Critical epitaxial layers of different kinds of polyethylene on highly oriented isotactic poly(propylene) substrates

Epitaxial crystallization of high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) in the epitaxial system with highly oriented isotactic poly(propylene) (iPP) has been investigated by electron microscopy. The results obtained by bright field electron microscopy and electron diffraction confirmed that all of the three kinds of PE (HDPE, LLDPE and LDPE) have the ability of epitaxy on iPP surfaces. There exists a critical thickness of the PE layer in which the PE can grow epitaxially on iPP substrates. When the PE/iPP layered films are heat-treated at a temperature above the melting temperature (T_m) of PE and below T_m of iPP for 10 min and then quenched to room temperature, the critical layer thickness of epitaxial HDPE, LLDPE and LDPE is about 250, 120 and 30 nm, respectively. For PE further apart from the interface, the epitaxial orientation becomes disturbed due to simultaneous spherulitic nucleation or lamellar twisting of the epitaxially grown lamellae.