Nonspherical Nanoparticles with Controlled Morphologies via Seeded

Mar 22, 2015 - This work reports a facile novel approach to prepare asymmetric poly(vinylidene fluoride)/polystyrene (PVDF/PS) composite latex particl...
2 downloads 11 Views 3MB Size
Download PDF
Article pubs.acs.org/Langmuir

Nonspherical Nanoparticles with Controlled Morphologies via Seeded Surface-Initiated Single Electron Transfer Radical Polymerization in Soap-Free Emulsion Jinfeng Yuan,† Lixia Wang,† Lei Zhu,‡ Mingwang Pan,*,† Wenjie Wang,† Ying Liu,† and Gang Liu† †

Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States

‡

ABSTRACT: This work reports a facile novel approach to prepare asymmetric poly(vinylidene fluoride)/polystyrene (PVDF/PS) composite latex particles with controllable morphologies using one-step soap-free seeded emulsion polymerization, i.e., surface-initiated single electron transfer radical polymerization (SET-RP) of styrene (St) at the surface of PVDF seed particles. It was observed that the morphology was influenced mainly by the St/PVDF feed ratio, the polymerization temperature, and the length of the catalyst Cu(0) wire (Φ 1.00 mm). When the feed ratio was St/PVDF = 5.0 g/1.0 g, snowman-like Janus particles were exclusively obtained. Raspberry-like and popcorn-like composite particles were observed at a higher reaction temperature or a shorter length of the catalyst wire. The reaction kinetics plots demonstrated some unique features. The formation of nonspherical composite nanoparticles can be ascribed to the surface nucleation of PS bulges following the SET-RP. like Janus particles with pH-responsive properties were finally obtained. Most of the experimental methods to prepare anisotropic particles via the above work is relatively complex. Among them, although conventional seeded emulsion polymerization has the advantages of being simple and easy to scale up, it is not “living” and cannot be used to control molecular weight and to synthesize block or graft copolymers. In this work, single electron transfer radical polymerization (SET-RP) is used to prepare asymmetric particles in the presence of seed particle. Compared with other living radical polymerization methods such as ATRP, the SET-RP method generally displays its advantages, including mild reaction conditions, less amount and ease in removal of catalyst, and the lack of coloration of the product.23−27 These superior attributes have made the SET-RP a preferable approach for the synthesis of homopolymers,28,29 block copolymers,30 dendritic macromolecules,31 and polymer with complex architecture.32 A variety of monomers, including acrylates, methacrylates, and vinyl halides, have been used in SET-RP, while the polymerization of styrene (St) in the presence of seed latex is rarely reported in the literature.26 In present study, the feasibility of SET-RP for the seeded emulsion polymerization of styrene in the presence of PVDF latex particles is investigated with objectives to control the molecular weight of PS and the morphology of composite

1. INTRODUCTION Nonspherical colloidal particles with asymmetric structures and different chemical functionalities have been widely researched because of their development prospects as functional surfactants,1,2 compatibilizers,3 coatings,4 and catalysts.5 To date, various methods for the syntheses of asymmetric particles have been proposed, including microfluidic technique,6 toposelective modification,7 Pickering emulsion polymerization,8 seeded emulsion polymerization,9 self-assembly,10 controlled phase separation,11,12 and surface nucleation.13 Among these, seeded emulsion polymerization has been proved to be efficient and flexible to prepare snowman-like,14 raspberry-like,15 and other nonspherical composite particles.16 From the perspective of polymerization mechanism, previous work in the field shows that conventional free radical polymerization, 17 atom transfer radical polymerization (ATRP),18 and reversible addition−fragmentation chain transfer polymerization (RAFT)19 have been commonly used to prepare anisotropic particles. These techniques possess their own advantages and disadvantages. Liu et al. have successfully synthesized Janus particles by surface-initiated ATRP at Pickering emulsion interface from initiator groups fixed on one side of particles surface.20 Okubo et al. described an extension of the method by Liu et al. to prepare Janus particles, which did not need any interfaces for masking particles.21,22 In Okubo’ work, the spherical Janus particles with ATRP initiator at one-half of the surface as macroinitiator relied on internal phase separation generated by solvent evaporation. Mushroom© XXXX American Chemical Society

