Well-defined fluoro- and carbazole-containing diblock copolymers: Synthesis, characterization and immobilization onto Au-coated silicon surfaces

Article abstract of DOI:10.1039/c2ra20650a

A series of well-defined diblock copolymers consisting of 2-(N-carbazolyl)ethyl methacrylate (CbzEMA) and 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (HFBMA) (CbzEMA_x-b-HFBMA_y) was synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. All polymers were characterized in terms of molecular weights, molecular weight distributions and chemical compositions using Size Exclusion Chromatography (SEC), Fourier Transform Infrared (FTIR) spectroscopy and Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy, respectively. The thermal properties (glass transition and decomposition temperatures) of the CbzEMA_x and HFBMA_x homopolymers and the CbzEMA_x-b-HFBMA_y diblock copolymers were determined by Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA). As demonstrated by photoluminescence measurements, immobilization of the dithioester-ended functionalized CbzEMA_x-b-HFBMA_y chains onto Au-coated silicon surfaces has been accomplished via anchoring of the sulfur-containing end-groups onto the Au surfaces. The presence of the low-surface energy fluorinated block, combined with the electro-active carbazole-containing segment within CbzEMA_x-b-HFBMA_y diblock copolymers imparts to these immobilized thin layers useful properties towards their potential applicability in gas sensing technologies.

Full text of DOI:10.1039/c2ra20650a

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PAPER
Well-defined fluoro- and carbazole-containing diblock copolymers: synthesis,
characterization and immobilization onto Au-coated silicon surfaces
Ioanna Savva,a Maria Demetriou,a Andreas Othonos,b Rodica Turcu,c Adriana Popa,c Sergiu Macaveic and
Theodora Krasia-Christoforou*a
Received 24th October 2011, Accepted 23rd July 2012
DOI: 10.1039/c2ra20650a
A series of well-defined diblock copolymers consisting of 2-(N-carbazolyl)ethyl methacrylate
(CbzEMA) and 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (HFBMA) (CbzEMAx-b-HFBMAy) was
synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. All
polymers were characterized in terms of molecular weights, molecular weight distributions and
chemical compositions using Size Exclusion Chromatography (SEC), Fourier Transform Infrared
(FTIR) spectroscopy and Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy,
respectively. The thermal properties (glass transition and decomposition temperatures) of the
CbzEMAx and HFBMAx homopolymers and the CbzEMAx-b-HFBMAy diblock copolymers were
determined by Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA).
As demonstrated by photoluminescence measurements, immobilization of the dithioester-ended
functionalized CbzEMAx-b-HFBMAy chains onto Au-coated silicon surfaces has been accomplished
via anchoring of the sulfur-containing end-groups onto the Au surfaces. The presence of the low-
surface energy fluorinated block, combined with the electro-active carbazole-containing segment
within CbzEMAx-b-HFBMAy diblock copolymers imparts to these immobilized thin layers useful
properties towards their potential applicability in gas sensing technologies.
modification reactions for the introduction of different sub-
stituents into the carbazole ring.13–16
Introduction
Polymer-based materials exhibiting electro-active functionalities
Polymers with fluorinated groups present either in the main
have attracted significant attention due to their potential use in
backbone or in the side chain, exhibit some outstanding
various optoelectronic applications including organic-based
properties, including high thermal stability, chemical inertness,
photovoltaics,1,2 polymer light emitting diodes,3,4 thin film
low dielectric constants and dissipation factors, low water
transistors and sensors,5,6 optical switching,7,8 optical data
absorptivities, and good resistance to surface properties.14,17–20
storage and information processing.9,10
Due to the ability of fluoropolymers to provide a unique
Carbazole-containing polymers belong to this class and are
combination of chemical resistance, toughness, and purity, they
highly attractive due to their potential applications as photo-
are used in a wide variety of fluid and device handling
conductors and photorefractive or charge transporting materials.
