Row nucleated structure cylindrites

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

The row nucleated structure contains two types of crystals a small fraction of fibrillar crystals (row nuclei) with partially or even fully extended chains and the normal type folded chain lamellae. The existence of two types of crystals is detectable by calorimetry and the resistance to filmic nitric acid attack, high in the row nuclei and low in the surface layers of lamellae. The number of tie molecules between consecutive 

Schematic diagram illustrating row nucleated crystallisation under (a) low stress and (b) high stress for polyethylene (Keller and Machinl ), 

Since the row nuclei in the crystallising strained melt act as reinforcing frame structure carrying most of the load in the strained liquid, the not yet crystallised melt inside the frame is able to relax almost completely. It hence either crystallises in conventional manner on the surface of the row nuclei yielding the cylindrites or forms new conventional primary nuclei yielding spherulites as shown by electron microscopy of thick nylon 6 fibres as spun. The coexistence of both structural elements in the fibre as spun with a very small fraction of material in row nuclei explains the poor mechanical properties of such a material and the similarity of fibrous structure obtained after drawing with that obtained from purely spherulitic films. 

Under special thermal and strain conditions, the radial growth of lamellae of a cylindrite favours the formation of highly localised tie molecule bridges (Reneker web) between two adjacent lamellae. With sufficiently strong bridges the lamellae are as stapled (Fig. 7). Under tensile stress perpendicular to the lamellae the whole structure may deform like a honeycomb with the free sections of lamellae acting 

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