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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02121 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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ACS Sustainable Chemistry & Engineering

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, Wei Yang*

College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, No. 24 South Section 1, Yihuan Road, Chengdu, 610065, China

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 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 environmental 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 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.

*

Corresponding authors. Tel.: + 86 28 8546 0130; fax: + 86 28 8546 0130.

E-mail addresses: [email protected] (W Yang)and [email protected] (RY Bao)

ACS Paragon Plus Environment


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Introduction Hierarchical porous materials have attracted enormous attention mostly as functional materials in recent years because of their diverse performance1-3. Generally, the micro (50 nm) would reduce the transport limitation within the material and increase the accessible space4-5. The combination of multiple length scales of pores could confer unique balanced properties to the materials, which provides great potential in the fields of catalysis6-7, photocatalytic H2 production8, adsorption9-12, separation13-14, energy storage15-17 and conversion18-19, and biomedicines

20-21

.

Nowadays, numerous approaches to obtain hierarchical porous materials have been developed. The dual-templating18, 22-23and a combination of multiple-template methods24 are most commonly used to fabricate materials with hierarchical porous structures. Some other important strategies to obtain hierarchical porous structures included lithography25-26, colloidal assembly27-28, mechanochemistry29 and phase separation30-34. Inorganic nano-scale 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. Favis et al.35found that the removal of the polystyrene (PS) phase from poly(L-lactide) (PLLA)/PS blends prepared by melt mixing could produce porous polylacide (PLA) materials 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


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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 co-continuous binary blends, and the PLLA constituents expelled out from the POM banded spherulite crystals were fully included in the nanoscaled interlamellar and/or interfibrillar regions of POM crystals. Consequently, they obtained nanoporous POM materials with hierarchical 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 condition. 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 macro-chain transfer agent in the presence of poly(ethylene oxide) (PEO) and a PLA-b-P(S-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 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, and 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 biocompatibility38-43. For PLA, a specific crystal form, the stereocomplex (sc) crystallites, can be formed in virtue of the tight integration between the two opposite chiral PLLA and poly(D-lactide) (PDLA) molecular chains44-48. One of the most attractive features of sc crystallites is their high melting point, almost 50 °C higher than that of homo-crystallites (hc), so it



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can provide greater heat resistance to the material49-52. Sc crystallization was also reported to be able to enhance the properties of PLA markedly, including mechanical performance, hydrolysis resistance, and heat resistance44, 53-61. Sc crystallites of PLA could be formed effectively during processing with a low temperature approach as described in our previous work49. 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 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 co-crystallization 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 sc crystallite network 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 the potential application in oil-water separation.

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


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melting temperatures of PLLA and PDLA are 178

°

C (DSC, 10

°

C/min). PEO with a

viscosity-average 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 condition. The PEO phase was used as the porogen which 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 hours. After extraction, the specimens were dried in a vacuum oven at 40 °C for 24 hours. 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 influence the porous structure, the weight ratio of PEO was kept at 40 wt% to maintain the co-continuous phase structure for PLLA/PDLA/PEO blends, and the weight ratio of PDLA varied from 0, to 10, 20, 30 to 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 varied from 0, to 10, 20, 30, 40 to 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



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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) to 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 melt compounding process were cryofractured in liquid nitrogen, and the fractured surfaces were immersed into deionized water for 3 hours to remove PEO. Finally, the dried fractured surfaces were sputter-coated with a layer of 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,


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samples were degassed for 6 hours 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, respectively.

Results and discussion Stereocomplex crystallite formation In our previous work, sc crystallites were obtained in PLLA/PDLA blends with a low-temperature approach49. 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 co-continuous 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 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 showed the WAXD patterns and DSC melting curves of the samples after removal of PEO,



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and the characteristic data from WAXD and DSC tests were included in Table 1. As shown in Figure 1(a), the WAXD patterns exhibited diffraction peaks at 14.8°, 16.9°, 19.1° and 22.5°, assigned to the (010), (200) and/or (110), (203) and (210) plane of α-form hc crystallites for the sample 0% PDLA63. 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) and/or (030) and (220) plane of sc crystallites64. 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 work49.

