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Theoretical and Experimental Studies on the Surface
Structures of Conjugated Rod−Coil Block Copolymer Brushes
Wen-Chung Wu, Yanqing Tian, Ching-Yi Chen, Chun-Sheng Lee, Yu-Jane Sheng, Wen-Chang Chen, and
Langmuir, 2007, 23 (5), 2805-2814 • DOI: 10.1021/la0631769
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Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Theoretical and Experimental Studies on the Surface Structures of
Conjugated Rod-Coil Block Copolymer Brushes
Wen-Chung Wu,†Yanqing Tian,‡Ching-Yi Chen,‡Chun-Sheng Lee,†Yu-Jane Sheng,†,§
Wen-Chang Chen,*,†,§and Alex K.-Y. Jen*,‡
Department of Chemical Engineering and Institute of Polymer Science and Engineering,
National Taiwan UniVersity, Taipei, Taiwan 106 and Department of Materials Science and Engineering,
UniVersity of Washington, Seattle, Washington 98195-2120
ReceiVed October 31, 2006. In Final Form: December 7, 2006
The theoretical study for the surface structures of rod-coil block copolymer brushes was established based on the
ratio of the polymer brushes on the surface structures were examined. The rod blocks of polymer brushes were found
to be well-dispersed on the surface in their good solvents. On the other hand, aggregative domains of the rod blocks
were formed in their poor solvents with the conformations of isolated islands or worm-like structures depending on
the grafting density of the polymer brushes. The aggregative domains tend to stay on top of the coil blocks for small
preferred for large enough rod-to-coil block ratio. New multifunctional amphiphilic rod-coil block copolymers, poly-
[2,7-(9,9-di-n-hexylfluorene)]-block-poly-[poly(ethylene glycol) methyl ether methacrylate]-block-poly-[3(tripro-
poxysilyl)propyl methacrylate] (PF-b-PPEGMA-b-PPOPS), with two different block ratios were synthesized and
used to prepare the corresponding polymer brushes via the grafting- method. The effects of stimuli factors on the
surface structures characterized by the atomic force microscopy images were consistent with the theoretical results.
Furthermore, the photophysical properties of PF-b-PPEGMA-b-PPOPS brushes were significantly varied by the
solvent stimuli. The emission peaks originated from the aggregation and/or excimer formation of PF blocks were
observed after methanol treatment. The photoluminescence intensity and its efficiency were well correlated to the
surface structure and the methanol content in mixed solvents. Our study demonstrates how the surface structures and
photophysical properties of rod-coil block copolymer brushes response to environmental stimuli.
Polymer brushes have been recognized as a fascinating
synthetic target because of their ability in surface modification
and potential applications in many fields.1-18The chemical
characteristics of polymer segments give thermal and solvent
stabilities in various environments and processing conditions,
which are potential for certain applications such as switching
membranes, sensors, and cell growth control.19-26Two primary
approaches for preparation of polymer brushes are grafting-to
in the polymer backbones. The grafting-from method, or
the initiators bound to surfaces to prepare polymer brushes from
precision. However, conjugated polymer rod-coil brushes via
surface-initiated polymerization have seldom been explored
because of the synthetic difficulty of conjugated backbones.
Rod-coil block copolymers consisting of a π-conjugated
polymer as the rod and a flexible block as the coil are an unique
and interesting class of nanomaterials because it opens the way
Several fluorene-based rod-coil and coil-rod-coil block
* Towhomallcorrespondenceshouldbeaddressed:Tel.: 886-2-23628398.
Fax: 886-2-23623040. E-mail: (W.C.C) email@example.com; (A.K.Y.J)
†Department of Chemical Engineering, National Taiwan University.
‡University of Washington.
§Institute of Polymer Science and Engineering, National Taiwan
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Langmuir 2007, 23, 2805-2814
10.1021/la0631769 CCC: $37.00© 2007 American Chemical Society
Published on Web 01/24/2007
and chemical stability and high fluorescence quantum yields in
both solution and solid state.33-38The equilibrium properties of
are determined by the environmental stimuli (solvent stimuli for
it is worthwhile to investigate the relationship between the
optoelectronic characteristics and the surface structures of
polyfluorene-based rod-coil block copolymer brushes. In
be a possible approach for the formation of polyfluorene-based
rod-coil block copolymer brushes.
