Charge-transfer interaction between poly(9-vinylcarbazole) and 3,5-dinitrobenzamido group or 3-nitrobenzamido group

Article abstract of DOI:10.1021/la9030409

We studied the charge-transfer (CT) interaction between poly(9-vinylcarbazole) (PVK) and 3,5-dinitrobenzamido (DBA) group or 3-nitrobenzamido (NBA) group. The complexation equilibrium constant of PVK and N-butyl-3,5dinitrobenzamide was found to be larger

Full text of DOI:10.1021/la9030409

pubs.acs.org/Langmuir
© 2009 American Chemical Society
Charge-Transfer Interaction between Poly(9-vinylcarbazole) and
3,5-Dinitrobenzamido Group or 3-Nitrobenzamido Group
Ying Yu,† Yuan Yao,‡ Liyan Wang,*,† and Zesheng Li‡
†State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China,
and ‡Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology,
Harbin 150080, China
Received August 15, 2009. Revised Manuscript Received September 30, 2009
We studied the charge-transfer (CT) interaction between poly(9-vinylcarbazole) (PVK) and 3,5-dinitrobenzamido
(DBA) group or 3-nitrobenzamido (NBA) group. The complexation equilibrium constant of PVK and N-butyl-3,5-
dinitrobenzamide was found to be larger than that of PVK and N-butyl-3-nitrobenzamide using UV-vis spectroscopy.
The desorption process of PVK from a DBA-modified substrate was studied with single molecule force spectroscopy. A
lot of force-extension curves with a plateau were observed, which indicated the existence of trainlike conformation. The
average desorption force of a PVK chain from the DBA-modified substrate was 28 pN. For comparison, the desorption
process of PVK from an NBA-modified substrate was also studied, and an average desorption force of 18 pN was
obtained. The apparent interaction energy of a CT complex was calculated on the basis of the desorption force and the
surface density of electron acceptors. The obtained apparent interaction energies were 14 and 8 kJ mol-1 for the
3
carbazolyl-DBA complex and carbazolyl-NBA complex, respectively. On the basis of the energy of the frontier
molecular orbital of carbazole, 3,5-dinitrobenzamide, and 3-nitrobenzamide, the difference in strength between the
interactions was interpreted as the influence of the substituent group on the energy of the frontier molecular orbital.
Introduction
technique for investigating nanomechanical properties and ad-
sorption processes of single polymer chains.12-18 Different types
of interactions, for example, electrostatic19 or hydrogen-bond-
ing,20 that determined adsorption processes of polymers have
been characterized by SMFS. These research studies provide
valuable information about the nature of noncovalent interac-
tions and facilitate the control of polymer layer-by-layer
assembly.19-21 Besides polymer adsorption, SMFS is effective
for detecting noncovalent interaction between small molecules,
such as molecular recognition,22,23 CT interaction,24,25 multiple
hydrogen bonding,26 and π-π stacking,27 through measuring
rupture forces.
Polymer adsorption at the liquid-solid interface is the most
important step for the layer-by-layer (LbL) assembling technique,
which has proved to be a versatile method for fabricating a
multilayer film with controlled composition and architecture.1-6
The driving force for LbL assembly has been extended from
electrostatic force7 to other secondary intermolecular inter-
actions.8-11 For example, LbL assembly via hydrogen bonding
was developed by Stockton and Rubner8 and Wang and co-
workers,9 separately. Yamamoto et al. reported LbL assembly
based on the charge-transfer (CT) interaction between the dini-
trobenzoyl group and carbazolyl group.11 Polymer adsorption
based on electrostatic force, hydrogen bonding, and so on is
already studied by many kinds of methods, while research on
polymer adsorption based on CT interaction has not been
reported yet.
(12) Rief, M.; Gautel, M.; Oesterhelt, F.; Fernandez, J. M.; Gaub, H. E. Science
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G. T. Macromol. Rapid Commun. 1998, 19, 609.
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Langmuir 1999, 15, 2120.
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W. K.; Zhang, X. Prog. Polym. Sci. 2003, 28, 1271.
