Polylactic Acid and Polylactic Acid-Based Nanocomposite

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Polylactic Acid and Polylactic Acid-Based Nanocomposite Photooxidation Sergio Bocchini,* Kikku Fukushima, Alessandro Di Blasio, Alberto Fina, Alberto Frache, and Francesco Geobaldo Dipartimento di Scienze dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy Received June 17, 2010; Revised Manuscript Received September 16, 2010

The importance of photooxidation in promoting formation of anhydride functional groups and thus promoting hydrolysis/biodegradation of polylactic acid and PLA nanocomposites were elucidated. PLA-based nanocomposites were prepared by adding 5% wt filler content of sodium montmorillonite (ClNa), sodium montmorillonite partially exchanged with Fe(III) (ClFe), organically modified montmorillonite (Cl20A), unmodified sepiolite (SEP), and fumed silica (SiO2). The pure PLA and nanocomposites were UV-light irradiated in artificial accelerated conditions representative of solar irradiation (λ > 300 nm) at 60 °C in air. The chemical modifications resulting from photooxidation were followed by IR and UV-visible spectroscopies. The infrared analyses of PLA photooxidation show the formation of a band at 1845 cm-1 due to the formation of anhydrides. A photooxidation mechanism based on hydroperoxide decomposition is proposed. The mechanism proposed is confirmed by an increase in anhydride formation rate: the main responsible for this acceleration was identified as transition metals contained in the nanofillers as impurities and involved in the catalytic hydroperoxide decomposition.

Introduction Polylactic acid (PLA) has one of the most important positions on the market of biodegradable polymers due to various fields of application including a wide variety of biomedical products, food packaging, films for agro-industry, fiber production, and, as one of the last tendencies, as composites for technical durable applications, such as electronic and electrical devices and mechanical parts.1-3 PLA has reasonably good optical, physical, mechanical, and barrier properties compared to existing oil-based polymers.4-8 However, PLA properties are often not good enough for some demanding applications,2,9 such as in high barrier packaging and in transport, where a high level of mechanical, thermal, and flame retardant properties is required. The above drawbacks could be overcome or limited by the addition of nanosized particles.8,10 Various nanoreinforcements in polymer matrices (including PLA) are being developed, but the most extensively studied polymer nanocomposites so far are those based on layered silicate clays, due to their availability, low cost, and enhancements that they induce in mechanical, thermal, barrier, and flame retardant properties of the polymer matrix, in comparison to more conventional microcomposites.9,11-14 Several authors15-20 have reported the preparation and characterization of biodegradable polymer nanocomposites with modified and unmodified montmorillonites prepared by in situ polymerization, melt mixing, or solvent-casting methods, achieving remarkable thermal and mechanical improvements of the polymer properties observed even at nanofiller content as low as 3-5% wt. In the past few years, the impact of light on polymer/clay nanocomposites has been studied,21-29 being the resistance to UV-light is a key factor for most outdoor applications of * To whom correspondence should be addressed. E-mail: sergio. [email protected].

polymeric materials. It was shown that all the nanocomposites prepared with montmorillonite nanoclay in polypropylene,19,22,23 polyethylene,20 and polycarbonate21 display similar photodegradation behaviors under exposure to UV light. These nanocomposites degrade faster than the pristine polymers as a consequence of a significant reduction of the oxidation induction time (OIT). In a previous series of papers concerning the photooxidation of montmorillonite/polypropylene22,24 and nanoboehmite/polypropylene nanocomposites,26 several hypotheses were proposed to explain the oxidative degradation of these materials. Indeed, the reduction of the OIT may be attributed to the adsorption of antioxidants onto the nanofiller, to a catalytic effect of transition metal impurities of nanofillers, as well as to additional initiation routes due to the oxidation of the nanofillers organic modifiers. The aim of the present work is to study the photooxidation behavior of polylactic acid and the influence on it by the addition of both natural or organomodified layered clay (montmorillonite), unmodified needlelike clay (sepiolite), and fumed silica.

