Study of interfaces in polymer-metal oxide films and free-volume hole using low-energy positron lifetime measurements

Journal of Science: Advanced Materials and Devices 4 (2019) 413e419

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Original Article

Study of interfaces in polymer-metal oxide films and free-volume hole
using low-energy positron lifetime measurements
Aman Deep Acharya a, b, Bhawna Sarwan a, b, *, Ratnesh Sharma a, S.B. Shrivastava a,
Manoj Kumar Rathore c
a
b
c

Vikram University, Ujjain, 456010, MP, India
Lovely Professional University, Jalandhar, Punjab, India
M.P. Council of Science and Technology, Bhopal, India

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 5 February 2019
Received in revised form
28 July 2019
Accepted 10 August 2019
Available online 16 August 2019

To reveal how the distribution of different nano fillers affect the UV-shielding efficiency of their polymerbased composites and to further develop a simple strategy to refrain the erection of the composites, we
prepared ZnO doped polystyrene (PS/ZnO) and TiO2 doped polystyrene (PS/TiO2) films by the solution
cast technique with different concentrations of ZnO and TiO2 (0.25%, 0.5%, 0.75% and 1%.). Contrary to the
common observation, the better tunability for UV shielding efficiency was found in the case of TiO2 as
compared to ZnO. This is mainly due to the appearance of a rod like structure on neat PS which has
improved the dispersion as well as provides a higher interface area that enhanced the UV-absorption
efficiency of the PS matrix. This analysis is equally supported by the PALS study where the free volume was closely associated with the interfacial interaction between the filler and the PS matrix. These
observations recommend that the better dispersion of filler particles leads a stronger interfacial interaction and enhances the UV-protection efficiency of the composite materials.
© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
Polystyrene thin films
ZnO
TiO2
Positron annihilation
Free volume hole
Interfacial interaction
Solution cast method

1. Introduction
Previously, our group has successfully prepared TiO2/PS films
with concentrations up to 1 wt % by the solution cast method in the
development of photo-protective polymeric materials for the protection against ultraviolet radiation. Interestingly, the as-prepared
thin films have shown a tremendous UV-shielding proficiency
[1,2]. These results suggested to pursue a further study on the relationships among the atomic free volume and the interfacial
interaction between the filler particles and the PS matrix. Contrary
to the common observations where numerous approaches have
been made for the development of ZnO doped composites as a
better UV protective material, in our previous study TiO2 has

expressed a better harmony for the efficient UV shielding which we
will incorporate in the present work as a comparative study. To
analyze the imperfections produced at the early stage of the

* Corresponding author. Lovely Professional University, Jalandhar, Punjab, India.
E-mail addresses: (A.D. Acharya), sarbhawna@
gmail.com (B. Sarwan).
Peer review under responsibility of Vietnam National University, Hanoi.

process in engineering materials it is important to predict the
weakness and failure of the material. This work is consequential for
the final understanding of the UV-shielding efficiency by
comparing its results with those of a widely studied material as
ZnO. The very extensively studied inorganic materials ZnO and TiO2
with a wide band-gap energy of 3 eV have been expansively used as
inorganic UV absorbers due to their significant optical properties
[3]. Consequently, such polymer nanocomposites have been
regarded as excellent candidates for UV shielding applications. As a
matter of fact, the extraordinary properties of the polymer nanocomposite include the dispersion of the nanoparticles in the matrix
and the subsequent growth of enormous interfacial areas. This
complete dispersion allows the exploration of the available
matrixeparticle interface and then the optimization of the
organiceinorganic interaction which is accountable for the
improved properties of the final material. Nevertheless, there have
been less reported on the research efforts in this area especially
those dealing with the effect of the free volume hole and the
interfacial interaction on the UV-shielding efficiency [4e6]. Moreover, most of the works have adopted higher dopant concentrations
to conquer a better UV-shielding efficiency of the films [4,7e10]

