Crystallite with drawing chains

The break-up of crystallites and the reformation of the lamellar fragments into microfibrUs is the basis of a theory for the cold-drawing of isotropic semi-crystalline polymers due to Peterlin. " (See also Hosemann et and Robertson .) Both Peterlin and Hosemann assert that the main mechanism is the break-up of each crystallite into approximately twenty smaller units which lie like pearls on a string with their chain axes parallel to the IDD. Many aspects of these theories would seem to be relevant to the deformation of oriented polymers of modest draw ratios. 

Through the use of multiple experimental techniques, we have shown how both the NXL and XL phases of PILE interact and respond to applied tensile deformation. Strains transmitted to PILE crystals lead to two distinct slip modes and, at higher strains, to the breakup and alignment of lamellar fragments. In our experiments, crystallites in PTFE orient fuUy with respect to the draw direction at strains between 70 to 200%. With increasing strain, some chains originally in the XL phase are transformed to NXL material. Noncrystalline chains continue to orient until macroscopic failure is reached. This could be a fairly general microstructural response for semicrystalline polymers. 

Figure 29-3 Representation of attractive interactions between the hydrogens in a crystallite of polyethene. This drawing is incomplete in that it does not show the interactions of the depicted chains with the other chains in front and behind.

Figure 5. Replica of platinum shadowed, fractured, (010) hydrogen-bonded surface showing chain folded crystallites aligned with their molecular axis along the draw direction arrowed...

A new transversal correlation length is established at about 37 % strain. By shearing the lamellar blocks break down to a size which later represents the dimensions of the fibrils. According to the relatively low internal mobility the aligned chains are not able to form new crystallites. Their mean dimension in transversal direction is with 8.4 nm clearly below the lamellar thickness. This correlates with the WAXS results, which also indicate the final disappearance of the lamellar transversal order during cold drawing. Ultimately, at high strains, this transversal correlation dominates the whole CDF. 

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