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Molecular changes on drawing isotopic blends of polyethylene and ethylene copolymers. Part 2. MCIRS studies

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The technique of mixed crystal FTIR spectroscopy has been applied to uniaxially drawn isotopic blends of linear polyethylene and ethylene copolymers. Following the drawing of melt-quenched samples at 1 mm minK1, clamped samples were cooled with liquid nitrogen. Analysis of the CD2 bending vibration of the ‘guest’ perdeuterated species enabled the arrangement of crystal stems to be characterised. Changes in band components observed were identified with the processes of polymer crystallisation, the untwisting of lamellar ribbons perpendicular to the draw direction and coarse slip. For example, the appearance of new doublets or increased splittings is related to crystallisation, while an increase in singlet band area, at least at higher draw ratios, is indicative of coarse slip. Crystallisation appears to be complete at a draw ratio of 4 for a linear polyethylene sample with low guest molecular weight. For similar copolymer ‘guest’ molecular weights, the processes identified above are delayed to higher levels of deformation. This was attributed to copolymer branches hindering both crystallisation and chain translation through lamellae. Estimates of the crystallite block size present after drawing enable a quantitative description of coarse slip to be obtained. A progressive increase in width of the central singlet for two of these samples is tentatively ascribed to variations in molecular strain.

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