A Green and Facile Melt Approach for Hierarchically Porous

Research Article pubs.acs.org/journal/ascecg

A Green and Facile Melt Approach for Hierarchically Porous Polylactide Monoliths Based on Stereocomplex Crystallite Network Xiao-Rong Sun, Zhi-Qiang Cao, Rui-Ying Bao,* Zhengying Liu, Bang-Hu Xie, Ming-Bo Yang, and Wei Yang* State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China S Supporting Information *

ABSTRACT: A facile and green route to fabricate sustainable and biodegradable tridimensional interconnected hierarchically meso- and macroporous polylactide (PLA) monoliths was developed. The tunable morphologies and controllable pore sizes of the monoliths based on the stereocomplex (sc) crystallization of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) during the melt blending process were caused by the macro- and microphase separation between PLA and poly(ethylene oxide) (PEO). The proposed approach can be easily scaled-up and is environmentally sustainable, and it involves neither any toxic chemical reagents nor templates. The hierarchical morphologies of the porous materials contain mesopores regulated by the sc crystallite network formed during melt processing and macropores induced by macrophase separation. The porous structure was influenced by PDLA contents seriously, and the construction of interconnected pores made up of sc crystallite tridimensional network appears at a low PEO content of 20 wt %. The porous polymer monolith featured very good hydrophobicity with a water contact angle over 135°, as well as the strong lipophilicity, endowing the materials with potential applications in selective oil−water separation. Moreover, the much higher thermal and chemical resistance of the porous PLA monoliths based on sc crystallite network allowed us to significantly broaden the applications of the adsorbent in some harsh conditions and catalytic systems compared to those based solely on PLLA. KEYWORDS: Hierarchical porous materials, Polylactide, Stereocomplex crystallization

■

INTRODUCTION Hierarchical porous materials have attracted enormous attention mostly as functional materials in recent years because of their diverse performance.1−3 Generally, the micro- (50 nm) would reduce the transport limitation within the material and increase the accessible space.4,5 The combination of multiple length scales of pores could confer unique balanced properties to the materials, which provide great potential in the fields of catalysis,6,7 photocatalytic H2 production,8 adsorption,9−12 separation,13,14 energy storage15−17 and conversion,18,19 and biomedicines.20,21 Nowadays, numerous approaches to obtain hierarchical porous materials have been developed. The dualtemplating18,22,23 and a combination of multiple-template methods24 are most commonly used to fabricate materials © 2017 American Chemical Society

with hierarchical porous structures. Some other important strategies to obtain hierarchical porous structures included lithography,25,26 colloidal assembly,27,28 mechanochemistry,29 and phase separation.30−34 Inorganic nanoscale porous materials have exhibited remarkable superiority in their applications, but the development of polymer porous monoliths still deserve great attention because of their plasticity and the ability to be fabricated into specific shapes and dimensions for special applications. Sarazin and Favis35 found that the removal of the polystyrene (PS) phase from poly(L-lactide) (PLLA)/PS blends prepared by melt mixing could produce porous polylactide (PLA) materials Received: June 27, 2017 Revised: July 23, 2017 Published: August 14, 2017 8334

DOI: 10.1021/acssuschemeng.7b02121 ACS Sustainable Chem. Eng. 2017, 5, 8334−8343

Research Article

ACS Sustainable Chemistry & Engineering

between PLLA and PDLA was found to show crucial effects on the morphologies of the resulting porous materials. The very good hydrophobicity and excellent solvent resistance of the porous monoliths endowed the material with potential application in oil−water separation.

with completely interconnected porosity, and the micrometer scale pores derived from the thermodynamic immiscibility of PLLA and PS could be tuned by annealing and compatibilization. Tsuji et al.36 obtained macroporous PLLA films by extraction of poly(ethylene oxide) (PEO) from solution-cast PLLA/PEO blend films, and the pore size and porosity could be controlled by blend ratio of the components and molecular weight of the porogen. Ye et al.37 found that the poorly crystallizable PLLA chains remained amorphous in the poly(oxymethylene) (POM)/PLLA cocontinuous binary blends, and the PLLA constituents expelled out from the POM banded spherulite crystals were fully included in the nanoscale interlamellar or interfibrillar regions of POM crystals. Consequently, they obtained nanoporous POM materials with hierarchically patterned surface and 3D interpenetrated internal channels by removal of the amorphous PLLA phase, and the hierarchically patterned structure could be adjusted by the isothermal crystallization conditions. Saba et al.32 developed a synthetic route for nanoporous polymer monoliths based on controlled polymerization of styrene (St) and divinylbenzene (DVB) from a PLA macrochain transfer agent in the presence of poly(ethylene oxide) (PEO), and a PLA-b-P(St-co-DVB) diblock polymer was generated by RAFT copolymerization. After copolymerization, the nonreactive PEO additive was macrophase separated from the microphase separated diblock polymer. Etching in basic solution simultaneously degraded PLA and dissolved PEO, producing mesopores and macropores, respectively. However, organic solution used for extraction of the porogen phase and complicated procedures for the synthesis of these materials would limit the wide application of these materials. It is of great importance to reduce the cost, simplify the process, and increase the yield in green and environmentally sustainable approaches. PLA is one of the most promising biodegradable polymers owing to its excellent mechanical properties and biocompatibility.38−43 For PLA, a specific crystal form, the stereocomplex (sc) crystallites, can be formed by virtue of the tight integration between the two opposite chiral PLLA and poly(D-lactide) (PDLA) molecular chains.44−48 One of the most attractive features of sc crystallites is their high melting point, almost 50 °C higher than that of homocrystallites (hc), so it can provide greater heat resistance to the material.49−52 Sc crystallization was also reported to be able to enhance the properties of PLA markedly, including mechanical performance, hydrolysis resistance, and heat resistance.44,53−61 Sc crystallites of PLA could be formed effectively during processing with a low temperature approach as described in our previous work.49 Moreover, the melt rheological properties of asymmetric PLLA/PDLA blends could be tuned effectively even with the addition of a small amount of PDLA, and the sc crystallite network has been confirmed to appear at a PDLA content of 2 wt %.62 It was expected that the tridimensional network of sc crystallites formed during processing would influence the morphology of polymer blends and would be beneficial to obtain hierarchical porous structure materials. In this work, a facile and green method to construct a novel degradable and high-temperature-resistant hierarchical porous material with tunable pore size was developed. The porous materials formed by hierarchical phase separation were obtained via the cocrystallization of PLLA and PDLA in the presence of PEO additive, which was the most commonly used green porogen owing to its good water-solubility. During the melt blending process, the formation of a sc crystallite network

