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Intriguing morphology evolution from non-crosslinked poly(tert-butyl acrylate) seeds with polar functional groups in soap-free emulsion polymerization of styrene Lu Wang, Mingwang Pan, Shaofeng Song, Lei Zhu, Jinfeng Yuan, and Gang Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01179 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Intriguing

morphology

evolution

from

non-

crosslinked poly(tert-butyl acrylate) seeds with polar functional

groups

in

soap-free

emulsion

polymerization of styrene Lu Wang,† Mingwang Pan,*,† Shaofeng Song,† Lei Zhu‡, Jinfeng Yuan,† 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 KEYWORDS: anisotropic nanoparticles; soap-free seeded emulsion polymerization; poly(tertbutyl acrylate); polystyrene, morphology evolution; phase separation

ACS Paragon Plus Environment

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ABSTRACT: Herein,

we

demonstrate

a

facile

approach

to

prepare

anisotropic

poly(tert-butyl

acrylate)/polystyrene (PtBA/PS) composite particles with controllable morphologies by soap-free seeded emulsion polymerization (SSEP). In the first step, non-crosslinked PtBA seeds with selfstabilizing polar functional groups (e.g., ester groups and radicals) are synthesized by soap-free emulsion polymerization. During subsequent SSEP of styrene (St), PS bulges are nucleated on the PtBA seeds due to microphase separation confined in the latex particles. The morphology evolution of PtBA/PS composite particles is tailored by varying the monomer/seed feed ratio, polymerization time, and polymerization temperature. A number of intriguing morphologies, including hamburger-like, litchi-like, mushroom-like, strawberry-like, bowl-like, and snowmanlike, have been acquired for PtBA/PS composite particles. The polar groups on the PtBA seed surface greatly influence the formation and further merging of PS/St bulges during polymerization. A possible formation mechanism is proposed on the basis of experimental results. These complex

composite particles are promising for the application of

superhydrophobic coatings.


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Introduction Recently, uniform anisotropic colloidal particles with a variety of morphologies have triggered great interest in consideration of promising applications in self-assembly,1-5 compatibilizers,6 coatings,7,8 and so on.9,10 For example, Walther et al. discovered that Janus particles were superior to block and graft copolymers as stabilizers in polymer blends. Additionally, under high temperature and shear conditions, they can be located exclusively at the interface of two polymer phases, which efficiently compatibilize immiscible polymer blends.11 As we know, applications of composite materials are decided by corresponding properties, which intimately depend on the composition, domain size and size distribution, and morphology. Therefore, controlling morphology of colloids is crucial to gaining better performances of composite materials. Until now, a vast variety of anisotropic particles have been demonstrated, such as snowman-like,12,13 dumbbell-shaped,14 popcorn-like,15 cone-like,16 hamburger-like,17 mushroom-like,18,19

raspberry-like,20,21

acorn-shaped,22

and

many

others.23-25

These

morphologies are generally determined by both thermodynamic and kinetic factors.26-28 Among many preparation methods, seeded emulsion polymerization has been demonstrated to be an effective approach. There are different ways to prepare seeds, including dispersion polymerization,29-31 emulsion polymerization,32-34 interfacial polymerization,35 and suspension polymerization.36 From these seeds, anisotropic particles can be prepared using many methods. However, from the view point of productivity and industrialization, soap-free seeded emulsion polymerization (SSEP) is considered to be one of the most important preparation methods because of its effectiveness and clean product free of surfactant additives. The seeds can be categorized into


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inorganic and organic particles, each having its own advantages and drawbacks. Many remarkable achievements have been obtained so far. Among inorganic seeds such as SiO237-39 and Fe3O4,40 SiO2 is widely used. However, they could not be swollen by monomers, and this has restricted the formation of diverse morphologies of composite particles. Organic seeds are mainly composed of polystyrene (PS),26,41-44 poly(vinyl chloride) (PVC),27,45 poly(vinylidene fluoride) (PVDF),13,28 poly(methyl methacrylate) (PMMA),46 polytetrafluoroethylene (PTFE),47 poly(glycidyl methacrylate) (PGMA),30,48 or polyacrylates. Pan et al.49 used PVDF seeds to prepare snowman-like and acorn-shaped PVDF/PS composite particles by soap-free emulsion polymerization based on a surface nucleation mechanism, and studied the relationship between morphology and confined crystallization. However, the second monomer St could not swell the PVDF seeds, which limits the access to various morphologies for composite particles. Lately, modified seeds are often introduced. For instance, Yang et al.50 fabricated plum-shaped, urchinshaped, and walnut-shaped amphiphilic nonspherical composite particles utilizing sulfonated polystyrene (SPS) as the seed. However, highly crosslinked SPS seeds may lead to incomplete phase separation between the seeds and new bulges, and also limit further processing for the coating. In our previous work, Niu et al.27 prepared non-crosslinked and hydrophilicitycontrollable

