Fluorinated polymer/organo-modified magnetic nanoparticle composites

14 March 2018
Manami Iizuka and Atsuhiro Fujimori
Organo-modified magnetic nanoparticles can be evenly dispersed within a fluoropolymer matrix by melt compounding for the development of transparent, heat-resistant, flexible nanocomposite films.

When magnetic nanoparticles (MNPs) are incorporated into polymeric matrices, they can improve the physical properties (e.g., chemical resistance, dimensional stability, mechanical characteristics, and heat resistance) of the resulting nanocomposites, as well as introduce new features (e.g., magnetism and flame retardance). However, MNPs have a remarkable ability to clump together, or ‘aggregate,’ within polymer matrices. This aggregation significantly inhibits the introduction of beneficial characteristics to the nanocomposite and is the main barrier to the use of MNPs as nanofiller material.

Fluorinated polymers are a class of polymer that are both water and oil repellent. Although the functionality of these polymers is exceptionally high, very few techniques have so far been found to improve their characteristics. Introducing MNPs into the fluoropolymer matrix could be one route toward achieving this. However, due to the hydrophobic nature of fluoropolymers, obtaining a uniform distribution of MNPs within the matrix is highly challenging. Hydrogenated materials and inorganic particles are both phase-separated from fluorinated polymers, with no miscibility.1 Additionally, perfluorinated and partially fluorinated crystalline polymers—such as polytetrafluoroethylene (PTFE);2 poly(tetrafluoroethylene-co-[perfluoroalkylvinylether]) (PFA),3 perfluoro(ethylene-co-[propylene]) (FEP),4 and poly(ethylene-co-[tetrafluoroethylene]) (ETFE)5—have high melting points and are insoluble, or sparingly soluble,6 in organic solvents. The formation of nanohybrids by polymer solution casting is therefore almost impossible. Furthermore, it is difficult to employ melt-compounding methods because the organo-modifier (which is required to improve the wettability of the filler toward organic polymers) thermally decomposes below the melting point of the polymer matrix.

In this study, we aimed to overcome these problems by using organo-modified iron(II,III) oxide (Fe3O4) MNPs to obtain a well-dispersed fluoropolymer/MNP nanocomposite. We modified the MNP surfaces with a long-chain fatty acid (i.e., stearic acid),7 which can withstand high temperatures, before incorporating the organo-modified MNPs into poly(vinylidene fluoride-co-tetrafluoroethylene)—P(VDF-TeFE)—by melt compounding. The samples were subsequently subjected to drawing (i.e., manual stretching performed at a temperature just below the melting point of the material). We thus obtained a new heat-resistant, transparent, flexible fluorinated polymer film: see Figure 1. Films with these properties, produced by the formation of a high-density amorphous polymer, are expected to have a wide range of industrial applications (e.g., as artificial muscles, actuators, and plastic magnets).

Schematic illustration of the transparent and flexible crystalline nanohybrids. The organo-modified iron(II,III) oxide (Fe3O4) magnetic nanoparticles (MNPs) are well dispersed within the high-density amorphous fluoropolymer poly(vinylidene fluoride-co-tetrafluoroethylene), or P(VDF-TeFE).

To investigate the properties of our P(VDF-TeFE)/organo-Fe3O4nanocomposite, as well as those of the neat constituents, we obtained wide-angle x-ray scattering (WAXD) profiles: see Figure 2(a). By analyzing the diffraction profiles, we determined that a transition in the crystal system of the matrix polymer did not occur after formation of the nanohybrid. The profile of P(VDF-TeFE) clearly shows a (110), (200) convoluted peak; a (020) reflection; and a (111), (201) convoluted reflection. The crystal system of neat P(VDF-TeFE) is orthorhombic (β-form: a= 9.13Å; b= 5.27Å; and c= 2.51Å).8 Only the β-form of the PVDF copolymer crystals shows ferroelectric properties.9 It would therefore be advantageous to maintain the β-form (i.e., to add ferroelectric properties) in the resultant composite. Indeed, the diffraction profile of the composite suggests that the β-form is maintained in the P(VDF-TeFE) matrix, and hence its ferroelectricity is also maintained. Furthermore, peaks that indicate the formation of layered organo-Fe3O4 structures are not apparent in the nanohybrid. This suggests that the organo-MNP is uniformly dispersed in the matrix by the surface modification and melt compounding.

(a) Wide-angle x-ray scattering (WAXD) profiles of neat P(VDF-TeFE), organo-modified Fe3O4, and the P(VDF-TeFE)/organo-Fe3O3nanocomposite (with 0.2wt% filler content). (b) Atomic force microscope and (c) transmission electron microscope images of the surface of the nanocomposite. (d) Photographs of P(VDF-TeFE) film before and after high-temperature drawing.

