Customizing the Hydrolytic Degradation Rate of Stereocomplex PLA

Article pubs.acs.org/Biomac

Customizing the Hydrolytic Degradation Rate of Stereocomplex PLA through Different PDLA Architectures Sofia Regnell Andersson,† Minna Hakkarainen,† Saara Inkinen,‡ Anders Södergård,‡ 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 ‡ Laboratory of Polymer Technology, Center for Functional Materials (FUNMAT), Åbo Akademi University, Piispankatu 8, 20100 Turku, Finland ABSTRACT: Stereocomplexation of poly(L-lactide) (PLLA) with star shaped D-lactic acid (D-LA) oligomers with different architectures and end-groups clearly altered the degradation rate and affected the degradation product patterns. Altogether, nine materials were studied: standard PLLA and eight blends of PLLA with either 30 or 50 wt % of four different D-LA oligomers. The influence of several factors, including temperature, degradation time, and amount and type of D-LA oligomer, on the hydrolytic degradation process was investigated using a fractional factorial experimental design. Stereocomplexes containing star shaped D-LA oligomers with four alcoholic end-groups underwent a rather slow hydrolytic degradation with low release of degradation products. Materials with linear D-LA oligomers exhibited similar mass loss but released higher concentrations of shorter acidic degradation products. Increasing the fraction of D-LA oligomers with a linear structure or with four alcoholic end-groups resulted in slower mass loss due to higher degree of stereocomplexation. The opposite results were obtained after addition of D-LA oligomers with carboxylic chain-ends. These materials demonstrated lower degree of stereocomplexation and larger mass and molar mass loss, and also the release of degradation products increased. Increasing the number of alcoholic chain-ends from four to six decreased the degree of stereocomplexation, leading to faster mass loss. The degree of stereocomplexation and degradation rate were customized by changing the architecture and end-groups of the D-LA oligomers.

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INTRODUCTION Polylactide stereocomplexes, formed between the two enantiomers poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), have many favorable characteristics compared to pure PLLA. These include elevated glass transition and melting temperatures, higher degree of crystallinity, and increased resistance toward hydrolytic degradation.1,2 These features are derived from the strong interactions between the complementary chain structures of the D- and the L-forms of polylactide (PLA). A homocrystallite of PLLA has a 103 helical structure, while the PLLA and PDLA enantiomers crystallize together forming 31 helices. This results in a more compact side by side crystallization for the stereocomplex crystallite.3 In concentrated solutions, stereocomplex crystallites are formed; these will act as cross-links which will induce increased viscosity, leading to the formation of three-dimensional gelation, which is irreversible.4 A higher degree of crystallinity can be obtained by, for example, solution casting, where further crystallization will take place during slow solvent evaporation. The average size of the stereocomplex crystallites decreases with increasing molar mass.5 The maximum interaction, and hence the largest degree of stereocomplexation, occurs between a 1:1 mixture of PLLA and PDLA. © 2012 American Chemical Society

PLLA molded by ordinary processing techniques becomes amorphous due to slow crystallization rate; therefore, it drastically softens above the Tg. This limits the applications of PLLA as a plastic material. To enhance the thermal stability, a faster crystallization rate is needed.6 Addition of PDLA increases the number of nucleation sites and the crystallization rate.7,8 Stereocomplex crystallization occurs rapidly, and this results in enhanced thermal stability, leading to improved processability. Direct melt processing of the stereocomplex is not desirable due to the high temperatures needed, which can cause degradation of the polymer.9 However, melt blending of homopolymers below the melting temperature for the stereocomplex allows for formation of stereocomplex crystallites during processing, and in this case, temperatures for processing of pure PLLA can be used.10 Block copolymers of PLLA and PDLA give more effective stereocomplex crystallization and reduce the single polymer crystallization for chains with a molecular mass of above 100 000 g/mol.11 If the molar mass is higher than 100 000 g/mol, homocrystallites prevail.6 Received: February 6, 2012 Revised: March 5, 2012 Published: March 6, 2012 1212

dx.doi.org/10.1021/bm300196h | Biomacromolecules 2012, 13, 1212−1222

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Table 1. Materials Used in the Study material ID

fraction of D-LA oligomer

DLA-30 DLA-50 DLA-4OH-30 DLA-4OH-50 DLA-6OH-30 DLA-6OH-50 DLA-4COOH-30 DLA-4COOH-50 PLLA

0.3 0.5 0.3 0.5 0.3 0.5 0.3 0.5 high molar mass PLLA

D-LA

molar mass (g/mol) of D-LA by NMR

oligomer core

D-LA

molar mass Mn (g/mol) of D-LA by SEC

3100 3100 9200 9200 16200 16200 not calculated not calculated

D-LA

pentaerythritol pentaerythritol dipentaerythritol dipentaerythritol 1,2,3,4-butanetetracarboxylic acid 1,2,3,4-butanetetracarboxylic acid

6200 7000 14200 12900 25100 27700 8500 7900

± ± ± ± ± ± ± ±

1400 600 500 1100 2200 1600 1000 1600

alcoholic or carboxylic end-groups, to hydrolytic degradation in water using a fractional factorial experimental design.

