International Journal of Polymer Science
Volume 2015 (2015), Article ID 510390, 10 pages
http://dx.doi.org/10.1155/2015/510390

Research Article

Synthesis of New Thiophene Derivatives and Their Use as Photostabilizers for Rigid Poly(vinyl chloride)

Asim A. Balakit,¹ Ahmed Ahmed,² Gamal A. El-Hiti,³ Keith Smith,⁴ and Emad Yousif²

¹College of Pharmacy, Babylon University, Babylon, Iraq
²Department of Chemistry, College of Science, Al-Nahrain University, Baghdad, Iraq
³Cornea Research Chair, Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia
⁴School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK

Received 23 September 2014; Revised 16 December 2014; Accepted 19 December 2014

Academic Editor: Qinglin Wu

Copyright © 2015 Asim A. Balakit et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Five new thiophenes, namely, N-[(3-bromo-2-methylthiophen-5-yl)methylene]-4-methoxyaniline (4a), N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethoxyaniline (4b), N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethylaniline (4c), 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-methylquinazolin-4(3H)-one (4d), and 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-isopropylquinazolin-4(3H)-one (4e), have been synthesized. All of these materials brought about a reduction in the level of photodegradation of poly(vinyl chloride) (PVC) films containing the synthesized thiophenes (0.5%; by weight). The results obtained showed that the extent of photostabilization of PVC in the presence of an additive was in the order 4e > 4d > 4b > 4a > 4c. For the most favorable additive (4e), the rate of appearance of infrared absorption bands of degradation products was reduced by around two-thirds, while the quantum yield of chain scission was calculated to be reduced by a factor of more than one thousand. It is suggested that the additives may help stabilize PVC by direct absorption of UV radiation and dissipation of the energy as heat or that electrostatic attraction between the additives and PVC may assist transfer of energy from excited state PVC to the additive, from where it can be dissipated.

1. Introduction

Poly(vinyl chloride) (PVC) is second to polyethylene among the highest selling types of plastic materials, which are widely used in a wide range of industries including architecture, electronics, packaging, and transportation [1]. However, low photostability of PVC leads to hydrogen chloride loss, discoloration, and finally serious corrosion phenomena, accompanied by changes of physical and chemical properties of the PVC. The low cost and the good performance of PVC products have increased the utilization of this polymer in building, mainly in exterior applications such as window profiles and cladding [2]. However, ultimate user acceptance of the PVC products for outdoor building applications depends on their ability to resist photodegradation over long periods of sunlight exposure. To ensure weatherability, the PVC resin needs to be compounded and processed properly using suitable additives, leading to a complex material whose behavior and properties are quite different from those of the PVC resin itself [3]. Because of this, it is important to perform reliable weathering methods to test factors, like light and temperature, which influence the degradation of PVC based materials in accelerated service conditions. Due to the growing importance of the PVC, various studies have been reported that deal with the rheological behavior and mechanical properties of its blends [4], the properties of PVC/thermoplastic polyester elastomer blend systems [5], and fabrication of new antimicrobial PVC plastic [6]. The effect of UV light on the molecular structure of the PVC is an important issue that has attracted the attention of researchers [7].

As part of on-going research on the synthesis of polymeric materials [8], lithiation reactions [9], and the synthesis of various substituted heteroaromatic compounds [10] and on the stabilization of polymers [11], we became interested in the synthesis of thiophene derivatives. Here we report the syntheses of various imines derived from thiophenes, namely, N-[(3-bromo-2-methylthiophen-5-yl)methylene]-4-methoxyaniline (4a), N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethoxyaniline (4b), N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethylaniline (4c), 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-methylquinazolin-4(3H)-one (4d), and 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-isopropylquinazolin-4(3H)-one (4e) and studies of their activity in the photostabilization of PVC. To the best of our knowledge this is the first attempt to investigate the photostabilization of PVC films containing such compounds.

