Preparation, characterization, and properties ofnanofibers based on poly(vinylidene fluoride)and polyhedral oligomeric silsesquioxaneErika Simona Cozzaa, Orietta Monticellia*, Ornella Cavallerib

and Enrico Marsanoa

Nanostructered nanofibers based on poly(vinylidene fluoride) (PVDF) and polyhedral oligomeric silsesquioxane(POSS) have been prepared by electrospinning process. The starting solutions were prepared by dissolving boththe system components in the mixture N,N-dimethylacetamide/acetone. The characteristics of the fiber prepared,studied by scanning electron microscopy, atomic force microscopy, and wide angle X-ray diffraction, have beencompared with those of PVDF fibers. Morphological characterization has demonstrated the possibility to obtaindefect-free PVDF/POSS nanofibers by properly choosing the electrospinning conditions, such as voltage, polymerconcentration, humidity, etc. Conversely, in the case of fibers based on the neat polymer, it was not possible toattain the complete elimination of beads in the electrospun nanofibers. The different behavior of the two typesof solutions has been ascribed to silsesquioxane molecules, which, without influencing the solution viscosity orconductivity, favor the formation of uniform structures by decreasing the system surface tension. Concerning POSSdistribution in the fibers, the morphological characterization of the electrospun films has shown a submicrometricdispersion of the silsesquioxane. It is relevant to underline that cast films, prepared by the same solutions, havebeen found to be characterized by POSS aggregation, thus demonstrating a scarce affinity between the two-systemcomponents. Indeed, the peculiar solvent evaporation of the electrospun solution, which is much faster than thatoccurring during the cast process, prevents POSS aggregation, thus leading to the formation of nanofibers characterizedby a silsesquioxane dispersion similar to that present in solution. Finally, the presence of POSS improves the electrospunfilm mechanical properties. Copyright © 2011 John Wiley & Sons, Ltd.

Keywords: electrospinning; nanofibers; POSS; nanocomposites

INTRODUCTION

Nanocomposite fibers have attracted much interest because oftheir often enhanced electrical, electronic, optical, and chemicalcharacteristics and wide potential use in applications such assensors, filtration membranes, microelectronics and photonicdevices, structural reinforcements, biomedical applications,defense and security, and energy generation.[1–8] Among thedifferent approaches that have been reported to producenanofibers, electrospinning is the handiest, lowest-cost, andhighest speed method to produce nanocomposite fibers.[9,10]

Concerning poly(vinylidene fluoride) (PVDF), the object of thepresent work, electrospun composite fibers based on differentfillers and nanofillers, such as carbon nanotubes,[11] silica,[12]

and clays,[13–15] have been developed. In general, PVDF has beenwidely investigated because of its peculiar electroactive proper-ties, namely piezo-, pyro-, and ferro-electric activities, as well asexcellent mechanical properties, high chemical resistance, goodthermal stability, and processability. Because of the above char-acteristics, PVDF nanofibers are potential candidates as polymerelectrolytes or separators in rechargeable batteries and metalcells.[16–19]

Although there is wide interest in this polymer, no workconcerning the preparation of PVDF nanofibers based on poly-hedral oligomeric silsesquioxanes (POSS) has been reported sofar. POSS is characterized by a polyhedral siloxane skeleton

(general formula [RSiO3/2]n), surrounded by several organicgroups linked to silicon atoms by covalent bonds.[20–24] The in-corporation of POSS into polymeric materials, which can be per-formed via in situ polymerization, grafting, blending, and soforth, leads in some case to relevant improvement in polymerproperties, including thermal behavior, flammability, mechanicalstrength, and oxygen permeability.[25–30]

Recently, we explored the possibility to prepare nanostruc-tured nanofibers by adding POSS to electrospinning solutions.[31]

Indeed, it was demonstrated that the above technique is anefficient method able to disperse, in one step, POSS at ananometric level into a polymer matrix characterized by a lowaffinity for silsesquioxane molecules. Moreover, as far as theelectrospinning of PVDF is concerned, we studied the effect ofthe experimental conditions on the electrospun nanofibers

* Correspondence to: Orietta Monticelli, Dipartimento di Chimica e ChimicaIndustriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy.E-mail: orietta.monticelli@chimica.unige.it

a E. S. Cozza, O. Monticelli, E. MarsanoDipartimento di Chimica e Chimica Industriale, Università di Genova, ViaDodecaneso 31, 16146 Genova, Italy

b O. CavalleriDipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146Genova, Italy

Polym. Adv. Technol. (2011) Copyright © 2011 John Wiley & Sons, Ltd.