Tuning the hydrolytic degradation is of critical importance for utilizing polylactide in different commercial applications. The effect of, for example, surface modifications,12 plasticizers,13,14 and copolymerization15 on hydrolytic degradation has previously been studied. Polymer stars offer an additional option for tuning the hydrolysis rate. Polymer stars have an increased concentration of functional end-groups compared to linear polymers of equal mass. Consequently, polymer stars can have short PLA arms but still have high molecular mass and elevated hydrophilicity. Arm length, number of arms, endgroups, and tacticity affect the thermal and physical properties of polymer stars.16 With increasing molar mass, the increased chain length contributes to reduced chain mobility whereas the decreased branching density elevates mobility and lowers the amount of defect points which have to be excluded from the crystallite nuclei; these two factors compete in determining the thermal and crystalline properties.17 Reduced mobility results in a higher Tg, and branching disturbs spherulite growth and makes the structure of the spherulite centra more disordered, consequently disturbing thickening of crystallites, which results in lower degree of crystallinity.17 Star structures also have increased solubility and different hydrodynamic volumes. Many properties are more dependent on the molecular mass of the arms and not on the total molecular mass.18 The end-groups play an important role on the hydrolytic stability.19 A comparison between polylactide stars terminated with either OH, Cl, NH2, or COOH showed that COOH end-groups were the least hydrolytically stable. The hydrolytic stability and the degree of crystallinity decreased with an increasing number of arms.20 Previously the possibility to affect the thermal properties of PLLA materials by combining the properties of star structures and those of stereocomplexes was investigated by blending PLLA with D-lactic acid (D-LA) oligomer stars with different arm lengths and blending ratios. The melting temperature and the melting enthalpy of the final materials were shown to be strongly affected by arm length and blending ratio. Tm and ΔHm were maximized with an arm length of 2−3 kDa and increased as a function of the D-LA oligomer contents.21 Not only the thermal properties are affected by stereocomplexation with D-LA oligomers of varying star structures but also the hydrolytic degradation process will be affected. Knowledge of how the amount, geometry, and type of endgroups in the D-oligomer affect the hydrolytic degradation rate can be used to tune the degradation rate, which will consequently widen the application range of PLLA/PDLA stereocomplexes. This possibility was evaluated by subjecting PLLA materials, with different blending ratios of D-LA oligomers with different numbers of end-groups and with

