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Solid-phase photodegradation of polystyrene by nano TiO2 under ultraviolet radiation
Environmental Nanotechnology, Monitoring & Management 12 (2019) 100229
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Solid-phase photodegradation of polystyrene by nano TiO2 under ultraviolet radiation
T
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Dinoop lal Sa, Sunil Jose Ta, , Rajesh Cb a b
Dept. of Chemistry, St. Thomas’ College, Thrissur, Kerala, 680 001, India Dept of Chemistry, MES Keveeyam College, Valanchery, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Sol-gel method Nano titanium dioxide Ultraviolet irradiation Photooxidative degradation Chain scission
Solid phase photodegradation of polystyrene (PS) and Polystyrene – titanium dioxide nano composite (PS-TiO2) was studied under ultraviolet radiation of wavelength of 253 nm. Titanium dioxide (TiO2) photocatalyst was synthesized via sol-gel method and characterized using SEM, EDX and XRD. Photodegradation of PS and PS-TiO2 was monitered using GPC, FTIR spectroscopy, UV-DRS, SEM and weight loss measurements. An appreciable decrease in the values of average molecular weights were observed from the GPC data with the increase in the UV exposure time for PS as well as PS-TiO2 composites. FTIR and UV–vis spectra revealed that the photodegradation of PS and PS-TiO2 composite was accelerated by the increase in UV irradiation time and also with the increase in the percentage of TiO2. It was also evident from the FTIR spectra that photooxidative degradation was the possible mechanism of degradation. Band gap energies of PS and PS-TiO2 determined using Tauc plot decreased upon increase in UV irradiation. The change in surface morphology of the irradiated samples compared to the pristine ones were clearly observed in their SEM images. Weight loss of the polymer samples increased upon increase in UV irradiation time. Mechanical properties (tensile and flexural) of raw as well as irradiated samples of PS and PS-TiO2 composites showed an appreciable decrease in their values with increase in irradiation time. The values of dielectric constant of the UV irradiated polymer specimens showed higher values compared to that of non irradiated specimens at varying frequencies. Dielectric strength of the polymer specimens decreased with the increase in UV irradiation time.
1. Introduction Polystyrene (PS), made up of the monomer styrene has been playing a significant role in our day to day life for the past few decades. The rigid and foamed forms of polystyrene find its application in varied areas. Polystyrene has touched many areas of human need including medical products, appliances, toys, packaging, electronic insulations and still many more as they satisfies the optimum condition for the mode of their use as they are light, durable, strong, good thermal and electrical insulators, transparent and also they could be moulded into desired shape(Chaukura et al., 2016). In some cases, products made of polystyrene can be recycled and can be used for other applications (Maharana et al., 2007; Thakur et al., 2018). Nowadays the concerns regarding polystyrene as pollutants have arisen due to the use and throw system practiced by the human community. Polystyrene, as a major component of debris found in land and marine system, threats the bio system and leads to the transfer of toxic chemicals to the food chain. Polystyrene which are blown in the wind
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can be lethal to birds and many creatures. Burning polystyrene debris is not an appropriate solution for their eradication because during the ignition process, many harmful gases which include certain reactive radical specious are evolved. These gases may cause suffocation and many sorts of disease to the creatures consuming them. They may also be carcinogenic(Gibbs and Mulligan, 1997; Kwon et al., 2014). Polystyrene undergoes photo oxidative degradation when exposed to ultraviolet (UV) radiation. Naturally occurring photodegradation is of course a long process. During photodegradation, radiations especially in the UV region are absorbed by polystyrene and causes change in its original properties. Photodissociation (chain scission), formation of carbonyl groups, isolated and conjugated double bonds and slight yellowing occurs due to radical initiated mechanism. Photodegradation results a change in the original properties of the polymer with depleted mechanical properties resulting in weight loss, brittleness etc (Al Safi et al., 2014; Ani and Ramadhan, 2008; Jane et al., 1953; Rincón, 1997; Timóteo et al., 2007). Nano TiO2 synthesized by sol-gel route was used as photocatalyst by loading it to PS at various weight percentages. The
Corresponding author. E-mail address: [email protected] (S.J. T).
https://doi.org/10.1016/j.enmm.2019.100229 Received 18 March 2019; Received in revised form 10 May 2019; Accepted 14 May 2019 2215-1532/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. XRD pattern (B) SEM image and (C) EDAX of nano TiO2 synthesized by sol-gel route.
Rajendran, 2012).
samples were irradiated and analysed at regular intervals of 500 h. A comparison of photodegradation of pure PS as well as PS-TiO2 blends at regular intervals was done. Mechanical and electrical properties of PS and PS-TiO2 blends were also measured before and after irradiation.
2.2. Characterization of nano TiO2 The particle size and morphology of the synthesized nano TiO2 was studied using Scanning Electron Microscope (FESEM) Hitachi SU6600 Variable Pressure Field Emission. The elemental composition in the synthesized sample was determined by Energy Dispersive X-Ray (EDX) analysis. XRD pattern of synthesized TiO2 was done using X-Ray Powder Diffractometer Bruker AXS D8.
2. Materials and methods Materials: Titanium(IV)isopropoxide (TTIP) (Sigma Aldrich), Polystyrene(PS) (LG Polymer India Pvt. Ltd). Ultraviolet (UV) tube 30 W 253 nm (Philips Hollend), ethanol, toluene and deionized water were used.
3. Preparation of PS and PS-TiO2 composite films 2.1. Preparation of nano TiO2 5g of PS pellets were dissolved in 20 ml of toluene and sonicated for an hour using 750 W probe sonicator. The temperature of the system was maintained below 45 °C so as to eliminate chances of thermal degradation at higher temperatures. This was then poured into petridishes, taken to a vacuum oven and kept under a vacuum of 700 mm Hg for 24 h and then allowed to dry naturally for another 48 h. PS-TiO2 composites were prepared in similar manner as discussed above by loading various weight percentages of nano TiO2 into toluene solution of PS. Thin dry films of PS and PS-TiO2 so obtained were subjected to photodegradation studies.
Sol-gel method was adopted for the synthesis of TiO2. The precursor used was Titanium(IV) isopropoxide (TTIP). About 0.5 ml of TTIP was added to a mixture of 2 ml ethanol and 2 ml deionized water in a boiling tube with constant stirring using a magnetic stirrer.. The temperature of the system was set to 50 °C. pH 4 was maintained by adding drops of HNO3, tested by litmus indicator. The stirring was continued for three hours after which it was centrifuged. The settled white precipitate was separated and washed with deionized water three times. Nano particles of TiO2 collected were dried and calcined at 400 °C for 2 h. The synthesized nano TiO2 was characterized and used as the photocatalyst for the photodegradation of PS(Alam and Cameron, 2002; Ba-Abbad et al., 2012; Karami, 2010; Kavitha et al., 2012; Malekshahi Byranvand et al., 2013; Parra et al., 2007; Shang et al., 2003; Simonsen and Søgaard, 2010; Terabe et al., 1994; Thomas et al., 2013; Vijayalakshmi and
4. Photodegradation of PS films Photodegradation of PS films were studied by exposing them to ultraviolet (UV) radiation. The UV irradiation chamber contained a UV 2
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Fig. 2. (A) Reflectance spectra of nano TiO2 obtained from UV-Vis Spectrophotometer. (B) Plot of F(R) versus hʋ of nano TiO2. (C) (F(R)hʋ)1/2 versus hʋ representing the indirect allowed transition of nano TiO2 .(D) (F(R)hʋ)2 versus hʋ representing the direct allowed transition of nano TiO2.
5. Preparation of PS and PS-TiO2 composites for determination of mechanical properties and electrical properties Specimens for determination of mechanical (tensile and flexural) properties were prepared by injection moulding as per ASTM standard. Polymer specimens in the form of circular discs (1 mm thick and 7.5 cm diameter) were moulded in a hydraulic hot press at 180 ͦC for measuring dielectric strength. The samples were then subjected to UV irradiation and tested at regular interval of 500 h. Mechanical measurements were recorded using a universal testing machine (Autograph AGX plus, 10 kN, Shimadzu). Dielectric strength was measured by keeping the polymer disc between 2 electrodes immersed in transformer oil in a specially designed chamber connected to a high power electric supply. Fig. 3. PS-TiO2 Composite before UV irradiation (A) and after UV irradiation of 1500 h (B).
6. Results and discussion 6.1. Characterisation of synthesized nano TiO2
tube (30 W, 253 nm, Philips Holland) and the samples were irradiated keeping the distance between the UV tube and the samples at exactly 8 cm.A small portion of the sample films were cut out at regular intervals of every 500 h (i.e., 500 h, 1000 h and 1500 h) and subjected to IR studies using FTIR spectrometer IR Affinity-1, Shimadzu, Japan, UV–vis spectroscopic analysis using UV–vis spectrophotometer (UV2600 spectrometer, Shimadzu, Japan) and SEM analysis using (JEOL Model JSM - 6390 L V).
The XRD pattern of TiO2 synthesized by sol-gel method was shown in Fig. 1A. Strong diffraction peaks observed at 2θ = 24.940°, 47.696°, 53.636° and 62.404° reveals that the synthesized nano TiO2 is in anatase phase. The diffraction peaks obtained at 2θ = 27.069° and 54.727° indicates the presence of rutile phase in a lower percentage (Hassanjani-Roshan et al., 2011; Thamaphat et al., 2008). Crystallite size of the synthesized TiO2 particles was calculated using the Scherrer’s formula. 3
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Table 1 PDI,S and Nt calculated from the GPC data of PS and PS-TiO2 composite. Sample
UV irradiation time (h)
Mw (Da)
Mn (Da)
PDI (Mw/Mn)
Nt
S (Mn)
1
0 ⎡ − 1⎤ ⎣ (Mn)t ⎦
⎡ − ⎣ (Mn)t
1 ⎤ (Mn)0 ⎦
PS
0 500 1000 1500
92031 87228 86100 85476
75346 69470 67235 65916
1.221445 1.255621 1.280583 1.296741
0 0.084583 0.120637 0.143061
0 1.12 × 10−6 1.6 × 10−6 1.9 × 10−6
PS-TiO2
0 500 1000 1500
92723 85900 82500 81419
76329 68731 63508 61480
1.214781 1.2498 1.299049 1.324317
0 0.110547 0.20188 0.241526
0 1.45 × 10−6 2.64 × 10−6 3.16 × 10−6
Fig. 4. A) Weight average molecular weight (Mn), B) Number average molecular weight (Mw), C) Number of chain scission per molecule (S) and D) number of scission events per gram (Nt) of PS and PS+3%TiO2 composite.
