A counter-intuitive asymmetric habit of spherulites in polyethylene blends

An unusual but characteristic asymmetric development of immature spherulites of high-density polyethylene, forming isothermally in blends with the low-density polymer at temperatures when the latter is unable to crystallize, has been studied using scanning electron microscopy. Two interpenetrating sheaves of unequal length grow roughly at right angles within a single object. The longer sheaf with an open texture of individual lamellae forms later, and grows faster, than the other with its more compact texture, which directs kinetic measurements confirm. The phenomenon is explained as a consequence of differing initial concentrations of segregated low-density polyethylene at the growth front. Individual lamellae in a more open structure are expected to grow faster than those in a more-densely packed array because segregated molecules are then able to diffuse away more easily whereas their increasing greater concentration slows the growth rate of the latter monotonically.