/>2468-2179/© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license

( />

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A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419

whereas low content dosaging was constantly disregarded causing
a shortage of understanding of the mechanism of the UV-shielding
enhancement. It would therefore be interesting to find out at which
doping concentration the composite film starts to absorb the UV
radiation to a significantly large extent. In this contribution, we
have prepared ZnO/PS and TiO2/PS films with dopant concentrations up to 1 wt % by the solution cast method. This is a green as
well as simple method. It is also useful in dissolving the polymers to
have the material as finely divided as possible and to have each
particle thoroughly wetted by the solvent. Numerous techniques
have been employed to analyze the impact of the interfacial
interaction on the thermal conductivity, the thermal expansion, the
viscoelastic properties and the density [11,12]. In view of the applications the technique employed should have a strong influence
on the estimation of the free volume. The quantitative depiction of
the free volume properties of polymers in the amorphous state can
be accomplished by the use of the positron annihilation lifetime
spectroscopy technique (PALS). This technique involves the insertion of positrons into the material and then recording the individual positron lifetimes until the annihilation with electrons of the
sample takes place [11,12]. Since the fraction of the positrons annihilates from the state of an orthopositronium (o-Ps) and the
lifetime of the orthopositronium depends on the size of the free
volume cavity where it is placed, hence, it can be employed to
characterize the free volume size in amorphous polymers.
Following the above described study concern, the main motivation
of the present work is to investigate what would be the effects of
doping ZnO and TiO2 into PS on the atomic free volume, the
interfacial contact among the filler particles and the PS matrix, and

the UV-absorption efficiency of PS at low content of fillers by the
positron annihilation lifetime spectroscopy.
2. Experimental
ZnO/PS and TiO2/PS thin films with different concentrations viz
0.25%, 0.5%, 0.75% and 1% have been prepared by using the solution
casting method. The polystyrene was procured from the market
which was in the granular form. The PS solution was prepared in
the dichloromethane, the requisite amount of semiconductors (ZnO
or TiO2) was then added into the solution under rapid stirring for
uniform dissolution. The resultant was then poured on to a cleaned
petri dish to cast the film and the solvent was subsequently allowed
to evaporate gradually over a period of 12e24 h in a dry atmosphere. The membrane was then physically peeled off from the
surface. The area of the cast surface, the material quantity and the
density of the material can determine the thickness of the membrane. We have prepared the polymer thin films of ~50 mm thickness. For preparing the doped PS thin films, the dopant
concentration was calculated from the following equation [1,13].

Wðwt%Þ ¼

wf
 100
wp þ wf

where,wf and wp represent the weight of the dopant and the
polymer, respectively.
X-ray diffraction patterns of ZnO/PS and TiO2/PS thin films were
recorded on an X-ray diffractometer (Bruker D8 ADVANCE) with
Cu-Ka radiation having the wavelength of 1.5418 Å in the range of
2q ¼ 200 - 700. Atomic force microscopy (AFM) measurements were
carried out on a digital instrument of Nanoscope E with the Si3N4
100 mm cantilever and 0.58 N/m force constant. The transmittance

of the films has been measured with a UV-Vis Spectrophotometer
(PerkinElmer Lambda 950). Measurements of the positron lifetime
in ZnO/PS and TiO2/PS thin films have been done by using the slow
e fast coincidence method.

Positron lifetime experiments are capable of distinguishing
different kinds of defects. As a spectroscopic technique the positron
annihilation spectroscopy is a sensitive tool for the study of openvolume type defects which include vacancies, vacancy agglomerates, and dislocations. A conventional fast-slow coincidence spectrometer was used to carry out the positron lifetime measurements
having a resolution of 280 ps (FWHM) for Na22 source. The Na22
source with strength of 20 mCi was deposited onto a nickel foil with
the thickness of 1 mg/cm2 and sandwiched between two similar
pieces of the sample. Nearly 105e106 counts were recorded in the
PAL spectra for each sample. Standard PATFIT program was used to
analyze the positron lifetime spectra with their relative intensities.
3. Results and discussion
3.1. Structural and surface analysis
The XRD patterns of the films are shown in Fig. 1. As it is
perceived from the XRD patterns the pure PS film (Fig. 1 a,b) shows
an amorphous polymeric structure and the diffraction peaks of PS
do not appearein the patterns. The pattern of the PS thin films
loaded with 0.5 wt% TiO2 and ZnO (Fig. 1 a,b), however, shows
diffraction peaks of low intensity suggesting improved crystalinity
of the PS. At the higher filler content, the peak positions of the 1 wt
% sample is slightly shifted towards lower diffraction angles. The
most likely reason for this shift is the interaction between the filler
particle and the polymer structure that leads to a rearrangement of
the PS chains (See Fig. 1a and b (Inset)). The increased intensity of
the reflections from the diffracted planes with the higher amount of
filler loadings suggests that a slowering of the crystallization rate
arrised due to the enclosure of filler particles. It can be concluded