■

EXPERIMENTAL SECTION

Materials. PLLA (Mn = 9.1 × 104 g mol−1, Mw = 2.1 × 105 g mol−1, Mw/Mn = 2.3, measured by gas permeation chromatography (GPC)) and PDLA (Mn = 7.4 × 104 g mol−1, Mw = 2.0 × 105 g mol−1, Mw/Mn = 2.7, measured by GPC) were kindly provided by Zhejiang Hisun Biomaterials Co., Ltd., China, and the melting temperatures of PLLA and PDLA are 178 °C (DSC, 10 °C/min). PEO with a viscosityaverage molecular weight of 3.0 × 105 g mol−1 was purchased from Aladdin Company. Sample Preparation. PLLA and PDLA were vacuum-dried at 60 °C for 12 h before use and PEO was dried in a vacuum oven at 40 °C for 12 h. PLLA/PDLA/PEO blends were prepared with a Haake rheometer (Haake Rheomix 600, Germany) at 180 °C and 60 rpm for 5 min and then cooled at the ambient conditions. The PEO phase was used as the porogen and was extracted with deionized water after melt compounding. The samples with the sizes of 2 cm × 1 cm × 1 cm were extracted in deionized water at room temperature for 3 days under stirring, and the deionized water was changed every 8 h. After extraction, the specimens were dried in a vacuum oven at 40 °C for 24 h. The extraction was completed until the dried extracted specimens maintained at a constant weight. To find out how the sc crystallite network in the blends influenced the porous structure, the weight ratio of PEO was kept at 40 wt % to maintain the cocontinuous phase structure for PLLA/PDLA/PEO blends, and the weight ratio of PDLA was varied from 0 to 10, 20, 30, and 40 wt % of the weight of PLA. For convenience, the specimens were denoted as x % PDLA, where x represents the content of PDLA in PLA, and the content of PEO was 40 wt % in the PLLA/PDLA/PEO blends. The effect of the content of PEO on the pore structure of the material was also studied. The weight ratio of PEO was varied from 0 to 10, 20, 30, 40, and 50 wt % of the weight of the blends, and the weight ratio of PLLA to PDLA was fixed at 70:30, which can form a stable sc crystallite network. The specimens were denoted as y wt % PEO briefly, where y represents the weight content of PEO in the PLLA/PDLA/PEO blends. Characterizations. Differential Scanning Calorimeter (DSC). Thermal analysis of all the blends before and after extraction of PEO was performed with DSC tests at a heating rate of 10 °C/min from 40 to 270 °C using a DSC Q20 (TA Instruments, USA). The temperature and heat flow were calibrated by pure indium under nitrogen atmosphere before use. Wide-Angle X-ray Diffraction (WAXD). WAXD profiles were recorded on an Ultima IV X-ray diffractometer (Rigaku Corporation, Japan) using a Cu Kα radiation source (λ = 0.154056 nm, 40 kV, 25 mA) in the scanning angle range of 2θ = 3−50° at a scan speed of 3°/ min. The crystallinities of hc crystallites (Xc(hc)) and sc crystallites (Xc(sc)) were calculated by dividing the sum of hc diffraction peak area and sc diffraction peak area to the total peak area after peak fitting, respectively. The fraction of sc crystallites ( fsc), could be obtained by dividing the Xc(sc) by the total crystallinity (Xc). Thermomechanical Analyzer (TMA). The heat resistance of the porous materials was measured on a thermomechanical analyzer (TMA, TA Q200, USA). The temperature ramp was recorded at a rate of 2 °C/min over the temperature range of 20−230 °C in a purified nitrogen atmosphere, and the constant normal force was set to be 0.1 N. Scanning Electron Microscopy (SEM). The morphologies of the porous materials were examined with an FEI-INSPECTF scanning electron microscopy (SEM, Hillsboro, OR, USA) at an accelerating voltage of 10 kV. The samples obtained from the melt compounding process were cryofractured in liquid nitrogen, and the fractured surfaces were immersed into deionized water for 3 h to remove PEO. Finally, the dried fractured surfaces were sputter-coated with a layer of 8335


Research Article


Figure 1. WAXD patterns (a) and DSC heating curves (b) for the samples after removal of PEO. gold prior to examination. The sample after extraction process was also fractured for observation of the inner pore structure. Nitrogen Sorption. Nitrogen sorption isotherms were obtained on a Quantachrome Autosorb SI (Quantachrome Instruments, USA) at liquid nitrogen temperature (77 K). Prior to measurement, samples were degassed for 6 h at 100 °C using a turbo molecular pump. Brunauer−Emmett−Teller (BET) specific surface areas were obtained from the adsorption branch from P/P0 = 0.05−0.35. Wettability Measurement. A Krüss DSA100 (Krüss Compony, Germany) apparatus was used to measure the contact angle (CA) of water and diiodomethane droplets (3 μL) at 25 °C.

Table 1. WAXD and DSC Data of Blends with Different Contents of PDLAa WAXD samples 0% PDLA 10%PDLA 20% PDLA 30% PDLA 40% PDLA

■

RESULTS AND DISCUSSION Stereocomplex Crystallite Formation. In our previous work, sc crystallites were obtained in PLLA/PDLA blends with a low-temperature approach.49 Here, PLA/PEO blends with various weight ratio of PLLA/PDLA were prepared with this approach at 180 °C, a temperature above the melting temperature of hc crystallites of PLA, and only sc crystallization can occur. In all the melt-compounded blends, the melting peaks of PEO were observed upon heating in DSC tests, indicated that PEO was introduced into the blends (Figure S1). Subsequent extraction of PEO was confirmed by gravimetric analysis and Fourier transform infrared (FTIR) spectroscopy. All the samples showed a high continuity no matter the weight ratio of PLLA/PDLA, indicating that a cocontinuous phase structure for the PLLA/PDLA/PEO blends was obtained during melt compounding (Experimental section in Supporting Information and Figure S2). It is also interesting to note that the extracted samples maintained nanostructural integrity (as evidenced by nitrogen sorption analysis), and the visible appearance of the blends changed from opaque faint yellow to pure white because of the light scattering (Figure S2). In addition, the characteristic methylene stretching band of the PEO (2880 cm−1) phase was absent after water extraction of PEO, corresponding to the complete removal of PEO in the blends (Experimental section in Supporting Information and Figure S3). Figure 1 shows the WAXD patterns and DSC melting curves of the samples after removal of PEO, and the characteristic data from WAXD and DSC tests are included in Table 1. As shown in Figure 1a, the WAXD patterns exhibited diffraction peaks at 14.8°, 16.9°, 19.1°, and 22.5°, assigned to the (010), (200) or (110), (203), and (210) plane of α-form hc crystallites for the

a

DSC

Xc(hc) (%)

