ultrafiltration membrane synthesis by nanoscale templating
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Journal of Membrane Science 198 (2002) 173186
Ultrafiltration membrane synthesis by nanoscale templating ofporous carbon
Michael S. Strano a,1, Andrew L. Zydney a,1, Howard Barth b,Gilber Wooler b, Hans Agarwal a,1, Henry C. Foley c,
a Colburn Laboratory, Department of Chemical Engineering, University of Delaware, Academy Street, Newark, DE 19716, USAb DuPont Company, Central Research and Development, Experimental Station, P.O. Box 80228, Wilmington, DE 19880-0228, USA
c Department of Chemical Engineering, Center for Catalytic Science and Technology,
Pennsylvania State University, University Park, PA 16802, USA
Received 22 March 2001; received in revised form 28 June 2001; accepted 9 July 2001
Abstract
A novel method for producing carbon membranes for ultrafiltration applications is presented using a spray deposition and
pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures on macroporous stainless steel supports. The poly(ethylene
glycol) or PEG employed as a carbonization template creates a mesoporosity that leads to pores in the ultrafiltration range.
Scanning electron microscopy (SEM) shows that the membranes consisted of 12- to 15-m thick carbon films. Gas permeation
and water permeability data were used for the calculation of mean pore sizes, which were found to decrease with decreasingaverage molecular weight of the PEG template. Ultrafiltration of a polydisperse dextran solution was used to quantify the
retention properties of the membranes. Molecular weight cutoffs determined from dextran retention data were shown to vary
with template molecular weight: values of 2 104, 3.5 104, and 6 104 gmol1 dextran were measured for respective
templates of 2000, 3400, and 8000g mol1 PEG. For PEG molecular weights of 2000 or below, the templating effect was
ill defined, membrane film cracking became more prominent, and membrane selectivity and reproducibility were adversely
affected. 2002 Published by Elsevier Science B.V.
Keywords: Ultrafiltration; Carbon membranes; Membrane formation; Inorganic membranes
1. Introduction
Membrane filtration technologies are critical to a
variety of industrial process applications including
cell harvesting, sterile filtration, protein enrichment,
and removal of particulate matter [1]. Ultrafiltration
Corresponding author. Tel.: +1-814-865-2574;
fax: +1-814-865-7846.
E-mail addresses: strano@che.udel.edu (M.S. Strano),
foley@che.udel.edu (H.C. Foley).1 Tel.: +1-302-831-2345; fax: +1-302-831-2085.
in particular, where the membrane retains compo-
nents with kinetic diameters from 1 to 100 nm, haswidespread utility for industrial separation processes.
Traditionally, membranes used for ultrafiltration have
been polymeric in nature. Asymmetric ultrafiltration
membranes are commonly synthesized using phase
inversion, where a polymer solution consisting of
a base and pore-former in a solvent is induced to
form two interdispersed liquid phases. Membranes
synthesized in this manner include the bilayer type
containing slit shaped fissures or cracks [2] and those
that contain plasticizers and are stable while dry [3,4].
0376-7388/02/$ see front matter 2002 Published by Elsevier Science B.V.
P I I : S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 7 4 - 9
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Nomenclature
B0 porous structure factor
(mol ideal gas m
1
)Cb concentration of dextran in
bulk solution (g l1)Cp concentration of dextran in
permeate (g l1)
Cw concentration of dextran at
membrane surface (g l1)dm average membrane pore size from
gas permeation (m)
J flux of gas (mol m2 s1)
Jv filtrate flux during ultrafiltration (m s1)
k solute mass transfer coefficient (m s1)K0 Knudsen permeability
(mol m1 s1 Pa1)
l carbon layer thickness (m)
M molecular weight (g mol1)p average trans-membrane pressure
difference (Pa)
p pressure driving force under gas
permeation (Pa)
rpore cylindrical pore radius (m)
R ideal gas constant (Pa m3 mol1)
Rc rejection coefficient for dextran
ultrafiltrationT temperature of gas permeation (K)
Greek symbols
geometric constant for porous materials
geometric constant for porous materials
proportionality constant in mass
transfer correlation
stagnant film thickness above
membrane in stirred-cell (m) membrane porosity
gas viscosity (Pa s) viscosity of solvent (Pa s)
pore tortuosity
Despite the widespread use of these types of
membrane materials, they suffer from several disad-
vantages. The membranes have limited mechanical
integrity which can lead to deformation during op-
eration and adversely affect membrane performance.