that a suitable, but not excessive, amount of dopant is responsible
for observed good dispersion of the inorganic filler particles in the
PS matrix.
To get a further insight, we extended our approach to another
important analysis using the atomic force microscopy (AFM) of the
doped polymer samples. Figs. 2 and 3 show the AFM images of thin
sections of the ZnO/PS and TiO2/PS composite surfaces loaded with
the dopant content of 0.25, 0.5, 0.75 and 1.0 wt %. It can be observed
from AFM images (Fig. 2) of PS/ZnO that the grain size increases
with the increase in the ZnO concentration upto 0.5 wt% (See
Fig. 2c) leading to the aggregation. The addition of ZnO particles at
about 0.75 wt % does not encourage the faster crystallization (See
Fig. 2d). In case of the 1.0 wt % sample (See Fig. 2e), the efficiency of
ZnO for enhancing the matrix crystallization get reduced due to the
high particle density and obstructed the development of crystalline
sections. This illustrates that the small amount of ZnO particles i.e.
0.5 wt% located in the PS matrix corresponds to the primary particles and the extent of the agglomeration was found to be quite
negligible. The PS matrix having 0.75 wt% and 1.0 wt% ZnO particles
changed to large size aggregates where several primary ZnO
nanocrystallites were gathered. Furthermore, the entire
morphology was deformed when 1.0 wt% ZnO was employed.
By comparing the AFM images, an obvious difference can be
seen between the neat PS and the TiO2/PS film. Fig. 3a shows an
AFM image of thin sections of the TiO2/PS thin film at the dopant
content of 0.25 wt %. It signifies that the TiO2 particles were setup in
the form of aggregates of slackly linked paramount particles
showing areas which are homogeneously implanted in the PS
matrix. After addition of 0.5 wt% TiO2 into the matrix some rods
appeared (See Fig. 3c) on the surface of the neat PS presuming that
the formation of these rods mainly depends on the growth and

nucleation conditions. Moreover, a fractal type of aggregation of
TiO2 particles has been observed in Fig. 3d, such situation may arise
due to the high concentration of nucleates that were formed by


A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419

415

Fig. 1. X-ray diffraction patterns: (a) ZnO/PS and (b) TiO2/PS thin films.

Fig. 2. AFM images of the ZnO/PS film.

adding the 0.75 wt% filler content. Moreover, looking into Fig. 3e,
the nucleates randomly agglomerate in the continuous phase and
cause the increase of the number of TiO2 particles, thus, making the
interface area larger and the overlap of these led to opaquely
appearing TiO2/PS films [9,13]. This area is notably higher than that
of the 0.5 wt% TiO2/PS films. This observation suggests that the
0.75 wt% TiO2/PS films have large particle agglomerates, while the
0.5 wt % TiO2/PS films have an improved dispersion as well as a
higher interface area and therefore exhibit a higher UV- absorption
efficiency. From this analysis, it may be inferred that to speed up the
matrix crystallization and for altering the synthesized nanostructure morphology, a low concentration of dopant as such of

0.5 wt % is enough. Herewith, the AFM results suggest that the
inorganic semiconductor particles were well incorporated in the
PS, which consequently modify significantly the morphology of the
PS films.
3.2. ZnO/PS and TiO2/PS UVevis shielding

The transmittance characteristics of the pure PS, the ZnO/PS and
TiO2/PSfilms are visualized in Fig. 4. It is found that almost 99% of
the light was passed-on by the pure PS in the UVevis region of
wavelengths from 300 to 700 nm. As shown in Fig. 4a, the
maximum value of transmittance of the ZnO/PS films containing


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Fig. 3. AFM images of the TiO2/PS film.