Received: January 13, 2015 Revised: March 13, 2015

A

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 1. SEM micrographs of PVDF/PS composite particles at the polymerization time of (A) 0 , (B) 1, (C) 2, (D) 3, (E) 4, and (F) 5 h when the St/PVDF feed ratio being 1.0 g/1.0 g. The polymerization temperature was 25 °C, and the length of the Cu(0) wire was 1 cm. Shengmiao Fine Chemical Co. Ltd.) were activated by hydrochloric acid (8 wt %) before polymerization. Preparation of PVDF/PS Janus Colloidal Particles. The PVDF/PS composite particles were prepared by single electron transfer radical polymerization (SET-RP) at the surface of dialyzed PVDF latex seeds in soap-free seeded emulsion system. All experiments were carried out in a 100 mL four-necked round-bottom flask equipped with a Teflon mechanical stirring blade, a condenser, a thermometer, and a nitrogen gas inlet. First, 6.67 g of dialyzed PVDF latex dispersion with a solid content of 15.0 wt % was added directly into the four-necked round-bottom flask at ambient temperature. The content was ultrasonicated for 40 min to avoid the agglomeration of any colloidal particle. Then 1.0 g (or 0.5, 3.0, and 5.0 g) of purified St monomer containing the initiator (MBP, 2 wt % of the St monomer) was added into the dispersion containing 1.0 g of PVDF seed particles. After that, the mixture was placed in a water bath thermostated at 25 °C (or 35 and 40 °C) with stirring. Meanwhile, the side arm of the flask was purged with nitrogen during the entire reaction process to remove O2. The mixture was stirred for 1.5 h to allow for St and MBP initiator to swell the surfactant-stabilized PVDF particles. Subsequently the ligand (PMDETA, 2 wt % of the St monomer) and 1 cm (or 0.5 and 0.15 cm) long Cu(0) wire or Cu powder (13 wt % of the St monomer) were added into the system. Then the system was deoxygenated with five pump−nitrogen aeration cycles. The polymerization was continued for 5 h under a nitrogen atmosphere and stopped by taking the Cu(0) wire or Cu powder out of the flask. Sample aliquots were taken for both morphology observation and determination of the St conversion gravimetrically. The final product was dried in a vacuum desiccator for 2 days and a vacuum oven at 50 °C for 3 days until a constant weight was reached. Then, the St conversion could be calculated based on the known weight percentage of PVDF and the obtained solid content. Characterization. Morphology of the PVDF/PS composite particles was characterized by a scanning electron microscope (SEM, Nano 450 at 10 kV, FEI, USA) and a transmission electron microscope (TEM, H-7650B at 80 kV, Hitachi, Japan). To prepare the samples for SEM and TEM, a drop of PVDF/PS latex was diluted into deionized water to obtain a translucent suspension and was ultrasonicated for 30 min. For SEM observation, a drop of the suspension was cast onto a conductive silicon wafer and sputter-coated with Pt after being dried in air. For TEM observation, a drop of the suspension was drop-cast onto

particles. Although there is a rough control of the numberaverage molecular weight (M n), the molecular weight distribution (Mw/Mn, where Mw is the weight-average molecular weight) is relatively broad, indicating certain deviation from living or controlled radical polymerization. Similar to conventional seeded emulsion polymerization reported before,11−13 asymmetric poly(vinylidene fluoride)/ polystyrene (PVDF/PS) composite particles with various morphologies are observed. By systematically changing the St/PVDF feed ratio, reaction temperature, and length of the catalyst Cu(0) wire, the viscosity of polymerization loci and the chance of collision between initiators and the Cu(0) catalyst can be affected. As a consequence, various morphologies of PVDF/PS composite particles, such as raspberry-like and snowman-like, are obtained. These asymmetric fluoropolymer particles may find broad prospects for development of selfcleaning, weather/chemical resistance, and tough coatings. In addition, due to the reversible termination nature of the chain ends in SET-RP, it is possible to synthesize block copolymers if a second comonomer is added at a later stage. This research is currently underway.

2. EXPERIMENTAL SECTION Materials and Treatment. Styrene (St, Tianjin Chemical Reagent Co., Ltd.) was distilled under elevated temperature (65 °C) and reduced pressure to remove inhibitors. Purified St monomer was stored in refridgerator (5 °C) prior to use. Poly(vinylidene fluoride) (PVDF) latex, Kynar Latex 32, was kindly supplied by Arkema Inc. The PVDF latex, before using, was dialyzed for 14 days by a dialysis tube with a molecular weight cutoff (MWCO) of 7500 Da (Spectrum Laboratories, Inc.). The aqueous solution in the dialysis beaker was substituted by clean deionized water each day to remove free surfactants as much as possible. At last, the solid content of the dialyzed PVDF latex was measured gravimetrically. Methyl 2bromopropionate (MBP, above 98 wt %) and N,N,N′,N″,N″pentamethyldiethylenetriamine (PMDETA, above 98 wt %) were purchased from J&K Chemical Reagent Co. and used as received. Cu(0) wires (Φ 1.00 mm) and Cu powder (40 μm) (from Tianjin B

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir a 400-mesh copper grid coated with carbon and stained by osmium tetroxide (OsO4) for 3 h at room temperature. The size distributions of PVDF seed particles and PVDF/PS particles in aqueous dispersions were analyzed with Zeta-Sizer 90 type of dynamic laser scattering particle size analyzer (DLS, Malvern, England). The wavelength of incident light was 532 nm, and the scattering angle was 90°. Fourier transform infrared (FTIR) spectra were recorded with a Bruker Tensor-27 spectrometer using the pressed KBr pellets. GPC analysis of the polymer samples were performed on a Polymer Laboratories (PL) Series 220 high temperature chromatograph, equipped with an column oven and two PL gel columns (10 μm, mixed bed, 300 × 7.5 mm). THF (HPLC-grade; Kermel) was used as an eluent at a flow rate of 1 mL/min. The molecular weights (MW) and number-average molecular weight (Mn) were determined using PS standards. The samples preparation was as follows. The PVDF/PS composites were Soxhlet’s extracted using toluene for 36 h. The dissolved PS was precipitated in methanol. The obtained PS was dried in vacuum for 3 days before GPC measurements.

Table 1. Polymerization Recipe for Preparation of PVDF/PS Particles sample no.

