Amphiphilic Conetworks and Gels Physically Cross-Linked via

Article pubs.acs.org/Langmuir

Amphiphilic Conetworks and Gels Physically Cross-Linked via Stereocomplexation of Polylactide Xiaoshan Fan,† Mian Wang,‡ Du Yuan,† and Chaobin He*,†,§ †

Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, 117576 Singapore School of Life Science and Technology, Xinxiang Medical University, Xinxiang, China 453003 § Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ‡

ABSTRACT: Amphiphilic conetworks (APCNs), consisting of hydrophilic poly[poly(ethylene glycol) methyl ester acrylate] (PPEGMEA) and hydrophobic stereocomplex of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were prepared by free radical copolymerization of PEGMEA with acrylate macromonomer of the PLA stereocomplex. The effects of stereocomplexation and the amount of PLA stereocomplex on the rheology properties of APCNs were investigated. The results indicated that the APCNs was stronger in the presence of stereocomplexation compared with the that of nonstereocomplex system, and the strength of the APCNs increased with the increasing of the amount of PLA stereocomplex. The storage modulus of the APCNs could be easily tuned from 1200 to 4300 Pa by incorporating 2−10% of stereocomplex PLA. On the other hand, the swelling behavior of APCNs decreased with the increasing content of hydrophobic PLA cross-linker.

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INTRODUCTION Polylactide (PLA), obtained from renewable resources, is one of the most commonly used polymer materials in the biomedical area for its facile biodegradability.1−3 It is also well-known that the two enantiomers of PLA, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), can form a racemate, socalled stereocomplex.4−6 Compared with the individual enantiomers, PLA stereocomplexes exhibit better physical properties, such as higher melting point, higher mechanical strength, and improved thermal and hydrolytic stability.7−11 This unique characteristic benefits the use of PLA in biomedical applications. Recently, several research groups have prepared novel hydrogels from water-soluble PLLA and PDLA based block or star-like copolymers, in which physical cross-linkages were formed by stereocomplexation between the enantiomeric PLLA and PDLA blocks.12−22 Most importantly, PLA stereocomplex acting as a cross-linker has several advantages over other cross-linking methods, such as reaction under mild conditions and avoiding the use of catalyst, auxiliary crosslinking agents, and other active molecules. As rapidly emerging novel materials, amphiphilic conetworks (APCNs) consist of not only hydrophilic but hydrophobic polymer segments as well, which are covalently bonded together in a cross-linked macromolecular assembly.23−47 Due to their unique structure, APCNs have some extraordinary properties, such as swelling with independence on solvent polarity, existence of nanophase structure, excellent mechanical strength, and good biocompatibility, which make them promising for applications in a wide variety of areas.23−54 Up © 2013 American Chemical Society

to now, a number of APCNs with various compositions have been developed, such as poly(N-isopropylacrylamide)-l-poly(methyl methacrylate) (PNiPAAm-l-PMMA),45 poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF),46 and (Nisopropylacrylamide)-l-polyisobutylene (PNiPAAm-l-PIB) (“l” stands for “linked by”).47 However, most of the APCNs reported in the literature are formed via chemical cross-linking, and few APCNs with physical cross-linking are reported. In this study, we successfully synthesized biocompatible APCNs, poly[poly(ethylene glycol) methyl ester acrylate]-lstereocomplex of poly(L-lactide) and poly(D-lactide) (PPEGMEA-l-S-(PLLA-PDLA)), by compolymerizing PEGMEA with a well-defined acrylate-telechelic S-(PLLA-PDLA) macromonomer (as shown in Scheme 1). The resulting conetworks are physically connected by the stereocomplexes of PLLA and PDLA block segments, which are drastically distinct from those chemically cross-linked APCNs reported. Most importantly, these materials possess biocompatible characteristic, which may facilitate their applications in biomedical area.