P(VC-co-AAEM)

seeds

by

copolymerization

of

vinyl

chloride

and

acetoacetoxyethyl methacrylate. However, the polar functional groups on the seed surface were mostly nonuniform. Tian et al.30,48 employed PGMA containing abundant epoxide groups as seeds and synthesized a variety of anisotropic micron-sized particles. The preparation procedure was relatively complex due to the transformation from dispersion polymerization for the seeds to the second step SSEP. Based on the above method, the range of seeds, no matter highly crosslinked, modified, or added in another hydrophilic comonomer, is relatively narrow. As a


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result, it is necessary to enrich the types of seeds and discover a simple and feasible method. Until now, direct usage of PtBA as the seeds in SSEP for preparation of composite particles having various controlled morphologies has not yet been reported. In this paper, we present a facile approach to synthesize PtBA/PS composite particles with different morphologies, including hamburger-like, litchi-like, mushroom-like, bowl-like, and snowman-like, via SSEP-induced phase separation in confined spaces. Moreover, the morphology of the PtBA/PS composite particles could be systematically tailored by adjusting the monomer/seed feed ratio, reaction temperature, and polymerization time. On the basis of experimental results, the formation mechanism of PtBA/PS composite particles in SSEP is proposed. The presented synthesis method in this work is easy to scale-up and can be applied to other systems.

Experimental Section Materials. The monomers, styrene (St, >98%, Tianjin Chemical Reagent Co., Ltd.) and tert-butyl acrylate (tBA, 99%, J&K Chemical Reagent Co., Ltd.), were distilled under elevated temperature (65 and 40 °C, respectively) and reduced pressure (~0.01 MPa) to remove inhibitors. Before purification, hydroquinone was added into the monomers to avoid thermal polymerization. The purified St and tBA monomers were stored in a refrigerator at 5 °C prior to use. The initiators, potassium persulfate (KPS, > 99.5%, China Medicine Group Chemical Reagent Co., Ltd.) and sodium bisulfite (SBS, Analytically pure, Tianjin Fengchuan Chemical Co., Ltd.) were used without further purification. Deionized water was used as the polymerization medium throughout all experiments.


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Synthesis of PtBA Latex Seeds using SSEP. Spherical PtBA seeds were prepared by soap-free emulsion polymerization, which was carried out at 70 °C in a 500 mL four-necked round-bottom flask equipped with a condenser, a Teflon -blade mechanical stirrer, a nitrogen gas inlet, and a thermometer. First, 20.0 g of tBA and 200 g of deionized water were added to the flask under a mild stirring. Subsequently, the mixture was deoxygenated by bubbling nitrogen gas at room temperature for 15 min, followed by heating to 70 °C. A 5.0 g of aqueous solution containing 0.12 g KPS (i.e., 0.6 wt.% of the monomer) was added into the mixture, and the stirring speed was adjusted to 250 rpm, and polymerization reaction was kept at 70 °C for 5 h. Although the tBA conversion was as high as 99.6%, there was still some residual tBA. To avoid the remaining tBA copolymerizing with St monomer in the next step, the PtBA latex was treated by rotary evaporation under reduced pressure for 30 min to remove residue tBA. Synthesis of PtBA/PS Composite Particles. PtBA/PS composite particles with diverse morphologies were synthesized by SSEP using the PtBA particles as seeds. A typical procedure is shown as follows. The latex containing 1.0 g of PtBA particles was diluted in a 250 mL flask using 80.0 g of deionized water. The mixture was ultrasonicated for 40 min to avoid any agglomeration. The dispersion was then bubbled with nitrogen gas to remove oxygen, while being stirred at a speed of 120 rpm. After 15 min, a certain amount of St was added into the system. After swelling for 30 min at room temperature, the system was heated to a preset temperature (i.e., 60, 65, 70, and 75 °C, respectively). Next, 3.0 g of aqueous solution containing the predetermined amount of KPS (i.e., 0.6 wt.% of the St monomer) was added, and the polymerization was carried out for 5 h. Sample aliquots were


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taken from the reaction mixture at preset time intervals for morphology observation of the PtBA/PS composite particles. The detailed experimental recipes in this work are listed in Table 1. Table 1. Recipes for synthesis of PtBA/PS composite colloidal particles Sample no. S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9

PtBA / g 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

St / g 1.0 2.0 3.0 4.0 5.0 7.0 4.0 4.0 4.0

water / g 80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0

KPS / g 0.006 0.012 0.018 0.024 0.030 0.042 0.024 0.024 0.024

Temperature / °C 70 70 70 70 70 70 60 65 75

Instrumentation and Characterization. The size distribution of the PtBA seed particles and PtBA/PS composite particles in aqueous suspension were analyzed using a Zeta-Sizer Nano 90 dynamic laser scattering (DLS) particle size analyzer (Malvern, United Kingdom). The wavelength of incident light was 532 nm, and the scattering angle was 90o. Morphology of the PtBA seed particles and PtBA/PS composite particles was characterized by a scanning electron microscopy (FEI Nova NanoSEM 450, operated at 10 kV). To prepare samples for the SEM study, a drop of PtBA or PtBA/PS latex was diluted with deionized water to obtain a translucent suspension and ultrasonicated for 40 min. Then, a drop of the suspension was coated on a conductive silicon wafer and then dried for two days using a vacuum freeze-drying equipment (FD-1-50, Beijing Boyikang Laboratory Instruments Co., Ltd.) at -45 °C. A transmission electron microscope (Hitachi H-7650B transmission electron microscope (TEM), operated at 80 kV) was employed to investigate the internal structure of PtBA/PS


