Photoluminescent epoxy microspheres: Preparation, surface functionalization via grafting polymerization and photophysical properties

Article abstract of DOI:10.1039/c5ra15804d

In this contribution, we reported the preparation of photoluminescent epoxy microspheres. First, diglycidyl ether of tetraphenylethene (DGETPE), a novel epoxy monomer bearing tetraphenylethene was synthesized and then it was incorporated into a commercial

Full text of DOI:10.1039/c5ra15804d

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PAPER
Photoluminescent epoxy microspheres:
preparation, surface functionalization via grafting
Cite this: RSC Adv., 2015, 5, 77922
polymerization and photophysical properties
*
Wenjun Peng, Lei Li and Sixun Zheng
In this contribution, we reported the preparation of photoluminescent epoxy microspheres. First, diglycidyl
ether of tetraphenylethene (DGETPE), a novel epoxy monomer bearing tetraphenylethene was synthesized
and then it was incorporated into a commercial epoxy (viz. diglycidyl ether of bisphenol A) to obtain the
epoxy microspheres with sizes of 1–3 mm via a phase-inverted reaction-induced phase separation
approach. It was found that the epoxy microspheres were luminescent under UV light radiation. The
surfaces of the photoluminescent epoxy microspheres were further functionalized with poly(N-vinyl
pyrrolidone) (PVPy) with a grafting-from methodology via reversible addition–fragmentation chain
transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) process. The
PVPy-grafted epoxy microspheres can be well dispersed into aqueous solutions. The morphologies and
photophysical properties of the epoxy microspheres were investigated by means of scanning electron
microscopy, and UV-vis and fluorescence spectroscopy.
Received 7th August 2015
Accepted 8th September 2015
DOI: 10.1039/c5ra15804d
www.rsc.org/advances
the multiple aryl rings of the AIE molecules can rotate around
the single-bond axles linking the peripheral aryl rotor and the
Introduction
central stator. The free intramolecular rotation would convert
photonic energy into heat and deactivates the excited states no-
radioactively, thus making the molecules non-emissive. In the
aggregates, however, the intramolecular free rotation is
restricted and the non-radiative relaxation channel is obstruc-
ted and thus the molecules become emissive. The mechanism
that restriction of intramolecular rotation (RIR) of multiple aryl
groups in the above molecules is responsible for the AIE effect
inspires one of designing other constraints to suppress intra-
molecular rotation to make these compounds emissive.
Epoxy microspheres are a class of important materials owing
to their excellent thermal, dimensional stability and chemical
resistance; they have been applied as coatings, adhesives,
molding compounds, chromatographic column packing and
substrates for medical diagnostics.25–29 Generally, epoxy micro-
spheres are prepared via dispersion polymerization30,31 in the
blends of epoxy with a liquid polyether (e.g., polypropylene
glycol, PPG). In this case, epoxy microspheres are formed and
phase-separated out as the curing reaction proceeds.32–38 The
driving forces for the reaction-induced phase separation are: (i)
the increased molecular weight owing to polymerization (viz.
curing reaction), which gives rise to the decreased contribution
of mixing entropy (DSm) to the free energy of mixing (DGm) and
(ii) the alteration of the inter-component interaction parame-
ters (c) with conversion of monomers. The use of the liquid
polyether is to facilitate the isolation of the resulting epoxy
microspheres from the mixtures. In this approach, nonetheless,
epoxy microspheres are the minor component whereas the
For most organic aromatic compounds, uorescence intensity
would be weakened upon increasing the concentration of their
solutions. Such a concentration-quenching effect is attributable
to the formation of sandwich-shaped excimers and exciplexes
aided by the collisional interactions between the aromatic
molecules in the excited and ground states.1–3 In marked
contrast to this common knowledge, nonetheless, Tang et al.4–7
recently reported a class of novel molecules, the aggregation of
which can induce and even intensify the emission of uores-
cence. For instance, silole molecules and their derivatives are
generally non-luminescent in the solution state but they
became emissive in the aggregation state.4,5 This novel
phenomenon has been named as “aggregation-induced emis-
sion” (AIE) effect.4,5 In the past years, many compounds such as
tetraphenylethene (TPE),4,6,8–11 benzobis(thiadiazole)12,13 and
1,4-di[(E)-styryl]benzene14 have been found (or/and synthe-
sized), which can display such AIE effect. These compounds are
potential to nd a variety of applications in OLEDs, chemo-
sensors and bioprobes15–24 because the luminescent properties
via AIE effect are exhibited in the aggregation state, which is in
marked contrast to those luminogens to possess concentration-
quenching effect. The mechanistic investigation shows that
restriction of intramolecular rotation (RIR) in the aggregates is
the main cause of the AIE effect. It is proposed that in solution,
Department of Polymer Science and Engineering, State Key Laboratory of Metal Matrix
Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. E-mail:
szheng@sjtu.edu.cn; Fax: +86-21-54741297; Tel: +86-21-54743278
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liquid polyether is the major component. Recently, it was real- morphologies of the epoxy microspheres were characterized by
ized that epoxy microspheres can be alternatively prepared via means of scanning electron microscopy (SEM), UV-vis and
so-called phase-inverted reaction-induced phase separation uorescence spectroscopy.
