Controlled fabrication, characterization and comparison of porous poly(L-lactide) and poly(D-lactide) films by electric breath figure

Abstract

Porous poly(L-lactide), PLLA, and poly(D-lactide), PDLA, films were fabricated by the electric breath figure (EBF) method. The pore sizes of both the PLLA and PDLA films were reduced with the increase in electric voltage; however, the former followed a non-linear fit, whereas the latter followed a linear fit. Under 0 V condition, the average diameter, d_a, was 15.0 ± 0.50 μm for the PLLA film and 6.66 ± 0.54 μm for the PDLA film, whereas under 3 kV condition, d_a was 1.65 ± 0.35 μm for the PLLA film and 0.73 ± 0.07 μm for the PDLA film. FTIR spectra analysis showed that the EBF process did not change the structure of PLLA or PDLA.

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Introduction

Poly(lactide), PLA, is a biodegradable and biocompatible thermoplastic that is broadly applied in some bio-cases.¹ Due to the presence of two chiral centres, LA exists as two optical isomers, i.e. D-LA and L-LA.¹ These opposite chiral properties can be reasonably extended to the application of PLA. For example, recently, we applied both PLLA and PDLA with the same molecular weight as two soft chiral templates to controllably fabricate one-dimensional polyaniline (PANI) micro-/nanostructures.^2,3 Under the same conditions, we interestingly found that the PDLA-guided PANI presented a joint-like 1D-structure, whereas the PLLA-guided PANI presented a vertebra-like 1D-structure.³ Obviously, this strongly suggested that the opposite chiral property of PLLA and PDLA would influence their applications.

Ways to fabricate a porous PLA film have been reported by several groups. Shimomura et al.⁴ fabricated honeycomb films comprising PLLA with amphiphilic polymers using chloroform and benzene as solvents. These researchers found that irregular patterned films were produced from the PLLA/chloroform system without amphiphilic polymers.⁴ Zhao et al.⁵ fabricated honeycomb microporous films by a poly(D,L-LA-co-glycolide)/CHCl₃ system and modulated the hydrophilic copolymers by varying the ratio of the D,L-LA/glycolide constituents. In terms of the studies by these researchers, they found that no regular pattern was formed on the film surface from using copolymers with a low content of glycolide.⁵ Zhao et al.⁶ fabricated porous PLLA films with a honeycomb structure by evaporating the PLLA solution in THF under humid conditions. By applying the breath figure method, BF, Jiang et al.⁷ fabricated honeycomb PLLA films using chloroform with dioleoylphosphatidylethanolamine (DOPE). It can be noted that in this case, DOPE is a surfactant that played an important role in inducing the coalescence of water droplets in the BF process.⁷ Very recently, Hu et al.⁸ reported a case for the fabrication of porous PLLA membrane by phase separation using water microdroplets produced by an ultrasonic atomizer as a coagulation bath. These researchers formed S-type clusters on PLA films by controlling the slow exchange rate between the solvent and coagulant to provide time for movement of the polymer molecules.⁸

In addition to the formation of a porous PLA film, the wettability is another key aspect of PLA and has been focused on by many researchers due to this surface property strongly influencing the bioapplication.^9–11 In terms of reports in the literature, the water contact angle on the common surface of both PLLA and PDLA is reported as being below 90°,^12–14 except for the surface formed by some special methods such as the electro-spinning.¹⁵ In order to modify the wettability of PLA, it was noted that several methods have been developed, e.g. blending with other materials,⁹ copolymerization,¹⁶ grafting,¹⁷ coating and plasma treatment.¹⁸ According to the reported cases, the physical methods could cause radical changes on the bulk properties of PLA, e.g. the melting temperature and crystallinity,¹⁹ while the chemical methods could modify the PLA at its short fluorocarbon segments,²⁰ α-glucose,²¹ lactose,²² aminoethanol²³ and dodecyl ester or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (MEEE) ester.¹⁵