Single molecule force spectroscopy (SMFS), which is based on
atomic force microscopy (AFM), has proved to be a powerful
*Corresponding author. Telephone: þ86-431-85168479. Fax: þ86-431-
85193421. E-mail: wangliyan@vip.sina.com.
(1) Decher, G.; Schlenoff, J. B. Multilayer Thin Films - Sequential Assembly of
Nanocomposite Materials; Wiley-VCH: Weinheim, Germany, 2003.
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Chem. Commun. 1994, 1055.
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2005, 44, 2723.
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Seitz, M. Macromolecules 2001, 34, 1039.
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S2369.
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(8) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717.
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Macromol. Rapid Commun. 1997, 18, 509. (b) Wang, L. Y.; Cui, S. X.; Wang, Z. Q.;
Zhang, X. Langmuir 2000, 16, 10490.
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Langmuir 2010, 26(5), 3275–3279
Published on Web 11/17/2009
DOI: 10.1021/la9030409 3275

Article
Yu et al.
Scheme 1. Illustration of Single Chain Stretching and Magnified
View of Part of a PVK Chain Adsorbed in Trainlike Conformation at
the Quartz Surface Modified with DBA Groups
withthe solventmixture dichloromethane/methanol =10:1asthe
eluent. The product, BNBA, was a light yellow solid with a yield
of 51%. 1H NMR (500 MHz, CDCl3): δ 8.57 (1H, s, ArH), 8.36
(1H, d, ArH), 8.16 (1H, d, ArH), 7.65 (1H, t, ArH), 6.22 (1H, s,
NH) 3.50 (2H, q, NCH2), 1.64 (2H, m, CH2), 1.44 (2H, m, CH2),
0.98 (3H, t, CH3).
Substrate Modification and SMFS Experiment. A quartz
slide covered with electron acceptors (Scheme 1) was prepared as
follows. A quartz slide was cleaned with a hot “piranha” solution
(H2SO4/30% H2O2 = 7:3, (v/v)), followed by ultrasonic cleaning
thoroughly with deionized water and drying overnight in oven.
(Caution! Piranha solution is highly corrosive. Extreme care should
be taken when handling piranha solution, and only small quantities
should be prepared.) Then the slide was silanized in the xylene
solution of APS (0.1vol %) for 20 h and rinsed with benzene and
dichloromethane successively. The self-assembly of APS led to an
amino-covered quartz slide.28 The slide was immersed into 0.1 M
xylene solution of DBC (or NBC) for 12 h and then ultrasonic
cleaned thoroughly with benzene and dichloromethane succes-
sively. After these modifications, the slide showed a characteristic
UV-vis absorbance of the DBA (or 3-nitrobenzamido, NBA)
group, indicating that the surface of the quartz slide was modified
with plenty of electron acceptors. A time of 10 h proved enough
for the modification to reach saturation. The freshly modified
quartz slides were used in SMFS experiments.
A home-built SMFS instrument was set up in cooperation with
€
Prof. Hermann E. Gaub in Ludwig-Maximilians-Universitat
Munchen. SMFS experiments were carried out in a particular
€
way as reported before.29 The spring constants of Si3N4 tips
(Veeco, Santa Barbara, CA) ranged between 20 and 35 pN nm-1
,
3
which were measured before each experiment using a thermal
noise method.30 The details of the SMFS experiment were
described elsewhere.14,17 Briefly, PVK chains were adsorbed onto
the tip from the dichloromethane solution, the concentration of
Herein, inspired by Yamamoto’s CT-based LbL assembly, we
investigated CT interaction between poly(9-vinylcarbazole)
(PVK) and 3,5-dinitrobenzamido (DBA) groups, aiming for
obtaining direct information of polymer adsorption based on
CT interaction. SMFS was used to study the desorption of a PVK
chain from a substrate modified with DBA groups (Scheme 1).
For comparison, we also studied the desorption of PVK from 3-
nitrobenzamido (NBA)-modified substrate, which bore one nitro
group instead of two nitro groups. The influence of substituent on
CT interaction was discussed on the basis of results from several
different approaches.
which in repeat units was 1.27 ꢀ 10-5 mol L-1. A substrate
3
modified with DBA was placed on the piezo of the SMFS
instrument. Then the modified substrate was brought to contact
the PVK-adsorbed tip and some polymer chains would adsorb
onto the substrate. A contacting time of 1 s for the tip and the
substrate proved enough to achieve the equilibrium of polymer
adsorption based on CT interaction. During the separation of the
tip and the substrate, PVK was desorbed from the substrate and
the cantileverwas deflected. At thesame time, a deflection-exten-
sion curve was recorded and converted into a force-extension
curve. All the force-extension curves were obtained in dichlor-
omethane.