(PIRIPS) approach while thermoplastics are used instead of
liquid polyethers. It is noted that phase inversion occurs at the
low concentration of thermoplastics (e.g., <20%). In marked
Experimental
contrast to dispersion polymerization approach, epoxy is a
Materials
major component whereas thermoplastic polymers are the
minor component in this approach. For instance, Woo et al.39
have reported the preparation of epoxy microspheres in the
thermosetting blends of epoxy with poly(methyl methacrylate)
(PMMA). In the thermosetting blends containing up to 30 wt%
of PMMA, dispersed epoxy microdomains were formed with the
diameter of 10–80 mm aer continuous PMMA matrices were
removed via extraction with dichloromethane. Recently, Zheng
et al.40 reported that the preparation of epoxy microspheres with
the size of 5–10 mm in diameter with the blends of epoxy with
poly(3-caprolactone) with PIRIPS approach. The PIRIPS
behavior has been interpreted on the basis of spinodal
decomposition mechanism,32,34 which is involved with several
competitive kinetics such as curing reaction, phase separation
and changes of visco-elastomeric properties of component
polymers. Compared to dispersion polymerization, the effi-
ciency of preparation is signicantly enhanced with PIRIPS
approach.
Generally, epoxy microspheres are not luminescent. It is of
interest to prepare novel epoxy microspheres with photo-
luminescent properties. It is expected that photoluminescent
epoxy microspheres could nd more applications such as ink
ller for currency anti-counterfeiting, luminescent coating and
bioprobes. In addition, luminescent epoxy microspheres
dispersed in water could be used for the detection of wastewater
on the basis of uorescent quenching.41 To the best of our
knowledge, there has been no previous report on the prepara-
tion of photoluminescent epoxy microspheres. In this work, we
explored to prepare luminescent epoxy microspheres via PIRIPS
p-Methoxybenzophenone was purchased from TCI Co.,
Shanghai, China. Titanium(IV) chloride (TiCl4), zinc powder,
boron tribromide and 2-bromopropionyl bromide were
purchased from Aldrich Co., Shanghai, China. Epichlorohydrin,
sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3),
magnesium sulfate (MgSO4), 4,4-diaminodiphenylsulfone
(DDS) were obtained from Shanghai Reagent Co., China.
Diglycidyl ether of bisphenol A (DGEBA) was supplied from
Shanghai Resin Co., China and it has a quoted epoxide equiv-
alent weight of 185–210. Poly(3-caprolactone) (PCL) was
purchased from Aldrich Co., USA and it had a quoted molecular
weight of Mn ¼ 50 000 Da. 2,2-Azobisisobutylnitrile (AIBN) was
purchased from Shanghai Reagent Co., China and was recrys-
tallized from acetone twice before use. N-Vinyl-2-pyrrolidinone
(NVP) was purchased from Aldrich Co., Shanghai, China and
it was puried by passing alkaline aluminum oxide column to
remove the inhibitor. Potassium ethyl xanthate was synthesized
by following the method of literature.42 The organic solvents
such as tetrahydrofuran (THF) and triethylamine (TEA) were
obtained from commercial source. Before use, THF was reuxed
over metal sodium and then distilled; trimethylamine was
reuxed with p-toluenesulfonyl chloride and then distilled.
Dichloromethane (DCM) was dried over CaH2 and then
distilled. All other reagents and solvents used in this work were
purchased from Shanghai Reagent Co., China.
