PVDF–MWNT interactions control process induced β-lamellar morphology and orientation in the nanocomposites†
The effect of methylene blue (MB) modified multiwall carbon nanotubes (MWNTs) on the nucleation and morphology of polyvinylidene fluoride (PVDF) in comparison with the effect of MWNTs was systematically assessed by DSC, 13C NMR, FT-IR, TEM, WAXS and SAXS analysis. TEM analysis of ultra-microtomed samples revealed that MB modification enhanced the dispersibility of MWNTs in PVDF. Further, the nanocomposites were subjected to mechanical rolling and the synergistic effect of processing and fillers on the PVDF morphology (before and after rolling) at different length scales was studied. Both FT-IR and WAXS analyses suggested that mechanical rolling transforms α-PVDF to β-PVDF (ca. 88%). TEM and two-dimensional WAXS analyses revealed that the MWNTs and β-crystallites are oriented preferentially along the rolling direction and the degree of orientation is not influenced by the fillers suggesting that crystallite orientation is fully controlled by mechanical rolling. On the other hand, β-lamellae showed perpendicular orientation with respect to the rolling direction. Unlike β-crystallites, the β-lamellar morphology and orientation are highly governed by the fillers as evident from SAXS analysis. Using MWNTs and the MWNT–MB π-complex, we demonstrate that the β-lamellar morphology and degree of orientation are controlled by the extent of interaction of fillers with PVDF. Interestingly, both β-lamellar morphology and degree of orientation correlate well with the mechanical properties of the rolled PVDF. More specifically, the dynamic storage modulus of the samples in the rolling direction increases with increasing β-lamellar morphology and degree of orientation. The present work demonstrates that the polymer–filler interaction plays a crucial role in regulating the processed polymer morphology and can be tuned by appropriately modifying the surface of fillers through either covalent or non-covalent interactions.