polymer nanocomposite films based on poly-ε-caprolactone and terephthalic acid

Materials Science and Engineering C 70 (2017) 85–93

Contents lists available at ScienceDirect

Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Development of biodegradable metaloxide/polymer nanocomposite films based on poly-ε-caprolactone and terephthalic acid Kokkarachedu Varaprasad a,⁎, Manuel Pariguana a,b, Gownolla Malegowd Raghavendra c, Tippabattini Jayaramudu d, Emmanuel Rotimi Sadiku e a

Centro de Investigación de Polímeros Avanzados (CIPA), Avenida Collao 1202, Edificio de Laboratorios, Concepción, Chile Centro de Innovación Tecnológica Agroindustrial CITE Agroindustrial, Panamericana Sur Km, 293.3, Ica, Peru c Department of Packaging, Yonsei University, Wonju, Gangwon-do 220 710, South Korea d Center for Nano Cellulose Future Composites, Department of Mechanical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402–751, South Korea e Department of Polymer Technology, Tshwane University of Technology, CSIR-Campus, Pretoria 0040, South Africa b

a r t i c l e

i n f o

Article history: Received 31 March 2016 Received in revised form 18 August 2016 Accepted 22 August 2016 Available online 25 August 2016 Keywords: Composite films ZnO-CuO nanoparticles Terephthalic acid Disposed poly(ethylene terephthalate) oil bottles Biodegradable poly-ε-caprolactone

a b s t r a c t The present investigation describes the development of metal-oxide polymer nanocomposite films from biodegradable poly-ε-caprolactone, disposed poly(ethylene terephthalate) oil bottles monomer and zinc oxide-copper oxide nanoparticles. The terephthalic acid and zinc oxide-copper oxide nanoparticles were synthesized by using a temperature-dependent precipitation technique and double precipitation method, respectively. The terephthalic acid synthesized was confirmed by FTIR analysis and furthermore, it was characterized by thermal analysis. The as-prepared CuO-ZnO nanoparticles structure was confirmed by XRD analysis and its morphology was analyzed by SEM/EDS and TEM. Furthermore, the metal-oxide polymer nanocomposite films have excellent mechanical properties, with tensile strength and modulus better than pure films. The metal-oxide polymer nanocomposite films that were successfully developed show a relatively brighter colour when compared to CuO film. These new metal-oxide polymer nanocomposite films can replace many non-degradable plastics. The new metaloxide polymer nanocomposite films developed are envisaged to be suitable for use in industrial and domestic packaging applications. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Over the past few years, polymers utilization has been increasing in several fields, due to their significantly important features, such as: lightweight, low-cost and easier to design to the desired shape, by customers, than metals [1]. These special characteristics make them attractive for applications in the domestic, automobile, packaging and bottling fields, etc. [2,3]. However, disposal of these polymeric materials leads to serious environmental problems for the earth [3]. Owing to their stable network, it takes a very long time to degrade in the soil. Polyethylene terephthalate (PET) is one of the most commonly used synthetic polymers for disposable, packaging, bottles and other applications due to its light weight, durability, chemical resistance and for its good transparency and low market price when compared to other high-performance polymers [4]. Owing to these factors, PET consumption increases on a day-to-day basis [4]. At the same time, PET garbage is also proportionately increasing, thereby bringing a substantial problem to the green environment due to their non-biodegradable nature [4]. To resolve this difficulty researchers have focused on the synthesis of monomers from the trash polymers and their re-applicability with ⁎ Corresponding author at: Center for Advanced Polymer Research, Concepcion, Chile. E-mail addresses: [email protected], [email protected] (K. Varaprasad).

http://dx.doi.org/10.1016/j.msec.2016.08.053 0928-4931/© 2016 Elsevier B.V. All rights reserved.

suitable biodegradable polymers for practical [5] and technical applications [6,7]. Terephthalic acid (used in the synthesis of PET) is one of the organic compounds that is non-irritating to the skin and eyes and it is non-toxic to aquatic organisms at concentrations lower than its water solubility and lower toxicity (15 mg/L at 10 °C) when compared to other synthetic monomers [8,9]. Due to these characteristics, it is widely used as a raw material to produce bio-plastics, synthetic perfumes, medicines, additives and other engineering materials [10,11], which are widely used in our daily life, for example, drinking bottles made of PET [9]. It is well-known that terephthalic acid is an important industrial chemical compound for the synthesis of engineering materials. Poly-ε-caprolactone is a semi-crystalline linear aliphatic synthetic polymer, which is non-toxic, biocompatible and biodegradable polyester. It is a water, solvents and oil resistance polymer [12–14]. Further aspects of the polymer that are worthy of consideration include: its flexibility, hydrophobicity and the ease of processing with several other polymers [15,16]. Due to its desirable properties, it has been used in packaging, tissue engineering, agriculture and other applications [16–18]. However, its suitability in advanced applications requires the combined utilization of different inorganic oxide and their coreshell materials through effective impregnation or functionalization [14].

