Synthesis and self-assembly of polyhydroxylated and electropolymerizable block copolymers

Article abstract of DOI:10.1002/pola.27234

A facile synthetic strategy for preparing hydroxylated polymethacrylate amphiphilic block copolymers (PCzMMA-b-PBMMA, PFlMMA-b-PBMMA) incorporated with primary and secondary hydroxyl groups and electroactive moieties along the polymer backbone is reported

Full text of DOI:10.1002/pola.27234

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Synthesis and Self-Assembly of Polyhydroxylated and
Electropolymerizable Block Copolymers
Satyananda Barik, Suresh Valiyaveettil
Department of Chemistry, 3 Science Drive 3, National University of Singapore, 10 Kent Ridge Crescent, Singapore 117543
Correspondence to: Dr. S. Valiyaveettil (E-mail: chmsv@nus.edu.sg)
Received 2 January 2014; accepted 26 April 2014; published online 14 May 2014
DOI: 10.1002/pola.27234
ABSTRACT: A facile synthetic strategy for preparing hydroxy-
lated polymethacrylate amphiphilic block copolymers
in thin films. The network structure was characterized with a
range of spectroscopic techniques. Such highly processable
polymers may be of interest to applications in which a con-
(PCzMMA-b-PBMMA, PFlMMA-b-PBMMA) incorporated with
primary and secondary hydroxyl groups and electroactive moi-
eties along the polymer backbone is reported. Full characteriza-
tion, structure-property relationship and self-assembly of these
polymers are discussed. Due to interplay of hydrophobic/
hydrophilic interactions, PCzMMA-b-PBMMA formed a layered
lattice and PFlMMA-b-PBMMA showed a vesicular morphol-
ogy. Electropolymerization of the electroactive units led to the
formation of cross-conjugated polymer network in solution and
ducting amphiphilic films with strong adhesion to various sub-
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2014 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2014, 52, 2217–2227
KEYWORDS: amphiphilic block copolymers; atom transferoˆ radi-
cal polymerization (ATRP); electropolymerization; polyhydroxy-
lated polymers; self-assembly
PBMMA) with one block bearing primary/secondary
hydroxyl groups and the second block with electroactive car-
bazole or fluorene moieties (Fig. 1). Interplay of hydrophilic
and hydrophobic interaction leading to interesting self-
assembled structures is expected. In addition, electropolyme-
rization of carbazole or fluorene groups along the polymer
backbone present on the hydrophobic block will lead to
cross-CPN formation.
INTRODUCTION The self-assembly of block copolymers has
gained much attention owing to the presence of long range
ordering in the polymer lattice to form highly ordered micelles,1
cylindrical rods,2 long fibers, tubes or vesicles.1–6 Multiple fac-
tors such as hydrophilic or hydrophobic nature, block lengths,
and interaction of various functional groups leading to miscibil-
ity or phase separation inside the polymer lattice are responsi-
ble for the formation of interesting morphologies.7–10
Hydrophilic polymers based on telechelic polyethylene glycol,
2-ethyl-(2-pyrrolidine)methacrylate, poly(2-hydroxyethyl meth-
acrylate), and other polymers bearing primary hydroxyl groups
on the side chain have been reported.4 Poly(vinyl alcohol) and
its copolymers have been extensively used for preparing stable
hydrogels and interpenetrating networks.11
EXPERIMENTAL
Materials and Measurements
CuBr, CuCl, PMDETA, HMTETA, ethyl 2-bromoisobutyrate,
and 9-hydroxy fluorene were purchased from Sigma Aldrich
and used without further purification. Solketal, tosyl chlo-
ride, carbazole, 3-hydroxy benzyl alcohol, and methacryloyl
chloride were purchased from Sigma-Aldrich and 18-crown
Amphiphilic block copolymers are interesting owing to their
applications in drug encapsulation,12 adhesives,13 coatings,14
and stable gels.15,16 Block copolymers have been extensively
studied due to their unusual morphologies in solution and in
solid state.17–22 Solketal-based monomers offer a facile route
for obtaining multihydroxylated hydrophilic polymers21 and
incorporation of electroactive groups helps to prepare conju-
gated polymer network (CPN) for enhancing the conductivity
of the polymer lattice through electropolymerizaton.22,23
1
ether-6 from Merck. H and 13C NMR spectra were recorded
using Bruker ACF 300/AMX 500 MHz instrument. Molecular
weight of polymers was determined by using gel permeation
chromatograph (GPC, Shimadzu LC vt 10AT) instrument
equipped with UV and refractive index detectors and polysty-
rene as standard. THF was used as an eluent at a flow rate
of 0.3 mL/min. FT-IR spectra were recorded using Bio-Rad
FT-IR spectrophotometer. Thermogravimetric analyses (TGA)
were done on a TA-SDT2960 instrument using a heating rate
of 10 ꢀC/min under nitrogen atmosphere. The self-assembly
Here, we report synthesis, characterization of polyhydroxy-
lated block copolymers (PCzMMA-b-PBMMA and PFlMMA-b-
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 Molecular structure of target amphiphilic block
copolymers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
of block copolymers was investigated according to the fol-
lowing procedure. Deionized water (2 mL) was added drop-
wise to block copolymer solution in THF (2 mL, 0.05 mg/
mL) under vigorous stirring. After 2 h, a drop of the disper-
sion was dried on a glass slide (Scanning electron micro-
graph, SEM) or carbon coated grid (TEM) and used for
morphological studies. The morphology of the polymer in
solid state was analyzed using JEOL 2010F at an accelerating
voltage of 200 kV. SEM images were taken in JEOL JSM 6700
instrument. The wide angle X-ray scattering patterns were
obtained using a D5005 Siemens X-ray diffractometer with
Cu Ka (1.54 Å) radiation (40 kV, 40 mA). Sample was
scanned between 2h 5 2ꢀ and 50ꢀ with step size of 2h 5
0.01ꢀ/m. Cyclic voltammogram (CV) studies were performed
using an Autolab PGSTAT30 Potentistat/Galvanostat with a
standard three electrode electrochemical cell containing a
0.1 M tetrabutylammonium hexafluorophosphate solution in
acetonitrile at room temperature under nitrogen atmosphere
using a scanning rate of 50 mV/s. A platinum/ITO working
electrode, platinum counter electrode, and SCE (saturated
KCl aqueous solution) reference electrode were used. Atomic
force microscopy (AFM) imaging was done under ambient
conditions with a PicoSPM II (PicoPlus, Molecular imaging)
in the Magnetic AC mode (MAC mode). MAC mode uses a
magnetic field to drive a magnetically coated cantilever in
the top-down configuration.