D=
k.λ β.Cos θ
SEM image of the synthesized TiO2 reveals its spherical morphology with particle diameter ≈25 nm Fig. 1B. Energy Dispersive X-Ray (EDAX) clearly confirms that the synthesized nano TiO2 particles were pure Fig. 1C. Band gap energy of the synthesized nano TiO2 samples were calculated from UV–vis Reflectance spectra (Fig. 2A) applying Kubelka–Munk (KeM or F(R)) function (Eq. (2)) in Tauc method(Chandran et al., 2010; López and Gómez, 2012; Reddy et al., 2003).
(1)
Where D = Crystallite size (nm) K = Shape-sensitive coefficient (0.89- For spherical spheres) λ = Wavelength of the X-Ray beam (0.15406 nm for Cu-Kα radiation) β = Full width at half maximum (FWHM) of the peak under consideration θ = Diffracting angle The average crystallite size of the particles calculated from Scherrer’s formula was found to be 17.57 nm.
F(R) = (1-R)2/2R
(2)
where ‘R’ is the reflectance and ‘F(R)’ is proportional to the extinction coefficient (α). 4
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Fig. 5. (A) &(B).The FTIR spectra of pristine PS sample before UV irradiation(a), after 500 hr UV irradiation(b), 1000 hr UV irradiation(c),1500 hr UV irradiation(d).
Fig. 6. (A)& (B). The FTIR spectra of PS-TiO2 composite (PS loaded with 3 wt% of TiO2) before UV irradiation(a), after 500 hr UV irradiation(b), 1000 hr UV irradiation(c),1500 hr UV irradiation(d).
exponential factor to the final equation (Eq. (3)).
Table 2 Important observations made from FTIR spectra of PS and PS-TiO2 composites. Peaks
Groups
−1
3700–3600 cm 2631 cm−1 1740–1700 cm−1 1680–1650 cm−1 1452 cm−1, 1600 cm−1 1030, 905, 758, 700 cm−1 830 cm−1 650 cm−1
Free eOH/eOOH eOH (acid) stretch eC]O stretch eC]Ce Stretch Aromatic eC]Ce stretch Ar CeH (Out of plane bend) Conjugated eC]Ce ]CeH bend
(F(R)hʋ)n
Change in intensity up on UV irradiation
(3)
The plot of Eq. (3) versus energy (hʋ) in eV was used to calculate the band gap energy. Such a type of plot is called as Tauc plot. The plot (F (R)hʋ)1/2 versus hʋ represents the indirect allowed transition (Fig. 2C) were as (F(R)hʋ)2 versus hʋ represents the direct allowed transition (Fig. 2D). The Eg corresponding to indirect allowed and direct allowed transitions were found to be 3.86 eV and 3.36 eV respectively.
Intensity Increased Intensity Increased Intensity Increased Intensity Increased No change No change
6.2. Photo catalytic degradation of solid PS films using nano TiO2
Intensity Increased Intensity Increased
A slight yellowing and loss of transparency was observed in UV exposed PS and PS-TiO2 composite films. UV exposure of the specimens also resulted in increased brittleness. These physically observed facts suggested that a change in optical and mechanical properties of PS as well as PS-TiO2 composite had occurred as a consequence of UV irradiation (Fig. 3). Gel permeation Chromatography analysis on PS as well as PS-TiO2 was used to measure their average molecular weights mainly weight average molecular weight (Mw), Number average molecular weight (Mn) and Z average molecular weight (Mz). These average molecular weights decreased as the time of UV irradiation of the polymer
F(R) was plotted against energy (hʋ) in eV and the band gap energy (Eg) was determined from the plot obtained by extrapolating the linear portion of the curve to the X axis (Fig. 2B). The band gap energy determined from this plot was 3.6 eV, irrespective of transitions (direct or indirect). Band gap of TiO2 were also calculated using modified K–M function. For this, function F(R) was multiplied by hʋ and coefficient (n) associated with corresponding electronic transition were also introduced as
5
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Fig. 7. UV-Visible spectra of :-Pristine PS irradiated under UV lamp for 0hr,1000 hr and 1500hr((a), (b) and (c) respectively).PS (a) & PS-TiO2 composite (0.5%TiO2 (b) & 1%TiO2 (c)) irradiated under UV lamp for 1000 hr.PS-TiO2 composite (0.5%TiO2) irradiated under UV lamp for 0hr (a), 500hr (b)1000 hr (c) and 1500hr(d).
specimens increased. The following calculations were also made from the average molecular weight data obtained from GPC.
Average chain scission per polymer macro
molecule
(Mn)0 =⎡ − 1⎤ ⎢ ⎥ ⎣ (Mn)t ⎦
S (4)
Number of scission events per gram of polymer Nt 1 1 ⎤ =⎡ − ⎢ (Mn)0 ⎥ ⎣ (Mn)t ⎦ Polydispersity index PDI =
(5)
Mw Mn
(6)
Where (Mn)0 is the number average molecular weight of non irradiated polymer, (Mn)t is the number average molecular weight of polymer after t hours of irradiation The results obtained from GPC is tabulated below Table 1. The decrease in the average molecular weights (Mw, Mn and Mz) of the polymer on UV irradiation was due to polymer chain scission Fig. 4A The increasing value of S and Nt on irradiation gave the extent of chain scission at different UV exposure time Fig. 4B and C. The PDI of the polymer increased with the increase in UV exposure time suggesting a random scission of polymer chains. All the above facts clearly reveal the fact that the polymer chains have underwent random chain cleavage upon UV irradiation due to photodegradation. Another observed fact was that the decrease in the average molecular weights was
Fig. 8. Plot showing the degradation percentage (D%) of PS and PS-TiO2 composite (3 wt % TiO2) before and after UV irradiation.
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Fig. 9. Plot of (αhʋ)2 v/s hʋ (A): PS films irradiated for 0,500,1000 and 1500 hrs and (B): PS-TiO2 composite films irradiated for 0,500,1000 and 1500 hrs.
Fig. 10. Plot of (αhʋ)1/2 v/s hʋ (A): PS films irradiated for 0,500,1000 and 1500 hrs and (B): PS-TiO2 composite films irradiated for 0,500,1000 and 1500 hrs.
Fig. 11. SEM image of PS+3%TiO2 composite film irradiated for 0 hr (a) and 1500hr (b).
−OOH, eC]O, −COOH, eC]Ce etc. The variation in other peak intensities also emphasized the formation of conjugated double bond, carbonyl groups in the polymer backbone chain, breakage of polymer chain etc. From the FTIR spectra, it was found that all the samples of pristine PS before and after UV irradiation (0 h, 500 h, 1000 h and 1500 h) showed characteristic peaks of phenyl ring at around 695 cm−1 750 cm−1, 906 cm−1 and 1028 cm−1 with no change in their intensities (Fig. 5A). These bands attributed to the C–H out of plane bending
predominant in PS-TiO2 composite compared to pristine PS upon UV irradiation. The parameters S, Nt showed more increase in PS-TiO2 composite than pristine PS up on irradiation. All these facts suggested that the extant of photodegradation was predominant in PS-TiO2 composite compared to pristine PS. Photodegradation of PS (pristine and composite) was further confirmed by FTIR spectroscopy. FTIR spectra of pristine PS at different irradiation time intervals clearly showed the variation in peak intensities corresponding to various functional groups such as −OH, 7
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phenyl rings of PS remain intact after UV irradiation. An increase in the intensity of bands corresponding to carbonyl frequencies ranging between 1750 to 1650 cm−1 were observed upon increase in UV irradiation time (Fig. 5B). These observations clearly reveal the fact that there was formation and increase in the intensities of carbonyl (eC]O) stretching bands (1740–1700 cm−1) as well as carbon- carbon double bond (eC]Ce) stretching bands (1680−1600 cm−1) upon UV irradiation. The intensity of bands corresponding to hydroxyl and/or hydroperoxy groups (3700–3600 cm−1) also showed an appreciable increase during UV irradiation. Formation of new −OH /−OOH groups upon UV irradiation were established from this observation. Photodegradation of PS-TiO2 composites were also monitered using FTIR spectroscopy. FTIR spectra of PS-TiO2 composites with different UV irradiation periods (500 h, 1000 h and 1500 h) (Fig. 6) as well as PS loaded with varying weight percentages of TiO2 (0%, 0.5%, 1%, 2% and 3% of TiO2) were analysed. The FTIR spectra PS-TiO2 composite resemble that of pristine PS. The only differences was that the intensities of absorption bands corresponding to eC]O, eC]Ce, −OH, −OOH, PheC]O etc of PS-TiO2 composite showed a higher increase with the increase in UV irradiation time It was also observed that the increase in absorption bands of the functional groups mentioned above were higher for the PS-TiO2 composite with higher percentage of TiO2. The following conclusions were drawn from the above observations. The increase in intensities of to eC]O,−OH, −OOH, PheC]O etc., functional groups suggested that photo oxidative mechanism has taken place upon UV irradiation. Increase in the intensities of bands corresponding to eC]Ce stretch, Conjugated eC]Ce, =C–H bend etc suggests that chain and/or bond scission(due to eCeCe or –C–H bond breakage) had taken place. Since there was no change in aromatic eC]Ce (stretch), aromatic-C–H (out of plane bend) etc, it was clear that the phenyl ring remained intact upon UV irradiation. It was also concluded that for PS-TiO2, the rate of degradation was more effective compared to pristine PS. Rate of photodegradation of PS-TiO2 composite increased with the increase in the weight percent of loaded TiO2 and also with the increase in UV irradiation time. The important observations made from FTIR spectral analysis of PS and PS-TiO2 composite is tabulated in Table 2. UV-DRS spectroscopy of pristine PS as well as PS-TiO2 composite further supports photodegradation. Characteristic absorption peak of PS was observed in the UV region (between 220–400 nm) (Fig. 7). The intensity of these UV absorption bands showed an appreciable decrease with the increase in irradiation time. As UV irradiation time of the specimens increased, peak broadening towards higher wavelength was observed with an increase in the band intensities corresponding to visible region (380–600 nm for PS and 380–725 nm for PS-TiO2). The decrease in absorption bands corresponding to the UV region suggest that PS chain had underwent chain breakage causing a decrease in its characteristic absorption bands. New conjugated double bonds have formed as a consequence of photooxidation resulting in the increase of absorption bands in the visible region. UV-DRS spectra revealed that the increase in concentration of loaded TiO2 in PS-TiO2 composite as well as the increase in UV exposure time of the composite resulted in increased rate of photodegradation. Degradation percentages of PS and PS-TiO2 composites were calculated from the UV–vis absorption spectra using Eq. (7).
