the optical constants of poly methyl methacrylate pmma...
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ISSN (Print): 2328-3777, ISSN (Online): 2328-3785, ISSN (CD-ROM): 2328-3793
American International Journal of Research in Formal, Applied & Natural Sciences
AIJRFANS 16-206; © 2016, AIJRFANS All Rights Reserved Page 13
AIJRFANS is a refereed, indexed, peer-reviewed, multidisciplinary and open access journal published by International Association of Scientific Innovation and Research (IASIR), USA
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The Optical Constants of Poly Methyl Methacrylate PMMA Polymer
Doped by Alizarin Red Dye Fadhil A. Tuma
Department of Physics,
College of Education for Pure Sciences,
University of Basrah, Basrah, Iraq
I. Introduction In recent years, polymeric materials has gained widespread attention by the scientific and technological
researchers, because of their important industrial applications. Such materials play today a very great role in
numerous fields of everyday life due to their unique advantages over conventional materials (e.g. wood, clay
and metals) like lightness, resistance to corrosion, ease of processing and low cost production. Furthermore, the
physical properties of polymers may be affected by doping using various materials as an additives. Such
additives are used in polymers for a variety of reasons, for example: improved processing, density control,
optical effects, thermal conductivity, control of the thermal expansion, electrical properties that enable charge
dissipation or electro-magnetic interference shielding, magnetic properties, flame resistance and improved
mechanical properties, such as hardness, elasticity and tear resistance [1-4]. Currently, the doped polymers have
been the subject of interest for both theoretical and experimental studies, because of the physical and chemical
properties needed for specific application may be obtained by adding or doping with some dopant [5]. Detailed
studies of doped polymer with different dopant concentrations allow the possibility of choice of the desired
properties [6]. General poly (methacrylates) are polymers of the esters of methacrylic acid. The most commonly
used among them is poly (methyl methacrylate) PMMA. Poly (methyl methacrylate) PMMA is one of the
earliest and best known polymers, with chemical formula (C5H8O2)n [7]. It is naturally transparent and colorless
polymer with density (1.15-1.19 g/cm3), and available on the market in both pellet (granules) and sheet form
under the names Plexiglas, Acrylite, Perspex, Plazcryl, Acryplast, Altuglas, Lucite etc. It is commonly called
acrylic glass or simply acrylic [8]. Table.1 shows some of optical characteristics of PMMA. Table.1 optical characteristics of PMMA
Optical property Value
Haze 1-96%
Transmission, Visible 80-93%
Refractive Index 1.49-1.489
PMMA is produced by free-radical polymerization of methyl methacrylate in mass (when it is in sheet form) or
suspension polymerization according to the following chart [9] Fig.(1).
Fig.(1) Preparation of the polymer PMMA
Abstract: In this work, the optical constants: refractive index (n), extinction coefficient (k), real and
imaginary parts of dielectric constants (εr ,εi) and the optical energy gap (Eg) of poly methyl methacrylate
PMMA doped by Alizarin red AR dye with thickness in the range ((40-60)µm have been investigated in the
wavelength range (190-900)nm. The AR dye was added by different concentrations (1,2 and 3) wt.%. The
study of the optical properties of the doped films have done in order to identify the possible change that
happen to the PMMA films due to doping. The results show that the optical constants change with
increasing dye doping concentration, and it has been found that optical energy gap (Eg) decreases with the
increasing of dye concentration.
Keywords: PMMA polymer, Alizarin red AR dye, Optical constants, Optical energy gap.
Fadhil A. Tuma, American International Journal of Research in Formal, Applied & Natural Sciences, 15(1), June-August, 2016, pp. 13-18
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Poly (methyl methacrylate) PMMA have been widely used due to its remarkable properties, such as, excellent
mechanical properties where it is one of the hardest thermoplastic and is also highly scratch resistant, very
suitable for electrical engineering purposes and it is dielectric properties is very good, thermal capability where
it is resistance to temperature changes is very good, exhibits unique optical properties, simple synthesis and low
cost, high transparency in the visible region, possible to use in nonlinear optics [10-14].
