Polyelectrolyte templated polymerization in langmuir films: Nanoscopic control of polymer chain organization

Article abstract of DOI:10.1002/chem.200500830

Polyelectrolytes introduced in the aqueous subphase are shown to have a profound impact on the kinetics of polymerization of N-octadecylaniline at the air/water interface. This can be attributed to changes effected in molecular organization and reorientation behavior in the Langmuir film. The polyelectrolyte templates lead to considerable modification of the morphology of the monomer and polymer Langmuir films. Polyelectrolyte complexation is found to be an elegant and efficient methodology to achieve enhanced alignment of the polyaniline chains in the transferred Langmuir-Blodgett (Langmuir-Schafer) film.

Full text of DOI:10.1002/chem.200500830

DOI: 10.1002/chem.200500830
Polyelectrolyte Templated Polymerization in Langmuir Films:
Nanoscopic Control of Polymer Chain Organization
M. Sharath Chandra and T. P. Radhakrishnan*[a]
Abstract: Polyelectrolytes introduced
in the aqueous subphase are shown to
have a profound impact on the kinetics
of polymerization of N-octadecylani-
line at the air/water interface. This can
be attributed to changes effected in
molecular organization and reorienta-
tion behavior in the Langmuir film.
The polyelectrolyte templates lead to
considerable modification of the mor-
phology of the monomer and polymer
Langmuir films. Polyelectrolyte com-
plexation is found to be an elegant and
efficient methodology to achieve en-
hanced alignment of the polyaniline
chains in the transferred Langmuir–
Blodgett (Langmuir–Schafer) film.
Keywords: interfaces
· Langmuir
films · polyaniline · polyelectrolytes ·
polymerization · template synthesis
Introduction
logical extension of the studies cited above suggests that the
covalent assembly of molecules through polymerization at
the air/water interface, modulated by noncovalent interac-
tions with polyelectrolytes introduced in the subphase, could
be an efficient approach to the controlled organization of
polymers in ultrathin films.
Molecular assembly mediated by covalent and noncovalent
interactions and its control at the nanoscopic level are of
fundamental relevance to supramolecular chemistry, poly-
mer chemistry, and molecular-scale electronics, photonics,
and patterning. The Langmuir–Blodgett (LB) technique is
especially suited to exercise such control; this technique also
facilitates efficient monitoring of the assembly process. A
fine example is the synthesis of conjugated polymers at the
air/water interface[1,2] and investigation of the kinetics
through the measurement of surface-pressure/monolayer-
area changes.[1,3–6] We have previously demonstrated the util-
ity of polyelectrolyte complexation in stabilizing otherwise
unstable Langmuir films[7] and the role of polyelectrolyte
templates in efficient deaggregation leading to enhanced[8]
and stable[9] second-harmonic generation in LB films of
hemicyanines. Polyelectrolytes have been extensively em-
ployed as dopants and templates in the synthesis of conduct-
ing polymers;[10,11] they impart higher solubility and process-
ability to the polymers and modify the morphology and con-
ducting properties.[11] Polyion-complexed diacetylene has
We have chosen to investigate the polymerization of N-
octadecylaniline (NOA) assembled as a Langmuir film on
the surface of water and demonstrate the influence of poly-
electrolytes introduced from the subphase on the morpholo-
gy of the Langmuir film, rates of polymerization, and the
polymer-chain organization in the transferred Langmuir–
Schafer film. Polymerized N-alkylaniline cannot sustain the
conventional emeraldine base (neutral quinonoid) and the
corresponding salt (protonated) forms because of the substi-
tution on the nitrogen atom. Polymers with alkyl groups,
butyl to dodecyl, synthesized in the bulk phase were found
to exhibit low conductivity.[13] However, NOA possesses the
advantages of easy synthesis, stable protonated monomer/
polymer states, and steric/structural features conducive for
organization at the air/water interface; therefore it should
serve as a suitable candidate to demonstrate phenomena as-
sociated with the molecular assembly and reaction. We en-
visaged that, under the usual aniline polymerization condi-
tions, polyelectrolytes would exercise strong control on the
orientation of the protonated monomer and the organiza-
tion of the polymer, which is likely to form in the fully pro-
tonated (with ammonium groups) or further oxidized (bipo-
laronic) form. Brewster angle microscopy (BAM) was em-
ployed to observe the monolayer at the air/water interface
through the polymerization process and investigate the in-
been photopolymerized following LB film deposition.[12]
A
[a] M. S. Chandra, Prof. T. P. Radhakrishnan
School of Chemistry, University of Hyderabad
Hyderabad, 500 046 (India)
Fax: ( +91)40-2301-2460
E-mail: tprsc@uohyd.ernet.in
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
fluence of the polyelectrolytes
in the subphase on the domain
morphologies. A new protocol
that we have developed facili-
tated accurate monitoring of
the rates of polymerization at
the air/water interface and the
impact of the polyelectrolytes.
