Polyethylene hexagonal phase

Keywords Polymer crystallization NMR of polymers Polyethylene Hexagonal phases Nanostructured materials Confined polymers Crystal engineering Nanochannels... 

As already stated, we shall not explain the details but refer the reader to the literature for further developments. However, we would stress that there are very large intrinsic differences between one- and two-dimensional nucleation, and these are likely to be important for highly mobile phases such as the hexagonal phase in polyethylene. 

Fig. 1 The differing optical textures, between crossed polars, of linear polyethylene after crystallization from the melt at pressures close to the triple point, 0.3 GPa (a) the conventional spherulitic texture of the orthorhombic phase (b) the coarse lamellar texture formed as the hexagonal phase then transformed to orthorhombic during return to ambient temperature and pressure from [ 14]...

Insufficiently precise data do not allow the theoretical possibility to be decided whether thin polyethylene lamellae are more stable at atmospheric pressure in the hexagonal phase rather than the orthorhombic though it appears not to be the case. 

The orthorhombic and hexagonal phases of polyethylene crystallize independently in accordance with the phase diagram and kinetic competition during growth. 

Another way to disentangle linear polyethylenes, and thus control the interphase without using a solvent, is to anneal the polymer in the hexagonal phase. Bassett has discussed the role of the hexagonal phase in the crystallization of polyethylene extensively in an earlier chapter in this book Briefly, polyethylene exhibits a number of different crystal structures, with the hexagonal phase being observed in linear polyethylenes at elevated pres-sure/temperature in isotropic samples or at ambient pressure in oriented samples. For this reason, we have to distinguish between these two situations, namely isotropic and oriented polyethylene. However, we will focus only on isotropic polyethylene and will refer readers to reference [18,19] for an overview of oriented polyethylene. 

For isotropic polyethylene, the hexagonal phase is usually observed at elevated pressure and temperature, in fact, above the triple point Q located at 3.4 kbar and 220 °C according to the pioneering work of Bassett et al. [20] and Wunderlich et al. [21]. Later, more detailed studies involving in situ light microscopy and X-ray studies showed that the equilibrium point, Q0, is located at even higher temperatures and pressures, 250 °C and 5.3 kbar, re-... 

In a similar manner, the ethylene-octene copolymer crystallized directly via the orthorhombic phase without the intervention of the anticipated hexagonal phase as would be anticipated in linear polyethylenes at these high pressures and temperatures (at approximately 3.8 kbar and around 200 °C). At 100 °C, see Fig. 15, the d values for (110) and (200) orthorhombic reflections are 4.08 A and 3.71 A. When the sample is cooled below 100 °C, a new reflection adjacent to the (110) orthorhombic peak appears at 80 °C. The position of the new reflection is found to be 4.19 A and so corresponds to a new phase. No change in the intensity of the existing (110) and (200) reflections is observed, however the intensity of the amorphous halo decreases, which suggests that the appearance of the new reflection (d = 4.19 A) is solely due to the crystallization of a noncrystalline component. On cooling further as the new reflection intensifies, the (110) and (200) orthorhombic reflections shift gradually. However, at 50 °C, the (100) monoclinic reflection appears with a concomitant decrease in the intensity of the (110) orthorhombic reflec-... 

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