St [g]

PVDF [g]

reaction time [h]

Dpa [nm]

PDIb

1 2 3 4 5 6 7 8

1.0 1.0 1.0 1.0 1.0 0.5 3.0 5.0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1 2 3 4 5 5 5 5

239.1 247.4 250.1 262.0 264.3 252.9 295.8 271.6

0.017 0.016 0.041 0.011 0.038 0.067 0.381 0.007

a

Dp means sizes of composite particles determined by DLS technique. PDI is polydispersity index of particle sizes measured by DLS technique.

b

3. RESULTS AND DISCUSSION In this work, morphology evolution of PVDF/PS composite particles during SET-RP was first studied by SEM for samples at preset time intervals. Here, we fixed the St/PVDF weight ratio as 1.0 g/1.0 g and total reaction time as 5 h. The polymerization temperature was 25 °C, and the length of the Cu(0) wire was 1 cm (1.0 mm diameter). The specific preparation procedure has been described in the Experimental Section. The morphology development of PVDF/PS composite particles at different times is shown in Figure 1. In the beginning, a regular spherical shape is seen for the seed particles (average diameter ∼193 nm). At 1 h, a number of fuzzy protrusions were nucleated at the surface of PVDF seeds (∼212 nm, see Figure 1B). Here, the bulge phase was proved to be PS (see results in Figure 3).13 When the polymerization time reached 2 h, it was clearly observed in Figure 1C that the bulges on the surface of seed particles became larger (∼225 nm for the average size along the particle long axes). After reaction for 3−4 h, it was noted that the number of bulges reduced to 2−4 (see Figure 1D,E). Finally, at 5 h (Figure 1F) the bulges became bigger; however, the composite particles (about 262 nm in size) remained nonuniform in shape. On the basis of the above result, we propose that the oilsoluble initiator will decompose into primary free radicals to initiate the polymerization when the particles collide with the catalyst Cu(0) wires under stirring.33 The polymerized PS, which is still swollen with St monomer, eventually nucleates into bulges on the seed surface (Figure 1B). As the polymerization proceeds, the multiple PS/St bulges grow larger through absorbing the St monomer surrounding the particle and start to fuse together in order to diminish the free energy of the system (see Figure 1D−F). To further investigate overall grown situation of the composite particles during the polymerization, the average size and size distributions of PVDF seeds and PVDF/PS composite particles were analyzed by DLS. The results are shown in Table 1 (no. 1−5) and Figure 2. The Z-average particle size of the PVDF seeds was determined to be ∼220 nm, and the PDI was 0.014. When the average size of PVDF/PS composite particles increased to 264 nm at 5 h of polymerization, the size distribution became a little bit broader (Figure 2). Note that these resultant nonspherical composite particles are asymmetric in shape compared with the spherical seed particles, and thus there

Figure 2. Size distributions of (a) PVDF seeds and (b) PVDF/PS particles. The St/PVDF feed ratio was 1.0 g/1.0 g. The polymerization time was 5 h. The polymerization temperature was 25 °C, and the length of the Cu(0) wire was 1 cm.

might be some error in measurement of hydrodynamic dimension. As we can see in Table 1 (no. 1−5), the average sizes of PVDF/PS particles increased with the polymerization time, and the size distribution still showed a single peak until 5 h. In addition, the mean particle size and PDI of the composite particles obtained at different St/PVDF feed ratios are also summarized in Table 1, no. 5−8. The smallest particle size was observed when the St/PVDF feed ratio of 0.5 g/1.0 g, the least amount of St monomer, was used in the polymerization. When the St/PVDF feed ratio was 3.0 g/1.0 g, the average particle size was largest and the PDI value was relatively high. The reason for the high PDI may be attributed to the fact that the morphology of the resulting composite particles was most irregular and asymmetric (see Figure 6C,G). It should be mentioned here that the average sizes of the PVDF/PS particles in Table 1 were inconsistent with the results from SEM and TEM photographs. This is because the composite particles in Figures 1 and 6 were observed by SEM and TEM in a dry state. However, those in Table 1 were measured in distilled water with unreacted monomer swelling in the latex particles, obviously demonstrating larger hydrate sizes. To confirm that the newly grown bulge in the composite particles is PS phase, Figure 3A−C shows all SEM photographs for PVDF seeds, PVDF/PS composite particles, and the remainder particles after Soxhlet extraction with toluene for 36 h. The remainder particles showed a spherical shape (Figure C

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. (A) SEM micrograph and (D) FTIR spectrum of PVDF seed particles. (B) SEM micrograph and (E) FTIR spectrum of PVDF/PS composite particles. (C) SEM micrograph and (F) FTIR spectrum of the remainder particles after Soxhlet extraction by toluene for 36 h. The experiment conditions for preparing the PVDF/PS particles: St/PVDF feed ratio 1.0 g/1.0 g, the polymerization temperature 25 °C, and the length of the Cu(0) wire 1 cm.

Figure 4. Kinetics plots (a) and Mn and Mw/Mn versus conversion (b) of the PVDF/PS composite particles at the St/PVDF feed ratio of 1.0 g/1.0 g. The polymerization temperature was 25 °C, and the length of the Cu(0) wire was 1 cm.

that the bulge phases on the PVDF/PS composite particles are the PS phase. The absorption peaks in Figure 3F are almost the same as those in Figure 3D. The characteristic peaks of the benzene ring at 1450−1600 and 699 cm−1 all disappear, which implies that these particles are pure PVDF without PS. Obviously, the adhesion force between the PVDF core and the PS bulges is physical, not chemical grafting. The band at around 3440 cm−1 is assigned to the −OH absorption of trace amount of water in KBr and during the sample preparation step. The SET-RP method allows for great superiority in reaction rate tuning and recyclability of catalyst, and SET-RP kinetics of St monomer in the presence of PVDF latex seed should have different character. Then the reaction kinetics with the above standard experimental recipe are studied herein. Figure 4a shows the St conversion and ln([M]0/[M]) as a function of polymerization time, where [M]0 is the initial concentration of St monomer and [M] is instant St monomer concentration.