Atomic
force
microscopy
(AFM) images of the transfer-
red Langmuir–Schafer films re-
vealed the utility of polyelectro-
lyte templating in directing the
formation of organized chains
Figure 1. Pressure–area (p vs. A) isotherms of protonated a) NOA and b) PNOA monolayers without and with
of poly(N-octadecylaniline) in polyelectrolytes in the subphase; note the different y scales.
these films.
at the air/water interface by the amphiphile monolayer and
Results and Discussion
the isotherm shift is a consequence of the amphiphile–poly-
electrolyte complexation.[7–9] The complexation also effects a
cooperative motion of the headgroups leading to a smooth-
ening of the transition. A combination of electrostatic and
hydrogen-bonding interactions, depending on the level of
ionization of the polyacids, is likely to lead to the complexa-
tion.
NOA was synthesized by the reaction of 1-bromooctadecane
with aniline (Scheme 1). It does not show any tendency to
Previous studies of polymerization of anilines at the air/
water interface involved spreading the monolayer on a sub-
phase already charged with the oxidizing agent.[3–5] We have
investigated the polymerization of NOA using such a proce-
dure.[16] However, we found that the kinetics is affected by
the barrier speed and delays in the initial compression;
hence, a new protocol was developed (Scheme 2). The mo-
Scheme 1.
spread evenly and form a monolayer on the surface of pure
water. Acidification of the subphase with hydrochloric acid
facilitated spreading; however, the monolayer was found to
be unstable to compression. Interestingly, introduction of
sulfuric acid in the subphase imparted appreciable stability
to the monolayer as revealed by the pressure–area isotherm
and the collapse pressure of ~33 mNmꢀ1 (Figure 1a). It ap-
pears that the protonated NOA layer is stabilized by the bi-
dentate dianion; this stabilization is reminiscent of earlier
observations on pyridinium-based amphiphiles.[7,8] The iso-
therm shows a transition at ~23 mNmꢀ1; based on the ex-
2
trapolated areas below and above, 101.6 and 75.8 Š per
molecule, respectively, the transition can be ascribed to a re-
orientation of the headgroup.[14,15] The polyelectrolytes used
in this study are the salts of poly(vinylsulfate) (PVS) and
carboxymethylcellulose (CMC); they are likely to be mostly
in the polyacid form under the acidic conditions employed
in this work. Introduction of polyelectrolytes in the sub-
phase increased the residual pressure slightly, but more sig-
nificantly it shifted the isotherms to higher areas and
smoothed the transition. The extrapolated areas above the
Scheme 2.
nomer was spread on the acidic subphase and compressed
to the desired pressure. Subsequently, the oxidizing agent
(ammonium peroxydisulfate as aqueous solution) was inject-
ed into the subphase with a surgical needle inserted through
the monolayer; this had no adverse effect on the film. In
this procedure, the monomers are organized before initiat-
2
transition are 96.7 and 94.1 Š per molecule with PVS and
CMC, respectively. The increase of residual pressure possi-
bly arises as a result of the adsorption of the polyelectrolyte
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T. P. Radhakrishnan and M. S. Chandra
ing the polymerization and the rate is monitored from the
start; conceptually it is similar to the double-compartment
method reported previously.[6] Reactions were carried out at
258C with constant target pressures of 20, ~23 and
30 mNmꢀ1 (below, near, and above the transition). The rate
of the reaction at the interface can be formulated within the
spirit of the 2D “ideal-gas equation” [Eq. (1)], in which p,
A, and n are the surface pressure, area, and number of mon-
omers during the course of the reaction. A1 is the final area
and C is a constant.
that the reaction is aided by the molecular reorientation at
this point. Significantly, the polyelectrolytes reduce the rates
in general and the peak rate decreases with increasing pres-
sure. These observations point to the control of monomer
motion and reorientation by the cooperative impact of the
polyelectrolyte.