86

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

Lately, core-shells of metal-oxide nanomaterials have attracted huge interest due to their advanced chemical, physical and biological properties [19,20]. Due to their characteristics, they are widely used in several fields, such as: biomedical, energy and other industrial applications [19, 21,22]. The selection and composition of new metal-oxide materials are more important in order to optimize their physical, chemical and biological characteristics, with cost-effective and minimal colour, which can enhance their applicability in the packaging industries, principally for health care and food packing applications [23]. Keeping in view the above mentioned points, in the present scientific research, the synthesis of terephthalic acid from the disposed poly(ethylene terephthalate) oil bottles by a simple method is explored. In addition, the as-synthesized terephthalic acid is technically utilized to develop metal-oxide polymer nanocomposite films with the aid of biodegradable poly-ε-caprolactone. Dual metaloxide nanoparticles (ZnO-CuO), synthesized via double precipitation technique, were used as antibacterial agents. The structure of the synthesized CuO-ZnO nanoparticles' was confirmed by XRD analysis and its morphology was analyzed by SEM/EDS and TEM. And, the mechanical properties of the films developed based on these nanoparticles were evaluated and compared with respect to pure films. It was reported that the ZnO and CuO nanoparticle-based composites can kill certain bacteria tested and in addition CuO can minimize the emission from food, vegetables and other fruits [24]. Therefore, the composite films developed can withstand (without any significant damage) long period of normal usage. Such type of composite films will be very useful for packaging applications. The details of the investigation are presented in this article. The terephthalic acid synthesized was confirmed by FTIR analysis and was characterized by thermal analysis. Furthermore, the metaloxide polymer nanocomposite films have excellent mechanical properties, with tensile strength and modulus better than pure films.

In the formation of the nano CuO via the precipitate method, the following reaction mechanisms were formulated in terms of the chemical equations. 2CuSO4 þ 2NH2 OH  HCl þ 6NaOH → Cu2 O þ N2 þ 2NaCl þ 2Na2 SO4 þ7H2 O

Δ

2Cu2 O þ O2 → 4CuO 2.2.2. Synthesis of ZnO-CuO nanoparticles by double precipitation technique 2.2.2.1. Zinc oxide-copper oxide nanoparticles. The metal-oxide nanoparticles were prepared by double precipitation technique. In this technique, 9.1 g of zinc nitrate hexahydrate and 0.7 g of hydroxylamine hydrochloride were initially dissolved in 25 mL distilled water. Secondly, 5 mL of copper oxide (at concentrations: 0.1, 0.25 and 0.5 g) solution was introduced. Thereafter, it was precipitated with 60 mL of sodium hydroxide (12 g) solution and a pH 12 was maintained for 30 min. The precipitated solution obtained was washed with distilled water in order to remove the salts and other elements. Finally, the resulting precipitate was boiled for 5 min and the core-shell was filtered and dried at 100 °C for 2 h and heat-treated at 250 °C for 1 h. The reaction Scheme 1 is as follows: 2.3. Preparation of metal-oxide polymer nanocomposite film

Zinc nitrate hexahydrate, anhydrous copper sulfate, hydroxylamine hydrochloride, sodium hydroxide, poly-ε-caprolactone, ethylene glycol, sodium hydroxide, dimethyl sulfoxide, tetrahydrofuran chemicals were purchased from Sigma-Aldrich, Inc., (Chile) and used without further purification. Poly(ethylene terephthalate) flakes were obtained from the disposed Poly(ethylene terephthalate) oil bottles, which were collected from trash bins in Concepcion Street, Bio-Bio Region, Chile.

2.3.1. Depolymerization of poly(ethylene terephthalate) Waste poly(ethylene terephthalate) from used oil bottles were cut into small pieces and thoroughly washed with soap water and distilled water. The cleaned PET pieces were dried in an oven for 48 h at 80 °C. The de-polymerization reaction was achieved, with a slight modification, in accordance with previous reports [25,26]. For the synthesis of terephthalic acid, 2.7 g of NaOH was initially dissolved in 12 g of ethylene glycol at 70 °C. Thereafter, 5 g of poly(ethylene terephthalate) was added and stirred. Then, the temperature was slowly increased to 200 ± 2 °C in 10 min in order to completely dissolve the PET in the ethylene glycol and also to completely avoid the formation of the oligomers [26]. To this solution, 100 mL of distilled water was added slowly and then the solution was cooled to room temperature following which, H2SO4 was added drop-wise to this solution for the formation of a white terephthalic acid precipitate, with a pH 2.4. The resulting terephthalic acid was washed several times, with distilled water in order to remove unwanted elements. Finally, it was filtered and dried at room temperature. The yield obtained was ~ 99%. The yield (%) of TA was calculated using the following equation:

2.2. ZnO-CuO nanoparticles preparation

TA Yield ð%Þ ¼

2.2.1. Copper oxide (CuO) nanoparticles synthesis CuO was synthesized by precipitation technique. 5 g of anhydrous copper sulfate and 2.27 g of hydroxylamine hydrochloride were completely dissolved in 14 mL distilled water in a 250 mL beaker under constant stirring condition (250 rpm) for 5 min at room temperature. To this reaction mixture, sodium hydroxide solution (3.63 g in 70 mL in distilled water) was added drop-wise with vigorous stirring until a brown colour precipitate was formed and the pH was adjusted to 12. After stirring for 30 min, the precipitate formed was washed several times with distilled water in order to remove the unwanted salt and other impurities and the pH of the precipitate was reduced to 7. To this solution, sufficient amount of ethanol was added and kept over a hot plate to boil for 5 min in order to obtain a good crystallization of the oxide nanoparticles. Finally, it was filtered and dried at 100 °C for 2 h and subsequently the temperature was increased to 250 °C (for 1 h), which led to the formation of better oxide nanoparticles.