SCHEME 1 Synthetic procedures for monomers: (i) p-TsCl,
THF, 0 ꢀC, 12 h; (ii) 2, K2CO3, DMF, 18-crown-6, 75 ꢀC, 5 days;
(iii) THF/EtOH, NaCNBH3, AcOH, rt, 12 h; (iv) methacryloyl chlo-
ride, triethylamine, THF, rt, 12 h; (v) KOH, DMSO,
1-bromohexanol, rt, 24 h.
were immersed in an ice bath and tosyl chloride (32.4 g,
170 mmol) in THF (100 mL) was added drop wise. The mix-
ture was stirred overnight, concentrated, mixed with water,
and extracted with ether. The organic layer was dried over
anhydrous sodium sulfate, filtered, and concentrated to give
a low melting solid in quantitative yield (31 g, 98%). 1H
Syntheses
The primary synthetic goal was to synthesize amphiphilic
methacrylate block copolymers from monomers functional-
ized with hydroxyl groups or electroactive groups. Block
copolymers (PCzMMA-b-PBMMA and PFlMMA-b-PBMMA)
were prepared using synthetic routes shown in Schemes 1
and 2. The monomers were synthesized by esterification
reaction between hydroxyl group containing intermediates
(5, 7, and 10) with methacryloyl chloride in dry THF and
triethylamine at room temperature (Scheme 1).23(c,d)
Synthesis of (Toluene-4-sulfonic acid 2, 2-(dimethyl-[1, 3]
dioxolan-4-ylmethyl ester) (2)
To a solution of solketal (15 g, 113 mmol) in THF (120 mL),
a solution of NaOH (5N, 60 mL) was added. The contents
SCHEME 2 Synthetic procedures: (i) CuBr/PMDETA, Ethyl 2-
bromoisobutyrate, toluene, 75 ꢀC; (ii) CuCl, HMTETA, anisole,
75 ꢀC, 24 h; (iii) THF, 2M HCl, 70 ꢀC, 2 h.
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NMR (300 MHz, CDCl3, d ppm): 1.33 (s, 3H, ACH3), 1.3 (s,
3H, ACH3), 2.45 (s, 3H, Ar (ACH3), 3.74 (m, 4H, ACH2), 4.25
(m, 1H, ACH), 7.33 (d, 2H, J 5 8.04Hz, ArAH) 7.78 (d, 2H,
J 5 8.43 Hz, ArAH). 13C NMR (75.4 MHz, CDCl3, d ppm):
145.0 (ArACASO2A), 132.4, 129.8, 127.8 (ArAC), 109.9
(ACA), 72.7 (ACH), 70.6 (ACH2), 65.9 (ACH2), 30.7 (ACH3),
26.4 (ACH3), 24.9, 21.4 (ACH3). MS-ESI: 287.1 (M 1 1).
ELEM. ANAL cal. C13H18O5S: C, 54.53; H, 6.34; S, 11.20. Found:
C, 54.47; H, 6.28; S, 11.14.
(M 1 1): 239.12; ELEM. ANAL cal. C13H18O4: C, 65.53; H, 7.61;
Found: C, 65.49; H, 7.63.
