HIGH PRESSURE CRYSTALLISATION

A further development, discussed by Moritoki(91), is high-pressure crystallisation, which is considered in Section 15.9. 

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). 

There are essentially four steps or unit operations in the manufacture of fatty acids from natural fats and oils (/) batch alkaline hydrolysis or continuous high pressure hydrolysis (2) separation of the fatty acids usually by a continuous solvent crystallisation process or by the hydrophilisation process (J) hydrogenation, which converts unsaturated fatty acids to saturated fatty acids and (4) distillation, which separates components by their boiling points or vapor pressures. A good review of the production of fatty acids has been given (1). 

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

The presence of these branch points is bound to interfere with the ease of crystallisation and this is clearly shown in differences between the polymers. The branched high-pressure polymers have the lowest density (since close-packing due... 

Natural diamonds used for jewellery and for industrial purposes have been mined for centuries. The principal diamond mining centres are in Zaire, Russia, The Republic of South Africa, and Botswana. Synthetic diamonds are made by dissolving graphite in metals and crystallising diamonds at high pressure (12-15 GPa) and temperatures in the range 1500-2000 K [6] see section 3. More recently, polycrystalline diamond films have been made at low pressures by... 

To save space the following abbreviations have been generally used in Chapters 3, 4 and 5 abs (absolute), anhyd (anhydrous), aq (aqueous), atm (atmospheric), crystd (crystallised), crystn (crystallisation), crysts (crystallises), dec (decomposes), dil (dilute), distd (distilled), distn (distillation), evap (evaporate), evapd (evaporated), evapn (evaporation), filtd (filtered), h (hour[s]), pet ether (petroleum ether, ligroin), ppte (precipitate), ppted (precipitated), pptn (precipitation), satd (saturated), soln (solution), TLC (thin layer chromatography), HPLC (high pressure liquid chromatography), vac (vacuum), vol (volume). Other abbreviations used occasionally are self evident in meaning. 

The solubility of N in liquid Ga was experimentally determined [7,8] for the conditions corresponding to the equilibrium pN2 - T curve. In the high pressure experimental system (20 kbar, 2000 K), the nitrogen content in Ga can be increased up to -1 at.% which is sufficient for effective crystallisation from the solution. 

In practice, many fabrication processes take place under non-isothermal, non-quiescent and high-pressure conditions. Mechanical deformation and pressure can enhance the crystallisation as well as the crystal morphology, by aligning the polymer chains. This leads to pressure-induced crystallisation and to flow-induced or stress-induced crystallisation, which in fact is the basis for fibre melt-spinning (see Sect. 19.4.1)... 

Important investigations in this field have been made by Wunderlich (1964/1972) and Basset (1973/1974). The effect of high pressure on the crystallisation process is threefold ... 

Some polymerisations do not continue in a linear fashion, and branching occurs, as shown in Fig. 1.7(a). When branching is prevalent, it can have a serious effect on properties. Thus the polymerisation of ethylene under high-pressure conditions gives a product that has so many side chains on each main chain that crystallisation is appreciably suppressed. This material is softer than the highly crystalline linear form from a catalytic low-pressure process. The two forms of polyethylene may be distinguished by a difference in density between them, the more crystalline material being denser. 

---