(a) (b) Figure 1. WAXD patterns (a) and DSC heating curves (b) for the samples after removal of PEO

From Figure 1(b), 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,


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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 non-equimolar ratio, thicker lamella and more perfect crystals could be formed if the composition was nearly symmetric, resulting in higher Tm(sc) with the increasing ratio of PDLA. On the contrary, 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 extracting, while the materials possessed good hydrolysis resistance with the incorporation of PDLA. Table 1.WAXD and DSC data of blends with different contents of PDLA (after removal of PEO)

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

Xc(hc)% 45.0 43.6 24.7 18.1 3.9

WAXD Xc(sc)% X c% 45.0 13.1 56.7 35.0 59.7 51.6 69.7 61.2 65.1

fsc% 23.1 58.6 74.0 94.0

DSC Tm(hc) C Tm(sc)°C 176.6 177.5 228.8 173.4 235.2 171.1 239.8 167.9 240.5 °

Tailored porous structure with stereocomplex crystallite network The morphology of the hierarchical porous materials was directly characterized by SEM. As shown in Figure 2(a-e and a1-e1), the introduction of PDLA influenced the crystalline morphology seriously. From Figure 2(a and a1), the hc spherulites of PLLA with the 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 2(b and b1), due to that the sc crystallites formed between PLLA and PDLA acted as the nucleating agent for the hc crystallization of PLLA. With further increasing



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content of PDLA, obvious changes took place in the surface morphology. From Figure 2(c and 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 2(d and 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 2 (a and 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 2(b and 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 2(c and 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 2(d and d1), the strips of pores could be hardly observed, and the uniformity of size and distribution of the macropores and mesopores was further enhanced, due to that a stable sc crystallite tridimensional network was established. The nano-scale mesopores could be identified at a higher magnification as inserted in Figure 2d. With PDLA content increasing to 40%, from Figure 2(e and e1), it was observed that isolated macropores distributed in the monolith, and mesopores were present within the tridimensional rigid skeleton


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constructed by sc crystallites clearly. In addition, the sample after 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.

Figure 2.SEM micrographs at (a-e) 10,000× and (a1-e1) 40,000× 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 (80,000×) of the PLA monolith with 30% PDLA was inserted in (d).

The detailed hierarchical porous structure of the materials was further characterized by nitrogen sorption and desorption. As shown in Figure 2(a2-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 was included in Table 2. It should be noted that the pore volume measured by nitrogen sorption analysis does not contain the macropores 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.



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There was a slightly decrease in the specific surface area for the monolith with 40% PDLA (Table 2), due to the appearance of larger pore possibly. The increased PDLA content led to the formation of 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 2(e and 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). Table 2. Pore volume and surface area measured by nitrogen sorption analysis. Samples

Specific surface area(m2/g)

Pore volume(cm3/g)

0% PDLA

7.4

0.064

10% PDLA

17.1

0.145

20% PDLA

17.9

0.165

30% PDLA

19.9

0.211

40% PDLA

15.4

0.138

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 were shown in Figure 3.

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


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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. It 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 work62, the sc crystallite network can 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 perfect with the further increasing of PDLA content, result in overall good thermostability. As expected, the tridimensional network of sc crystallites was not destroyed at high ambient temperature and performed 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



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above. It was because that the certain value of the porogen phase would lead to porous materials with a certain pore size and volume36. From Figure S7, the sample with 10 wt% PEO showed a small continuity. It was interesting to note that the sample showed a high continuity when the PEO content was beyond 20 wt%, indicating a co-continuous structure was formed (Figure S7). The formation of sc crystallite tridimensional network made it possible that the co-continuous structure occurred at such a low critical PEO contents.

Figure 4. SEM micrographs at (A-F) 10,000× and (inset) 40,000×magnification of the etched monolith with a ratio of PLLA/PDLA (70/30), prepared with 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt% PEO additive, respectively.

The morphology affected by PEO content was shown in SEM results (Figure 4). Interestingly, in Figure 4(A), 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 increased number of nanopores in Figure 4(B). 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 be beyond 30 wt% resulted in the formation of both macro- and mesoporous, and an increase in the


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apparent connectivity of the macro- and mesoporous network. As shown in Figure 4(D-F), macropores with the size 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 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.

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

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 co-crystallization of PLLA and PDLA, and similar phenomenon was also mentioned by Gao65, Hu66 and Brzezinski67. The specific surface area and pore volume increased with increasing PEO content, which was consistent with the increase in volume fraction of the porogen component. Interestingly,



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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 inter-lamellae were full of porogen, the remaining parts of porogen would aggregate and macrophase separated from the matrix, resulting in the increasing swelling of the PEO domains and formation of macropores. Table 3. Pore size and surface area measured by nitrogen sorption analysis. Samples

DFT pore width (nm)

Specific surface area (m2/g)

Pore volume (cm3/g)

