan dialysis Donnan equilibrium 361 269... 

Because of its regularity it would be expected that the polymer would be capable of crystallisation. In practice, however, the X-ray pattern characteristics of crystalline polymer is absent in conventionally fabricated samples. On the other hand films which have been prepared by slow evaporation from solvent or by heating for several days at 180°C do exhibit both haziness and the characteristic X-ray diagram. The amount of crystallisation and the size of the crystallite structures decrease with an increase in the molecular weight of... 

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

In high polymers crystallisation means the formation of areas of regularity in chain aggregation rather than the formation of discrete crystals, as in simple chemical compounds. Crystallite... 

The difference in must be due to the previous history. The first sample has been quenched from the melt, and thus it is crystallised at a lower temperature, e g. 130 °C. The crystallites are, therefore, smaller (more nuclei at a lower temperature) and less perfect than in the second sample, which was cooled more slowly and which crystallised at, e.g. 145 °C. 

The lower the temperature of crystallisation, the lower the melting point of the best crystal will be, but also the more imperfect crystallites are formed. The melting range of the sample with = 161 °C is, therefore, broader. 

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

Deuterated n-octane was used as a probe of the amorphous regions in a stretched PDMS network cooled down below Tm (octane molecules do not penetrate into crystallites). It was observed that the local chain orientation in the amorphous regions decreases as the system crystallises [81]. 

Roe, R. J., K. J. Smith jr., and W. Krigbaum Equilibrium degrees of crystallisation predicted for "single pass" and folded chain crystallite models. J. Chem. Phys. 35, 1306-1311 (1961). 

Since polymers cannot be completely crystalline (i.e. cannot have a perfectly regular crystal lattice) the concept "crystallinity" has been introduced. The meaning of this concept is still disputed (see Chap. 2). According to the original micellar theory of polymer crystallisation the polymeric material consists of numerous small crystallites (ordered regions) randomly distributed and linked by intervening amorphous areas. The polymeric molecules are part of several crystallites and of amorphous regions. 

There is a striking resemblance between Permeation (Chap. 18) and Crystallisation. Just as Permeability is the product of Solubility and Diffusivity (P = SD), the rate of crystallisation is the product of Nucleability (or probability of Nucleation, also called "nucleation factor") and Transportability (Self-diffusivity of chains or chain fragments, also called "transport factor"). This statement is valid as well for the primary nucleation in melt or solution, as for the growth of the crystallites (which is a repeated sequence of surface nucleation and surface growth). 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

Table 19.2 gives a survey of the morphology of polymer crystallisation. The survey is self-explanatory it demonstrates an almost continuous transition from the pure folded chain to the pure extended-chain crystallite. 

Most of the pressure crystallisation research has been done on polyethylene. Pressures of 3 kbar or higher are required to obtain crystallite thicknesses of 10-7 m. Some other polymers have a much stronger tendency towards extended chain formation. Poly(chloro trifluoro) ethylene shows this effect at about 1 kbar poly(tetrafluoro)ethene already at about 0.3 kbar. 

The mechanical properties of pressure-crystallised polymers are disappointing, indeed they are very poor The main disadvantage of pressure crystallisation is that it results in a quasi-isotropic brittle product, a mosaic of randomly oriented crystallites without much interconnection. 

It is clear that the extension must be maintained until the melt has solidified by crystallisation. For spherulitic crystallisation this would require residence times of 0.5-50 s (0.5 s for PE and 50 s for PETP), which excludes normal spherulitic crystallisation for most polymers, since only fractions of seconds are available in practice. It is therefore fortunate that another type of crystallisation, that of the microfibrillar crystallite, is the dominant mode. This is due to the fact that under tension threadlike nucleation is favoured. 

Critical Oxygen Index (COI), 853 Critical size, 704-705 Critical spherical nucleus, 710, 711 Critical strain, 867, 868 Critical stress energy factor, 474 Critical surface tension of wetting, 232 Critical temperature, 655 Cross-linked polymers, 29 Cross-linking, 148 Cross model, 731 Cross polarisation, 376, 377 Crystallinity, 728, 732, 815 Crystallites/Crystallisation, 690, 725 of rigid macromolecules, 739 Cyclical chain length, 782... 

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