ZnO Nanocomposite Films

11 Sep 2012 - This study reports the effect of light on PLA/ZnO nanocomposites films produced by melt-extrusion. The attention focused on the discrimi...
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Photochemical Behavior of Polylactide/ZnO Nanocomposite Films Sandrine Therias,*,†,‡ Jean-François Larché,†,‡ Pierre-Olivier Bussière,§,‡ Jean-Luc Gardette,†,‡ Marius Murariu,∥ and Philippe Dubois∥ †

Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France ‡ CNRS, UMR 6296, ICCF, BP 80026, F-63171 Aubiere, France § Clermont Université, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France ∥ Centre of Innovation and Research in Materials & Polymers (CIRMAP), Laboratory of Polymeric and Composite Materials (LPCM), University of Mons − UMONS & Materia Nova Research Centre, Place du Parc 20, 7000 Mons, Belgium ABSTRACT: This study reports the effect of light on PLA/ ZnO nanocomposites films produced by melt-extrusion. The attention focused on the discrimination between the photocatalytic degradation of PLA provoked by ZnO and the UV screening effect of the ZnO nanoparticles. The chemical modifications of PLA induced by UV light irradiation were analyzed using infrared spectroscopy and completed through the analysis of the low-molecular-weight photoproducts using IC and SPME and the characterization of chain scissions with SEC. A comprehensive mechanism for the photooxidation of PLA was then proposed. The results indicated that the photocatalytic activity of ZnO nanoparticles induces the oxidation of PLA. Because ZnO limits the penetration of light inside the samples, this effect mainly concerns the first micrometers at the surface of the exposed samples. Cross-sectional analysis using micro-IR and ATR-IR spectroscopies was performed to highlight the degradation profile in the PLA/ZnO nanocomposites.

1. INTRODUCTION As the environmental impact of plastic wastes becomes a global concern and because of the increasing price of petrochemical based polymers, there is great interest in developing biodegradable polymers and even more interest in developing biosourced biodegradable polymers.1 Because poly(lactide) or poly(lactic acid) (PLA) is not only biodegradable but is also produced from renewable resources, such as sugar and corn starch,2−4 PLA is undoubtedly one of the most promising candidates for future developments in conventional applications such as packaging5−7 and textile fibers8,9 and more recently, as (nano)composites10−12 for technical applications. The utilization of nanoscale materials is indeed an emerging area for several applications. Recently, awareness of general sanitation, contact disease transmission, and personal protection has resulted in the development of proper candidates as selfcleaning and antimicrobial materials, for example, using nanoparticles with photocatalytic properties such as zinc oxide (ZnO) 2 and titanium dioxide (TiO2).13,14 PLA/ZnO nanocomposites are even more interesting for medical applications because of the bioresorbable and biocompatible properties of PLA-based materials within human bodies.15−17 Several studies have reported that when ZnO nanoparticles were mixed with various polymers, nanocomposites with effective properties, such as antibacterial activity 2, intense ultraviolet, and infrared (IR) absorption were obtained.18−21 Besides the hydrolytic degradation of PLA,22,23 the photo© 2012 American Chemical Society

chemical behavior of pristine PLA has already been reported,24−28 and a few papers29−31 have noted the effects of ZnO nanoparticles on the degradation, particularly the photodegradation, of polymers. Indeed, it has been demonstrated that the photocatalytic effect of semiconductors such as ZnO, which are useful for their antibacterial activity due to the formation of reactive species that act as oxidants, mainly results in the formation of radicals that are able to degrade the polymer matrix.32 Thus, previous works32 have shown that the photodegradability of nanocomposites is efficiently promoted. Moreover, by considering some similarities between the photocatalytic effects of TiO2 and ZnO, it is important to remind that according to the results reported by Nakayama et al.14 the rate of PLA degradation can be controlled by the loading of TiO2 nanofiller, whereas an efficient photodegradability is finally allowing the eco-friendly disposal of polymer waste. However, other works33 have concluded that ZnO nanoparticles have a strong UV-light screening effect and thus drastically reduce polymer photodegradation. The aim of this work was to understand fully the photooxidation mechanism of PLA and to investigate the role played by ZnO nanoparticles (with discrimination of the UVlight screening effect from the photocatalytic effect) on the Received: July 11, 2012 Revised: August 30, 2012 Published: September 11, 2012 3283