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EXPERIMENTAL SECTION

Materials. Poly(ethylene glycol) methyl ester acrylate (Mn = 480) and poly(ethylene glycol) acrylate (Mn = 375) were purchased from Sigma and purified by passing through a column with neutral alumina oxide to remove inhibitor. 2,2-Azoisobutyronitrile (AIBN) was purified Received: September 5, 2013 Revised: October 16, 2013 Published: October 22, 2013 14307

dx.doi.org/10.1021/la403432y | Langmuir 2013, 29, 14307−14313

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Scheme 1. Synthesis Illustration of Amphiphilic Conetworks Physically Cross-Linked by PLA Stereocomplex

Figure 1. 1H NMR spectrum of the copolymer A-PEG-PLLA in CDCl3. by recrystallization from methanol. L-Lactide and D-lactide (PURAC biochem) were purified by recrystallization from ethyl acetate. Dichloromethane (DCM) was dried by distillation from calcium hydride before use. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (98%, Aldrich) was used as received. Characterization. Dynamic rheology was performed using a coneand-plate configuration (20 mm diameter, 1° cone geometry) on a stress-controlled rheometer. The gap distance between the cone and the plate was fixed at 1 mm. Stress-amplitude sweeps were performed at a constant oscillation frequency of 1 Hz for the strain range of 0.01− 100 at 25 °C. 1H nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker Ultrashield 600 MHz/54 mm NMR spectrometer at room temperature using CDCl3 as the solvent. Xray diffraction patterns were obtained on a Bruker AXS D8 Advance thin film XRD instrument operating under a voltage of 40 kV and a current of 40 mA using Cu Kα radiation (λ = 0.15418 nm). Gel permeation chromatographic (GPC) analyses were performed on Waters 2690 equipped with an evaporative light scattering detector (Waters 2420) and three phenomenox linear 5 mm styragel columns (500, 104, and 106 Å) . THF was used as eluent at a rate of 1.0 mL/ min at 25 °C. Nonodispersed poly(methyl methacrylate) (PMMA) was used as standard. The differential scanning calorimetry (DSC)

analysis was performed on a TA Instrument Q100 under N2, following the method: equilibrate at 25 °C; ramp 10 °C min−1 to 250 °C; isothermal for 5 min; ramp 10 °C min−1 to 250 °C. Swelling Studies. The experiments were carried out as follows: a piece of APCNs sample was transferred to a vial containing deionized water or organic solvents at room temperature and was left to swell until constant weight. The weight of the hydrogel sample was recorded regularly after removing the hydrogel from solvent and drying the surface with a filter paper. Synthesis of the Acrylate-Terminated Copolymer A-PEGPLLA. Under an atmosphere of dry nitrogen, poly(ethylene glycol) acrylate (0.438g, 1.17 mmol), L-lactide (4.136g, 28.72 mmol), and dry DCM (50 mL) were added to a dried 100 mL round-bottom flask. The mixture was stirred until all monomer dissolved, and then DBU (20 μL 0.13 mmol) was injected into the system. After stirring at room temperature overnight, the reaction was quenched by adding benzoic acid. The reaction mixture was concentrated, then precipitated in hexane for two times. The resulting polymer was dried under vacuum at 35 °C. Yield: 92% (Mn,NMR = 2950 g/mol; Mn,GPC = 2630 g/mol, PDI = 1.07). 14308


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The synthesis procedure of the acrylate-terminated copolymer APEG-PDLA was similar to that of A-PEG-PLLA. Yield: 90% (Mn,NMR = 2880 g/mol; Mn,GPC = 2710 g/mol, PDI = 1.06). Synthesis of Amphiphilic Conetworks with PLA Stereocomplex as Cross-Linkage. The physically cross-linked amphiphilic conetworks were prepared by free radical copolymerization of the active PLA stereocomplex with poly(ethylene glycol) methyl ester acrylate. A typical preparation procedure was described as follows: The copolymers of A-PEG-PLLA (0.150 g, 0.0051 mmol) and A-PEGPDLA (0.150 g, 0.0052 mmol) were dissolved in 3 mL of THF, respectively. Then, the solutions were mixed together and stirred overnight to ensure the complete formation of the stereocomplex. PEGMEA (2.705 g, 5.63 mmol) was added to the mixed solution above. After removing the THF under reduced pressure, AIBN (5.0 mg) was added to the system. Argon flow was bubbled through the solution for 15 min to eliminate oxygen, and the polymerization was carried out at 70 °C for 1 h.

