Influence of oleylamine–functionalized boron nitride nanosheets on the crystalline phases, mechanical and piezoelectric properties of electrospun PVDF nanofibers
Graphical abstract
Introduction
Poly(vinylidene fluoride) (PVDF) have been actively applied in piezoelectric nanogenerators, sensors and actuators because of their excellent flexibility and unique electroactive properties [[1], [2], [3], [4]]. PVDF exhibits three main crystal phases (α, β and γ), among them, α-phase (TGTG′ conformation) is non–electroactive due to antiparallel packing of the molecular dipoles within the unit cell. In contrast, β (TTTT conformation) and γ (TTTGTTTG′ conformation) phases are the electrically active phases, thus, promotion of these phases within PVDF has attracted greatest research interests [5,6]. There have been many works reported in the literature related to the formation of β–phase, however, promotion of γ–phase is often less explored. Interestingly, as compared to β–phase, γ–phase offers higher breakdown strength and discharge energy density, therefore, receiving greater attention as a ferroelectric material [7]. Furthermore, mechanical and thermal properties of PVDF plays a vital role in many applications, for instance, thermally conductive polymer composites offer new opportunities for thermal management applications in electronics and electrical systems [[8], [9], [10]]. Thus, developing PVDF with good mechanical and thermal properties and as well as high electroactive phases may broaden its applications in various fields.
So far, various techniques such as, mechanical stretching [11,12], high electric field poling [13], electrospinning [14,15] and incorporation of nanofillers [16,17] has been developed in order to produce highly electroactive PVDF. Among them, electrospinning is considered as an ideal technique for producing PVDF with high content of electroactive phases due to its simplicity and a single step electrical poling and mechanical stretching process, which directly produces PVDF nanofibers with piezoelectric properties. Moreover, as compared to PVDF thin films, electrospun nanofibers display larger piezoelectric coefficient [18,19], and therefore, receiving greater attention in piezoelectric sensors and energy harvesting applications. Furthermore, the addition of nano–fillers can simultaneously improve the mechanical, thermal properties and as well as promote the formation of desired electroactive phases owing to the interaction between the dipoles of the PVDF chains and the surface of the fillers. Thus, various fillers including carbon–based, (carbon nanotubes and graphene) [[20], [21], [22]], ceramic-based [23,24] and metallic-based [25,26] fillers have been incorporated into PVDF matrix for altering its properties. For instance, Ke et al. investigated the effects of different surface functional groups of multi–walled carbon nanotubes (MWCNTs) on the β–phase formation in PVDF nanocomposites [27]. They fabricated PVDF nanocomposites filled with three different kinds of surface functionalized MWCNTs (hydroxyl functionalized MWCNTs, carboxyl functionalized MWCNTs and amino functionalized MWCNTs) by melt mixing process. The results revealed that the composites filled with amino functionalized MWCNTs have the highest β–phase due to the strongest filler–polymer interaction.
Recently, mono or few–layered two–dimensional (2D) boron nitride (BN) nanosheets (BNNS) have attracted greatest interest in polymer composites, nanoelectromechanical systems, and electronics because of their striking properties such as bandgap (5.5–6.0 eV), thermal conductivity (200–300 Wm−1K−1), thermal stability (monolayer BN can withstand up to 850 ∘C) and Young's modulus (0.865 TPa for monolayer BN) [[28], [29], [30], [31]]. In addition, monolayer BN possess strong in–plane piezoelectric properties [[32], [33], [34]], which make it as ideal reinforcing nanofiller for PVDF. To date, a variety of techniques including chemical vapor deposition, micromechanical cleavage and liquid–phase exfoliation (LPE) have been developed for producing atomically thin 2D materials [35,36]. In comparison, LPE [37,38] is a more suitable method for polymer nanocomposites applications because it offers 2D materials with versatile chemical properties and solubility, which is more beneficial for their effective hybridization with various polymers in solutions. Among various LPE methods [[39], [40], [41], [42], [43], [44]] (solvent–assisted direct exfoliation, surfactant assisted exfoliation, chemical functionalization, ion–intercalation assisted exfoliation), chemical functionalization [[45], [46], [47]] is an effective way for tuning the surface properties of BNNS to specific solvents, improving their solution processability and enhancing their compatibility and interaction with polymers. The choice of organic compounds used to functionalize BNNS play a vital role in determining its overall exfoliation efficiency, scalability and yield of the process. Nevertheless, the fabrication of high-performance PVDF composites with BNNS as nanofillers are still in high demand.