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2.2. Irradiation. The samples were irradiated under accelerated artificial conditions. The aging device was a SEPAP 12/24 unit34 from Atlas, which was equipped with four medium-pressure mercury lamps (Novalamp RVC 400W) located in a vertical position at each corner of the chamber. Wavelengths below 295 nm were filtered by the glass envelopes of the sources. The temperature at the surface of the samples was fixed at 60 °C. 2.3. Characterization. Transmission Electron Microscopy (TEM). TEM images of selected PLA/ZnO nanocomposites were obtained with a Philips CM200 apparatus using an accelerator voltage of up to 120 kV. For TEM investigations, the nanocomposite samples (70−80 nm thick) were prepared with a Leica Ultracut UCT ultracryomicrotome by cutting at −100 °C. Reported microphotographs represent typical morphologies as observed at, at least, three different locations of the sample. Infrared Spectrometry and Infrared Microspectrometry. IR spectra were recorded in transmission mode using a Nicolet 760FTIR spectrophotometer (nominal resolution of 4 cm−1, 32 scans summations) and in reflection mode using a Nicolet 380-FTIR spectrophotometer equipped with a thunderdome-ATR (4 cm−1, 32 scans) accessory. The thunderdome was a single reflection ATR accessory containing a diamond or germanium crystal. In reflection mode, the penetration depth (dp)35 of the IR beam into the sample is given in eq 1.

mechanism and the oxidation kinetics of PLA under UV-light irradiation. The approach is based on the identification of many photoproducts formed, macromolecular and molecular, trapped in the film or released in the atmosphere. Moreover, one of the objectives of this work was to characterize the oxidation profiles and the dependency on the light absorption profile due to ZnO nanoparticles to understand the global effect resulting from photocatalytic activity and the UV absorbing properties. The first results are focused on the influence of the ZnO nanoparticles on the mechanism and oxidation rate of PLA. The second part is dedicated to a photodegradation crosssectional study for quantifying the impact of the screening effect of the ZnO nanoparticles on PLA oxidation.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(L,L-lactide), hereafter called PLA (supplier NatureWorks), was a grade designed for realization of films (4032D) having the following characteristics: relative viscosity = 3.94; D isomer =1.4%; residual monomer = 0.14%. PLA chemical structure (aliphatic polyester) is recalled in Scheme 1.

Scheme 1. Chemical Structure of PLA

1

dp = 2π × ν ̅ × n1 ×

+ + + + +

0.3% 0.3% 0.3% 0.3% 0.3%

U626A U626A U626A U626A U626A

ZnO content

code

0% 0% 0.5% 1% 2% 3%

PLA PLA0 PLA0.5 PLA1 PLA2 PLA3

n2 n1

(1)

Table 2. Penetration Depth during the ATR Analysis for PLA crystal ATR diamond (n1 = 2.38) germanium (n1 = 4.02)

ν̅ (cm−1)

dp (μm)

3 × dp (μm)

3600 700 3600 700

0.30 1.52 0.14 0.73

0.89 4.57 0.42 2.18

The distribution profiles of the photoproducts through the film thickness were determined using microinfrared spectroscopy, as previously described in the literature.37 The measurements were performed on a Nicolet 6700-FTIR equipped with a CONTINUμM microscope (liquid-nitrogen-cooled MCTA detector, 128-scan summations). The films were embedded into an epoxy resin and then sliced using a Leica RM2165 microtome. The slices, which had a thickness of 5 μm, were analyzed using the FTIR microscope. The spectra were recorded every 2 μm with an analysis window of 10 μm × 20 μm. UV−Visible Spectrometry. Changes in the UV−visible spectra were monitored with a Shimadzu UV-2101PC spectrophotometer equipped with an integrating sphere. The UV−visible spectra were recorded from 140 μm films in transmission mode. Ionic Chromatography (IC). IC analyses were performed by the Service Central d’Analyze du CNRS, Vernaison, France. The IC analyses were performed in a water immersion solution of a photooxidized PLA film. Solid-Phase Microextraction (SPME). The PLA and PLA/ZnO 3% films were also irradiated in sealed vials to collect the volatile photodegradation products. A Carboxen−PDMS fiber (75 μm) purchased from Supelco (Bellefonte, PA) was used to extract the volatile products. The extraction time was 5 min at 80 °C. The volatile compounds were analyzed using gas chromatography/mass spectrom-

Table 1. Compositions of the PLA/ZnO Nanocomposite Samples composition

2

( )

where n1 and n2 are the refractive indices of the crystal and the polymer (PLA ≈ 1.40), respectively, ν̅ is the wavenumber of analysis, and θ is equal to 45°. According to the literature,36 the actual dp should be considered to be 3 × dp. The dp and 3 × dp values for PLA, using either a diamond or germanium ATR crystal, are reported in Table 2.

