Effects of graphene oxide (GO) and GO modified with dodecylamine (org-GO) on glass transition and segmental dynamics of poly(L-lactic acid) (PLLA) were investigated by differential scanning calorimetry (DSC) and two dielectric techniques, broadband dielectric spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC) techniques. Measurements were performed on initially amorphous and on crystallization annealed samples. Isothermal crystallization in the neat matrix and the nanocomposites was followed by DSC and BDS with the same thermal protocol. No change in the glass transition temperature is observed by DSC, whereas the heat capacity step decreases in the nanocomposites. The segmental a relaxation becomes faster and broader in the nanocomposites and its dielectric strength decreases. The results were rationalized in terms of a fraction of interfacial polymer becoming immobilized in the nanocomposites. Changes of this interfacial polymer fraction with composition and functionalization show similar trends in DSC and BDS, absolute values are, however, larger by BDS. (C) 2015 Elsevier B.V. All rights reserved.