The supercooling dependence of the initial fold length of polyethylene crystallized from the melt: unification of melt and solution crystallization

Abstract

Following the previous recognition [1], reached with the aid of real time low angle X-ray diffraction (using a synchrotron X-ray source) that in melt crystallized polyethylene the initial (primary) lamellar thickness is much smaller than hitherto envisaged, we have proceeded to construct the full relationship between primary fold length (I _g ^*) and supercooling (ΔT) covering a wide range of crystallization temperature (T _c). The principal result of this work is the identification of supercooling as the sole factor which determinesI _g ^*. Comparison with crystallization from solution [2–4] has revealed that theI _g ^* against ΔT curves are completely superposable thus removing the gap which has existed up to the present between melt and solution crystallization, bringing about a welcome unification of these two separate (at least as far as fold length was concerned) aspects of polymer crystallization. Further, we show that while ΔT determinesI _g ^*, subsequent thickening is determined by the absolute temperature. Isothermal thickening in particular proceeds first by a large discontinuous step followed by a continuous logarithmic increase with time. The importance of these findings and in particular the affirmation of the unique role of supercooling for chain folding and lamellar crystallization in general is emphasized.