Commercially available ZnO nanofillers (rod-like, particle size of ∼30 nm) were kindly supplied by Umicore Zinc Chemicals (Belgium) as Zano 20 Plus (surface-coated with a silane especially suitable for the treatment of metal oxides, that is, triethoxy caprylylsilane; ZnO content: 96.2 ± 0.5%, bulk density: 360 g/L, loss on drying at 105 °C (2 h): max 1%). A thermal stabilizer, Ultranox 626A (bis(2,4-di-tbutylphenyl)pentaerythritol diphosphite), was used at preferred percentage of 0.3 wt % in PLA. Throughout this contribution, all percentages are given as wt %. To allow the realization of films, PLA/ZnO nanocomposites were produced by melt-compounding PLA with up to 3% ZnO nanofiller in Leistritz twin-screw extruders (type ZSE 18 HP-40D, diameter of screws (D) = 18 mm, L/D = 40). The previously dried granules of PLA and additives were first mixed in a Rondol turbo-mixer (2000 rpm, 2 min) step, followed by the dosing and melt compounding into the twin-screw extruder (throughput of 1.5 kg/h, speed of the screws = 100 rpm, temperatures of extrusion adapted to the rheological characteristics of PLA, temperatures of the molten polymer ∼185 °C). The granules of PLA nanocomposites were dried (at 80 °C overnight, under vacuum) and used for the realization of films by extrusion. Films with a thickness of ∼140 μm were obtained using a DSM twin-screw microextruder (batch-volume: 15 mL, speed of screws: 70 rpm, temperature of molten polymer: 185−190 °C) equipped with a special die (width 35 mm, die opening: 0.4 mm) and a DSM Xplore microfilm device (speed: 200 mm/min, torque of winding unit: 40−50 N mm). Various formulations of PLA and PLA/ZnO nanocomposites with different ZnO contents were prepared, and their compositions are provided in Table 1.

PLA PLA PLA PLA PLA PLA

sin 2 θ −

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Figure 1. (a,b) TEM pictures at different magnifications of PLA2 nanocomposites to illustrate the good distribution of ZnO nanoparticles within the polyester matrix. etry (GC/MS) with a 6890N Agilent GC coupled to a 5973 Agilent mass detector. The GC was equipped with a Supelcowax 10 column (30 m × 0.25 mm × 0.25 μm) from Supelco. Splitless injections were used (2 min). The temperature was ramped from 35 (10 min hold) to 60 °C at a rate of 5 °C/min and then to 200 °C (15 min hold) at a rate of 10 °C/min. Helium was used as a carrier gas at a constant pressure of 38 kPa. The temperature of the splitless injector was 280 °C (2 min duration), and the flow rate was 50 mL/min. The transfer line temperature was 280 °C, and the ion source temperature was maintained at 230 °C. Ionization occurred with a kinetic energy of the impacting electrons of 70 eV. The detector voltage was 70 eV. The mass spectra and the reconstructed chromatograms (total ion current, TIC) were acquired using the electron ionization (EI) mode at 70 eV and were recorded from 20 to 400 m/z. The compounds were identified either by comparison of the retention times and mass spectra with standards or by using the spectral library. Size Exclusion Chromatography (SEC). The changes in the molecular weight for the PLA and PLA/ZnO nanocomposites were obtained by SEC using a Waters 600 chromatograph working with a differential refractometer detection (Waters 2410) and two linear columns (PL, Polymer Laboratories). The analyses were performed using THF as an eluent at a flow rate of 1 mL/min at room temperature (25 °C). The PLA solutions were prepared in THF (10 mg polymer/5 mL solvent) and were filtered before injection. The equipment was calibrated with a PS standard.