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EXPERIMENTAL SECTION

Materials. The high molar mass poly(L-lactide) was obtained from Hycail Finland Oy. The D-lactic acid (D-LA) was a 90% aqueous solution produced by Purac. The other chemicals used were 1,2,3,4butanetetracarboxylic acid (BTCA, 99%, Aldrich), chloroform (HPLC grade Hibersolv, BDH Prolabo, VWR), chloroform-d (Aldrich), dipentaerythritol (90%, Acros Organics), isopropanol (puriss p.a., Sigma-Aldrich), pentaerythritol (98%, Acros Organics), tin(II)ethyl hexanoate (95%, Sigma-Aldrich), water (LC-MS grade, Merck), and methanol (LC-MS grade, Fluka). Preparation of D-LA Oligomers. The types of the D-LA oligomers prepared are described in Table 1. The synthesis was conducted in a round flask using a rotary evaporator equipped with an oil bath, and the total batch size was ca. 350 g for the linear oligomer and 250−260 g for the branched oligomers and 0.1 wt % of catalyst. The oil bath was preheated to 100 °C, and the temperature of the oil bath was raised to 170−180 °C within 1 h after lowering the bottle into the oil. The pressure was stepwise lowered to 30−20 mbar within 3.5 h after reaching the final reaction temperature. For the copolymers, the theoretical number of D-LA units per branch was set to ca. 40 by the amount of comonomer used. The type of the comonomer also determined whether the branches of the copolymers became hydroxyl or carboxyl terminated. The D-LA oligomers were purified by dissolving in chloroform and precipitating in isopropanol. The obtained polymer powders were first dried in air at room temperature for 1−2 days. After this they were put into a vacuum oven at ca. 40 °C for 7 days, at which point no solvent evaporation was detected gravimetrically. Preparation of Stereocomplex. The oligomer and high molar mass PLLA were first dissolved in chloroform in separate 100 mL bottles. After 1 day, the oligomer solution was transferred into the bottle containing the molar mass PLLA. The mixture was shaken vigorously and poured onto a clean Petri-dish thereafter. The evaporation of the solvent was done as slowly as possible in order to favor stereocomplex crystal formation over homocrystallization of PLLA or PDLA. The polymer solutions were covered with aluminum foil containing small holes and the glass lid of the Petri-dish. The glass lids were removed after 2 days, after which the chloroform could only evaporate through the holes in the aluminum foil. A rubber band was placed around the Petri-dish in order to prevent solvent evaporation from the sides. After 5 days the aluminum foils were removed and the drying was continued in air for 2 days. The films were further dried in vacuum at ca. 40 °C for a minimum of 7 days. Two blends were made of PLLA with each oligomer, with 30 and 50 wt % of D-LA oligomer (Table 1). For comparison, films were also prepared of plain PLLA. 1 H NMR Analysis. The samples were dissolved in chloroform-d, and the spectra were collected using Bruker NMR AV600 equipment. The molar masses of the D-LA oligomers were estimated by relating the methine protons of the end-units appearing at 4.34−4.44 ppm to the methine protons of the PLA repeating unit at 5.08−5.35 ppm (Table 1). 1H NMR was not used for the calculation of the BTCA 1213


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Figure 1. (a) Modified CCF experimenal design, 11 experiments for each material. (b) Coefficient plot for remaining weight. containing copolymer and the high molar mass PLLA, since the copolymer was carboxyl terminated and the comonomer peaks in the spectrum were too small for quantification purposes. The lactide content in the different oligomers after purification was 0.4−0.5 mol %, which is typical for step-growth polymerization of lactic acid. Size Exclusion Chromatography (SEC). The molar masses of the polymers were determined by SEC. The polymers were analyzed with a Verotech PL-GPC 50 Plus system equipped with a PL-RI Detector and two PolarGel-M Organic (300 mm × 7.5 mm) columns from Varian. The samples were injected with a PL-AS RT Autosampler for PL-GPC 50 Plus, and chloroform was used as mobile phase (1 mL/min, 35 °C). The calibration was made using polystyrene standards with a narrow molecular mass distribution. Corrections for the flow rate fluctuations were made using toluene, with a retention time of 19.5 min, as an internal standard. CirrusTM GPC Software was used to process data. The molar masses (Mn) of the D-LA part of the materials determined by SEC are given in Table 1. Hydrolysis. Circular plates with a diameter of 10 mm were cut from the polymer films. The polylactide films were exposed to hydrolytic degradation at 22, 37, and 52 °C according to an experimental plan prepared using the software MODDE 9.0 (Umetrics). In addition, experiments at 60 °C were performed. Samples were placed in 20 mL glass vials with 5 mL of LC-MS grade water. The vials were sealed with butyl/PTFE septa and aluminum lids and placed in thermostatically controlled ovens. After predetermined periods of time, ranging from 1 to 25 weeks, samples were withdrawn from the incubator and subsequently withdrawn from the test medium, dried under vacuum, and subjected to the various analyses. Triplicate samples were performed after 13 weeks at 37 and 60 °C. Mass Loss. The course of degradation was followed by determining the remaining mass of the samples after different periods of time. After withdrawing the samples from the degradation medium, the samples were, if possible, gently wiped with a tissue and the wet weight was determined. The percentage of remaining mass of the solid polymer was determined after drying the samples to constant weight for 2 weeks under vacuum and comparing the dry weight (md) with the initial weight (m0) according to eq 1.

m remaining mass = 100 d m0

analyzed at each temperature and time. The melting temperatures and crystallization temperatures were noted as, respectively, the maximum value and the minimum value of the peaks; the glass transition temperatures were taken as the midpoint of the glass transition. Determination of Tg, Tm, and Tc was made from both the first and second heating scans. The approximate degree of crystallinity of the samples was calculated according to eq 2, where wc is the degree of crystallinity, ΔHf is the heat of fusion of the sample, and ΔH°f is the heat of fusion for a 100% crystalline polymer. The value used for ΔH°f was 93 J/g for PLLA (homocrystals)22 and 142 J/g for the stereocomplex (stereocomplex crystals).23

wc = 100

ΔHf ΔH °f

(2)