In this study, a combined theoretical and experimental
investigation of conjugated fluorene based rod-coil block
copolymer brushes is reported. The equilibrium properties of
Carlo (MC) simulations,39molecular-dynamics (MD),40,41and
dissipative particle dynamics (DPD).42-46Because the DPD
method has the ability to treat a wider range of length and time
of soft forces, it is used for the present study. In this work, the
effects of solvent stimuli, grafting density, and rod-coil block
ratio of the polymer brushes on the surface structures were
brushes were prepared, as shown in Figure 1. The block
(ethylene glycol) methyl ether methacrylate]-b-poly-[3(tripro-
poxysilyl)propyl methacrylate] (PF-b-PPEGMA-b- PPOPS),
block, poly-[poly(ethylene glycol) methyl ether methacrylate]
(PPEGMA) as the hydrophilic coil block, and poly-[3(tripro-
poxysilyl)propyl methacrylate] (PPOPS) as the reactive block
the block copolymers were assembled onto a glass substrate
with the silanol groups anchored on the surface. The surface
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2806 Langmuir, Vol. 23, No. 5, 2007Wu et al.
structures and photophysical properties of the polymer brushes
the PF and PPEGMA blocks) and then with methanol (poor
solvent for PF blocks, whereas good solvent for PPEGMA
and photophysical properties of rod-coil block copolymer
brushes response to environmental stimuli.
DPD Simulation Method. The simulation method used to
technique that can be used to study systems over greater length
and time scales than are accessible in classical atomistic
simulations like MC and MD. It has been successfully used to
The polymer model and simulation details are described as
Model and Parameters for Rod-Coil Block Copolymer
Brushes. In our system (volume V ) Lx × Ly × Lz with N
particles), the polymer brush is modeled as a copolymer with
sections of rod and coil blocks. The end-beads of the coil blocks
are attached to an impenetrable surface at z ) 0. The lengths of
The total number of particles in the system is N ) NS+ NP(nA
+ nB) in which NSand NPare the numbers of solvent particles
and polymer brushes. We define grafting (surface) density FS)
of motion. In addition to the three different pairwise-additive
in DPD simulations, the spring force (FS) and angle force (Fθ)
describing the chemical bonding and angle bending effects of
the polymer chain are also taken into account in this system and
are given by
In this work, we have chosen c ) 100 and kθ) 20.
Our system volume is set to be V ) Lx× Ly× Lz) 30 × 30
× Lz, and the total number density (F ) N/V) is 3. The depth
of the system, Lz, ranges from 15 to 25 depending on the length
of the polymer brush. The length of the rod block is fixed at 5
ratio of rod to coil ranges from 1:1 to 1:6. The effect of different
degrees of grafting on the surface structures also is studied and
the grafting density (FS) spans from 0.1 to 1.0.
The conservative force FCfor nonbonded beads is a soft
repulsive force and the repulsion parameter aijis the maximum
repulsion between particles i and j. Groot and Warren47showed
that the repulsion parameter of 25 corresponds to a highly
compatible pair. As the repulsion parameter increases, the
compatibility between i and j particles decreases. In our study,
rod block and solvent particle, coil block and solvent particle,
and rod and coil blocks, respectively. We chose aAS) 26 to
symbolize the solvophilic nature of the rod block and aAS) 40
to denote the solvophobic character of the rod block. The aBS
and aABwere set to be 26 and 40, respectively, indicating good
solvent condition for the coil block and incompatible charac-
the cutoff radius rc, and kBT were put at 1 for simplicity. A
(47) Groot, R. D.; Warren, P. B. J. Chem. Phys. 1997, 107, 4423.
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Scheme 1. Synthetic Scheme of PF-b-PPEGMA-b-PPOPS
S) c(rij- 0.7) (1)
θ)kθ(θ - π)2
Conjugated Rod-Coil Block Copolymer BrushesLangmuir, Vol. 23, No. 5, 2007 2807
modified version of the velocity-Verlet algorithm was adapted
with a time step of ∆t ) 0.05 and λ ) 0.65.49
Materials. 2,7-Dibromofluorene, 2-bromophenethyl alcohol,
2-bromoisobutyryl bromide, 1,1,4,7,10,10-Hexamethyltriethylene-
were purchased from Aldrich and were used without purification.
was dried under vacuum. Poly(ethylene glycol) methyl ether
methacrylate (PEGMA, Aldrich, Mn ≈ 300) was purified by
distillation under vacuum before use. Tetrahydrofuran (THF) and
ether were distilled from sodium/benzophenone under nitrogen.
acid (2)50were prepared according to known procedures.
