Pressure-controlled growth of piezoelectric low-dimensional structures in ternary fullerene C60/carbon nanotube/poly (vinylidene fluoride) based hybrid composites

Abstract

Hybrid composites, consisting of polymers and nanostructured carbon allotropes, can exhibit different properties than their constituent components. In this work, the synergistic action of fullerene C60 and carbon nanotube (CNT) on poly (vinylidene fluoride) (PVDF) was evaluated, in terms of the dispersion of the carbonaceous fillers in the polymer matrix during the composite preparation, and the control of crystalline morphology of the polymer at high pressure. The synergistic effect of the zero-dimensional and one-dimensional carbon materials resulted in a well dispersed ternary C60/CNT/PVDF based composite, which was simply fabricated by a physical and mechanical route. Furthermore, the pressure-controlled growth of piezoelectric low-dimensional crystalline structures of PVDF, including hollow nanowires and extended-chain lamellae, was achieved by the simultaneous introduction of C60 and CNT. Especially, PVDF nanowires with folded- and extended-chain lamellae as their substructures were obtained respectively by controlling the crystallization conditions of the ternary composite at high pressure. Under specific conditions, composite samples, which crystalline structures were totally with extended-chain β- or γ-form lamellae, further self-reinforced with extended-chain β-form nanowires, were successfully crystallized. The present study provides a facile and effective approach for the fabrication and the multi-level control of polar crystalline structures of a polymer based hybrid composite, with an overall good dispersion of zero- and one-dimensional nanostructured carbonaceous materials.

Introduction

Nanostructured carbon allotropes have been intensively investigated in the past two decades, including single- or multi-walled carbon nanotubes (CNT), fullerenes, graphene, and their chemical derivatives [1], [2], [3], [4], [5], [6]. Exhibiting different properties than their constituent components, various hybrid nanostructured carbon materials have also been fabricated, and can enable decoupled engineering of energy conversion and transport functions [3], [4], [5], [6]. Especially, the combinational use of the nanostructured carbonaceous fillers, such as zero-dimensional fullerenes and one-dimensional carbon nanotubes, in the production of hybrid polymer composites to enhance their properties has attracted considerable industrial attention [7].

The combination of polymers with zero- and one-dimensional carbon materials offers an attractive route to combine the merits of organic and inorganic materials into novel hybrid nanocomposites, with synergistic improvement observed in mechanical, electrical and thermal properties [8], [9]. Fang et al. [10] decorated multi-walled carbon nanotubes with fullerene C60 via a three-step chemical functionalization. Compared with pristine CNTs, C60-decorated CNTs further reduced flammability of polypropylene (PP), due to the free-radical-trapping effect of C60 and the barrier effect of the CNT network. Guo et al. [7] reported the design of a grape-cluster-like conductive network in a polypropylene matrix. Oriented multi-walled carbon nanotubes (MWCNTs) served as branches and provided charge transport over large distances, and grape-like carbon black (CB) aggregates enriched around the MWCNTs and linked them through the charge transport over small distances. The experimental results showed that such grape-cluster-like network provided a low percolation threshold for PP/CB/MWCNT composites because of the synergistic effect of CB and oriented MWCNTs. Ellis et al. [8], [9] evaluated the electrical and mechanical properties of single-walled carbon nanotube (SWCNT) reinforced poly (phenylene sulphide) (PPS) composites. The wrapping of SWCNTs in polyetherimide (PEI) and the addition of inorganic fullerene-like tungsten disulfide (IF-WS₂) nanoparticles were found to be effective in dispersing the SWCNTs. Significant enhancements were demonstrated in stiffness, strength and toughness of the hybrid composites by the addition of both nanofillers, and the electrical conductivity of PPS was improve drastically at low SWCNT content.

Furthermore, the synergistic action of the zero- and one-dimensional carbonaceous nanofillers has been utilized in polymer for the conversion and storage of energy [11], [12], [13]. Katz et al. [11] studied electrospun sub-micron fibers containing conjugated polymer (poly (3-hexylthiophene), P3HT) with a fullerene derivative, phenyl-C61-butyric acid methylester (PCBM) or a mixture of PCBM and SWCNTs. The results provided experimental evidence of electron transfer between PCBM and P3HT components in two-component (P3HT/PCBM) and three-component (P3HT/PCBM/SWCNT) fibers, and suggested that the presence of the dispersing block-copolymer did not prevent the efficiency of the electron transfer at the P3HT–PCBM interface in PCBM–P3HT and SWCNT–PCBM–P3HT fibers. These findings suggested a research perspective towards utilization of fibers of functional nanocomposites in fiber-based organic optoelectronic and photovoltaic devices. Hong et al. [12] fabricated composites for a photo-active layer in an organic photovoltaic device using homogeneously dispersed CNTs in a polymer:fullerene bulk-heterojunction matrix. The composites showed considerable improvements of their optical and electrical properties due to the effects of the wideband photo-absorption and high charge carrier mobility of the CNTs. The organic solar cell assembled from these composites showed a remarkable increase of the power conversion efficiency compared to its counterpart using a photo-active layer without CNTs. Pieta et al. [13] devised a (carbon nanotube)-(fullerene–ferrocene dyad polymer) composite, pyr-SWCNTs/(C60Fc-Pd), and then tested as an active material of a symmetrical device for electrical energy storage. The composite was redox conducting at both positive and negative potentials due to the Fc/Fc⁺ and C60⁻/C60 electrode process of the ferrocene and fullerene moiety of the dyad, respectively.