These materials often have limited temperature and
chemical stability, prohibiting their use in applica-
tions involving harsh organic solvents or at elevated
temperature [4]. The use of caustic and hypochlorite
solutions as cleaning agents can also lead to chemicaldegradation of the polymeric materials, significantly
reducing the life of these membranes. In addition,
membranes made from hydrophobic polymers like
polysulfone and polyethersulfone must contain a
humectant such as glycerol to avoid problems asso-
ciated with re-wetting, while membranes made from
hydrophilic polymers like cellulose must remain wet
at all times to avoid collapse of the pore structure.
While inorganic ultrafiltration membranes have
attracted increasing attention in recent years [57],
pore size modified carbon membranes have been
relatively unexplored. Yet compared to ceramic and
metallic ultrafiltration membrane materials, carbon
can be considerably less expensive. Nanoporous car-
bon is also a promising material because it is chem-
ically inert under typical processing conditions and
thermally stable at temperatures well above 200 C
[8,9]. Formed from the pyrolysis of carbonizing nat-
ural or synthetic polymeric precursors, this material
is a disordered solid having a pore size approach-
ing molecular dimensions and has been shown to
possess highly shape-selective molecular transport
properties [10]. Poly(furfuryl alcohol) (PFA)-derivednanoporous carbons have a mean pore size about
0.5 nm as measured from N2 and methyl chloride ad-
sorption isotherms [11]. Attempts to use this material
in the synthesis of defect free, micron scale films on
structurally stable macroporous supports have been
successful. Nanoporous carbon membranes have been
fabricated with high selectivity for some gas phase
separations of small molecules [1214].
While such membranes have a mean pore size on
the molecular scale (
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M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173186 175
and 0.28 cm3 g1 for 2000, 3400, and 8000 g mol1
templates, respectively. These results have been used
in the synthesis of various nanoporous carbon cat-
alysts with greatly enhanced effectiveness factorswhen compared to catalysts prepared from native
nanoporous carbon alone [16,17].
Studies involving the use of porous carbon for ul-
trafiltration membrane formation in the literature are
scarce. Schindler and Maier [4] claimed to have syn-
thesized such a membrane through the carbonization
of a pre-existing polymer membrane with identical
pore structure. Although pyrolysis of a polymeric
microfiltration membrane can reasonably produce a
carbon membrane with similar macroporosity, there is
little theoretical or empirical evidence to suggest that
pores in the entire ultrafiltration range (1100 nm)
would be preserved during the carbonization of a
pre-existing film. Additionally, ultrafiltration range
pores were undetectable using the bubble-point test-
ing methodology employed by the authors [4].
In this paper, a novel method for producing carbon
ultrafiltration membranes is presented. Spray deposi-
tion and pyrolysis on macroporous stainless steel of
poly(furfuryl alcohol)/poly(ethylene glycol) mixtures
is the straightforward fabrication technique employed.
The methodology is demonstrated in Fig. 1. As in our
earlier work, the PEG is used as a non-carbonizingtemplate molecule to introduce controllable meso-
porosity into the carbon film. Permeation of a poly-
disperse dextran solution was used to quantify the
Fig. 1. Mesoporous carbon membranes via nanoscale templating.
retention properties of the membranes as a function
of template size. A correlation of partial rejection
coefficients for 90% of the dextran mixture with tem-
plate molecular weight was observed. Membraneswere also characterized using scanning electron
microscopy (SEM) and gas permeation.
