Migration and Hydrolysis of Hydrophobic Polylactide Plasticizer

Nov 23, 2009 - with hydrophobic acetyl tributyl citrate (ATC) plasticizer showed a slower ... immersion of the materials, and the relative abundance o...
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Biomacromolecules 2010, 11, 277–283

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Migration and Hydrolysis of Hydrophobic Polylactide Plasticizer Anders Ho¨glund, Minna Hakkarainen, and Ann-Christine Albertsson* Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, S-100 44, Stockholm, Sweden Received October 9, 2009; Revised Manuscript Received November 4, 2009

Hydrophobic plasticizer protects polylactide (PLA) against hydrolytic degradation but still migrates to aging medium and there undergoes further hydrolysis contributing to the spectrum of degradation products. PLA plasticized with hydrophobic acetyl tributyl citrate (ATC) plasticizer showed a slower degradation rate compared with pure PLA because of the increased hydrophobicity of the material. The enhanced bulk hydrophobicity also overcame the degradation enhancing effect of hydrophilic surface grafting. In addition to plasticization with ATC, some of the samples were also surface grafted with acrylic acid. The materials were subjected to hydrolysis at 37 and 60 °C for up to 364 days to compare the effect of hydrophobic and hydrophilic bulk and surface modifications. Although considered insoluble in water, the plasticizer was detected in the water solutions immediately upon immersion of the materials, and the relative abundance of the ATC degradation products increased with hydrolysis time.

Introduction Controlling the degradation rate and release of degradation products and other migrants are key issues during design of degradable polymers. Among the most interesting degradable materials is polylactide (PLA), which is also derivable from renewable resources. PLA has, mainly because of its high cost, historically been used primarily in biomedical applications, where it has proven good biocompatibility and favorable mechanical properties.1 Continuous progress in the manufacturing methods has, however, led to economically feasible production of PLA for packaging applications as well.2 The homopolymer of PLA has some drawbacks compared with the traditional polyolefins, for example, somewhat inferior barrier properties and inherent brittleness, which could be an issue in some cases. The possibility of synthesizing copolymers of PLA with controlled composition and advanced architectures has provided a variety of materials with specific properties and predetermined degradation rates.3,4 Modification of PLA with different functional groups and incorporation of drugs has also been demonstrated to effect the degradation rate.5,6 We have in previous work shown the effect of copolymer composition and macromolecular architecture on the release of monomeric and oligomeric degradation products.7-10 We have also shown that surface modification affects the degradation process and the degradation product patterns.11 However, despite the versatility provided by copolymerization and surface modification, these techniques are not always sufficient to fulfill all demands. PLA suffers from the drawback of being rigid and hard. Plasticization is a common method to improve the flexibility of a polymer and overcome inherent brittleness and processing limitations. In a previous work, we showed migration and hydrolysis of polyester-based plasticizers from PVC.12,13 The effect of citrate ester plasticizers on the PLA material properties has been previously studied, and there are also a few reports on the hydrolytic degradation of these materials.14,15 However, no attention has been paid to the lowmolar-mass products formed during degradation and especially * Corresponding author. Tel: +46-8-790 82 74. Fax: +46-8-20 84 77. E-mail: [email protected].

the migration of the plasticizer and its further fate in the surrounding medium. The aim of the present work was therefore to reveal the water-soluble migrant patterns of modified PLA during hydrolysis. Our hypothesis was that hydrophobic plasticizers, despite their low water solubility, migrate from the polymer matrix to the surrounding water, where they subsequently hydrolyze and thereby contribute to the spectrum of degradation products. To confirm this, PLA was plasticized with acetyl tributyl citrate (ATC), and some of the samples were surface grafted with acrylic acid (AA), and the materials were thereafter subjected to hydrolysis in deionized water for up to 364 days. The water-soluble product patterns after different time periods were analyzed by electrospray ionization mass spectrometry (ESI-MS), and mass loss, molar mass changes, and changes in thermal properties of the materials were determined in parallel.

