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Structural changes and nano-TiO2 migration of poly(lactic acid)-based food packaging film contacting with ethanol as food simulant
International Journal of Biological Macromolecules 139 (2019) 85–93
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Structural changes and nano-TiO2 migration of poly(lactic acid)-based food packaging film contacting with ethanol as food simulant Chen Yang a, Bifen Zhu b, Jianming Wang a,⁎, Yuyue Qin b,⁎ a b
State Key Laboratory of Food Nutrition and Safety, Tianjin University of Science & Technology, Tianjin 300457, PR China Institute of Agriculture and Food Engineering, Kunming University of Science and Technology, Kunming 650550, PR China
a r t i c l e
i n f o
Article history: Received 2 July 2019 Received in revised form 16 July 2019 Accepted 24 July 2019 Available online 29 July 2019 Keywords: PLA Nano-TiO2 Migration
a b s t r a c t The poly(lactic acid) (PLA)/nano-TiO2 composite films with different nano-TiO2 loading (0 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, and 20 wt%) were prepared and contacted with 50% (v/v) ethanol solution as food simulant to study the behavior of nano-TiO2 migration. The structural changes and intermolecular interactions were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The migration amount increased with the increase of initial nano-TiO2 content. SEM images demonstrated that the microstructure of PLA nanocomposite films became rougher as exposure to ethanol solution in a few days. XRD spectra indicated that a decrease in the intensity of specific diffraction peak occurred as the decrease in nano-TiO2 content of PLA nanocomposite films during exposure to ethanol simulant. DSC analysis confirmed that the higher crystallinity percentage obtained during the different degradation times. The reasonable food packaging application of the nano-TiO2 composite films with restrained nano-TiO2 migration could be accomplished by controlling the nano-TiO2 loading. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Food packaging is intended to protect food products from external influences, such as air, water, microbial growth, and chemical contamination [1]. Biodegradable polymer synthesized from natural renewable resource has recently gained growing attentions. Poly(lactic acid) (PLA) is one of the most promising polymers because of its biodegradability, biocompatibility, relatively low cost, and ready availability [2–5]. In recent years, ‘Nanocomposite food packaging’ with antimicrobial property represents a new generation of active packaging based on metal nanocomposites [6]. These packaging materials can be prepared to exhibit antimicrobial activity by adding different types of inorganic nanoparticles, such as nano-Ag, nano-ZnO, and nano-TiO2 [7–11]. Titanium dioxide (TiO2) is a kind of non-toxic, stable, environmentally-friendly, and relatively inexpensive nanofiller with strong antimicrobial activity against a wide spectrum of microorganisms [12]. It has been approved by the US Food and Drug Administration (FDA) for use in the food industry [13]. TiO2 can generate reactive oxygen species (ROS) and hydroxyl radicals (-OH) on the surface of TiO2 nanoparticles [13]. Polymer/nano-TiO2 composite has been used as an active antimicrobial packaging material in food industry. For example, Gumiero et al. reported that a high density polyethylene incorporated with calcium carbonate and TiO2 could maintain the cheese structure ⁎ Corresponding authors. E-mail addresses: [email protected] (J. Wang), [email protected] (Y. Qin).
https://doi.org/10.1016/j.ijbiomac.2019.07.151 0141-8130/© 2019 Elsevier B.V. All rights reserved.
than a commercial rigid container because of the inhibition of coliforms and lactic acid bacteria [14]. Fang et al. indicated that polyethylene packaging materials containing nano-Ag, nano-TiO2, and nano-SiO2 could prolong postharvest life of mushrooms (Flammulina velutipes) [15]. After 14 days of storage period, cap opening, weight loss, stipe elongation, and respiration of mushrooms were effectively inhibited. Like other inorganic nanoparticles, TiO2 would migrate from packaging materials to the packed food products [9,13,14]. The antimicrobial activity of nanocomposite packaging materials could be exerted by direct contact rather than continuous release of the active component to food stuff [16]. For antimicrobial purpose, a small amount of TiO2 must be presented on the surface of packaging materials to inhibit the growth of microorganisms [17]. However, TiO2 migration amount should not exceed the migration limit of 10 mg/kg as defined by EFSA for food contact materials [9]. The migration of nanoparticles would lead to changes in water barrier property, mechanical property, thermal property, and antimicrobial activity of nanocomposite [18,19]. This would affect the protective function of food packaging materials. So, the migration of TiO2 would cause relevant structural change in the nanocomposite and subsequent reduction of food packaging performance [20]. The aim of this study was to analyze the relationship between the structural changes and migration of TiO2 during contacting with ethanol solution as alcoholic food simulant. The structural changes and intermolecular interactions were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC).
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2. Material and methods 2.1. Materials PLA (Mw = 280 kDa, Mw/Mn = 1.98) used in this work was obtained from Natureworks LLC (Nebraska, USA). Nano-TiO2 powder with an average particle size b100 nm and purity of 99.5% was obtained from Sigma (St. Louis, MO, USA) and used as received. Dichloromethane was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). 2.2. PLA-based film preparation
Fig. 1. The migration amount of nano-TiO2 from the different PLA nanocomposite films stored for 45 days.
A series of PLA/nano-TiO2 composite films were prepared by a solvent casting method through varying the nano-TiO2 content from 0 wt % to 20 wt% [21]. PLA film was prepared as control. The film forming solution was prepared by dissolving 2 g of PLA in 50 mL of dichloromethane by magnetic stirring overnight. Then, nano-TiO2 was dispersed in the film forming solution with the help of high-speed homogenization (14,000 r/min) for 5 min. The solution was ultrasonic dispersed for 30 min to get the final nano-TiO2 loading (0 wt%, 1 wt%, 5 wt%, 10 wt %, 15 wt%, and 20 wt%). After the solution was treated by ultrasound, it was cast onto a 20 cm × 20 cm glass dishes and dried at room temperature. After drying, the PLA/nano-TiO2 composite films were peeled and kept in a desiccator at 23 ± 2 °C and relative humidity of 50 ± 5% for at
Fig. 2. The SEM image of (a) PLA, (b) PLA/T5, (c) PLA/T10, and (d) PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively.
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Fig. 2 (continued).
least 48 h prior to characterization. The PLA/nano-TiO2 composite films with 0 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, and 20 wt% nano-TiO2 loading were named as PLA, PLA/T1, PLA/T5, PLA/T10, PLA/T15, and PLA/T20 film, respectively.
voltage of 5.00 kV and the magnification was 10,000×. Before observation, the fractured surface of samples was sputter-coated with a thin conductive gold layer.
2.3. Migration test
2.5. X-ray diffraction (XRD)
The nano-TiO2 migration test was performed at 40 °C. According to the recommendation from European Union and FDA, 50% (v/v) ethanol was used as alcoholic food simulant. The samples were cut into squares (40 × 40 mm) and weighed. Then, the samples were immersed in 30 mL of alcoholic food simulant and incubated at 40 °C until the time of equilibration. Subsequently, the simulant in contact with samples was taken out from the bottle at regular time interval in order to investigate the migration of nano-TiO2 as time proceeding. The films were taken out on day 0, 5, 10, 15, 25, 30, and 45. The amount of nano-TiO2 in the alcoholic food simulant was determined by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES, Optima 8000, Perkin Elmer, USA). The migration test was performed in triplicate and the result was expressed as mg/kg.
XRD spectra were carried out in a computer-controlled diffractometer (D8 Advance, Brucker, Germany) with Cu Kα radiation generated at 40 mA and 40 kV. The scanning speed was 2°/min and the samples were examined in the 2θ range from 10° to 40°.
2.4. Scanning electron microscopy (SEM) The SEM measurement was operated by a field emission scanning electron microscope (Hitachi S-4800, Tokyo, Japan) with an accelerating
2.6. Differential scanning calorimetry (DSC) DSC experiment was conducted by using a Netzsch DSC 214 instrument (Berlin, Germany) under inert nitrogen stream. An empty aluminum pan was used as the reference. 10 mg of sample was introduced in an aluminum pan and exposed to the following thermal cycle: heating from 20 °C to 200 °C at 10 °C/min (5 min hold), followed by quenching to 20 °C (5 min hold), and further heating to 200 °C at 10 °C/min. The first heating scan was used to eliminate any prior thermal history of the sample. The cold crystallization temperature (T c), glass transition temperature (T g ), and melting point (Tm) were recorded.