Experimental Section Materials. The poly(lactic acid), PLA 2002D (PLA), was commercial grade and purchased from NatureWorks (U.S.A.). Sodium montmorillonite, CLOISITE Na (ClNa), and modified montmorillonite, CLOISITE 20A (Cl20A), were purchased from Southern Clay (U.S.A.). Unmodified sepiolite, PANGEL S9 (SEP), was supplied by Tolsa, S.A (Spain). Hydrophilic fumed silica, HDKN20 (SiO2), was purchased by Wacker S.A. (U.S.A.). The characteristics of the nanoparticles used in this work are listed in Table 1. Montmorillonite partially exchanged with Fe(III) was prepared by dispersion of 3.5 g of ClNa in 350 mL at 80 °C; 150 mL of a solution 2 M of FeCl3 was added at the same temperature under vigorous stirring at 10 mL min-1 and the mixture was stirred for 24 h. The Fe(III) exchanged clay was filtered, washed until free from chloride ion as controlled using 0.01 M AgNO3, dried at 80 °C for 24 h, and ground

10.1021/bm1006773  2010 American Chemical Society Published on Web 10/13/2010

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Table 1. Characteristics of Nanoparticles Used

to a fine powder using a mortar and pestle. The Fe(III)-loaded MMT was designated as ClFe. Prior to the melt blending, polymer matrix was dried at 70 °C under vacuum for at list 4 h to a Karl Fischer titration moisture content below 190 ppm. Nanoparticles were dried at 100 °C under vacuum for 10 h to a Karl Fischer titration moisture content below 1000 ppm. Nanocomposites were obtained at 5% filler loading by melt blending using a Microextruder DSM Micro 15CC Twin Screw Compounder, with a mixing time of 5 min, at 180 °C, and in a nitrogen flow. The mixing was performed at two different rotor speeds: 60 rpm in the loading step and 100 rpm during mixing. Films (e40-50 µm) were prepared from pellets using a hot-plate hydraulic press at 190 °C using hydraulic pressure of 100 bar for 1 min. Film thickness was selected to allow oxygen saturation during film photo-oxidation. Specimens for XRD 30 · 30 · 1 mm3 were prepared by compression molding at 5 MPa, 190 °C for 2 min. Irradiations of specimens at λ > 300 nm were carried out in air in a SEPAP 12/24 unit at 60 °C. This apparatus is equipped with four medium-pressure mercury lamps with borosilicate envelope which filters wavelengths below 300 nm and it is designed for the study of polymer photodegradation in artificial conditions that are relevant to natural outdoor weathering.30 Photooxidation of PLA was followed by the intensity 1847 cm-1 peak υCdO of anhydride, which was plotted as a function of time (Figure 11); to avoid differences due to film thickness, it was normalized by dividing for the IR absorption band at 2997 cm-1 υC-H characteristic vibration stretching band of PLA. (asymmetric υ(CH3) band of PLA). Reactions of films with gaseous NH3 were performed in NH3 saturated atmosphere obtained under glass bell in presence of concentrated NH3 solution.

Characterization Techniques. Wide-angle X-ray spectra (WAXS) were recorded at room temperature in the range 1-30° (2θ; step size ) 0.02°, scanning rate ) 2 s/step) by using filtered Cu KR radiation (λ ) 1.54 Å). Scanning electron microscopy (SEM) was carried out on the cryogenic fracture surfaces of the 0.6 mm specimens previously coated by sputtering with gold, using a Leo 14050 VP SEM apparatus. UV-visible spectra (UV-vis) of films were recorded on a Shimadzu UV-2101 PC spectrometer equipped with an integrating sphere. Infrared spectra (FT-IR) of films were recorded using a Perkin-Elmer Spectrum GX Infrared spectrometer. Spectra were obtained using eight scans and a 4 cm-1 resolution.