applications.21 In particular, block copolymers containing
Further applications of these materials include the development
fluoropolymer segments receive considerable attention nowa-
of light-emitting diodes (LEDs), electrochemical and gas
days, since such materials combine the properties of the fluoro-
sensors11,12 and photovoltaic devices.13 Carbazolyl groups easily
containing segment with special characteristics deriving upon the
form relatively stable radical cations (holes), present compara-
introduction of other functional units within the copolymer.22–28
tively high charge carrier mobilities and exhibit high thermal and
Herein, a simple and cost-effective synthetic approach invol-
photochemical stability. Moreover, carbazole is a cheap raw
ving Reversible Addition Fragmentation chain Transfer (RAFT)
material that can be very easily subjected into chemical
controlled radical polymerization29,30 has been employed for
aDepartment of Mechanical and Manufacturing Engineering, School of
Engineering, University of Cyprus, P. O. Box 20537, 1678 Nicosia,
Cyprus. E-mail: isavva02@ucy.ac.cy; demmaria@hotmail.com;
krasia@ucy.ac.cy; Fax: +357 22895081; Tel: +357 22892288
bResearch Center of Ultrafast Science, Department of Physics, Faculty of
Pure and Applied Sciences, University of Cyprus, P. O. Box 20537, 1678
Nicosia, Cyprus. E-mail: othonos@ucy.ac.cy
the synthesis of a series of well-defined 2-(N-carbazolyl)ethyl
methacrylate-block-2,2,3,3,4,4,4-heptafluorobutyl methacrylate
(CbzEMAx-b-HFBMAy) diblock copolymers. For the first time
HFBMA is combined with CbzEMA to yield well-defined
functional diblock copolymers presenting electro-active and
fluorinated moieties. Hence, the present work describes the
unique combination of a rare example of a fluorinated methacry-
late monomer polymerized by RAFT, with an electro- and
cNational Institute R&D for Isotopic and Molecular Technologies, Cluj-
Napoca 400293, Romania. E-mail: rodica.turcu@itim-cj.ro;
adriana.popa@itim-cj.ro; sergiu.macavei@itim-cj.ro
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photoactive polymethacrylate, the controlled polymerization of
which has been rarely reported.31–34 The RAFT process was
readily employed to prepare well-defined CbzEMAx homopoly-
mers and CbzEMAx-b-HFBMAy diblock copolymers. The
molecular and compositional characteristics of the polymers
were determined by Size Exclusion Chromatography (SEC),
FTIR spectroscopy and 1H NMR spectroscopy respectively. Dif-
ferential Scanning Calorimetry (DSC) and Thermal Gravimetric
Analysis (TGA) provided information on the thermal properties
(glass transition and decomposition temperatures) of these
materials.
(CTA), cumyl dithiobenzoate (CDTB), was synthesized accord-
ing to a well-known procedure reported in the literature.35
Syntheses
2-(N-carbazolyl)ethyl methacrylate (CbzEMA). The synthesis
of 2-(N-carbazolyl)ethyl methacrylate was accomplished in two
steps by following a modified methodology based on previous
literature reports.31,36,37 2-(N-Carbazolyl)ethanol was synthesized
first and subsequently modified to generate 2-(N-carbazolyl) ethyl
methacrylate. For the synthesis of 2-(N-carbazolyl)ethanol,
carbazole (19.0 g, 0.114 mol) was reacted with ethylene carbonate
(11.0 g, 0.125 mol) in DMF (200.0 mL) under reflux (110–130 uC)
for 8 h in the presence of KOH (1.9 g, 0.034 mol). After being
cooled to room temperature, the reaction mixture was filtered and
then poured into a large amount of water (4.0 L). The precipitated
2-(N-carbazolyl)ethanol (white crystalline solid) was collected and
dried in a vacuum furnace for 12 h at 40 uC. Subsequently, it was
purified twice by recrystallization in a 1 : 1 benzene/cyclohexane
mixture (35 mL). Recrystallization involved the dissolution of
2-(N-carbazolyl)ethanol in the above-mentioned solvent mixture
under reflux and continuous stirring, followed by cooling down to
room temperature and storage at 220 uC. Subsequently, the
mixture was filtered off and the resulting solid (white powder) was
left to dry in a vacuum furnace at room temperature.
