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C60 down-shifting nanocomposite films
Accepted Manuscript AC/DC electrical conduction and dielectric properties of PMMA/PVAc/C60 downshifting nanocomposite films S.M. El-Bashir, N.M. Alwadai, N. AlZayed PII:
S0022-2860(17)31222-X
DOI:
10.1016/j.molstruc.2017.09.043
Reference:
MOLSTR 24292
To appear in:
Journal of Molecular Structure
Received Date: 1 June 2017 Revised Date:
14 September 2017
Accepted Date: 15 September 2017
Please cite this article as: S.M. El-Bashir, N.M. Alwadai, N. AlZayed, AC/DC electrical conduction and dielectric properties of PMMA/PVAc/C60 down-shifting nanocomposite films, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.09.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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GRAPHICAL ABSTRACT
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TEM image of PMMA/PVAc/C60 nanocomposite film doped with 200 ppm fullerene C60.
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AC/DC Electrical Conduction and Dielectric Properties of
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PMMA/PVAc/C60 Down-Shifting Nanocomposite Films
3 S.M. El-Bashir 1,2 *, N.M. Alwadai 3, N. AlZayed 1
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Department of Physics & Astronomy, Science College, King Saud University,
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Riyadh, Saudi Arabia.
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Department of Physics, Faculty of Science, Benha University, Benha, Egypt.
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Physics Department, Science College, Princess Nora Bint Abdul Rahman University,
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Riyadh, KSA.
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Abstract
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Polymer nanocomposite films were prepared by doping fullerene C60 in polymer
blend
composed
of
polymethacrylate/polyvinyl
acetate
blends
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(PMMA/PVAc) using solution cast technique. The films were characterized by
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differential scanning calorimeter (DSC), Transmission electron microscope (TEM),
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DC/AC electrical conductivity and dielectric measurements in the frequency range
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(100 Hz- 1 MHz). The glass transition temperature, Tg, was increased by increasing
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the concentration of fullerene C60; this property reflects the increase of thermal
20
stability by increasing the nanofiller content. The DC and AC electrical conductivities
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were enhanced by increasing C60 concentration due to the electron hopping or
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tunneling between filled and empty localized states above Tg. The relaxation time was
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determined from the αβ -
24
relaxations and found to be attenuated by increasing the temperature as a typical
25
behavior of amorphous polymers. The calculated values of thermodynamic
26
parameters revealed the increase of molecular stability by increasing the doping
27
concentration;
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nanocomposite films in a wide scale of solar energy conversion applications such as
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luminescent down-shifting (LDS) coatings for photovoltaic cells.
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Keywords: PMMA/PVAc/C60 nanocomposites, AC/DC fields, luminescent down-
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shifting coatings.
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this
feature
supports
the
application
of
PMMA/PVAc/C60
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*
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Astronomy, Science College, King Saud University, Riyadh, Saudi Arabia.
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Tel.: 966 565850487; fax: 96614673656.
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E-mail addresses: [email protected]
Corresponding author: S.M. El-Bashir, Associate Prof. at Department of Physics &
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37 1. Introduction
Polymer nanocomposites are applied for various areas of our life as they have
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a stimulating function for the development of future applications [1]. Many polymers
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are considered as suitable matrices for the development of nanocomposite structure
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due to their ease of preparation, good adhesion with reinforcing fillers, weathering
43
resistance, low weight, and excellent mechanical strength [2-5]. The most popular and
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economical method of the modification of polymer properties is by blending two or
45
more kind having different properties. This method is attractive for producing new
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polymeric materials with tailored properties for totally new materials with different
47
features [6]. It has been reported that polymethylmethacrylate (PMMA) and polyvinyl
48
acetate (PVAc) are unique pairs of polymers as their blends and nanocomposites have
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a wide range of applications due to their thermal, rheological, electrical, mechanical
50
and optical properties [7-9]. Among different types of nanofillers, fullerenes have
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been widely studied due to their unique structure, electrical and optical properties,
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which make it unique material for miscellaneous applications such as biological,
53
chemical, electronic, optical and solar energy conversion [10-16]. The incorporation
54
of fullerene C60 into the polymer structure gives various nanocomposite materials,
55
films, and fibers which are suitable for various purposes [17-19]. In a previous work,
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we have studied the structure and photophysical properties of PMMA/PVAc/C60
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nanocomposite films as luminescent down-shifting coatings (LDS) for PV cells[20].
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These nanocomposite films can efficiently overcome the short-wavelength limit of
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solar cells by absorbing of incident UV-photons and re-emitting them at longer
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wavelengths that match the spectral response of the photovoltaic (PV) cell under
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consideration [21-24]. The use of LDS to improve the performance of PV cells has
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been first reported by Hovel et al.[25] since then LDS materials have been
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investigated using theoretical and
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materials considered for LDS coatings in PV cells are polymers such as
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polymethylmethacrylate (PMMA) doped with luminescent molecules [22]. Over the
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last few years, significant efforts were made to obtain an appropriate LDS material to
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split a high energy photon into two lower energy photons using different designs[26].
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One of the most famous designs for LDS is the luminescent solar concentrators
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(LSCs)[27]. Here, a highly transparent polymer plate doped by luminescent materials;
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the luminescence is isotropically emitted and guided to PV cells via total internal
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reflection (TIR)[28]. LSCs present many advantages such as that heat generated can
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be well dissipated into the concentrator plate area, direct and diuse light is collected
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to improve the performance of the PV device[29-31]. The present work concerns on
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studying the influence C60 concentration on thermal, electrical and dielectric
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properties of PMMA/PVAc/C60 LSC downshifting nanocomposite films prepared by
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low-cost solution casting technique. The importance of this study is that the physical
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properties of polymers are strongly affected by the presence of molecular relaxation
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mechanisms which are directly related the polymer structure, composition,
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thermodynamic parameters and weathering stability for outdoor applications such as
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LDS coatings for PV cells.
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experimental methods [24]. Conventional
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2. Experimental Techniques
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2.1. Materials. polymethylmethacrylate (PMMA) and polyvinyl acetate (PVAc) were
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obtained from Sigma-Aldrich (USA) and were reported to have molecular weights of
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996,000 and 167,000 g.mol−1, respectively. Buckminsterfullerene C60 powder has a
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purity of 99.5 % was obtained from Sigma-Aldrich (USA). Chloroform (CHCl3) was
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used as the casting solvent with purity of 99.8% (HPLC grade
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2.2. Preparation of PMMA/PVAc/C60 nanocomposite films.
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PMMA/PVAc/C60 nanocomposite films were prepared by using solution-cast
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technique [32]. Firstly, polymer solutions were prepared by dissolving PMMA and
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PVAc separately in chloroform with concentration 5g/100 mL and stirred after mixing
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with the ratio (50/50) for 48 h at 40 oC. After that, C60 was doped in the solution of the
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mixed homopolymers with different concentrations 50, 100 and 200 ppm and then
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cast onto glass Petri dishes until being dried at room temperature. After drying, the
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films were carefully removed and heated at 100 oC for 6 h to evaporate the confined
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solvent molecules. The thickness of the prepared nanocomposite films was in the
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range of 60 ± 10 µm as measured optically used air wedge experiment [33].
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2.3. Measurements and devices
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Differential scanning calorimetry was performed by a calibrated Setaram
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DSC 131 to record thermograms in the presence of high purity nitrogen as an inert
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atmosphere at a heating rate 293 K0/min. over a temperature range (298- 473 K). The
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glass transition temperature Tg for PMMA/PVAc/C60 nanocomposite films was
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determined as described in the literature[34-36].