Experimental Section Materials. PLA pellets (95% L-LA, 5% D-LA) were used as received. Natureworks PLA was chosen to obtain a homogeneous and reproducible material and to make the results comparable to previous work.11,16 PLA pellets (∼8 g) were dissolved in 200 cm3 chloroform (Fischer Scientific, HPLC grade) and subsequently solution-casted into films on silanized glass molds. The solvent was allowed to evaporate, after which the films were dried for 1 week at reduced pressure. We obtained plasticized PLA films with 10% (w/w) plasticizer in the same way by dissolving 7.2 g of PLA pellets and 0.8 g of ATC (g99%, Fluka). Specimens in the form of circular discs with a diameter of 10 mm and a thickness of ∼200 µm were punched from the films, washed with ethanol (99.5%), and dried under vacuum prior to grafting or degradation. AA (99.5%, Acros) was distilled under vacuum just before use and stored cold. Benzophenone (99%, Alpha Aesar) was used as received. Vapor-Phase Grafting. The grafting procedure has been described in detail elsewhere.17 In brief, a glass reactor with two interconnected cylindrical compartments was used in the grafting process. The PLA substrate specimens were placed in one of the reactor compartments, and AA monomer and benzophenone (M/I ) 10:1) were added in the other reactor compartment. The reactor was evacuated and thereafter filled with nitrogen according to a freeze-pump-thawing procedure

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Table 1. Polymer Properties before Hydrolysis Including Number-Average Molar Mass, Polydispersity, Melting Temperature, Glass-Transition Temperature, and Degree of Crystallinity sample

Mn

PLA PLA-ATC PLA-AA PLA-ATC-AA

100 800 ( 1900 92 800 ( 2800 72 500 ( 3000 67 600 ( 3500

PDI 1.68 1.76 1.66 1.76

( ( ( (

0.03 0.02 0.06 0.06

that was repeated three times. After the last evacuation, the reactor was sealed and immersed into a water bath at 35 or 40 °C for the plasticized and nonplasticized PLA samples, respectively. Following an equilibration time of 5 min, the reactor was irradiated with UV light for 20 min. The entire procedure was repeated after the films were turned over to obtain grafted layers on both sides of the sample substrates. The surface-grafted PLA samples were thereafter thoroughly washed with deionized water and ethanol (99.5%) and dried under reduced pressure prior to hydrolysis. Hydrolysis. Four different PLA samples, pure PLA (PLA), plasticized PLA (PLA-ATC), acrylic acid-grafted PLA (PLA-AA), and plasticized acrylic acid-grafted PLA (PLA-ATC-AA), were subjected to hydrolytic degradation in deionized water at 37 and 60 °C. Samples were hydrolyzed in 20 mL vials containing 10 mL of water. Deionized water was chosen over a phosphate-buffered solution as degradation medium because salts are known to be detrimental to the sensitive components of the ESI instrument mass analyzer. The sample vials were sealed with septa and placed in a thermostatically controlled incubator at 37 °C and 60 rpm rotation or in an oven at 60 °C. After different time periods between 1 and 364 days, duplicate samples of each material were withdrawn from the test environment, dried under reduced pressure, and analyzed by various techniques. In addition, the water-soluble products in the sample solutions were analyzed after each hydrolysis time. Electrospray Ionization Mass Spectrometry. The water-soluble products were analyzed with a Finnigan LCQ ion trap mass spectrometer (Finnigan, San Jose, CA). Methanol (Fischer Scientific, super gradient) was added to the samples (2:1 v/v), and the solutions were continuously infused into the ESI ion source at a rate of 5 µL/min using the instrument syringe pump. The LCQ ion source was operating at 5 kV, and the capillary heater was set to 175 °C. Nitrogen was used as nebulizing gas, and helium was used as damping gas and collision gas in the mass analyzer. Positive ion mode was used for all analyses. Size Exclusion Chromatography (SEC). The molar mass of the polymers was determined by SEC. Chloroform (Fischer Scientific, HPLC grade) with 5% methanol (v/v) was used as the eluent at a flow rate of 1.0 mL/min, and the injection volume was 50 µL. The instrument comprised a Waters 717 Plus autosampler and a Waters model 510 solvent pump equipped with a PL-ELS 1000 light scattering evaporative detector and three PLgel 10 µm mixed B columns (300 × 7.5 mm) from Polymer Laboratories. Calibration was performed with narrow molar mass polystyrene standards, and Millennium software version 3.20 was used to process the data. Mass Loss. Samples were withdrawn from the test environment after different hydrolysis times, washed with deionized water, and gently wiped with a tissue. The mass loss was determined after the samples were dried for 2 weeks under reduced pressure (0.5 × 10-3 mbar) and the dry mass (md) was compared at a specific time with the initial mass (m0) according to eq 1