Synthesis of dimethoxytetraphenylethylene (DMOTPE)
approach. First, we synthesized diglycidyl ether of tetrapheny- Dimethoxytetraphenylethylene (DMOTPE) was synthesized with
lethene (DGETPE), a novel epoxy monomer bearing tetraphe- McMurry coupling reaction.43,44 Typically, to a ask equipped
nylethene (TPE). Thereaer, DGETPE together with diglycidyl with a magnetic stirrer, zinc powder (6.540 g, 100.0 mmol) and
ether of bisphenol A (DGEBA) was used to obtain the lumines- THF (80 mL) were charged under an argon atmosphere with
cent epoxy microspheres with the thermosetting blends of vigorous stirring. This mixture was cooled to 0 ꢀC, and then
epoxy with poly(3-caprolactone) via PIRIPS approach. It has titanium(IV) chloride (TiCl4) (2.200 mL, 20.0 mmol) was drop-
been known that tetraphenylethene (TPE) and its derivatives are wise added. The system was reuxed for 3 hours and cooled to
capable of displaying AIE effect.4,6,9–11 It is expected that the
0
ꢀC and then p-methoxybenzophenone (2.120 g, 10.0 mmol)
incorporation of TPE structural units into epoxy thermosets dissolved in THF (60 mL) was dropwise added into the system.
could achieve the restriction of intramolecular rotation (RIR) of The mixture was reuxed overnight. Aer reaction, the system
TPE via the formation of crosslinking networks (viz., vitrica- was cooled down to room temperature and the reaction was
tion of polymer), making the moieties emissive. This strategy is quenched by adding the saturated aqueous solution of sodium
in marked contrast to the case that RIR was fullled via the bicarbonate (NaHCO3) until no bubbles were released. The
aggregation of TPE moieties.4,6,9–11 The purpose of this work is to reacted mixture was then ltered and the ltrate was extracted
demonstrate this situation. To investigate the photophysical with dichloromethane (DCM). The organic layer was dried with
behavior of the epoxy microspheres in water, we further func- anhydrous magnesium sulfate (MgSO4) overnight and then
tionalized the surfaces of the luminescent epoxy microspheres ltered. With rotary evaporation, the crude product was
with poly(N-vinyl pyrrolidone) (PVPy), a water-soluble polymer obtained. The crude product was puried by recrystallization
with a graing-from methodology via reversible addition–frag- from the mixture of dichloromethane with methanol (2 : 1 vol)
mentation chain transfer/macromolecular design via the inter- and the product (1.629 g) was obtained with the yield of 83%. 1H
change of xanthate (RAFT/MADIX) process. In this work, the NMR (CDCl3, ppm): 7.10–7.05 (m, 10H, –C6H5), 6.92 (d, 4H,
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m-C6H2, J ¼ 8.8 Hz), 6.63 (d, 4H, o-C6H2, J ¼ 8.8 Hz) and 3.73 (m, remove PCL. Aer drying in a vacuum oven at 30 ꢀC for
6H, –OCH3).
24 hours, the epoxy microspheres were obtained.
Synthesis of dihydroxytetraphenylethylene (DHTPE)
Surface graing polymerization of epoxy microspheres
To a ask equipped with a magnetic stirrer, DMOTPE (2.000 g,
5.1 mmol) and DCM (30 mL) were charged with vigorous stir-
ring. The system was cooled to ꢁ78 ꢀC and then boron tri-
bromide (5.110 g, 20.4 mmol) dissolved in 20 mL of DCM was
added with vigorous stirring for 15 min. Aer the system was
warmed to room temperature, the reaction was performed for
additional 12 hours. The ask was then immersed into an ice-
water bath and then deionized water (40 mL) was dropwise
added with vigorous stirring. The hydrolysis reaction was carried
out at room temperature with vigorous stirring for one hour. The
organic layer was isolated and washed with deionized water
three times. Aer dried with anhydrous MgSO4, the solvent was
eliminated via rotary evaporation. The crude product was further
puried with recrystallization from the mixture of acetone with
water (9 : 1 vol) and the product (1.634 g) was obtained with the
yield of 88%. 1H NMR (CDCl3, ppm): 7.13–7.01 (m, 10H, –C6H5),
6.90 (d, 4H, m-C6H2, J ¼ 9.2, 13.6 Hz), 6.59 (d, 4H, o-C6H2, J ¼ 8.4,
12.0 Hz) and 4.64 (s, 2H, –OH).
First, the surfaces of the epoxy microspheres were functional-
ized with 2-bromopropionyl groups. To a ask equipped with a
magnetic stirrer, the epoxy microspheres (1.500 g), THF (20 mL),
and triethylamine (5 mL) were charged with vigorous stirring.
To promote the dispersion of the epoxy microspheres, the
mixture was subjected to ultrasonic irradiation for 20 min.
Thereaer, 2-bromopropionyl bromide (8.240 g, 38.2 mmol)
was dropwise added at 0 ꢀC with vigorous stirring. The reaction
was carried out at room temperature for 24 hours and the
microspheres were isolated with a centrifugeꢀ. Aer washing
with THF three times and dried in vacuo at 30 C for 24 hours,
the 2-bromopropionyl-functionalized epoxy microspheres were
obtained. The content of bromine was measured to be 11.2 wt%
by means of elemental analysis.
