Surface-phase separation of PEO-containing biodegradable PLLA blends and block copolymers

Applied Surface Science 255 (2008) 2360–2364

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Surface-phase separation of PEO-containing biodegradable PLLA blends and block copolymers Joo-Woon Lee a, Euh Duck Jeong b, Eun Jeong Cho c, Joseph A. Gardella Jr.d,*, Wesley Hicks Jr.e, Robert Hard f, Frank V. Bright d a

Chemistry – School of Liberal Arts and Sciences, Chungju National University, Chungju, Chungbuk 380-702, Republic of Korea High-Technology Component & Material Research Center and Busan Center, Korea Basic Science Institute, Busan 618-230, Republic of Korea Department of Chemistry & Biochemistry, University of Texas at Austin, Austin, TX 78712, USA d Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260, USA e Department of Head & Neck Surgery, Roswell Park Cancer Institute, Buffalo, NY 14263, USA f Department of Anatomy & Cell Biology, State University of New York at Buffalo, Buffalo, NY 14214, USA b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 March 2008 Received in revised form 3 July 2008 Accepted 16 July 2008 Available online 31 July 2008

The surface chemistry of two series of poly(ethylene oxide) (PEO)-containing biodegradable poly(L-lactic acid) (PLLA) matrix systems has been investigated using time-of-flight SIMS (ToF-SIMS) and XPS. The two systems are (1) PLLA blend matrices with an amphiphilic Pluronic1 P-104 surfactant, P(EO)27-b-P(PO)61b-P(EO)27, and (2) PLLA-b-PEO diblock and PLLA-b-PEO-b-PLLA triblock copolymers. The phase separation is analyzed in determining the surface enrichment of the component and chemical composition at the polymer–air interface. The PEO component is surface-dominant in the blend system in contrast to the surface excess of poly(propylene oxide) (PPO) in pure Pluronic1 P-104. The block copolymer system shows the surface enrichment of PLLA component. These results can be explained in terms of the change in surface free energy for the block copolymers and the better miscibility of PLLA and PPO against amphiphilic PEO for the blends, respectively. ß 2008 Elsevier B.V. All rights reserved.

PACS: 68.35.B 68.35.p Keywords: Biodegradable Block copolymer PLLA PEO Secondary ion mass spectrometry XPS

1. Introduction Poly(ethylene oxide) (PEO) is a neutral, highly biocompatible, and pharmacologically inactive water-soluble polymer [1]. The physicochemical incorporation of PEO into biodegradable poly(Llactic acid) (PLLA)-based drug delivery implant systems would be expected to improve the interfacial biocompatibility of these polymeric devices. Blend matrices of PEO and relatively hydrophobic PLLA or PEO-b-PLLA block copolymers should improve the three dimensional stability and the biological activity of watersoluble macromolecular drugs such as proteins or enzymes in the delivery systems, which are incorporated in the matrix [2] for two reasons: first, if surface-segregated PEO in the aqueous environment provides a diffusive hydrophilic layer [3] facilitating the interaction between cells and polymeric biodevices, and secondly,

* Corresponding author. Tel.: +1 716 645 6800x2111; fax: +1 716 645 5994. E-mail address: [email protected] (J.A. Gardella Jr.). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.121

if hydrophilic PEO segments in the bulk protect water-soluble drugs from hydrophobic polymers in the form [4] of a micelle or encapsulation. Two model systems of PEO-containing polymer systems are considered in the present study as candidates for seeding proteinbased drugs into a PLLA matrix. One approach is to use PLLA blend matrices with PEO-b-poly(propylene oxide) (PPO)-b-PEO triblock copolymers (Pluronics1, BASF Corp.), Pluronics1 exhibit a wide range of hydrophilicity/hydrophobicity as a function of PEO/PPO ratio [5]. The blend concept is based on the assumption that blends can exhibit advantageous physicomechanical properties that each individual polymer does not have [6,7]. The other model system involves PLLA-b-PEO diblock and PLLA-b-PEO-b-PLLA triblock copolymers [8]. Recently, these nonionic amphiphilic block copolymers have been studied as a new biodegradable hydrogel in the application of injectable drug delivery systems due to the novel thermosensitive sol–gel transition [4,9,10]. The chemistry in bulk of two polymer systems above has been extensively studied. However, few attempts have been made for the surface char-