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Fig. 2 (continued).
2.7. Statistical analysis The statistical analysis of the data was analyzed through analysis of variance (ANOVA) using SPSS 13.0 software. Duncan's multiple range tests were used to determine significant difference among mean values. 0.05 was the significant limit.
3. Results and discussion 3.1. Migration test The amount of nano-TiO2 migration was determined by the quantity of nano-TiO2 found in the food simulant versus the initial quantity of nano-TiO2 for the PLA/nano-TiO2 composite films. Iñiguez-Franco et al. investigated the hydrolytic degradation of PLA nanocomposites in pure water, 50% and 95% ethanol solution at 40 °C. The hydrolytic degradation of PLA nanocomposite films was the fastest in 50% ethanol solution [22]. So, 50% (v/v) ethanol was chosen as the alcoholic food simulant to evaluate the release behavior of nano-TiO2. The migration amount of nano-TiO2 from the different PLA nanocomposite films was shown in Fig. 1. After the PLA/nano-TiO2 composite films were exposed to 50% (v/v) ethanol simulant, the amount of nano-TiO2 in simulant increased over
contact time indicating the release of nanoparticles from the nanocomposites. The migration amount depended on migration time, food simulant, and nano-TiO2 content in PLA nanocomposite films. The release behavior displayed a sharp increase in the first few days and a steady state from 20 to 45 days (Fig. 1). Nano-TiO2 in the outer layer of film surface was easily accessible by food simulant. This allowed a rapid release of nanoparticles from films. However, the rest of nanoparticles were inside the films and food simulant was not easily to penetrate into the polymer matrix. The nanoparticles slowly migrated from the films or remained in the films even after a long period of contacting with food simulant [23]. The migration amount increased as the initial quantity of nano-TiO2 for the PLA/nano-TiO2 composite films rose. In all tested films, the highest migration amount was 0.54 ± 0.04 mg/kg for PLA/T20 film on day 45, and it was still lower than the limit of nano-TiO2 quantity. Migration was a result of mass transfer of nanoparticles from polymer matrix to food simulant [24]. This indicated that nanoparticles might be released via dissolution from the surface and cut edge of the PLA/ nano-TiO2 composite films (solid phase). Lin et al. also reported that the migration amount of Ti from nano-TiO2-polyethylene composite packaging films gradually increased until it reached equilibrium [25]. The migration of nanoparticles was a competitive process that depended on the compatibility of nanoparticles to the liquid phase (food simulant) and the solid phase (nanocomposite film) when the
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Fig. 2 (continued).
film contacted with the liquid phase. The steady state from 20 to 45 days (0.31 ± 0.02 mg/kg for PLA/T1 film and 0.54 ± 0.04 mg/kg for PLA/T20 film) could be because of the process of solvent induced crystallization. In this stage, the crystallization rate was slow and the release of nanoTiO2 would be restricted by the secondary crystallization process. 3.2. Microstructure Changes in microstructure of the nanocomposite films were studied by SEM technique. The fractured surfaces of PLA, PLA/T5, PLA/T10, and PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, were shown in Fig. 2. As could be seen from Fig. 3a, the neat PLA showed smooth and flat surface before contacting with ethanol simulant. As time proceeding, the fractured surface became rougher. Before contacting with ethanol simulant, the addition of nano-TiO2 made the fractured surface rougher than the neat PLA film. A few of small particles could be seen in Fig. 2b and c. However, a lot of particles existed in Fig. 3d. After 5 days of exposure of PLA/T5 nanocomposite film to 50% (v/v) ethanol simulant, the fractured surface became rougher (Fig. 2b). No significant change was observed on day 30. Only a very few of small particles were observed on the end of contacting with ethanol solution. In the case of the PLA/T10 and PLA/T20 nanocomposite film, nano-TiO2 agglomeration gradually reduced as time proceeding. This might be
because of nanoparticles migration from the PLA composite films. The fractured surface of films became rougher. As the porosity of films increased, the absorption of water or ethanol within the PLA matrix increased [26]. The water or ethanol entrapment was directly proportional to porosity, which might provide a high tendency for migration. Some voids and islands were obvious on the 30th day (Fig. 3c and d). From the SEM images of fractured surface, it could be observed that developed crystal morphology was different among the PLA, PLA/ T5, PLA/T10, and PLA/T20 nanocomposite films. 3.3. Crystalline structural changes The XRD spectra is one of the fundamental tools for micro analytical characterization available for thin films owing to its quantitative, simplicity, more reliability, and nondestructive nature [27]. The XRD curve of PLA, PLA/T5, PLA/T10, and PLA/T20 nanocomposite films contacting with ethanol as food simulant at 40 °C was shown in Fig. 3. There was no specific diffraction peak observed in the XRD of PLA without nano-TiO2. This revealed the amorphous or semicrystalline nature of PLA. The XRD curve for PLA/nano-TiO2 composites indicated that the incorporation of nano-TiO2 to PLA matrix would lead to the appearance of a diffraction peak at 25.4°. As could be seen from Fig. 3a–d, the diffraction peak at 25.4° became sharper as the nano-TiO2 loading increased from 0 wt% to 20 wt%. The nano-TiO2 showed peaks at 25.4°,
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38.1°, 48.1°, 54.0°, and 55.2°, which correspond to the anatase phase of TiO2 nanoparticles [26]. The XRD curve of pure PLA and PLA/nano-TiO2 composites (Fig. 3a– d) displayed that polymer matrix kept an amorphous structure before hydrolytic degradation [28]. The character peak of nano-TiO2 (2θ = 25.4°) decreased as time proceeding (Fig. 1b–d). For the PLA/T5 film, the character peak of nano-TiO2 almost disappeared on day 30. However, the character peak of nano-TiO2 for the PLA/T10 film remained half of the intensity and that for the PLA/T20 film was the highest among the samples on the 30th day. When the PLA/nano-TiO2 composite films contacted with 50% (v/v) ethanol food simulant, the migration of nano-TiO2 for all samples increased with the migration time. The migration of nanoparticles would directly result in enhanced interactions among polymer molecules chains. The decrease in nano-TiO2 content of nanocomposite films would lead to the decrease in the intensity of specific diffraction peak. The migration content for the PLA/T20 film was the lowest. The trend in XRD patterns was consistent with that in migration test. When the PLA film was exposed to the ethanol solution for 5 days (Fig. 3a), two strong diffraction peaks emerged at 2θ = 16.9° and 2θ = 19.2° corresponding to the 100/203 and 110/200 plane reflections, respectively. This confirmed that crystalline structure was formed. Similarly, Fig. 3b–d showed that the formation of α-crystal of PLA/nanoTiO2 composites took place on day 5. During the solvent induced crystallization and hydrolytic degradation of amorphous region of PLA polymer matrix, the same form of crystals was present in PLA/nano-TiO2 composites. When the polymer chain in amorphous region degraded, the number of amorphous region would decrease and result in the ratio of crystalline to amorphous region increasing [28]. However, the intensity of α-crystal diffraction peaks for the PLA/T20 film was the
weakest. The presence of nano-TiO2 increased the sorption of ethanol simulant and acted as an anchor to restrict the movements of the polymer chains [22]. 3.4. DSC analysis for molecular order Changes in crystallization degree during PLA/nano-TiO2 composite films contacting with ethanol as food simulant were very important, especially if considering the influence of solvent induced degradation of amorphous phase over the crystalline portions [29]. The DSC curve obtained from the second run is performed to analyze the thermal transition of the PLA/nano-TiO2 composites (Fig. 4). DSC data in Table 1 listed Tg, Tc, Tm, and Xc of the different thermal events. The crystallinity percentage (Xc) of the PLA phase in the PLA/nano-TiO2 composites was calculated by formula (1): Xcð%Þ ¼ ΔHm=ΔH0m w 100
ð1Þ
where ΔHm is the melting enthalpy (J/g), ΔH0m is the theoretical enthalpy for 100% crystalline PLA (93.7 J/g), and w is the weight fraction of PLA in the nanocomposites [30]. There was no significant difference in Tg and Tm value with the incorporation of nano-TiO2 to PLA matrix. This was in consistent with previous studies [31]. The Xc value for the PLA, PLA/T5, PLA/T10, and PLA/T20 nanocomposites was 14.2%, 15.8%, 18.2%, and 17.4%, respectively. The Xc value gradually increased and then slightly decreased. This result indicated that incorporation of nano-TiO2 into PLA matrix led to some improvements in the crystallization degree in PLA polymer. However, excess amount of nano-TiO2 (20 wt% loading) probably caused the
Fig. 3. The XRD curve of (a) PLA, (b) PLA/T5, (c) PLA/T10, and (d) PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively.
