Preparation and Properties of Plasticized Poly (lactic acid) Films

Biomacromolecules 2005, 6, 1789-1796

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Preparation and Properties of Plasticized Poly(lactic acid) Films Nadia Ljungberg*,† and Bengt Wessle´ n Department of Polymer Science & Engineering, Lund Institute of Technology, P.O. Box 124, SE-221 00 Lund, Sweden Received February 7, 2005; Revised Manuscript Received March 12, 2005

Poly(lactic acid), PLA, was blended with monomeric and oligomeric plasticizers in order to enhance its flexibility and thereby overcome its inherent problem of brittleness. Differential scanning calorimetry, dynamic mechanical analysis, transmission electron microscopy, and tensile testing were used to investigate the properties of the blends. Monomeric plasticizers, such as tributyl citrate, TbC, and diethyl bishydroxymethyl malonate, DBM, drastically decreased the Tg of PLA, but the blends showed no morphological stability over time since rapid cold crystallization caused a size reduction of the amorphous domains in PLA. Consequently, the ability of PLA to accommodate the plasticizer diminished with the increase in crystallinity and migration of the plasticizer occurred. Increasing the molecular weight of the plasticizers by synthesizing oligoesters and oligoesteramides resulted in blends that displayed Tg depressions slightly smaller than with the monomeric plasticizers. The compatibility with PLA was dependent on the molecular weight of the oligomers and on the presence or not of polar amide groups that were able to positively interact with the PLA chains. Aging the materials at ambient temperature revealed that the enhanced flexibility as well as the morphological stability of the films plasticized with the oligomers could be maintained as a result of the higher molecular weight and the polar interactions with PLA. Introduction Many interesting monomers with properties that are favorable to the environment can be found in the metabolisms and cycles of nature1. One such substance is lactic acid, that can be obtained from waste streams from food processing2. Lactic acid can be polymerized into poly(lactic acid), PLA, a biodegradable polyester. The main synthetic route is ringopening polymerization3 of the cyclic dimer of lactic acid. PLA possesses many physical characteristics that make it suitable for replacing commodity polymers.4 Among its advantages are high strength, biocompatibility, thermoplastic fabricability, good crease-retention, grease and oil resistance and excellent aroma barrier.5-8 PLA exists with various stereo-regularities as a result of the two enantiomeric forms (D and L) of lactic acid. Enantiomerically pure PLA results in a semicrystalline polymer. Its amorphous state has a glass transition temperature (Tg) of around 55 °C and its crystalline entities present a melting point (Tm) of around 175 °C. PDLLA, which contains both the D and L form of lactic acid, is generally amorphous with a somewhat lower Tg.9 Investigations on the crystallization of the enantiomerically pure PLLA have shown that the cooling rate of PLA from the melt is of great importance for the crystallinity. Cooling rates of less than 2 °C/min resulted in an effective crystallization with large spherulites, whereas a cooling rate of 20 °C/min or higher left the material completely amorphous.10 * Corresponding author. Tel.: +33 4 76037608. Fax: +33 4 76547203. E-mail: [email protected]. † Present address: Centre de Recherches sur les Macromole ´ cules Ve´ge´tales (CERMAV-CNRS), Universite´ Joseph Fourier, P.O. Box 53, F-38041 Grenoble, France.

It was also found that a lower molecular weight of the polymer resulted in an increased crystallization rate as well as in a more complete crystallization because of increased chain mobility.11 A large amount of crystallinity in PLA results in it having a high modulus and strength, but also a brittle nature and a lack in toughness. These drawbacks are especially disadvantageous in the film extrusion industry. However, there are means of improving the flexibility and toughness in PLA by copolymerizing or blending with other substances. Blending is much more cost-effective than copolymerization and thus the more frequently used method. The compatibility of the two components in the blend affects physical properties such as Tg, Tm, crystallinity, and morphology. Consequently, these properties determine the properties of the macroscopic material, e.g., processability, rigidity, impact and tensile strength, barrier properties, and degradation. A large number of substances have been copolymerized or blended with PLA in order to increase its flexibility, and among them can be mentioned poly(ethylene oxide),12-15 poly(hydroxy butyrate),16-24 and poly(-caprolactone).14,25-32 Various low molecular weight compounds have also been investigated as potential plasticizers for PLA. The role of the plasticizer is to reduce the modulus of elasticity in PLA, and it is of great importance that the plasticizer be compatible with the polymer in order to be evenly distributed in its matrix. There are numerous reports on the use of citrate esters as plasticizers for PLA for biomedical applications.33-36 PLA has also been blended with citrates at concentrations between 10 and 30 wt % for film applications. DSC measurements