where NTA represents the number of moles weighed and NTA,0 represents the number of moles weighed theoretical number of TA moles that will be formed upon complete decomposition of disposed PET. The (%) degradation of disposed PET was calculated using the following equation:

2. Materials and method 2.1. Materials

N TA  100 NTA;0

PET Degradationð%Þ ¼

W 0 −W 1  100 W0

where W0 represents the initial weight of disposed PET and W1 represents the weight of undepolymerized PET. The preparative equations (Scheme 2) are as presented below: 2.3.2. Preparation of poly-ε-caprolactone-terephthalic acid metal oxide nanocomposite film The poly-ε-caprolactone-terepthalic acid nanocomposite films were obtained via solvent evaporation method.

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

87

Scheme 1. Illustrative formation processes of ZnO-CuO nanoparticles.

In the typical process, primarily 0.5 g of terephthalic acid was dissolved in a solution containing 2 mL of dimethyl sulfoxide and 5 mL of tetrahydrofuran. To this terephthalic acid solution, 20 mg of oxide nanoparticles was dispersed in 2 mL of tetrahydrofuran and was thoroughly mixed. Secondly, the poly-ε-caprolactone pellets (2 g) were dissolved in 15 mL of tetrahydrofuran (THF) at the 40 °C under continuous stirring (250 rpm). Finally, the solutions of oxide nanoparticles terephthalic acid and poly-ε-caprolactone were homogeneously mixed and poured into a Petri dish and dried at room temperature in order to evaporate the solvent and to obtain metal-oxide polymer nanocomposite films (Fig. 1). Table 1 illustrates the feed composition of the various components used in the preparation of biodegradable metal-oxide poly-εcaprolactone-terephthalic acid nanocomposite films. 2.4. Characterizations FTIR spectra of disposed PET oil bottles, terephthalic acid, oxide nanoparticles, polymer films and metal-oxide polymer nanocomposite films were obtained on a Perkin Elmer, UATR two, FTIR spectrometer (Beaconsfield, Bucks, UK) in the wave number range of between 4000 and 400 cm−1. Signal averages were obtained from 25 scans at a resolution of 1 cm−1. Thermal characteristics of the samples developed were

determined on a thermogravimetric analyzer (TGA), using the TGA Q 50 thermal analyzer (T.A. instruments-water LLC, Newcastle, DE, USA), at a heating rate of 10 °C/min, a passing nitrogen gas at a flow rate of 100 mL/min in the temperature range of 25–600 °C. DSC thermograms of films and metal-oxide polymer nanocomposite films developed were recorded using a DSC882e (Mettler Toledo) instrument at a heating rate of 10 °C/min under a constant nitrogen flow (100 mL/min) in the temperature range of between 25 and 350 °C. The morphology and elemental analysis of the gold-coated samples were observed by using the scanning electron microscope (SEM, JEOL 6460 LV) at an accelerating voltage of 10 kV. Wide angle X-ray diffraction profile of oxide nanoparticles were studied with a Rigaku diffractometer with a Cu-Kα radiation at a voltage of 40 kV and a current of 40 mA and using a scan rate of 0.02°s−1. The size and shape of the nanoparticles were measured by using the transmission electron microscopy (TEM, FEI Technai G2 20S-TWIN, USA). For this observation, the sample was dispersed in a 1:1 mixture of methanol and water solution and deposited on a 3 mm copper grid and dried at ambient temperature. The tensile (tensile strength, modulus and % elongation-atbreak) properties were determined using the INSTRON 3369 Universal Testing Machine (Buckinghamshire, England). The samples were cut into dimensions 10 mm × 100 mm and the mechanical properties were studied using a 10 kg load cell, by maintaining a gauge length of 50 mm

HO 2 NaOH

+

OH

Sodium hydroxide

Ethylene glycol

70 oC NaO ONa

+

.

O

O

C

C

O

Sodium salt of ethylene glycol

CH2

CH2

O

O

n ONa C

C

ONa

Depolymerization process

+

n

HO

CH2

CH2

OH

Ethylene glycol Disodium salt of terephthalic acid

HO

H2SO4

. n

poly(ethylene terephthalate)

200 oC

O

+

H2O Water

Precipitation process

O

O

C

C

OH

+

Na2SO4 Sodium sulfate

Terephthalic acid

Scheme 2. Depolymerization of the disposed poly(ethylene terephthalate) oil bottles.