Synthesis of (2-Methyl-acrylic acid 3-(2, 2-dimethyl-[1, 3]
dioxolan-4-ylmethoxy)-benzyl ester) (6)
Compound 5 (2.5 g, 10.5 mmol) was dissolved in dry THF (25
mL). To the solution, triethylamine (2 g, 21 mmol) was added
and cooled to 0 ꢀC in an ice bath. Methacryloyl chloride (2.2 g,
21 mmol) dissolved in dry THF (15 mL) was added drop wise
and stirred at room temperature for 12 h. The solvent was
removed under reduced pressure and the residue was
extracted using diethyl ether (3 3 50 mL). The organic layer
was washed with NaHCO3 solution (2N, 3 3 50 mL) and com-
bined organic layers were dried over anhydrous sodium sul-
fate. Filtered and concentrated under reduced pressure to give
the crude compound 6. The product was purified using col-
umn chromatography on silica gel and hexane: ethyl acetate
(8:2, v/v) mixture as eluent. The compound 6 was obtained as
Synthesis of ([3-(2,2-Dimethyl-[1, 3] dioxolan-4-
ylmethoxy)-phenyl]-methanol) (4)
Compound 3 (6.88 g, 56 mmol), potassium carbonate (23.5 g,
170 mmol), catalytic amount of 18-crown- 6, and DMF (120
mL) were mixed to get slurry. The reaction mixture was heated
ꢀ
to 60 C for 1 h under nitrogen atmosphere. Compound 2 (24
g, 83.8 mmol) in dry DMF (100 mL) was added drop wise in
30 min and the temperature was maintained at 75 ꢀC for 5
days. The remaining K2CO3 was filtered off and the filtrate was
poured into 1 L water and extracted with diethyl ether (3 3
50 mL). The organic layer was washed with water (3 3 50
mL) and dried over anhydrous sodium sulfate, filtered, and
concentrated under reduced pressure to give a brown colored
crude product. The compound was purified using flash silica
column chromatography using hexane and ethyl acetate as elu-
ent (7:3 v/v) to give compound 4 as colorless solid (10.3 g,
1
colorless oil (2.2 g, 68%). H NMR (300 MHz, CDCl3, d ppm):
1.40 (s, 3H, ACH3), 1.46 (s, 3H, ACH3), 1.81 (s, 3H, ACH3),
3.86 (m, 4H, ACH2), 4.14 (m, 1H, ACH), 5.15 (s, 2H, ACH2O),
5.58 (s, 1H, @CH), 6.15 (s, 1H, @CH), 6.85 (m, 3H, ArAH), 7.24
(m, 1H, ArAH). 13C NMR (75.4 MHz, CDCl3, d ppm): 167.0
(C@O), 158.6, 137.6 (AC@CH2), 136.0, 129.5, 125.7 (@CH2),
120.5, 114.0, 109.6 (ArAC), 103.4 (ACA), 73.8 (ACH), 68.6
(ACH2AO), 66.6 (ACH2), 66.0 (ACH2), 26.6 (CH3), 25.2 (CH3),
17.8 (CH3). FT-IR (KBr, cm21): 2986, 1726, 1678, 1635, 1587,
1491, 1449, 1372, 1040, 943, 844, 783, 695. MS-ESI: 307.15
(M11). ELEM. ANAL cal. C17H22O5: C, 66.65; H, 7.24; Found: C,
66.59; H, 7.23.
1
78%). H NMR (300 MHz, CDCl3, d ppm): 1.39 (s, 3H, ACH3),
1.41 (s, 3H, ACH3), 3.86 (s, 2H, ACH2), 4.40 (m, 1H, ACH),
4.08 (d, 2H, ACH2AO), 6.88 (s, 1H, ArAH), 7.32–7.37 (m, 3H,
ArAH), 10.01 (m, 1H, ArACHO). 13C NMR (75.4 MHz, CDCl3, d
ppm): 190.1, 158.5, 141.7, 129.6, 121.3, 113.4, 112.6 (ArAC),
109.6, 77.8 (ACHA), 66.4 (ACH2A), 64.2 (ACH2A), 26.5
(ACH3), 25.1 (ACH3). FT-IR (cm21): 3255, 2987, 2872, 1605,
1491, 1370, 1151, 1091, 973, 842, 787, 694. MS-ESI (M 1 1):
237.1. ELEM ANAL cal. C13H16O4: C, 66.09; H, 6.83; Found: C,
66.12; H, 6.81.
Synthesis of Macroinitiator PBMMA-Br (12)
Compound 6 (0.96 g, 3.1 mmol) in toluene (10 mL) was
transferred to a Schlenk flask and purged with nitrogen for
15 min. To the solution, CuBr (0.004 g, 0.027 mmol), and
PMDETA (0.009 g, 0.052 mmol) were added. The solution
was degassed three times, ethyl 2-bromoisobutyrate (ꢀ0.098
g, 0.5 mmol) was added and mixture was heated to 75 C. At
different intervals of polymerization, the samples were taken,
diluted with THF and injected into GPC for molecular weight
determination. The final polymer was precipitated from large
amount of hexane, filtered and dried. 1H NMR (300 MHz,
CDCl3, d ppm): 1.32 (s, 3H, ACH3), 1.42 (s, 3H, ACH3), 1.82
(3H, ACH3), 3.83 (m, 4H, ACH2), 4.15 (broad, 1H, ACH),
4.47 (broad, 2H, ACH2O), 6.81 (broad 1H, ArAH), 6.97
(broad, 2H, ArAH), 7.12 (broad, 1H, ArAH). 13C NMR (75.4
MHz, CDCl3, d ppm): 158.5, 142.7, 129.3, 119.3, 113.4, 112.6
(ArAC), 109.6 (ACA), 73.8 (ACHA), 68.4 (ACH2OH), 66.4
(ACH2A), 64.2 (ACH2A), 26.5 (ACH3), 25.1 (ACH3). FT-IR
(cm21): 2934, 2881, 1728, 1587, 1491, 1451, 1371, 1269,
1157, 1049, 930, 851, 783, 692. GPC (THF, PS standard): Mn
5 6600 (Mw/ Mn 5 1.12). ELEM. ANAL cal. C19H27BrO5: C, 54.