Figure 3. Wide angle X-ray diffraction patterns of (A) hydrogel of APCN-5%-L and (B) hydrogel of APCN-5%-S.

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RESULTS AND DISCUSSION Preparation of Amphiphilic Conetworks with PLA Stereocomplex as Cross-Linkage. The preparation proceTable 1. Compositions of the Amphiphilic Conetworks and Swelling Degrees of the Corresponding Gels feed composition (wt%) sample

PEGMA

S

APCN-2%-S APCN-5%-S APCN-10%-S APCN-5%-L

98 95 90 95

2 5 10

PLLA

swelling degree (%) H2O

CH3OH

THF

1100 890 590

842 754 633

838 803 734

Figure 4. DSC thermograms of (A) APCN-10%-L and (B) APCN10%-S.

5

S, stereocomplex of PLLA and PDLA; L, PLLA.

process. The 1H NMR spectrum of A-PEG-PLLA is shown in Figure 1. The resonance signals (a) at 6.13 ppm and (d) at 5.18 ppm are attributed to proton of the active double bonds and methine proton of LA repeat units, respectively. The molecular weight of A-PEG-PLLA was calculated by 1H NMR spectra according to the following formula:

dure of amphiphilic conetworks physically cross-linked by PLA stereocomplex is illustrated in Scheme 1. Acrylate-terminated copolymers A-PEG-PLLA and A-PEG-PDLA, were first synthesized by DBU catalyzing ring-opening polymerization of L-lactide and D-lactide, respectively. Compared with the Sn(II) catalyst, DBU can catalyze the ring-opening polymerization of lactide at room temperature, and the side reaction of active acrylate groups can be avoided during the polymerization

⎛A ⎞ M n,NNR = ⎜ d + 1⎟ × 72 + 375 ⎝ Aa ⎠

Figure 2. Storage modulus (G′) and loss modulus (G″) of APCNs-5%-S and APCNs-5%-L performed at a constant oscillation frequency of 1 Hz for a strain range of 0.01−100 at 25 °C. 14309


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Figure 5. Storage modulus (G′) and loss modulus (G″) of amphiphilic conetworks with different PLA stereocomplex content performed at a constant oscillation frequency of 1 Hz for a strain range of 0.01−100 at 25 °C.

Figure 6. Effect of increasing amount of PLA stereocomplex on the degree of swelling of the gels.

Figure 7. Transparency of hydrogels formed by APCNs containing different PLA stereocomplex content (A) APCN-2%-S, (B) APCN-5%-S, and (C) APCN-10%-S.

where Aa and Ad stand for the integral area of peaks (a) and (d), respectively. The values of 72 and 375 are the molecular

weights of repeat units of PLLA and the macroinitiator PEGA, respectively. The molecular weights of A-PEG-PLLA and A14310