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acterization of two polymeric matrix systems. For examples, Park et al. [11] postulated from the SEM study of PLLA/Pluronics1 blend systems that Pluronics1 might be entangled within the amorphous domains of PLLA, while the PPO component might be anchored on PLLA spherulites. Shard et al. [12] qualitatively reported from the block copolymers of PEO and poly(lactic-co-glycolic acid) (PLGA) that the PLGA component preferentially resided at the surface, but quantitative surface composition profiles were not examined. In this report, we present in detail the phase separation phenomenon at the surface of PEO-containing PLLA blends and block copolymers using time-of-flight SIMS (ToF-SIMS) for both low and high mass portion analysis of the spectra and angle-dependent XPS for surface composition profiles. 2. Experimental Three kinds of PLLA-b-PEO (Mn: 1122–5000 and 1730–5000) and PLLA-b-PEO-b-PLLA (Mn: 1050–4600–1050) block copolymers were synthesized through the ring opening polymerization of Llactide (LLA) onto monomethoxy end-capped PEO (for diblock) or PEO (for triblock) [8]. Mn of them was estimated using 1H NMR (Bruker, DPX 300 MHz). PLLA (MW 100,000) was purchased from Polysciences, Inc. Pluronic1 P-104, P(EO)27-b-P(PO)61-b-P(EO)27, was kindly donated by BASF Corp. [5]. A series of 2% (w/v) polymer solutions, i.e., Pluronic1 P-104, its PLLA blends (PLLA/ Pluronic1 P-104 = 90:10, 80:20, and 70:30 ratio in wt.%), and three PLLA-b-PEO(-b-PLLA) block copolymers in chloroform, were spun cast onto 10 mm  10 mm glass substrates at 2000 rpm for 60 s. The substrates were ultrasonically cleaned in n-hexane prior to the casting. To avoid specimen hydrolysis and contamination, exposure to atmospheric conditions was minimized by enclosing samples in a desiccator filled with argon under vacuum. Chemical compositions at the surface of the matrices were obtained using a Physical Electronic/PHI 5300 XPS operated at 15 kV and 20 mA. Stationary Mg Ka radiation (1253.6 eV) and pass energy of 89.45 eV for survey acquisitions as well as 17.90 eV for high-resolution acquisitions of the C 1s were used for all angledependent acquisitions. A hemispherical analyzer was used. The energy resolution was 0.1 eV for high-resolution spectra and 1.0 eV for survey spectra. Binding energies were calibrated by setting C–H peak in C 1s envelope at 285.0 eV. Photoelectron emission takeoff angles of 158, 308, 458, and 908 were used for all samples and led to the sampling depths of 2.7, 5.0, 7.3, and 10.3 nm for C 1s envelope, respectively [13]. Curve-fitting was performed for each carbon fraction of C–H, C–O, and O C–O functionalities using AugerScan software (RBD Enterprises, OR). ToF-SIMS analysis was performed using a Physical Electronics 7200 ToF-SIMS equipped with a Cs+ ion gun, a reflectron assembly, and a channel plate detector. The static mode was used in all acquisitions with primary ion current of 0.3 pA. The pulse width of primary ion current was 1.0 ns. The extractor was operated in the positive ion mode. The total ion dosage in each spectral acquisition was no more than 1  1011 ion/cm2. An electron neutralizer was operated during all spectral acquisitions in pulsed mode at low electron energy with a target current under 1 mA for charge compensation. A time resolution of 1.25 ns per step was used for good S/N ratio at high m/z range. The pressure of main chamber was kept between 108 and 1010 torr for each analysis. The spectra were analyzed using the data reduction software, Physical Electronics TOFPak. 3. Results and discussion An XPS study was carried out at various takeoff angles. No foreign element was detected except very little amount of sodium

Table 1 Carbon fractions in bulk of PEO-containing polymeric matrices used

1

C–O (%)

O

C–O (%)