10.1021/bm050098f CCC: $30.25 © 2005 American Chemical Society Published on Web 04/08/2005

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displayed single Tg values for all studied compositions far below that of neat PLA.37,38 There was also an enhanced crystallinity as a result of the increased molecular mobility of the PLA chains. Oligomeric lactic acid, OLA, has been found to induce a significant depression of Tg as compared to neat PLA, and the depression was larger with an increasing amount of OLA. It was also found that the crystallization temperature decreased with an increasing amount of plasticizer since mobility of the PLA chains was facilitated as a consequence of the lowered Tg.37 Melt-mixing of PLA and PEG at various concentrations displayed that PEG acted as an efficient plasticizer for PLA. A blend containing 30 wt % PEG resulted in a material with a Tg well below room temperature and allowed for higher elongation at break and lower modulus values as compared to neat PLA.37,39-42 However, the low Tg rendered the blend unstable over time resulting in an increase in modulus accompanied by a decrease in the elongation at break.41 This was attributed to the enhanced crystallization of the PEG chains, which caused phase separation in the blend, gradually enriching the amorphous phase in PLA and increasing its Tg.40,42 Cooling the blend from the melt state at a rate of 5 °C/min or less caused crystallization of both PLA and PEG to occur. This also resulted in a gradual increase of the Tg. However, both the aging and the crystallization ceased when the Tg reached ambient temperature.39 The present study is focused on successful means of plasticizing PLA in order to render it suitable for packaging applications. Important issues are the compatibility of the plasticizer in the PLA matrix, the morphological stability of the plasticized material and the prevention of the plasticizer migrating from the material bulk. Experimental Section Materials. Tributyl citrate (TbC) was purchased from Acros Organics (Springfield, NJ). Diethylene glycol (DEG) and the catalyst tetrabutyl titanate were obtained from Aldrich (Milwaukee, WI). Diethyl bishydroxymethyl malonate (DBM), adipoyl dichloride (AdCl), succinyl dichloride (SuCl), and triethylene glycol (TEG) were purchased from Aldrich (Steinheim, Germany). Triethylene glycol diamine (TA) with a molecular weight of 148 g/mol was kindly supplied by Huntsman Corporation (Rozenburg, The Netherlands). All reactants, along with other chemicals and solvents, were used as received. PLA, with a molecular weight (Mw) of approximately 100 000 g/mol and a polydispersity index of 2.4, was supplied by Fortum, Finland. The melting temperature (Tm) and the glass transition temperature (Tg), as given by the supplier, were 175 and 52 °C, respectively. The enantiomeric form of the PLA was 100% L. The polymer was dried for 30 h at 40 °C in a Piovan H31M drier with a Piovan DS403 control unit (Venice, Italy) and then stored in sealed PE-bags wrapped in aluminum foil in a desiccator at ambient temperature. Oligomer Synthesis. The two monomeric units TbC and DBM were separately used to synthesize three groups of plasticizers denoted TbC-oligoesters, DBM-oligoesters, and DBM-oligoesteramides. A simplified reaction scheme dis-

Ljungberg and Wessle´ n

Figure 1. Synthesis path for (a) the transesterification of TbC with DEG producing TbC-oligomers and (b) the esterification of DBM with either AdCl (m ) 4) or SuCl (m ) 2) giving DBM-oligomers as a first step and the DBM-oligoesteramide after adding TA.

