Evidence for isothermal lamellar thickening at and behind the growth front as polyethylene crystallizes from the melt

The poorly characterized and little understood phenomenon of isothermal lamellar thickening, central to melt crystallization, has been studied morphologically in polyethylene rows, grown around high-melting fibres as linear nuclei revealing that thickening is a function of position within the morphology as well as of elapsed time. In contrast to polyethylene spherulites whose central lamellae are the thickest, in rows the first lamellae to form remain the thinnest because, being close-packed, they have no space into which to thicken. The thickness of lamellae at the growth front increases linearly with the logarithm of elapsed time but, as the thickest lamellae are found at finite radius, thickening must also occur behind the growth front. The data are consistent with a uniform rate of thickening throughout spherulitic polyethylene but melt crystallization must now be envisaged as occurring not at an interface in steady-state condition but at one whose thickness increases asymptotically and where interference will reduce thickening.