Carbon nanotube-conductive add

ctr

zie

ronau

ton V

Resea

ised f

line 2

mate

potential applications on Gossamer spacecraft. In addition to these properties, sufficient electrical conductivity is required in order to

color film. The 3 is a measure of the films ability to radiate

Polymer 45 (2004)

protection in the US.0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.dissipate electrostatic charge (ESC) build-up brought about by the charged orbital environment. One approach to achieve sufficient electrical

conductivity for ESC mitigation is by the incorporation of single-walled carbon nanotubes (SWNTs). However, when SWNTs are dispersed

throughout the polymer matrix, the nanocomposite films tend to be significantly darker than the pristine material resulting in a higher a. The

incorporation of conductive additives in combination with a decreased SWNT loading level is one approach for improving a while retaining

conductivity. Taken individually, the low loading level of conductive additives and SWNTs was insufficient in achieving the percolation

level necessary for electrical conductivity. When added concurrently to the film, conductivity was achieved. The chemistry, physical and

mechanical properties of the nanocomposite films will be presented.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Low color polyimides; Nanocomposites; Electrostatic charge mitigation

1. Introduction

Several space mission concepts proposed by NASA are

based on the use of large, deployable, and ultra-lightweight

vehicles (e.g. Gossamer spacecraft) consisting of both

structural and polymer film components [1]. One example

of a proof of concept of this approach was the inflatable

antennae experiment (IAE) deployed from the space shuttle

Endeavor (STS-77) in 1996 [2]. This experiment involved

the successful deployment of a 14 m Mylarw film based

antennae from a compact vehicle (Spartan satellite). Since

this experiment was a proof of concept, the space

environmental durability of the film material was not of

concern.

However, future Gossamer spacecraft will require films

that are durable to the space environment and compliant.

Compliance is needed so that the film can be folded into

compact volumes found on conventional launch vehicles.

Once in orbit, the folded film is deployed to create structures

that are many square meters in size. To be space durable, the

film must exhibit resistance to many environmental factors

such as atomic oxygen (AO) in low earth orbit, UV and

vacuum UV radiation, and electron and proton attack.

Additionally, some applications require that the film exhibit

low solar absorptivity (a) and high thermal emissivity (3).

The a pertains to the fraction of incoming solar energy that

is absorbed by the film and is typically low (w0.1) for a lowThis paper is work of the US Government and is not subject to copyrightnanocomposite films for ele

Joseph G. Smith Jra,*, Donavon M. Delo

aAdvanced Materials and Processing Branch, National Ae

Mail Stop 226, HampbNational Institute of Aerospace, 144

Received 12 April 2004; received in rev

Available on

Abstract

Thin film membranes of space environmentally stable polymericitive-space durable polymer

ostatic charge dissipation*

ra,1, John W. Connella, Kent A. Watsonb

tics and Space Administration, Langley Research Center,

A 23681-2199, USA

rch Drive, Hampton VA 23666, USA

orm 1 July 2004; accepted 2 July 2004

4 July 2004

rials possessing low color/solar absorptivity (a) are of interest for

61336142

www.elsevier.com/locate/polymerdesirable since these films would effectively absorb little

radiation. Many of these material requirements can be met

by aromatic polyimides, due to the inherent excellent

doi:10.1016/j.polymer.2004.07.004

E-mail address: joseph.g.smith@nasa.gov (J.G. Smith).1 National Research Council research associate located at NASA Langley

Research Center.energy from the film surface. Low a and high 3 values are* Corresponding author. Tel.: C1-757-864-4297; fax: C1-757-864-8312.

A typical preparative method of the alkoxysilane

terminated amide acid (ASTAAs) is described. Into a

lymerphysical and mechanical properties of the polymers as well

as radiation resistance. Through the proper choice of

monomers, low color (which is related to a) and AO

resistance have been achieved [3,4].

