Photostability and Performance of Polystyrene

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Dec 9, 2016 - photostability of polystyrene films against irradiation compared with the result obtained in the ... plastic produced from polymerization of styrene.
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Photostability and Performance of Polystyrene Films Containing 1,2,4-Triazole-3-thiol Ring System Schiff Bases Gassan Q. Ali 1 , Gamal A. El-Hiti 2, *, Ivan Hameed R. Tomi 1 , Raghad Haddad 3 , Alaa J. Al-Qaisi 4 and Emad Yousif 4, * 1 2 3 4

*

Department of Chemistry, College of Science, University of Al-Mustansiriyah, Baghdad 10052, Iraq; [email protected] (G.Q.A.); [email protected] (I.H.R.T.) Cornea Research Chair, Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia College of Pharmacy, Privet Uruk University, Baghdad 10069, Iraq; [email protected] Department of Chemistry, College of Science, Al-Nahrain University, Baghdad 64021, Iraq; [email protected] Correspondence: [email protected] (G.A.E.-H.); [email protected] (E.Y.); Tel.: +966-11469-3778 (G.A.E.-H.); Fax: +966-11469-3536 (G.A.E.-H.)

Academic Editor: Derek J. McPhee Received: 5 November 2016; Accepted: 7 December 2016; Published: 9 December 2016

Abstract: Series of 4-(4-substituted benzylideneamino)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole3-thiols were synthesized and their structures were confirmed. The synthesized Schiff bases were used as photostabilizers for polystyrene against photodegradation. Polystyrene polymeric films containing synthesized Schiff bases (0.5% by weight) were irradiated (λmax = 365 nm and light intensity = 6.43 × 10−9 ein·dm−3 ·s−1 ) at room temperature. The photostabilization effect of 1,2,4-triazole3-thiols Schiff bases was determined using various methods. All the additives used enhanced the photostability of polystyrene films against irradiation compared with the result obtained in the absence of Schiff base. The Schiff bases can act as photostabilizers for polystyrene through the direct absorption of UV radiation and/or radical scavengers. Keywords: photostabilization; UV light; polystyrene; 1,2,4-triazole-3-thiol; functional group index; Schiff bases

1. Introduction Polystyrene (PS) is an aromatic synthetic polymer that is produced in millions of tons every year. It was reported that 55 million tons of polystyrene was produced in 2013 [1]. It is a widely used plastic produced from polymerization of styrene. Various types of polystyrene are known that mainly depend on the positions of phenyl groups along the polymeric chain [2–4]. Polystyrene is naturally colorless and transparent and can be used in the manufacture of containers, bottles, electronics, and insulation [2]. One of the main problems associated with the use of polymeric materials, either synthetic or semisynthetic, is their photodegradation [5]. Polystyrene undergoes photodegradation when exposed to harsh environments such as high temperatures and sunlight [6]. Polystyrene is not biodegradable, meaning that it will undergo very slow degradation which may require many years [7]. Photodegradation of polystyrene can lead to chain scission, cross-linking, and discoloration [8]. Therefore, polystyrene polymeric materials should be stabilized against photodegradation and photo-oxidation to prevent and/or reduce the effect weathering conditions. In addition, stabilization would maximize the long-term use of polystyrene to increase its economic viability. Various

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additives have been used to enhance the photostability of polymeric films such as plasticizers [9], heterocycles [10–13], aromatics [14–16], and organometallics [17–19]. Such photostabilizers in most cases can act as direct UV absorbers, radical scavengers, excited state quenchers and/or peroxide decomposers [8]. Molecules 2016, 1699 reported successful polymeric films photostabilization processes 2 of 12 the use Recently, we 21, have by of various additives at low concentrations [20–25] as part of our continuing interest in the field of organometallics [17–19]. Such photostabilizers in most cases can act as direct UV absorbers, radical polymeric materials [26–30]. Now, we report the photostabilization of polystyrene films in the presence scavengers, excited state quenchers and/or peroxide decomposers [8]. of 1,2,4-triazole-3-thiol bases. Recently, we ring have system reported Schiff successful polymeric films photostabilization processes by the use of various additives at low concentrations [20–25] as part of our continuing interest in the field of

2. Results and Discussion polymeric materials [26–30]. Now, we report the photostabilization of polystyrene films in the presence of 1,2,4-triazole-3-thiol ring system Schiff bases.

