Crystallisation from the melt

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

PMP can also crystallise in several crystalline forms (28). Form I is produced during crystallisation from the melt. This is the only crystalline form present in commercially manufactured articles. Other crystalline modifications ate formed when the polymer is crystallised from solution (28). 

The ct-form which forms on rapid crystallisation from the melt and which has a helical conformation. 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

Chloroacetic acid [79-11-8] M 94.5, m 62.8°, b 189", pK s 2.87. Crystallise the acid from CHCI3, CCI4, benzene or water. Dry it over P2O5 or cone H2SO4 in a vacuum desiccator. Further purification is by distillation from MgS04, and by fractional crystallisation from the melt. Store it under vacurnn or under dry N2. [Bemasconi et al. J Am Chem Soc 107 3621 1985, Beilstein 2 IV 474.]... 

Purify the styrene by fractional crystallisation from the melt and by distillation in vacuo. [Beilstein 5 in 1174.]... 

Isoquinoline [119-65-3] M 129.2, m 24 , 25.5-26 , h 120 /18mm, 243.25 /760mm, d " 1.0986, Ud 1.6148, pK 5.40. Dry isoquinoline with Linde type 5A molecular sieves or Na2S04 and fractionally distil at reduced pressure. Alternatively, it can be refluxed with, and distilled from, BaO. It is also purified by fractional crystallisation from the melt and distilled from zinc dust. It forms a phosphate (m 135 ) and a picrate (m 223°), which are purified by crystallisation, and the free base can be recovered and distilled. [Packer et al. J Am Chem Soc 80 905 795S.] The procedure for purification via the picrate comprises the addition of quinoline to picric acid dissolved in the minimum volume of 95% EtOH to yield yellow crystals which are washed with EtOH and air dried before recrystaUising from acetonitrile. The crystals are dissolved in dimethyl sulfoxide (previously dried over 4A molecular sieves) and passed through a basic alumina column, on which picric acid is adsorbed. The free base in the effluent is extracted with -pentane and distilled under vacuum. Traces of solvent from small quantities are removed by vapour phase chromatography. The hydrochloride crystallises from EtOH with m 193°. [Mooman Anton J Phys Chem 80 2243 1976, Beilstein 20 II 236, 20 III/IV 3410, 20/7 V 333.]... 

Fig. 18.10. The effect of crystallite phase on PVDF CPMAS spectra. Top. sample crystallised from the melt (a form). Bottom 9-p.m thick biaxially drawn film ()3 form with a little a). Spectra were obtained using the Tip filter (precontact H spin lock 40 ms, contact time 50 p-s).

At present, isotactic polypropylene (i-PP) is commercially by far the most important system of the three modifications mentioned above. During crystallisation from the melt, i-PP is usually in the a form, which has a monoclinic crystal lattice with a Tm-value of about 160°C. The occurrence of a S form (with a hexagonal lattice and a Tm-value of about 152°C) is also possible during crystallisation under stress. Besides, a third (gamma) form with a triclinic crystal lattice is possible under exceptional circumstances [11]. 

The carbonyl absorption at around 280 nm can be used to see oxidation in polypropylene directly but this is only possible in heavily oxidised samples which are also too brittle to be sectioned for microscopy. It is possible in this way to look only at samples which have been directly crystallised from the melt as thin films. In order to work with samples with more normal oxidation levels we must stain the oxygen containing groups to make them uv absorbing. In principle it should be possible to separately stain carbonyl or peroxide groups but we have concentrated on 2,4-dinitrophenylhydrazine (I IPH) as a stain for carbonyl groups. Dansyl hydrazine was also tried as a fluorescent stain but was limited in its ability to penetrate the sample so that only the surface was stained. 

Fig. 2.20 Examples of electron micrographs of polymers, (a) A defocussed bright-field image of a thin film of isotactic polystyrene annealed and crystallised at about 170 °C (b) An image of a fracture surface replica from a sample of linear polyethylene crystallised from the melt at 4.95 kbar. ((a) Adapted by permission of Masaki Tsuji and (b) adapted from Principles of Polymer Morphology by D. C. Bassett. Cambridge University Press 1981.)...

There is evidence from neutron-scattering experiments that lamellar crystallisation from the melt can occur without significant change of overall molecular dimensions, such as the radius of gyration. A model that fits this evidence and incorporates the ideas described above is shown in fig. 5.7. Lamellar stacks are discussed further in section 5.4.3. 

Extended, or fully extended, chain crystallites contain straight chains at least 200 nm long and have been obtained for only a few polymers, such as polytetrafluoroethylene (PTFE), polyethylene and polychlorotrifluoro-ethylene, using special crystallisation techniques. Extended-chain polytetrafluoroethylene can be obtained by slow crystallisation from the melt the other two are obtained by crystallisation from the melt under elevated pressure. Solution crystallisation has so far not been shown to give rise to extended-chain crystals. 

Spherulites, which consist of aggregates of crystal lamellar stacks, are important structural features found in very many polymers crystallised from the melt. The simplest ways of observing them are directly in the optical (or electron) microscope and indirectly by light scattering. Other polycrystalline structures observed under certain circumstances are axia-lites and shish-kebabs, which are described briefly in section 5.5.4. 

Fig. 8. Pattern during recrystallisation of a stretched iPP sample at temperatures below 140 °C Isotropic pattern after crystallisation from the melt, a slight preferential orientation may be observed (creation of crystallites with a wide range of orientation) crystallisation of the common stretched lamellar fragments generation of a meridional double-reflex (daughter lamellae) creation of a new series of (h k l)-crystallites (from left to right). The stretching direction is vertical.

Diphenylmethane [101-81-5] M 168.2, m 25.4", b 74"/0.4mm, 260.5"/764.5mm. Sublime it under vacuum, or distil it at 72-75°/0.4tnm. Recrystallise it from cold EtOH. It has also been purified by fractional crystallisation from the melt. [AiiamegoAustJChem 13 95 1960, Beilstein 5II498,5IV 1841.]... 

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