Polyethylene orthorhombic phase

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]...

Fig. 3 A plot of the supercoolings as a function of pressure at which exotherms appear during the crystallization of linear polyethylene during cooling from the melt at the rates shown. Crosses show the start of the exotherms filled circles show the peak temperatures for orthorhombic crystallization filled triangles show the sequential peak temperatures (where resolved) corresponding first to hexagonal crystallization then its conversion to the orthorhombic phase. Redrawn from [9]...

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-... 

From cyclooctatetracontane orthorhombic crystals with less well defined folds grow from the melt and have a pseudohexagonal condis-phase transition before transition into the isotropic melt. This condis phase is closely related to the paraffin and polyethylene condis phase and has similar dynamics. 

Fig. 2 Static deuterium NMR spectra at 4900 bar of polyethylene (a) in the hexagonal phase and (b) in the orthorhombic phase, showing the mobility of the hexagonal phase, as indicated by the shrinkage of the singularities to about 50 kHz. The isotropic signal at the center of the spectrum represents the mobile amorphous phase...

The monoclinic phase is formed at high crystallization pressure in addition to commonly observed orthorhombic phase in polyethylene. The nucleation of monoclinic phase is encotrraged by nanoclay even at low crystallization pressmes. ... 

Keller et al. (1994) proposed a new scheme for polymer crystallization, also most probably applicable to melt-crystallization of polyethylene at normal pressure. They proposed that crystallization starts with the formation of thin hexagonal phase crystals, which in this crystal size range are more stable than the orthorhombic crystals. The high mobility of the chains in the hexagonal phase leads to rapid crystal thickening which proceeds until the orthorhombic phase becomes more stable than the hexagonal phase. 

Figure 13.18 Schematic picture showing temperature against reciprocal size of polyethylene. Subscripts refer to phases (o, orthorhombic h, hexagonal) or phase transition (tr, transition from hexagonal phase to orthorhombic phase) of polyethylene. Keller et al. [102]. Reproduced with permission of The Royal Society.

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

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