Since most polymeric films are inherently insulating,

they can become charged during ground-based manufactur-

ing and handling as well as in space due to the orbital

environment. The material can then behave like a capacitor

and discharge in a single event causing considerable damage

to surrounding materials and electronics on the vehicle.

Also, static charge build-up on the ground creates problems

in handling the film during subsequent processing steps or

fabrication of components. To mitigate electrostatic charge

(ESC) build-up, a surface resistivity in the range of 106

1010 U/square is needed without degrading other desiredmaterial properties (e.g. flexibility, a, 3, transparency). The

current state-of-the-art to impart electrical conductivity

while maintaining a low a and high optical transparency has

been through the use of conductive coatings such as

indiumtin oxide (ITO). While exhibiting high surface

conductivity, these coatings are rather brittle and make

handling difficult. Once the coating is broken (cracked) by

handling or on orbit, the conductive pathway is lost.

Another way to achieve electrical conductivity is through

the incorporation of uniformly dispersed additives in the

polymer. One such additive that has received much attention

over the last several years is single-walled carbon nanotubes

(SWNTs) [512]. Based on their high aspect ratio and

electrical properties, SWNTs are excellent candidates for

obtaining the surface resistance necessary for ESC dissipa-

tion. However, good and uniform dispersal of SWNTs has

been difficult due to their insolubility and/or incompatibility

with the host resin. Typically, SWNTs tend to agglomerate

as bundles in solution and if dispersed, reagglomerate soon

thereafter due to electrostatic attraction.

Uniform dispersions of SWNTs have been reported in

space durable polymers (e.g. LaRCe CP2) using an in situpolymerization approach [5,7,8] and by the addition to

amide acid polymers terminated with alkoxysilane moieties

[7,11,12]. Both approaches have afforded bulk conductivity

sufficient for ESC mitigation at a wt loading of 0.05%

relative to the polymer. However, there was a loss in the

optical transparency of the film and an increase in a

compared to the virgin material. Another approach has

involved the spray coating of SWNT onto the film surface

affording only surface, not volume, conductivity [9,10]. By

this approach, the optical properties were not significantly

affected compared to the pristine material. These

approaches provided conductive films that were robust to

harsh manipulation.

One means of improving the optical and thermo-optical

properties while retaining sufficient bulk electrical conduc-

tivity would involve the introduction of a low amount of a

second conducting agent with simultaneous reduction of the

quantity of SWNTs. Ideally, this second conductive additive

J.G. Smith Jr et al. / Po6134would be anticipated to have a negligible effect upon the500 ml three neck round-bottom flask equipped with a

mechanical stirrer, nitrogen gas inlet, and drying tube filled

with calcium sulfate were placed APB (22.8551 g,

0.0782 mol), APTS (0.8552 g, 0.0040 mol), and DMAc

(50 ml). Once dissolved, the flask was immersed in a room

temperature water bath to regulate the temperature. 6FDA

(35.6215 g, 0.0802 mol) was added in one portion as a

slurry in DMAc (45 ml) and rinsed in with 25 ml of DMAc

to afford a solution with a solids content of w34.4%. Thereaction was stirred forw24 h at ambient temperature undernitrogen. The inherent viscosity (hinh) in DMAc at 25 8C fora 0.5% (w/v) solution was 0.57 dl/g.

2.3. Blending of SWNT and ASTAAproperties of interest. It has been reported that electrical

conductivity was achieved using conductive carbon black as

a filler at a volume loading of 0.06% in an epoxy by the

addition of a small amount of an inorganic salt [13]. This

was proposed to be due to the agglomeration of the carbon

black into networks throughout the matrix by increasing the

ionic concentration (strength) of the epoxy. Thus, it was

postulated that by increasing the ionic strength of the

polyimide matrix by the addition of an inorganic salt (a

second conducting agent), sufficient network formation of

SWNTs at a lower loading level could be achieved. This

would in-turn improve the optical and thermo-optical

properties of the film due to decreased SWNT loading

levels. The preparation and characterization of nanocompo-

site films based on this approach is described herein.