21.02

78.98

N/A

PLLA/Pluronic1 P-104 blends (wt.%) 90:10 31.90 80:20 30.53 70:30 29.14

38.63 43.74 48.68

29.47 25.73 22.13

PLLA-b-PEO(-b-PLLA) (Mn) 1122–5000 1730–5000 1050–4600–1050

88.62 83.94 80.32

5.69 8.03 9.84

Pluronic

P-104

C–H (%)

5.69 8.03 9.84

with less than 0.12 at.% concentration in the survey spectra (not shown), so no adjustment was made in the analysis (typical detection limits: 0.25 at.%). The chemical composition at the surface was investigated using a curve-fitting method for the C 1s line, where quantitative analysis was presented by the C–H fraction for pure Pluronic1 P-104 and the O C–O fraction applied for PLLA/Pluronic1 P-104 blend matrices and PLLA-b-PEO(-b-PLLA) block copolymers. This is because the C–H fraction is only attributed from the methyl group in PPO component of Pluronics1 and the O C–O fraction only from the ester functionality in PLLA component of both the blends and the block copolymers. In order to compare degrees of surface-phase separation, each carbon fraction in bulk is theoretically calculated for the blends and block copolymers systems in Table 1. Fig. 1 shows the surface composition for (A) pure Pluronic1 P-104 and (B) PLLA/Pluronic1 P-104 blends as a function of takeoff angle. The data points represent the average area % up to four times measurements and the error bars reflect the standard deviation at each takeoff angle (x-axis). In Fig. 1(A), the C–H fraction represented for PPO component exponentially decreases with increasing takeoff angle. The changing extent in C–H fraction is the largest between 158 and 308. The bulk C–H fraction is theoretically 21% (see, Table 1). This observation indicates that the PPO component of Pluronic1 P-104 is segregated to the topmost surface of the matrix. In Fig. 1(B) for PLLA/Pluronic1 P-104 blends, the O C–O fraction represented for PLLA component gradually increases in the aspect of increase in takeoff angle and the value at each takeoff angle is reflective of that particular PLLA blend matrix. Comparing the theoretical bulk fractions in Table 1, these results indicate Pluronic1 P-104 is surface-segregated in the blend system. However, present XPS study for the blend system has the limitation in determining which component of Pluronic1 P-104 is surface-dominant, due to the overlapping of C–H (or C–O) fraction attributed from PLLA with the PPO (or PEO) component of Pluronic1 P-104. To determine the surface-dominant component of Pluronic1 P-104 in the blend system, low and high mass analyses were done of positive ToF-SIMS spectra. A small amount of sodium was detected in the low mass range of ToF-SIMS (not shown). Fig. 2 shows the ToF-SIMS spectra obtained from PLLA/Pluronic1 P-104 blend system, where (A–C) represent the low mass analysis for pure Pluronic1 P-104, pure PLLA, and the PLLA/Pluronic1 P-104 (70:30 wt.%) blend, respectively, and (D) and (E) represent the high mass analysis for pure Pluronic1 P-104 and the PLLA/Pluronic1 P-104 (80:20 wt.%) blend, respectively, for the comparison in surface-dominant components. The ToF-SIMS spectrum obtained from pure Pluronic1 P-104 in Fig. 2(A) shows two significant fragment ion peaks at 45 m/z, [CH2CH2OH]+, for PEO component and at 59 m/z, [CH3CH2OCH2]+ and [CH2CH(CH3)OH]+, for both PEO and PPO components, respectively. It is difficult from the spectrum, however, to distinguish which component was predominant at the surface in comparing

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Fig. 1. Surface composition from the C 1s envelopes of (A) pure Pluronic1 P-104 and (B) PLLA/Pluronic1 P-104 blend matrices ((& ) 90:10, (*) 80:20, and (~ ) 70:30 wt.%).

with the results from XPS in Fig. 1(A). The fragments of pure PLLA in Fig. 2(B) were normally observed at 45 m/z, [HOCHCH3]+, from the hydroxyl end group and at 56 m/z, [CH3CHC O]+, from the backbone structure [14,15]. The peaks at 45 m/z from both Fig. 2(A) and (B) overlapped with each other, so the peak at

