Newly Designed Copolymers for Fabricating Particles with Highly

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Newly Designed Copolymers for Fabricating Particles with Highly Porous Architectures Chia-Chen Li,*,† Sheng Yang,† Yu-Ju Tsou,† Jyh-Tsung Lee,‡,§ and Chang-Ju Hsieh‡ †

Department of Materials & Mineral Resources Engineering, and Institute of Materials Science and Engineering, National Taipei University of Technology, Taipei 10608, Taiwan ‡ Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan § Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan S Supporting Information *

ABSTRACT: A new type of designed copolymers has been synthesized for the formation of porous polymeric microspheres. The copolymers contain hydrophobic (styrene, methyl methacrylate, vinylbenzyl chloride, or vinylbenzyl ethyl ether) and hydrophilic (vinylbenzyl alcohol) repeating units. Since the specially designed copolymers have unique chemical and physical properties, the porous structure can be easily accomplished by a facile single-step process. The resulting porous microspheres exhibit good morphological quality, showing open pore structure with a pore size ranging from submicrometer to micrometer, by neither use of porogens nor the requirement of complicated multistep emulsifications. The discovery for the exceptional performance of pores in microspheres is exciting and groundbreaking. The chemical features of the proposed copolymers for the availability in the formation of porous architecture provide important insights into the design principle of high quality porous structures.

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that contain discrete phase droplets composed of monomeric species, initiators, and a porogen suspended in a continuous phase by the presence of soluble surfactants. Completion of the polymerization of the monomeric species results in a solid/ liquid suspension containing porous particles. This technique benefits from low cost; however, the simultaneous polymerization and formation of a porous structure is not easily handled, resulting in low quality of morphologies. Moreover, most performed pores on microspheres are too tiny and insignificant, which may restrict future applications, especially for some fields such as energy and biomaterials.27,28 To date, there is little research reporting the fabrication by conducting a preformed polymer system, that is, to directly dissolve a preformed polymer in an organic solvent, instead of forming the polymer from a monomeric species, to simplify the process. Even though the as-mentioned method is taken, the result for the morphology of the porous structures is not good and the fabrication procedure is still too complicated.27−29 It is believed that the lack of suitable polymers with exact physicochemical features is the major crux. An ideal polymer for forming porous architectures should basically have proper stoichiometry and distribution of hydrophobic and hydrophilic units in the chemical structure.30 Therefore, a new design of copolymers that is able to provide required amphiphilic property and polarity was proposed in this investigation. By the use of the designed copolymers, demonstrating porous microspheres with exceptional architecture becomes easy, and

ompared to conventional dense materials, porous materials exhibit special features such as relatively low density, high surface area, light weight, sound and thermal insulation, and good permeating selectivity.1−5 These remarkable properties have made porous materials of great scientific and technological interest, enabling their use in a wide range of industrial applications and products, including efficient adsorbents for storage and controlled release, carriers for medicines and biomaterials, supports for conversion reactions, supercapacitors, batteries, solar power, and fuel cells.4−14 With an increased demand for new materials in surface-related applications, research into developing fabrication techniques for porous materials has increased. Among material types and architectures, polymeric porous spheres have been the highest developed, and they are also common precursors and templates for other materials like carbons, metals, and ceramics in the fabrication of porous structures.15−18 In general, there are two main techniques for the preparation of polymeric porous microspheres: utilizing templates and heterogeneous polymerizations.4,19−26 In the former technique, a number of organic and inorganic compounds are available to serve as templates. After removing the template from the polymeric matrix, the voids left in the polymer result in pores. The primary advantage of this method is control over the pore size and shape. Disadvantages and drawbacks of this method are the challenging conditions needed to control the template molecules and the corrosive chemicals required for their removal. Heterogeneous polymerizations include several approaches such as suspension, dispersion, emulsion, precipitation, and microfluidic polymerizations. In these polymerization methods, reactions begin with liquid/liquid mixtures © XXXX American Chemical Society

Received: April 10, 2016 Revised: August 23, 2016

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Figure 1. Schemes of (a) the polymerization and (b) the resulting chemical structure of PVBC. (c) Schematic mechanism for the formation of a porous microsphere. SEM and TEM images of the porous microspheres prepared from the as-synthesized (d, h) PVBC, (e, i) PSV, and (f, j) PMSV and as-received commercial PVBCα (g, k).