selected samples assess once again that the silane-treated ZnO nanoparticles are well-distributed and dispersed through the PLA matrix because the presence of aggregates is difficult to be observed in the analyzed nanocomposites. IR and UV Visible Spectroscopies. The IR transmission spectra of PLA, PLA0 (0.3% U626A), and PLA/ZnO nanocomposite at 3% ZnO content (PLA3) were compared; the peaks in the neat PLA have already been assigned in previous papers.27,38 The spectra of all samples are similar, and no specific absorption bands resulting from the ZnO nanoparticles or the thermal stabilizer (U626A) were observed. The UV−visible transmission spectra of the PLA and PLA/ ZnO nanocomposite films before irradiation are presented in Figure 2. Note that in the UV−visible spectrum of the neat PLA, a very weak absorption occurs in the UV region 300−400 nm. In the case of PLA0 with the thermal stabilizer, an absorption band with a maximum at 280 nm is present in the UV−visible spectrum, and it can be attributed to the phenolic function of the Ultranox 626. For all of the samples with ZnO

3. RESULTS AND DISCUSSION 3.1. Characterization of PLA-ZnO Nanocomposites. TEM Characterization. ZnO adequately treated by selected silanes can lead via melt-compounding to PLA nanocomposites characterized by good preservation of related physicochemical characteristic features and intrinsic molecular parameters of PLA. Noteworthy, minerals surface-covered with hydroxyl such as ZnO are generally very receptive to the bonding with alkoxysilanes, and more than one (mono)layer of silane is usually applied onto the surface of the filler, which could also play the role of compatibilizer and dispersion agent. The surface-coated ZnO nanoparticles proved to finely and regularly disperse within the PLA matrix under different shear conditions (twin-screw extruder, internal kneaders 2). Moreover, the TEM images realized at different magnifications (Figure 1a,b) on

Figure 2. UV−visible spectra of the (ca. 140 μm) PLA and PLA/ZnO nanocomposite films. 3285

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more intense for the PLA/ZnO nanocomposites than for the PLA. To complete the previously proposed photooxidation mechanism,12 the low-molecular-weight photoproducts formed upon the irradiation of PLA were identified using IC (results reported in Table 3) and SPME. By comparing with standards,

nanoparticles, an intense absorption resulting from the presence of ZnO is observed in the UV domain up to 400 nm. These results highlight for the specific anti-UV properties of PLA-ZnO nanocomposites the increase in ZnO loading leading to an evident decrease in transmittance of UV radiation. 3.2. Photooxidation of PLA-ZnO Nanocomposites. The various PLA/ZnO nanocomposites were exposed to light in the presence of air, and the chemical modifications resulting from photooxidation were monitored using IR spectroscopy. Figure 3 presents the modifications of the IR spectra of PLA and PLA3 after irradiation.

Table 3. Concentrations of the Carboxylic Acids Extracted from the Irradiated PLA from Ionic Chromatography Analysis compound

formula

molecular weight (g·mol−1)

C (mg·L−1)

carbonic acid formic acid oxalic acid

HO−COOH H−COOH HOOC−COOH

62 46 90

12.025 0.0019 0.140

the primary released products were identified as carbonic and oxalic acids. Formic acid was also detected but in minor proportion; the major extracted product was carbonic acid. Several products were identified using SPME, and they were similar for both samples. Among these products, acetic acid and methyl acetate were the main volatile products formed. The SPME analyses of the gas phase were compared after irradiating the PLA for 300 h and the PLA/ZnO 3% for 20 h. These irradiation times were chosen to obtain similar extents of oxidation. (See Figure 6 below.) Acetic acid and methyl acetate were identified as the main volatile products formed for PLA/ ZnO 3% and neat PLA. Similar observations resulted from the comparative analysis of PLA/ZnO 3% and PLA by IC, which produced the same products with similar relative concentrations. The variations in the molar weights and PDI of pristine PLA (PLA0) and PLA/ZnO (1 and 3%) nanocomposites (PLA1 and PLA3) were determined, and the obtained values are reported in Table 4. Table 4. Variations in the Molar Weight (Mw (PLA)) and Polydispersity Index (PDI) of PLA0, PLA1, and PLA3 after Photooxidation

Figure 3. (a) Infrared transmission spectra of PLA from 0 to 500 h of photooxidation. The inset shows the subtraction spectra. (b) Infrared spectra of PLA3 from 0 to 350 h.

