Langmuir 2000, 16, 3401-3406
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Hydrolytic Kinetics of Biodegradable Polyester Monolayers Won-Ki Lee† and Joseph A. Gardella, Jr.* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000 Received June 22, 1999. In Final Form: December 9, 1999 The rate of hydrolysis of Langmuir monolayer films of a series of biodegradable polyesters was investigated at the air/water interface. The present study investigated parameters such as degradation medium, pH, and time. The hydrolysis of polyester monolayers strongly depended on both the degradation medium used to control subphase pH and the concentration of active ions. Under the conditions studied here, polymer monolayers showed faster hydrolysis when they were exposed to a basic subphase rather than that of acidic or neutral subphase. The basic (pH )10) hydrolysis of [poly(l-lactide)/polycaprolactone] (l-PLA/PCL 1/1 by mole) blend was faster than that of each homopolymer at the initial stage. This result is explained by increasing numbers of base attack sites per unit area owing to the very slow hydrolysis of PCL, a “dilution effect” on the concentration of l-PLA monolayers. Conversely the hydrolytic behavior of l-lactide-cocaprolactone (1/1 by mole) was similar to that of PCL even though the chemical compositions of the blend and the copolymer are very similar to each other. The resistance of the copolymer to hydrolysis might be attributed to the hydrophobicity and the steric hindrance of caprolactone unit in the copolymer.
Introduction In recent years, there are increasing demands for the use of degradable polymers, in particular to minimize polymer waste management caused by synthetic nondegradable polymers1 and for various biomedical applications.2-4 Biodegradation usually occurs through a hydrolyzable linkage along the main polymer chain, leading to a lower molecular weight and eventually to monomer production. Well-known synthetic hydrolyzable polymers are polyesters,1,5,6 polyanhydrides,7,8 and poly(amino acids).1 Medical applications of these materials have led to significant developments, such as the controlled release of drugs, surgical implants, and bone plates, because of the controlled degradation. Many variables affecting degradability have been revealed, including morphology, molecular weight, hydrophobic/hydrophilic properties, and type of hydrolyzable linkage.1,9 Many workers have attempted to control the range of mechanical properties and hydrolytic degradability of many types of polyesters by blending and copolymerization for biomedical applications because they break down to common metabolites such as lactic or glycolic acid.10-13 * To whom correspondence should be sent. † Present address: Polymer Chemistry Laboratory, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wakoshi, Saitama 351-0198, Japan. (1) Scott, G.; Gilead, D. Degradable Polymers; Chapman & Hill: London 1995. (2) Nijenhuis, A. J.; Grijpma, D. W.; Pennings, A. J. Macromolecules 1992, 25, 6419. (3) Tsuji, H.; Ikada, Y. Polymer 1995, 36, 2709. (4) Iwata, T.; Doi, Y. Macromolecules 1998, 31, 2461. (5) Fredericks, R. J.; Melveger, A. J.; Dolegiewtz, L. J. J. Polym. Sci. Polym. Phys. Ed. 1984, 22, 57. (6) Chu, C. C. J. Appl. Polym. Sci. 1981, 26, 1727. (7) Ron, E.; Mathiowitz, E.; Mathiowitz, A.; Domb, A.; Langer, R. Macromolecules 1991, 24, 2278. (8) Hayashi, T. Prog. Polym. Sci. 1994, 19, 663. (9) Pitt, C. G., Gratzel, M. M.; Kimmel, G. L. Biomaterials 1981, 2, 215. (10) Mathisen, T.; Masus, K.; Albertsson, A. Macromolecules 1989, 22, 3842. (11) Davies, M. C.; Shakesheff, K. M.; Shard, A. G.; Domb, A.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Macromolecules 1996, 29, 2205. (12) Mallarde, D.; Valiere, M.; David, C.; Menet, M.; Guerin, Ph. Polymer 1998, 39, 3387.
