Polymeric Crystallisation

Commercially, the PMDA mixtures are normally treated with phosgene to produce the corresponding isocyanates. These isocyanate mixtures, commonly called polymeric MDI (PMDI), are sold direcdy and have varied chemical compositions. The 4,4 -MDI can be separated from the PMDI products by distillation or crystallisation (31,32). The amount of 4,4 -MDI that is removed depends on marketing conditions. The residues are also viable commercial products. 

The stmcture of kernite consists of parallel infinite chains of the Bion (6) composed of six membered rings (80). The polymeric nature of the anion is consistent with the slow rate of dissolution and crystallisation observed for kernite. 

These three factors influence the ability of the polymer to crystallise, the melting point of any resulting crystalline regions, and also the glass transition temperature. It is the last of these features of polymeric materials which we will concentrate on for the rest of this chapter. 

Acetaldehyde also polymerises gradually on keeping, yielding the liquid paraldehyde (CH3.CHO)3, which boils without decomposition at 124°. If a little hydrogen chloride is passed, with cooling, into acetaldehyde the likewise polymeric metaldehyde crystallises out. 

The sol is either then treated or simply left to form a gel over time by dehydrating and/or polymerizing. To obtain the final product, the gel is heated. This heating serves several purposes—it removes the solvent, it decomposes anions such as alkoxides or carbonates to give oxides, it allows rearrangement of the structure of the solid, and it allows crystallisation to occur. Both the time and the temperature needed for reaction in sol-gel processes can reduced from those in the direct ceramic method in favourable cases, the time from days to hours, and the temperature by several hundred degrees. 

In cases where metals or metal ions can contaminate the products, reaction vessels fabricated from inert polymeric materials restrict that possibility. A significant example involved the reaction of maltol with aqueous methylamine to give l,2-dimethyl-3-hydroxypyrid-4-one. The product is a metal chelator employed for the oral treatment of iron overload. Consequently, it is an excellent metal scavenger but must be produced under stringent conditions that preclude metal complexation. Literature conditions involved heating maltol in aqueous methylamine at reflux for 6 h, the product was obtained in 50% yield, but required decolourisation with charcoal135. With the CMR, the optimal reaction time was 1.3 min, and the effluent was immediately diluted with acetone and the near colourless product crystallised from this solvent in 65% yield (Scheme 9.18). A microwave-based batch-wise preparation of 3-hydroxy-2-methylpyrid-4-one from maltol and aqueous ammonia was also developed. 

Partial crystallisation may take place in polymeric materials, especially when stretched and/or cooled. From the mechanical standpoint, the introduction of crystalline regions in a polymer is equivalent to increasing the degree of cross-linking, and a partial loss of elasticity results. 

If crystallisation and orientation go together, the strength can be further improved. The strongest polymeric materials (synthetic fibres) are oriented crystalline polymers. 

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. 

As a model of the nucleus in polymer crystallisation one often takes a rectangular prism. A breakthrough in this respect was the discovery and exploration of polymer single crystals (Schlesinger (1953) and Keller (1957)) which are indeed small prisms, platelets of polymeric chains, folded back and forth in a direction perpendicular to the basal plane (see Fig. 19.1)... 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

INDUCED CRYSTALLISATION OF FLEXIBLE POLYMERIC MOLECULES BY PRESSURE AND STRESS... 

The interplay of orientation and crystallisation leads to a wide range of super-molecular structures or morphologies. Each different morphology represents to the user a different compromise in physical properties, so that characterisation and control of morphology becomes very important for the efficient application of polymeric materials. 

The synthesis of [Rh2(OAc)4(H20)2] from RhCl3-3H20, HOAc and NaOAc first gives [Rh2Cl2(OAc)4]2, which has now been crystallised and found to have the expected paddle-wheel structure the cations of the hydrated sodium salt are co-ordinated to one Cl and one OAc oxygen on adjacent dimers and to two water molecules.152 [Rh2(02CCF3)4(thf)] exists as two isomers, one being polymeric with... 

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