3C), and the size was close to that of PVDF seed particles (Figure 3A), indicating that the removed bulges were PS phase because PVDF phase cannot be dissolved by toulene.13 FTIR spectra of PVDF seeds, PVDF/PS composite particles, and the remainder particles are shown in Figure 3D−F. The peaks at 3027 and 2979 cm−1 in Figure 3D can be atttibuted to stretching vibrations (νC−H) of PVDF, and the characteristic peak at 1417 cm−1 is caused by the C−H bending vibrations of PVDF. The band at 1185 cm−1 can be assigned to the stretching vibration (νC−F) of PVDF. The strong absorption band at 883 cm−1 is attributed to the C−C skeletal vibration of PVDF. The sharp peaks at 841, 758, and 614 cm−1 can be assigned to vibration absorption from crystalline PVDF phases. The absorption peaks at 699 and 758 cm−1 in Figure 3E can be assigned to flexural vibrations (γC−H) of benzene ring, and the bands at 1450, 1492, and 1604 cm−1 can be assigned to the benzene ring vibrations (CC) in PS. The characteristic peak at 3024 cm−1 can be attributed to stretching vibrations (νC−H) of unsaturated C−H groups in benzene ring, which indicates D

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir The conversion increases with an extension of polymerization time. The dependence of ln([M]0/[M]) vs polymerization time shows an approximate line with an induction period of ca. 1 h. In the beginning, the dialyzed PVDF seeds were dispersed in the reaction mixture, and St monomer needed to diffused onto the surface of the PVDF seeds. The ligand was soluble in water, but the initiator was oil-soluble. Thus, the chain initiation should mainly happen at the interface between absorbed St and water. So, it took a period of time for the initiator, ligand, catalyst, and monomer to meet at the interface (see the formation mechanism below). Therefore, it is reasonable to observe an induction period in the kinetic analysis plot.34 Figure 4b shows nonlinear Mn vs conversion and dependence of Mw/Mn vs conversion. These results indicate that the polymerization deviates from the living nature. This can be explained by the following. Generally speaking, a living polymerization requires fast initiation (chains start at the same time), no termination, and slow or controlled growth. Deviation from any of these three requirements will not guarantee the living nature. In our SET seeded emulsion polymerization, initiation is realized by collisions of the monomer-bearing PVDF latex particles with the Cu(0) wires due to the heterogeneous distribution of monomers (i.e., on the surface of PVDF latex particles). In addition, the same latex particle can collide with Cu(0) wires multiple times (this is inferred from the observation of PVDF latex particles with multiple PS bulges). Apparently, these collisions cannot all happen at the same time. This is different from conventional SET-RP, where the halogenated species can “collide” with Cu(0) nearly at the same time due to the homogeneous distribution of the monomer, the halogenated initiator, and the ligand in the polymerization solution. As a consequence, the molecular weight distribution of PS is fairly broad in the beginning of polymerization. As the polymerization proceeds, the molecular weight distribution gradually decreases because the differences in the initiation stage become less significant as compared to the ultimate high molecular weight. This is exactly seen in the experimental results shown in Figures 4b and 5. When the PVDF/PS composite particles were extracted by toluene, those dissolved protrusions were the PS domains because PS can be dissolved by toluene but PVDF cannot. GPC traces of the extracted PS are shown in Figure 5. It is observed from the data in Figure 5 that the molecular weights of the PS