95; H, 6.55; Found: C, 55.01; H, 6.63.
Synthesis of ([3-(2,2-Dimethyl-[1, 3] dioxolan-4-
ylmethoxy)-phenyl]-methanol) (5)
Compound 4 (8.5 g, 36 mmol) was dissolvedꢀin a mixture of
THF/EtOH (70 mL: 25 mL) and cooled to 0 C. Sodium cya-
noborohydride (3.85 g, 62 mmol) was added at 0 ꢀC fol-
lowed by the addition of acetic acid (4.5 g, 75 mmol). The
reaction mixture was stirred at room temperature for 12 h.
NaHCO3 solution (2M, 50 mL) was added slowly to it and
product was extracted in diethyl ether (2 3 50 mL). The
organic layer was washed with water (2 3 50 mL) and dried
over anhydrous sodium sulfate. Filtered and concentrated
under reduced pressure to give compound 5 in quantitative
1
yield and used without further purification (4.75 g). H NMR
(300 MHz, CDCl3, d ppm): 1.37 (s, 3H, ACH3), 1.43 (s, 3H,
ACH3), 3.83 (m, 4H, ACH2), 4.40 (m, 1H, ACH), 4.59 (s, 2H,
ACH2OH), 6.78 (m, 1H, ArAH), 6.89 (m, 2H, ArAH), 7.19 (m,
1H, ArAH). 13C NMR (75.4 MHz, CDCl3, d ppm): 158.5,
142.7, 129.3, 119.3, 113.4, 112.6 (ArAC), 109.6 (ACA), 73.8
(ACHA), 68.4 (ACH2OH), 66.4 (ACH2A), 64.2 (ACH2A),
26.5 (ACH3), 25.1 (ACH3). FT-IR (cm21): 3255, 2987, 2872,
1605, 1491, 1370, 1151, 1091, 973, 842, 787, 694. MS-ESI
General Procedure for Synthesis of Precursor Polymers
(13 and 14)
Macro-initiator 12 (0.2 g, 0.03 mmol) in anisole (10 ml) was
transferred to a Schlenk flask and purged nitrogen for
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30 min. CuCl (0.004 g, 0.03 mmol) and PMDETA (0.12 g, 0.6
mmol) were added to the flask. The flask was degassed and
refilled with nitrogen three times using freeze-pump-thaw
cycles. Nitrogen purged monomer 11 or 8 (0.03 mmol) was
added to the Schlenk tube with macroinitiator, which was
preheated in oil bath at 75 ꢀC. Polymerization was carried
out for 24 h and polymer 13 or 14 was precipitated from
hexane, filtered, and dried to get a quantitative yield. The
polymer was purified using re-precipitation method (three
times) by dissolving in minimum amount of DCM, mixing
with excess hexane, and centrifuged. The polymer was dried
13C NMR (75.4 MHz, CDCl3, d ppm): 206.9 (AC@O), 158.5,
142.7, 140.3, 129.3, 125.6, 125.2, 122.8, 120.3, 118.7, 113.4,
112.6 (ArAC), 108.2, (ACA), 64.8 (ACH2AOA), 64.7
(AOACH2A), 64.5 (ACH2ACH2A) 60.3 (ANACH2), 44.8,
44.2, 31.9, 28.6, 26.8, 25.9, (ACH2), 14.1, 14.0 (ACH3). ELEM.
ANAL: calculated; C, 72.24; H, 7.82; N, 2.22; Found; C, 72.13;
H, 7.71; N, 2.14.
PFlMMA-b-PBMMA
1H NMR (500 MHz, CDCl3, d ppm): 0.88 (b, 1H, ACHA),
1.43–1.67 (b, 2H, ACH2A), 2.04 (s, 3H, ACH3), 5.05 (b, 2H,
AOACH2A), 5.12 (broad, 2H, ACH2AO), 6.54 (b, 1H, FlAH9),
6.95–7.16 (broad 5H, ArAH), 7.46 (broad, 4H, FlAH), 7.72
(broad, 2H, FlAH). 13C NMR (75.4 MHz, CDCl3, d ppm):
206.9 (AC@O), 158.5, 142.7, 141.8, 129.3, 129.4, 127.8,
126.8, 125.9, 120.0, 119.3, 113.4, 112.6 (ArAC), 109.6
(ACA), 75.4 (ACHFlA), 66.4 (ACH2OH), 64.8 (ACH2A), 64.2
(ACH2A), 26.5 (ACHA), 18.4 (ACH3). ELEM. ANAL: Calculated;
C, 72.51; H, 7.01; Found; C, 73.48; H, 6.98.
ꢀ
in vacuum oven at 60 C for 1 day, before characterization.