The IR spectrum of the pristine PLA indicates that 500 h of irradiation results in the formation of two absorption bands. Two bands can be clearly observed in the subtracted spectra, which are presented in the inset of Figure 3a. The first absorption band is a narrow band with a maximum at 1845 cm−1, and the second one is a broad band centered at 3420 cm−1. The IR band at 1845 cm−1 has been previously attributed to anhydride groups11,12 and the broad absorption band in the hydroxyl region has been attributed to alcohol and carboxylic acid groups.12 The absorbance of the ester band is too intense to be quantified from the IR transmission spectra of the 140 μm thick PLA films. The IR analysis of the PLA/ZnO nanocomposites during photo-oxidation (Figure 3b) also presents the formation of the two absorption bands with maxima at 1845 and 3420 cm−1, as observed in the case of pristine PLA (Figure 3a). This observation indicates that the same PLA oxidation photoproducts are formed in the presence of ZnO nanoparticles. However, one can observe a dramatic difference in terms of photoproduct concentrations because under identical exposure times the absorbance measured at 1845 cm−1 is considerably

irradiation time (h)

Mw PLA0

PDI PLA0

Mw PLA1

PDI PLA1

Mw PLA3

PDI PLA3

0 100 200 300

207 000 180 000 172 000 162 000

1.8 1.8 1.8 1.8

150 000 125 000 113 000 101 000

1.7 1.8 1.8 1.8

162 000 130 000 77 000 69 000

1.7 1.8 2.0 2.0

Before irradiation, the SEC analyses revealed a slight decrease in the molar weight of PLA in the nanocomposite samples, which reflects the degradation of PLA during the meltmixing and processing in the presence of ZnO nanoparticles, as it was previously reported by some of us.2 The photooxidation of pristine PLA results in a decrease in the molar weight, ∼20% after 300 h irradiation in a SEPAP 12/24 device. As previously reported,12 this decrease indicates that photooxidation of PLA provokes chain scissions, which are responsible for forming the low-molecular-weight products. The analysis of the evolution of the molar mass of the PLA matrix in the PLA/ZnO nanocomposites during photooxidation also indicates a decrease in the molar weights but to a higher extent. After 300 h of irradiation, the molar weight of PLA was reduced by approximately 30 and 60% in the presence of 1 and 3% of ZnO nanoparticles, respectively. The value of the polydispersity 3286

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Figure 4. Photooxidation mechanism of PLA induced by ZnO. (e−cb represents an electron and h+vb represents a positive hole, a vacancy formed in the previously full valence band.)

index (PDI) is not modified upon irradiation for PLA0 and PLA1, whereas in the case of PLA3, the PDI value increases from 1.7 to 2.0 after 200 h of irradiation, whereas Mw displays a dramatic decrease. The ability of pigments to catalyze the photooxidation of polymers has received significant attention with regard to their mechanistic behavior. In this regard, much of the information originates from works examining the photocatalytic oxidation of polymers by TiO229,39,40 and for other white pigments such as ZnO. UV-light absorption by the ZnO nanoparticles results in the promotion of an electron (e−cb) into the conduction band, which leaves a positive hole (h+vb) in the valence band. The excited-state conduction-band electrons and the valence-band holes can recombine and dissipate the input energy with electron donors and electron acceptors adsorbed onto the semiconductor surface or within the surrounding electrical double layer of the charged particles. The exciton resulting from the irradiation of ZnO reacts with the surface hydroxyl groups to form a hydroxyl radical.41 Oxygen anions are also produced, which are adsorbed onto the surface of the pigment particle and produce active perhydroxyl radicals. The reactions of e− with O2 and h+ with H2O are most likely to yield O2−, HOO, and HO. The active oxygen species described above initiate the degradation reaction by attacking the neighboring polymer chains. On the basis of these results and our previous work,12 one can propose a mechanism for the photooxidation of PLA with an initiation step arising from the photocatalytic effect of ZnO nanoparticles, as depicted in Figure 4. In a previous paper concerning the photooxidation mechanism of neat PLA,12 we have shown that despite the fact that PLA does not absorb radiation with wavelengths greater than 300 nm, this polymer is sensitive to photo-

