Surface Modification Changes the Degradation Process and

Aug 24, 2009 - Polymers for Advanced Technologies 2013 24 (10.1002/pat.v24.8), ... Polymers for Advanced Technologies 2011 22 (10.1002/pat.v22.12), ...
0 downloads 0 Views 1MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

Surface Modification Changes the Degradation Process and Degradation Product Pattern of Polylactide Anders H€oglund, Minna Hakkarainen, Ulrica Edlund, and Ann-Christine Albertsson* Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), S-100 44, Stockholm, Sweden Received June 16, 2009. Revised Manuscript Received August 3, 2009 The effect of surface modification on the degradation process and degradation product patterns of degradable polymers is still a basically unexplored area even though a significant effect can be expected. Polylactide (PLA) and PLA grafted with acrylic acid (PLA-AA) were, thus, subjected to hydrolytic degradation, and water-soluble degradation products were determined by electrospray ionization-mass spectrometry (ESI-MS) after different time periods. Low molar mass compounds migrated from surface-grafted PLA already during the first 7 days at 37 °C, while it took 133 days in the case of nongrafted PLA before any low molar mass compounds were detected in the aging water. In addition, the degradation product pattern of surface-grafted PLA showed significant variation as a function of hydrolysis time with the evolution of short and long AA-grafted lactic acid oligomers as well as plain lactic acid oligomers after different time periods. The degradation product pattern of plain PLA consisted of lactic acid and its oligomers with up to 13 lactic acid units. Surface grafting, thus, changed the degradation product patterns and accelerated the formation of water-soluble degradation products.

Introduction Surface modification of polymers is a way to tailor the initial interaction between a polymeric material and its environment. This interaction is especially important in the case of bioresorbable materials, where surface modification is increasingly being used to influence protein adsorption, cell adhesion and proliferation. Surface modification could also be a tool to tune the degradation rate of environmentally degradable polymers. However, the effect of surface grafting on the degradation products is an important issue to take into consideration when designing surface grafted polymers. The high sensitivity of degradable polymers makes them susceptible to severe degradation during the commonly used surface modification techniques. To overcome this, a nondestructive single-step gas-phase surface modification technique has been developed for covalent surface modification of degradable polymers.1-3 This method was applied for surface grafting of polylactide (PLA), polycaprolactone and other degradable materials with compounds such as N-vinyl pyrrolidone, acrylamide and maleic anhydride. Poly-L-lactide films grafted with N-vinyl pyrrolidone, acrylamide and acrylic acid (AA) showed faster degradation rate in terms of mechanical characteristics and mass loss,4 while retardation of enzymatic erosion was observed for microbial polyesters plasma treated with CF3H.5 *Corresponding author. Phone: þ46-8-790 82 74. Fax: þ46-8-20 84 77. E-mail: [email protected]. (1) Edlund, U.; K€allrot, M.; Albertsson, A.-C. J. Am. Chem. Soc. 2005, 127, 8865–8871. (2) Janorkar, A. V.; Metters, A. T.; Hirt, D. E. Macromolecules 2004, 37, 9151– 9159. (3) K€allrot, M.; Edlund, U.; Albertsson, A.-C. Biomaterials 2006, 27, 1788– 1796. (4) K€allrot, M.; Edlund, U.; Albertsson, A.-C. Biomacromolecules 2007, 8, 2492–2496. (5) Ryou, J.-H.; Ha, C.-S.; Kim, J.-W.; Lee, W.-K. Macromol. Biosci. 2003, 3, 44–50. (6) Grizzi, I.; Garreau, H.; Li, S.; Vert, M. Biomaterials 1995, 16, 305–311. (7) Hakkarainen, M. Adv. Polym. Sci. 2002, 157, 113–138. (8) Hakkarainen, M.; Albertsson, A.-C.; Karlsson, S. Polym. Degrad. Stab. 1996, 52, 283–291.

