Influence of Alcl3 on the optical properties of new synthesized 3-Armed poly(methyl methacrylate) films

Article abstract of DOI:10.3906/kim-0903-29

Three-armed poly(methyl methacrylate)[PMMA] was synthesized by atom transfer radical polymerization at 100 ° C using benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate as 3-armed initiator and a cuprous(I) bromide/2,2' -bipyridine catalytic system. The characterization of PMMA was confirmed by FT-IR, ¹HNMR, GPC, DSC, and TGA techniques. Optical absorption measurements were also studied by UV-VIS technique. Transmittance and reflectance measured in the wavelength range 300-700 nm were used to calculate the optical constants. The changes in dispersion and Urbach parameters were investigated as a function of AlCl 3 content. The wide band tails increased in width and the optical energy gap decreased from 2.39 to 1.28 eV as the AlCl ₃ content increased from 0% to 15%. Analysis revealed that the type of transition is the indirect allowed one. TUeBITAK.

Full text of DOI:10.3906/kim-0903-29

Turk J Chem
34 (2010) , 67 – 79.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-0903-29
Influence of AlCl3 on the optical properties of new
synthesized 3-armed poly(methyl methacrylate) films
Adnan KURT
Department of Chemistry, Faculty of Arts and Science, University of Adıyaman,
02040, Adıyaman-TURKEY
e-mail: adnkurt@gmail.com
Received 11.03.2008
Three-armed poly(methyl methacrylate)[PMMA] was synthesized by atom transfer radical polymeriza-
tion at 100 ◦ C using benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate as 3-armed initiator and a cuprous(I)
bromide/2,2ꢀ -bipyridine catalytic system. The characterization of PMMA was confirmed by FT-IR, 1 H-
NMR, GPC, DSC, and TGA techniques. Optical absorption measurements were also studied by UV-VIS
technique. Transmittance and reflectance measured in the wavelength range 300-700 nm were used to calcu-
late the optical constants. The changes in dispersion and Urbach parameters were investigated as a function
of AlCl3 content. The wide band tails increased in width and the optical energy gap decreased from 2.39
to 1.28 eV as the AlCl3 content increased from 0% to 15%. Analysis revealed that the type of transition is
the indirect allowed one.
Key Words: Optical constants, optical band gap, dispersion parameters, thin films, ATRP, PMMA.
Introduction
The progress of living radical polymerization for the synthesis of polymers with controlled architecture, molecular
weight, and molecular weight distribution is among the most important achievements in polymer chemistry in the
last 2 decades.1,2 One of the representative achievements obtained in the area of living radical polymerization
should be atom transfer radical polymerization.3,4 Atom transfer radical polymerization is a living radical
polymerization system that has been indicated to successfully polymerize a series of vinyl monomers such as
(meth)acrylate, (meth)acrylamide, and styrene as well as its derivative and acrylonitrile.5−8 ATRP has been
shown to be more versatile with respect to the novel polymer architectures such as graft copolymers, block
copolymers, and (hyper)branched polymers.9−11 It is also one of the most effective synthetic methods that can
be applied to prepare well-defined polymers with controlled chain topology, composition, microstructure, and
67