2. Experimental

2.1. General

Bromine-lithium exchange reactions were performed under an inert atmosphere. Glassware was oven dried, assembled hot, and allowed to cool under a stream of nitrogen gas. All chemicals and reagents were purchased from commercial sources and used without further purification. THF was distilled from sodium benzophenone ketyl and other solvents were purified by standard procedures [12]. IR spectra were recorded as KBr discs for solid materials or by applying droplets of liquid materials on a NaCl plate. ¹H and ¹³C NMR spectra were recorded on a spectrometer operating at 400 MHz for ¹H and 100 MHz for ¹³C measurements, respectively. Chemical shifts are reported in parts per million (ppm) relative to tetramethylsilane (TMS). Low- and high-resolution mass spectra were recorded on a time-of-fight mass spectrometer using electron impact (EI). Column chromatography was carried out using Fischer Scientific silica 60A (35–70 micron). n-Butyllithium was obtained from Aldrich Chemical Company and was estimated prior to use by the method of Watson and Eastham [13].

2.2. Synthesis of 3,5-Dibromo-2-methylthiophene (2)

A solution of bromine (12.00 mL, 37.20 g, 232.8 mmol) in acetic acid (16 mL) was added in a dropwise manner over 10 min to a stirred solution of 2-methylthiophene (1; 9.82 g, 100.0 mmol) in acetic acid (40 mL) at (0°C). The reaction mixture was stirred at 0°C for 2 h and at room temperature for 16 h. The mixture was quenched with water (20 mL) and solid Na₂CO₃ was then added to bring the pH to ≥9. The product was extracted with Et₂O (3 × 60 mL) and the combined organic extracts were dried (MgSO₄). The solvent was removed under reduced pressure to give the crude product as a reddish liquid, which was purified by distillation under reduced pressure to give pure 2 (23.04 g, 90.0 mmol; 90%) as a colorless oil (lit. [14] colorless oil). ¹H NMR (400 MHz, CDCl₃) : 6.87 (s, 1H) and 2.36 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) : 136.0, 131.9, 108.8, 108.6, and 14.9. EI-MS (m/z, %): 258 ([M⁸¹Br₂]⁺, 62), 257 (30), 256 ([M⁸¹Br⁷⁹Br]⁺ 98), 255 (60), 254 ([M⁷⁹Br₂]⁺, 80), 253 (32), 175 (100) and 96 (84). HRMS (EI): Calcd for C₅H₄Br₂S [M⁷⁹Br₂]⁺ 253.8409; found, 253.8400. FTIR (, cm⁻¹): 3097, 2918, 1534, 1449 and 1305.

2.3. Synthesis of 3-Bromo-2-methylthiophene-5-carboxaldehyde (3)

A solution of n-butyllithium (64.0 mL, 1.60 M; 64.0 mmol) in hexane was added in a dropwise manner over 5 min to a stirred solution of 3,5-dibromo-2-methylthiophene (2; 16.0 g, 62.5 mmol) in anhydrous THF (100 mL) at −78°C under nitrogen. The solution turned orange then greenish yellow and finally yellowish orange. The reaction mixture was stirred at −78°C for 30 min and DMF (4.80 g, 65.7 mmol) was then added. The cooling bath was removed and the mixture was stirred at room temperature for 16 h. The mixture was quenched with aqueous HCl (20 mL, 2 M) and the product was extracted with Et₂O (3 × 100 mL). The organic layer was washed with saturated aqueous NaHCO₃ solution (100 mL) and brine (100 mL) and then dried (MgSO₄). The solvent was removed under reduced pressure to give pure 3 (10.88 g, 53.05 mmol; 85%) as a yellow solid, m.p. 56-57°C (lit. 57-58°C) [15]. ¹H NMR (400 MHz, CDCl₃) (ppm): 9.69 (s, 1H), 7.51 (s, 1H) and 2.41 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) (ppm): 181.6, 145.9, 140.1, 138.8, 111.3 and 15.9. EI-MS (m/z, %): 206 ([M⁸¹Br]⁺, 75], 205 (98), 204 ([M⁷⁹Br]⁺, 77), 203 (100), 177 (25), 125 (20), 96 (40), 84 (32) and 69 (49). HRMS (EI): Calcd for C₆H₄BrOS [M⁷⁹Br, 1]⁺ 202.9172; found, 202.9166. FTIR (, cm⁻¹): 3080, 2859, 1678 and 1521.