The presence of the low-surface energy fluorinated block,
combined with the electro-active carbazole-containing segment
within CbzEMAx-b-HFBMAy diblock copolymers imparts to
these materials useful properties for the fabrication of high
quality and high performance functional thin films. In that
respect, anchoring of the CbzEMAx-b-HFBMAy chains onto
Au-coated surfaces has been accomplished via the dithioester end
groups that are retained onto the polymer chains grown by
RAFT. Photoluminescence measurements demonstrated that the
diblock copolymer chains have been successfully immobilized
onto the Au-coated silicon surfaces. These new systems may be
promising for the fabrication of novel macromolecular thin films
towards gas sensing. The latter relies on the fact that ultrathin
polymer films possessing electro-active moieties demonstrate
enhanced performance translated into fast response, good
reproducibility and high sensitivity and that the adsorption of
a gas onto the surface of an electro-active polymer may cause
significant changes in either the electrical or optical proper-
ties.15,21
The obtained 2-(N-carbazolyl)ethanol (8.5 g, 0.0403 mol), was
then dissolved in dichloromethane (134.4 mL). In the reaction
flask, triethylamine (6.7 mL, 0.047 mol), and methacryloyl
chloride (4.7 mL, 0.047 mol) were added in a 20% excess, to
ensure a more quantitative reaction. The methacryloyl chloride
was added dropwise during stirring at 0 uC, (exothermic
reaction). During this process, the formation of a by-product
was observed as a sediment namely hydrochloric triethylamine
(Et3NHCl). The reaction mixture was stirred for 24 h at room
temperature. The hydrochloric triethylamine was removed from
the reaction mixture by filtration.
Experimental section
Materials
Benzene (Fluka, Merck ¢99.5%) and tetrahydrofuran (THF)
(HPLC grade, LabScan) were stored over CaH2 (Merck, 99.9%)
and distilled under reduced pressure immediately prior to the
polymerization reactions. Methanol (LabScan, Scharlau 99.9%),
n-hexane (Scharlau, 99%), ethyl acetate (Scharlau, 99%),
cyclohexane (Scharlau, 99%), chloroform (Scharlau, 99%),
acetone (analytical grade, Scharlau), dimethylformamide
(DMF) (Sigma-Aldrich, ¢99.8%), dichloromethane (Scharlau,
99%), triethylamine (Merck, ¢99.8%), hydrochloric acid
(Merck, 37% solution) and diethyl ether (LabScan, 99.5%) were
used as received by the manufacturer. Regarding deuterated
solvents, CDCl3 (Scharlau) was used in 1H NMR studies.
Methacryloyl chloride (Merck, ¢97%), carbazole (Sigma
¢99.5%), 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (Sigma-
Aldrich, 97%), ethylene carbonate (Sigma-Aldrich, 98%), benzyl
chloride (Sigma-Aldrich, 99%), a-methylstyrene (Sigma-Aldrich,
99%), sodium methoxide (Sigma-Aldrich, 30% solution in
methanol), sodium hydroxide pellets (Scharlau), potassium
hydroxide pellets (HiMedia, 85%), sulfur (Sigma-Aldrich,
powder #100 mesh), anhydrous magnesium sulfate (Scharlau),
sodium bicarbonate (99.5%, Sigma-Aldrich) and silica gel
After filtration, water (10 mL) was added for the hydrolysis of
the excess of methacryloyl chloride into methacrylic acid. The
methacrylic acid resigned into the aqueous phase whereas the
product remained in the organic phase.
Subsequently, dichloromethane (40.0 mL) was added in the
solution and the mixture was extracted two times with water (5.0
and 10.0 mL), then three times with NaHCO3 solution (5% in
water, addition of 5.0 mL each time) and again three times with
water. After the extractions, anhydrous MgSO4 was added in the
organic phase in order to remove the water traces. Subsequently,
the organic solvent was removed under reduced pressure; the
product was dried and recrystallized twice in methanol. The
recrystallization process was carried out at room temperature, in
order to avoid thermal polymerization of the monomer. The
resulting solid was then filtered and dried in a vacuum furnace at
room temperature. The synthetic route followed for the synthesis
of the CbzEMA monomer is presented in Fig. 1, whereas the
chemical structure of the monomer is depicted in Fig. 2.
1H NMR (300 MHz, CDCl3) d (ppm): 8.1–8.0 (br, 2H (e)), 7.5–
7.34 (m, 6H (f)), 5.92–5.85 (br, 1H (b1)), 5.48–5.44 (br, 1H (b2)),
4.62–4.52 (m, 4H (c, d)), 1.8 (s, –CH3 (a)).
˚
(Sigma-Aldrich, 60 A, 70–230 mesh) were used as received.
2,2-Azobis(isobutylnitrile) (AIBN) (Sigma-Aldrich, 95%), was
recrystallized twice from ethanol. The chain transfer agent
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Fig. 1 Two-step synthetic methodology followed for the preparation of the 2-(N-carbazolyl)ethyl methacrylate (CbzEMA).