Influence of AlCl3 on the optical properties of..., A. KURT
functionality.2,12−14 The slow growth of polymer chains mediated by fast and reversible activation/deactivation
reactions, and the fast initiation process make the polymerization system proceed in a controlled/living form. 15
The stability of polymer thin films on solid substrates is of great technological importance in applications
ranging from protective coatings to paintings, semiconductors, and micro- and optoelectronic devices.16,17
Optical polymers have attracted considerable attention in recent years because of their important industrial
applications. PMMA is one of the earliest and best known polymers. PMMA was seen as a replacement for
glass in a variety of applications and is currently used extensively in glazing applications. The material is one
of the hardest polymers, and is rigid, glass-clear with glossy finish and good weather resistance. PMMA is
naturally transparent and colorless. The transmission for visible light is very high. Polymeric composites of
PMMA are known for their importance in technical applications.18 Studies of doping transition metal halides
into PMMA are important for determining and controlling the operational characteristic of the different PMMA
composites. The addition of transition metal halides to the PMMA network will cause a remarkable change in
their properties.19 In the study of physical properties of polymers, the optical absorption spectrum is one of the
most important tools for understanding band structure, electronic properties, and optical constants (refractive
and absorption indices) of pure and doped polymers.
The objective of this study was to synthesize and characterize the 3-armed poly(methyl methacrylate)
and to investigate the AlCl3 dependence of some optical properties of polymer films. For this purpose, the
Urbach parameters, the optical energy gap, and single-oscillator parameters were evaluated as a function of
AlCl3 concentration.
Experimental
Materials
Methyl methacrylate (MMA) was distilled under vacuum after washing with 5% NaOH aqueous solution just
before polymerization. Cuprous(I) bromide (CuBr), 2,2ꢀ -bipyridine, diphenyl ether, dichloromethane, methanol,
and trihydroxybenzene (analytical reagent) were used as received. The benzene-1,3,5-triyl tris(4-(bromomethyl)
benzoate used as initiator for ATRP was synthesized in the laboratory.
Instrumental techniques
UV-visible spectrums of the thin films were recorded by a T80 + UV/VIS Spectrometer (PG Instruments Ltd.)
in the wavelength range 300-700 nm at room temperature. Gel permeation chromatography (GPC) analyses
were carried out using a high pressure liquid chromatography pump with an Agilent 1100 system equipped with
a vacuum degasser and a refractive index detector. The eluting solvent was tetrahydrofuran (THF) and the
flow rate was 1 mL min−1 . Calibration was achieved with polystyrene. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) measurements were carried out using a Shimadzu TGA-50 and DSC-50
under nitrogen flow with heating rate of 10 and 20 ◦ C min−1 , respectively. Infrared spectra were recorded on a
Perkin Elmer Spectrum One and obtained by polymer film on a salt plate. 1 H-NMR spectra were recorded on
a Bruker AC-300 MHz Fourier Transform NMR spectrometer at room temperature using CDCl3 as a solvent
and TMS as an internal standard. The films of the 3-armed PMMA polymer were prepared by the solution
68

Influence of AlCl3 on the optical properties of..., A. KURT
casting method.20 In this method, the polymer is dissolved in a suitable solvent and poured onto a flat, carefully
cleaned substrate, and then the solvent evaporates from a solution of the polymer samples. For this purpose,
the dichloromethane solvent, (CH2 Cl2), pro analysis, from Merck, was used to produce polymer films. Before
being used as substrates, the glass disks, with a thickness of 1.27 mm, a length of 80.20 mm, and a width of 23.22
mm, were carefully washed with detergent and afterwards with ethyl alcohol vapors and dried in a vacuum oven
at room temperature. Several drops of the polymer solution (the concentration, PMMA/dichloromethane = 50
mg/mL) were poured onto the substrate. To avoid thermal gradients and minimize the effect of the evaporation
of the solvent immediately after deposition, both the polymer solution and the substrates were kept at the
same temperature and an evaporation barrier was placed on the substrates. The polymer films were placed in a
vacuum oven at room temperature, for 5 h. Afterwards, the temperature of the oven was gradually raised to a
value slightly under the glass transition temperature of the PMMA, 100 ◦ C, in order to remove completely any
residual solvent. The films were kept at this temperature for 1 day. After being cooled down slowly, the films
were stored at room temperature. The thicknesses of the films were adjusted to 200 nm for pure and doped
polymers. The optical parameters of pure and doped films were calculated according to the methods given in
the literature.21−26
Br
Ha
Hd
CH2
OH
Hc
O
Br
0 - 5 °C
+
OH
Cl
Hb
THF, TEA
O
O
O
HO
Br
O
O
Br
O
Br
O
MMA / CuBr / bpy
110 °C
O
O
O
Br
O
H3C
C
O
CH2
Br
n
C
O
O
CH3
Scheme. Synthesis of 3-armed PMMA.
69

