Synthesis and properties of comb polymers with semirigid mesogen-jacketed polymers as side chains

Article abstract of DOI:10.1002/pola.25844

A series of novel comb polymers, poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl] styrene}-g-polystyrene (PMPCS-g-PS), with mesogen-jacketed rigid side chains were synthesized by the "grafting onto" method from α-yne-terminated PMPCS (side chain) and poly(vinyl

Full text of DOI:10.1002/pola.25844

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Synthesis and Properties of Comb Polymers with Semirigid
Mesogen-Jacketed Polymers as Side Chains
Ziyue Ma, Cui Zheng, Zhihao Shen, Dehai Liang, Xinghe Fan
Beijing National Laboratory for Molecular Sciences, The Key Laboratory of Polymer Chemistry and Physics of Ministry
of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Correspondence to: Z. Shen (E-mail: zshen@pku.edu.cn) or X. Fan (E-mail: fanxh@pku.edu.cn)
Received 10 September 2011; accepted 31 October 2011; published online 28 November 2011
DOI: 10.1002/pola.25844
ABSTRACT: A series of novel comb polymers, poly{2,5-bis[(4-
methoxyphenyl)oxycarbonyl]styrene}-g-polystyrene (PMPCS-g-
PS), with mesogen-jacketed rigid side chains were synthesized
by the ‘‘grafting onto’’ method from a-yne-terminated PMPCS
laser light scattering. Both surface morphologies and solution
behaviors were investigated. Surface morphologies of PMPCS
combs on different surfaces were investigated by scanning
probe microscopy. PMPCS combs showed different aggrega-
tion morphologies when depositing on silicon wafers with/
without chemical modification. The PMPCS comb polymers
transferred to polymer-modified silicon wafers using the Lang-
muir-Blodgett technique showed a worm-like chain conforma-
tion. VC 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
(
side chain) and poly(vinylbenzyl azide) (backbone) by Cu(I)-
catalyzed 1,3-dipolar cycloaddition click reaction. The a-yne-ter-
minated PMPCS was synthesized by Cu(I)-catalyzed atom trans-
fer radical polymerization initiated by a yne-functional initiator.
Poly(vinylbenzyl azide) was prepared by polymerizing vinylben-
zyl chloride using nitroxide mediated radical polymerization to
obtain poly(vinylbenzyl chloride) as the precursor which was
then converted to the azide derivative. The chemical structure
and architectures of PMPCS comb polymers were confirmed
Chem 50: 918–926, 2012
KEYWORDS: atom transfer radical polymerization (ATRP); block
1
by H NMR, gel permeation chromatography, and multiangle
copolymers; liquid-crystalline polymers (LCP)
1
0
INTRODUCTION Polymer brushes with well-defined struc-
tures have attracted much attention in recent years. Comb
morphologies in neutral and charged states. Moreover, the
dynamic processes, for example, the collapse/decollapse of
polymethacrylate-g-polybutylacrylate comb polymers, have
1
polymers, which are densely grafted polymer brushes, have
distinct properties compared with sparsely grafted polymer
1
1,12
also been visually observed.
The molecular weights and
MW distributions of comb polymers can be determined by
multiangle laser light scattering (MALLS). Besides the ex-
perimental studies, theoretical simulation and modeling on
comb polymers have also been established recently.
2
–4
brushes and linear homologues.
Comb polymers have
1
3
wide applicability by tailoring their chemical components
and structural parameters, which endow them potential
functionalities in various fields. In a comb polymer, side
chains are densely packed around the backbone, resulting in
a fully stretched backbone. Even a soft backbone, for exam-
ple, polystyrene, in a comb polymer exhibits a rigid or
1
4,15
Controllable methods, such as atom transfer radical polymer-
ization (ATRP), ring-opening polymerization (ROP), and
ionic polymerization, have been widely applied in the syn-
thesis of comb polymers, while click reaction and single-
1
6
17
1
8
5
,6
worm-like conformation rather than random-coil. The per-
1
9
sistence length of polymethacrylate-g-polymethylmethacry-
2
0–22
7
electron-transfer LRP
have also been introduced in
late combs can reach 120 nm, and the backbone stiffness is
8
recent years. Different strategies, classified as ‘‘grafting
through,’’ ‘‘grafting from,’’ and ‘‘grafting onto,’’ have been well
developed for synthesizing comb polymers. In the grafting
through strategy, comb polymers are obtained from radical
or ionic polymerization of vinyl-terminated macromono-
mers, or from ring-opening polymerization of norbornenyl-
strongly affected by solvent quality. Thus, a comb polymer
usually acts like a rigid or worm-like semirigid cylinder,
which is also called a ‘‘cylindrical polymer’’ or ‘‘bottle brush.’’