Second, the surfaces of the above 2-bromopropionyl-
functionalized epoxy microspheres were subjected to the reac-
tion with potassium ethyl xanthate to obtain the xanthate-
functionalized epoxy microspheres. Typically, to a ask equip-
ped with a magnetic stirrer, the above 2-bromopropionyl-
functionalized epoxy microspheres (1.000 g) and acetone
(10 mL) were charged. With ultrasonic irradiation for 30 min,
potassium ethyl xanthate (0.700 g) was added. The reaction was
carried out at room temperature for 24 hours and then the
reacted epoxy microspheres were isolated with centrifuge. The
as-obtained solids were then washed with THF three times.
Aer drying in vacuo at 30 ꢀC for 24 hours, the product was
obtained.
Third, the surface graing polymerization of N-vinyl pyrro-
lidone (NVP) was carried out with the above ethyl xanthate-
functionalized epoxy microspheres as the chain transfer agent
via a RAFT/MADIX process. To a ask equipped with a magnetic
stirrer, the above xanthate-functionalized epoxy microspheres
(0.200 g), NVP (0.600 g, 5.400 mmol), 1,4-dioxane (5 mL) and
AIBN (8 mg, 0.049 mmol) were charged with vigorous stirring.
The ask was connected onto a Schlenk line to degas via three
freeze–pump–thaw cycles. The polymerization was performed at
70 ꢀC for 24 hours, and the reacted epoxy microspheres were
isolated via centrifuge. The as-obtained solids were then
washed with the mixture of tetrahydrofuran with water
(3 : 1 vol) three times. Aer drying in a vacuum oven at 30 ꢀC for
24 hours, the product, PVPy-graed epoxy microspheres were
obtained by controlling the conversion of NVP to be ca. 11.5%.
Synthesis of diglycidyl ether of tetraphenylethene (DGETPE)
To a ask equipped with a mechanical stirrer, a constant
pressure dropping funnel and a graham condenser, DHTPE
(1.000 g, 2.74 mmol) and epichlorohydrin (2.530 g, 27.4 mmol)
were charged with vigorous stirring and then the saturated
solution of NaOH (2.5 mL) was slowly added. The mixture was
reuxed for 2 hours. Cooled to room temperature, the reacted
mixture was diluted with DCM (50 mL) and washed with the
saturated aqueous solution of NaCl (20 mL) and deionized
water (20 mL) to neutrality. The organic layer was dried with
anhydrous magnesium sulfate and ltered. With rotary evapo-
ration, the crude product was obtained. The crude product was
puried by passing a silica gel column with the mixture of
hexane with ethyl acetate (4 : 1 vol) as the eluent. With rotary
evaporation, the product (1.072 g) was obtained with the yield of
82%. 1H NMR (CDCl3, ppm): 7.01–6.96 (m, 10H, –C6H5), 6.82 (d,
4H, m-C6H2, J ¼ 10.4 Hz), 6.57 (d, 4H, o-C6H2, J ¼ 8.4 Hz), 4.03 (s,
2H, –CH2C2H3O of trans-DGEDPE), 3.80 (s, 2H, –CH2C2H3O of
cis-DGEDPE), 3.22 (s, 2H, –CH– of epoxide group), 2.79 (s, 2H,
2–CH2– of epoxide group of trans-DGETPE), 2.64 (s, 2H, –CH2–
of epoxide group of cis-DGETPE).
Preparation of epoxy microspheres
DGEBA (2.700 g), PCL (1.730 g) and DGETPE (0.300 g) were
mixed at 120 C with continuous stirring until PCL was totally
Measurement and characterization
ꢀ
dissolved, and then DDS (1.050 g) was added with continuous
Nuclear magnetic resonance (NMR) spectroscopy. The NMR
stirring until the systems became homogeneous. The mixtures measurements were carried out on a Varian Mercury Plus
were cured at 150 ꢀC for 3 hours and 180 ꢀC for one hour to 400 MHz NMR spectrometer at 25 ꢀC. The samples were dis-
attain a complete curing reaction. The thermosetting blend of solved in deuterium chloroform and the solutions were
epoxy with PCL was ground and added to a beaker and 200 mL measured with tetramethylsilane (TMS) as an external
of THF was added with vigorous stirring until all of the pellets reference.
were dispersed in THF. The mixture was centrifuged to afford
Fourier transform infrared (FTIR) spectroscopy. FTIR
the solids. The solids were washed with THF three times to measurements were performed on a Perkin-Elmer Paragon 1000
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FTIR spectrometer. The samples were granulated, and the
powders were mixed with KBr and then pressed into small
akes for FTIR measurement. Before the measurements, the
ꢀ
specimens were dried in vacuo at 60 C for 48 hours. All of the
specimens used in the study were sufficiently thin to be within a
range where the Beer–Lambert law was obeyed. The spectra
were obtained at a resolution of 2 cmꢁ1 and are reported as the
averages of 64 scans.
