Recrystallisation from the melt

Fractional solidification and its applications to obtaining ultrapure chemical substances, has been treated in detail in Fractional Solidification by M.Zief and W.R.Wilcox eds, Edward Arnold Inc, London 1967, and Purification of Inorganic and Organic Materials by M.Zief, Marcel Dekker Inc, New York 1969. These monographs should be consulted for discussion of the basic principles of solid-liquid processes such as zone melting, progressive freezing and column crystallisation, laboratory apparatus and industrial scale equipment, and examples of applications. These include the removal of cyclohexane from benzene, and the purification of aromatic amines, dienes and naphthalene. 

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PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

A -Allylthiourea (thiosinamine) [109-57-9] M 116.2, m 70-73 , 78 . Reciystallise it from H2O. It is soluble in 30 parts of cold H2O, and it is soluble in EtOH but insoluble in C6H6. It has also been recrystallised from acetone, EtOH or ethyl acetate, after decolorising with charcoal. The white crystals have a bitter taste with a slight garlic odour and are TOXIC. An unstable crystalline form is obtained by recrystallising from the melt. [McCrone et al. Anal Chem 21 421 1949, Beilstein 4 IV 1072.]... 

Dimethylanthracene [781-43-1] M 206,3, m 180-181°, Purify 9,10-dimethylanthracene by crystallising from EtOH, and by recrystallising from the melt. [Beilstein 5 IV 2329.]... 

A fused sample is often subsequently cooled, to follow the recrystallisation from the melt. The resulting exothermic recrystallisation effect is usually described by the temperature of the minimum heat flow (the Tc-value) and by the total heat effect involved (Hc-value). Some advance knowledge is necessary, however, to arrive at reproducible data. Incompletely fused crystal residues remain present when the temperature of the fused sample has been too low. These residues cause the nucleation process to start at higher temperatures than would normally be the case, resulting in higher Tc-values. 

RECRYSTALLISATION FROM THE MELT OF SHOP LINEAR RLPHR-OLEFINE BASED POLYMERS... 

A series of non-equilibrium Tm-values were measured on samples recrystallised from the melt under standard conditions (the so-called Tm2-values, see 1.1.4) and a number of nonequilibrium literature values were used to look for an improved correlation between Tm/Tg relations. We tried to improve the results of such a relation by distinguishing different groups of polymers instead of looking for one relation for all types of polymers. Three groups of polymers offering the best fitting correlations, were selected finally ... 

The experimental results in Table 9.4 indeed show that the optimum annealing temperature shifts from 250°C/255°C for a non-fused powder sample to about 225°C for a first fused sample. Annealing during less than one minute at 250°C/255°C was already sufficient for the formation of an or-crystalline phase with a maximum Tm - and Hct- value in a non-fused powder sample. This time increased for the fused samples from less than one minute into about six minutes. Moreover, the maximum Tm -value reached now proved to be 106°C instead of the 120°C measured for the powder sample. This illustrated that after recrystallisation from the melt either less perfect B-crystallites or smaller E-crystallites are obtained than after crystallisation from a liquid. The annealing process is clearly not able to eliminate this difference completely. 

Figure 9.6 shows the (shifted along the E" axis) dynamic loss modulus (E")/temperature curves of the cold compressed powder sample and that of the compression moulded sheet sample. The crystallinity of the cold compressed sample is about 57%, while the crystallinity of the sample after compression moulding at 240°C i.e. after recrystallisation from the melt, is decreased to about 42%. Both samples are measured at a frequency of 1 Hertz and a heating rate of 2°C/minute. The relaxation behaviour proved to be clearly changed due to this difference in crystallinity of about 15 %. 

The lower crystallinity of the sample in Figure 9.11 after recrystallisation from the melt is also reflected in the higher Cp values in the temperature region between Tg and Tm,... 

FIGURE 9.10B The (DSC) Tg effect for a PK terpolymer sample after recrystallisation from the melt... 

This is amorphous material hindered to such a degree by the crystalline fraction that it behaves rigid [12, 13]. Assuming that a ACp-step at Tg of 0.115 J/g.C corresponds with a polymer weight fraction of 0.14, the extent of the possible rigid amorphous phase in PK terpolymer can be estimated. The sum of x(c), x(a) and x(r,a) is 1.00, thus might hold for PK terpolymer (after recrystallisation from the melt) ... 

A purified DPM reference sample (p,p-DPM > 80 %wt.) showed two endothermic fusion effects (Tmi l58°C/Hfi = 124 J/g, Tm2 -108°C/Hf2 = 14 J/g) and a clear recrystallisation effect (Tc = 120°C/Hc = 88 J/g) during heating and cooling scans at 20°C/minute in the DSC. The difference between the Hfl-value (124 J/g) and the Hc-value (88 J/g) shows already that recrystallisation from the melt for this relative pure sample clearly occurs slower than recrystallisation from solution. 

Butanediol (tetramethylene glycol) [110-63-4] M 90.1, f 20.4 , b 107-108 /4mm, 127 /20mm, d 4 1.02, n 1.4467. Distil the glycol and store it over Linde type 4A molecular sieves, or crystallise it twice from anhydrous diethyl ether/acetone, and redistil it. It has been recrystallised from the melt and doubly distilled in vacMo in the presence of Na2S04. [Beilstein 1IV 2515.]... 

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