**8. Composites and characterization**

**7. Electro-spinning of polymers**

152 Advances in Nanofibers

solution) is of some importance.

above.

**Figure 4.** Details of a simple "home brew" electro-spinning apparatus.

Electro-spinning or electrostatic deposition is one of the easiest and most inexpensive methods of generating nanoscopic polymer / composite fibers of macroscopic lengths. In other words, the ease of processing, the low learning curve and inexpensive instrumentation render this technique extremely popular and versatile. One electro-spin viscous liquids, so having the right resin / solvent ratio, or in the case of composites, the right filler / binder / solvent proportions (and perhaps access to a good rheometer in order to characterize the polymer

The "home brew" configuration utilized in electro-spinning usually consists of a small (low power), high voltage power supply capable of voltage outputs in the range to several tens of kilovolts at very low current levels (below µA is desirable, see figure 4). A syringe pump, (to controlled the rate of fluid outcome from the syringe), syringe (containing the polymer solution), and an electrically grounded collector, complete the "home brew" set-up as shown

The morphology of the out coming fibers or mats will depend of several experimental conditions including: viscosity of the solution, applied electric field (applied voltage/needle to collector distance), the pump rate, temperature and pressure, solvent vapor pressure (evapo‐ ration rate), and collection time. Another method employed to control the fibers morphology (alignment) during electro-spinning is using a rotating collector or an oscillating needle [15]. These make electro-spinning a good tool to produce mats with a better-controlled morphology. A common morphological feature in electro-statically deposited fiber and mats is the formation of beads or beading. This particular feature involve the formation of random beads, preferen‐ tially where fibers cross each other, often due to the visco-elastic nature of the electro-spinned

In our studies, solutions of PLLA and PLLA- multi walled carbon nano-tubes (MWCNTS) composites were used. It is know that the addition of carbon nano-tubes to a polymeric solution can improved the mechanical properties of the resulting fibers and/or mat [16]. The PLLA was dissolved in 1:3 acetone/chloroform to prepare solutions of 13, 15 and 17 % w/v concentrations. The viscosity of the solutions were measured in a Bolin Rheometer with the following results, namely (1.1 +/\_ 0.1, 2.8 +/\_ 0.4 and 3.2 +/\_0.6) Pa.s, respectively. As expected, viscosity increased as the content of polymer in solution is increased. These initial solutions were spun using the standard electro-spinning set-up, where the resulting fibers were collected in silicon wafers for subsequent structural analysis. The applied electric field during deposition was 2.3 kV/ cm, with a collection time of 10 s and a pump rate of 0.5 mL/hr. Usually for large anode-cathode distance, in excess of let's say 5 cm and a homogeneous solution, fibers diameters are randomly distributed usually following a log-normal distribution.

One problem when working with very small fibers, with diameters comparable to the wavelength of the visible light, is you inability to see or clearly image structural details such as diameters, asperities, and general morphological features. It is in such cases that scanning electron microscopy (SEM) can be a formidable characterization tool. Even when one ignores the differences in magnification between optical and SEM instruments (500,000 x, that is 250 times better magnification than a good optical microscope), the depth of field is what makes SEM such a versatile instrument. In optical microscopy, you can see the top of a micrometric diameter fiber, but not its sides or bottom, unless you move the focus. In SEM one sees around the complete fiber in focus, that is the top, sides and bottom. Of course, SEM requires a vacuum for the electron optics, and liquid samples, or samples involving liquids present serious imaging challenges. In our experimental work, SEM has been the instrument of choice for imaging and measure.

Analysis of SEM data for the fiber diameter reveals that as the solution viscosity increased, the diameter of the obtained fibers was smaller. One can see the effect by examining the distribu‐ tion with a higher mean value of the fiber diameter (4 µm) for the 13 % w/v solution, and compare it to those with smaller mean diameters, i.e. (1 µm), for the 17 % w/v solution. Figure 5, show the SEM images of deposited fibers from different solutions and a histogram with the diameters frequency.