prepared from PVBC. For the porous PMSV microsphere (Figure 1f), the particle size distribution was the same as that of PSV; however, the pore morphology was very different from that of PVBC and PSV. The PMSV microsphere appeared to have a structure with pores densely distributed on the surface and in the interior (Figure S1) and an average pore size larger than 1 μm. The TEM images shown in Figure 1h−j indicate the presence of connected pore channels in the three microspheres; that is, the pores are open and connected. The porosities of the PVBC, PSV, and PMSV microspheres measured by mercury porosimetry are 81%, 70%, and 91%, respectively. Furthermore, the surface area of the PMSV microsphere having higher porosity is 90.60 ± 1.19 m2·g−1. We believe that this result of obtaining microspheres with an exceptional architecture by single-step emulsification from preformed polymers is groundbreaking. As mentioned previously, only polymers with an amphiphilic architecture and appropriate polarity are good candidates for the fabrication of porous microspheres. When microspheres were prepared via the use of as-received commercial PVBC, denoted as PVBCα, solid dense particles resulted, without the formation of pores. The SEM and TEM images of PVBCα are shown in Figure 1g,k, demonstrating the lack of internal and external pores. Clearly, partial hydrolysis of the polymer is a key factor in the formation of porous microspheres. Chemical Structures of Designed Copolymers. The chemical structures of the as-synthesized copolymers were characterized by Fourier transformed infrared (FT-IR) and

the available fabrication can also be simplified to an unsophisticated single-step.

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RESULTS AND DISCUSSION Formation of Porous Architectures. On the basis of the above, three partially hydrolyzed polymers, poly(vinylbenzyl chloride) (PVBC), poly(styrene-co-vinylbenzyl chloride) (PSV), and poly(methyl methacrylate-co-styrene-co-vinylbenzyl chloride) (PMSV), were synthesized and used for the preparation of porous microspheres. As shown in Figure 1a,b, polymerization was conducted by in situ hydrolysis; thus, the product is expected to contain the structure like a copolymer. By dissolving the as-synthesized polymer in an organic solvent and emulsifying in an aqueous phase, as shown in Figure 1c, microspheres spontaneously form with a porous architecture. Formation of the porous spheres results from the amphiphilic, partially hydrolyzed polymer that is able to trap water, stabilizing it inside the oil droplets and resulting in the formation of pores. Figure 1d−f shows the microstructures of the porous microspheres from the as-synthesized PVBC, PSV, and PMSV, respectively. Figure 1h−j shows their corresponding TEM images. Among the three porous microspheres, the PVBC microsphere (Figure 1d) exhibited the broadest particle size distribution, ranging from 1 to 100 μm with a pore size of less than 0.5 μm. For the porous PSV microsphere (Figure 1e), the particle size distribution was narrower, in the range of 2 to 25 μm, with a pore size and size distribution similar to those B

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Chemistry of Materials carbon-13 nuclear magnetic resonance (13C NMR) spectroscopies. Figure 2a compares the FT-IR spectra of the as-

VBC units in the as-synthesized PVBC have been hydrolyzed, whereas hydrolysis was not observed in the commercial PVBCα. Figure 2b compares the 13C NMR spectra of the assynthesized PVBC and PVBCα; the important chemical shifts have been assigned as shown in the figure. In addition to residual monomers, there were no other peaks in the PVBCα attributable to other compounds. In contrast, the 13C NMR spectra of the as-synthesized PVBC contained peaks from three types of monomer units in the polymer chain attributed to VBC, vinylbenzyl alcohol (VBA), and vinylbenzyl ethyl ether (VBEE). Since VBC is a primary chloride, the substitution of the −Cl by an −OH or −OCH2CH3 (−OEt) group would be favorable and generally proceeds through a second-order nucleophilic substitution mechanism.33 Additionally, the IR absorptions at 1093 and 1014 cm−1 correspond well to the presence of a VBEE unit, likely formed through reaction with ethanol during polymerization. In addition to PVBC, partially hydrolyzed structures from the as-synthesized PSV and PMSV have been assigned using FT-IR and NMR spectroscopies (Figure S2). While the VBA unit is hydrophilic, VBC, styrene, methyl methacrylate (MMA), and VBEE are all hydrophobic. According to further NMR analysis, the content and distribution of the monomer units, VBC, VBA, and VBEE, in the chemical structures of the as-synthesized copolymers were determined by the analyses of hydrolysis kinetics. The experiment for hydrolyzing VBC at 80 °C in the cosolvent of deionized water and ethanol in a volume ratio of 3:2 was carried out, and the hydrolysis products were tracked by sampling the intermediates at different time periods and analyzing the samples by 1H NMR (Figure S3). As shown in Figure 3a, the VBC was easily hydrolyzed under 80 °C; 28% of VBA and 14% of VBEE were obtained within 2 h. Generally, smaller molecules react kinetically faster; hence, the free monomer VBC is more easily hydrolyzed than when in its polymeric form. That is, most of the hydrolyzed monomer units in the polymer chain are more probable from hydrolyzed monomers in the solution, rather than from hydrolysis of the polymer (Figures S4−S7). During the polymerization, hydrolysis of VBC monomers continues to proceed, and thus, the content of VBA in the polymer chain will increase. Therefore, the relative distribution between hydrophilic and hydrophobic units in the as-synthesized polymers may be similar to the structure shown in Figure 1b. Regarding formation of the porous structure, an important question is if a diblock or pseudodiblock copolymer is necessary or if a random copolymer can result in the same structure. To further clarify the effect of the relative distribution of monomer units on the formation of porous structures, a copolymer with randomly distributed VBC, VBA, and VBEE was synthesized (Figure S8). Figure 3b shows the microstructure of the resulting microspheres. No pores are observed on the surface; however, the broken sphere shown in Figure 3c indicates that the interior structure should be porous. This result is different from those obtained when the copolymers were synthesized by in situ hydrolysis. As a result, a diblock structure may be not necessary, but different types of polymer blocks will lead to varied quality and architecture of the resulting pores. Effects of Surface Tension. In addition, the surface tension of solvents should also be important for the formation of porous microspheres. The morphology of PVBC microspheres prepared from aqueous emulsion systems containing various concentrations of surfactant−sodium dodecyl sulfate ([SDS]) was studied. Figure 4a−c shows the SEM images of