Scanning electron microscopy (SEM). The specimens were
examined with a JEOL JSM 7401 F FESEM scanning electron
microscope (SEM) at an activation voltage of 5 kV. Before the
morphological observation, the surfaces of epoxy microspheres
˚
were coated with thin layers of gold of about 100 A.
Elemental analysis. The content of bromine element was
determined by means of a technique of oxygen ask combus-
tion followed by titration with mercuric nitrate. The samples
were rst combusted in a 1.0 L oxygen-lled combustion ask
Scheme
(DGETPE).
1
Synthesis of diglycidyl ether of tetraphenylethene
containing 0.01 M nitric acid. Aer combustion, the ask was dihydroxytetraphenylethylene (DHTPE) by the use of baron tri-
cooled, and the solution was titrated with a dilute standard bromide (BBr3), followed by treatment with water. Finally, the
silver nitrate solution to calculate the content of bromine. The condensation between DMOTPE and epichlorohydrin was per-
value of bromine content was taken as the average of three formed to afford diglycidyl ether of tetraphenylethene
measurements with a deviation of 5%. The contents of sulfur (DGETPE). Shown in Fig. 1 are the 1H NMR spectra of DMOTPE,
were determined by means of a technique of oxygen ask DHTPE and DGETPE. For DMOTPE, the signal of resonance at
combustion. Samples were rst catalytic oxidized in the high- 3.73 ppm is assignable to the protons of methoxyl groups
temperature oxygen-rich environment. Gas mixture were sepa- whereas the peaks in the range of 6.5–7.5 ppm are attributable
rated effectively by a special adsorption–desorption device, and to the protons of phenyl groups. Upon reacting with BBr3, the
sulfur dioxide was collected to calculate the content of sulfur.
The value of sulfur content was taken as the average of three
measurements with a deviation of 5%.
Thermogravimetric analysis (TGA). A TA Instruments ther-
mogravimetric analyzer (Q-5000) was used to investigate the
thermal stability of the samples. The samples were heated in a
ꢀ
nitrogen atmosphere from ambient temperature to 800 C at a
ꢁ1
ꢀ
heating rate of 20 C min in all cases.
Ultraviolet-visible (UV-vis) spectroscopy. UV-vis spectroscopy
was carried out on a Cary 60 spectrometer, Agilent Technolo-
gies, USA. The concentration of DGETPE in THF solution or the
mixed solution of THF with water (THF : water ¼ 1 : 99 vol) was
set as 10.0 mM. The concentration of PVPy-graed epoxy
microspheres in the suspensions of water were set as 0.01, 0.03,
0.10, 0.33, 0.50 and 1.00 g Lꢁ1, respectively.
Fluorescent (FL) spectroscopy. Fluorescent (FL) spectroscopy
was carried out on a Perkin-Elmer LS 55 spectrometer. The FL
spectra were obtained with a 450 W Xe lamp as a continuous UV
light source at the wavelength of l ¼ 330 nm. Fluorescence
quantum yield was measured with anthracene as the standard
sample with its ethanol solutions.45
Results and discussion
Synthesis of DGETPE
The route of synthesis for diglycidyl ether of tetraphenylethene
(DGETPE) is shown in Scheme 1. First, dimethoxytetraphenyl-
ethylene (DMOTPE) was synthesized via McMurry coupling
reaction43,44 of p-methoxybenzophenone in the presence of
titanium(IV) chloride and metal zinc. Second, the demethylation
Fig. 1 1H NMR spectra of: DMOTPE (down), DHTPE (middle) and
DGETPE (up). The signals of resonance labeled with asterisks (*)
reaction of DMOTPE was carried out to afford resulted from a trace of water in the deuterated solvent.
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Fig. 2 SEM micrographs of luminescent epoxy microspheres: (A)
unmodified epoxy microspheres, (B) PVPy-grafted epoxy
microspheres.