Block Copolymer 13
1H NMR (300 MHz, CDCl3, d ppm): 0.87 (t, 3H, ACH3), 1.25
(broad, 2H, ACH2), 1.43 (s, 1H, ACHA), 1.70 (b, 2H,
ACH2ACH2A), 1.85 (b, 2H, ACH2), 3.72 (s, 6H, ACH3), 3.76
(broad, 2H, NACH2), 4.02 (broad, 2H, ACH2AOA), 4.21 (b,
4H, OACH2), 6.97 (broad, 1H, ArAH), 7.10 (b, 1H, ArAH),
7.18 (broad, 1H, ArAH), 7.41 (b, 4H, CzAH), 8.07 (b, 2H,
CzAH); 13C NMR (75.4 MHz, CDCl3, d ppm): 206.2 (AC@O),
158.5, 142.7, 140.3, 129.3, 125.6, 125.2, 122.8, 119.3, 118.7,
113.4, 112.6 (ArAC), 109.6 (ACA), 108.5, 73.7 (ACHA),
69.4 (ACH2AOA), 67.4 (AOACH2A), 67.2 (ACH2A), 64.5,
60.3 (ANACH2), 26.5 (ACH3), 42.8, 30.6, 26.9, 25.1 (ACH3),
25.5, 24.7, 18.1. ELEM. ANAL: calculated; C, 73.14; H, 7.67; N,
2.13; Found; C, 73.03; H, 7.71; N, 2.04.
RESULTS AND DISCUSSIONS
Design and Synthesis of Monomers and Diblock
Copolymers
The amphiphilic block copolymers were prepared from
hydrophilic (hydroxyl polymethacrylate) and hydrophobic
(electroactive aromatic) monomers. We chose groups such as
carbazole and fluorene as side chains because they undergo
electropolymerization under suitable conditions to afford a
conducting polymer network (CPN). Commercially available
solketal was reacted with tosyl chloride to give the protected
compound 2. Reaction of compound 2 with 3-hydroxy benz-
aldehyde gave 4, which were reduced to alcohol 5 with
sodium cyanoborohydride. The methacrylate monomer 6, 8,
and 11 was prepared from the corresponding alcohols and
methacryloyl chloride, in presence of triethylamine as a base
in THF at room temperature. Macro-initiator 12 (Mn 5 6600,
Mw/Mn 5 1.12) was obtained from homopolymerization of 6
using atom transfer radical polymerization (ATRP) using
ethyl 2-bromoisobutyrate as the initiator and CuBr/PMDETA
as the catalyst. The macro-initiator 12 was used to copolym-
erize the fluorenyl/carbazolyl methacrylate using ATRP
method of polymerization to afford precursor block copoly-
mers 13 and 14 (Scheme 2). The block copolymers were
readily soluble in common organic solvents such as CHCl3,
CH2Cl2, and THF. Hydroxylated polymethacrylate block
copolymers (PFlMMA-b-PBMMA and PCzMMA-b-PBMMA)
were achieved by deprotection of the dioxalane groups (pre-
cursor polymers 13 or 14) using 2M HCl solution in THF at
70 ꢀC. The molecular weights of diblock copolymers were
determined using GPC with polystyrene standards. The GPC
curves of block copolymers (PFlMMA-b-PBMMA and
PCzMMA-b-PBMMA) are shown in Figure 2 and data are
summarized in Table 1.
Block Copolymer 14
1H NMR (300 MHz, CDCl3, d ppm): 0.88 (t, 3H, ACH3), 1.25
(broad, 1H, ACH), 2.38 (broad, 6H, ACH3), 5.0 (broad, 2H,
ACH2), 5.12 (broad, 2H, ACH2AO), 6.89–7.22 (broad 5H,
ArAH), 7.46 (broad, 4H, FlAH), 7.80 (broad, 2H, FlAH); 13C
NMR (75.4 MHz, CDCl3, d ppm): 206.8 (AC@O), 158.5, 142.7,
141.8, 129.3, 129.4, 127.8, 126.8, 125.9, 120.0, 119.3, 113.4,
112.6 (ArAC), 109.6 (ACA), 75.4 (ACHA), 73.8 (ACHA),
68.4 (ACH2OH), 66.4 (ACH2A), 64.2 (ACH2A), 26.5 (ACH3),
25.1 (ACH3), 18.4. ELEM. ANAL: Calculated; C, 73.53; H, 6.88;
Found; C, 73.48; H, 6.91.
General Procedure for Synthesis of Hydroxylated Block
Polymers
The protected precursor block copolymers (13/14) were
dissolved in THF (10 mL). To the solution, 2 M HCl (2.5 mL)
solutions were added and the reaction mixture stirred at 70
ꢀC for 2 h. The mixture was cooled, concentrated, and pre-
cipitated from excess amount of hexane. The polymer was
purified by dissolving in minimum amount of DCM, added
excess hexane, and centrifuged. The solid was then filtered
and dried. The corresponding block copolymers, PCzMMA-b-
PBMMA and PFlMMA-b-PBMMA were afforded in good yield.
