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

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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.

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Nội dung Text: 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 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd 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 Vikram University, Ujjain, 456010, MP, India b Lovely Professional University, Jalandhar, Punjab, India c 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: To reveal how the distribution of different nano ﬁllers affect the UV-shielding efﬁciency of their polymer- Received 5 February 2019 based composites and to further develop a simple strategy to refrain the erection of the composites, we Received in revised form prepared ZnO doped polystyrene (PS/ZnO) and TiO2 doped polystyrene (PS/TiO2) ﬁlms by the solution 28 July 2019 cast technique with different concentrations of ZnO and TiO2 (0.25%, 0.5%, 0.75% and 1%.). Contrary to the Accepted 10 August 2019 Available online 16 August 2019 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 Keywords: Polystyrene thin ﬁlms efﬁciency of the PS matrix. This analysis is equally supported by the PALS study where the free vol- ZnO ume was closely associated with the interfacial interaction between the ﬁller and the PS matrix. These TiO2 observations recommend that the better dispersion of ﬁller particles leads a stronger interfacial inter- Positron annihilation action and enhances the UV-protection efﬁciency of the composite materials. Free volume hole © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. Interfacial interaction This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Solution cast method 1. Introduction process in engineering materials it is important to predict the weakness and failure of the material. This work is consequential for Previously, our group has successfully prepared TiO2/PS ﬁlms the ﬁnal understanding of the UV-shielding efﬁciency by with concentrations up to 1 wt % by the solution cast method in the comparing its results with those of a widely studied material as development of photo-protective polymeric materials for the pro- ZnO. The very extensively studied inorganic materials ZnO and TiO2 tection against ultraviolet radiation. Interestingly, the as-prepared with a wide band-gap energy of 3 eV have been expansively used as thin ﬁlms have shown a tremendous UV-shielding proﬁciency inorganic UV absorbers due to their signiﬁcant optical properties [1,2]. These results suggested to pursue a further study on the re- [3]. Consequently, such polymer nanocomposites have been lationships among the atomic free volume and the interfacial regarded as excellent candidates for UV shielding applications. As a interaction between the ﬁller particles and the PS matrix. Contrary matter of fact, the extraordinary properties of the polymer nano- to the common observations where numerous approaches have composite include the dispersion of the nanoparticles in the matrix been made for the development of ZnO doped composites as a and the subsequent growth of enormous interfacial areas. This better UV protective material, in our previous study TiO2 has complete dispersion allows the exploration of the available expressed a better harmony for the efﬁcient UV shielding which we matrixeparticle interface and then the optimization of the will incorporate in the present work as a comparative study. To organiceinorganic interaction which is accountable for the analyze the imperfections produced at the early stage of 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 * Corresponding author. Lovely Professional University, Jalandhar, Punjab, India. interfacial interaction on the UV-shielding efﬁciency [4e6]. More- E-mail addresses: acharyaphysics2011@gmail.com (A.D. Acharya), sarbhawna@ over, most of the works have adopted higher dopant concentrations gmail.com (B. Sarwan). to conquer a better UV-shielding efﬁciency of the ﬁlms [4,7e10] Peer review under responsibility of Vietnam National University, Hanoi. https://doi.org/10.1016/j.jsamd.2019.08.003 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 (http://creativecommons.org/licenses/by/4.0/).
