Emulsion Lyophilization as a Facile Pathway to Fabricate Stretchable

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Applications of Polymer, Composite, and Coating Materials

Emulsion Lyophilization as a Facile Pathway to Fabricate Stretchable Polymer Foams Enabling Multi-shape Memory Effect and a Clip Application Yukun Hou, Guangqiang Fang, Yongbo Jiang, Huijie Song, Yuhua Zhang, and Qian Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11424 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Materials & Interfaces

Emulsion Lyophilization as a Facile Pathway to Fabricate Stretchable Polymer Foams Enabling Multi-shape Memory Effect and a Clip Application Yukun Hou,1 Guangqiang Fang,4,5 Yongbo Jiang,1 Huijie Song,1 Yuhua Zhang,2,3* Qian Zhao1* 1 State

Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China 2 Zhejiang

3 People's

4 Institute

5 Space

Provincial People's Hospital, Hangzhou 310014, China

Hospital of Hangzhou Medical College, Hangzhou 310014, China

of Aerospace System Engineering Shanghai, Shanghai 201109, China

Structure and Mechanism Technology Laboratory of China Aerospace Science and Technology Group Co.Ltd, Shanghai 201109, China

ABSTRACT: Solvent freezing is an important method to produce polymer foams with highly tunable pore structure. However, foams prepared from aqueous solution precursors

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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

commonly suffer from poor water resistance, whereas those organo-phase systems are not environmental-friendly. Here we present that using an emulsion lyophilization method can overcome such a contradiction and synthesize multi-functional polymer foams. Commercially available polyacrylate-based emulsions with various targeted glass transition temperatures (Tgs) were applied. Adipodihydrazide molecules contained in the water phase of the emulsions reacted with the acetyl groups on the polymers during the freeze-drying, forming elastic networks to maintain the pore structure. The foams can tolerant a 650% elongation without failure and are notch insensitive. The porosity of the foams can be tuned from approximately 45% to 90% via lyophilization of diluted emulsions. The facile blending of emulsions with different targeted Tg enabled foams with multi-shape memory capability. Moreover, the foams showed an excellent mechanical damping property, and the slow recovery nature enabled a clip application of clamping extremely weak objects. Keywords: emulsion, lyophilization, porous polymers, shape memory polymers, vessel clip Introduction Porous polymer materials, so-called polymer foams, are of low density, large specific surface area, and tunable permeability.1 Due to the unique structural and mechanical properties, they have been extensively used in various applications including separation,25

sound absorption,6 mechanical damping,7 heat insulation,8 etc. Thanks to the good

processability of polymers, porous polymer monoliths can be produced via many methods including direct synthesis,9 direct templating,10-12 foaming,13,14 particle/fiber stacking,15-17 3D printing,18,19 solvent freezing,20-32 and the like. Each method provides its own value to


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result in various pore morphology (e.g. open or closed cells) and to meet different preparation requirements (e.g. large-scale industrial manufacturing or small-scale laboratory preparation). In other words, none of these methods can solve all problems. For example, foaming13,14 as the most widely used porogenation method refers to utilize the bubbles generated by physical or chemical methods as the template to prepare porous materials. However, such a method in most cases results in closed-cell structure and is generally applicable to high-viscosity precursors, restricting the multi-functionality of the material systems. Direct templating10-12 is a universal and easily applicable method for nearly all polymers. However, removal of the incompatible templating agents from the polymer matrix always requires a solvent washing process which is not only time consuming but also environmentally unfriendly. The solvent freezing method uses the solvent crystals generated in-situ during the freezing process as a template to create pores. Cryo-gelation20-30 or lyophilization31,32 is commonly required to counter the collapse of the pores after melting of the crystals. The method is simple to be conducted, and structural parameters of the resulted foams such as pore size, porosity, and pore morphology can be conveniently adjusted. Water and organic solvents have both been applied to fabricate polymer foams via the solvent freezing method. While the utilization of organic solvents is not an environmental-friendly nor an economic pathway, the foams prepared from water solutions of polymers or monomers are typically of poor water resistance. Such a contradiction severely restricts the method from being used more widely. The utilization of emulsion might be a pathway to solve the contradiction. As a system which can use water to finely disperse hydrophobic polymers, emulsion provides an environmental-friendly nature and the incorporated polymers can be



