Kinetics of Crystallisation

With regard to the crystallisation, polymers stand in sharp contrast to other materials such as metals that crystallise completely at the melting temperature, Tm the topological 

Reproduced from L. Dujourdy, J.P. Bazile and J.R Cohen Addad, Polymer International, 1999, 48, 561. Copyright Society of the Chemical Industry. Reproduced with permission. Permission is granted by John Wiley and Sons on behalf of the SCI 

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Fig. 9.10. Sulphur, glasses and polymers turn into viscous liquids at high temperature. The atoms in the liquid ore arranged in long polymerised chains. The liquids ore viscous because it is difficult to get these bulky chains to slide over one another. It is also hard to get the atoms to regroup themselves into crystals, and the kinetics of crystallisation are very slow. The liquid can easily be cooled past the nose of the C-curve to give a metastable supercooled liquid which can survive for long times at room temperature.

At the onset of biomineralization the mechanism of phosphate and silica deposition is essentially the same. Both start with a highly hydrated amorphous phase having glass-like physical-chemical properties. The kinetics of crystallisation of the two differs. ACP will rapidly alter in the direction of apatite in hours or days, whereas amorphous silica requires thousands of years or higher temperatures to yield quartz. 

Wide-line and 2H NMR spectra and T2 relaxation experiments have been used to determine the composition of the phases in semi-crystalline polymers [133, 136, 138-144]. The experiments were also used to obtain real-time information on the kinetics of crystallisation and melting [143-148]. The use of high-resolution NMR methods to characterise semi-crystalline polymers is reviewed elsewhere [17, 18, 30, 34, 149]. 

Boon (1966/1968) investigated the kinetics of crystallisation of isotactic polystyrene. This polymer is extremely interesting as a model substance for crystallisation work. Its rate of growth is so low that the crystallisation can be studied in the whole region from Tg to Tm. Due to the low growth rate the fundamental processes of nucleation and growth can be studied almost separately. 

The above disadvantage of the lack of spatial information can be overcome by a combination of NMR data and other techniques. From the aH pulse NMR, the fraction and the molecular mobility of different molecular environment can be obtained as free induction decays (FIDs) within a short time, which is suitable for a practical, better understanding of the morphology-property relationship. Wide angle X-ray diffraction (WAXD) and small angle X-ray diffraction (SAXD) as well as electron microscopy provide direct information between the nano- and micrometre scale. A combination of NMR data with those from X-ray diffraction and electron microscopy should be able to analyse the structure from the atomic level to the macro scale. In this review, the morphology-property relationship, the dynamics of morphological transition, the kinetics of crystallisation, etc. analysed by a combination of NMR and other tools are introduced. 

Kinetics of Crystallisation of Solids from Aqueous Solution... 

A Rodenstock RM600 has been used to follow the shape change of chocolate and obtain an understanding of the nature of the kinetics of crystallisation of chocolates of different temper (Fryer and Pinschower 2000). The equipment consists of an optical distance sensor, with sub-micron vertical resolution, together with a traverse table which holds the sample and allows the sensor to be tracked across it. 

Figure 22.13 compares the kinetics of crystallisation of tempered and untempered material in more detail, here for a continuous cooling rate of 0.1 C/min. The difference in the rate and extent of the shape change is clear. No experiments were done to identify the final crystal form in these experiments the final shrinkage in untempered chocolate is however less than for tempered material. 

Kinetics of crystallisation of the blends have also been studied by following crystallisation isotherms using DSC. Figure 7 presents an Avrami plot (16) for PA 66 and for the nylon matrix blends containing either EPR or functionalised EPR. From these data, half-times for crystallisation at 240 deg C were calculated. For PA 66, the half time is 1.6 minutes. Unmodified EPR in the nylon lowers this to 1.4 minutes, while with functionalised EPR in the blend, the value is 1.3 minutes. As reproducibility is only +/- 0.1 minutes, the difference between the two blends may be insignificant. However, it appears that the presence of EPR may increase the crystallisation rate of the nylon matrix. 

Rates of radial growth of spherulites, constant with time, were highest for pure PCL and decreased with increasing SAN content and with increasing temperature in the range 34-50 C [1061 he. the presence of SAN decreased the rate of crystallisation. The variation in extent of crystallinity with time was sigmoidal and the kinetics of crystallisation were consistent with the Avrami equation (Eq. 26) with an exponent of 3 0.02, consistent with three-dimensional growth... 

For the amorphous sample, the dry Tg was easily observed close to the hterature value of 116°C [35]. Despite the fast scan rates up to 500°C/min, crystallisation exotherms were still observed, indicating the rapid mobility and kinetics of crystallisation of small molecules compared with polymers [36]. Investigating the relationship between crystallisation and heating rate may offer an opportunity to characterise, or at least rank order, the mobility within devitrified amorphous pharmaceuticals. 

The presence of these different forms and the fact that they crystallise at different rates and have different levels of supersaturation at any one temperature affords the opportunity for some very complex behaviour. However, some simple principles can give a reasonable understanding. The kinetics of crystallisation tend to be studied less frequently than that of the equilibrium phase behaviour, but is often more important industrially [63]. Here we reproduce work carried out in our laboratory [64]. It can be seen in Figure 9.17 that there... 

Kinetic information maybe determined including isothermal and non-isothermal kinetics of crystallisation [19,20] and measurement ofCCRfor glass formation (see Section 10.4.3.2) for liquidus temperatures below 1000 K [21]. 

Within the crystallisation field of a given zeolite, increasing the alkalinity has a similar effect upon the kinetics of crystallisation as a rise in temperature. This is illustrated in Fig. 8 [69] for mordenite synthesis from gels at a series of increasing pH values. This figure is to be compared with Fig. 1 for the same zeolite at a series of increasing temperatures. Fig. 8 also shows that at pH 13.3 the mordenite first formed dissolves. Its succesor is analcime. 

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