Xc(sc) (%)

Xc (%)

fsc (%)

Tm(hc) (°C)

Tm(sc) (°C)

45.0 43.6 24.7

13.1 35.0

45.0 56.7 59.7

23.1 58.6

176.6 177.5 173.4

228.8 235.2

18.1

51.6

69.7

74.0

171.1

239.8

3.9

61.2

65.1

94.0

167.9

240.5

After removal of PEO.

sample 0% PDLA.63 The high Xc(hc) of 45.0% in sample 0% PDLA prepared by melt blending was owing to the good chain mobility of PLLA in the presence of a large amount of PEO. With the introduction of 10% PDLA, another three visible diffraction peaks appeared at 12.0°, 20.9°, and 24.1°, assigned to the (110), (300) or (030), and (220) plane of sc crystallites. 64 The formed sc crystallites could act as heterogeneous nuclei for the crystallization of PLLA matrix, resulting in a high crystallinity in the sample 10% PDLA. With the increase of PDLA content, the diffraction peaks corresponding to sc crystallites became stronger, and the peaks corresponding to hc crystallites became weaker, indicating greatly increased content of sc crystallites. For the sample 40% PDLA, the fraction of sc crystallites (fsc) could be as high as 94%. These results indicated that the sc crystallites could be successfully obtained during melt processing of the samples with the addition of PDLA, which was well consistent with our previous work.49 From Figure 1b, the DSC melting curve of the sample without PDLA showed only one peak at around 176.6 °C, indicating the exclusive formation of hc crystallites. With the addition of PDLA, another melting peak from sc crystallites appeared at around 230 °C. The melting point of sc crystallites, Tm(sc), increased with the increasing content of PDLA, and it came with the decrease in the melting point of hc crystallites, Tm(hc). When PLLA and PDLA were mixed at a nonequimolar ratio, thicker lamella and more perfect crystals could be formed if the composition was nearly symmetric, resulting in higher 8336


Research Article


Figure 2. SEM micrographs at (a−e) 10000× and (a1−e1) 40000× magnification and (a2−e2) the corresponding nitrogen sorption isotherm of the etched PLA monolith with 0%, 10%, 20%, 30%, and 40% PDLA, respectively, prepared with 40 wt % PEO additive. Filled circles (●) correspond to the adsorption branch, and the open circles (○) to the desorption branch. SEM macrograph (80000×) of the PLA monolith with 30% PDLA was inserted in panel d.

Tm(sc) with the increasing ratio of PDLA. In contrast, the hc crystallization was confined by the preformed sc crystallite network. The restricted chain mobility led to difficulties in the thickening of hc lamella and imperfect hc crystals with lower melting points formed. It was also interesting to note that Tm(hc) in 0% PDLA decreased about 1.4 °C after the extraction of PEO (compared with Tm(hc) of 0% PDLA in Figure S1) because of the hydrolysis during extraction, while the materials possessed good hydrolysis resistance with the incorporation of PDLA. Tailored Porous Structure with Stereocomplex Crystallite Network. The morphology of the hierarchical porous materials was directly characterized by SEM. As shown in Figure 2a−e,a1−e1), the introduction of PDLA influenced the crystalline morphology seriously. From Figure 2a,a1, hc spherulites of PLLA with a size of about 10 μm were observed on the etched surface for the sample without PDLA. With the incorporation of 10% PDLA, relatively small spherulites appeared in Figure 2b,b1, due to the sc crystallites formed between PLLA and PDLA acting as a nucleating agent for hc crystallization of PLLA. With further increasing content of PDLA, obvious changes took place in the surface morphology. From Figure 2c,c1, the crystallite network consisting of irregular lamellar textures of sc crystallites was established, and only several smaller spherulites could be identified (marked with yellow circles). This network became complete with the incorporation of more PDLA. As shown in Figure 2d,d1, when the PDLA composition was increased to 30%, a complete sc crystallite tridimensional network was established. SEM micrographs also showed that the etched samples had a rough surface with visible pores. The shape and size of the pores showed obvious differences among the samples with different weight ratio of PLLA/PDLA. From Figure 2a,a1, disordered narrow strips of pores existed in the gap between the stacking lamella of PLLA, probably caused by the macrophase separation during the crystallization of PLLA. In Figure 2b,b1, there were still some narrow strips of pores, and the surface roughness increased. With further increasing content of PDLA, obvious changes took place in the surface

morphology. In Figure 2c,c1, most of the pores represented spherical shape and the uniformity of size and distribution of the pores was enhanced. This variation indicated that the formation of abundant sc crystallites would greatly affect the morphology of the monoliths, and the PEO molecular chains could be confined within the sc crystallite tridimensional network. When the PDLA content was increased to 30%, as shown in Figure 2d,d1, the strips of pores could hardly be observed, and the uniformity of size and distribution of the macropores and mesopores was further enhanced, due to the stable sc crystallite tridimensional network being established. The nanoscale mesopores could be identified at a higher magnification as inserted in Figure 2d. With PDLA content increasing to 40%, from Figure 2e,e1, it was observed that isolated macropores distributed in the monolith and mesopores were present within the tridimensional rigid skeleton constructed by sc crystallites clearly. In addition, the sample after the extraction process was also fractured and the inner pore structure of the porous materials was shown in Figure S4, which exhibited the same structure as shown in Figure 2. The detailed hierarchical porous structure of the materials was further characterized by nitrogen sorption and desorption. As shown in Figure 2a2−e2, all the porous samples with different contents of PDLA or different degree of perfection of sc tridimensional network showed type IV isotherms with H2 hysteresis, characteristic of mesoporous materials. The detailed pore volume estimated by nitrogen sorption is included in Table 2. It should be noted that the pore volume measured by nitrogen sorption analysis does not contain the macropores Table 2. Pore Volume and Surface Area Measured by Nitrogen Sorption Analysis

8337

samples

specific surface area (m2/g)

pore volume (cm3/g)

0% PDLA 10% PDLA 20% PDLA 30% PDLA 40% PDLA

7.4 17.1 17.9 19.9 15.4

0.064 0.145 0.165 0.211 0.138 DOI: 10.1021/acssuschemeng.7b02121 ACS Sustainable Chem. Eng. 2017, 5, 8334−8343

Research Article

ACS Sustainable Chemistry & Engineering larger than about 300 nm as shown in SEM images because of the limitation of the testing instrument. The specific surface area and pore volume of the porous monoliths with sc crystallites was significantly higher than that of 0% PDLA. There was a slight decrease in the specific surface area for the monolith with 40% PDLA (Table 2), due to the appearance of larger pores possibly. The increased PDLA content led to the formation of a more perfect and compact sc crystallite network and finally resulted in increasing swelled PEO domains, as evidenced by the significantly increased macropore size in Figure 2e,e1. Moreover, the mesopores formed by microphase separation showed similar size when different content of PDLA was incorporated (Figure S5). The formation of the sc crystallite network tailored hierarchical porous structure was also verified by recording the rheological properties in such a dynamic processing state (Figure S6). The formation of the sc crystallite tridimensional network could greatly improve the heat resistance of the monolith. The heat resistance of the porous materials with different weight ratio of PLLA and PDLA was evaluated by TMA analysis and the curves of dimension change (%) vs temperature are shown in Figure 3.