Fig. 12. Percentage of weight loss of pristine PS and PS loaded with 3% TiO2 with UV irradiation time.
Fig. 13. Tensile strength of PS as well as PS-TiO2 composite (3 wt % TiO2) with different irradiation times (0,500,1000 & 1500 hours respectively).
D% = [(Aₒ − A)/Aₒ] × 100
(7)
Where D% is the degradation percentages of PS and PS-TiO2 ; Aₒ and A are absorbance of the polymer samples before and after irradiation respectively. From the graphical representation of D% (Fig. 8) it was observed that the D% of PS-TiO2 composite was higher than that of PS after UV irradiation. Optical band gap energy of the PS and PS-TiO2 composite films were determined using Tauc relation given below (Abdelghany et al., 2016;
Fig. 14. Flexural strength of PS as well as PS-TiO2 composite (3 wt % TiO2) with different irradiation times (0,500,1000 & 1500 hours respectively).
frequencies of the phenyl ring. The peak at 1452 cm-1 corresponding to aromatic carbon-carbon double bond stretch (AreC]Ce stretch) also showed no change in peak intensity. The above observations show that 8
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Fig. 15. Plot of dielectric constants versus frequency of PS (A) and PS-TiO2 composite (3 wt % TiO2) (B) with varying irradiation times (0 hr (a),500 hr (b) & 1500 hr (c) respectively).
Alwan, 2010; Jaleh et al., 2011; Sangawar and Golchha, 2013; Sirohi and Sharma, 1999). αhʋ = A(hʋ-Eg)n
(8)
Were α is the absorption coefficient, hʋ is the energy of photon in eV (h = Plank’s constant and ʋ = frequency of radiation), A is a constant (different for different transition), Eg is the band gap energy and the index n is assumed to have different values corresponding to different electronic transitions (n = ½ for direct allowed transition and n = 2 for indirect allowed transition). A plot of (αhʋ)2 versus hʋ gave the direct allowed band gap energy (Eg) of PS as well as PS-TiO2 composite films, on extrapolating the linear portion of the curve to the X axis (where α = 0) (Fig. 9). The indirect allowed Eg of PS as well as PS-TiO2 composite films was determined from the plot of (αhʋ)1/2 versus hʋ and extrapolating the linear portion of the curve to the X axis (Fig. 10). The value of direct allowed and indirect allowed optical Eg for PS as well as PS-TiO2 composites showed a decrease in value upon increase in the time of UV irradiation. It was found that the decrease in Eg of PS-TiO2 composites were higher compared to that of PS samples upon UV irradiation. It was also observed that the Eg of PS-TiO2 composite (direct Eg = 3.25 eV,
Fig. 16. Dielectric breakdown of PS and PS-TiO2 composite (3 wt % TiO2).
Table 3 Observations and conclusions made from GPC, FTIR spectroscopy, UV-DRS spectroscopy, electrical studies, weight loss measurements and SEM. Observations (with the increase in UV exposure time)
Conclusions or eCeH bond of the polymer chain has ruptured upon UV exposure • eCeCe • Scission of polymer chains was random
GPC Decrease in Mw,Mn,Mz Increase in No: of chain Scission per macro molecule (S) Increase in Scission events per gram (Nt) Decrease in Mechanical Properties of PS (Tensile & Flexural strengths) Electrical Studies Dielectric strength (BDV) has decreased Dielectric constant have increased FTIR Increase in the intensity of peaks corresponding to eC]O, eOH, eOOH, eC]Ce, conjugated eC]Ce etc No change in the intensity of peaks corresponding to phenolic stretch or bend.
• • • •
of charge centers • Formation • Decrease in the capacitance property of new C]C linkages • Formation of new eC]O groups • Formation of new eOH, eOOH etc groups • Formation of new eC]CeC]CeC]C (conjugation) • Formation ring remains intact • Phenolic of characteristic absorption of PS due to photodegradation. • Loss shift (Which could also be observed as slight yellowing of PS after • Red irradiation) • The ability of polymer specimens to absorb UV radiation has decreased. of volatile specious/gases formed as a consequence of photodegradation • Loss of PS • Change in surface morphology of PS
• •
UV-DRS spectroscopy Decrease in characteristic absorption peaks of PS. Peaks have shifted towards higher wavelength. Band gap energy(Eg) calculations Reduces to lower energy Weight loss measurements Weight loss was observed SEM
• • • •
9
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Fig. 17. xxxPossible -C-C- and/or -C-H- bond scissions at various sites of PS.
Fig. 18. Formation of C=C double bond and conjugated double bonds.
TiO2 composite film. After 2000 h of UV irradiation, the surface morphology of the composite film seemed to be rough. The observed roughness in the SEM image attributed to the degradation process taken place on the surface of the composite film. Weight loss of both pristine PS as well as 3 wt% TiO2 loaded PS
indirect Eg = 3.98 eV) were lower than that of pristine PS (direct Eg = 4.12 eV, indirect Eg = 4.25 eV). Fig. 11 show the SEM image of 3 wt % TiO2 loaded PS before UV irradiation and after 2000 h of UV irradiation. The SEM image clearly shows the surface morphology of the irradiated and non irradiated PS10
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Fig. 19. Formation of –OOH,-C=O and -OH double bond and conjugated double bonds.
films; εₒ is the dielectric constant of free space (8.854 × 10−12 F/m); A and t are area and thickness of the polymer films respectively. The plots of εr against frequency of both PS and PS-TiO2 composite is illustrated in Fig. 15A and B. From the plots it was observed that as the time of UV irradiation increased there was an increase in the value of εr for a particular frequency for both PS as well as PS-TiO2 composite. Dielectric breakdown of the polymer films were measured at a frequency of 50 Hz and it was found that the value of break down voltage (BDV) decreased upon increase in uv exposure time (Fig. 16). The value of breakdown voltage was found to be higher for the PS-TiO2 composite before irradiation. UV irradiation resulted in the decrease of BDV for both the polymer samples (PS and PS-TiO2) due to the formation of new charge centers caused by the degradation of the samples. These charge centers allow electric current to pass through the sample more easily. Table 3 highlights the important observations and their conclusions drawn from the above studies of PS and PS-TiO2 photodegradation. It was found that the observations given in Table 3 were most predominant in PS-TiO2 composite compared to pristine PS. This envisages that photodegradation was most predominant in PS-TiO2 proving TiO2 to be a good photocatalyst in the UV region. Mechanism of photodegration of PS followed random pathways due to the multiple possibility of bond breakage, bond recombination, addition or elimination of various atoms. Based on all the above observations summerised in Table 3 the following mechanism could be considered. The photodegradation starts with the phenolic rings of PS absorbing UV radiation and getting excited into singlet states and then undergoing an inter system crossing (ISC) to form excited triplet states. Various photochemical reactions could be originated from excited triplet from of benzene rings belonging to PS. As evident from GPC data, eCeCe and/or eC–He bond scissions at various sites could be initiated (Fig. 17).
where measured regularly after 500 h of UV exposure time under same conditions. A considerable weight loss was observed in the samples with increase in the time of exposure. It was also found that the weight loss of 3 wt % TiO2 loaded PS was found to be higher compared to that of pristine PS as shown in Fig. 12. The observed weight loss may be due to the formation of volatile species that have been formed as a result of photo-oxidation and chain scission of the polymer chain during UV exposure. 6.3. Mechanical properties of irradiated and non-irradiated PS and PS-TiO2 composites Tensile property of PS and PS-TiO2 composite was measured and it was found that tensile strength of PS as well as PS-TiO2 composite has decreased with the increase in UV irradiation time. Fig. 13, shows the variation of tensile strength in MPa of PS and PS-TiO2 composite (3 wt% TiO2) irradiated under UV lamp for 0 h, 500 h and 1000 h respectively. Flexural strength of both PS and PS-TiO2 decreased with the increase in time of UV irradiation (Fig. 14). The decrease in the value of tensile and flexural properties was a clear evidence of degradation of polymer chain. 6.4. Electric property of irradiated and non-irradiated PS and PS-TiO2 composites Dielectric constant of both pristine PS as well as nano PS-TiO2 composite films was determined by measuring their capacitance using the following equation (Eq. (9))(Grove et al., 2005; Rao et al., 2000). C = εrεₒA/t
(9)
Were C is the Capacitance; εr is the dielectric constant of the polymer 11
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FTIR spectra shows the evident of formation of alkenic eC]Ce double bonds and conjugated double bonds. This could be possible when cleavage –C–H bonds belonging to adjacent carbons occur as represented in Fig. 18. Formation of eC]O, −OOH, −OH etc groups were also evident from the FTIR spectra. This could only be possible due to the adsorption of H2O and O2 on the surface of PS. H2O and O2 could also be trapped on PS as impurities during the time of its manufacture. These molecules form reactive radicals or ions when interacted with incoming UV radiations. The possible mechanism is as illustrated in Fig. 19. The results of various analyses discussed above supports the fact that the rate of photodegradation of PS increased in the presence of nano TiO2 photocatalyst. It was also found that the increase in the percentage of TiO2 loaded to PS matrix showed no left or right shift in the characteristic IR bands of PS. This observation made it clear that there existed no chemical bond between PS and TiO2. The interaction between PS and TiO2 was just physical. The mechanism of photodegradation in the case of PS-TiO2 composite occurs not only by PS absorbing UV radiations to get excited but also by TiO2 particles absorbing UV radiations creating electron-hole pair(Shang et al., 2003). Electrons get excited to conduction band leaving behind holes in valence band. Photocatalysis is possible only when the newly created electron- hole pair interacts with external atoms or molecules before recombining. Adsorbed O2 molecules over the surface of TiO2 interacts with the excited electrons in conduction band to form ions such as O−, O2−etc. H2O or OH− ions adsorbed on the surface of TiO2 results in the formation of H+, OH%, OH− etc specious. These newly created ions and radicals further interacts with PS chain forming hydroxides, hydrperoxides, carbonyl compounds etc and the degradation process get propagated as illustrated in Figs. 17, 18 and 19.