Research Article

Received: 18 March 2011, Revised: 27 May 2011, Accepted: 7 June 2011, Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI: 10.1002/pat.2037

morphology. In particular, the possibility to attain b nanofiberswas assessed, by finely tuning the features of the polymer. Al-though it was demonstrated that the above crystal phase, whichis responsible of the enhanced piezo-electric and ferro-electricPVDF activities, can be obtained in the electrospinning nanofi-bers without additives, the presence of fillers canpotentially improve the characteristics of the electrospun films,allowing more extensive applications. In this respect, it isrelevant to underline that the dispersion of nanofillers stronglyaffects the electrospun film mechanical, thermal, and surfaceproperties.[10]

In the present work, we have explored the influence ofPOSS on PVDF nanofiber features, in terms of morphology, crys-tallinity, and mechanical properties. Electrospun nanofibers,based on PVDF and PVDF/POSS, have been prepared and fullycharacterized.

EXPERIMENTAL

Materials

Poly(vinylidene fluoride), Foraflon 1000 HD (from Elf Atochem S.A., Puteaux, France) (MW=4.5�105), in powder form, was used asreceived. Generally, the solutions were prepared by dissolving15wt% of PVDF in the solvent mixture N,N-dimethylacetamide(DMAc, from Aldrich) and acetone (from Aldrich) with a weightratio of 70:30. In order to accelerate PVDF solubilization, the poly-mer was first dissolved in DMAc and the PVDF/DMAc solution wasstirred for 24h at 70�C. Successively, acetone was added and thesolution was stirred again for 24h at room temperature. POSS-based solutions were prepared by adding epoxycyclohexylisobutylPOSS (from Hybrid Plastics Co., Hattiesburg, MS, USA), 3wt% withrespect to PVDF, in the solvent mixture containing PVDF. The mix-tures were stirred for 12h at room temperature.

Preparation of electrospun films

Electrospun films were prepared using a conventional electro-spinning system.

The solutions were loaded in a syringe (model Z314544, diam-eter d=11.6mm, Aldrich Fortuna Optima) placed in the horizontaldirection. A gamma high-voltage research power supply (ModelES30P-5W) was used to charge the solution in the syringe with apositive DC voltage. The positive electrode was connected to theneedle (diameter d=0.45mm) of the syringe, and the negativeelectrode was attached to the grounded collector—an aluminumsheet wrapped on a glass cylinder (height 4cm, diameter 14.5cm).The distance between the tip and the collector was 20cm.

A syringe pump (Harvard Apparatus Model 44 ProgrammableSyringe Pump) was used to feed the needle.

The needle tip and the ground electrode were contained in ahollow plastic cylinder (height 30.5cm, inner diameter 24cm,and thickness 3.5mm), internally coated with a polytetrafluor-oethylene sheet (thickness 1mm), which was supplied witha XS Instruments digital thermohygrometer (model UR100,accuracy �3% relative humidity, and �0.8�C) as humidity andtemperature sensor to monitor and control the ambient para-meters (temperature around 21�C). A glass Brooks rotameterwas used to keep constant the airflow (Fa) in the enclosedelectrospinning space. The airflow was fed in the chamber atatmospheric pressure from an inlet placed behind the collector.The electrospinning parameters were modified in order to assess

the most suitable conditions. These established conditions are:voltage tension (V) 20kV, solution flow rate (f) 0.0025ml/min,tip–collector distance (d) 20cm, airflow rate (Fa) 3.5l/min, relativehumidity 50%, and temperature 21�C.

Preparation of cast films

Dense films with a thickness of ca 200mmwere prepared by cast-ing the solutions, the same as used above for electrospinning, ona glass slide by a brass knife and evaporating the solvent mixturein vacuum at 80�C for 12h.