Figure 2. (a) FT-IR and (b) 13C NMR spectra of as-synthesized PVBC (a1, b1) and commercial PVBCα (a2, b2).

synthesized PVBC and PVBCα. In both spectra, the peaks at ∼3000 cm−1 belong to various C−H stretches. For organic chlorine compounds,31 the C−Cl stretch generally appears in the range of 760−505 cm−1 with strong intensity, and a terminal −CH2Cl group will show a medium to strong absorption related to C−H wagging at 1315−1215 cm−1. Hence, the IR absorbance at 1265 cm−1 can be attributed to the −CH2Cl vibration of the vinylbenzyl chloride (VBC) unit of PVBC.32 With the exception of the appearance of a peak at 1265 cm−1, clear differences exist between the FT-IR spectra of the as-synthesized PVBC and PVBCα; most significantly, a new broad band centered at 3380 cm−1 is present for the assynthesized PVBC, generally attributable to an O−H stretch. Additionally, the FT-IR peaks at 1093 and 1014 cm−1 are attributed to C−O bendings of −CH2−OH and −CH2−O− CH2CH3, respectively. In summary, the reduced intensity of the peak at 1265 cm−1 and the appearance of additional peaks at 3380, 1093, and 1014 cm−1 strongly suggest that some of the C

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when 0.3 wt % of SDS was added. The surface tension of toluene was measured to be 29.4 dyn·cm−1 and increased to 35.0 and 34.5 dyn·cm−1 when 7 wt % of the as-synthesized PVBC and PVBCα was dissolved, respectively. Pure toluene and two solutions of PVBC/toluene and PVBCα/toluene in contact with deionized water are shown in Figure 4e, and the three liquid/liquid interfaces show distinct results (toluene phases have been dyed red for clearer observation of the interfaces). The interfaces are concave for both systems of toluene/water (left) and (PVBCα/toluene)/water (middle), whereas it is flat for the system of (PVBC/toluene)/water (right). The concave interfaces shown in the left and middle are likely caused primarily by the differences in surface tension and polarity between toluene and water. The flat interface in the case of toluene dissolved with the as-synthesized PVBC demonstrates that the compatibility between the two liquids becomes better. Since the surface tension of the PVBC/toluene solution (35.0 dyn·cm−1) is still as low as that of the PVBCα/toluene solution (34.5 dyn·cm−1), the better compatibility of the PVBC/toluene solution with water is not likely to be due to the increased surface tension. The likely cause of the flat interface is better interactions due to H-bonding between water and the VBA unit of the synthesized PVBC. That is, chemical affinity/polarity, rather than surface tension, dominated the behavior of the polymer solution. In addition, the better affinity of the PVBC solution to water also supports the rationality of the proposed pore formation mechanism, where the as-synthesized polymers trap water to enable formation of the pores. Applications of Designed Copolymers and Porous Microspheres. To illustrate the usefulness of the porous microspheres and the as-synthesized copolymers, some of their potential applications have been evidenced preliminarily in this work. Figure 5a,b shows the morphologies of the microspheres prepared from the as-synthesized PSV and PMSV after swelling in ethanol for 2 h. The PSV microsphere showed an enlarged pore size of ∼0.5−1.5 μm after treatment. The swelled PMSV microsphere showed the same range of pore size; nevertheless, its pores were more densely distributed. These gigaporous microspheres are especially useful in highly efficient chromatography for large biomolecules, such as proteins, DNA, and virus-like species. Figure 6a demonstrates that as-synthesized PMSV can be used to form a porous free-standing film. A membrane with the porous architecture was successfully produced by a simple fabrication process. In addition, the asprepared porous PSV and PMSV microspheres were carbonized, and the resulted porous carbon spheres were shown in Figure 6b,c. It should be noted that acid pretreatment has to be conducted before carbonization. The carbonization of the PMSV microspheres was not that favorable. From the collapsed microstructures, a partial melting of the polymeric microspheres that likely occurred during the thermal treatment process reveals. The unfavorable result for the carbonization of the PMSV microsphere may be attributed to the particular thermal property of the chain section containing MMA units that generally degrade through an end-chain scission mechanism (unzipping process).34 Additionally, the MMA unit is not as rigid as a benzene ring and may tend to flow under heating. In contrast, the carbonization is shown to be very successful for the swelled PSV microsphere. Figure 6c,d shows the morphology and corresponding Raman spectrum. The microstructure of the carbon microspheres remained intact after thermal treatment. By comparing its Raman spectrum to that of commercial carbon black (Super-P, Timcal A+G Sins,