Science and Technology, NIT-Calicut; Department of Polymer Science and Technology, CUSAT and SAIF-STIC, CUSAT for their technical support. References Abdelghany, A.M., Abdelrazek, E.M., Badr, S.I., Morsi, M.A., 2016. Effect of gamma-irradiation on (PEO/PVP)/Au nanocomposite: materials for electrochemical and optical applications. Mater. Des. 97, 532–543. https://doi.org/10.1016/j.matdes.2016. 02.082. Al Safi, S.A., Al Mouamin, T.M., Al Sieadi, W.N., Al Ani, K.E., 2014. Irradiation effect on photodegradation of pure and plasticized poly (4-methylstyrene) in solid films. Mater. Sci. Appl. 5 (05), 300. Alam, M.J., Cameron, D.C., 2002. Preparation and characterization of TiO2 thin films by sol-gel method. J. Sol–Gel Sci. Technol. 25 (2), 137–145. https://doi.org/10.1023/ A:1019912312654. Alwan, T.J., 2010. Refractive index dispersion and optical properties of dye doped polystyrene films. Malays. Polym. J. 5 (2), 204–213. https://doi.org/10.3906/fiz1107-5. Ani, K.E.Al, Ramadhan, A.E., 2008. Photodegradation kinetics of poly(para-substituted styrene) in solution. Polym. Degrad. Stab. 93 (8), 1590–1596. https://doi.org/10. 1016/j.polymdegradstab.2008.04.010. Ba-Abbad, M.M., Kadhum, A.A.H., Mohamad, A.B., Takriff, M.S., Sopian, K., 2012. Synthesis and catalytic activity of TiO2 nanoparticles for photochemical oxidation of concentrated chlorophenols under direct solar radiation. Int. J. Electrochem. Sci. 7, 4871–4888. Chandran, A., Francis, N., Jose, T., George, K., 2010. Synthesis, structural characterization and optical bandgap determination of ZnS nanoparticles. SB Acad. Rev. 17, 17–21. Chaukura, N., Gwenzi, W., Bunhu, T., Ruziwa, D.T., Pumure, I., 2016. Potential uses and value-added products derived from waste polystyrene in developing countries: a review. Resour. Conserv. Recycl. 107, 157–165. https://doi.org/10.1016/j.resconrec. 2015.10.031. Gibbs, B.F., Mulligan, C.N., 1997. Styrene toxicity: an ecotoxicological assessment. Ecotoxicol. Environ. Saf. 38 (3), 181–194. https://doi.org/10.1006/eesa.1997.1526. Grove, T.T., Masters, M.F., Miers, R.E., 2005. Determining dielectric constants using a parallel plate capacitor. Am. J. Phys. 73 (1), 52–56. Hassanjani-Roshan, A., Kazemzadeh, S.M., Vaezi, M.R., Shokuhfar, A., 2011. Effect of sonication power on the sonochemical synthesis of titania nanoparticles. J. Ceram. Process. Res. 12 (3), 299–303. Jaleh, B., Madad, M.S., Tabrizi, M.F., Habibi, S., Golbedaghi, R., Keymanesh, M.R., 2011. UV-degradation effect on optical and surface properties of polystyrene-TiO2 nanocomposite film. J. Iran. Chem. Soc. 8 (1), S161–S168. https://doi.org/10.1007/ BF03254293. Jane, M., Tryon, M., Achhammer, B.G., 1953. Study of degradation of polystyrene, using ultraviolet spectrophotometry. J. Res. Bur. Stand. 51 (3). Karami, A., 2010. Synthesis of TiO2 nano powder by the sol-gel method and its use as a photocatalyst. J. Iran. Chem. Soc. 7 (2), S154–S160. https://doi.org/10.1007/ BF03246194. Kavitha, T., Rajendran, A., Durairajan, A., 2012. Synthesis, characterization of TiO2 nano powder and water based nanofluids using two step method. Eur. J. Appl. Eng. Sci. Res. 1, 235–240. Kwon, B.G., Saido, K., Koizumi, K., Sato, H., Ogawa, N., Chung, S.-Y., et al., 2014. Regional distribution of styrene analogues generated from polystyrene degradation along the coastlines of the North-East Pacific Ocean and Hawaii. Environ. Pollut. 188, 45–49. https://doi.org/10.1016/j.envpol.2014.01.019. López, R., Gómez, R., 2012. Band-gap energy estimation from diffuse reflectance measurements on sol–gel and commercial TiO2: a comparative study. J. Sol–Gel Sci. Technol. 61 (1), 1–7. https://doi.org/10.1007/s10971-011-2582-9. Maharana, T., Negi, Y.S., Mohanty, B., 2007. Recycling of polystyrene. Polym. Plast. Technol. Eng. 46 (7), 729–736. Malekshahi Byranvand, M., Nemati Kharat, A., Fatholahi, L., Malekshahi Beiranvand, Z., 2013. A review on synthesis of nano-TiO2 via different methods. J. Nanostruct. 3 (1), 1–9. https://doi.org/10.7508/jns.2013.01.001. Parra, R., Góes, M.S., Castro, M.S., Longo, E., Bueno, P.R., Varela, J.A., 2007. Reaction pathway to the synthesis of anatase via the chemical modification of titanium isopropoxide with acetic acid. Chem. Mater. 20 (1), 143–150. Rao, Y., Qu, J., Marinis, T., Wong, C.P., 2000. A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory. IEEE Trans. Compon. Packag. Technol. 23 (4), 680–683. Reddy, K.M., Manorama, S.V., Reddy, A.R., 2003. Bandgap studies on anatase titanium dioxide nanoparticles. Mater. Chem. Phys. 78 (1), 239–245. https://doi.org/10. 1016/S0254-0584(02)00343-7. Rincón, G.A., 1997. Photodegradation of chain halogenated polystyrene: poly(c-chloro and c-bromostyrene)s. Polym. Bull. 38 (2), 191–196. https://doi.org/10.1007/ s002890050037. Sangawar, V., Golchha, M., 2013. Evolution of the optical properties of polystyrene thin films filled with zinc oxide nanoparticles. Int. J. Sci. Eng. Res. 4 (6), 2700–2705. Shang, J., Chai, M., Zhu, Y., 2003. Solid-phase photocatalytic degradation of polystyrene plastic with TiO2 as photocatalyst. J. Solid State Chem. 174 (1), 104–110. https:// doi.org/10.1016/S0022-4596(03)00183-X. Simonsen, M.E., Søgaard, E.G., 2010. Sol–gel reactions of titanium alkoxides and water: influence of pH and alkoxy group on cluster formation and properties of the resulting products. J. Sol–Gel Sci. Technol. 53 (3), 485–497.
7. Conclusions Nano TiO2 was synthesized via sol-gel route and characterised by SEM, EDX and XRD. The synthesized TiO2 particles composed of mostly anatase phase with particle diameter ≈25 nm. Photodegradation had taken place on the surface of solid phase PS films when subjected to UV irradiation (253 nm) under normal condition. Photodegradation of PS was proportional to UV irradiation time. The rate of photodegradation was higher in PS-TiO2 composite and the rate of degradation increased with the increase in the percentage of TiO2 loaded into PS matrix. The degradation of PS and PS-TiO2 composite had taken place through photooxidative mechanism. This fact was supported by the increase in FTIR spectral bands of carbonyl groups, hydroxyl group etc., with the increase in the time of UV irradiation. Changes in regular absorption bands in the UV region were also observed for both PS and PS-TiO2 in their UV–vis spectra upon irradiation. Weight loss of PS-TiO2 composite films was more compared to pristine PS films upon increase in UV irradiation time. Tensile and flexural properties of PS as well as PS-TiO2 composite had decreased with the increase in the time of irradiation. Dielectric strength of the specimens increased with UV exposure time with maximum increase for PS-TiO2 composite. The decrease in the dielectric breakdown measured at a frequency of 50Hz with a maximum decrease for the PS-TiO2 composite suggested the formation of charged centers upon UV irradiation. These results highlighted the fact that degradation in the mechanical, electrical and chemical properties has taken place as result of UV irradiation, rendering an effective methodology of environment friendly disposal of polystyrene. Declarations of interest None. Acknowledgements The authors thank UGC for financial support, School of Nano 12
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D.l. S, et al. Sirohi, S., Sharma, T.P., 1999. Bandgaps of cadmium telluride sintered film. Opt. Mater. 13 (2), 267–269. https://doi.org/10.1016/S0925-3467(98)00084-6. Terabe, K., Kato, K., Miyazaki, H., Yamaguchi, S., Imai, A., Iguchi, Y., 1994. Microstructure and crystallization behaviour of TiO2 precursor prepared by the solgel method using metal alkoxide. J. Mater. Sci. 29 (6), 1617–1622. https://doi.org/ 10.1007/BF00368935. Thakur, S., Verma, A., Sharma, B., Chaudhary, J., Tamulevicius, S., Thakur, V.K., 2018. Recent developments in recycling of polystyrene based plastics. Curr. Opin. Green Sustain. Chem. 13, 32–38. https://doi.org/10.1016/j.cogsc.2018.03.011. Thamaphat, K., Limsuwan, P., Ngotawornchai, B., et al., 2008. Phase characterization of
TiO2 powder by XRD and TEM. Kasetsart J. (Nat. Sci.) 42 (5), 357–361. Thomas, R.T., Nair, V., Sandhyarani, N., 2013. TiO2 nanoparticle assisted solid phase photocatalytic degradation of polythene film: a mechanistic investigation. Colloids Surf. A: Physicochem. Eng. Asp. 422, 1–9. https://doi.org/10.1016/j.colsurfa.2013. 01.017. Timóteo, G.A.V., Fechine, G.J.M., Rabello, M.S., 2007. Stress cracking and photodegradation: the combination of two major causes of HIPS failure. Macromol. Symp. 258, 162–169. Vijayalakshmi, R., Rajendran, V., 2012. Synthesis and characterization of nano-TiO2 via different methods. Arch. Appl. Sci. Res. 4 (2), 1183–1190.