Comb polymers have been well-characterized by a variety of
physical techniques. More importantly, because of its high
molecular weight (MW) and large diameter, an individual
comb polymer can be visualized by scanning probe micros-
1
6
1
8
1
7
terminated macromonomers. In the grafting from strat-
9
23,24
copy (SPM). A poly(2-vinylpyridine)-co-polymethylmethacry-
egy,
the polymer backbone is first synthesized, followed
late-g-polymethacrylate comb shows completely different
by the polymerization of side chains. In the grafting onto
Additional Supporting Information may be found in the online version of this article.
2011 Wiley Periodicals, Inc.
V
C
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2
5,26
strategy,
the polymer backbone and side chains are sepa-
vertically polarized solid state laser (GNI, Changchun GXC-III,
532 nm, 100 mW) was used as the light source.
rately polymerized and then coupled, and the parameters of
both the backbone and side chains can be individually
tailored. To attach side chains onto the backbone efficiently,
highly efficient, quantitative reactions, such as nucleophilic
Other Experimental Methods
Gel permeation chromatographic (GPC) experiments were
conducted on a Waters 2410 instrument equipped with a
Waters 2410 refractive index (RI) detector and two Waters
l-Styragel columns (103 and 104 Å). THF with a flow rate of
2
6,27
25,28
reaction,
click reaction,
and atom transfer nitroxide
2
9
radical coupling (ATNRC), have been widely used in the
grafting onto strategy. So far, most of the reported works on
comb polymers use flexible, coil-like polymers, such as poly-
styrene (PS), polybutylacrylate, poly(ethylene oxide), as
the side chains, either linear or multibranched self-assembly
1
.0 mL/min was used as the eluent. The calibration curve
1
was obtained with linear polystyrenes as standards. H NMR
spectra were obtained with a Varian 300 MHz spectrometer.
Thermogravimetric analysis (TGA) was performed on a TA
Q600 SDT instrument in a nitrogen atmosphere. Differential
scanning calorimetric (DSC) examination was carried out on
a TA Q100 calorimeter in a nitrogen atmosphere.
2
7
30
31
3
2–39
dendrons.
However, comb polymers with rod-like side
4
0
chains are rarely reported due to the difficulty in synthesis
caused by the immense steric hindrance of such crowded
side chains.
Materials
Mesogen-jacketed liquid crystalline polymers (MJLCPs),
which usually contain bulky mesogenic side groups, have
been extensively studied by our group.
polymers, mesogenic side groups are considered to be
densely packed around the backbone; therefore, the MJLCP
molecules may act as semirigid rod-like objects and form
ordered phases in certain conditions. The rigidity of MJLCP
rods can be easily tailored by varying the length, shape,
rigidity, and tails of side groups. Therefore, MJLCP is an
excellent rod-like candidate to build block copolymers with
2-Bromo-2-methylpropionyl bromide (98%, Acros), ethyl 2-
bromoisopropionyl (98%, Acros), 2,2,6,6-tetramethylpiperidi-
nyloxy (TEMPO, 98%, Acros), propargyl alcohol (99%,
4
1,42
In this kind of
0
00 00
Aldrich), sodium azide (99%), and N,N,N ,N ,N -pentamethyl
diethylenetriamine (PMDETA, 98%, TCI) were used as
received. N,N-Dimethylformamide (DMF), tetrahydrofuran
(
THF), and triethylamine (98%, Beijing Chemical Reagents,
A.R.) were used after distillation. Chlorobenzene was succes-
sively washed with concentrated sulfuric acid to remove re-
sidual thiophenes, 5% sodium carbonate solution, and deion-
ized water, and then dried with anhydrous calcium chloride
before finally distilled for use. All other reagents were pur-
chased from Beijing Chemical Reagents and used as received.
