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Abstract

A suspension of ultrasonically agitated Titanium dioxide micrometer-sized powder has been introduced into a poly(methyl methacrylate) (PMMA) matrix by solvent (CHCl₃) mixing under slow mechanical stirring, and the resulting suspension was used to cast free-standing PMMA/TiO₂ composite films. Filler contents from 0 wt% to 7.5 wt% were selected, since TiO₂ particles could not be previously suspended in chloroform at higher concentrations by ultrasonic agitation. Optical and dielectric properties of composites were determined by ultraviolet (UV)-visible and dielectric spectrometry. In the UV region, 450% improvement of UV absorption was achieved in the TiO₂/PMMA composite compared with neat PMMA. The structural properties and hole-free volume of the composites were determined by X-ray diffractometry and positron annihilation lifetime spectroscopy, respectively. The particle size and homogeneous dispersion of the particles in the polymer matrix were related to the hole-free volume of the PMMA.

Keywords

PMMA composite dielectric properties UV-visible free volume positron annihilation lifetime spectroscopy

Introduction

Inorganic–organic hybrid composite and nanocomposite materials based on functional inorganic particles embedded into organic networks represent widely explored field of investigation.¹ Such hybrid materials join interesting characteristics of polymer such as flexibility, processability, and high thermal and mechanical stability with specific size-dependent properties of inorganic nanocrystalline oxides. Fabrication of hybrid materials can be accomplished by dispersing inorganic fillers in host polymer, “in-situ” synthesis of particles in polymer matrix or reaction of polymer precursor in the presence of inorganic fillers.² Poly(methyl methacrylate) (PMMA) is an amorphous thermoplastic and widely used as a host material in hybrid composites.^1

-9 PMMA has specific electrical properties with high resistance and low dielectric constants. Therefore, it is widely used in high-voltage applications as dielectric film.¹ Besides, it transmits light in the range of 360–1000 nm and resists oxidative photodegradation.

Inorganic oxides with high refractive indices are generally characterized by poor workability; however, lightweight, flexible, and good processable organic materials have lower refractive indices than inorganic materials.³ TiO₂ has attracted a high research interest due to photochemical activity and optical properties.⁴ It has also high dielectric constant. As an inorganic filler in the polymer matrix, it can increase the dielectric constant of composite compared with the neat polymer. Because of wide variety applications of research interests, many researchers have studied PMMA/TiO₂ hybrid materials to improve their thermal and mechanical^5,6 surface properties,^7,8 sensor and battery application,^2,9 and optical and dielectric properties.^3,10

-14

Agglomeration of inorganic fillers and particle-size distribution in polymer matrix prevent the improvements of composite properties. The agglomeration problem can be handled by using different techniques such as ultrasonic agitation, polymerization, and solution mixing condition. Ultrasound-assisted emulsion polymerization and ultrasonic agitation of inorganic fillers were well described.¹⁵ One can also change the solution mixing conditions such as temperature, mixing duration, or speed. Agglomeration of inorganic fillers can also cause a change in the “free volume” of the polymer matrix. For this reason, it is important to determine the free volume in the polymer matrix. The free volume model in polymers may give an explanation about some significant physical quantities such as viscosity, ionic conductivity, glass transition temperature, and mechanical and thermodynamic properties. Bamford et al.¹⁶ and Cohen and Turnbull¹⁷ interpret the transport phenomena in polymers in terms of the free volume arisen from structural disorders in amorphous polymers. On the other hand, the size of the hole-free volume and its intensity are measured by ortho-positronium (o-Ps) lifetime ( $τ_{3}$ ) and intensity (I ₃) using positron annihilation lifetime spectroscopy (PALS), a unique tool to probe the holes. The PALS, a non-destructing powerful technique, yields invaluable information about the structural hole-free volume and the free volume fraction.¹⁸

The aim of this study is to obtain the structural and dielectric properties of PMMA/TiO₂ composites prepared by ultrasonic agitation and low mixing speeds. The structural and dielectric properties of PMMA/TiO₂ composites were determined and free volume of PMMA with TiO₂ in different weights (10, 30, 50, and 75 mg) was studied under vacuum in the temperature range from 293 K to 373 K. Furthermore, a relation between optical and dielectric properties as well as the hole-free volume and free volume fraction of the samples was presented for each TiO₂ proportion.