414 A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419 whereas low content dosaging was constantly disregarded causing Positron lifetime experiments are capable of distinguishing a shortage of understanding of the mechanism of the UV-shielding different kinds of defects. As a spectroscopic technique the positron enhancement. It would therefore be interesting to ﬁnd out at which annihilation spectroscopy is a sensitive tool for the study of open- doping concentration the composite ﬁlm starts to absorb the UV volume type defects which include vacancies, vacancy agglomer- radiation to a signiﬁcantly large extent. In this contribution, we ates, and dislocations. A conventional fast-slow coincidence spec- have prepared ZnO/PS and TiO2/PS ﬁlms with dopant concentra- trometer was used to carry out the positron lifetime measurements tions up to 1 wt % by the solution cast method. This is a green as having a resolution of 280 ps (FWHM) for Na22 source. The Na22 well as simple method. It is also useful in dissolving the polymers to source with strength of 20 mCi was deposited onto a nickel foil with have the material as ﬁnely divided as possible and to have each the thickness of 1 mg/cm2 and sandwiched between two similar particle thoroughly wetted by the solvent. Numerous techniques pieces of the sample. Nearly 105e106 counts were recorded in the have been employed to analyze the impact of the interfacial PAL spectra for each sample. Standard PATFIT program was used to interaction on the thermal conductivity, the thermal expansion, the analyze the positron lifetime spectra with their relative intensities. viscoelastic properties and the density [11,12]. In view of the ap- plications the technique employed should have a strong inﬂuence 3. Results and discussion on the estimation of the free volume. The quantitative depiction of the free volume properties of polymers in the amorphous state can 3.1. Structural and surface analysis be accomplished by the use of the positron annihilation lifetime spectroscopy technique (PALS). This technique involves the inser- The XRD patterns of the ﬁlms are shown in Fig. 1. As it is tion of positrons into the material and then recording the indi- perceived from the XRD patterns the pure PS ﬁlm (Fig. 1 a,b) shows vidual positron lifetimes until the annihilation with electrons of the an amorphous polymeric structure and the diffraction peaks of PS sample takes place [11,12]. Since the fraction of the positrons an- do not appearein the patterns. The pattern of the PS thin ﬁlms nihilates from the state of an orthopositronium (o-Ps) and the loaded with 0.5 wt% TiO2 and ZnO (Fig. 1 a,b), however, shows lifetime of the orthopositronium depends on the size of the free diffraction peaks of low intensity suggesting improved crystalinity volume cavity where it is placed, hence, it can be employed to of the PS. At the higher ﬁller content, the peak positions of the 1 wt characterize the free volume size in amorphous polymers. % sample is slightly shifted towards lower diffraction angles. The Following the above described study concern, the main motivation most likely reason for this shift is the interaction between the ﬁller of the present work is to investigate what would be the effects of particle and the polymer structure that leads to a rearrangement of doping ZnO and TiO2 into PS on the atomic free volume, the the PS chains (See Fig. 1a and b (Inset)). The increased intensity of interfacial contact among the ﬁller particles and the PS matrix, and the reﬂections from the diffracted planes with the higher amount of the UV-absorption efﬁciency of PS at low content of ﬁllers by the ﬁller loadings suggests that a slowering of the crystallization rate positron annihilation lifetime spectroscopy. arrised due to the enclosure of ﬁller particles. It can be concluded that a suitable, but not excessive, amount of dopant is responsible 2. Experimental for observed good dispersion of the inorganic ﬁller particles in the PS matrix. ZnO/PS and TiO2/PS thin ﬁlms with different concentrations viz To get a further insight, we extended our approach to another 0.25%, 0.5%, 0.75% and 1% have been prepared by using the solution important analysis using the atomic force microscopy (AFM) of the casting method. The polystyrene was procured from the market doped polymer samples. Figs. 2 and 3 show the AFM images of thin which was in the granular form. The PS solution was prepared in sections of the ZnO/PS and TiO2/PS composite surfaces loaded with the dichloromethane, the requisite amount of semiconductors (ZnO the dopant content of 0.25, 0.5, 0.75 and 1.0 wt %. It can be observed or TiO2) was then added into the solution under rapid stirring for from AFM images (Fig. 2) of PS/ZnO that the grain size increases uniform dissolution. The resultant was