4
3
44
various architectures. Researches on rod-coil, rod–rod,
4
5
46
triblock, and multiarmed polymers containing MJLCPs as
the rod component have been investigated. However, comb
polymers containing MJLCPs as rigid side chains have never
been reported.
Synthesis of MJLCP Comb
Synthesis of Monomer {2,5-bis[(4-Methoxyphenyl)
oxycarbonyl]Styrene} (MPCS)
Herein, we report a new kind of comb polymers with poly-
styrene (PS) as the backbone and an MJLCP, poly{2,5-bis[(4-
methoxyphenyl)oxycarbonyl]styrene} (PMPCS), as the rod-
like side chain. By combining ATRP, nitroxide mediated
radical polymerization (NMRP), and Cu(I)-catalyzed classic
click reaction, we successfully synthesized a series of MJLCP-
containing comb polymers (PMPCS-g-PS) with different
backbone length via the grafting onto strategy. The surface
morphologies of comb polymers on different substrates were
investigated by SPM, respectively.
MPCS was synthesized from 2,5-dimethylbenzonic acid and
47
p-methyloxy phenol by the method described in literature.
Synthesis of Propargyl 2-Bromoisobutyrate
Propargyl alcohol (1.1 g, 19.6 mmol) and triethylamine
(
(
(
(
3.5 g, 23.5 mmol) were dissolved in anhydrous THF
30 mL), and the solution was cooled down to 0 C. THF
30 mL) containing 2-bromo-2-methylpropionyl bromide
ꢀ
3.0 g, 23.5 mmol) was added dropwise. The reaction mix-
ꢀ
ture was stirred at 0 C for 30 min and at ambient tempera-
ture overnight. An equal amount of water was then added,
and the mixture was extracted with 100 mL dichloromethane
for three times. After the removal of solvents, the raw
product was purified by silica column chromatography. The
product was a yellowish liquid with a yield of 73%.
EXPERIMENTAL
Characterization Methods
SPM Measurements
SPM Images were obtained at ambient conditions with a
Seiko SPI3800/SPA400 instrument (Seiko Instrument, Japan)
operating in DFM mode. SI-DF3 cantilever was used as a
probe with a resonance frequency of about 24 kHz and a
spring constant of 1.4 N/m. Both height and phase images
were recorded.
1
H NMR (CDCl , 300 MHz, d): 4.78 (d, 2H, CH O), 2.53
3
2
(t, 1H,CBCH), and 1.96 (s, 6H, (CH ) C).
3
2
Polymerization of MPCS Initiated by
Yne-Functional ATRP Initiator
Light Scattering Measurements
In a typical experiment for polymerization, MPCS (0.5 g,
1.2 mmol), propargyl 2-bromoisobutyrate (4.2 mg, 20 lmol),
CuBr (2.9 mg, 20 lmol), PMDETA (3.5 mg, 20 lmol), and
chlorobenzene (2.8 g) were added into a polymerization
tube. The concentration of MPCS was 15 wt %. After
degassed by three freeze-pump-thaw cycles, the tube was
A commercial laser light scattering spectrometer (Brook-
haven, Holtsville, NY) equipped with a BI-200SM goniometer
and a BI-TurboCorr digital correlator was used to perform
both static light scattering (SLS) and dynamic light scattering
ꢀ
(
DLS) over scattering angles ranging from 20 to 150 . A
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sealed under vacuum. Subsequently, the tube was immerged
into an oil bath thermostated at 90 C for 12 h. At the end
22 lmol) were introduced into a Schlenk flask and dissolved
in DMF (4.0 g). The ratio of yne moiety and azide moiety
was 1.5:1 (mol/mol). The Schlenk flask was degassed by the
freeze-pump-thaw sequence for three cycles, sealed under
vacuum, stirred at ambient temperature for 4 h, and then
ꢀ
of polymerization, the tube was quenched in liquid nitrogen.
The polymer solution was diluted by an equal amount of
cold THF and passed through a neutral alumina column to
remove the Cu(I) catalyst, and then the polymer was precipi-
tated out from methanol to obtain a white solid. The conver-
sion was about 90%, and the polydispersity index (PDI) was
determined to be about 1.1 by GPC.
ꢀ
immerged into an oil bath thermostated at 80 C for 24 h.
