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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 [r]
(1)Original Article
Study of interfaces in polymer-metal oxidefilms and free-volume hole
using low-energy positron lifetime measurements
Aman Deep Acharyaa,b, Bhawna Sarwana,b,*, Ratnesh Sharmaa, S.B Shrivastavaa,
Manoj Kumar Rathorec
aVikram University, Ujjain, 456010, MP, India bLovely Professional University, Jalandhar, Punjab, India cM.P Council of Science and Technology, Bhopal, India
a r t i c l e i n f o Article history:
Received February 2019 Received in revised form 28 July 2019
Accepted 10 August 2019 Available online xxx
Keywords:
Polystyrene thinfilms ZnO
TiO2
Positron annihilation Free volume hole Interfacial interaction Solution cast method
a b s t r a c t
To reveal how the distribution of different nanofillers affect the UV-shielding efficiency of their polymer-based composites and to further develop a simple strategy to refrain the erection of the composites, we prepared ZnO doped polystyrene (PS/ZnO) and TiO2doped 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 case of TiO2as 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 vol-ume was closely associated with the interfacial interaction between thefiller and the PS matrix These observations recommend that the better dispersion offiller particles leads a stronger interfacial inter-action 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 (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Previously, our group has successfully prepared TiO2/PSfilms with concentrations up to wt % by the solution cast method in the development of photo-protective polymeric materials for the pro-tection 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 re-lationships among the atomic free volume and the interfacial interaction between thefiller 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
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 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 nano-composite 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 thefinal 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] More-over, most of the works have adopted higher dopant concentrations to conquer a better UV-shielding efficiency of thefilms [4,7e10] *Corresponding author Lovely Professional University, Jalandhar, Punjab, India
E-mail addresses:acharyaphysics2011@gmail.com(A.D Acharya),sarbhawna@ gmail.com(B Sarwan)
Peer review under responsibility of Vietnam National University, Hanoi
Contents lists available atScienceDirect
Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.08.003
(2)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 inser-tion of positrons into the material and then recording the indi-vidual positron lifetimes until the annihilation with electrons of the sample takes place[11,12] Since the fraction of the positrons an-nihilates 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 thefiller particles and the PS matrix, and the UV-absorption efficiency of PS at low content offillers by the positron annihilation lifetime spectroscopy
2 Experimental procedure
ZnO/PS and TiO2/PS thinfilms 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 thefilm and the solvent was subsequently allowed to evaporate gradually over a period of 12e24 h in a dry atmo-sphere 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 mem-brane We have prepared the polymer thinfilms of ~50mm thick-ness For preparing the doped PS thin films, the dopant concentration was calculated from the following equation[1,13]
Wwt%ị ẳ wf wpỵwf
100
where,wf and wp represent the weight of the dopant and the polymer, respectively
X-ray diffraction patterns of ZnO/PS and TiO2/PS thinfilms were recorded on an X-ray diffractometer (Bruker D8 ADVANCE) with Cu-Karadiation having the wavelength of 1.5418Åin the range of 2q¼200- 700 Atomic force microscopy (AFM) measurements were carried out on a digital instrument of Nanoscope E with the Si3N4 100mm cantilever and 0.58 N/m force constant The transmittance of thefilms has been measured with a UV-Vis Spectrophotometer (PerkinElmer Lambda 950) Measurements of the positron lifetime in ZnO/PS and TiO2/PS thinfilms have been done by using the slow efast coincidence method
3.1 Structural and surface analysis
The XRD patterns of the films are shown in Fig As it is perceived from the XRD patterns the pure PSfilm (Fig 1a,b) shows an amorphous polymeric structure and the diffraction peaks of PS not appearein the patterns The pattern of the PS thinfilms loaded with 0.5 wt% TiO2 and ZnO (Fig a,b), however, shows diffraction peaks of low intensity suggesting improved crystalinity of the PS At the higherfiller content, the peak positions of the wt % sample is slightly shifted towards lower diffraction angles The most likely reason for this shift is the interaction between thefiller particle and the polymer structure that leads to a rearrangement of the PS chains (SeeFig 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 offiller particles It can be concluded that a suitable, but not excessive, amount of dopant is responsible for observed good dispersion of the inorganicfiller 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 and 3show 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 (SeeFig 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 particlesi.e
0.5 wt% located in the PS matrix corresponds to the primary par-ticles 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
(3)adding the 0.75 wt%filler content Moreover, looking intoFig 3e, the nucleates randomly agglomerate in the continuous phase and cause the increase of the number of TiO2particles, thus, making the interface area larger and the overlap of these led to opaquely appearing TiO2/PSfilms[9,13] This area is notably higher than that of the 0.5 wt% TiO2/PSfilms This observation suggests that the 0.75 wt% TiO2/PSfilms have large particle agglomerates, while the 0.5 wt % TiO2/PSfilms 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 nano-structure 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 PSfilms
3.2 ZnO/PS and TiO2/PS UVevis shielding
The transmittance characteristics of the pure PS, the ZnO/PS and TiO2/PSfilms are visualized inFig 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 Fig 1.X-ray diffraction patterns: (a) ZnO/PS and (b) TiO2/PS thinfilms