Samples 0% PDLA and 10% PDLA disintegrated at around 160 °C under a constant normal force of 0.1 N. The dimensional stability of sample 10% PDLA with a higher crystallinity was worse than that of 0% PDLA, which might be attributed to higher pore continuity in 10% PDLA. With increasing PDLA content, the content of sc crystallites increased and the sc crystallite tridimensional network became complete gradually. Thus, sample 20% PDLA maintained structural integrity to a great extent even at a test temperature up to 180 °C. When the content of PDLA was higher than 30%, the ability of the materials to resist high ambient temperature was greatly improved. TMA curves clearly revealed that the samples with 30% and 40% PDLA maintained a good dimensional stability even at 210 °C. This was consistent with the results of SEM observation that a stable sc crystallite tridimensional network was established when the PDLA content was higher than 20%. In our previous work,62 the sc crystallite network could be formed by bridging molecules with the addition of a small amount of PDLA (2 wt %). High heat resistance appeared at a PDLA content of 20%; in this case, the network was formed by sc crystallites connected directly with each other. The sc crystallite network became even better with the further increasing of PDLA content, resulting in overall good thermostability. As expected, the tridimensional network of sc crystallites was not destroyed at high ambient temperature and had superior heat resistance even if the materials were porous. Effect of PEO Content on the Porous Structure. The above results revealed that the porous structure of the materials largely depended on the crystalline morphology and whether the perfect sc crystallite tridimensional network had formed. At the same time, it was important for such a system to know the composition for the phase inversion at a fixing ratio of PLLA/ PDLA (70/30), which could form a stable sc crystallite network as mentioned above. It was because a certain value of the porogen phase would lead to porous materials with a certain pore size and volume.36 From Figure S7, the sample with 10 wt % PEO showed small continuity. It was interesting to note that the sample showed high continuity when the PEO content was above 20 wt %, indicating a cocontinuous structure was formed (Figure S7). The formation of the sc crystallite tridimensional network made it possible that the cocontinuous structure occurred at such a low critical PEO content.

Figure 3. Dimension change of the hierarchical porous materials as a function of temperature.

Figure 4. SEM micrographs at (A−F) 10000× and (inset) 40000× magnification of the etched monolith with a ratio of PLLA/PDLA (70/30), prepared with 0, 10, 20, 30, 40, and 50 wt % PEO additive, respectively. 8338


Research Article


Figure 5. Pore size distribution based on DFT analysis of the desorption branch for samples.

The morphology affected by PEO content is shown in SEM results (Figure 4). Interestingly, in Figure 4A, the sample 0 wt % PEO showed a rough surface with plenty of micropapillae in the size of tens of nanometers, indicating that there was some visible pore structure in the PLLA/PDLA blends even without PEO. The sample 10 wt % PEO showed well-defined pores and apparently an increased number of nanopores in Figure 4B. In this case, the etched sample represented a low continuity (24%), and the sample contains abounding uniform closed pores. When the PEO content was increased to 20 wt %, the continuity of the sample improved significantly, and the residual materials formed a tridimensional interconnected porous network. Further increasing PEO to above 30 wt % resulted in the formation of both macro- and mesopores and an increase in the apparent connectivity of the macro- and mesoporous network. As shown in Figure 4D−F, macropores with sizes of 0.2−0.8 μm counted by Image Pro Plus were observed along with the mesopores in the porous skeleton. In addition, the sample after the extraction process was also fractured and the inner pore structure of the porous materials was shown in Figure S8, which exhibited the same structure as shown in Figure 4 when the samples had a high continuity of PEO phase. The morphology affected by PEO content was also verified by nitrogen sorption and desorption. The mesopore size distribution of the extraction samples was estimated based on a density functional theory (DFT) algorithm applied to the desorption branch using a cylindrical pore model (Figure 5). The detailed pore sizes estimated by nitrogen sorption were included in Table 3 and would contribute to a more accurate understanding on the mechanism of porous structure formation. From the pore size distribution and the pore volume data of the PLLA/PDLA blends without PEO, it can be concluded that the sample without PEO contained plenty of closed pores. The DFT pore width estimated to be about 4.1 nm was probably because of the rough surface caused by the cocrystallization of PLLA and PDLA, and a similar phenomenon was also mentioned by Gao,65 Hu66 and Brzezinski.67 The specific surface area and pore volume increased with increasing PEO content, which was consistent with the increase in volume

Table 3. Pore Size and Surface Area Measured by Nitrogen Sorption Analysis samples 0 wt % PEO 10 wt % PEO 20 wt % PEO 30 wt % PEO 40 wt % PEO 50 wt % PEO

DFT pore width (nm)

specific surface area (m2/g)

pore volume (cm3/g)

4.1 4.3

0.3 0.6

0.001 0.001

5.9

1.7

0.008

29.4

7.7

0.063

29.4

19.9

0.211

29.4

24.6

0.304

fraction of the porogen component. Interestingly, when the content of PEO was higher than 30 wt %, the mesopores formed in all the samples showed a DFT pore width of 29.4 nm. This fact once again demonstrated the mechanism of the formation of perfect sc crystallite network in the monoliths. When the same perfect sc crystallite network formed in the samples (i.e., at fixed weight ratio of PLLA/PDLA), the pore size was determined by PEO embedded in the compact sc crystallite network skeleton. After the stacking interlamellar spaces were full of porogen, the remaining parts of porogen would aggregate and macrophase separate from the matrix, resulting in increasing swelling of the PEO domains and formation of macropores. Mechanism of Porous Structure Formation. Now, tridimensional interconnected hierarchical meso- and macroporous PLA monoliths with tunable morphology and controlled pore size by macro- and microphase separation were fabricated. As illustrated in Scheme 1, PLA and PEO were melt blended and a cocontinuous phase structure was generated by the macrophase separation of the two components, and each phase is highly continuous in three-dimensions. For pure PLLA, PEO was expelled from PLLA crystal lamellae, and highly porous PLLA monoliths were obtained after extracting the PEO component from the spherulitic hc crystallites. With the introduction of PDLA, the formation of the tridimensional sc 8339