playing the chemical structures of the three groups is shown in Figure 1. The polymerization for the TbC-oligoesters (Figure 1a) was performed as a transesterification reaction between TbC and DEG. The temperature was kept at 130 °C, and there was a constant flow of N2 gas through the reaction vessel. Butanol (BuOH) was distilled from the reaction mixture and collected in a trap device. The second series of oligoesters had DBM as the monomeric unit, and esterification reactions were carried out with DBM and an acid dichloride (Figure 1b). The acid dichloride was either AdCl (m ) 4) or SuCl (m ) 2), and the polymerizations were performed in chloroform at 60 °C with a constant flow of N2 gas through the reaction vessel. Hydrochloric acid (HCl) was emitted from the reaction mixture and collected in a trap device containing distilled water. The products from the syntheses were denoted DBM-A when the reaction was carried out with AdCl and DBM-S when SuCl was used. The third group of potential plasticizers consisting of an oligoesteramide and an oligoester was polymerized as a twostep reaction where the first step produced an ester similar to the DBM-A ones, terminated with AdCl rather than DBM in order for it to be able to react further. In the second step, this DBM-A ester was then linked together by either TA, giving the oligoesteramide DBMATA, or by TEG, giving the oligoester DBMAT. Figure 1b shows the chemical structure for DBMATA. Detailed information of the synthesis routes has been given earlier.43-45 Size Exclusion Chromatography (SEC). The oligomers from the syntheses were characterized by means of SEC. SEC analyses were run at room temperature in THF (Labscan Ltd, Ireland, concentration 1-2 wt %) on Waters’ Styragel columns (105, 104, 103, 500 Å) or two Waters’ Ultrastyragel

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Preparation and Properties of Plasticized PLA Films

linear columns, using differential refractive index and viscometry detectors (Dual detector, 250, Viscotek). Blending. Blending experiments, with a blend composition of 15 wt % plasticizer and 85 wt % PLA, were performed in a Midi 2000 co-rotating twin screw extruder from DSM Research (Heerlen, The Netherlands). The extruder had a chamber volume of 15 cm3 and was chosen because effective blending of small volumes was especially important since the syntheses only produced limited amounts of the oligomers. The temperature profile ranged from 220 °C in the feeding zone down to 180 °C in the die, and the screw speed was 100 rpm. The melt was blended for 2 min and then extruded as a strand through a single-filament die with the dimension 10 × 1 mm. The strand was wound and cooled on a glass cylinder with a diameter of 15 cm and stored at ambient temperature in sealed PE bags. Also the neat PLA was passed through the extruder so as to have the same thermal history as the blends. Film Preparation. Films were prepared from the blends and neat PLA by heat pressing at 200 °C. Short strands of the blends were placed in a template frame to ensure a constant film thickness (either 0.35 mm or 0.08 mm) and covered with aluminum foil sheets to prevent sticking to the press plates. This assembly was then placed between the press plates for 3.5 min, without applying pressure, until the material was properly melted, and then pressed for 30 s at a pressure of 9 × 105 Pa. The samples were removed from the press plates and cooled in air until they reached ambient temperature (approximately 20 s). The specimens were then stored in sealed plastic bags in air awaiting analysis. Differential Scanning Calorimetry (DSC). DSC was conducted on a Q1000 from TA Instruments (New Castle, Delaware) on the 0.35 mm thick film materials as well as on the pure plasticizing agents. All DSC scans were carried out in hermetic pans, under N2 atmosphere on approximately 5 mg of material. The scans of the films were run from -60 to +210 °C (10 °C/min) and back down to -60 °C (10 °C/ min). During the heating ramp, the glass transition, cold crystallization, and melting of the material occurred and during the cooling the crystallization of the material could be observed. The DSC traces of the pure plasticizing agents were recorded from -90 to +50 °C at 10 °C/min in order to observe their glass transitions. Dynamic Mechanical Analysis (DMA). DMA was performed on a DMA 2980 from TA Instruments (New Castle, Delaware). The experiments were conducted in tensile mode under isochronal conditions at a frequency of 1 Hz. Curves displaying storage (E′) and loss (E′′) moduli were recorded as a function of temperature between -60 and +150 °C at a heating rate of 3 °C/min. The amplitude was chosen at 5 µm, a pre-load force of 0.010 N was applied, and the autostrain was set to 115%. The shape of the film samples was rectangular, approximately 15 × 5 × 0.35 mm. The DMA 2980 was also used to perform tensile testing on the film material. The measurements were performed in controlled force mode on rectangular samples approximately 7.5 × 3 × 0.08 mm under isothermal conditions at 30 °C. The force was ramped at 10 N/min from 0 to 17 N, unless the sample broke before. 10 tests were carried out on each

Table 1. Molecular Weights (Mn and Mw), No. of Repeating Units (n), Glass Transition Temperatures (Tg), and Solubility Parameters (δ) for PLA and 10 Plasticizing Agents substance

Mn (g mol-1)

Mw (g mol-1)

