So in these conditions are you sure that the GNPs are not agglomerated in the polymer, thereby leading to a much lower contact area ? If only 10 GNPs are stacked, the surface area will be divided by one order of magnitude, i.e. will be at the level of CNTs. Another possibility is that, because they are so thin, GNPs don't give strong perturbation to the polymer, whereas CNTs are curled and finally induce a much higher disorder.
polymer chains interact with the filler surface and get immobilized on the filler surface, this immobilization facilitate and thus accelerate the further nucleation. immobilized surface polymer is also called interfacial polymer.