378 DOI: 10.1021/la902166j

Degradation of aliphatic polyesters has been extensively studied,6-9 and mathematical models have been developed to predict the hydrolytic degradation process.10,11 However, the effect of different polyester structures, macromolecular architectures, and especially surface modifications on the degradation process and degradation product patterns is still not well established.12,13 The advances in chromatographic and mass spectrometric techniques have also opened up for molecular level characterization of the microstructures and degradation processes.13-15 The effect of macromolecular architecture and hydrophilicity on the degradation rate and degradation product patterns of different caprolactone (CL) and 1,5-dioxepan-2-one (DXO) homopolymers and copolymers was clearly shown by gas chromatography-mass spectrometry (GC-MS)16,17 and electrospray ionization-mass spectrometry (ESI-MS).18 The influence of composition and chain microstructure on the hydrolytic degradation of glycolide and CL copolymers was also shown by ESI-MS.19,20 Surface grafting is increasingly being used to control the biomaterial-cell interaction. We anticipate, however, that surface modification also significantly affects the degradation process and, even more importantly, (9) Li, S.; Garreau, H.; Vert, M. J. Mater. Sci.: Mater. Med. 1990, 1, 123–130. (10) Antheunis, H.; van der Meer, J.-C.; de Geus, M.; Kingma, W.; Koning, C. E. Macromolecules 2009, 42, 2462–2471. (11) Lyu, S.; Schley, J.; Loy, B.; Lind, D.; Hobot, C.; Sparer, R.; Untereker, D. Biomacromolecules 2007, 8, 2301–2310. (12) Hakkarainen, M.; Albertsson, A.-C. Adv. Polym. Sci. 2008, 211, 85–116. (13) Montaudo, G.; Lattimer, R. P. Mass Spectrometry of Polymers; CRC Press: Washington, DC, 2002. (14) Burman, L.; Albertsson, A.-C.; Hakkarainen, M. Adv. Polym. Sci. 2008, 211, 1–22. (15) Nielen, M. W. F.; Buijtenhuijs, F. A. Anal. Chem. 1999, 71, 1809–1814. (16) Hakkarainen, M.; H€oglund, A.; Odelius, K.; Albertsson, A.-C. J. Am. Chem. Soc. 2007, 129, 6308–6312. (17) H€oglund, A.; Odelius, K.; Hakkarainen, M.; Albertsson, A.-C. Biomacromolecules 2007, 8, 2025–2032. (18) Hakkarainen, M.; Adamus, G.; H€oglund, A.; Kowalczuk, M.; Albertsson, A.-C. Macromolecules 2008, 41, 3547–3554. (19) Kasperczyk, J.; Li, S.; Jaworska, J.; Dobrzynski, P.; Vert, M. Polym. Degrad. Stab. 2008, 93, 990–999. (20) Li, S.; Dobrzynski, P.; Kasperczyk, J.; Bero, M.; Braud, C.; Vert, M. Biomacromolecules 2005, 6, 489–497.

Published on Web 08/24/2009

Langmuir 2010, 26(1), 378–383

H€ oglund et al.

Article

dramatic changes in the degradation product patterns could take place. The aim of the present work was, thus, to show the effect of surface modification on the hydrolytic degradation process and degradation product patterns of PLA. In the future, simultaneously with the development of new materials, the effect of surface grafting on the degradation process and on the degradation product patterns should be taken into account.

Experimental Section Materials. PLA pellets were used as received. Natureworks PLA was chosen in order to obtain a homogeneous and reproducible material and to make the results comparable with previous work.1,2 Polymer films were prepared by dissolving the pellets in chloroform (Fischer Scientific, HPLC grade) and subsequent solution-casting on a silanized glass mold. The solvent was evaporated, and the films were dried under vacuum for 1 week. Circular sample discs with a diameter of 10 mm and a thickness of approximately 250 μm were punched from the films, washed with ethanol (99.5%), and dried under reduced pressure prior to grafting and degradation. AA (99.5%, Acros) was distilled under reduced pressure just before use and stored cold. Benzophenone (99%, Alpha Aesar) was used as received. Vapor-Phase Grafting. The grafting procedure has been described thoroughly elsewhere.1 Briefly, grafting was performed in a glass reactor comprising two interconnected cylindrical compartments. The PLA substrate samples were placed in one of the reactor compartments, and the monomer (AA) and initiator (benzophenone) (M:I = 10:1) were added in the other reactor compartment. The reactor was alternately evacuated and filled with nitrogen three times according to a freeze-pump-thawing protocol. In the last round, the reactor was evacuated, sealed, and immersed into a water bath at 40 °C. After a 5 min equilibration time, the reactor was irradiated with UV light for 20 min. After completed grafting, the films were turned over, and the procedure was repeated in order to obtain grafted layers on both sides of the sample substrates. Thereafter, the surface modified films were thoroughly washed with deionized water and ethanol (99.5%) and subsequently dried under vacuum prior to hydrolysis. Hydrolysis. PLA and AA-grafted PLA (PLA-AA) samples were subjected to hydrolytic degradation in deionized water at 37 and 60 °C. Each specimen was placed in a vial containing 10 mL of water, and the sample vials were sealed with septa. The vials were placed in a thermostatically controlled incubator at 37 °C and 60 rpm rotation or in an oven at 60 °C. After different periods of degradation between 1 and 364 days, duplicate samples of each material were withdrawn from the test environment, dried under vacuum, and subjected to the various analyses. In addition, the water-soluble degradation products in the sample solutions were analyzed after each hydrolysis time. Atomic Force Microscopy (AFM). The surface morphology was analyzed with a CSM Instrument Nano indentor with combined atomic force microscope. Samples were placed on a copper plate and aligned under the AFM tip. The measurements were performed in non contact mode in air using a Pointprobe plus probe with a nominal spring constant of ∼46.5 N/m and a resonance frequency of 181-200 kHz. The length of the cantilever was 223 μm. Image analysis was performed in CSM Instruments ImagePlus v.3.1.10. Size Exclusion Chromatography (SEC). The molar mass of the polymers after different hydrolysis times were determined by SEC. CHCl3 (Fischer Scientific, HPLC grade) was used as the eluent at a flow rate of 1.0 mL/min, and the injection volume was 50 μL. The apparatus consisted of 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. Narrow molar mass polystyrene standards were used for calibration. The data was processed with Millennium software version 3.20. Langmuir 2010, 26(1), 378–383