The solution was diluted by an equal amount of cold THF
and passed through a neutral alumina column to remove
Cu(I) catalyst. The comb polymer was precipitated out from
methanol. The raw product of the comb polymer was then
reprecipitated.
Synthesis of TEMPO-Cl
4
8
TEMPO-Cl was prepared by following the reported method.
0
Reprecipitation
[
N,N -Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediami-
In a general procedure, 400 mg of the raw comb polymer
was dissolved in 20 mL of THF. An equal amount of metha-
nol was added dropwise to the solution until it was turbid.
The suspension was centrifuged at 4000 rpm for 15 min,
and the supernatant was carefully removed. The gel-like
solid was washed with THF quickly, dissolved in fresh THF,
and then precipitated in methanol. This procedure was
repeated for several times until no signal of unreacted
PMPCS was detected by GPC measurements.
nato]manganese(III) chloride (1.9 g), followed by di-tert-
butyl peroxide (2.94 g) and sodium borohydride (1.5 g),
was added to a solution of p-chloromethylstyrene (3.0 g,
1
9.7 mmol) and TEMPO (3.0 g, 19.2 mmol) in 1:1 toluene/
ethanol (v/v, 90 mL). The suspension obtained was stirred
at ambient temperature for 24 h and then poured into
2
00 mL of deionized water. The mixture was extracted with
dichloromethane (200 mL) three times. The combined
organic layers were then dried, and the crude product was
purified by flash chromatography, eluted with a mixed
solvent of dichloromethane/petroleum ether (1:9 gradually
increased to 1:1, v/v). The product (TEMPO-Cl) was
obtained as a light yellowish solid (4.20 g, 52.5% yield).
1
Surface Modification of Silicon Wafer
Initiator Grafting
Silicon disks were cut into 1 cm ꢁ 1 cm pieces, cleaned
49
ꢀ
according to a four-step RCA method, and dried at 90 C
for 2 h (Marked as SiAOH). The SiAOH wafers were put in a
beaker and immerged into 100 mL of anhydrous THF con-
H NMR (CDCl , 300 MHz, d): 0.66, 1.02, 1.16, 1.28 (s,12H,
3
CH ), 1.25-1.54 (m, 6H, CH ), 1.45 (d, 3H, CH ), 4.59 (s, 2H,
3
2
3
CH Cl), 4.80 (q,1H, CH), 7.26-7.34 (m, 4H, ArH). ELEM. ANAL.
3
taining 1 mL Et N. An additional 20 mL of THF solution con-
2
Calcd. for C H ClNO: C, 69.8%; H, 9.11%; N, 4.52%. Found:
C, 69.24%; H, 9.10%; N, 4.23%.
taining 1 mL of 2-bromo-2-methylpropanoyl bromide was
added dropwise under stirring. The mixture was stirred for
another 4 h and then poured into 100 mL of deionized water
to remove excess 2-bromo-2-methylpropanoyl bromide. The
silicon wafers were washed with deionized water and
acetone and dried at ambient temperature in a nitrogen
atmosphere (Marked as SiABr).
1
8 28
Polymerization of Vinylbenzylchloride
In a typical experiment for polymerization, vinylbenzylchlor-
ide (VBC) (1.0 g, 6.5 mmol), TEMPO-Cl (6.8 mg, 23 lmol),
and chlorobenzene (2.3 g) were added into a polymerization
tube. The concentration of VBC was 30 wt %. After degassed
by three freeze-pump-thaw cycles, the tube was sealed under
vacuum. Subsequently, the tube was immerged into an oil
PMPCS Grafting
MPCS (0.2 g, 0.48 mmol) and a SiABr wafer were put in a
polymerization tube, which was then added with predeter-
mined amounts of ethyl 2-bromopropionate free initiator,
PMDETA, and CuBr. Chlorobenzene (0.46 g) was added as a
solvent, and the final concentration of MPCS was 30 wt %.
The tube was treated by freeze-pump-thaw for three cycles,
sealed under vacuum, and then put into an oil bath thermo-
ꢀ
bath thermostated at 120 C for 48 h. At the end of polymer-
ization, the tube was quenched in liquid nitrogen. The poly-
mer solution was diluted by an equal amount of cold THF,
and the polymer was precipitated out from methanol to
obtain poly(vinylbenzyl chloride) (PVBC) as a white solid.