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The DC electrical resistance R for the investigated sample was measured using a
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KEITHLEY 616 electrometer in a shielded electric furnace. The temperature was
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measured using thermocouple Chromel Alumel (USA). AC electrical conductivity and dielectric properties (dielectric constant, dielectric
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loss and the dielectric loss tangent) have been carried out in the frequency range
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(100 Hz -1MHz), using Wayne Kerr-6440B RLC Impedance Meter (USA), with
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accuracy (± 0.005%) in the temperature range (298–383 K). The samples used were
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cut in the form of rectangular discs of area 1.6 cm2 and thickness about 0.3 mm. The
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desired area used in the AC measurements was coated by silver conducting paint
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which showed ohmic contact with the samples. The dielectric constant of the
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sample is calculated using the following relation [37, 38],
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= .
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(1)
where C is the capacitance of the sample, is the free space permittivity, t and A are
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the thickness and area of the sample. In addition, the dielectric loss was calculated
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from the following relation [37, 38],
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= tan
where = (90 − ) and is the phase angle.
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3. Results & Discussion
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3.1. Thermal Analysis of PMMA/PVAc/C60 nanocomposite films.
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(2)
Differential scanning calorimetry (DSC) is one of the most convenient
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methods to determine the homogeneity and stability of PMMA/PVAc/C60
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nanocomposite films. Fig.1. illustrates the effect of fullerene C60 doping on the value
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of Tg for PMMA/PVAc/ C60 nanocomposite films; it is observed that the value of Tg
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is shifted to higher temperatures from 340 K for pure PMMA/PVAc blend to 350 K
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after doping with 200 ppm fullerene C60. Regarding Tg values listed in Table.1, it is Page 5 of 15
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noted Tg is increased by increasing fullerene C60 content; this behavior is ascribed to
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the decrease of the polymer free volume and the increase of chain rigidity by
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increasing nanofiller [39-41]. This behavior suggests the outstanding thermal
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resistance of PMMA/PVAc/ C60 nanocomposite films towards hot climates.
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3.2. DC electrical conductivity of PMMA/PVAc/C60 nanocomposite films.
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The electrical conductivity of PMMA/PVAc/C60 nanocomposite films have
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been studied in the temperature range (303-383 K), a representative plot is shown in
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Fig.2. The plot of direct-current (DC) electrical conductivity σdc versus 103/T
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represents a semilogarithmic plot which suggests an Arrhenius type thermally
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activated process according to [38, 42]:
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= exp (
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)
(3)
where T is the absolute temperature, kB is Boltzmann's constant, σo is a pre-
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exponential factor, and ∆Edc is the activation energy. It is clearly observed that the
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electrical conductivity is thermally activated up to a peak value occurred at a certain
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temperature Tmax depending on the Tg value of each sample [38, 43]. The activated
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part in σ-103/T relation can be attributed to the fact that as the temperature increases,
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the polymer chains acquire faster internal modes in which bond rotations produce
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segmental motions and the conductivity increases [38, 43]. Table1 shows the values
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of ∆Edc which have been evaluated from the slope of Arrhenius plot in the thermally
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activated part (T < Tmax) of σ-103/T relation. It is noted that the value of ∆Edc for pure
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PMMA/PVAc film (0.52 eV) is lower than that calculated in the literature for solvent
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cast PMMA (0.64 eV) [38]; this can be due to the increase of carrier concentration
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resulting from the addition of acetate group. In addition, it is noted that the values of
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∆Edc are decreased by increasing the concentration of C60, this can be ascribed to the
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increase of the interaction between C=O donor groups and fullerene C60 acceptor
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molecules [44, 45]. The conduction process usually occurs by electron hopping and/or
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tunneling between donor atoms and empty sites located in the energy band gap [46].
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The conduction mechanism will be confirmed in the next section by the study of
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alternating-current (AC) conductivity measurements. On the other hand, the
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attenuated part of the temperature dependence of σdc (T > Tmax) can be attributed to
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the large deformations characteristic of the viscoelastic state permitted by dipole
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segmental motions [38]. This lead to the decrease of the electron mobility due to the
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charge carrier scattering caused by electron- phonon coupling at high temperatures
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(T >Tg) [38].
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3.3. AC Conductivity of PMMA/PVAc/C60 nanocomposite films.
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Fig.3. shows the frequency dependence of the total conductivity σtot(ω) for
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200 ppm fullerene doped PMMA/PVAc/C60 nanocomposite film at different
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temperatures as a representative plot for all the samples under investigation. The
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frequency dependence of σtot(ω) obeys the following relation [37, 38],
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( ) = + " = + # $
(4)
where ω is the angular frequency which equals to 2πf, σac is the AC electrical
170
conductivity, A is the independent frequency factor, and s is the frequency power
171
determining the conduction mechanism. According to Eq.(2), it is clear that σtot(ω) is
172
frequency dependent in the high-frequency range; this linear dependence of AC
173
electrical conductivity with frequency could account for the electron conduction via a
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hopping process between coordinating sites, local structural relaxations and segmental
175
motions of the polymer films [47]. In order to determine the conduction mechanism,
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the values of the exponent, s, were deduced for all samples at different temperatures
177
and plotted against temperature in Fig. 4. It is observed that the value of s is decreased
178
to a minimum value by increasing temperature T~Tg, this behavior is corresponding
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to the hopping of electrons according to the correlated barrier hopping model (CBH)
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[38, 48]. At temperatures, higher than T~Tg , it is noted that s , is increased by
181
increasing temperature referring to the change of the conduction mechanism as the
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electrons can transport between the localized states according to quantum-mechanical
183
tunneling model (QMT) [38, 48]. From this study, it can be confirmed that at higher
184
temperatures above the glass transition the increase of the electron mobility decreases
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the width of the potential energy barrier and causes electron tunneling between
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localized states [47, 49, 50].
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The temperature dependence of σtot(ω) for the investigated sample is shown in
188
Fig.5.; it is clear that σtot(ω) is thermally activated up to a maximum peak which is
189
shifted to higher temperatures by increasing frequency and then attenuated as the
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same behavior observed for the temperature dependence of σdc. The values of AC
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activation energy ∆Eac are obtained at different frequencies as previously calculated
192
from Arrhenius equation and listed in Table.2. It is noted that the values of σtot(ω) are
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higher than σdc and ∆Eac are lower than ∆Edc. This can be due to the fact that the
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increase in the applied field frequency enhanced the electron mobility and,
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subsequently, the conductivity value [38, 50]. In addition, the loss factor dominates at
196
higher frequencies and, therefore the conductivity increases up to comparatively
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higher values for higher frequency ranges [51]. The decrease in the total conductivity
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at higher temperatures may be due to the scattering of electrons which is the dominant
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behavior by increasing temperature [43]. For an accurate analysis of dielectric
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response data, consideration of just conductivity is not sufficient; more information
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can be obtained from other data presentations, as we will proceed to show.
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3.4. Dielectric Permittivity of PMMA/PVAc/C60 nanocomposite films. Fig.6. Illustrates the frequency dependence of the dielectric permittivity ε ׳for
204
200 ppm fullerene doped PMMA/PVAc/C60 nanocomposite films at different
205
temperatures. From the plots, it is observed that the dielectric permittivity is decreased
206
by increasing frequency and going to a constant value at higher frequencies as the
207
behavior observed in most of the polar polymers following Debye dispersion
208
relation[52], ׳
'
׳
(5)
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where, ׳is the dielectric permittivity at angular frequency ω , $ is the static
211
dielectric permittivity, % is the infinite dielectric permittivity and τ is the relaxation
212
time. According to Debye model, polar dielectric materials have high initial value of
213
dielectric permittivity ׳, but as the frequency of the field is raised this value begins to
214
drop. This could be due to the disability of the diploes to follow the field variations at
215
high frequencies and also due to electrode polarization effects [53]. At low
216
frequencies, the high values of dielectric constant can be due to the space charge
217
effects arising from the electrodes [42] .At high frequencies, the polarization due to
218
the charge accumulation decreases, leading to the decrease in the value of dielectric
219
permittivity [38, 54]. Fig.7. shows the temperature dependence of the dielectric
220
permittivity ׳for the investigated sample at different frequencies, it is clearly
221
observed that the trend of ( ׳.) is a typical behavior for a polar dielectric [38, 54].