Synthesis. The preparation of the triblock copolymer is shown
in Scheme 1.
rene) (3). 1.0 g (2.18 mmol) of compound 2, 1.34 g of sodium
palladium(0) were dissolved in 10 mL of anhydrous N,N-dimethyl-
acetamide (DMAc). The solution was degassed and flushed with
mixture for 30 h, 1.5 mL (7.5 mmol) of 2-bromophenethyl alcohol
was added as an end capping reagent. Heating overnight at 120 °C
followed by reprecipitation of the reaction mixture into 300 mL of
methanol afforded a solid. The solid was washed with water and
then was dissolved into 5 mL of methylene chloride and was
precipitated into 200 mL of methanol to afford 630 mg of polymer
3. Mn (GPC)) 2700, Mw/Mn) 1.80. Mn (NMR)) 5500. The degree
of polymerization (DP) of polyfluorene is 16 based on the NMR
(s, b, 352H), 2.05-2.26 (m, 64H), 2.92 (t, 2H), 3.94 (t, 2H), 7.34-
7.39 (m, 2H), 7.42-7.60 (m, 2H), 7.58-7.92 (m, 100H).
was added dropwisely to a solution of 620 mg of 3 and 2.0 mL of
triethyl amine in 10 mL of dry THF, and the reaction mixture was
stirred at room temperature for 24 h. After the mixture was poured
into 200 mL of cold methanol, a solid was obtained. The solid then
was redissolved into 5 mL of THF and reprecipitated into 200 mL
of cold methanol to afford 590 mg of white powder with yield of
94%. Mn (GPC)) 2800, Mw/Mn) 1.81. Mn (NMR)) 5600. The DP
of polyfluorene is 16.1H NMR (300 MHz, CDCl3): δ)0.60-0.98
and 1.05-1.31 (s, b, 352H), 1.96 (s, 6H), 2.05-2.26 (m, 64H), 3.04
7.92 (m, 100H).
methyl ether methacrylate]-b-poly-[3(tripropoxysilyl)propyl meth-
triblock copolymers of PF-b-PPEGMA-b-PPOPS were prepared
1. In detail, 180 mg (∼0.03 mmol) of PF macroinitiator 4 and 8.7
mg of CuBr (0.06 mmol) were added into a Schlenk tube and then
vacuumed for 10 min. Under nitrogen atmosphere, a solution of
PEGMA [450 mg (∼1.5 mmol) for P1; 900 mg (∼3 mmol) for P2]
into the Schlenk tube. The mixture was degassed three times and
then filled with Argon. After stirring at ambient temperature for 30
min, the Schlenk tube was immersed into an oil bath at 80 °C for
5 h. A small amount of the polymerization mixture was taken out
1H NMR result shows the conversion is >95%. Then, 498 mg (1.5
mmol) of degassed POPS was added into the polymerization tube
via microsyringe. The polymerization was continuously kept at 80
Oxford, England, 1987.
(50) Ozaki, H.; Hirao, A.; Nakahama, S. Macromolecules 1992, 25, 1391.
WO 97/05184, 1997.
(52) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614.
Figure 2. Simulated surface structures of rod-coil block copolymer brushes with the parameters of FS) 0.7, nA) 5, nB) 20, aBS) 26,
and aAB) 40: (a) top view (for aAS) 26); (b) side view (for aAS) 26); (c) top view (for aAS) 40); and (d) side view (for aAS) 40).
The yellow and red spheres correspond to the rod and coil blocks, respectively. The blue spheres are the anchoring points of the copolymers
on the surface. The system volume is set to be V ) Lx× Ly× Lz) 30 × 30 × 20.
2808 Langmuir, Vol. 23, No. 5, 2007Wu et al.
°C overnight. After being cooled down to room temperature, the
mixture was passed through an Al2O3column to remove the copper
and then was precipitated into an excess amount of ether resulting
in a PF-b-PPEGMA-b-PPOPS triblock copolymer. The gas
of POPS shows a shift of the peak position to a higher molecular
weight region, showing the successful preparation of the functional
triblock copolymer. Through the controlling of the polymerization
times and feed ratios of PPOPS or PPEGMA to PF, two triblock
copolymers (P1, and P2) were synthesized. The1H NMR spectra
of the purified triblock block copolymers indicate the DPs of
PPEGMA and POPS of P1 are 44 and 16, respectively, and those
of P2 are 92 and 11, respectively. The rod-coil block ratios (PF/
PPEGMA) of P1 and P2 are about 1:3 and 1:6, respectively. Mn
(GPC)Mw/Mnand Mn (NMR)of P1 and P2 are 13200, 1.52, and 24400,
and 18600, 1.36, and 37100, respectively. Because the PF macro-
the prepared PF macroinitiator has a high polydispersity of 1.81.