88

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

Fig. 1. Original images of A) metal-oxide nanoparticles, B) poly-ε-caprolactone-terephthalic acid composite films and C) metal-oxide poly-ε-caprolactone-terephthalic acid (PCL-TA- ZnOCuO0.5) nanocomposite films.

and by operating the machine at a crosshead speed of 5 mm/min and at 23 °C. All Universal Testing Machine data from three repeated tests were averaged. 3. Results and discussions Fig. 2A shows the FTIR spectra of disposed PET (Fig. 2Aa) and terephthalic acid (Fig. 2Ab). In the case of disposed PET spectra, a peak at 1715 cm−1, which is the vital band of the ester group of PET, was observed [27]. The band at 1410 cm−1 is attributed to the phenylene ring vibration of PET [28]. The band at 1344 cm− 1 indicates the \\CH2 groups of the glycol present in the PET. The bands at 1243 and 1095 cm−1 are indicative of the ester C_O stretching and the bands at 1021, 973 and 845 cm−1 are the in-plane vibrations of benzene, the C\\O stretching vibration of glycol and the CH2 rocking of glycol unit

in PET, respectively [29]. The bands at 724 and 872 cm−1 are indicative of the out-of-plane vibrations of the benzene group. Furthermore, the selected disposed poly(ethylene terephthalate) oil bottles were depolymerized via a temperature-dependent technique. From this technique, terephthalic acid was obtained and characterized by FTIR. Fig. 2Ab shows the FTIR spectrum of the terephthalic acid synthesized. The characteristic peaks between 2551 and 3200, 1679 cm−1 and 1574–1425 cm−1 are assigned to the\\COOH,\\C_O and an aromatic ring of the terephthalic acid, respectively [30]. The peaks in the region 1281–1000 cm−1 are indicative of\\C\\OH,\\C_O,\\C\\CH, and \\C\\H groups present in the terephthalic acid [30]. The band in the region 700 to 800 cm−1 indicates the aromatic ring of terephthalic acid [31]. Fig. 2B shows the Fourier transform infrared spectroscopy spectra of poly-ε-caprolactone (Fig. 2Ba) and poly-ε-caprolactone-terephthalic

Table 1 Feed composition of metal-oxide poly-ε-caprolactone-terepthalic acid nanocomposite films. Films codes

Poly-ε-caprolactone (g)

Terephthalic acid (g)

ZnO (mg)

CuO (mg)

ZnO-CuO 0.1 g (mg)

ZnO-CuO 0.5 g (mg)

PCL PCL-TA1 PCL-TA2 PCL-TA-ZnO PCL-TA-CuO PCL-TA-ZnO-CuO0.1 PCL-TA-ZnO-CuO0.5

2 2 2 2 2 2 2

– 0.5 1 0.5 0.5 0.5 0.5

– – – 20 – – –

– – – – 20

– – – – – 20 –

– – – – – – 20

–

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

89

Fig. 2. FTIR spectra of A: a) disposed poly(ethylene terephthalate) oil bottle, b) terephthalic acid. B: a) poly-ε-caprolactone, b) poly-ε-caprolactone-terephthalic acid composite films and C: a) metal-oxide nanoparticles, b) poly-ε-caprolactone-terephthalic acid nanocomposite films.

acid films (Fig. 2Bb). As reported in the literature, poly-ε-caprolactone shows characteristic peaks at 3341, 1737, 1716, 850–1480 and 720 cm−1 [32]. On the other hand, in the case of poly-ε-caprolactoneterephthalic acid (PCL-TA) films, these peaks are slightly shifted further and new peaks are observed at 1574, 1430 and 787 cm−1, corresponding to the aromatic ring of terephthalic acid, which confirms clearly, a composite that has functional groups of terephthalic acid and poly-εcaprolactone. Similar to PCL-TA, the metal-oxide polymer nanocomposite films (PCL-TA-ZnO, PCL-TA-CuO, PCL-TA-ZnO-CuO0.1 and PCL-TA-ZnOCuO0.5) exhibit characteristic peaks of PCL-TA with a shift in frequency (Fig. 2Ca,b). In addition, depending on the metal oxide present, the films exhibit new characteristic peaks at 483 and 593 cm−1 (Cu\\O), 365 cm−1 (Zn\\O) and at around 452 cm−1 for the bimetallic nanoparticles (Zn-O/Cu-O) [33–36]. ZnO-CuO0.5 and PCL-TA-ZnO-CuO0.5 nanocomposite films were chosen as representative samples for the structural discussion. Fig. 3A shows the XRD pattern of the ZnO-CuO nanomaterials. The resulting diffraction peaks are explained thus: that the ZnO-CuO nanoparticles have

a hexagonal wurtzite ZnO (JCPDS card no. 36–1451) and monoclinic CuO (JCPDF card no. 073–6234) crystal structures [37]. The peaks at 2θ 38.75° belong to the monoclinic CuO, while all other peaks relate to the ZnO crystal structure [19]. However, these results suggest that the CuO nanoparticles are strongly anchored on the ZnO nanoparticles. In the XRD patterns of PCL-TA-ZnO-CuO0.5 nanocomposite film (Fig. 3B), the specific intensity peaks of ZnO-CuO nanoparticles can be observed besides the two main peaks (2θ = 21.38°, 23.66°) of poly-εcaprolactone [38], which confirms that nanooxide particles were introduced in the polymer films and the crystal structure of ZnO-CuO nanoparticles is not modified in the polymer film. In order to obtain a detailed information on the morphology of synthesized nanocore-shell oxides, TEM studies were carried out and Fig. 3C shows TEM image of ZnO-CuO. TEM examination reveals that the ZnO nanoparticles have flowery flake-like structure that is impregnated with small copper oxide nanoparticles, which are spherical in shape. Conversely, the oxide nanoparticles obtained have an average length of 50 nm and a width of 10 nm. In order to obtain a detailed information on the morphology and elemental analysis of the metal-

Fig. 3. XRD diffraction pattern of A) ZnO-CuO0.5 nanoparticles, B) PCL-TA-ZnO-CuO0.5 composite film and C) TEM image of ZnO-CuO0.5 nanoparticles.

