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Emulsion Templating Cyclic Polymers as Microscopic Particles with Tunable Porous Morphology Dingguan Wang,† Lifen Xiao,‡ Xinyue Zhang,† Ke Zhang,*,‡ and Yapei Wang*,† †

Department of Chemistry, Renmin University of China, Beijing, 100872, China State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing, 100190, China

‡

ABSTRACT: Cyclic polymers are a particular class of macromolecules without terminal groups. Most studies has involved their physical properties and polymer composition, while attention has rarely been paid to their emulsification in an oil−water system. Herein we synthesized a cyclic polymer with polystyrene side chains via ring-expansion metathesis polymerization and click-chemistry. This cyclic polymer was compared with linear polystyrene in order to investigate the effect of cyclic topology on preparing porous particles by emulsion templating methods. The contribution of cyclic topology to emulsification originates from the formation of hollow microspheres with the use of cyclic polymer while linear polymer only afforded solid microspheres. With addition of hexadecane as soft template, both cyclic polymer and linear polymer emulsions were successfully converted into porous particles. Superior to linear polymer, cyclic polymer enables the stabilization of emulsion droplets and the tuning of porous morphology. It is revealed that cyclic polymer with nanoring shape tends to assemble at the interfacial area, leading to the Pickering effect that decelerates the macrophase separation. Furthermore, the unique porous feature of polymer particles affords a convenient application for the detection of trace explosive.

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INTRODUCTION The importance of macromolecular architecture to material properties has stimulated research interest in exploiting new polymer topologies.1−4 In addition to linear polymers, the emergence of brushed polymers, branched polymers, and crosslinked polymers has expanded macromolecular topology from one dimension to two and three dimensions.5,6 Cyclic polymers without terminal groups have also come into focus owing to their distinct nanoring structures.7−10 This particular class of 2dimensional polymers could even be regarded as 2.5-dimensional materials if they are grafted with side polymer chains.11,12 In comparison with linear polymers at the same molecular weight, cyclic polymers have been demonstrated to possess different physical properties,13,14 such as reduced volume of fluid mechanics, higher glass transition temperature, and lower viscosity. Recent research involving several new discoveries suggested that cyclic polymers could also afford opportunities for creating advanced materials with unprecedented functions. Unlike the crawling way by which linear polymers pass through nanopores, cyclic polymers have to deform because of the absence of chain ends. Therefore, the cyclic topology could hamper the elimination of polymer carriers by the kidney, thus facilitating prolonged blood circulation and accumulation of drug at the tumor site.15,16 Cyclic polymers could act as host to restrict the intermolecular rotation of pendant fluorescent substrates, remarkably enhancing their emission intensity and extending the fluorescence lifetime.17−21 Self-assembly behavior © XXXX American Chemical Society

between cyclic polymers and linear polymers in aqueous solution was preliminarily compared, showing that cyclic topology had a positive effect to improve the thermal stability of nanoaggregates.22−25 With the development of polymeric methodology, especially “living” polymerization and “click” chemistry, the precise modulation of cyclic main chains and side grafting chains with particular excipients became possible.26−28 Yet the straightforward applications of cyclic polymers are still at an early stage to date. Methods and techniques for turning these macromolecules into substantial and scalable polymer materials are limited and urgently needed. In order to integrate macromolecules to be a macroscopic entity, self-assembly has received preferential attention due to its ability to control molecular arrangement.29−31 However, special excipients of polymers are required to ensure the formation of intermolecular interactions between macromolecules. Emulsion techniques afford great convenience for condensing macromolecules into macroscopic materials because of their simplicity and generality.32−34 Polymers dissolved in a dispersed phase can be rapidly solidified into microscopic particles with diverse morphologies upon solvent evaporation. The dispersed phase can be simultaneously loaded with a number of other Received: November 11, 2015 Revised: December 17, 2015