PCzMMA-b-PBMMA
1H NMR (500 MHz, CDCl3, d ppm): 0.87 (broad, 2H, ACH2),
1.16 (s, 1H, ACHA), 1.70 (b, 2H, ACH2ACH2A), 1.80 (b, 2H,
ACH2), 2.16 (s, 3H, ACH3), 3.84 (broad, 2H, NACH2), 4.03
(broad, 2H, ACH2AOA), 4.23 (b, 4H, OACH2), 6.85 (broad,
1H, ArAH), 7.04 (b, 1H, ArAH), 7.18 (broad, 1H, ArAH),
7.36 (b, 2H, CzAH), 7.42 (b, 2H, CzAH), 8.07 (b, 2H, CzAH);
The chemical structures of the block copolymers PCzMMA-b-
PBMMA and PFlMMA-b-PBMMA were confirmed from 1H
1
NMR and FT-IR spectra. The representative H NMR of block
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peak at 1250 cm21 in the spectrum of PCzMMA-b-PBMMA
shows NAC bond stretching, which is absent in the spectrum
of PFlMMA-b-PBMMA. The molecular weights and physical
properties of block copolymers are summarized in Table 1.
Photophysical Properties
The normalized absorption and emission spectra of
PCzMMA-b-PBMMA and PFlMMA-b-PBMMA in THF are
shown in Figure 3. The absorption band of PCzMMA-b-
PBMMA below 350 nm is attributed to the absorption by the
p-p*, n-p*, and benzenoid transitions of carbazole and phenyl
groups. The absorption band of PFlMMA-b-PBMMA below
300 nm is attributed to the absorption by the p-p*, n-p*
transitions of fluorene groups.18 The block copolymers did
not exhibit any new bands in the absorption spectra, indicat-
ing that there is no chemical changes in chromophores dur-
ing the polymerization.
FIGURE 2 Gel Permeation Chromatography (GPC) plots of
block copolymers: PFlMMA-b-PBMMA (~) and PCzMMA-b-
PBMMA ($) in THF against PS standards using a refractive
index detector.
The emission spectra of the block copolymers are shown in
Figure 3(b). The emission peaks at 353 and 366 nm for
PCzMMA-b-PBMMA and 315 nm for PFlMMA-b-PBMMA indi-
cates the emission from carbazole and fluorene groups,
implying that the formation of intermolecular exciplex can
be ruled out in solution. This supports the previous conclu-
sion that the chromophores were not in the stacked form.19
The UV–vis and fluorescence spectra of the corresponding
homopolymers are shown in Supporting Information
Figure S4.
copolymers is shown in Figure S5 (see Supporting Informa-
tion) and corresponding FT-IR spectra are shown in Figure
S6 (see Supporting Information). Both copolymers contains a
strong C@O stretching band at ꢁ1724 cm21 and a broad
CAO band due to the ester group at ꢁ1150 cm21. The sharp
TABLE 1 Physical Properties of Block Copolymers
Molar
fraction
Decomposition
Temperature
Glass Transition
Temperature
(Tg; ꢀC)d
Hydrophobic
group
Polymer
aMn 3 103 PDIa Monomers m:n (mol %)b (Td; ꢀC)c
PFlMMA-b-PBMMA Fluorene
PCzMMA-b-PBMMA Carbazole
33
22
1.57 50.50
1.17 50.50
47:22
36:12
250
337
92
51
a
c
Determined from GPC (THF, Polystyrene standard).
Determined from TGA curve (N2 atmosphere, 10 ꢀC/min scan rate).
Determined from DSC (N2 atmosphere, 5 ꢀC/min scan rate).
b
d
Monomers ratio present on block copolymers calculated from NMR.
FIGURE 3 Absorption (a) and emission spectra (b: excitation at 290 nm) of PCzMMA-b-PBMMA (~) and PFlMMA-b-PBMMA (᭜) in
THF.
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FIGURE 4 TGA of protected (a) and deprotected (b) block copolymers: PFlMMA-b-PBMMA (~) and PCzMMA-b-PBMMA ($) in N2
atmosphere with a heating rate of 10 ꢀC/min.
Thermal Properties
polar block is same in both cases, such differences may be
due to the subtle differences among carbazole and fluorene
groups, where fluorene incorporated block induce more
hydrophobicity than carbazole incorporated blocks.
The thermal properties of block copolymers were deter-
mined by TGA. Figure 4 represents the TGA curves of the
deprotected and protected block copolymers under N2
atmosphere. The hyroxylated copolymers (PCzMMA-b-
PBMMA and PFlMMA-b-PBMMA) were stable up to 300 ꢀC
and showed only single stage decomposition. The observed
thermal stability of a block copolymer bearing carbazole
moiety can be explained by the presence of spacer linkage
between the polymer backbone and the chromophore unit. It
was noticed that the residual weights of the deprotected
copolymers were reduced to zero in the TGA thermograms
around 500 ꢀC, indicating a complete degradation of the
polymer chain [Fig. 5(b)]. The protected block copolymers
(13 and 14) showed two degradation temperatures and sta-
Water is a good solvent for hydrophilic block (i.e., hydroxy-
lated polymethacrylate), but a nonsolvent for hydrophobic
block (i.e., carbazole or fluorene functionalized). In case of
PCzMMA-b-PBMMA, the flexibility of long alkyl spacer, molec-
ular dipole, and p–p stacking tends to play role in the forma-
tion of lamella morphology. In the case of block copolymer,
PFlMMA-b-PBMMA, the hydrophobic fluorene groups are
ꢀ
ble up to 250 C [Fig. 5(a)]. The glass transition temperature
(Tg) of block copolymers PCzMMA-b-PBMMA and PFlMMA-b-
PBMMA were 51 and 92 ꢀC, respectively. The pendant
groups tend to decrease the glass transition temperature of
the block copolymers owing to disordered packing of bulky
groups in the polymer lattice. The DSC curves for block
copolymers are shown in Supporting Information Figure S3.