Figure 3. (a) Molar percentages of the hydrolyzing products of VBC as a function of reaction time. (b, c) SEM images of the microspheres prepared from a random copolymer of VBC, VBA, and VBEE.

the resulting microspheres from the as-synthesized PVBC, and the insets show their corresponding TEM images. Without the addition of SDS, mechanical emulsification has to be provided for at least 1 h to prevent demulsifying of the oil phase from the aqueous phase. After evaporation of toluene, no pores are observed on the surfaces; however, cavities are observed in the interior (Figure 4a). With the addition of 0.15 wt % SDS, only 2 min of mechanical emulsification was required for the prevention of de-emulsification and some pore formation is observed (Figure 4b). With an [SDS] of 0.3 wt % and 2 min of mechanical emulsifying, pore formation is observed on almost all microspheres, and the size and distribution of pores are more uniform (Figure 4c). Beyond 0.3 wt % of [SDS], the morphology of the porous microspheres remains almost unchanged due to the achievement of a constant surface tension as shown in Figure 4d. The surface tension appears to be of critical importance in determining the resulting morphology. Formation of porous microspheres is not observed in commercial PVBCα. An irregular fractal was formed in the absence of SDS, and only solid dense spheres were obtained D

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Figure 4. Microstructures of PVBC microspheres prepared in the presence of SDS at concentrations of (a) 0, (b) 0.15, and (c) 0.30 wt % in an aqueous solution. (d) Surface tension of aqueous solutions with various contents of SDS. (e) Interfaces between water and three toluene solutions (dyed in red color) with the absence (left) and presence of dissolved PVBCα (middle) and as-synthesized PVBC (right), respectively.

Figure 5. SEM images of microspheres prepared from the as-synthesized (a) PSV and (b) PMSV after swelling in ethanol for 2 h.

the porous architectures. Additionally, the hydrophobicity from the other monomer units is also essential for help in sculpting the skeleton of the porous microspheres. That is, the hydrophobic units adjust the solubility of the copolymers, avoiding phase microseparation that occurs if the content of the hydrophilic units is too high. Therefore, specific stoichiometry of the monomer units is essential for the as-synthesized copolymers to have the appropriate polarity, which leads to successful formation of the porous microstructure with good architectural quality.

Switzerland), it is obvious that the porous carbon spheres have a good quality of carbonization, which can have a high potential for the application in lithium−sulfur batteries.