2. Experimental

2.1. Starting materials

Aminophenyltrimethoxysilane (APTS, Gelest Inc.,

w90% meta, w10% para) was purified by vacuumdistillation. 4,4 0-Hexafluoroisopropylidene diphthalic anhy-dride (6FDA, Hoechst Celanese Inc., mp 241243 8C) wassublimed prior to use. 1,3-Bis(3-aminophenoxy)benzene

(APB, Mitsui Chemicals America, Inc. mp 107108.5 8C)and N,N-dimethylacteamide (DMAc) were used as received

without further purification. SWNTs prepared by the HiPco

process were obtained from Tubes at Rice and purified by

heating at 250 8C for w16 h in a high humidity chamberfollowed by Soxhlet extraction with hydrochloric acid

(22.2 wt%) for w24 h. All other chemicals were obtainedfrom commercial sources and used as received without

further purification.

2.2. Polymer synthesis

45 (2004) 61336142The following example is representative of the

reflectometer with measurements taken between 250 andw

lymerpreparative method employed. To a 100 ml three neck

round-bottom flask equipped with nitrogen inlet, mechan-

ical stirrer, and drying tube filled with calcium sulfate was

charged 11.06 g of an ASTAA solution (w33.5% solids). Ina separate vial, SWNTs (0.0011 g) were placed in 3 ml of

DMAc and the mixture sonicated forw2.5 h in a Degussa-Ney ULTRAsonik 57X cleaner operated at w50% powerand degas levels. The initial temperature of the water bath

was ambient and w40 8C at the conclusion of sonication.The suspended tubes were then added to the stirred mixture

of ASTAA at room temperature and rinsed in with 4.5 ml of

DMAc to afford a solids content of w20%. The SWNTloading was 0.03% by wt. The mixture was subsequently

stirred for w16 h under a nitrogen atmosphere at roomtemperature prior to film casting.

2.4. Blending of salt and ASTAA

The following example is representative of the prepara-

tive method employed. To a 100 ml three neck round-

bottom flask equipped with nitrogen inlet, mechanical

stirrer, and drying tube filled with calcium sulfate was

charged 5.8 g of an ASTAA solution (w33.7% solids).Copper sulfate (0.0003 g) was added and rinsed in with 4 ml

of DMAc to afford a solids content of w20%. The saltloading was 0.014% by wt. The mixture was then stirred for

w16 h under a nitrogen atmosphere at room temperatureprior to film casting.

2.5. Blending of SWNT, salt, and ASTAA

The following example is representative of the prepara-

tive method employed. To a 100 ml three neck round-

bottom flask equipped with nitrogen inlet, mechanical

stirrer, and drying tube filled with calcium sulfate was

charged 9.82 g of an ASTAA solution (w34.4% solids). In aseparate vial, SWNTs (0.0010 g) and copper sulfate

(0.0007 g) were placed in 3 ml of DMAc and the mixture

sonicated for w2.5 h in a Degussa-Ney ULTRAsonik 57Xcleaner operated at w50% power and degas levels. Theinitial temperature of the water bath was ambient and

w40 8C at the conclusion of sonication. The suspendedSWNT mixture was then added to the stirred ASTAA

solution at room temperature and rinsed in with 4 ml of

DMAc to afford a solids content of w20%. The SWNTloading was 0.03% and the salt loading was 0.014%,

respectively, by wt. The mixture was stirred for w16 hunder a nitrogen atmosphere at room temperature prior to

film casting.