∆md )

m0 - md × 100 m0

(1)

Differential Scanning Calorimetry (DSC). The thermal properties of the materials were determined with a DSC (Mettler Toledo DSC 820 module) under nitrogen atmosphere. Approximately 5 mg of the polymer was encapsulated in a 40 µL aluminum crucible without pin. Samples were heated from 0 to 200 °C at a rate of 10 °C/min under a nitrogen gas flow of 50 mL/min. Then, the samples were cooled from

Tm (°C) 144.0 146.5 147.9 144.0

( ( ( (

Tg (°C)

1.1 0.5 1.7 2.0

49.4 36.2 53.9 46.9

( ( ( (

0.3 1.0 1.6 3.6

wc (%) 24 24 24 26

( ( ( (

4 2 6 3

200 to 0 °C at a rate of 10 °C/min before being heated again from 0 to 200 °C at a rate of 10 °C/min. The melting temperatures, Tm, were noted as the maximum values of the melting peaks from the first heating scan. The glass-transition temperature, Tg, was taken at the midpoint temperature of the glass transition. The approximate degree of crystallinity of the PLA samples was calculated according to eq 2

wc )

∆Hf ∆Hf0

× 100

(2)

where wc is the degree of crystallinity, ∆Hf is the heat of fusion of the sample, and ∆Hf0 is the heat of fusion of 100% crystalline polymer. The value for ∆Hf0 was 93 J/g.18

Results and Discussion We assessed the effect of plasticization and surface grafting on the degradation rate and the release of water-soluble products from PLA by analyzing samples and the sample solutions during hydrolytic degradation in deionized water for up to 364 days. The graft yield was ∼5%, and successful grafting was verified by Fourier transform infrared spectroscopy (FTIR), contact angle measurement, scanning electron microscopy (SEM), and atomic force microscopy (AFM).11 The materials used and their properties prior to degradation are presented in Table 1. Previous studies have shown that the molar mass of PLA films are not significantly affected during vapor phase grafting, regardless of the grafting time and the presence of grafting monomer.17 The AA-surface grafted PLA chains were, however, no longer soluble in chloroform, which could have influenced the molar mass values of PLA-AA and PLA-ATC-AA. The polymer names are denotations of their respective modifications, for example, PLA-ATC-AA is plasticized and surface modified PLA. Analysis of Water-Soluble Migrants. The water-soluble migrants in the water fractions after different hydrolysis times and at different degradation temperatures were monitored by electrospray ionization mass spectrometry (ESI-MS) to determine the effect of plasticization, hydrolysis time, and temperature on the degradation product patterns. The water-soluble degradation products released from pure and surface grafted PLA have been described in detail in previous work.11 In general, water-soluble lactic acid oligomers were detected for pure PLA after 28 days of hydrolysis at 60 °C and after 133 days of hydrolysis at 37 °C. In the case of surface-grafted PLA, grafted oligomers were detected already after 1 day at 60 °C and after 7 days at 37 °C, and nongrafted oligomers were observed after 1 day at 60 °C and after 28 days at 37 °C. Moreover, the degradation product patterns of surface-grafted PLA showed significant variation with hydrolysis time with the formation of short and long AA-grafted lactic acid oligomers as well as plain lactic acid oligomers. Migration of Water-Soluble Products from Plasticized Polylactide. Figure 1 shows the positive ESI-MS spectra of the compounds that had migrated from plasticized PLA after 28 days of hydrolytic degradation at 60 °C in the mass range m/z 150-1000.