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Solid-phase photodegradation of polystyrene by nano TiO2 under ultraviolet radiation
T
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Dinoop lal Sa, Sunil Jose Ta, , Rajesh Cb a b
Dept. of Chemistry, St. Thomas’ College, Thrissur, Kerala, 680 001, India Dept of Chemistry, MES Keveeyam College, Valanchery, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Sol-gel method Nano titanium dioxide Ultraviolet irradiation Photooxidative degradation Chain scission
Solid phase photodegradation of polystyrene (PS) and Polystyrene – titanium dioxide nano composite (PS-TiO2) was studied under ultraviolet radiation of wavelength of 253 nm. Titanium dioxide (TiO2) photocatalyst was synthesized via sol-gel method and characterized using SEM, EDX and XRD. Photodegradation of PS and PS-TiO2 was monitered using GPC, FTIR spectroscopy, UV-DRS, SEM and weight loss measurements. An appreciable decrease in the values of average molecular weights were observed from the GPC data with the increase in the UV exposure time for PS as well as PS-TiO2 composites. FTIR and UV–vis spectra revealed that the photodegradation of PS and PS-TiO2 composite was accelerated by the increase in UV irradiation time and also with the increase in the percentage of TiO2. It was also evident from the FTIR spectra that photooxidative degradation was the possible mechanism of degradation. Band gap energies of PS and PS-TiO2 determined using Tauc plot decreased upon increase in UV irradiation. The change in surface morphology of the irradiated samples compared to the pristine ones were clearly observed in their SEM images. Weight loss of the polymer samples increased upon increase in UV irradiation time. Mechanical properties (tensile and flexural) of raw as well as irradiated samples of PS and PS-TiO2 composites showed an appreciable decrease in their values with increase in irradiation time. The values of dielectric constant of the UV irradiated polymer specimens showed higher values compared to that of non irradiated specimens at varying frequencies. Dielectric strength of the polymer specimens decreased with the increase in UV irradiation time.
1. Introduction Polystyrene (PS), made up of the monomer styrene has been playing a significant role in our day to day life for the past few decades. The rigid and foamed forms of polystyrene find its application in varied areas. Polystyrene has touched many areas of human need including medical products, appliances, toys, packaging, electronic insulations and still many more as they satisfies the optimum condition for the mode of their use as they are light, durable, strong, good thermal and electrical insulators, transparent and also they could be moulded into desired shape(Chaukura et al., 2016). In some cases, products made of polystyrene can be recycled and can be used for other applications (Maharana et al., 2007; Thakur et al., 2018). Nowadays the concerns regarding polystyrene as pollutants have arisen due to the use and throw system practiced by the human community. Polystyrene, as a major component of debris found in land and marine system, threats the bio system and leads to the transfer of toxic chemicals to the food chain. Polystyrene which are blown in the wind
⁎
can be lethal to birds and many creatures. Burning polystyrene debris is not an appropriate solution for their eradication because during the ignition process, many harmful gases which include certain reactive radical specious are evolved. These gases may cause suffocation and many sorts of disease to the creatures consuming them. They may also be carcinogenic(Gibbs and Mulligan, 1997; Kwon et al., 2014). Polystyrene undergoes photo oxidative degradation when exposed to ultraviolet (UV) radiation. Naturally occurring photodegradation is of course a long process. During photodegradation, radiations especially in the UV region are absorbed by polystyrene and causes change in its original properties. Photodissociation (chain scission), formation of carbonyl groups, isolated and conjugated double bonds and slight yellowing occurs due to radical initiated mechanism. Photodegradation results a change in the original properties of the polymer with depleted mechanical properties resulting in weight loss, brittleness etc (Al Safi et al., 2014; Ani and Ramadhan, 2008; Jane et al., 1953; Rincón, 1997; Timóteo et al., 2007). Nano TiO2 synthesized by sol-gel route was used as photocatalyst by loading it to PS at various weight percentages. The
Corresponding author. E-mail address: [email protected] (S.J. T).
https://doi.org/10.1016/j.enmm.2019.100229 Received 18 March 2019; Received in revised form 10 May 2019; Accepted 14 May 2019 2215-1532/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. XRD pattern (B) SEM image and (C) EDAX of nano TiO2 synthesized by sol-gel route.
Rajendran, 2012).
samples were irradiated and analysed at regular intervals of 500 h. A comparison of photodegradation of pure PS as well as PS-TiO2 blends at regular intervals was done. Mechanical and electrical properties of PS and PS-TiO2 blends were also measured before and after irradiation.
2.2. Characterization of nano TiO2 The particle size and morphology of the synthesized nano TiO2 was studied using Scanning Electron Microscope (FESEM) Hitachi SU6600 Variable Pressure Field Emission. The elemental composition in the synthesized sample was determined by Energy Dispersive X-Ray (EDX) analysis. XRD pattern of synthesized TiO2 was done using X-Ray Powder Diffractometer Bruker AXS D8.
2. Materials and methods Materials: Titanium(IV)isopropoxide (TTIP) (Sigma Aldrich), Polystyrene(PS) (LG Polymer India Pvt. Ltd). Ultraviolet (UV) tube 30 W 253 nm (Philips Hollend), ethanol, toluene and deionized water were used.
3. Preparation of PS and PS-TiO2 composite films 2.1. Preparation of nano TiO2 5g of PS pellets were dissolved in 20 ml of toluene and sonicated for an hour using 750 W probe sonicator. The temperature of the system was maintained below 45 °C so as to eliminate chances of thermal degradation at higher temperatures. This was then poured into petridishes, taken to a vacuum oven and kept under a vacuum of 700 mm Hg for 24 h and then allowed to dry naturally for another 48 h. PS-TiO2 composites were prepared in similar manner as discussed above by loading various weight percentages of nano TiO2 into toluene solution of PS. Thin dry films of PS and PS-TiO2 so obtained were subjected to photodegradation studies.
Sol-gel method was adopted for the synthesis of TiO2. The precursor used was Titanium(IV) isopropoxide (TTIP). About 0.5 ml of TTIP was added to a mixture of 2 ml ethanol and 2 ml deionized water in a boiling tube with constant stirring using a magnetic stirrer.. The temperature of the system was set to 50 °C. pH 4 was maintained by adding drops of HNO3, tested by litmus indicator. The stirring was continued for three hours after which it was centrifuged. The settled white precipitate was separated and washed with deionized water three times. Nano particles of TiO2 collected were dried and calcined at 400 °C for 2 h. The synthesized nano TiO2 was characterized and used as the photocatalyst for the photodegradation of PS(Alam and Cameron, 2002; Ba-Abbad et al., 2012; Karami, 2010; Kavitha et al., 2012; Malekshahi Byranvand et al., 2013; Parra et al., 2007; Shang et al., 2003; Simonsen and Søgaard, 2010; Terabe et al., 1994; Thomas et al., 2013; Vijayalakshmi and
4. Photodegradation of PS films Photodegradation of PS films were studied by exposing them to ultraviolet (UV) radiation. The UV irradiation chamber contained a UV 2
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Fig. 2. (A) Reflectance spectra of nano TiO2 obtained from UV-Vis Spectrophotometer. (B) Plot of F(R) versus hʋ of nano TiO2. (C) (F(R)hʋ)1/2 versus hʋ representing the indirect allowed transition of nano TiO2 .(D) (F(R)hʋ)2 versus hʋ representing the direct allowed transition of nano TiO2.
5. Preparation of PS and PS-TiO2 composites for determination of mechanical properties and electrical properties Specimens for determination of mechanical (tensile and flexural) properties were prepared by injection moulding as per ASTM standard. Polymer specimens in the form of circular discs (1 mm thick and 7.5 cm diameter) were moulded in a hydraulic hot press at 180 ͦC for measuring dielectric strength. The samples were then subjected to UV irradiation and tested at regular interval of 500 h. Mechanical measurements were recorded using a universal testing machine (Autograph AGX plus, 10 kN, Shimadzu). Dielectric strength was measured by keeping the polymer disc between 2 electrodes immersed in transformer oil in a specially designed chamber connected to a high power electric supply. Fig. 3. PS-TiO2 Composite before UV irradiation (A) and after UV irradiation of 1500 h (B).
6. Results and discussion 6.1. Characterisation of synthesized nano TiO2
tube (30 W, 253 nm, Philips Holland) and the samples were irradiated keeping the distance between the UV tube and the samples at exactly 8 cm.A small portion of the sample films were cut out at regular intervals of every 500 h (i.e., 500 h, 1000 h and 1500 h) and subjected to IR studies using FTIR spectrometer IR Affinity-1, Shimadzu, Japan, UV–vis spectroscopic analysis using UV–vis spectrophotometer (UV2600 spectrometer, Shimadzu, Japan) and SEM analysis using (JEOL Model JSM - 6390 L V).
The XRD pattern of TiO2 synthesized by sol-gel method was shown in Fig. 1A. Strong diffraction peaks observed at 2θ = 24.940°, 47.696°, 53.636° and 62.404° reveals that the synthesized nano TiO2 is in anatase phase. The diffraction peaks obtained at 2θ = 27.069° and 54.727° indicates the presence of rutile phase in a lower percentage (Hassanjani-Roshan et al., 2011; Thamaphat et al., 2008). Crystallite size of the synthesized TiO2 particles was calculated using the Scherrer’s formula. 3
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Table 1 PDI,S and Nt calculated from the GPC data of PS and PS-TiO2 composite. Sample
UV irradiation time (h)
Mw (Da)
Mn (Da)
PDI (Mw/Mn)
Nt
S (Mn)
1
0 ⎡ − 1⎤ ⎣ (Mn)t ⎦
⎡ − ⎣ (Mn)t
1 ⎤ (Mn)0 ⎦
PS
0 500 1000 1500
92031 87228 86100 85476
75346 69470 67235 65916
1.221445 1.255621 1.280583 1.296741
0 0.084583 0.120637 0.143061
0 1.12 × 10−6 1.6 × 10−6 1.9 × 10−6
PS-TiO2
0 500 1000 1500
92723 85900 82500 81419
76329 68731 63508 61480
1.214781 1.2498 1.299049 1.324317
0 0.110547 0.20188 0.241526
0 1.45 × 10−6 2.64 × 10−6 3.16 × 10−6
Fig. 4. A) Weight average molecular weight (Mn), B) Number average molecular weight (Mw), C) Number of chain scission per molecule (S) and D) number of scission events per gram (Nt) of PS and PS+3%TiO2 composite.