90

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

oxide nanoparticles and metal-oxide polymer nanocomposite films synthesized, SEM and EDS were carried out. Figs. 4 and 5 show SEM and EDS of oxide nanoparticles and nanooxide films, respectively. SEM images reveal that ZnO, CuO and ZnO-CuO particles have flower petals structure (Fig. 4A), spherical shape (Fig. 4B) and that the spherical particle is impregnated in the flower petals sutures (Fig. 4C), respectively. However, when the CuO content increases in the ZnO-CuO nanoparticles, it was observed that the colour of the nanoparticles slightly increased in intensity (Fig. 1). In here, a phenomenon of limited colour of oxide nanoparticles can be useful for certain biomedical and engineering-related industrial applications. Furthermore, these nanoparticles were added to the PCL-TA film in order to increase their applicability in industrial applications. Fig. 4(a– f) shows the SEM images of PCL, PCL-TA film and the ZnO, CuO and ZnO-CuO impregnated PCL-TA films, respectively. The SEM images of pure PCL-TA exhibits smooth surface structure, whereas the metaloxide polymer nanocomposite films of PCL-TA-ZnO, PCL-TA-CuO, PCLTA-ZnO-CuO0.1 and PCL-TA-ZnO-CuO0.5 show irregular surfaces with some pore structures. The irregular surfaces are due to the oxide nanoparticles with different morphology and the porous structures occurred due to the evaporation of solvent during the process. The EDS spectra were recorded for the products of all the oxide nanoparticles, pure PCL-TA and metal-oxide polymer nanocomposite films (Fig. 5a1). The EDS spectra of the pure film show that the presence of C and O, which can be attributed to the structure of PCL and TA. From the EDS spectrum shown in Fig. 5(A1–C1), it is confirmed that ZnO, CuO, ZnO-CuO oxide nanoparticles and metal-oxide polymer nanocomposite films (ZnO films (Fig. 5b1), CuO films (Fig. 5c1) and ZnO-CuO oxide films (Fig 5d1)) exhibit peaks corresponding to their respective constituent elements. The thermal behaviour of polymers and monomers is usually investigated by TGA/DSC. The disposed PET and terephthalic acid TGA curves are shown in Fig. 6A. The initial decomposition temperatures of TA and PCL occurred at 246 and 350 °C. This is due to the evaporation

of organic molecules, dihydroxylation and decomposition of terephthalate (246 °C) and the inner atmospheric changes that took place through the rupture of the polyester chains via ester pyrolysis reaction, with the release of CO2 and water and the carboxylic acid groups in PCL (350 °C), respectively. However, there is the total weight loss that occurred at 600 °C, whereas the disposed PET shows an 80% weight loss occurred at 600 °C, since PET has strong ester groups, hence it can increase the thermal stability of the polymers. From the TGA studies, it is clearly shown that the terephthalic acid has low thermal stability, followed by the disposed PET and PCL. The PCL-TA exhibits a three-stage degradation: the first is related to a significant water loss between 60 and 120 °C, the second and third degradation weight losses are of the TA and PCL, which occurred at 250 and 350 °C. The final weight loss observed at 600 °C is ~98.4%. Similar phenomena were observed in the case of PCL-TA-ZnO-CuO0.1 and PCL-TA-ZnO-CuO0.5, where their weight losses are 92% and 93.28%, respectively, at final the temperature. These weight loss changes are due to the incorporation of oxide metal and because of the conductive nature of oxide nanoparticles. The DSC curves of the disposed PET material and TA sample are illustrated in Fig. 6B. The PET exhibits its melting point at 258 °C, whereas TA cannot exhibit any peak in the temperature range 25 to 300 °C [39]. According to Noritake et al., PET endothermic peak disappeared when its terephthalic acid is formed [39]. A similar phenomenon was obtained in this investigation. However, DSC and TGA results show a progressive decrease in the thermal stability and molecular weight, in anticipation of the presence of terephthalic acid. In addition Fig. 6B shows the thermograms of neat PCL, PCL-TA, PCL-TA-ZnO-CuO0.1 and PCL-TA-ZnOCuO0.5 metal-oxide polymer nanocomposite films. In addition, Fig. 6B shows the thermograms of: neat PCL, PCL-TA, PCL-TA-ZnO-CuO0.1 and PCL-TA-ZnO-CuO0.5 metal-oxide polymer nanocomposite films. All the materials exhibit endothermic melting between 73 and 77 °C, which is characteristics of the semi-crystalline structure of PCL [40]. However, a significant decrease in the melting peaks of films incorporated with TA

Fig. 4. SEM images of pure metal-oxide nanoparticles: A) ZnO, B) CuO, C) ZnO-CuO and polymer composites [a) PCL, b)PCL-TA, c) PCL-TA-ZnO, d) PCL-TA-CuO, e) PCL-TA-ZnO-CuO0.1, and f) PCL-TA-ZnO-CuO0.5].

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

91

Fig. 5. EDS images of metal-oxide nanoparticles A1) ZnO, B1) CuO, C1) ZnO-CuO and polymer composites [a1) PCL-TA, b1) PCL-TA-ZnO, c1) PCL-TA-CuO0.1 and d1) PCL-TA-ZnO-CuO0.5].