56 m/z from PLLA and the peak at 59 m/z from Pluronic1 P-104 were used in the low mass analysis of the blend system. Fig. 2(C) represents the low mass analysis of ToF-SIMS obtained from the PLLA/Pluronic1 P-104 (70:30 wt.%) blend. The peak at 59 m/z from Pluronic1 P-104 showed the strongest intensity in the low

Fig. 2. Comparison of low (left) and high (right) mass portions in positive ToF-SIMS spectra: (A) and (D) Pluronic1 P-104, (B) PLLA, (C) PLLA/Pluronic1 P-104 (70:30 wt.%), and (E) PLLA/Pluronic1 P-104 (80:20 wt.%).

J.-W. Lee et al. / Applied Surface Science 255 (2008) 2360–2364

mass range, while the peak at 56 m/z was observed as low as seen in Fig. 2(A) of pure Pluronic1 P-104. These results from the low mass analysis in ToF-SIMS confirmed the XPS results in Fig. 1(B); Pluronic1 P-104 was predominant at the surface of the PLLA blend matrices system. The high mass portion analysis of the corresponding ToF-SIMS spectrum showed two series of fragment ion distributions over the range from 400 m/z. Fig. 2(D) obtained from pure Pluronic1 P-104 shows two series of the distributions derived from PPO component, () for [(PO)n + Na]+ and (! ) for [(PO)n + O + Na]+ with 7  n  20, where sodium participated in the ionization process as an ionization assisting agent [14]. This suggests that PPO component was surface-segregated and PEO is embedded in the matrix, which was well consistent with the results from XPS study in Fig. 1(A). Such a phenomenon was attributed to the changing surface free energy at the polymer–air interface: i.e., to minimize the free energy, polar PEO component (high energy component) was buried below the surface and hydrophobic PPO with nonpolar CH3 groups was oriented toward the free air surface. Meanwhile, two series of interesting distributions were observed in the high mass range of ToF-SIMS obtained from PLLA/Pluronic1 P-104 blend matrices system. Fig. 2(E) is the representative spectrum of the PLLA/Pluronic1 P104 (80:20 wt.%) blend, where two series of PEO fragments are detected, ($) for [(EO)n + Na]+ and (#) for [(EO)n + O + Na]+ with 9  n  31. Hence, it can be concluded that the PPO component of Pluronic1 P-104 in PLLA/Pluronic1 P-104 blend matrices anchored into PLLA and the PEO component was surfacesegregated contrary to the enrichment of PPO component at the surface of pure Pluronic1 P-104 matrix. This phenomenon can be explained by the phase separation based on the better

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miscibility of PLLA and PPO against amphiphilic PEO. In addition, the results from both ToF-SIMS and XPS in the present blend system proved the previous postulation [11] from SEM study on the phase separation of PLLA/Pluronics1 blends. Three kinds of PLLA and PEO block copolymers were investigated in terms of: (1) block types, AB vs. ABA, and (2) MWs of PLLA (1122 vs. 1730  2  1050) relative to that of PEO (5000  4600). Typical ToF-SIMS spectra and angle-dependent XPS results are presented in Fig. 3. Low mass analysis of the spectra in Fig. 3(A–C) were obtained from each of the block copolymer matrix, where three significant fragments were designated with a triangle; [C2H5O]+ at 45 m/z for both PLLA and PEO, [CH3CHC O]+ at 56 m/z for PLLA, and [CH3CH2OCH2]+ at 59 m/z for PEO. Also, a small amount of sodium was detected (not shown). The change in relative peak intensities between 56 and 59 m/z qualitatively indicates that the amount of PLLA at the surface roughly depends on the Mn of PLLA component in block copolymers regardless of the block types; however, it does not suggest which component is surface-dominant. A typical high mass analysis of ToF-SIMS is shown in Fig. 3(D) obtained from PLLA-b-PEO-b-PLLA (1050– 4600–1050) triblock copolymer. Two series ($ and #) of PEO fragment ion distributions were observed in the mass range over 400 m/z, ($) for [(EO)n + Na]+ and (#) for [(EO)n + O + Na]+ with 9  n  31, as likely as in Fig. 2(E) of the blend system. However, the development of PEO fragment distributions in Fig. 3(D) was not surprising according to our high mass analysis in ToF-SIMS for two decades. This is because PEO itself has a tendency to be easily desorbed in fragments during ToF-SIMS ionization process, especially in case of the low MW of PEO, commonly poly(ethylene glycol) (PEG) [16]. The ToF-SIMS spectrum of pure PLLA shows very