Influence of AlCl3 on the optical properties of..., A. KURT
Synthesis of benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate
The synthesis and characterization of benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate were as described in
the literature.27 This synthesis was accomplished with the reaction of 2-bromobenzoyl chloride (0.3 mol) with
trihydroxybenzene (0.1 mol) between 0 and 5 ◦ C using triethylamine (0.31 mol) and tetrahydrofuran (100 mL),
and then it was crystallized from methanol.
Atom transfer radical polymerization of MMA
A typical procedure for the ATRP synthesis of MMA is as follows: deoxygenated CuBr (0.01 g), 2,2ꢀ -bipyridine
(0.022 g), and tri-drops diphenyl ether were added to a dried glass tube to produce ATRP complex. Benzene-
1,3,5-triyl tris(4-(bromomethyl) benzoate (0.0165 g) as 3-armed initiator and then MMA (0.7 g) were added to
this deep brown mixture under argon atmosphere. The glass tube was then closed with a rubber septum and
immersed in a preheated oil bath at 100 ◦ C. The polymerization was continued for 4 h. The polymerization
mixture was then diluted with dichloromethane, filtered, and poured into excess ethyl alcohol with 1% aqueous
hydrochloric acid. The polymer so purified was isolated and dried in a vacuum oven at 45 ◦ C for 24 h. The
polymer yield was calculated gravimetrically.
Results and discussion
Atom transfer radical polymerization of 3-armed PMMA was carried out using an initiator with tri-active C-Br
end group function and cuprous(I) bromide/2,2ꢀ -bipyridine catalytic system in the molar ratio of 1:3:6:300 for
3 armed initiator: Cu(I)Br:bpy:MMA, respectively.28−30 The schematic representation of polymer synthesis
is illustrated in Scheme 1. Figure 1a shows the FT-IR spectra of benzene-1,3,5-triyl tris(4-(bromomethyl)
benzoate. The characteristic bands are assigned to C-H stretching of the aromatic ring at 3126-3038 cm−1
,
aliphatic C-H stretching at 2967-2868 cm−1 , C=O stretching vibration at 1739 cm−1 , and aromatic ring C=C
stretching vibration at 1610 cm−1
.
(a)
(b)
1775
1607
4000
3200
2400
1800
cm–1
1400
1000
600
Figure 1. FT-IR spectra of (a) benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate and (b) 3-armed PMMA.
The 1 H-NMR spectra of benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate are illustrated in Figure 2a.
The signals at 8.2 ppm (proton integrals = 4.56) and 7.5 ppm (proton integrals = 4.56) are attributed to aromatic
70

Influence of AlCl3 on the optical properties of..., A. KURT
ring protons next to ester carbonyl (Hc) and next to bromine methyl (Hd), respectively. The aromatic ring
protons in the core (Hb) are shown at 7.2 ppm (proton integrals = 2.29). The signal attributed to bromine
methyl protons is also shown at 4.6 ppm (proton integrals = 4.58). Figure 1b shows the FT-IR spectra for
3-armed PMMA. The bands at 3054-3005 cm−1 and 2925-2846 cm−1 are assigned to aromatic (from initiator
units) and aliphatic C-H stretching, respectively. The small peaks at 1775 cm−1 and 1607 cm−1 are also
attributed to ester carbonyl and aromatic ring C=C stretching reasoned from 3-armed initiator. The strong
peak at 1730 cm−1 is characteristic for ester carbonyl in MMA units. Figure 2b shows the 1 H-NMR spectra
of PMMA in which the signals at 1.95-1.2 ppm are assigned to the CH3 and CH2 protons in the main chain.
The signal at 3.67 ppm is characteristic for CH3 protons next to ester carbonyl in MMA units. The very small
signals seen at ≈7-8 ppm are attributed to aromatic ring protons reasoned from the 3-armed initiator unit on
the head of polymer chains.
Figure 2. 1 H-NMR spectra of (a) benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate and (b) 3-armed PMMA.
The theoretical number-average molecular weight was calculated according to the relation Mn,theo
=
([M]0 /[I]0)× conversion × MWmon + MWinit , where [M]0 and [I]0 are initial concentrations of monomer and
initiator, while MWmon and MWinit are molecular weights of monomer and initiator, respectively. Theoretical
number-average molecular weight for purified 3-armed PMMA at 82% monomer conversion was calculated as
Mn,theo = 25,300 using the above relation. The molecular weight of polymer was also determined according to
1 H-NMR. The signals of the chemical shifts at 7-8 ppm (sum of aromatic proton integrals = 1.12) correspond to
the aromatic ring protons from the 3-armed initiator unit on the head of polymer chains and the chemical shifts
at 3.67 ppm (proton integrals = 46.8) correspond to the CH3 protons next to ester carbonyl in MMA units
and can be used as the internal standard to calculate the molecular weight. The number of repeating monomer
units was estimated from the ratio of integral peak areas of CH3 protons to that of the aromatic ring protons.
Thus, molecular weight determined by 1 H-NMR technique was calculated as Mn,NMR = 21,250. When the
number-average molecular weight of the purified polymer was measured on a gel permeation chromatography
(GPC) system, this value was determined as Mn,GPC = 14,250 and molecular weight distribution was 2.1.
While the Mn measured with the GPC system is close to that of the NMR system, this value does not agree
with the theoretical prediction of Mn,theo = 25,300. For a controlled/“living” polymerization the observed
71