D=
k.λ β.Cos θ
SEM image of the synthesized TiO2 reveals its spherical morphology with particle diameter ≈25 nm Fig. 1B. Energy Dispersive X-Ray (EDAX) clearly confirms that the synthesized nano TiO2 particles were pure Fig. 1C. Band gap energy of the synthesized nano TiO2 samples were calculated from UV–vis Reflectance spectra (Fig. 2A) applying Kubelka–Munk (KeM or F(R)) function (Eq. (2)) in Tauc method(Chandran et al., 2010; López and Gómez, 2012; Reddy et al., 2003).
(1)
Where D = Crystallite size (nm) K = Shape-sensitive coefficient (0.89- For spherical spheres) λ = Wavelength of the X-Ray beam (0.15406 nm for Cu-Kα radiation) β = Full width at half maximum (FWHM) of the peak under consideration θ = Diffracting angle The average crystallite size of the particles calculated from Scherrer’s formula was found to be 17.57 nm.
F(R) = (1-R)2/2R
(2)
where ‘R’ is the reflectance and ‘F(R)’ is proportional to the extinction coefficient (α). 4
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Fig. 5. (A) &(B).The FTIR spectra of pristine PS sample before UV irradiation(a), after 500 hr UV irradiation(b), 1000 hr UV irradiation(c),1500 hr UV irradiation(d).
Fig. 6. (A)& (B). The FTIR spectra of PS-TiO2 composite (PS loaded with 3 wt% of TiO2) before UV irradiation(a), after 500 hr UV irradiation(b), 1000 hr UV irradiation(c),1500 hr UV irradiation(d).
exponential factor to the final equation (Eq. (3)).
Table 2 Important observations made from FTIR spectra of PS and PS-TiO2 composites. Peaks
Groups
−1
3700–3600 cm 2631 cm−1 1740–1700 cm−1 1680–1650 cm−1 1452 cm−1, 1600 cm−1 1030, 905, 758, 700 cm−1 830 cm−1 650 cm−1
Free eOH/eOOH eOH (acid) stretch eC]O stretch eC]Ce Stretch Aromatic eC]Ce stretch Ar CeH (Out of plane bend) Conjugated eC]Ce ]CeH bend
(F(R)hʋ)n
Change in intensity up on UV irradiation
(3)
The plot of Eq. (3) versus energy (hʋ) in eV was used to calculate the band gap energy. Such a type of plot is called as Tauc plot. The plot (F (R)hʋ)1/2 versus hʋ represents the indirect allowed transition (Fig. 2C) were as (F(R)hʋ)2 versus hʋ represents the direct allowed transition (Fig. 2D). The Eg corresponding to indirect allowed and direct allowed transitions were found to be 3.86 eV and 3.36 eV respectively.
Intensity Increased Intensity Increased Intensity Increased Intensity Increased No change No change
6.2. Photo catalytic degradation of solid PS films using nano TiO2
Intensity Increased Intensity Increased
A slight yellowing and loss of transparency was observed in UV exposed PS and PS-TiO2 composite films. UV exposure of the specimens also resulted in increased brittleness. These physically observed facts suggested that a change in optical and mechanical properties of PS as well as PS-TiO2 composite had occurred as a consequence of UV irradiation (Fig. 3). Gel permeation Chromatography analysis on PS as well as PS-TiO2 was used to measure their average molecular weights mainly weight average molecular weight (Mw), Number average molecular weight (Mn) and Z average molecular weight (Mz). These average molecular weights decreased as the time of UV irradiation of the polymer
F(R) was plotted against energy (hʋ) in eV and the band gap energy (Eg) was determined from the plot obtained by extrapolating the linear portion of the curve to the X axis (Fig. 2B). The band gap energy determined from this plot was 3.6 eV, irrespective of transitions (direct or indirect). Band gap of TiO2 were also calculated using modified K–M function. For this, function F(R) was multiplied by hʋ and coefficient (n) associated with corresponding electronic transition were also introduced as
5
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Fig. 7. UV-Visible spectra of :-Pristine PS irradiated under UV lamp for 0hr,1000 hr and 1500hr((a), (b) and (c) respectively).PS (a) & PS-TiO2 composite (0.5%TiO2 (b) & 1%TiO2 (c)) irradiated under UV lamp for 1000 hr.PS-TiO2 composite (0.5%TiO2) irradiated under UV lamp for 0hr (a), 500hr (b)1000 hr (c) and 1500hr(d).
specimens increased. The following calculations were also made from the average molecular weight data obtained from GPC.
Average chain scission per polymer macro
molecule
(Mn)0 =⎡ − 1⎤ ⎢ ⎥ ⎣ (Mn)t ⎦
S (4)
Number of scission events per gram of polymer Nt 1 1 ⎤ =⎡ − ⎢ (Mn)0 ⎥ ⎣ (Mn)t ⎦ Polydispersity index PDI =
(5)
Mw Mn
(6)
Where (Mn)0 is the number average molecular weight of non irradiated polymer, (Mn)t is the number average molecular weight of polymer after t hours of irradiation The results obtained from GPC is tabulated below Table 1. The decrease in the average molecular weights (Mw, Mn and Mz) of the polymer on UV irradiation was due to polymer chain scission Fig. 4A The increasing value of S and Nt on irradiation gave the extent of chain scission at different UV exposure time Fig. 4B and C. The PDI of the polymer increased with the increase in UV exposure time suggesting a random scission of polymer chains. All the above facts clearly reveal the fact that the polymer chains have underwent random chain cleavage upon UV irradiation due to photodegradation. Another observed fact was that the decrease in the average molecular weights was
Fig. 8. Plot showing the degradation percentage (D%) of PS and PS-TiO2 composite (3 wt % TiO2) before and after UV irradiation.
6
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Fig. 9. Plot of (αhʋ)2 v/s hʋ (A): PS films irradiated for 0,500,1000 and 1500 hrs and (B): PS-TiO2 composite films irradiated for 0,500,1000 and 1500 hrs.
Fig. 10. Plot of (αhʋ)1/2 v/s hʋ (A): PS films irradiated for 0,500,1000 and 1500 hrs and (B): PS-TiO2 composite films irradiated for 0,500,1000 and 1500 hrs.
Fig. 11. SEM image of PS+3%TiO2 composite film irradiated for 0 hr (a) and 1500hr (b).
−OOH, eC]O, −COOH, eC]Ce etc. The variation in other peak intensities also emphasized the formation of conjugated double bond, carbonyl groups in the polymer backbone chain, breakage of polymer chain etc. From the FTIR spectra, it was found that all the samples of pristine PS before and after UV irradiation (0 h, 500 h, 1000 h and 1500 h) showed characteristic peaks of phenyl ring at around 695 cm−1 750 cm−1, 906 cm−1 and 1028 cm−1 with no change in their intensities (Fig. 5A). These bands attributed to the C–H out of plane bending
predominant in PS-TiO2 composite compared to pristine PS upon UV irradiation. The parameters S, Nt showed more increase in PS-TiO2 composite than pristine PS up on irradiation. All these facts suggested that the extant of photodegradation was predominant in PS-TiO2 composite compared to pristine PS. Photodegradation of PS (pristine and composite) was further confirmed by FTIR spectroscopy. FTIR spectra of pristine PS at different irradiation time intervals clearly showed the variation in peak intensities corresponding to various functional groups such as −OH, 7
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phenyl rings of PS remain intact after UV irradiation. An increase in the intensity of bands corresponding to carbonyl frequencies ranging between 1750 to 1650 cm−1 were observed upon increase in UV irradiation time (Fig. 5B). These observations clearly reveal the fact that there was formation and increase in the intensities of carbonyl (eC]O) stretching bands (1740–1700 cm−1) as well as carbon- carbon double bond (eC]Ce) stretching bands (1680−1600 cm−1) upon UV irradiation. The intensity of bands corresponding to hydroxyl and/or hydroperoxy groups (3700–3600 cm−1) also showed an appreciable increase during UV irradiation. Formation of new −OH /−OOH groups upon UV irradiation were established from this observation. Photodegradation of PS-TiO2 composites were also monitered using FTIR spectroscopy. FTIR spectra of PS-TiO2 composites with different UV irradiation periods (500 h, 1000 h and 1500 h) (Fig. 6) as well as PS loaded with varying weight percentages of TiO2 (0%, 0.5%, 1%, 2% and 3% of TiO2) were analysed. The FTIR spectra PS-TiO2 composite resemble that of pristine PS. The only differences was that the intensities of absorption bands corresponding to eC]O, eC]Ce, −OH, −OOH, PheC]O etc of PS-TiO2 composite showed a higher increase with the increase in UV irradiation time It was also observed that the increase in absorption bands of the functional groups mentioned above were higher for the PS-TiO2 composite with higher percentage of TiO2. The following conclusions were drawn from the above observations. The increase in intensities of to eC]O,−OH, −OOH, PheC]O etc., functional groups suggested that photo oxidative mechanism has taken place upon UV irradiation. Increase in the intensities of bands corresponding to eC]Ce stretch, Conjugated eC]Ce, =C–H bend etc suggests that chain and/or bond scission(due to eCeCe or –C–H bond breakage) had taken place. Since there was no change in aromatic eC]Ce (stretch), aromatic-C–H (out of plane bend) etc, it was clear that the phenyl ring remained intact upon UV irradiation. It was also concluded that for PS-TiO2, the rate of degradation was more effective compared to pristine PS. Rate of photodegradation of PS-TiO2 composite increased with the increase in the weight percent of loaded TiO2 and also with the increase in UV irradiation time. The important observations made from FTIR spectral analysis of PS and PS-TiO2 composite is tabulated in Table 2. UV-DRS spectroscopy of pristine PS as well as PS-TiO2 composite further supports photodegradation. Characteristic absorption peak of PS was observed in the UV region (between 220–400 nm) (Fig. 7). The intensity of these UV absorption bands showed an appreciable decrease with the increase in irradiation time. As UV irradiation time of the specimens increased, peak broadening towards higher wavelength was observed with an increase in the band intensities corresponding to visible region (380–600 nm for PS and 380–725 nm for PS-TiO2). The decrease in absorption bands corresponding to the UV region suggest that PS chain had underwent chain breakage causing a decrease in its characteristic absorption bands. New conjugated double bonds have formed as a consequence of photooxidation resulting in the increase of absorption bands in the visible region. UV-DRS spectra revealed that the increase in concentration of loaded TiO2 in PS-TiO2 composite as well as the increase in UV exposure time of the composite resulted in increased rate of photodegradation. Degradation percentages of PS and PS-TiO2 composites were calculated from the UV–vis absorption spectra using Eq. (7).