Electrospray Ionization-Mass Spectrometry (ESI-MS). The water fractions after each hydrolysis temperature and time were analyzed with a Finnigan LCQ ion trap mass spectrometer (Finnigan, San Jose, CA). Methanol (LC-MS grade) was added to the water fractions (water/MeOH = 2:1 v/v), the solutions were filtered through PTFE filters (13 mm × 0.45 μm) and continuously infused by the instrument syringe pump at a rate of 5 μL/min. For the analysis of the low molar mass fractions in the samples before hydrolysis, a 5 mg piece of the film was placed in 1 mL of the solvent. After 10 min, the solvent was removed and analyzed by ESI-MS. Positive ion mode was used for all analyses. The LCQ ion source was operating at 5 kV, and the capillary heater was set to 175 °C. Nitrogen was used as sheet gas, and helium was used as damping gas and collision gas in the mass analyzer. No cationizing agents were needed. Scanning Electron Microscopy (SEM). The surfaces of the samples after different degradation times were studied using a tabletop Hitachi TM-1000 SEM with an acceleration voltage of 15 kV.

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RESULTS AND DISCUSSION acid (D-LA) oligomers with different end-groups and number of arms were blended with high molar mass poly(Llactide) (PLLA) to evaluate how different D-oligomer structures and architectures affect the stereocomplex formation and following hydrolytic degradation. Altogether, nine materials were studied: standard poly(L-lactide) and eight blends of PLLA with either 30 or 50 wt % of four different D-LA oligomers. The influence of several factors, including temperature (22−52 °C), degradation time (2−25 weeks), and amount of D-LA oligomer (0−50%), on the hydrolytic degradation process was investigated using a fractional factorial experimental design (Figure 1a). The experiments were planned according to a modified CCF experimental design, and the results were analyzed using MODDE 9.0 (Umetrics). A total of 11 experiments including 3 center points were performed for each material (Figure 1a). Models of the D-Lactic

(1)

pH. The pH measurements in the degradation medium were carried out using a pH-meter equipped with an Ag/AgCl electrode. Differential Scanning Calorimetry. The thermal properties of the PLA samples were investigated using a differential scanning calorimeter (DSC) (Mettler Toledo DSC 820 module) under nitrogen atmosphere. 2−6 mg of the sample was placed in a 40 μL aluminum cap and sealed with a lid. Samples were heated under a nitrogen gas flow of 50 mL/min from −30 to 240 °C at a rate of 10 °C, held at 240 °C for 2 min, thereafter cooled to −30 °C at a rate of 10 °C, and held at the lowest temperature for 2 min. Finally the samples were heated from −30 to 240 °C at a rate of 10 °C. Duplicate samples were 1214


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Figure 2. ESI-MS mass spectra after 1 week of hydrolysis at 60 °C: (a) DLA-4COOH-30 showing various adducts of linear oligomers; (b) DLA4OH-30 showing only baseline noise.

Figure 3. ESI-MS mass spectra after 13 weeks of hydrolysis at 60 °C showing linear degradation products and traces of the star D-LA oligomers (marked with arrows) for (a) DLA-4OH-50 and (b) DLA-6OH-50.

oligomers (DLA) resulted in the same early release of degradation products as that for DLA-4COOH. A larger fraction of D-LA oligomer gave a larger number of short chains released from the materials after 1 week of degradation at 60 °C. In addition, materials with linear D-LA oligomers appeared to release larger amounts of short chains compared to the cases of other materials after longer degradation times (13 weeks, 60 °C); this can be explained by the higher degree of stereocomplex crystallinity for the materials with linear Doligomers, which was earlier shown to result in shorter degradation products due to the large number of short tie chains connecting the stereocomplex crystallites.2 These short tie chains are susceptible to hydrolysis. The number of branches with −OH end-groups did not seem to have a substantial effect on the degradation product patterns. No migration of degradation products could be observed in the samples kept at 22 °C. Traces of the star D-LA oligomers were seen in the low mass region of the spectra for the samples hydrolyzed for extended times (13 weeks, 60 °C) (Figure 3). The star D-LA oligomers appeared as peaks at 159 and 231 m/z for materials DLA-4OH and 277 m/z for materials DLA-6OH; for DLA-4COOH the core has the same molecular mass as the linear chains. In less hydrolyzed samples, the D-LA star oligomers are most likely locked by the stable stereocomplex crystallites or by the amorphous parts of the material, with strong interactions between the L- and D-configured chains, and therefore are not released as easily as the linear chains. Mass Loss. Mass loss of the polymer matrix was measured during the hydrolysis to follow the degradation process. At 22