These experimental results on fibers diameter distribution suggested continuing further experimentation using the 15 % w/v solution. This particular solution produced fiber with an average diameter of (2.2±0.5) µm. It also, have no beading and more uniform morphology than

Note, how the diameters distribution shift to nano-metric size fibers for the solutions contain‐ ing s-MWCNTs (figure 6, a-b) in comparison to the solutions containing l-MWCNTs (figure 6, c-d). From SEM images it can also be observed that the addition of l-MWCNTs seems to promote the formation of beads within the fibers. In order to study the suitability of PLLA/ MWCNTs composites in the production of membranes, the electro-spinning set-up was modified, using as a collector a rotating cylinder. The electro-spinning conditions were kept as before (applied electric field was 2.3 kV/ cm, and a pump rate of 0.5 mL/hr), with the exception of an extended collection time of 1 hr, while the cylinder angular speed was kept at

**Viscosity, Pa.s**

15% w/v PLLA (0.2mg/mL s-MWCNT)

1.1+ 0.1 2.8 + 0.4 3.2 + 0.6 1.6 + 0.2 1.3 + 0.1 1.5 + 0.3 2.1 + 0.6

15% w/v PLLA (0.2mg/mL l-MWCNT)

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15% w/v PLLA (1.0mg/mL s-MWCNT

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13% w/v PLLA 15% w/v PLLA 17% w/v PLLA

**Table 1.** Viscosity as a function of composition for PLLA / MWCNT composites.

**Figure 6.** SEM images and histograms for A) 15 % w/v PLLA with 0.2 mg/mL s-MWCNTS B) 15 % w/v PLLA with 1.0 mg/mL s-MWCNTS, C) 15 % w/v PLLA 0.2 mg/mL l-MWCNTS, and D) 15 % w/v PLLA 1.0 mg/mL l-MWCNTS. All in a

69 rpm.

solution of 1:3 acetone/chloroform.

**Figure 5.** SEM images and data analysis for sample A) 13 % w/v PLLA, B) 15 % w/v PLLA, and C) 17 % w/v PLLA. All in 1:3 acetone/chloroform

the fibers collected from the other two solutions. These sets of facts are important, as later on they can be helpful in controlling the membranes morphology. As we proceed further, (0.2 mg/mL and 1.0 mg/mL) utilizing multi-walled carbon nano-tubes of different sizes, small (s), or large (l) (s-MWCNTs, 30-50 nm diameter, 0.5-2 µm length, and l-MWCNTs 60-100 nm diameter, 5-15 µm length), were added to the solution (15 % w/v PLLA). The viscosity of these solutions is lower than that of the pure PLLA solutions as expected, since the CNT's can only chemically interact through atoms lying in the prismatic planes (both ends of the tube) and defects, not through atoms in cylinder surface (the non-bonding π bands). Table 1 shows the viscosity data for all the solutions studied.

Figure 6 shows the SEM images and histograms for fiber diameter size distribution of 15 % w/ v PLLA/ MWCNTs composites. From the SEM images we can observe that even when the viscosities of these solutions are similar, the morphology of the obtained fibers is affected by the size of the carbon nano-tubes.


**Table 1.** Viscosity as a function of composition for PLLA / MWCNT composites.

the fibers collected from the other two solutions. These sets of facts are important, as later on they can be helpful in controlling the membranes morphology. As we proceed further, (0.2 mg/mL and 1.0 mg/mL) utilizing multi-walled carbon nano-tubes of different sizes, small (s), or large (l) (s-MWCNTs, 30-50 nm diameter, 0.5-2 µm length, and l-MWCNTs 60-100 nm diameter, 5-15 µm length), were added to the solution (15 % w/v PLLA). The viscosity of these solutions is lower than that of the pure PLLA solutions as expected, since the CNT's can only chemically interact through atoms lying in the prismatic planes (both ends of the tube) and defects, not through atoms in cylinder surface (the non-bonding π bands). Table 1 shows the

**Figure 5.** SEM images and data analysis for sample A) 13 % w/v PLLA, B) 15 % w/v PLLA, and C) 17 % w/v PLLA. All in

Figure 6 shows the SEM images and histograms for fiber diameter size distribution of 15 % w/ v PLLA/ MWCNTs composites. From the SEM images we can observe that even when the viscosities of these solutions are similar, the morphology of the obtained fibers is affected by

viscosity data for all the solutions studied.

the size of the carbon nano-tubes.