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PEG-PDLA calculated by 1H NMR spectra were 2880 g/mol and 2950 g/mol, respectively. Copolymerization of the acrylate macromonomer of the PLA stereocomplex with PEGMEA comonomer will allow the acrylate groups on the macromonomer to be incorporated into the propagating chain of the latter, which leads to physically cross-linked amphiphilic conetworks. The composition of the conetworks can be controlled by varying the feed ratios of the PEGMEA monomer to the stereocomplex of PLA. In order to investigate the effect of composition on rheology properties, a series of conetworks with various compositions were synthesized (see Table 1). Effect of Composition on the Properties of Conetworks. To understand the effect of stereocomplxation, APCNs consisting of 5% PLA stereocomplex (APCN-5%-S) and 5% PLLA (APCN-5%-L) were evaluated by a rheometer to characterize their bulk mechanical properties. From Figure 2, it can be seen that the rheological properties of these two APCNs were greatly different though containing the same amount of hydrophobic (PLA) component. APCN-5%-S was found to be stronger than the corresponding conetworks APCN-5%-L with its higher storage moduli (G′) present. This behavior was further attributed to the effect of physical crosslinkage formed from the PLA stereocomplex, which is more stable due to the existence of additional hydrogen bond.55,56 The existence of PLA stereocomplx in the polymer conetworks was verified with X-ray diffraction (XRD) (Figure 3). The gel formed by APCN-5%-S showed diffraction peaks (a) and (b) at 2θ = ∼12° and 21°, which reflects the presence of PLA stereocomplex crystal.57 This provides confirmation on the existence of PLA stereocomplex crystalline domains in the PPEGMEA-l-S-(PLLA-PDLA) conetworks, where the crystalline domain size was estimated to be ∼11.3 nm via Scherrer’s fomula. However, the hydrogel containing PLLA showed no apparent diffraction peaks, implying amorphous structure of gel formed by APCN-5%-L. The stereocomplex formation was further confirmed by DSC. In Figure 4B, the melting peak at around 190 °C corresponding to the PLA stereocomplex crystallites can be observed clearly. The effect of the amount of PLA stereocomplex on the properties of the APCNs was also investigated. It was found that both storage and loss moduli of the APCNs increased with an increase of the amount of the PLA stereocomplex in the cross-linked macromolecular assemblies (Figure 5). This is mainly due to the increase of “physical cross-linking”. The elastic modulus of the APCNs could be fine-tuned from 1200 to 4200 Pa by changing the stereocomplex concentration from 2% to 10%. Swelling Studies of the Amphiphilic Conetworks. Swelling behavior of these gels was probed gravimetrically by recording the water uptake by the gels over time until their weight reached equilibrium. From Figure 6, it was observed that an increase in the amount of stereocomplex of PLA in the feed ratio led to a decrease in the swelling ability of the gels. This behavior could be due to increased cross-linking as a result of increasing amount of PLA stereocomplex in the conetworks. The equilibrium swelling ratio (Q) was obtained by the following formula: m − mo Q= mo

organic solvents such as methanol and THF was also studied according to the same method, and the results are listed in Table 1. Due to the amphiphilicity of APCNs, the hydrophilic PEGMEA component swells in water media, while the hydrophobic PLA component aggregates, leading to the formation of hydrogels with nanophase separated. The water uptake for APCNs-2%-S was 1100%, while that for APCN-10%S was at about 600%. It is clearly observed that the transparency of APCN gels decreased with increasing amount of the hydrophobic PLA (Figure 7), which may be closely associated with the nanostructure in the stereocomplex as revealed by XRD.

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CONCLUSION APCNs were successfully synthesized by free radical copolymerization of acrylate PLLA and PDLA stereocomplex with PEGMEA comonomer. A series of conetworks with different compositions were obtained by tuning the ratio of the hydrophobic macromonomer to the hydrophilic monomer. The resulting conetworks were characterized in terms of their bulk properties and swelling behavior. The results indicated that the properties of these stereocomplexed conetworks can be tuned by changing the feed ratios. Due to the amphiphilic character, these physically cross-linked conetworks can form nanophase separated structure with PEGMEA and PLA domains in water medium. These PACNs may find potential applications in the biomedical area due to their unique structure and properties, such as encapsulating and delivering hydrophobic drug molecules.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from Science and Engineering Research Council (SERC), Agency of Science, Technology and Research (A*STAR) to National University of Singapore under Grant Number 1123004036. The authors also appreciate the kind help from Mr. Sun Yang in XRD experiments.

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REFERENCES

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where m and mo stand for the weight of swollen and dry conetworks, respectively. Swelling behavior of the gels in 14311


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