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Fig. 4. The DSC curve of (a) PLA, (b) PLA/T5, (c) PLA/T10, and (d) PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively.
aggregation of nanoparticles in the polymer matrix and led to the decrease in the crystallization degree. As the PLA/nano-TiO2 composite films contacted with 50% (v/v) ethanol simulant, Tg value for the PLA, PLA/T5, and PLA/T10 films gradually increased from day 0 to day 30. However, Tg value for the PLA/T20 film significantly (P b 0.05) increased on day 5 and then slightly decreased on day 15. After the nanocomposites immersed into food simulant, the increase in the Tg value indicated that the amorphous phase gradually degraded from the very beginning and the presence of more polymeric chains involved in the crystallization process. The nucleating ability of nanoparticles and the hydrolytic chain cleavage preferentially proceeded in the amorphous region [25]. The solvent induced degradation could act as crosslinking and hinder the motion of the polymer chains [27]. The migration test was performed at 40 °C. The degradation temperature was below the Tg value of PLA and PLA nanocomposites. The degradation process slowly occurred because of the inactivity of the polymer molecules changing from glass-like state to rubber-like state. Moreover, the degradation of PLA polymer was restricted to its surface if the degradation temperature remained below the Tg value [28]. Tc value for the PLA film increased from day 0 to day 15. Tc value for the PLA/nano-TiO2 composite films initially increased on day 5 and then decreased from the 15th day. However, there was no significant (P N 0.05) difference in Tm value for all of the samples as the time proceeding. Degradation in ethanol solution led to a formation of double melting specific peaks in PLA film on day 30 and PLA/T5 film after degradation of 15 days and longer. PLA/T10 and PLA/T20 samples emerged double melting peaks from day 5. The presence of double melting peaks in PLA is well known and attributed to the melting crystalline phase formed during the second heating run [32]. Furthermore, the degraded
macromolecules were shorter than the original undegraded PLA of amorphous phase which might crystallize faster than the neat PLA [29]. As could be seen from Table 1, the crystallinity degree of the films contacting with ethanol simulant gradually increased from day 0 to day 30. The increase in Xc value showed that ethanol and water acted on amorphous regions where hydrolytic degradation was prevalent. Other researchers also reported that the higher Xc value obtained during the different degradation times by DSC analysis [29–34]. As for the PLA/nano-TiO2 composites, when the nano-TiO2 was homogeneously distributed into the polymer matrix by proper nanoparticle loading, the water molecule would more easily penetrated within the nanocomposites to trigger the degradation process. The presence of nanoparticles also increased the sorption of ethanol, which was might be absorbed in the gaps between the conglomeration of TiO2 due to the agglomeration of nano-TiO2. The absorption of water and ethanol would make them spend more time to diffuse into the polymer matrix [28]. Along with the 50% (v/v) ethanol solution diffused into the amorphous phase which was less organized, the crystallinity degree of PLA changed. The combined effects of nanoparticles migration, time, morphology (preferential degradation of amorphous phase compared to crystalline phase), and the interactions between polymeric matrix and nanoparticles would control the degradation of PLA, including the observed effects on melting peaks and crystallinity degree of PLA/nano-TiO2 composites. LA oligomers and shorter PLA chains were not easily soluble in the 50% (v/v) ethanol simulant even though they could absorb much water [28]. Only lactic acid itself as well as dimmer or trimmer of lactic acid was soluble in the ethanol solution. The results revealed that although the degradation process started for the PLA/nano-TiO2 composite films from the very beginning, a long time was still need to have a
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Table 1 The thermal performance parameters of the PLA/nano-TiO2 composite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively. Sample
Time
PLA
Day 0 Day 5 Day 15 Day 30 Day 0 Day 5 Day 15 Day 30 Day 0 Day 5 Day 15 Day 30 Day 0 Day 5 Day 15 Day 30
PLA/T5
PLA/T10
PLA/T20
Tg (°C)
Tc (°C)
Tm (°C)
Xc (%)
48.5 ± 1.3a 55.3 ± 3.7b 57.8 ± 5.8b 58.9 ± 4.5b 47.2 ± 2.7a 56.6 ± 0.9b 57.4 ± 4.8b 58.5 ± 2.9b 46.5 ± 2.0a 60.2 ± 0.9b 61.5 ± 0.8b 61.8 ± 1.6b 50.3 ± 3.3a 62.7 ± 2.9b 61.9 ± 0.7b 62.5 ± 2.4b
115.2 ± 1.5a 120.0 ± 4.9b 124.3 ± 4.7b 121.5 ± 1.7b 108.4 ± 1.8a 126.2 ± 5.3b 122.5 ± 4.3b 121.0 ± 7.4b 105.6 ± 0.2a 124.2 ± 6.1bc 115.0 ± 1.1ab 119.3 ± 3.9b 112.2 ± 3.9a 127.7 ± 6.2b 119.1 ± 2.7ab 121.0 ± 2.0b
170.3 ± 3.3a 170.1 ± 1.1a 170.0 ± 8.5a 169.4 ± 3.4a 170.0 ± 5.3a 170.5 ± 3.1a 169.1 ± 8.1a 171.2 ± 0.9a 170.4 ± 4.2a 171.2 ± 2.8a 171.5 ± 4.4a 170.9 ± 3.6a 170.3 ± 3.0a 169.4 ± 3.1a 170.1 ± 3.4a 170.5 ± 8.2a
14.2 ± 0.1a 15.5 ± 0.4b 17.3 ± 0.3c 18.5 ± 0.3d 15.8 ± 0.5a 16.3 ± 0.4a 18.2 ± 0.2b 19.8 ± 0.1c 18.2 ± 0.4a 19.7 ± 0.4b 21.5 ± 0.1c 22.7 ± 0.6d 17.4 ± 0.4a 20.5 ± 0.7b 21.3 ± 0.5b 23.6 ± 0.2c
a–d Values followed by different letters in the same column for the same sample were significantly (P b 0.05) different, where a was the lowest value.
complete degradation during contacting with 50% (v/v) ethanol solution as food simulant. 4. Conclusion After the PLA/nano-TiO2 composites were immersed into ethanol solution, the film structural features gradually changed and nano-TiO2 would migrate from polymer matrix to food simulant. The observed effects on microstructure, crystalline structure, crystallinity degree, and molecular order of PLA/nano-TiO2 composites were verified by SEM graphs, XRD spectra, and DSC analysis. In the stage of solvent induced crystallization, the crystallization rate was slow and the release of nano-TiO2 would be restricted by the secondary crystallization process. The increase in crystallinity degree of the films during contacting with ethanol simulant showed that ethanol and water acted on amorphous regions where hydrolytic degradation was prevalent. As time proceeding, the PLA nanocomposite films became rougher. This provided more available space for contacting with ethanol food simulant, which might induce a higher tendency for nanoparticle migration. Although the study on migration test and structural changes in this paper was not exhaustive, the result provided important information about the relationship between the structural changes and migration of nanoparticles during contacting with ethanol solution as food simulant. Further work was still need to do, especially on the migration of nanoparticles into water, acidic food simulant, fatty food simulant, and real food products. Acknowledgement The study was financially supported by the Open Project Program of State Key Laboratory of Food Nutrition and Safety, Tianjin University of Science & Technology, China (No. SKLFNS-KF-201815) and Tianjin Natural Science Foundation, China (No. 18JCYBJC43700). References [1] M. Peltzer, J. Wagner, A. Jimenez, Migration study of carvacrol as a natural antioxidant in high-density polyethylene for active packaging, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 26 (6) (2009) 938–946. [2] S.W. Hwang, J.K. Shim, S. Selke, H. Soto-Valdez, L. Matuana, M. Rubino, R. Auras, Migration of α-tocopherol and resveratrol from poly(L-lactic acid)/starch blends films into ethanol, J. Food Eng. 116 (4) (2013) 814–828. [3] I. Spiridon, C.E. Tanase, Design, characterization and preliminary biological evaluation of new lignin-PLA biocomposites, Int. J. Biol. Macromol. 114 (2018) 855–863.