1:3 acetone/chloroform

154 Advances in Nanofibers

Note, how the diameters distribution shift to nano-metric size fibers for the solutions contain‐ ing s-MWCNTs (figure 6, a-b) in comparison to the solutions containing l-MWCNTs (figure 6, c-d). From SEM images it can also be observed that the addition of l-MWCNTs seems to promote the formation of beads within the fibers. In order to study the suitability of PLLA/ MWCNTs composites in the production of membranes, the electro-spinning set-up was modified, using as a collector a rotating cylinder. The electro-spinning conditions were kept as before (applied electric field was 2.3 kV/ cm, and a pump rate of 0.5 mL/hr), with the exception of an extended collection time of 1 hr, while the cylinder angular speed was kept at 69 rpm.

**Figure 6.** SEM images and histograms for A) 15 % w/v PLLA with 0.2 mg/mL s-MWCNTS B) 15 % w/v PLLA with 1.0 mg/mL s-MWCNTS, C) 15 % w/v PLLA 0.2 mg/mL l-MWCNTS, and D) 15 % w/v PLLA 1.0 mg/mL l-MWCNTS. All in a solution of 1:3 acetone/chloroform.

Other significant characterization techniques, of particular importance in the compositional and structural characterization of carbon allotropes and polymers are infrared (IR) and Raman spectroscopy. In the case of IR spectroscopy (see figure 7), the electric field associated with the photon; excite vibrations of existing dipoles in the material. If the material is crystalline, phonons of differing modes will propagate carrying part of the degenerated energy as heat (motion). In the case of Raman, a laser, with the intense electric field associated to its photons, induce a dipole in the material, and excite dipole oscillations with similar consequences as in IR. Of course, the light emitted by the vibrating charges in both cases (IR and Raman) is used as a signature of the material under study, collected by suitable optics, send to a photo-detector and transduced as a voltage to the processing unit and displayed. Most polymers are IR active, that is, they possess the pertinent dipoles in their structure. For these materials IR spectroscopy is a useful characterization tool, as most spectra are tabulated and indexed. In the case of carbonaceous materials, Raman spectroscopy is the technique of choice, as their spectrum, shows two prominent peaks around 1350 and 1600 cm-1. (see figure 8) The peak at ~ 1350 wave numbers is a disorder induced response, and the other, ~1600 cm-1, is a Raman-allowed mode found in highly oriented pyrolytic graphite (HOPG). The ratio of the intensity of both peaks often correlates with order and crystallinity [17]. In order to have a large SSA material, its crystallinity (order) is compromised and these spectroscopic tools become excellent local structural characterization instruments.

The interaction between mathematical modeling and experiments often can be mutually beneficial. A theoretical framework can provide experiments with the "backbone" of predict‐ ability. Clear correlations by fitting experimental results to a suitable theory helps the scientist decide how and where to do the next experiment, and to "understand" results, errors and fluctuations. The problem is that at times, analytical results are difficult or impossible to obtain. In that case the scientist often utilize numerical methods with the aid of a plethora of com‐ mercially available software packages of ease implementation in a personal computer. The software of our choice is COMSOLTM. We like this software for its versatility, friendliness, and economy [18]. This software is capable of coupling the PDE's for mechanics, heat, E&M, acoustic, electro-chemistry and others. We have used it for about a decade now, and really

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157

**Figure 8.** Raman spectrum for a carbonized electro-spun PAN fiber. The first peak, labeled D, correlates with disorder