The conversion was about 70%, and the PDI was around 1.3
determined by GPC.
ꢀ
stated at 90 C. After 24 h, the tube was quenched by liquid
nitrogen, and the solution was dissolved in THF. The polymer
solution was passed through a neutral alumina column, and
the polymer was precipitated out from methanol, and char-
acterized by GPC. The silicon wafer was successively washed
thoroughly with DMF to remove adsorbed Cu(I) catalyst and
then with deionized water and acetone, and it was dried at
ambient temperature in a nitrogen atmosphere (Marked as
SiAPolymer).
Azide Substitution of PVBC
PVBC (500 mg) was dissolved in 20 mL of anhydrous DMF,
and NaN (1.0 g) was added to the solution. The suspension
3
was stirred at ambient temperature for 24 h. When the reac-
tion was completed, 100 mL of water was added. Pure poly
(
vinylbenzyl azide) (PVBA) was collected by filtration as a
white solid and dried overnight.
Click Reaction of PVBA and a-Yne PMPCS
Langmuir-Blodgett Film Preparation
A typical procedure is described as follows. a-Yne-terminated
Samples were dissolved in chloroform to obtain a 1 mg/mL
solution. The injected volume was 50 lL. Each sample had
20 min to reach equilibrium before compression. The
PMPCS (400 mg, M ¼ 18,000 g/mol, 22 lmol yne moiety),
n
PVBA (2.3 mg, 14.8 lmol azide moiety), and CuBr (3.1 mg,
920
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tions. However, only methacryl-terminated PMPCS macro-
monomer could be polymerized to afford oligomers (Fig. 1).
This might be caused by the bulkiness of the PMPCS side
chain and the relatively low concentration of the terminal
group.
Alternatively, we chose the grafting onto strategy taking its
advantages over control of backbone length, side-chain
length, and polydispersity. The key step for this strategy is
to choose an effective reaction that ensures maximum graft-
5
0
ing efficiency. Click reaction has been widely used in the
synthesis of brush polymers and proven to be quantitative.
Herein we designed a synthetic scenario shown in Scheme 1.
Because vinylbenzyl azide was unstable in the polymeriza-
tion process, the PVBC backbone was polymerized from VBC
as a precursor and then converted to PVBA by azide substi-
tution with high efficiency (Supporting Information Fig. S4).
Polymerization of MPCS was initiated by an ATRP initiator
FIGURE 1 GPC traces of methacryl-terminated PMPCS before
(
blue line) and after (red line) polymerization. Peaks on the red
line refer to monomer, dimer, and trimer.
containing yne group. The polymerization was carried out in
1
a dilute solution to prevent yne–yne coupling. In the
H
2
compression speed was 20 cm /min. The monolayer film
NMR spectrum the signal of the hydrogen on terminal
ACBCH appeared at d ¼ 2.20 ppm (Fig. 2, curve 1). Finally,
the backbone and the side chains were linked by the classic
click reaction, and PMPCS combs were obtained. After click
reaction and reprecipitation, the resonance of yne at d ¼
was transferred onto a silicon wafer (dipping up) at the
selected pressure. During the transfer process the pressure
remained constant. All sample wafers were kept dry for 24 h
before the SPM measurement was conducted.
2
.20 ppm disappeared (Fig. 2, curve 2), indicating that no
RESULTS AND DISCUSSION
residual side chains remained in the PMPCS combs.
Comb Synthesis
The CuBr catalyst worked so well in our experiment that no
ligand (e.g., PMDETA) was needed for this reaction. Para-
meters for the click reaction were carefully chosen. Higher
temperature, higher concentration, or longer reaction time
led to inevitable gelation. Figure 3 shows the influence of
molar feeding ratio of yne to azide. Sufficient feeding of
side chains (yne moiety) resulted in a broad and asymmetric
peak, which might be due to side reactions such as
To obtain a comb polymer there are three strategies classi-
fied as grafting through, grafting from, and grafting onto. We
first attempted to use the grafting through method and
synthesized two kinds of PMPCS macromonomers with
methacryl and vinylbenzyl terminal groups. Polymerizations
of both kinds of macromonomers with different molecular
weights were subsequently attempted under various condi-
SCHEME 1 Synthesis routine of comb polymers with a semirigid MJLCP as side chains.