(order of endothermic melting: PCL N PCL-TA N PCL-TA-ZnO-CuO0.5 N PCL-TA-ZnO-CuO0.1) was observed. This is because incorporation of TA monomer can control the periodic arrangement of PCL chains into its networks, leading to some loss in the polymer structure of the bio-composites than in pure PCL, which can decrease the melting point of the films [13]. Similarly, in the case of metal-oxide polymer nanocomposite films, a decrease of the melting point was observed and this is due to the

metal-oxide nanoparticles which is associated with the decrease in the size of spherulites and their surface free energies [41]. The tensile stress–strain curves of PCL, PCL-TA and metal-oxide polymer nanocomposite films are presented in Fig. 7. The curve illustrates the mechanical properties: Young's modulus (Fig. 7A), ultimate tensile strength (Fig. 7B) and % elongation-at-break (Fig. 7C) of all the films. In the case of PCL-TA, there are slight increments in the

Fig. 6. TGA curve of A) TA, waste PET and metal-oxide polymer nanocomposite films, DSC curve of B) trash PET, terephthalic acid and metal-oxide polymer nanocomposite films.

92

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93

Fig. 7. Uniaxial stress–strain curves of PCL-TA, PCL-TA-ZnO, PCL-TA-CuO, PCL-TA-ZnO-CuO0.1, PCL-TA-ZnO-CuO0.5 A) Young's modulus, B) maximum stress and C) elongation-at-break.

mechanical properties with reinforcement with TA. This is due to the presence of TA crystallites structure within the polymeric composite [42]; these crystallites can act to absorb strain energy during deformation, through an unfolding of crystalline structure or the break-up of its physical crystalline structure. It is well established that the crystalline structure of materials can play a vital role in the improvement of the mechanical properties of the polymeric films [43]. The results indicate that the Young's modulus and the ultimate tensile strength of the PCL-TA films increased when metal-oxide nanomaterials were added to the composite films, which translates into the films having good stiffness that can affect the strength of the composite films. Improvements of these properties are due to the strong interaction and better dispersion of the oxide nanoparticles. The Young's modulus and the ultimate tensile strength enhancements are consistent with previous studies [44]. However, the % elongation-atthe break decreased with the addition of metal-oxide nanoparticles. It seems that the presence of metal-oxide nanoparticles restricts the movement of the polymer chains (physical interaction) and they provided some degree of rigidity to the polymer films. Similarly, Reza Salehiyan et al. reported increases in the Young's modulus and the ultimate tensile strength and a decrease the elongation-at-the break of PLA/ PCL/MMT and PLA/PCL/mLLDPE/MMT nanocomposites [45]. In contrast, the reference PCL-TA films, showed a difference in comparison with the metal-oxide polymer nanocomposite films, because the metal-oxide polymer nanocomposite films have an effective metal-oxide nanoparticles reinforcement. Overall, metal-oxide polymer nanocomposite films showed a better Young's modulus and an improved ultimate tensile strength than the neat film (PCL-TA). 4. Conclusion Metal-oxide polymer nanocomposite films were developed by a solvent evaporation process, utilizing terephthalic acid, poly-εcaprolactone and bi-metallic zinc oxide-copper oxide nanoparticles. The terephthalic acid used was synthesized from disposed poly(ethylene terephthalate) oil bottles via a temperature-dependent precipitation technique. The experimental results showed that enhanced mechanical strength of the composite CuO-ZnO films developed over ZnO or CuO films alone resulted. In addition, enhanced brightness was found than CuO films. On the basis of the experimental findings, it is

shown that the disposed PET bottles can be successfully utilized to develop composite films with enhanced properties. Furthermore, the findings of this investigation are more useful from the point of view of a possible pilot scale application, since it is envisaged that the adopted approach should reduce the overall cost of manufacture of films, with improved properties. The properties of the PCL films obtained from disposed PET bottles, with this simple method, were improved by the addition of TA. However, costing and a pilot scale plant need to be done to confirm the possible cost-effectiveness and the feasibility of up-scaling. Overall, the current investigation has given a new direction on the utilization of disposed PET bottles for the development of composite films that are envisaged to find applications in the packaging sector. Acknowledgement The author, Kokkarachedu Varaprasad wishes to acknowledge the Programa de Atracción e Inserción de Capital Humano Avanzado (PAI) Proyecto No. 78130211 Conicyt, Chile and the Centro de Investigación de Polímeros Avanzados (CIPA), CONICYT Regional, and GORE BIO-BIO PRFC0002. References [1] S. Akçaözoğlu, C. Ulu, Recycling of waste PET granules as aggregate in alkali-activated blast furnace slag/metakaolin blends, Constr. Build. Mater. 58 (2014) 31–37. [2] C.-q. Wang, H. Wang, J.-g. Fu, Y.-n. Liu, Flotation separation of waste plastics for recycling—a review, Waste Manag. 41 (2015) 28–38. [3] M. Ozdemir, C.U. Yurteri, H. Sadikoglu, Physical polymer surface modification methods and applications in food packaging polymers, Crit. Rev. Food Sci. Nutr. 39 (1999) 457–477. [4] N. George, T. Kurian, Recent developments in the chemical recycling of postconsumer poly(ethylene terephthalate) waste, Ind. Eng. Chem. Res. 53 (2014) 14185–14198. [5] E. van der Harst, J. Potting, C. Kroeze, Comparison of different methods to include recycling in LCAs of aluminium cans and disposable polystyrene cups, Waste Manag. 48 (2016) 565–583. [6] T. Yoshioka, N. Okayama, A. Okuwaki, Kinetics of hydrolysis of PET powder in nitric acid by a modified shrinking-core model, Ind. Eng. Chem. Res. 37 (1998) 336–340. [7] A. Ulrici, S. Serranti, C. Ferrari, D. Cesare, G. Foca, G. Bonifazi, Efficient chemometric strategies for PET–PLA discrimination in recycling plants using hyperspectral imaging, Chemom. Intell. Lab. Syst. 122 (2013) 31–39. [8] Z. Yi-Mei, S. Yao-Qun, W. Zhi-Jiang, Z. Jie, Degradation of Terephthalic Acid by a Newly Isolated Strain of Arthrobacter sp.0574, 2013.