Though the abovementioned evaluated methods are available for the fabrication of porous PLA films, it must be addressed that some reported cases have presented a non-ordered pore structure,⁸ and it is also truly remarkable that the formation of porous PDLA is not yet known. The aim of this study is to apply our recently developed electric BF method (EBF)^24–26 to fabricate porous PLLA and PDLA films, then to study the effect of voltage on their pore size, structure and wettability. As known previously, EBF is a simple and economical method due to using water droplets as the template and usually only requiring the voltage as a unique controlling parameter.^24–26

Results and discussion

The FESEM images of porous PLLA and PDLA films in relation to applied voltages at 0, 1 and 3 kV are compared in Fig. 1. It can be initially observed that both PLLA and PDLA films show honeycomb structures effectively at all the used voltages, indicating that EBF is a useful method for the fabrication of ordered porous PLA films. Following, we found that the voltage increase induced the pore size reduction for both PLLA and PDLA films; however, the pore size was obviously different because the average diameter, d_a, corresponding to the 0 V condition was at 15.0 ± 0.50 μm for the PLLA film but at 6.66 ± 0.54 μm for the PDLA film, whereas under the 3 kV condition, it was at 1.65 ± 0.10 μm for the PLLA film and at 0.73 ± 0.07 μm for the PDLA film.

Fig. 1 FESEM images of porous PLLA and PDLA films formed by EBF at 0, 1 and 3 kV, respectively, and the relationship between the pore size, d_a, and applied voltage, ρ.

Since the pore size was always smaller for PDLA film and greater for PLLA films, even under the normal electric-free condition, the presented pore size difference for the PLLA and PDLA films is interesting. Taking into account the fact that recently it was found that PANI guided by PLLA and PDLA also presented different structures,³ the fact that the PDLA film showed a smaller pore size than that of the PLLA film is therefore considered due to the natural opposite chiral properties of these two PLAs. On the basis of Fig. 1 and on the normal BF-process-related mechanism,⁶ it was assumed that the solution with the formed D-LA structure has a different evaporation rate than that of the solution with the formed L-LA structure during the pore structure formation. Moreover, these L/D-chiral structures caused the different evaporation rates to follow the different electric-responses, thus leading to the different the pore sizes, as presented in Table 1.

Table 1 Comparison of the pore size of PLA films prepared in this study and reported in literature, where X represents the unknown conditions

da (μm)	Polymer	Mw × 10^4 (g mol^−1)	Solvents (mg ml^−1)	kV	RH (%)	Refs
1.65 ± 0.35	PLLA	11.6	CHCl3(1)	3	75	This work
10.0 ± 0.50	PLLA	11.6	CHCl3(1)	1	75	This work
15.0 ± 0.50	PLLA	11.6	CHCl3(1)	0	75	This work
0.73 ± 0.07	PDLA	11.6	CHCl3(1)	3	75	This work
2.41 ± 0.37	PDLA	11.6	CHCl3(1)	1	75	This work
6.66 ± 0.54	PDLA	11.6	CHCl3(1)	0	75	This work
2.20	PLLA	21.6	THF(0.05)	0	50	6
5.29	PLLA	21.6	THF(0.01)	0	60	6
2.90	PLLA	17.0	THF(0.05)	0	60	6
3.71	PLLA	21.6	THF(0.05)	0	60	6
3.65	PLLA	21.6	THF(0.075)	0	60	6
5.25	PLLA	21.6	THF(0.05)	0	70	6
1–13	PLLA/0.1%DOPE	20.0	CHCl3(5)	0	80	9
5.5 ± 0.5	PLLA/0.2%DOPE	20.0	CHCl3(5)	0	80	9
4.5 ± 0.5	PLLA/0.5%DOPE	20.0	CHCl3(5)	0	80	9
5.5 ± 0.5	PLLA/1%DOPE	20.0	CHCl3(5)	0	80	9
7 ± 1	PLLA/2%DOPE	20.0	CHCl3(5)	0	80	9
8 ± 2	PLLA/5%DOPE	20.0	CHCl3(5)	0	80	9
7 ± 1	PLLA/10%DOPE	20.0	CHCl3(5)	0	80	9
8 ± 2	PLLA/20%DOPE	20.0	CHCl3(5)	0	80	9
6.5	PLLA/PEO–PPO–PEO	18.0	CHCl3(X)	0	80	10
4.3	PLLA/PEO–PPO–PEO	18.0	CHCl3(X)	0	75	10
3.9	PLLA/PEO–PPO–PEO	18.0	CHCl3(X)	0	65	10
1.75 ± 0.24	PLLA	10.0	CH2Cl2(3)	0	43	11
11.50 ± 1.43	PLLA	10.0	CH2Cl2(3)	0	91	11