Research Article


monoliths were fabricated by removal of PEO phase which showed hierarchical macro- and microphase separation with PLA phase. Wettability of the Hierarchical Porous Monoliths. Figure 6a shows that all the porous monoliths 0% PDLA to 40% PDLA exhibited hydrophobicity with water contact angles (WCA) above 135° because of the rough hierarchical porous micro−nanomorphology. It was worth noting that the hydrophobicity of the porous monoliths would not be destroyed if the surface of the blends was abraded or cut by a knife. It indicated that the hierarchical porous morphology extended throughout the whole volume of the materials, which endowed the material with very good hydrophobicity. In contrast, diiodomethane was immediately drawn into the monoliths, showing undeniable superoleophilicity. This opposite wetting behavior inspired us to consider the hierarchical porous materials with good hydrophobicity as adsorbents for oil leak cleanup.68 The oil−water separation experiments were performed, and the digital photographs of the sorption processes are shown in Figure 6. The experiments were performed at room temperature; 0.5 mL of solvent was dropped into 20 mL water for sorption and 5 min was used to make sure that all of the adsorbents could finish the sorption process. Samples 0% PDLA to 40% PDLA with the same weight were used to adsorb the chloroform sinking in water, as shown in Figure 6b−d. After a few minutes, the adsorbents were taken out. The chloroform was adsorbed successfully by the monoliths 30% PDLA and 40% PDLA, leaving a glass of transparent water, and the adsorbent remained dry and clean. For the samples 0% PDLA to 20% PDLA, the external structure of the monolith collapsed. This indicated that the perfect sc crystallite tridimensional network formed in the samples 30% PDLA and 40% PDLA could endow the material with excellent solvent

Scheme 1. Schematic Illustration of the Formation of Interconnected Hierarchical PLA Monoliths Based on sc Crystallite Network

crystallite network in PLA phase and the controllable rheological response of the PLA phase would affect the phase morphology and the macroporous structure obviously. On the other hand, irregular lamellar textures of sc crystallites formed isothermally under the strong shear during melt processing. PEO was also located in the superdense sc crystallite rigid network due to the crystallization induced microphase separation. Then, the nanopores were obtained with the removal of the PEO chains embedded in the nanoscale interlamellar space of PLA sc crystallites. To be noted, the crystallization process in PLLA also induced microphase separation of the two components, and the mesopores were present in 0% PDLA. As a consequence, the tridimensional interconnected hierarchical meso- and macroporous PLA

Figure 6. (a) CAs of monoliths with 0%−40% PDLA from left to right, the top and bottom images correspond to the water and oil droplet, respectively. (b−d) Images of the removal processes of chloroform (dyed with Sudan III) from water using porous polymer monoliths. The samples from left to right correspond to the content of PDLA being 0%−40%, respectively. The whole sorption processing is presented in (e−e3) 0% PDLA and (f−f3) 40% PDLA, and the insets are the CAs toward the corresponding samples. 8340


Research Article


Figure 7. Images of the (a, a1) 0% PDLA and (b, b1) 40% PDLA before and after boiling in acetonitrile at 80 °C. (b2) WCA results of the 40% PDLA after boiling (WCA = 136°).

owing to the macro- and microphase separation during the cocrystallization of PLLA and PDLA in melt blending process. No toxic chemical reagents were used, and the simple melt processing process demonstrated an environmentally sustainable and easily scaled-up preparation technique. The morphology of the porous material could be tuned innovatively by the varied crystal forms and the rheological network constructed by sc crystallites. Moreover, the very good hydrophobicity and excellent solvent resistance of the porous polymer monoliths endowed the material with potential applications in selective oil−water separation. We envisioned that the specific structures would greatly broaden the applications that required such an ordered hierarchical structure in some extreme temperature and solvent ambient environment.

resistance. The adsorbing capacity of the samples 30% PDLA and 40% PDLA was determined to be 1.39 ± 0.28 and 1.87 ± 0.20 g g−1, respectively. The detailed sorption processes of the sample 0% PDLA and 40% PDLA are presented in Figure 6e−e3,f−f3. In Figure 6f−f3, the chloroform was immediately taken up when the monolith got closed to it and the whole process was completed in seconds. In Figure 6e−e3, the chloroform was also taken up quickly and then the monolith dissolved partly. The open pores on the surface closed because of the dissolution, and the samples could not be used as adsorbent anymore. The results demonstrated that the porous monoliths with perfect sc crystallite network could be used to remove the oily reagent sinking in water, even though pure PLLA porous adsorbent could be dissolved in it. Besides, the porous monolith could still adsorb the oil floating on the water. As shown in Figure S9, all of the samples showed a superb sorption capacity. The cyclohexane floating on water could be taken up by the porous monoliths within a few seconds. To reveal the thermal and chemical resistance of the porous materials, samples 0% PDLA and 40% PDLA were refluxed with acetonitrile at a temperature as high as 80 °C. PLA cannot dissolve in acetonitrile at room temperature. The pictures taken during the process of refluxing are presented in Figure 7. Interestingly, the sample 0% PDLA dissolved in 25 min, while the 40% PDLA maintained structural integrity even after 24 h, which was well consistent with the reported results in ref 69, in which PLLA or PDLA acetonitrile solution was prepared at 80 °C and stereocomplexes were formed in acetonitrile at 80 °C, meaning that sc crystallites are stable in this condition. Moreover, after high temperature refluxing, sample 40% PDLA still possessed good hydrophobicity and showed a WCA over 135°. The much higher thermal and chemical resistance of the porous PLA monoliths based on sc crystallite network with respect to those based solely on PLLA allowed us to significantly broaden the applications of the adsorbent in some harsh conditions and catalytic systems.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02121. Continuity measurement, Fourier transform infrared spectroscopy (FTIR) measurement, DSC heating curves, SEM micrographs, pore size distribution, mixing torque curves, and images of the removal processes of cyclohexane from water (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Tel: + address: *Tel: + address:

86 28 8546 0130. Fax: + 86 28 8546 0130. E-mail [email protected] (W. Yang) 86 28 8546 0130. Fax: + 86 28 8546 0130. E-mail [email protected] (R.-Y. Bao).

ORCID

Wei Yang: 0000-0003-0198-1632 Notes

The authors declare no competing financial interest.

