Fast growth rates of polyethylene single crystals grown at high temperatures and their relevance to crystallization theories

The rates of growth of polyethylene single crystals grown from dilute solution in hexadecane and tetradecanol have been measured over the temperature range T_c = 98–120°C by following the change in turbidity during crystallization of a suspension of crystals of known shape and final size. The rates decrease similarly with T_c in each solvent, but for a given supercooling crystals grow much faster in tetradecanol where the corresponding crystallization temperature is higher. Similarly, the rates are much higher in hexadecane than those previously reported from xylene at equivalent supercoolings but lower T_c. Changes in the corresponding crystal morphologies as T_c is raised are quantified in terms of the axial ratio and the degree of curvature of the nominally {100} faces, both of which increase with T_c. The results can be interpreted as showing a transition from regime I to regime II growth in both solvents, which agrees both qualitatively and quantitatively with the predictions of the nucleation-based kinetic theories. Such a transition has never before been reported for solution crystallization. Using this analysis, reasonable values are obtained for the crystal side-surface energy σ of 7.4–7.5 erg cm^?2 and for the regime I substrate length L of 0.14 μm. No correlation is found between crystal morphology and growth rate and there are no discontinuous changes in morphology at the proposed transition points. The occurrence of curved crystal edges raises the fundamental issue of how to reconcile noncrystallographic growth surfaces with nucleation-controlled growth. A new approach to polymer crystal growth based on equilibrium surface roughening, which does not require nucleation, is therefore very pertinent in this respect and this is discussed.