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°C for 16 h. The solution was exposed to air to stop the reaction and precipitated in methanol three times to obtain a pure cyclic polymer with PS side chains. The GPC characterization produced Mn = 706 800 g/mol and a PDI = 1.61. The absolute Mn was measured as 2380 kg/mol from SEC/LLS. Preparation of Oil-in-Water Emulsion Droplets and Polymer Microspheres. The CH2Cl2-in-H2O emulsion droplets were prepared via high-speed homogenization. First, a polymer solution (dissolved in CH2Cl2, 0.4 mL, 0.03 g/mL) was dispersed in a PVA aqueous solution (2.0 mL, 0.05 g/mL) under high-speed homogenization (120 s, 10 000 rpm, Fluko FA 25). As a result, an oil-in-water emulsion was obtained. Second, the emulsion droplets were heated in a water bath at 40 °C for 20 min to completely remove the organic solvent. Hollow microspheres were acquired when the polymer was C-PS2380k, while solid microspheres came from linear PS, including L-PS4k, L-PS230k, and LPS2000k. Third, the porous microspheres were washed with deionized water three times to remove the surfactant. Preparation of Porous Microspheres. Emulsion droplets were obtained by means of the same way as described above. Hexadecane (dissolved in CH2Cl2, v/v, 3.75%) was added to the aforementioned solution. C-PS2380k and L-PS2000k were selected to dissolve in CH2Cl2, respectively. Then the oil-in-water emulsion (0.25 mL) was transferred into ethanol solution (4.0 mL, v/v, 60%) to quickly extract the organic solvent. Finally, the porous microspheres were washed with pure ethanol three times to remove the hexadecane template and surfactant. Tuning Average Pore Diameter via Polymer Topologies and Rates of Removing Solvent. Emulsion droplets were acquired via the same way as described above, except for the addition of hexadecane in polymer solution (dissolved in CH2Cl2, v/v, 3.75%). For comparison, three polymers of C-PS2380k, L-PS230k, and L-PS4k were separately included in the emulsion. The solvent of the oil phase was removed at various speeds. The evaporation speed was controlled by bulk heating in a water bath at different temperatures, 40 °C, 60 °C, and 80 °C, respectively, or by extraction in ethanol solution at various concentrations, v/v, 20% ethanol, 40% ethanol, 60% ethanol, respectively. The solidified microspheres with porous morphology were washed with pure ethanol three times to completely remove the hexadecane template and surfactant. TNT Sensing Tests. Porphyrin was dissolved in polymer solution (dissolved in CH2Cl2, 2.50 mg/mL). Other steps including emulsification and the removal of solvent were the same as the preparation for the porous microspheres as stated above. The asprepared porous microspheres were adhered to paper by a double sticky tape to make a paper-supported sensor. Then the paper was inserted into a sealed fluorescent pool with solid TNT at the bottom. Fluorescence spectra were obtained at room temperature under an excitation wavelength of 420 nm.

components, rendering the opportunity to assign intriguing functions to the polymer particles.35−37 As far as we know, rarely has attention been paid to this emulsion methodology with the use of cyclic polymers. It is worth investigating the effect of cyclic topology on the thermodynamic stability of emulsions and morphology of polymer particles, which may be beneficial to expanding the application of cyclic polymers in material science. Herein we synthesized cyclic polymer with polystyrene side chains (C-PS) via ring-expansion metathesis polymerization (REMP) and copper(I)-catalyzed azide−alkyne click chemistry (CuAAC) following our previous publications.38−40 In order to emphasize the contribution of cyclic topology to the emulsification and particle morphology, linear polystyrene (LPS) was involved as a control material in this research. The Pickering effect from cyclic polymer allowed the formation of hollow microspheres as long as they condensed from emulsion droplets. Consequently, porous microspheres with tunable pore size were produced in a large scale by loading solvent template in the emulsion droplets. The cyclic polymer exhibited interfacial activity superior to that of linear polymers to stabilize interfaces and prevent phase coalescence. As a result, microspheres with smaller pores and larger specific surface area were obtained with the use of cyclic polymers.