As a result of the above characteristics, PMMA has been extensively used in various industrial fields, like,
optical devices such as optical lenses and optical fiber [15], photonic of nanotechnology [16], as an optical
diffuser in a liquid crystal display backlighting unit (BLU) [17], humidity sensing after surface modification of
PMMA by argon-oxygen plasma processing [18], as a gas sensors [19], in memory materials [20].
Many studies have been carried out using different materials as an additive in PMMA such as methylene blue,
methyl orange and methyl red [21-23] as a pigments (dyes), and many of other studies which using metal
chloride such as MgCl2 and CrCl2 [5,6], or Kaolinte [12], and Erbium and Erbium/Ytterbium [13]. All the
above studies reveal that the optical constants change with increasing dopant concentrations.
In this study an attempt was introduced to obtain the effect of Alizarin red AR dye as additive on the optical
constants of PMMA.
Alizarin is an organic compound is known since ancient times an important red pigment. Anthraquinone, a
compound derived from the roots of the madder plant. It is the first natural pigment produced industrially in
1869 [24]. Alizarin red (AR) is a solid appearing reddish-orange as crystals and brownish-yellow as powder,
with chemical formula (C14H8O4), and used chiefly in the synthesis of other dyes [25]. Fig.(2) illustrates in (a)
Sample of AR and (b) Skeletal formula of AR.
(a)
(b)
Fig.(2) (a) Sample of AR. (b) Skeletal formula of AR
It must be noted that alizarin name is derived from the Arabic word al-usara [26]. Table. 2 shows some
properties of AR [25].
Table.2 The main characteristics of AR dye Quantity Value
Molecular Weight 240.21 g/mol.
Exact Mass 239.93 g/mol.
Density 1.540 g/cm3
Purity 98.97
II. Materials and Methods
Materials:
The materials used in this work are Poly (methyl methacrylate) PMMA, Alizarin red dye AR and
Tetrahydrofuran THF, which is an organic compound with the chemical formula C4H8O, and is a versatile
solvent [27].Table. 3 shows some properties of the solvent THF.
Table. 3 Some properties of the solvent THF Quantity Value
Molar Mass 72.11 g/mol.
Density 0.89 g/cm3 at 20oC
Appearance Colorless Liquid
Purity 99.96 %
Films Preparation: Poly (methyl methacrylate) PMMA films doped with AR dye were prepared using solution casting technique.
Poly (methyl methacrylate) solution was prepared by dissolving PMMA granules in glass beaker (30ml) by
Tetrahydrofuran THF solvent using magnetic stirrer in mixing process for more than an hour until the polymer
granules became completely soluble, and AR dye was dissolved easily in another glass beaker by water.
The chosen weight percentages of AR dye relative to pure PMMA (1, 2 and 3) wt.% were added and mixed for
(30min) to get homogeneous solution. The mixture was poured into clean flat glass dishes placed on a platform
which can be leveled with the aid of spirit level fixed on the platform, and then kept inside dry-oven under
temperature (40oC ) for at least (24hr) to ensure removal of solvent traces, then left to cool slowly to room
temperature (25oC ). Several similar films have been prepared in order to ensure dried samples without bubbles
and thermal damage. The thickness of the prepared films were measured with the help of digital micrometer and
was found to be in the range (40-60)µm, these films were clear, without any remarkable defects and showing
light brownish color.
Fadhil A. Tuma, American International Journal of Research in Formal, Applied & Natural Sciences, 15(1), June-August, 2016, pp. 13-18
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Optical Measurements:
The absorbance and transmittance spectra were recorded utilizing double beam UV/VIS-7200 (England)
spectrophotometer in wavelength range (190-900) nm, all the measurements were carried out at room
temperature.