Characterization

The viscosity of the solutions was measured using a Brookfielddigital viscometer (model DV-II+Programmable Viscometer) at15�C. The solutions conductivity was determined using a con-ductivity meter (Orion Research, model 101) at 20�C.To study the sample surface morphology, a Leica Stereoscan

440 scanning electron microscope was used. All the sampleswere thinly sputter-coated with carbon using a Polaron E5100sputter coater. The fiber diameters and their distribution weremeasured using an image analyzer, namely IMAGEJ 1.41 software.Atomic force microscopy (AFM) measurements were performed

using a Dimension 3100 (Veeco Instruments Inc., Plainview,New York, USA). The images were acquired in tapping mode inair using commercial Si cantilevers (Olympus) with resonancefrequencies in the range of 300–350Hz.Wide angle X-ray diffraction (WAXD) was carried out on a Philips

PW 1830 powder diffractometer (Ni-filtered CuKa radiation).Mechanical properties of the electrospun films were obtained

using an Instron 5565 at a strain rate of 5mm/min. From eachelectrospun film, five rectangular specimens were cut and tested.The tensile stress at yield was calculated at 1% of strain.

RESULTS AND DISCUSSION

Electrospun and cast films based on PVDF and POSS have beenprepared starting from DMAc/acetone solutions. The choice ofthe aforementioned solvent was mainly because of the goodsolubility of the chosen silsesquioxane.In order to evaluate the morphology of the material prepared

and POSS dispersion in the PVDF matrix, scanning electronmicroscopy (SEM) investigations, coupled to energy-dispersiveX-ray spectroscopic analysis, were carried out.In Fig. 1, SEM micrographs, obtained by back scattering emis-

sion, of PVDF/POSS cast film (Fig. 1a), of neat PVDF (Fig. 1b), andof PVDF/POSS (Fig. 1c) electrospun films as well as the histo-grams of fiber diameters are reported. The films shown in theFigure are those obtained by applying the most suitable electro-spinning conditions, which minimizes the presence of beads onthe fibers. Indeed, although the fibers based on PVDF/POSS turnout to be uniform and defect-free (Fig. 1c), those prepared start-ing from the neat PVDF solution and applying the same experi-mental conditions show a typical bead-on-string morphologywith a fibrous structure characterized by the presence of numer-ous droplets (Fig. 1b). Moreover, the nanofiber diameter distribu-tions were calculated by using an image software. Indeed, in thecase of the neat PVDF, different nanofiber diameters have asimilar frequency of occurrence, thus indicating a relevantheterogeneity, with an average diameter of 157nm. Conversely,the electrospun PVDF/POSS nanofibers show a slight increase

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of the average diameter (197nm). Moreover, the variability coef-ficient of the PVDF/POSS nanofiber distribution is 20%, whereasthat of neat PVDF nanofiber distribution is 25%. In this light,the introduction of POSS in the polymer system not onlyincreases the nanofiber diameter but also seems to improvetheir dimensional homogeneity. In order to elucidate the specificrole of POSS on the nanofiber formation, conductivity and vis-cosity measurements have been carried out. As widely reported,conductivity and viscosity are the main factors, which influencethe transformation of polymer solution into electrospun fibers.In this respect, Zhong et al. demonstrated that the addition offillers, namely salts, to solutions based on poly(D,L-lactic acid)results in fibers free of beads.[32] They argued that the additionof salts resulted in a higher charge density on the surface ofthe solution jet during the electrospinning. The addition of saltin order to modify the fiber morphology was used also for otherpolymers, such as polyvinyl alcohol,[33] polyacrylic acid,[34] andpolyamide 6.[35] More recently, Wang et al. found that the disper-sion of clays in nanocomposites, of methyl methacrylate, andmethacrylic acid improved the electrospinnability of the nano-composite dispersions, through increasing the apparent ex-tensional viscosity and conductivity.[36,37] In our system, thepresence of POSS in the electrospinning solutions does not

affect the conductivity; the conductivity of PVDF and PVDF/POSSsolutions are 0.310 and 0.330 mS, respectively. Also, the viscositymeasurements of the above two electrospinning solutionsdemonstrate the small influence of POSS on this parameter;the viscosity of PVDF electrospinning solution is 480cP, whereasthat of POSS-containing solution is 450cP.