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EXPERIMENTAL SECTION

Materials. Hexadecane, poly(vinyl alcohol) (PVA, 1788, Mw ∼ 22 000 g/mol), Nile red, copper(I) bromide (CuBr), β-alanine, 4(dimethylamino)pyridine (DMAP), N,N-dicyclohexylcarbodiimide (DCC), pentafluorophenol, propargylamine, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), ruthenium-alkylidene catalyst (UC-6), cis-5-norbornene-exo-2,3-dicarboxylic anhydride, and L-PS with Mn ∼ 4 kg/mol (L-PS4k), Mn ∼ 230 kg/mol (L-PS230k), and Mn ∼ 2000 kg/mol (L-PS2000) were purchased from Alfa Aesar and used as received unless otherwise noted. All organic solvents were provided by Beijing Chemical Reagent Company. Cyclic ruthenium-alkylidene catalyst (UC-6),38−40 monomer 1,39 and azide end-functionalized PS (PS-N3) side chains39,40 were synthesized according to our previous publication. Preparation of Cyclic Polymer Backbone with Alkyne Side Groups (Cyclic Poly-2). The preparation of Cyclic Poly-2 followed our published procedure.39,40 Monomer 1 (400.0 mg, 1.0 mmol) and UC-6 (3.6 mg, 5.0 × 10−3 mmol) were each dissolved in 3.0 mL of chloroform. After degassing through four freeze−evacuate−thaw cycles, the catalyst solution was added all at once to the monomer solution at room temperature under N2 while stirring vigorously. The polymerization was carried out in an oil bath at 55 °C for 12 h to produce Cyclic Poly-1. The GPC characterization produced Mn = 536 300 g/mol with a PDI of 1.57. After exposing the polymerization solution in air for 24 h at room temperature, propargylamine (109.6 mg, 1.99 mmol) in 7.0 mL of DMF was added to postfunctionalize Cyclic Poly-1 in situ. The reaction mixture was stirred at room temperature for 24 h. The resultant Cyclic Poly-2 with alkyne side groups was purified by dialysis from THF/DMF (4/1, v/v). The molecular weight cut-off of the dialysis membrane was 10 000 g/mol. The GPC characterization of Cyclic Poly-2 produced Mn = 628200 g/ mol and a PDI = 1.58. Preparation of Cyclic Polymer with PS Side Chains (C-PS). CPS was prepared by grafting PS-N3 side chains (Mn = 2290 g/mol, PDI = 1.06, degree of polymerization = 18) onto the Cyclic Poly-2 backbone by CuAAC chemistry. Specifically, Cyclic Poly-2 (130.0 mg, 0.48 mmol alkyne group), PS-N3 (860.2 mg, 0.43 mmol of azide group), and PMDETA (82.0 mg, 0.5 mmol) were dissolved in DMF (40.0 mL). The mixture was degassed by three freeze−evacuate−thaw cycles and backfilled with N2. After adding CuBr (68.0 mg, 0.48 mmol), the mixture was degassed by three evacuation cycles and sealed under vacuum. The reaction was carried out in an oil bath at 50

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RESULTS AND DISCUSSION Synthesis and Emulsification of Cyclic Polymers. Cyclic polymer with PS side chains was synthesized following the previous work.39,40 The detailed synthetic procedures are outlined in Figure 1. Cyclic Poly-1 with activated ester side groups was typically synthesized from REMP using compound

Figure 1. Synthesis of cyclic polymer with polystyrene side chains. B

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index between particle shell and interior (Figure 2e). The hollow interior of C-PS particles was further verified by scanning electron microscopy (SEM). As shown in Figure 2h− j, the cross-section was successfully viewed by opening the polymer particles. Notably, the C-PS particle possesses a hollow structure with a distinct shell that is distinguished from the LPS particle. Molecular weight contributed little to the formation of hollow morphology. As shown in Figure 2i,j, L-PS as control material condensed only into solid particles regardless of the molecular weight. C-PS presents a 1H NMR spectrum similar to that of polystyrene while the signal of the polynorbornene backbone is almost fully hidden. It is assumed that polynorbornene acts only as a cyclic backbone to increase the polymer size while its chemical performance can be neglected in emulsions or particles. Pickering Effect of Cyclic Topology in O/W Emulsions. Grafting the pendant chains of PS further increases the polymer size (Figure 3a). Unlike the linear polymers that are unfolded