oxidation. There is considerable literature reporting that the degradation of nonabsorbing polymers can be initiated by chromophoric impurities. These chromophors readily absorb UV light and produce radicals that then react with the polymer. The irradiation of PLA resulted in the formation of several photoproducts; macromolecular anhydrides, P1, were identified as the main products detected during the analysis of the solid PLA films. An oxidation mechanism that accounts for the main degradation routes was proposed, as summarized in Figure 5. This mechanism involves a classical hydrogen abstraction on the polymeric backbone at the tertiary carbon in the α-position of the ester function, which results in the formation of macroradicals R0. These macroradicals react with oxygen, resulting in peroxy radicals that give hydroperoxides by abstraction of a labile hydrogen atom, which propagates the chain reaction oxidation. Once formed, the hydroperoxides can decompose leading to the formation of alkoxy and hydroxyl radicals. The hydroxyl radicals readily propagate the chain oxidation by hydrogen abstraction at a tertiary carbon atom, producing water and a macroalkyl radical R0. The alkoxy macroradical R1 is the key intermediate in the reaction and can decompose through β-scission. Among the three different βscissions that can occur, only one reaction produces anhydrides P1: the one involving the homolysis of the C−C bond on the macromolecular chain, scission of the C−CH3 bond, would produce a methyl radical CH3 that can be oxidized to give methanol. This reaction was considered to be the major mechanism occurring during the photooxidation of PLA under irradiation at wavelengths above 300 nm. The analysis of the photooxidation by SPME and IC allowed us to complete the previously proposed mechanism.12 The mechanism is provided in Figure 5. 3287

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Figure 5. Completed photooxidation mechanism of PLA with the formation of low-molecular-weight photoproducts.

acid, and the reaction of R5 with hydroxyl radicals HO produces carbonic acid. To evaluate the influence of the amount of ZnO nanoparticles on the PLA oxidation, we plotted the increase in absorbance at 1845 cm−1 versus irradiation time in Figure 6 for the six formulations.

The hydrolysis of the macromolecular anhydride, P1, formed by the β-scission of the macroalkoxy radical, R1, produces acetic acid and acid terminated chains. The oxidation at the tertiary carbon of anhydride P1 can produce an alkoxyl radical by a similar mechanism to that leading to R1. A β-scission of this radical by homolysis of the C−C bond yields the radical R2 and the macromolecular anhydride P1. This mechanism explains why the anhydride P1 is the main macromolecular photoproduct observed during the IR analysis of the PLA films. This reaction will progressively shorten the macromolecular chain (as observed by SEC analysis). Radical R2 can abstract a hydrogen atom on the macromolecular backbone to form R0 and a molecular anhydride P1′. The hydrolysis of P′1 produces acetic acid and formic acid. The radical R2 can also evolve by decarbonylation. The loss of CO produces a radical R2′ that can lead to the formation of acetic acid and to methyl acetate by reaction with a methyl radical CH3. Methyl acetate could also be obtained by the reaction of methanol with P1. Radical R1 can also decompose by a β-scission involving the homolysis of the C−O bond. This reaction produces P4 and a radical R3. The abstraction of a hydrogen atom on the macromolecular backbone by radical R3 propagates the chain reaction and forms carboxylic acid P5. The oxidation of P5 through the classical mechanism produces an alkoxy radical R4, where β-scission of the C−C bond leads to R5 and permits P1 to be formed again. The dimerization of R5 produces oxalic

Figure 6. Variation of absorbance at 1845 cm−1 as a function of irradiation time for PLA and PLA/ZnO nanocomposites (0 to 3% ZnO). 3288