Poly(2-(N-carbazolyl) ethyl methacrylate) (CbzEMAx)
Poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate) (HFBMAx)
A series of well-defined CbzEMAx homopolymers, with various
polymerization degrees (DP), was prepared by RAFT. To a
round-bottom flask (25.0 mL) maintained under a dry nitrogen
atmosphere, CbzEMA (1 g, 3.58 6 1023 mol) was added. CDTB
(8.12 mg, 2.98 6 1025 mol) and AIBN (2.68 mg, 1.63 6 1025 mol)
were dissolved in freshly distilled THF (2.0 mL) and were
transferred into the flask with the aid of a syringe. Subsequently,
the resulting solution was degassed by three freeze-evacuate-thaw
cycles, placed under a dry nitrogen atmosphere and heated in a
thermostated oil bath at 63 uC for 18 h. The polymerization was
terminated by cooling the reaction down to room temperature. The
produced CbzEMAx (0.634 g, 64% polymerization yield) (Fig. 3)
was retrieved by precipitation in n-hexane and was left to dry in
vacuo at room temperature for 24 h.
To a round-bottom flask (50.0 mL) maintained under a dry
nitrogen atmosphere, HFBMA (3.0 mL, 1.5 6 1022 mol) was
added. CDTB (20.5 mg, 7.52 6 1025 mol) and AIBN (5.53 mg,
3.37 6 1025 mol) were dissolved in freshly distilled THF
(12.0 mL) and were transferred into the flask with the aid of a
syringe. Subsequently, the resulting solution was degassed by
three freeze-evacuate-thaw cycles, placed under a dry nitrogen
atmosphere and heated in a thermostated oil bath at 63 uC for
18 h. The polymerization was terminated by cooling the reaction
down to room temperature. The produced HFBMAx (1.6974 g,
42% polymerization yield) (Fig. 4) was retrieved by precipitation
in methanol and was left to dry in vacuo at room temperature for
24 h. Further purification, of the polymer was carried out by re-
precipitation in n-hexane.
1H NMR (300 MHz, CDCl3) d (ppm): 7.9 (br, 2H (e)), 7.22–
7.07 (m, 6H (f)), 4.3–3.6 (m, br 4H (c, d)), 1.85 (br, –CH2 (b)),
0,8–1.0 (br, –CH3 (a)).
1H NMR (300 MHz, CDCl3) d (ppm): 4.5–4.3 (s, br, 2H (g)),
1.95 (br, 2H (b9)), 1.10 (br, –CH3 (a9)).
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Fig. 4 The chemical structure of the poly(2,2,3,3,4,4,4-heptafluorobutyl
methacrylate) (HFBMAx) homopolymer.
Fig. 2 The chemical structure of the 2-(N-carbazolyl)ethyl methacrylate
(CbzEMA) monomer.
Fig. 5 The chemical structure of the CbzEMAx-b-HFBMAy diblock
copolymers.
in freshly distilled THF (4.0 mL) were placed together with HFBMA
(0.5 mL, 2.51 6 1023 mol). The reaction mixture was degassed by three
freeze-evacuate-thaw cycles, placed under a dry nitrogen atmosphere and
heated in a thermostated oil bath at 63 uC for 18 h. The polymerization
was terminated by cooling the reaction down to room temperature. The
produced CbzEMAx-b-HFBMAy diblock copolymer (0.881 g, 38%
polymerization yield) (Fig. 5) was retrieved by precipitation in n-hexane
or methanol and was left to dry in vacuo at room temperature for 24 h.
1H NMR (300 MHz, CDCl3) d (ppm): 7.9 (br, 2H (e)), 7.21–7.06
(br, 6H, (f)), 4.41–4.3 (s, br, 2H (g)), 4.3–4.05 (br, 2H (c)), 4.05–3.75
(br, 2H (d)), 1.95 (br, –CH2 (b, b9)), 1.0–1.2 (br, –CH3 (a, a9)).
Fig. 3 The chemical structure of the poly(2-(N-carbazolyl)ethyl metha-
crylate) (CbzEMAx) homopolymer.