relations between the factors and responses (remaining weight and Mn) were fitted using partial least squares (PLS).24 Model parameters that did not have a significant effect were removed one by one until all remaining model terms were either main terms or statistically significant terms (p < 0.05). The resulting significant coefficients were as follows: time, temperature, addition of D-LA oligomers, temperature∧2, temperature*time, time*DLA-4COOH, temperature*DLA-6OH, and temperature*DLA-4COOH (Figure 1b). Additional experiments outside the experimental design were performed at 60 °C, which was above the Tg of the studied materials. Degradation Products. The water-soluble degradation products were analyzed by ESI-MS to study the effect of D-LA oligomers on the degradation product patterns. ESI-MS was previously proven to be an ideal tool for studying migrants released from polymers into water.25,26 A series of peaks representing cyclic oligomers were present before aging in all original samples, observed by analysis of methanol extracts. Linear D-LA oligomers were detected as degradation products from all the materials as the main series of peaks (Figure 2). However, depending on the D-LA oligomer, these degradation products were detectable after different aging times. When materials with D-LA oligomers with the same number of end-groups were compared, the −COOH end-groups had an accelerating effect on the degradation compared to the −OH end-groups. Lower hydrolytic stability for −COOH terminated PLA stars has been observed before.20 Water-soluble products were detected after 1 week at 60 °C and after 13 weeks at 37 °C in the case of DLA-4COOH (Figure 2). The corresponding periods for DLA-4OH were 2 and 25 weeks. Linear D-LA 1215


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Figure 4. Contour plots showing remaining mass as a function of temperature and time for PLLA with addition of D-LA oligomers with different architectures and in different amounts.

°C, the total mass loss was less than 4% for all the materials. Considering the effect of different D-oligomer structures and architectures on the degradation process, the addition of D-LA oligomer with acidic end-groups had the most significant influence. In Figure 4 models are presented as response contour plots for each material. From these plots it is apparent that the extent of mass loss increased with temperature and time. In addition, the materials containing D-LA oligomers with carboxylic acid end-groups exhibited the largest mass loss. A larger fraction of DLA-4COOH resulted in increased mass loss, both at higher temperatures and after longer periods of time (Figure 4). A reason for this can be acid-catalyzed hydrolysis of ester bonds.20 Also, the large carboxylic chain-ends may hinder close packing, thus facilitating water penetration. A comparison with four alcoholic chain-ends shows an opposite effect. The

hydrolytic stability increased with increasing amount of DLA4OH, both at higher temperatures and after extended hydrolysis times. The same results were obtained for materials with linear D-LA oligomer. This is due to the increased stereocomplex crystallinity. Materials containing D -LA oligomers with six OH end-groups had a higher rate of degradation at higher temperatures compared to the materials with the D-LA oligomers with lower number of OH end-groups. This is likely a result from low Tg and lower total degree of crystallinity. This in turn is a consequence of the large number of arms in the D-LA oligomer, making it difficult for the linear PLLA to penetrate the star structure. pH Variations. The pH of the degradation medium was measured for all samples to further evaluate the amount of hydroxy acids released. pH decreased rapidly for all materials, 1216


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Figure 5. pH change at (a) 37 °C; (b) 37 °C, enlarged; and (c) 60 °C.

Figure 6. Mn of (a) the first peak, representing the PLLA matrix, and (b) the second peak, representing the D-LA oligomer, during degradation at different temperatures.

Figure 7. Decrease of molar mass (Mn) in percentage of the first peak during degradation at 22 °C for different fractions of the four D-LA oligomers added. Coefficients used: time, DLA, DLA-4OH, DLA-6OH, DLA-4COOH, and DLA∧2.

