Micropore Geometry Manipulation by Macroscopic Deformation Based

Subscriber access provided by READING UNIV

Letter

Micro-pore Geometry Manipulation by Macroscopic Deformation Based on Shape Memory Effect in Porous PLLA Membrane and its Enhanced Separation Performance Jingxin Zhao, Qiucheng Yang, Tao Wang, Lian Wang, Jichun You, and Yongjin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16648 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 3, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Micro-pore Geometry Manipulation by Macroscopic Deformation Based on Shape Memory Effect in Porous PLLA Membrane and its Enhanced Separation Performance Jingxin Zhao†, Qiucheng Yang†, Tao Wang, Lian Wang, Jichun You*, Yongjin Li* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou, 310036, P. R. China

Abstract An effective strategy to tailor the micro-porous structures has been developed based on the shape memory effect in porous poly(L-lactic acid) membranes in which tiny crystals and amorphous matrix play the roles of shape-fixed phase and reversible-phase respectively. Our results indicate that not only PLLA membranes but micro-pores exhibit shape memory properties. The proportional deformations on two scales have been achieved by uniaxial or biaxial tension, providing a facile way to manipulate continuously the size and the orientation degree of pores on micro-scale. The enhanced separation performance has been validated by taking polystyrene colloids with varying diameters as an example. Keywords: Porous membranes; Shape memory effect; Separation; Deformation; Pore size;

† The authors contribute equally to this work. * Corresponding author [email protected] (Prof. You J.) or [email protected] (Prof. Li Y.)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 10

Porous membranes are promising candidate for the separation of mixture or emulsion based on the “size-sieving effect”, in which only materials of certain sizes can pass through its pores 1-3. In the membrane separation, the micro-pore geometry plays a key role, determining both separation efficiency and flux 4. For one thing, the simultaneous improvement of them in conventional porous materials remains as a great challenge. The oriented pores, however, provide solutions to this issue since the width and the length of them can serve as the size selective barriers and channels for higher flux respectively. For another thing, the pores with continuously controllable size are significant in many fields. In the synthesis and application of nanoparticles, for instance, the optoelectronic properties of them depend crucially on the particle size and uniformity. Filtration through porous membranes has been proved as an efficient way for the purification and size-fractionation of them, requiring porous membrane with tunable pore size

5-6

. However, the

precise and continuous manipulation strategy for micro-pore geometry is far from being established. Shape memory polymers exhibit shape changeability in response to external stimuli 7-8

. By incorporating porous structures with shape memory effect, porous shape memory polymers

(PSMPs) can provide an opportunity for the manipulation of micro-pore geometry, thus enabling the design of membranes with oriented and size-tunable pores by stiffening the demanded porous structures as the temporary shape. For instance, Zhao and his co-workers developed a freezing method to synthesize porous shape memory foam or hydrogel, enriching the fabrication strategy of PSMPs

9-10

. In addition to the shape memory effect, both interconnected pores and good

mechanical properties are necessary to achieve the separation by PSMP membranes because they account for the interpenetrated micro-channels for permeation and the deformability at elevated temperature respectively. In the developed fabrication strategies, however, either the former or the latter is absent. On one hand, gas foaming has been utilized to prepare porous shape memory foams, producing close-celled porous structures

11-12

. On the other hand, it is facile to obtain

porous scaffolds with inter-connected pores by particle leaching and polymerized high internal phase emulsions (PolyHIPEs)

13-16

. The profound decrease of mechanical performance resulted

from the poor continuity of PSMPs does limit the draw ratio during biaxial or uniaxial tension and the resultant variation of pore geometry 17. In this work, therefore, porous shape memory PLLA membranes have been fabricated by incorporating phase separation in PLLA/PEO blend system with the crystallization of PLLA. The


Page 3 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


combination of shape memory effect and the micro-pores endows the porous membranes and the micro-pores with deformability. The proportional deformation and recovery both on the microscopic pore and macroscopic membrane levels have been achieved. The tunable micro-pore geometry makes it effective to control the separation performance by the deformation of the porous shape memory membranes on macro-scale.