Fig. 4. The transmittance spectra of (a) ZnO/PS and (b) TiO2/PS films.

0.25 wt % ZnO was found as pretty as 95%. By a careful consideration, it can be seen that the continuous inclusion of ZnO induces a
systematic decline of the transmitted light, lowering the transmittance. The transparency is also dependent on the dispersion/
aggregation of the nanoparticles into the polymer matrix. The
fractal distribution of discretely dispersed nanoparticles favors the
optical transparent intensity loss of the transmitted light because
the scattering abruptly rises with the particle size. This causes a
significant drop in the transparency of the films [10,14]. In line with
this, the gradual decrease in the visible-light transmission from 95
to 70% in the films containing 0.25e1 wt % ZnO was observed and
highlighted by the shaded area in Fig. 4a. The thin films with 1 wt %
ZnO dopant shows a non-uniform distribution of the ZnO particles
within the polymer matrix. This could be endorsed by the AFM
results (Fig. 2) where no substantial ZnO agglomerations were

found. The obtained experimental results provide a visual illustration to the UV-shielding effect in ZnO/PS. When ZnO/PS film is

irradiated with the incident radiation, the visible light perfectly
passes through the material as ZnO particles are apparent for the
wavelengths greater than 375 nm while the UV-spectrum is
obstructed depending on the ZnO concentration. For the reason
that the ZnO nanoparticles build a physical obstacle that the UV
light cannot cross since they act as a protective network. When the
dopant concentration is further increased, the scattering mean free
path gets decreased. Due to this reason, the light traveled strongly
inside the film with increased obstacle leading to the reduction in
UV-light transmission to 63% with 1 wt% ZnO concentration (see
Fig. 4a).
The UVeVis transmittance of the TiO2/PS film is plotted in
Fig. 4b. High transparency in both the visible and UV region is


A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419

Fig. 5. Plots of (a∙h∙y)

2

417

v/s photon energy (h∙y): (a) ZnO/PSand (b) TiO2/PS thin films.

observed in the pure PS film (see Fig. 4b), which is not competent to
filter out the UV radiations, whereas the addition of TiO2 content
leads to the increase in the UV shielding efficiency due to the empty
conduction band and the filled valence band. However, the UV
blocking consequence is seen in the films with TiO2 contents as low

as 0.25 wt%, while the high transparency in the visible range is
maintained. The concentration of 0.5 wt % TiO2 could be assumed as
the optimal one for the better UV shielding effect as evidenced by
the graphical situation in the region bellow 355 nm, where more
than 70% transparency is observed. This evidences that the introduction of TiO2 particles into the PS matrix is compatible to increase the UV protecting proficiency of the PS film in the region
from 300 to 355 nm. The further increment of TiO2 (0.75 wt %)
results in the opaque appearance with the increased absorption in
both the visible and UV region. In this state, the apparent nature of
the material as a UV filter is decreased. This behavior can be
interpreted by the fact that the increased amount of TiO2 enhances
the interface scattering causing the reduction in the transmittance.
This reduction might be ascribed to the growing cluster size [6]. In
addition, the cluster size of the film becomes more non-uniform,
and irregular with the increasing TiO2 content up to 1 wt% leading to the reduction in the transmittance as it is confirmed on the
AFM images (Fig. 3e) of the composite films. Here, the shape of the
PS latex is almost demolished and then totally vanished because of
the interdiffusion process between the polymer chains. From this
result, it might apparently be easy to load the interstices of the thin
PS template with a low concentration of dopant, but it is difficult to
fill the interstices at a higher concentration. So, the dopant content
can be considered as a key parameter for the permeation of the PS
templates [15].

