Polyethylene crystallisation

Fig. 22 Transmission electron micrograph of permanganic-etched branched polyethylene crystallised at 120 °C for 1 min showing dominant S-shaped lamellae. From Patel and Bassett [54] with permission from Elsevier, UK...

Fig. 24 Transmission electron micrograph of linear polyethylene crystallised at 127 °C for 2.5 min showing spiral terraces in the 110 sectors. Scale bar represents 1 pm. From Toda and Keller [37] with permission from Springer, Berlin Heidelberg New York...

Fig. 29 Cumulative melting and dissolution (in p-xylene) curves of a linear polyethylene crystallised at 401 K to completeness and then rapidly cooled to room temperature. Drawn after data of Gedde et al. [152]...

Logarithm of crystallisation half time vs. logarithm of molecular weight, for polyethylene crystallised isothermally at the temperatures indicated (from Mandelkern, L. J. Mater. ScL, 6,615, 1968). 

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.)...

Fig. 5.3 Electron micrographs of single crystals of polyethylene crystallised from dilute solution in xylene (a) diamond-shaped crystals and (b) truncated crystals. (Reprinted by permission of John Wiley Sons, Inc.)...

Miller et al. have recently reported infra-red dichroic data obtained for high density polyethylene crystallised under the orientation and pressure effects of a pressure capillary viscometer. Their data for a number of crystalline bands (including the 1894 cm" absorption) showed that the crystal c-axes were almost perfectly oriented (f 1) in the initial extrusion direction. The amorphous orientation functions were generally lower, but corresponded to an extension ratio between 2 and 7 when compared with the above results of Read and Stein and of Glenz and Peterlin. Further evidence was also obtained for the relatively high orientation of the amorphous component of the 2016 cm" band (U = 0-66-0-72). 

Fig. 5.6 The long periods for a series of polyethylenes crystallised at differing temperatures for polyethylenes GX with and without DBS as indicated in the key...

Olley RH, Mitchell GR, Moghaddam Y (2014) On row-structures in sheared polypropylene and a propylene-ethylene copolymta-. Eur Polym J 53 37-49 Pople JA, Mitchell GR, Sutton SJ, Vaughan AS, Chai C (1999) The development of organised structures in polyethylene crystallised liom a sheared melt, analyzed by WAXS and TEM. Polymer 40 2769-2777... 

P. J. Barham, R. A. Chivers, and A. Keller, The supercooling dependence of the initial fold length of polyethylene crystallised from the melt Unification of melt and solution crystallisation. Journal of Mat. Science 20 (5), 1625-1630 (1985). M. Avrami, Kinetics of phase changes 1. J. Chem. Phys. 7,1103-1112 (1939). M. Avrami, Kinetics of phase changes II. /. Chem. Phys. 8,212-224 (1940). 

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

If a polymer molecule has a sufficiently regular structure it may be capable of some degree of crystallisation. The factors affecting regularity will be discussed in the next chapter but it may be said that crystallisation is limited to certain linear or slightly branched polymers with a high structural regularity. Well-known examples of crystalline polymers are polyethylene, acetal resins and polytetrafluoroethylene. 

From a brief consideration of the properties of the above three polymers it will be realised that there are substantial differences between the crystallisation of simple molecules such as water and copper sulphate and of polymers such as polyethylene. The lack of rigidity, for example, of polyethylene indicates a much lower degree of crystallinity than in the simple molecules. In spite of this the presence of crystalline regions in a polymer has large effects on such properties as density, stiffness and clarity. 

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 more recently developed so-called linear low-density polyethylenes are virtually free of long chain branches but do contain short side chains as a result of copolymerising ethylene with a smaller amount of a higher alkene such as oct-1-ene. Such branching interferes with the ability of the polymer to crystallise as with the older low-density polymers and like them have low densities. The word linear in this case is used to imply the absence of long chain branches. 

The first patent on the chlorination of polyethylene was taken out by ICI in 1938. In the 1940s scientists of that company carried out extensive studies on the chlorination process. The introduction of chlorine atoms onto the polyethylene backbone reduces the ability of the polymer to crystallise and the material becomes rubbery at a chlorine level of about 20%, providing the distribution of the chlorine is random. An increase in the chlorine level beyond this point, and indeed from zero chlorination, causes an increase in the Tg so that at a chlorine level of about 45% the polymer becomes stiff at room temperature. With a further increase still, the polymer becomes brittle. 

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

A further approach is used by Bayer with their polyesteramide BAK resins. A film grade, with mechanical and thermal properties similar to those of polyethylene is marketed as BAK 1095. Based on caprolactam, adipic acid and butane diol it may be considered as a nylon 6-co-polyester. An injection moulding grade, BAK 2195, with a higher melting point and faster crystallisation is referred to as a nylon 66-co-polyester and thus presumably based on hexamethylene diamine, adipic acid and butane diol. 

Processing temperatures should not exceed 180°C, and the duration of time that the material is in the melt state should be kept to a minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding, the blow pin and mould should be at about 60°C to optimise crystallisation rates. Similarly, injection moulds are recommended to be held at 60 5 C. 

---