Figure 1. Preparation process and property characterizations of the porous shape memory PLLA membranes. (A) Diagram of the preparation process. The photos of the obtained membranes of S-4-1000 (B) and B-4-20 (C). The membranes are marked using green squares due to the good transparency. (D) and (E) are SEM images of the indicated specimen respectively.

As illustrated in Figure 1A, spin-coating and blade-coating were employed to fabricate blend films of PLLA/PEO. After the preparation, there is no any structure on film surface (Figure S1). When the specimen was immersed in water, the free-standing porous PLLA membrane can be obtained because of the water-solubility of PEO and the excellent wettability between water and glass (Figure 1). The occurrence of micro-pores can be interpreted as follow. Due to the phase separation in PLLA/PEO blend (it has been discussed in detail in reference 18 and 19) and the existence of strong shear field during spin-coating or blade-coating, phase-separated PEO-rich phases are interconnected with each other

20-21

. They play the role of pore-forming agent upon

water etching, creating the interpenetrated micro-channels in the membranes. The successful removal of PEO by water etching can be validated by both weight calculation and differential scanning calorimetry (DSC) results (Figure S2). The photos in Figure 1B and Figure 1C show the good transmittance (94.3% and 93.8%) of the porous membranes from spin-coating and



blade-coating, respectively. In SEM images, the pores with various diameters and pore densities distribute over the whole porous PLLA membranes. In the spin-coated samples (Figure 1D), the pores with average diameter of 2.8 µm are closely packed within the membrane. The blade-coating samples exhibit lower pore densities and higher pore size (average diameter=5.2 µm, Figure 1E). Obviously, it is facile to manipulate the pore size and density by means of phase separation of PLLA/PEO blend under the strong shear field. Therefore, the pore geometries exhibit profound dependence on the preparation parameters including spin-coating parameters (Figure S3, Figure S4 and Figure S5) as well as blade-coating conditions (Figure S6). The thicknesses and pore sizes in the porous PLLA membranes measured by atomic force microscopy (AFM) (Figure S7A) or SEM (Figure S7B) are listed in Table S1 and Table S2.

Figure 2. The shape memory cycles (A and D), SEM images (B and E) during shape memory and the correlations between deformations on micro- and macro-scales (C and F) of S-4-4000 (A, B and C) and B-4-20 (D, E and F).

In the prepared PLLA membranes, the tiny crystals and amorphous matrix act as the shape fixed phase and reversible phase respectively, leading to the shape memory property shown in Figure S8 22

. In conjunction with this effect, the micro-pore geometries in the membrane are expected to be


Page 4 of 10

Page 5 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sensitive to macroscopic deformation. On one hand, uniaxial tension was carried out in the porous PLLA membranes. On macro-scale (Figure 2A), the rectangular specimen with the length of 1.45 cm was stretched to 3.61 cm in the water bath (70oC). This temporary shape can be fixed by cooling to room temperature with the constant strain. Then, the permanent shape (1.41 cm) can be recovered upon the exposure to heat. On micro-scale (Figure 2B), the micro-pores are isotropic before uniaxial tension. When the specimen was stretched to 2.23 cm (draw ratio=1.5) and 3.61 cm (draw ratio=2.5), the oriented micro-pores can be observed. After the recovery at 70oC, there are only round pores in the membranes. In this shape memory cycle, the macroscopic and microscopic deformations can be quantified in terms of draw ratio and length-width-ratio respectively. The former is determined from the ratio of lengths after and before uniaxial tension while the latter is the ratio of length over width of the oriented micro-pores. As shown in Figure 2C, the plot of them matches well the linear fitting with the slope close to 1, indicating the proportional deformation on micro- and macro-scales. The oriented micro-pores are greatly desired in the separation membranes since it is an efficient way to improve the flux at constant separation efficiency. With the help of shape memory effect, the micro-pores with identical width but different length (relative to the isotropic pores before tension) have been fabricated successfully. The former corresponds to the similar separation efficiency while the latter results in the higher flux relative to isotropic pores. The gravity-driven flux of the deformed separation membrane was assessed by taking water as an example. Upon uniaxial tension, the flux of the porous