The p versus A isotherms of poly(N-n-octadecylaniline)
(PNOA) (Figure 1b) show steeper curves, higher collapse
2
pressures, and lower extrapolated areas (~40 Š per mole-
cule), relative to those of the monomer. The contraction of
area results from covalent-bond formation. The steeper
curves and higher collapse pressures reveal the enhanced
stiffness and stability of the polymer monolayer. Interesting-
ly, the isotherms are nearly independent of the presence or
type of polyelectrolyte, indicating that polyaniline back-
bones are identical in all cases. BAM of the monolayer at
different points along the p versus A curve, through the
polymerization process, reveals the influence of the poly-
electrolytes. Images for the polymerization carried out at
20 mNmꢀ1 are presented in Figure 3. The polyelectrolytes
pðAꢀA1Þ ¼ Cn
ð1Þ
At constant temperature, the rate is given by Equa-
tion (2),[16] in which p0, A0, and n0 are the initial values.
n0
d
dt
Rate ¼
½pðA1ꢀAÞꢁ
ð2Þ
p0ðA0ꢀA1Þ
This expression takes into account possible minor pres-
sure fluctuations (p~p0) in the “constant-pressure” experi-
ment. The rates were determined by following the change of
the monolayer area with time at a constant target pressure
and employing the above equation at each point (Figure 2).
The rate varies through the reaction; the peak rates ob-
Figure 2. Change of monolayer area and rate of reaction with time during
the polymerization of NOA (sulfate in subphase, target pressure=
25 mNmꢀ1); zero on the time axis indicates the point when the oxidizing
agent was injected in.
Figure 3. BAM images of Langmuir films. Panels 1 and 2: protonated
NOA monolayer at 0 and 20 mNmꢀ1, panels 3 and 4: PNOA monolayer
at 20 and 0 mNmꢀ1, with a) only sulfate, and additionally b) PVS, and
c) CMC in the subphase. The images represent a 220”220 mm2 area.
tained for NOA polymerization carried out at different
target pressures, with and without the polyelectrolytes in the
subphase are summarized in Table 1. Without polyelectro-
lytes, the peak rate is highest near the transition, indicating
support domain formation prior to polymerization (panel 1).
The change of morphology on polymerization (panels 2 and
3) is marked in the absence of polyelectrolytes; the poly-
electrolytes homogenize the monomer and polymer mono-
layers smearing out contrast. The most significant impact of
the polyelectrolytes is evident upon expansion of the poly-
mer monolayer (panel 4). The entangled network structures
present in the case of PNOA give way to short broken
chainlike features when polyelectrolytes are present in the
subphase during polymerization.
Table 1. Peak rate of polymerization of NOA at 258C with only sulfate
and with different polyelectrolytes in the subphase.
Pressure [mNmꢀ1
]
Rate [1013 molecules sꢀ1
PVS
]
Sulfate
CMC
20
~23
30
2.24
2.61
1.40
1.66
1.30
1.06
1.40
1.15
0.89
Optical-absorption spectra were recorded by using 24-
layer LB films of PNOA polymerized at 20 mNmꢀ1; they
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Templated Polymerization
FULL PAPER
show a broad absorption with peaks at ~305, 445, and
565 nm[16] that can be assigned to a p–p* transition, a polar-
onic state with ammonium sites,[17] and pernigraniline
form,[5] respectively. Negligible absorption above 625 nm in-
dicates a low population of quinonoid structures.[13,17] PNOA
is inferred to be in the fully protonated state with mostly
benzenoid rings.
Monolayer films of PNOA deposited on mica by the
Langmuir–Schafer method were examined by AFM. In the
absence of polyelectrolytes (Figure 4a), a complexnetwork
Figure 5. Optimized geometries of decamers of a) fully protonated
PNOA, b) PVS, and c) CMC. The estimated average monomer length is
0.51, 0.22, and 0.51 nm, respectively. C (gray), H (white), N (blue), O
(red), and S (yellow).
threads are segmented with lengths in the range 0.6–1.6 mm.