[4] Y. Byun, K. Rodriguez, J.H. Han, Y.T. Kim, Improved thermal stability of polylactic acid (PLA) composite film via PLA-beta-cyclodextrin-inclusion complex systems, Int. J. Biol. Macromol. 81 (2015) 591–598. [5] A.L.P.d.L. Freitas, L.R. Tonini Filho, P.S. Calvão, A.M.C.d. Souza, Effect of montmorillonite and chain extender on rheological, morphological and biodegradation behavior of PLA/PBAT blends, Polym. Test. 62 (2017) 189–195. [6] B. Panea, G. Ripoll, J. González, Á. Fernández-Cuello, P. Albertí, Effect of nanocomposite packaging containing different proportions of ZnO and Ag on chicken breast meat quality, J. Food Eng. 123 (2014) 104–112. [7] Y. Huang, S. Chen, X. Bing, C. Gao, T. Wang, B. Yuan, Nanosilver migrated into foodsimulating solutions from commercially available food fresh containers, Packag. Technol. Sci. 24 (5) (2011) 291–297. [8] Y. Yang, H. Zhang, P. Wang, Q. Zheng, J. Li, The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci. 288 (1–2) (2007) 231–238. [9] H. Song, B. Li, L. Q-B, W. H-J, Y. Chen, Migration of silver from nanosilverpolyethylene composite packaging into food simulants, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 28 (12) (2011) 1758–1762. [10] A. Youssef, Polymer nanocomposites as a new trend for packaging applications, J. Macromol. Sci. D Rev. Polym. Process. 52 (7) (2013) 635–660. [11] Y. Zhou, L. Lei, B. Yang, J. Li, J. Ren, Preparation and characterization of polylactic acid (PLA) carbon nanotube nanocomposites, Polym. Test. 68 (2018) 34–38. [12] A.M. Díez-Pascual, A.L. Díez-Vicente, Effect of TiO2nanoparticles on the performance of polyphenylsulfone biomaterial for orthopaedic implants, J. Mater. Chem. B 2 (43) (2014) 7502–7514. [13] H. Bodaghi, Y. Mostofi, A. Oromiehie, Z. Zamani, B. Ghanbarzadeh, C. Costa, A. Conte, M.A. Del Nobile, Evaluation of the photocatalytic antimicrobial effects of a TiO2 nanocomposite food packaging film by in vitro and in vivo tests, LWT Food Sci. Technol. 50 (2) (2013) 702–706. [14] M. Gumiero, D. Peressini, A. Pizzariello, A. Sensidoni, L. Iacumin, G. Comi, R. Toniolo, Effect of TiO2 photocatalytic activity in a HDPE-based food packaging on the structural and microbiological stability of a short-ripened cheese, Food Chem. 138 (2–3) (2013) 1633–1640. [15] F. Donglu, Y. Wenjian, B.M. Kimatu, A.M. Mariga, Z. Liyan, A. Xinxin, H. Qiuhui, Effect of nanocomposite-based packaging on storage stability of mushrooms (Flammulina velutipes), Innovative Food Sci. Emerg. Technol. 33 (2016) 489–497. [16] C. Lantano, I. Alfieri, A. Cavazza, C. Corradini, A. Lorenzi, N. Zucchetto, A. Montenero, Natamycin based sol-gel antimicrobial coatings on polylactic acid films for food packaging, Food Chem. 165 (2014) 342–347. [17] N. von Goetz, L. Fabricius, R. Glaus, V. Weitbrecht, D. Gunther, K. Hungerbuhler, Migration of silver from commercial plastic food containers and implications for consumer exposure assessment, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 30 (3) (2013) 612–620. [18] Y. Tang, M. Lewin, New aspects of migration and flame retardancy in polymer nanocomposites, Polym. Degrad. Stab. 93 (11) (2008) 1986–1995. [19] S.R. Andersson, M. Hakkarainen, A.-C. Albertsson, Long-term properties and migration of low molecular mass compounds from modified PLLA materials during accelerated ageing, Polym. Degrad. Stab. 97 (6) (2012) 914–920. [20] J. Zhu, X. Li, C. Huang, L. Chen, L. Li, Structural changes and triacetin migration of starch acetate film contacting with distilled water as food simulant, Carbohydr. Polym. 104 (2014) 1–7. [21] J. Yang, Y. Qin, M. Yuan, J. Xue, J. Cao, Y. Wu, M. Yuan, Preparation and characterization of poly(L-lactide)-co-poly(trimethylene carbonate)/talc film, Int. J. Biol. Macromol. 62 (2013) 411–417. [22] H. Chi, W. Li, C. Fan, C. Zhang, L. Li, Y. Qin, M. Yuan, Effect of high pressure treatment on poly(lactic acid)/nano(-)TiO(2) composite films, Molecules 23 (10) (2018). [23] F. Iñiguez-Franco, R. Auras, M. Rubino, K. Dolan, H. Soto-Valdez, S. Selke, Effect of nanoparticles on the hydrolytic degradation of PLA-nanocomposites by waterethanol solutions, Polym. Degrad. Stab. 146 (2017) 287–297. [24] F. Liu, C.Y. Hu, Q. Zhao, Y.J. Shi, H.N. Zhong, Migration of copper from nanocopper/ LDPE composite films, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 33 (11) (2016) 1741–1749. [25] A. Ozaki, E. Kishi, T. Ooshima, A. Hase, Y. Kawamura, Contents of Ag and other metals in food-contact plastics with nanosilver or Ag ion and their migration into food simulants, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 33 (9) (2016) 1490–1498. [26] Q.-B. Lin, H. Li, H.-N. Zhong, Q. Zhao, D.-H. Xiao, Z.-W. Wang, Migration of Ti from nano-TiO2-polyethylene composite packaging into food simulants, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 31 (7) (2014) 1284–1290. [27] S. Mallick, Z. Ahmad, F. Touati, J. Bhadra, R.A. Shakoor, N.J. Al-Thani, PLA-TiO2 nanocomposites: thermal, morphological, structural, and humidity sensing properties, Ceram. Int. 44 (14) (2018) 16507–16513. [28] A.J. Pai, B.K. Sarojini, K.R. Harshitha, B. Shivarama Holla, A.G. Lobo, Spectral, morphological and optical studies on bischalcone doped polylactic acid (PLA) thin films as luminescent and UV radiation blocking materials, Opt. Mater. 90 (2019) 145–151. [29] Y.-B. Luo, X.-L. Wang, Y.-Z. Wang, Effect of TiO2 nanoparticles on the longterm hydrolytic degradation behavior of PLA, Polym. Degrad. Stab. 97 (5) (2012) 721–728. [30] M. Sednickova, S. Pekarova, P. Kucharczyk, J. Bockaj, I. Janigova, A. Kleinova, D. Jochec-Moskova, L. Omanikova, D. Perdochova, M. Koutny, V. Sedlarik, P. Alexy, I. Chodak, Changes of physical properties of PLA-based blends during early stage of biodegradation in compost, Int. J. Biol. Macromol. 113 (2018) 434–442. [31] C. Fan, H. Chi, C. Zhang, R. Cui, W. Lu, M. Yuan, Y. Qin, Effect of multiscale structure on the gas barrier properties of poly(lactic acid)/Ag nanocomposite films, Polym. Adv. Technol. 30 (2019) 1709–1715.
C. Yang et al. / International Journal of Biological Macromolecules 139 (2019) 85–93 [32] C. Fan, R. Cui, W. Lu, H. Chen, M. Yuan, Y. Qin, Effect of high pressure treatment on properties and nano–Ag migration of PLA-based food packaging film, Polym. Test. 76 (2019) 73–81. [33] A.A. Cuadri, J.E. Martín-Alfonso, Thermal, thermo-oxidative and thermomechanical degradation of PLA: a comparative study based on rheological, chemical and thermal properties, Polym. Degrad. Stab. 150 (2018) 37–45.