Fig. 3 Photographs of PVPy-grafted epoxy microspheres under: (A)
incandescence and (B) ultraviolet (l ¼ 365 nm) illumination.
epoxy monomer. Before the curing reaction, the mixture
composed of DGEBA, DDS and PCL were homogenous trans-
parent at the curing temperature (viz., 150 ꢀC). As the curing
reaction proceeded, the initially transparent mixtures gradually
became cloudy, indicating the occurrence of reaction-induced
phase separation. Aer the continuous phase (viz. PCL) was
removed via dissolution, the epoxy microspheres were thus
obtained. The morphology of the as-obtained epoxy micro-
spheres was examined by means of scanning electron micros-
copy (SEM) and the SEM micrographs were shown in Fig. 2. It
is seen that the crosslinked epoxy microspheres with the size of
1–3 mm were successfully obtained. Fig. 3 shows the photo-
graphs of the epoxy microspheres under incandescence and
ultraviolet (l ¼ 365 nm) illumination. Under incandescence, the
epoxy microspheres were not luminescent (Fig. 3A). Under
ultraviolet (l ¼ 365 nm), the materials emitted blue light
(Fig. 3B). The above results indicate that the luminescent epoxy
microspheres were successfully obtained.
intense peak of resonance at 3.73 ppm disappeared whereas the
peaks the range of 6.5–7.5 ppm remained unchanged. Concur-
rently, there appeared a minor peak at 4.64 ppm, which is
assignable to the proton resonance of phenolic hydroxyl groups.
The 1H NMR spectroscopy indicates that the demethylation
reaction has been performed to completion. With the occur-
rence of DHTPE with epichlorohydrin, there appeared several
new signals of resonance at 2.63, 2.79, 3.22, 3.78 and 4.04 ppm,
assignable to the protons of methylene and methine in glycidyl
groups. According to the integral intensity of the methylene (or
methine) protons to phenyl protons, it is judged that the all the
phenolic hydroxyl groups of DHTPE were capped with glycidyl
groups, i.e., DGETPE was successfully obtained.
Preparation of epoxy microspheres
It has been reported that the reaction-induced phase separation
occurred in the thermosetting blends of epoxy and poly(3-cap-
rolactone) (PCL) while 4,40-diaminodiphenylsulfone (DDS) was
used as the curing agent.46,47 Notably, the phase inversion
Surface graing polymerization via RAFT/MADIX process
occurred while the content of PCL was 30 wt% or higher. The The above epoxy microspheres were functionalized with a
results of SEM showed that in the thermosetting blend, PCL was surface graing-from methodology via reversible addition–
the continuous phase whereas epoxy was the dispersed phases fragmentation chain transfer/macromolecular design via the
in the form of spherical particles.41 In this work, the thermo- interchange of xanthate (RAFT/MADIX) approach as depicted in
setting blend containing 30 wt% of PCL was thus utilized to Scheme 3. First, the epoxy microspheres were subjected to a
prepare epoxy microspheres as descripted in Scheme 2. To surface reaction with 2-bromopropionyl bromide to afford the
obtain the luminescent epoxy microspheres, the mixture of modied epoxy microspheres, the surfaces of which bore
DGETPE with DGEBA at the mass ratio of 1 : 9 was used as the 2-bromopropionyl groups. The result of the elemental analysis
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Fig. 4 FTIR spectra of: (A) unmodified, (B) 2-bromopropionyl-func-
tionalized, (C) xanthate-functionalized, and (D) PVPy-grafted epoxy
microspheres.
Scheme 2 Preparation of luminescent epoxy microspheres via phase-
inverted phase separation-induced polymerization.
stretching vibration of the carbonyl groups in the moiety of 2-
bromopropionyl groups. The FTIR spectroscopy indicates that
the surfaces of the epoxy microspheres were successfully func-
tionalized with 2-bromopropionyl groups. Upon reacting with
potassium ethyl xanthate, there appeared a new band at 1040
cmꢁ1 compared to 2-bromopropionyl-functionalized epoxy
microspheres. This band was assignable to the stretching
vibrations of thiocarbonyl groups in the moiety of ethyl
xanthate groups. The FTIR spectroscopy indicated that the
surfaces of the epoxy microspheres were successfully function-
alized with ethyl xanthate groups.
showed that the content of bromine was measured to be
1.4 mmol gꢁ1. These 2-bromopropionyl groups were allowed
further to react with potassium ethyl xanthate to obtain the new
epoxy microspheres, whose surfaces bore ethyl xanthate groups.
