Insight into growth details and characteristics of windmill-like polyethylene crystals

The crystallization detail of polyethylene (PE) has been scarcely studied via in-situ approach since it is an extremely fast process. In this work, optical microscopy is used to investigate crystallization details and characteristics of windmill-like polyethylene crystals. It has been shown that the straight edges of the petals appear firstly and grow in pairs from their central junctions, which subsequently induce the surrounding domains in between each pairs of petals to nucleate and crystallize into twisted lamellar overgrowths. The remaining terrace-stacked lamellae which form curved edges of the petals start to develop only after the straight edges of the petals together with the twisted lamellar overgrowths have completed their growth. It is confirmed that the preferential growth direction of these petals are along crystallographic [113] axis, which has an angle of 65° with the typical direction along b-axis adopted also by the twisted lamellar overgrowths. The crystallization kinetics is analyzed in terms of Avrami equation and an Avrami exponent n = 2 is thus obtained, which is consistent with the morphological observations of heterogeneous nucleation and 2-dimensional growth for these PE crystals. The melting behaviors of these crystals are exactly the reversed process of crystallization.