Preparation, characterization, and properties of nanofibers based on poly(vinylidene fluoride) and polyhedral oligomeric silsesquioxane

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<ul><li><p>Preparation, characterization, and properties ofnanobers based on poly(vinylidene uoride)and polyhedral oligomeric silsesquioxaneErika Simona Cozzaa, Orietta Monticellia*, Ornella Cavallerib</p><p>and Enrico Marsanoa</p><p>Nanostructered nanobers based on poly(vinylidene uoride) (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 ber prepared,studied by scanning electron microscopy, atomic force microscopy, and wide angle X-ray diffraction, have beencompared with those of PVDF bers. Morphological characterization has demonstrated the possibility to obtaindefect-free PVDF/POSS nanobers by properly choosing the electrospinning conditions, such as voltage, polymerconcentration, humidity, etc. Conversely, in the case of bers based on the neat polymer, it was not possible toattain the complete elimination of beads in the electrospun nanobers. The different behavior of the two typesof solutions has been ascribed to silsesquioxane molecules, which, without inuencing the solution viscosity orconductivity, favor the formation of uniform structures by decreasing the system surface tension. Concerning POSSdistribution in the bers, the morphological characterization of the electrospun lms has shown a submicrometricdispersion of the silsesquioxane. It is relevant to underline that cast lms, prepared by the same solutions, havebeen found to be characterized by POSS aggregation, thus demonstrating a scarce afnity 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 nanobers characterizedby a silsesquioxane dispersion similar to that present in solution. Finally, the presence of POSS improves the electrospunlm mechanical properties. Copyright 2011 John Wiley &amp; Sons, Ltd.</p><p>Keywords: electrospinning; nanobers; POSS; nanocomposites</p><p>INTRODUCTION</p><p>Nanocomposite bers have attracted much interest because oftheir often enhanced electrical, electronic, optical, and chemicalcharacteristics and wide potential use in applications such assensors, ltration membranes, microelectronics and photonicdevices, structural reinforcements, biomedical applications,defense and security, and energy generation.[18] Among thedifferent approaches that have been reported to producenanobers, electrospinning is the handiest, lowest-cost, andhighest speed method to produce nanocomposite bers.[9,10]</p><p>Concerning poly(vinylidene uoride) (PVDF), the object of thepresent work, electrospun composite bers based on differentllers and nanollers, such as carbon nanotubes,[11] silica,[12]</p><p>and clays,[1315] 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 nanobers are potential candidates as polymerelectrolytes or separators in rechargeable batteries and metalcells.[1619]</p><p>Although there is wide interest in this polymer, no workconcerning the preparation of PVDF nanobers based on poly-hedral oligomeric silsesquioxanes (POSS) has been reported sofar. POSS is characterized by a polyhedral siloxane skeleton</p><p>(general formula [RSiO3/2]n), surrounded by several organicgroups linked to silicon atoms by covalent bonds.[2024] 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, ammability, mechanicalstrength, and oxygen permeability.[2530]</p><p>Recently, we explored the possibility to prepare nanostruc-tured nanobers by adding POSS to electrospinning solutions.[31]</p><p>Indeed, it was demonstrated that the above technique is anefcient method able to disperse, in one step, POSS at ananometric level into a polymer matrix characterized by a lowafnity for silsesquioxane molecules. Moreover, as far as theelectrospinning of PVDF is concerned, we studied the effect ofthe experimental conditions on the electrospun nanobers</p><p>* Correspondence to: Orietta Monticelli, Dipartimento di Chimica e ChimicaIndustriale, Universit di Genova, Via Dodecaneso 31, 16146 Genova, Italy.E-mail: orietta.monticelli@chimica.unige.it</p><p>a E. S. Cozza, O. Monticelli, E. MarsanoDipartimento di Chimica e Chimica Industriale, Universit di Genova, ViaDodecaneso 31, 16146 Genova, Italy</p><p>b O. CavalleriDipartimento di Fisica, Universit di Genova, Via Dodecaneso 33, 16146Genova, Italy</p><p>Polym. Adv. Technol. (2011) Copyright 2011 John Wiley &amp; Sons, Ltd.</p><p>Research Article</p><p>Received: 18 March 2011, Revised: 27 May 2011, Accepted: 7 June 2011, Published online in Wiley Online Library</p><p>(wileyonlinelibrary.com) DOI: 10.1002/pat.2037</p></li><li><p>morphology. In particular, the possibility to attain b nanoberswas assessed, by nely 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 nano-bers without additives, the presence of llers canpotentially improve the characteristics of the electrospun lms,allowing more extensive applications. In this respect, it isrelevant to underline that the dispersion of nanollers stronglyaffects the electrospun lm mechanical, thermal, and surfaceproperties.[10]</p><p>In the present work, we have explored the inuence ofPOSS on PVDF nanober features, in terms of morphology, crys-tallinity, and mechanical properties. Electrospun nanobers,based on PVDF and PVDF/POSS, have been prepared and fullycharacterized.</p><p>EXPERIMENTAL</p><p>Materials</p><p>Poly(vinylidene uoride), Foraon 1000 HD (from Elf Atochem S.A., Puteaux, France) (MW=4.5105), 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 rst dissolved in DMAc and the PVDF/DMAc solution wasstirred for 24h at 70C. 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.</p><p>Preparation of electrospun lms</p><p>Electrospun lms were prepared using a conventional electro-spinning system.</p><p>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 collectoran aluminumsheet wrapped on a glass cylinder (height 4cm, diameter 14.5cm).The distance between the tip and the collector was 20cm.</p><p>A syringe pump (Harvard Apparatus Model 44 ProgrammableSyringe Pump) was used to feed the needle.