UV-Vis Absorption Spectroscopy and the Calculation of
Interaction Energy. UV-vis absorption spectroscopy was used
to determine the complexation equilibrium constant of BDBA
and PVK in dichloromethane. All the UV-vis measurements
were carried out on a Lambda 800 UV/vis spectrometer, Perkin-
Elmer Instrument.
Experimental Section
Materials. Poly(9-vinylcarbazole) (PVK, Mw ∼ 1.1 ꢀ 106 g
3
mol-1, PDI 3.04) and 3-nitrobenzoyl chloride (NBC, 98%) were
purchased from Aldrich. 3,5-Dinitrobenzoyl chloride (DBC,
98%) was purchased from Fluka. 3-Aminopropyldimethyl-
methoxysilane (APS, 97%) was purchased from Acros.
UV-vis absorption spectroscopy was also used to characterize
the modified quartz slides and to calculate the surface density of
DBA groups. UV-vis spectra of solutions of BDBA with differ-
ent concentrations were measured, and the molar absorptivity of
BDBAat250nmwascalculated. Sincetheabsorption ofdichloro-
methane between 230 and 260 nm was strong, the mixture of
dichloromethane and cyclohexane (v/v = 5:8) was used as
solvent. In addition, we measured UV-vis spectra of several
DBA-modified substrates. It was assumed that the molar absorp-
tivity of DBA groups at the surface of the substrate was approxi-
mately equal to the molar absorptivity of BDBA in solution. The
N-Butyl-3,5-dinitrobenzamide (BDBA) was prepared through
a condensation reaction of 1-butylamine and DBC. 1-Butylamine
(7 mmol, 0.7 mL) was dissolved in 10 mL of xylene, and DBC (2
mmol, 0.53 g) dissolved in 20 mL of xylene was added dropwise.
Excessive 1-butylamine was used to remove HCl generated in the
condensation. The reaction mixture was stirred at room tempera-
ture for 2 h. The reaction mixture was washed with water three
times and then was evaporated to remove xylene to get the raw
product. The raw product was purified via silica gel column
chromatography with the solvent mixture cyclohexane/ethyl acet-
ate = 2:1 as the eluent. The product, BDBA, was a yellow solid
with a yield of 60%. 1H NMR (500 MHz, CDCl3): δ 9.17 (1H, s,
ArH), 8.95 (2H, d, ArH), 6.22 (1H, s, NH), 3.54 (2H, q, NCH2),
1.67 (2H, m, CH2), 1.45 (2H, m, CH2), 0.99 (3H, t, CH3).
surface density was calculated by dS = 0.5A250 250
-1, where dS is
ε
(28) Haller, I. J. Am. Chem. Soc. 1978, 100, 8050.
(29) Zhang, W. K.; Zou, S.; Wang, C.; Zhang, X. J. Phys. Chem. B 2000, 104,
10258.
(30) (a) Butt, H. J.; Jaschke, M. Nanotechnology 1995, 6, 1. (b) Florin, E. L.;
Ludwig, M.; Rief, M.; Gaub, H. E. Biosens. Bioelectron. 1995, 10, 895.
N-Butyl-3-nitrobenzamide (BNBA) was prepared through a
similar condensation reaction using NBC instead of DBC. The
raw product was purified via silica gel column chromatography
3276 DOI: 10.1021/la9030409
Langmuir 2010, 26(5), 3275–3279

Yu et al.
Article
Figure 3. UV-vis spectra for dichloromethane solutions of PVK,
BNBA, and their mixtures in different concentrations. The con-
centrations of PVK solutions are described in repeating units.
Figure 1. UV-vis spectra for dichloromethane solutions of PVK,
BDBA, and their mixtures in different concentrations. The con-
centrations of PVK solutions are described in repeating units.