222
The value of ׳increases up to a maximum value and then drops at a certain
223
temperature, this drop in ׳can be attributed to the intensified thermal oscillations of
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the polymer which disturb the orderliness of dipole orientations. Moreover the sample
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exhibits a relatively low temperature gradient ∂ε'/∂T compared to literatures which
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׳
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refers to the complete vaporization of molecules of the casting solvent (chloroform)
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and reflects the quality and homogeneity of PMMA/PVAc/C60
228
films[38, 54].
229
3.5. Dielectric Loss of PMMA/PVAc/C60 nanocomposite films.
nanocomposite
Fig.8 shows the temperature dependence of ε ׳׳at different frequencies for the
231
investigated sample. It is observed that the position of ε׳׳max (T) shifts downward
232
right toward higher temperatures by increasing frequency due to the presence of
233
different polarization mechanisms; which is a typical behavior of a dielectric material
234
[38, 54, 55]. The explanation for the appearance of this peak loss in ε( ׳׳T) curves can
235
be ascribed to the fact that the β-relaxation is much broader than the α-relaxation
236
[38, 54]. This is due to the fact that the local barrier to rotation of the side groups is
237
different along the chain as a consequence of the irregularity of the glassy state [38,
238
54]. In the contour map described in the literature, α and β relaxations are only well
239
separated below 1 kHz, so ε׳׳max (T) can be ascribed to an αβ relaxation in which the
240
side groups cooperate with the limited backbone motions in a micro-Brownian motion
241
which have properties similar to a pure α-relaxation [38, 54, 55].
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3.6 Thermodynamic Parameters
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Eyring [38, 56]; the Gibbs free energy of activation, ∆G for dipole relaxation was
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calculated using relation [38, 56],
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∆0 = 1. 23 [
5
]
(6)
247
where R is the universal gas constant, T the temperature k is Boltzmann constant, τ is
248
relaxation time, and h is Plank´s constant. ∆G is directly dependent on the enthalpy,
249
∆H, and entropy ∆S by the following equation [38, 56], Page 10 of 15
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(7)
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The obtained relaxation time τ =1/ωmax showed an attenuation with increasing
252
temperature, obeyed an exponential relation of the form [38, 56],
253
:=
5
∆;
exp ( )
(8)
<
The thermodynamic parameters can be obtained by plotting ln(Tτ) versus 103/T that
255
give a linear behavior with slope ∆H/R [38, 56] as the value of τ is systematically
256
decreased with temperature in well agreement with the concept of molecular
257
relaxation in polymers [38]. This means that the dominated type of the polarization in
258
this sample is orientation- polarization since this type of polarization is very sensitive
259
to temperature. The thermodynamic parameters were evaluated at the temperature
260
below Tg of each sample by abovementioned equations for PMMA/PVAc/C60
261
nanocomposite films and listed in Table3. The molecular explanation of this
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temperature dependence is that the relative influence of the segmental molecular
263
interaction energy is decreased with increasing temperature. The decrease of the ∆G
264
by increasing temperature reflects the role of fullerene C60 doping for restricting the
265
rearrangement of polymer and enhancement in the molecular stability of organic host.
266
Moreover, the values of ∆S are increased when ∆G decreased by increasing
267
temperature; this can be ascribed to the increase of the disorder degree [38] which
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attributed to the increase of polymer free volume and chain mobility caused by the
269
anchored fullerene C60 molecules as confirmed by DSC measurements.
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4. Conclusions
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A low-cost solution cast technique was used to prepare PMMA/PVAc/C60
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nanocomposite LDS films; TEM studies designated the homogeneity and good
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distribution of C60 nanoparticles in the polymeric matrix. DSC measurements showed
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the increase of the glass transition temperature, Tg, by increasing the concentration of
275
C60. This reflects the impact of nanofillers on the enhancement of the thermal stability
276
of the nanocomposite films by controlling the main chain relaxation processes which
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greatly affect the solidification of polymeric LDS PV coatings in hot countries like
278
KSA. The study of the DC and AC electrical conductivities showed the temperature
279
dependence of two conduction mechanisms which are directly related to Tg of each
280
sample. The first mechanism is correlated barrier hopping (CBH) below T~Tg, and the
281
second is quantum-mechanical tunneling model (QMT) at temperatures higher than
282
T~Tg. The study of the dielectric properties showed the typical behavior molecular
283
relaxation of amorphous polar polymers. Finally, the calculated values of the
284
thermodynamic parameters showed the increase of the free energy and the decrease of
285
the entropy by increasing the concentration of C60; this clarified the improvement of
286
the molecular stability of the prepared nanocomposites for optoelectronic and
287
applications solar energy conversions.
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Acknowledgements
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This research project was supported by a grant from “The Research Center of
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the Female Scientific and Medical Colleges,” Deanship of Scientific Research, King
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Saud University.
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References
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[5] S.M. El-Bashir, M.A. Binhussain, N.A. Al-Thumairi, N. AlZayed, Preparation and characterization of PMMA/stone waste nanocomposites for marmoreal artificial stone industry, Journal of Reinforced Plastics and Composites, 33 (2014) 350-357. [6] J.A. Manson, Polymer blends and composites, Springer Science & Business Media2012. [7] S.Q. Ding, L.Q. Zhang, S.W. Sun, J. Ou-Yang, B.G. Han, Nano-Engineered Strong, Durable and Multifunctional/Smart Concretes, Key Engineering Materials, Trans Tech Publ, 2017, pp. 1084-1088. [8] Y. Li, H. Shimizu, Compatibilization by homopolymer: significant improvements in the modulus and tensile strength of PPC/PMMA blends by the addition of a small amount of PVAc, ACS applied materials & interfaces, 1 (2009) 1650-1655. [9] O. Gritsenko, A. Nesterov, Segmental adsorption energy and phase behavior of filled polymer blends, European polymer journal, 27 (1991) 455-459. [10] A.W. Jensen, S.R. Wilson, D.I. Schuster, Biological applications of fullerenes, Bioorganic & medicinal chemistry, 4 (1996) 767-779. [11] R. Bakry, R.M. Vallant, M. Najam-ul-Haq, M. Rainer, Z. Szabo, C.W. Huck, G.K. Bonn, Medicinal applications of fullerenes, International journal of nanomedicine, 2 (2007) 639. [12] V. Georgakilas, J.A. Perman, J. Tucek, R. Zboril, Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures, Chemical reviews, 115 (2015) 4744-4822. [13] B. Yadav, R. Kumar, Structure, properties and applications of fullerenes, International Journal of Nanotechnology and Applications, 2 (2008) 15-24. [14] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of