Homogeneous crystal nucleation in short polyethylenes

The homogeneous nucleation of the solid from the melt is studied in. low mo lecular weight (MW) polyethylene samples with a narrow MW distribution and average carbon numbers in the range 38 less than or equal to n less than or equal to 144. The measurements employ scanning calorimetry on both emulsif ied and non-emulsified samples, and low- and high-angle X-ray diffraction. The shortest polyethylenes show undercoolings agreeing well with those of m onodisperse n-alkanes of similar chain lengths. The longer samples extrapol ate smoothly between the undercoolings measured for monodisperse alkanes an d high-MW polyethylenes with n greater than or equal to 654. The results su pport the earlier conclusions that any transition from a full-molecule nucl eation to a 'bundle' nucleation must occur well below n approximate to 25. The absence of significant features around n = 120 where chain folding is e xpected to set in, implies that it may not occur at the nucleation stage, o r may have no significant contribution to the nucleation energetics. (C) 20 01 Elsevier Science Ltd. All rights reserved.