then poured on to a cleaned with the increase in the ZnO concentration upto 0.5 wt% (See petri dish to cast the ﬁlm and the solvent was subsequently allowed Fig. 2c) leading to the aggregation. The addition of ZnO particles at to evaporate gradually over a period of 12e24 h in a dry atmo- about 0.75 wt % does not encourage the faster crystallization (See sphere. The membrane was then physically peeled off from the Fig. 2d). In case of the 1.0 wt % sample (See Fig. 2e), the efﬁciency of surface. The area of the cast surface, the material quantity and the ZnO for enhancing the matrix crystallization get reduced due to the density of the material can determine the thickness of the mem- high particle density and obstructed the development of crystalline brane. We have prepared the polymer thin ﬁlms of ~50 mm thick- sections. This illustrates that the small amount of ZnO particles i.e. ness. For preparing the doped PS thin ﬁlms, the dopant 0.5 wt% located in the PS matrix corresponds to the primary par- concentration was calculated from the following equation [1,13]. 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 wf changed to large size aggregates where several primary ZnO Wðwt%Þ ¼ 100 wp þ wf nanocrystallites were gathered. Furthermore, the entire morphology was deformed when 1.0 wt% ZnO was employed. where,wf and wp represent the weight of the dopant and the By comparing the AFM images, an obvious difference can be polymer, respectively. seen between the neat PS and the TiO2/PS ﬁlm. Fig. 3a shows an X-ray diffraction patterns of ZnO/PS and TiO2/PS thin ﬁlms were AFM image of thin sections of the TiO2/PS thin ﬁlm at the dopant recorded on an X-ray diffractometer (Bruker D8 ADVANCE) with content of 0.25 wt %. It signiﬁes that the TiO2 particles were setup in Cu-Ka radiation having the wavelength of 1.5418 Å in the range of the form of aggregates of slackly linked paramount particles 2q ¼ 200 - 700. Atomic force microscopy (AFM) measurements were showing areas which are homogeneously implanted in the PS carried out on a digital instrument of Nanoscope E with the Si3N4 matrix. After addition of 0.5 wt% TiO2 into the matrix some rods 100 mm cantilever and 0.58 N/m force constant. The transmittance appeared (See Fig. 3c) on the surface of the neat PS presuming that of the ﬁlms has been measured with a UV-Vis Spectrophotometer the formation of these rods mainly depends on the growth and (PerkinElmer Lambda 950). Measurements of the positron lifetime nucleation conditions. Moreover, a fractal type of aggregation of in ZnO/PS and TiO2/PS thin ﬁlms have been done by using the slow TiO2 particles has been observed in Fig. 3d, such situation may arise e fast coincidence method. 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, 0.5 wt % is enough. Herewith, the AFM results suggest that the the nucleates randomly agglomerate in the continuous phase and inorganic semiconductor particles were well incorporated in the cause the increase of the number of TiO2 particles, thus, making the PS, which consequently modify signiﬁcantly the morphology of the interface area larger and the overlap of these led to opaquely PS ﬁlms. 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 3.2. ZnO/PS and TiO2/PS UVevis shielding 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 The transmittance characteristics of the pure PS, the ZnO/PS and higher interface area and therefore exhibit a higher UV- absorption TiO2/PSﬁlms are visualized in Fig. 4. It is found that almost 99% of efﬁciency. From this analysis, it may be inferred that to speed up the the light was passed-on by the pure PS in the UVevis region of matrix crystallization and for altering the synthesized nano- wavelengths from 300 to 700 nm. As shown in Fig. 4a, the structure morphology, a low concentration of dopant as such of maximum value of transmittance of the ZnO/PS ﬁlms containing
416 A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419 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 consider- found. The obtained experimental results provide a visual illus- ation, it can be seen that the continuous inclusion of ZnO induces a tration to the UV-shielding effect in ZnO/PS. When ZnO/PS ﬁlm is systematic decline of the transmitted light, lowering the trans- irradiated with the incident radiation, the visible light perfectly mittance. The transparency is also dependent on the dispersion/ passes through the material as ZnO particles are apparent for the aggregation of the nanoparticles into the polymer matrix. The wavelengths greater than 375 nm while the UV-spectrum is fractal distribution of discretely dispersed nanoparticles favors the obstructed depending on the ZnO concentration. For the reason optical transparent intensity loss of the transmitted light because that the ZnO nanoparticles build