Influence of AlCl3 on the optical properties of..., A. KURT
molecular weight should coincide with the theoretical relation. A discrepancy between observed and theoretical
molecular weights indicates a transfer reaction to some low molecular weight species. 31 Here, polydispersity was
partially observed at a high value due to the variable quantity of initiating sites per chain. The high value in
the molecular weight distribution for the MMA polymerization may have originated from slow ATRP initiation
of MMA from the primary alkyl halide sites in the 3-armed initiator. It can also be caused by deactivation of
chain ends and termination reactions that are more evident at higher monomer conversion.32
The glass transition temperature value (Figure 3) was 116 ◦ C at a heating rate of 20 ◦ C/min for pure
3-armed PMMA. The decomposition of pure 3-armed PMMA obtained from ambient temperature to 500 ◦ C at
a heating rate of 10 ◦ C/min under nitrogen flow occurs in a single step at about 291 ◦ C, and its thermogram
is shown in Figure 4. The residue at 450 ◦ C was 2%.
TGA
[%]
100.00
50.00
0.00
0.00
100.00
200.00
300.00
400.00
500.00
0.00
50.00
100.00
150.00
200.00
Temp[C]
Temp[C]
Figure 3. DSC curve of 3-armed PMMA at a heating
rate of 20 ◦ C/min.
Figure 4. TGA curve of 3-armed PMMA at a heating
rate of 10 ◦ C/min.
The optical absorption measurements were carried out in the UV/VIS region (300-700 nm) for films
of 3-armed PMMA and doped with different contents of AlCl3 . The absorption spectra at about 300 nm of
pure and doped PMMA films may be attributed to n-π * transition of the carbonyl group in the polymeric
macromolecule.21 The transmittance and reflectance of pure and doped PMMA polymer films recorded in the
applied wavelength range are shown in Figure 5. It is clear from this figure that transmittance spectra for all
films increased with increasing wavelength while reflectance decreased. Increasing the AlCl3 content of the
films decreases transmittance and increases reflectance for a lower wavelength range. This means that there is
some absorption in that wavelength range. For each composition typical spectral behavior of transmittance and
reflectance are given for pure and doped PMMA films.
The refractive index, n(λ), was determined from the absolute values of the transmittance and reflectance
of the investigated films using the following formula:22,33
ꢀ
ꢁ
ꢀ
ꢁ
1/2
1 + R
1 − R
4R
(1 − R)2
n =
+
− k2
(1)
where k is the extinction coefficient and R is the optical reflectance. The extinction coefficient can be obtained
from the relation where k = αλ/4π . Plots in Figure 6 represent the dispersion in the refractive index for pure
72

Influence of AlCl3 on the optical properties of..., A. KURT
and doped PMMA thin films in the investigated range of wavelengths. Inspection of Figure 6 indicates for all
compositions that the refractive index decreases with increasing wavelength as well as it increases as AlCl 3
content increases from 0% to 15%. The values of the refractive index at 300 nm and room temperature for 0%,
5%, 10%, and 15% AlCl3 content were 4.10; 4.56; 4.77; and 5.04, and at 700 nm they were 1.83; 2.14; 2.22, and
2.37, respectively. Among these contents, the magnitude of the refractive index of 15% AlCl3 content has the
highest value (n = 5.04 at 300 nm). A more rapid decrease in n values may be seen at the low wavelength range
300-450 nm. Towards a higher wavelength range, the decrease in the n values is slower and reaches a nearly
constant value.
100
90
80
70
60
50
50
40
30
20
10
0
T(0%)
T(5%)
T(10%)
T(15%)
R(0%)
R(5%)
R(10%)
R(15%)
300
350
400
450
500
550
600
650
700
Wavelength(nm)
Figure 5. The transmittance and reflectance spectra of films.
5.5
0%
4.5
3.5
2.5
1.5
5%
10%
15%
300
350
400
450
500
550
600
650
700
Wavelength(nm)
Figure 6. The refractive index dispersion curves of films.
The real εr and imaginary εi parts of the dielectric constant were obtained using the formula εr = n2
–
k2 and εi = 2nk. The variation in the real (εr) and imaginary (εi) parts of the dielectric constant for different
AlCl3 contents are shown in Figure 7 (a, b). The values of the real part are higher than those of the imaginary
73