fullerenes and carbon nanotubes: their properties and applications, Academic press1996. [15] L. Chibante, A. Thess, J. Alford, M. Diener, R. Smalley, Solar generation of the fullerenes, Journal of Physical Chemistry;(United States), 97 (1993). [16] Y. Lin, J. Wang, Z.G. Zhang, H. Bai, Y. Li, D. Zhu, X. Zhan, An electron acceptor challenging fullerenes for efficient polymer solar cells, Advanced Materials, 27 (2015) 1170-1174. [17] A. Patil, G. Schriver, B. Carstensen, R. Lundberg, Fullerene functionalized polymers, Polymer Bulletin, 30 (1993) 187-190. [18] M. Prato, N. Martín, F. Giacalone, Fullerene polymers: synthesis, properties and applications, John Wiley & Sons2009. [19] F. Giacalone, Fullerene-Containing Polymers, Fullerenes2011, pp. 125-161. [20] R. Ahmed, S. El-Bashir, Structure and physical properties of polymer composite films doped with fullerene nanoparticles, International Journal of Photoenergy, 2011 (2010). [21] G. Griffini, F. Bella, F. Nisic, C. Dragonetti, D. Roberto, M. Levi, R. Bongiovanni, S. Turri, Multifunctional Luminescent Down‐Shifting Fluoropolymer Coatings: A Straightforward Strategy to Improve the UV‐Light Harvesting Ability and Long ‐ Term Outdoor Stability of Organic Dye ‐ Sensitized Solar Cells, Advanced Energy Materials, 5 (2015). [22] S. El-Bashir, I. Yahia, F. Al-Harbi, H. Elburaih, F. Al-Faifi, N. Aldosari, Improving photostability and efficiency of polymeric luminescent solar concentrators by PMMA/MgO nanohybrid coatings, International Journal of Green Energy, 14 (2017) 270-278. [23] S.-M. Lee, P. Dhar, H. Chen, A. Montenegro, L. Liaw, D. Kang, B. Gai, A.V. Benderskii, J. Yoon, Synergistically Enhanced Performance of Ultrathin
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Nanostructured Silicon Solar Cells Embedded in Plasmonically Assisted, Multispectral Luminescent Waveguides, ACS nano, DOI (2017). [24] M. de la Mora, O. Amelines-Sarria, B. Monroy, C. Hernández-Pérez, J. Lugo, Materials for downconversion in solar cells: Perspectives and challenges, Solar Energy Materials and Solar Cells, 165 (2017) 59-71. [25] H. Hovel, R. Hodgson, J. Woodall, The effect of fluorescent wavelength shifting on solar cell spectral response, Solar Energy Materials, 2 (1979) 19-29. [26] A. Solodovnyk, B. Lipovsek, D. Riedel, K. Forberich, E. Stern, M. Batenschuk, J. Krc, C.J. Brabec, Key parameters of efficient phosphor-filled luminescent downshifting layers for photovoltaics, Journal of Optics, DOI (2017). [27] B. McKenna, R.C. Evans, Towards Efficient Spectral Converters through Materials Design for Luminescent Solar Devices, Advanced Materials, DOI (2017). [28] M. Gajic, F. Lisi, N. Kirkwood, T.A. Smith, P. Mulvaney, G. Rosengarten, Circular luminescent solar concentrators, Solar Energy, 150 (2017) 30-37. [29] W. Weber, J. Lambe, Luminescent greenhouse collector for solar radiation, Applied Optics, 15 (1976) 2299. [30] S. El-Bashir, F. Al-Harbi, H. Elburaih, F. Al-Faifi, I. Yahia, Red photoluminescent PMMA nanohybrid films for modifying the spectral distribution of solar radiation inside greenhouses, Renewable Energy, 85 (2016) 928-938. [31] S. El-Bashir, F. Barakat, M. AlSalhi, Metal-enhanced fluorescence of mixed coumarin dyes by silver and gold nanoparticles: Towards plasmonic thin-film luminescent solar concentrator, Journal of Luminescence, 143 (2013) 43-49. [32] W.-K. Lee, W.-J. Cho, C.-S. Ha, A. Takahara, T. Kajiyama, Surface enrichment of the solution-cast poly (methyl methacrylate)/poly (vinyl acetate) blends, Polymer, 36 (1995) 1229-1234. [33] H.-M. Shabana, Determination of film thickness and refractive index by interferometry, Polymer testing, 23 (2004) 695-702. [34] J. Rieger, The glass transition temperature T g of polymers—comparison of the values from differential thermal analysis (DTA, DSC) and dynamic mechanical measurements (torsion pendulum), Polymer testing, 20 (2001) 199-204. [35] M.P. Sepe, Thermal analysis of polymers, iSmithers Rapra Publishing1997. [36] E. Turi, Thermal characterization of polymeric materials, Elsevier2012. [37] C.C. Ku, R. Liepins, Electrical properties of polymers, MacMillan Publishing Company.1993. [38] M. El‐Shaarawy, A. Mansour, S. El‐Bashir, M. El‐Mansy, M. Hammam, Electrical conduction and dielectric properties of poly (methyl methacrylate)/perylene solar concentrators, Journal of applied polymer science, 88 (2003) 793-805. [39] R.D. Priestley, P. Rittigstein, L.J. Broadbelt, K. Fukao, J.M. Torkelson, Evidence for the molecular-scale origin of the suppression of physical ageing in confined polymer: fluorescence and dielectric spectroscopy studies of polymer–silica nanocomposites, Journal of Physics: Condensed Matter, 19 (2007) 205120. [40] C. Müller, On the glass transition of polymer semiconductors and its impact on polymer solar cell stability, Chemistry of Materials, 27 (2015) 2740-2754. [41] P. Rittigstein, J.M. Torkelson, Polymer–nanoparticle interfacial interactions in polymer nanocomposites: confinement effects on glass transition temperature and suppression of physical aging, Journal of Polymer Science Part B: Polymer Physics, 44 (2006) 2935-2943. [42] R. Baskaran, S. Selvasekarapandian, N. Kuwata, J. Kawamura, T. Hattori, Conductivity and thermal studies of blend polymer electrolytes based on PVAc– PMMA, Solid State Ionics, 177 (2006) 2679-2682. [43] D.A. Seanor, Electrical properties of polymers, Elsevier2013.
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[44] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Advanced functional materials, 11 (2001) 15-26. [45] P.P. Edwards, H.B. Gray, M.T. Lodge, R.J. Williams, Electron transfer and electronic conduction through an intervening medium, Angewandte Chemie International Edition, 47 (2008) 6758-6765. [46] D. Emin, Electronic and vibrational hopping transport in boron carbides, AIP Conference Proceedings, AIP, 1991, pp. 65-76. [47] A. Ghosh, Ac conduction in iron bismuthate glassy semiconductors, Physical Review B, 42 (1990) 1388. [48] M. Buraidah, L. Teo, S. Majid, A. Arof, Ionic conductivity by correlated barrier hopping in NH 4 I doped chitosan solid electrolyte, Physica B: Condensed Matter, 404 (2009) 1373-1379. [49] J.C. Dyre, A simple model of ac hopping conductivity in disordered solids, Physics Letters A, 108 (1985) 457-461. [50] S. Elliott, A theory of ac conduction in chalcogenide glasses, Philosophical Magazine, 36 (1977) 1291-1304. [51] A.K. Jonscher, Dielectric relaxation in solids, Journal of Physics D: Applied Physics, 32 (1999) R57. [52] K.S. Cole, R.H. Cole, Dispersion and absorption in dielectrics I. Alternating current characteristics, The Journal of chemical physics, 9 (1941) 341-351. [53] T.W. Nee, R. Zwanzig, Theory of dielectric relaxation in polar liquids, The Journal of Chemical Physics, 52 (1970) 6353-6363. [54] H. Mark, Encyclopedia of Polymer Science and Technology, 12 Volume Set, John Wiley & Sons, Inc2005. [55] Z. Ahmad, Polymer dielectric materials, Dielectric material, InTech2012. [56] H. Eyring, Viscosity, plasticity, and diffusion as examples of absolute reaction rates, The Journal of chemical physics, 4 (1936) 283-291.
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List of Tables
Table.1.
blends doped with different concentrations of fullerene C60.
0
340
50
343
100
346
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200
∆Edc, (eV) 0.52
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Tg, (K)
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The glass transition temperature, Tg , and DC activation energy, ∆Edc , PMMA/PVAc
350
0.47
0.40 0.31
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Table.2. The calculated values of ac activation energy, ∆Eac , at different frequencies for fullerene
50 ppm
100 ppm
200 ppm
1 kHz
0.41
0.39
0.36
0.28
5 kHz
0.39
0.36
0.31
0.24
10 kHz
0.34
0.31
0.27
0.20
50 kHz
0.29
0.28
0.20
0.18
100 kHz
0.27
0.22
0.19
0.16
500 kHz
0.25
0.19
0.16
0.13
1 MHz
0.20
0.16
0.13
0.10
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∆Eac, (eV) Frequency
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C60 doped in PMMA/PVAc blends.