2.6. Films

DMAc solutions of the neat ASTAA and nanoparticle

containing ASTAAs were doctored onto clean, dry plate

glass and dried to tack-free state in a low humidity chamber

J.G. Smith Jr et al. / Poat ambient temperature. The films were cured in a forced air2800 nm using a vapor deposited aluminum on Kapton

film (1st surface mirror) as a reflective reference for air mass

0 per ASTM E903. An AZ Technology Temp 2000A

infrared reflectometer was used to measure 3. High

resolution scanning electron microscopy (HRSEM) was

obtained on a Hitachi S-4700 and S-5200 field emission

scanning electron microscopy systems operating below

2.0 kV. The composite images were obtained in the low

voltage mode in order to set up a stable local electric field on

the sample while minimizing beam-induced damage. Sur-

face resistivity was measured using a Prostatw PSI-870Surface Resistance and Resistivity Indicator per ASTM D-

257 operating at 9 V and reported as an average of three

readings. Volume resistivity was measured using a ProstatwPRS-801 Resistance System with a PRF-911 Concentric

Ring Fixture operating at 10100 V per ASTM D-257.

3. Results and discussion

3.1. Polymer synthesis

The objective of this work was to impart sufficient

conductivity to mitigate ESC build-up in low color, space

durable polymer films while not detracting from other

desirable properties (e.g. low color, low a, transparency,

flexibility). The approach involved the addition of nano-

particles (SWNTs and/or inorganic salts) to a premade

amide acid polymer terminated with alkoxysilane (ASTAA)

groups as previously reported [7,11,12]. The ASTAA, based

on LaRCe CP2, was prepared by the reaction of 6FDA withAPB and end-capped with APTS at a 2.5% molar offset

(Scheme 1), which corresponded to a calculated number

average molecular weight Mn of w27,700 g/mol. Tomitigate any temperature increase of the solution when

6FDA was added, the flask was immersed in a roomoven at 100, 200 and 300 8C for 1 h each. The films weresubsequently removed from the glass plate and

characterized.

2.7. Other characterization

Melting point ranges (tangent of onset to melt and the

endothermic peak) were determined by either DSC at a

heating rate of 10 8C/min or visually on a Thomas-Hoovercapillary melting point apparatus (uncorrected). Glass

transition temperatures (Tgs) were determined on thin

films at a heating rate of 20 8C/min and were taken as theinflection point of the DH vs. temperature curve. Opticalmicroscopy was performed on an Olympus BH-2 at a

magnification of 200!. The percentage transmission (%T)at 500 nm was obtained on thin films using a Perkin-Elmer

Lambda 900 UV/VIS/near-IR spectrometer. The a was

measured on an AZ Technology Model LPSR-300 spectro-

45 (2004) 61336142 6135temperature water bath. This was done to alleviate

following dimensions:w0.71.5 nm in diameter and in the

-term

lymermicron range for length [7,12].

Due to the purification process, oxygenated containing

species such as carboxylic acid and hydroxyl groups can

form at defect sites along the tube or at the tube ends [14,

15]. These groups present on the SWNTs could presumably

react with the silanol groups of the endcapper as depicted inpremature reaction of the alkoxysilane groups. The mixture

was then stirred overnight under a nitrogen atmosphere at

ambient temperature. Several batches of the endcapped

polymer were prepared with hinhs ranging from 0.57 to

0.78 dl/g.

3.2. Blending of nanoadditives with LaRCe CP2 ASTAA

The nanoadditives evaluated were HiPco SWNTs,

inorganic salts, and a combination of one of the inorganic

salts with the SWNTs (Scheme 2). The HiPco SWNTs

utilized in this study were purified to remove amorphous

carbon and residual catalyst [14,15]. This purification

involved a chemical oxidative process to remove amor-

phous carbon followed by soxhlet extraction with hydro-

chloric acid to remove residual metal (i.e. iron) catalyst.

Prior analyses by atomic force microscopy, HRSEM, and

Raman spectroscopy have shown that the purified tubes to

be consistent with other HiPco prepared materials with the

Scheme 1. Synthesis of alkoxysilane

J.G. Smith Jr et al. / Po6136Fig. 1 to form a covalent bond thus aiding in the dispersion.