Influence of AlCl3 on the optical properties of..., A. KURT
part. The values of real and imaginary parts of the dielectric constant at 2 eV for 0%, 5%, 10%, and 15% AlCl3
content were 3.34; 4.58; 4.94; and 5.60 and 0.084; 0.157; 0.178, and 0.220, respectively.
25
0%
(a)
20
15
10
5
5%
10%
15%
0
1
.
75
2
.
25
2
.
75
3
.
25
3
.
75
4.25
E(eV)
0%
(b)
0
0
0
.
.
.
6
4
2
0
5%
10%
15%
1
.
75
2
.
25
2
.
75
3
.
25
3
.
75
4.25
E(eV)
Figure 7. The spectra of real (a) and imaginary (b) parts of the dielectric constant of films.
The refractive index dispersion data were evaluated according to the single-effective-oscillator model34,35
using the following relation:
EoEd
Eo2 − (hν)2
n2(hν) = 1 +
(2)
The physical meaning of the single-oscillator energy Eo is that it simulates all the electronic excitation
involved and Ed is the dispersion energy related to the average strength of the optical transitions.23 In practice
the dispersion parameters Ed and Eo can be obtained according to Eq. (2) by a simple plot of (n2− 1)−1
versus (hν)2 as shown in Figure 8. The values of Ed and Eo can be directly determined from the slope and the
intercept on the vertical axis. The values obtained for the dispersion parameters Eo and Ed are listed in the
Table. The values of Eo and Ed for 0%, 5%, 10%, and 15% AlCl3 content were 4.24; 4.21; 4.03; and 4.20 eV,
and 7.42; 9.98; 10.78, and 11.29 eV, respectively. According to the single-oscillator model, the single oscillator
parameters Eo and Ed are related to the imaginary part of the complex dielectric constant; the moments of
74

Influence of AlCl3 on the optical properties of..., A. KURT
and M
moments34,36 can be derived from the following
−3
the imaginary part of the optical spectrum M
relations and are given in the Table:
−1
M3
M
−1
−3
−1
−3
Eo2 =
,
Ed2 =
(3)
M
M
For the definition of the dependence of the refractive index non the light wavelength (λ), the single-term
Sellmeier relation can be used:37
So.λ2o
1 − (λo/λ)2
n2(λ) − 1 =
(4)
where λo is the average oscillator position and So is the average oscillator strength. The parameters So and
λo in Eq. (4) can be obtained experimentally by plotting (n2 – 1)−1 against λ−2 . From Figure 9, the slope of
the resulting straight line gives 1/So , and the infinite-wavelength intercept gives 1/( Soλ2o). These values can
be seen in the Table.
0.4
0%
5%
0.3
10%
15%
0.2
0.1
0
7.5
8.5
9.5
10.5
11.5
12.5
(hν)2(eV)2
Figure 8. Variation in (n2 – 1)−1 as a function of (hυ)2 of films.
Table. Optical parameters of 3-armed PMMA thin films.
AlCl3
(%wt) Eo(eV) Ed(eV) So.1013(m−2
)
λo(nm)
M
M
−3(eV)−2 Et(eV)
σ
Eg(eV)
2.39
−1
0
5
4.24
4.21
4.03
4.20
7.42
9.98
2.04
2.73
2.82
3.07
292.79 1.75
294.46 2.37
295.63 2.67
307.77 2.69
0.097
0.134
0.165
0.152
1.252
1.407
1.485
1.597
0.0205
0.0183
0.0173
0.0161
2.02
10
15
10.78
11.29
1.52
1.28
75