Diblock copolymers
Poly[2-(N-carbazolyl)ethyl methacrylate]-block-poly[(2,2,3,3,4,4,4-
heptafluorobutyl methacrylate)] (CbzEMAx-b-HFBMAy). To
a
Film fabrication
round-bottom flask (25.0 mL) maintained under a dry nitrogen
atmosphere, AIBN (1.45 mg, 8.85 6 1026 mol) and CbzEMAx
Polymer films were generated on gold-coated silicon substrates
(gold film thickness 25 nm) upon immersion of the latter in a
(0.626 g, Mn = 31 711 g mol21, 2.246 1023 mol, macro-CTA) dissolved
¯
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dilute THF solution of CbzEMA52-b-HFBMA69 (2 g L21) at
room temperature.38 After 24 h, the solution was removed via a
syringe and the substrate with adsorbed polymer was washed
with fresh solvent (THF) until no polymer could be detected by
UV-vis analysis in the washing medium. The films were then left
to dry on the bench for 24 h and were not supplementary dried
prior to photoluminescence and atomic force microscopy (AFM)
measurements.
Photoluminescence (PL) measurements were performed using
a UV source based on a Ti-sapphire ultrafast laser generating
75 fs pulses at repetition rate of 250 kHz. In this system the
fundamental light at 800 nm generated by the ultrafast amplifier
was frequency doubled and used as the source of excitation for
an Optical Parametric Amplifier (OPA) which was tuned for
these experiments at 510 nm. The OPA pulses were frequency
doubled using a BBO non-linear crystal to produce 255 nm. The
UV beam was then separated and directed on the sample using
several wavelength selective dielectric mirrors. The average
power that was incident on the sample was approximately
10 mW with a measured spot size at the excitation region of
2 mm. The PL emission was collected and analyzed through a
double pass spectrometer equipped with a photon counting
photomultiplier system.
Instrumentation
Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy
was used for confirming the expected chemical structure of the
polymers. 1H NMR spectra were recorded in CDCl3 using an
Avance Brucker 300 MHz spectrometer equipped with an
Ultrashield magnet. The CDCl3 contained traces of tetramethyl-
silane (TMS), which was used as an internal reference. For
classifying the signals in the 1H NMR spectra, abbreviations
such as s (singlet), d (doublet), t (triplet), m (multiplet) and br
(broad) are used.
The gold-coated silicon substrates used (Au film thickness:
25 nm) were prepared by sputtering using argon at a pressure of
approximately 1022 mBar.
Results and discussion
The molecular weights and polydispersity indices of the
polymers were determined by Size Exclusion Chromatography
(SEC). All measurements were carried out at room temperature
using Styragel HR 3 and Styragel HR 4 columns. The mobile
phase was THF, delivered at a flow rate 1 mL min21 using a
Waters 515 isocratic pump. The refractive index was measured
with a Waters 2414 refractive index detector. The instrumentation
was calibrated using poly(methyl methacrylate) (PMMA) stan-
dards with narrow polydispersity indices (MWs of 102, 450, 670,
1580, 4200, 14 400, 31 000, 65 000, 126 000, 270 000, 446 000,
739 000 g mol21) supplied by Polymer Standards Service (PSS).
The refractive index of THF, a HFBMAx homopolymer and a
CbzEMAx-b-HFBMAy diblock copolymer was determined using
an ABBE refractometer.
Molecular characterization
Well-defined CbzEMAx and HFBMAx homopolymers and
CbzEMAx-b-HFBMAy diblock copolymers were successfully
synthesized following typical RAFT methodologies already
described in the Experimental Section. The molecular character-
1
ization of these materials was carried out by means of SEC, H
NMR and FTIR. Table 1 summarizes the chemical structures of
all homopolymers and diblock copolymers prepared in this study
along with their molecular weight (MW) and compositional
characteristics.
1H NMR spectroscopy confirmed the expected chemical
structure of the diblock copolymers. In Fig. 6 the 1H NMR
spectrum of the CbzEMA113-b-HFBMA52 diblock copolymer is
exemplarily given. The peak assignments are shown in the
spectrum. The degrees of the polymerization of the HFBMA
comonomer were determined from the ratio of the areas under
the characteristic signals of the CbzEMA and HFBMA,
appearing at 4.2–3.9 ppm (c, d) and 4.4 ppm (g) respectively.
The signal at 4.4 ppm corresponds to the methylene protons
Differential Scanning Calorimetry (DSC) was used to measure
the glass transition temperatures (Tg) of the homopolymers and
the diblock copolymers using a Q100 TA Instrument with a
heating rate of 10 uC min21. Each sample was scanned twice
between 2100 uC to +170 uC. The second run (heat) was used for
data analysis.