a large pH drop, partly due to the large number of carboxylic acid chain-ends. These materials also had a relatively low degree of stereocomplex crystallinity. The large pH drop despite of the low amount of stereocomplex crystallites implies a fast overall degradation rate; this is in accordance with the large mass loss. Four carboxylic end-groups clearly resulted in a larger pH drop compared to that of four alcoholic end-groups, while the effect on the pH of four or six alcoholic end-groups is similar. However, the pH for the materials with D-LA oligomers with four OH end-groups decreased slowly and was correlated with the small mass loss. Materials containing a D-LA oligomer with six OH arms also had a rather small pH drop, while the mass loss especially at 52 and 60 °C was rather high. These materials had a low degree of stereocomplex crystallinity which could facilitate water uptake, and consequently, scission of chains in larger and less acidic segments is possible. The longer chains could also more easily be released from the material, as they were not tied by the stereocomplex structure. The formation

and the lack of resolution over the time axis made it difficult to build a useful model for pH using experimental design. The pH decrease was somewhat faster with increasing amount of D-LA oligomer and stereocomplexation. After 1 week of degradation at 60 °C, clear differences between the materials were seen (Figure 5). Consistent with the results from the ESI-MS analysis, the fastest decrease was observed for the materials with DLA and DLA-4COOH. The type of D-LA oligomer clearly affected the correlation between mass loss and pH change. Materials with linear D-LA oligomers exhibited a large pH drop but a relatively small mass loss. This can be explained by the high degree of stereocomplex crystallization, seen by differential scanning calorimetry analysis. Stereocomplexation protects the material against hydrolysis, resulting in lower mass loss. However, the stereocomplex crystallites are joined together by many short tie chains which are susceptible to hydrolysis, leading to formation of shorter more acidic D-LA oligomers.2 Materials containing D-LA oligomer with acidic end-groups had 1217


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Figure 8. SEM micrographs showing the surface of stereocomplex with linear D-LA oligomer: (a) DLA-30, before degradation; (b) DLA-30, after 5 weeks of degradation at 60 °C; (c) DLA-50, before degradation; (d) DLA-50, after 5 weeks of degradation at 60 °C.

At higher temperatures, the molar mass loss was almost 100% after 13 weeks at 37 °C and after 5 weeks at 52 and 60 °C. Materials with 30% of DLA and DLA-4OH had a smaller molar mass loss compared to the other material; this behavior can be explained by their higher Tg (above 52 °C) and larger degree of crystallinity compared to the other materials. Before degradation, the DLA-6OH oligomer had the largest molar mass. The molar masses for both peaks decreased almost analogously, with a slightly slower decrease for the second peak representing the D-LA oligomers, which are most likely bound by stereocomplexation to a larger degree. This was also evident by the ESI-MS analysis where peaks for the D-LA oligomer were only visible after long degradation times and at high temperatures. Consistent with the results from the other analysis, the material with DLA-4COOH had the largest molar mass loss, probably due to the catalytic effect of the acidic chain-ends. Visual Examination/Changes in Surface Morphology. The materials had quite different surface morphologies. This was visible both by visual examination and by scanning electron microscopy. The materials with DLA and DLA-4OH were opaque and consisted of spheres (Figures 8 and 9). These spheres might be a result of rapid stereocomplex crystallite formation in solution before solution casting. Similar spherical participates have been observed before.27 With low temperature and high polymer concentration, more complete spherulite-like particles can be formed. In the materials DLA-50 and DLA4OH, the spheres contained smaller holes. These holes may be formed by incompleteness in the stereocomplex micronetwork structure and might be formed as the microcrystallites shrink

and migration of the degradation products of longer chain lengths give a slow pH decrease. Molar Mass Changes. Changes of the molar mass during degradation were determined with SEC. The molar mass (Mn) before degradation for PLLA was 116 900 ± 2 600 g/mol. Two peaks were observed in the chromatograms for the material with D-LA oligomers. The later eluting peak was assumed to be the shorter D-LA oligomer; the original molar masses of D-LA oligomers are given in Table 1. The pure PLLA material also displayed bimodal peaks with degradation time, due to increased crystallinity and faster molar mass decrease for the amorphous regions. Building a functional model was possible after aging at 22 °C but not after aging at the other temperatures, since the molar masses decreased too fast at higher temperatures (Figure 6). The results for the material with linear D-LA oligomers were somewhat conflicting, since the material with 30% D-LA oligomer seems to have increased stability while the addition of 50% D-LA oligomer decreased the stability (Figure 7). This might be caused by different Tg values. The end-group structures had different effects on the molar mass decrease. Materials containing increasing amount of D-LA with carboxylic end-groups (DLA-4COOH) had a larger decrease (Figures 6 and 7). The decrease was the largest among the materials at all temperatures. This is in agreement with the mass loss and pH results, where these materials showed less resistance against hydrolytic degradation. On the contrary, varying amounts of alcoholic end-groups showed little effect on the molar mass decrease. Increasing the number of alcoholic end-groups resulted in increased hydrolytic stability, and the stability increased with the amount of D-LA oligomer. 1218