Influence of AlCl3 on the optical properties of..., A. KURT
0.4
0%
5%
10%
15%
0.3
0.2
0.1
0
5.0E-06
5.5E-06
6.0E-06
6.5E-06
7.0E-06
7.5E-06
8.0E-06
λ-2(nm)-2
Figure 9. Variation in (n 2 – 1)−1 as a function of (λ)−2 of films.
In the high absorption region (where absorption is associated with interband transitions), the absorption
coefficient, α, can be calculated from observed absorbance data using Beer Lambert’s formula: α = 2.303A/d,
where A is the optical absorbance and d is the thickness of the film. It is usual to analyze the optical absorption
at the fundamental edge in terms of band-to-band transitions theory in polymers.38 In this treatment, the
absorption data follow a power law, which is given by39
(αhν)m = g(hν − Eg)
(5)
and yield values of the optical energy band (Eg). Here g is a constant and mis the power that characterizes
the transition process and it can take values such as 2, 1/2, 1/3, or 3/2. The investigation of the spectrum of
the absorption coefficient in the fundamental region and near the fundamental edge provides us with valuable
information about the energy band structure of the material.23 The experimental data were fitted to the
theoretical Eq. (5) for different values of m and the best fit was obtained for m = 1/2. This behavior indicated
that the transitions are allowed indirect transitions.
Plots of (αhν)1/2 versus the photon energy (hν) are shown in Figure 10 in order to achieve the band
gap for thin films. The values of the energy gap for each composition were calculated by extrapolating the
straight portion to zero absorption. The band gap of the PMMA films decreases with the increase in AlCl3
content from 0% to 15%. The variations in optical band gap with different AlCl3 contents are shown in the
Table. The E g values of films were 2.39; 2.02; 1.52; and 1.28 eV, respectively. This decrease in band gap may
be attributed to the presence of unstructured defects, which increase the density of localized states in the band
gap and consequently decrease the energy gap.40
The tail of the absorption edge is exponential, indicating the presence of localized states in the energy
band gap. The amount of tailing can be predicted to a first approximation by plotting the absorption edge data
in terms of an equation originally given by Urbach.41 The absorption edge of non-metallic materials gives a
measure of the energy band gap and the exponential dependence of the absorption coefficient, ln(α), on photon
energy, hν , is found to hold over several decades for a polymeric material and takes the following form:41
α = α0 exp(hν/Et)
(6)
76

Influence of AlCl3 on the optical properties of..., A. KURT
where α 0 is a constant and Et is interpreted as the width of the tails of localized states in the gap region. To
evaluate the values of α 0 and Et , it was α in logarithmic scale as a function of photon energy hν as shown in
Figure 11. The reciprocal of the slope of each line yields the magnitude of Et and its values for different AlCl3
content of films are listed in the Table. It is clear that AlCl3 dopant increases the width of the tail of localized
states and decreases the energy gap of PMMA thin films. Using Eq. (6) at a constant temperature graph
representing ln(α) on the y-axis and hν on the x-axis in the range of the Urbach tail would yield a straight
line with a slope equal to σ /kT, where σ , known as the steepness parameter, is a temperature-dependent
parameter characterizing the broadening of the absorption edge due to electron–phonon or exciton–phonon
interactions,26,42 k is the Boltzmann constant, and T is the temperature. If temperature is taken as 298 K, the
steepness parameters for 0%, 5%, 10%, and 15% AlCl3 content can be calculated as 0.0205, 0.0183, 0.0173, and
0.0161, respectively. These values can be seen in the Table.
330
0%
5%
10%
15%
280
230
180
130
80
1.5
2
2.5
3
3.5
4
E(eV)
Figure 10. Plots of (αhν)1/2 vs. E of films.
10
9.6
9.2
8.8
8.4
8
0%
5%
10%
15%
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
3.6
E(eV)
Figure 11. Plots of lnα vs. E of films.
77

Influence of AlCl3 on the optical properties of..., A. KURT
Conclusions
Three-armed poly(methyl methacrylate)[PMMA] was synthesized by atom transfer radical polymerization at 100
◦ C using benzene-1,3,5-triyl tris(4-(bromomethyl) benzoate as 3-armed initiator and a cuprous(I) bromide/2,2ꢀ -
bipyridine catalytic system. The characterization of PMMA was confirmed with FT-IR, 1 H-NMR, GPC, DSC,
and TGA techniques. The optical absorption measurements were carried out in the UV/VIS region (300-700 nm)
for films of 3-armed PMMA and doped with different contents of AlCl3 . It was determined that transmittance
spectra for all films increased with increasing wavelength while reflectance decreased. The refractive index
decreased with increasing wavelength and it increased as AlCl3 content increased from 0% to 15%. The value
of refractive index at 300 nm and room temperature for 0%, 5%, 10%, and 15% AlCl3 content were 4.10; 4.56;
4.77; and 5.04, and at 700 nm the values were 1.83; 2.14; 2.22, and 2.37, respectively. The values of the real part
were higher than those of the imaginary part. The single–oscillator parameters were determined. The change
in dispersion and Urbach parameters were investigated as a function of AlCl3 content. The wide band tails
increased in width and optical energy gap decreased from 2.39 to 1.28 eV as the AlCl3 content increased from
0% to 15%. Analysis revealed that the type of transition is the indirect allowed one.
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