Experimental

Materials

PMMA, from Alfa Aesar (molecular weight = 120,000), was dissolved in 25 mL chloroform (Merck) for 3 h. TiO₂ powders (anatase phase), selected in micrometer size (<33 µm), in 25 mL chloroform were subjected to ultrasonic agitation for 4 h. Then, TiO₂ solutions were added to polymer solutions and mechanically stirred at a slow stirring rate (60 r min⁻¹) for 30 min.

Slow stirring rate and duration were selected to prevent the agglomeration of TiO₂ particles in the polymer matrix. In all samples, PMMA was 1 g and TiO₂ masses were chosen to be 10, 30, 50, and 75 mg, respectively. Over the 100 mg of TiO₂, TiO₂ particles were not well dispersed in the ultrasonic agitator. Therefore, the maximum TiO₂ mass was selected as 75 mg. All solutions were cast on Petri dishes to obtain composite films.

Characterization

Ultraviolet (UV)-visible transmittance spectra of pure PMMA and its composites were taken by PerkinElmer (United Kingdom) Lambda 35 (190–1100 nm) UV-visible spectrometer. Dielectric spectroscopy studies of composites were carried out by NovoControl (Germany) Alpha-A Dielectric, Conductivity and Impedance Analyzer (1 Hz to 1 MHz).

Real (∊′) and imaginary (∊″) parts of the dielectric constant were calculated by

ε' = \frac{C_{p}}{C_{0}} and ε ″ = ε' tan δ

where C ₀ is the vacuum capacitance, C _p is the capacitance, and tan δ is the loss factor. The real part (∊′) is related to the stored energy by the external field and the imaginary (∊″) part (dielectric loss) is related to loss of energy.

The PALS measurements were carried out using a fast–fast conventional coincidence system measuring the spectrum of time interval between prompt γ-ray, as a start signal of positrons accompanied by a γ-ray of 1274 keV, and subsequent γ-ray, as a stop signal due to the fact that positrons annihilated with electrons from the sample resulting in 511 keV of γ-rays. The positron source was prepared by depositing about 25 μCi of ²²NaCl aqueous solution on a thin aluminum foil (5 µm thick) and sandwiched between two sets of samples. Each set of samples was kept about 2 mm thickness piling up three films of samples. Each film thickness is about 0.7 mm.

The plastic scintillators mounted onto Hamamatsu (Photonics Deutschland GmbH, Germany) 2059 photomultiplier tubes based by Ortec-265 (AMETEK GmbH, Meerbusch, Germany) operated at negative 2050 V have been employed for γ-ray detection. For windows settings of 1274 and 522 keV and timing signals, two constant fraction differential discriminators (Ortec CFDD 583B) were used. A time to amplitude converter used to convert pulses of different heights to a time-to-pulse-height signal was fed to a multichannel analyzer (Ortec Model 919E Ethernim MCA). The spectroscopic data yielded from Multi-Channel Analyzer were analyzed by using the code of Life Time-polymer¹⁹ to obtain the lifetime and its intensity revealing the information about the free volume. The resolution of the system is about 350 ps and a million counts have been taken for each run.

Kobayashi purposed the free volume fraction to be proportional to the product of hole-free volume calculated from o-Ps lifetime and o-Ps intensity. It is supposed to be related with the hole fraction as a measure of free volume computed by the Simha–Somcynsky (SS) hole theory. ²⁰ The SS theory, based on the lattice-hole model, is formulated in terms of reduced pressure, volume, and temperature variables with the reduced characteristic parameters of each variable.²⁰