Thermal Gravimetric Analysis (TGA) measurements were
used in order to determine the thermal stability of the polymers
and they were based on continuous measurement of weight on a
sensitive balance (thermobalance) upon temperature increase.
TGA measurements were performed on a Q-500 TA Instrument
Table 1 Characteristics of the homopolymers and diblock copolymers
based on CbzEMA and HFBMA obtained by RAFT (polymerization
yields, molecular weights and polydispersity indices)
SEC Resultsc
under an argon flow at heating rate of 10 uC min21
.
Polymera
Yield (%)b
PDI
¯
Mn
¯
Mw
Ultra Violet-visible spectroscopy (UV-vis) was carried out on
a Jasco UV-vis spectrophotometer (V 630). Measurements were
performed in THF and solution concentration was approxi-
CbzEMA13
HFBMA95
58
42
52
80
n.d.
38
3602
4174
1.16
1.25
1.28
1.22
1.59
1.26
25 861
25 437
25 781
22 106
34 245
32 283
32 623
31 440
35 105
43 406
mately 1 g L21
.
CbzEMA13-b-HFBMA244
CbzEMA13-b-HFBMA316
CbzEMA52-b-HFBMA69
CbzEMA113-b-HFBMA52
a
Fourier Transform Infrared Spectroscopy (FTIR) was carried
out on a Jasco FTIR spectrometer (4100).
Determined by SEC and 1H NMR. [(g monomer)/(mol RAFT
b
The AFM investigations have been performed with an UHV-
3500 Scanning Probe Microscope from RHK Technology. The
system is operating in ultra high vacuum (UHV) and the samples
were analyzed in the non-contact operating mode using a scan
speed of 2.71 micrometer/s and a tip frequency of 74.7 kHz.
n-Type silicon (phosphorus doped) tips with a conical shape were
used to acquire the AFM images.
agent)] 6 (polymerization yield) + MW of CTA (for homopolymers)
and [(g monomer)/(mol macro-CTA agent)] 6 (polymerization yield) +
c
Mn of macro-CTA (for diblock copolymers). SEC calibrated with
¯
¯
PMMA standards; Mn = number average molecular weight; PDI =
polydispersity index; CbzEMA = 2-(N-carbazolyl) ethyl methacrylate;
2,2,3,3,4,4,4-heptafluorobutyl methacrylate; n.d.: not
HFBMA
determined.
=
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Fig. 7 The FTIR spectra of the HFBMA95 and CbzEMA52 homo-
polymers, and the CbzEMA113-b-HFBMA52 diblock copolymer.
to be 1.4065, which is lower than that of THF. In contrary, the
refractive index of the CbzEMA113-b-HFBMA52 diblock copo-
lymer was determined to be higher than that of the pure solvent
as expected, due to the higher content of the CbzEMA units
within the diblock copolymer.
Fig. 6 1H NMR spectrum of the CbzEMA113-b-HFBMA52 diblock
copolymer. *Solvent (CHCl3).
adjacent to the oxygen of the ester group in HFBMA.39,40 The
signals appearing at 4.2–3.9 ppm correspond to the –CH2–CH2–
protons of the carbazole group (adjacent to nitrogen in
CbzEMA).41
In Fig. 8(b) the SEC traces of the CbzEMA13 homopolymer
and the CbzEMA13-b-HFBMA244 diblock copolymer are illu-
strated. As seen in the Figure, the molecular weight distribution
(MWD) of the latter is shifted towards higher MWs compared to
that of the corresponding homopolymer, demonstrating the
block efficiency from homopolymer to block copolymer.