Prior work had shown that the product from the model

reaction of the purified HiPco SWNTs with 3-aminopro-

pyldimethyl ethoxysilane indicated that the tubes were not

significantly affected as determined by Raman spec-

troscopy. Additionally, analysis of the reaction product by

HRSEM showed that the tubes had a light organic surface

coating [12]. The SWNTs in DMAc were sonicated to afford

a suspension prior to addition to the ASTAA solution. The

SWNTs were incorporated at loadings of 0.03 and 0.04 wt%

based on prior work that showed the percolation threshold

was between 0.03 wt% (non-conductive) and 0.05 wt%

(conductive) loading using this batch of SWNTs [12].

The inorganic salts investigated were copper sulfate

(CuSO4) and copper chloride (CuCl2) at a loading of0.014 wt%. This loading level corresponded to 3!10K6 molthat was determined to be the quantity needed to achieve

conductivity in an epoxy matrix containing conductive

carbon black [13]. Copper(II) chloride was chosen based on

the epoxy study [13] while CuSO4 was chosen based on its

relative absorption in the region of interest (the solar

maximum, 500 nm). Since the salts were soluble in DMAc,

they were added directly to the stirred ASTAA solution.

Blends of CuSO4 with the SWNTs were evaluated at two

different SWNT and salt loadings. The loading levels

examined were 0.030.04 SWNT and 0.0140.042 CuSO4wt%, respectively. Copper(II) chloride was not studied in

detail based on its effect upon %T as discussed in the Optical

Transparency section. For the SWNT-CuSO4 mix, the salt

was added to the SWNTs and the mix sonicated prior to

addition to the ASTAA.

In all cases, solutions were stirred overnight under a

nitrogen atmosphere at room temperature prior to film

casting. Due to the acidic nature of the amide acid, no

additional acid catalyst was required for the hydrolysis and

condensation of the alkoxysilane endgroups [16]. Once

dried to a tack free state, the films were cured to 300 8C inflowing air to effect imidization and condensation of the

silanol endgroups.

The Tg and room temperature tensile properties of the

neat polymer (P1) and nanocomposite films (P2P8) are

inated amide acid of LaRCe CP2.45 (2004) 61336142reported in Table 1. All the cured films exhibited a Tg of

w206 8C regardless of the amount of nanoadditiveincorporated. Additionally, no significant differences in

the room temperature tensile properties were observed

compared to the control film (P1). Similar results were

previously observed using this same method and SWNTs

[12].

3.3. Optical microscopy

To assess the effect of salt addition upon SWNT

dispersion, P4P8 were examined by optical microscopy

at 200! (Fig. 2). The pristine polymer (P1), shown forcomparison, was featureless. Films P4 and P7, containing

0.03 and 0.04 wt% SWNTs, respectively, and P5,

comp

J.G. Smith Jr et al. / Polymer 45 (2004) 61336142 6137Table 1

Neat polymer and nanocomposite characterization

Scheme 2. Nanocontaining 0.03 wt% SWNTs and 0.014 wt% CuSO4,

exhibited uniform dispersion of SWNTs. Film P4 contain-

ing 0.03 wt% SWNT exhibited a peppered appearance of

SWNTs with no apparent connectivity. With the addition of

0.014 wt% CuSO4 (P5), connectivity or network formation

of the SWNTs was observed. The micrograph of P5 showed

a matted appearance of the SWNTs. A similar effect was

reported for the addition of salt to conductive carbon black

in an epoxy [13]. Increasing the salt concentration to

0.042 wt% (P6) resulted in enhancing this effect. In P7

containing a loading of 0.04 wt% SWNTs, a network or

Film ID SWNT (wt%) Additive (wt%) Tg (8C)

P1 207

P2 CuSO4, 0.014 207

P3 CuCl2, 0.014 206

P4 0.03 205

P5 0.03 CuSO4, 0.014 206

P6 0.03 CuSO4, 0.042 206

P7 0.04 207

P8 0.04 CuSO4, 0.014 206

Fig. 1. Postulated reaction betwosite synthesis.matted appearance was evident. The addition of CuSO4(P8) to this SWNT loading level resulted in enhancing this

network and was comparable in appearance to that of P6.