Results and Discussion Morphology. The WAXS pattern of PLA (Figure 1) is characterized by a broad peak with maximum approximately at 2θ ) 17°, indicating a completely amorphous structure. WAXS of the polymer matrix was not significantly affected by the presence of the nanoparticles. ClNa is characterized by a main diffraction peak at 7.8° corresponding to an interlayer distance (d001) of 1.1 nm. In the case of ClNa-based material there are no significant variations of the main clay diffraction peak, indicating the absence of an intercalated structure. Scanning electron microscopy (Figure 2a) reveals a low degree of dispersion of ClNa in the polymer matrix, showing large number of silicate agglomerates by about 10-20 µm on the surface fracture sample (encircled particles in Figure 2a). Thus, there is low interaction between PLA and ClNa as previously reported in the literature.31

Figure 1. WAXS patterns of (a) ClNa, PLA, and nanocomposites PLAClNa and (b) Cl20A, PLA, and nanocomposites PLACl20A. Y-axis shifted spectra.

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Figure 2. Scanning electron micrographs for (a) PLAClNa, (b) PLACl20A, (c) PLASiO2, and (d) PLASEP. Table 2. Band Assignments of PLA (cm-1) assignment νasCH3 νsCH3 νCH νCO δasCH3 δsCH3 δ1CH + δsCH3 δ2CH Figure 3. UV spectra of PLA and nanocomposites.

Cl20A is characterized by a diffraction peak at 2θ ) 3.5° corresponding to an interlayer distance (d001) related to the presence of bulky organic modifier molecules between clay layers and accounting for a d001 ) 2.5 nm. When Cl20A is blended in PLA a shift of the main clay diffraction peak to lower angles is observed (Figure 1), corresponding to an increase of the interlayer distance of 1.3 nm, indicating the formation of an intercalated structure. SEM analyses showed no micrometric clay aggregates in PLACl20A (Figure 2b), confirming improved compatibility with the polymer matrix. The dispersion of fumed silica nanoparticles and needleshaped sepiolite in the PLA matrix was assesses by SEM only, provided that X-ray diffraction does not deliver any information, owing to the lack of periodic stacking for these nanoparticles. PLASiO2 and PLASEP (respectively, Figure 2c and d) present fair nanoparticle dispersion, despite some residual aggregates by about 3-10 µm were observed on the sample surface (circles in Figure 2c,d). Figure 3 shows the UV-vis spectra of pure PLA and nanocomposites. For PLA there is a saturation of the spectra below about 230 nm due to the absorbance of PLA ester groups. The absorption is not significantly affected by the presence of fumed silica (PLASiO2), whereas, in the presence of sepiolite and montmorillonite, nanocomposites show an absorbance below 300 nm that is related to the presence of transition metals impurities such as iron.32 Sepiolite also shows an absorbance at higher λ that could be attributed to the haze arising from the scattering of light as it encounters regions with different refractive index such as micrometric filler particles.33 The haze is reduced when particles are small34 and refractive indices are

assignment 2997 2945 2881 1759 1454 1348-1384 1368-1360 1315-1300

δCH + νCOC νasCOC rasCH3 rasCH3 νsCOC νsC-CH3 rCH3 + νCC νC-COO δCO

1265 1209-1186 1132 1090 1046 960-925 870 756

similar. Indeed, despite the presence of similar micrometric aggregates, this effect is not observed with SiO2, owing to the refractive index (≈1.4635) very similar to that of PLA (≈1.4536), whereas refractive index of sepiolite is higher (≈1.50).37 Infrared Characterization. The peak assignments for neat PLA are shown in Table 2, accordingly with values provided in the literature.38 The IR spectrum of the different nanocomposites (Figure 4) corresponds to those expected from blending of the nanofillers with PLA. However, the main absorbance band of nanofillers (Si-O-Si stretching at about 950-1050 cm-1) is overlapped and often covered by the saturation of the PLA bands. In PLAClNa, the bands that are readily recognizable are 520 and 463 cm-1 that are due to absorbance of, respectively, the Si-O-Si and Si-O-Al bending.39 In PLACl20A these two bands are more evident and at the same time it is possible to recognize the νasCH2 and νsCH2 of the organic modifier,40 respectively, at 2922 and 2853 cm-1. In the case of PLASEP, the main difference is a structured band of the inorganic part with a maximum at 983 and 469 cm-1, respectively, to Si-O-Si stretching and bending.35,41 Finally, PLASiO2 present a large band at about 472 cm-1 Si-O-Si bending.35 PLA Photooxidation. The infrared analysis of PLA photooxidation shows the formation of a band with a maximum at 1845 cm-1 (Figure 5), attributed to the formation of anhydride groups.42 In the hydroxyl region, a broad absorption band is observed with a maximum at 3400 cm-1 that corresponds to products such as hydroperoxides or alcohols. Anhydride presence was confirmed by exposition of the photooxidated film to NH3 saturated atmosphere. Infrared analysis of exposed films shows the disappearance of the 1845

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Figure 4. FTIR spectrum of PLA and PLA-based nanocomposites. Y-axis shifted spectra.