Moreover, FTIR spectroscopy was employed to verify the
chemical structure of the polymers. Fig. 7 presents the FTIR
spectra of the HFBMA95 and CbzEMA52 homopolymers and of
the CbzEMA113-b-HFBMA52 diblock copolymer. In the spectra
the vibration bands appearing at 1597 cm21 correspond to the
CLO bond that is present in the systems42 whereas the vibration
frequency of the C–O bond of the ester group appears at
1731 cm21. In the FTIR spectrum of the HFBMA95 homo-
polymer, the signal appearing at 1351 cm21 corresponds to the
vibration frequency of the C–F bond. Moreover, the two signals
at 1203 cm21 and 1156 cm21, correspond to the symmetric and
antisymmetric stretching vibrations of the –CF2 group respec-
tively, whereas two medium bands appear at 749 cm21 and
722 cm21, corresponding to a combination of cocking and
wagging vibrations of the –CF3 group.40
Thermal properties
DSC and TGA measurements were carried out for the
determination of the glass transition (Tg) and decomposition
temperatures of the polymers, respectively. The glass transition
temperature (Tg) of CbzEMA52 and HFBMA95 prepared by
RAFT was 142 and 52 uC respectively (Fig. 9(a)) which is close
to literature values.43,44 The diblock copolymers exhibited two
Tgs, at y128 uC (corresponding to the CbzEMAx block) and
y58 uC (corresponding to the HFBMAy block). These results
may suggest that the CbzEMAx-b-HFBMAy diblock copolymers
tend to microphase separate in the bulk. As far as thermal
stability is concerned, all polymers start losing weight at y300–
350 uC and decompose losing most of their weight between 400–
450 uC as indicated by TGA (Fig. 9(b)).
The average MWs and PDIs of the homopolymers and
corresponding diblock copolymers were determined by SEC. All
data are provided in Table 1. All polymers (with the exception of
the CbzEMA52-b-HFBMA69 system) exhibited low PDI (,1.3),
which are well-comparable with PDI values of previously
reported polymers prepared by RAFT,29 demonstrating the
well-defined character of these new materials.
Film fabrication and characterization
As already described in the experimental section, a polymer layer
was generated on a gold-coated silicon substrate upon immersion
of the latter in a dilute THF solution of CbzEMA52-b-HFBMA69
at room temperature. The strong specific interaction between the
sulfur atom and the gold surface45 induced the spontaneous
assembly of the polymer chains onto the gold interface as
schematically presented in Fig. 10.
Fig. 8(a) displays the SEC traces of the HFBMA95 and
CbzEMA108 homopolymers, and of the CbzEMA113
-
b-HFBMA52 and CbzEMA13-b-HFBMA244 diblock copolymers.
Due to the lower refractive index of the HFBMA95 homo-
polymers compared to that of THF, the signal corresponding to
the SEC eluogram of the HFBMA95 homopolymer and
CbzEMA13-b-HFBMA244 diblock copolymer is negative. As
provided in Table 2, the experimental refractive index value
corresponding to the HFBMA95 homopolymer was determined
According to C.-A. Fustin and A.-S. Duwez,46 the dithioester
group is chemisorbed on a gold substrate with both sulfur atoms
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Fig. 8 SEC eluograms of (a) HFBMA95 and CbzEMA108 homopolymers and of the CbzEMA113-b-HFBMA52 and CbzEMA13-b-HFBMA244 diblock
copolymers and (b) CbzEMA13 homopolymer and the CbzEMA13-b-HFBMA244 diblock copolymer.
attached onto its surface, forming monolayers, whereas based on
X-ray photoelectron spectroscopy (XPS) data the formation of
multilayers is excluded.
intense band at around 300 nm being assigned to the p–p* local
excitation of carbazole moieties, whereas HFBMA95 presents an
absorption band in the UV region.
Room temperature photoluminescence (PL) measurements
were carried out on the Au-coated silicon substrates prior and
after the immersion of the latter in the polymer solution for 24 h,
in order to verify the anchoring of the polymers onto the Au
surfaces. Fig. 12(a) presents the PL emission spectra recorded in
the absence and presence of anchored polymer chains after 24 h
immersion of the substrates in the polymer solution. Furthermore,
time resolved PL at the peak of the signal (i.e. at 450 nm) is
presented in Fig. 12(b). Clearly seen from the data, the Au appears
to have almost zero PL whereas the CbzEMA52-b-HFBMA69 PL
emission is much stronger under identical excitation conditions
and depends on the sample position (see PL for spot 1 and spot 2).