increased with the polymerization time, while the PDI values decreased from 6.74 to a relatively low value of 1.57. The high PDI values clearly indicate that the polymerization is not living. This reason has been explained as above. In our previous study,13 we conclude that the feed ratio of reactive monomer versus seeded polymer was important to the morphology of composite particles. In this work, we also studied the influence of the St/PVDF feed ratio on the morphology evolution of PVDF/PS particles produced by SETRP. It is necessary to mention that the composite particles in the following TEM images were stained with OsO4 for improving the contrast. Herein, PVDF is denser than PS, and the PVDF seed has a larger diameter as compared with the newly grown PS bulges. As a result, the PVDF phase was discerned as the darker area while the PS as the gray area, as shown in Figure 6. On the basis of the aforementioned standard formula, we changed the St/PVDF feed ratio from 0.5 g/1.0 g to 3.0 g/1.0 and 5.0 g/1.0 g. At the same time, other experiment conditions were kept the same. The corresponding SEM and TEM micrographs are shown in Figure 6. When the feed ratio was low (i.e., 0.5 g/1.0 g), the resulting morphology showed small but more bulges with a raspberry-like morphology (Figure 6A,E). This is largely because there is not enough St supply to lower the viscosity of the PS/St bulges and improve their mobility on the seed surface, so the newly nucleated bulges will develop independently. When the feed ratio was 1.0 g/1.0 g as mentioned above, composite particles with two, three, or four bulges are seen in Figure 6B,F (see arrows in Figure 6B). With increasing the feed ratio to 3.0 g/1.0 g, in addition to one large PS bulge, one or two new small PS bulges further nucleated on the exposed surface of the seed particle and the whole particle shape became somewhat irregular (see arrows in Figure 6C,G). Further increasing the feed ratio to 5.0 g/1.0 g, most composite particles exhibited only one PS bulge (see Figure 6D,H), showing uniform snowman-like morphology. These composite particles could be termed as Janus particles. In this case, the amount of monomer is adequate; then it is easy for the multiple PS/St bulges to fuse into one single protrusion because of the low viscosity of the bulges. And meanwhile, it can reduce the unfavorable surface tension. More detailed formation mechanism of the PVDF/PS composite particles is discussed later (see Figure 9). To study the influence of polymerization temperature on the morphology of composite particles, we fixed the feed ratio, reaction time, and the length of the Cu(0) wire in accordance with the standard experimental conditions but varied the reaction temperature. When the temperature increased to 35 °C, the SEM and TEM photographs of the composite particles exhibited more PS protrusions on the seed particle (see Figure 7B,E). Further increasing the temperature to 40 °C, the composite particles (Figure 7C,F) had similar morphology as that polymerized at 35 °C, but with different details. From these images, it was observed that the PS bulges on the seed particle were more and smaller. From the point of mechanism of SET-RP, the initiation step is mediated by an outer-sphere electron transfer (OSET) from Cu(0) electron donor to acceptor MBP initiator. Subsequently, the radical anion [R-Br]•− is decomposed into the electrophilic (R•) radical and the anion (Br‑), and the Cu(I) generated in situ instantaneously disproportionates into Cu(0) and Cu(II) species in the presence of H2O and ligand PMDETA. And then the oxidation and reduction reaction between R• and Cu(II) generates Cu(I)

Figure 5. GPC traces of the PS extracted by toluene for PVDF/PS composite particles at different polymerization times. The St/PVDF feed ratio was 1.0 g/1.0 g. The polymerization temperature was 25 °C, and the length of the Cu(0) wire was 1 cm. E

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 6. SEM (A, B, C, D) and TEM (E, F, G, H) micrographs of PVDF/PS composite particles at the St/PVDF feed ratio of (A, E) 0.5 g/1.0 g, (B, F) 1.0 g/1.0 g, (C, G) 3.0 g/1.0 g, and (D, H) 5.0 g/1.0 g when the polymerization time being 5 h. The polymerization temperature was 25 °C, and the length of the Cu(0) wire was 1 cm.

Figure 7. SEM (A−C) and TEM (D−F) micrographs of PVDF/PS composite particles at the polymerization temperature of (A, D) 25, (B, E) 35, and (C,F) 40 °C when the St/PVDF feed ratio being 1.0 g/1.0 g. The polymerization time was 5 h, and the length of the Cu(0) wire was 1 cm.

and dormant species Pn-Br while R• initiates St monomer. The formation of active radicals via heterolytic cleavage of C−X ([R--Br]•−), which is a thermopositive process, requires much lower bond dissociation energy.23,35 Room temperature is enough to assist this process. When the temperature increases, the mobility of the radicals is enhanced to result in higher collision rate. As a result, the nucleating sites became more and the bulges developed quickly without merging before quick consumption of the monomer. Eventually, raspberry-like composite particles can be obtained at higher temperatures.

Finally, the effect of the length of the catalyst Cu(0) wire (Φ 1.00 mm) on the morphology of composite particles was studied, but the total amount of the catalyst wire was kept the same. When the length decreased from 1 to 0.5 cm, popcornlike composite particles (Figure 8B) were seen by SEM. A similar flower-like morphology (Figure 8C) was observed when 0.15 cm long Cu(0) wire was used. Activated Cu powder (40 μm) was also tried for comparison; however, the morphology (Figure 8D) was also similar to that in Figure 8C for the 0.15 cm long Cu(0) wire. Because of the mechanism of SET-RP and the surface nucleation of PS/St bulges, the collision between F

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 8. SEM micrographs of PVDF/PS composite particles at the length of the Cu(0) wire (A) 1, (B) 0.5, (C) 0.15, and (D) 40 μm (Cu powder) when the St/PVDF feed ratio being 1.0 g/1.0 g. The polymerization time was 5 h, and the polymerization temperature was 25 °C.

Figure 9. Schematic morphology evolution of the PVDF/PS composite particles via SET-RP in soap-free emulsion polymerization.

the monomer swollen seed particles and the catalyst Cu(0) wire was crucial to the seeded polymerization and final particle morphology.34 When the Cu(0) wire is cut into small pieces, the collision frequency between the seed particles and the catalyst wires is increased. Therefore, many more protrusions can be nucleated on the seed surface. For the Cu powder, the dispersity was not good and particles are easy to aggregate. As a result, no significant change in morphology is observed (Figure 8D). It is worth mentioning that the Cu powder for catalysis was difficult to remove at the postprocessing stage. On the basis of the above experimental results, we present a formation mechanism of the composite particles as shown in Figure 9. In the St-swelling stage before SET-RP, the PVDF latex particles are stabilized by surfactants on the seed surface. As we all know, the oil-soluble St is highly immiscible with semicrystalline PVDF. Thus, it cannot diffuse or swell into the PVDF seed particles at all. It may exist as either of the following scenarios at the swelling stage. (1) St may exist between the seed surface and the surfactant layer, and/or (2) it forms a monomer protrusion on the particle surface. Because the initiator is oil-soluble, it is certainly soluble in the monomer. Under the agitation condition, the catalyst Cu(0) wires contact the oil-soluble initiator at the surface of seed particles. Then, the outer electron of Cu(0) transfers to initiator MBP, resulting in a primary free radical which can initiate the polymerization. Because of random collision, many of the newly polymerized PS (swollen with St monomer) eventually nucleate into small bulges on the seed surface (Figure 1B). The new PS/ St bulges grow bigger by continuously absorbing the St monomers around the seed and start to merge together (Figure 1F) to decrease the free energy of the system (the driving force for the transport of monomers and fusion of PS/St bulges). Furthermore, the amount of particles with a linear triplet structure is more than that of particles having many protrusions. Generally, anisotropic nanoparticles produced by

