ffects of cooling rates on the epitaxy of high-density polyethylene(HDPE) on isotactic polypropylene(iPP)substrate film have been studied by TEM and electron diffraction. The results of electron microscopy and electron diffraction indicate that epitaxial nucleation and growth of HDPE on oriented iPP substrate take place only on the direct contact plane of HDPE with iPP.There exists a critical epitaxial thickness of HDPE on iPP film.The thickness of epitaxial interface layer is strongly affected by the cooling rate of HDPE after heat treatment. During slow cooling process

three layers of crystalline structure of HDPE are formed. The thickness of critical epitaxial layer is about 100nm. Fast cooling rate results in the formation of two layer crystalline structure. The thickness of critical epitaxial layer is about 250nm.