Fig. 12. Percentage of weight loss of pristine PS and PS loaded with 3% TiO2 with UV irradiation time.
Fig. 13. Tensile strength of PS as well as PS-TiO2 composite (3 wt % TiO2) with different irradiation times (0,500,1000 & 1500 hours respectively).
D% = [(Aₒ − A)/Aₒ] × 100
(7)
Where D% is the degradation percentages of PS and PS-TiO2 ; Aₒ and A are absorbance of the polymer samples before and after irradiation respectively. From the graphical representation of D% (Fig. 8) it was observed that the D% of PS-TiO2 composite was higher than that of PS after UV irradiation. Optical band gap energy of the PS and PS-TiO2 composite films were determined using Tauc relation given below (Abdelghany et al., 2016;
Fig. 14. Flexural strength of PS as well as PS-TiO2 composite (3 wt % TiO2) with different irradiation times (0,500,1000 & 1500 hours respectively).
frequencies of the phenyl ring. The peak at 1452 cm-1 corresponding to aromatic carbon-carbon double bond stretch (AreC]Ce stretch) also showed no change in peak intensity. The above observations show that 8
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Fig. 15. Plot of dielectric constants versus frequency of PS (A) and PS-TiO2 composite (3 wt % TiO2) (B) with varying irradiation times (0 hr (a),500 hr (b) & 1500 hr (c) respectively).
Alwan, 2010; Jaleh et al., 2011; Sangawar and Golchha, 2013; Sirohi and Sharma, 1999). αhʋ = A(hʋ-Eg)n
(8)
Were α is the absorption coefficient, hʋ is the energy of photon in eV (h = Plank’s constant and ʋ = frequency of radiation), A is a constant (different for different transition), Eg is the band gap energy and the index n is assumed to have different values corresponding to different electronic transitions (n = ½ for direct allowed transition and n = 2 for indirect allowed transition). A plot of (αhʋ)2 versus hʋ gave the direct allowed band gap energy (Eg) of PS as well as PS-TiO2 composite films, on extrapolating the linear portion of the curve to the X axis (where α = 0) (Fig. 9). The indirect allowed Eg of PS as well as PS-TiO2 composite films was determined from the plot of (αhʋ)1/2 versus hʋ and extrapolating the linear portion of the curve to the X axis (Fig. 10). The value of direct allowed and indirect allowed optical Eg for PS as well as PS-TiO2 composites showed a decrease in value upon increase in the time of UV irradiation. It was found that the decrease in Eg of PS-TiO2 composites were higher compared to that of PS samples upon UV irradiation. It was also observed that the Eg of PS-TiO2 composite (direct Eg = 3.25 eV,
Fig. 16. Dielectric breakdown of PS and PS-TiO2 composite (3 wt % TiO2).
Table 3 Observations and conclusions made from GPC, FTIR spectroscopy, UV-DRS spectroscopy, electrical studies, weight loss measurements and SEM. Observations (with the increase in UV exposure time)
Conclusions or eCeH bond of the polymer chain has ruptured upon UV exposure • eCeCe • Scission of polymer chains was random
GPC Decrease in Mw,Mn,Mz Increase in No: of chain Scission per macro molecule (S) Increase in Scission events per gram (Nt) Decrease in Mechanical Properties of PS (Tensile & Flexural strengths) Electrical Studies Dielectric strength (BDV) has decreased Dielectric constant have increased FTIR Increase in the intensity of peaks corresponding to eC]O, eOH, eOOH, eC]Ce, conjugated eC]Ce etc No change in the intensity of peaks corresponding to phenolic stretch or bend.
• • • •
of charge centers • Formation • Decrease in the capacitance property of new C]C linkages • Formation of new eC]O groups • Formation of new eOH, eOOH etc groups • Formation of new eC]CeC]CeC]C (conjugation) • Formation ring remains intact • Phenolic of characteristic absorption of PS due to photodegradation. • Loss shift (Which could also be observed as slight yellowing of PS after • Red irradiation) • The ability of polymer specimens to absorb UV radiation has decreased. of volatile specious/gases formed as a consequence of photodegradation • Loss of PS • Change in surface morphology of PS
• •
UV-DRS spectroscopy Decrease in characteristic absorption peaks of PS. Peaks have shifted towards higher wavelength. Band gap energy(Eg) calculations Reduces to lower energy Weight loss measurements Weight loss was observed SEM
• • • •
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Fig. 17. xxxPossible -C-C- and/or -C-H- bond scissions at various sites of PS.
Fig. 18. Formation of C=C double bond and conjugated double bonds.
TiO2 composite film. After 2000 h of UV irradiation, the surface morphology of the composite film seemed to be rough. The observed roughness in the SEM image attributed to the degradation process taken place on the surface of the composite film. Weight loss of both pristine PS as well as 3 wt% TiO2 loaded PS
indirect Eg = 3.98 eV) were lower than that of pristine PS (direct Eg = 4.12 eV, indirect Eg = 4.25 eV). Fig. 11 show the SEM image of 3 wt % TiO2 loaded PS before UV irradiation and after 2000 h of UV irradiation. The SEM image clearly shows the surface morphology of the irradiated and non irradiated PS10
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Fig. 19. Formation of –OOH,-C=O and -OH double bond and conjugated double bonds.
films; εₒ is the dielectric constant of free space (8.854 × 10−12 F/m); A and t are area and thickness of the polymer films respectively. The plots of εr against frequency of both PS and PS-TiO2 composite is illustrated in Fig. 15A and B. From the plots it was observed that as the time of UV irradiation increased there was an increase in the value of εr for a particular frequency for both PS as well as PS-TiO2 composite. Dielectric breakdown of the polymer films were measured at a frequency of 50 Hz and it was found that the value of break down voltage (BDV) decreased upon increase in uv exposure time (Fig. 16). The value of breakdown voltage was found to be higher for the PS-TiO2 composite before irradiation. UV irradiation resulted in the decrease of BDV for both the polymer samples (PS and PS-TiO2) due to the formation of new charge centers caused by the degradation of the samples. These charge centers allow electric current to pass through the sample more easily. Table 3 highlights the important observations and their conclusions drawn from the above studies of PS and PS-TiO2 photodegradation. It was found that the observations given in Table 3 were most predominant in PS-TiO2 composite compared to pristine PS. This envisages that photodegradation was most predominant in PS-TiO2 proving TiO2 to be a good photocatalyst in the UV region. Mechanism of photodegration of PS followed random pathways due to the multiple possibility of bond breakage, bond recombination, addition or elimination of various atoms. Based on all the above observations summerised in Table 3 the following mechanism could be considered. The photodegradation starts with the phenolic rings of PS absorbing UV radiation and getting excited into singlet states and then undergoing an inter system crossing (ISC) to form excited triplet states. Various photochemical reactions could be originated from excited triplet from of benzene rings belonging to PS. As evident from GPC data, eCeCe and/or eC–He bond scissions at various sites could be initiated (Fig. 17).
where measured regularly after 500 h of UV exposure time under same conditions. A considerable weight loss was observed in the samples with increase in the time of exposure. It was also found that the weight loss of 3 wt % TiO2 loaded PS was found to be higher compared to that of pristine PS as shown in Fig. 12. The observed weight loss may be due to the formation of volatile species that have been formed as a result of photo-oxidation and chain scission of the polymer chain during UV exposure. 6.3. Mechanical properties of irradiated and non-irradiated PS and PS-TiO2 composites Tensile property of PS and PS-TiO2 composite was measured and it was found that tensile strength of PS as well as PS-TiO2 composite has decreased with the increase in UV irradiation time. Fig. 13, shows the variation of tensile strength in MPa of PS and PS-TiO2 composite (3 wt% TiO2) irradiated under UV lamp for 0 h, 500 h and 1000 h respectively. Flexural strength of both PS and PS-TiO2 decreased with the increase in time of UV irradiation (Fig. 14). The decrease in the value of tensile and flexural properties was a clear evidence of degradation of polymer chain. 6.4. Electric property of irradiated and non-irradiated PS and PS-TiO2 composites Dielectric constant of both pristine PS as well as nano PS-TiO2 composite films was determined by measuring their capacitance using the following equation (Eq. (9))(Grove et al., 2005; Rao et al., 2000). C = εrεₒA/t
(9)
Were C is the Capacitance; εr is the dielectric constant of the polymer 11
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FTIR spectra shows the evident of formation of alkenic eC]Ce double bonds and conjugated double bonds. This could be possible when cleavage –C–H bonds belonging to adjacent carbons occur as represented in Fig. 18. Formation of eC]O, −OOH, −OH etc groups were also evident from the FTIR spectra. This could only be possible due to the adsorption of H2O and O2 on the surface of PS. H2O and O2 could also be trapped on PS as impurities during the time of its manufacture. These molecules form reactive radicals or ions when interacted with incoming UV radiations. The possible mechanism is as illustrated in Fig. 19. The results of various analyses discussed above supports the fact that the rate of photodegradation of PS increased in the presence of nano TiO2 photocatalyst. It was also found that the increase in the percentage of TiO2 loaded to PS matrix showed no left or right shift in the characteristic IR bands of PS. This observation made it clear that there existed no chemical bond between PS and TiO2. The interaction between PS and TiO2 was just physical. The mechanism of photodegradation in the case of PS-TiO2 composite occurs not only by PS absorbing UV radiations to get excited but also by TiO2 particles absorbing UV radiations creating electron-hole pair(Shang et al., 2003). Electrons get excited to conduction band leaving behind holes in valence band. Photocatalysis is possible only when the newly created electron- hole pair interacts with external atoms or molecules before recombining. Adsorbed O2 molecules over the surface of TiO2 interacts with the excited electrons in conduction band to form ions such as O−, O2−etc. H2O or OH− ions adsorbed on the surface of TiO2 results in the formation of H+, OH%, OH− etc specious. These newly created ions and radicals further interacts with PS chain forming hydroxides, hydrperoxides, carbonyl compounds etc and the degradation process get propagated as illustrated in Figs. 17, 18 and 19.