seeded emulsion polymerization are controlled by both thermodynamic and kinetic factors: the former determines the equilibrium morphology of nanoparticles, and the latter results in various metastable morphologies.36 The thermodynamically lowest free energy system shall be the snowman-like Janus particle, which is similar to the case of oil on water in a beaker. All other morphologies shall be metastable, which include the linear triplet (PS−PVDF−PS) particle and the particle having many PS protrusions.11−14,37 When the composite particles in the system grow as path 2 in Figure 9, there is not enough St supply to lower the viscosity of the PS/St domains and improve their mobility on the seed surface at a low St feed ratio (e.g., 0.5 g/1.0 g of St/PVDF). So, small bulges will grow independently and are difficult to merge together (Figure 6A,E). At a high St feed ratio (e.g., 5.0 g/1.0 g of St/PVDF), there is enough monomer supply. Therefore, the viscosity of the PS/St bulges is low, and the multiple bulges are easy to fuse into one large single protrusion. Hence, the snowman-like PVDF/PS Janus particle, which has the lowest free energy, is solely seen in Figure 6D,H. Nevertheless, the thermodynamically stable snowman-like morphology is not always observed in experiments. During the seeded emulsion polymerization, a number of kinetic factors, such as viscosity of system, speed of stirring, amount of catalyst, and monomer/ PVDF feed ratio, can greatly influence the metastable morphologies. As a result, kinetic factors enable the access of various composite particle morphologies.14,16,38 At an intermediate St concentration, i.e., 3.0 g/1.0 g of St/ PVDF, the large single protrusion grows persistently. At the same time, a couple small bulges may appear on the other side of exposed surface of the PVDF seed. Then, both snowman-like and irregular particles can be seen in Figure 6C,G. We notice that, during the investigation time, the PVDF seed particles cannot be wrapped completely by the PS phase because they are highly immiscible with each other.13 G

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

for incompatible polymer blends and self-cleaning coatings. Further, the overall synthesis procedure reported here could be extendable to other hydrophobic polymers incompatible with PVDF such as poly(4-chloromethylstyrene), poly(4-chlorostyrene), and poly(tert-butyl (meth)acrylate).

When the composite particles grow as path 1 in Figure 9, at ambient temperature (25 °C), SET-RP experiments conduct smoothly, and the resultant particles are demonstrated in Figure 7A,D. At a higher temperature (e.g., 35 or 40 °C), many more and smaller protrusions on the seed surface can be clearly observed in Figures 7B/E and 7C/F. From the mechanism of the SET-RP, the formation of active radicals via the heterolytic outer-sphere single electron transfer bond dissociation requires much lower energy. Then, this process can be conducted even at room temperature. When the reaction temperature is increased, the rate of collision between St-swollen PVDF seeds and Cu(0) wires increases. As a result, the nucleating centers become more than that at ambient temperature, and more PS/St bulgets are formed. Eventually the raspberry-like composite particles can be obtained by controlling the reaction temperature. Furthermore, the effect of the catalyst Cu(0) wire (Φ 1.00 mm) dimension on the morphology of the composite particles is investigated. Note that the variation of morphology in path 1 in Figure 9. When the length is decreased to 0.5 cm, the morphology of popcorn-like composite particles (Figure 8B) is confirmed by SEM. A similar flower-like morphology (Figure 8C) is also observed when 0.15 cm Cu(0) wire is used as a primary activating species. We had also tried the activated Cu powder (40 μm, the smallest size investigated). Nevertheless, the morphology (Figure 8D) was similar to that in Figure 8C for 0.15 cm Cu(0) wire. It can be seen that these morphologies are kinetics-controlled. Because of the method of surface nucleation and the mechanism of SET-RP, it is critical for the oil-soluble initiator in the monomer existed on the seed surface and the catalyst Cu(0) wire to contact each other, and SET-RP catalyzed by Cu(0)/PMDETA is believed to be a surfacemediated OSET process. So the accessible surface area of the initial copper source and the effective contact area between initiator and catalyst and the collision frequency should directly affect the morphology of resultant composite particles. Once the Cu(0) wire is cut into many pieces, not only the surface area of the catalyst is increased but also the frequency of collision between the particles and the catalyst is improved. Therefore, many more protrusions can be formed on the seed particle, and we call those particles as popcorn-like particles. For the Cu powder, due to its aggregation and worse dispersion, consequently there was no significant change in the final morphology of the composite particles. From the above discussion, it can be seen that increasing polymerization temperature or decreasing length of the catalyst Cu(0) wire has a similar influence on the morphology of composite particles.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.P.). Author Contributions

J.Y. and L.W. contribute equally to this work. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge support for this work from the Hebei Province Natural Science Fund (B2012202131) and the National Natural Science Foundation of China (Project no. 51373047).