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1
FIGURE 2 H NMR spectra of yne-PMPCS (curve 1), and PMPCS
FIGURE 3 GPC traces of click reaction with different yne:azide
comb after click reaction (curve 2).
ratios.
intermolecular coupling or degradation induced by unreacted
azide groups. Therefore, we added excess amount of yne
moiety to eliminate side reactions and to obtain a maximum
grafting density. The eluent peak became monomodal as the
concentration of the side chain increased. However, the re-
moval of unreacted side chains became a challenge (Fig. 3,
black curve). The optimal yne to azide ratio was determined
to be 1.5:1. After careful reprecipitation in a selective solvent
of 1:1 THF/methanol (v/v), a series of PMPCS combs with
different lengths of side chains and backbone were obtained.
The results are shown in Table 1.
The absolute molecular weights were obtained from SLS
results, and according to which the grafting densities were
2
4,28
calculated.
The Mw, LS of combs were in accordance with
the theoretical molecular weights (see Table 1) calculated
from the DPs of the backbone and the side chain. For Comb
1 and Comb 2, grafting densities were 100% and 92%,
respectively. Thus, almost each site had a PMPCS grafted. For
Comb 3 with the longest backbone, because its precursor
PVBA with a large molecular weight (M
n
¼ 150,000 g/mol)
had relatively poor solubility during the click reaction, Mw, LS
measured by SLS was lower than the theoretical value, indi-
cating a lower grafting density of about 75%.
GPC equipped with an RI detector was used to monitor the
reprecipitation process. Typical GPC eluent curves for a
PMPCS comb before and after reprecipitation are shown in
Figure 4. However, for comb polymers molecular weights
measured by GPC are not accurate. Side chains in PMPCS
combs were densely packed, and their molecular weights, cali-
brated by linear polystyrenes, were strongly underestimated.
In addition, anomalous elusive behavior of high MW combs in
All samples had glass transition temperatures of about 130
ꢀ
C, and the temperatures at which 5% weight loss occurred
ꢀ
were all over 300 C. Compared with linear PMPCS which is
soluble in common organic solvents, PMPCS combs were
only soluble in DMF, THF, dichloromethane, and chloroform
because of their large molecular weights.
7
GPC measurements also makes molecular weight deviate
from the true value. GPC traces showed monomodal peaks,
Surface Modification of Silicon Wafers
and hydrodynamic radius (R
gle narrow peak from DLS measurements; therefore,
unreacted PMPCS side chains had been completely removed.
h
) of each sample presented a sin-
SPM technique is widely used in probing comb polymers.
However, the imaging of PMPCS combs was rather challeng-
ing. Because PMPCS combs contained fewer ester groups
TABLE 1 Structural Parameters and Characterizations of PMPCS Combs
M
n, comb
6
M
w,LS
6
M
n,backbone
a
M
n,side chain
b
(10
(10
Y
graft
e
T
g
ꢀ
T
d, 5%
g
a
c
d
f
ꢀ
(g/mol)
(g/mol)
DPbackbone
DPside chain
g/mol)
PDI
g/mol)
(%)
( C)
( C)
Comb 1
Comb 2
Comb 3
Comb 4
a
8,500
20,000
20,000
19,000
30,000
54
283
943
54
50
50
47
74
0.18
0.21
0.37
0.24
1.4
1.4
1.3
1.3
1.50
7.15
16.8
2.91
ꢂ100
92
128
132
132
130
342
360
364
–
4,5000
150,000
8,500
75
–
c
Determined by the GPC (calibrated with polystyrene standards).
PDI ¼ M
w n
/M from GPC results.
Determined by SLS in DMF (for details see Supporting Information).
Calculated according to Mw, LS measured by SLS.
b
d
Obtained by the GPC value (calibrated with polystyrene standards)
44
e
f
multiplied by 1.52. In previous work we have proven that the absolute
molecular weight of linear PMPCS is approximately 1.52 times of the
Determined from DSC measurements in a nitrogen atmosphere.
Determined by TGA measurements in a nitrogen atmosphere.
g
n
GPC M value.