It is noteworthy to emphasize at this point that the spectral
profile is the same over the entire sample. The observed
differences in the intensity of the signal at different positions
onto the sample may suggest that the polymer is not evenly
distributed onto the substrate. The latter has been confirmed by
Prior to photoluminescence measurements (PL) performed on
the polymer-coated Au-silicon substrates, UV-Vis characteriza-
tion of the homopolymers and diblock copolymers was carried
out in THF. In Fig. 11 the UV-Vis spectra of (i) the CbzEMA52
and HFBMA95 homopolymers and (ii) the CbzEMA113
-
b-HFBMA52 diblock copolymer recorded in THF at room
temperature are shown. Several absorption maxima appear in
the spectra of the CbzEMA52 and CbzEMA113-b-HFBMA52
within the wavelength range of 240–350 nm with the strong
Table 2 Refractive index of HFBMA95 homopolymer, CbzEMA113
b-HFBMA52 diblock copolymer and THF
-
Polymer
Refractive Index
THF
HFBMA95
CbzEMA113-b-HFBMA52
1.4080
1.4065
1.4135
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Fig. 9 (a) DSC thermograms for the HFBMA95 and CbzEMA52 homopolymers and the CbzEMA113-b-HFBMA52 diblock copolymer (b) TGA
curves of the CbzEMA52 and HFBMA95 homopolymers and the CbzEMA113-b-HFBMA52 diblock copolymer.
Fig. 10 Schematic representation of the generation of a polymer film on gold-coated silicon substrate.
AFM characterization of the resulting films. Fig. 13 shows the
AFM measured surface features of the analyzed films covering
an area of 250 nm 6 250 nm and 500 nm 6 500 nm. The change
of colors over the image indicates the thickness variation. The
heights of the surface features are represented with the brighter
pixel whereas the dark pixel indicates the valleys. As shown in
Fig. 13, the distribution of the polymer onto the substrate is
uneven. The polymer film roughness varies between 6 and 12 nm.
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In the case of the Au-coated silicon substrate covered with
polymer chains it is clearly observed that the ‘‘monomer’’
emission peaks appearing at 348 and 365 nm in the case of the
CbzEMAx homopolymers47 do not exist, whereas an emission
peak at y435 nm appears instead. In general as stated by
Keyanpour-Rad et al.,47 the fluorescence emission expected for
‘‘monomer’’ emission is a mirror image of the 1Lb
r
1A
absorption band, resulting from the singlet state of an excited
carbazolyl chromophore that does not interact strongly with a
neighbouring chromophore. It is likely that the anchoring of
the dithioester-functionalized CbzEMA52-b-HFBMA69 diblock
copolymer chains onto the Au surface resulted in the generation
of a parallel overlap interchromophore geometry allowing for
excimer formation to occur. Similarly, Keyanpour-Rad and co-
workers have demonstrated the excimer formation in solution at
different lmax excimer wavelengths ranging between 418–440 nm
(depending on the solvent system used) for a polymethacrylate
containing side-chain electroactive carbazole moiety namely
poly([2-(9-ethyl)carbazolyl]methyl methacrylate), due to favourable
Fig. 11 UV-vis spectra of the CbzEMA52 and HFBMA95 homopolymers
and the CbzEMA113-b-HFBMA52 diblock copolymers recorded in THF.
Fig. 12 (a) PL spectra of Au-coated silicon substrate and CbzEMA52-b-HFBMA69 –S–CLS-/Au-coated silicon substrate (recorded at different sample
positions). (b) Time resolved PL at the peak of the signal (i.e. at 450 nm).
Fig. 13 AFM 3D-graph topography images of a gold substrate immersed in a THF solution of CbzEMA52-b-HFBMA69 (2 g L21) at room temperature for 24 h.
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substrate may assist their arrangement into a parallel geometry,
allowing for interchromophore overlapping and thus excimer
generation. The combination of a low-surface energy fluorinated
block, with an electro-active carbazole-containing segment
imparts to these immobilized thin layers useful properties
towards their potential applicability in gas sensing technologies.
Acknowledgements
This work was financially supported by the University of
Cyprus. We are grateful to Professors Costas S. Patrickios and
Panagiotis Koutentis (Chemistry Department, University of
Cyprus) for providing access to the DSC and TGA respectively.
We also thank Dr Matthew Zervos (Department of Mechanical
and Manufacturing Engineering, University of Cyprus) for the
fabrication of the Au-coated silicon substrates. Finally, we thank
the A. G. Leventis Foundation for a generous donation that
enabled the purchase of the NMR spectrometer of the University
of Cyprus.
Fig. 14 AFM 3D-graph topography images of
a gold substrate
immersed in a THF solution of CbzEMA52-b-HFBMA69 (2 g L21) at
room temperature for 48 h.
steric constrains. Moreover, it has been found that the latter
becomes much more pronounced when the polymer chain is
immobilized in a rigid matrix.47
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