Herein, we report the effect of oleylamine (OLA) functionalized BNNS (OLABN) on the performance of electrospun PVDF nanofibers (PVDFNFs). An efficient and simple LPE method to simultaneously exfoliate and functionalize BNNS by using OLA as functionalization/exfoliation agent and 1, 2–dichlorobenzene (DCB) as solvent have been accomplished. Due to its ability to act as electron donor at elevated temperatures (80 ∘C [48], 300 ∘C [49], 65–110 ∘C [50], 160 ∘C [51]) and the presence of long alkyl chain that can stabilize the OLA functionalized nanoparticles in different organic solvents with long-term colloidal stability, OLA is considered as an ideal candidate for functionalizing BNNS. Additionally, as compared to other long chain alkyl amines such as, hexadecylamine (HDA) and octadecylamine (ODA), OLA possess the double bond (C=C) in the middle of the chemical structure which is another special feature of this chemical. For instance, OLA capped metal nanoparticles exhibit different morphology and crystallinity as compared to ODA based ones [52,53]. Thus, it is exciting to investigate the interaction and effect of OLABN with PVDFNFs, which may offer advanced polymer composites for various applications. Even though there have been many works published on PVDF–BNNS based composites, in this study, we fabricated a novel composites based on OLABN and PVDFNFs and systematically investigated its characteristics. OLABN exhibited good solution processability in organic solvents with long-term colloidal stability. Interestingly, when used as reinforcing nanofiller, OLABN promote the formation of γ-phase in PVDFNFs. Moreover, the effect of OLABN on the mechanical and piezoelectric properties of PVDFNFs were investigated. The results revealed that the functionalized BNNS exhibited excellent compatibility with PVDFNFs matrix; as a result, the composites demonstrated significantly enhanced performances. Furthermore, the piezoelectric and energy harvesting ability of the composites were successfully demonstrated.
Section snippets
Materials
Boron nitride, OLA, PVDF (Mw,000, product number 182702, Sigma Aldrich, Republic of Korea), DCB, acetone and N, N–dimethylformamide (DMF) were purchased from Sigma Aldrich, Republic of Korea. All the chemicals and solvents were used without further purification.
Synthesis of OLABN
OLABN was synthesized from bulk boron nitride (BN) crystals by using OLA as an exfoliating/functionalizing agent and DCB as solvent, similar to LPE method reported for other 2D materials in the literature [54]. Briefly, 200 mg of
Results and discussion
PVDFNFs–OLABN composites were synthesized in two steps, as illustrated in Fig. 1a. In the first step, OLABN was prepared by sonicating bulk BN in the presence of OLA and DCB. In the second step, OLABN incorporated PVDFNFs composites were prepared by solution blending and electrospinning techniques. The possible interactions between BNNS, OLA and PVDFNFs were illustrated in Fig. 1b. In the case of interaction between OLA and BNNS, the amine group of OLA acts as Lewis base for electron deficient
Conclusions
In conclusion, we demonstrate an effective and simple method for the fabrication of PVDFNFs based composites with high γ–phase content, enhanced mechanical and piezoelectric properties by using OLABN as nanofillers. PVDFNFs with 92% γ–crystalline phase was achieved by incorporating 4 wt% OLABN. An obvious enhancement in mechanical properties (343% enhancement in ultimate tensile strength and 823% increment in Young's modulus for composite with 0.1 wt% OLABN) was observed for the composites due
Data statement
The measured and theoretical datasets for this study are available from the corresponding author on request.
CRediT authorship contribution statement
Madeshwaran Sekkarapatti Ramasamy: Conceptualization, Methodology, Data curation, Formal analysis, Writing - original draft, preparation. Ashiqur Rahaman: Software, Data curation, Investigation, Formal analysis, Writing - original draft. Byungki Kim: Conceptualization, Resources, Writing - review & editing, Validation, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Priority Research Program (Grant NRF-2018R1A6A1A03025526) through National Research Foundation (NRF) of Korea under Ministry of Education. The authors thank the Cooperative Equipment Center at KOREATECH for assistance with AFM and SEM-EDS analysis.
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