Modification of microfibrillar composites (MFCs) by nanofillers shows important effects of its localization on performance. This work deals with control of migration/localization of amine-modified graphite nanoplatelets (GNPAs) in the HDPE/PA6 system through their in situ coupling. GNPA/HDPE linking also leads to complex morphology of PA6 fibers. Although such control of GNPA localization and migration in MFC is limited, marked variations of properties, including decrease with high draw ratio, occurs. This is in spite of the fact that structure and crystallinity are comparable with that of the polymer/GNPA adduct. The reason is a more pronounced negative effect of presence of the GNP adducts in the fiber surface layer on HDPE crystallinity, in comparison with neat GNP, that is, formation of "soft" interface. The negative effects were markedly eliminated by pre-blending and coupling of GNPA in both polymer phases. Results confirm more effective control of MFC performance by in situ coupling of GNPA with polymer components in comparison with a nonreactive system. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47660.