These findings suggest that the specific effect of silsesquioxanemolecules on the nanofiber morphology can be ascribed to otherphenomena. Another solution parameter that may be influencedby silsesquioxane molecules is the surface tension that wasreported to play a critical role in the electrospinning process.Indeed, silsesquioxane molecules diminish the surface tension ofliquids,[38] and taking into account that generally, the decreaseof this parameter helps the formation of nanofiber withoutbeads,[39] it is possible to hypothesize that the modification ofthe surface tension in the PVDF/POSS system affects themorphology of the resulting fibers.

As far as POSS distribution is concerned, the surface of the castfilm is characterized by the presence of micrometer-sized POSScrystals (Fig. 1a). This demonstrates the poor affinity betweenthe two components, which leads, at a particular POSS concen-tration in the solution, namely during solvent evaporation, to aphase separation of silsesquioxane molecules and consequently

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Figure 1. Scanning electron microscopy micrographs by back scattering emission of (a) PVDF/POSS cast film, (b) PVDF electrospun nanofibers,and (c) PVDF/POSS electrospun nanofibers and histograms of the nanofibers diameters. Electrospinning conditions as follows: V=20kV, d=20cm,f=0.0025ml/min, RH=50%, Fa=3.5l/min.

PVDF/POSS Nanofibers

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to the formation of micron-sized aggregates. In contrast, analyzingFig. 1c, i.e. the micrographs of electrospun nanofibers based onPOSS, no POSS aggregates have been observed, whereas theelemental analysis has shown a uniform Si distribution withoutany visible aggregates. These results evidence a submicrometerPOSS dispersion as well as demonstrate that no loss of POSSoccurs during the solvent evaporation process.

Moreover, the different morphology of the PVDF cast andelectrospun films demonstrate that, as reported in the case ofcellulose acetate-based membranes,[31] electrospinning is a tech-nique capable of facilitating silsesquioxane dispersion in thepolymer matrix. Indeed, as previously reported, it is possible thatthe peculiar solvent evaporation of the electrospinning process[40]

prevents POSS molecule aggregation, thus leading to the forma-tion of nanofibers characterized by a silsesquioxane dispersionsimilar to that present in solution. In other words, the extremelyfast solvent evaporation, when the jet crosses the electrical fieldpath, prevents POSS aggregation in the polymer matrix.

In order to fully assess the topography of the fibers surface,AFM measurements were also performed on both PVDF andPVDF/POSS electrospun membranes, as reported in Fig. 2.

The Figure shows a zone of the PVDF nanofibers withoutdefects. The three-dimensional fibrous web consists of fiberswith an average diameter of ca 150nm in agreement with SEMmeasurements.

Comparing the images shown in Fig. 2, the neat PVDF nanofi-bers appear to be relatively smooth (Fig. 2a), whereas those con-taining POSS show a nodular (hills and valley) morphology(Fig. 2b). The roughness of the PVDF/POSS nanofiber surfacemay be ascribed to silsesquioxane nanoaggregates, which arenot detected by SEM measurements.

In order to evaluate the influence of POSS addition on thecrystal structure of PVDF, WAXD measurements were carriedout. Figure 3 shows WAXD patterns of POSS (Fig. 3a), of the neatPVDF (Fig. 3b), and of the PVDF/POSS electrospun films (Fig. 3c).