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Scheme 1. Illustration of Possible Hydrolysis Reactions for Acetyl Tributyl Citratea

a

Figure 1. Positive ESI-MS spectrum of the migrants from PLA plasticized with acetyl tributyl citrate after 28 days of hydrolytic degradation at 60 °C in the mass range m/z 150-1000.

Figure 2. Positive ESI-MS spectrum of the migrants from PLA plasticized with acetyl tributyl citrate after 1 day of hydrolytic degradation at 60 °C in the mass range m/z 150-1000.

This mass spectrum is similar to the corresponding mass spectrum of pure PLA.11 The main series of peaks, marked LAn, correspond to water-soluble lactic acid oligomers with n repeating units terminated by carboxyl and hydroxyl end groups. These peaks appear at m/z ) (1 + n × 72 + 17 + 23), and their general chemical structure is shown in Figure 1. Oligomers with up to 12 repeating units were observed in the spectrum, which is in correlation with previously reported results after the hydrolysis of pure PLA.11 In addition, three other distinct peaks at m/z 403, 425, and 827 were seen, which were not visible in the spectra from pure PLA. These peaks were detected already after 1 day of degradation at both 37 and 60 °C, as illustrated in Figure 2. In this stage, no water-soluble PLA oligomers were detected because of the short degradation time. The observed peaks instead originate from the added plasticizer ATC. ATC has a molar mass of 402.5 g/mol, and the additional peaks therefore most likely correspond to ATC (m/z 403), ATC with attached sodium (m/z 425), and two ATC molecules with attached sodium (m/z 827). The hydrophobic nature of the plasticizer probably favors the formation of noncovalently bound pairs of molecules in polar solvents. The formation of pairs of intact ATC

Top route may occur on all three butyl ester groups.

molecules was confirmed by GC-MS and 13C NMR, which showed two separate peaks with similar mass spectra in the GC chromatogram and intact C-C bonds, respectively (data not shown). Furthermore, the low energy involved in the electrospray ionization allows intact complexes to be directly detected in the mass spectrometer.19,20 Parallel ESI-MS on the related plasticizer acetyl triethyl citrate showed the exact same behavior with the formation of molecular pairs in both MeOH and MeOH/ H2O. The appearance of the peaks corresponding to the plasticizer already after 1 day of degradation clearly shows the migration of the plasticizer without any significant degradation of the bulk material. Moreover, two additional peaks with lower intensity and an m/z difference of 42 were also seen in Figure 2. These peaks indicate that some hydrolysis of the plasticizer had occurred. Scheme 1 illustrates possible hydrolysis pathways for ATC. The peaks at m/z 361 and m/z 785 in Figure 2 correspond to the products formed after hydrolysis of the ATC acetyl group with the formation of acetic acid according to the bottom route in Scheme 1. It is interesting to note that the plasticizer started to migrate from the material more or less immediately upon immersion into water and that hydrolysis of the ester arms was directly initiated. We confirmed the susceptibility of ATC toward hydrolysis in water by analyzing the pure plasticizer in a parallel control experiment. Pure ATC was treated in the same manner as during the film preparation and was dissolved in methanol, and this solution, with and without the addition of water (MeOH/ H2O 2:1), was subsequently analyzed by ESI-MS. Peaks corresponding to the ATC hydrolysis products were only detected in the case of the MeOH/H2O solution, which confirms that the hydrolysis of the ester arms was induced neither during contact with CHCl3 or MeOH nor during the ionization in the ESI instrument. When the hydrolysis time was increased, the migrated ATC was further hydrolyzed according to Scheme 1. This is shown in Figure 3, which shows the positive ESI-MS spectra of the migrants from plasticized PLA after 182 days of hydrolytic degradation at 60 °C in the mass range m/z 150-1000. In this stage, the hydrolysis products of ATC were dominating the ESI-MS spectrum. The peaks with the highest intensity correspond to products formed after hydrolysis of the ATC acetyl group (∆m/z ) 42). Moreover, two series of peaks with a mass-to-mass peak decline of 56 were also observed. These peaks correspond to products formed after the hydrolysis of various numbers of butyl ester groups with the formation of butanol (top route in Scheme 1). In addition, pure lactic acid oligomer peaks were also observed in the mass spectrum, which also showed degradation of the polymer matrix in this stage. The further hydrolysis of ATC with degradation time and the