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[34] J.Y. Wu, C.W. Li, C.H. Tsai, C.W. Chou, D.R. Chen, G.J. Wang, Synthesis of antibacterial TiO2/PLGA composite biofilms, Nanomedicine 10 (5) (2014) e1097–e1107.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Structural changes and nano-TiO2 migration of poly(lactic acid)-based food packaging film contacting with ethanol as food simulant Chen Yang a, Bifen Zhu b, Jianming Wang a,⁎, Yuyue Qin b,⁎ a b
State Key Laboratory of Food Nutrition and Safety, Tianjin University of Science & Technology, Tianjin 300457, PR China Institute of Agriculture and Food Engineering, Kunming University of Science and Technology, Kunming 650550, PR China
a r t i c l e
i n f o
Article history: Received 2 July 2019 Received in revised form 16 July 2019 Accepted 24 July 2019 Available online 29 July 2019 Keywords: PLA Nano-TiO2 Migration
a b s t r a c t The poly(lactic acid) (PLA)/nano-TiO2 composite films with different nano-TiO2 loading (0 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, and 20 wt%) were prepared and contacted with 50% (v/v) ethanol solution as food simulant to study the behavior of nano-TiO2 migration. The structural changes and intermolecular interactions were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The migration amount increased with the increase of initial nano-TiO2 content. SEM images demonstrated that the microstructure of PLA nanocomposite films became rougher as exposure to ethanol solution in a few days. XRD spectra indicated that a decrease in the intensity of specific diffraction peak occurred as the decrease in nano-TiO2 content of PLA nanocomposite films during exposure to ethanol simulant. DSC analysis confirmed that the higher crystallinity percentage obtained during the different degradation times. The reasonable food packaging application of the nano-TiO2 composite films with restrained nano-TiO2 migration could be accomplished by controlling the nano-TiO2 loading. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Food packaging is intended to protect food products from external influences, such as air, water, microbial growth, and chemical contamination [1]. Biodegradable polymer synthesized from natural renewable resource has recently gained growing attentions. Poly(lactic acid) (PLA) is one of the most promising polymers because of its biodegradability, biocompatibility, relatively low cost, and ready availability [2–5]. In recent years, ‘Nanocomposite food packaging’ with antimicrobial property represents a new generation of active packaging based on metal nanocomposites [6]. These packaging materials can be prepared to exhibit antimicrobial activity by adding different types of inorganic nanoparticles, such as nano-Ag, nano-ZnO, and nano-TiO2 [7–11]. Titanium dioxide (TiO2) is a kind of non-toxic, stable, environmentally-friendly, and relatively inexpensive nanofiller with strong antimicrobial activity against a wide spectrum of microorganisms [12]. It has been approved by the US Food and Drug Administration (FDA) for use in the food industry [13]. TiO2 can generate reactive oxygen species (ROS) and hydroxyl radicals (-OH) on the surface of TiO2 nanoparticles [13]. Polymer/nano-TiO2 composite has been used as an active antimicrobial packaging material in food industry. For example, Gumiero et al. reported that a high density polyethylene incorporated with calcium carbonate and TiO2 could maintain the cheese structure ⁎ Corresponding authors. E-mail addresses: [email protected] (J. Wang), [email protected] (Y. Qin).
https://doi.org/10.1016/j.ijbiomac.2019.07.151 0141-8130/© 2019 Elsevier B.V. All rights reserved.
than a commercial rigid container because of the inhibition of coliforms and lactic acid bacteria [14]. Fang et al. indicated that polyethylene packaging materials containing nano-Ag, nano-TiO2, and nano-SiO2 could prolong postharvest life of mushrooms (Flammulina velutipes) [15]. After 14 days of storage period, cap opening, weight loss, stipe elongation, and respiration of mushrooms were effectively inhibited. Like other inorganic nanoparticles, TiO2 would migrate from packaging materials to the packed food products [9,13,14]. The antimicrobial activity of nanocomposite packaging materials could be exerted by direct contact rather than continuous release of the active component to food stuff [16]. For antimicrobial purpose, a small amount of TiO2 must be presented on the surface of packaging materials to inhibit the growth of microorganisms [17]. However, TiO2 migration amount should not exceed the migration limit of 10 mg/kg as defined by EFSA for food contact materials [9]. The migration of nanoparticles would lead to changes in water barrier property, mechanical property, thermal property, and antimicrobial activity of nanocomposite [18,19]. This would affect the protective function of food packaging materials. So, the migration of TiO2 would cause relevant structural change in the nanocomposite and subsequent reduction of food packaging performance [20]. The aim of this study was to analyze the relationship between the structural changes and migration of TiO2 during contacting with ethanol solution as alcoholic food simulant. The structural changes and intermolecular interactions were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC).
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2. Material and methods 2.1. Materials PLA (Mw = 280 kDa, Mw/Mn = 1.98) used in this work was obtained from Natureworks LLC (Nebraska, USA). Nano-TiO2 powder with an average particle size b100 nm and purity of 99.5% was obtained from Sigma (St. Louis, MO, USA) and used as received. Dichloromethane was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). 2.2. PLA-based film preparation
Fig. 1. The migration amount of nano-TiO2 from the different PLA nanocomposite films stored for 45 days.
A series of PLA/nano-TiO2 composite films were prepared by a solvent casting method through varying the nano-TiO2 content from 0 wt % to 20 wt% [21]. PLA film was prepared as control. The film forming solution was prepared by dissolving 2 g of PLA in 50 mL of dichloromethane by magnetic stirring overnight. Then, nano-TiO2 was dispersed in the film forming solution with the help of high-speed homogenization (14,000 r/min) for 5 min. The solution was ultrasonic dispersed for 30 min to get the final nano-TiO2 loading (0 wt%, 1 wt%, 5 wt%, 10 wt %, 15 wt%, and 20 wt%). After the solution was treated by ultrasound, it was cast onto a 20 cm × 20 cm glass dishes and dried at room temperature. After drying, the PLA/nano-TiO2 composite films were peeled and kept in a desiccator at 23 ± 2 °C and relative humidity of 50 ± 5% for at
Fig. 2. The SEM image of (a) PLA, (b) PLA/T5, (c) PLA/T10, and (d) PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively.
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Fig. 2 (continued).
least 48 h prior to characterization. The PLA/nano-TiO2 composite films with 0 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, and 20 wt% nano-TiO2 loading were named as PLA, PLA/T1, PLA/T5, PLA/T10, PLA/T15, and PLA/T20 film, respectively.
voltage of 5.00 kV and the magnification was 10,000×. Before observation, the fractured surface of samples was sputter-coated with a thin conductive gold layer.
2.3. Migration test
2.5. X-ray diffraction (XRD)
The nano-TiO2 migration test was performed at 40 °C. According to the recommendation from European Union and FDA, 50% (v/v) ethanol was used as alcoholic food simulant. The samples were cut into squares (40 × 40 mm) and weighed. Then, the samples were immersed in 30 mL of alcoholic food simulant and incubated at 40 °C until the time of equilibration. Subsequently, the simulant in contact with samples was taken out from the bottle at regular time interval in order to investigate the migration of nano-TiO2 as time proceeding. The films were taken out on day 0, 5, 10, 15, 25, 30, and 45. The amount of nano-TiO2 in the alcoholic food simulant was determined by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES, Optima 8000, Perkin Elmer, USA). The migration test was performed in triplicate and the result was expressed as mg/kg.
XRD spectra were carried out in a computer-controlled diffractometer (D8 Advance, Brucker, Germany) with Cu Kα radiation generated at 40 mA and 40 kV. The scanning speed was 2°/min and the samples were examined in the 2θ range from 10° to 40°.
2.4. Scanning electron microscopy (SEM) The SEM measurement was operated by a field emission scanning electron microscope (Hitachi S-4800, Tokyo, Japan) with an accelerating
2.6. Differential scanning calorimetry (DSC) DSC experiment was conducted by using a Netzsch DSC 214 instrument (Berlin, Germany) under inert nitrogen stream. An empty aluminum pan was used as the reference. 10 mg of sample was introduced in an aluminum pan and exposed to the following thermal cycle: heating from 20 °C to 200 °C at 10 °C/min (5 min hold), followed by quenching to 20 °C (5 min hold), and further heating to 200 °C at 10 °C/min. The first heating scan was used to eliminate any prior thermal history of the sample. The cold crystallization temperature (T c), glass transition temperature (T g ), and melting point (Tm) were recorded.
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Fig. 2 (continued).