The relevance of mechanical properties to the performance of super-capacitors separating membranes stem from the forces and stresses these membranes are exposed during capacitor fabrication, when the material is cut and aligned, as well as during service, when Joule heating generated thermal stresses load the fibers. Since the fibers forming the matt are randomly deposited, they are fluctuating in both on its spatial distributions in the deposition plane, and in fiber diameter. The possibility of having simultaneously a particular spatial location with a particular concentration of small diameter fibers, generating a "soft spot" mechanically and easily deformed by the transported fluid, is finite. Ongoing analytical and simulations research

The relation between stresses and deformation, at least for small deformations or strain are linear and they have been systematized by solutions to Hookes' Law that, for an anisotropic substance, linearly relates the strain tensor to the stress tensor with the elastic moduli as the

testing this hypothesis is expected to soon yield a predictive model.

benefited from it.

while the second, labeled G correlates with crystallinity.

**9. Mechanical properties**

**Figure 7.** IR spectra for A) PLLA in 1:3 acetone/chloroform B) insert: bands at 1763 cm-1 and 1714 cm-1 assigned to the ester carbonyl and carboxylic acid carbonyl functional group respectively can be observed together to the OH bend at 1361 cm-1 and 1093 cm-1, the C-O stretch can be observed at 1223 cm-1. The strong band at 750 cm-1 is due to the S-C stretch because the samples were mounted in silicon wafers.

The interaction between mathematical modeling and experiments often can be mutually beneficial. A theoretical framework can provide experiments with the "backbone" of predict‐ ability. Clear correlations by fitting experimental results to a suitable theory helps the scientist decide how and where to do the next experiment, and to "understand" results, errors and fluctuations. The problem is that at times, analytical results are difficult or impossible to obtain. In that case the scientist often utilize numerical methods with the aid of a plethora of com‐ mercially available software packages of ease implementation in a personal computer. The software of our choice is COMSOLTM. We like this software for its versatility, friendliness, and economy [18]. This software is capable of coupling the PDE's for mechanics, heat, E&M, acoustic, electro-chemistry and others. We have used it for about a decade now, and really benefited from it.

**Figure 8.** Raman spectrum for a carbonized electro-spun PAN fiber. The first peak, labeled D, correlates with disorder while the second, labeled G correlates with crystallinity.

## **9. Mechanical properties**

Other significant characterization techniques, of particular importance in the compositional and structural characterization of carbon allotropes and polymers are infrared (IR) and Raman spectroscopy. In the case of IR spectroscopy (see figure 7), the electric field associated with the photon; excite vibrations of existing dipoles in the material. If the material is crystalline, phonons of differing modes will propagate carrying part of the degenerated energy as heat (motion). In the case of Raman, a laser, with the intense electric field associated to its photons, induce a dipole in the material, and excite dipole oscillations with similar consequences as in IR. Of course, the light emitted by the vibrating charges in both cases (IR and Raman) is used as a signature of the material under study, collected by suitable optics, send to a photo-detector and transduced as a voltage to the processing unit and displayed. Most polymers are IR active, that is, they possess the pertinent dipoles in their structure. For these materials IR spectroscopy is a useful characterization tool, as most spectra are tabulated and indexed. In the case of carbonaceous materials, Raman spectroscopy is the technique of choice, as their spectrum, shows two prominent peaks around 1350 and 1600 cm-1. (see figure 8) The peak at ~ 1350 wave numbers is a disorder induced response, and the other, ~1600 cm-1, is a Raman-allowed mode found in highly oriented pyrolytic graphite (HOPG). The ratio of the intensity of both peaks often correlates with order and crystallinity [17]. In order to have a large SSA material, its crystallinity (order) is compromised and these spectroscopic tools become excellent local

**Figure 7.** IR spectra for A) PLLA in 1:3 acetone/chloroform B) insert: bands at 1763 cm-1 and 1714 cm-1 assigned to the ester carbonyl and carboxylic acid carbonyl functional group respectively can be observed together to the OH bend at 1361 cm-1 and 1093 cm-1, the C-O stretch can be observed at 1223 cm-1. The strong band at 750 cm-1 is due to the S-C

structural characterization instruments.