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Figure 3. Positive ESI-MS spectrum of the migrants from PLA plasticized with acetyl tributyl citrate after 182 days of hydrolytic degradation at 60 °C in the mass range m/z 150-1000.

Figure 5. Positive ESI-MS spectrum of the migrants from acrylic acidgrafted PLA plasticized with acetyl tributyl citrate after 182 days of hydrolysis at 60 °C in the mass range m/z 200-2000.

Figure 4. Positive ESI-MS spectrum of the migrants from acrylic acidgrafted PLA plasticized with acetyl tributyl citrate after 7 days of hydrolytic degradation at 37 °C in the mass range m/z 150-2000.

increase in its hydrolysis products were also confirmed by analyzing pure ATC aged in water for ∼10 months at 60 °C with ESI-MS. Although a quantification of the amount of the respective degradation products was not performed, it is possible to conclude that the ATC plasticizer migrates from the PLA matrix and subsequently undergoes hydrolysis in the surrounding water. This, in turn, entails a large difference in the degradation product patterns of pure and plasticized PLA. Despite the fact that migration of the plasticizer occurred, the plasticizer also protected the material toward degradation leading to lower total mass loss. (See the Mass Loss section.) Migration of Water-Soluble Products from Acrylic Acid-Grafted and Plasticized PLA. The water-soluble product patterns of the acrylic-acid-grafted and plasticized PLA were quite different from the product patterns of pure and plasticized PLA. Figure 4 shows the ESI mass spectrum recorded from the water fraction of AA-grafted and plasticized PLA after 7 days of degradation at 37 °C.

In addition to the ATC plasticizer, peaks corresponding to water-soluble lactic acid oligomers with grafted AA, marked LAnAA, were also observed. The suggested general chemical structure of these oligomers is shown in Figure 4. In the case of pure and plasticized PLA, water-soluble oligomers were not detected until after 133 days of degradation at 37 °C. Therefore, there was a large difference in the degradation process and products of grafted and nongrafted plasticized PLA. The AAgrafted oligomers are more hydrophilic than the nongrafted oligomers and therefore migrate easier and faster to the surrounding water solution. This has also been shown in a previous study of pure PLA and AA-grafted PLA.11 In addition, some longer AA-grafted oligomers are also visible in the higher mass range. These peaks probably correspond to longer AAgrafted lactic acid oligomers that are water-soluble because of the increased hydrophilicity from the AA units. An assignment of the degradation products corresponding to each of these peaks was not performed because the chemical structure of the grafted surface layer results in the formation of multiply charged ions and ions with several sodium atoms in the ESI-MS. This, in turn, leads to complex mass spectra where each hydrolysis product can induce multiple peaks corresponding to ions with different charges and a different number of sodium atoms.11 When the hydrolysis time and hydrolysis temperature were increased, the intensity of these peaks increased in the mass spectra. This is illustrated in Figure 5, which shows the ESI mass spectrum recorded from the water fraction of grafted and plasticized PLA after 182 days of degradation at 60 °C. Prolonged degradation of the surface-grafted and plasticized PLA induced a shift in the water-soluble product patterns. After 182 days of hydrolysis at 60 °C, the longer AA-grafted LA oligomers dominated the mass spectrum, although peaks corresponding to the ATC plasticizer and its hydrolysis products (ATC′) were still clearly discerned. In summary, plasticizing with ATC, surface grafting with AA, and degradation temperature and degradation time all had a large

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Figure 7. Molar mass as a function of hydrolysis time at 37 °C for PLA, PLA plasticized with acetyl tributyl citrate (PLA-ATC), PLA grafted with acrylic acid (PLA-AA), and AA-grafted PLA plasticized with ATC (PLA-ATC-AA).