The band gap energies (Eg values) of the ZnO/PS and TiO2/PS
films could be estimated from a plot of (ahn)2 vs. the photon energy
(hn) in Fig. 5a,b. Band gap values of 3.00, 2.47 and 2.61 eV were
obtained for the pure PS, ZnO/PS and TiO2/PS films, respectively (for
the optimum content, i.e. 0.5%). However, two different mechanisms are accounted for the variation in the calculated optical band
including: (1) The inclusion of a tiny amount of dopant produces
charge transfer complexes in the host matrix which accelerate the

electrical conductivity by providing additional charges which cause
the reduction of the band gap [18,19]; (2) When the amount of
dopants increases, the dopant molecules initiate to linking the gap
between the localized states and thus lowering the potential barrier between them [16e22].
3.3. Positron annihilation lifetime studies
The measurement for the positron annihilation lifetime studies
(PALS) was carried out to examine the effect of TiO2 and ZnO on the
microstructure of the composite films. The positron lifetime spectra
of the pure PS,ZnO/PS and TiO2/PS films, respectively, are presented
in Fig. 6 . They show a systematic decreasing trend of the lifetime.
This indicates a decrease in the longest lifetime.
The free volume size (Vf), and the o-Ps lifetime (t3) as a function
of the TiO2 and ZnO content are shown in Fig. 7aeb, respectively.
From Fig. 7a, it can be observed that the t3 and Vf initially drop with
the TiO2 incorporation upto 0.5 wt%. In the range from 0.5 to
0.75 wt%, a slight increase in t3 and Vf is seen. They finally decrease
to the lowest value at higher doping, i.e. at 1 wt%. Looking again into
Fig. 7a, there is a decrease in t3 and Vf with the increasing TiO2
concentration (0.25e0.5 wt %) indicating that the additional

Fig. 6. Positron lifetime spectra: (a) ZnO/PSand (b) TiO2/PS thin films.


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Fig. 7. The PALS results: o-Ps lifetime t3 and free volume size Vf of (a) TiO2 (b) ZnO.

amount of TiO2 slows down the o-Ps formation. This can be

explained by the fact that firstly the TiO2 particles fill up some of
the free volume holes in the PS and so the values of t3 and Vf
decrease. Secondly, positrons may be annihilated from the TiO2
filler and there may be a lack of positrons which should be available
to form the positronium in PS [12]. On the other side, the increase of
o-Ps at the dopant concentration of 0.75 wt% TiO2 suggests the
formation of new positron trapping sites at the TiO2ePS interface.
As the filler concentration is increased to that corresponding to the
1 wt% concentration, the TiO2 filler inhibits the o-Ps formation and
the filler particles are scattered among the molecular chains of the
PS and thus reducing the free volumes size leading to the decrease
of the o-Ps lifetime in the PS. Quite the reversal, a small but systematic increase in the free volume size and in the o-Ps lifetime has
also been initially observed in the case of the low ZnO doping (i.e.
0.25e0.5 wt%) (see Fig. 7b). This is because of the development of
new positron trapping sites at the ZnO/PS interface. The highest
values of t3 and Vf have been found for the 0.5 wt% ZnO/PS film,
whereas when we have increased the ZnO concentration upto 1 wt
%, the values of t3 and Vf decrease showing that some of the free
volume holes in the PS are filled up by the ZnO particles. It is
interesting to note that the interfacial interaction between the filler
and the polymer matrix has caused a vital effect on the free volume
size and the o-Ps lifetime. This interaction dominates the delivery
of phonons between the matrix and the fillers [23e25] mainly at
0.5 wt% ZnO concentration where both the free volume size and the
o-Ps lifetime reach their maximum. We recall the main fact that the
film with a low dopant amount ZnO represents a high surface area,
thus, providing more positron trapping sites which scatter the
phonons at the interface [26e28]. However, at the high ZnO concentrations, the ZnO agglomerates and due to this interfacial
interaction, the induced disruption effect becomes limited,
reducing the free volume size and the o-Ps lifetime.