of good water resistance after drying. Although there are already a few works using emulsion lyophilization33,34 to prepare polymer foams, the advantages have not been extensively explored. Firstly, organic solvents such as chloroform were still involved to dissolve hydrophobic polymers in the disperse phase. Secondly, the resulted foams were typically rigid thermoplastics with few additional functionalities. Shape memory polymers (SMPs)35-43 can be fixed into temporary shapes after thermomechanical programming. Stimuli-responsive recovery to the permanent shapes will be induced via heating above their thermal transition temperatures. In comparison with traditional non-porous SMPs, porous SMPs44-46 provide additional capabilities such as large volume changes and energy dissipation. In this work, we discover a facile method to prepare porous SMPs enabling good stretchability and highly tunable properties via lyophilization of commercially available polyacrylate emulsions. Notably, the whole process involves no organic solvent. Such a green nature provides significant benefits for the potential large-scale manufacture.


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Results and Discussion

Figure 1. Chemical structure and crosslinking mechanism of the emulsions. (a) Chemical structure of monomers and crosslinker used in the emulsion polymerization. (b) Crosslinking reaction during lyophilizing. (c) Schematic illustration of the formation procedures of the foam. The emulsions used in this study are commercially available, which have been widely applied to form fine coatings in the textile printing industry. These commercial products were chosen because they contain no organic solvent and thus are environmental-friendly. In addition, their compositions and mechanical properties can be facilely tuned. Figure 1a presents the main monomers for synthesis of the emulsions. Ethyl acrylate (EA) and butyl acrylate (BA) are soft monomers which can result in polymers with Tgs lower than room



temperature after polymerization. Methyl methacrylate (MMA) and styrene (St) serve as hard monomers which can increase Tg of the resulted polymers. Diacetone acrylamide (DAAM) and adipodihydrazide (ADH) can form acylhydrazone-based crosslinking points due to the reaction between the ketone groups and the hydrazide groups (Figure 1b). The emulsions E1, E2, and E3 were composed of various soft and hard monomers, exhibiting different Tg as -14 °C, 34 °C, and 61 °C after the drying and crosslinking process (Figure S1). Notably, the ADH molecules as crosslinkers were added and existed in the water phase of the emulsions because of their hydrophilic nature. Although the ketone groups and the hydrazide groups provided sufficient reactivity at room temperature, the emulsions were stable without gelation because the reactants were isolated respectively in the organic phase and the water phase by the emulsifiers. Upon frozen, the emulsions will be highly concentrated because of the crystallization of the water phase, leading to a demulsification. At the same time, the ADH molecules will also be excluded from the water phase due to the ice crystallization. Subsequently, the crosslinking reaction will occur during the lyophilization process to maintain the porous structure (Figure 1c). Chemical structure of the lyophilized porous material made no remarkable difference from that of the air-dried nonporous sample proved by ATR-IR characterization (Figure S2). Solid contents of all three emulsions (E1, E2, and E3) were all 50%.


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Figure 2. The mechanical property of the foams. (a) Tensile tests of intact and notched F50-0 samples. (b) Photographs recording the tensile process of the notched F5-0-0 sample. (c) Tensile tests of foams prepared by the various composition of emulsions; (d) Young’s modulus and tensile strength of the foams. In addition to the environmental-friendly nature, emulsion polymerization can synthesize products with higher molecular weight and thus better mechanical property in comparison with common bulk or solution polymerizations. Therefore, the resulted foams exhibited a good toughness. For instance, the F5-0-0 sample can tolerate a tensile strain of more than 650% without failure (Figure 2a), which is challenging for polymer foams. The foam was also notch-insensitive. When an F5-0-0 sample with a notch at the edge was stretched to 400%, the notch was blunted and remained stable (Figure 2b). The F5-0-0 sample was