be attributed to the well-defined PPEGMA chains existing in the
copolymers through the ATRP process. Similar phenomenon was
observed by Lu et al.53Details such as the1H NMR spectra and the
GPC traces of PF macroinitiator, P1 and P2 are given in the
Self-Assembly of Triblock Copolymers on Oxidized Surfaces
and Solvent Stimuli. The polymer brushes were prepared by using
a new surface-reactive method developed by Thomas et al.14Glass
substrates were cleaned by water followed by acetone. And then,
the substrates were immersed into Piranha solution (H2O2/H2SO4
and then dried in vacuum oven for 2 h, the oxidized substrates were
immersed in a toluene solution with a concentration of polymer
ranging from 5 to 20 mg/mL and a small amount (∼20 µL/mL) of
triethylamine as catalyst for 24 h at room temperature followed by
repeated rinsing and ultrasonic washing with toluene three times.
Finally, the films were Soxhlet extracted with CH2Cl2for 46 h to
remove any possible physical absorbed polymers on the surfaces.
of both the PPEGMA block and the PF block) or methanol (good
at room temperature and then were dried under a flow of clean air
Characterization.1H NMR spectra were measured by using a
Bruker 300 instrument spectrometer operating at 300 MHz with
Molecular weights of polymers were determined by using a Lab
Alliance RI2000 instrument (two column, MIXED-C and D from
from Schambeck SFD Gmbh. All GPC analyses were performed on
polymer/THF solution at a flow rate of 1 mL/min at 40 °C and
calibrated with polystyrene standards.
Atomic force microscopy (AFM, NanoScope III, Digital Instru-
ment) equipped with an integrated silicon tip/cantilever with
resonance frequency ∼240 kHz in height and phase image models
showed no evidence of tip-induced modification during successive
scans. X-ray photoelectron spectroscopy (XPS) was analyzed by
using Surface Science Instruments (X-Probe). Photoluminescence
(PL) spectra were recorded on a Fluorolog-3 spectrofluorometer
Results and Discussion
Theoretical Investigation on the Surface Structures of
an alternative approach to gain more direct and microscopic-
level information than experiments. The building of mesoscale
the use of classical molecular dynamics at atomic resolution is
a challenge at present owing to the length and time scales at
which these phenomena can occur. However, mesoscale simula-
tions such as dissipative particle dynamics can overcome this
of solvent stimuli, grafting density, and block ratio on the
morphologies of surface structures formed by rod-coil block
copolymer brushes. The results are complementary to the
The Effect of SolVent Stimuli on the Surface Structures of
Rod-Coil Block Copolymer Brushes. It is well known that the
influence the resulting morphologies of the systems. Two
scenarios were considered here: first, the solvents were compat-
ible to both the rod and coil blocks of the rod-coil copolymer
and second, the solvents became incompatible to the rod blocks.
as shown in Figure 2a,c. The yellow and red spheres correspond
to the particles in the rod and coil blocks, respectively. The blue
spheres are the anchoring points of the rod-coil copolymers on
solvents (Figure 2a), the rod blocks are well dispersed on the
and the solvents. On the other hand, aggregative domains of the
rod blocks are observed in Figure 2(c). The incompatibility
between the rod blocks and the solvent particles drives the rod
blocks into forming aggregates to reduce the contacts between
rod blocks and solvents. In doing so, the free energy of the
system decreases and a thermodynamically stable structure is
From the side views of the systems, the rod blocks of the
polymers extend themselves into the good solvents (Figure 2b).
On the other hand, the rod blocks back away from the solvents
and even hide within the coil blocks to avoid contact with the
Figure 3. Top views of the surface structures of rod-coil block
copolymer brushes with the parameters of nA) 5, nB) 20, aAS)
40, aBS) 26, and aAB) 40: (a) FS) 0.1; (b) FS) 0.5; (c) FS)
0.7; (c) FS) 0.9. The system volume is set to be V ) Lx× Ly×
Lz) 30 × 30 × 20.