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Table.3.
The values of thermodynamic parameters for fullerene C60 doped in PMMA/PVAc
∆S ∆G (kJ/mol.) (JK-1mol.-1) 41.35 101.76 38.64 34.12 30.25 27.65 24.19 21.46
115.89 135.92 146.56 159.65 172.43 183.66
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328 333 338 343 348 353
100 ppm
200 ppm
∆G (kJ/mol.) 50.65
∆S (JK-1mol.-1) 95.68
∆G (kJ/mol.) 60.77
∆S (JK-1mol.-1) 90.21
53.92 49.68 46.32 43.21 40.13 39.29
103.65 129.10 134.23 147.65 156.28 168.12
59.20 59.03 55.19 53.96 49.89 47.51
95.34 116.54 126.12 131.56 146.81 157.43
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50 ppm
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Temperature, (K)
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blends.
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0 ppm 50 ppm 100 ppm 200 ppm Tg
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Tg
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E x o .(a .u .)
List of Figures
E n d o .(a .u .)
Tg
TE D
Tg
280
320
360
400
440
Temperature (K)
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Fig.1. DSC thermograms of PMMA/PVAc/C60 nanocomposite films.
480
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10-8
10-9
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2.8
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σ dc (Ω
-1cm-1)
10-7
3.0
3.2
3.4
103/T (K-1)
Fig.2. Temperature dependence of DC electrical conductivity σdc for PMMA/PVAc/C60
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film doped with 200 ppm fullerene C60.
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10-6
10-7
10-8
102
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σ to t ( Ω -1 cm -1 )
10-5
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313 K 323 K 333 K 353 K 363 K 373 K
103
104
105
106
TE D
Frequency (Hz)
Fig.3. Frequency dependence of the total conductivity σtot for PMMA/PVAc/C60
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nanocomposite film doped with 200 ppm of fullerene C60.
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1.1
1.0
s
QMT
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0.9
CBH
0.7 300
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0.8
320
340
360
380
400
Temperature (K)
TE D
Fig.4. Plot of the frequency exponent, s, for PMMA/PVAc/C60 nanocomposite film doped with 200 ppm of fullerene C60, the conduction mechanisms are illustrated on the
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10-4 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
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10-6
10-7
10-8
2.6
2.7
2.8
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σ tot ( Ω -1 cm -1 )
10-5
2.9
3.0
3.1
3.2
3.3
3.4
3.5
103/T (K-1)
TE D
Fig.5. Temperature dependence of the total conductivity σtot(ω) for PMMA/PVAc/C60
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313 K 323 K 333 K 353 K 363 K 373 K
12
ε`
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10
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8
4
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102
103
104
105
106
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Frequency (Hz)
Fig.6. Frequency dependence of dielectric permittivity ε´ for PMMA/PVAc/C60
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9 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
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ε`
7
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6
4 300
320
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5
340
360
380
400
Temperature (K)
TE D
Fig.7. Temperature dependence of dielectric permittivity ε´ for PMMA/PVAc/C60
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nanocomposite film doped with 200 ppm of fullerene C60.
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1.6
ε"
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1.2
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0.8
0.0
300
320
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0.4
340
360
380
400
TE D
Temperature (K)
Fig.8. Temperature dependence of the dielectric loss ε´´ for PMMA/PVAc/C60
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High Lights PMMA/PVAc/C60 nanocomposite films were prepared by using low cost solution-cast technique.
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The addition of nanofillers improved the molecular stability of the nanocomposite films.
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These nanocomposites are promising for luminescent down-shifting (LDS) PV coatings.
S0022-2860(17)31222-X
DOI:
10.1016/j.molstruc.2017.09.043
Reference:
MOLSTR 24292
To appear in:
Journal of Molecular Structure
Received Date: 1 June 2017 Revised Date:
14 September 2017
Accepted Date: 15 September 2017
Please cite this article as: S.M. El-Bashir, N.M. Alwadai, N. AlZayed, AC/DC electrical conduction and dielectric properties of PMMA/PVAc/C60 down-shifting nanocomposite films, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.09.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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200 nm
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GRAPHICAL ABSTRACT
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TEM image of PMMA/PVAc/C60 nanocomposite film doped with 200 ppm fullerene C60.
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AC/DC Electrical Conduction and Dielectric Properties of
2
PMMA/PVAc/C60 Down-Shifting Nanocomposite Films
3 S.M. El-Bashir 1,2 *, N.M. Alwadai 3, N. AlZayed 1
4 5 1
7
Department of Physics & Astronomy, Science College, King Saud University,
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6
Riyadh, Saudi Arabia.
8
2
Department of Physics, Faculty of Science, Benha University, Benha, Egypt.
9
3
Physics Department, Science College, Princess Nora Bint Abdul Rahman University,
10
Riyadh, KSA.
13
Abstract
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Polymer nanocomposite films were prepared by doping fullerene C60 in polymer
blend
composed
of
polymethacrylate/polyvinyl
acetate
blends
15
(PMMA/PVAc) using solution cast technique. The films were characterized by
16
differential scanning calorimeter (DSC), Transmission electron microscope (TEM),
17
DC/AC electrical conductivity and dielectric measurements in the frequency range
18
(100 Hz- 1 MHz). The glass transition temperature, Tg, was increased by increasing
19
the concentration of fullerene C60; this property reflects the increase of thermal
20
stability by increasing the nanofiller content. The DC and AC electrical conductivities
21
were enhanced by increasing C60 concentration due to the electron hopping or
22
tunneling between filled and empty localized states above Tg. The relaxation time was
23
determined from the αβ -
24
relaxations and found to be attenuated by increasing the temperature as a typical
25
behavior of amorphous polymers. The calculated values of thermodynamic
26
parameters revealed the increase of molecular stability by increasing the doping
27
concentration;
28
nanocomposite films in a wide scale of solar energy conversion applications such as
29
luminescent down-shifting (LDS) coatings for photovoltaic cells.
30
Keywords: PMMA/PVAc/C60 nanocomposites, AC/DC fields, luminescent down-
31
shifting coatings.
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this
feature
supports
the
application
of
PMMA/PVAc/C60
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32
-------------------------------------------------------------------------------------------------------
33
*
34
Astronomy, Science College, King Saud University, Riyadh, Saudi Arabia.
35
Tel.: 966 565850487; fax: 96614673656.
36
E-mail addresses: [email protected]
Corresponding author: S.M. El-Bashir, Associate Prof. at Department of Physics &
38
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37 1. Introduction
Polymer nanocomposites are applied for various areas of our life as they have
40
a stimulating function for the development of future applications [1]. Many polymers
41
are considered as suitable matrices for the development of nanocomposite structure
42
due to their ease of preparation, good adhesion with reinforcing fillers, weathering
43
resistance, low weight, and excellent mechanical strength [2-5]. The most popular and
44
economical method of the modification of polymer properties is by blending two or
45
more kind having different properties. This method is attractive for producing new
46
polymeric materials with tailored properties for totally new materials with different
47
features [6]. It has been reported that polymethylmethacrylate (PMMA) and polyvinyl
48
acetate (PVAc) are unique pairs of polymers as their blends and nanocomposites have
49
a wide range of applications due to their thermal, rheological, electrical, mechanical
50
and optical properties [7-9]. Among different types of nanofillers, fullerenes have
51
been widely studied due to their unique structure, electrical and optical properties,
52
which make it unique material for miscellaneous applications such as biological,
53
chemical, electronic, optical and solar energy conversion [10-16]. The incorporation
54
of fullerene C60 into the polymer structure gives various nanocomposite materials,
55
films, and fibers which are suitable for various purposes [17-19]. In a previous work,
56
we have studied the structure and photophysical properties of PMMA/PVAc/C60
57
nanocomposite films as luminescent down-shifting coatings (LDS) for PV cells[20].