2.7. Statistical analysis The statistical analysis of the data was analyzed through analysis of variance (ANOVA) using SPSS 13.0 software. Duncan's multiple range tests were used to determine significant difference among mean values. 0.05 was the significant limit.
3. Results and discussion 3.1. Migration test The amount of nano-TiO2 migration was determined by the quantity of nano-TiO2 found in the food simulant versus the initial quantity of nano-TiO2 for the PLA/nano-TiO2 composite films. Iñiguez-Franco et al. investigated the hydrolytic degradation of PLA nanocomposites in pure water, 50% and 95% ethanol solution at 40 °C. The hydrolytic degradation of PLA nanocomposite films was the fastest in 50% ethanol solution [22]. So, 50% (v/v) ethanol was chosen as the alcoholic food simulant to evaluate the release behavior of nano-TiO2. The migration amount of nano-TiO2 from the different PLA nanocomposite films was shown in Fig. 1. After the PLA/nano-TiO2 composite films were exposed to 50% (v/v) ethanol simulant, the amount of nano-TiO2 in simulant increased over
contact time indicating the release of nanoparticles from the nanocomposites. The migration amount depended on migration time, food simulant, and nano-TiO2 content in PLA nanocomposite films. The release behavior displayed a sharp increase in the first few days and a steady state from 20 to 45 days (Fig. 1). Nano-TiO2 in the outer layer of film surface was easily accessible by food simulant. This allowed a rapid release of nanoparticles from films. However, the rest of nanoparticles were inside the films and food simulant was not easily to penetrate into the polymer matrix. The nanoparticles slowly migrated from the films or remained in the films even after a long period of contacting with food simulant [23]. The migration amount increased as the initial quantity of nano-TiO2 for the PLA/nano-TiO2 composite films rose. In all tested films, the highest migration amount was 0.54 ± 0.04 mg/kg for PLA/T20 film on day 45, and it was still lower than the limit of nano-TiO2 quantity. Migration was a result of mass transfer of nanoparticles from polymer matrix to food simulant [24]. This indicated that nanoparticles might be released via dissolution from the surface and cut edge of the PLA/ nano-TiO2 composite films (solid phase). Lin et al. also reported that the migration amount of Ti from nano-TiO2-polyethylene composite packaging films gradually increased until it reached equilibrium [25]. The migration of nanoparticles was a competitive process that depended on the compatibility of nanoparticles to the liquid phase (food simulant) and the solid phase (nanocomposite film) when the
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Fig. 2 (continued).
film contacted with the liquid phase. The steady state from 20 to 45 days (0.31 ± 0.02 mg/kg for PLA/T1 film and 0.54 ± 0.04 mg/kg for PLA/T20 film) could be because of the process of solvent induced crystallization. In this stage, the crystallization rate was slow and the release of nanoTiO2 would be restricted by the secondary crystallization process. 3.2. Microstructure Changes in microstructure of the nanocomposite films were studied by SEM technique. The fractured surfaces of PLA, PLA/T5, PLA/T10, and PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, were shown in Fig. 2. As could be seen from Fig. 3a, the neat PLA showed smooth and flat surface before contacting with ethanol simulant. As time proceeding, the fractured surface became rougher. Before contacting with ethanol simulant, the addition of nano-TiO2 made the fractured surface rougher than the neat PLA film. A few of small particles could be seen in Fig. 2b and c. However, a lot of particles existed in Fig. 3d. After 5 days of exposure of PLA/T5 nanocomposite film to 50% (v/v) ethanol simulant, the fractured surface became rougher (Fig. 2b). No significant change was observed on day 30. Only a very few of small particles were observed on the end of contacting with ethanol solution. In the case of the PLA/T10 and PLA/T20 nanocomposite film, nano-TiO2 agglomeration gradually reduced as time proceeding. This might be
because of nanoparticles migration from the PLA composite films. The fractured surface of films became rougher. As the porosity of films increased, the absorption of water or ethanol within the PLA matrix increased [26]. The water or ethanol entrapment was directly proportional to porosity, which might provide a high tendency for migration. Some voids and islands were obvious on the 30th day (Fig. 3c and d). From the SEM images of fractured surface, it could be observed that developed crystal morphology was different among the PLA, PLA/ T5, PLA/T10, and PLA/T20 nanocomposite films. 3.3. Crystalline structural changes The XRD spectra is one of the fundamental tools for micro analytical characterization available for thin films owing to its quantitative, simplicity, more reliability, and nondestructive nature [27]. The XRD curve of PLA, PLA/T5, PLA/T10, and PLA/T20 nanocomposite films contacting with ethanol as food simulant at 40 °C was shown in Fig. 3. There was no specific diffraction peak observed in the XRD of PLA without nano-TiO2. This revealed the amorphous or semicrystalline nature of PLA. The XRD curve for PLA/nano-TiO2 composites indicated that the incorporation of nano-TiO2 to PLA matrix would lead to the appearance of a diffraction peak at 25.4°. As could be seen from Fig. 3a–d, the diffraction peak at 25.4° became sharper as the nano-TiO2 loading increased from 0 wt% to 20 wt%. The nano-TiO2 showed peaks at 25.4°,
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38.1°, 48.1°, 54.0°, and 55.2°, which correspond to the anatase phase of TiO2 nanoparticles [26]. The XRD curve of pure PLA and PLA/nano-TiO2 composites (Fig. 3a– d) displayed that polymer matrix kept an amorphous structure before hydrolytic degradation [28]. The character peak of nano-TiO2 (2θ = 25.4°) decreased as time proceeding (Fig. 1b–d). For the PLA/T5 film, the character peak of nano-TiO2 almost disappeared on day 30. However, the character peak of nano-TiO2 for the PLA/T10 film remained half of the intensity and that for the PLA/T20 film was the highest among the samples on the 30th day. When the PLA/nano-TiO2 composite films contacted with 50% (v/v) ethanol food simulant, the migration of nano-TiO2 for all samples increased with the migration time. The migration of nanoparticles would directly result in enhanced interactions among polymer molecules chains. The decrease in nano-TiO2 content of nanocomposite films would lead to the decrease in the intensity of specific diffraction peak. The migration content for the PLA/T20 film was the lowest. The trend in XRD patterns was consistent with that in migration test. When the PLA film was exposed to the ethanol solution for 5 days (Fig. 3a), two strong diffraction peaks emerged at 2θ = 16.9° and 2θ = 19.2° corresponding to the 100/203 and 110/200 plane reflections, respectively. This confirmed that crystalline structure was formed. Similarly, Fig. 3b–d showed that the formation of α-crystal of PLA/nanoTiO2 composites took place on day 5. During the solvent induced crystallization and hydrolytic degradation of amorphous region of PLA polymer matrix, the same form of crystals was present in PLA/nano-TiO2 composites. When the polymer chain in amorphous region degraded, the number of amorphous region would decrease and result in the ratio of crystalline to amorphous region increasing [28]. However, the intensity of α-crystal diffraction peaks for the PLA/T20 film was the
weakest. The presence of nano-TiO2 increased the sorption of ethanol simulant and acted as an anchor to restrict the movements of the polymer chains [22]. 3.4. DSC analysis for molecular order Changes in crystallization degree during PLA/nano-TiO2 composite films contacting with ethanol as food simulant were very important, especially if considering the influence of solvent induced degradation of amorphous phase over the crystalline portions [29]. The DSC curve obtained from the second run is performed to analyze the thermal transition of the PLA/nano-TiO2 composites (Fig. 4). DSC data in Table 1 listed Tg, Tc, Tm, and Xc of the different thermal events. The crystallinity percentage (Xc) of the PLA phase in the PLA/nano-TiO2 composites was calculated by formula (1): Xcð%Þ ¼ ΔHm=ΔH0m w 100
ð1Þ
where ΔHm is the melting enthalpy (J/g), ΔH0m is the theoretical enthalpy for 100% crystalline PLA (93.7 J/g), and w is the weight fraction of PLA in the nanocomposites [30]. There was no significant difference in Tg and Tm value with the incorporation of nano-TiO2 to PLA matrix. This was in consistent with previous studies [31]. The Xc value for the PLA, PLA/T5, PLA/T10, and PLA/T20 nanocomposites was 14.2%, 15.8%, 18.2%, and 17.4%, respectively. The Xc value gradually increased and then slightly decreased. This result indicated that incorporation of nano-TiO2 into PLA matrix led to some improvements in the crystallization degree in PLA polymer. However, excess amount of nano-TiO2 (20 wt% loading) probably caused the
Fig. 3. The XRD curve of (a) PLA, (b) PLA/T5, (c) PLA/T10, and (d) PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively.