Figure 4. Plot of lCBNBA/A421CT versus the inverse concentration
of PVK. A linear fit of the data yields a slope of (6.0 ( 0.5) ꢀ 10-2
cm mol2 L-2 and a y-axis intercept of (8.4 ( 0.5) ꢀ 10-2
Figure 2. Plot of lCBDBA/A460CT versus the inverse concentration
of PVK. Following the method of Benesi and Hildebrand,31 the
absorbance of the CT complex can be related to the concentration
of carbazolyl groups by lCB/AλCT = 1/ελ þ 1/ελKeqCP, when CB ,
CP. ελ is the molar absorptivity of the complex at the selected
wavelength, l is the path length, and CB and CP are the initial
concentrations of BDBA and PVK, respectively. The absorbance
oftheCTcomplexattheselectedwavelength,AλCT, iscalculatedby
subtracting the absorbance of PVK and that of BDBA from the
absorbance of the mixed solution of PVK and BDBA. Keq is the
equilibrium constant, Keq = [complex]eq/[BDBA]eq[PVK]eq. A
linear fit of the data yields a slope of (9.6 ( 0.5) ꢀ 10-3
cm mol2 L-2 and a y-axis intercept of (3.5 ( 0.5) ꢀ 10-2
3
3
3
cm mol L-1, which gives ε421 = 12 ( 4 cm-1 L mol-1 and
3
3 3
K
eq = 1.4 ( 0.4 L mol-1
.
3
in the range from 400 to 500 nm when they exist separately in
solution, while the spectra of mixtures show strong absorption,
and the absorbance is much stronger than the sum of the
absorbance of pure PVK and BDBA at the same concentration.
The increase of absorbance in the range between 400 and 500 nm
indicates the formation of the CT complex. The series of spectra
of BDBA mixed with different concentrations of PVK allow us to
calculate the equilibrium constant, Keq, of the CT complexation
of BDBA and PVK, using the Benesi-Hildebrand function,31 as
shown in Figure 2. In this way, Keq for the BDBA-PVK complex
3
3
3
cm mol L-1, whichgivesε460 = 29( 4 cm-1 L mol-1 andKeq
=
3
3 3
3.7 ( 0.4 L mol-1. The results obtained at different wavelengths
are close to each other (Keq calculated from the data of 450 and
3
470 nm are 3.5 ( 0.3 and 3.8 ( 0.4 L mol-1, respectively).
is determined to be 3.7 ( 0.4 L mol-1
.
3
3
Similarly, we studied the CT interaction between BNBA and
PVK in dichloromethane, intending to study the effect of the
substituent group on CT interaction. Figure 3 shows UV-vis
absorption spectra of PVK, BNBA, and their mixtures in differ-
ent concentrations. The increase of absorbance in the range
between 400 and 500 nm in the spectra of PVK-BNBA mixtures
indicates the formation of a CT complex. Using the Benesi-
Hildebrand function, Keq for the BNBA-PVK complex is
the surface density, A250 is the absorbance of the modified
substrate at 250 nm, and ε250 is the corresponding molar absorp-
tivity of the acceptor in solution.10 The linear density, dL, was
calculated by dL = dS0.5, using a simple square packing. Then the
interaction energywas calculated byE = NAFdL-1, where E is the
interaction energy, NA is the Avogadro constant, and F is the
average desorption force.
Theoretical Calculation. The frontier molecular orbitals of
model compounds were calculated at the B3LYP/6-31G(d, p)
level using the Gaussian program.
determined to be 1.4 ( 0.4 L mol-1, as shown in Figure 4. It is
3
clear that Keq for the BDBA-PVK complex is larger than that for
BNBA and PVK, indicating the CT interaction between BDBA
and PVK is stronger. We wonder if the difference in strength
between the interactions can be probed at the molecular level by
SMFS.
Results and Discussion
BDBA was synthesized and had the same electron-acceptor
group that Yamamoto et al. used in LbL assembly.11 BNBA was
also synthesized for investigating the influence of the substituent
of the electron acceptor on the CT interaction.