Figure 6. Remaining mass as a function of hydrolysis time at (a) 37 and (b) 60 °C for PLA, PLA plasticized with acetyl tributyl citrate (PLAATC), PLA grafted with acrylic acid (PLA-AA), and AA-grafted PLA plasticized with ATC (PLA-ATC-AA).

influence on the degradation rate and the water-soluble product patterns of PLA. The ATC plasticizer and its hydrolysis products were detected by ESI-MS already after 1 day of degradation at both 37 and 60 °C. With increased temperature and time, the migrated ATC was further hydrolyzed, and water-soluble lactic acid oligomers were released from the PLA matrix. Grafting with AA increased the hydrophilicity of the PLA substrate, and grafted lactic acid oligomers were detected even at shorter degradation times. Finally, longer AA-grafted oligomers were predominantly formed after prolonged degradation of grafted and plasticized PLA. Mass Loss. We also studied the degradation process by monitoring the mass loss after different hydrolysis times. Figure 6a,b shows the remaining mass as a function of hydrolysis time at 37 and 60 °C, respectively. As expected, the remaining mass decreased with hydrolysis time for all four materials, and the process was substantially faster at 60 °C than at 37 °C. The plasticized PLA showed the lowest mass loss of all materials. This is most likely due to the increased hydrophobicity after the addition of ATC. The hydrophobic nature of ATC suppresses water absorption and thereby decreases the degradation rate. This is in line with earlier results on hydrolytic degradation of PLA with ATC as plasticizer.14 However, the water-soluble migrants were not previously analyzed. On the contrary, the AA-grafted PLA showed the

highest mass loss of the various PLA materials. Analogously, this is due to the increased hydrophilicity induced by the grafted surface layer. The AA side chains facilitate water absorption and accelerate the cleavage of the ester bonds in the PLA main chain. Grafted and plasticized PLA had an intermediate mass loss profile at 37 °C, which is somewhat slower than pure PLA but somewhat faster than plasticized PLA. Apparently, the increased bulk hydrophobicity caused by the plasticization had a larger influence on the degradation rate compared with the increased surface hydrophilicity caused by surface grafting. Molar Mass Changes. The molar mass changes during hydrolysis were determined with SEC. Figure 7 shows the molar mass of the various PLA materials as a function of hydrolysis time. In general, the molar mass decreased with hydrolysis time for all four materials. The surface-grafted PLA chains were, however, no longer soluble in chloroform, which influenced the molar mass values, and the grafted and nongrafted PLA are therefore not directly comparable. Nevertheless, in conformity with the mass loss results, the lowest molar mass decrease was obtained for the plasticized PLA. It is interesting to note that a relatively large decrease in molar mass was already detected after 7 days of degradation at 37 °C, especially for the surfacegrafted PLA. The mass loss was considerably slower, and over 90% of the original mass remained in all materials after 28 days of degradation. This is due to the hydrophobic nature of PLA. The degradation products formed during hydrolysis are not water-soluble until they have a molar mass of ∼1000 g/mol (Figure 1) and therefore remain in the polymer bulk. Thermal Properties. DSC was used to evaluate the changes in thermal properties during hydrolysis. Figure 8 shows the melting temperature of the different PLA materials as a function of hydrolysis time. All four materials had approximately the same melting temperature prior to degradation. The melting temperatures increased slightly during the first 49 days of degradation, after which they decreased more rapidly. Similar to mass loss and changes in molar mass, the lowest decrease in melting temperature was observed for the plasticized PLA. The initial increase in Tm is explained by continued crystallization during hydrolysis, where the crystal thickness increased.21 When the degradation time was increased, the melting temperatures decreased because of the formation of shorter polymer chains resulting in lower Tm. In parallel to the decrease in Tm, an increase in the degree of crystallinity, wc, was also observed (Figure 9).