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Fig. 4. The DSC curve of (a) PLA, (b) PLA/T5, (c) PLA/T10, and (d) PLA/T20 nanocomposite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively.
aggregation of nanoparticles in the polymer matrix and led to the decrease in the crystallization degree. As the PLA/nano-TiO2 composite films contacted with 50% (v/v) ethanol simulant, Tg value for the PLA, PLA/T5, and PLA/T10 films gradually increased from day 0 to day 30. However, Tg value for the PLA/T20 film significantly (P b 0.05) increased on day 5 and then slightly decreased on day 15. After the nanocomposites immersed into food simulant, the increase in the Tg value indicated that the amorphous phase gradually degraded from the very beginning and the presence of more polymeric chains involved in the crystallization process. The nucleating ability of nanoparticles and the hydrolytic chain cleavage preferentially proceeded in the amorphous region [25]. The solvent induced degradation could act as crosslinking and hinder the motion of the polymer chains [27]. The migration test was performed at 40 °C. The degradation temperature was below the Tg value of PLA and PLA nanocomposites. The degradation process slowly occurred because of the inactivity of the polymer molecules changing from glass-like state to rubber-like state. Moreover, the degradation of PLA polymer was restricted to its surface if the degradation temperature remained below the Tg value [28]. Tc value for the PLA film increased from day 0 to day 15. Tc value for the PLA/nano-TiO2 composite films initially increased on day 5 and then decreased from the 15th day. However, there was no significant (P N 0.05) difference in Tm value for all of the samples as the time proceeding. Degradation in ethanol solution led to a formation of double melting specific peaks in PLA film on day 30 and PLA/T5 film after degradation of 15 days and longer. PLA/T10 and PLA/T20 samples emerged double melting peaks from day 5. The presence of double melting peaks in PLA is well known and attributed to the melting crystalline phase formed during the second heating run [32]. Furthermore, the degraded
macromolecules were shorter than the original undegraded PLA of amorphous phase which might crystallize faster than the neat PLA [29]. As could be seen from Table 1, the crystallinity degree of the films contacting with ethanol simulant gradually increased from day 0 to day 30. The increase in Xc value showed that ethanol and water acted on amorphous regions where hydrolytic degradation was prevalent. Other researchers also reported that the higher Xc value obtained during the different degradation times by DSC analysis [29–34]. As for the PLA/nano-TiO2 composites, when the nano-TiO2 was homogeneously distributed into the polymer matrix by proper nanoparticle loading, the water molecule would more easily penetrated within the nanocomposites to trigger the degradation process. The presence of nanoparticles also increased the sorption of ethanol, which was might be absorbed in the gaps between the conglomeration of TiO2 due to the agglomeration of nano-TiO2. The absorption of water and ethanol would make them spend more time to diffuse into the polymer matrix [28]. Along with the 50% (v/v) ethanol solution diffused into the amorphous phase which was less organized, the crystallinity degree of PLA changed. The combined effects of nanoparticles migration, time, morphology (preferential degradation of amorphous phase compared to crystalline phase), and the interactions between polymeric matrix and nanoparticles would control the degradation of PLA, including the observed effects on melting peaks and crystallinity degree of PLA/nano-TiO2 composites. LA oligomers and shorter PLA chains were not easily soluble in the 50% (v/v) ethanol simulant even though they could absorb much water [28]. Only lactic acid itself as well as dimmer or trimmer of lactic acid was soluble in the ethanol solution. The results revealed that although the degradation process started for the PLA/nano-TiO2 composite films from the very beginning, a long time was still need to have a
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Table 1 The thermal performance parameters of the PLA/nano-TiO2 composite films contacting with ethanol as food simulant on day 0, 5, 15, and 30, respectively. Sample
Time
PLA
Day 0 Day 5 Day 15 Day 30 Day 0 Day 5 Day 15 Day 30 Day 0 Day 5 Day 15 Day 30 Day 0 Day 5 Day 15 Day 30
PLA/T5
PLA/T10
PLA/T20
Tg (°C)
Tc (°C)
Tm (°C)
Xc (%)
48.5 ± 1.3a 55.3 ± 3.7b 57.8 ± 5.8b 58.9 ± 4.5b 47.2 ± 2.7a 56.6 ± 0.9b 57.4 ± 4.8b 58.5 ± 2.9b 46.5 ± 2.0a 60.2 ± 0.9b 61.5 ± 0.8b 61.8 ± 1.6b 50.3 ± 3.3a 62.7 ± 2.9b 61.9 ± 0.7b 62.5 ± 2.4b
115.2 ± 1.5a 120.0 ± 4.9b 124.3 ± 4.7b 121.5 ± 1.7b 108.4 ± 1.8a 126.2 ± 5.3b 122.5 ± 4.3b 121.0 ± 7.4b 105.6 ± 0.2a 124.2 ± 6.1bc 115.0 ± 1.1ab 119.3 ± 3.9b 112.2 ± 3.9a 127.7 ± 6.2b 119.1 ± 2.7ab 121.0 ± 2.0b
170.3 ± 3.3a 170.1 ± 1.1a 170.0 ± 8.5a 169.4 ± 3.4a 170.0 ± 5.3a 170.5 ± 3.1a 169.1 ± 8.1a 171.2 ± 0.9a 170.4 ± 4.2a 171.2 ± 2.8a 171.5 ± 4.4a 170.9 ± 3.6a 170.3 ± 3.0a 169.4 ± 3.1a 170.1 ± 3.4a 170.5 ± 8.2a
14.2 ± 0.1a 15.5 ± 0.4b 17.3 ± 0.3c 18.5 ± 0.3d 15.8 ± 0.5a 16.3 ± 0.4a 18.2 ± 0.2b 19.8 ± 0.1c 18.2 ± 0.4a 19.7 ± 0.4b 21.5 ± 0.1c 22.7 ± 0.6d 17.4 ± 0.4a 20.5 ± 0.7b 21.3 ± 0.5b 23.6 ± 0.2c
a–d Values followed by different letters in the same column for the same sample were significantly (P b 0.05) different, where a was the lowest value.
complete degradation during contacting with 50% (v/v) ethanol solution as food simulant. 4. Conclusion After the PLA/nano-TiO2 composites were immersed into ethanol solution, the film structural features gradually changed and nano-TiO2 would migrate from polymer matrix to food simulant. The observed effects on microstructure, crystalline structure, crystallinity degree, and molecular order of PLA/nano-TiO2 composites were verified by SEM graphs, XRD spectra, and DSC analysis. In the stage of solvent induced crystallization, the crystallization rate was slow and the release of nano-TiO2 would be restricted by the secondary crystallization process. The increase in crystallinity degree of the films during contacting with ethanol simulant showed that ethanol and water acted on amorphous regions where hydrolytic degradation was prevalent. As time proceeding, the PLA nanocomposite films became rougher. This provided more available space for contacting with ethanol food simulant, which might induce a higher tendency for nanoparticle migration. Although the study on migration test and structural changes in this paper was not exhaustive, the result provided important information about the relationship between the structural changes and migration of nanoparticles during contacting with ethanol solution as food simulant. Further work was still need to do, especially on the migration of nanoparticles into water, acidic food simulant, fatty food simulant, and real food products. Acknowledgement The study was financially supported by the Open Project Program of State Key Laboratory of Food Nutrition and Safety, Tianjin University of Science & Technology, China (No. SKLFNS-KF-201815) and Tianjin Natural Science Foundation, China (No. 18JCYBJC43700). References [1] M. Peltzer, J. Wagner, A. Jimenez, Migration study of carvacrol as a natural antioxidant in high-density polyethylene for active packaging, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 26 (6) (2009) 938–946. [2] S.W. Hwang, J.K. Shim, S. Selke, H. Soto-Valdez, L. Matuana, M. Rubino, R. Auras, Migration of α-tocopherol and resveratrol from poly(L-lactic acid)/starch blends films into ethanol, J. Food Eng. 116 (4) (2013) 814–828. [3] I. Spiridon, C.E. Tanase, Design, characterization and preliminary biological evaluation of new lignin-PLA biocomposites, Int. J. Biol. Macromol. 114 (2018) 855–863.