■

■

CONCLUSIONS In summary, we developed a facile and green route to fabricate sustainable, degradable, high-temperature-resistance, and goodsolvent-resistance tridimensional interconnected porous PLA materials. The tunable morphology and controlled pore size of hierarchical meso- and macroporous PLA monoliths were

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NNSFC Grants 21374065, 51503132, and 51422305), Sichuan Provincial Science Fund for Distinguished Young Scholars (2015JQO003), and China Postdoctoral Science Foundation (Grant No. 2015M580789). 8341


Research Article


■

(18) Cho, C. Y.; Moon, J. H. Hierarchically porous TiO2 electrodes fabricated by dual templating methods for dye-sensitized solar cells. Adv. Mater. 2011, 23 (26), 2971−2975. (19) Zhao, Z.; Liu, G.; Li, B.; Guo, L.; Fei, C.; Wang, Y.; Lv, L.; Liu, X.; Tian, J.; Cao, G. Dye-sensitized solar cells based on hierarchically structured porous TiO2filled with nanoparticles. J. Mater. Chem. A 2015, 3 (21), 11320−11329. (20) He, W.; Min, D.; Zhang, X.; Zhang, Y.; Bi, Z.; Yue, Y. Hierarchically Nanoporous Bioactive Glasses for High Efficiency Immobilization of Enzymes. Adv. Funct. Mater. 2014, 24 (15), 2206− 2215. (21) Tsujimura, S.; Murata, K.; Akatsuka, W. Exceptionally high glucose current on a hierarchically structured porous carbon electrode with ″wired″ flavin adenine dinucleotide-dependent glucose dehydrogenase. J. Am. Chem. Soc. 2014, 136 (41), 14432−14437. (22) Liang, C.; Dai, S. Dual Phase Separation for Synthesis of Bimodal Meso-/Macroporous Carbon Monoliths. Chem. Mater. 2009, 21 (10), 2115−2124. (23) Dhainaut, J.; Dacquin, J.-P.; Lee, A. F.; Wilson, K. Hierarchical macroporous−mesoporous SBA-15 sulfonic acidcatalysts for biodiesel synthesis. Green Chem. 2010, 12 (2), 296−303. (24) Sai, H.; Tan, K. W.; Hur, K.; Asenath-Smith, E.; Hovden, R.; Jiang, Y.; Riccio, M.; Muller, D. A.; Elser, V.; Estroff, L. A.; Gruner, S. M.; Wiesner, U. Hierarchical porous polymer scaffolds from block copolymers. Science 2013, 341 (6145), 530−534. (25) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Nanoengineered Multiscale Hierarchical Structures with Tailored Wetting Properties. Langmuir 2006, 22 (4), 1640−1645. (26) Morariu, M. D.; Voicu, N. E.; Schaffer, E.; Lin, Z.; Russell, T. P.; Steiner, U. Hierarchical structure formation and pattern replication induced by an electric field. Nat. Mater. 2003, 2 (1), 48−52. (27) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Superhydrophobic Bionic Surfaces with Hierarchical Microsphere/SWCNT Composite Arrays. Langmuir 2007, 23 (4), 2169−2174. (28) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. Template-Assisted SelfAssembly: A Practical Route to Complex Aggregates of Monodispersed Colloids with Well-Defined Sizes, Shapes, and Structures. J. Am. Chem. Soc. 2001, 123 (36), 8718−8729. (29) Crawford, D. E.; Casaban, J. Recent Developments in Mechanochemical Materials Synthesis by Extrusion. Adv. Mater. 2016, 28 (27), 5747−5754. (30) Nakanishi, H.; Norisuye, T.; Tran-Cong-Miyata, Q. Formation of Hierarchically Structured Polymer Films via Multiple Phase Separation Mediated by Intermittent Irradiation. J. Phys. Chem. Lett. 2013, 4 (22), 3978−3982. (31) Li, Y.; Zhang, Z.; Ge, B.; Men, X.; Xue, Q. One-pot, templatefree synthesis of a robust superhydrophobic polymer monolith with an adjustable hierarchical porous structure. Green Chem. 2016, 18 (19), 5266−5272. (32) Saba, S. A.; Mousavi, M. P.; Buhlmann, P.; Hillmyer, M. A. Hierarchically Porous Polymer Monoliths by Combining Controlled Macro- and Microphase Separation. J. Am. Chem. Soc. 2015, 137 (28), 8896−8899. (33) Hwang, J.; Jo, C.; Hur, K.; Lim, J.; Kim, S.; Lee, J. Direct access to hierarchically porous inorganic oxide materials with threedimensionally interconnected networks. J. Am. Chem. Soc. 2014, 136 (45), 16066−16072. (34) Lubbad, S. H.; Buchmeiser, M. R. Highly cross-linked polymeric capillary monoliths for the separation of low, medium, and high molecular weight analytes. J. Sep. Sci. 2009, 32 (15−16), 2521−2529. (35) Sarazin, P.; Favis, B. D. Morphology Control in Co-continuous Poly(l-lactide)/Polystyrene Blends: A Route towards Highly Structured and Interconnected Porosity in Poly(l-lactide) Materials. Biomacromolecules 2003, 4 (6), 1669−1679. (36) Tsuji, H.; Smith, R.; Bonfield, W.; Ikada, Y. Porous biodegradable polyesters. I. Preparation of porous poly(L-lactide) films by extraction of poly(ethylene oxide) from their blends. J. Appl. Polym. Sci. 2000, 75 (5), 629−637.