The optical constants are very important because they characterize the optical behavior of the materials. The
spectroscopic behavior of materials is utilized to determine their optical constants refractive index (n), extinction
coefficient (k), real and imaginary parts of dielectric constants (εr,εi). Such constants could be determine by
using the following procedure which include measurements of absorbance (A), reflectance (R) and transmittance
(T). The relationship between absorbed light intensity (I) by material and the intensity of incident light (Io) is given
by [28]
(1)
Where (t) is the thickness of the matter and (α) is the absorption coefficient.
(2)
Where the amount of log( I/Io) represents the absorbance (A).
The absorption coefficient (α) is defined as the ability of material to absorb light of the given wavelength can
be calculated by [29]
(3)
The relation between the optical energy gap (Eg), absorption coefficient (α) and the photon energy (h𝜐) can be
written as [28,29]
(4)
Where (B) is a constant related to the properties of the valance band and conduction band, the exponent (r) is an
empirical index which determines the type of optical transition and can be assumed to have values of (1/2, 3/2, 2
and 3) depending on the nature of the electronic transition responsible for the absorption. r = 1/2 and 3/2 for
direct allowed and forbidden transition respectively, r = 2 and 3 for indirect allowed and forbidden transition
respectively [28,30].
The refractive index (n) of a material is the ratio of the velocity of light in vacuum to that of the specimen, and
can be measured by using the following equation [1,29]
(5)
(6)
The extinction coefficient (k) can be defined as the amount of energy losing as a result of interaction between
the light and the charge of medium, and can be calculated by the following relation [31]
(7)
Where (λ) is the wavelength. The extinction coefficient (k) is directly proportional to the absorption coefficient
as seen from Eq.(7).
The complex dielectric constant (ε*= εr + i εi) characterizes the optical properties of the solid material, where (εr)
real part and (εi) imaginary part of the dielectric constant, and can be calculated by the following relations [31]
(8)
(9)
The real (εr) and imaginary (εi) parts of the dielectric constant are related to (n) and (k) values as seen from Eq.
(8) and Eq. (9). The calculation of these two parts supply information about the loss factor.
III. Results and Discussion
Optical characterization: The optical measurements of absorbance for doped PMMA films by AR dye in different concentrations
deposited on glass substrate have been studied through recording the absorption spectrum in the range (190-
900)nm as shown in Fig.(3). It can be observed that all the doped PMMA films showed two absorption bands at
the same position nearly, the first one in the UV region, and the other in the visible region, which are related to
energy absorption [32].
Fadhil A. Tuma, American International Journal of Research in Formal, Applied & Natural Sciences, 15(1), June-August, 2016, pp. 13-18
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0
0.1
0.2
0.3
0.4
0.5
0.6
0 200 400 600 800 1000
PMMA+3% AR
PMMA+1% AR
PMMA+2% AR
Ab
sorb
ance
(nm) Fig.(3) Absorption spectrum of PMMA doping with AR at different concentrations
The absorption increase with increasing concentration of AR dye. The best absorption was for the doped PMMA
film by 3% weight percentage of dye, which is attributed to the localized state of doping material in the energy
gap. The localized state causes decrease in energy gap, then increasing in absorption value [33].
Refractive index and extinction coefficient:
Fig.(4) shows the variation of refractive index (n) for doped PMMA films with different concentrations of AR
dye with the photon energy, the values of (n) are increased with increasing each of the photon energy and dye
concentration, this due to increase the density of doped PMMA films [22].
0
0.5
1
1.5
2
2.5
3
0 1 2 3 4 5
PMMA+3%AR
PMMA+2%AR
PMMA+1%AR
(eV)
Fig.(4) Variation in refractive index (n) as a function of photon energy of doped PMMA with AR dye
Fig.(5) shows the variation of the extinction coefficient (k) with the photon energy. It is noticed that (k) values
has reduced, this behavior has systematic with photon energy, where decrease with increasing photon
energy[21].