[4] Y. Byun, K. Rodriguez, J.H. Han, Y.T. Kim, Improved thermal stability of polylactic acid (PLA) composite film via PLA-beta-cyclodextrin-inclusion complex systems, Int. J. Biol. Macromol. 81 (2015) 591–598. [5] A.L.P.d.L. Freitas, L.R. Tonini Filho, P.S. Calvão, A.M.C.d. Souza, Effect of montmorillonite and chain extender on rheological, morphological and biodegradation behavior of PLA/PBAT blends, Polym. Test. 62 (2017) 189–195. [6] B. Panea, G. Ripoll, J. González, Á. Fernández-Cuello, P. Albertí, Effect of nanocomposite packaging containing different proportions of ZnO and Ag on chicken breast meat quality, J. Food Eng. 123 (2014) 104–112. [7] Y. Huang, S. Chen, X. Bing, C. Gao, T. Wang, B. Yuan, Nanosilver migrated into foodsimulating solutions from commercially available food fresh containers, Packag. Technol. Sci. 24 (5) (2011) 291–297. [8] Y. Yang, H. Zhang, P. Wang, Q. Zheng, J. Li, The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci. 288 (1–2) (2007) 231–238. [9] H. Song, B. Li, L. Q-B, W. H-J, Y. Chen, Migration of silver from nanosilverpolyethylene composite packaging into food simulants, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 28 (12) (2011) 1758–1762. [10] A. Youssef, Polymer nanocomposites as a new trend for packaging applications, J. Macromol. Sci. D Rev. Polym. Process. 52 (7) (2013) 635–660. [11] Y. Zhou, L. Lei, B. Yang, J. Li, J. Ren, Preparation and characterization of polylactic acid (PLA) carbon nanotube nanocomposites, Polym. Test. 68 (2018) 34–38. [12] A.M. Díez-Pascual, A.L. Díez-Vicente, Effect of TiO2nanoparticles on the performance of polyphenylsulfone biomaterial for orthopaedic implants, J. Mater. Chem. B 2 (43) (2014) 7502–7514. [13] H. Bodaghi, Y. Mostofi, A. Oromiehie, Z. Zamani, B. Ghanbarzadeh, C. Costa, A. Conte, M.A. Del Nobile, Evaluation of the photocatalytic antimicrobial effects of a TiO2 nanocomposite food packaging film by in vitro and in vivo tests, LWT Food Sci. Technol. 50 (2) (2013) 702–706. [14] M. Gumiero, D. Peressini, A. Pizzariello, A. Sensidoni, L. Iacumin, G. Comi, R. Toniolo, Effect of TiO2 photocatalytic activity in a HDPE-based food packaging on the structural and microbiological stability of a short-ripened cheese, Food Chem. 138 (2–3) (2013) 1633–1640. [15] F. Donglu, Y. Wenjian, B.M. Kimatu, A.M. Mariga, Z. Liyan, A. Xinxin, H. Qiuhui, Effect of nanocomposite-based packaging on storage stability of mushrooms (Flammulina velutipes), Innovative Food Sci. Emerg. Technol. 33 (2016) 489–497. [16] C. Lantano, I. Alfieri, A. Cavazza, C. Corradini, A. Lorenzi, N. Zucchetto, A. Montenero, Natamycin based sol-gel antimicrobial coatings on polylactic acid films for food packaging, Food Chem. 165 (2014) 342–347. [17] N. von Goetz, L. Fabricius, R. Glaus, V. Weitbrecht, D. Gunther, K. Hungerbuhler, Migration of silver from commercial plastic food containers and implications for consumer exposure assessment, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 30 (3) (2013) 612–620. [18] Y. Tang, M. Lewin, New aspects of migration and flame retardancy in polymer nanocomposites, Polym. Degrad. Stab. 93 (11) (2008) 1986–1995. [19] S.R. Andersson, M. Hakkarainen, A.-C. Albertsson, Long-term properties and migration of low molecular mass compounds from modified PLLA materials during accelerated ageing, Polym. Degrad. Stab. 97 (6) (2012) 914–920. [20] J. Zhu, X. Li, C. Huang, L. Chen, L. Li, Structural changes and triacetin migration of starch acetate film contacting with distilled water as food simulant, Carbohydr. Polym. 104 (2014) 1–7. [21] J. Yang, Y. Qin, M. Yuan, J. Xue, J. Cao, Y. Wu, M. Yuan, Preparation and characterization of poly(L-lactide)-co-poly(trimethylene carbonate)/talc film, Int. J. Biol. Macromol. 62 (2013) 411–417. [22] H. Chi, W. Li, C. Fan, C. Zhang, L. Li, Y. Qin, M. Yuan, Effect of high pressure treatment on poly(lactic acid)/nano(-)TiO(2) composite films, Molecules 23 (10) (2018). [23] F. Iñiguez-Franco, R. Auras, M. Rubino, K. Dolan, H. Soto-Valdez, S. Selke, Effect of nanoparticles on the hydrolytic degradation of PLA-nanocomposites by waterethanol solutions, Polym. Degrad. Stab. 146 (2017) 287–297. [24] F. Liu, C.Y. Hu, Q. Zhao, Y.J. Shi, H.N. Zhong, Migration of copper from nanocopper/ LDPE composite films, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 33 (11) (2016) 1741–1749. [25] A. Ozaki, E. Kishi, T. Ooshima, A. Hase, Y. Kawamura, Contents of Ag and other metals in food-contact plastics with nanosilver or Ag ion and their migration into food simulants, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 33 (9) (2016) 1490–1498. [26] Q.-B. Lin, H. Li, H.-N. Zhong, Q. Zhao, D.-H. Xiao, Z.-W. Wang, Migration of Ti from nano-TiO2-polyethylene composite packaging into food simulants, Food Addit. Contam. Part A Chem. Anal. Control Expo. Risk Assess. 31 (7) (2014) 1284–1290. [27] S. Mallick, Z. Ahmad, F. Touati, J. Bhadra, R.A. Shakoor, N.J. Al-Thani, PLA-TiO2 nanocomposites: thermal, morphological, structural, and humidity sensing properties, Ceram. Int. 44 (14) (2018) 16507–16513. [28] A.J. Pai, B.K. Sarojini, K.R. Harshitha, B. Shivarama Holla, A.G. Lobo, Spectral, morphological and optical studies on bischalcone doped polylactic acid (PLA) thin films as luminescent and UV radiation blocking materials, Opt. Mater. 90 (2019) 145–151. [29] Y.-B. Luo, X.-L. Wang, Y.-Z. Wang, Effect of TiO2 nanoparticles on the longterm hydrolytic degradation behavior of PLA, Polym. Degrad. Stab. 97 (5) (2012) 721–728. [30] M. Sednickova, S. Pekarova, P. Kucharczyk, J. Bockaj, I. Janigova, A. Kleinova, D. Jochec-Moskova, L. Omanikova, D. Perdochova, M. Koutny, V. Sedlarik, P. Alexy, I. Chodak, Changes of physical properties of PLA-based blends during early stage of biodegradation in compost, Int. J. Biol. Macromol. 113 (2018) 434–442. [31] C. Fan, H. Chi, C. Zhang, R. Cui, W. Lu, M. Yuan, Y. Qin, Effect of multiscale structure on the gas barrier properties of poly(lactic acid)/Ag nanocomposite films, Polym. Adv. Technol. 30 (2019) 1709–1715.
C. Yang et al. / International Journal of Biological Macromolecules 139 (2019) 85–93 [32] C. Fan, R. Cui, W. Lu, H. Chen, M. Yuan, Y. Qin, Effect of high pressure treatment on properties and nano–Ag migration of PLA-based food packaging film, Polym. Test. 76 (2019) 73–81. [33] A.A. Cuadri, J.E. Martín-Alfonso, Thermal, thermo-oxidative and thermomechanical degradation of PLA: a comparative study based on rheological, chemical and thermal properties, Polym. Degrad. Stab. 150 (2018) 37–45.
93
[34] J.Y. Wu, C.W. Li, C.H. Tsai, C.W. Chou, D.R. Chen, G.J. Wang, Synthesis of antibacterial TiO2/PLGA composite biofilms, Nanomedicine 10 (5) (2014) e1097–e1107.