a physical obstacle that the UV the scattering abruptly rises with the particle size. This causes a light cannot cross since they act as a protective network. When the signiﬁcant drop in the transparency of the ﬁlms [10,14]. In line with dopant concentration is further increased, the scattering mean free this, the gradual decrease in the visible-light transmission from 95 path gets decreased. Due to this reason, the light traveled strongly to 70% in the ﬁlms containing 0.25e1 wt % ZnO was observed and inside the ﬁlm with increased obstacle leading to the reduction in highlighted by the shaded area in Fig. 4a. The thin ﬁlms with 1 wt % UV-light transmission to 63% with 1 wt% ZnO concentration (see ZnO dopant shows a non-uniform distribution of the ZnO particles Fig. 4a). within the polymer matrix. This could be endorsed by the AFM The UVeVis transmittance of the TiO2/PS ﬁlm is plotted in results (Fig. 2) where no substantial ZnO agglomerations were 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 417 Fig. 5. Plots of (a∙h∙y) 2 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 The band gap energies (Eg values) of the ZnO/PS and TiO2/PS ﬁlter out the UV radiations, whereas the addition of TiO2 content ﬁlms could be estimated from a plot of (ahn)2 vs. the photon energy leads to the increase in the UV shielding efﬁciency due to the empty (hn) in Fig. 5a,b. Band gap values of 3.00, 2.47 and 2.61 eV were conduction band and the ﬁlled valence band. However, the UV obtained for the pure PS, ZnO/PS and TiO2/PS ﬁlms, respectively (for blocking consequence is seen in the ﬁlms with TiO2 contents as low the optimum content, i.e. 0.5%). However, two different mecha- as 0.25 wt%, while the high transparency in the visible range is nisms are accounted for the variation in the calculated optical band maintained. The concentration of 0.5 wt % TiO2 could be assumed as including: (1) The inclusion of a tiny amount of dopant produces the optimal one for the better UV shielding effect as evidenced by charge transfer complexes in the host matrix which accelerate the the graphical situation in the region bellow 355 nm, where more electrical conductivity by providing additional charges which cause than 70% transparency is observed. This evidences that the intro- the reduction of the band gap [18,19]; (2) When the amount of duction of TiO2 particles into the PS matrix is compatible to in- dopants increases, the dopant molecules initiate to linking the gap crease the UV protecting proﬁciency of the PS ﬁlm in the region between the localized states and thus lowering the potential bar- from 300 to 355 nm. The further increment of TiO2 (0.75 wt %) rier between them [16e22]. results in the opaque appearance with the increased absorption in both the visible and UV region. In this state, the apparent nature of 3.3. Positron annihilation lifetime studies the material as a UV ﬁlter is decreased. This behavior can be interpreted by the fact that the increased amount of TiO2 enhances The measurement for the positron annihilation lifetime studies the interface scattering causing the reduction in the transmittance. (PALS) was carried out to examine the effect of TiO2 and ZnO on the This reduction might be ascribed to the growing cluster size [6]. In microstructure of the composite ﬁlms. The positron lifetime spectra addition, the cluster size of the ﬁlm becomes more non-uniform, of the pure PS,ZnO/PS and TiO2/PS ﬁlms, respectively, are presented and irregular with the increasing TiO2 content up to 1 wt% lead- in Fig. 6 . They show a systematic decreasing trend of the lifetime. ing to the reduction in the transmittance as it is conﬁrmed on the This indicates a decrease in the longest lifetime. AFM images (Fig. 3e) of the composite ﬁlms. Here, the shape of the The free volume size (Vf), and the o-Ps lifetime (t3) as a function PS latex is almost demolished and then totally vanished because of of the TiO2 and ZnO content are shown in Fig. 7aeb, respectively. the interdiffusion process between the polymer chains. From this From Fig. 7a, it can be observed that the t3 and Vf initially drop with result, it might apparently be easy to load the interstices of the thin the TiO2 incorporation upto 0.5 wt%. In the range from 0.5 to PS template with a low concentration of dopant, but it is difﬁcult to 0.75 wt%, a slight increase in t3 and Vf is seen. They ﬁnally decrease ﬁll the interstices at a higher concentration. So, the dopant content to the lowest value at higher doping, i.e. at 1 wt%. Looking again into can be considered as a key parameter for the permeation of the PS Fig. 7a, there is a decrease in t3 and Vf with the increasing TiO2 templates [15]. concentration (0.25e0.5 wt %) indicating that the additional Fig. 6. Positron lifetime spectra: (a) ZnO/PSand (b) TiO2/PS thin ﬁlms.
