Fast crystallisation

Economic Aspects. The first company to introduce nucleated PET mol ding resins was Ak2o Plastics BV with their Amite range. This business has been taken over by DSM NV as part of a portfoHo exchange between the two companies. Du Pont introduced their Rynite fast crystallising materials in 1978, and other manufacturers followed. 

It goes without saying that also the growth speeds as functions of temperature were determined in Linz for a number of the rather fast crystallising industrial polymers (see the mentioned reviews.)... 

Polyester is not a fast crystallising polymer. The rate of crystallisation is maximum for polyester at around 180 C. The maximum crystallisation rate for polyester is about 0.016/sec, while it is 0.14/sec for nylon 6. The following changes take place as a function of increasing temperature in polyester when heat-set under free to relax conditions (i.e. free annealing) and when held taut at constant length (i.e. taut annealing) ... 

A clear distinction between crystallisation and precipitation is not always possible from a practical point of view [57] hence, it is more convenient to consider precipitation as a very fast crystallisation process. Crystallisation is a result of the combined effects of nucleation, nucleus growth and secondary processes inside the suspension such as agglomeration, ageing and recrystallisation. Depending on the reaction conditions, the above processes can occur together or sequentially during the crystallisation period. 

The synthesis of all these properties in a single compound appears at first to lead to contradictions. The fast crystallisation implies that the structure of the amorphous and crystalline phases should not be too dissimilar, but the contrast in properties and the stability of the amorphous over long times implies strong differences in the nature of the crystal and amoprhous phase, as well as a significant activation barrier to recrystallisation. 

Assuming fast crystallisation relative to chain scission... 

If fast crystallisation is assumed, then the degree of crystallinity is given by... 

Figure 4.5 Best fits of the experimental data obtained by Weir et al. (2004) for PLLA using the model assuming fast crystallisation (solid lines) and the model without the assumption (dashed lines), respectively. The degradation data (discrete symbols) of molecular weight and degree of crystalfinity as functions of time were obtained at three temperatures (a) 37°C (b) 50°C and (c) 70°C. Source Taken from Gleadall et al., 2012.)...

Distil the filtered ethereal solution, using a 100 ml. flask fitted with a dropping-funnel and a side-arm for the condenser observe all the normal precautions for ether distillation (p. 162) and run the ethereal solution into the flask as fast as the ether distils over. When all the ether has distilled off, detach and cool the flask, when the oily colourless residue of saligenin will rapidly crystallise. Weight of product, 5-0 g. m.p. 75-82°. Recrystallise either from a mixture of benzene and petroleum (b.p. 60-80°), or from a minimum of water, allowing the stirred aqueous solution to cool to 65-70° before chilling. The dry crystalline saligenin has m.p. 85-86°. 

Hexamethylenetetramine. Pure hexamethylenetetramine [100-97-0] (also called hexamine and HMTA) is a colorless, odorless, crystalline sohd of adamantane-like stmcture (141). It sublimes with decomposition at >200° C but does not melt. Its solubiUty in water varies Htde with temperature, and at 25°C it is 46.5% in the saturated solution. It is a weak monobase aqueous solutions are in the pH 8—8.5 range (142). Hexamethylenetetramine is readily prepared by treating aqueous formaldehyde with ammonia followed by evaporation and crystallisation of the soHd product. The reaction is fast and essentially quantitative (142). 

Liquid clathrates offer a great advantage over solid-state separations (e.g. by formation of Hoffman-type inclusion compounds, Section 9.4) because of the extremely fast mixing kinetics, the avoidance of the need to wait for crystallisation to occur and the easy separation of the two liquid phases. It should also prove possible to run liquid clathrate separations in a continuous extraction manner. The avalues of a number of liquid clathrate-based separations have been reported and are summarised in Table 13.1. 

Crystallisation from a solution largely depends on the rate of cooling and on the rate of change in solubility connected with it. Again, the polymers with a regular molecular chain without side groups crystallise fast. 

By rapid cooling-down of the filament - under stress - the obtained orientation is frozen in, combined with a very fast fibrillar crystallisation or recrystallisation. 

We can go beyond the case of fast-ion conductors and apply these principles to understand the role of templating species in inorganic and organic synthesis. A number of examples of zeolite frameworks have been shown in Chapter 2 to follow closely IPMS. The crystallisation of zeolites invariably requires the presence of templating species. A wide variety of templates have been used, from sodium ions, tetra-alkyl ammonium ions to crown ethers. 

The elucidation of so many structures has allowed a successful classification of protein structures [7,8] it has laid the basis for certain predictive methods [9,10], and it has given insight into the possible evolutionary origins of proteins [11-13]. Our understanding of biological function in terms of structure has not increased so fast. This has turned out to be a more difficult problem. In the case of enzymes, a description of several different states of the protein complexed with substrate, intermediates and products together with necessary co-factors and activators is required. Often these different states can only be achieved by co-crystallisation, and even then it may be difficult to trap the necessary conformation. To date, it is... 

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