■

REFERENCES

(1) Tu, F.; Park, B. J.; Lee, D. Thermodynamically stable emulsions using Janus dumbbells as colloid surfactants. Langmuir 2013, 29, 12679−12687. (2) Tu, F.; Lee, D. Shape-changing and amphiphilicity-reversing Janus particles with pH-responsive surfactant properties. J. Am. Chem. Soc. 2014, 136, 9999−10006. (3) Walther, A.; Matussek, K.; Müller, A. H. E. Engineering nanostructured polymer blends with controlled nanoparticle location using Janus particles. ACS Nano 2008, 2, 1167−1178. (4) Berger, S.; Ionov, L.; Synytska, A. Engineering of ultrahydrophobic functional coatings using controlled aggregation of bicomponent core/shell Janus particles. Adv. Funct. Mater. 2011, 21, 2338−2344. (5) Seh, Z. W.; Liu, S. H.; Low, M.; Zhang, S. Y.; Liu, Z. L.; Mlayah, A.; Han, M. Y. Janus Au-TiO2 photocatalysts with strong localization of plasmonic near-fields for efficient visible-light hydrogen generation. Adv. Mater. 2012, 24, 2310−2314. (6) Kim, J.; Joo, J.; Park, S. Y. Preparation of asymmetric porous Janus particles using microfluidics and directional UV curing. Part. Part. Syst. Charact. 2013, 30, 981−988. (7) Bradley, M.; Rowe, J. Cluster formation of Janus polymer microgels. Soft Matter 2009, 5, 3114−3119. (8) Suzuki, D.; Tsuji, S.; Kawaguchi, H. Janus microgels prepared by surfactant-free Pickering emulsion-based modification and their selfassembly. J. Am. Chem. Soc. 2007, 129, 8088−8089. (9) Suzuki, D.; Kobayashi, C. Raspberry-shaped composite microgel synthesis by seeded emulsion polymerization with hydrogel particles. Langmuir 2014, 30, 7085−7092. (10) Zhang, K.; Jiang, M.; Chen, D. Y. Self-assembly of particles-The regulatory role of particle flexibility. Prog. Polym. Sci. 2012, 37, 445− 486. (11) Niu, Q.; Pan, M. W.; Yuan, J. F.; Liu, X.; Wang, X. M.; Yu, H. F. Anisotropic nanoparticles with controllable morphologies from noncross-linked seeded emulsion polymerization. Macromol. Rapid Commun. 2013, 34, 1363−1367. (12) Liu, X.; Pan, M. W.; Yuan, J. F.; Niu, Q.; Wang, X. M.; Zhang, K. C. Popcorn-like morphology of asymmetric nanoparticles coadjusted by hydrophilicity and crosslinking degree of the seeds. RSC Adv. 2014, 4, 4163−4169. (13) Pan, M. W.; Yang, L. Y.; Guan, B.; Lu, M. S.; Zhong, G. J.; Zhu, L. Surface nucleation-induced fluoropolymer Janus nanoparticles via emulsifier-free batch-seeded emulsion polymerization. Soft Matter 2011, 7, 11187−11193.

4. CONCLUSIONS We have presented a simple synthetic method to prepare asymmetric PVDF/PS composite particles with tunable morphologies via seeded surface-initiated SET-RP in a soapfree emulsion system. Morphological evolution of the PVDF/ PS composite particles revealed that the St/PVDF feed ratio, polymerization temperature, and the length of the catalyst Cu(0) wire played important roles in controlling the asymmetric shapes of the composite particles. By increasing the St/PVDF feed ratio, snowman-like Janus particles could be obtained. The raspberry-like composite particles could be prepared at a higher polymerization temperature or a smaller length of the Cu(0) wire. The obtained fluoropolymer hybrid particles having uniform size at nanoscales are potential to future applications such as particulate surfactant/compatibilizer H