shape

memory

membranes

increases

from

2.8*103

to

2.0*104

(draw

ratio=length-width-ratio=1.5) and 4.2*104 (L*m-2*h-1, draw ratio=length-width-ratio=2.5). The profound improvement of flux can be attributed to the increase of the length-width-ratio and the resultant overlap of them in the membrane thickness directions. On the other hand, in conjunction with shape memory effect, biaxial tension was employed to manipulate the pore size. As shown in Figure 2D, the length of the square specimen increases from 1.95cm to 2.91cm and 3.85cm, determining the draw ratio of 1.5 and 2 respectively. The full recovery to permanent shape and original size can be triggered by the exposure to hot water bath (70oC). On micro-scale, the pore size exhibits linearly response to the macroscopic draw ratio. The average diameter of micro-pores increases from 5.2 to 7.9 (macroscopic draw ratio=1.5) and 11.3µm (macroscopic draw ratio=2.0). After recovery, it decreases to 5.3µm which has good agreement with the original value. To



Page 6 of 10

describe the deformation on micro-scale, the ratio of D/D0 was introduced, where D0 and D are the pore diameters before and after biaxial tension respectively. The plot of this ratio as a function of macroscopic draw ratio is shown in Figure 2F. D/D0 increases linearly with increasing draw ratio, suggesting the proportional deformation on macro- and micro-scales. Upon biaxial tension, the flux increases from 4.7*104 to 2.9*105 (draw ratio=1.5) and 6.0*105 (L*m-2*h-1, draw ratio=2). After recovery to the original macroscopic shape and micro-pore size, the value goes back to 5.1*104 (L*m-2*h-1). In addition to the uniaxial and biaxial tension, it is also facile to control the pore size in the recovery process (Figure S9 and S10). The average pore size locates at 45.1 µm at the draw ratio of 2. This value decreases to 33.4, 28.7 and 22.3 µm when the porous membrane recovers to the draw ratio of 1.5, 1.2 and permanent shape respectively. This result confirms the proportional deformations on macro- and micro-scales which makes it effective to control the micro-pore size by means of macroscopic deformation.

Figure 3. The stress-strain curves (A), contact angle (B) of the obtained membrane and the home-made setup for separation (C). (D) to (F) show the optical microscope images of PS latex filtered by the porous membrane (B-4-20) with the draw ratio (biaxial tension) of 1 (before biaxial tension), 1.5 and 2 respectively. (G) is the colloid separated by the membrane with draw ratio=1 and 1.5 successively. Scale bars in D to G is 100µm.

The porous PLLA membranes from spin-coating and blade-coating exhibit good mechanical properties with the elongation at break of ~ 40%. This value is much higher than the result in solid PLLA (Figure 3A and Figure S11). The remarkable improvement can be attributed to the toughness effect of micro-pores 23. The glass transition temperature of the porous PLLA measured by dynamical mechanical analysis (DMA) locates at 62.3oC (Figure S12). This temperature acts as the switching temperature for shape memory performance, which is the reason for that the temperature of 70oC was adopted to trigger the shape memory effect


