Chain tilt and slip

Fig. I. Chain tilt and slip (a) in a crystal lamella leading to the formation of a crack, (b) bridged by the partially unfolding macromolecules (Peterlirfi).

Still larger stresses cause a destruction of the substructure, involving reorientation of chain segments and of lamellar crystals (crystal rotation, chain tilt and slip),... 

These observations imply that above 40 lun the crystals thickness is no longer the decisive factor for the yield and that some other mechanism takes over the control of the yielding process. Any coarse slip and other inhomo-geiieities in the course of compression which could decrease the yield stress did not occur. Instead, as evidenced in scanning electron microscope (SEM) examination, chain tilt and homogeneous lamellae thiiming occur up to a compression ratio above 2 (see Figm-e 1.9). 

The rapid disappearance of these lamellae at EDR >4.6 suggests that when the chain tilt reaches at an angle, the lamellae suddenly broke into smaller blocks due to shear slip along the chain direction and are reorganized in microfibrils oriented along the draw direction. 

Fig. 1. (a) Short, flat aromatic planes (seen end-on), when bonded to 5 A-wide alkane chains, pack poorly if not tilted (b) the same aromatic planes, if tilted and side-slipped, will achieve better packing, at the expense of good overlap, (c) Longer aromatic planes, e.g. perylene, can tilt and also overlap better, with best packing. 

A recent study by van Aerie and Braam states that the process of ultradrawing seems to be a combination of tilting/breaking up/slipping of the chain folded crystals and converting them into chain extended micro fibrils as shown in the picture of the broken fiber. For polymer scientists, there is still an exciting area to study the enormous molecular rearrangements upon deformation. 

Due to chain entanglements the self diffusion coefficient of the labeled macromolecules is very small (<10 mVs), and the transport of the photobleached tracers takes place predominantly through convection. If there is no slip at the polymer - prism interface, the fluorescence intensity decreases steadily with time as the concentration pattern is progressively tilted by the shear. If slip occurs, the concentration pattern is translated (and tilted) in front of the interference fringes creating a damped periodic oscillation of the fluorescence intensity with a period T = iA s, if Vs is the average slip velocity within the distance A from the prism surface. Measuring T and i yields directly Vj. 

Block slippage and chains pulled out. Continued tilt, slip, and twist... 

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