The results of elemental analysis showed that the content of
sulfur element was measured to be 3.76 mmol gꢁ1. Deducting
the content of sulfur element in the curing agent (viz. DDS), the
content of sulfur element in the form of ethyl xanthate groups at
the surfaces of the epoxy microspheres were calculated to be
2.7 mmol gꢁ1. The ethyl xanthate groups could behave as the
sites to grow the graed polymer chains from the surfaces of the
epoxy microspheres. In this work, the ethyl xanthate-
functionalized epoxy microspheres were then used as the
chain transfer agent to grow poly(N-vinyl pyrrolidone) (PVPy)
chains via reversible addition–fragmentation chain transfer/
macromolecular design via the interchange of xanthate
(RAFT/MADIX) process. Shown in Fig. 4 are the FTIR spectra of
the epoxy microspheres, the surface of which had different
functional groups. For the control epoxy microspheres, the
band at 3380 cmꢁ1 is assignable to the stretching vibration of
hydroxyl groups in hydroxyl ether structural units of amine-
crosslinked epoxy. In addition, there appeared a minor band
at 1726 cmꢁ1. This band is ascribed to the carbonyl groups of a
small amount of PCL entrapped into the epoxy microspheres in
the process of reaction-induced phase separation. Upon func-
tionalization with 2-bromopropionyl bromide, there appeared a
new band at 1742 cmꢁ1. This band is assignable to the
The epoxy microspheres bearing ethyl xanthate groups were
used as the chain transfer agent for the RAFT polymerization of
Scheme 3 Surface functionalization of epoxy microspheres with a
grafting-from polymerization methodology via
a RAFT/MADIX
process.
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Fig. 6 UV absorption spectra of DGETPE in THF and the mixture of
THF with water (THF : water ¼ 1 : 99 vol).
Fig.
5 TGA curves of the unmodified and PVPy-grafted epoxy
microspheres.
surfaces of the epoxy microspheres was measured to be 1.79
mmol gꢁ1. According to this value, the mass fraction of PVPy to
epoxy was 33.8% in the PVPy-graed epoxy microspheres.
N-vinyl pyrrolidone (NVP). With the polymerization, PVPy
chains were grown from the surfaces of the epoxy microspheres,
i.e., the PVPy-graed epoxy microspheres were obtained. The
solubility test showed that the PVPy-graed epoxy microspheres
were easily dispersed in water, which was in marked contrast to
the unmodied epoxy microsphere. This observation indicates
that the water-soluble polymer (viz. PVPy) was successfully
obtained. The morphology of the PVPy-graed epoxy micro-
spheres was investigated by means of scanning electron
microscopy (SEM) and the SEM image is also presented in Fig. 2
(see Fig. 2B). For the unmodied epoxy microspheres, the
surfaces were smooth; there was not the adhesion among the
adjacent particles (see Fig. 2A). For the PVPy-graed epoxy
microspheres, the surfaces were coated with a layer of polymer
(viz. PVPy) and some wrinkles were found at the surfaces of
some particles. In addition, there was the adhesion among the
adjacent particles (see Fig. 2B). These observations indicate that
PVPy chains have been successfully graed onto the surfaces of
the epoxy microspheres. The control and PVPy-graed epoxy
microspheres were subjected to thermogravimetric analysis
(TGA) and the TGA curves are shown in Fig. 5. The unmodied
epoxy microspheres displayed one-step degradation prole and
Photophysical properties
Tetrahydrofuran (THF) was a good solvent of DGETPE whereas
water is a non-solvent of DGETPE. Therefore, DGETPE would
display an aggregation-induced emission phenomenon in the
mixture of THF with water as a function of the concentration of
water. The UV absorption and uoresce spectra of DGETPE
ꢀ
the initial degradation occurred at ca., 400 C. In contrast, the
PVPy-graed epoxy microspheres exhibited a two-step degra-
dation prole at ca., 236 and 400 ꢀC, respectively. It is plausible
to propose that the rst one is attributable to PVPy chains which
were graed from the surfaces of the epoxy microspheres
whereas the second to the epoxy. The results of FTIR, solubility
test and TGA show that the PVPy-graed epoxy microspheres
were successfully obtained. In order to determine the mass
fraction of the graed PVPy chains, we further measured the
content of the sulfur element by means of elemental analysis.
The content of sulfur element at the ends of PVPy chains at the
Fig. 7 FL spectra of DGETPE in THF and the mixture of THF with water
(THF : water ¼ 1 : 99 vol).
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Fig. 8 Photographs of aqueous suspensions of unmodified and PVPy-grafted epoxy microspheres with various concentrations under ultraviolet
illumination (l ¼ 365 nm).