Results and discussion

UV-visible study

Figure 1 shows the UV-visible absorbance spectrum of neat PMMA film. There was intense absorption below 260 nm in the UV region which is a typical characteristic of PMMA. Figure 2 shows the UV-visible absorbance spectra of TiO₂/PMMA composites. The absorbance onset shifted to the higher wavelengths with an increase of TiO₂ compared with neat PMMA. The absorbance was also increased with an increase of TiO₂ content. Maximum absorbance (4.5%) was observed in 50 mg TiO₂-loaded composite in the UV region. Interestingly, the absorbance of 75 mg TiO₂ content which was found to be the limit of good dispersion in the solvent by ultrasonic agitation is lower than the absorption of 50 mg TiO₂ content (UV region). In the UV region, 450% improvement of UV absorption was achieved in the TiO₂/PMMA composite according to the neat PMMA. Similar results were observed in TiO₂/PMMA nanocomposites.^5,21 Increase in the absorption was related to the chemical interaction between TiO₂ and PMMA.⁵ In this region, the reduction in the absorption value for the 75 mg TiO₂ containing composite can be related to a decrease in the chemical interaction between TiO₂ and PMMA. The reason for the loss of absorbance for this filler rate (7%, weight/weight (w/w)) may be that the TiO₂ particles are agglomerated in the polymer matrix and thus the hole fraction in the polymer matrix may have increased. In the visible region (400–700 nm), the absorbance values were also increased by an increase in the TiO₂ content. Low filler rates (7% w/w), low solution mixing speed (60 r/min⁻¹), and ultrasonic agitation of micro-sized fillers before the composite preparation can improve the UV absorption properties of the TiO₂/PMMA hybrid materials.

Figure 1.

UV-visible spectrum of neat PMMA film.

Figure 2.

UV-visible spectra of TiO₂/PMMA composites.

X-ray Diffractometer

Figure 3 shows X-ray Diffractometer spectra of TiO₂ and PMMA. In this figure, sharp peaks, especially at high-angle values, represent crystalline regions. At small angles, broadened peaks with low amplitude represent scattering from amorphous regions of PMMA. In Figure 3, a typical amorphous polymer behavior is also observed. The observed peak at 13.7° indicates the packing order of the polymer chains.²² The observed peak at 29.5° represents the arrangement within the main chains, and this arrangement is observed to be rather limited due to its high level of broadening and low amplitude.²² Similarly, X-ray diffraction of TiO₂ is shown in Figure 3. The values of 2θ of the peaks and corresponding planes they are scattered in are given in Table 1. The peaks observed at 25° and 55° are an indication that TiO₂ is in anatase form. The notable difference for the rutile form is the shift from 25° to 27–28°. Similar results for TiO₂ have been pointed out in the literature.^10,12,23,24

Figure 3.

X-ray diffraction of natural PMMA and TiO₂.

Table 1.

X-ray diffraction maxima and corresponding planes of TiO₂ (anatase structure).

2θ (°)	25	37	48	54	55	63
d _hkl	101	004	200	105	211	204

Figure 4 shows the X-ray diffractograms of TiO₂/PMMA composites. TiO₂ peak intensities are very strong in 30 and 50 mg TiO₂-doped composites. In 10 and 75 mg TiO₂-doped composites, these peaks are weakened. The peak at 13.7° (Figure 4) in powder samples of pure PMMA has shifted to 17° in the film sample. This behavior can be explained by the formation of the crystalline phase, which is observed in very small amounts in the powder sample. In the 30 and 50 mg TiO₂-added composites, the TiO₂ peak (d₁₀₁), observed at 22° compared to the other composites, is due to the amorphous behavior of PMMA.¹² The traces of the amorphous behavior of the polymer are explained by the shift of the peak (17°) seen in the PMMA film sample to the lower 2θ values in 10 and 75 mg TiO₂-loaded composites. In the 10 and 75 mg cases, this peak was weakened and broadened. The particle size and homogeneous dispersion of the particle in the polymer matrix can also be related to peak intensities and peak areas.²⁵ In the 30 and 50 mg TiO₂-added samples, the peak observed at 25° (d₁₀₁ plane of TiO₂) is sharp. Thus, it can be concluded that the filler grain size is low and the homogeneity is high in the 30 and 50 mg TiO₂-added composites where severe peaks are observed compared to other composites.

Figure 4.

XRD diffractograms of PMMA/TiO₂ composites.

Dielectric study

Figure 5 shows the dielectric permittivity of neat PMMA and TiO₂/PMMA composites. It can be seen that dielectric permittivity was increased with an increase of TiO₂ content in the low-frequency region. In the high-frequency region, an increase in dielectric permittivity was also observed, but 50 mg TiO₂-filled composite has the lower dielectric permittivity. Maximum dielectric permittivity which is 25% higher than neat PMMA was observed in 75 mg TiO₂-filled composite.