K. Varaprasad et al. / Materials Science and Engineering C 70 (2017) 85–93 [9] J. Pang, M. Zheng, R. Sun, A. Wang, X. Wang, T. Zhang, Synthesis of ethylene glycol and terephthalic acid from biomass for producing PET, Green Chem. 18 (2016) 342–359. [10] X.H. Lin, S.N. Lee, W. Zhang, S.F.Y. Li, Photocatalytic degradation of terephthalic acid on sulfated titania particles and identification of fluorescent intermediates, J. Hazard. Mater. 303 (2016) 64–75. [11] K.K. Garg, B. Prasad, Development of box Behnken design for treatment of terephthalic acid wastewater by electrocoagulation process: optimization of process and analysis of sludge, J. Environ. Chem. Eng. 4 (2016) 178–190. [12] A. Muñoz-Bonilla, M. Cerrada, M. Fernández-García, A. Kubacka, M. Ferrer, M. Fernández-García, Biodegradable polycaprolactone-titania nanocomposites: preparation, characterization and antimicrobial properties, Int. J. Mol. Sci. 14 (2013) 9249. [13] A. Valdés García, M. Ramos Santonja, A.B. Sanahuja, M.d.C.G. Selva, Characterization and degradation characteristics of poly(ε-caprolactone)-based composites reinforced with almond skin residues, Polym. Degrad. Stab. 108 (2014) 269–279. [14] S. Saravanamoorthy, M. Muneeswaran, N. Giridharan, S. Velmathi, Solvent-free ring opening polymerization of ?-caprolactone and electrical properties of polycaprolactone blended BiFeO3 nanocomposites, RSC Adv. 5 (2015) 43897–43905. [15] J. Song, H. Gao, G. Zhu, X. Cao, X. Shi, Y. Wang, The preparation and characterization of polycaprolactone/graphene oxide biocomposite nanofiber scaffolds and their application for directing cell behaviors, Carbon 95 (2015) 1039–1050. [16] S. Sayyar, E. Murray, B.C. Thompson, S. Gambhir, D.L. Officer, G.G. Wallace, Covalently linked biocompatible graphene/polycaprolactone composites for tissue engineering, Carbon 52 (2013) 296–304. [17] S. Alix, A. Mahieu, C. Terrie, J. Soulestin, E. Gerault, M.G.J. Feuilloley, R. Gattin, V. Edon, T. Ait-Younes, N. Leblanc, Active pseudo-multilayered films from polycaprolactone and starch based matrix for food-packaging applications, Eur. Polym. J. 49 (2013) 1234–1242. [18] A. Hejna, K. Formela, M.R. Saeb, Processing, mechanical and thermal behavior assessments of polycaprolactone/agricultural wastes biocomposites, Ind. Crop. Prod. 76 (2015) 725–733. [19] K. Varaprasad, K. Ramam, G.S. Mohan Reddy, R. Sadiku, Development and characterization of nano-multifunctional materials for advanced applications, RSC Adv. 4 (2014) 60363–60370. [20] J. Fei, J. Zhao, C. Du, A. Wang, H. Zhang, L. Dai, J. Li, One-pot ultrafast self-assembly of autofluorescent polyphenol-based core@shell nanostructures and their selective antibacterial applications, ACS Nano 8 (2014) 8529–8536. [21] K. Varaprasad, G.M. Raghavendra, T. Jayaramudu, J. Seo, Nano zinc oxide–sodium alginate antibacterial cellulose fibres, Carbohydr. Polym. 135 (2016) 349–355. [22] S. Agnihotri, G. Bajaj, S. Mukherji, S. Mukherji, Arginine-assisted immobilization of silver nanoparticles on ZnO nanorods: an enhanced and reusable antibacterial substrate without human cell cytotoxicity, Nanoscale 7 (2015) 7415–7429. [23] P. Rai, S.-H. Jeon, C.-H. Lee, J.-H. Lee, Y.-T. Yu, Functionalization of ZnO nanorods by CuO nanospikes for gas sensor applications, RSC Adv. 4 (2014) 23604–23609. [24] R. Manimaran, K. Palaniradja, N. Alagumurthi, S. Sendhilnathan, J. Hussain, Preparation and characterization of copper oxide nanofluid for heat transfer applications, Appl. Nanosci. 4 (2014) 163–167. [25] A. Oku, L.C. Hu, E. Yamada, Alkali decomposition of poly(ethylene terephthalate) with sodium hydroxide in nonaqueous ethylene glycol: a study on recycling of terephthalic acid and ethylene glycol, J. Appl. Polym. Sci. 63 (1997) 595–601. [26] J. Anwar, M.A. Munawar, W. uz-Zaman, Z. Abbas, J.M. Anzano, Production of terephthalic acid from waste polyethylene terephthalate) materials, J. Polym. Eng. (2008) 129.

