Homogeneous crystal nucleation in short polyethylenes

The homogeneous nucleation of the solid from the melt is studied in low molecular weight (MW) polyethylene samples with a narrow MW distribution and average carbon numbers in the range 38≤n≤144. The measurements employ scanning calorimetry on both emulsified and non-emulsified samples, and low- and high-angle X-ray diffraction. The shortest polyethylenes show undercoolings agreeing well with those of monodisperse n-alkanes of similar chain lengths. The longer samples extrapolate smoothly between the undercoolings measured for monodisperse alkanes and high-MW polyethylenes with n≥654. The results support the earlier conclusions that any transition from a full-molecule nucleation to a ‘bundle’ nucleation must occur well below n≈25. The absence of significant features around n=120 where chain folding is expected to set in, implies that it may not occur at the nucleation stage, or may have no significant contribution to the nucleation energetics.