2.4. Synthesis of Imines 4a–e

In a 10 mL microwave vial charged with a magnetic bar, 3-bromo-2-methylthiophene-5-carboxaldehyde (3; 0.30 g, 1.46 mmol) and the appropriate amine (1.46 mmol) were dissolved in absolute methanol (6 mL) and a drop of HCl was added to the mixture. The vial was sealed and subjected to microwave irradiation at 100°C for 5 min. The mixture was cooled and then decanted into a 50 mL round bottomed flask and the solvent was evaporated under reduced pressure. The solid crude product was recrystallized from ethanol to give the desired product.

2.4.1. N-[(3-Bromo-2-methylthiophen-5-yl)methylene]-4-methoxyaniline (4a)

Produced from 4-methoxyaniline (0.18 g, 1.46 mmol); yield: 0.41 g, 1.32 mmol; 91%; m.p. 73-74°C. ¹H NMR (400 MHz, CDCl₃) : 8.36 (s,  1H), 7.20–7.10 (m, 4H), 6.83 (s, 1H), 3.76 (s, 3H), and 2.38 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) : 158.5, 149.6, 143.8, 139.8, 139.4, 133.8, 122.4, 114.4, 109.9, 55.5, and 15.5. EI-MS (m/z, %): 311 ([M⁸¹Br]⁺, 88), 309 ([M⁷⁹Br]⁺, 90), 296 (97), 294 (100), 86 (51), 84 (96). HRMS (EI): calcd. for C₁₃H₁₂BrNOS [M⁷⁹Br]⁺ 308.9823; found, 308.9825. FTIR (, cm⁻¹): 2960, 2836, 1614, 1505, 1469.

2.4.2. N-[(3-Bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethoxyaniline (4b)

Produced from 3,4-dimethoxyaniline (0.224 g, 1.46 mmol); yield: 0.44 g, 1.30 mmol; 89%; m.p. 118-119°C. ¹H NMR (400 MHz, CDCl₃) : 8.47 (s, 1H), 7.28 (s, 1H), 6.90–6.81 (m, 3H), 3.94 (s, 3H), 3.92 (s, 3H), and 2.48 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) : 150.0, 149.3, 147.9, 144.2, 139.7, 139.6, 134.0, 112.1, 111.2, 110.0, 105.8, 56.1, 55.9, and 15.52. EI-MS (m/z, %): 341 ([M⁸¹Br]⁺, 53), 339 ([M⁷⁹Br]⁺, 54), 326 (51), 324 (54), 86 (94), 84 (100). HRMS (EI): calcd. for C₁₄H₁₄BrNO₂S [M⁷⁹Br]⁺ 338.9929; found 338.9924. FTIR (, cm⁻¹): 2998, 2933, 2833, 1612, 1587, 1510.

2.4.3. N-[(3-Bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethylaniline (4c)

Produced from 3,4-dimethylaniline (0.177 g, 1.46 mmol); yield: 0.40 g, 1.30 mmol; 89%; m.p. 51-52°C. ¹H NMR (400 MHz, CDCl₃) : 8.34 (s, 1H), 7.19 (s, 1H), 7.10–6.90 (m, 3H), 2.38 (s, 3H), 2.21 (s, 3H), and 2.19 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) : 150.7, 148.7, 139.8, 139.6, 137.4, 134.9, 133.9, 130.3, 122.5, 118.2, 109.9, 19.9, 19.4, and 15.5. EI-MS (m/z, %): 309 ([M⁸¹Br]⁺, 46), 308 (32), 307 ([M⁷⁹Br]⁺, 47), 306 (24), 206 (41), 205 (44), 204 (33), 203 (42), 86 (64), 84 (100). HRMS (EI): calcd. for C₁₄H₁₄Br NS [M⁷⁹Br]⁺ 307.0030; found, 307.0029. FTIR (, cm⁻¹): 3084, 3014, 2966, 2917, 1615, 1591.