Figure 5.

Dielectric permittivity neat PMMA and TiO₂/PMMA composites.

Figure 6 shows the loss factor (tangent loss) of neat PMMA and TiO₂/PMMA composites. Relaxation peaks were observed in the low-frequency region (<40 Hz). The relaxations were slightly shifted to lower frequencies with an increase of TiO₂ concentration. Loss factors were increased with an increase of TiO₂ concentration and interestingly maximum loss factor values were observed in 50 mg TiO₂-filled composite. Similar behaviors were observed in some studies.^10,11,26

Figure 6.

Loss factor of neat PMMA and TiO₂/PMMA composites.

It was reported that both dielectric permittivity and loss factor increase with an increase of oxide fillers. Higher tan (δ) at low frequencies is generally attributed to the higher current leakage in the system. In the case of TiO₂ nanocomposites, an increase in tan (δ) was observed below 10 Hz that could be a sign of an increase in electrical conductivity because of the higher possibility of charge carriers to reach the surface of the electrodes.²⁶ In our study, similar results were observed.

TiO₂ particles having high surface energy cause a high particle–particle attraction resulting in the aggregation of TiO₂ particles. This aggregation can cause the incompatibility between the TiO₂ particle and polymer chain. Free volume increases at the interface between particle and polymer.²⁶ The decrease in dielectric constants with increasing free volume is the origin of electronic polarizations.²⁷ On the other hand, the increase in the dielectric constants with increasing free volume is the origin of the dipole orientation.^27,28 Anomaly in a decrease of dielectric permittivity for 75 mg TiO₂ content can be explained by this phenomenon; 75 mg TiO₂-added composites have low free volume value with respect to 30 and 50 mg TiO₂-filled composites. In composites filled with 30 and 50 mg TiO₂, the homogeneity of the particles is stable. With the increase of the TiO₂ content in the PMMA matrix (75 mg), the particles are aggregated in the matrix and the homogeneity is degraded. The contribution of the metal oxide to the composite dielectric constants is limited by the agglomeration of the oxide particles in the polymer matrix. Thus, the maximum filler rate of TiO₂ to PMMA is around 5% (w/w).

The scaling parameters of the SS theory and the hole fraction

The experimental Presure-Volume-Temperature data for PMMA polymer for about 100 kg molar masses are taken from 29.7°C to 230.4°C temperature and from ambient to 200 MPa pressure by Zoller and Walsh.²⁹ The scaling parameters P*, V*, and T*, of the SS theory in the melt state from about 121°C to 220°C temperature and from ambient to 200 MPa pressure ranges are calculated as $V^{*} = 0.83574 {cm}^{3} / g$ , $T^{*} = 11, 845 K$ , and $P^{*} = 989.8 MPa$ with the total degrees of freedom 3c = s + 3 = 2520 as the structural flexibility parameter. Using these parameters, the average relative percentage error in specific volume is 0.053% and the maximum relative percentage error is 0.29%. Hence, the hole fraction can be computed both for the melt and for the glassy states. In the glassy state as a nonequilibrium state at 20°C, 40°C, 60°C, 80°C, and 100°C, the predicted hole fractions may be computed as 4.86%, 5.41%, 6.11%, 6.77%, and 7.46%, respectively, using the above-mentioned parameters in the liquid phase.

PALS results

The longest-lived component, o-Ps lifetime ( $τ_{3}$ ) and its intensity (I ₃)^18,30 are obtained from the analysis of the positron lifetime spectra. The o-Ps lifetime can serve as a measure of the hole-free volume size. In Table 2, o-Ps lifetime ( $τ_{3}$ ), its intensity (I ₃), and hole-free volume radius and free volume fraction as the PALS parameters are given in terms of temperature for the various weight of TiO₂ contents. Figure 7 shows a variation of the o-Ps lifetime (left vertical axis) with the corresponding hole-free volume(right vertical axis), and the o-Ps intensity as a function of temperature for PMMA with neat, 10, 30, 50, and 75 mg TiO₂ addendum. The o-Ps lifetime (or the hole-free volume) increases linearly with temperature, however, raising TiO₂ amount in PMMA does not result in an equal change of increase in the o-Ps lifetime. On the other hand, the o-Ps intensity, associated with the number of hole-free volumes, decreases up to about 60°C and then increases.