24

. The porous membranes

Page 7 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


prepared by spin-coating and blade-coating exhibit the similar water contact angle of 85o (Figure 3B), indicating the good wettability of them and the resultant permeability of water. The size of micro-pores plays a key role in the precise separation based on size-sieving effect. Therefore, the micro-pores with tunable size are greatly desired. According to the result in Figure 2E and 2F, the micro-pore size can be manipulated continuously by macroscopic deformation (e.g. biaxial tension to certain draw ratio). Using the home-made setup (Figure 3C), we assessed the separation performance of porous PLLA membranes by taking polystyrene colloids as an example 25-26. Three latexes (from Sigma-Aldrich) containing colloids with the diameter of 5, 10, 30µm were mixed. Before biaxial tension, only the smallest colloid (5µm) can permeate through the membrane (Figure 3D) although plenty of water was used to elute them. Upon biaxial tension (draw ratio=1.5), there are colloids with the diameter of 5 and 10µm in the filtrate (Figure 3E). In the case of draw ratio=2, the membrane cannot serve as a separation membrane for these colloids since all of them are obvious in Figure 3F. Especially, it is facile to obtain separated colloids with uniform diameters by filtering the mixed colloids by the membranes with the draw ratio=1 (before biaxial tension) and 1.5 successively. A typical result is shown in Figure 3G, in which only colloids of 10µm can be found. Obviously, the separation performance depends crucially on the micro-pore size which is under the specific control of draw ratio during biaxial tension on macro-scale. In this work, a new strategy for manipulating micro-pore geometry in porous membranes by macroscopic deformation has been developed by taking PLLA as an example. Not only the porous PLLA membranes but also the micro-pores in them exhibit shape memory properties. The proportional deformations of them have been accomplished by employing uniaxial or biaxial tension to adjust and then fix the temporary shape based on the shape memory effect. During these processes, macroscopic deformation can act as a key element to stiffen the desired microscopic pore geometry (including orientation degree and size). Consequently, the successful separation of PS colloids with different diameters has been achieved using PLLA membranes with varying micro-pores discussed above, suggesting the enhanced separation performance. Our results open a new avenue for tuning the microscopic pore geometry by the macroscopic deformation based on the shape memory properties on two scales.



Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21234007). Reference 1.

Shi, Z.；Zhang, W. B.；Zhang, F.；Liu, X.；Wang, D.; Jin, J. L. Ultrafast Separation of Emulsified Oil/Water Mixtures by Ultrathin Free-Standing Single-Walled Carbon Nanotube Network Films. Jiang. Adv. Mater. 2013, 25, 2422-2427.

2.

Rana, D.; Matsuura, T. Surface Modifications for Antifouling Membranes. Chem. Rev. 2010, 110, 2448-2471.

3.

Pendergast, M. M.; Hoek, E. M. V. A Review of Water Treatment Membrane Nanotechnologies. Energy Environ. Sci. 2011, 4, 1946-1971.

4.

Zhang, W. B.; Zhu, Y. Z.; Liu, X.; Wang, D.; Li, J. Y.; Jiang, L.; Jin, J. Salt-Induced Fabrication of Superhydrophilic and Underwater Superoleophobic PAA-g-PVDF Membranes for Effective Separation of Oil-in-Water Emulsions. Angew. Chem. Int. Ed. 2014, 53, 856-860.

5.

Sweeney, S. F.; Woehrle, G. H.; Hutchison, J. E. Rapid Purification and Size Separation of Gold Nanoparticles via Diafiltration. J. Am. Chem. Soc. 2006, 128, 3190-3197.

6.

Kowalczyk, B.; Lagzi, I.; Grzybowski, B. A. Nanoseparations: Strategies for Size and/or Shape-Selective Purification of Nanoparticles. Curr. Opin. Colloid Interface Sci. 2011, 16, 135-148.

7.

Hager,M. D.; Bode, S.; Weber, C.; Schubert, U. S. Shape Memory Polymers: Past, Present and Future Developments. Prog. Polym. Sci. 2015, 49-50, 3-33.

8.

You, J.; Fu, H.; Dong, W.; Zhao, L.; Cao, X.; Li, Y. Shape Memory Performance of Thermoplastic Polyvinylidene Fluoride/Acrylic Copolymer Blends Physically Cross-Linked by Tiny Crystals. ACS Appl. Mater. Interfaces 2012, 4, 4825-4831.

9.

Chen, D.; Xia, X.; Wong, T. W.; Bai, H.; Behl, M.; Zhao, Q.; Lendlein, A.; Xie, T. Omnidirectional Shape Memory Effect via Lyophilization of PEG Hydrogels. Macromol. Rapid Commun, 2017, 38, 1600746.