2. Experimental
2.1. Membrane synthesis
Carbon based ultrafiltration membranes were syn-
thesized using an adaptation of a spray coating proce-
dure used for creating supported nanoporous carbon
membranes [13]. Furfuryl alcohol resin in the form of
Durez Resin #16,470 (PFA) from Occidental Chem-
ical Corporation diluted in reagent grade acetone
(Aldrich Chemical) was combined with poly(ethylene
glycol) (PEG) and spray deposited upon a macrop-
orous stainless steel support. The PFA resin has a
specific gravity of 1.21 and a viscosity of 200 cP
at 25 C that decreases to about 5 cP at 80 C. PEG
samples of average molecular weights of 1000, 1500,
2000, 3400, and 8000 g mol1 were obtained from
Aldrich Chemical Company.
This PFA/PEG/acetone precursor was prepared bycombining PFA resin with a given average molecular
weight of PEG in a 50/50 wt.% ratio at 70 C and sub-
sequent dilution in acetone forming a stable mixture
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Table 1
Summary of synthesis and performance characteristics for membranes used in this worka
Initial weight
support (g)
Wet weight
support (g)
Net carbon
weight (g)
Pressure
applied (psig)
Water flux
(m s
1
)
Water permeance
(m s
1
Pa
1
)M8000-1-0.021 10.5107 10.6393 0.021 50.5 1.8 106 5.1 1012
M8000-2-0.018 10.8591 10.9768 0.018 54 1.1 105 2.9 1011
M8000-3-0.022 10.4478 10.58 0.022 54 1.8 106 4.7 1012
M1500-1-0.019 10.4659 10.5728 0.019 55 5.9 107 1.5 1012
M3400-1-0.019 10.7327 10.8409 0.0188 75 8.3 107 1.6 1012
M3400-2-0.016 10.286 10.3696 0.0161 75 8.3 107 1.6 1012
M3400-3-0.018 10.5444 10.6481 0.018 75 8.3 107 1.6 1012
M2000-1-0.019 10.6382 10.7455 0.0189 50 2.0 108 5.6 1014
M2000-2-0.019 10.4353 10.534 0.019 50 1.3 107 3.8 1013
M2000-3-0.019 10.3555 10.4623 0.019 50 8.8 108 2.5 1013
M2000-4-0.019 10.6816 10.7785 0.0189 50 1.0 107 3.0 1013
M1000-1-0.029 10.5297 10.6695 0.0288 50 1.7 107 5.0 1013
M1000-2-0.023 10.7057 10.8189 0.0227 50 5.7 108 1.6 1013
a Labels are as follows: M(PEG MW)-(sample #)-(carbon mass (in g)).
at room temperature with a viscosity near 5 cP. The
resulting solution was subsequently spray deposited
onto circular porous stainless steel supports (0.2m
pore size, 11.4 cm2 area, and 1 mm thickness) sup-
plied by Mott Metallurgical Co. Before spray coating
the stainless steel supports were cleaned by sonica-
tion in acetone. After spray coating approximately
200 mg of the polymer blend solution (wet weight)was deposited on the support. The resulting system
was subsequently pyrolyzed in a stream of flowing
He (50sccm) at 5 Cmin1 to 600 C, held for 2h
at this temperature, and then allowed to cool to room
temperature. The final carbon mass on each support
was approximately 20 mg. Table 1 presents data on
the membranes used in this work. Membrane labels
contain the PEG template molecular weight and total
coat mass of carbon for each sample.
2.2. Scanning electron microscopy
SEM was used to characterize the internal mor-
phology of the membranes. Membrane cross-sections
were cut orthogonal to the support surface using a
diamond-wafering saw. These sections were mounted
in an epoxy resin, polished, and given a coating of
Au for imaging with a Hitachi S-4000 field emis-
sion scanning electron microscope. Imaging was
performed on areas of these cross-sections external
to and within the macroporosity of the stainless steel
support and at various radii from the center of the
disk-shaped membranes.