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Figure 9. SEM micrographs showing the surface of stereocomplex with 4-armed D-LA oligomer with alcoholc end-groups: (a) DLA-4OH-30, before degradation; (b) DLA-4OH-30, after 5 weeks of degradation at 60 °C; (c) DLA-4OH-50, before degradation; (d) DLA-4OH-50, after 5 weeks of degradation at 60 °C.

hydrolysis and migration of the amorphous low molar mass chains, working as plasticizers, from the parts of the material not protected by crystallites. Tg in the second scan decreased during hydrolysis due to formation of shorter chains with high mobility. Also, when tie-chains are broken, short chains might be formed which are part of the crystallites in the first scan but remain in the amorphous region when cooling the material. For materials with DLA and DLA-4OH oligomers, the increase was smaller because of the lower hydrolysis rate. Degree of Crystallinity. In the first heating scan the stereocomplex materials had both melting peaks for homo- (at approximately 140 °C) and stereocrystallites (at approximately 185 °C). This value for melting of stereocrystallites was lower than that reported before, which is explained by the less perfect crystallites due to the branched D-oligomers. The materials with the largest degree of stereocomplex crystallinity had the highest melting temperatures, indicating that those materials also had the largest crystallites. For the materials with a larger amount of D-LA oligomer, the degree of sterocomplex crystals was approximately 2% higher compared to the lower amount of D-LA oligomer. In the second heating, only sterocomplex crystals were formed, and in a higher concentration compared to that of the first scan (Table 2). For some materials, stereocomplexation also occurred during cooling. This is due to the higher growth rate of the sterocomplex crystallites.29 Also, residual stereocomplex crystallites may act as nucleation sites for stereocomplex crystallization.30 The degree of homocrystals was naturally highest for PLLA and lowest for materials with a high degree of stereocomplex crystallites. The degree of

during drying of the films. The degradation of the amorphous phase between the stereocrystallites might proceed faster compared to the case of amorphous PLLA, since the density of end-groups in the amorphous regions is higher for crystalline stereocomplex PLA compared to amorphous PLLA.28 The presence of spheres is probably connected with the degree of stereocomplex crystallites, as the initial stereocomplex crystallinity was highest for materials with DLA and DLA-4OH. In materials with linear D-LA oligomers, the spheres had a larger diameter, probably related to the higher degree of strereocomplex crystallinity and faster stereocomplexation rate, which is a consequence of, for example, the lower molar mass of the D-LA oligomer. Cracks appeared in the spheres at prolonged degradation times at higher temperatures (5 weeks, 60 °C). Materials with DLA-6OH and DLA-4COOH had a smoother surface, with more and more holes forming with hydrolysis time as the amorphous fraction, not included in the stereocomplex crystallites, degraded (Figures 10 and 11). Also, as the crystallinity increases, the amorphous material can be pushed out from the crystalline regions and be more susceptible to hydrolysis. In the pure PLLA material, spherulites appeared during degradation. Over time, cracks were formed caused by degradation of the disordered spherulite centra (Figure 12). Thermal Analysis. The glass transition temperature was generally lower for the materials with higher amount of added D-LA oligomer, which is explained by the lower molar mass of D-oligomers and increasing number of chain-ends (Table 2). In the first heating scan, Tg for the materials increased with degradation time and temperature. This is caused by the 1219


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Figure 10. SEM micrographs showing the surface of stereocomplex with 6-armed D-LA oligomers with alcoholc end-groups: (a) DLA-6OH-30, before degradation; (b) DLA-6OH-30, after 5 weeks of degradation at 60 °C; (c) DLA-6OH-50, before degradation; (d) DLA-6OH-50, after 5 weeks of degradation at 60 °C.

homocrystallinity for PLLA increased rapidly over time, increasing more than the stereocomplex crystallinity for the other materials, probably making the degradation slower with time. Materials with linear D-LA oligomers had the highest degree of sterocomplex crystallinity and the largest crystals, because of easier crystallization (Table 2). The lowest degree of sterocomplex crystals was measured for the materials with Doligomers with more crowded six armed branching point (DLA-6OH) and highest overall molar mass. These materials also had additional melting peaks indicating different sized crystallites, implicating difficulties in forming crystals. This can also be explained by a melting-recrystallization process during heating of the relatively unstable crystals, causing crystals of low perfection to have time to melt and recrystallize a few degrees above and remelt.31 With degradation time and with increased temperature, the degree of crystallinity increased for all materials, as the amorphous material is more susceptible to hydrolysis (Figure 13). In addition, the shorter chains formed are more mobile and allow for reorientation of the crystalline phase, and the water uptake and higher temperature create more space and mobility. Also, the arms of the star D-LA oligomers are hydrolyzed and detached from the core. Materials with the higher amount of D-LA oligomer had the largest increase of the degree of crystallinity (Figure 13). The melting temperatures for the homocrystallites generally decreased with hydrolysis time. As the crystals degraded, degraded chains could crystallize, forming smaller crystals, and the chains could also rearrange into stereocrystallites. This decrease did not occur for