Figure 5. FTIR spectrum of PLA versus photooxidation time.

Figure 6. FTIR spectrum of PLA after 670 h of photooxidation, before (gray) and after the reaction (black) with gaseous NH3.

Figure 7. Reaction of anhydride with ammonia.

cm-1 band of anhydride and the formation of a peak at 1624 cm-1 and a shoulder at about 1670 cm-1 (Figure 6) that can be ascribed to the formation of carboxylic salt and primary amide group on the basis of the well-known reaction between amines and anhydrides (Figure 7). The photodegradation of PLA was previously described in literature by Ikada,43 proposing a Norrish II mechanism of carbonyl polyester (Figure 8). However, significant difference in the used UV spectra applies with the present paper. Indeed, Ikada et al. used a UV light source having high emission starting

from 220 nm, thus, in a region where the carbonyl groups of aliphatic polyester can absorb energy and consequently lead to photoreaction. In the present paper the UV light wavelengths below 300 nm were filtered to simulate natural outdoor exposure. Few other studies of photodegradation were reported in literature.44,45 Copinet et al.40 found that UV irradiation promotes molecular weight reduction in moist environment (50% RH), but without investigating the mechanism; however, in these conditions it is likely that photodegradation involves anhydride groups, which are easily decomposed in carboxylic acids by hydrolysis. Belbachir et al.41 confirmed the molecular weight reduction moreover the photodegradation leads to a decrease of mechanical properties. In particular, a loss of stiffness and strength was observed.



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Figure 8. Norrish II reaction for PLA acid.

Figure 9. Radical oxidation process of irradiated PLA samples: Hydroperoxide chain propagation and formation of anhydrides by photolysis of hydroperoxide.

To the best of the authors’ knowledge, the formation of anhydride groups was not reported before: a photooxidation radical mechanism of PLA, with hydroperoxide as intermediates, is proposed here (Figure 9). Photooxidation usually begins by radical formed from impurities by UV-irradiation or thermal decomposition. The reaction with higher probability is the abstraction of tertiary hydrogen from PLA chain with the formation of a tertiary radical P• (1). This radical can react with oxygen to form a peroxide radical (2), which may easily abstract another hydrogen from a tertiary carbon with the formation of an hydroperoxide and the initial radical P• (3). Then, the hydroperoxide undergoes photolysis (4) with the formation of the HO• and a PO• radical that can further evolve by β-scission (5). Taking into account the stability of the different fragments the most probable β-scission appears to be the (5b) reaction, leading to the formation of anhydride groups. Nanocomposites Photooxidation. Different oxidation rates are measured on PLA-based nanocomposites, depending on the type of nanofiller. For all nanocomposites the appearances of the 1845 cm-1 band and of a broad band in the 4000-3000

cm-1 region are confirmed. The addition of nanofillers without organic modifier does not induce significant qualitative changes in the infrared spectra evolution, compared to PLA. On the other hand, in PLACl20A the CH2 stretching bands at 2922 and 2853 cm-1 assigned to the organic modifier completely disappear in the first hours of oxidation (Figure 10), evidencing that the organic modifier of Cl20A is easily photodegraded. Moreover, after the first 170 h of irradiation, PLACl20A films were cracked, whereas no significant damages were observed for reference PLA and other PLA composites. Provided that no evidence of low photostability of alkyl chains in onium salts are reported in literature, this effect is ascribed to a catalytic effect of montmorillonite, possibly linked to the presence of transition metals in the clay structure. The rapid decomposition of the organic modifier accounts for a photodegradation activity of montmorillonite surface therefore it is obvious to predict a higher oxidation rate for Cloisite containing nanocomposites such as PLAClNa and PLACl20A. The rate of formation of anhydride is approximately constant vs time and strictly depends on the type of nanofiller, the ranking

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Figure 10. FTIR spectrum of PLACl20A vs photooxidation time.