Morphology Studies of Block Copolymer Self-Assembly
The self-assembly of block copolymers was investigated
using SEM and TEM. FE-SEM micrographs of block copoly-
mers PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b) are
given in Figure 5. The micrographs show the formation of
smooth nanorod structures for PCzMMA-b-PBMMA with a
thickness of 90 nm on a glass surface. The block copolymer,
PFlMMA-b-PBMMA formed vesicle type structure through
self-assembly. The morphological characteristic was further
examined using transmission electron microscopy (TEM).
The TEM images of block copolymers are shown in Figure
5(c,d). The block copolymer, PCzMMA-b-PBMMA showed a
lamella type packing along the nanorod (c) axis, in which
black domains (hydrophilic) alternates with white domains
(hydrophobic). Similarly, TEM image of block copolymer
PFlMMA-b-PBMMA (d) showed vesicular assemblies with a
membrane thickness of 30–80 nm. Considering that the
FIGURE 5 SEM (top) and TEM (bottom) images of block
copolymers: PCzMMA-b-PBMMA (a, c) and PFlMMA-b-PBMMA
(b, d) in THF: H2O (1:1) with concentration of 0.05 mg/mL.
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Figure 6 shows the X-ray diffraction pattern of PCzMMA-b-
PBMMA and PFlMMA-b-PBMMA. PCzMMA-b-PBMMA results
a diffraction peak at lower angle at 2h 5 3.72ꢀ correspond
to the thickness of lamella formed by the blocks. It shows a
long-range order and sharp peak at 2h 5 28.54ꢀ correspond
to the d-spacing of 3.13 Å is close to the typical distance
(ꢁ3.5 Å) for an effective p – p stacking between the aro-
matic molecules.26 In the diffraction pattern of PFlMMA-b-
PBMMA, no low angle diffraction peaks were observed and
broad diffraction peaks at high angle region at 2h 5 29.35ꢀ
correspond to d-spacing of 3.0 Å is close to an effective p –
p stacking between the aromatic molecules inside the bilayer
structure of the vesicles.24 The vesicles and lamella structure
of block copolymers is schematically represented in Figure 7.
Electrochemical Polymerization
Electropolymerization of block copolymers was investigated
using cyclic voltammetry with Ag/AgCl as reference elec-
trode, platinum as counter electrode, ITO substrate as work-
ing electrode and tetrabutylammonium hexafluorophosphate
(TBAP, 0.1 M) as supporting electrolyte. The block copoly-
mers PCzMMA-b-PBMMA and PFlMMA-b-PBMMA solution
(10 wt % in DCM) were spin coated onto ITO substrate and
used for the electropolymerization at a scan rate of 50 mV/s.
FIGURE 6 X-ray diffraction pattern of block copolymer thin
films with step size of 2h 5 0.01ꢀ/m. PCzMMA-b-PBMMA (~)
and PFlMMA-b-PBMMA ($) on a glass slide.
Figure 8 shows the CV traces for the block copolymers
PCzMMA-b-PBMMA and PFlMMA-b-PBMMA. No changes in
peak positions or intensities in the polymer free scan shows
that all polymerizable groups on the polymer backbone have
been polymerized with no changes in peak intensities. For
block copolymer PCzMMA-b-PBMMA, oxidation potential of
the tethered carbazole monomer was observed at 0.9 V on
the first cycle. Starting from second anodic scan, one new
anodic peak at 1.05 V appeared with a reduction peak at
directly connected to the backbone and more rigid and a
bilayer structure would be preferred with hydrophilic block
mixing with water phase. Formation of nanostructures for
PCzMMA-b-PBMMA and PFlMMA-b-PBMMA was further con-
firmed using wide angle X-ray diffraction (WAX) experiment.
The self-assembled solution of block copolymers was drop
casted on a glass cover slip and dried at room temperature.
FIGURE 7 Schematic representation of lamella and vesicle structure formation. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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FIGURE 8 Ten cycle CV for electrochemical polymerization of block copolymers (a) PCzMMA-b-PBMMA, (c) PFlMMA-b-PBMMA.
Polymer free scans for block copolymers after electropolymerization of PCzMMA-b-PBMMA (b) and PFlMMA-b-PBMMA (d). Tetra-
butylammonium hexafluorophosphate (TBAP, 0.1M) is used as supporting electrolyte in acetonitrile. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
0.85 V, which corresponds to the radical cation of dimer unit
[Fig. 8(a)].22 Subsequent cycles showed the same onset val-
ues of oxidation and reduction peaks with increase in cur-
rent values, suggesting the electropolymerization of
carbazole unit. The polymer free scan [Fig. 8(b)] of the
crosslinked polymer film shows complete electropolymeriza-
tion of carbazole groups to form stable polycarbazole. The
crosslinking mechanism of carbazole unit is explained in
Supporting Information Figure S9.27
inside the polymer lattice. Crosslinked polymer network for-
mation was further confirmed from the linear relationship
between the current and the number of scans for both poly-
mers [Fig. 9(a,b)] which shows increase in current with
number of scans.