Many different analytical methods have been applied to determine the degradation rate of polyesters. One suitable technique to study the hydrolysis behavior is to use a Langmuir film balance to study polymers at the air/water interface, since the hydrolysis of a polyester usually occurs though the cleavage of ester groups and eventually produces water-soluble oligomers and monomers. The hydrolysis of polyester monolayers would result in a change in the occupied area when the monolayer is maintained at a constant surface pressure. The study of polyester monolayers at the air/water interface will, therefore, give a fundamental understanding of the hydrolytic mechanism of polyesters, such as polymers and copolymers derived from lactic acid (LA), glycolic acid (GA), and -caprolactone (CL). Previously, there have been several reports on the structural study of polyester monolayers at the air/water interface.14,15 Recently, Ivanova et al. reported the hydrolytic behavior of poly(dl-lactide) monolayers spread on acidic (pH 1.9 by HCl) and basic (pH 11.4 by Na2HPO4 and NaOH) subphases for short times.16 They calculated the hydrolysis rates by assuming that the reaction products from fragment hydrolysis are soluble when the number of lactic units in sequence is below 4. In this study, the hydrolytic degradation of dl- and l-PLA, PCL, LA-co-CL, and a blend (1/1 by mole) has been systematically investigated at the air/water interface as a function of degradation medium, pH, and time. Experimental Section Materials. l- and dl-Lactide were obtained from Aldrich and recrystallized from anhydrous ethyl acetate. Glycolide (Polysciences), -caprolactone (Aldrich), and stannous octoate (Sigma) were used as received. Poly(dl-lactide-co-glycolide) (dl-LA-coGA 7/3 by weight) was purchased from Polysciences Co. All other (13) Madden, L. A.; Anderson, A. J.; Asrar, J. Macromolecules 1998, 31, 5660. (14) Lambeek, G.; Vorenkamp, E. J.; Schouten, A. J. Macromolecules 1995, 28, 2023. (15) Vila, N.; Minones, J.; Iribarnegaray, E.; Conde, O.; Casas, M. Colloid Polym. Sci. 1997, 275, 580. (16) Ivanova, T.; Panaiotov, I.; Boury, F.; Benoit, J. P.; Verger, R. Colloids Surf., B 1997, 8, 217.
10.1021/la990800r CCC: $19.00 © 2000 American Chemical Society Published on Web 02/04/2000
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Table 1. Characterization of the Materials Used in This Study Mw
Mw/Mn Tg (K) Tm (K)
l-PLA 57 000 dl-PLA 15 000 dl-LA-co-GA
2.9 1.4
l-LA-co-CL
8 200
1.8
16 000
1.6
PCL a
324 318
sources
440 synthesized for this study n.d.a synthesized for this study polysciences (7/3 by weight) n.d. synthesized for this study (1/1 by mole) 321 synthesized for this study
Not determined.
chemicals were of reagent grade and were used without further purification. Bulk polymerizations of homopolymers and copolymer were carried out in vacuum-sealed glass ampules under N2 at a given temperature using stannous octoate as a catalyst. The products in the ampules were dissolved in chloroform and then precipitated with excess of methanol. The copolymer composition was confirmed by 1H NMR spectroscopy (GEMINI 300, CDCl3). The molecular characteristics of homopolymers and the copolymers used in this study are listed in Table 1. To investigate the surface composition of the blend film and the copolymer before and after hydrolysis by X-ray photoelectron spectroscopy (XPS), thick films (thickness: ca. 5 µm) were prepared by dissolving each component in chloroform and solventcasting onto aluminum foil dishes. The bulk composition of the blend (l-PLA/PCL 1/20 by weight) was chosen to obtain the thick film of 1/1 surface composition by mole in order to compare the hydrolytic behavior of blend film to the blend monolayer. The thick films were dried slowly at 298 K and then kept in vacuo to constant weight. Hydrolysis was performed by exposing the film to 10 mL of NaOH solution (pH )11.4) at 293 K for 15 min. After hydrolysis, films were washed three times, followed by drying in vacuo for 2 days. XPS analysis was done after this exposure. Reagent-grade HCl and NaOH were used to adjust the pH of the water, unless otherwise specified. A pH meter equipped with an electrode (Orion Research) was used to measure the pH of solutions. Langmuir Trough. Monolayer properties were studied by using a computer-controlled KSV 2200 film balance held at 293 K. A compression rate of 30 cm2/min was used throughout. The surface pressure could be measured with an accuracy of 0.1 mN/ m. The water subphase was purified with a Mega-Pure system, MP-6A. The purified water of ca. 1600 mL was used as a subphase liquid. The spreading solvent used in this study was chloroform (Fisher, 99%+). The concentrations of all solutions were ca. 2 µmol/mL. After spreading (100 and 150 µL for isotherm and kinetic curve measurements, respectively, unless otherwise stated), the solvent was allowed to evaporate over 1 min (the residual solvent is evaporated during compression), to minimize the hydrolysis during the solvent evaporation and the compression. The time for solvent evaporation was determined on the basis of the reproducibility of PCL and l-PLA monolayers on pure water taking advantage of very slow hydrolysis, under measured conditions. XPS. The surface chemical compositions of thick polyester films before and after hydrolysis were obtained using XPS (Perkin-Elmer Physical Electronic Model 5100 ESCA). XPS measurements were performed with an achromatic Mg KR X-ray source operated at 15 kV and 20 mA. High-resolution scans of the C1s and O1s were acquired at the takeoff angles of 10, 15, and 90°.