58
These nanocomposite films can efficiently overcome the short-wavelength limit of
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59
solar cells by absorbing of incident UV-photons and re-emitting them at longer
60
wavelengths that match the spectral response of the photovoltaic (PV) cell under
61
consideration [21-24]. The use of LDS to improve the performance of PV cells has
62
been first reported by Hovel et al.[25] since then LDS materials have been
63
investigated using theoretical and
64
materials considered for LDS coatings in PV cells are polymers such as
65
polymethylmethacrylate (PMMA) doped with luminescent molecules [22]. Over the
66
last few years, significant efforts were made to obtain an appropriate LDS material to
67
split a high energy photon into two lower energy photons using different designs[26].
68
One of the most famous designs for LDS is the luminescent solar concentrators
69
(LSCs)[27]. Here, a highly transparent polymer plate doped by luminescent materials;
70
the luminescence is isotropically emitted and guided to PV cells via total internal
71
reflection (TIR)[28]. LSCs present many advantages such as that heat generated can
72
be well dissipated into the concentrator plate area, direct and diuse light is collected
73
to improve the performance of the PV device[29-31]. The present work concerns on
74
studying the influence C60 concentration on thermal, electrical and dielectric
75
properties of PMMA/PVAc/C60 LSC downshifting nanocomposite films prepared by
76
low-cost solution casting technique. The importance of this study is that the physical
77
properties of polymers are strongly affected by the presence of molecular relaxation
78
mechanisms which are directly related the polymer structure, composition,
79
thermodynamic parameters and weathering stability for outdoor applications such as
80
LDS coatings for PV cells.
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experimental methods [24]. Conventional
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81
2. Experimental Techniques
82
2.1. Materials. polymethylmethacrylate (PMMA) and polyvinyl acetate (PVAc) were
84
obtained from Sigma-Aldrich (USA) and were reported to have molecular weights of
85
996,000 and 167,000 g.mol−1, respectively. Buckminsterfullerene C60 powder has a
86
purity of 99.5 % was obtained from Sigma-Aldrich (USA). Chloroform (CHCl3) was
87
used as the casting solvent with purity of 99.8% (HPLC grade
88
2.2. Preparation of PMMA/PVAc/C60 nanocomposite films.
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PMMA/PVAc/C60 nanocomposite films were prepared by using solution-cast
90
technique [32]. Firstly, polymer solutions were prepared by dissolving PMMA and
91
PVAc separately in chloroform with concentration 5g/100 mL and stirred after mixing
92
with the ratio (50/50) for 48 h at 40 oC. After that, C60 was doped in the solution of the
93
mixed homopolymers with different concentrations 50, 100 and 200 ppm and then
94
cast onto glass Petri dishes until being dried at room temperature. After drying, the
95
films were carefully removed and heated at 100 oC for 6 h to evaporate the confined
96
solvent molecules. The thickness of the prepared nanocomposite films was in the
97
range of 60 ± 10 µm as measured optically used air wedge experiment [33].
98
2.3. Measurements and devices
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99
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Differential scanning calorimetry was performed by a calibrated Setaram
100
DSC 131 to record thermograms in the presence of high purity nitrogen as an inert
101
atmosphere at a heating rate 293 K0/min. over a temperature range (298- 473 K). The
102
glass transition temperature Tg for PMMA/PVAc/C60 nanocomposite films was
103
determined as described in the literature[34-36].
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104
The DC electrical resistance R for the investigated sample was measured using a
105
KEITHLEY 616 electrometer in a shielded electric furnace. The temperature was
106
measured using thermocouple Chromel Alumel (USA). AC electrical conductivity and dielectric properties (dielectric constant, dielectric
108
loss and the dielectric loss tangent) have been carried out in the frequency range
109
(100 Hz -1MHz), using Wayne Kerr-6440B RLC Impedance Meter (USA), with
110
accuracy (± 0.005%) in the temperature range (298–383 K). The samples used were
111
cut in the form of rectangular discs of area 1.6 cm2 and thickness about 0.3 mm. The
112
desired area used in the AC measurements was coated by silver conducting paint
113
which showed ohmic contact with the samples. The dielectric constant of the
114
sample is calculated using the following relation [37, 38],
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107
= .
115
(1)
where C is the capacitance of the sample, is the free space permittivity, t and A are
117
the thickness and area of the sample. In addition, the dielectric loss was calculated
118
from the following relation [37, 38],
119
TE D
116
= tan
where = (90 − ) and is the phase angle.
121
3. Results & Discussion
122
3.1. Thermal Analysis of PMMA/PVAc/C60 nanocomposite films.
AC C
123
EP
120
(2)
Differential scanning calorimetry (DSC) is one of the most convenient
124
methods to determine the homogeneity and stability of PMMA/PVAc/C60
125
nanocomposite films. Fig.1. illustrates the effect of fullerene C60 doping on the value
126
of Tg for PMMA/PVAc/ C60 nanocomposite films; it is observed that the value of Tg
127
is shifted to higher temperatures from 340 K for pure PMMA/PVAc blend to 350 K
128
after doping with 200 ppm fullerene C60. Regarding Tg values listed in Table.1, it is Page 5 of 15
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noted Tg is increased by increasing fullerene C60 content; this behavior is ascribed to
130
the decrease of the polymer free volume and the increase of chain rigidity by
131
increasing nanofiller [39-41]. This behavior suggests the outstanding thermal
132
resistance of PMMA/PVAc/ C60 nanocomposite films towards hot climates.
133
3.2. DC electrical conductivity of PMMA/PVAc/C60 nanocomposite films.
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129
The electrical conductivity of PMMA/PVAc/C60 nanocomposite films have
135
been studied in the temperature range (303-383 K), a representative plot is shown in
136
Fig.2. The plot of direct-current (DC) electrical conductivity σdc versus 103/T
137
represents a semilogarithmic plot which suggests an Arrhenius type thermally
138
activated process according to [38, 42]:
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= exp (
139
)
(3)
where T is the absolute temperature, kB is Boltzmann's constant, σo is a pre-
141
exponential factor, and ∆Edc is the activation energy. It is clearly observed that the
142
electrical conductivity is thermally activated up to a peak value occurred at a certain
143
temperature Tmax depending on the Tg value of each sample [38, 43]. The activated
144
part in σ-103/T relation can be attributed to the fact that as the temperature increases,
145
the polymer chains acquire faster internal modes in which bond rotations produce
146
segmental motions and the conductivity increases [38, 43]. Table1 shows the values
147
of ∆Edc which have been evaluated from the slope of Arrhenius plot in the thermally
148
activated part (T < Tmax) of σ-103/T relation. It is noted that the value of ∆Edc for pure
149
PMMA/PVAc film (0.52 eV) is lower than that calculated in the literature for solvent
150
cast PMMA (0.64 eV) [38]; this can be due to the increase of carrier concentration
151
resulting from the addition of acetate group. In addition, it is noted that the values of
152
∆Edc are decreased by increasing the concentration of C60, this can be ascribed to the
153
increase of the interaction between C=O donor groups and fullerene C60 acceptor
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molecules [44, 45]. The conduction process usually occurs by electron hopping and/or
155
tunneling between donor atoms and empty sites located in the energy band gap [46].
156
The conduction mechanism will be confirmed in the next section by the study of
157
alternating-current (AC) conductivity measurements. On the other hand, the
158
attenuated part of the temperature dependence of σdc (T > Tmax) can be attributed to
159
the large deformations characteristic of the viscoelastic state permitted by dipole
160
segmental motions [38]. This lead to the decrease of the electron mobility due to the
161
charge carrier scattering caused by electron- phonon coupling at high temperatures
162
(T >Tg) [38].