The electropolymerized thin films on ITO substrate were fur-
ther characterized by UV–vis, fluorescence, FT-IR spectros-
copy, and AFM measurements. The UV–vis spectra of
crosslinked film of PCzMMA-b-PBMMA and PFlMMA-b-
PBMMA showed an absorption maximum at higher wave-
length compared to the precursor. This confirms the
formation of conjugated polymer chain through electropoly-
merization of electroactive units. For block copolymer
PCzMMA-b-PBMMA [Fig. 10(a)], the absorption peak at 300
nm wavelength can be assigned as polaron bonding to p*
conduction band and the 520 nm peak appears due to tran-
sition in bonding to antibonding state i.e. p - p* transition.25
However, for block copolymer PFlMMA-b-PBMMA [Fig.
10(c)], the observed red shift of absorption peak at 320 nm
indicates an extended conjugation along the polymer chain.
The emission spectra of the electropolymerized films showed
For PFlMMA-b-PBMMA, oxidation potential of the tethered
fluorene monomer was at 1.24 V for the first three cycles.
Starting from fourth anodic scan, two oxidation peaks (Epa1
and Epa2) appeared at 1.24 and 1.43 V and two reduction
peaks (Epc1 and Epc2) at 0.9 and 1.10 V [Fig. 8(c)], which
corresponds to the formation of radical cation and bication
species of the dimer unit, respectively.23 The anodic and
cathodic peak potentials were increased in the subsequent
cycles. The polymer free scan of crosslinked polymer film
showed the redox potential peaks [Fig. 8(d)] with no
changes in repeated scans.23 This confirms complete poly-
merization of fluorene groups to form a CPN of polyfluorene
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FIGURE 9 Linearity of current with number of scans of electropolymerization of block copolymers: (a) PCzMMA-b-PBMMA and (b)
PFlMMA-b-PBMMA. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 10 Absorption (a, c) and emission (b, d) spectra of electropolymerized films on ITO with its precursor block copolymers
PCzMMA-b-PBMMA (a, b) and PFlMMA-b-PBMMA (c, d); Precursor polymer (~) and electropolymerized film ($).
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FIGURE 11 FT-IR spectra of block copolymers; precursor polymer (a) and after electropolymerization (b); PCzMMA-b-PBMMA (~)
and PFlMMA-b-PBMMA ($). The spectra were recorded in KBr matrix at R.T.
red shift compared to the precursor polymers [Fig. 10(b,d)].
The electropolymerized polymers showed the appearance of
extra peaks due to polymer formation in the FT-IR spectrum
at 846 cm21 [Fig. 11(b)], which is absent for precursor poly-
mer [Fig. 11(a)] and corresponds to the CAH out- of- plane
bending vibrations of 1, 2, 3, 4-substituted aromatic units.26
pared and polymerized using ATRP to form diblock copoly-
mers with hydrophobic and hydrophilic blocks. The
macroinitiator (PBMMA-Br) was synthesized from 2,2-
dimethyl-1,3-dioxolane functionalized methacrylate monomer.
The carbazole incorporated block copolymer formed lamella,
while fluorene incorporated copolymer showed vesicle mor-
phologies via self-assembly in solution. The interplay of
weak interactions such as hydrogen bonding and van der
Wall’s interactions are responsible for the formation of nano-
structures from these block copolymers. A range of spectro-
scopic and electron microscopic techniques were used to
characterize the structure and properties of the polymers.
The cyclic voltammetry and AFM were used to electropoly-
merize the carbazole and fluorene groups to from CPN inside
the polymer lattice.
The electropolymerization of electroactive units to form a
polymer film was also monitored with different number of
scans at a scan rate of 50 mV/s. The absorption peaks at
lower (300 nm) and higher (520 nm) wave lengths in the
absorption spectra showed a linear increase with increase in
number of CV cycles leads the formation of longer polymer
chains [Supporting Information Fig. S7(a,b)]. As expected,
polymerization of carbazole groups was much faster than
the fluorene groups. The calculated band gaps for electropo-
lymerized block copolymers were 1.76 eV (PCzMMA-b-
PBMMA) and 1.64 eV (PFlMMA-b-PBMMA). The surface mor-
phology and molecular orientation of crosslinked films on
ITO substrate was studied using AFM.22 Polymerized
PCzMMA-b-PBMMA film showed sheets stacked layers with
250 nm length and 100 nm of width with an interlayer sepa-
ration of 3.5 nm and formed large crystals (Supporting Infor-
mation Fig. S8). However, for polymerized PFlMMA-b-
PBMMA block copolymer film showed ring like nanostruc-
tures [Supporting Information Fig. S8(b)]. These rings were
of 25 nm in thickness and 80–100 nm internal diameters.
The observed dimensions are complimentary to the data col-
lected from TEM images of nonpolymerized self-assembled
structures.
ACKNOWLEDGMENTS
The authors are grateful to Department of Chemistry and
National University of Singapore Nanoscience and Nanotech-
nology Initiative (NUSNNI), for technical support. S. Barik
acknowledges
Education.
a graduate scholarship from Ministry of
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