Shear interlamellar

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

The results of Figure 11 indicate that the polymers studied were subject to different microstructural deformation mechanisms. In this connection it must be borne in mind that the maximum nominal deformation of POM and PA66 was only 1% whereas PP and PTFE were deformed up to 1.8% and 3.3% respectively. Therefore it may be assumed that for POM and PA66 only an instantaneously reversible deformation of the amorphous matrix of the spherulitic microstructure occurred (18) whereas for PP and PTFE some irreversible effects, like interlamellar shearing or reorientation of the lamellae may have taken place. 

The relaxation associated with interlamellar shear has been further investigated by Stachurski and Ward, who confirmed and extended Takayanagi s earlier measurements. The anisotropy of the a peak in annealed samples, with tan 5q > tan 45 > tan 9o, was similar to that of the P relaxation in low density polyethylene, being attributed to an inter-lamellar shear mechanism. Evidence of this process was present also in cold drawn samples, but was less clearly defined. 

Fig. 24. Schematic representation of the possible deformation processes of a stack of crystal lamellae (a) the initial state, (b) interlamellar shear, (c) interlamellar separation, (d) intralamellar block shear, (e) intralamellar fine shear (not shown bending and rotation of lamellae), and (f) cavitation within the amorphous regions.

Figure 8.12 Schematic structure diagrams of mechanical loss spectra and 10 s isochronal creep moduli (a), (d) and (g) for be sheet (b), (e) and (h) for parallel lamellae sheet (c), (f) and (i) for ab sheet. P, interlamellar shear process Q, c-shear process (note absence of c-shear process in (f)) R, small-angle X-ray diagram, beam along X... 

The identification of the a process as a c-shear relaxation and the process as interlamellar shear in a drawn and annealed LDPE sheet was nicely confirmed by measurements of the anisotropy of dielectric relaxation [27], Pure polyethylene... 

Interlamellar shear in anisotropic polyethylene sheets. /. Macromol. Sci. B, 6, 215 Davies, G.R. and Ward, I.M. (1972) Anisotropy of mechanical and dielectric-relaxation in oriented poly(ethylene terephthalate). /. Polym. Sci. B, 6, 215. 

Key results for the mechanical anisotropy of LDPE sheets have already been discussed in Section 9.5.3. Oriented and annealed sheets can be considered as composite solids, where the relaxation is an interlamellar shear process, consistent with its assignment by Boyd to an amorphous process. Figure 10.10 shows results for cold-drawn and annealed... 

HDPE, where no p relaxation is observed [29], In these sheets, the crystal lamellae make an acute angle of about 40° with the initial draw direction [30], Applying the stress along the initial draw direction then gives the maximum resolved shear stress parallel to lamellar planes. We see from Figure 10.10 that the maximum loss is tan 3o, confirming that the a relaxation in HDPE is primarily an interlamellar shear process from a macroscopic mechanical viewpoint. 

The identification of the a process as a c-shear relaxation and the p process as interlamellar shear in a drawn and annealed LDPE sheet was nicely confirmed by measurements of the anisotropy of dielectric relaxation [32], Pure polyethylene shows no dielectric response, so experiments were made on specimens that had been lightly decorated with dipoles by means of oxidation, to such a small extent that the overall relaxation behaviour was not significantly affected. The dielectric relaxation data showed marked anisotropy for the relaxation, consistent with its assignment to the c-shear relaxation, but the P relaxation... 

As we have seen, the strain rate dependence does suggest that yield behaviour often indicates the presence of two thermally activated processes, as discussed above. In some cases, notably polyethylene, a double yield point is observed. Ward and co-workers [64], Seguala and Darras [65] and Gupta and Rose [66] concur that these two deformation processes are essentially interlamellar shear and intra lamellar shear (or c-slip). They are akin to the dynamic mechanical relaxation processes identified in Chapter 10.7.1 for the specially oriented PE sheets, and Seguala and Darras have related them to the a and o 2 transitions reported by Takayanagi [67]. This establishes a direct link between yield and viscoelastic behaviour. 

The mechanisms of tensile deformation of semicrystalline polymers was a subject of intensive studies in the past [8-20]. It is believed that initially tensile deformation includes straining of molecular chains in the interlamellar amorphous phase which is accompanied by lamellae separation, rotation of lamellar stacks and interlamellar shear. At the yield point, an intensive chain slip in crystals is observed leading to fragmentation but not always to disintegration of lamellae. Fragmentation of lamellae proceeds with deformation and the formation of fibrils is observed for large strains [21-24]. 

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