PLA TbC TbC-3 TbC-7 DBM DBM-A-8 DBM-A-18 DBM-S-4 DBM-S-7 DBMAT DBMATA

41500 360 1000a 2200a 220 2500a 5300a 1300a 2100a 1800a 1600a

100 000 4550a 63 600a 4200a 8900a 1800a 3500a 3200a 2300a

n 1 3b 7b 1 8b 18b 4b 7b

Tgc (°C)

δd (J cm-3)1/2

52 -89 -62 -32 -50 -52 -42 -26 -29 -47 -53

20.1 19.6 18.6 18.6 20.2 18.7 18.7 18.4 18.4 19.1 20.0

a Measured by SEC according to polystyrene standards. b Calculated number of repeating units according to Figure 1. c Measured by DSC at the inflection point of the change in the ∆Cp baseline. d Calculated with group molar attraction constants from the Hoy series.47

film sample and average values of stress and strain at break were calculated. Transmission Electron Microscopy (TEM). The film samples were cut into 70 nm sections in a Leica Ultracut T cryo ultra-microtome, with a sample temperature of -70 °C and a knife temperature of -40 °C. Water with 50% dimethyl sulfoxide was used as trough liquid. All sections were stained in gas phase with ruthenium tetroxide for 5 min and were then examined in a Philips CM 10 transmission electron microscope. Magnification was either 11 500× or 105 000×, and the micrographs were recorded on negative films which were subsequently scanned. Aging. To study the effect of storing the plasticized material, film samples of the blends were left to age at ambient conditions for 6 weeks. The investigation of naturally aged samples was deemed of interest in order to reproduce the current storage conditions used in the packaging material industry. Results and Discussion Oligomer Characterization. The choice of plasticizers to be used as modifiers for PLA is limited by the requirements of the packaging application. Only nontoxic substances approved for food contact can be considered as plasticizing agents in materials for food packaging. In addition, the substances must be biodegradable. The monomeric units used in this study were chosen since they had previously44,46 displayed good plasticizing abilities when blended with PLA. It was also of importance that they had functional groups in order for them to react easily and create oligomers. SEC analysis was used to estimate the apparent molecular weights (Mn and Mw) of all reaction products according to polystyrene standards, and the results can be found in Table 1. The number of repeating units (n) in the oligomers was calculated based on the chemical structures in Figure 1 and the esters were denoted accordingly, i.e., TbC-3, TbC-7, DBM-A-8, DBM-A-18, DBM-S-4, and DBM-S-7. Table 1 also displays the Tg of the substances as determined by DSC measurements. It can be seen that the glass transition

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Figure 2. DSC trace recorded for neat PLA. Table 2. Thermal Data Obtained by DSC Measurements for Films of Neat and Plasticized PLA substance

Tg

cold crystallization temp. (°C)

melting temp. (°C)

∆Hf (J/g)

neat PLA PLA/TbC PLA/TbC-3 PLA/TbC-7 PLA/DBM PLA/DBM-A-8 PLA/DBM-A-18 PLA/DBM-S-4 PLA/DBM-S-7 PLA/DBMATA PLA/DBMAT

52 25 33 43 29 39 42 36 40 35 39

91 68 76 78 72 74 76 75 80 63 71

173 166 168 170 165 170 170 167 169 167 170

17a 20 19 18 21 20 20 20 20 26 22

a Corresponds to a degree of crystallinity of 18% assuming that ∆H f for 100% crystalline PLA equals 93 J/g.49

temperature increased with increasing molecular weight of the plasticizers within each series. The solubility parameter of any chemical substance can be calculated according to the equation δ)

∑ FF M0

where δ represents the solubility parameter, F the density, M0 the molecular weight, and F the group molar attraction constants according to the Hoy series.47 As also displayed in Table 1, the calculations showed that all substances had solubility parameters close to that of PLA but that the three substances with the nearest values were DBM, DBMATA, and TbC. Thermal Properties. The thermal properties of the film materials were investigated by means of differential scanning calorimetry. Figure 2 shows a typical DSC trace for neat PLA displaying a shift in the signal baseline related to the glass transition, two exothermic peaks of cold and regular crystallizations, respectively, and an endothermic melting peak. The thermal characteristics from the DSC runs of all of the blends are summarized in Table 2. The Tg values identified in the DSC traces were lower in the blended materials than in neat PLA. This can be visualized in Figure 3 which displays DSC traces for blends of (a) PLA/TbC, PLA/TbC-3, and PLA/TbC-7; (b) PLA/DBM, PLA/DBM-