2.3. Characterization using gas permeation
Gas phase transport of He, Ar, N2, O2, SF6 and CO2was used to characterize the selective porosity and
integrity of the carbon membrane. The disk-shapedmembranes were sealed using VitonTM gaskets into a
stainless steel module setup to measure the transport
of a single gas through the membrane. The pure com-
ponent flux of each gas was measured as a function
of pressure. Details concerning the experimental setup
and technique can be found in [18].
2.4. Ultrafiltration of polydisperse dextrans
Ultrafiltration of polydisperse dextrans was used to
characterize the retention properties of the templatedcarbon films. Dextrans having average molecular
weights of 2 106, 1.7 105, 7.0 104, 3.9 104,
9.9 103 gmol1 (Pharmacia) were obtained from
Aldrich Chemical Company. The dextrans were added
in equal mass ratios to create a 10 g l1 solution in a
phosphate buffer prepared from monobasic (NaHPO4)
and dibasic (NaH2PO4) sodium phosphate to achieve
a solution pH of 7.40.3. When applicable, 1 wt.% of
methanol was added to prevent bacterial contamina-
tion.
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Retentates and permeates were analyzed using size
exclusion chromatography (SEC) with a model 1050
HP chromatograph (Hewlett Packard) and a Waters
410 differential refractometer. SEC analysis wasperformed using a 7.5-mm i.d. 30-cm TSK G4000
SW column (Tosoh Corp.), thermostated at 30 C,
with 0.1 M sodium sulfate used as the mobile phase
at a flow rate of 0.6 ml min1. Samples were injected
with an autosampler without additional dilution using
a 100-l sample loop. The above mentioned dex-
trans were injected individually at a concentration of
1mgml1, and retention volume data were used to
construct a molecular weight calibration curve. Data
was processed using Waters Millennium 32 HPLC
software.
Membrane separation was carried out using a
dead end filtration setup. A 50 ml volume of dextran
solution was loaded above the carbon membrane in
a 30 mm diameter Amicon stirred-cell. An attached
Ar line supplied gas pressure to the solution above
the membrane. The permeate was collected in 30 ml
vials, with the total permeate mass weighed contin-
uously by placing the vial directly on a laboratory
balance. From this time series data mass flow through
the membrane was computed. A series of permeate
and retentate samples were collected over time for
off-line dextran analysis.
2.5. Batch depletion dextran adsorption
Granular carbon membrane samples were prepared
using the identical procedure described above for
making thin carbon films, except that the solution
was poured into a quartz boat and pyrolyzed as a
bulk solution. These samples were analyzed for total
dextran adsorption capacity using a batch depletion
method. Each carbon was ground to 60/140 mesh
and suspended in a water solution. After settling, theremaining solution was decanted to remove fine parti-
cles. The process was repeated until all fine particles
were removed. A mass of 0.5 g of carbon was sealed
with 30 ml of the polydisperse dextran solution in
a Pyrex vial as described above. Sealed vials were
vigorously agitated for 32 h, after which the solutions
were passed through a 1m filter and analyzed using
SEC as described above. The average concentration
of dextran remaining in the solution above the carbon
was integrated over the molecular weight distribu-
tion, with the results compared to the corresponding
integral for the original solution.
3. Results and discussion
Table 1 lists the initial support weight, the weight
after spray coating (wet), and the weight after car-
bonization (dry) for each membrane studied in this
work. All samples except M8000-3-0.022 were given
a single coat and thermal treatment. This particular
sample was fabricated in two coating/carbonization
cycles. During the carbonization process, the evolu-
tion of gases reduced the polymer mass on the support
leaving behind the solid carbon film. For poly(furfuryl
alcohol) carbonization at 600 C, the carbon yield pro-duced in this way is approximately 2530% for sim-
ilar thin films [14]. The PEG template is expected to
produce very little residual carbon as seen in the syn-
thesis of mesoporous catalyst supports (
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Fig. 3. Water permeance through carbon membranes as a functionof template molecular weight.
the entire template being emitted as gaseous prod-
ucts. This suggests that volatilization of the template
does not cause any significant loss of poly(furfuryl
alcohol) prior to carbonization.