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prepared via lyophilization of the neat E1 emulsion, the Tg of which was lower than room temperature. To enable ordinary shape memory capability, the E2 emulsion which provided a higher Tg was blended with the E1 emulsion to prepare the foams. Thanks to the facile blending of emulsions, the mechanical property of the foams can be easily tuned. With the increase of the E2 ratio, the foams contained more amount of rigid component. Accordingly, Young’s modulus and tensile strength increased remarkably although the strain at break decreased. Young’s modulus of F5-0-0, F4-1-0, F3-2-0, and F2-3-0 were respectively 249.7 kPa, 328.7 kPa, 1835.0 kPa, and 8466.7 kPa. Tensile strength of F2-3-0 reached 750 kPa in comparison with 150 kPa provided by F5-0-0. It is noted that a further increase of the E2 ratio (> 60%) in the blended emulsion can hardly result in intact foams via lyophilization. Although demulsification may still occur upon freezing, the rigid particles cannot fuse together due to the low mobility of the polymer chains. Table 1. Mechanical properties of the foam with different solid contents. Solid content/%

Porosity/%

Young’s modulus/kPa

Tensile strength/kPa

Elongation at break/%

5

90.9±0.7

-a

-a

-a

10

84.0±0.8

-a

-a

-a

20

74.9±1.3

235.3±34.5

72.0±4.5

165.8±24.6

30

67.1±0.4

260.8±20.4

96.8±7.2

237.4±26.8

40

54.3±1.8

293.3±6.0

160.0±13.8

291.8±13.5

50b

45.9±2.7

328.7±14.7

174.0±20.3

341.7±14.5

aThe

foams with high porosity were too weak to be measured.

bThe

foam with 50% solid content represents the foam F4-1-0.

While blending different emulsions can achieve foams with various mechanical property,


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dilution of the emulsions can result in monoliths with tunable porosity after lyophilization.47-49 We used the blended emulsion containing 80% E1 and 20% E2, namely the precursor of F4-1-0, as a model system to produce foams with various porosity via the dilution. As shown in Table 1, the porosity of the foams can be adjusted from approximately 45% to 90% when the solid content of the precursor emulsion was formulated from 50% to 5%. The resulted porosities were slightly lower than the ideally targeted porosities, e.g. the emulsion providing a 5% solid content was targeted at a 95% porosity. It indicates that there might be a slight volume shrinkage during the lyophilization. Tensile tests were carried out on the foams with different porosities, and the results are also shown in Table 1. At a porosity of approximately 75%, Young’s modulus was about 230 kPa. The tensile strength and the strain at break were respectively higher than 70 kPa and 160%, which would be adequate as a soft foam for many applications. However, the foams prepared by emulsions with a solid content of 5% and 10% provided insufficient mechanical properties for the tensile tests. Representative stress-strain curves of the foams are presented in Figure S3.

Figure 3. SEM microimages of the foams with various solid contents. (a) 5%, (b) 10%, (c)



20%, (d) 30%, (e) 40%, and (f) 50%. The scale bars represent 100 μm. The foam with 50% solid content represents the foam F4-1-0. Figure 3 shows the scanning electron microscopy (SEM) microphotos of the foams with various porosities. With the decrease of the porosity, the pores become smaller, and the walls of the pores become thicker. This morphology result is also consistent with that of the studies on the mechanical properties. In comparison with polyacrylate-based shape memory foams synthesized by organo-phase cryo-polymerization, the pore morphology in this study is isotropically providing no remarkable alignment.26 Even under an isotropicfreezing condition, the pore structure would easily fall into a globally disordered yet locally ordered orientation. Such a polydomain structure is unfavorable to the stretchability of the materials since it would more likely to bring on stress concentration. It is noted that a small amount of poly(vinyl alcohol) (PVA) was involved to dilute the emulsion in this study. The PVA molecules can significantly promote the ice nucleation, which would lead to a more uniform microphase separation during freezing.30 The resulted isotropic pore orientation is beneficial for the overall mechanical property of the foams. If pure water were applied as the dilution instead of PVA aqueous solution, the pore morphology would also exhibit locally ordered orientation which would reduce the stretchablity. These discussions are supported by the characterizations on morphology and mechanical property of the foams using PVA aqueous solution and pure water as the dilutions presented in Figure S4 and Figure S5.


