Solution grown crystals of polyethylene

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. 

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... 

The single crystals of polyethylene grown from solution showed a number of other important features ... 

Figure 12. (F—E) Characteristic FTIR spectrum of 110 chain folds in solution-grown single crystals of n-alkane Ci98H3g8 (spectrum of extended-chain crystals subtracted from that of once-folded crystals) (PESC) solution-grown single crystals of polyethylene (bulk PE) melt-crystallized linear polyethylene. All bands are CH2 wagging defect modes except the CH3 band at 1378 cm-1. Spectra recorded at 110 K (from ref 68 by permission of Elsevier Science Publ.)...

When they are grown at sufficient dilution, the crystallites approximate to lamellae with a uniform thickness of about 12 nm, the precise value depending on the temperature of growth. Electron diffraction shows that the chain axes are approximately perpendicular to the planes of the lamellae. The crystals are not exactly flat, but have a hollow-pyramidal structure, with the chain axes parallel to the pyramid axis. This pyramidal structure is seen clearly in fig. 5.5, which shows a single crystal of polyethylene floating in solution. This should be compared with fig. 5.3(b), which shows similar crystals flattened on an electron-microscope grid. The dark lines on the crystals in fig. 5.3(b) show where the pyramid has broken when the crystal flattened. 

Fig. 5.8 A schematic diagram of chain folding in a solution-grown single crystal of polyethylene. (Reproduced from The Vibrational Spectroscopy of Polymers by D. I. Bower and W. F. Maddams.

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... 

One method of investigating molecular motion in polymer physics is the observation of the temperature dependence of the line width of broad-line NMR spectra. However, since UPEC is composed of polyethylene and urea molecules, the protons in urea molecules must be replaced by deuterons in order to observe the behavior of the polyethylene chain by proton magnetic resonance. For this purpose, deuterated urea molecules were used in the preparation of UPEC (d-UPEC). In the preparation of d-UPEC, deuterated methanol has been used as a solvent in order to prevent proton exchange. In order to compare the new data with the data of bulk polymers, solution-grown polyethylene and extended-chain crystals of polyethylene were also used in the NMR study. 

Wunderlich et al. have also reported the preparation and sepeiration of extended chain crystals of polyethylene from the melt at 510 K and 500 MN m" and separating them by etching with fuming nitric acid. The product was, of course, an a,(o-dicarboxylic acid with a molecular length of about 900 —CH — units. Cavello et have grown crystals of random isotactic copolymers of propylene and but-l-ene from isoamyl acetate solution. By comparison with melt-crystallized materials they concluded that both morphologies, and hence crystallizations, were fundamentally identical, and that the co-monomer unit co-crystallizes. There is a depression of the melting point over that of the homopolymer and a eutectic is observed at 48% butene content. 

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). 

Figure 7.IS Annealing (heat treatment) of a solution-grown monolayer single crystal of polyethylene (schematic drawing).

Spells, Keller and Sadler (1984) showed by infrared spectroscopy that 75% of the folds in solution-grown single crystals of polyethylene led to adjacent re-entry (tight folds) and that single molecules were diluted by 50% along the lIO fold plane. Both observations are consonant with the super-fold model. 

Extended-chain crystals of polyethylene are produced by high-pressure crystallization at elevated temperatures, typically at 0.5 MPa and 245 °C. These micrometre-thick crystals display a distinctly different melting behaviour from that of the thin folded-chain single crystals grown from solution. The recorded... 

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