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (14) Mock, E. B.; Bruyn, H. D.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Synthesis of anisotropic nanoparticles by seeded emulsion polymerization. Langmuir 2006, 22, 4037−4043. (15) Sun, Y. Y.; Yin, Y. Y.; Chen, M.; Zhou, S. X.; Wu, L. M. Onestep facile synthesis of monodisperse raspberrylike P(S−MPS−AA) colloidal particles. Polym. Chem. 2013, 4, 3020−3027. (16) Yang, M.; Wang, G.; Ma, H. T. An efficient approach for production of polystyrene/poly(4-vinylpridine) particles with various morphologies based on dynamic control. Chem. Commun. 2011, 47, 911−913. (17) Shah, A. A.; Schultz, B.; Kohlstedt, K. L.; Glotzer, S. C.; Solomon, M. J. Synthesis, assembly, and image analysis of spheroidal patchy particles. Langmuir 2013, 29, 4688−4696. (18) Yang, H. L.; Liang, F. X.; Wang, X.; Chen, Y.; Zhang, C. L.; Wang, Q.; Qu, X. Z.; Li, J. L.; Wu, D. C.; Yang, Z. Z. Responsive Janus composite nanosheets. Macromolecules 2013, 46, 2754−2759. (19) Zhang, J.; Jin, J.; Zhao, H. Y. Surface-initiated free radical polymerization at the liquid-liquid interface: A one-step approach for the synthesis of amphiphilic Janus silica particles. Langmuir 2009, 25, 6431−6437. (20) Liu, B.; Wei, W.; Qu, X. Z.; Yang, Z. Z. Janus colloids formed by biphasic grafting at a Pickering emulsion interface. Angew. Chem. 2008, 120, 4037−4039. (21) Tanaka, T.; Okayama, M.; Kitayama, Y.; Kagawa, Y.; Okubo, M. Preparation of “mushroom-like” Janus particles by site-selective surface-initiated atom transfer radical polymerization in aqueous dispersed systems. Langmuir 2010, 26, 7843−7847. (22) Tanaka, T.; Okayama, M.; Minami, H.; Okubo, M. Dual stimuliresponsive “mushroom-like” Janus polymer particles as particulate surfactants. Langmuir 2010, 26, 11732−11736. (23) Percec, V.; Guliashvili, T.; Ladislaw, J. S.; Wistrand, A.; Stjerndahl, A.; Sienkowska, M. J.; Monteiro, M. J.; Sahoo, S. Ultrafast synthesis of ultrahigh molar mass polymers by metal-catalyzed living radical polymerization of acrylates, methacrylates, and vinyl chloride mediated by SET at 25 °C. J. Am. Chem. Soc. 2006, 128, 14156− 14165. (24) Nguyen, N. H.; Rosen, B. M.; Ligadas, G.; Percec, V. Surfacedependent kinetics of Cu(0)-wire-catalyzed single-electron transfer living radical polymerization of methyl acrylate in DMSO at 25 °C. Macromolecules 2009, 42, 2379−2386. (25) Lligadas, G.; Rosen, B. M.; Bell, G. A.; Monteiro, M. J.; Percec, V. Effect of Cu(0) particle size on the kinetics of SET-LRP in DMSO and Cu-mediated radical polymerization in MeCN at 25 °C. Macromolecules 2008, 41, 8365−8371. (26) Rosen, B. M.; Percec, V. Single-electron transfer and singleelectron transfer degenerative chain transfer living radical polymerization. Chem. Rev. 2009, 109, 5069−5119. (27) Guliashvili, T.; Percec, V. A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET-LRP of vinyl monomers. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607−1618. (28) Lligadas, G.; Percec, V. SET-LRP of acrylates in the presence of radical inhibitors. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3174− 3181. (29) Nguyen, N. H.; Levere, M. E.; Kulis, J.; Monteiro, M. J.; Percec, V. Analysis of the Cu(0)-catalyzed polymerization of methyl acrylate in disproportionating and nondisproportionating solvents. Macromolecules 2012, 45, 4606−4622. (30) Anastasaki, A.; Waldron, C.; Wilson, P.; Boyer, C.; Zetterlund, P. B.; Whittaker, M. R.; Haddleton, D. High molecular weight block copolymers by sequential monomer addition via Cu(0)-mediated living radical polymerization (SET-LRP): An optimized approach. ACS Macro Lett. 2013, 2, 896−900. (31) Percec, V.; Barboiu, B.; Grigoras, C.; Bera, T. K. Universal iterative strategy for the divergent synthesis of dendritic macromolecules from conventional monomers by a combination of living radical polymerization and irreversible terminator multifunctional initiator (TERMINI). J. Am. Chem. Soc. 2003, 125, 6503−6516.

(32) Voepel, J.; Edlund, U.; Albertsson, A. C.; Percec, V. Hemicellulose-based multifunctional macroinitiator for single-electron-transfer mediated living radical polymerization. Biomacromolecules 2011, 12, 253−259. (33) Levere, M. E.; Nguyen, N. H.; Leng, X. F.; Percec, V. Visualization of the crucial step in SET-LRP. Polym. Chem. 2013, 4, 1635−1647. (34) Nguyen, N. H.; Kulis, J.; Sun, H. J.; Jia, Z. F.; Beusekom, B. V.; Levere, M. E.; Wilson, D. A.; Monteiro, M. J.; Percec, V. A comparative study of the SET-LRP of oligo(ethylene oxide) methyl ether acrylate in DMSO and in H2O. Polym. Chem. 2013, 4, 144−155. (35) Zhang, N.; Samanta, S. R.; Rosen, B. M.; Percec, V. Single electron transfer in radical ion and radical-mediated organic, materials and polymer synthesis. Chem. Rev. 2014, 114, 5848−5958. (36) Du, Y. Z.; Tomohiro, T.; Kodaka, M. Synthesis of hemispherical poly(2-hydroxylethyl methacrylate-co-methyl methacrylate)/poly(styrene-co-glycidyl methacrylate) composite particles with heterobifunctional groups by soap-free seeded emulsion polymerization. Macromolecules 2004, 37, 803−812. (37) Chen, Y. C.; Dimonie, V.; El-Aasser, M. S. Effect of interfacial phenomena on the development of particle morphology in a polymer latex system. Macromolecules 1991, 24, 3779−3787. (38) Kraft, D. J.; Hilhorst, J.; Heinen, M. A. P.; Hoogenraad, M. J.; Luigjes, B.; Kegel, W. K. Patchy polymer colloids with tunable anisotropy dimensions. J. Phys. Chem. B 2011, 115, 7175−7181.

I

DOI: 10.1021/acs.langmuir.5b00132 Langmuir XXXX, XXX, XXX−XXX