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CONCLUSIONS In this work, we have demonstrated the use of specially designed copolymers possessing amphiphilic properties for the preparation of porous microspheres. Compared to conventional fabrications, the porous microspheres in this investigation were obtained by an easy, simple procedure of dissolving the assynthesized polymer, rather than conducting the polymerization during the formation of microspheres. The resulting microspheres exhibit large pores of good morphological quality. The hydrophilicity from the hydrolyzed monomer units played an important role in the ability to function as a porogen, helping to trap water from the aqueous phase and leading to

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EXPERIMENTAL SECTION

Syntheses of Copolymers. First, 50 mL of cosolvent containing deionized water and ethanol (95%, Echo, Toufen, Taiwan) in a volume ratio of 3:2 was placed in a 100 mL single-necked flask, into which 0.05 g of initiator, 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA; 97%, Sigma-Aldrich, Saint Louis, USA) was dissolved. Then,

E

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Figure 6. (a) Free standing porous film prepared from the as-synthesized PMSV. (b, c) SEM images of carbon spheres prepared from the assynthesized PMSV (b) and swelled PSV (c); inset of (c) shows the zirconia crucible containing the black powder carbonized from the swelled PSV porous microspheres. (d) Raman spectrum of porous carbon spheres prepared from swelled PSV microspheres (compared with commercial carbon black). 7.5 mL of the required monomer mixture, such as 4-vinylbenzyl chloride (VBC; 90%, Acros, New Jersey, USA), a mixture of styrene (99.5%, Showa, Tokyo, Japan) and VBC in a volume ratio of 2:1, or the mixture of VBC, styrene, and methyl methacrylate (MMA; 99%, Sigma-Aldrich, Zwijndrecht, Netherlands) in a volume ratio of 1:1:1, was added into the flask for the syntheses of PVBC, PSV, and PMSV, respectively. After sealing the flask and purging with nitrogen gas for 20 min, the flask was heated in an oil bath at 80 °C for 16 h. The product was purified by precipitation from methanol (99.8%, Grand, Pathum Thani, Thailand) after redissolving in toluene (100%, Mallinckrodt, USA) at least 3 times.35 Finally, the product was dried at 60 °C in a vacuum oven for 1 day and then stored in a desiccator. Preparation of Porous Structures. For the formation of the microspheres, 0.1 g of the as-synthesized polymer was dissolved in 1.2 g of toluene and then emulsified in a 40 mL aqueous solution with the addition of 0.3 wt % sodium dodecyl sulfate (SDS; 99%, Acros, New Jersey, USA) by a homogenizer (T25, IKA, Staufen, Germany) at a stirring speed of 3400 rpm for 2 min. Next, the mixture was magnetically stirred at 55 °C for 1 h until a white emulsion formed. The white emulsion was then separated by centrifugation and washed with deionized water several times. In the fabrication of porous standing film, a mixture of 0.1 g of PSV, 0.07 g of styrene butadiene rubber (SBR; Asahi Kasei, Japan), 1 mL of deionized water, and 1.2 mL of toluene was stirred under 400 rpm for 2 min and cast on a poly(propylene) (PP) plate and dried at room temperature. A free membrane was obtained by peeling off the cast material from the PP plate. For carbonization, the porous microsphere was pretreated by immersion in a warm concentrated sulfuric acid (95−97%, Scharlau, Sentmenat, Spain) for 1 h. Then, the acid-treated powder was washed repeatedly with deionized water and separated from the supernatant for drying. Next, the powder was placed in a quartz furnace saturated with nitrogen gas and heated from room temperature to 850 °C at a rate of 10 °C/min. Finally, a black powder was obtained. Characterizations. The chemical structures of the as-synthesized polymers were characterized by FT-IR (DA8.3, Bomem, Canada) and NMR (NUITY INOVA-500, Varian, USA) spectroscopies. The microstructures of the as-prepared porous microspheres were characterized by field emission SEM (S-470, Hitachi, Tokyo, Japan) and TEM (JEM-2100, Jeol, Japan) images. The porosity of the as-

prepared porous microspheres was measured using a mercury porosimetry (AutoPore IV 9500, Micromeritics, GA, USA). The surface area of the as-prepared porous microsphere was measured by the gas adsorption method (ASAP 2020, Micrometrics Instrument Co., USA). The microsphere was pretreated under vacuum at 60 °C for 48 h. The chemical composition of the carbonized microspheres was identified by Raman spectroscopy (HR800, Jobin Yvon, Kyoto, Japan).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01434. TEM images, FT-IR spectra, 13C NMR and 1NMR spectra, and optical microscopic image (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial aid from National Taipei University of Technology. ABBREVIATIONS VBC, vinylbenzyl chloride; VBA, vinylbenzyl alcohol; VBEE, vinylbenzyl ethyl ether; PVBC, poly(vinylbenzyl chloride); PSV, poly(styrene-co-vinylbenzyl chloride); PMSV, poly(methyl methacrylate-co-styrene-co-vinylbenzyl chloride); TEM, transmission electron microscope; FT-IR, Fourier transformed infrared; 13C NMR, carbon-13 nuclear magnetic F

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resonance; 1H NMR, proton nuclear magnetic resonance; PP, poly(propylene)

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