Fig. 3. Comparison of low (left) mass portions in positive ToF-SIMS spectra: (A) PLLA-b-PEO = 1122–5000, (B) PLLA-b-PEO = 1730–5000, and (C) PLLA-b-PEO-b-PLLA = 1050– 4600–1050. (D) High (top right) mass portion in positive ToF-SIMS spectrum of PLLA-b-PEO-b-PLLA = 1050–4600–1050. (E) Surface composition from the C 1s fractions of PLLA-b-PEO(-b-PLLA) block copolymers ((& ) 1122–5000, (*) 1730–5000, and (~ ) 1050–4600–1050).

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little fragments except noisy background in the mass range over 400 m/z, due to the strong entanglement of the polymer itself [15]. Subsequent XPS study for O C–O fraction in C 1s envelopes confirms that PLLA component is enriched at the surface of the block copolymers in Fig. 3(E), where the change of the fraction was presented by plotting the average area % (y-axis)  standard deviation (S.D.) for a maximum of four times measurements at each takeoff angle (x-axis). The O C–O fraction decreased while the takeoff angle for each of the block copolymers was increased. The relative concentration in area % was roughly dependant (not proportional) on the Mn of PLLA in the block copolymers. Compared with the corresponding theoretical bulk fraction in Table 1, this observation reflects the enrichment of PLLA component at the surface, which can be generally explained based on the change in surface free energy at the polymer–air interface. It is worth noting that simple surface energetics is not always enough to predict segregation and surface excess in a system with PLLA, PEO and PPO available. This is because the comparison between the blend and the block copolymer systems is much more complex than revealed by simple considerations of surface energy difference between components. For example, the MW of the PLLAb-PEO(-b-PLLA) block copolymers is small, and there is good mixing between components, further the length of the PEO component in the block copolymer system is longer than that of the PLLA, making this copolymer water soluble. Thus, these factors also play a role in surface excess and surface segregation phenomena in this system. Therefore, experimental measurements of multicomponent systems are necessary to understand mixing, phase separation, and other factors in these important materials for drug delivery matrices. 4. Conclusion The phase separation of polymeric matrices was investigated at the surface of PEO-containing biodegradable PLLA blends and block copolymers; PLLA blend matrices with an amphiphilic Pluronic1 P104, P(EO)27-b-P(PO)61-b-P(EO)27, and PLLA-b-PEO(-b-PLLA) block copolymers. The enrichment of the component was determined in detail at the polymer–air interface, which was possible through the complementary ToF-SIMS and XPS studies. For the blend system, the combination of the hydrophobic PPO component in Pluronic1 P-104

drives the formation of a surface excess of PEO component in the blends with PLLA in contrast to the enrichment of PPO at the surface of pure Pluronic1 P-104. This indicated that the PPO component of Pluronic1 P-104 was well mixed with the PLLA component residing in the bulk of the blend matrices. In the block copolymer system, PLLA was predominant at the surface due to the change in surface free energy. These results suggest for the future work that the presence of PPO component could selectively control the surfacephase separation of PLLA and PEO based matrix formulations. The comparison of the PLLA/PEO blends with the PLLA-b-PEO(-b-PLLA) block copolymers is under investigation, especially in terms of the similar MW of each component.

Acknowledgements The authors acknowledge financial supports from the National Science Foundation (CHE-0079114 and CHE-0316735), Korea Basic Science Institute, and the Academic Research Program of Chungju National University in 2007. The authors also acknowledge Professor Taihyung Chang of the Department of Chemistry at POSTECH, Pohang, Korea, for his help in facilitating the collaboration.

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