It is noteworthy that a similar trend is observed for
the water permeability (Fig. 3) plotted as a function of
Fig. 4. Scanning electron micrograph of a porous carbon ultrafiltration membrane (M8000-1-0.021) cross-section showing external carbon
layer and stainless steel support.
template size although there is considerable variabil-
ity between samples formed from the same template
size. Generally, water permeability decreases with
decreasing template size and this may correspond toa decrease in membrane pore size and porosity as dis-
cussed below. In Table 1, the values for the hydraulic
permeabilities range from 1014 to 1011 m s1 Pa1,
which are considerably lower than commercial ul-
trafiltration membranes (typical values are around
1010 m s1 Pa1. These reduced permeabilities are
attributable to both the much greater thickness of the
membranes and the decrease in the film porosity that
occurs with decreasing template size. These factors
are discussed in detail below.
3.1. Scanning electron microscopy
Fig. 4 is a scanning electron micrograph of a
cross-section of the external porous carbon layer and
the stainless steel support for a membrane synthesized
using a 50% mixture of PFA: 8000 g mol1 PEG at
600 C. The demarcation between the stainless steel
support and external carbon layer is clearly visible,
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M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173186 179
and there are also localized along with localized sep-
arations or cracks between the two layers. These film
separations apparently have a negligible impact on
coating integrity and mechanical stability, as therewas no observed carbon mass loss during testing or
property variations after repeated testing. The com-
bination of the diamond saw cutting and surface
polishing of the SEM sample has a smoothing affect
on the morphology of the membrane cross-sections.
The actual porosity of the steel support is roughly
0.6 as reported by the manufacturer. The carbon layer
is 12- to 15-m thick and relatively uniform over
the membrane surface, consistent with observations
of similarly prepared carbon films for gas separation
[14]. The calculated thickness range based on the
mass of carbon deposited (Table 1) and assuming
a porous carbon density of 1.6 g cm3 is 915m,
which agrees well with this measurement.
Fig. 5 is a 500 nm scale micrograph of the mi-
crostructure of the PFA/PEG carbon taken from
the material external to the support surface. This
cross-section was prepared by fracturing the carbon
surface while attached to the support using a scalpel.
Fig. 5. Scanning electron micrograph of the pore structure of the carbon ultrafiltration membrane showing grain boundaries between
nanoporous carbon domains (M8000-1-0.021).
The microstructure suggests pores with diameters in
the nanometer range. As expected from the inferred
hypothesis for pore formation, these pores are in the
form of fissures and voids between purely nanoporouscarbon domains. Porous carbon was also observed to
be present on the opposite side of the support (the
side opposite from the initial deposition) and may
fill some portion of the macroporosity of the sup-
port. During pyrolysis the polymer blend drops in
viscosity prior to carbonization, making deeper pore
penetration more facile.
3.2. Porosity characterization by gas permeation
Fig. 6(a) is a plot of the flux versus pressure for
several gases through an uncoated stainless steel
0.2m support. The data show both a high rate of
permeation and low gas selectivity. A nanoporous
carbon film pyrolized in the absence of polymer tem-
plate shows a linear relationship between the flux and
applied pressure for He, O2, N2 and Ar (Fig. 6(b)).