3.4. The correlation of PALS results and UV-shielding
From the calculated results of the PALS and the morphological
studies, the UV-shielding efficiency of the ZnO/PS and TiO2/PS films
could be clearly understood. From the PALS results of TiO2/PS, it is
noticed that the free volume hole size and the o-Ps lifetime initially
drop with the TiO2 incorporation. It might be an evidence for the
gradual formation of neutral aggregates at the initial level of filler
concentrations which creates blockages and reduces the free volume holes, enhances the crystallization of the matrix as it was
clearly confirmed by the AFM results. This decrease in the free
volume holes may also contribute to the increase in the UV
shielding effect. Furthermore, this factor reduces the ion and the
segmental mobility through the unified matrix and hence, leads to
the reduction of the free volume size. At the high filler

concentrations, a random distribution of filler particles might
initiate the formation of free volume holes in the PS matrix. This
process of the free volume formation gradually dominates the
creation of neutral aggregates which fairly agrees with the AFM
results and confirms the transition from the crystalline state to the
amorphous one at higher TiO2 concentrations where the regretable
interaction between the loaded TiO2 particles and the PS matrix
have slight limitations on the segmental mobility because of less
contact area and so contributing to the increase in t3 and Vf as
shown in Fig. 7a. An explanation based on the PALS results and
detailed literature survey [27e29] implies that the o-Ps mainly
annihilates in the interfacial regions. In fact, there is some information indicating that the interfacial free volume is a vital factor for
determinating the variation in the o-Ps annihilating lifetime
because the interfaces have an excellent electronic density
compared to the bulk phase.
It is worth noting that in the case of ZnO/PS, the initial increment in t3 with increasing ZnO concentration upto 0.5 wt% (see

Fig. 7b) suggests the formation of the free volume and amorphous
phases in the blend matrix due to the sufficient separation between the filler particles at low filler concentrations. Our tentative
elucidation is that the dispersion of ZnO particles can cause the
disorder of the molecular configuration leading to the morphological change in chains and thus increase in the free volume
concentration and the o-Ps lifetime. The above discussion corroborates that the better the dispersion of the lifetime the stronger
will be the distraction of the molecular morphology. On the other
hand, as the ZnO content further increases, this distraction of ZnO
weakens as being caused by the aggregation of ZnO leads to the
decrease in the free volume concentration and the o-Ps lifetime.
The analysis also confirms that the free volume hole decreases
with the increased filler concentration, because the filler limits the
moving space of the molecular chains.
From the above explanation it can be clear that the calculated
free volume hole size does not show a drastic variation with the
ZnO content, but revealing a very little amount being adequate to
accelerate the matrix crystallization which assures a less tunability
for the UV radiation. This could be probable because an excessive
amount of ZnO can obstruct the formation of well crystallized regions and lead to the turbidity/translucency of the composite materials. This illustrates that the particle size of the ZnO in the PS
matrix corresponds to that of the primary particles and the extent
of the agglomeration is moderately negligible, whereas in the case
of TiO2 particles the reduction in the free volume hole size leads to
the increase in the UV shielding effect. It is evident that the more
exposure of the PS to the UV causes the increase in the size of the
free volume hole while the hole density remains unchanged. The
destruction of the PS matrix sets in when TiO2 is added leading to
the formation of voids in the region of the TiO2 particles aggregates.


A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419

This is attributed to the desorption and dispersion of the active
oxygen species produced on TiO2 surface engraving the polymer
matrix. Furthermore, the rod shaped aggregation of the TiO2
nanoparticles (as shown in Fig. 3c) acting as the nucleating agents
leads to the increase in the UV shielding. Moreover, doping with
TiO2 results in a considerable increase in grain size due to the rod
shape aggregation that leads to the reduction of the grain boundary
scattering and this enhances the UV absorption and increases the
visible transparency of the films.
To the best of our knowledge, the correlation between the UVabsorption behavior and the free volume hole was for the first
time scrutinized. Our experimental results clearly show that the
PALS are useful to understand the UV-absorption efficiency of
doped polymer film mixtures.
4. Conclusion
The present research has explored the potential enhancement of
polymer's UV-shielding properties. The attention of our study has
been focused on: (i) the analysis of properties of the atomic free
volume defect, (ii) the filler-PS interfacial interaction and (iii) its
impact on the UV-protecting adequacy of the films which have
been investigated and examined by PALS. Concussively, the shrink
of free volume hole size due to the high filler concentrations is a
supporting positron lifetime parameter. The calculated value for
the free volume hole size does not show any dramatic disparity
with the high ZnO concentrations revealing the less tunability of
the material for the UV radiation. In the case of the TiO2 particles,
however, the decrease in the free volume hole size has been
observed because of the rod shaped aggregation of the TiO2 particles which act as nucleating agents contributing to the UV shielding
efficiency of the PS. The results as obtained suggest that TiO2 and
ZnO acting as active fillers for PS can be used for improving the
tremendous photo-protective shielding quality of the polymeric