Results and Discussion Surface Pressure-Area Isotherm Behaviors of Polyesters. The Langmuir technique has been used to measure the hydrolytic degradation of polyester monolayers on a molecular scale, since most polyesters are capable of forming monolayers owing to their hydrophilic/ hydrophobic balance.17 The low molecular weight oligomers and monomers generated by hydrolysis dissolve into water. Despite extensive investigation of the hydrolytic behavior of thick polyester films,9-13,18,19 few studies have been made on the hydrolysis of monolayers at the air/
water interface.16,20 Figure 1A shows the surface pressurearea isotherms for both dl-PLA and PCL monolayers on subphases of pH 7.3, 10.4, and 10.5. At low surface pressure, the areas occupied by monolayers of dl-PLA on subphases of pH 10.4 and 10.5 are larger than that at pH 7.3; this can be explained by inter- and intramolecular repulsion due to the ionization of monolayers.16 Similar behaviors were observed in the surface pressure-area isotherms of PCL monolayers spread on a basic subphase (pH 10.7) with hold times of 0 and 60 min as shown in Figure 1B. As the hold time increased, the overall area occupied by monolayers increased, regardless of a constant surface pressure. The increase in area is attributable to several factors, including the interaction between ester groups and sodium and hydroxyl ions at the air/water interface, if those ions exist at the interface. This also indicates that PCL is formed as a relatively expanded monolayer compared to PLA owing to its strong hydrophobicity. The ester groups of PCL are the only hydrophilic part of the monomer that could contact the subphase. Thus, the hydrolysis of PCL at the air/water interface is much slower. At surface pressures above 5 mN/m, however, the area per repeating unit of dl-PLA decreased with increasing pH of subphase. This result can be explained by the fact that the measured surface pressure-area isotherm reflects the dissolution of some oligomers generated by hydrolysis during the compression and that interfacial counterions at the air/water interface are submerged into subphase by compression of monolayers, as schematically shown in Figure 1A. This behavior is supported by results from the surface pressure-area isotherm of dl-PLA as measured with a hold time of 60 min, as shown in Figure 1A (dotted line). The dramatic decrease in the area is complicated by the fact that the dissolution of hydrolytic product into the subphase is increased during the pause time. A similar behavior was observed in the surface pressure-area isotherm of dl-LA-co-GA, which shows faster hydrolysis than dl-PLA (not shown here). Since the hydrolysis of the ester bond in polyesters produces carboxyl acid and alcohol end groups, octadecanoic acid and octadecanol were used as model systems to study the solubility of the hydrolytic product in the subphase. As shown in Figure 2A, the area per molecule for octadecanoic acid significantly decreased with increasing pH, and eventually no monolayer was formed at pH 10.6. Since the shape of the isotherms at both pH 7.3 and 8.5 are similar to each other, this area change would be related to the solvation of some octadecanoic acids into subphase during the evaporation of solvent and the barrier compression. No change, however, was observed in the areas of octadecanol monolayers on both subphases of pH 7.3 and 9.4. This behavior implies that the carboxyl acid group, not the alcohol group generated by the chain cleavage of the ester bond, is mainly related to the molecular loss likely due to the formation of carboxyl acid salts with sodium ion and then the increase of the solubility into subphase.21 Therefore, the diffusion of acid fragments away from the reacting monolayer may well be the ratedetermining step. Figure 3A shows the isotherms for l-PLA monolayers on subphases of different pH’s between 7.3 and 10.7 at a hold time of zero. The plateau region at ca. 8.5 mN/m (17) Fukuzaki, H.; Yoshida, M.; Asano, M.; Kumakura, M. Eur. Polym. J. 1989, 25, 1019. (18) Li, S.; Anjard, S.; Rashkov, I.; Vert M. Polymer 1998, 39, 5412. (19) Tsuji, H.; Ikada, Y. J. Appl. Polym. Sci. 1998, 67, 405-415. (20) O’Brien, K. C.; Lando, J. B. Langmuir 1985, 1, 533. (21) Li, S. M.; Garreau, H.; Vert, M. J. Mater. Sci.: Mater. Med. 1991, 1, 198.