163
3.3. AC Conductivity of PMMA/PVAc/C60 nanocomposite films.
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Fig.3. shows the frequency dependence of the total conductivity σtot(ω) for
165
200 ppm fullerene doped PMMA/PVAc/C60 nanocomposite film at different
166
temperatures as a representative plot for all the samples under investigation. The
167
frequency dependence of σtot(ω) obeys the following relation [37, 38],
168
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( ) = + " = + # $
(4)
where ω is the angular frequency which equals to 2πf, σac is the AC electrical
170
conductivity, A is the independent frequency factor, and s is the frequency power
171
determining the conduction mechanism. According to Eq.(2), it is clear that σtot(ω) is
172
frequency dependent in the high-frequency range; this linear dependence of AC
173
electrical conductivity with frequency could account for the electron conduction via a
174
hopping process between coordinating sites, local structural relaxations and segmental
175
motions of the polymer films [47]. In order to determine the conduction mechanism,
176
the values of the exponent, s, were deduced for all samples at different temperatures
177
and plotted against temperature in Fig. 4. It is observed that the value of s is decreased
178
to a minimum value by increasing temperature T~Tg, this behavior is corresponding
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to the hopping of electrons according to the correlated barrier hopping model (CBH)
180
[38, 48]. At temperatures, higher than T~Tg , it is noted that s , is increased by
181
increasing temperature referring to the change of the conduction mechanism as the
182
electrons can transport between the localized states according to quantum-mechanical
183
tunneling model (QMT) [38, 48]. From this study, it can be confirmed that at higher
184
temperatures above the glass transition the increase of the electron mobility decreases
185
the width of the potential energy barrier and causes electron tunneling between
186
localized states [47, 49, 50].
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The temperature dependence of σtot(ω) for the investigated sample is shown in
188
Fig.5.; it is clear that σtot(ω) is thermally activated up to a maximum peak which is
189
shifted to higher temperatures by increasing frequency and then attenuated as the
190
same behavior observed for the temperature dependence of σdc. The values of AC
191
activation energy ∆Eac are obtained at different frequencies as previously calculated
192
from Arrhenius equation and listed in Table.2. It is noted that the values of σtot(ω) are
193
higher than σdc and ∆Eac are lower than ∆Edc. This can be due to the fact that the
194
increase in the applied field frequency enhanced the electron mobility and,
195
subsequently, the conductivity value [38, 50]. In addition, the loss factor dominates at
196
higher frequencies and, therefore the conductivity increases up to comparatively
197
higher values for higher frequency ranges [51]. The decrease in the total conductivity
198
at higher temperatures may be due to the scattering of electrons which is the dominant
199
behavior by increasing temperature [43]. For an accurate analysis of dielectric
200
response data, consideration of just conductivity is not sufficient; more information
201
can be obtained from other data presentations, as we will proceed to show.
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3.4. Dielectric Permittivity of PMMA/PVAc/C60 nanocomposite films. Fig.6. Illustrates the frequency dependence of the dielectric permittivity ε ׳for
204
200 ppm fullerene doped PMMA/PVAc/C60 nanocomposite films at different
205
temperatures. From the plots, it is observed that the dielectric permittivity is decreased
206
by increasing frequency and going to a constant value at higher frequencies as the
207
behavior observed in most of the polar polymers following Debye dispersion
208
relation[52], ׳
'
׳
(5)
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׳
& ( = ׳% + )*+ , -,
209
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203
210
where, ׳is the dielectric permittivity at angular frequency ω , $ is the static
211
dielectric permittivity, % is the infinite dielectric permittivity and τ is the relaxation
212
time. According to Debye model, polar dielectric materials have high initial value of
213
dielectric permittivity ׳, but as the frequency of the field is raised this value begins to
214
drop. This could be due to the disability of the diploes to follow the field variations at
215
high frequencies and also due to electrode polarization effects [53]. At low
216
frequencies, the high values of dielectric constant can be due to the space charge
217
effects arising from the electrodes [42] .At high frequencies, the polarization due to
218
the charge accumulation decreases, leading to the decrease in the value of dielectric
219
permittivity [38, 54]. Fig.7. shows the temperature dependence of the dielectric
220
permittivity ׳for the investigated sample at different frequencies, it is clearly
221
observed that the trend of ( ׳.) is a typical behavior for a polar dielectric [38, 54].
222
The value of ׳increases up to a maximum value and then drops at a certain
223
temperature, this drop in ׳can be attributed to the intensified thermal oscillations of
224
the polymer which disturb the orderliness of dipole orientations. Moreover the sample
225
exhibits a relatively low temperature gradient ∂ε'/∂T compared to literatures which
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226
refers to the complete vaporization of molecules of the casting solvent (chloroform)
227
and reflects the quality and homogeneity of PMMA/PVAc/C60
228
films[38, 54].
229
3.5. Dielectric Loss of PMMA/PVAc/C60 nanocomposite films.
nanocomposite
Fig.8 shows the temperature dependence of ε ׳׳at different frequencies for the
231
investigated sample. It is observed that the position of ε׳׳max (T) shifts downward
232
right toward higher temperatures by increasing frequency due to the presence of
233
different polarization mechanisms; which is a typical behavior of a dielectric material
234
[38, 54, 55]. The explanation for the appearance of this peak loss in ε( ׳׳T) curves can
235
be ascribed to the fact that the β-relaxation is much broader than the α-relaxation
236
[38, 54]. This is due to the fact that the local barrier to rotation of the side groups is
237
different along the chain as a consequence of the irregularity of the glassy state [38,
238
54]. In the contour map described in the literature, α and β relaxations are only well
239
separated below 1 kHz, so ε׳׳max (T) can be ascribed to an αβ relaxation in which the
240
side groups cooperate with the limited backbone motions in a micro-Brownian motion
241
which have properties similar to a pure α-relaxation [38, 54, 55].
242
3.6 Thermodynamic Parameters
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The thermodynamic parameters were obtained using the method suggested by
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244
Eyring [38, 56]; the Gibbs free energy of activation, ∆G for dipole relaxation was
245
calculated using relation [38, 56],
246
∆0 = 1. 23 [
5
]
(6)
247
where R is the universal gas constant, T the temperature k is Boltzmann constant, τ is
248
relaxation time, and h is Plank´s constant. ∆G is directly dependent on the enthalpy,
249
∆H, and entropy ∆S by the following equation [38, 56], Page 10 of 15
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250
(7)
251
The obtained relaxation time τ =1/ωmax showed an attenuation with increasing
252
temperature, obeyed an exponential relation of the form [38, 56],
253
:=
5
∆;
exp ( )
(8)
<
The thermodynamic parameters can be obtained by plotting ln(Tτ) versus 103/T that
255
give a linear behavior with slope ∆H/R [38, 56] as the value of τ is systematically
256
decreased with temperature in well agreement with the concept of molecular
257
relaxation in polymers [38]. This means that the dominated type of the polarization in
258
this sample is orientation- polarization since this type of polarization is very sensitive
259
to temperature. The thermodynamic parameters were evaluated at the temperature
260
below Tg of each sample by abovementioned equations for PMMA/PVAc/C60
261
nanocomposite films and listed in Table3. The molecular explanation of this
262
temperature dependence is that the relative influence of the segmental molecular
263
interaction energy is decreased with increasing temperature. The decrease of the ∆G
264
by increasing temperature reflects the role of fullerene C60 doping for restricting the
265
rearrangement of polymer and enhancement in the molecular stability of organic host.
266
Moreover, the values of ∆S are increased when ∆G decreased by increasing
267
temperature; this can be ascribed to the increase of the disorder degree [38] which
268
attributed to the increase of polymer free volume and chain mobility caused by the
269
anchored fullerene C60 molecules as confirmed by DSC measurements.