This permeation behavior is typical of PFA derived
carbon membranes [18]. Fig. 7 is characteristic of gas
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Fig. 6. Flux as a function of driving force pressure for: (a) uncoated stainless steel support; (b) 0% PEG/PFA carbon membrane carbonized
at 600 C; (c) 50% 8000 amu PEG/PFA templated membrane carbonized at 600 C.
permeation through a templated carbon membrane
(50% PFA/8000 g mol
1 PEG) showing a quadraticdependence of the flux versus pressure and Knudsen
gas selectivity at low pressure. This behavior is
Fig. 7. Ultrafiltration membrane (M3400-1-0.019) gas permeability
as a function of trans-membrane pressure for mean pore size
characterization.
observed on all PEG/PFA carbon membranes regard-
less of coating deposition mass or template size emplo-yed. Hence, the permeation results cannot necessarily
be attributed to partially coated or partially formed
nanoporous films such as that shown in Fig. 6(b).
For a membrane with a mean pore size between
100 and 1 nm, the pore dimension can be estimated
by examining data for the membrane gas perme-
ability versus trans-membrane pressure. The linear
relationship yields an intercept equal to the Knud-
sen permeability with slope inversely proportional to
the gas viscosity (Fig. 7). The gas flux through the
membrane is written as
J=
K0 +B0
p
p
l(1)
The structure factor B0 is proportional to the square
of the mean pore size
B0 =
2d2m (2)
with = 2.5 for consolidated media [20]. The
Knudsen permeability K0 can be written as
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M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173186 181
Table 2
Mean pore sizes of membranes measured by gas permeation
dm (m)
PEG1500 PEG3400 PEG8000
He 9.92 109 1.19 108
N2 1.03 108 1.28 108 2.83 108
Ar 1.37 108 6.81 109 2.59 108
O2 9.15 109 2.39 108 2.21 108
CO2 1.43 108 1.22 108 2.87 108
SF6 1.13 108 1.10 108
Mean 1.15 108 1.31 108 2.63 108
S.D. 2.10 109 5.69 109 3.05 109
K0 =4
32 dm8RTM (3)
where is 0.8 for consolidated media [20]. The gas
permeation data can be used to evaluate both B0 and
K0, with the ratio of these two parameters used to
evaluate the mean pore size of the membrane, dmwithout any assumptions regarding the membrane
porosity, or tortuosity factor, . Here, we do assume
that the pore size of the carbon film is significantly
less than that of the support (i.e.
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Fig. 8. Observed (dotted line) and actual (solid line) partial rejection coefficients vs. dextran molecular weight for carbon membranes
synthesized from poly(ethylene glycol) template: (a) 1000 MW PEG; (b) 1500 MW PEG; (c) 2000 MW PEG; (d) 3400 MW PEG; (e)
8000 MW PEG.
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M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173186 183
as a function of dextran molecular weight for mem-
branes synthesized using the five different template
PEG sizes. The results for 1000 MW PEG shown
in Fig. 8(a) demonstrate a lack of dextran retentionwith respect to molecular weight indicating that large,
non-selective pores exist across the film. The polar-
ization correction reveals that the broad, non-selective
behavior of the membrane cannot be attributed to po-
larization, but rather that the membrane is defective.
As the template size tends toward zero, the carbon
membrane behaves as an untemplated film with a sig-
nificant tendency towards cracking. It has been well
established that furfuryl alcohol resin-based mem-
branes require several coatings before a crack-free
layer can be constructed [12,14]. The curves for
1500 (Fig. 8(b)) and 2000 PEG templates (Fig. 8(c))
show an increase in membrane selectivity. However,
a tendency towards film cracking is retained and
membrane reproducibility is affected. Fig. 8(c) in
particular shows a lack of reproducibility for four
membranes synthesized at the same carbon deposition
mass. Membrane M2000-4-0.019 (Fig. 8(c)) shows a
significantly enhanced retention compared to those in
Fig. 8(a), which may represent the properties of the
film in the absence of any significant cracking. Such
retention could not be attained using a 1000 MW pre-
cursor. The templating of nanoporous carbon usingPEG has been observed to yield accessible meso-
porosity only at molecular weights of 2000 g mol1
and higher for granular materials [15], although
this study represents the most comprehensive work
undertaken to determine the exact threshold.