2.4.4. 3-[(3-Bromo-2-methylthiophen-5-yl)methyleneamino]-2-methylquinazolin-4(3H)-one (4d)

Produced from 3-amino-2-methylquinazolin-4(3H)-one (0.256 g, 1.46 mmol). Yield: 0.48 g, 1.33 mmol; 91%; m.p. 231-232°C. ¹H NMR (400 MHz, CDCl₃) : 9.12 (s, 1H, CH), 8.22 (d, J = 9.0 Hz, 1H), 7.72–7.66 (m, 1H), 7.60 (d, J = 8 Hz, 1H), 7.40 (app. t, J = 8.0 Hz, 1H), 7.20 (s, 1H), 2.60 (s, 3H, CH₃), and 2.42 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) : 159.1, 157.4, 154.2, 146.3, 141.3, 136.3, 134.6, 134.5, 127.2, 126.9, 126.5, 121.4, 110.5, 22.9, and 15.6. EI-MS (m/z, %): 363 ([M⁸¹Br]⁺, 8), 361 ([M⁷⁹Br]⁺, 9), 203 (100), 160 (98), 122 (97), 86 (92), 84 (94), 70 (88). HRMS (EI): calcd. for C₁₅H₁₂Br N₃OS [M⁷⁹Br]⁺ 360.9884; found, 360.9877. FTIR (, cm⁻¹): 2917, 1673, 1601, 1568.

2.4.5. 3-[(3-Bromo-2-methylthiophen-5-yl)methyleneamino]-2-isopropylquinazolin-4(3H)-one (4e)

Produced from 3-amino-2-isopropylquinazolin-4(3H)-one (0.297 g, 1.46 mmol). Yield: 0.50 g, 1.28 mmol; 88%; m.p. 165-166°C. ¹H NMR (400 MHz, CDCl₃) : 9.15 (s, 1H), 8.28 (d, J = 8 Hz, 1H), 7.78–7.76 (m, 2H), 7.51–7.45 (m, 1H), 7.36 (s, 1H), 3.57 (sept, J = 7 Hz, 1H), 2.51 (s, 3H), and 1.40 (d, J = 7 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃)δ: 160.6, 159.4, 158.1, 141.4, 136.2, 134.7, 134.4, 127.2, 127.2, 126.6, 121.1, 110.5, 32.0, 20.4, and 15.6 (one quaternary carbon signal was of insufficient intensity to allow confident discrimination from noise, but may have been at 147.8 ppm). EI-MS (m/z, %): 391 ([M⁸¹Br]⁺, 32), 389 ([M⁷⁹Br]⁺, 39), 203 (90), 174 (100), 160 (95), 122 (98), 103 (95), 77 (97). HRMS (EI): calcd. for C₁₇H₁₆N₃OS⁷⁹Br [M⁷⁹Br]⁺ 389.0197; found, 389.0198. FTIR (, cm⁻¹): 2987, 2932, 1671, and 1599.

2.5. Preparation of Polymer Films

The polymer matrix used in this study was PVC (K value = 67, degree of polymerization = 800) supplied by SABIC (Saudi Arabia). It was reprecipitated from a THF solution (THF is the best solvent for PVC) by alcohol several times and finally dried under vacuum at room temperature for 24 h. PVC films were prepared by dissolving PVC (5 g) in THF (100 g) under vigorous stirring for 30 min. It was necessary to control the hygrometry and the rate of evaporation of solvent during casting to maintain good optical quality and very limited turbidity. The film transmission should be greater than 80% in the near-UV range. After 3 h, the solution was spread on a slide of stainless steel (250 × 120 × 0.5 mm) and air-dried for 24 h. After the solvent evaporation, the samples were dried in a vacuum at room temperature for 30 h. The thickness of the resulting PVC film (30 μm) was measured by a micrometer type 2610 A, Germany.

2.6. Irradiation Experiments

2.6.1. Accelerated Testing Technique

A QUV Accelerated Weather Tester (Q-Lab, USA) was used for irradiation of polymers films. The accelerated weathering tester contained a stainless steel plate, which had two holes in the front side and a third one behind. Each side contained a 40 watt fluorescent ultraviolet light. These lamps were of the type UV-B 313, giving a spectrum range between 290 and 360 nm with a maximum wavelength at 313 nm. The polymer film samples were vertically fixed parallel to the lamps to make sure that the UV incident radiation was perpendicular to the samples. The irradiated samples were rotated from time to time to ensure that the intensity of light incident on all samples was the same.