S-4, and PLA/DBM-S-7; and (c) PLA/DBM-A-8, PLA/ DBM-A-18, PLA/DBMAT, and PLA/DBMATA. In terms of efficiency as plasticizers for PLA, it was observed that blends containing the monomeric units showed the lowest Tg values. The heat of fusion (∆Hf) of the materials after blending and film pressing (Table 2) was determined by subtracting the enthalpies for premelt crystallization and cold crystallization from the melting enthalpy (Figure 2). In almost all of the blends, the ∆Hf increased only slightly as compared to neat PLA. However, for the blend containing 15 wt % DBMATA, the ∆Hf showed a larger increase. This result was believed to be directly related to an increased ability for short-range motion of the PLA chains in this blend, as compared to the others. As a consequence, a larger portion of the amorphous phase was allowed to crystallize during the cooling of the plasticized films. It should, however, be mentioned that a slight shift was noticed in the baseline of all DSC traces (Figure 3), suggesting that crystallization occurred continuously between the cold crystallization and the premelt crystallization, i.e., in the temperature region 80-150 °C. This could not be taken into account when calculating the ∆Hf since it was very difficult to assess the baseline, and thus, the ∆Hf values may lack in accuracy. The DSC measurements did not reveal any additional Tg’s at low temperature indicating phase separation for any of the blends. This was somewhat unexpected when considering the relatively high molecular weights of TbC-7 and DBMA-18. It was assumed that the sensitivity of the DSC measurements was too low for detecting such transitions, especially since the size of a potential separate phase would be very small. Dynamic mechanical analysis, on the other hand, is known to have a higher sensitivity when it comes to detecting phase separation in blends, and thus, DMA measurements were performed in order to further investigate the blended film material. TbC-Plasticizers. Figure 4 displays the temperature dependence of storage and loss moduli curves from dynamic mechanical measurements at 1 Hz comparing neat PLA with blends of PLA and plasticizers from the TbC group. The TR values of the films are here defined as the temperature location of the maximum of the loss modulus, E′′, obtained for the R-relaxation (associated with the glass transition) at the frequency of 1 Hz. In accordance with the DSC measurements, it was found that both TbC-3 and TbC-7 decreased the TR of PLA, however, not to the extent of the monomeric unit, TbC, that decreased the TR with 25 °C. It could be seen from the storage modulus curves that the cold crystallization of the materials occurred at a lower temperature the lower the TR. The blend containing TbC-7 showed a depression of the TR but also a secondary peak around -20 °C indicating partial phase separation in the system. This may result from the large polydispersity of this oligomer (Table 1). It is believed that the low molecular weight species of TbC-7 induced the plasticization effect in the PLA whereas the higher molecular weight species were more incompatible with the PLA and thus caused the phase separation to occur. The storage modulus curves of the PLA/TbC and PLA/TbC-3



Figure 3. DSC traces recorded for blends of (a) PLA/TbC, PLA/TbC-3, and PLA/TbC-7; (b) PLA/DBM, PLA/DBM-S-4, and PLA/DBM-S-7; and (c) PLA/DBM-A-8, PLA/DBM-A-18, PLA/DBMAT, and PLA/DBMATA.

Figure 4. Temperature dependence of storage and loss modulus curves from DMA runs comparing blends of PLA/TbC, PLA/TbC-3, and PLA/TbC-7 with neat PLA.

samples displayed no secondary peaks at temperatures around the respective Tg values of the neat plasticizers (-89 for TbC and -62 for TbC-3, cf Table 1). The curves in Figure 4 were thus truncated at -60 °C in order to maintain the symmetry of the other curves. The PLA/TbC and PLA/TbC-3 samples were naturally aged at ambient temperature (approximately 22 °C) for 6 weeks to investigate their morphological stability. Figure 5 displays loss modulus curves as a function of temperature comparing the aged and unaged material. The blend containing TbC was seemingly compatible with PLA without any signs of phase separation in the unaged state. The aged

Figure 5. Temperature dependence of loss modulus curves from DMA runs comparing aged and unaged blends of PLA/TbC and PLA/ TbC-3.

sample, however, showed signs of significant broadening of the R-transitional peak and a shift for the cold crystallization peak toward the cold crystallization temperature of neat PLA. This indicates that the material had phase separated as a result of cold crystallization due to the depressed glass transition temperature. The Tg around room temperature provided for an increased segmental mobility of the PLA chains allowing them to rearrange and slowly crystallize. DSC measurements confirmed this suggestion as it was found that the heat of fusion in the films had increased upon aging: from 19 to 32 J/g for the PLA/TbC blend.48 In addition, the shift to higher

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Figure 7. Temperature dependence of (a) storage and (b) loss modulus curves from DMA runs displaying blends of PLA and the DBM plasticizers.