Problems of film cracking and reproducibility
diminish for higher molecular weight templates as
transport through these films becomes dominated by
that which takes place through the porosity created via
the templates rather than through adventitious cracks
and fissures. Fig. 8(d) and (e) show reproduciblebehavior for 3400 and 8000 MW PEG templated
membranes, respectively. Retention properties tend to
increase from those seen in Fig. 8(b) and (c) and are
apparently independent of carbon thickness and water
permeance as expected. Membrane M8000-3-0.022 is
the result of two successive coating and carbonization
cycles and shows an increase in retention behavior
likely from a reduction in pore size after the initial
coat. Here, the pore structure of the first coat may be-
come impregnated with a second coating of precursor
Fig. 9. Dextran R90 value and calculated pore size as a function
of template molecular weight of carbon membrane.
material that modifies the retention characteristics
as shown. This procedure, which is analogous to a
slip casting design, may in fact provide a route to
the synthesis of ultra-thin nanofiltration membranes.
With successive layer deposition, the retentive prop-
erties decrease with only a marginal increase in film
thickness and deposition mass.
In Fig. 9, the molecular weight of dextran corre-sponding to 90% rejection is plotted as a function of
the molecular weight of the PEG template. This R90value is obtained by interpolation or extrapolation of
the data in Fig. 8(a)(e) as needed. The extrapolated
value for the 1000 g mol1 template determined from
Fig. 8(a) is actually an underestimate of this molecular
weight. As observed above, the analysis suggests that
these films possess large cracks and defects that are
non-retentive. The trend shows an increase in dextranR90 with increasing PEG molecular weight starting
from the 2000 MW PEG template size. Below thiscutoff, the selectivity decreases sharply with decreas-
ing template size suggesting the formation of cracks
and defects. Fig. 9 also plots the calculated pore size
based upon a simple partitioning model assuming a
cylindrical pore structure and using the Stokes radius
of the dextran molecule for the corresponding R90molecular weight [1]
Rc = 1
1rdextran
rpore
2(6)
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184 M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173186
Pore sizes estimated in this way for the membranes
formed from the larger PEG templates are generally
smaller than those calculated from gas permeability
data. For example, the values of 15 and 12.4 nm cor-responding to 8000 and 3400 molecular weight PEG
compare to 26.3 6.1 and 13.1 11.4 nm calculated
from gas permeation, respectively. In contrast, the
pore size calculated from the dextran data for the
1500 PEG membrane was 28.2 nm, which is signifi-
cantly greater than the 11.54.2 nm determined from
gas permeation. This large discrepancy is due to the
greater effect of the pore defects on dextran transport
than on the gas permeation results.
The water permeability can also be used to provide
an estimate of the average pore size or porosity using
the HagenPoiseuille equation. Here, the membrane
porosity/tortuosity ratio, /, can be expressed as a
function of the pore size and water permeability Lpassuming a cylindrical pore structure [1]
=
8lLp
(rpore)2(7)
Using pore sizes from dextran rejection data (Eq. (6)),
application of Eq. (7) predicts that the ratio of porosity
to tortuosity decreases with decreasing template size.
Average values are 0.024, 0.005, 0.001 and 0.001 for
8000, 3400, 2000, and 1000 g mol1 template. Thisdecrease in / is consistent with the greater carbon
yield, and thus, lower porosity, of the membranes
formed using the smaller PEG templates.
Results of the batch depletion of dextran over gran-
ular templated carbons yield relative values of the
accessible pore volumes of the various carbons. This
allows for a comparison between properties resulting
from the templating process and those resulting from
pores developed as a result of film cracking. Fig. 10
is a plot of relative dextran amount adsorbed versus
PEG molecular weight used during carbon synthesis.The results show negligible uptake at values of 2000
MW PEG and below with substantial increases in
accessible area at larger template molecular weights
(sizes). The data suggest a maximum in adsorption
uptake near 8000 MW PEG. At 18,500 MW PEG, the
extent of adsorption falls to values below that of the
3400 MW PEG. The initial portion of the curve agrees
well with previous gas adsorption characterization,
where it has also been noted that accessibility of the
mesopore distribution becomes hampered for lower
Fig. 10. Dextran uptake from batch depletion on granular carbons
synthesized using templates of 30018,500 g mol1 of PEG.