2.7. Photodegradation Measuring Methods

2.7.1. Measuring the Photodegradation Rate of Polymer Films Using Infrared Spectrophotometry

The degree of photodegradation of polymer film samples was followed by monitoring FTIR spectra in the range 4000–400 cm⁻¹ using a Shimadzu FTIR 8300 spectrophotometer. Absorptions due to an alkene group at 1631 cm⁻¹, a carbonyl group at 1724 cm⁻¹, and a hydroxyl group at 3400 cm⁻¹ were specifically monitored. The progress of photodegradation during different irradiation times was followed by observing the changes in these peaks. Then hydroxyl (), carbonyl (), and polyene () indices were calculated by comparison of these absorption peaks with a reference peak at 1328 cm⁻¹, which is not significantly affected by the degradation process. The method used was the band index method, which includes the following features [16]:

As = absorbance of peak under study, Ar = absorbance of reference peak, and Is = index of group under study.

Actual absorbance, the difference between the absorbance of top of peak and base line (A Top of Peak − A Base Line), is calculated using the base line method [16].

2.8. Determination of Average Molecular Weight Using Viscometry

The viscosity property was used to determine the average molecular weight of polymer, using the Mark Houwink relation [17]:

where = the intrinsic viscosity and K and α are constants that depend upon the polymer-solvent system at a particular temperature. The intrinsic viscosity of a polymer solution was measured with an Ostwald U-tube Viscometer. Solutions were made by dissolving the polymer in a solvent (0.2 g/100 mL) and the flow times of polymer solution and pure solvent are and , respectively. Specific viscosity () was calculated as follows:

where = relative viscosity

The single-point measurements were converted to intrinsic viscosities by the following relationship:

where = concentration of polymer solution (g/100 mL).

Molecular weights of PVC with and without additives were calculated from intrinsic viscosities measured in THF solution using the following equation:

The quantum yield of main chain scission () [18] was calculated from viscosity measurement using the following relationship:

where C = concentration; A = Avogadro’s number; = the initial viscosity − average molecular weight; [] = intrinsic viscosity of PVC before irradiation; = incident intensity (1.052 × 10⁻⁸ ein dm⁻³ s⁻¹); and t = irradiation time in seconds.

3. Results and Discussion

3.1. Synthesis of the Target Imines 4a–e

The target imines (4; Schiff bases) were synthesized by the condensation reactions of 3-bromo-2-methylthiophene-5-carboxaldehyde with the appropriate amine. The required aldehyde 3 was synthesized from 3-methylthiophene in two steps. The first one involved bromination of 1 to give 3,5-dibromo-2-methylthiophene which was then converted to the aldehyde 3 in the second step by selective bromine-lithium exchange of the α-bromo group, followed by treatment with dimethylformamide (DMF) as shown in Scheme 1.

Scheme 1: Synthesis of 3-bromo-2-methylthiophene-5-carboxaldehyde (3).

For the synthesis of the target imines 4, aldehyde 3 (1 molar equivalent) was allowed to react with various amines, namely, 4-methoxyaniline, 3,4-dimethoxyaniline, 3,4-dimethylaniline, 3-amino-2-methylquinazolin-4(3H)-one, and 3-amino-2-isopropylquinazolin-4(3H)-one (Scheme 2). Reactions were carried out under microwave irradiation for 5 min in absolute methanol. The solvent was removed and the crude products were recrystallized to give pure imines 4 in high yields. The structures of the synthesized imines 4a–e were confirmed by FTIR, ¹H NMR, and ¹³C NMR spectroscopy and by mass spectrometry (EI-MS and HRMS).

Scheme 2: Synthesis of imines 4a–e.

The FTIR spectra of 4a–e showed the presence of a characteristic band for the C=N stretching vibration within the 1599–1615 cm⁻¹ region. The ¹H NMR spectra showed a characteristic singlet signal at δ = 8.34–9.15 ppm corresponding to H–C=N. The structures of 4a–e were confirmed further by EI-mass spectrometry, which showed the presence of molecular ion peaks, the elemental compositions of which were proved by high resolution mass spectrometry (HRMS). See Section 2 for details.