Figure 11. Evolution of 1845 cm-1 anhydride peak vs time.

Figure 12. Photocatalytic decomposition of hydroperoxides by transition metal ions: Transition metal ion catalyzes the hydroperoxide decomposition.

in oxidation rates of the different samples being PLACl20A > PLAClNa > PLASEP > PLASiO2 > PLA (Figure 11). These differences can be explained taking into account the difference between the filler, that is, chemical composition, UV-vis absorbance, and dispersion. PLASiO2 shows photooxidation rate quite similar to that of PLA, having a low absorbance in the UV and visible range and no metal impurities. The higher rate of oxidation for PLASEP may be ascribed to the higher UV-vis spectra absorbance compared to PLASiO2, as well as to the presence of transition metals impurities in the natural mineral, especially iron as previously reported.46 The catalytic effect of metallic compounds has been widely reported and is well-known.47 Metal ions can cause an acceleration of the oxidation of polymers by various processes including the decomposition of hydroperoxides (Figure 12). This effect of iron impurities is, thus, likely to apply in PLA decomposition by increasing the decomposition rate of hydroperoxide, thus, accelerating the overall radical process. Catalytic effects appear to play the most important role in PLA nanocomposites containing montmorillonite, which is supposed to have a higher amount of transition metal impurities or at least more accessible to PLA macromolecular chains, owing to the higher polymer contact surface area. The importance of catalytic sites accessibility is evidenced by the

difference between PLAClNa and PLACl20A. Indeed, the better dispersion in PLACl20A results in a higher interface surface, thus, the overall photodegradation efficiency of transition metal impurities is increased. An additional contribution to the higher oxidation rate for PLACl20A compared to PLAClNa is in the initiation activity of catalyzed decomposition of organic modifier. It constitutes a supplementary source of radicals that are likely to initiate the oxidation of PP, leading to an increase of the overall rate. To confirm the catalytic role of transition metal impurities, a sample of PLA containing ClFe, the montmorillonite enriched in Fe(III) content, was prepared (PLAClFe) at the same inorganic filler content of PLAClNa. Both samples were exposed to photooxidation. The rate of formation of anhydride is approximately constant versus time for both samples. The oxidation rates of the sample containing ClFe is higher than the sample containing ClNa, as reported in Figure 13. In this case, the increase in oxidation rate can be directly ascribed to the increase in transition metal impurities concentration. Similar results at photodegradation behavior were obtained using transition metal ions and a prooxidant agent by Berthe´ et al.48

Conclusions The main degradation mechanism of PLA exposed to photooxidation under outdoor-representative conditions was investigated and showed polymer degradation by the formation of anhydride groups. A mechanism based on β-elimination from hydroperoxides formed on the tertiary carbon atom of PLA is proposed. The rapid consumption of a clay organic modifier is the first direct evidence of the photodegradation activity of



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(17) Figure 13. Comparison between PLAClNa and PLAClFe: Evolution of 1845 cm-1 anhydride peak vs time.

montmorillonite in the polymer matrix. An increase in the rate of anhydride formation is induced by addition of nanofillers, suggesting a radical mechanism on the basis of a hydroperoxide decomposition catalytic effect of the transition metals present in the nanofiller as impurities. The photooxidation conducted on sample addicted with Fe(III) confirms the role of transition metals on PLA photooxidation. The influence of nanofillers must therefore be taken into account for durable outdoor applications of PLA nanocomposites. On the other hand, when the environmental biodegradability is desired the addition of nanofillers could help in accelerate PLA decomposition. Acknowledgment. The authors thank Prof Giovanni Camino of Politecnico di Torino (Politecnico di Torino sede di Alessandria) for the feedback and useful comments.

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Polylactic Acid and Polylactic Acid-Based Nanocomposite

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