2.1. Synthesis of Schiff Bases 1–4 2. Results and Discussion

4-(4-Substituted benzylideneamino)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole-3-thiols 1–4 were synthesized inof68%–79% 2.1. Synthesis Schiff Basesyields 1−4 (Table 1) from reaction of 4-amino-5-(3,4,5-trimethoxyphenyl)-4H1,2,4-triazole-3-thiol and various aromatic aldehydes (4-methylbenzaldehyde, 4-nitrobenzaldehyde, 4-(4-Substituted benzylideneamino)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole-3-thiols 1−4 were 4-methoxybenzaldehyde, and 4-(dimethylamino)benzaldehyde) in anhydrous ethanol in the presence synthesized in 68%−79% yields (Table 1) from reaction of 4-amino-5-(3,4,5-trimethoxyphenyl)-4H1,2,4-triazole-3-thiol and acid various (4-methylbenzaldehyde, 4-nitrobenzaldehyde, of few drops of hydrochloric as aaromatic catalystaldehydes under reflux for 4 h (Scheme 1). 4-methoxybenzaldehyde, and 4-(dimethylamino)benzaldehyde) in anhydrous ethanol in the presence of few dropsTable of hydrochloric acid as a catalyst 4 h (Scheme 1. Physical properties andunder somereflux FT-IRforspectral data1). for Schiff bases 1–4. Table 1. Physical properties and some FT-IR spectral data for Schiff bases 1−4.

Schiff Base

R

Schiff Base

1 2 3 4

1Me 2NO2 OMe 3 NMe2 4

m.p. (◦ C)

Color R

Color

m.p. (°C)

yellowyellow Me orange orange NO2 yellowyellow OMe orange NMe2 orange

265–268 265−268 219–222 219−222 238–240 238−240 221–224 221−224

Yield (%) Yield (%)

71 71 79 79 68 68 75

75

FR-IR (υ, cm−1 )

FR-IR (υ, cm−1) CH=N NH CH=N C=S 3122 1599 1599 3122 1244 3257 3257 1589 1589 1240 3120 1606 1606 3120 1244 3120 1614 3120 1614 1240

NH

C=S 1244 1240 1244 1240

Scheme 1. Synthesis of 4-(4-substituted benzylideneamino)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole-

Scheme 1. Synthesis of 4-(4-substituted benzylideneamino)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole3-thiols 1−4. 3-thiols 1–4. 2.2. Characterization of Schiff Bases 1−4

2.2. Characterization of Schiff Bases 1–41−4 have been confirmed by the IR and 1H-NMR spectral data. The The structures of Schiff bases spectra showed characteristics stretching vibration bands for the CH=N bonds that resonate TheFT-IR structures of Schiff bases 1–4 have been confirmed by the IR and 1 H-NMR spectral data. within the 1589–1614 cm−1 region. Also, they showed absorption bands at the 1240–1244 cm−1 region The FT-IR spectra showed characteristics stretching vibration bands for the CH=N bonds that resonate corresponding to the C=S bonds. Moreover, the FT-IR spectra of 1−4 showed the absence of the NH2 −1 region. Also, they showed absorption bands at the 1240–1244 cm−1 region within the 1589–1614 streching bandscm for aminotriazole or the C=O bands for aromatic aldehydes [31]. Some of the most corresponding the C=S bonds. Moreover, the FT-IR spectra 1–4 showed the absence of the NH2 common to and abundant absorption bands for Schiff bases 1−4 areofrepresented in Table 1. The 1H-NMR spectra for 1−4 presence two different types of [31]. aromatic protons. streching bands for aminotriazole orconfirmed the C=Othe bands for of aromatic aldehydes Some of the most They two doublets (two protons J = 8.1–8.4 Hz)1–4 thatare resonated within the 7.73–8.30 common andshow abundant absorption bandseach; for Schiff bases represented in Table 1. ppm region, corresponding the aromatic protons from the aldehyde moeity. They also showed singlet 1 The H-NMR spectra for 1–4 confirmed the presence of two different types of aromatic protons. signals (two protons) that are corresponding to the other aromatic protons (7.20–7.24 ppm), from the They show two doublets (two protons each; J = 8.1–8.4 Hz) that resonated within the 7.73–8.30 ppm amine moeity. Also, they show singlet signals that resonate in the 9.56–9.13 ppm region due to the region, CH corresponding aromatic from the aldehyde moeity. They also showed proton. This is athe clear indicationprotons for the formation of the Schiff bases 1−4. However, the SH proton singlet signals was (twoonly protons) are corresponding to as theanother aromatic protons apparentthat in compound 4 and resonates exchangeable singlet signal (7.20–7.24 at 14.09 ppm.ppm), The from 1H-NMR spectral data for 1–4 are shown in Table 2. the amine moeity. Also, they show singlet signals that resonate in the 9.56–9.13 ppm region due to

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the CH proton. This is a clear indication for the formation of the Schiff bases 1–4. However, the SH proton was only apparent in compound 4 and resonates as an exchangeable singlet signal at 14.09 ppm. The 1 H-NMR spectral data for 1–4 are shown in Table 2. Molecules 2016, 21, 1699 Molecules 2016, 21, 1699

Table 2. 1 H-NMR spectral data for Schiff bases 1–4.