Table 2.

o-Ps lifetime ( $τ_{3}$ ), its intensity (I ₃), and free volume radius and fraction in terms of temperature for the various weight of TiO₂ additives.

T (°C)	Wt (mg)	τ ₃ (ns)	I ₃	R (Å)	f_υ (%)
20	0	2.11 ± 0.02	10.4 ± 0.3	2.94 ± 0.02	4.7 ± 0.2
	10	2.35 ± 0.03	10.8 ± 0.3	3.15 ± 0.02	6.1 ± 0.2
	30	2.15 ± 0.02	10.9 ± 0.3	2.98 ± 0.02	5.2 ± 0.2
	50	2.01 ± 0.02	12.0 ± 0.3	2.85 ± 0.01	5.0 ± 0.2
	75	2.09 ± 0.02	11.4 ± 0.3	2.93 ± 0.02	5.1 ± 0.2
40	0	2.26 ± 0.01	10.1 ± 0.2	3.07 ± 0.01	5.3 ± 0.1
	10	2.46 ± 0.03	10.3 ± 0.3	3.24 ± 0.02	6.3 ± 0.3
	30	2.30 ± 0.03	10.3 ± 0.3	3.11 ± 0.02	5.6 ± 0.2
	50	2.09 ± 0.01	11.8 ± 0.2	2.93 ± 0.01	5.3 ± 0.1
	75	2.15 ± 0.02	11.1 ± 0.2	2.98 ± 0.01	5.3 ± 0.2
60	0	2.46 ± 0.01	9.8 ± 0.2	3.24 ± 0.01	6.0 ± 0.2
	10	2.57 ± 0.03	9.8 ± 0.3	3.32 ± 0.02	6.5 ± 0.3
	30	2.36 ± 0.03	10.3 ± 0.3	3.16 ± 0.02	5.9 ± 0.2
	50	2.13 ± 0.01	11.7 ± 0.2	2.97 ± 0.01	5.5 ± 0.2
	75	2.27 ± 0.01	10.9 ± 0.1	3.08 ± 0.01	5.7 ± 0.1
80	0	2.63 ± 0.02	9.9 ± 0.2	3.37 ± 0.01	6.8 ± 0.2
	10	2.63 ± 0.03	10.1 ± 0.3	3.37 ± 0.01	7.0 ± 0.3
	30	2.51 ± 0.03	10.3 ± 0.3	3.27 ± 0.02	6.5 ± 0.3
	50	2.23 ± 0.02	11.9 ± 0.2	3.05 ± 0.01	6.1 ± 0.2
	75	2.39 ± 0.03	10.8 ± 0.2	3.18 ± 0.02	6.3 ± 0.2
100	0	2.70 ± 0.03	10.6 ± 0.2	3.41 ± 0.02	7.6 ± 0.3
	10	2.80 ± 0.03	9.5 ± 0.3	3.49 ± 0.02	7.2 ± 0.3
	30	2.61 ± 0.03	11.0 ± 0.3	3.35 ± 0.02	7.5 ± 0.3
	50	2.32 ± 0.02	12.1 ± 0.2	3.12 ± 0.01	6.7 ± 0.2
	75	2.47 ± 0.02	11.2 ± 0.2	3.25 ± 0.01	6.9 ± 0.2

o-Ps: ortho-positronium.

Figure 7.

The o-Ps lifetime with the corresponding hole-free volume and intensity versus temperature for some different weight of TiO₂ added to PMMA.

Similarly, at 20°C, 60°C, and 100°C, the o-Ps lifetime and intensity are plotted with respect to TiO₂ content in Figure 8. Increasing the TiO₂ content, the o-Ps lifetime (or the hole-free volume) increases at 10 mg TiO₂ additive, and then it has a decrease up to 50 mg and a slight increase at 75 mg. Unlike the lifetime, the change of intensity (I ₃) behaves opposite to the lifetime. The decrease in the intensity may suggest a decrease in the holes’ numbers with increasing TiO₂ additive since the additives occupy the preexisting free volume holes in the PMMA structure. On the other hand, the free volume hole size increases since the size of the TiO₂ is much larger than the holes.