In the next part of our study, we investigated the morphology of our composite samples before and after drawing. An atomic force microscopy image of the undrawn composite surface—see Figure 2(b)—shows that the size of the aggregated nanoparticles is about 50–250nm, which is considerably smaller than the wavelength of visible light. Further, Figure 2(c) shows a transmission electron microscope image of the internal morphology of the nanocomposite that reveals that the agglomeration size inside the fluorinated matrix is suppressed to under 100nm. The transparency of the composite material is therefore presumed to be maintained.

One target of our study was to ensure that transparency was maintained in the nanohybrids after they were subjected to high-temperature drawing. We found that neat P(VDF-TeFE) has a high transparency after 5 drawing cycles at 110°C: see Figure 2(d).5 We also found that, in the case of the composite, the uniformly dispersed MNPs do not form aggregates with sizes above the wavelength of visible light during the transparency treatment (i.e., high-temperature drawing) of the matrix polymer. In addition, as illustrated in the photograph in Figure 1, this transparent plastic—fabricated by amorphous densification—is flexible, owing to the properties of the unmodified amorphous regions. Although transparent films tend to suffer damage from bending, we have shown that it is possible to create a flexible film that does not undergo such damage. This is because, in spite of the high density of our nanocomposite, the amorphous part strictly remains.

The photographs in Figure 3(a) show the changes to the transparency before and after the nanohybrids undergo the drawing process. The nanocomposites are comprised of P(VDF-TeFE) and organo-MNPs of different species (i.e., Fe3O4 and cobalt ferrite, CoFe2O4) and sizes (5 and 30nm). Our results show that fluorinated nanocomposites containing MNPs can exhibit transparency after drawing and nanohybridization. However, by using organo-modified Fe3O4 with a 5nm particle size as the nanofiller, we were able to attain a relatively high transparency even by nanocomposite formation alone (i.e., without high-temperature drawing). When the particle diameter was increased to 30nm, the composites exhibited a matrix-derived white color. In contrast, when organo-CoFe2O4 with a 30nm particle diameter was integrated into the polymer, a brownish film was obtained immediately after nanocomposite formation. Flexibility and bending strength, which are macroscopic physical properties based on the high-density amorphous phase, were retained in all cases.

(a) Photographs of the P(VDF-TeFE)/organo-MNP (containing 0.2wt% organo-MNPs), before and after the high-temperature drawing process. (b) Schematic illustration of the changes in lamellae arrangement as a result of high-temperature drawing.

Previous studies8 have shown that when high-temperature drawing is carried out for crystalline fluorinated polymers with switchboard-type lamellae—see Figure 3(b)—the amorphous region is densified, thereby suppressing refraction of the transmitted light at the crystalline/amorphous interface. In our work, we confirmed that high-temperature drawing near the melting point of the polymer leads to the densification of the amorphous region, resulting in transparency. Indeed, when we used organo-Fe3O4 particles with a 5nm diameter as filler, an extremely high transparency was obtained in the resulting nanocomposite. Moreover, we found that the transparency was maintained even when the particle size was increased to 30nm. In the case of organo-CoFe2O4, however, there is a tendency for the film to attain a brown color (although transparency is preserved). The presence of aggregates with sizes below the wavelength of visible light cannot be confirmed by optical microscopy. We are therefore only considering our observations of macroscopic transparency in the material here.

In summary, we have developed transparent, heat-resistant, flexible plastic nanocomposite films by incorporating organo-modified MNPs in P(VDF-TeFE) and employing a high-temperature drawing process. The formation and mechanisms of these films are summarized in Figure 4. We found that partially fluorinated crystalline polymers became transparent and formed high-density amorphous (i.e., flexible) regions after drawing. Additionally, we found that nanohybrid formation with organo-MNPs enables a uniformly dispersed state, which leads to an increased thermal degradation temperature.10 This suggests that hierarchical changes have been induced within the material. In the next stage of our research, we intend to replace the modified chain of the organo-MNP with fluorocarbon chains to improve the particle dispersibility. We thus aim to improve the transparency and versatility of the material.

Schematic illustration showing the formation of the transparent and well-dispersed nanohybrid films fabricated in this study, and the change to the lamellae arrangement (i.e., the introduction of organo-MNPs increases the crystallite size in the ab-plane.)


Manami Iizuka
Graduate School of Science and Engineering, Saitama University

Atsuhiro Fujimori
Graduate School of Science and Engineering, Saitama University


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DOI:  10.2417/spepro.006995