3.2. Photostabilization Study

Compounds 4a–e were used as additives for the photostabilization of PVC films. In order to study the photochemical activity of these additives for the photostabilization of PVC films; the carbonyl and alkene indices were monitored as a function of irradiation time using IR spectrophotometry. The irradiation of PVC films with UV light of wavelength, λ = 313 nm, led to a clear change in their FTIR spectra, as shown in the FTIR spectra of the PVC before (Figure 1) and after (Figure 2) irradiation. Appearance of bands at 1770 cm⁻¹ and 1724 cm⁻¹ was attributed to the formation of carbonyl groups related to chloroketone and to aliphatic ketone, respectively. A third band was observed at 1631 cm⁻¹, which implied formation of a C=C double bond conjugated to a carbonyl group. The hydroxyl band that appeared at 3400 cm⁻¹ was attributed to the OH of the hydroperoxide and alcohol groups, as shown in Scheme 3 [16, 19].

Scheme 3: Some of the photooxidation processes of PVC: reactions of radical B with molecular oxygen, akin to those of radical A, would give rise to chloroketone, chloroalcohol, and chlorohydroperoxide moieties; further reactions of such reactive functionalities may lead to chain cleavage and reduction in molecular weight.

Figure 1: FTIR spectrum of PVC film (control) (30 m) at zero time.

Figure 2: FTIR spectrum of PVC film (control; 30 m) after 250 hours.

The changes in the intensities of the stretching absorption bands of the hydroxyl, carbonyl, and alkene groups were used to follow the extent of polymer degradation during irradiation, because the intensities of these bands are directly proportional to the extent of degradation [11]. The degradation was calculated in terms of the hydroxyl index (, as OH formation indicator), carbonyl index (, as C=O formation indicator), and polyene index (, as C=C formation indicator). Figure 3 shows the effects of adding the new imines 4a–e to the PVC (0.5% by weight) on the growth of the , which reflects the extent of degradation of the PVC. It is clear that the addition of any of the compounds 4a–e lowered the growth of the carbonyl index of the PVC in comparison with the control sample. So, it is reasonable to conclude that these additives might be considered as photostabilizers of PVC polymer.

Figure 3: The relationship between the carbonyl index and irradiation time for PVC films (30 m thickness) containing each of 4a–e (0.5% by weight) compared to PVC without additive (blank).

Since an efficient photostabilizer shows a longer induction period, compound 4e is considered as the most active photostabilizer, followed by 4d, 4b, 4a, and 4c. Just as formation of carbonyl groups indicates degradation of polymer, so does the formation of alkene units. Therefore, the polyene index () was also monitored with irradiation time in the presence of each of 4a–e. As shown in Figure 4 the results are comparable with those of the effect on the ; compounds 4a–e showed the same order in their efficiency at reducing the growth of .

Figure 4: The relationship between the alkene index and irradiation time for PVC films (30 m thickness) with or without additives 4a–e (0.5% by weight).

The hydroxyl index () growth versus irradiation time was also monitored for the PVC in the presence of each of compounds 4a–e against the pure PVC. All of the compounds 4a–e showed a positive effect by lowering the growth of the hydroxyl index in the same order of efficiency as for both and (Figure 5).

Figure 5: The relationship between the hydroxyl index and irradiation time for PVC films (30 m thickness) with or without 4a–e (0.5% by weight).

3.3. Variation of PVC Molecular Weight during Photolysis in the Presence of 4a–e

The photodegradation of polymers causes changes in viscosity average molecular weight [20]. Accordingly, we decided to monitor the photodegradation process by monitoring the change in and other parameters that are related to it, particularly the average number of chain scissions (average number of cuts per single chain) (N) and the quantum yield of the chain scission process ().

Figure 6 shows the change of the values with the irradiation time for PVC films in the presence of each of the selected additives 4a–e 0.5% (wt/wt) and for the pure PVC as control ( was calculated using (2) with THF as solvent at 25°C). It is clear that addition of 4a–e to the PVC films reduces the decrease in the which means that the additives inhibit the photodegradation process and stabilize the PVC. Again the efficiencies of 4a–e were in the same order found in the previous tests.

Figure 6: Changes in the viscosity-average molecular weight during irradiation of PVC films (30 m) with or without 4a–e (0.5% by weight).