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0 1 2 3 4 5
PMMA+3%AR
PMMA+2%AR
PMMA+1%AR
(eV)
Fig.(5) Variation in extinction coefficient (k) as a function of photon energy of doped PMMA with AR dye
Determination of the dielectric constant:
The variation of the real (εr) and imaginary (εi) parts of the dielectric constant for doped PMMA films by AR
dye with photon energy are shown in Fig.(6) and Fig.(7) respectively. They almost indicate the same pattern
curves but the values of real part are higher than those of the imaginary part. On the other hand the increase of
the real and imaginary parts of the dielectric constant with increasing the weight percentages of AR dye
attributed to increase the density of doped PMMA films and number of carries charges [22].
Fadhil A. Tuma, American International Journal of Research in Formal, Applied & Natural Sciences, 15(1), June-August, 2016, pp. 13-18
AIJRFANS 16-206; © 2016, AIJRFANS All Rights Reserved Page 17
0
1
2
3
4
5
6
7
8
9
0 1 2 3 4 5
PMMA+3%ARPMMA+2%ARPMMA+1%AR
(eV)
Fig.(6) Variation of real part (εr) of dielectric constant for doped PMMA by AR dye with photon energy
0
0.001
0.002
0.003
0.004
0.005
0.006
0 1 2 3 4 5
PMMA+3%ARPMMA+2%ARPMMA+1%AR
(eV)
Fig.(7) Variation of imaginary part (εi) of dielectric constant for doped PMMA by AR dye with photon energy
It is conclude that the variation of(εr) mainly depends on (n2) because of small values of (k2), while (εi) mainly
depends on (k) values which are related to the variation of absorption coefficients.
The real part of the dielectric constant is associated with the term that shows how much it will slow down the
speed of light in the material and the imaginary part shows how a dielectric absorbs energy from an electric field
due to dipole motion [34].
Optical energy gap:
The absorption coefficient helps to know the nature of electronic transition, when the values of the above
coefficient is high (α > 104 cm-1) at high energies, it is expected that direct transition of electron occur, the
energy and moment are preserved by the electrons and photons. On the other hand when the values of the
absorption coefficient is low (α < 104 cm-1) at low energies, it is expected that indirect transition of electron
occur, and the momentum of electron and photon are preserved with the assistance of a phonon [35]. The
findings indicated that the values of the absorption coefficient is less than (104 cm-1) , that is clear how the
electronic transition is indirect.
The optical energy gap (Eg) for the doped PMMA films with AR dye can be estimated by plotting (αh𝜐)1/2
versus photon energy (h𝜐), then extrapolating the straight line part of the curve to a point ((αh𝜐)1/2 = 0) on the
photon energy axis. Fig.(8) shows the variation of the optical energy gap for doped PMMA films.
0
10
20
30
40
50
0 1 2 3 4 5
PMMA+3%ARPMMA+2%ARPMMA+1%AR
(eV)
( ℎ
)1/2
(c
m-1
eV)1
/2
Fig.(8) The optical energy gap (Eg) for the doped PMMA films with AR dye
Fadhil A. Tuma, American International Journal of Research in Formal, Applied & Natural Sciences, 15(1), June-August, 2016, pp. 13-18
AIJRFANS 16-206; © 2016, AIJRFANS All Rights Reserved Page 18
The calculation of (Eg) of doped PMMA films showed which decreased with increasing of AR concentration,
this is ascribed to increase in absorption coefficient as a result of introducing dopant atoms and hence (Eg) will
be decreasing [21].The values of (Eg) for doped PMMA with AR dye are listed in Table.4.
Table.4 The values of (Eg) for doped PMMA Sample Eg (eV)
PMMA + 1% AR 1.6
PMMA + 2% AR 1.4
PMMA + 3% AR 0.8
IV. Conclusions
1. The absorbance of prepared films of doped PMMA with AR dye is increased with increasing the AR
concentration.
2. The optical constants: refractive index, extinction coefficient, real and imaginary parts of dielectric constants
are changed with increasing weight percentages of AR dye.
3. The doped PMMA with AR dye have indirect energy band gap (optical energy gap) which decrease with
increasing the AR concentration.
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