materials to be applied in ultraviolet radiation protection. Howbeit,
due to the surface free energy of the nanocrystals, ZnO particles
tend to aggregate making them obscured to attain a homogeneous
dispersal and that results in opaque composite films. Therefore, the
main efforts should be focused on the nanocrystals without aggregation in the PS.
Acknowledgements
The authors are grateful to Dr. Y.K. Vijay (Prof.) at University of
Rajasthan, Jaipur for providing the experimental facilities to study
the positron lifetime and Dr. Balram Tripathi for helping in the
analysis of the experimental data. The authors also thank the
anonymous reviewers for their extremely insightful comments. We
would like to convey our excellent gratitude to Prof. Nguyen The
Hien Vietnam National University, Hanoi, Vietnam for the language
editing and formatting of the article. The financial support from
MPCST Bhopal is gratefully acknowledged.
References
[1] A.D. Acharya, Positron annihilation characterization of TiO2 doped polystyrene, Mater. Sci. Forum 771 (2014) 169e177.
[2] A.D. Acharya, B. Sarwan, R. Sharma, S. Moghe, S.B. Shrivastava, V. Ganesan,
UV-shielding efficiency of TiO2-polystyrene thin films prepared by solution
cast method, J. Phys. Conf. Ser. 836 (2017) 012048.
[3] A.D. Acharya, S. Moghe, R. Panda, S.B. Shrivastava, M. Gangrade, T. Shripathi,
D.M. Phase, V. Ganesan, Effect of Cd dopant on electrical and optical properties
of ZnO thin films prepared by spray pyrolysis route, Thin Solid Films 525
(2012) 49e55.

419

[4] V.S. Sangawar, M.C. Golchha, Evolution of the optical properties of Polystyrene
thin films filled with Zinc Oxide nanoparticles, I.J.S.E. Res. 4 (2013)
2700e2705.

[5] W. Zhou, J. Wang, Z. Gong, J. Gong, N. Qi, B. Wang, Investigation of interfacial
interaction and structural transition for epoxy/nanotube composites by
positron annihilation lifetime spectroscopy, Appl. Phys. Lett. 94 (2009)
021904.
[6] S. Sharma, J. Prakash, K. Sudarshan, P. Maheshwari, D. Sathiyamoorthy,
P. Pujari, Effect of interfacial interaction on free volumes in phenolformaldehyde resine carbon nanotube composites: positron annihilation lifetime and
age momentum correlation studies, Phys. Chem. Chem. Phys. 14 (2012)
10972e10978.
[7] C. Pirlot, I. Willems, A. Fonseca, J.B. Nagy, J. Delhalle, Preparation and characterization of carbon nanotube/polyacrylonitrile composites, Adv. Eng.
Mater. 4 (2002) 109.
[8] Y. Wang, T. Li, P. Ma, H. Bai, Y. Xie, M. Chen, W. Dong, Simultaneous enhancements of UV-shielding properties and photostability of poly(vinyl
alcohol) via incorporation of sepia eumelanin, ACS Sustain. Chem. Eng. 4
(2016) 2252e2258.
[9] X. Meng, Z. Zhang, N. Luo, S. Cao, M. Yang, Transparent poly(methyl methacrylate)/TiO2 nanocomposites for UV shielding applications, Polym. Sci. A 53
(2011) 977e983.
[10] E. Lizundia, L.R. Rubio, J.L. Vilas, L.M. Leon, Poly(L-lactide)/ZnO nanocomposites as efficient UV-shielding coatings for packaging applications,
Appl. Polym. 133 (2016) 1e7.
[11] L. Khounlavong, V. Ganesan, Influence of interfacial layers upon the barrier
properties of polymer nanocomposites, J. Chem. Phys. 130 (2009) 104901.
[12] J. Algersa, R. Suzukib, T. Ohdairab, F.H.J. Maurer, Characterization of free
volume and density gradients of polystyrene surfaces by low-energy positron
lifetime measurements, Polymer 45 (2004) 4533e4539.
[13] B. Scrosati, B.V.R. Chowdari, S. Radhakrishna, Solid States Ionic Devices, World
Scientific Publishing Co, 1988.
[14] H. Althues, J. Henle, S. Kaskel, Functional inorganic nanofillers for transparent
polymers, Chem. Soc. Rev. 36 (2007) 1454.
[15] M. Selin Sunay, O. Pekcan, S. Ugur, The effect of film thickness and TiO2
content on film formation from PS/TiO2 nanocomposites prepared by dipcoating method, J. Nanomater. (2012) 1e17.
[16] J. Rozra, I. Saini, A. Sharma, N. Chandak, S. Aggarwal, R. Dhiman, P.K. Sharma,
Cu nanoparticles induced structural, optical and electrical modification in