418 A.D. Acharya et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 413e419 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 concentrations, a random distribution of ﬁller particles might explained by the fact that ﬁrstly the TiO2 particles ﬁll up some of initiate the formation of free volume holes in the PS matrix. This the free volume holes in the PS and so the values of t3 and Vf process of the free volume formation gradually dominates the decrease. Secondly, positrons may be annihilated from the TiO2 creation of neutral aggregates which fairly agrees with the AFM ﬁller and there may be a lack of positrons which should be available results and conﬁrms the transition from the crystalline state to the to form the positronium in PS [12]. On the other side, the increase of amorphous one at higher TiO2 concentrations where the regretable o-Ps at the dopant concentration of 0.75 wt% TiO2 suggests the interaction between the loaded TiO2 particles and the PS matrix formation of new positron trapping sites at the TiO2ePS interface. have slight limitations on the segmental mobility because of less As the ﬁller concentration is increased to that corresponding to the contact area and so contributing to the increase in t3 and Vf as 1 wt% concentration, the TiO2 ﬁller inhibits the o-Ps formation and shown in Fig. 7a. An explanation based on the PALS results and the ﬁller particles are scattered among the molecular chains of the detailed literature survey [27e29] implies that the o-Ps mainly PS and thus reducing the free volumes size leading to the decrease annihilates in the interfacial regions. In fact, there is some infor- of the o-Ps lifetime in the PS. Quite the reversal, a small but sys- mation indicating that the interfacial free volume is a vital factor for tematic increase in the free volume size and in the o-Ps lifetime has determinating the variation in the o-Ps annihilating lifetime also been initially observed in the case of the low ZnO doping (i.e. because the interfaces have an excellent electronic density 0.25e0.5 wt%) (see Fig. 7b). This is because of the development of compared to the bulk phase. new positron trapping sites at the ZnO/PS interface. The highest It is worth noting that in the case of ZnO/PS, the initial incre- values of t3 and Vf have been found for the 0.5 wt% ZnO/PS ﬁlm, ment in t3 with increasing ZnO concentration upto 0.5 wt% (see whereas when we have increased the ZnO concentration upto 1 wt Fig. 7b) suggests the formation of the free volume and amorphous %, the values of t3 and Vf decrease showing that some of the free phases in the blend matrix due to the sufﬁcient separation be- volume holes in the PS are ﬁlled up by the ZnO particles. It is tween the ﬁller particles at low ﬁller concentrations. Our tentative interesting to note that the interfacial interaction between the ﬁller elucidation is that the dispersion of ZnO particles can cause the and the polymer matrix has caused a vital effect on the free volume disorder of the molecular conﬁguration leading to the morpho- size and the o-Ps lifetime. This interaction dominates the delivery logical change in chains and thus increase in the free volume of phonons between the matrix and the ﬁllers [23e25] mainly at concentration and the o-Ps lifetime. The above discussion cor- 0.5 wt% ZnO concentration where both the free volume size and the roborates that the better the dispersion of the lifetime the stronger o-Ps lifetime reach their maximum. We recall the main fact that the will be the distraction of the molecular morphology. On the other ﬁlm with a low dopant amount ZnO represents a high surface area, hand, as the ZnO content further increases, this distraction of ZnO thus, providing more positron trapping sites which scatter the weakens as being caused by the aggregation of ZnO leads to the phonons at the interface [26e28]. However, at the high ZnO con- decrease in the free volume concentration and the o-Ps lifetime. centrations, the ZnO agglomerates and due to this interfacial The analysis also conﬁrms that the free volume hole decreases interaction, the induced disruption effect becomes limited, with the increased ﬁller concentration, because the ﬁller limits the reducing the free volume size and the o-Ps lifetime. moving space of the molecular chains. From the above explanation it can be clear that the calculated 3.4. The correlation of PALS results and UV-shielding free volume hole size does not show a drastic variation with the ZnO content, but revealing a very little amount being adequate to From the calculated results of the PALS and the morphological accelerate the matrix crystallization which assures a less tunability studies, the UV-shielding efﬁciency of the ZnO/PS and TiO2/PS ﬁlms for the UV radiation. This could be probable because an excessive could be clearly understood. From the PALS results of TiO2/PS, it is amount of ZnO can obstruct the formation of well crystallized re- noticed that the free volume hole size and the o-Ps lifetime initially gions and lead to the turbidity/translucency of the composite ma- drop with the TiO2 incorporation. It might be an evidence for the terials. This illustrates that the particle size of the ZnO in the PS gradual formation of neutral aggregates at the initial level of ﬁller matrix corresponds to that of the primary particles and the extent concentrations which creates blockages and reduces the free vol- of the agglomeration is moderately negligible, whereas in the case ume holes, enhances the crystallization of the matrix as it was of TiO2 particles the reduction in the free volume hole size leads to clearly conﬁrmed by the AFM results. This decrease in the free the increase in the UV shielding effect. It is evident that the more volume holes may also contribute to the increase in the UV exposure of the PS to the UV causes the increase in the size of the shielding effect. Furthermore, this factor reduces the ion and the free volume hole while the hole density remains unchanged. The segmental mobility through the uniﬁed matrix and hence, leads to destruction of the PS matrix sets in when TiO2 is added leading to the reduction of the free volume size. At the high ﬁller the formation of voids in the region of the TiO2 particles aggregates.
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