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Figure 4. Thermomechanical property of the foams. (a) Shape memory cycles of the foam F4-1-0. (b) Temperature sweep of storage modulus of various foams. (c) Temperature sweep of tan δ of various foams. (d) Photos showing the quadruple shape memory effect of the foam F3-1-1. The scale bars represent 1 cm. Quantitative shape memory performance of the foams was evaluated using the F4-1-0 as a representative. Consecutive shape memory cycles are shown in Figure 4a. The shape fixity ratio and the shape recovery ratio for each cycle are higher than 98%, showing a stable and almost ideal shape memory performance. To tune the shape transition temperature of the foams, as well as to enable multi-shape memory effect via adding more transition temperature, emulsions with various compositions were applied for the lyophilization. Temperature sweep of storage modulus on the resulted foams was conducted (Figure 4b). The correlated Tan δ curves were presented in Figure 4c. The



various glass transitions respectively emerged at around 20 °C, 75 °C, and 110 °C, correspondingly provided by the emulsions E1, E2, and E3. In comparison with the Tgs measured by DSC, the Tgs here determined by DMA were nearly 50 °C higher for all the three components. Such a distinction should be attributed to the different sample form for the two measurements. The DSC study only required a small amount of powder sample which could shortly reach the temperature equilibrium. On the contrary, the thermal equilibrium of the foam samples would take a very long time due to the low thermal conductivity of polymer foams. As such, there would be a large temperature hysteresis during the temperature sweep of the DMA measurements. In this case, the shape recovery can be triggered at temperatures those are mildly higher than Tgs determined by DSC. For instance, the shape recovery of the F4-1-0 foam in Figure 4a was triggered at 45 °C which was higher than 34 °C (Tg of the E2 component determined by DSC) but much lower than 75 °C (tan  peak of the E2 component determined by DMA), merely took a long time to achieve the thermal equilibrium and the full recovery. It can be seen from the tan  curves that the F3-1-1 foam provides three separated peaks, implying that the material may enable a quadruple-shape memory effect. As shown in Figure 4d, a quadruple shape-memory cycle is performed using the F3-1-1 foam. A strip sample was sequentially programmed into three temporary shapes upon temperature steps of 90 °C to 50 °C, 50 °C to 20 °C, and 20 °C to -18 °C. Stepwise recovery was achieved when the foam was sequentially placed at 20 °C, 50 °C, and 90 °C. Although multi-shape polymers50-53 have already been developed by a number of methods including blending materials with different transition temperature and synthesizing materials with a broad transition temperature, porous multi-shape polymers are still rare. Nevertheless, the


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lyophilization of blended emulsion presented here provides a facile solution.

Figure 5. Falling-ball experiment on (a) an F5-0-0 foam and (b) an EVA foam. In addition to the shape memory capability, the foams were expected to provide excellent mechanical damping property since they provided high values of tan  over an extremely large temperature range. For instance, tan  of F5-0-0 exhibits a peak value as high as 2.5 around room temperature, whereas F3-1-1 maintains a relatively high damping performance from -10 °C to 120 °C in which range the tan  is larger than 0.3.7 Such a property can be attributed to the foam structure as well as the nature of polyacrylate network since the pendent alkyl chains on the acrylate enable significant energy dissipation when the main chains are sliding under mechanical force. A falling-ball experiment was carried out to demonstrate the damping performance (Figure 5a and Supplemental video1). When a steel ball fell on the F5-0-0 foam from a height of 50 cm, it didn’t show any visible bounce. In comparison, a remarkable bounce can be observed when an EVA-based commercial damping foam was applied (Figure 5b). In another demonstration, a raw egg maintained intact when dropping on the F5-0-0 foam from the height of 50 cm (Supplemental video-2).



Figure 6. Clip application. (a) Permanent shape; (b) temporary shape; (c) clamping tofu at the temporary shape; (d) clamping tofu after shape recovery; (e) measurement of the peeling-off force using silicone rubbers with various thickness as models. The scale bars represent 1 cm. The large tan  value also implies a slow recovery rate. While most researches on shape memory polymers are aiming at developing stronger materials with fast recovery rate,54 we demonstrate here that a soft material with slow recovery provides unique merits of its own. Vessel clips have been widely used in minimally invasive surgery. If the clamping force were too large, the fragile vessels might be cut, especially for the relatively thick vessels. On the contrary, the clips suffer from the high risk of coming off when clamping thin vessels. Up to now, all commercially available clips are made of rigid polymers, which cannot fully resolve the contradiction. We combined the soft shape memory foams F4-1-0 onto the two arms of a clip as shown in Figure 6a. The unlocked clip before and after the combination is shown in Figure S6 and Figure S7. In a permanently locked shape, the two foams just touched each other without intended compression, and there was almost no gap between the two arms. The foams were compressed at 50 °C and temporarily fixed in a compact shape at room temperature, and the clip was afterwards applied to clamp objects.


