Polyethylene solution-grown crystals

Figure 6.10 uses polyethylene as the model material. The orthorhombic cell structure and the a- and 6-axes are illustrated. The c-axis runs parallel to the chains. The dimension is the thickness of the crystal. The predominant fold plane in polyethylene solution-grown crystals is along the (110) plane. Chain folding is also supported by NMR studies (see Section 6.7) (49-51). 

In order to confirm the anisotropy due to the a-hydrogen, Salovey et al. [9] and Shimada et al. [10] studied the patterns of the ESR spectra from irradiated solution grown crystals of polyethylene. The crystal c-axis was oriented perpendicular to the plane of the sample while the a- and b-axes were randomly oriented in the plane as shown in Fig. 7.8. Six- and ten-line spectra were observed when the c-axis of the crystal was set to be parallel (Fig. 7.9(a)) and perpendicular (Fig. 7.9(b)) to the direction of the applied magnetic field, respectively. From these results, the anisotropic hyperfine splitting due to the a-hydrogen Ay = 0.75 mT, Ax = 1.72 mT and Az = 3.70 mT were determined and were related to the molecular orientation of the crystal. The x-, y- and z-axes coincide with the directions of the p-orbital, the (Ca )— Ha bond, and the main chain axis, respectively, as shown in Fig. 7.10. 

An answer to the question posed above can be found in the following study of free radicals trapped in a urea-polyethylene complex (UPEC). Figure 7.7 in Section 7.4.1 showed ESR spectra observed at 320 K which demonstrated the difference between the spectra of the alkyl radicals trapped in UPEC and those in solution grown crystals. The conclusion was that the radical sites in the UPEC were more mobile than in the crystals. The difference between the decay behavior of the alkyl radicals trapped in the complex and in the solution grown crystals is shown in Fig. 7.16. It can be said that the free radicals in the complex have a very long life time at 318 K and the decay rate in the complex at 411 K is of the same order as that in... 

In Sects. 5 and 6, a few investigations of urea-polyethylene complexes (UPEC) were discussed. The UPEC is an interesting material because a single polyethylene chain is located in an hexagonal canal of urea molecules and it must be expected that the polyethylene chain can behave differently from the bulk systems like solution-grown crystals or materials recrystallized from the melt. The inclusion complex system composed of short hydrocarbon molecules and urea molecules was studied more than 30 years ago. The crystalline structures of urea-hydrocarbon complexes are known The urea-polyethylene complex system was prepared rather recently by Monobe et al. , replacing the hydrocarbon molecules in the urea-hydrocarbon complex by... 

Fig. 9.2. Correlation time estimated from the spectrum of the spin-probe molecule at surface area of solution-grown crystals of polyethylene (based on Ref. ).

Figure 10.12 Electron diffraction pattern from a solution-grown crystal of polyethylene.

In solution-crystallized polyethylene fractions, Raman spectra have demonstrated that crystalline structure is invariant with molecular mass and that crystallinity is far from complete. The interfadal region is relatively small, as expected from theoretical considerations. Densities of solution-grown crystals of linear polyethylene show that the crystals are 8(C90% crystalline [145 147]. This conclusion is supported by measurements of the enthalpy of fusion, infrared and Raman spectroscopy, and other physical properties [148]. Consequently there is a small but appreciable... 

Polyethylene may be extmded below its melting point if sufficiently high pressures are exerted, in which case the process is known as solid-state extrusion. If the starting material is a reduced entanglement precursor, such as solution-grown crystals [56] or as-polymerized granules [57], fibers and tapes with ultrahigh... 

Solid-state tensile drawing to yield highly oriented polyethylene products may be performed on melt-crystallized samples and reduced entanglement precursors such as solution-grown crystal mats, dried gels, as-polymerized films, and solid-... 

Our broad-line XH NMR analysis showed that this type of sample generally consists of the phase structure of lamellar crystallites and noncrystalline overlayer with a negligible amount of the noncrystalline amorphous phase [16,62]. In broad-line H NMR spectra of solution-grown linear polyethylene samples, a narrow component that suggests the existence of a liquid-like amorphous phase is hardly recognized. In Table 2, the three-component analysis of the broad-line XH NMR spectra of linear polyethylene samples with different molecular weights that were crystallized isothermally from 0.08% toluene solution at 85 °C for 24 hours under a nitrogen atmosphere is summarized. 

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