Figure 8.

The o-Ps lifetime and intensity versus weight percentages of TiO₂ added to PMMA for some temperatures of 20°C, 60°C, and 100°C.

The free volume fraction, $f_{v}$ , linearly related to the intensity and the free volume, is given by^18,30

f_{v} = A I_{3} υ_{f} (τ_{3})

where A is a constant. Using the values of hole fraction at 20°C, 40°C, 60°C, 80°C, and 100°C calculated by means of the SS theory and then equating them to equation (2) with $υ_{f} (τ_{3})$ in Å³, we may find A = (4.30 ± 0.15)10⁻³ Å⁻³ on the simultaneous fitting of both data. Free volume fraction of neat PMMA with respect to temperature is plotted in Figure 3. The solid line is the free volume (hole) fraction calculated from the SS theory in the melt and glassy states, and the red dots are calculated from experimental PALS data using equation (2).

In Figure 10, the free volume fraction ( $f_{v}$ ) is plotted with respect to the TiO₂ content for all the temperature considered. The free volume fraction increases with temperature since the increasing temperature has an effect on the structural expansion of the free volume, and thus an increase of the o-Ps lifetime. Neat PMMA in all the temperature range considered is in the glassy state so that the free volume fraction is not in equilibrium as depicted in Figure 9. However with respect to the weight percentages, there is a bump with a highest free volume fraction at 10 mg TiO₂ addition, and then the free volume fraction decreases down to 50 mg addition, but at 75 mg TiO₂ addition, there is again a slight increase for all the temperature range. As seen from Figure 8, the increase of the free volume fraction at 10 mg case is not in favor of creating new free volume holes, but increasing the size of the free volume holes together with combining some free volume holes. In this case, the free volume fraction increase has a tendency for the orientation of main polymer as to a higher disorder as confirmed by the X-ray analysis shown in Figure 4. On the other hand, TiO₂ additives at 30 and 50 mg cause a decrease in free volume fraction occupying the preexisting holes, due to the free volume hole size to decrease even though the number of holes has an increase. In this case, the free volume fraction decrease has a tendency for the main polymer to reorient itself to a higher order as confirmed by the X-ray analysis in Figure 4. However when we increase TiO₂ content to 75 mg, we observe agglomeration of TiO₂ so that the main polymer loses its orientation and tends to amorphous form.

Figure 9.

Free volume fraction of PMMA with respect to temperature is plotted. The solid line is the free volume fraction calculated from the SS theory. The red dots are calculated from experimental PALS data using equation (2).

Figure 10.

The free volume fraction versus TiO₂ added to PMMA for all the temperatures considered.

Conclusions

In this study, optical and dielectric properties of PMMA/TiO₂ composites were investigated by selecting filler contents to be as 10, 30, 50, and 75 mg (<7 wt%). Since aggregation of fillers prevents the contributions to the composites, ultrasonic agitation was used for the TiO₂ particles in chloroform. Four hours was selected as the best time for this filler masses. It was also selected low mixing duration (30 min) and speed (60 r min⁻¹). Although the thick films were selected to investigate the optical properties of composites, in the UV region, 450% improvement of UV absorption was achieved in the PMMA/TiO₂ composite according to neat PMMA. Dielectric permittivity was increased with an increase of filler concentrations. It was observed 25% improvement for dielectric permittivity with respect to neat PMMA.

Low free volume values in composites were associated with the doping rate and particle agglomeration and no contribution to dielectric constants was observed above 5% TiO₂ doping.

As a result, desired improvements in optical and dielectric properties of the PMMA/TiO₂ composites can be achieved by appropriate ultrasonic agitation process, low fillers contents, and low mixing speeds. Therefore, such hybrid materials can be used as a gate dielectric in organic field-effect transistors with desired dielectric permittivity or UV shielding materials with high absorbance in UV region and low transmission loss in the visible region.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

K Esmer

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Structural and dielectrical properties of PMMA/TiO 2 composites in terms of free volume defects probed by positron annihilation lifetime spectroscopy