For films P6 and P8, the SWNTs appeared uniformly

dispersed but more bundles were evident.

3.4. High resolution SEM

Films P5P7 were further examined by HRSEM (Fig. 3)

to assess SWNT dispersion. Regions of dark contrast in the

images were due to SWNTs residing at or near the polymer

Room temperature tensile Elong. at break (%)

Strength (MPa) Modulus (GPa)

116G7 3.4G0.1 5G1

118G9 3.4G0.1 5G1

123G2 3.5G0.1 5G1

122G6 3.5G0.2 5G1117G8 3.4G0.2 5G1

124G5 3.4G0.1 5G1

121G2 3.4G0.1 6G1

118G8 3.4G0.1 5G1

een polymer and SWNT.

surface disturbing the beam-induced electric field [17]. The

images show that the tubes were fairly well dispersed

throughout the polymer with no large agglomerations.

However, there were regions of high and low concentrations

3.5. Optical transparency

The effect of nanoadditive inclusion upon the optical

transparency of polymer films at 500 nm, the solar

Fig. 2. Optical micrographs at 200X of nanocomposite films P1 and P4P8.

J.G. Smith Jr et al. / Polymer 45 (2004) 613361426138of SWNTs evident. The side view of P7 showed tube ends

jutting out from the polymer surface with the size of the

bundles being w1020 nm in diameter. Images of P1P3were not obtained since they were non-conductive. Film P8

was not examined since the optical micrograph was

comparable to P6.Fig. 3. HRSEM images of nanomaximum, was determined with the results presented in

Table 2. It is known that optical properties are thickness

dependent. In this study, the sensistivity of %T to film

thickness was not investigated. However, care was taken to

obtain films that were of comparable thickness. Film

thickness ranged from 45 to 48 mm allowing for a directcomposite films P5P7.

comparison of the results. An initial screening was solvent and air was the blank. The following series were

I

inte

analogous to the extinction coefficient obtained from a

Table 2

Optical transparency at 500 nm

Film ID Film thickness (mm) SWNT (wt%) Additive (wt%) %T at 500 nm A at 500 nma

P1 48 85 0.0706

P2 45 CuSO4, 0.014 76 0.1367

P3 48 CuCl2, 0.014 69 0.1612

P4 45 0.03 66 0.1805

P5 48 0.03 CuSO4, 0.014 63 0.2007

P6 45 0.03 CuSO4, 0.042 53 0.2757

P7 48 0.04 58 0.2366

P8 45 0.04 CuSO4, 0.014 59 0.2291

a AZ2K log10%T

J.G. Smith Jr et al. / Polymer 45 (2004) 61336142 6139performed to determine the effect of inorganic salt upon

transparency. Polymer solutions containing CuSO4 and

CuCl2 were prepared using the same master batch of base

polymer (hinhZ0.57 dl/g) and cast as thin films. The curedcopper containing films (P2 and P3) exhibited an orange-

brown color. It was found that the addition of the inorganic

salts at a 0.014 wt% decreased the %T compared to the base

polymer (P1) in the order of P1OP2OP3. Due to the lower%T exhibited by P3 compared to P1 and P2, no further work

was performed with CuCl2.

As observed in earlier studies, increasing SWNT

concentration led to a decrease in %T with P1OP4OP7[5,7,8,11,12]. When CuSO4 was added to the 0.03 wt%

SWNT composition, the %T decreased with increasing salt

concentration in the order: P4OP5OP6. Comparable %Tswere obtained for P7 and P8 containing 0.04 wt% SWNTs

even though P8 contained 0.014 wt% CuSO4. For films

containing 0.014 wt% CuSO4, the %T decreased with

increasing SWNT incorporation as expected: P2OP5OP8.Since the films were of comparable thickness, %T was

converted to absorbance (A) to determine if the SWNT

and/or copper sulfate inclusions adhered to Beers law

(Table 2). In a typical Beers law experiment, the material of

interest is dissolved in a solvent at various concentrations

and compared to the neat solvent in a matching cell.