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Figure 1. Pressure-area isotherms of dl-PLA (A) and PCL (B) monolayer films onto subphases between pH 7.3 and 10.7. Dashed lines in figure indicate the isotherms measured with a pause time of 60 min. The arrows represent the collapse point of monolayers. Schematic representation for the change of surface pressure-area of polyester monolayer films on basic subphase is shown as (b) polyester film; (O) low molecular weight oligomer generated by hydroysis; (O) ion in subphase. The hydrolysis of monolayers is sterically hindered by water-soluble oligomer salts under monolayers.
Figure 2. Pressure-area isotherms of octadecanoic acid (A) and octadecanol (B) monolayer films on subphases of different pH’s between 7.3 and 10.1.
appears below pH 10.3 and disappeared above pH 10.3. This plateau region in the isotherm of the l-PLA film on pure water was interpreted as a phase transition and a formation of three-dimensional structure by Ivanova et al.16 Below the surface pressure of the plateau region, the change in the area of l-PLA is similar to that of dl-PLA when the subphase pH is increased. The areas occupied by films, however, were larger than those on pure water above the surface pressure of the plateau region. This means that the monolayer does not form this threedimensional structure at a high surface pressure because the hydrolysis of the film is accompanied by the increase of its hydrophilicity (anchoring effect). On increasing pH, therefore, the areas occupied by monolayers are decreased. Figure 3B represents that the isotherms of l-PLA monolayers maintained for 60 min before the compression on subphases of various basic pH’s. As expected, the area per repeating unit considerably decreased with increasing subphase pH after the pause time of 60 min, regardless of surface pressure. From the results in Figures 1 and 3, the compressibility of the monolayer increases with increasing pH or time of hydrolysis (increasing the surface pressure at the collapse) and the existence of plateau region in the isotherm of l-PLA monolayers is related to the hydrolysis rate. Hydrolysis of Polyester Monolayers. Figure 4A shows the change of areas occupied by dl-PLA monolayers with time at various constant surface pressures on subphases of pH 3 and 10.5. The extent of area reduction appears to increase upon lowering the surface pressure
at pH 10.5. This trend would result from increasing number of the base attack sites per ester bond unit. Under the condition of this measurement, however, no significant difference in the area with time up to 120 min was observed when dl-PLA was exposed to acidic subphase, pH 3.5, which has been adjusted by addition of HCl. This latter result is in accordance with other reported work,20,22,23 which also showed that basic pH’s yielded significant hydrolysis. Ivanova et al.16 reported that hydrolysis of the dl-PLA monolayer at pH ) 1.9 occurred, but used a very different experimental procedure than that used here. Figure 4B shows a plot of the area ratio, A/A0, vs time of various polyester monolayers on the subphase of pH 10.5 at a constant surface pressure of 7 mN/m, where A0 and A represent the areas occupied by the film at time 0 and t, respectively. The initial time, t ) 0, was considered when the surface pressure reaches a desired surface pressure, meaning that the effect of dissolving low molecular oligomers due to hydrolysis was neglected during the compression. This effect is considered to be marginal, if any, since there is little difference in A/A0 ratios with hydrolysis time. The measured kinetic curves follow a typical sigmoid shape, that is, the faster the reduction of A/A0 ratio, the smaller the real initial area. The extent of hydrolysis, under the condition studied here, follows the order dl-LA-co-GA > dl-PLA > l-PLA > PCL, which is consistent with the relative reactivity of the bulk polymers.1,24,25 The data in Figure 4B indicate that the reduction fraction of the original area was approximately 0.3, 0.13, 0.05, and 0.01 (experimental error of (1%) for dl-LA-co-GA, dl-PLA, l-PLA, and PCL, respectively, after a hydrolysis time of 18 min exposed to pH 10.5. To investigate the effect of activity of ions in the subphase, experiments to investigate hysteresis were performed by respreading the polymer solution after carefully removing the monolayer under a constant of pH 10.5 for a certain time by sweeping the compression barrier. Figure 5A is a plot of A/A0 vs time at pH 10.5 (by adding NaOH) for as-spread and respread dl-PLA monolayers. The pH of the subphase was changed negligibly during this measurement within an experimental error of (0.1. The extent of the decrease of the A/A0 ratio of respread dl-PLA monolayer was not the same as that of the as-spread one; the A/A0 ratio of respread dl-PLA (22) Ginde, R. M.; Gupta, R. K. J. Appl. Polym. Sci. 1987, 33, 2411. (23) Younes, H.; Nataf, P. R.; Cohn, D. Biomater. Art. Cells, Art. Org. 1988, 16, 705. (24) Gilding, D. K.; Reed, A. M. Polymer 1979, 20, 1459. (25) Browning, A.; Chu, C. C. J. Biomed. Mater. Res. 1986, 20, 613.
