Polyethylene orthorhombic crystal

Figure 7 Polyethylene orthorhombic crystal habit, (a) Orthogonal view (b) view along the c axis.

Attempts were made to include all hydrogen atoms explicitly in the simulations. This computationally demanding explicit-atom model shows (Fig. 1) that the crystal symmetry is orthorhombic, in agreement with the well-known experimental result for polyethylene single crystals, instead of the hexagonal symmetry seen in united-atom model simulations. 

The orthorhombic crystal phase of polyethylene has, like most polymer crystals, pronounced anisotropic properties. The elastic moduli along the three orthogonal crystallographic directions of the orthorhombic unit cell are at 23 °C 3.2 GPa along a and 3.9 GPa along b according to Sakaruda et al. [14] and 240-360 GPa along c [14, 15, 16]. The thermal expansion coeffi-... 

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]...

Nevertheless, it has subsequently been suggested that, for polyethylene, there could be a phase inversion at small lamellar thickness which could make the hexagonal the precursor of orthorhombic crystallization from the melt even at atmospheric pressure [4]. The basis of this proposal will now be outlined and the conclusion reached that it is inapplicable, in part because the effective fold surface energy during growth of hexagonal lamellae is not sufficiently low. 

Polyethylene conforms to the situation of Figure la, l.e. under normal conditions it does not display a mesophase, only the familiar orthorhombic crystal form (o). Nevertheless, there exists a virtual mesophase which can be "uncovered" the ways in which this can be achieved is the subject of what follows. 

Figure 1.8. The orthorhombic crystal structure of polyethylene. Adapted from Strobl (1996).

Orthorhombic crystals fromRb. sphaeroides RCs are obtained in sitting drops by the vapour diffusion method using polyethylene glycol 4000 (PEG " ) as precipitant and 1,2,3-heptanetriol as additive (Allen... 

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. 2.16 A sketch of a 3D view through a polyethylene single crystal showing the planar zig-zag conformation of molecules referred to the orthorhombic lattice of PE (from Lin and Argon (1994b) courtesy of Springer-Verlag).

Temperature-resolved X-ray experiments on cooling are presented in Fig. 7.36. The first scans show only an amorphous halo. The copolymer is melted. At 321 K, the 110 and 200 orthorhombic crystal interferences appear. The peaks are shifted to lower values compared to the polyethylene homopolymer. The calculated lattice constant a of 0.766 nm is expanded, as discussed with Fig. 7.32. At slightly lower temperatures, the shape of the amorphous halo changes abruptly and acquires a... 

Fig. 10.10 Illustration of polyethylene chains viewed as ovals along the chains, alternately changing orientations to pack into the orthorhombic crystal structure, which determines the quantity of cell parameter a and b...

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