A Brief Review on Nanocomposites based on PVDF with Nanostructured TiO2 as Filler

Nanocomposites are based on a combination of two or more nanomaterials. Depending on the selection of the constituents forming the composite, the end properties may be tuned as per requirements. Moreover, the combination of organic and inorganic materials can help improve the composite properties a step further, and thus provide exceptional functionalities. Owing to their exceptional mechanical and thermal stability, titania (TiO2) nanostructures are one of the most commonly used reinforcements. Thus, considerable research has focused on the development and characterization of TiO2 reinforced composites based on several different polymer matrices. Amongst the matrix materials used in different nanocomposites, Polyvinylidene Fluoride (PVDF) is the choicest matrix because of its stability, ease of processing and good mechanical properties relative to other thermoplastic polymers. This review focuses on recent progress in the incorporation of TiO2 nanostructures into a PVDF matrix so as to develop nanocomposites having outstanding optical, mechanical and electrical properties. A summary of the diverse potential applications of the PVDF/TiO2 nanocomposites has also been given.


Introduction
Small clusters of atoms from 1 to 100 nanometers long fall under the category of nanoparticles and may be compared to smaller visible living creature -an ant which is millions of nanometers. They are not conformed to absolute quantum chemistry or laws of classical physics and hold strikingly exciting properties in comparison to their bulk counterpart due to increased surface area and quantum confinement effects leading to greater chemical reactivity and enhanced strength [1][2][3][4][5]. Quantum effects become much more important in determining the resultant properties of nanomaterials in terms of unusual electrical, magnetic and optical characteristics [6][7][8]. Nanoparticles play vital role in broad range of fields for instance pharmaceuticals, electronics and environmental remediation [9][10].
Nanoparticles may be identified as nanocomposites, aerosols, nanopowders, nanoceramics, and colloids contingent to their use of interest. These can include carbon based nanomaterials e.g., fullerenes, nanodiamonds, carbon nanotubes, graphene, metal clusters (Pt, Pd, Au, Ag) and metal oxide nanoparticles like TiO 2 , ZnO, Al 2 O 3 and SiO 2 biomaterials or organic materials [11]. Out of these different types of nanomaterials, metal oxide nanoparticles have been extensively explored as they are already being utilized in many commercial processes and products. They are widely employed not Madridge J Nanotechnol Nanosci ISSN: 2638-2075 only as everyday items including paints and varnishes, sunscreens, cosmetics, stain-resistant clothing, sporting goods and electronics but also find applications in the field of nanomedicine as diagnostic, drug delivery and imaging agent [12][13][14][15]. Titanium dioxide nanoparticles (TiO 2 NPs) have received utmost interest among other metal oxide NPs owing to its exciting properties for instance cost effectiveness, stability, commercial handiness, exceptional photocatalytic, UV-cleaning, antibacterial properties and biocompatibility [14].
Nano TiO 2 usually exists in different morphological forms for instance either as spherical NPs or as nanotubes. Siegel [16] classified nanostructured materials in general as Zero dimensional (clusters), one dimensional (nanotubes), two dimensional (films) and three dimensional (polycrystalline) nanostructures as represented in figure 1.

Cluster
Tubular Films Polycrystalline Properties of the nanoparticles can be exploited for their utilization in various exciting future technologies by controlling their morphology. For instance, nanotubes are preferred in applications including need to create electrical bridges in conducting materials or to store/ process information in magnetic devices [17].

Features of TiO 2 -NPs vs Nanotubes
Critical properties of TiO 2 which should be considered for its specific application include geometry, morphology and microstructure. These properties are summarized in Table 1.  [18][19].
TNTs exist as anatase phase at/ below 450°C Where as above this temperature converts to rutile phase. However, TNTs deform at elevated temperatures [20].

Morphology
TNPs are spherical in nature having zero dimension as shown below.  Apart from these, other synthesis methods for TNPs include solvothermal, direct oxidation, microwave, sonochemical and electrodeposition [21][22].
Following methods are employed for the synthesis of TNTs [22]. a) Sol-Gel b) Hydrothermal followed by alkaline treatment of anatase TiO 2 c) Electrochemical anodization of Ti foil.

Efficiency
NPs of TiO 2 are preferred to get maximum output efficiency for certain application e.g., in photocatalysis, NPs are quite efficient due to enhanced surface area [23].
1-D structures are reported to be more efficient than 0-D NPs due to high surface to volume and aspect ratio. For instance, S.
Hernandez et al., has demonstrated that 10 folds enhanced photocatalytic efficiency of TNTs in comparison to TNPs [23].