Conjugated Rod-Coil Block Copolymer Brushes Langmuir, Vol. 23, No. 5, 2007 2809
solvents (Figure 2d). The surface layer of the former is clearly
thicker than that of the latter.
The Effect of Grafting Density on the Surface Structures of
Rod-Coil Block Copolymer Brushes. The surface structure of
polymer chains. Thus, the grafting density of polymer brushes
on the surface becomes an important factor for determining the
surface structures of the polymer brushes. Our study found that
the resulting surface structures show no distinguished variation
is the surface coverage. Nevertheless, the surface structures are
greatly affected by the grafting density if the polymer brushes
are immersed in the poor solvents for the rod blocks. Figure 3
shows the surface structures of the rod-coil block copolymer
The rod blocks form a series of small isolated islands on the
surface when the grafting density is as small as FS) 0.1, as
shown in Figure 3a. The size of these isolated islands increases
with increasing grafting density (FS) 0.5). Then, worm-like
disclose (FS) 0.9).
The Effect of Block Ratio on the Surface Structures of Rod-
Coil Block Copolymer Brushes. Two different rod-to-coil block
ratios, 1:2 (nA) 5; nB) 10), and 1:6 (nA) 5; nB) 30), were
simulated under different solvent stimuli and grafting densities
to illustrate the effect of block ratio on the surface structures.
Again, the resulting surface structures show no distinguished
in the good solvent for both the rod and coil blocks. However,
it does reveal evident influence on the surface structures in the
the surface structures and their corresponding side views of the
rod-coil block copolymer brushes with rod-to-coil block ratio
As the grafting density increases, the surface structures vary
from isolated islands to wormlike conformation and finally the
networklike configuration for all the polymer brushes with
different block ratio. However, the increase in the length of the
coil blocks delays the onset of the wormlike conformation. For
example, as shown in Figure 4a at Fs) 0.7, the system with nB
) 30 has aggregates shaped like isolated islands but for system
with nB) 10, the worms are noticeably interconnected.
It also is interesting to find that the isolated islands tend to
float on the top of the coil blocks if the length of the coil blocks
in Figure 4b for Fs) 0.5 and nB) 10. As nBincreases, the
flexible coil blocks start to cover the islands. Finally, the
aggregations formed by the rod blocks are entirely surrounded
by the flexible coil blocks (for Fs ) 0.5 and nB ) 30). The
flexible coil blocks, which are compatible with the solvent
particles, protect the solvophobic aggregates from the energeti-
the aggregative domains into the coil blocks is thus thermody-
Figure 4. Simulated surface structures of rod-coil block copolymer brushes with the parameters of nA) 5, aAS) 40, aBS) 26, aAB) 40,
FS) 0.1-0.9 and nB) 10 and 30. (a) Top view and (b) side views. The system volume is set to be V ) Lx× Ly× Lz) 30 × 30 × Lz.
Lzare 15 and 25 for nB) 10 and 30, respectively.
2810 Langmuir, Vol. 23, No. 5, 2007Wu et al.
of the coil blocks. By doing so, the entropy of the system will
be greatly reduced. When the gain in energy cannot compensate
The system suffers less entropic loss as nBincreases and the
energetic effect becomes the dominant factor. Therefore, the
aggregates reside within the solvophilic coil blocks.
Coil Block Copolymer Brushes. Surface Structures of the PF-
b-PPEGMA-b-PPOPS Polymer Brushes. The polymer brushes
of P1 are named as PB1-1, PB1-2, and PB1-3 corresponding to
the concentration of polymer solution equals to 20, 10, and 5
mg/mL, respectively. Similarly, the polymer brushes of P2 are
named as PB2-1∼PB2-3. The thickness (5-6 nm in dry state)
of the tethered block copolymer was nearly independent of
of Thomas et al., indicating the brush layer set up a self-limiting
barrier to prevent further diffusion of the rod-coils to surface.14
Figure 5. AFM images (5 × 5 µm) of the polymer brushes after solvent treatment. (a) and (b): PB2-1 after toluene treatment; (c) and (d):
PB2-1 after methanol treatment; (e) and (f): PB2-2 and PB2-3 after methanol treatment. (a), (c), (e), and (f) are height images; (b), and
(d) are phase images.