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Since Fig. 1 shows that the pore size of both PLLA and PDLA films reduced with the voltage increase, this confirms that the EBF method can be used to controllably form ordered porous PLA films, which is in good agreement with our previous studies.^24–26 In fact, this was also proven by comparison of the literature reports on pore size values as presented in Table 1. For example, the PLLA and PDLA films prepared at 3 kV both showed a smaller pore size as compared with the literature reported values (Table 1). According to Table 1, our results are acceptable because the 0 V sample presented pore size is similar to that reported in the literature.^6,9–11

In terms of the presented relationship between the pore size, d_a, and applied voltage, ρ, as seen at the bottom of Fig. 1, the d_a reduction with the ρ increase for PLLA and PDLA followed two different patterns, e.g. the former following a linear fit and the latter following a non-linear fit. This difference may also be ascribed to the opposite chiral structure of PLLA and PDLA. It can be noted from this plot (Fig. 1 bottom) that PLLA and PDLA both would form similar pore sizes when ρ approaches about 3 kV, indicating that the further increase of ρ is unimportant for the formation of a porous PDLA film.

The wettability of EBF-prepared porous PLLA and PDLA films was studied by performing water contact angle, θ_W, measurements and the results are presented in Fig. 2. It can be observed that the θ_W on the non-porous PLLA and PDLA films was the smallest, corresponding to good hydrophilicity, and on the porous PLLA and PDLA films, it increased with the ρ increase, especially for the 3 kV-based samples, which suggests it can be tuned into becoming hydrophobic. A comparison of two series of PDLA samples further found that the θ_W on the surface peeled samples linearly increased to approach the superhydrophilicity range.^24–26 In terms of Fig. 2, the θ_W increase with the ρ increase followed two different patterns, e.g. the PLLA film followed a linear fit and the PDLA film a non-linear fit, suggesting the EBF formation processes of porous PLLA and PDLA are different, probably due to the two evaporation rates. It is also possible that these two opposite chiral structures have different electric-responses.

Fig. 2 Wettability of porous PLLA and PDLA films in relation to various voltages and one series of PDLA films after surface peeling.

According to Fig. 2, the as-received 0 V-formed porous PLLA and PDLA films both have similar θ_W of about 72°. This indicates that the common PLLA and PDLA samples have the same wettability, ignoring their opposite chiral behaviours. In terms of Fig. 2, the voltage increase from zero to 3 kV led the θ_W on the porous PLLA film surface to change from about 80° to 120°, and on the porous PDLA film surface from about 80° to 100°, and on the peeled porous PDLA film surface from about 115° to 145°. This wettability indicated that the EBF method can be used to enhance the hydrophobicity of the PLA film, and the additional use of surface peeling can tune it from hydrophobic to superhydrophobic, which is in good agreement with our recently reported case for PS films.^24–26

Since the porous PDLA films, i.e. the as-(EBF) formed and after surface peeling, presented two linear fits that formed an area as seen in Fig. 2 right, it is considered that this area is perhaps an effective wetting area for porous PDLA films, and similarly this should also be available for porous PLLA films. Therefore, it is primarily understood that the EBF method can be used to form porous PLLA and PDLA films with a controlled pore size and wettability, and the enhancement of the hydrophobicity can be performed by firstly increasing the voltage then by applying surface peeling.