REFERENCES

(1) Bae, W. G.; Kim, H. N.; Kim, D.; Park, S. H.; Jeong, H. E.; Suh, K. Y. 25th anniversary article: scalable multiscale patterned structures inspired by nature: the role of hierarchy. Adv. Mater. 2014, 26 (5), 675−700. (2) Bradshaw, D.; El-Hankari, S.; Lupica-Spagnolo, L. Supramolecular templating of hierarchically porous metal-organic frameworks. Chem. Soc. Rev. 2014, 43 (16), 5431−5443. (3) Sun, M. H.; Huang, S. Z.; Chen, L. H.; Li, Y.; Yang, X. Y.; Yuan, Z. Y.; Su, B. L. Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine. Chem. Soc. Rev. 2016, 45 (12), 3479−3563. (4) Wu, Z.; Zhao, D. Ordered mesoporous materials as adsorbents. Chem. Commun. 2011, 47 (12), 3332−3338. (5) Linares, N.; Silvestre-Albero, A. M.; Serrano, E.; Silvestre-Albero, J.; Garcia-Martinez, J. Mesoporous materials for clean energy technologies. Chem. Soc. Rev. 2014, 43 (22), 7681−7717. (6) Bronstein, L. M.; Goerigk, G.; Kostylev, M.; Pink, M.; Khotina, I. A.; Valetsky, P. M.; Matveeva, V. G.; Sulman, E. M.; Sulman, M. G.; Bykov, A. V.; Lakina, N. V.; Spontak, R. J. Structure and Catalytic Properties of Pt-Modified Hyper-Cross-Linked Polystyrene Exhibiting Hierarchical Porosity. J. Phys. Chem. B 2004, 108 (47), 18234−18242. (7) Chen, L.-H.; Li, X.-Y.; Tian, G.; Li, Y.; Rooke, J. C.; Zhu, G.-S.; Qiu, S.-L.; Yang, X.-Y.; Su, B.-L. Highly Stable and Reusable Multimodal Zeolite TS-1 Based Catalysts with Hierarchically Interconnected Three-Level Micro-Meso-Macroporous Structure. Angew. Chem., Int. Ed. 2011, 50 (47), 11156−11161. (8) Zhou, X.; Chen, H.; Sun, Y.; Zhang, K.; Fan, X.; Zhu, Y.; Chen, Y.; Tao, G.; Shi, J. Highly efficient light-induced hydrogen evolution from a stable Pt/CdS NPs-co-loaded hierarchically porous zeolite beta. Appl. Catal., B 2014, 152−153, 271−279. (9) Shi, W.; Tao, S.; Yu, Y.; Wang, Y.; Ma, W. High performance adsorbents based on hierarchically porous silica for purifying multicomponent wastewater. J. Mater. Chem. 2011, 21 (39), 15567− 15574. (10) Xue, C.; Wang, J.; Tu, B.; Zhao, D. Hierarchically Porous Silica with Ordered Mesostructure from Confinement Self-Assembly in Skeleton Scaffolds. Chem. Mater. 2010, 22 (2), 494−503. (11) Wang, S.; Peng, X.; Zhong, L.; Tan, J.; Jing, S.; Cao, X.; Chen, W.; Liu, C.; Sun, R. An ultralight, elastic, cost-effective, and highly recyclable superabsorbent from microfibrillated cellulose fibers for oil spillage cleanup. J. Mater. Chem. A 2015, 3 (16), 8772−8781. (12) Kimling, M. C.; Chen, D.; Caruso, R. A. Temperature-induced modulation of mesopore size in hierarchically porous amorphous TiO2/ZrO2beads for improved dye adsorption capacity. J. Mater. Chem. A 2015, 3 (7), 3768−3776. (13) Nakanishi, K.; Tanaka, N. Sol−Gel with Phase Separation. Hierarchically Porous Materials Optimized for High-Performance Liquid Chromatography Separations. Acc. Chem. Res. 2007, 40 (9), 863−873. (14) Ma, T. Y.; Li, H.; Tang, A. N.; Yuan, Z. Y. Ordered, mesoporous metal phosphonate materials with microporous crystalline walls for selective separation techniques. Small 2011, 7 (13), 1827−1837. (15) Long, C.; Chen, X.; Jiang, L.; Zhi, L.; Fan, Z. Porous layerstacking carbon derived from in-built template in biomass for high volumetric performance supercapacitors. Nano Energy 2015, 12, 141− 151. (16) Ding, J.; Wang, H.; Li, Z.; Cui, K.; Karpuzov, D.; Tan, X.; Kohandehghan, A.; Mitlin, D. Peanut shell hybrid sodium ion capacitor with extreme energy−power rivals lithium ion capacitors. Energy Environ. Sci. 2015, 8 (3), 941−955. (17) Zhou, L.; Cao, H.; Zhu, S.; Hou, L.; Yuan, C. Hierarchical micro-/mesoporous N- and O-enriched carbon derived from disposable cashmere: a competitive cost-effective material for highperformance electrochemical capacitors. Green Chem. 2015, 17 (4), 2373−2382. 8342


Research Article

ACS Sustainable Chemistry & Engineering (37) Ye, L.; Shi, X.; Ye, C.; Chen, Z.; Zeng, M.; You, J.; Li, Y. Crystallization-Modulated Nanoporous Polymeric Materials with Hierarchical Patterned Surfaces and 3D Interpenetrated Internal Channels. ACS Appl. Mater. Interfaces 2015, 7 (12), 6946−6954. (38) Raquez, J.-M.; Habibi, Y.; Murariu, M.; Dubois, P. Polylactide (PLA)-based nanocomposites. Prog. Polym. Sci. 2013, 38 (10−11), 1504−1542. (39) Jiang, L.; Wolcott, M. P.; Zhang, J. Study of Biodegradable Polylactide/Poly(butylene adipate-co-terephthalate) Blends. Biomacromolecules 2006, 7 (1), 199−207. (40) Kuang, T.; Chen, F.; Chang, L.; Zhao, Y.; Fu, D.; Gong, X.; Peng, X. Facile preparation of open-cellular porous poly (l-lactic acid) scaffold by supercritical carbon dioxide foaming for potential tissue engineering applications. Chem. Eng. J. 2017, 307, 1017−1025. (41) Zhang, W.; Chen, B.; Zhao, H.; Yu, P.; Fu, D.; Wen, J.; Peng, X. Processing and characterization of supercritical CO2 batch foamed poly(lactic acid)/poly(ethylene glycol) scaffold for tissue engineering application. J. Appl. Polym. Sci. 2013, 130 (5), 3066−3073. (42) Peng, X.-F.; Mi, H.-Y.; Jing, X.; Yu, P.; Qu, J.-P.; Chen, B.-Y. Preparation of highly porous interconnected poly(lactic acid) scaffolds based on a novel dynamic elongational flow procedure. Mater. Des. 2016, 101, 285−293. (43) Yin, G.; Zhao, D.; Ren, Y.; Zhang, L.; Zhou, Z.; Li, Q. A convenient process to fabricate gelatin modified porous PLLA materials with high hydrophilicity and strength. Biomater. Sci. 2016, 4 (2), 310−318. (44) Saeidlou, S.; Huneault, M. A.; Li, H.; Park, C. B. Poly(lactic acid) crystallization. Prog. Polym. Sci. 2012, 37 (12), 1657−1677. (45) Tsuji, H. Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications. Macromol. Biosci. 2005, 5 (7), 569−597. (46) Xie, Q.; Han, L.; Shan, G.; Bao, Y.; Pan, P. Polymorphic Crystalline Structure and Crystal Morphology of Enantiomeric Poly(lactic acid) Blends Tailored by a Self-Assemblable Aryl Amide Nucleator. ACS Sustainable Chem. Eng. 2016, 4 (5), 2680−2688. (47) Na, B.; Zhu, J.; Lv, R.; Ju, Y.; Tian, R.; Chen, B. Stereocomplex Formation in Enantiomeric Polylactides by Melting Recrystallization of Homocrystals: Crystallization Kinetics and Crystal Morphology. Macromolecules 2014, 47 (1), 347−352. (48) Tsuji, H. Poly(lactic acid) stereocomplexes: A decade of progress. Adv. Drug Delivery Rev. 2016, 107, 97−135. (49) Bao, R.-Y.; Yang, W.; Jiang, W.-R.; Liu, Z.-Y.; Xie, B.-H.; Yang, M.-B.; Fu, Q. Stereocomplex formation of high-molecular-weight polylactide: A low temperature approach. Polymer 2012, 53 (24), 5449−5454. (50) Han, L.; Pan, P.; Shan, G.; Bao, Y. Stereocomplex crystallization of high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) racemic blends promoted by a selective nucleator. Polymer 2015, 63, 144− 153. (51) Zhang, Z.-C.; Sang, Z.-H.; Huang, Y.-F.; Ru, J.-F.; Zhong, G.-J.; Ji, X.; Wang, R.; Li, Z.-M. Enhanced Heat Deflection Resistance via Shear Flow-Induced Stereocomplex Crystallization of Polylactide Systems. ACS Sustainable Chem. Eng. 2017, 5 (2), 1692−1703. (52) Tan, B. H.; Muiruri, J. K.; Li, Z.; He, C. Recent Progress in Using Stereocomplexation for Enhancement of Thermal and Mechanical Property of Polylactide. ACS Sustainable Chem. Eng. 2016, 4 (10), 5370−5391. (53) Tsuji, H.; Ikada, Y. Stereocomplex formation between enantiomeric poly(lactic acid)s. XI. Mechanical properties and morphology of solution-cast films. Polymer 1999, 40 (24), 6699−6708. (54) Tsuji, H. In vitro hydrolysis of blends from enantiomeric poly(lactide)s Part 1. Well-stereo-complexed blend and non-blended films. Polymer 2000, 41 (10), 3621−3630. (55) Ma, C.; Pan, P.; Shan, G.; Bao, Y.; Fujita, M.; Maeda, M. Core− Shell Structure, Biodegradation, and Drug Release Behavior of Poly(lactic acid)/Poly(ethylene glycol) Block Copolymer Micelles Tuned by Macromolecular Stereostructure. Langmuir 2015, 31 (4), 1527−1536.