template molecular weights [15]. It is this lack of ac-
cessibility that may be responsible for the trapping
of template material within the carbonizing system
and hence the increase in carbon yield at low template
molecular weights (Fig. 2). This retention of material
would explain the observed decrease in membrane
porosity. In Fig. 10, the subsequent decrease in capac-
ity with 18,500 PEG may be the effect of a decrease inthe rate of pyrolysis of this larger polymeric template.
The subsequent decrease in the rate of evolution of the
pyrolysis gases could also lead to a retention of tem-
plate carbon within the forming solid, as in the case
of the diffusion limitations associated with smaller
template sizes.
The internal pore volume of membranes made from
the smaller templates is largely inaccessible, leading
to a small amount of dextran uptake. As the template
size increases, the accessibility of the pore volume
increases, but at the expense of an increasing aver-age pore radius, which ultimately reduces the internal
surface area.
The emerging picture of the actual templating
process from the above observations is one that is
kinetically driven by the diffusion of template decom-
position products from the forming solid. At small
template pore sizes (
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M.S. Strano et al. / Journal of Membrane Science 198 (2002) 173186 185
resistance is exceedingly high. These diffusion lim-
itations increase carbon yield and decrease carbon
porosity as observed. The lack of mesopore intercon-
nection also means that the membrane behaves as apurely nanoporous thin film, and hence is subject to
cracking and the formation of defects after a single
coating. As the template size increases, this diffusion
length scale increases and membrane properties be-
come a function of the templated carbon itself with a
subsequent improvement in retention properties and
reproducibility.
The results presented here underscore several
potential improvements that can be made through
modification of the synthesis procedure. The most
notable of these is improvement in water perme-
ability by decreasing carbon film thickness. The
spray deposition on macroporous support was readily
adapted from an analogous method of gas-separation
membrane fabrication [15], although this may not be
the optimal system for liquid separation applications.
The results of microscopy and water permeation mea-
surements suggest that decreasing the carbon deposi-
tion mass while maintaining coating uniformity can
accomplish this goal. Additionally, this work presents
indirect evidence that lower molecular weight tem-
plates have a discernable affect on pore structure and
may not be entirely flashed off during carbonizationas originally proposed [17]. This warrants a more
complete investigation of the templating process with
a particular emphasis on the composition and molec-
ular weight dependence of the template as well as the
pyrolysis ramp rate during carbonization.
4. Conclusions
A novel method for producing ultra and nanofil-
tration carbon membranes is presented using spraydeposition on macroporous stainless steel and pyrol-
ysis of poly(furfuryl alcohol)/poly(ethylene glycol)
mixtures. Membranes characterized by SEM show
12- to 15-m thick carbon films. Pores in the ultrafil-
tration range clearly vary with the average molecular
weight of the template for PEG with molecular weight
of 2000 g mol1 and above. Below this threshold, the
film behaves similar to an untemplated carbon mem-
brane with crack formation a significant factor for the
single coated membranes. Transport of a polydisperse
dextran solution was used to quantify the retention
properties of the membranes. Partial rejection coef-
ficients for 90% of the dextran mixture were shown
to vary with template molecular weight: 90% cutoffsof 2 104, 3.5 104, 6 104 gmol1 of dextran
were measured for 2000, 3400, and 8000 g mol1 of
template employed, respectively. Although generally
of less accuracy, the mean pore sizes as measured
from gas permeation data also demonstrate a similar
decrease in value with decreasing template size.
Acknowledgements
Support for this research was provided by the
Department of Energy Office of Basic Energy Science
and the DuPont Co. Michael Strano is grateful for fi-
nancial support in the form of a Presidential Graduate
Fellowship from the University of Delaware.
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