93

[27] Z. Chen, J.N. Hay, M.J. Jenkins, FTIR spectroscopic analysis of poly(ethylene terephthalate) on crystallization, Eur. Polym. J. 48 (2012) 1586–1610. [28] A. Ajji, J. Guèvremont, K.C. Cole, M.M. Dumoulin, Orientation and structure of drawn poly(ethylene terephthalate), Polymer 37 (1996) 3707–3714. [29] J.-M. Andanson, S.G. Kazarian, In situ ATR-FTIR spectroscopy of poly(ethylene terephthalate) subjected to high-temperature methanol, Macromol. Symp. 265 (2008) 195–204. [30] S.G. Kazarian, G.G. Martirosyan, ATR-IR spectroscopy of superheated water and in situ study of the hydrothermal decomposition of poly(ethylene terephthalate), Phys. Chem. Chem. Phys. 4 (2002) 3759–3763. [31] S. Arias, J.G. Eon, R.A.S. San Gil, Y.E. Licea, L.A. Palacio, A.C. Faro, Synthesis and characterization of terephthalate-intercalated NiAl layered double hydroxides with high Al content, Dalton Trans. 42 (2013) 2084–2093. [32] C.-S. Wu, Physical properties and biodegradability of maleated-polycaprolactone/ starch composite, Polym. Degrad. Stab. 80 (2003) 127–134. [33] A.K. Zak, R. Razali, W.H.A. Majid, M. Darroudi, Synthesis and characterization of a narrow size distribution of zinc oxide nanoparticles, Int. J. Nanomedicine 6 (2011) 1399–1403. [34] N. Batra, M. Tomar, V. Gupta, ZnO–CuO composite matrix based reagentless biosensor for detection of total cholesterol, Biosens. Bioelectron. 67 (2015) 263–271. [35] F. Fathollahi, M. Javanbakht, H. Omidvar, M. Ghaemi, LiFePO4/C composite cathode via CuO modified graphene nanosheets with enhanced electrochemical performance, J. Alloys Compd. 643 (2015) 40–48. [36] J.K. Sharma, M.S. Akhtar, S. Ameen, P. Srivastava, G. Singh, Green synthesis of CuO nanoparticles with leaf extract of Calotropis gigantea and its dye-sensitized solar cells applications, J. Alloys Compd. 632 (2015) 321–325. [37] A. Zainelabdin, G. Amin, S. Zaman, O. Nur, J. Lu, L. Hultman, M. Willander, CuO/ZnO nanocorals synthesis via hydrothermal technique: growth mechanism and their application as humidity sensor, J. Mater. Chem. 22 (2012) 11583–11590. [38] L. Ji, W. Wang, D. Jin, S. Zhou, X. Song, In vitro bioactivity and mechanical properties of bioactive glass nanoparticles/polycaprolactone composites, Mater. Sci. Eng. C 46 (2015) 1–9. [39] A. Noritake, M. Hori, M. Shigematsu, M. Tanahashi, Recycling of polyethylene terephthalate using high-pressure steam treatment, Polym. J. 40 (2008) 498–502. [40] B. Wu, R.W. Lenz, B. Hazer, Polymerization of methyl methacrylate and its copolymerization with ε-caprolactone catalyzed by isobutylalumoxane catalyst, Macromolecules 32 (1999) 6856–6859. [41] M. Ravi, S. Song, K. Gu, J. Tang, Z. Zhang, Electrical properties of biodegradable poly(ɛ-caprolactone): lithium thiocyanate complexed polymer electrolyte films, Mater. Sci. Eng. B 195 (2015) 74–83. [42] M.B.a.C. Brown, The crystal structure of terephthalic acid, Acta Crystallogr. 22 (1967) 387–391. [43] J.-s. You, S.-t. Noh, Thermal and mechanical properties of poly(glycidyl azide)/ polycaprolactone copolyol-based energetic thermoplastic polyurethanes, Macromol. Res. 18 (2010) 1081–1087. [44] N. Dardmeh, A. Khosrowshahi, H. Almasi, M. Zandi, Study on effect of the polyethylene terephthalate/nanoclay nanocomposite film on the migration of terephthalic acid into the yoghurt drinks simulant, J. Food Process Eng. (2015), http://dx.doi. org/10.1111/jfpe.12324 n/a-n/a. [45] R. Salehiyan, A.A. Yussuf, N.F. Hanani, A. Hassan, A. Akbari, Polylactic acid/ polycaprolactone nanocomposite: influence of montmorillonite and impact modifier on mechanical, thermal, and morphological properties, J. Elastomers Plast. (2013), http://dx.doi.org/10.1177/0095244313489906.