Figure 1. Surface topography of PLA and PLA-AA prior to hydrolysis, as observed by AFM.

Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra of the samples were recorded on a PerkinElmer Spectrum 2000 FTIR spectrometer equipped with an attenuated total reflectance (ATR) accessory (golden gate) from Graseby Specac (Kent, U.K.). Specimens in the form of cut pieces from the polymer discs were placed directly on the crystal on the top plate of the golden gate. The analysis depth of the surface was approximately 1 μm, and all spectra were calculated means from 16 scans. Differential Scanning Calorimetry (DSC). The thermal properties of the PLA samples were investigated using a DSC (Mettler Toledo DSC 820 module) under nitrogen atmosphere. Approximately 5 mg of the polymer was encapsulated in a 40-μL aluminum cap without pin. Samples were heated under a nitrogen gas flow of 50 mL/min from 0 to 200 °C at a rate of 10 °C/min. Thereafter, the samples were cooled from 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, and the midpoint temperature of the glass transition was taken as the glass transition temperature, Tg. The approximate degree of crystallinity of the PLA samples was calculated according to wc ¼

ΔHf  100 ΔHf0

ð1Þ

where wc is the degree of crystallinity, ΔHf is the heat of fusion of the sample, and ΔH0f is the heat of fusion of 100% crystalline polymer. The value used for ΔH0f was 93 J/g.21

Results and Discussion The effect of surface grafting on the following degradation process and degradation product patterns was evaluated by subjecting PLA and PLA-AA to hydrolysis in deionized water for different time periods up to 364 days. The graft yield was approximately 5%, and successful grafting was also verified by FTIR and AFM. The surface morphology prior to hydrolysis is shown in Figure 1. The pure PLA substrate had quite a smooth surface, whereas the PLA-AA surface was somewhat rougher. The difference in surface roughness was, however, in the nanometer scale, which shows that the surface layer is fairly thin. This was confirmed by scanning electron microscopy (data not shown) and has also been reported in previous work.3 The polymer properties prior to degradation are presented in Table 1. The surface-grafted PLA chains were, however, no longer soluble in chloroform, which influences the molar mass values of PLA-AA. (21) Fischer, E. W.; Sterzel, H. J.; Wegner, G. Kolloid Z. Z. Polym. 1973, 251, 980–990.

DOI: 10.1021/la902166j

379

Article

H€ oglund et al.

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

Mn

PDI

Tm (°C)

Tg (°C)

wc (%)

PLA 100 800 ( 1900 1.68 ( 0.03 144.0 ( 1.1 49.4 ( 0.3 24 ( 4 PLA-AA 72 500 ( 3000 1.66 ( 0.06 147.9 ( 1.7 53.9 ( 1.6 24 ( 6

Figure 3. Positive ESI-MS spectrum of water-soluble degradation products of PLA-AA after 7 days of hydrolytic degradation at 37 °C and the suggested chemical structure of the species corresponding to the detected peaks. LAnAA are here drawn schematically, i.e., the grafted unit may be attached anywhere along the oligomer chain.