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Here, a strip of tofu with a width of 2 mm was used as a model object. It is noted that the tofu providing a compressive strength of 9 kPa (Figure S8) was much more challenging to be clamped than blood vessels since it was more fragile. If the tofu won’t break during the clamping, it can make insurance that various vessels will be all intact. Upon shape recovery of the foams when heated under 50 °C, the tofu can be clamped and remain intact. Notably, the shape memory effect is essential for clamping tofu. The tofu would still be cut when it was clamped directly by the snap locking of the clip without the assistance of the shape memory effect. Such a phenomenon can be explained according to the momentum conservation law (Ft=mv). The thermally induced shape recovery took much longer time than the directly snap locking, so, the output clamping force was much smaller to ensure that the object wouldn’t be damaged. Such a concept can further be extended to robotic applications for clamping extremely weak objects. Although the clamping force was not large, the composite clip can still guarantee that the clamped objects would not come off since the porous structure provided large friction between the materials. Silicone rubber films with various thicknesses were used as model objects to measure the peeling-off force (Figure 6b). When the films were thicker than 0.5 mm, the force increased remarkably with the film thickness. For the thin films (< 0.5 mm), the peeling-off force can maintain at 0.5 N which is an acceptable value for practical clip applications. Conclusion In this study, emulsion lyophilization was explored as a facile pathway to fabricate stretchable and multi-functional polymer foams. The crosslinking took place during the lyophilization process. When the applied emulsion was composed of low Tg polyacrylates,



the resulted foams were soft yet mechanically stable providing a strain at break of 677%. The emulsion can be diluted to prepare foams with various porosity ranging from 45% to 90%. Blending emulsions those were composed of polyacrylates with various Tgs can fabricate foams with tunable mechanical properties. When the blended emulsion contained a large proportion of high Tg polyacrylates, however, intact foams cannot be formed. Quadruple shape memory effect can be achieved using a foam fabricated from the blending of three different emulsions. The foams exhibited high tan  values over a large temperature range which was conducive to mechanical damping. The porous structure also enabled a slow shape recovery property which was utilized to clamp extremely weak objects in a clip application. Experimental descriptions Materials. Ethyl acrylate (EA, > 98%), butyl acrylate (BA, > 98%), methyl methacrylate (MMA, > 99.5%), diacetone acrylamide (DAAM, > 99%), adipodihydrazide (ADH, > 99%), poly(vinyl alcohol) (PVA, Mn = 75,000, alcoholysis degree = 98~99%)，benzoyl peroxide (BPO, AR), triethylamine (99.0%) were purchased from Aladdin Reagent; ammonium persulfate (> 98%), tert-Butyl hydroperoxide solution (70 wt% in H2O) were purchased from Sinopharm Reagent. Polycaprolactone diol (PCL, Mn = 10,000) was purchased from Sigma Aldrich, and acryloyl chloride (> 98 %) was purchased from TCI. All the reagents were used as received. Polycaprolactone diacrylate (PCLDA) was synthesized according to the method reported in the literature (35). Three emulsions labeled as E1, E2, and E3 with different targeted Tg were supplied by Jiangshan Caida Textile