PVA, Mater. Chem. Phys. 134 (2012) 1121e1126.
[17] C.U. Devi, A.K. Sharma, V.V.R.N. Rao, Electrical and optical properties of pure
and silver nitrate-doped polyvinyl alcohol films, Mater. Lett. 56 (2002)
167e174.
[18] G. Fussell, J. Thomas, J. Scanlon, A. Lowman, M. Marcolongo, The effect of
protein-free versus protein-containing medium on the mechanical properties
and uptake of ions of PVA/PVP hydrogels, J. Bio. Sci. 16 (2005) 489e503.
[19] A. Nimrodh Ananth, S. Umapathy, On the optical and thermal properties of in
situ/ex situ reduced Ag NP's/PVA composites and its role as a simple SPRbased protein sensor, Appl. Nanosci. 1 (2011) 87e96.
[20] A.J. Marzocca, S. Cerveny, W. Salgueiro, A. Somoza, L. Gonzalez, Characterization of free volume during vulcanization of styrene butadiene rubber by
means of positron annihilation lifetime spectroscopy and dynamic mechanical
test, Phys. Rev. 65 (2002) 021801.
[21] R. Gulotty, M. Castellino, P. Jagdale, A. Tagliaferro, A.A. Balandin, Effects of
functionalization on thermal properties of single-wall and multi-wall carbon
nanotube polymer nanocomposites, ACS Nano 7 (2013) 5114e5121.
[22] D. Konatham, A. Striolo, Thermal boundary resistance at the graphene-oil
interface, Appl. Phys. Lett. 95 (2009) 163105.
[23] C. Liu, S. Fan, Effects of chemical modifications on the thermal conductivity of
carbon nanotube composites, Appl. Phys. Lett. 86 (2005) 123106.
[24] G. Xue, J. Zhong, S. Gao, B. Wang, Correlation between the free volume and
thermal conductivity of porous poly(vinyl alcohol)/reduced graphene oxide
composites studied by positron spectroscopy, Carbon 96 (2016) 871e878.
[25] F.H. Gojny, M.H.G. Wichmann, B. Fiedler, I.A. Kinloch, W. Bauhofer,
A.H. Windle, Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites, Polymer 47 (2006)
2036e2045.
[26] H. Im, J. Kim, Thermal conductivity of a graphene oxide carbon nanotube
hybrid/epoxy composite, Carbon 50 (2012) 5429e5440.
[27] U. Rana, P.M. Nambissan, S. Malik, K. Chakrabarti, Effects of process parameters on the defects in graphene oxide-polyaniline composites investigated by
positron annihilation spectroscopy, Phys. Chem. Chem. Phys. 16 (2014)
3292e3298.

[28] Y. Li, S. Wang, H. Wu, R. Guo, Y. Liu, Z. Jiang, Z. Tian, P. Zhang, X. Cao, B. Wang,
High-performance composite membrane with enriched CO2-philic groups and
improved adhesion at the interface, ACS Appl. Mater. Interfaces 6 (9) (2014)
6654e6663.
[29] C. Ohrt, K. Ratzke, N. Oshima, Y. Kobayashi, B.E. O'Rourke, R. Suzuki,
A. Uedono, F. Faupe, Free volume profiles at polymer solid interfaces probed
by focused slow positron beam, Macromolecules 48 (5) (2015) 1493e1498.