Based on the average molecular weights of PVS and CMC
(170000 and 90000, respectively) and the computed mono-
mer lengths (0.22 and 0.51 nm, respectively), these polymer
chains can be estimated to be ~0.2 mm long. Therefore the
segment length appears to be built from 3–8 polyelectrolyte
chains, along with the templated PNOA chain. If the aver-
age length of the PNOA chains is assumed to be ~1 mm, the
computed monomer length of 0.51 nm suggests that the
degree of polymerization is ~1960.
Figure 4. AFM topography images of monolayer Langmuir–Schafer films
of PNOA fabricated with a) only sulfate, and additionally b) PVS, and
c) CMC in the subphase. Top panel: 2D image (5”5 mm2); bottom panel:
3D image (2 mm”2 mm”10 nm).
Conclusion
is observed with threads typically 3.5–4.0 nm high and 30–
50 nm wide. Molecular modeling of the decamer unit of
fully protonated PNOA shows the polyaniline chain with
the hydrocarbon groups splayed out in a wedge shape; the
cross-sectional dimensions are ~2.4”2.6 nm2 (Figure 5a).
Based on this, the threads observed in the AFM image can
be visualized as a monolayer of 12–20 PNOA chains with
sulfate counterions assembled laterally forming a tapelike
structure. Films fabricated in the presence of polyelectro-
lytes are dramatically different, showing enhanced align-
ment of extended polymer chains. Templating by amphi-
philes is known to align arrays of conjugated polymers;[18] in
the present case, the preorganized NOA amphiphiles possi-
bly align the complexed polyelectrolytes, which in turn en-
force orderly growth of the PNOA chains. Figure 4b and c
show threads 4.0–4.5 nm high and 40–50 nm wide with PVS,
and 3.8–4.2 nm high and 50–60 nm wide with CMC, respec-
tively. Molecular modeling of the polyelectrolyte decamers
(Figure 5b and c)[16] indicates cross-sectional dimensions of
~0.9”0.9 nm2 for PVS and ~0.7”1.1 nm2 for CMC; the
latter dimensions are in good agreement with a previous
model.[15] These estimates and the observed dimensions of
the threads suggest plausible structures for the templated
systems, consisting of PNOA chains on top of PVS and lat-
erally placed alongside CMC. Figure 4b and c show that the
The present study has demonstrated that polyelectrolytes in-
troduced in the subphase can have a profound impact on
the morphology of Langmuir films, kinetics of polymeri-
zation at the air/water interface, and the formation of highly
aligned polymer chains in the transferred ultrathin films.
While the impact of PVS and CMC are beneficial, prelimi-
nary studies using poly(4-styrenesulfonate) indicate ambigu-
ous effects—unique isotherms, absorption spectral shifts,
and poor chain alignments—possibly due to additional and
undesirable p-stacking interactions with polyaniline. There-
fore, a judicious choice of the polyelectrolyte appears to be
critical. Control of polymerization in monolayers at the air/
water interface using the simple and efficient technique of
polyelectrolyte complexation has significant implications for
the fabrication of organized polymer assemblies for molecu-
lar electronics and photonics applications.
Experimental and Computational Section
Synthesis and characterization of NOA: The synthesis of NOA was car-
ried out following Scheme 1. 1-Bromo-n-octadecane (1.0 g, 3 mmol) in
toluene (20 mL) was added to
a stirred solution of aniline (0.32 g,
3.3 mmol) in 30% aqueous NaOH containing a catalytic amount of
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2985

T. P. Radhakrishnan and M. S. Chandra
N,N,N,N-tetrabutylammonium bromide( PTC, phase-transfer catalyst).
The reaction mixture was stirred at 70–808C for seven days. The toluene
phase was washed with sodium chloride solution and evaporated. The re-
sultant solid was purified by several recrystallizations from hexane.
Yield: 0.82 g, 80%; m.p. 50–528C; FT-IR (KBr): n˜ =3396.9, 2916.6,
chosen as follows: PNOA with ammonium functionalities—the hydropho-
bic hydrocarbon chains oriented in the same direction to simulate their
organization away from the aqueous subphase; PVS and CMC—the hy-
drophilic acid groups alternately on either side of the polymer backbone.