emission, notably, there was a red shi from 397 to 468 nm with
the inclusion of a large amount of water. Owing to the inclusion
of water, the rotation of aromatic ring in DGETPE was restricted
in its aggregates, which facilitated the formation of the
excimers.48
The above DGETPE was further incorporated into epoxy to
prepare the luminescent epoxy microspheres via phase-inverted
reaction-induced phase separation approach. It is expected that
in the crosslinked network, the rotation of the aromatic rings of
DGETPE would be restricted due to the constraint from the
environmental crosslinked network. It is expected that the
DGETPE moieties entrapped in the crosslinked network would
convert photonic energy to emission, i.e.,
a so-called
crosslinking-induced emission occurred. To facilitate the
dispersion of the epoxy microspheres in water, the surfaces of
the microspheres were functionalized with PVPy, a water-
soluble polymer. Shown in Fig. 8 are the photographs of the
unmodied and PVPy-graed epoxy microspheres dispersed in
water at variable concentrations taken under UV illumination
(l ¼ 365 nm). Notably, the epoxy microspheres were emissive in
all the cases. The epoxy microspheres whose surfaces were not
functionalized with PVPy oated above water in the form of
aggregation. In marked contrast, the PVPy-graed epoxy
microspheres can well disperse in water; the intensity of the
uorescence increased with increasing the concentration of the
microspheres in water. This observation indicates that the
luminescent epoxy microspheres have been successfully func-
tionalized with PVPy via surface graing polymerization. The
suspensions of the PVPy-graed epoxy microspheres in water
were further subjected to FL spectroscopy and the FL spectra are
shown in Fig. 9. In all the cases, the PVPy-graed epoxy
microspheres displayed the intense emission. For the suspen-
sion with the concentration of 0.01 g Lꢁ1, a few sharp bands
were detected at l ¼ 373, 383, 394 and 415 nm, respectively. The
separate FL bands could reect the different restriction of
intramolecular rotation (RIR) in TPE moiety entrapped in the
environmental networks, i.e., there existed the conformational
anisotropy of TPE moiety. In the thermosetting polymer, the
RIR process was activated owing to the formation of the cross-
linked network, which exerted the constraint on the variation of
the molecular conformation of TPE moiety. The TPE moieties in
the crosslinked network were emissive owing to the
Fig. 9 FL spectra of PVPy-grafted epoxy microspheres with various
concentrations at an excitation wavelength of l ¼ 300 nm.
dispersed in THF and its mixture of water (THF : water ¼
1 : 99 vol) are shown in Fig. 6 and 7. In the UV-vis absorption
spectrum, DGETPE displayed an absorption band at l ¼ 322 nm
in THF solution whereas the absorption occurred at l ¼ 332 nm
in the mixture of THF with water. The bathochromic phenom-
enon reected the change in conformation of the compound
with the inclusion of water. In the THF solution, this compound
displayed a feeble uorescent emission band at ca. l ¼ 397 nm
while it was excited with ultraviolet radiation of l ¼ 330 nm
(see Fig. 7). Upon adding water to the THF solution at the
concentration of 99 vol%, notably, a very strong uorescent
emission appeared at l ¼ 468 nm. This is a typical aggregation-
induced emission (AIE) phenomenon. The increased intensity
of uorescent emission is attributable to the aggregation of
DGETPE in the system owing to the incorporation of a large
amount of non-solvent (viz., water). For the uorescent
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environmental constraints although there is hardly the aggre-
gation of the TPE moieties in the crosslinked network owing to
the low ratio of DGETPE to DGEBA. This case is in marked
contrast to the case that DGETPE was dissolved in THF (Fig. 7).
The conformational anisotropy of TPE moieties in the cross-
linked network gave rise to the polarized emission with
different wavelengths. While the concentration of the PVPy-
graed epoxy microspheres was increased to 0.03 g Lꢁ1 or
higher, the FL bands shied to the position with a longer
wavelength (i.e., l ¼ 469 nm). The intensity of the bands
increased with increasing the concentration of PVPy-graed
epoxy microspheres in the suspensions; in the meantime, the
intensity of the bands in the range of 350–420 nm was signi-
cantly decreased. Generally, a bathochromic phenomenon of FL
spectra with increasing the concentration of luminogen could
be interpreted on the basis of: (i) the formation of excimer (and/
or exciplex) and (ii) the light scattering effect of the epoxy
microspheres. In the AIE systems, the restriction of intra-
molecular rotation (RIR) would be achieved via gathering of
luminogens. The aggregation of luminogens would additionally
promote the formation of excimers and thus the observed
emission would result from the excimers with long wavelengths,
i.e., bathochromic phenomenon appeared.49–53 In the present
case, the aggregation of TPE moieties hardly occurred owing to
the low concentration of DGETPE. The increase in the concen-
tration of PVPy-graed epoxy microspheres in the suspensions
did not give rise to the aggregation of the luminogens. The
bathochromic phenomenon could result from the light scat-
Acknowledgements
The nancial supports from Natural Science Foundation of
China (No. 51133003 and 21274091) were gratefully acknowl-
edged. The authors thank the Shanghai Synchrotron Radiation
Facility for the support under the projects of No. 10sr0260 &
10sr0126.
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