Polyethylene crystallite thickness

Figure 20 Normalized frequency distributions of crystallite thickness for linear polyethylene fraction, Mn = 76,700, Mw = 80,800 crystallized at 118 °C ( ), electron micrograph histogram ...

Figure 21 Plot of thickness values as a function of molecular weight for linear polyethylene fractions quenched to —78°C. (A), crystallite thickness, Lc (O), interlamellar thickness, La ( ), interfacial thickness, Lb. Reprinted with permission from Ref. [277]. Copyright 1990 American Chemical Society.

Most of the pressure crystallisation research has been done on polyethylene. Pressures of 3 kbar or higher are required to obtain crystallite thicknesses of 10-7 m. Some other polymers have a much stronger tendency towards extended chain formation. Poly(chloro trifluoro) ethylene shows this effect at about 1 kbar poly(tetrafluoro)ethene already at about 0.3 kbar. 

As was pointed out earlier, the determination of the melting temperature of solution formed crystals of linear polyethylene by differential scanning calorimetry is complicated by melting, or partial melting followed by recrystallization. In essence, what happens is that, because of the very small crystallite thicknesses... 

It should be noted that the dimensions of lattice parameters, determined by x-ray diffraction, have been commonly used to establish the purity of the crystalline phase. Extensive studies of this kind have been carried out with polyethylene copolymers.(21,88-94) The basic assumption is made that the expansion of the lattice reflects the inclusion of the co-unit. However, Bunn has pointed out that this interpretation is not unique.(95) The crystallite thicknesses of such copolymers are relatively small, being less than 100 A, depending on the composition.(74) The strain that develops in the thick interfacial region of such thin crystallites could easily cause the lattice expansion. Hence, the analysis of lattice parameters does not necessarily yield definitive information with respect to the issue of interest. In some cases this analysis has led to incorrect conclusions. 

An intense P transition is universally observed for all branched polyethylenes (short and long chains). However, this transition is found only for a very high molecular weight linear polyethylene. The data plotted in Fig. 4.48 show that, in contrast to the a transition, the temperature of the P transition does not depend on the crystallite thickness. The location of the P transition, Tp, depends on the chemical nature and concentration of the co-unit. Thus, each copolymer has its own p transition [244]. 

When actual experimental data is analyzed further complications can develop. For low molecular weight polyethylene and poly(ethylene oxide) T is essentially independent of as would be expected from the curves of Figure 24. Extended chain crystals are formed and the extrapolation is straightforward. Unusual TJ vs. relations are found for polyethylenes of 3000-5000 molecular weight and poly(ethylene oxide) in a similar range. Here there is a well-defined temperature at which there is a jump in the crystallite thickness accompanied by a discontinuity in Above this temperature extended chain crystals are formed. Hence it is important that experiments be conducted in this latter temperature range to insure a consistent extrapolation. 

A wide range in densities can thus be obtained at 25 °C for the same crystaUine homopolymer. The values depended directly on the crystallization temperature. Other thermodynamic, physical, and mechanical properties are also sensitive to the manner in which the crystallization is conducted. Another example of the importance of crystallization conditions in governing properties is illustrated in Fig. 9.2.(4) Here the crystallite thickness of a molecular weight fraction of linear polyethylene (Mw = 1.89 X 10, Mn = 1.79 X 10 ) is plotted as a function of the crystallization temperature. There is a dramatic increase in these values at about 125 °C. A profound effect is observed in the maximum crystallite thickness. 

Fig. 9.2 Plot of crystallite thickness as a function of crystallization or quenching temperature for a molecular weight fraction of linear polyethylene, Mw = 1.89 x 10 Mn = 1.79 X 10 (4)...

It does not necessarily follow that the crystalUte thickness can be identified with the length of the completely ordered chain.(16) Details of the structure and morphology of such polyethylene crystallites will be discussed in detail in Volume 3. 

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