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Table 2. 1H-NMR spectral data for Schiff bases 1−4. Table 2. 11H-NMR dataDMSO-d for Schiff6 ,bases 1−4.J in Hz) Compound H-NMRspectral (300 MHz: δ, ppm, 1H-NMR (300 MHz: DMSO-d6, δ, ppm, J in Hz) Compound 1H-NMR 9.56 (s, 1H, CH), 7.84 (d, J = 8.1 Hz,MHz: 2H, Ar), 7.39 (d, J =ppm, 8.1 Hz, (300 DMSO-d 6, δ, J in2H, Hz)Ar), 7.24 (s, 2H, Ar), Compound 1 9.56 (s, 1H, CH), 7.843.74 (d, (s, J =3H, 8.1 OMe), Hz, 2H, Ar), (d, J = 8.1 Hz, 2H, Ar), 7.24 (s, 2H, Ar), 3.76 6H, 2 OMe), 2.41 (s,7.39 3H, 9.56 (s,(s, 1H, CH), 7.84 (d, J = 8.1 Hz, 2H, Ar), 7.39 (d, Me) J = 8.1 Hz, 2H, Ar), 7.24 (s, 2H, Ar), 1 1 3.76 (s, 6H, 2 OMe), 3.74 (s, 3H, OMe), 2.41 (s, 3H, Me) 3.76 (s,(s, 6H, 2 OMe), 3.74(d, (s,J 3H, 2.41Ar), (s, 3H, 9.69 1H, CH), 8.30 = 8.2OMe), Hz, 2H, 8.02 Me) (d, J = 8.2 Hz, 2H, Ar), 7.20 (s, 2H, Ar), 2 9.69 1H, CH), 8.30 (d, J 8.2 =3H, 8.2 Hz, Ar), 8.02 J = 8.2 (s, 2H, 3.80 (s, 6H, 2 OMe), 3.75 OMe) 9.69 (s,(s, 1H, CH), 8.30 (d, J =(s, Hz, 2H,2H, Ar), 8.02 (d, (d, J = 8.2 Hz,Hz, 2H,2H, Ar),Ar), 7.207.20 (s, 2H, Ar), Ar), 2 2 3.80 (s, 6H, 2 OMe), 3.75 (s, 3H, OMe) 3.80 (s,(s, 6H, 2 OMe), 3.75(d, (s,J 3H, 9.44 1H, CH), 7.90 = 8.4OMe) Hz, 2H, Ar), 7.23 (s, 2H, Ar), 7.12 (d, J = 8.4 Hz, 2H, Ar), 3 9.44(s,(s,1H, 1H,CH), CH), 7.90 = 8.4 Hz, Ar), 7.23 (s, 2H, J = Hz, 8.4 Hz, 9.44 7.90 (d,(d, J(s,=J6H, 8.4 Hz, 2H,2H, Ar), (s, OMe) 2H, Ar),Ar), 7.127.12 (d, J(d, = 8.4 2H, 2H, Ar), Ar), 3.85 (s, 3H, OMe), 3.76 2OMe), 3.71 7.23 (s, 3H, 3 3 3.85(s,(s,3H, 3H,OMe), OMe), 3.76 6H, 2OMe), 3.71 OMe) 3.85 3.76 (s,(s, 6H, 2OMe), 3.71 (s, (s, 3H,3H, OMe) 14.09 (s, exch., 1H, SH), 9.13 (s, 1H, CH), 7.73 (d, J = 8.2 Hz, 2H, Ar), 7.24 (s, 2H, Ar), 14.09(s,(s,exch., exch., 1H, SH), 9.13 (s, 1H, CH), 7.73 = 8.2 Hz, 2H, 7.24 (s, 2H, 4 14.09 1H, 7.73 (d, (d, J = J8.2 Ar),Ar), 7.24(s, (s,6H, 2H, Ar),Ar), 6.81 (d, J = 8.21H, Hz,SH), 1H,9.13 Ar),(s, 3.76 (s,CH), 6H, 2OMe), 3.71 (s,Hz, 3H,2H, OMe), 3.04 NMe 2) 44 6.81 (d, J = 8.2 Hz, 1H, Ar), 3.76 (s, 6H, 2OMe), 3.71 (s, 3H, OMe), 3.04 (s, 6H, NMe 2) 6.81 (d, J = 8.2 Hz, 1H, Ar), 3.76 (s, 6H, 2OMe), 3.71 (s, 3H, OMe), 3.04 (s, 6H, NMe 2)