(56) Zhao, Z.; Zhang, Z.; Chen, L.; Cao, Y.; He, C.; Chen, X. Biodegradable Stereocomplex Micelles Based on Dextran-blockpolylactide as Efficient Drug Deliveries. Langmuir 2013, 29 (42), 13072−13080. (57) Tsuji, H.; Fukui, I. Enhanced thermal stability of poly(lactide)s in the melt by enantiomeric polymer blending. Polymer 2003, 44 (10), 2891−2896. (58) Sun, Y.; He, C. Biodegradable “Core−Shell” Rubber Nanoparticles and Their Toughening of Poly(lactides). Macromolecules 2013, 46 (24), 9625−9633. (59) Muiruri, J. K.; Liu, S.; Teo, W. S.; Kong, J.; He, C. Highly Biodegradable and Tough Polylactic Acid−Cellulose Nanocrystal Composite. ACS Sustainable Chem. Eng. 2017, 5 (5), 3929−3937. (60) Ma, P.; Shen, T.; Xu, P.; Dong, W.; Lemstra, P. J.; Chen, M. Superior Performance of Fully Biobased Poly(lactide) via Stereocomplexation-Induced Phase Separation: Structure versus Property. ACS Sustainable Chem. Eng. 2015, 3 (7), 1470−1478. (61) Oyama, H. T.; Abe, S. Stereocomplex Poly(lactic acid) Alloys with Superb Heat Resistance and Toughness. ACS Sustainable Chem. Eng. 2015, 3 (12), 3245−3252. (62) Wei, X.-F.; Bao, R.-Y.; Cao, Z.-Q.; Yang, W.; Xie, B.-H.; Yang, M.-B. Stereocomplex Crystallite Network in Asymmetric PLLA/PDLA Blends: Formation, Structure, and Confining Effect on the Crystallization Rate of Homocrystallites. Macromolecules 2014, 47 (4), 1439−1448. (63) Hoogsteen, W.; Postema, A. R.; Pennings, A. J.; Ten Brinke, G.; Zugenmaier, P. Crystal structure, conformation and morphology of solution-spun poly(L-lactide) fibers. Macromolecules 1990, 23 (2), 634−642. (64) Cartier, L.; Okihara, T.; Lotz, B. Triangular Polymer Single Crystals: Stereocomplexes, Twins, and Frustrated Structures. Macromolecules 1997, 30 (20), 6313−6322. (65) Gao, A.; Zhao, Y.; Yang, Q.; Fu, Y.; Xue, L. Facile preparation of patterned petal-like PLA surfaces with tunable water micro-droplet adhesion properties based on stereo-complex co-crystallization from non-solvent induced phase separation processes. J. Mater. Chem. A 2016, 4 (31), 12058−12064. (66) Hu, J.; Tang, Z.; Qiu, X.; Pang, X.; Yang, Y.; Chen, X.; Jing, X. Formation of Flower- or Cake-Shaped Stereocomplex Particles from the Stereo Multiblock Copoly(rac-lactide)s. Biomacromolecules 2005, 6 (5), 2843−2850. (67) Brzeziński, M.; Biedroń, T.; Tracz, A.; Kubisa, P.; Biela, T. Spontaneous Formation of Colloidal Crystals of PLA Stereocomplex Microspheres and their Hierarchical Structure. Macromol. Chem. Phys. 2014, 215 (1), 27−31. (68) Yin, G.; Zhao, D.; Zhang, L.; Ren, Y.; Ji, S.; Tang, H.; Zhou, Z.; Li, Q. Highly porous 3D PLLA materials composed of nanosheets, fibrous nanosheets, or nanofibrous networks: Preparation and the potential application in oil−water separation. Chem. Eng. J. 2016, 302, 1−11. (69) Tsuji, H.; Hyon, S. H.; Ikada, Y. Stereocomplex formation between enantiomeric poly(lactic acids). 5. Calorimetric and morphological studies on the stereocomplex formed in acetonitrile solution. Macromolecules 1992, 25 (11), 2940−2946.

8343


A Green and Facile Melt Approach for Hierarchically Porous

Recommend Documents