Nanocomposite of PVDF with TiO2 nanostructures as nanofiller and applications
Nanoscale innovative material design and synthesis holding unusual properties is important in fabrication of advanced devices for optics, electronics and biotechnology. In this perspective, polymers are considered as a usual choice based on their high surface area and low cost. However the polymeric material lack in mechanical and thermal stability. Thus, in polymer science, there is a dire need to widen the application window by enhancing their mechanical, thermal, and electrical properties. Among the polymeric material, Polyvinylidene fluoride (PVDF) is a semi crystalline polymer posses incredible properties, such as excellent resistance to chemicals, good thermal stability, high mechanical strength and inflammable, etc. Figures 2 and 3 PVDF has been extensively employed in various fields, such as nuclear-waste processing, waste water treatment, pulp and paper industry, chemical processing industry and as piezoelectric material [24][25][26]. Reinforcement of polymers with incorporation of whisker, platelets, fibers or nanoparticles may be considered as analternative approach to attain improved properties in nanocomposites [27]. Polymer nanocomposites have shown promising ability to maintain balance among customary properties including ease of fabrication and low weight of polymers with the toughness and strength of reinforcing material [28]. TiO 2 is extensively used as filler material for PVDF matrix owing to its promising role in enhancement of mechanical, thermal, electrical and optical properties of PVDF/TiO 2 nanocomposite for potential applications in variety of fields [29][30].
Nanocomposites of PVDF with inclusion of different contents of TiO 2 nanofiller along with their synthesis protocol and applications have been collected in table 1.
Madridge J Nanotechnol Nanosci ISSN: 2638-2075  [49]. Similarly, an improvement in the tensile strength of PVDF hollow fibre membranes from 1.71 MPa to 3.74 MPa was observed upon incorporation of a combination of 1 wt % Al 2 O 3 and 2 wt % TiO 2 in PVDF matrix. The authors attributed this improvement to the formation of compact macro void in the nanocomposite [50]. There is considerable impact of addition TiO 2 on the thermal and electrical properties of PVDF/ HfP as reported by K. Prabakaran et al [51]. They reported about two fold enhancement in the dielectric constant of nanocomposite upon inclusion of surface modified TiO 2 NPs. Moreover, significant changes in optical properties of PVDF could be observed by incorporation of TiO 2 .
NPs e.g., Zhang et al and A. Munoz-Bonilla et al have reported the transmittance spectra of PVDF/ ZrO 2 -TiO 2 and PVDF/TiO 2 nanocomposite [52][53]. They showed an increase in the absorption and refractive index of the nanocomposite in the presence of TiO 2 NPs. In fact, enhancement in different properties of PVDF/TiO 2 nanocomposite is attributed to the interaction between the filler material and polymer matrix. It has been reported that the fluorine atom of the PVDF polymer can interact strongly with Ti 4+ ions via Vander Waal forces of interaction to induce mechanical stability to nanocomposites for various exciting applications as shown pictorially in figure 4 [54].

Environmental application
Extensive reports are available in literature for the utilization of PVDF/ TiO 2 nanocomposite as water treatment membranes, removal of heavy metals and photocatalytic degradation of dyes in textile industry [55]. TiO 2 nanofiller attributes to the enhancement of hydrophilic and antifouling properties of PVDF due to the presence of OH surface groups [56][57][58][59][60][61][62][63] PVDF membrane and 3% TiO 2 / PVDF membrane. An excellent anti-fouling performance exhibited by TiO 2 -HNTs/ PVDF membrane was attributed to hydrophilic nanoparticles which resisted the hydrophobic contaminants in waste water [64]. Safarpour M et al. [43] reported rGO/ TiO 2 blended PVDF membrane prepared by the phase inversion method. The hydrophilicity and permeability of the blended membranes were enhanced due to the addition of the rGO/ TiO 2 nanocomposite containing various oxygenated hydrophilic groups. Also, the rGO/ TiO 2 blended PVDF membranes had higher flux recovery ratios compared with the bare PVDF membrane. On the basis of the achieved hydrophilicity, pure water flux, and antifouling results, the best content of rGO/ TiO 2 nanocomposite was 0.05 wt % in the casting solution. Hence, rGO/ TiO 2 nanocomposite is an excellent antifouling additive having promising applications in the membrane field [43]. In another study it is demonstrated that PVDF/TiO 2 nanocomposite membranes displayed outstanding permeability performance tests even higher than commercially available PVDF membrane due to the extraordinary pore connectivity between the filler material and matrix [60]. Incorporation of TiO 2 also enhanced the protein resistance of membranes significantly due to hydrophilic nature of the NPs [60]. Nanocomposite membranes based on PVDF/ TiO 2 have been reported by different researchers for the degradation of dyes owing to photocatalytic effect of TiO 2 NPs. N.A.M. Nor et al., recently reported nanocomposite membrane of PVDF/TiO 2 for decomposition of organic pollutant from waste water [45]. H.P. Ngang et al developed PVDF-TiO 2 mixed-matrix ultrafiltration membrane for the degradation of methylene blue (MB). Although, the performance of the nancomposite membrane was little affected at higher concentration of MB yet it exceeded the neat PVDF membrane due to the extra adsorption sites provided by TiO 2 NPs for MB dye [37].