0 wt% PEO

4.1

0.3

0.001

10 wt% PEO

4.3

0.6

0.001

20wt% PEO

5.9

1.7

0.008

30wt% PEO

29.4

7.7

0.063

40wt% PEO

29.4

19.9

0.211

50 wt% PEO

29.4

24.6

0.304

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 co-continuous phase structure was generated by the macrophase separation of the two components, and each phase is highly continuous in three-dimensional. For pure PLLA, PEO was expelled out from PLLA crystal lamellae, and highly porous PLLA monoliths were obtained after extracting PEO component from the spherulitic hc crystallites. With the introduction of PDLA, the formation of the tridimensional sc 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


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lamellar textures of sc crystallites formed isothermally under the strong shear during melt processing. PEO was also located in the super-dense 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 nano-scaled interlamellar of PLA sc crystallites. To be noted, the crystallization process in PLLA also induced microphase separation of the two components, and the mesopores were presented in 0% PDLA. As a consequence, the tridimensional interconnected hierarchical meso- and macroporous PLA monoliths were fabricated by removal of PEO phase which showed hierarchical macro- and microphase separation with PLA phase.

Scheme 1.Schematic illustration of the formation of interconnected hierarchical PLA monoliths based on sc crystallite network.

Wettability of the hierarchical porous monoliths Figure 6(a) showed that all the porous monoliths 0% PDLA~40% PDLA exhibited hydrophobicity with water contact angles (WCA) above 135° because of the rough hierarchical porous micro-nano morphology. 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



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material with very good hydrophobicity. On the contrary, the diiodomethane used was immediately drawn into the monoliths, showing unexceptionable super-oleophilicity. This opposite wetting behavior inspired us to consider the hierarchical porous materials with good hydrophobicity as adsorbent for oil leaking cleanup68.

Figure 6 (a) CAs of monoliths with 0%-40% PDLA from left to right, the upper and underlying images corresponded to the water and oil droplet, respectively. (b-d) Images of the removal processes of chloroform (dyed with SudanⅢ) from water using porous polymer monoliths. The samples from left to right corresponded to the content of PDLA being 0%-40%, respectively. The whole sorption processing was presented in (e-e3) 0% PDLA and (f-f3) 40% PDLA, and the insets were the CAs toward the corresponding samples.

The oil-water separation experiments were performed and the digital photographs of sorption processes were shown in Figure 6. The experiments were performed at room temperature. 0.5 ml solvent was dropped into 20 ml water for sorption and 5 minutes were used to make sure that all of the adsorbents could finish the sorption process. The samples 0% PDLA~40% PDLA with the same weight were used to adsorb the chloroform sinking in water, as shown in Figure 6(b-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 maintained dry and clean. For the samples 0% PDLA~20% PDLA, the external structure of the monolith collapsed.


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It indicated that the perfect sc crystallite tridimensional network formed in the sample 30% PDLA and 40% PDLA could endow the material with excellent solvent resistance. The adsorbing capacity of the sample 30% PDLA and 40% PDLA were 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 were presented in Figure 6(e-e3) and (f-f3). In Figure 6(f-f3), the chloroform was immediately sucked up when the monolith got closed to it and the whole process completed in seconds. While in Figure 6(e-e3), the chloroform was also sucked 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 the water could be sucked up by the porous monoliths within a few seconds.

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°).

To reveal the thermal and chemical resistance of the porous materials, sample 0% PDLA and 40%



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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 the refluxing were presented in Figure 7. Interestingly, the sample 0% PDLA dissolved in 25 minutes, while the 40% PDLA maintained structural integrity even after 24 hours, which was well consistent with the reported results in ref.69, in which PLLA or PDLA acetonitrile solution was prepared at 80 °C and stereocomplex were formed in acetonitrile at 80 °C, and means that sc crystallites are stable in this condition. Moreover, after high temperature refluxing sample 40% PDLA still possessed a 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 to significantly broaden the applications of the adsorbent in some harsh conditions and catalytic systems.

Conclusions In summary, we developed a facile and green route to fabricate sustainable, degradable, high-temperature-resistance and good-solvent-resistance tridimensional interconnected porous PLA materials. The tunable morphology and controlled pore size of hierarchical meso- and macroporous PLA monoliths were owing to the macro- and microphase separation during the co-crystallization 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


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structures would greatly broaden the applications which required such an ordered hierarchical structure in some extreme temperature and solvent ambient environment.

ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Continuity measurement, Fourier transform infrared spectroscopy (FTIR) measurement, DSC heating curves, continuity, FTIR spectra, SEM micrographs, pore size distribution, mixing torque curves and images of the removal processes of cyclohexane from water. Acknowledgements 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).

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A facile and environmentally sustainable route to fabricate the degradable tridimensional interconnected hierarchical meso- and macroporous polylactide (PLA) monoliths was developed.


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A Green and Facile Melt Approach for ... - ACS Publications

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