1 as the monomer and UC-6 as the catalyst, respectively. By virtue of the efficient activated ester chemistry, Cyclic Poly-1 was then postfunctionalized by propargylamine to produce the Cyclic Poly-2 backbone with clickable alkyne side groups. Subsequently, the cyclic polymer with PS side chains was efficiently produced by CuAAC coupling Cyclic Poly-2 backbone and PS-N3 side chains. Emulsification is a choice to condense polymers into particular solid materials. The C-PS has solubility similar to that of L-PS, as its cyclic backbone has been fully screened by PS side chains, allowing it to be conveniently involved in emulsion system. Typical O/W single emulsions were prepared by dispersing the dichloromethane phase containing polymers into the water phase with the help of water-soluble surfactant poly(vinyl alcohol) (PVA). As shown in Figure 2b−d, the

Figure 3. Characterization of C-PS. (a) GPC results of Cyclic Poly-1 (black), Cyclic Poly-2 (blue), and C-PS (red) PS standards for calibration and DMF as eluent. (b) DLS plots of C-PS in CH2Cl2 (0.01 g/mL); the inset is an illustration of the shape of C-PS. (c) The 1 H NMR spectrum of C-PS in CDCl3.

and exist as random coils in good solvents, PS chains are constrained on a cyclic backbone, as illustrated in Figure 3b. CPS with a molecular weight of 2380 kg/mol generally acts like a nanoparticle in dichloromethane with an average hydrodynamic diameter of 21 nm. Regardless of the chemical contribution, the intrinsic cyclic topology can benefit the improvement of interfacial activity of C-PS in the emulsion system. The interfacial energy at the oil− water interface was ascertained by a pendant drop method (Figure 4a). An oil phase of dichloromethane with dissolved polymers was suspended in an aqueous solution. Interference with the interfacial energy does not occur upon the addition of L-PS, regardless of the molecular weight. However, the interfacial energy drops remarkably in the presence of C-PS. The interfacial energy between pure dichloromethane and water is 27.8 mN/m and drops to 18.3 mN/m at a C-PS concentration of 10 mg/mL. We attribute the interfacial activity of C-PS to its Pickering effect that routinely exists in emulsion systems with the use of nanoparticle emulsifiers.41−47 C-PS

Figure 2. (a) A schematic illustration of the formation of C-PS particles and L-PS particles via the emulsion method. Microscopic images of oil-in-water emulsions (b−d) and polymer particles (e−g) after the removal of dichloromethane at 40 °C. Scanning electron microscope images (h−j) of the interior of polymer particles. The polymers are C-PS (b, e, h), L-PS230k (c, f, i), and L-PS2000k (d, g, j), respectively. Poly(vinyl alcohol) (5.0 mg/mL) is used as emulsifier to improve the stability of emulsion droplets.

polymer has no remarkable effect on the formation of emulsion droplets. Under a given homogenization condition, the emulsion droplets with the disperse phase containing either C-PS or L-PS are almost the same. However, these emulsion droplets afforded different polymer particles upon the evaporation of dichloromethane. Microscopic examination under bright field suggested the hollow structure of C-PS particles unlike L-PS particles due to the different refractive C

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strengthened upon solvent evaporation owing to the increase in polymer concentration. C-PS polymers are mobile and reorganized at the early evaporation stage, indicating that the average particle size is smaller than the size of initial oil droplets. However, the polymers become entangled and tend to be immobilized at the interfacial area when the local concentration is high enough. They are solidified into a shelllike morphology after the complete removal of organic solvent. Emulsion templating L-PS cannot form hollow particles because the linear polymers are well dispersed in the oil droplet during the whole process of solvent evaporation. Loading Inert Solvents in the Emulsion for the Preparation of Porous Particles. An additional phase of hexadecane was loaded in the emulsion to introduce micropores in the polymer particles. Hexadecane is codissolved with polymer in the dichloromethane phase while it separates from polymer when dichloromethane is removed (Figure 6a). The

Figure 4. (a) A typical microscopic image of a dichloromethane droplet suspended on a microneedle for pendant drop measurement. (b) The interfacial energy between water and dichloromethane containing polymers measured by the pendant drop method.

rather than L-PS displaces the interface like the nanoparticle so that they are not easily detached once adsorbed at the interface. The intriguing interfacial performance allows C-PS to stabilize emulsion droplets even without the help of additional surfactants. As shown in Figure 5, dichloromethane and water

Figure 5. Optical photos of an oil−water mixture at a volume ratio of 1:1 before (a) and after (b) vigorous emulsification. Each oil phase contains a specific polymer and Nile red as a probing dye. The polymer concentration is 30.0 mg/mL. The microscopic images (c) and fluorescent images (d) of emulsions containing C-PS in the oil phase.