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Printing Materials Co. Ltd. Detailed materials and method for the laboratory-level synthesis are presented in the Supporting information. Preparation of foams. A neat or blended emulsion was poured into a paper cup and frozen in a refrigerator at -10 °C for 4 h. The frozen emulsion was then lyophilized for 12 h to produce a foam. The resulted foam was labelled as Fa-b-c, where a, b, and c respectively represented the proportion of the emulsions E1, E2, and E3 in the corresponding blended emulsion precursor. For instance, F4-1-0 represented that the foam was obtained from a blended emulsion the proportion of the emulsions E1, E2, and E3 in which were 4/5 (80 vol%), 1/5 (20 vol%), and 0/5 (0 vol%), respectively. To tune the porosity, the precursor emulsion of F4-1-0 was lyophilized after diluted into various solid contents by adding a given amount of 1 wt% PVA aqueous solution. Characterization. Chemical composition was characterized by ATR-IR (Nicolet 5700) test. The solid content of various emulsions, including the neat, blended, and diluted emulsions was calculated as (m1/m2) ×100%, where m1 refers to the mass of all the monomers, emulsifiers, initiators and other additives except deionized water, and m2 is the mass of the whole emulsion. The pore morphology of the foams was observed by scanning electron microscopy (SEM, JEOL SU-3500). Before the observation, the foam samples were quenched in liquid nitrogen and fractured, and subsequently sputter coated with gold. The tensile tests were conducted under a tensile speed of 20 mm/min by a universal



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material testing machine (Zwick/Roell Z005). The samples were cut into a standard dumbbell type by a laser cutter (Trotec Speedy100R) before the tests. The compression strength of tofu was tested by a universal material testing machine (Instron 5900) under a compression speed of 0.5 mm/min. Dynamic mechanical analysis (DMA) was conducted using DMA Q800 (TA instruments). Tan δ of the foams was recorded by the temperature sweep under a multifrequency-strain mode at a heating rate of 10 °C/min and an amplitude of 20 μm. The shape memory cycles were performed under a controlled-force mode. Thermal property of the emulsion after air drying was measured using a differential scanning calorimeter (DSC, TA Q200) within the temperature range from -50 to 120 °C at a heating rate of 5 °C/min. The porosity was determined as follows. A foam was cut into a cuboid shape (7×7 mm) by the laser cutter. Volume V was calculated, and the weight m was measured. The density ρ of the corresponding nonporous polyacrylate sample is determined as 1.12 g/cm3. The porosity P of the foams can be gain with the formula: 𝑃=

𝑉―

𝑚 𝜌

𝑉

(1)

Falling-ball experiment A steel ball with a diameter of 15 mm was dropped onto the F5-0-0 foam with a thickness of 10 mm from the height of 50 cm. A commercial EVA foam was used as a control material. Preparation and characterization of the clip


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The material for making the vessel clip was synthesized by the bulk copolymerization of MMA (8.0 g) and PCLDA (2.0 g). The precursor was poured into a mold defined by two glass slides separated by a silicone rubber spacer with a thickness of 2.0 mm. The polymerization took for 2 h at 80 °C using BPO (0.1 g) as the initiator. The sheet was then cut into the geometry of the main body of the vessel clip using laser cutting (Figure S6) providing two slots on the arms with the dimension of 6.6×1.5×2.0 mm. Two shape memory foams of F4-1-0 with the thickness of 1.5 mm was just fitted into the slots and adhered by adhesive tape. Silicone rubber films with a width of 5 mm and different thicknesses were clamped in the clip. Films were pulled at a tensile speed of 20 mm/min and the force was recorded by the universal material testing machine. The setup was presented in Figure S9.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications websites. Additional information on the experimental descriptions, DSC and ATR-IR characterization, representative stress-strain curves, photos of the vessel clip, and setup of the peeling-off experiment.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]



ORCID: Yuhua Zhang: 0000-0002-1308-1992 Qian Zhao: 0000-0002-6931-9859 Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the following programs for the financial support: National Natural Science Foundation of China (51822307 and 51673169); Natural Science Foundation of Zhejiang Province (LR18E030001); Shanghai Aerospace Technique Innovation Foundation (SAST2018-019), and Fundamental Research Funds for the Central Universities (2018RC020). The authors also thank Mrs. Li Xu for her assistance in performing DSC analyses, Mrs. Na Zheng for her assistance in performing SEM analyses, and Mrs. Jing He for her assistance in performing ATR-IR analyses, at State Key Laboratory of Chemical Engineering (Zhejiang University).

REFERENCES (1) Wu, D.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Design and Preparation of Porous Polymers. Chem. Rev. 2012, 112, 3959-4015. (2) Landsman, T. L.; Touchet, T.; Hasan, S. M.; Smith, C.; Russell, B.; Rivera, J. A Shape


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