(4)0.25 wt % ZnO was found as pretty as 95% By a careful consider-ation, it can be seen that the continuous inclusion of ZnO induces a systematic decline of the transmitted light, lowering the trans-mittance 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 thefilms[10,14] In line with this, the gradual decrease in the visible-light transmission from 95 to 70% in thefilms containing 0.25e1 wt % ZnO was observed and highlighted by the shaded area inFig 4a The thinfilms with 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 illus-tration to the UV-shielding effect in ZnO/PS When ZnO/PSfilm 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 thefilm with increased obstacle leading to the reduction in UV-light transmission to 63% with 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 Fig 3.AFM images of TiO2/PSfilm
(5)observed in the pure PSfilm (seeFig 4b), which is not competent to
filter out the UV radiations, whereas the addition of TiO2content 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 thefilms with TiO2contents as low as 0.25 wt%, while the high transparency in the visible range is maintained The concentration of 0.5 wt % TiO2could 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 intro-duction of TiO2particles into the PS matrix is compatible to in-crease the UV protecting proficiency of the PSfilm 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 TiO2enhances 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 thefilm becomes more non-uniform, and irregular with the increasing TiO2content up to wt% lead-ing to the reduction in the transmittance as it is confirmed on the AFM images (Fig 3e) of the compositefilms 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 (Egvalues) of the ZnO/PS and TiO2/PS
films could be estimated from a plot of (ahn)2vs the photon energy (hn) inFig 5a,b Band gap values of 3.00, 2.47 and 2.61 eV were obtained for the pure PS, ZnO/PS and TiO2/PSfilms, respectively (for the optimum content, i.e 0.5%) However, two different mecha-nisms 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 bar-rier 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 TiO2and ZnO on the microstructure of the compositefilms The positron lifetime spectra of the pure PS,ZnO/PS and TiO2/PSfilms, respectively, are presented inFig 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 TiO2and ZnO content are shown inFig 7aeb, respectively FromFig 7a, it can be observed that thet3andVfinitially drop with the TiO2 incorporation upto 0.5 wt% In the range from 0.5 to 0.75 wt%, a slight increase int3andVfis seen Theyfinally decrease to the lowest value at higher doping,i.e.at wt% Looking again into Fig 7a, there is a decrease in t3and Vfwith the increasing TiO2 concentration (0.25e0.5 wt %) indicating that the additional Fig 5.Plots of (a∙h∙y)2v/s photon energy (h∙y): (a) ZnO/PSand (b) TiO
2/PS thinfilms
(6)amount of TiO2 slows down the o-Ps formation This can be explained by the fact thatfirstly the TiO2particlesfill 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 thefiller concentration is increased to that corresponding to the wt% concentration, the TiO2filler inhibits the o-Ps formation and thefiller 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 sys-tematic 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%) (seeFig 7b) This is because of the development of new positron trapping sites at the ZnO/PS interface The highest values oft3andVfhave been found for the 0.5 wt% ZnO/PSfilm, whereas when we have increased the ZnO concentration upto wt %, the values oft3andVfdecrease 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 thefiller 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 thefillers[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 con-centrations, 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/PSfilms 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 TiO2incorporation It might be an evidence for the gradual formation of neutral aggregates at the initial level offiller concentrations which creates blockages and reduces the free vol-ume 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 TiO2concentrations where the regretable interaction between the loaded TiO2particles and the PS matrix have slight limitations on the segmental mobility because of less contact area and so contributing to the increase in t3 andVf as shown inFig 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 infor-mation 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 incre-ment int3with 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 be-tween thefiller particles at lowfiller concentrations Our tentative elucidation is that the dispersion of ZnO particles can cause the disorder of the molecular configuration leading to the morpho-logical change in chains and thus increase in the free volume concentration and the o-Ps lifetime The above discussion cor-roborates 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 increasedfiller concentration, because thefiller limits the moving space of the molecular chains
(7)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 inFig 3c) acting as the nucleating agents leads to the increase in the UV shielding Moreover, doping with TiO2results 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 thefilms
To the best of our knowledge, the correlation between the UV-absorption 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 polymerfilm mixtures
4 Conclusions
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) thefiller-PS interfacial interaction and (iii) its impact on the UV-protecting adequacy of thefilms which have been investigated and examined by PALS Concussively, the shrink of free volume hole size due to the highfiller 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 TiO2particles, however, the decrease in the free volume hole size has been observed because of the rod shaped aggregation of the TiO2 parti-cles which act as nucleating agents contributing to the UV shielding efficiency of the PS The results as obtained suggest that TiO2and ZnO acting as activefillers 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 compositefilms Therefore, the main efforts should be focused on the nanocrystals without ag-gregation in the PS
Acknowledgement
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 Thefinancial support from MPCST Bhopal is gratefully acknowledged
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