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Figure 8. Melting point as a function of hydrolysis time at 37 °C for PLA, PLA plasticized with acetyl tributyl citrate (PLA-ATC), PLA grafted with acrylic acid (PLA-AA), and AA-grafted PLA plasticized with ATC (PLA-ATC-AA).

Figure 9. Degree of crystallinity as a function of hydrolysis time at 37 °C for PLA, PLA plasticized with acetyl tributyl citrate (PLA-ATC), PLA grafted with acrylic acid (PLA-AA), and AA-grafted PLA plasticized with ATC (PLA-ATC-AA).

It is well known that the amorphous regions are more susceptible to hydrolysis than the crystalline regions, which leads to an increase in wc during hydrolysis of semicrystalline polyesters. The higher mobility of the shorter chains formed during hydrolysis also allows for a reorientation of the crystalline phase with a subsequent increase in wc. As expected, larger differences between the materials were observed when comparing the glass-transition temperatures, as seen in Figure 10. As expected, the plasticized PLA materials had a lower glasstransition temperature, Tg, than their nonplasticized analogues. (See also Table 1.) Grafting with AA increased the Tg of the PLA. This is probably due to the polarity of the grafted acid groups, which increase the secondary interactions between the polymer chains and restrict their mobility. The Tg values of all four materials decreased with increased degradation time. This is also due to the formation of shorter chains with higher mobility. The Tg values were taken from the second heating scan because the peak from the first heating scan was too illdefined to be evaluated.

Conclusions Plasticizing PLA with ATC significantly altered the watersoluble product patterns during hydrolysis. Despite its hydrophobic nature, the plasticizer started to migrate from the PLA

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Figure 10. Glass transition-temperature as a function of hydrolysis time at 37 °C for PLA, PLA plasticized with acetyl tributyl citrate (PLAATC), PLA grafted with acrylic acid (PLA-AA), and AA-grafted PLA plasticized with ATC (PLA-ATC-AA).

material immediately upon immersion in water. In addition, subsequent hydrolysis of the ATC ester arms was immediately initiated upon contact with water leading to the formation of new degradation products from the plasticizer. The migrated plasticizer underwent continuous hydrolysis, and the relative abundance of ATC degradation products increased with hydrolysis time. The addition of ATC decreased the degradation rate of PLA as a result of the increased overall hydrophobicity of the material. Water-soluble lactic acid oligomers were, however, detected in the water solutions after the same degradation periods as those for nonplasticized PLA, although in lower amounts because the total mass loss caused by degradation products and plasticizer migration was lower for plasticized PLA. This was valid for both pure PLA and AAgrafted PLA. The increased bulk hydrophobicity caused by the plasticizer thus overcame the degradation-enhancing effect from the hydrophilic surface layer. The fate of the plasticizer with respect to the migration and formation of new degradation products as well as the possible effect on the degradation rate should be taken into consideration when designing new materials. Acknowledgment. We gratefully acknowledge the European Community Sixth Framework Programme Sustainable Microbial and Biocatalytic Production of Advanced Functional Materials (BIOPRODUCTION) under the contract number NMP2-CT2007-026515 for financial support of this work.

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Hydrophobic PLA Plasticizer (10) Ho¨glund, A.; Odelius, K.; Hakkarainen, M.; Albertsson, A.-C. Biomacromolecules 2007, 8, 2025–2032. (11) Ho¨glund, A.; Hakkarainen, M.; Edlund, U.; Albertsson, A.-C. Langmuir, published online Aug 24, http://dx.doi.org/10.1021/la902166j. (12) Lindstro¨m, A.; Hakkarainen, M. Biomacromolecules 2007, 8, 1187– 1194. (13) Lindstro¨m, A.; Hakkarainen, M. J. Appl. Polym. Sci. 2007, 104, 2458– 2467. (14) Kranz, H.; Ubrich, N.; Maincent, P.; Bodmeier, R. J. Pharm. Sci. 2000, 89, 1558–1566. (15) Labrecque, L. V.; Kumar, R. A.; Dave, V.; Gross, R. A.; McCarthy, S. P. J. Appl. Polym. Sci. 1997, 66, 1507–1513.

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