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Figure 3. Pressure-area isotherms of l-PLA monolayer films on subphases of different pH’s between 7.3 and 10.7 without (A) and with a pause time of 60 min (B).
Figure 4. Kinetic curves with time for dl-PLA monolayer films on subphases of pH’s 3.5 and 10.5 at various surface pressures (A) and area ratio vs time for various polyester monolyer films maintained at a constant surface pressure of 7 mN/m on subphase of pH 10.5 (B).
Figure 5. Area ratio vs time for dl-PLA monolayer films maintained at 7 mN/m on subphases of pH 10.5 with NaOH (A) and pH 9.95 with Na2CO3 and NaHCO3 (B).
monolayer decreases much more slowly than that of the as-spread one. This phenomenon may be attributed to the decreasing numbers of active base attack sites such as ester groups in the subphase and/or the steric hindrance by the dissolved oligomer salts located near the monolayers (see Figure 1A). Na2CO3 and NaHCO3 were used to adjust the pH of subphase to study the effect of degradation medium. Figure 5B shows A/A0 vs time curves for asspread and respread dl-PLA monolayers at pH 9.95 (by Na2CO3 and NaHCO3). The hydrolysis of the dl-PLA monolayer at pH 9.95 is faster than that at pH 10.5, which was controlled by addition of NaOH. Also, a small difference in the change of A/A0 ratio was observed between as-spread and respread dl-PLA monolayers. The concentration of Na+ ions in the subphase of pH 9.95 controlled
by adding Na2CO3 and NaHCO3 is nearly 80 times that in NaOH-controlling subphase of pH 10.5. This result suggests that the hydrolysis of polyesters in this work is more strongly affected by the concentration of the active sodium ion rather than the pH because the carboxyl acid can easily react to the sodium ion in water and also the solubility of the oligimer-Na+ salts into water is much higher than that of carboxyl acid end oligomers. Comparison of Monolayer Hydrolysis of Copolymer and Blend. For bulk polymer degradation, it is wellknown that the morphology of the polymers strongly affects hydrolysis rates. Amorphous regions of many biodegradable semicrystalline polymers are preferentially hydrolyzed since the tightly packed crystalline regions are less accessible to water. Gilding et al. carried out a
Hydrolysis of Biodegradable Polyester Monolayers
Langmuir, Vol. 16, No. 7, 2000 3405 Table 2. Typical C1s/O1s Ratios of Thick (l-PLA/PCL, 1/20 by Weight) Blend Films before and after Hydrolysis at pH 11.4 for 15 min C1s/O1s ratio takeoff angle (deg)
before hydrolysis
after hydrolysis
10 15 90
2.1 ( 0.05 2.1 ( 0.03 2.2 ( 0.03
2.4 ( 0.05 2.3 ( 0.05 2.3 ( 0.03
Thus, the fraction of the dissolved molecules into subphase is given by
Xt,blend ) [(A0t,blend - At,blend)/A0t,blend] Figure 6. Area ratio vs time for l-PLA, PCL, l-LA-co-CL, and (l-PLA/PCL) blend monolayer films maintained at 7 mN/m on subphase of pH 10.5. The calibration curve was calculated by eq 4.