The FTIR spectra of both porous PLLA and PDLA films corresponding to the voltages of 0 and 3 kV were compared and the results are presented in Fig. 3. It can be observed that the characteristic bands at about 1755–1760 cm⁻¹ due to the carbonyl C=O stretching and at 2945 cm⁻¹ due to the O–H stretching vibration appeared for both PLLA and PDLA films corresponding to the voltages at 0 and 3 kV. Since the band at 1755–1760 cm⁻¹ represents the chiral aspect of PLA,^27–29 the presented same FTIR spectra indicated that the main structure of both PLLA and PDLA are not influenced by the applied voltages. Therefore, the mechanism on EBF-formed porous PLA film can be reasonably considered to be the same as for the previously EBF-formed porous PS film.²⁴

Fig. 3 FTIR spectra of porous PLLA and PDLA films prepared by EBF at 0 and 3 kV, respectively.

Experimental

Materials

Both PLLA (L149) and PDLA (D155) with the same molecular weight of about 11.6 × 10⁴ (g mol⁻¹) were provided by the Zhejiang Hisun Biomaterials Co. Ltd.^2,3,30 In this study, both these PLAs were used as-received without furthermore purification or treatment. Laboratory made distilled water was always used in the experiments.

Fabrication of porous PLLA and PDLA films by the electric breath figure method

The as-received PLLA and PDLA were initially dissolved in chloroform to form a polymer solution with a concentration of 1 wt% and then these two PLA solutions were cast onto the glass substrate to allow the air to flow across the surface with an electrostatic generator used to provide various voltages, as in the literature.^24–26 During this EBF process, the total time for the preparation of each sample was about 3 min. It is obvious that this is a novel electric BF process, i.e. EBF. In this study, the voltage was varied at 0, 100, 200, 600 and 1000 V and the other conditions were fixed, e.g. the distance between two coppers of about 1 cm, related humidity of about 75%, drop flow velocity of about 50 m min⁻¹ and temperature at about 25 °C, respectively.

In order to understand the surface peeling-induced effects on the wettability of the formed PLA film, several EBF-fabricated porous PDLA films were peeled, same as in the literature.²⁴ The film surface peeling was performed using an adhesive tape (Scotch Tape, 3M), as previously described.²⁴

Characterization

The surface morphology and pore size were analyzed by field emission scanning electron microscopy, FESEM (S-4800, Hitachi Co. Ltd).

The wettability was studied by measuring the drop water contact angle on PLA film surface by using an OCA40 Micro goniometer (Dataphysics Co. Ltd). The advancing contact angle was employed and each given value was estimated according to the associated grayscale images software by averaging three independent measured values with a mean error of less than 2°.

Fourier transform infrared, FTIR, spectrum of each sample was obtained using the NEXUS 8700 (Nicolet, UK) in the range of 400–4000 cm⁻¹ with the resolution of 4 cm⁻¹. The KBr pellet technique was adopted to prepare all the samples.

Conclusions

This study demonstrated that porous PLLA and PDLA films can be controllably fabricated using the EBF method. Our results showed that the EBF-formed porous PLLA films have a greater pore size than that of the porous PDLA films under the same conditions, including the normal BF process. This suggests that the opposite chiral structures of PLLA and PDLA have different evaporation rates during normal BF and EBF processes. This study proves that the pore sizes of both the PLLA and PDLA films were reduced with the voltage increase, and this led the film surface to turn from hydrophilic to hydrophobic.

Acknowledgements

This study was financially supported by the Donghua University, China.

Notes and references

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