The neat PVDF electrospun film exhibits a very sharp peak atca 20� 2θ, thus evidencing the dominance of the b phase.Indeed, as reported, the electrospinning promotes the formationof this conformation.[41,42] Moreover, electrospun PVDF/POSSfilm shows a peak at 8� 2θ, typical of the POSS crystal structure(Fig. 3a), which underlines some degree of short-range aggrega-tion of POSS molecules. This finding seems to be in agreementwith AFM morphological observations. On the contrary, unlikethe morphology, the polymorphic behavior of the electrospun

PVDF films does not vary with adding POSS molecules to thestarting solution.It was reported that fillers and nanofillers added to PVDF electro-

spinning solutions promote the formation the of b phase. In par-ticular, very recently, Cebe et al. demonstrated that the additionof an organic-modified nanoclay to the solution totally elimi-nated the non-polar a crystal conformers in the composite elec-trospun nanofibers, thanks to ion–dipole interactions betweenthe PVDF chains and the nanoclay platelets.[14] The effect ofNi-Zn ferrite nanoparticles was studied by Andrew et al., whodemonstrated that the above nanoparticles were able to en-hance b and g phases in the composite electrospun fibers.[43]

In the case of our POSS-based system, the lack of influenceof the nanofiller on the PVDF crystallinity may be ascribed tothe peculiar polymer used, which allows to obtain almost pureb nanofibers.The mechanical properties of the electrospun films have been

evaluated, paying attention to the Young's modulus and thetensile stress at yield (Table 1). In Fig. 4, the stress–strain curvesobtained for PVDF and PVDF/POSS electrospun films are shown.In the case of the nanofibers prepared from DMAc/acetone

solutions, the tensile tests are not reproducible, probably becauseof the presence of morphological defects. In this respect, recently,Haung et al. reported that the beads on the fiber surface reduce

Figure 2. Atomic force microscopic height images of (a) PVDF electrospun nanofibers and (b) PVDF/POSS electrospun nanofibers. This figure is avail-able in colour online at wileyonlinelibrary.com/journal/pat

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the cohesive force among the fibers, thus resulting in poorermechanical properties of the electrospun films.[44]

Therefore, in order to evaluate the influence of POSS on themechanical properties of the electrospun PVDF membranes, itwas necessary to compare the tensile properties of PVDF/POSSelectrospun films with those of PVDF electrospun membraneswith a defect-free morphology. Thus, mechanical experimentswere carried out on PVDF nanofibrous mats, prepared startingfrom N,N-dimethylformamide/acetone solutions, studied in pre-vious work, which were characterized by a homogenous and de-fect-free structure. Moreover, it is worth underlining that thedimensions of the above nanofibers are similar to those of thePVDF/POSS nanofibers. Considering the data reported in Table 1,, POSS molecules improve the structural features of the electro-spun membrane, increasing the modulus from ca 35MPa in neatelectrospun PVDF to ca70MPa in the case of PVDF/POSS electro-spun membranes. Similarly, the addition of POSS enhances theelectrospun membrane tensile stress at yield, increasing this pa-rameter almost three times in the PVDF/POSS films.

CONCLUSIONS

It has been demonstrated that by electrospinning solutions con-taining both POSS and PVDF, it is possible to attain nanostruc-tered nanofibers. Indeed, compared with the classical casting ap-proach, this technique allows the improvement of silsesquioxanedistribution in the electrospun films. The peculiar solvent evapo-ration of the electrospun solution, which is much faster thanthat occurring during the cast process, prevents POSS molecule

aggregation, thus leading to the formation of nanofibers charac-terized by a silsesquioxane dispersion similar to that present insolution.

Moreover, the presence of POSS in solution ameliorates theelectrospinnability. This phenomenon has been ascribed to thesilsesquioxane molecules, which, without influencing the solu-tion viscosity or conductivity, favor the formation of uniformstructures by decreasing the system surface tension.

Moreover, the presence of POSS improves the electrospun filmmechanical properties.

Acknowledgement

The present study was financed by MIUR under the scheme ofPRIN 2008 projects.

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Table 1. Mechanical properties of poly(vinylidene fluoride)and poly(vinylidene fluoride)/polyhedral oligomeric silses-quioxane films

Type of film Modulus(MPa)

Tensile stressat yield (MPa)

Poly(vinylidene fluoride) 35�8 1.3�0.2Poly(vinylidene fluoride)/polyhedraloligomeric silsesquioxane

70�9 3.0�0.5

Figure 4. Tensile stress–strain curve: (a) PVDF electrospun film and(b) PVDF/POSS electrospun film.

PVDF/POSS Nanofibers

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