However, in this study the solid polymer matrix was theFig. 4. Absorbance at 500 nm vs. nanoadditive loading.Beers law plot and corresponds to contributions from

both the matrix and nanoadditives.are accounted for in this study. The slope of the line isandrcept of the lines is presumably due to scattering

reflection by the film surface(s); neither of whichconn a typical Beers law plot, A is zero at zero

centration. In the present study, the non-zero(3) A at constant SWNT loadingZ(2.3255)(CuSO4 wt%)K1

(CuSO4 wt%)C0.1191 R2Z0.98.(2)R Z0.99.A at constant CuSO4 loadingZ(2.7404)(SWNT wt%)

K1

(SWNT wt%)C0.1191 R2Z1.00.(1) AZ(4.0377)(SWNT wt%)K1(SWNT wt%)C0.06842coefinear least-squares fit of the data and correlation

ficient are shown below.plotted against A at 500 nm affording Beers law relation-

ships (Fig. 4):

(1) wt% SWNT inclusion (P1, P4, and P7);

(2) wt% SWNT inclusion at constant copper sulfate loading

(P2, P5, and P8);

(3) wt% CuSO4 loading at constant SWNT loading (P4

P6).

LFig. 5. a vs. nanoadditive loading.

3.6.

exhibited a peppered appearance of the SWNTs with no

Table 3

Thermo-optical properties

Film ID Film thickness (mm) SWNT (wt%) Additive (wt%) a 3

P1 48 0.09 0.58

P2 45 CuSO4, 0.014 0.10 0.56

P3 48 CuCl2, 0.014 0.12 0.58

P4 45 0.03 0.22 0.61

P5 48 0.03 CuSO4, 0.014 0.28 0.63

P6 45 0.03 CuSO4, 0.042 0.31 0.64

P7 48 0.04 0.30 0.65

P8 45 0.04 CuSO4, 0.014 0.31 0.64

J.G. Smith Jr et al. / Polymer 45 (2004) 613361426140increased by w30% with inclusion of 0.014 wt% CuCl2with minimal effect upon 3 compared to P1. As previously

reported, increasing SWNT loading increased a and 3 in the

order: P7OP4OP1 [5,7,8,11,12]. When CuSO4 was addedto the 0.03 wt% SWNT composition, a increased with

increasing salt concentration: P6OP5OP4. The 3 for thenanocomposites containing 0.03 wt% SWNTs was essen-

tially the same regardless of salt concentration. At a

0.04 wt% loading of SWNTs (P7 and P8), the inclusion of

salt had a negligible effect upon a and 3.

Since a is an absorptive property, a vs. SWNT and/or

CuSO4 wt% loading was plotted to afford Beers law

relationships (Fig. 5). This differs from a typical Beers

law relationship in that a is obtained over the spectral region

of 2502800 nm. Linear least-squares fit and correlation

coefficient for each of the relationships, previously

described, are shown below.

(1) aZ(5.04)(SWNT wt%)K1(SWNT wt%)C0.09 R2Z0.98.

(2) a at constant CuSO4 loadingZ(5.43)(SWNT wt%)K1

(SWNT wt%)C0.10 R2Z0.99.(3) a at constant SWNT loadingZ(1.99)(CuSO4 wt%)

K1

(CuSO4 wt%)C0.23 R2Z0.86.The

effesion applications. The results are shown in Table 3.

addition of CuSO4 at 0.014 wt% (P2) had minimal

cts upon a and 3 compared to P1. The a thoughinte

mishe effect upon a and 3 by nanoadditive inclusion was of

rest since these properties are important for someTThermo-optical properties of nanocompositesFig. 6. Surface and volume resistivities vs. SWNT loading.connectivity whereas P7 exhibited a matted appearance

suggestive of a network. Prior work suggested the percola-

tion threshold necessary for conductivity was between 0.03

and 0.05 wt% SWNT loading [7,12]. Theoretical predic-

tions have placed the percolation level needed for attainingAs seen from the graph, the correlation for the

relationship of a vs. wt% CuSO4 loading (relationship 3)

was not as good as that for the other two relationships and

maybe due to effects associated with the large spectral

region examined.