Figure 2. Positive ESI-MS spectra of water-soluble degradation products of PLA after 28 days of hydrolytic degradation at 60 °C in the mass range (a) m/z 150-2000 and (b) m/z 50-200.

Degradation Products. The water fractions after different hydrolysis times and at different degradation temperatures were analyzed by ESI-MS to identify the water-soluble degradation products formed during the hydrolysis and to evaluate the effect of surface grafting, hydrolysis time, and temperature on the degradation product patterns. Degradation products that could be expected include oligomers with hydroxyl and carboxyl end groups, lactic acid (LA), and additionally AA surface-grafted LA oligomers. ESI-MS Analysis of Water-Soluble Hydrolysis Products of PLA. Figure 2a,b shows positive ESI-MS spectra of the watersoluble degradation products of PLA after 28 days of hydrolytic degradation at 60 °C in the mass range m/z 150-2000 and m/z 50-200, respectively. One series of peaks with a mass-to-mass peak increment of 72 Da, corresponding to the molar mass of the LA repeating unit, is observed in the mass range m/z 150-1000 in the spectrum. 380 DOI: 10.1021/la902166j

These peaks correspond to water-soluble LA oligomers with up to 13 repeating units terminated by carboxyl and hydroxyl end groups. The general chemical structure of these oligomers is shown in Figure 2a. Longer oligomers are probably also formed during the hydrolysis, but they are not water-soluble and hence not detected by ESI-MS. The peak intensity in Figure 2a increases with increasing m/z ratio up to 545, whereafter it decreases quite rapidly. This is due to the hydrophobic nature of PLA, which makes the longer oligomers less water-soluble. In the lower mass range m/z 50-200 in Figure 2b, the monomer LA and its dimer are observed. It is known that the intensity of the signal corresponding to the monomeric hydroxyacid is lower compared to the intensities of the signals of hydroxyacid oligomers in the ESI mass spectra. Thus, an estimation of the quantitative relation between hydroxyacid and its oligomers would require separate calibration for LA and the oligomers. However, the observation of the peak at m/z 113 corresponding to the LA monomer confirmed that both the monomeric hydroxyacid and oligomers were formed during the hydrolytic degradation of PLA. Similar mass spectra as in Figure 2 were obtained when analyzing the water fractions of PLA aged at 37 °C from 133 days and forward, although with a higher relative intensity of the peaks in the lower mass range. Prior to this time period, and prior to 28 days of hydrolysis at 60 °C, no detectable signals were observed. The considerable difference in the aging time required for the detection of water-soluble degradation products at 37 and 60 °C clearly shows the large influence of degradation temperature on the hydrolysis rate of PLA. ESI-MS Analysis of Water-Soluble Hydrolysis Products of PLA-AA. The water-soluble degradation product patterns of the grafted PLA were quite different from the degradation product patterns of nongrafted PLA, both with respect to the nature of the degradation products and the time frame at which they appeared. As seen in Figure 3, water-soluble oligomers were observed already after 7 days of degradation at 37 °C compared to after 133 days for pure PLA. Also here one main series of peaks with a mass difference of 72 Da is observed in the mass range m/z 150-1000. However, in this case, the results indicate that the peaks correspond to LA oligomers grafted with AA. The suggested general chemical structure of these oligomers is shown in Figure 3. The absolute Langmuir 2010, 26(1), 378–383

H€ oglund et al.