2.3. Measuring Photostabilization of ofPolystyrene Films byby IRIR Spectroscopy 2.3.Measuring Measuring Photostabilization of Polystyrene Films Spectroscopy 2.3. Photostabilization Polystyrene Films by IR Spectroscopy Ultraviolet radiation has harmful effects on polystyrene that lead chemical changes within Ultraviolet radiation has harmful effects on polystyrene that lead to to chemical changes within the Ultraviolet radiation has harmful effects on polystyrene that lead to chemical changes within the the polymeric chains. As aresult, result, the polymeric materials could lose their mechanical properties and polymericchains. chains.As As the polymeric materials could lose their mechanical properties and become polymeric a aresult, the polymeric materials could lose their mechanical properties and become discolored [32]. Photo-oxidation ofof polystyrene leads to to the formation of several functional discolored [32].[32]. Photo-oxidation polystyrene leads the formation of several groups become discolored Photo-oxidation of polystyrene leads to the formation of functional severalgroups functional [33,34]. ItIthas been reported that irradiation of of polystyrene in the presence of oxygen leadlead to the [33,34]. has been reported that irradiation polystyrene in the of oxygen can to theto groups [33,34]. It has been reported that irradiation of polystyrene inpresence the presence of can oxygen can lead production of carbonyl and hydroxyl group moieties, for example, as shown in Figure 1 [35]. Initial production of carbonyl and hydroxyl group moieties, for example, as shown in Figure 1 [35]. Initial the production of carbonyl and hydroxyl group moieties, for example, as shown in Figure 1 [35]. Initial degradation ofofPS a apolystyrene radical [–CH 2CH(Ph)]. Such radicals cancan abstract a proton degradation PSleads leads polystyrene radical [–CH 2CH(Ph)]. Such radicals abstract a proton degradation of PS leads to atoto polystyrene radical [–CH 2 CH(Ph)]. Such radicals can abstract a proton from other polymeric chains to produce various other radicals [–CH 2 C(Ph)–CH 2 –, –CH 2 C(Ph)–CH 3 from other polymeric chains to produce various other radicals [–CHC(Ph)–CH 2C(Ph)–CH2–, –CH2C(Ph)–CH 3 from other polymeric chains to produce various other radicals [–CH 2 2 –, –CH2 C(Ph)–CH3 and –CH 2 CHC(H)–CH(Ph)–]. Theses radicals react with oxygen to produce the corresponding peroxy and –CH2CHC(H)–CH(Ph)–]. Theses radicals react with oxygen to produce the corresponding peroxy and –CH Theses in radicals react withofoxygen to produce the corresponding peroxy 2 CHC(H)–CH(Ph)–]. radicals the IRIR spectrum polystyrene duedue to irradiation can can be used radicals[8]. [8].Therefore, Therefore,the thechanges changes in the spectrum of polystyrene to irradiation be used radicals [8]. Therefore, the changes in the IR spectrum of polystyrene due to irradiation can be used as as thethe polymeric materials. TheThe FT-IR spectra for for PS films asaameasure measureofofphotodegradation photodegradationwithin within polymeric materials. FT-IR spectra PS films a measure of photodegradation within the polymeric materials. Therepresented FT-IR spectra for PS2films before before UV light (λmax = 365 nm) are in Figures and 3, beforeand andafter afterirradiation irradiation(250 (250h)h)byby UV light (λmax = 365 nm) are represented in Figures 2 and 3, and after irradiation (250 h) by UV light (λ = 365 nm) are represented in Figures 2 and 3, respectively. respectively. max respectively.

Figure 1. Photooxidative degradation of PS.

Figure 1. Photooxidative degradation Figure 1. Photooxidative degradationofofPS. PS.

Figure 2. FTIR spectrum for PS film before irradiation.

Figure 2. FTIR spectrum film beforeirradiation. irradiation. Figure 2. FTIR spectrum forfor PSPS film before

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Figure 3. FTIR spectrum film after irradiation(250 (250h). h). Figure 3. FTIR spectrum forfor PSPS film after irradiation Figure 3. FTIR spectrum for PS film after irradiation (250 h).