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where Wreal and Wexp are the real and apparent diameters of
the object, R is tip radius of the probe (10 nm for SI-DF3
t
probe used in our case), h is the height of the sample (par-
ticles or plateaus), and d is the illusional length which
should be subtracted. Equation 1 applies to large spherical
5
2
53
particles, while eq 2 to flat plateau samples. All diame-
ters given below were calibrated using these equations.
In the first attempt to deposit PMPCS combs on wafers, 30
lL of a dilute DMF solution with a concentration of 0.1 mg/
mL (DLS results indicated that PMPCS combs existed as sin-
gle polymer chains under such conditions) was added drop-
wise onto different wafers. Imaging results showed that
PMPCS combs aggregated into uniform nanospheres with an
average diameter of 350 nm on unmodified SiAOH wafer.
These nanospheres further assembled into crowded
‘‘necklace’’ pattern [Fig. 6(a,b)]. In contrast, the spherical
FIGURE 4 GPC traces of PMPCS side chain (blue curve), crude
nanospheres were mostly individually distributed on the
SiAPolymer wafers, and no further aggregation was
observed [Fig. 6(c,d)]. This phenomenon indicated a ‘‘two-
step aggregation’’ mechanism. During solvent evaporation,
PMPCS combs aggregated and formed free-standing nano-
spherical aggregates (first step) in the remaining solution.
This step of aggregation occurred on both types of wafers.
When the solvent continued to evaporate, on SiAOH wafers,
further aggregation occurred due to the incompatibility
between PMPCS nanospheres and the silica surface, and dif-
ferent patterns of spherical aggregation were formed (second
step). However, we could not explain the formation of the
necklace pattern observed in various locations. On the other
hand, the situation was different on the PMPCS-modified sur-
face. The modified surface was more PMPCS-philic than
silica; therefore, primary nanospheres were ‘‘stuck’’ onto the
surface that prohibited the second-step aggregation, resulting
in the formation of well distributed nanospheres. Correlated
to the results from light scattering measurements, the aggre-
comb (red curve), and reprecipitated comb (black curve).
than polybutylacrylate combs, they could not stick on mica
or silica surface firmly. Moreover, neither silica nor silicon
wafer is PMPCS-philic, PMPCS combs tended to form clusters
on these wafers. Therefore, surface modification was first
performed to improve compatibility.
An ATRP initiator based on 2-bromo-2-methylpropanoyl was
introduced onto silica surface. Because surface-initiated
ATRP has the same controllable features as in solution,
5
1
free initiator ethyl 2-bromopropionate was added for
the easy characterization of free polymers by conventional
methods. The Mn, GPC of the surface-grafted polymer was
regulated to be 20,000 g/mol. After polymerization, free
PMPCS was precipitated out from methanol and analyzed by
GPC. SiAPolymer wafer was immersed into pure DMF and
washed thoroughly to remove Cu(I) residues adsorbed on
surface. XPS spectra (Fig. 5) showed that the surface C com-
position increased from 14.5% on the SiAOH to 35.5% on
the SiAPolymer, and Si composition decreased from 54.3%
to 35.7%, confirming that PMPCS polymers were successfully
grafted onto the silicon wafer.
3
3
gation number was estimated to be 60 (Nagg ¼ R /R ).
SPM
g;LS
Surface Morphologies
Because Comb 3 had the largest aspect ratio with a DP of
943 in the backbone and a DP of 44 in each PMPCS side
chain, it was chosen for SPM measurements. The physical
shape of the probe will affect the SPM imaging process. The
height measurement in SPM is relatively reliable. However,
the diameter of an object is, to a certain degree, larger than
its real value depending upon the tip radius of the SPM
probe and the shape of the object. Therefore, we used the
following equations to correct the diameters:
2
2
ðW þ 4h Þ
exp
Wreal=2 ¼
(1)
(2)
8
h
FIGURE 5 XPS spectra of SiAOH (Curve 1, Si 54.3%, C 14.5%),
SiABr (Curve 2, Si 48.1%, C 22.3%), and SiApolymer (Curve 3,
Si 35.7%, C 35.5%).
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
Wreal ¼ Wexp ꢃ 2d ¼ Wexp ꢃ 2 R ꢃ ðRt ꢃ hÞ
t
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FIGURE 6 PMPCS comb aggregations of Comb 3 on different types of surfaces: SiAOH wafer (a,b); SiApolymer wafer (c,d).