DLA-30 and DLA-4OH-30, which is possibly a result of the slow molar mass decrease. Tm for the stereocrystallites decreased slightly during the degradation process, except for the materials with the six armed OH D-LA oligomers, while the degree of crystallinity increased, indicating the formation of more crystallites.

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CONCLUSIONS

The degree of stereocomplexation was customized by utilizing different D-LA oligomer architectures. This will subsequently tune the material properties, hydrolytic stability, and release rate of acidic degradation products. Addition of D-LA oligomers with a linear structure or with four alcoholic end-groups led to the highest degree of stereocomplexation and the hydrolytically most stable materials. Stereocomplexation with standard linear D-LA oligomers resulted in equivalent mass loss compared to that of the material with D-LA oligomers with four alcoholic chain-ends, but a higher concentration of short acidic degradation products. Increasing the amount of these Doligomers from 30 to 50 wt % additionally increased the stereocomplexation, which subsequently decreased the hydrolysis rate. Increasing the number of chain-ends from four to six decreased the degree of stereocomplexation and increased the susceptibility to hydrolytic degradation. The largest degradation rate was observed when the D-LA oligomer had carboxylic chain-ends. This led to a lower degree of stereocomplexation and faster mass loss, which was additionally accelerated by the catalyzing effect of the carboxylic chain-ends. 1220


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Figure 11. SEM micrographs showing the surface of stereocomplex with 4-armed D-LA oligomer with carboxyl end-groups: (a) DLA-4COOH-30, before degradation; (b) DLA-4COOH-30, after 5 weeks of degradation at 60 °C; (c) DLA-4COOH-50, before degradation; (d) DLA-4COOH-50, after 5 weeks of degradation at 60 °C.

Figure 12. SEM micrographs showing the surface of PLLA: (a) before degradation; (b) after 5 weeks of degradation at 60 °C.

Table 2. Values for Tg and wc from the First (1) and Second (2) Heating Scan material DLA-30 DLA-50 DLA-4OH-30 DLA-4OH-50 DLA-6OH-30 DLA-6OH-50 DLA-4COOH-30 DLA-4COOH-50 PLLA

Tg1 (°C) 55.3 46.6 55.3 50.3 42.0 38.4 28.2 39.1 34.8

± ± ± ± ± ± ± ± ±

0.3 0.1 0.5 1.5 0.4 0.2 5.0 0.7 2.1

Tg2 (°C) 51.4 42.9 50.4 44.9 48.5 43.4 50.5 46.5 55.1

± ± ± ± ± ± ± ± ±

0.6 0.8 0.6 1.4 2.5 0.7 5.2 3.6 4.0

wc1 PLLA (%) 1.6 2.9 2.0 4.7 9.6 4.0 13.7 11.3 24.1

1221

± ± ± ± ± ± ± ± ±

0.1 0.1 0.1 1.3 0.1 0.1 1.0 0.0 0.4

wc1 PLLA/PDLA (%) 27.5 29.7 22.2 24.1 6.5 8.9 14.8 16.2

± ± ± ± ± ± ± ±

1.5 4.0 1.3 1.5 0.5 0.0 1.7 0.4

wc2 PLLA/PDLA (%) 27.1 36.5 22.3 28.3 11.5 23.0 21.3 27.7

± ± ± ± ± ± ± ±

2.8 0.4 1.1 2.1 5.2 1.2 1.2 1.7


Biomacromolecules

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Figure 13. Degree of stereocomplex crystallinity (wc PLLA/PDLA) during hydrolysis at 37 °C (a) from the first scan and (b) from the second scan.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46-8-790 82 74. Fax: +46-8-20 84 77. E-mail: aila@ polymer.kth.se. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the European Community Sixth Framework Programme Sustainable Microbial and Biocatalytic Production of Advanced Functional Materials (BIOPRODUCTION) under Contract Number NMP2-CT2007-026515 for financial support of this work.

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Customizing the Hydrolytic Degradation Rate of Stereocomplex PLA

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