The crystallization behavior of polyethplene (PE) between oriented isotactic polypropylene (iPP) and poly(tetrafluoroethylene) (PTFE) substrates in sandwich triple layered samples at various crystallization conditions was studied by means of transmission electron microscopy and electron diffraction. It is well known that both iPP and PTFE oriented substrates are active nucleation agents for PE, but result in different crystallization manners of PE. While the iPP substrate leads to an epitaxial crystallization of PE in a way, with the molecular chain directions of both polymers +/-50 degrees apart, an oriented overgrowth of PE on the PTFE substrate with parallel chains of both polymers is identified. By using sandwich samples with the PE in between the two different substrates, i.e. iPP and PTFE, the nucleation efficiency of both polymers for PE can be compared. The results indicate that at high undercoolings, the iPP and PTFE oriented crystals have about the same nucleation ability onto PE, but at lower undercoolings, the PE crystallizes all on the PTFE side. This indicates unambiguously that the nucleation of PE on the PTFE substrate starts at a higher temperature than that on the iPP oriented substrate. (C) 2000 Elsevier Science Ltd. All rights reserved.