Synergistic effects of graphene nanoplatelets/multi-walled carbon nanotubes hybrids and thermal annealing on crystalline phase transition and properties of electrospun poly(vinylidene fluoride) nanofibers

A B S T R A C T Multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) were embedded into poly(vinylidene fluoride) (PVDF) electrospun fibers with varying mass fractions (0–4 wt%). In addition, the hybrid combinations at 1.5 wt.% GNP/MWCNT/PVDF were fabricated to evaluate potential synergistic effects. The electrospun fiber mats were thermally post-treated at 145°C for 3 and 6 h to assess the influence of annealing on the microstructure, crystalline phase composition, and functional performance. The morphological, structural, mechanical, and piezoelectric properties were examined in relation to both the distinct geometries of the fillers and the thermal annealing. The combination of annealing and nanofiller-induced nucleation significantly improved the crystallinity of the fibers. The tensile strength increased up to 11.1 MPa for 0.5 wt.% MWCNT/PVDF and 15.7 MPa for 1.5 wt.% GNP/PVDF after 6 h of annealing. Structural analysis revealed a pronounced α→β phase transition in the 1.5 wt. % hybrid compositions, reaching a β/α ratio up to 7.8 substantially greater than the transitions observed for mono-filler systems: 4.7 for 1.5 wt.% GNP/PVDF and 3.0 for 1.5 wt.% MWCNT/PVDF. The effect of the high β-phase content was further confirmed at the nanoscale by piezoresponse force microscopy (PFM), which showed a consistent piezoelectric response through the fibers: a coercive voltage of approximately ± 40 V, which decreased to ± 20 V, ascribed to the dispersed nanofillers within the fiber. These results demonstrate that combining hybrid carbon fillers with controlled annealing enables tunable crystalline structures and enhanced electromechanical performance in electrospun PVDF nanofibers.