2847.2, 1604.9, 1510.4, 1468.0, 1315.6, 748.5, 690.6 cmꢀ1
;
1H NMR
(200 MHz, CDCl3, 258C, TMS): d=0.90 (t, J=6.81 Hz, 3H), 1.27 (s,
30H), 1.60 (m, 2H), 3.10 (t, J=6.89 Hz, 2H), 6.72 (m, 3H), 7.13 ppm (m,
2H) (amine proton is not observed); 13C NMR (200 MHz, CDCl3, 258C):
d=148.6, 129.2, 117.0, 112.7, 44.0, 31.9, 29.7, 27.2, 22.7, 14.1 ppm.
Acknowledgements
We thank the DST (NSTI program), New Delhi and UPE program of
the UGC, New Delhi for financial and infrastructure support and Prof.
M. Durga Prasad for fruitful discussions. MSC thanks CSIR, New Delhi
for a senior research fellowship. Details of polymerization rates, absorp-
tion spectroscopy, line profile analysis of AFM images, and computed
molecular structures are given in the Supporting Information.
Fabrication of Langmuir and LB/Langmuir–Schafer film: The LB experi-
ments were carried out in a Nima Model 611M trough with a surface
area of 30”10 cm2. High-purity water (Millipore Milli Q) and chloroform
(Uvasol grade, EMerck) were used for the subphase and spreading solu-
tion, respectively in all experiments; the volume of the subphase was
~220 mL. A quantity of 0.04 mmol of NOA was spread on the aqueous
subphase containing sulfuric acid (0.10m) or sulfuric acid and polyelectro-
lyte (~0.20 mmol based on the acid groups). The polyelectrolytes used
were the potassium salt of PVS (average MW 170000) and the sodium
salt of CMC (average MW 90000). Ammonium peroxydisulfate (typically
1.0 mL of a 1.7m solution) was used as the oxidizing agent for polymeri-
zation. Reactions carried out at the air/water interface at 258C under
constant pressure were monitored using the change in monolayer area
(or barrier motion). Glass substrates for LB film deposition were cleaned
with detergent, rinsed several times in water, and then sonicated in sever-
al batches of fresh water for 10–15 min each. A hydrophobic surface was
prepared by exposing the slides to vapors of hexamethyldisilazane for
12 h. The LB film was deposited by vertical dipping of the substrate at a
speed of 5 mmminꢀ1; transfer ratios were ~0.50. The Langmuir–Schafer
film was transferred onto freshly cleaved mica by using a horizontal-dip-
ping method.
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Spectroscopy: Optical absorption spectra of multilayer LB films deposit-
ed on hydrophobic glass substrates by vertical dipping, were recorded on
a Shimadzu model UV-3100 UV/Vis spectrometer. The absorption wave-
length cut-off of the glass substrate is 270 nm.
Microscopy: The morphology of the Langmuir films at the air/water in-
terface was observed by BAM. Images of the monolayer were recorded
by using a Nanofilm Model BAM2Plus microscope. A 532 nm laser beam
with a power of 20 mW was used. The monomer film was examined at
different stages of compression. Following polymerization at a specific
target pressure, the barriers were opened and closed again. Images were
recorded throughout the whole procedure. The length scales of the
images were corrected for the angle of incidence of the beam. Monolay-
ers transferred to freshly cleaved mica plates by horizontal dipping, were
imaged by using a Seiko Model SPA 400 atomic force microscope. All
images presented were recorded in the dynamic-force (noncontact)
mode; the tip had a force constant of 20 Nmꢀ1. Line profiles were ana-
lyzed by using the software supplied by the microscope manufacturer.[16]
Molecular modeling: Computations were carried out by using the Accel-
rys MS Modeling 3.0.1 program. Monomer geometries were optimized
and extended in steps to decamer units by using the “polymer build”
option with geometry optimization at each stage carried out by using the
Forcite module and Universal force field. Initial conformations were
[18] F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. Schenning,
Chem. Rev. 2005, 105, 1491.
Received: July 17, 2005
Revised: October 19, 2005
Published online: February 2, 2006
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