Energy
Much attention is paid to develop solid state lithium ions battery in order to overcome the issues like leakage of solution electrolyte, longer life cycle, high energy state density, electrochemical stability and mechanical strength [63]. Highly electronegative fluorine in the backbone of PVDF and its high dielectric constant make it a perfect choice as polymer matrix to be used as solid polymer electrolyte (SPE).Y-J. Wang et al reported SPE based on PVDF/ LiClO 4 / TiO 2 with enhanced conductivity upon inclusion of up to 10 wt% addition of TiO 2 nanoparticles [34].
Another group reported lithium bis (oxalato) borate (LiBOB) based PVDF/PVC composite polymer electrolyte (CPE) membrane with 2.5 wt% TiO 2 exhibiting comparatively high ambient-temperature conductivity than ZrO 2 with even porous network to accommodate liquid electrolyte. Mobility of Li + ions could be facilitated due to the amorphicity of the highly conductive CPE having potential to be useful as lithium-ion electrolyte batteries [64]. Similarly, electrolyte membrane based on PEO/PVDF/TiO 2 was fabricated by H.

Electronics
Development of high energy density polymer-based dielectric materials has become a challenging topic of research interest [63]. Polymer matrices containing homogeneous Dispersion of metal oxide nanoparticles are of particular attention because of their potentially high breakdown strength and high dielectric permittivity. Such combinations may be used as an alternative approach to develop dielectric materials for energy storage and sensing applications. K. Prabakaran et al., reported PVDF/ HFP/ TiO 2 nanocomposite with two fold enhancement in dielectric constant upon incorporation of TiO 2 NPs [51]. In another study conducted by J. Iwagoshi et al., it is demonstrated that incorporation of rutile TiO 2 NPs into PVDF host shown promising results for their use as capacitor due to enhancement in electrical properties of the nanocomposite [66]. W. L. Ong et al focused on the development of a hybrid organic-inorganic TiO 2 nanocomposite, which were used as the volatile organic compound (VOC) sensing and photocatalytic degradation with production of hydrogen gas. These properties were enhanced by the well-structured TiO 2 nanotubes, metal nanoparticles and reduced graphene oxide loading for enhanced light absorption and charge-transfer kinetics. Crosslinking networks were induced due to functionalization of TiO 2 nanocomposite with a polyvinylidene fluoride (PVDF) matrix which produced the mechanical reinforcement-flexibility [67]. Anjum Qureshi et al investigated ac/dc electrical properties of (lithium and titanium doped nickel oxide) LTNO/ PVDF nanocomposites film as a function of temperature and as NH 3 gas sensor. It was observed that electrical conductivity obeys the power law. Spin coating method was used to fabricate LTNO/ PVDF nanocomposite which was used for the sensing of ammonia gas [68].
Haixiong Tanget at fabricated barium titanate (BaTiO 3 ) nanowires (NWs) in a poly (vinylidene fluoride-trifluoro ethylene-chloro fluoro ethylene) (PVDF-TrFE-CFE)) matrix. High breakdown strength and high dielectric permittivity were reported for these nanocomposites [70]. Lyly Nyl Ismail et al reported the dielectric properties of multilayer PVDF-TrFE/ PMMA:TiO 2 thin film. Spin coating method was used to fabricate these films .Due to the high dielectric properties, of these films are found to have wide applications in electronics industry [71].

Conclusion
In this mini review article we have discussed the morphology of TiO 2 nanostructures. It is shown that 1-D TiO 2 nanostructures have high aspect ratio having quite high efficiency in certain applications as for instance photocatalytic activity of TNTs is found to be 10 folds that of TNPs [23]. Apart from this, nanocomposites based on PVDF and TiO 2 demonstrate versatile applications especially in the field of environmental remediation, dielectric materials and solid state polymer electrolytes. There is still room to further explore the effect of TiO 2 doped with different metals, metal oxides and carbon nanomaterials to be used as filler materials for PVDF matrix. Also, little information is available in literature on the nanocomposites of TNTs with PVDF. TNTs can be more effective than TiO 2 nanoparticles due to the ordered structure and hence can open gate for exciting application.