Figure 6. (a) The schematic diagram of phase separation between hexadecane and polymer upon the removal of dichloromethane. (b, d) The SEM observation of the surface and interior of C-PS particles. The insets are the detailed images at higher magnification. (c, e) The SEM observation of the surface and interior of L-PS particles with a molecular weight of 2000 kg/mol.

at a volume ratio of 1:1 were emulsified under vigorous shaking. The polymer and a probing dye Nile red had been added to the oil phase. The emulsion was only formed between dichloromethane and water in the presence of C-PS. Such a Pickering emulsion is kinetically stable, and no remarkable phase separation happens within 4 h. The microscopic study reveals that water is dispersed in the oil phase, suggesting that oilsoluble C-PS facilitates the formation of the W/O emulsion (Figure 5c,d). However, the control polymers with linear topology could not stabilize water droplets in the oil phase. The oil and water phases were completely separated in a few minutes once vigorous shaking stopped. The formation of hollow structure (Figure 2e,h) should also be attributed to the Pickering effect as a result of interfacial adsorption of C-PS. It is believed that the interfacial assembly of C-PS is gradually

phase separation undergoes the processes of isolation and coalescence of hexadecane droplets in the dispersed phase. The former relies on the evaporation of dichloromethane, and the latter can be hindered if the polymer is able to stabilize the hexadecane droplets. Both processes have great effect on the porous feature of polymer particles. Taking into account the Pickering effect of polymer topology, C-PS is superior to L-PS for preventing coalescence of hexadecane droplets in the emulsion droplets, leading to polymer microspheres with controllable porous morphology. As shown in Figure 6b, dichloromethane was extracted by ethanol from the emulsion droplets consisting of C-PS and hexadecane, leading to C-PS particles with open pores on the D

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Langmuir surface. An examination at the cross-section reveals that the uniform pores are well dispersed throughout the whole particle. In contrast, L-PS afforded porous particles with irregular porous morphology (Figure 6c,e). It is worth noting that the outer shell of C-PS particles is thicker than that of L-PS particles, indicating that C-PS stabilizes not only the inner polymer−hexadecane interface but also the outer water−oil interface. A group of controlled trials was implemented to confirm the importance of hexadecane for generating pores in C-PS particles. As stated above, C-PS particles having a hollow interior were acquired without addition of hexadecane (Figure 2h and Figure 7g). As shown in Figure 7a, the hollow interior is

Figure 8. (a) Schematic illustration of the isolation and coalescence of the hexadecane phase in the emulsion droplets and the dependence on the removal of dichloromethane. SEM images of C-PS particles (a1− a6), L-PS particles with molecular weight of 230 kg/mol (b1−b6) and 4 kg/mol (c1−c6). 1−3 refer to the condition of removing dichloromethane in ethanol solution at volume concentrations of 60 v/v%, 40 v/v%, and 20 v/v%, respectively. 4−6 refer to the condition of removing dichloromethane under thermal treatment at temperatures of 80 °C, 60 °C, and 40 °C, respectively. The initial polymer concentration is 30.0 mg/mL. Scale bar: 10 μm; insert scale bar: 2 μm. Figure 7. Microscopic images and SEM images of C-PS particle template with hexadecane after solidification by diffusing dichloromethane into the ethanol solution (60 v/v%). The volume concentration of hexadecane in oil phase is (a, d, g) 0 v/v%, (b, e, h) 3.75 v/v%, (c, f, i) 7.50 v/v%, respectively.