CT interaction between BDBA and PVK was studied in
dichloromethane, which was exactly the same solvent that Ya-
mamoto et al. used in LbL assembly.11 Figure 1 shows UV-vis
absorption spectra of PVK, BDBA, and their mixtures in differ-
ent concentrations. Both PVK and BDBA have weak absorption
We utilized SMFS to study the adsorption of PVK at the
surface modified byDBA groups. Typical force-extension curves
are shown in Figure 5. A long plateau is the common feature in
these curves. Curves with a similar shape induced by other kinds
of interaction instead of CT interaction were reported before.19,20
(31) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
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DOI: 10.1021/la9030409 3277

Article
Yu et al.
Figure 7. Histogram of the desorption forces of PVK from NBA-
Figure 5. Representative force-extension curves of PVK desorb-
ing from DBA-modified substrate at a stretching velocity of
modified substrates at a stretching velocity of 500 nm s-1. The
solid line is a Gaussian fit of the histogram.
3
500 nm s-1. These curves are shifted vertically for convenient
observation.
3
electron-poor, so that it is rational to think that the main driving
force for the adsorption of PVK is CT interaction. Moreover, we
used a PVK-adsorbed tip to perform a control experiment on a
bare substrate without DBA groups, and the obtained force
curves have no plateau.
For comparison, we studied the desorption of PVK from the
substrate modified with NBA groups. Similarly, typical force-
extension curves feature a long plateau, indicating that part of the
PVK chain exists as a trainlike conformation on the substrate.
Figure 7 shows a histogram of the desorption forces of PVK from
NBA-modified substrates. The distribution of the plateau heights
can also be described by a Gaussian function, indicating the
desorption of single polymer chains.17 The average desorption
force determined by Gaussian function is 18 ( 6 pN. The
desorption force changes significantly when PVK is adsorbed
at substrate surfaces modified with different chemical groups,
making it clear that the PVK chain is desorbed from the substrate
rather than from the AFM tip.
Figure 6. Histogram of the desorption forces of PVK from DBA-
modified substrates at a stretching velocity of 500 nm s-1. The
solid line is a Gaussian fit of the histogram.
3
The plateau of the force curve suggests that there is a constant
force for the desorption of the polymer chain from the surface,
and the external energy used to separate each adsorptive point is
similar. According to the plateau feature of the force curves, we
can deduce that part of the PVK chain adsorbs as a trainlike
conformation on the substrate.17b The plateau length reflects
the length of the detached part of the polymer chain from the
substrate, while the height of a plateau corresponds to the
desorption force that is required to detach polymers from
the modified surface.26b The lengths of plateaus of most curves
are less than 100 nm, implying that only a small part of a polymer
chain is adsorbed in a trainlike conformation. In this experiment,
the forcecurves were obtained in dichloromethane, a goodsolvent
of PVK. The polymer chain prefers to stay in the solvent, and a
small part of the polymer chain is adsorbed onto the substrate in a
trainlike conformation.
In order to compare the strength of the interaction between
PVK and different electron-acceptor-modified substrates, we
tried to calculate the apparent interaction energy from the
desorption force and the surface density of electron acceptors.
According to ref 10, the surface density and linear density of
electron acceptors were calculated on the basis of UV-vis spectra
of both the modified substrate and the solutions. Details of the
UV-vis experiment were briefly introduced in the Experimental
Section. The linear density of electron acceptors on DBA-modi-
fied and NBA-modified substrates was estimated to be 1.2 and
1.4 nm-1, respectively. The linear density of carbazolyl groups
along the polymer chain is about 4.0 nm-1, larger than that of
electron acceptors; that is to say, the carbazolyl groups are
excessive for CT interaction. Therefore, the interaction energy
should be determined by the linear density of electron acceptors
rather than that of carbazolyl groups along the polymer chain.
Based on the desorption force and linear density of electron
acceptors, the interaction energies were calculated to be 14 and 8
According to the statistic analysis of the plateau heights, the
desorption force of PVK from the DBA-modified surface is 28 (
7 pN, as shown in Figure 6. The distribution of the plateau heights
can be described by a Gaussian function, indicating the desorp-
tion of single polymer chains.17 Even when changing the stretch-
kJ mol-1 for the carbazolyl-DBA and carbazolyl-NBA com-
3
plexes, respectively. The results indicate that the interaction
between carbazolyl and the DBA group is stronger than that
between carbazolyl and the NBA group. What we need to point
out here is that we carried out the energy calculation with the only
intention of comparing the two different interactions at the
surface. It is not appropriate to compare this calculated interac-
tion energy with the thermodynamic parameters. The interaction
energy we calculated from the desorption force is higher than the
energy based on force quantum in ref 24 by Skulason and Frisbie.