Compounds 1−4 (0.5% by weight) were mixed with PS to produce the polymeric films (40 μm Compounds 1–41−4 (0.5% by by weight) were mixed with PSPS to produce thethe polymeric films (40(40 µmμm Compounds (0.5% weight) were mixed with to produce polymeric films thickness). The efficiency of the additives as photostabilizers for the photostabilization of PS films thickness). The efficiency of of thethe additives as as photostabilizers forfor thethe photostabilization of of PSPS films thickness). The efficiency additives photostabilizers photostabilization films was investigated under irradiation for 250 h. The PS films were irradiated for 250 h and the carbonyl was investigated under irradiation forfor 250250 h. The PS PS films were irradiated forfor 250250 h and thethe carbonyl was investigated under irradiation h. The films were irradiated h and carbonyl (ICO) and hydroxyl (IOH) indices were monitored using an IR spectrophotometer. The IR absorption (ICO )COand hydroxyl (IOH ) indices were monitored using anan IRIR spectrophotometer. The IR IR absorption (Ibands ) and hydroxyl (IOH indices were monitored using spectrophotometer. The absorption that appeared at) ca. 1720 and 3400 cm−1 − can be assigned to the carbonyl and hydroxyl groups, 1 −1 bands that appeared at ca. and 3400 cm can be assigned to the carbonyl and hydroxyl bands that appeared at ca. 1720 and 3400 cm can be assigned to the carbonyl and hydroxyl respectively [36]. The increases in both ICO and IOH indexes, compared to the reference peak (1328 groups, cm−1) groups, respectively [36]. The increases in both I and I indexes, compared to the reference peak respectively [36]. The increases in both I CO and I OH indexes, compared to the reference peak (1328 cm−1) CO can be used as an indicator for PS photodegradation. It OH should be noted that IC=O does not starts from − 1 (1328 cm ) can as anfor indicator for PSplace photodegradation. should be that IC=O does can bebecause used asbe anused indicator PS photodegradation. It should beItnoted that C=O does not starts from zero some photodegradation takes during the preparation the PSInoted films. The changes in notzero starts from zero because some photodegradation takes place during the preparation the PS films. because some photodegradation takes place during the preparation the PS films. The changes the IC=O and IOH indices on irradiation of PS in the presence of Schiff bases 1−4 are represented in in Thethe changes Irespectively. indices on of PS in of theSchiff presence of1−4 Schiff 1–4 are in IC=O and Ithe OH onIOH irradiation of irradiation PS in the presence bases arebases represented C=O and Figures 4 in and 5,indices represented Figures 4 and 5, respectively. Figures 4 in and 5, respectively.

Figure 4. The effect of irradiation on the ICO index for PS films.

Figure 4. The effect of irradiation on the index films. Figure 4. The effect of irradiation on the ICOICO index forfor PS PS films.

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Figure 5. The effect of irradiation on the IOH index for PS films. Figure 5. The effect of irradiation on the IOH index for PS films. Figure 5. Theour effect of irradiation on additives the IOH index for PS films. As demonstrated through findings, all the used stabilized the PS film against

As demonstrated The through ourinfindings, alland theIOH additives used stabilized thefilms PS containing film against photodegradation. changes both the ICO indices were lower for the PS demonstrated through our all used lower stabilized thethe PSfilms film containing against photodegradation. The compared changes intoboth the IOH indices were for the PS the As additives 1−4 thefindings, onesICO forand thethe PSadditives (blank). Compound 1 was most efficient photodegradation. The changes in both the ICO IOH indices were lower forthe themost PS films containing additive 1–4 for the photostabilization offor PS film. the additives compared to the ones the PSand (blank). Compound 1 was efficient additive the additives 1−4 compared to the ones for the PS (blank). Compound 1 was the most efficient for the photostabilization of PS film. additive for the Photostabilization photostabilization PS film. Films by Weight Loss 2.4. Measuring of of Polystyrene

2.4. Measuring Photostabilization Polystyrene by Weight Lossfragments along with volatiles and Photodegradation of PSofproduces lowFilms molecular weight 2.4. Measuring Photostabilization of Polystyrene Films by Weight Loss leads to weight loss The efficiency of additive (0.5%fragments by weight)along photostabilizer for and PS was Photodegradation of [37]. PS produces low molecular weight with volatiles leads Photodegradation ofPS PSweight produces low molecular weight fragments alongofwith volatilesonand determined in terms of loss under irradiation for 250 h. The effect irradiation the to weight loss [37]. The efficiency of additive (0.5% by weight) photostabilizer for PS was determined leads to loss weight loss [37]. efficiency additive (0.5% by weight)loss photostabilizer for PS was weight of PS films is The represented in of Figure 6. The PS film increases gradually in terms of PS weight loss under irradiation for 250 h. The effectweight of irradiation on the weight with loss of determined in terms of PS weight loss under irradiation for 250 h. The effect of irradiation on the the irradiation time. Evidently, the weight loss was low for the PS films containing compounds 1−4 PS films is represented in Figure 6. The PS film weight loss increases gradually with the irradiation weight loss to of that PS films is represented Figure 6. The PS absence film weight loss additives. increases gradually with compared obtained for the PSin film (blank) in the of such The weight loss time.the Evidently, thetime. weight loss was low for the PS films compounds 1–4compounds compared 1−4 to that irradiation Evidently, lowcontaining for the PS films containing was lowest when compound 1 the wasweight used asloss thewas additive. obtained for the PS film (blank) in the absence of such additives. The weight loss was lowest when compared to that obtained for the PS film (blank) in the absence of such additives. The weight loss compound 1 was used as the additive. was lowest when compound 1 was used as the additive.

