Interlamellar slip

Another theory of the reason for increased friction in the presence of moisture was proposed by Gao et al . They found that in a humid environment molybdenum disulphide films were more readily thinned by sliding contact, due to increased ease of interlamellar slip. They suggested that adsorption of water softened the films, and that resulting increased deformation by plowing in sliding contact led to a poorly oriented film and thus to increased friction. However, they considered that this was a short-term reversible effect which was not in conflict with theories of chemical breakdown. Gao et al also poiinted out the possibility that an increase In friction is caused by capillary pressure effects of moisture at asperity contacts. 

In view of the multitude of observed deformation mechanisms it is useful at this point to examine the effects of external variables, especially that of ambient temperature, on the deformation behavior of semi-crystalline thermoplastics. At room temperature many of these polymers are above their glass transition point and owe their strength and stiffness to the crystalline phases. The first displacements start in the relatively soft amorphous layers, but the stress-strain curve is largely determined by the presence and arrangement of the crystals. Interlamellar slip has been identified as an important mechanism, but, in addition, crystalline deformation mechanisms occur at moderate strains The corresponding stress-strain curve shows an... 

On the lamellar level, inside the spherulites, several different processes control the ductility of the polymer, which are sketched in Fig. 2.20 [6, 12]. Lamellae running parallel to the direction of strain can be deformed by interlamellar slip (Fig. 2.20(a)), tilted lamellae rotate into the deformation direction twisting of lamellae, Fig. 2.20(b)), and lamellae that are aligned perpendicular to the applied load (in the equatorial region) will be separated lamellar separation, Fig. 2.20(c)). 

There are three, currently recognized, principal modes of deformation of the amorphous material in semicrystalline polymers interlamellar slip, interlamellm-separation and lamellae stack rotation [84,85]. Interlamellar slip involves shem-of the lamellae parallel to each other with the amorphous phase undergoing shear. It is a relatively easy mechanism of deformation for the material above Tg. The elastic part of the deformation can be almost entirely attributed to the reversible interlamellar slip. 

The lamellae slip rigidly past one another. Lamellae parallel to the direction of draw cannot slip thus, spherulites become anisotropic. At this stage, at which necking begins, the strain is accommodated almost entirely by the interlamellar amorphous component. 

The failure of the Zener model is easily rectified by assuming that the mechanism is a set of relaxation processes with a band of relaxation times which are closely spaced. The a-process shown in Fig. 4.4 is the interlamellar (intercrystal) slip process, which is analogous to the grain boundary creep process in metals. The heterogeneity of the polymeric solid is the origin of the fact that the relaxation times occur in a distribution all relaxations in polymers are found to be described by distributed relaxation times. 

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]. 

In an earlier paper by Galeski, Argon and Cohen [39] the following scenario of crystalline polymer deformation imder uniaxial tension was outlined the packets of lamellae in the 45° fans of spherulites experience resolved shear stress that promotes chain slip in the lamellae and shear in the interlamellar amorphous regions (see Figure 1.1). 

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