Chain scissions occur as a result of photodegradation of polymers and increase in number with increasing irradiation time. Therefore, the average number of chain scissions (average number of cuts per single chain) (N) [20, 21], calculated using (8), is used to monitor photodegradation. Consider

To study the effect on its stability of adding compounds 4a–e to the PVC, the values of the average number of chain scissions were calculated at the irradiation time intervals that were used in this study. The plot of versus irradiation time is shown in Figure 7, which shows that the growth of the value of the chain scissions in the presence of 4a–e follows the same trend as the decrease in molecular weight.

Figure 7: Changes in chain scission (N) during irradiation of PVC films (30 m) containing 4a–e (0.5% by weight) or no additive.

Finally, the change in the quantum yield of main chain scission () was studied. After 250 h of irradiation the was calculated using (7) for the PVC films with and without 0.5% (wt/wt) of compounds 4a–e (Table 1).

Table 1: Quantum yield () for chain scission for PVC films (30 m) with and without additive (0.5%wt) after 250 h irradiation time.

Additive (0.5%wt)
Quantum yield of main chain scission ()

PVC + 4e

PVC + 4d

PVC + 4b

PVC + 4a

PVC + 4c

PVC (blank)

The values for PVC films in the presence of additive were of course less than that of pure PVC (control), and the values increased in the following order: 4e < 4d < 4b < 4a < 4c. The explanation for low value of of PVC (control) is that, in the macromolecules of PVC, the energy is absorbed at one site, and then the electronic excitation is distributed over many bonds so that the probability of a single bond breaking is small, or the absorbed energy is dissipated by nonreactive processes. However, the additives caused a further reduction of around 1000-fold in the value of , which is comparable with the reduction brought about by use of salicylidene imines of substituted aminothiadiazoles [22].

It is well established that the quantum yield () increases with increasing temperature [23] around the glass transition temperature (Tg) of amorphous polymers and around the melting temperature of crystalline polymers. In the study presented in this work, the photolysis of PVC film was carried out at a temperature 35–45°C, well below the glass transition temperature (Tg of PVC = 80°C). Therefore, dependency of on temperature was not expected to be significant.

3.4. Suggested Mechanisms of Photostabilization of PVC by 3-[(3-Bromo-2-methylthiophen-5-yl)methyleneamino]-2-isopropylquinazolin-4(3H)-one (4e)

The efficiency of the new compounds 4a–e as photostabilizers for PVC films decreases in the order 4e > 4d > 4b > 4a > 4c. These compounds may stabilize PVC by direct absorption of UV radiation and dissipation of the energy as heat (Scheme 4).

Scheme 4: A suggested mechanism of photostabilization of PVC by 4e through absorption of UV light and dissipation of the light energy as heat.

Thiophene moieties themselves may also be able to act in a similar way to absorb UV radiation and to dissipate the energy as heat, but obviously the nature of the nitrogen substituent on the imine group had a significant effect on the efficiency of the additives as stabilizers so any direct effect of the thiophene ring alone is likely to be minor. The polarity of the thiophene ring may also lead to attraction between the stabilizer and PVC, which again would be influenced by the nature of the nitrogen substituent on the imine group, as shown for additive 4e in Scheme 5. Such attraction may assist transfer of energy from excited state PVC to the additive, from where it can be dissipated. The observed trends may help in the design of even better photostabilizers for PVC.

Scheme 5: A suggested mechanism of photostabilization of PVC by 4e through interaction between the polymer chains and Schiff base compounds, leading to more effective energy transfer.

4. Conclusion

New thiophene derivatives 4a–e incorporating an imine group have been successfully synthesized and characterized. The level of photodegradation of (PVC) films containing the synthesized thiophenes (0.5%; by weight) was reduced in all cases, with effectiveness of the additives in the order 4e > 4d > 4b > 4a > 4c. For the most favorable additive (4e), the rate of appearance of infrared absorption bands of degradation products was reduced by around two-thirds, while the quantum yield of chain scission was calculated to be reduced by a factor of more than one thousand.

It is suggested that the additives may help stabilize PVC by direct absorption of UV radiation and dissipation of the energy as heat, or that electrostatic attraction between the additives and PVC may assist transfer of energy from excited state PVC to the additive, from where it can be dissipated.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

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-VPP-239.

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