believed that the cyclic topology improves the stability of hexadecane droplets owing to the outstanding Pickering effect. L-PS of high molecular weight also produced well-dispersed pores though the pore size is larger than that of C-PS particles (Figure 8b1). It is assumed that the entanglement of polymer chains upon the removal of dichloromethane prevents the coalescence of hexadecane droplets. However, L-PS of low molecular weight has poor entanglement that can still relax when dichloromethane is mostly removed. Phase separation between polymer and hexadecane is remarkable in that nonuniform pores with large size are formed in the polymer particles (Figure 8c1). The coalescence of hexadecane is enhanced when the extraction rate of dichloromethane is decreased with the use of less ethanol. The pore size of C-PS particles is increased while the number of open pores on the surface becomes less. Given a longer time for the removal of dichloromethane, C-PS could reorganize at the interface due to its high interfacial activity that is tailored to the close proximity of surface pores. L-PS particles of high molecular weight retained porous structure without control over particle shape. The porous morphology became worse and even disappeared within the L-PS particles of low molecular weight. The removal of dichloromethane could be significantly slowed via thermal evaporation. For example, the solvent evaporation lasted 20 min at a heating temperature of 40 °C, offering enough time for polymer relaxation and hexadecane coalescence. Only a few hexadecane droplets remained in the C-PS particles as evident from the porous structure shown in Figure 8a6. For L-PS, the polymer and hexadecane were fully phase separated and

distinguished from the particle shell under microscopic observation. In the presence of 3.75 v/v% hexadecane in the emulsion, small holes appeared on the surface and in the interior of C-PS particles, as shown in Figure 7e,h. The particles became dark in the bright field owing to the light scattering by the inner pores (Figure 7b). Upon addition of more hexadecane (7.50 v/v%), the size of the surface pores and inner pores increased, which should be attributed to the increasing volume of soft templates in the emulsion (Figure 7f,i). The porous morphology is decided by the thermodynamic isolation and coalescence of hexadecane droplets in the emulsion droplets. It is assumed that the two processes can be tuned by changing the removal rate of dichloromethane from emulsions in addition to the interfacial stability. As summarized in Figure 8, the organic solvent was extracted from emulsion droplets at various rates. The polymer is likely immobilized as a result of the quick release of dichloromethane when the emulsion was exposed to the solvent mixture of ethanol and water. At a high concentration of ethanol (60 v/v %), hexadecane rapidly separates out while their coalescence within the polymer matrix is hampered. The hexadecane droplets serve as templates for the formation of well-distributed micropores in polymer particles. Notably, the porous structure in the interior of C-PS particles is uniform (Figure 8a1). It is E

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Langmuir hexadecane coalesced into a single droplet, resulting in a hollow microstructure (Figure 8b6,c6). The average pore size of polymer particles is summarized in Figure 9. Under a given condition for removal of dichloro-

Figure 9. Statistical summary of average pore size of polymer particles with control over phase separation and coalescence.

methane, C-PS emulsion affords polymer particles with smaller pore size in comparison with L-PS. The Pickering effect of cyclic topology improves the interfacial activity of cyclic polymer that benefits the dispersion of hexadecane templates in the polymer matrix. The thermodynamic removal of dichloromethane is also important to regulate the phase separation between polymer and hexadecane, which routinely determines the porous morphology. With appropriate control of phase separation, a uniform porous structure with an average pore size of 450 nm has been successfully obtained with the choice of cyclic-polymer emulsion (Figure 8a1). Cyclic polymer particles are considered a family of promising building units for new functional materials. Loading Porphyrin in Porous C-PS Particles for Sensing Detection of Explosive. The high throughput of emulsion templating fabrication renders the possibility to serve porous C-PS particles as macroscopic materials. Functional agents miscible with C-PS can be loaded in the porous particles by codissolving them in the initial emulsion droplets, thus endowing the particles with particular functions. Porphyrin with fluorescent performance was loaded into porous particles for sensing detection of explosive based on fluorescence quenching. C-PS particles are expected to have better sensing function than L-PS particles owing to their distinct porous morphology. As shown in Figure 10, both C-PS and L-PS particles can efficiently load porphyrin by codissolving porphyrin in the dichloromethane phase. Porphyrin is almost entirely encapsulated in polymer particles after the removal of dichloromethane owing to its poor solubility in water. The particles exhibit red color under microscopic observation in a green channel. At an excitation wavelength of 420 nm, a strong emission within the visible window (maximum emission ∼650 nm) was harvested from polymer particles. The fluorescence of the porphyrin can be quenched if it touches trinitrotoluene (TNT), an explosive for military use. Such fluorescent quenching is attributed to the electron transfer that happens from the excited porphyrin to the electron acceptor of TNT. As shown in Figure 10c, the fluorescent intensity of porous C-PS particles quickly drops when the particles are exposed to saturated TNT vapor (10 ppb at room temperature and 1 atm). The graph insert shows that fluorescent intensity decreased 8.8% upon TNT exposure for