156 Advances in Nanofibers

stretch because the samples were mounted in silicon wafers.

The relevance of mechanical properties to the performance of super-capacitors separating membranes stem from the forces and stresses these membranes are exposed during capacitor fabrication, when the material is cut and aligned, as well as during service, when Joule heating generated thermal stresses load the fibers. Since the fibers forming the matt are randomly deposited, they are fluctuating in both on its spatial distributions in the deposition plane, and in fiber diameter. The possibility of having simultaneously a particular spatial location with a particular concentration of small diameter fibers, generating a "soft spot" mechanically and easily deformed by the transported fluid, is finite. Ongoing analytical and simulations research testing this hypothesis is expected to soon yield a predictive model.

The relation between stresses and deformation, at least for small deformations or strain are linear and they have been systematized by solutions to Hookes' Law that, for an anisotropic substance, linearly relates the strain tensor to the stress tensor with the elastic moduli as the proportionality constant. For a homogeneous isotropic substance, the stress, strain and elastic constants are all scalars with the value of the elastic constants known as the Young's modulus as depicted in equations 5.

$$
\sigma\_{ij} = \mathcal{c}\_{ijkl}\mathcal{e}\_{kl} \tag{5}
$$

sample are clamped. The upper end is vertically pulled up at a constant force and velocity, until the samples fracture. From the geometry and strain rate information one can infer the value of the composite Young's modulus. Table 2 summarize the results for mechanical test for mats obtained from the electro-spinning of 15 % w/v PLLA solutions in 1:3 acetone/ chloroform with MWCNTs in comparison with currently use membrane in our laboratory,

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159

**Figure 9.** SEM with corresponding histogram analysis for fibers diameter and pores diameter for A) 15 % w/v PLLA B) 15 % w/v PLLA with 1.0 mg/mL s-MWCNTS, C) 15 % w/v PLLA 1.0 mg/mL l-MWCNTS in 1:3 acetone/chloroform.

It can be observed from our results that the Young's modulus decreases from l-MWCNTs (29.73 PSI) > 15 % PLA (20.07 PSI) > s-MWCNTs (12.27 PSI), this can be correlated with the mats fiber diameters distribution, where the number of nanometric diameter fibers increases in the same order: l-MWCNTs > 15 % PLA > s-MWCNTs. However, the number and dimensions of pores, decreases in the following order, 15 % PLA > s-MWCNTs > l-MWCNTs (figure 9). It has been reported previously that the Young's modulus in nonwoven fabrics is affected by porosity, fiber diameter, radius of curvature, and the distance between the junctions where fibers cross [21]. Therefore, not a single factor is responsible for the mechanical properties, but we can see a trend in our results that smaller fiber composition, together with bigger pores seems to somewhat enhance the Young's modulus. In comparison, the GorTexTM membrane has a higher Young's modulus, as it is expected for woven fabrics. We test the performance of the membrane

GoreTexTM.

Equations 5, *σij* =*cijkl εkl* (anisotropic media), *σ* =*Yε*(isotropic media), where σ is the symbol for stress, c for elastic constants, Y for Youngs' modulus, and ε for strain.


**Table 2.** Mechanical properties of the mats studied.

There are multiple approximate solutions to Hooke's law for different experimental circum‐ stances. Since the thickness of these permeable separating mats is small compared to the diameter when utilized in super-capacitors, they are bona-fide membranes. There are two types of membranes often encountered in practice [19] namely, the thick and thin membranes. The first type received its name when the maximum deflection at its center (in this case of a circular membrane as often used in super-capacitors) under load, wo is far smaller than the membrane thickness dm. The membrane is thin when the deflection is larger than the mem‐ brane thickness. The approximations are such that a thick membrane maybe treated as thin as the pressure head increases, and so the deflection. The deflection shape for a thick membrane relates to the torques acting along the circumference where the membrane is clamped. The deflection of a circular thick membrane w, with radius of Rm is described by equation 6. The membranes thickness range between 50 and 150 µm, while the estimated deflections are much smaller (nano-metric), so equation 6 for a thick membrane model the behavior of our separating membranes reasonably well.