values of the masses corresponding to the two main series of peaks in Figure 2a and Figure 3 differ by 22 Da, i.e., the main series of peaks in the mass spectra of PLA-AA degradation products corresponds to ions with two sodium atoms. This is probably due to a sodium ion exchange at one of the carboxyl groups, as each such exchange would increase the molar mass of the molecule by 22 Da. Molecules with two sodium atoms were also observed at low intensities in some of the mass spectra of pure PLA degradation products, but these molecules were never observed as the main series of peaks. It is thus likely that the extra carboxyl groups in the grafted LA oligomers are responsible for the increased tendency to ion exchange. To further support this, another series of peaks with lower intensity corresponding to oligomers with three sodium atoms was observed in the mass spectra of PLA-AA (Figure 3). This series of peaks was not observed in the case of pure PLA. The AA-grafted oligomers are more hydrophilic than the nongrafted oligomers and therefore migrate easier and faster to the surrounding water solution, as shown also by the early appearance of these peaks in the mass spectra. Unfortunately, both the molar mass of the LA and AA repeating unit is 72 Da, and it is therefore not possible to determine the number of respective repeating units simply from the m/z values. Figure 4a,b shows positive ESI-MS spectra of the watersoluble degradation products of PLA after 28 days of hydrolytic degradation at 37 °C in the mass range m/z 150-2000 and m/z 150-1000 respectively. When the hydrolysis time was increased from 7 to 28 days, a new set of peaks with high intensity appeared in the spectrum in the mass range m/z 1000-2000. A large difference in the degradation product pattern is thus observed compared to the ESI-MS spectra of pure PLA and PLAAA at shorter degradation times where no peaks were seen above m/z 1000 (cf. Figure 2a and Figure 3). The detected peaks in the higher mass range probably correspond to longer AA-grafted LA oligomers, which are water-soluble as a result of the hydrophilicity of the AA units. As the upper mass range of the ESI-MS was m/z 2000, it is likely that even larger degradation products are water-soluble and present in the degradation medium. A suggested general chemical structure of these degradation products is also presented in Figure 4. An assignment of the specific hydrolysis products and the respective structures corresponding to each peak in the spectrum was not performed for the larger AA-grafted hydrolysis products. As a result of the chemical structure of the grafted surface layer, multiply charged ions and ions with several sodium atoms through ion exchange were formed in the ESI-MS. This led to complex mass spectra where each AA-grafted oligomer can be presented by several ions with different charges and different number of sodium atoms. The mass spectra clearly demonstrate the large differences in the degradation process and products of pure PLA and surfacegrafted PLA. Earlier studies on polycarboxylic acids and poly(acrylic acid) have shown the formation of multiply charged ions in the case of larger oligomers and polymers.22 For poly(acrylic acid), doubly charged, triply charged, as well as quadruply charged oligomers were detected.23 The lower mass range m/z 150-1000 of PLA-AA after 28 days of hydrolysis shown in Figure 4b also shows interesting differences in the degradation product patterns as a function of hydrolysis time. After 7 days of degradation (Figure 3), peaks corresponding to LA oligomers with presumably one grafted AA (22) Schmitt-Kopplin, P.; Kettrup, A. Electrophoresis 2003, 24, 3057–3066. (23) Leenheer, J. A.; Rostad, C. E.; Gates, P. M.; Furlong, E. T.; Ferrer, I. Anal. Chem. 2001, 73, 1461–1471.

Langmuir 2010, 26(1), 378–383

Article

Figure 4. Positive ESI-MS spectrum of water-soluble degradation products of PLA-AA after 28 days of hydrolytic degradation at 37 °C in the mass ranges (a) m/z 150-2000 and (b) m/z 150-1000, and the suggested chemical structure of the species corresponding to the peaks in the higher mass range m/z>1000. LAnAAm are here drawn schematically, i.e., the grafted units may be attached anywhere along the oligomer chain.

monomer, e.g., m/z 423, 495, 567, were visible in the mass spectrum. However, after 28 days of degradation (Figure 4b), peaks corresponding to nongrafted LA oligomers, e.g., 401, 473, 545, were also visible with more or less the same relative intensity as the AA-grafted oligomers. Several factors can contribute to the fact that LA oligomers are released from the material after much shorter hydrolysis time compared to hydrolysis of pure PLA. The increased hydrophilicity of grafted PLA makes it more prone to absorb water compared to pure PLA, accelerating the hydrolysis of ester bonds and the subsequent release of low molar mass products at an earlier degradation stage. The increased hydrophilicity of the grafted PLA molecules also makes them watersoluble at higher molar masses compared to pure LA oligomers. Once the grafted chains have migrated into the water phase, further hydrolysis will continue at a faster rate, leading to the formation of pure LA oligomers and shorter AA-grafted oligomers. This is also clearly seen in Figure 5 which shows the ESI-MS spectrum recorded from PLA-AA after 364 days of degradation at 37 °C. The mass spectrum clearly shows the development of a degradation product pattern as a function of hydrolysis time, DOI: 10.1021/la902166j

381

Article

Figure 5. Positive ESI-MS spectra of water-soluble degradation products of PLA in the mass range m/z 150-1000 after 364 days of hydrolytic degradation at 37 °C.

H€ oglund et al.

Figure 7. Positive ESI-MS spectrum of water-soluble degradation products of PLA-AA after 182 days of hydrolytic degradation at 60 °C.

