Carbon Nanotube-conductive Additive-space Durable Polymer
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Transcript of Carbon Nanotube-conductive Additive-space Durable Polymer
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Carbon nanotube-conductive add
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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: [email protected] (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
constitute an official endorsement of such products or
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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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