Considering different molecular structures and different data
analysis, the difference in energy is understandable. In our
experiment, we recorded the force at the plateau and performed
statistical analysis; by contrast, Skulason and Frisbie obtained a
ing velocity from 500 to 2500 nm s-1, the height of the plateau is
3
almost the same. According to refs. 17, 26b and 32, it is inferred
that the adsorption and the desorption of the polymer chains are
on a much shorter time scale than the stretching process, sugges-
ting that the experiment was performed in the equilibrium state.
Based on the chemical structures of PVK and DBA groups, we
can roughly exclude the possibility of hydrogen bonding, electro-
static interaction, and coordination bonding. The carbazolyl
group is relatively electron-rich and the DBA group is relatively
(32) Cui, S. X.; Liu, C. J.; Wang, Z. Q.; Zhang, X.; Strandman, S.; Tenhu, H.
Macromolecules 2004, 37, 946.
3278 DOI: 10.1021/la9030409
Langmuir 2010, 26(5), 3275–3279

Yu et al.
Article
Figure 8. Frontier molecular orbital diagram of carbazole, dinitrobenzamide, and nitrobenzamide.
force quantum on the basis of periodicity of a pull-off force
histogram.
From the results above, we can draw a conclusion that the CT
interaction between BDBA and PVK is stronger than that
between BNBA and PVK, and the different CT interactions can
be discriminated by SMFS at the molecular level.
calculated apparent interaction energy indicates that the interac-
tion between carbazolyl and the DBA group is stronger than that
between carbazolyl and the NBA group. From the energy of the
frontier molecular orbitals of the three model compounds, the
change in the interaction strength may be attributed to the
influence of the substituent group on the energy of the frontier
molecular orbital. The difference of desorption force of PVK
adsorbed at different chemical modified substrates demonstrates
that we can perform in situ detection of the driving force of
polymer adsorption at the molecular level with SMFS, which
provides valuable information for LbL assembly as well as other
self-assembly processes.
In order to get a better understanding to the mechanism of CT
interaction, we applied frontier molecular orbital theory to
provide more information on the different CT interactions. The
frontier molecular orbitals of three model compounds, carbazole,
dinitrobenzamide, and nitrobenzamide, were calculated.33 The
obtained highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMOs) are shown in
Figure 8. The LUMO of an acceptor molecule interacts with the
HOMO of a donor molecule (carbazole). The lower the LUMO
level of the acceptor molecule is, the stronger the association will
be.25,34 It is shown in Figure 8 that the LUMO level of dinitro-
benzamide (0.005 eV) is lower than that of nitrobenzamide (0.035
eV), which means that the association between dinitrobenzamide
and carbazole is stronger than that between nitrobenzamide and
carbazole. Thus, the difference in strength between the interac-
tions can be explained as the influence of the substituent group on
the energy of the frontier molecular orbital.
Acknowledgment. We thank Prof. Zhongyuan Lu (Jilin Uni-
versity, China) for his helpful discussion. This work was sup-
ported by the National Natural Science Foundation of China
(20674028, 20640420622), Program for New Century Excellent
Talents in University, and National Basic Research Program
(2009CB939701).
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
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Conclusion
We studied the CT interaction between poly(9-vinylcarbazole)
and 3,5-dinitrobenzamido groupor 3-nitrobenzamido group. The
equilibrium constant of the PVK-BDBA complex was found to
be larger than that of the PVK-NBA complex. The desorption
force of a PVK chain from the DBA-modified substrate was
determined to be 28 pN; by contrast, the desorption force of a
PVK chain from the NBA-modified substrate was 18 pN. The
(34) Landman, U.; Ledwith, A.; Marsh, D. G.; Williams, D. J. Macromolecules
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Langmuir 2010, 26(5), 3275–3279
DOI: 10.1021/la9030409 3279