Figure 6. Positive ESI-MS spectrum of water-soluble degradation products of PLA-AA after 1 day of hydrolytic degradation at 60 °C.

as, at this stage, only peaks corresponding to nongrafted PLA oligomers were detected in the lower mass range, whereas AA-grafted oligomers were visible only in the higher mass range. Degradation temperature also largely influenced the degradation product pattern of the PLA-AA. Figure 6 shows the positive ESI-MS spectrum of water-soluble degradation products of PLA-AA after 1 day of hydrolysis at 60 °C. This spectrum is similar to the spectrum recorded after 28 days at 37 °C (Figure 4a), although with a somewhat higher intensity of the oligomer peaks in the lower mass range. The combined effect of increased hydrophilicity and increased degradation temperature induced a release of water-soluble degradation products already after 1 day of hydrolysis. In addition, prolonged degradation of the surface-grafted PLA induced a shift in the degradation product patterns. This is illustrated in Figure 7, which shows the ESI-MS spectrum after 182 days of degradation at 60 °C. At this time point, the longer AA-grafted LA oligomers in the higher mass range clearly dominate the ESI-MS spectrum. This is due to the continuous cleavage of ester bonds in the main chain allowing for subsequent release of large amounts of longer hydrophilic chains. To sum up, it was evident that both surface grafting with AA and degradation temperature had a large influence on the degradation rate and the degradation product patterns. Figure 8 describes 382 DOI: 10.1021/la902166j

Figure 8. Time-frame for detection of specific degradation products during hydrolysis of pure PLA and PLA-AA at 37 and 60 °C. LA oligomers correspond to lactic acid oligomers, and LA-AA oligomers correspond to acrylic acid surface lactic acid oligomers of different lengths.

schematically when different types of degradation products became visible in the ESI mass spectra. In the case of pure PLA, water-soluble LA oligomers were detected by ESI-MS after 28 days of degradation at 60 °C and after 133 days of degradation at 37 °C. For the PLA-AA, nongrafted LA oligomers were detected already after 1 day at 60 °C and after 28 days at 37 °C, and shorter LA oligomers with presumably one grafted AA monomer were detected already after 1 day at 60 °C and after 7 days of hydrolysis at 37 °C. Finally, longer AA-grafted oligomers were also recorded from PLA-AA after 1 day and after 28 days of hydrolysis at 60 and 37 °C, respectively. Langmuir 2010, 26(1), 378–383

H€ oglund et al.

Article

the surrounding water medium. However, the additional CdO shoulder was visible even after 364 of degradation. Although the carboxyl end groups of residual LA oligomers also may contribute to the CdO shoulder, the results indicate that some PLA-AA chains remain at the surface throughout the entire degradation study. This further supports that the chains are bonded to the PLA main chain with hydrolysis-resistant carbon-carbon bonds. This was also observed in a previous study on the hydrolysis of PLA grafted with acryl amide, N-vinyl pyrrolidone, and PLA-AA.4

Conclusions

Figure 9. FTIR spectra of the surface layer of PLA, PLA-AA, and PLA-AA after 364 days of hydrolytic degradation at 37 °C.

Surface Properties of the PLA Samples. The surface layer of the PLA samples was determined by FTIR. Figure 9 shows the FTIR spectra recorded for the pure and AA-grafted PLA samples prior to degradation and for the PLA-AA after 364 days of hydrolysis at 37 °C. The PLA-AA shows an additional CdO band in the form of a shoulder on the characteristic PLA carbonyl peak at ∼1750 cm-1. This shoulder originates from the carboxylic acid of the AA. The intensity of this additional CdO shoulder decreased with increasing hydrolysis time. This is due to the increased hydrophilicity and water-solubility of these species which facilitate the migration to

Langmuir 2010, 26(1), 378–383

Surface modification significantly influenced the degradation process and resulting degradation product patterns, leading to rapid formation of water-soluble degradation products. In addition, the degradation product pattern of PLA-AA varied as a function of hydrolysis time with the evolution of short and long AA-grafted LA oligomers as well as plain LA oligomers after different time periods. When the aging temperature was raised from 37 to 60 °C similar degradation product patterns were detected, but the degradation process was accelerated. FTIR confirmed that some grafted PLA chains remained in the matrix throughout the entire degradation study, which is important for maintaining the increased hydrophilicity. Parallel with the development of new materials, the effect of surface grafting on the degradation product patterns should be taken into account. Acknowledgment. Financial support from the European Community Sixth Framework Programme Sustainable Microbial and Biocatalytic Production of Advanced Functional Materials (BIOPRODUCTION) under the contract number NMP2-CT2007-026515 is gratefully acknowledged.

DOI: 10.1021/la902166j

383