3.7. Surface and volume resistivity

Due to the issues associated with film handling and

fabrication and the charged orbital environment that many

of the Gossamer mission concepts will be exposed to,

intrinsic electrical conductivity in the material to mitigate

ESC build-up is important. Electrical conductivity was

determined as surface and volume resistivities under

ambient conditions with the results presented in Table 4.

As expected, the pristine material (P1) was insulative. Due

to the low salt loading, P2 and P3 were likewise insulative.

Film P4 containing 0.03 wt% SWNT was insulative, while

P7 containing 0.04 wt% SWNTs exhibited a surface and

volume resistivity of 108 U/square and 1010 U cm, respect-ively. This level of conductivity is sufficient for ESC

mitigation as previously mentioned. These results are

supported by the optical micrographs (Fig. 2) where P4Fig. 7. Surface and volume resistivities vs. CuSO4 loading at 0.03 wt%

SWNT loading.

Aeronautics and Space Administration.

t%)

14

4

P4 0.03

P5 0.03 CuSO4, 0.014

P6 0.03 CuSO4, 0.042

14

lymer 45 (2004) 61336142 6141conductivity at 0.05 vol% [8]. In another report, experi-

mental data using untreated catalytically grown SWNTs in

an epoxy matrix suggested that the electrical percolation

threshold was between 0.0225 and 0.04 wt% [18].

The addition of 0.014 wt% CuSO4 to a mix containing

0.03 wt% SWNTs resulted in a film (P5) exhibiting a

surface and volume resistivity of 108 U/square and1010 U cm, respectively. These values were comparable tothat of P7 that contained 0.04 wt% SWNT and no salt. The

addition of the salt has been proposed to increase the ionic

strength of the matrix [13]. This presumably caused the

SWNT to agglomerate and form a sufficient network to

provide conductivity. The optical micrograph of P5

previously shown (Fig. 2) tends to support this network

formation. Increasing the salt concentration (P6 compared

to P5) afforded one order of magnitude improvement in

surface resistivity. The volume resistivity though remained

unchanged.

When 0.014 wt% CuSO4 was added to a SWNT

composition (P8) that already displayed conductivity (P7),

the surface and volume resistivity remained unchanged.

However, there was more apparent SWNT agglomeration in

P8 as compared to P7 as previously discussed.

The data was plotted vs. SWNT (Fig. 6) and CuSO4(Fig. 7) wt% loading. As seen in Fig. 6, a sharp drop in the

surface and volume resistivities occurred between 0.03 and

0.04 SWNT wt% loading. This implied that the percolation

threshold necessary for conductivity resided between these

two points. In Fig. 7, it can be seen that the addition of a

small amount of salt in a film composition that was just

P7 0.04

P8 0.04 CuSO4, 0.0Table 4

Surface and volume resistivity characterization

Film ID SWNT (wt%) Additive (w

P1

P2 CuSO4, 0.0

P3 CuCl2, 0.01

J.G. Smith Jr et al. / Pobelow the percolation threshold, conductivity was achieved.

No further enhancement in the conductivity was observed

with increased salt loading.

4. Summary

Films with surface and volume resistivities sufficient to

mitigate ESC build-up (1061010 U/square) were preparedby the incorporation of a low loading level of SWNTs in

conjunction with a small amount of inorganic salt. The

inorganic salt was proposed to increase the ionic strength of

the matrix thereby resulting in sufficient network formationReferences

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The use of trade names of manufacturers does not

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Surface resistivity (U/

square)

Volume resistivity (U cm)

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O1012 9.1!1015

O1012 9.9!1014

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