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The kinetic curves reported in Figure 6 clearly indicate that the ZnO nanoparticles have a prodegradant effect on the PLA photooxidation. Indeed, the rate of photooxidation dramatically increases with the amount of ZnO, from 0.5 up to 3%. However, PLA 0.5 and PLA1 present similar oxidation rates but higher than those obtained for PLA and PLA0. Note that the presence of 0.3% antioxidant Ultranox 626A has no stabilizing effect on the photooxidation of PLA (curves PLA vs PLA0). The same tendency was observed during the analysis by SEC (Table 4), which showed a much more important decrease in Mw in the presence of ZnO. As observed from the UV−visible spectra in Figure 2, the ZnO nanoparticles strongly absorb in the near UV region. On the basis of this observation, one may argue that ZnO will screen the active UV radiation, which could result in an effective UV protection of the polymer. The results provided in Figure 6 and Table 4 indicate that despite the inner filtering effect, the presence of ZnO nanoparticles results in a higher oxidation rate of PLA in nanocomposites, which reflects the photocatalytic effect of ZnO. This result contradicts recent results concerning the effect of the incorporation of ZnO nanoparticles into a polypropylene PP matrix41 with contents at 1.5, 3.0, and 5.0 wt %. The striking observation was that for the nanocomposites with increasing ZnO particle content, the oxidation peak intensities decreased correspondingly. It was deduced that the ZnO nanoparticles played an important role in stabilizing the PP molecules and delayed the photodegradation process by acting as screens. The results from that research study indicated that the incorporation of ZnO nanoparticles into the PP matrix can impart significant improvements on the photodegradation resistance of PP to UV-irradiation. However, nothing was reported concerning the possible photocalytic effects of ZnO. Note that the experiments reported in that study were performed on relatively thick plates because the thickness of the irradiated items was ∼3 mm. To analyze the effects of irradiation, PP samples of ∼0.5 mm thickness were then cut from the surface of the plates and powdered, and the powders were mixed and cold-pressed with KBr, followed by analysis with IR transmission spectroscopy. On the basis of the fact that the presence of ZnO nanoparticles provokes a dramatic decrease in the light absorption from the surface to the bulk, the obtained result is not surprising and only reflects the dilution of the oxidized layer by the unoxidized layers in the bulk. Erroneous conclusions can be given in terms of the photocatalytic effect of ZnO on the basis of such experiments.41 The results provided in the present paper indicate that there are two antagonistic effects of ZnO (inner filter effect and photocatalytic effect), resulting in a more accentuated oxidation of PLA/ZnO nanocomposites. In other words, the ZnO nanoparticles display a photocatalytic effect that is predominant compared with the protecting effect resulting from the inner filtering effect of ZnO. 3.3. Cross-Sectional Study. To determine the light absorption profile resulting from the UV inner filtering effect of ZnO, a cross-sectional analysis was performed to characterize the distribution of the oxidation photoproducts in the bulk of the irradiated samples. Figure 7 presents the photooxidation profile of the various PLA nanocomposites obtained from the cross-sectional analysis of films with a thickness of ca. 140 μm, using IR microspectroscopy. The increase in absorbance at 1845 cm−1 was chosen for quantifying the PLA oxidation. The experiments were performed for the different PLA/ZnO

Figure 7. (a) Oxidation profiles (absorbance at 1845 cm−1 as a function of film thickness) in the PLA/ZnO nanocomposites for different ZnO contents. (b) Amount (%) of absorbed light at 350 nm versus distance from the exposed surface for PLA and PLA/ZnO nanocomposites (calculated from the UV absorption spectra given in Figure 3).

nanocomposite samples that were exposed during different times, which were chosen to obtain the same oxidation extent: ∼ 0.8 at 1845 cm−1 (100 h for PLA 3, 100 h for PLA 2, and 500 h for PLA 1 and PLA 0.5). For the PLA0, because the band at 1845 cm−1 did not reach 0.8 in absorbance even at the longest irradiation time, the oxidation profile was performed after 500 h of irradiation (Abs ∼0.18). One can observe in Figure 7a that ZnO has a strong UV-light screening effect at a concentration above 1% in the PLA matrix. Indeed, photooxidation profiles are clearly marked in the presence of ZnO. The thickness of the oxidized layer decreases with increasing amounts of ZnO and is only ∼80 μm when the concentration of ZnO nanoparticles reaches 2%. The oxidation is dependent on both the ZnO content, which controls the photocatalytic effect, and the penetration of light into the film. It is then worthy to compare the results obtained for the different samples taking into account the penetration of the light. The light absorption profiles can be determined by the application of the Beer−Lambert law based on the data provided in Figure 2, which presented the absorption spectra of the different samples. The profiles of absorbed light were calculated for the different samples and reported in Figure 7b, which presents the profiles of absorbed light at 350 nm. These curves provide evidence of the screening effect of ZnO. For PLA0.5 and PLA1, one observes a weak profile with 15 and 10% absorption, respectively, at 140 μm from the surface. For PLA2 and PLA3, one observes that the light is completely absorbed in the first 120 and 80 μm, respectively, from the exposed surface. For the PLA0, because of the very weak absorbance, there is no absorption profile. The comparison of Figures 6 (kinetic curves) and 7 (absorption profiles and the 3289