To obtain the unimolecular image, we employed the Lang-
muir-Blodgett technique to avoid the aggregation of PMPCS
combs. From the p-A curve (Supporting Information Fig. S5),
the zero-pressure area was ꢂ20 Å per MPCS unit which
behaved as a semirigid chain with the gauche/trans confor-
mation. The aspect ratio of combs obtained from SPM was
9.4 (LSPM/Ldiameter), which was close to 10.0, the theoretical
value calculated from DPbackbone/2DPside chain
.
2
equals to 10,000 nm per comb. The compression process
In common situations the repulsion between neighboring
groups becomes a minimum when the backbone takes all-
trans conformation. In PMPCS combs, however, it was not
the optimal conformation. In all-trans conformation the
neighboring groups point to the same direction. Considering
that the diameter of bulky PMPCS side chains (15.9 Å) was
overwhelmingly larger than the maximum value of lDP,backbone
(2.5 Å), there was not enough space for PMPCS side chains
to pack around an all-trans backbone. According to the high
grafting density obtained from light scattering measurements
and diameters measured by SPM, we propose that the back-
bones in PMPCS combs took a partially coiled conformation
to stagger side chains to decrease hindrance. On the other
hand, in PMPCS side chain the mesogenic groups was not so
bulky that in side chains the vinyl backbone took a confor-
mation which was closer to all-trans conformation, which
was irreversible. Owing to the relatively slow conformational
transition for such a rigid comb, combs were fixed on the
water surface after solvent evaporation. Because the film
was not quite stable at high pressures, the transfer pressures
were chosen as 1 and 5 mN/m.
From the SPM images in Figure 7, unimolecules were clearly
visualized. The height of the PMPCS comb was 3.8 nm, indi-
cating that the combs lied flat on the surface. The average di-
ameter (a) of PMPCS comb cylinders was 21.3 nm. This di-
ameter gave the length per PMPCS monomer lDP,PMPCS ¼ a/
2
DPside chain ¼ 2.3 Å, suggesting that the side chains of the
PMPCS comb were fully extended and rod-like. The average
length of the comb, LSPM, was 200 nm after calibration. Thus,
the length per DP was calculated as lDP,backbone ¼ LSPM/
DPbackbone ¼ 2.1 Å, which indicated that the comb backbone
FIGURE 7 SPM images of PMPCS Comb 3 on SiAPolymer wafers. The dipping speed was 2 mm/min with a transfer pressure of 1
2
N/m : (a) height image, vertical scale 10 nm; (b) height image, vertical scale 9 nm; (c) phase image of b; (d) 3D view of b; (e) alti-
tude measurement, vertical scale 10 nm; (f) cross-sectional analysis of (e), vertical scale 5 nm. The apparent distance between tri-
angles is 37.1 nm. After calibration using eq 2, the real distance is 21.3 nm.
924
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was why in PMPCS comb lDP,side chain was slightly larger than
DP,backbone in our measurements.
11 Gallyamov, M. O.; Tartsch, B.; Khokhlov, A. R.; Sheiko, S.
S.; B o¨ rner, H. G.; Matyjaszewski, K.; M o¨ ller, M. Chem. Eur. J.
l
2
004, 10, 4599–4605.
Samples prepared under different conditions were all ana-
lyzed. Although the transfer pressure and the dipping speed
were varied, the shape and average length of the single mol-
ecules remained almost the same (Supporting Information
Fig. S6). This observation was in accordance with the irre-
versible compression behavior in the Langmuir-Blodgett
technique. Because there was no significant selectivity on
dipping speed or transfer pressure, the comb molecules
could be totally fixed on water surface after solvent evapora-
tion before transfer. Moreover, the attempts to transfer
PMPCS Langmuir-Blodgett film onto silicon wafers without
modification were failed, which might be due to the incom-
patibility between PMPCS combs and silicon wafer.
1
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A series of new comb polymers containing rod-like side
chains, poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}-
g-polystyrene (PMPCS-g-PS), with different lengths of the
backbone and side chains were successfully synthesized by
the grafting onto method. By combining ATRP, NMRP, and
click reaction, lengths of both the backbone and side chains
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effective way to prepare rod-grafted comb polymers. PMPCS
combs exhibited a two-step aggregation behavior when
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1
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