Figure 6. Micrographs from TEM analysis displaying the blend of PLA/TbC-3 at (a) 11 500× and (b) 105 000× magnification.

temperature of the cold crystallization peak indicated a largerscale phase separation causing migration of the plasticizer to the film surface, as had been observed in previous studies.43-45,48 The aged sample of the blend containing 15 wt % TbC-3 displayed a slight additional low-temperature peak in the loss modulus curve suggesting that also this material had undergone phase separation to some extent as a consequence of the aging. However, the modulus increase caused by the cold crystallization remained at the same temperature (∼90 °C) as that of the unaged material. The fact that the cold crystallization temperature was unchanged would suggest that despite the phase separation the plasticizer was still more or less incorporated in the material and that it had not migrated to the surface of the film. TEM analysis was performed on the PLA/TbC-3 sample to further support the suggestion of microphase separation in the system. Figure 6 shows micrographs of finely cut sample sections at (a) 11 500× and (b) 105 000× magnification. The plasticizer phase appears black in the micrographs since the ether bonds in TbC-3 had been stained with ruthenium tetroxide. It can be seen that microphase separation had occurred and that the TbC-3 phases ranged in size between 10 and 100 nm (the elongation of the domains was

a result of the sample preparation by microtoming). The combination of results from DMA measurements and TEM analysis indicated that the higher molecular weight of TbC-3 as compared to TbC decreased its inclination to migrate to the film surface despite the occurrence of phase separation, thus enhancing the stability of the material in the investigated time period. DBM-plasticizers. In a similar approach to the TbCplasticizers, the blends of PLA with the DBM-plasticizers were investigated by means of DMA, and the temperature dependence of (a) storage and (b) loss modulus curves for the samples are displayed in Figure 7. It can be seen that the blend containing DBM-A-18 exhibited a small additional peak at -15 indicating the early stages of phase separation in the system. Thus, DBM-A-18 was not totally compatible with PLA as a result of the larger molecular weight of this substance as compared to the others. The remaining oligomers, however, showed no signs of phase separation and were considered compatible with PLA at the concentration of 15 wt %. To give an overview of the efficiency of the plasticizers, the depression of TR (∆TR) was plotted for the molecular weights (Mn) of each of the DBM plasticizers. Figure 8 displays the resulting curve. It can be seen that DBM-A-8 and DBM-S-4 contributed to the same decrease in TR although there was a large difference between their respective molecular weights. This would therefore suggest that an increase in the hydrophobicity of the chains linking the DBM monomers together would increase the compatibility of the resulting oligomer with PLA. The largest depression of TR was obtained by plasticization with the monomeric unit DBM, but there was only a slight difference between the ∆TR values of the PLA/DBM and the PLA/DBMATA blends. DBMAT also showed a signifi-



Figure 8. Depression of TR as a function of the plasticizer molecular weight for blends of PLA and the DBM plasticizers.

cant decrease in TR, but the effect was not as large as with DBMATA suggesting that the amide groups in the esteramide enhanced the plasticization by increasing the solubility of PLA by polar interactions and hydrogen bonding. Samples of blended materials containing DBM, DBMATA, and DBM-A-8 were naturally aged at ambient temperature (approximately 22 °C) in sealed plastic bags for 6 weeks to investigate their morphological stability. Figure 9 displays the temperature dependence of the loss modulus of aged and unaged films of the blends. The aged blend containing 15 wt % DBM displayed two separate R-relaxational peaks and a peak for the cold crystallization. The peak portraying the TR for the blend had been shifted toward higher temperatures as compared to the unaged sample. This shift along with the distinct additional peak at -30 °C was a strong indication of phase separation in the system upon aging. The cold crystallization peak had been shifted from 80 °C in the unaged sample to 100 °C in the aged one, thus coming very close to that of neat PLA. This indicates that the low molecular weight of DBM and the low glass transition temperature of the blend significantly facilitated the migration of the plasticizer from the bulk of the material to the film surface. For the blends containing DBM-A-8 and DBMATA, the aged samples showed no major differences from the unaged ones indicating that phase separation did not occur in these systems during the aging period of 6 weeks. The cold crystallization peaks for the aged samples were also unchanged in comparison to the unaged ones. According to the aging experiments, both the blend containing DBM-A-8 and the one with DBMATA displayed morphological stability over the aging period of 6 weeks. Seeing as the PLA/DBMATA blend showed better plasticization efficiency with regards to the depression of TR it was chosen for tensile testing experiments. Figure 10 displays the stress as a function of strain comparing both aged and unaged films of neat PLA with aged and unaged films of the blend containing DBMATA. It can be seen that there is a large difference between the blended material and neat PLA. The PLA/ DBMATA blend was very flexible with a strain at break above 200% whereas the neat PLA only showed a strain at break value around 20%. The tensile tests conducted on the aged samples revealed that the PLA film remained virtually