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4. CONCLUSIONS This work focused on the photooxidation mechanism of PLA and the effects of ZnO nanoparticles on the photooxidation of PLA. A comprehensive mechanism was proposed based on the identification of the macromolecular and low-molecular-weight photoproducts. We have shown that the photocatalytic effect of ZnO, which is useful for antibacterial activity producing highly reactive oxidant products, plays a prodegradant role during the oxidation of the PLA matrix. This prodegradant role arises from the generation of active species of ZnO that can initiate the radical oxidation of PLA. PLA/ZnO nanocomposites are responsible for the marked heterogeneities in the distribution of the oxidation photoproducts in the polymer bulk. From the comparison between the measured photooxidation profiles and the absorbed light profiles, we have shown that degradation is governed by light absorption. In conclusion, our results show that the degradation rates of the PLA-ZnO nanocomposites are strongly dependent on the ZnO content, which plays a key role both in the photocatalytic effect and in the UV-screen effect and, as a consequence, also in the distribution of the photoproducts into the polymer bulk. Further investigations on the influence of ZnO nanoparticles on the surface modifications of PLA/ZnO nanocomposites films are currently being performed using AFM analyses and will be reported in a forthcoming paper.

distribution of the oxidation products) helps to understand the effects of ZnO nanoparticles on the photooxidation of PLA. The results indicate that at first the presence of ZnO nanoparticles increases the rate of photooxidation. This increase results from the photocatalytic activity of ZnO. The oxidation rate increases with increasing concentration. Simultaneously, increasing the amount of ZnO results in the oxidation becoming increasingly superficial, which is a result of the inner filtering effect of ZnO that prevents light from reaching the bulk of the sample. The global effect results from the competition between these two effects that have opposite consequences. Understanding the global effect requires that the distribution of the chemical modifications produced by exposure to light are monitored. Otherwise, one could misinterpret the experimental results; for example, by diluting the oxidation at the surface with the bulk layers that are not reached by light. The curves provided in Figure 7a intersect at a depth that is ∼40 μm. For the samples with the lowest amounts of ZnO (0.5 and 1%), oxidation occurs in the deeper layers, which is not the case for the samples with 2 and 3% ZnO because these layers are not accessible to incident light. For the PLA0 sample, the extent of oxidation is very limited, which once again confirms that the oxidation is induced by the photocatalytic effect of the ZnO nanoparticles. To summarize these results, it appears that the ZnO nanoparticles have a strong prodegradant effect on PLA that arises from the photocatalytic activity of ZnO but in turn have a protective effect on the bulk layers. This effect could be generalized to other classes, such as polyolefins. This suggests that one has to be careful when analyzing the behavior of nanocomposites containing ZnO (or TiO2) because the modifications of the bulk do not reflect those at the surface. Moreover, considering the entire thickness for the thick samples results in erroneous conclusions. The cross-sectional analyses can be completed from the IR characterization of the modifications that occur at the surface by using IR in ATR. In this case, we monitored the extent of the oxidation at a constant irradiation time, which was 300 h. The results are reported in Figure 8. As expected based on the results presented above, the oxidation at the surface was considerably greater when the amount of ZnO was increasing. This was observed for both crystals: the diamond one that approximately measures the first two micrometers (at 1845 cm−1) and the germanium one that measures the first micrometer.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Alexis Colin (ICCF Eq. photochimie) and Cécile Esparcieux (ENSCCF) for SEC analysis. The authors from UMons & Materia Nova thank the Wallonia Region, Nord-Pas de Calais Region and European Community for the financial support in the frame of the INTERREG − NANOLAC project. They thank also to NANOLAC partners and all mentioned companies for supplying raw materials. CIRMAP acknowledges supports by the Région Wallonne in the frame of OPTI2MAT program of excellence, by the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 6/27) and by FNRS-FRFC.

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Figure 8. Absorbance at 1845 cm−1 after 300 h of irradiation for the different samples measured by ATR using germanium (left) and diamond (right) ATR crystals. 3290

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