Figure 6. The effect of irradiation on weight loss for PS films. Figure The effectofofirradiation irradiation on on weight Figure 6. 6. The effect weightloss lossfor forPS PSfilms. films.

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2.5. Photostabilization of of Polystyrene Films byby Variation in in Molecular Weight 2.5.Measuring Measuring Photostabilization Polystyrene Films Variation Molecular Weight The PSPS molecular weight during photolysis was investigated. Thevariation variationofof molecular weight during photolysis was investigated.Figure Figure7 7shows showsthe the relative changes inin the average molecular weight (M( V ) for relative changes the average molecular weight ) forPSPSfilms films(40 (40µm μmthickness) thickness)ininthe thepresence presence and 1–4 (0.5% (0.5% by byweight). weight).The The decrease for PS thefilms PS films was sharp andabsence absenceofofadditives additives 1−4 decrease in in M for was sharp during V the during the50first 50 h and less noticeable thereafter. Photodegradation of PSleads filmstoleads to a reduction in the first h and less noticeable thereafter. Photodegradation of PS films a reduction in viscosity viscosity due to the formation of degraded polymeric chains of low molecular weight [38]. All additives due to the formation of degraded polymeric chains of low molecular weight [38]. All additives were were efficient against the photodegradation of PS films, compared to films the PS films without (blank) without efficient against the photodegradation of PS films, compared to the PS (blank) additives. additives. Compound 1 was once most efficient at photostabilizing the PS films. Compound 1 was once again theagain mostthe efficient at photostabilizing the PS films.

Figure The effect irradiation the viscosity average molecular weight films. Figure 7. 7. The effect ofof irradiation onon the viscosity average molecular weight forfor PSPS films.

The measurements of the initial viscosity average molecular weight ( , ) and a specific The measurements of the initial viscosity average molecular weight (M ) and a specific irradiation time ( , ) will allow the calculation of the average number of the V,O chain scissions (S) as irradiation time (MV,t ) will allow the calculation of the average number of the chain scissions (S) shown in Equation (1) [38]. as shown in Equation (1) [38]. / /M – 1− 1 (1)(1) S== M,V,O , V,t Figure8 8shows showsthe theeffect effectofofirradiation irradiationtime timeon onthe theSSvalues. values.Irradiation Irradiation of of PS PS (blank) (blank) showed showed a Figure branching compared to the films the presence of additives a higher higher degree degreecross-linking cross-linkingand/or and/or branching compared toPS the PS in films in the presence of 1−4. There was a sharp increase in the value of S for control PS between 150 and 250 h. Also, additives 1–4. There was a sharp increase in the value of S for control PS between 150 and 250 h. significant insoluble residues being formed are an anindication indicationforfor Also, significant insoluble residues being formedduring duringthe theirradiation irradiation process process are polymeric chains crosslinking. On the other hand, the increase the S value containing Schiff polymeric chains crosslinking. On the other hand, the increase inin the S value forfor PSPS containing Schiff base 1 was negligible compared to the others and low proportion of insoluble polymeric materials base 1 was negligible compared to the others and low proportion of insoluble polymeric materials were observed. were observed. The degree deterioration (α) for films provides a measure for the rapid break randomly The degree ofof deterioration (α) for PSPS films provides a measure for the rapid break ofof randomly distributed weak bonds at the initial stage of photodegradation. Equation (2) can be used to calculate distributed weak bonds at the initial stage of photodegradation. Equation (2) can be used to calculate theα α value that depends viscosity average molecular weight chainscissions scissions(S), (S),and and the value that depends onon thethe viscosity average molecular weight (M(V ), ),chain molecular weight (m). molecular weight (m). αα== m×× /S/MV

(2)(2)

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Figure 8. The effect of irradiation on the main chain scissions (S) for PS films.

Figure 8. The effect of irradiation on the main chain scissions (S) for PS films. Figure The effect of irradiation onon thethe main chainofscissions (S) for PS films. Figure 9 shows the8.effect of irradiation time degree deterioration (α). The α values were

Figure 9 shows of irradiation time oncompared the degree of deterioration (α). The α values very high for thethe PSeffect films (blank) on irradiation to the samples containing additives 1−4.were Figure 9 shows the effect of irradiation time on the degree of deterioration (α). The α values were Therefor was a sharp increase in the of α when the irradiation was between 150−250 h. The 1–4. very high the PS films (blank) onvalue irradiation compared to thetime samples containing additives very high the PS containing films (blank) on irradiation compared to the samples containing additives 1−4. values forfor PSincrease films additives compared to PS (blank). The α values were Thereαwas a sharp in the value of α were whenvery the low irradiation time was between 150–250 h. The α There was a sharp increase in the value of α when the irradiation time was between 150−250 h. The PS containing films in the presence of were Schiff very base 1low under irradiation. valuesminimal for PS for films additives compared to PS (blank). The α values were α values for PS films containing additives were very low compared to PS (blank). The α values were minimal for PSfor films in the of Schiff base 1 1under minimal PS films in presence the presence of Schiff base underirradiation. irradiation.