Figure 10. Sensing detection of TNT based on fluorescence quenching of porphyrin-loaded porous particles. The cross-section SEM view and fluorescent images of L-PS particles (a) and C-PS particles (b) The initial concentrations of polymer and porphyrin are 30.0 and 2.5 mg/mL, respectively. (c) Time-dependent fluorescent spectra of porphyrin-containing C-PS particles upon exposure to TNT vapor. The graph insert is a plot of fluorescent quenching against exposure time. (d) The comparison of sensing tests when exposed to TNT vapor. The evaluation of specific surface area of porous L-PS (e) and C-PS particles (f) on a mercury injection apparatus.

15 s, and more than 40% decrease was achieved within 10 min. Porous L-PS particles were compared, and they exhibited slower response to TNT (Figure 10d). The change in their fluorescent intensity was always less than that of porous C-PS particles at a given exposure time. For example, the fluorescent intensity of porous L-PS particles decreased only 0.7% at an exposure time of 15 s. Porous C-PS particles have a significant advantage in detecting TNT because of excellent porous structure. According to SEM observation of the cross-section, C-PS particles possess smaller pores in comparison with L-PS particles, which may lead to higher specific surface area. The specific surface areas of C-PS and L-PS particles were typically analyzed on a mercury injection apparatus and were 82.0 m2/g and 18.0 m2/g, respectively. The higher specific surface area of C-PS particles provides more opportunity to improve fluorescent quenching. It is worth noting that the inner pores of C-PS particles are interconnected with “window” pores (Figure 10b). In comparison, the solidification of L-PS emulsion afforded closed-cell structures with isolated pores (Figure 10a). The open-cell microporous structure with “window” pores may improve the mass transport of TNT within porous C-PS particles. The formation of “window” pores is a result of the shrinkage of the polymer−hexadecane interfacial domain after the removal of dichloromethane. It is hypothesized that these “window” pores mainly arise from the Pickering effect of C-PS that is a positive factor for thinning the interfacial domain. It should also be noted that a peak ascribed to mesopores (∼13 nm) exists in the pore size distribution (Figure 10f). We speculate that these mesopores are derived F

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Langmuir

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from the nanoring topology of C-PS. The combination of mesoporous structures may be another positive factor to enhance the detection sensitivity.48−50

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CONCLUSION In this article, we demonstrated an efficient way to condense cyclic polymer with polystyrene side chains into porous particles via the emulsion method. Our results reveal that the cyclic topology leads to a Pickering effect and is helpful for improving the interfacial stability. Various porous morphologies were introduced in polymer particles through the choice of a soft template. We showed that the phase separation and coalescence of template solvents can be controlled by polymer topology as well as the removal rate of oil phase solvent. Loading fluorescent porphyrin endows the porous polymer particles with a function of sensing detection of TNT. Porous C-PS particles with larger specific surface area and open-cell structures exhibited better sensitivity in comparison with porous L-PS particles. Our research represents a pioneering study for understanding the topology effect of polymer building blocks on emulsion systems. We believe that more cyclic polymers with asymmetric structures or polymers with other particular topologies can be combined within emulsion systems. It is also envisioned that the emulsion approach can be readily extended to hybrid functional units within the cyclic polymer matrix, generating cyclic polymer materials with fascinating properties and functions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51373197, 21422407, 21374122), Program for New Century Excellent Talents in University by the State Education Commission (NCET-120530).

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b04159 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b04159 Langmuir XXXX, XXX, XXX−XXX