</p><p>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 polytetrauor-oethylene sheet (thickness 1mm), which was supplied witha XS Instruments digital thermohygrometer (model UR100,accuracy 3% relative humidity, and 0.8C) as humidity andtemperature sensor to monitor and control the ambient para-meters (temperature around 21C). A glass Brooks rotameterwas used to keep constant the airow (Fa) in the enclosedelectrospinning space. The airow was fed in the chamber atatmospheric pressure from an inlet placed behind the collector.The electrospinning parameters were modied in order to assess</p><p>the most suitable conditions. These established conditions are:voltage tension (V) 20kV, solution ow rate (f) 0.0025ml/min,tipcollector distance (d) 20cm, airow rate (Fa) 3.5l/min, relativehumidity 50%, and temperature 21C.</p><p>Preparation of cast lms</p><p>Dense lms 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 80C for 12h.</p><p>Characterization</p><p>The viscosity of the solutions was measured using a Brookelddigital viscometer (model DV-II+Programmable Viscometer) at15C. The solutions conductivity was determined using a con-ductivity meter (Orion Research, model 101) at 20C.</p><p>To study the sample surface morphology, a Leica Stereoscan440 scanning electron microscope was used. All the sampleswere thinly sputter-coated with carbon using a Polaron E5100sputter coater. The ber diameters and their distribution weremeasured using an image analyzer, namely IMAGEJ 1.41 software.</p><p>Atomic force microscopy (AFM) measurements were performedusing 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 300350Hz.</p><p>Wide angle X-ray diffraction (WAXD) was carried out on a PhilipsPW 1830 powder diffractometer (Ni-ltered CuKa radiation).</p><p>Mechanical properties of the electrospun lms were obtainedusing an Instron 5565 at a strain rate of 5mm/min. From eachelectrospun lm, ve rectangular specimens were cut and tested.The tensile stress at yield was calculated at 1% of strain.</p><p>RESULTS AND DISCUSSION</p><p>Electrospun and cast lms 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.</p><p>In order to evaluate the morphology of the material preparedand POSS dispersion in the PVDF matrix, scanning electronmicroscopy (SEM) investigations, coupled to energy-dispersiveX-ray spectroscopic analysis, were carried out.</p><p>In Fig. 1, SEM micrographs, obtained by back scattering emis-sion, of PVDF/POSS cast lm (Fig. 1a), of neat PVDF (Fig. 1b), andof PVDF/POSS (Fig. 1c) electrospun lms as well as the histo-grams of ber diameters are reported. The lms shown in theFigure are those obtained by applying the most suitable electro-spinning conditions, which minimizes the presence of beads onthe bers. Indeed, although the bers 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 brous structure characterized by the presence of numer-ous droplets (Fig. 1b). Moreover, the nanober diameter distribu-tions were calculated by using an image software. Indeed, in thecase of the neat PVDF, different nanober diameters have asimilar frequency of occurrence, thus indicating a relevantheterogeneity, with an average diameter of 157nm. Conversely,the electrospun PVDF/POSS nanobers show a slight increase</p><p>E. S. COZZA ET AL.</p><p>wileyonlinelibrary.com/journal/pat Copyright 2011 John Wiley &amp; Sons, Ltd. Polym. Adv. Technol. (2011)</p></li><li><p>of the average diameter (197nm). Moreover, the variability coef-cient of the PVDF/POSS nanober distribution is 20%, whereasthat of neat PVDF nanober distribution is 25%. In this light,the introduction of POSS in the polymer system not onlyincreases the nanober diameter but also seems to improvetheir dimensional homogeneity. In order to elucidate the specicrole of POSS on the nanober formation, conductivity and vis-cosity measurements have been carried out. As widely reported,conductivity and viscosity are the main factors, which inuencethe transformation of polymer solution into electrospun bers.In this respect, Zhong et al. demonstrated that the addition ofllers, namely salts, to solutions based on poly(D,L-lactic acid)results in bers 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 ber 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</p><p>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 inuence of POSS on this parameter;the viscosity of PVDF electrospinning solution is 480cP, whereasthat of POSS-containing solution is 450cP.</p><p>These ndings suggest that the specic effect of silsesquioxanemolecules on the nanober morphology can be ascribed to otherphenomena. Another solution parameter that may be inuencedby 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 nanober withoutbeads,[39] it is possible to hypothesize that the modication ofthe surface tension in the PVDF/POSS system affects themorphology of the resulting bers.</p><p>As far as POSS distribution is concerned, the surface of the castlm is characterized by the presence of micrometer-sized POSScrystals (Fig. 1a). This demonstrates the poor afnity 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</p><p>0.05.0</p><p>10.015.020.025.030.0</p><p>20 60 100 140 180 220 260 300 340 380 420</p><p>Freq</p><p>uenc</p><p>y [%</p><p>]Nanofibers Diameter [nm]</p><p>0.05.0</p><p>10.015.020.025.030.0</p><p>60 100 140 180 220 260 300 340 38020 420Nanofibers Diameter [nm]</p><p>Freq</p><p>uenc</p><p>y [%</p><p>]</p><p>Figure 1. Scanning electron microscopy micrographs by back scattering emission of (a) PVDF/POSS cast lm, (b) PVDF electrospun nanobers,and (c) PVDF/POSS electrospun nanobers and histograms of the nanobers diameters. Electrospinning conditions as follows: V=20kV, d=20cm,f=0.0025ml/min, RH=50%, Fa=3.5l/min.</p><p>PVDF/POSS Nanobers</p><p>Polym. Adv. Technol. (2011) Copyright 2011 John Wiley &amp; Sons, Ltd. wileyonlinelibrary.com/journal/pat</p></li><li><p>to the formation of micron-sized aggregates. In contrast, analyzingFig. 1c,...</p></li></ul>