Conjugated Rod-Coil Block Copolymer Brushes Langmuir, Vol. 23, No. 5, 2007 2811
Tapping mode AFM was utilized to examine surface structures
height and phase images, respectively, of the polymer brush
PB2-1 after the toluene treatment. The surface is relatively
smooth; the roughness is 1.45 nm in which roughness is defined
as root-mean-square of height deviations taken from the mean
area plan. The phase image (Figure 5b) provides additional
information on the surface structure. The contrast in the phase
image reflects the differences in hardness between different
phases. Considering that the glass transition (Tg) of the poly-
of its relative hardness as compared to the PPEGMA phase at
both the PF and PPEGMA phases are present on the surface,
which is reasonable because toluene is a good solvent for both
PF and PPEGMA blocks. Figure 5c,d are the height and phase
images, respectively, of the polymer brush PB2-1 after the
the surface becomes relatively rough with a roughness of 6.97
nm. Note that methanol is a good solvent for PPEGMA block
with methanol resulting in its moving up to the surface, whereas
the PF blocks are possibly covered by the PPEGMA resulting
in its falling down to the substrate. Both effects lead to opposite
movement of the blocks and, hence, aggregated islands of PF
blocks with large dimension and even interconnected, wormlike
image of PB2-1 after methanol treatment as compared to that
after toluene treatment. The possible explanation of the reduced
the aggregated domains of PF blocks. The variation of surface
structures of PB2-1 affected by solvent stimuli can be further
qualitatively verified by XPS as shown in Figure 6. The ratio of
the C1sof -CH2O- group at 287.2 eV to the total C1speak
intensity increases significantly from 24 to 41% after treating
with methanol, implying that the -CH2O- side-group of
PPEGMA block on the surface becomes richer in content after
3, the surface coverage estimated from the ratio bright region in
height images increases from about 5% of PB2-3 to about 50%
of PB2-1 indicating that the grafting density of polymer brushes
increases with increasing concentration of the polymer solution
used to prepare the polymer brush. It can be seen that the size
of surface structure with grafting density is exactly the same as
the theoretical prediction as shown in Figure 3.
Figure 7a,b are the height and phase images of the polymer
brush PB1-1 after methanol treatment, respectively. The height
(54) Lin, W. J.; Chen, W. C.; Wu, W. C.; Niu, Y. H.; Jen, A. K. Y.
Macromolecules 2004, 37, 2335.
(55) Wu, W.-C.; Liu, C.-L.; Chen, W.-C. Polymer 2006, 47, 527.
(56) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36, 8312.
with toluene (A) and with methanol (B). 1, Hydrocarbon; 2, ester
carbon of -CH2O-; 3, carbonyl carbon (e.g., -COO-).
Figure 7. AFM images (5 × 5 µm) of PB1-1 after methanol
treatment. (a) Height image; (b) phase image.
2812 Langmuir, Vol. 23, No. 5, 2007Wu et al.
and similar roughness (6.49 nm for PB1-1; 6.97 nm for PB2-1).
However, Figures 5d and 7b suggest significant difference in
brushes after methanol treatment. As shown in Figure 7b, the
are attributed to the difference of hardness between PF and
PPEGMA phases. On the other hand, small phase contrast as
shown in Figure 5d suggests that most of the aggregated PF
domains might be covered by a layer of PPEGMA blocks. The
possible reason resulting in the difference is the different rod-
if the polymer chain length of the coil blocks is short because
of the limited movement of coil blocks. This is consistent with
the phase image of polymer brush PB1-1 because of the short
coil blocks of P1 (rod/coil ) 1:3). As the polymer chain length
in Figure 5d where the copolymer with longer coil blocks (P2,
rod/coil ) 1:6) was used. The results of AFM images suggest
that the surface structures of the studied polymer brushes are in
response to the solvent stimuli and also are determined by the
grafting density and the chemical structures of the amphiphilic
rod-coil block polymers.