Piezoelectric low-dimensional structures have exciting applications in electronics, optoelectronics, sensors, and the biological sciences [14], [15], [16], [17], [18], [19]. For example, nanogenerators that use aligned nanowires, based on such piezoelectric materials as zinc oxide, for converting nanoscale mechanical energy into electric energy, have been described recently [14], [15], [16]. Particularly, the lightweight and conformable polymeric nanowires with piezoelectricity show certain advantages in organic-based electronic devices, and could eventually lead to the realization of all-organic instruments [20], [21], [22], [23], [24]. Moreover, the extended-chain crystals of piezoelectric polymers, with a combination of three-dimensional crystal ordering and long-chain molecular orientation ordering, are ideal systems for studies of low-dimensional physics, in addition to their potential applications as functional components [25].

Poly (vinylidene fluoride) (PVDF) is one of the limited known piezoelectric class of polymers, and it promises applicability in diverse field of technology due to their high piezoelectric activity and availability as flexible thin films [26], [27], [28], [29]. PVDF exhibits a pronounced polymorphism, i.e. α, β, γ, δ and ε, transforming between several crystal forms under certain conditions [30], [31], [32], [33]. The successful development of piezoelectric polymer devices depends on the effective fabrication of polar crystalline structures, such as β and γ [20], [23], [24], [26], [34], [35].

Patterned arrays of isolated γ-type domains, embedded in the non-polar α structure in thin PVDF films, were already fabricated by Park et al. [20], using micro-imprinting lithography, and a capacitor fabricated with the compressed PVDF thin film showed reasonably high remanent polarization of approximately 6 μC cm⁻², with a coercive voltage of approximately 11 V [20]. Wang et al. [24] showed recently that nanoporous arrays of PVDF, fabricated by a lithography-free, template-assisted preparation method, could be used for robust piezoelectric nanogenerators. The as fabricated porous PVDF nanogenerators produced the rectified power density of 0.17 mW/cm³ with the piezoelectric potential and the piezoelectric current enhanced to be 5.2 times and 6 times those from bulk PVDF film nanogenerators under the same sonic-input.

Controlling the crystal phase and morphology of the polymer at high temperatures was also tried by the researchers, with the introduction of organic or inorganic fillers, especially those with size in nano scale [36], [37], [38]. Cha and Yang [36] investigated the effect of high-temperature spinning and poly (vinyl pyrrolidone) (PVP) additive on PVDF hollow fiber membranes, together with the corresponding microfiltration performances such as water flux, rejection rate, and elongational strength. By PVDF crystallization during high-temperature spinning, porous hollow fiber membranes with particulate morphology were prepared, which were further modified by the addition of miscible PVP with PVDF. The results showed that the rejection rate and strength of the fibers were increased at the expense of reduced water flux and mean pore size. This indicates that high-temperature spinning and PVP addition are very effective to control the morphology of PVDF hollow fiber membranes for microfiltration. Li et al. [37] systematically investigated the cold crystallization temperature effects on the crystal morphologies and the shape memory properties for a PVDF/acrylic copolymer (ACP) blend. It was found that tiny crystals of PVDF formed by annealing served as the physical cross-link points and the amorphous regions among them acted as the reversible phase for the blend materials during the mechanical deformations. So the PVDF/ACP blends with tiny crystals showed not only high shape fixity but also excellent recovery ratios. Maiti et al. [38] demonstrated process and nanoparticle induced piezoelectric super toughened PVDF nanohybrids, which were prepared by incorporating organically modified nanoclay through melt extrusion and solution route. Compared to pure PVDF without any trade-off, the solution processed nanohybrid exhibit 1100% improvement in toughness as well as adequate stiffness. This was attributed to the unique crystallization behavior of PVDF that created an island type of structure on top of the silicate layers (β-phase, planar zigzag chain conformation, and subsequent polar γ-phase and α-phase as layered type). Furthermore, the extent of piezoelectric β-phase has been enhanced by controlled stretching of the nanohybrid at moderately high temperature for better disentanglement, and 90% of the piezoelectric phase has been stabilized, leading to a super toughened lightweight piezoelectric material. However, to the best of our knowledge, no such investigation was performed on ternary C60/CNT/PVDF composites at high pressure.

Achieved by creating the desired crystal morphologies with ideal molecular orientation during the processing, solid phase forming under high pressure is a more effective route to produce polymer products with greatly improved physical and mechanical properties [39]. As for PVDF, pressure treatment has been proved to be effective in enhancing the yielding of its β-or γ-form crystals [40], [41]. Also, nanostructured carbonaceous fillers, such as fullerene C60 and C70, were found to be promising in promoting the formation of new polymeric structures at high pressure [32], [33], [42], [43]. In this work, we evaluated for the first time the synergistic action of C60 and CNT on PVDF, in terms of the dispersion of the carbonaceous fillers in the polymer matrix during the composite preparation, and the control of crystalline morphology of the polymer at high pressure. Pressure-controlled growth of piezoelectric low-dimensional crystalline structures of PVDF, including hollow nanowires and extended-chain lamellae, was achieved by the simultaneous introduction of C60 and CNT. Particularly, composite samples, which crystalline structures were totally with extended-chain β- or γ-form lamellae, further self-reinforced with extended-chain β-form nanowires, were successfully crystallized under specific conditions.