systematic degradation study on a series of l-PLA, PGA, and their copolymers.24 The copolymerization considerably enhances the degradation rate owing to the decrease of the crystallinity. A similar result was observed when l-PLA is quenched and thus is amorphous.25 Another approach in the design of biodegradable products with a controllable lifetime is a polymer blend in which one component degrades substantially faster than the other. To study the biodegradation of the polymer blend, it is important to know the surface-layer composition of the blend, since the surface structure of polymer blends is clearly different from that in the bulk, mainly depending on the difference in the surface free energy of each component. Therefore, the degradation of blend can be controlled by varying blend composition. One possible application of a blend for drug release is to use phase separation, rather than a homogeneous state. At the same time, two kinds of drugs, which have different medical efficacy, can be delivered with different controlled release times if they can be partitioned into their different phases. Figure 6 shows the change of A/A0 ratio with time for l-PLA, PCL, and their copolymer and blend (1/1 by mole) monolayers recorded during the hydrolysis at a constant surface pressure of 7 mN/m. The dotted line represents the arithmetic average of 1/1 blend on the basis of the hydrolytic behavior of each homopolymer. Although the compositions of copolymer and blend are the same under the measured conditions, they show different hydrolytic behaviors at the air/water interface. The hydrolytic behavior of copolymer is similar to that of PCL. This is likely due to the hydrolytic resistance of caprolactone unit located near lactide unit. In the case of the blend, the hydrolysis occurs in two stages, which can be interpreted as follows; The first stage mainly consists of the hydrolysis of l-PLA phase due to preferentially hydrolytic degradation of l-PLA followed by the hydrolysis of PCL. This is clear if we consider that the heterogeneous hydrolysis of the blend takes place owing to faster hydrolytic behavior of l-PLA, and then A0 of blend (A0t,blend) is increased with hydrolytic time (t). A0t, blend can be interpreted by considering the change of each homopolymer with t
Xt,PLA ) (At,PLA/A0,PLA)
(1)
Xt,PCL ) (At,PCL/A0,PCL)
(2)
A0t,blend ) (A0,PLAXt,PLA + A0,PCLXt,PCL)/(Xt,PLA + Xt,PCL) (3)
(4)
Figure 6 shows that the extent of hydrolytic degradation of blend, which can be calculated by eq 4, is faster than that of each homopolymer. This can be explained by the fact that the first stage mainly consists of the hydrolysis of l-PLA phase due to preferentially hydrolytic degradation of l-PLA. By blending of l-PLA with PCL, which shows much slower hydrolysis than l-PLA under the same conditions, the concentration of l-PLA per unit area is relatively diluted (dilution effect); i.e., the amorphous and therefore more accessible basic attack sites in the subphase to l-PLA monlayers are increased and forced to the hydrolysis of ester bonds of l-PLA. A similar effect was observed in the study on the spreading concentration, that is, the lower the concentration, the faster the hydrolysis. However, the degradation of copolymer is relatively slow. It may be attributed to the following; the diffusion of water and the basic attack of ester groups are difficult owing to the hydrophobicity of caprolactone located near lactide group and the solubility of the product generated by hydrolysis into basic subphase is decreased owing to lower solubility of caprolactone unit. To investigate the hydrolytic behavior of thick polyester blend and copolymer films, XPS measurements were performed. The surface composition was evaluated on the basis of the ratio of XPS peak area for the carbon (C1s) and the oxygen (O1s) corrected with the sensitivity factor. As expected, the composition of copolymer showed no difference because of its chemical homogeneity. Table 2 shows the result of angle-dependent XPS of blend film before and after hydrolysis at pH 11.4. The hydrolysis time was 15 min in order to minimize the removal of PCL particles due to the hydrolysis of l-PLA. After the hydrolysis, the l-PLA concentration in the blend was decreased at all takeoff angles; that is, the C1s/O1s ratio was increased, whereas as the depth of analysis was increased, the concentration of l-PLA was increased. This behavior suggests that the hydrolysis of l-PLA component in the blend film mainly occurs at the initial stage. Conclusions The hydrolysis of polyester monolayers maintained at a constant surface pressure leads to the reduction in the area occupied by films when they are exposed to a basic subphase. The extent of hydrolysis was increased with increasing pH of the subphase or the concentration of degradation medium. This behavior was attributed to the increase of solubility of oligomers into subphase owing to the salt formation of carboxyl end groups generated by hydrolysis with sodium ions in subphase. Therefore, the hydrolysis of the polyester was strongly affected by the degradation medium of subphase, rather than the pH of
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subphase. The monolayer of a (l-PLA/PCL) blend showed faster hydrolysis than each homopolymer. This result revealed that most basic attack sites in subphase are related to the hydrolysis of l-PLA since l-PLA is much more easily hydrolyzed than PCL. This could be related to the “dilution effect” on PLA in the blend.
Lee and Gardella
Acknowledgment. This research was supported in part by the National Science Foundation, Chemistry Division, Analytical and Surface Chemistry Program, and the Korea Research Foundation. LA990800R