Figure 9. The effect of irradiation on the degree of deterioration (α) for PS films Figure 9. The effect of (DP) irradiation on the degree of deterioration (α)infor films The degree is the of monomeric units a PS homopolymer. It can Figureof 9. polymerization The effect of irradiation onnumber the degree of deterioration (α) for PS films be calculated using Equation (3) based on the average molecular weight (Mn) and molecular weight degreeunit of polymerization of theThe monomer (M0) [38,39]. (DP) is the number of monomeric units in a homopolymer. It can The degree of using polymerization thethe number monomeric units a homopolymer. It can be calculated Equation (3)(DP) basedison averageof molecular weight (Mnin ) and molecular weight = average = /molecular weight (M ) and molecular (3) of the monomer unit (M0)(3) [38,39]. be calculated using Equation based on the weight of n

the monomer unit Figure 10 (M shows the effect of irradiation The 0 ) [38,39]. = on=the /reciprocal degree of polymerization (1/Pt).(3) curve indicates a sharp increase in 1/PtDP with irradiation time for PS film (blank) compared to the ones = X = M /M (3) n n n 0 Figure 10 shows the effect of irradiation on the reciprocal degree of polymerization (1/Pt). The

curve indicates a sharp increase 1/Pt with irradiation time for PS film (blank) compared to the ones Figure 10 shows the effect ofinirradiation on the reciprocal degree of polymerization (1/Pt). The curve indicates a sharp increase in 1/Pt with irradiation time for PS film (blank) compared to the

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ones obtained in the presenceofofadditives additives 1–4. changes in were 1/Ptvery werelow very lowcompound when compound 1 obtained in21, the presence 1−4. TheThe changes in 1/Pt when 18 of (0.5% Molecules 2016, 1699 12 (0.5% by weight) was mixed with the PS. by weight) was mixed with the PS. obtained in the presence of additives 1−4. The changes in 1/Pt were very low when compound 1 (0.5% by weight) was mixed with the PS.

Figure 10. The effect of irradiation on the average degree of polymerization (1/Pt) for PS films. Figure 10. The effect of irradiation on the average degree of polymerization (1/Pt) for PS films.

2.6. Photostabilization Films Suggested Mechanisms Figure 10. The effect of Polystyrene irradiation on the average degree of polymerization (1/Pt) for PS films.

2.6. Photostabilization of Polystyrene Films Suggested Mechanisms

Schiff bases contain heterocycles and aryl moieties, in the presence of a chromophore (POO·), 2.6. Photostabilization of Polystyrene Films Suggested Mechanisms

Schiff basesstabilize contain aryl moieties, in the presence of a chromophore (POO·), that could theheterocycles PS samples byand acting as radical scavengers. A complexation between the additive Schiff bases contain heterocycles and aryl moieties, in the presence of a chromophore (POO·), that could stabilize the PScould samples by acting radical A complexation between and the chromophore be achieved in an as excited statescavengers. in which the energy can be transferred. The the that could stabilize the samples by could acting as radicalthe scavengers. A charge complexation between additive resonance within thePS aryl moieties stabilize unreactive complexes to a level additive and the chromophore could be achieved in an excited state transfer in which thethe energy can be and the could be achieved in (Scheme anmoieties excited in which the energy can be transferred. that is chromophore harmless to the polymeric chains 2).state transferred. The resonance within the aryl could stabilize the unreactive charge The transfer resonance within the aryl moieties could stabilize the unreactive charge transfer complexes to a level complexes to a level that is harmless to the polymeric chains (Scheme 2). that is harmless to the polymeric chains (Scheme 2).

Scheme 2. Additives 1−4 act as radical scavengers. Scheme Additives 1–4 1−4 act act as Scheme 2. 2.Additives as radical radicalscavengers. scavengers.

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additives embedded within the polystyrene polystyrene polymeric Heterocyclic and aryl moieties of the additives chains can absorb UV light directly [8]. Schiff bases 1−4 1–4 have both aryl and triazole ring systems that can act as UV absorbers. They can absorb the harmful UV light and convert it into heat that is harmless to polystyrene (Scheme 3).