10. Wong, T. W.; Wu, J.; Yang, M.; Kadir, M. R. A.; Wahit, M. U.; Zhao, Q. Multifunctional shape-memory foams with highly tunable properties via organo-phase cryo-polymerization. J. Mater. Chem. A 2017, 5, 9793-9800. 11. Singhal, P.; Rodriguez, J. N.; Small, W.; Eagleston, S.; Van de Water, J.; Maitland, D. J.; Wilson, T. S. Ultra low Density and Highly Crosslinked Biocompatible Shape Memory Polyurethane Foams. J. Polym. Sci. Part B Polym. Phys. 2012, 50, 724-737. 12. Salerno, A.; Clerici, U.; Domingo, C. Solid-State Foaming of Biodegradable Polyesters by Means of Supercritical CO2/Ethyl Lactate Mixtures: Towards Designing Advanced Materials by Means of Sustainable Processes. Eur. Polym. J. 2014, 51, 1-11. 13. Okamoto, M.; John, B. Synthetic Biopolymer Nanocomposites for Tissue Engineering Scaffolds. Prog. Polym. Sci. 2013, 38, 1487-1503. 14. M. S. Silverstein, PolyHIPEs: Recent advances in emulsion-templated porous polymers. Prog. Polym. Sci. 2014, 39, 199-234. 15. Destribats, M.; Faure, B.; Birot, M.; Babot, O.; Schmitt, V.; Backov, R. Tailored Silica Macrocellular Foams: Combining Limited Coalescence‐Based Pickering Emulsion and Sol–Gel Process. Adv. Funct. Mater. 2012, 22, 2642-2654. 16. Hariraksapitak, P.; Suwantong, O.; Pavasant, P.; Supaphol, P. Effectual Drug-Releasing Porous Scaffolds from 1, 6-Diisocyanatohexane-Extended Poly (1, 4-butylene succinate) for Bone Tissue Regeneration. Polymer. 2008, 49, 2678-2685. 17. Wang, D.; Möhwald, H. Template-Directed Colloidal Self-Assembly–The Route to ‘Top-Down’


Page 8 of 10

Page 9 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanochemical Engineering. J. Mater. Chem. 2004, 14, 459-468. 18. Yu, J.; Mahoney, C. M.; Fahey, A. J.; Hicks, W. L.; Hard, R.; Bright, F. V.; Gardella, J. A. Phase Separation at the Surface of Poly (ethylene oxide)-Containing Biodegradable Poly (l-lactic acid) Blends. Langmuir, 2009, 25, 11467-11471. 19. Rufino, T. C.; Felisberti, M. I. An Epigenetic Bioactive Composite Scaffold with Well-aligned Nanofibers for Functional Tendon Tissue Engineering. RSC Adv. 2016, 6, 30937-30943. 20. Lennon, N.; Selma, S.; Aurore, N. Flow-Parametric Regulation of Shear-Driven Phase Separation in Two and Three Dimensions. Phys. Rev. E. 2015, 91, 062127-062140. 21. Shotaro, N.; Mikihito, T.; Takashi, T. Computer Simulation Study on the Shear-Induced Phase Separation in Semi-Dilute Polymer Solutions by Using Ianniruberto–Marrucci Model. Polymer, 2010, 51, 1853-1860. 22. You, J.; Dong, W.; Zhao, L.; Cao, X.; Qiu, J.; Sheng, W.; Li, Y. Crystal Orientation Behavior and Shape-Memory Performance of Poly(vinylidene fluoride)/Acrylic Copolymer Blends. J. Phys. Chem. B 2012, 116, 1256-1264. 23. Cramer, M.; Sevostianov, I. Effect of Pore Distribution on Elastic Stiffness and Fracture Toughness of Porous Materials. Int. J. Fract. 2009, 160, 189-196. 24. Lendlein, A.; Kelch, S. Shape-Memory Polymers. Angew. Chem. Int. Ed. 2002, 41, 2034-2057. 25. Bikos, D.; Mason, T.G. Propagation and Separation of Charged Colloids by Cylindrical Passivated Gel Electrophoresis. J. Phys. Chem. B 2016, 120, 6160-6165.

26. Ji, Y. L.; Zhao, Q.; An, Q. F.; Shao, L. L.; Lee, K. R.; Xu, Z. K.; Gao, C. J. Novel Separation Membranes Based on Zwitterionic Colloid Particles: Tunable Selectivity and Enhanced Antifouling Property. J. Mater. Chem. A 2013, 1, 12213-12220.



Page 10 of 10

TOC C


Micropore Geometry Manipulation by Macroscopic Deformation Based

Recommend Documents