$$\mathbf{w}(r) = \mathbf{w}\_0 (\mathbf{l} - \frac{r^2}{R\_m})^2 \tag{6}$$

Various fibers and mats were prepared for mechanical test as well as for morphological analysis. The mechanical strength was measured using the INSTRON (Model 4206), the sample were cut it with "doggy bone" configuration [20]. During the experiment, both ends of the sample are clamped. The upper end is vertically pulled up at a constant force and velocity, until the samples fracture. From the geometry and strain rate information one can infer the value of the composite Young's modulus. Table 2 summarize the results for mechanical test for mats obtained from the electro-spinning of 15 % w/v PLLA solutions in 1:3 acetone/ chloroform with MWCNTs in comparison with currently use membrane in our laboratory, GoreTexTM.

proportionality constant. For a homogeneous isotropic substance, the stress, strain and elastic constants are all scalars with the value of the elastic constants known as the Young's modulus

> e

inches2 UTS, PSI Breaking

GoreTex TM 0.0011 0.197 0.00022 3508.97 2787.64 0.5 0.84 3818.18 653.73 15 % PLA 0.0018 0.236 0.00042 889.37 728.93 0.3 1.56 3714.29 20.07

s-MWCNT 0.0006 0.236 0.00013 1186.90 957.78 0.1 0.74 5692.31 12.27

l-MWCNT 0.0007 0.236 0.00017 1711.86 1482.08 0.3 0.36 2117.65 29.73

There are multiple approximate solutions to Hooke's law for different experimental circum‐ stances. Since the thickness of these permeable separating mats is small compared to the diameter when utilized in super-capacitors, they are bona-fide membranes. There are two types of membranes often encountered in practice [19] namely, the thick and thin membranes. The first type received its name when the maximum deflection at its center (in this case of a circular membrane as often used in super-capacitors) under load, wo is far smaller than the membrane thickness dm. The membrane is thin when the deflection is larger than the mem‐ brane thickness. The approximations are such that a thick membrane maybe treated as thin as the pressure head increases, and so the deflection. The deflection shape for a thick membrane relates to the torques acting along the circumference where the membrane is clamped. The deflection of a circular thick membrane w, with radius of Rm is described by equation 6. The membranes thickness range between 50 and 150 µm, while the estimated deflections are much smaller (nano-metric), so equation 6 for a thick membrane model the behavior of our separating

> 2 2

*<sup>r</sup> wr w <sup>R</sup>* (6)

*m*

Various fibers and mats were prepared for mechanical test as well as for morphological analysis. The mechanical strength was measured using the INSTRON (Model 4206), the sample were cut it with "doggy bone" configuration [20]. During the experiment, both ends of the

<sup>0</sup> <sup>2</sup> ( ) (1 ) = -

Point, PSI

*ij ijkl kl* = *c* (5)

Breaking Time, min

Breaking Time, min/inch2

Young's Modulus, PSI

(anisotropic media), *σ* =*Yε*(isotropic media), where σ is the symbol for

Speed, inches/min

s

stress, c for elastic constants, Y for Youngs' modulus, and ε for strain.

Area,

as depicted in equations 5.

*εkl*

**Table 2.** Mechanical properties of the mats studied.

membranes reasonably well.

Cross Section Length, inches

Equations 5, *σij* =*cijkl*

158 Advances in Nanofibers

Sample ID Thickness, inches

(1 mg/mL)

(1 mg/mL)

**Figure 9.** SEM with corresponding histogram analysis for fibers diameter and pores diameter for A) 15 % w/v PLLA B) 15 % w/v PLLA with 1.0 mg/mL s-MWCNTS, C) 15 % w/v PLLA 1.0 mg/mL l-MWCNTS in 1:3 acetone/chloroform.