270
4. Conclusions
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271
A low-cost solution cast technique was used to prepare PMMA/PVAc/C60
272
nanocomposite LDS films; TEM studies designated the homogeneity and good
273
distribution of C60 nanoparticles in the polymeric matrix. DSC measurements showed
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the increase of the glass transition temperature, Tg, by increasing the concentration of
275
C60. This reflects the impact of nanofillers on the enhancement of the thermal stability
276
of the nanocomposite films by controlling the main chain relaxation processes which
277
greatly affect the solidification of polymeric LDS PV coatings in hot countries like
278
KSA. The study of the DC and AC electrical conductivities showed the temperature
279
dependence of two conduction mechanisms which are directly related to Tg of each
280
sample. The first mechanism is correlated barrier hopping (CBH) below T~Tg, and the
281
second is quantum-mechanical tunneling model (QMT) at temperatures higher than
282
T~Tg. The study of the dielectric properties showed the typical behavior molecular
283
relaxation of amorphous polar polymers. Finally, the calculated values of the
284
thermodynamic parameters showed the increase of the free energy and the decrease of
285
the entropy by increasing the concentration of C60; this clarified the improvement of
286
the molecular stability of the prepared nanocomposites for optoelectronic and
287
applications solar energy conversions.
288
Acknowledgements
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This research project was supported by a grant from “The Research Center of
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the Female Scientific and Medical Colleges,” Deanship of Scientific Research, King
291
Saud University.
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References
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[44] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Advanced functional materials, 11 (2001) 15-26. [45] P.P. Edwards, H.B. Gray, M.T. Lodge, R.J. Williams, Electron transfer and electronic conduction through an intervening medium, Angewandte Chemie International Edition, 47 (2008) 6758-6765. [46] D. Emin, Electronic and vibrational hopping transport in boron carbides, AIP Conference Proceedings, AIP, 1991, pp. 65-76. [47] A. Ghosh, Ac conduction in iron bismuthate glassy semiconductors, Physical Review B, 42 (1990) 1388. [48] M. Buraidah, L. Teo, S. Majid, A. Arof, Ionic conductivity by correlated barrier hopping in NH 4 I doped chitosan solid electrolyte, Physica B: Condensed Matter, 404 (2009) 1373-1379. [49] J.C. Dyre, A simple model of ac hopping conductivity in disordered solids, Physics Letters A, 108 (1985) 457-461. [50] S. Elliott, A theory of ac conduction in chalcogenide glasses, Philosophical Magazine, 36 (1977) 1291-1304. [51] A.K. Jonscher, Dielectric relaxation in solids, Journal of Physics D: Applied Physics, 32 (1999) R57. [52] K.S. Cole, R.H. Cole, Dispersion and absorption in dielectrics I. Alternating current characteristics, The Journal of chemical physics, 9 (1941) 341-351. [53] T.W. Nee, R. Zwanzig, Theory of dielectric relaxation in polar liquids, The Journal of Chemical Physics, 52 (1970) 6353-6363. [54] H. Mark, Encyclopedia of Polymer Science and Technology, 12 Volume Set, John Wiley & Sons, Inc2005. [55] Z. Ahmad, Polymer dielectric materials, Dielectric material, InTech2012. [56] H. Eyring, Viscosity, plasticity, and diffusion as examples of absolute reaction rates, The Journal of chemical physics, 4 (1936) 283-291.
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List of Tables
Table.1.
blends doped with different concentrations of fullerene C60.
0
340
50
343
100
346
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∆Edc, (eV) 0.52
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Tg, (K)
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350
0.47
0.40 0.31
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Table.2. The calculated values of ac activation energy, ∆Eac , at different frequencies for fullerene
50 ppm
100 ppm
200 ppm
1 kHz
0.41
0.39
0.36
0.28
5 kHz
0.39
0.36
0.31
0.24
10 kHz
0.34
0.31
0.27
0.20
50 kHz
0.29
0.28
0.20
0.18
100 kHz
0.27
0.22
0.19
0.16
500 kHz
0.25
0.19
0.16
0.13
1 MHz
0.20
0.16
0.13
0.10
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∆Eac, (eV) Frequency
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C60 doped in PMMA/PVAc blends.
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Table.3.
The values of thermodynamic parameters for fullerene C60 doped in PMMA/PVAc
∆S ∆G (kJ/mol.) (JK-1mol.-1) 41.35 101.76 38.64 34.12 30.25 27.65 24.19 21.46
115.89 135.92 146.56 159.65 172.43 183.66
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328 333 338 343 348 353
100 ppm
200 ppm
∆G (kJ/mol.) 50.65
∆S (JK-1mol.-1) 95.68
∆G (kJ/mol.) 60.77
∆S (JK-1mol.-1) 90.21
53.92 49.68 46.32 43.21 40.13 39.29
103.65 129.10 134.23 147.65 156.28 168.12
59.20 59.03 55.19 53.96 49.89 47.51
95.34 116.54 126.12 131.56 146.81 157.43
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Temperature, (K)
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0 ppm 50 ppm 100 ppm 200 ppm Tg
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E x o .(a .u .)
List of Figures
E n d o .(a .u .)
Tg
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Tg
280
320
360
400
440
Temperature (K)
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480
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10-8
10-9
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2.8
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σ dc (Ω
-1cm-1)
10-7
3.0
3.2
3.4
103/T (K-1)
Fig.2. Temperature dependence of DC electrical conductivity σdc for PMMA/PVAc/C60
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10-6
10-7
10-8
102
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σ to t ( Ω -1 cm -1 )
10-5
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313 K 323 K 333 K 353 K 363 K 373 K
103
104
105
106
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Frequency (Hz)
Fig.3. Frequency dependence of the total conductivity σtot for PMMA/PVAc/C60
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1.1
1.0
s
QMT
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0.9
CBH
0.7 300
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0.8
320
340
360
380
400
Temperature (K)
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Fig.4. Plot of the frequency exponent, s, for PMMA/PVAc/C60 nanocomposite film doped with 200 ppm of fullerene C60, the conduction mechanisms are illustrated on the
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10-4 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
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10-6
10-7
10-8
2.6
2.7
2.8
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σ tot ( Ω -1 cm -1 )
10-5
2.9
3.0
3.1
3.2
3.3
3.4
3.5
103/T (K-1)
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Fig.5. Temperature dependence of the total conductivity σtot(ω) for PMMA/PVAc/C60
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313 K 323 K 333 K 353 K 363 K 373 K
12
ε`
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8
4
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102
103
104
105
106
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Frequency (Hz)
Fig.6. Frequency dependence of dielectric permittivity ε´ for PMMA/PVAc/C60
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9 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
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8
ε`
7
SC
6
4 300
320
M AN U
5
340
360
380
400
Temperature (K)
TE D
Fig.7. Temperature dependence of dielectric permittivity ε´ for PMMA/PVAc/C60
AC C
EP
nanocomposite film doped with 200 ppm of fullerene C60.
ACCEPTED MANUSCRIPT 1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 500 kHz 1 MHz
1.6
ε"
RI PT
1.2
SC
0.8
0.0
300
320
M AN U
0.4
340
360
380
400
TE D
Temperature (K)
Fig.8. Temperature dependence of the dielectric loss ε´´ for PMMA/PVAc/C60
AC C
EP
nanocomposite film doped with 200 ppm of fullerene C60.
ACCEPTED MANUSCRIPT
High Lights PMMA/PVAc/C60 nanocomposite films were prepared by using low cost solution-cast technique.
RI PT
The addition of nanofillers improved the molecular stability of the nanocomposite films.
AC C
EP
TE D
M AN U
SC
These nanocomposites are promising for luminescent down-shifting (LDS) PV coatings.