Photophysical Properties of the PF-b-PPEGMA-b-PPOPS
Polymer Brushes. Figure 8 represents the PL spectra of polymer
and methanol treatment excited at the wavelength of 380 nm of
1. All the polymer brushes after toluene treatment show two
fluorescence emission peaks around 420 and 440 nm, which are
dramatic fluorescence spectral changes were observed except
for PB1-3. Additional emission shoulders around 475-488 nm
were observed. Furthermore, this additional shoulder shows
increasing intensity with increasing grafting density in both
clearly indicates that the PF blocks aggregate and possibly form
excimers in the polymer brushes.57Although the appearance of
such excimer emission is very common for PF polymers upon
heating,57our findings suggest that the formation of excimers
may be facilitated in PF aggregates with tight intermolecular
packing only through the solvent stimuli. Therefore, the
as described above. The absence of this additional shoulder for
PB1-3 is probably attributed to limited mobility because of the
short chain length of flexible PPEGMA blocks and the low
grafting density that in turn prevent the formation of aggregated
Figure 9 represents the PL spectra of polymer brush PB2-1
after treatments with mixed solvents of toluene and methanol
emission peaks are summarized in Table 1. The additional
shoulders due to the aggregation of PF blocks show increasing
intensity with increasing methanol ratio of mixed solvents. The
PL spectra clearly suggest that the surface structures of
to solvent stimuli.
The relative PL efficiencies of these polymer brushes after
different treatments were calculated by assuming the integrated
PL intensity of polymer brush PB1-1 after toluene treatment as
(57) Weinfurtner, K. H.; Fujikawa, H.; Tokito, S.; Taga, Y. Appl. Phys. Lett.
2000, 76, 2502.
Figure 8. Photoluminescence spectra of polymer brushes (a) PB1-
and methanol treatment (dot lines) excited at the wavelength of 380
Table 1. Photoluminescence Characteristics of
T/M ) 5:1
T/M ) 1:1
T/M ) 1:5
418, 440, 478
420, 442, 488
417, 440, 475c
418, 440, 475c
418, 440, 486
420, 442, 488
418, 440, 482
418, 442, 475c
aConcentration of the polymer solution used to prepare the polymer
brush.bRelative PL efficiencies with the integrated PL intensity of
Conjugated Rod-Coil Block Copolymer BrushesLangmuir, Vol. 23, No. 5, 2007 2813
unity. The corresponding PL efficiencies are listed in Table 1.
1∼PB2-3 decreases with decreasing grafting density after the
treatment with both toluene and methanol due to the decrement
of the polymer brushes based on P1 than those based on P2
under the conditions of similar grafting density and after the
same solvent treatment are attributed to the higher block ratio
of PF in the backbone of P1. The PL efficiency of the polymer
brush after methanol treatment is generally lower than that of
toluene treatment because the aggregation in the former results
in excimer formation and reduces the efficiency. Besides, the
as compared to that after toluene treatment is higher for PB2
than that of PB1 (e.g., from 0.78 to 0.51 for PB2-1 and from 1
to 0.86 for PB1-1). This probably is resulting from the coverage
of aggregated PF domains with flexible PPEGMA blocks due
to the longer PPEGMA blocks of P2. The PL spectra and the
relative PL efficiencies suggest that the photophysical charac-
rod-coil block polymers. The present study could be further
explored to helical polymer brushes by incorporating chiral coil
blocks, which then generate biomimetic assembling structures
and emit polarized light.58
the aggregation of the rod blocks was triggered by their poor
solvents, and the conformation of the aggregation was affected
The aggregative domains tend to stay on top of the surface as
nBis small and submerge into coil blocks as nBincreases. The
two new rod-coil block copolymers, PF-b-PPEGMA-b-
PPOPS, and their corresponding polymer brushes were syn-
thesized by the grafting- method. The experimentally observed
surface effects of solvent stimuli, grafting density, and rod-coil
block ratio on the surface structure were in a good agreement
with the theoretical results. The photophysical characteristics of
the polymer brushes also were consistent with their surface
structures. The present study demonstrates that the surface
and rod-coil block ratio.
Acknowledgment. The work at National Taiwan University
was supported by the National Science Council, the Ministry of
Education, and the Ministry of Economic Affairs of Taiwan,
R.O.C. W.C.W. thanks the financial support from National
Science Council of Taiwan, R.O.C for carrying out part of the
present work at University of Washington.
Supporting Information Available:
diagrams, and corresponding molecular weights of PF macroinitiator
and PF-b-PPEGMA-b-PPOPS. This material is free of charge via the
Internet at http://pubs.acs.org.
1H NMR spectra, GPC
(58) Lam, J. W. Y.; Tang, B. Z. Acc. Chem. Res. 2005, 38, 745.
Figure 9. Photoluminescence spectra of polymer brush, PB2-1,
after treatment with mixed solvent of toluene and methanol excited
at the wavelength of 380 nm.
2814 Langmuir, Vol. 23, No. 5, 2007 Wu et al.