Science and Technology, NIT-Calicut; Department of Polymer Science and Technology, CUSAT and SAIF-STIC, CUSAT for their technical support. References Abdelghany, A.M., Abdelrazek, E.M., Badr, S.I., Morsi, M.A., 2016. Effect of gamma-irradiation on (PEO/PVP)/Au nanocomposite: materials for electrochemical and optical applications. Mater. Des. 97, 532–543. https://doi.org/10.1016/j.matdes.2016. 02.082. Al Safi, S.A., Al Mouamin, T.M., Al Sieadi, W.N., Al Ani, K.E., 2014. Irradiation effect on photodegradation of pure and plasticized poly (4-methylstyrene) in solid films. Mater. Sci. Appl. 5 (05), 300. Alam, M.J., Cameron, D.C., 2002. Preparation and characterization of TiO2 thin films by sol-gel method. J. Sol–Gel Sci. Technol. 25 (2), 137–145. https://doi.org/10.1023/ A:1019912312654. Alwan, T.J., 2010. Refractive index dispersion and optical properties of dye doped polystyrene films. Malays. Polym. J. 5 (2), 204–213. https://doi.org/10.3906/fiz1107-5. Ani, K.E.Al, Ramadhan, A.E., 2008. Photodegradation kinetics of poly(para-substituted styrene) in solution. Polym. Degrad. Stab. 93 (8), 1590–1596. https://doi.org/10. 1016/j.polymdegradstab.2008.04.010. Ba-Abbad, M.M., Kadhum, A.A.H., Mohamad, A.B., Takriff, M.S., Sopian, K., 2012. Synthesis and catalytic activity of TiO2 nanoparticles for photochemical oxidation of concentrated chlorophenols under direct solar radiation. Int. J. Electrochem. Sci. 7, 4871–4888. Chandran, A., Francis, N., Jose, T., George, K., 2010. Synthesis, structural characterization and optical bandgap determination of ZnS nanoparticles. SB Acad. Rev. 17, 17–21. Chaukura, N., Gwenzi, W., Bunhu, T., Ruziwa, D.T., Pumure, I., 2016. Potential uses and value-added products derived from waste polystyrene in developing countries: a review. Resour. Conserv. Recycl. 107, 157–165. https://doi.org/10.1016/j.resconrec. 2015.10.031. Gibbs, B.F., Mulligan, C.N., 1997. Styrene toxicity: an ecotoxicological assessment. Ecotoxicol. Environ. Saf. 38 (3), 181–194. https://doi.org/10.1006/eesa.1997.1526. Grove, T.T., Masters, M.F., Miers, R.E., 2005. Determining dielectric constants using a parallel plate capacitor. Am. J. Phys. 73 (1), 52–56. Hassanjani-Roshan, A., Kazemzadeh, S.M., Vaezi, M.R., Shokuhfar, A., 2011. Effect of sonication power on the sonochemical synthesis of titania nanoparticles. J. Ceram. Process. Res. 12 (3), 299–303. Jaleh, B., Madad, M.S., Tabrizi, M.F., Habibi, S., Golbedaghi, R., Keymanesh, M.R., 2011. UV-degradation effect on optical and surface properties of polystyrene-TiO2 nanocomposite film. J. Iran. Chem. Soc. 8 (1), S161–S168. https://doi.org/10.1007/ BF03254293. Jane, M., Tryon, M., Achhammer, B.G., 1953. Study of degradation of polystyrene, using ultraviolet spectrophotometry. J. Res. Bur. Stand. 51 (3). Karami, A., 2010. Synthesis of TiO2 nano powder by the sol-gel method and its use as a photocatalyst. J. Iran. Chem. Soc. 7 (2), S154–S160. https://doi.org/10.1007/ BF03246194. Kavitha, T., Rajendran, A., Durairajan, A., 2012. Synthesis, characterization of TiO2 nano powder and water based nanofluids using two step method. Eur. J. Appl. Eng. Sci. Res. 1, 235–240. Kwon, B.G., Saido, K., Koizumi, K., Sato, H., Ogawa, N., Chung, S.-Y., et al., 2014. Regional distribution of styrene analogues generated from polystyrene degradation along the coastlines of the North-East Pacific Ocean and Hawaii. Environ. Pollut. 188, 45–49. https://doi.org/10.1016/j.envpol.2014.01.019. López, R., Gómez, R., 2012. Band-gap energy estimation from diffuse reflectance measurements on sol–gel and commercial TiO2: a comparative study. J. Sol–Gel Sci. Technol. 61 (1), 1–7. https://doi.org/10.1007/s10971-011-2582-9. Maharana, T., Negi, Y.S., Mohanty, B., 2007. Recycling of polystyrene. Polym. Plast. Technol. Eng. 46 (7), 729–736. Malekshahi Byranvand, M., Nemati Kharat, A., Fatholahi, L., Malekshahi Beiranvand, Z., 2013. A review on synthesis of nano-TiO2 via different methods. J. Nanostruct. 3 (1), 1–9. https://doi.org/10.7508/jns.2013.01.001. Parra, R., Góes, M.S., Castro, M.S., Longo, E., Bueno, P.R., Varela, J.A., 2007. Reaction pathway to the synthesis of anatase via the chemical modification of titanium isopropoxide with acetic acid. Chem. Mater. 20 (1), 143–150. Rao, Y., Qu, J., Marinis, T., Wong, C.P., 2000. A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory. IEEE Trans. Compon. Packag. Technol. 23 (4), 680–683. Reddy, K.M., Manorama, S.V., Reddy, A.R., 2003. Bandgap studies on anatase titanium dioxide nanoparticles. Mater. Chem. Phys. 78 (1), 239–245. https://doi.org/10. 1016/S0254-0584(02)00343-7. Rincón, G.A., 1997. Photodegradation of chain halogenated polystyrene: poly(c-chloro and c-bromostyrene)s. Polym. Bull. 38 (2), 191–196. https://doi.org/10.1007/ s002890050037. Sangawar, V., Golchha, M., 2013. Evolution of the optical properties of polystyrene thin films filled with zinc oxide nanoparticles. Int. J. Sci. Eng. Res. 4 (6), 2700–2705. Shang, J., Chai, M., Zhu, Y., 2003. Solid-phase photocatalytic degradation of polystyrene plastic with TiO2 as photocatalyst. J. Solid State Chem. 174 (1), 104–110. https:// doi.org/10.1016/S0022-4596(03)00183-X. Simonsen, M.E., Søgaard, E.G., 2010. Sol–gel reactions of titanium alkoxides and water: influence of pH and alkoxy group on cluster formation and properties of the resulting products. J. Sol–Gel Sci. Technol. 53 (3), 485–497.
7. Conclusions Nano TiO2 was synthesized via sol-gel route and characterised by SEM, EDX and XRD. The synthesized TiO2 particles composed of mostly anatase phase with particle diameter ≈25 nm. Photodegradation had taken place on the surface of solid phase PS films when subjected to UV irradiation (253 nm) under normal condition. Photodegradation of PS was proportional to UV irradiation time. The rate of photodegradation was higher in PS-TiO2 composite and the rate of degradation increased with the increase in the percentage of TiO2 loaded into PS matrix. The degradation of PS and PS-TiO2 composite had taken place through photooxidative mechanism. This fact was supported by the increase in FTIR spectral bands of carbonyl groups, hydroxyl group etc., with the increase in the time of UV irradiation. Changes in regular absorption bands in the UV region were also observed for both PS and PS-TiO2 in their UV–vis spectra upon irradiation. Weight loss of PS-TiO2 composite films was more compared to pristine PS films upon increase in UV irradiation time. Tensile and flexural properties of PS as well as PS-TiO2 composite had decreased with the increase in the time of irradiation. Dielectric strength of the specimens increased with UV exposure time with maximum increase for PS-TiO2 composite. The decrease in the dielectric breakdown measured at a frequency of 50Hz with a maximum decrease for the PS-TiO2 composite suggested the formation of charged centers upon UV irradiation. These results highlighted the fact that degradation in the mechanical, electrical and chemical properties has taken place as result of UV irradiation, rendering an effective methodology of environment friendly disposal of polystyrene. Declarations of interest None. Acknowledgements The authors thank UGC for financial support, School of Nano 12
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D.l. S, et al. Sirohi, S., Sharma, T.P., 1999. Bandgaps of cadmium telluride sintered film. Opt. Mater. 13 (2), 267–269. https://doi.org/10.1016/S0925-3467(98)00084-6. Terabe, K., Kato, K., Miyazaki, H., Yamaguchi, S., Imai, A., Iguchi, Y., 1994. Microstructure and crystallization behaviour of TiO2 precursor prepared by the solgel method using metal alkoxide. J. Mater. Sci. 29 (6), 1617–1622. https://doi.org/ 10.1007/BF00368935. Thakur, S., Verma, A., Sharma, B., Chaudhary, J., Tamulevicius, S., Thakur, V.K., 2018. Recent developments in recycling of polystyrene based plastics. Curr. Opin. Green Sustain. Chem. 13, 32–38. https://doi.org/10.1016/j.cogsc.2018.03.011. Thamaphat, K., Limsuwan, P., Ngotawornchai, B., et al., 2008. Phase characterization of
TiO2 powder by XRD and TEM. Kasetsart J. (Nat. Sci.) 42 (5), 357–361. Thomas, R.T., Nair, V., Sandhyarani, N., 2013. TiO2 nanoparticle assisted solid phase photocatalytic degradation of polythene film: a mechanistic investigation. Colloids Surf. A: Physicochem. Eng. Asp. 422, 1–9. https://doi.org/10.1016/j.colsurfa.2013. 01.017. Timóteo, G.A.V., Fechine, G.J.M., Rabello, M.S., 2007. Stress cracking and photodegradation: the combination of two major causes of HIPS failure. Macromol. Symp. 258, 162–169. Vijayalakshmi, R., Rajendran, V., 2012. Synthesis and characterization of nano-TiO2 via different methods. Arch. Appl. Sci. Res. 4 (2), 1183–1190.
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