Scheme 3. Additives act as as UV UV absorbers. absorbers. Scheme 3. Additives 1−4 1–4 act

3. Experimental 3. Experimental Section Section 3.1. General

Chemicals and reagents were obtained from BDH BDH Chemicals Chemicals (Poole, (Poole, UK) UK) and and Sigma-Aldrich Sigma-Aldrich Chemicals Chemical Company hot stage stage Gallenkamp Gallenkamp Chemical Company (Gillingham, (Gillingham, UK). UK). Melting Melting points points were were recorded recorded on a hot 1H-NMR 1 melting point apparatus. The spectra (300 MHz) were recorded on Bruker Ultrashield (Bruker, point apparatus. The H-NMR spectra (300 MHz) were recorded on Bruker Ultrashield Coventry, UK) in DMSO-d 6 with tetramethylsilane as an internal standard. (Bruker, Coventry, UK) in DMSO-d as an internal standard. 6 with tetramethylsilane 3.2. Synthesis of Schiff Bases 1–4 A mixture of 4-amino-5-(3,4,5-trimethoxiyphenyl)-1,2,4-triazole-2-thiol [40] (0.2 (0.2 g, g, 0.7 mmol) 4-amino-5-(3,4,5-trimethoxiyphenyl)-1,2,4-triazole-2-thiol [40] and appropriate appropriate aromatic aromatic aldehyde aldehyde(0.07 (0.07mmol) mmol)ininabsolute absoluteethanol ethanol(10 (10 mL) containing one drop mL) containing one drop of of concentrated hydrochloric acid was refluxed for 4 h. The solid obtained on cooling was filtered, concentrated hydrochloric acid was refluxed for 4 h. The solid obtained on cooling was filtered, washed washed hot ethanol, and to give Schiff bases 1–4 in 68%–79% with hotwith ethanol, and dried to dried give Schiff bases 1−4 in 68%−79% yields. yields. 3.3. Films Preparation Preparation 3.3. Films A mixtureofofpolystyrene polystyrene Schiff bases was dissolved in chloroform films were A mixture andand Schiff bases was dissolved in chloroform and the and filmsthe were prepared ◦ prepared the evaporation at Digital 25 C. AVernier DigitalCaliper Vernier2610A Caliper 2610A micrometer (Vogel using the using evaporation techniquetechnique at 25 °C. A micrometer (Vogel GmbH, GmbH, Kevelaer, Germany) was used to fix the thickness of PS films as 40 µm. Kevelaer, Germany) was used to fix the thickness of PS films as 40 μm.

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3.4. Accelerated Testing Technique The PS films were irradiated with UV light (λmax = 365 nm and light intensity = 6.43 × 10−9 ein·dm−3 ·s− 1 ) at room temperature using an accelerated weather-meter QUV tester (Philips, Saarbrücken, Germany). 3.5. Photodegradation of PS Films by IR Spectrophotometer The FTIR spectra (4000–400 cm−1 ) were recorded on FTIR 8300 Spectrophotometer (Shimadzu, Tokyo, Japan) for the PS films. The carbonyl and hydroxyl group indices (Is) can be calculated using Equation (4) [41]. The value of Is depends on the peak absorbance (As) of C=O or OH group and the reference peak absorbance (Ar) at 1328 cm−1 . Is = As/Ar

(4)

3.6. Measuring the Photodegradation by Weight Loss The weight loss percentage of PS films in photodegradation process was calculated using from the weight of PS sample before (W 1 ) and after irradiation (W 2 ) using Equation (5) [42]. Weight loss % = [(W1 − W2 )/W1 ] × 100

(5)

3.7. Photodegradation of PS Films by Viscometery Method α

The average molecular weight (MV ) of PS films was measured using Mark–Houwink relation, α Equation (6), in which α and K are constants [41,43]. MV is directly proportional to the intrinsic viscosity, [η], of PS film. α (6) [η] = KMV Also, the average molecular weight (MV ) can be calculated using Equation (7).

[η] = 4.17 × 10−4 MV

0.66

(7)

4. Conclusions Four Schiff bases with 4H-1,2,4-triazole-3-thiol ring systems have been synthesized and characterized. Polystyrene films containing Schiff bases at a concentration of 0.5% were found to more stable on irradiation compared to polystyrene without the additives. Such additives can be used for long-term polystyrene photostabilization. The additives could act as UV absorbers and/or radical scavengers. Acknowledgments: The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for its funding for this research through the research group project RGP-239 and to Al-Mustansiriyah and Al-Nahrain Universities for continued support. Author Contributions: Emad Yousif conceived and designed the experiments. Gassan Q. Ali, Ivan Hameed R. Tomi, Raghad Haddad, and Alaa J. Al-Qaisi performed the experiments and analyzed the data. Gamal A. El-Hiti and Emad Yousif wrote the paper. Gamal A. El-Hiti provided the funds and revised the paper. All the authors discussed the results and improved the final text of the paper. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples from the Schiff bases are not available. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).