Metallocene catalyst based polyethylene-co-7-octenyldimethylphenylsilane (PE/Si-Ph) and its post-treated functional forms PE/Si-X (X = Cl, F, OCH3, OCH2CH3) were used as additives in PE/ATH composites. The impact strength of the composites was significantly increased after a small addition (0.5-3.0 wt %) of the functionalized form of the copolymer (PE/Si-X). The thermal study of the composites gave us more information about the additive's behavior at the filler/matrix interphase and correlation to the mechanical properties was found. According to this thermal data, the original untreated form of PE/Si-Ph also seemed to interact weakly with the ATH-filler particles, which was seen in an altered interphase at the filler/matrix boundary layer. The interaction was not strong enough to improve the impact strength of composites but an increase was observed in some other mechanical properties (tensile stress, yield strain). (c) 2005 Wiley Periodicals, Inc.