It can be observed from our results that the Young's modulus decreases from l-MWCNTs (29.73 PSI) > 15 % PLA (20.07 PSI) > s-MWCNTs (12.27 PSI), this can be correlated with the mats fiber diameters distribution, where the number of nanometric diameter fibers increases in the same order: l-MWCNTs > 15 % PLA > s-MWCNTs. However, the number and dimensions of pores, decreases in the following order, 15 % PLA > s-MWCNTs > l-MWCNTs (figure 9). It has been reported previously that the Young's modulus in nonwoven fabrics is affected by porosity, fiber diameter, radius of curvature, and the distance between the junctions where fibers cross [21]. Therefore, not a single factor is responsible for the mechanical properties, but we can see a trend in our results that smaller fiber composition, together with bigger pores seems to somewhat enhance the Young's modulus. In comparison, the GorTexTM membrane has a higher Young's modulus, as it is expected for woven fabrics. We test the performance of the membrane produced from the 15% w/v PLA solution to that of the GorTex for one of our devices. The decision to use the 15% w/v PLA membrane was done under the bases that this membrane posses more uniformity in the average pores' size than the others.

**Acknowledgements**

R08-32802.

Penn' s LRSM.

**Author details**

**References**

We would like to acknowledge support from the following sources:

DMR11-20901 for her support as a postdoctoral fellow.

and the ACS-Seed fellow, Ms. Rebecca Irizarry.

Rocío del A. Cardona and Jorge J. Santiago-Avilés\*

mica Acta, 2000; 45, 2483-2498

Today. 2008, 43-47

\*Address all correspondence to: santiago@seas.upenn.edu

The Penn's Nano-Bio Interface Center (NBIC) through the NSF sponsored grant NSEC DM

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161

R. Cardona would like to acknowledge the University of Pennsylvania Provost Office and the Laboratory for the Research on the Structure of Matter (LRSM) through the NSF grant

We like to acknowledge the SEM microscopy work of our collaborator Prof. Eva Campo from

We acknowledge the help offered by our graduate students Mr. Timothy Jones, and Hitesh Sahoo, as well as the NSF – REU sponsored undergraduates Mr. Matt Biggers, Mr. Esteban Villareal, Mr. Raymond Xu, Mr. Melvin Berrios and the two H.S. interns, Mr. Adam Flecher

Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, USA

[1] R.Kotz, M.Carlen, *Principles and Applications of Electrochemical Capacitors*, Electroche‐

[2] H.D. Abruña, Y.Kiya, J.C.Henderson *Batteries and electrochemical capacitors*. Physics

[3] C.A. Vincent, and B. Scrosati. Modern Batteries. In: Butterworth, Oxford; 1997.

[7] Gore Text. http://en.wikipedia.org/wiki/Gore-Tex (accessed August 2 2012)

[5] Yury Gogotsi. Carbon Nanomaterials. In: CRC Press; 2006.

Processing, Applications. In: Hanser Publishers, N.Y.; 1996.

[4] B.E. Conway. Electrochemical Supercapacitors. In: Plenum Publishers, N.Y. ;1999.

[6] E. P. Moore. Polypropylene Handbook. Polymerization, Characterization, Properties,

#### **10. Concluding remarks**

The device utilized to test the suitability of the PLLA based composite membrane was a pseudo-capacitor device, using as electrode material the oxidized and neutralized species of poly-3,4-propylenedioxythiophene (the process for the construction of this device have been previously reported by our group) [22,23]. The results indicated (figure 10) that over slow charging-discharging rates our membrane performed better than the bench mark membrane, but as we moved from moderate to fast charge-discharge rates, the performance of both membranes are comparable.

**Figure 10.** Specific capacitance as a function of scan rate and voltage window for poly-3,4-propylenedioxythiophene pseudo-capacitor using GorTexTM and PLLA separator membranes.