Figure 9. Temperature dependence of loss modulus curves from DMA runs comparing aged and unaged blends of PLA/DBM, PLA/ DBM-A-8, and PLA/DBMATA.

Figure 10. Stress/strain curves comparing aged and unaged films of PLA/DBMATA and neat PLA.

unchanged. For the blend, there was a slight drop in the stress at break as compared to the unaged film, whereas the strain at break value was equivalent to that obtained before the aging. Thus, increasing the molecular weight of the plasticizer (i.e., polymerizing DBM) as well as indroducing polar interactions in the shape of amide bonds resulted in a system with an increased flexibility as compared to neat PLA that also displayed morphological stability within the storage period.

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Conclusions Plasticization of the semicrystalline polymer PLA with monomeric compounds such as TbC or DBM was possible but resulted in an unstable system due to morphological changes caused by cold crystallization. The reduction of the glass transition temperature of PLA as a result of the presence of the plasticizer led to a system where the PLA chains had enough mobility to rearrange. Therefore, upon aging, cold crystallization occurred, having the effect that the amorphous domains in PLA were reduced in size. This resulted in a reduction of the critical amount of plasticizer and further on to phase separation. There seems to exist a competition between the efficiency of a plasticizer and the kinetics of the aging or cold crystallization in PLA. The more the material is plasticized, the larger the increase in mobility and the faster the cold crystallization process. It is therefore imperative to find an optimum where the PLA is sufficiently flexible for the desired application without the cold crystallization taking place too fast. The low molecular weight of the plasticizers TbC and DBM significantly facilitated their migration from the bulk of the material to the film surface, ultimately leading to the blend regaining the inherent brittle properties of neat PLA. A way to overcome the migration problem was to increase the molecular weight of the plasticizers, while maintaining the same chemical structure. Two series of oligomers were obtained by transesterification and esterification reactions. It was shown that with oligoesters, such as DBM-A-8 or TbC-3, it was possible to reduce the migration rates and thus obtain an enhanced stability of the material. By introducing hydrogen bonding between PLA and the plasticizer, e.g., by the polar amide groups in the oligoesteramide DBMATA, an increased compatibility was achieved in the material and its morphological stability was enhanced even further. Acknowledgment. The VINNOVA and Industry sponsored Centre for Amphiphilic Polymers (CAP) and Fortum Corporation are gratefully acknowledged for financial support. Many thanks are also expressed to Dr. Helen Hassander for performing the TEM analyses and to Dr. Didier Colombini for valuable discussions. References and Notes (1) Lindblad, M. S.; Liu, Y.; Albertsson, A.-C.; Ranucci, E.; Karlsson, S. AdV. Polym. Sci. 2002, 157, 139. (2) Bonsignore, P. V.; Coleman, R. D. NonwoVens Conference Proceedings; TAPPI Press: Atlanta, GA, 1992; p 129. (3) Lo¨fgren, A.; Albertsson, A.-C.; Dubois, P.; Je´roˆme, R. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1995, C35, 379. (4) Vert, M.; Schwarch, G.; Coudane, J. J. Macromol. Sci., Pure Appl. Chem. 1995, A32, 787. (5) Lunt, J. Polym. Degrad. Stabil. 1998, 59, 145. (6) Wehrenberg, R. H., II. Mater. Eng. 1981, 94, 63. (7) Drumright, R. E.; Gruber, P. R.; Henton, D. E. AdV. Mater. 2000, 12, 1841.

Ljungberg and Wessle´ n (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49)

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Preparation and Properties of Plasticized Poly (lactic acid) Films

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