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composite particles. For the TEM observation, a drop of the suspension was drop-cast onto a 400-mesh carbon-coated copper grid, followed by drying for 2 days at 30 °C in a vacuum oven. Alternatively, particle powders were embedded in standard epoxy resin, and cured at 60 °C for 2 days. The sample was sectioned using an ultramicrotome (EM UC6, Leica, Germany) with a freshly cleaved glass knife. The thickness of the ultrathin section was about 70 nm. The carbon and oxygen contents on the surface of the PtBA/PS composite particles were confirmed by X-ray photoelectron spectroscopy (XPS, K-Alpha, ThermoFisher, United Kingdom). The infrared spectra were recorded using a Fourier transform infrared spectrometer (FTIR, Tensor-27, Bruker, Germany). Prior to analysis, the dried particle powder was mixed with dry KBr powder and pressed into pellets. The contact angle of water droplet on the surface of polymer films was measured using a DSA30S instrument (KRÜSS Co., Germany) at room temperature. A few drops of the latex suspension were drop-cast onto a glass slide, followed by drying in a vacuum oven at room temperature for 12 h before measurements. Thermogravimetric analysis (TGA) was performed using a SDT/Q-600 analyzer (TA Instruments, USA). The dried sample was heated in a ceramic crucible at a heating rate of 10 °C/ min from room temperature up to 600 °C under a N2 flow at 100 mL/ min.

Results and Discussion PtBA Seed Particles from Soap-Free Emulsion Polymerization. Figure 1 shows the size distribution and particle morphology of the PtBA seed particles obtained by DLS and SEM, respectively. The number-average particle size (APS) was determined to be ~321.5 nm and the


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polydispersity index (PDI) was 0.072. As shown in Figure 1B, it is clear that the PtBA seed particles show a uniform spherical shape. It is commonly accepted that the anionic sulfate radicals from thermally decomposed KPS could initiate tBA polymerization and thus stabilize the latex particles without the help of surfactants. However, under the same soap-free emulsion polymerization condition for St, micron-sized PS particles with a broad particle size distribution were obtained.27 This different result indirectly suggested that the polar groups (i.e., ester and anionic sulfate groups) on the PtBA particles must play an additional role in stabilizing the latex particles during the soap-free emulsion polymerization.

Figure 1 (A) DLS size distribution histogram and (B) SEM micrograph of the PtBA seed particles from soap-free emulsion polymerization.

Morphology Evolution of Anisotropic PtBA/PS Complex Particles during SSEP. In this study, morphology evolution of PtBA/PS complex particles during SSEP was systematically investigated for sample aliquots at different time intervals using SEM and TEM techniques. In the preparation of the complex particles, we fixed the PtBA seed weight at 1.0 g and varied the St monomer feed amount as 3.0, 4.0, or 5.0 g. The synthetic process of the PtBA/PS complex particles has been described in the Experimental Section. When the St/PtBA feed ratio was 4.0 g/ 1.0 g, the morphologies of PtBA/PS complex particles at different reaction times are shown in Figure 2. At 0.5 h, strawberry-like particles with


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many uniform small bulges on the seed surface were formed, because PtBA is highly immiscible with PS, as shown in Figures 2A-i and 2B-i. When the reaction time reached 1 h (Figures 2A-ii and 2B-ii), these small bulges became larger and their number decreased to about ten. These bulges (see arrow in Figure 2A-ii) were ascribed to the PS domains due to their smooth surface, while PtBA seeds had a relatively rough surface, probably due to electron beam damage during SEM observation. The PS bulges having a smooth surface was also confirmed in our previous work,13,27 and further evidence will be provided later. When the polymerization time was 3 h, as presented in Figures 2A-iii and 2B-iii, the number of PS protrusions reduced to two or three (see arrows in Figure 2A-iii) owing to the continued growth and fusion of adjacent St-swollen PS bulges. With further prolonging the polymerization time to 4 h, the PS protrusions became much bigger, and at their seams appeared some small bulges (see arrows in Figure 2A-iv) because the newly formed PS domains were not reconcilable with the polar groups on the seams. Finally, at 5 h, a seam was formed near the “equator” of the composite particles, where many crowded small bulges were seen (Figure 2A-v). The main reason is the polar tert-butoxy ester groups on the seed surface, hindering further fusion of adjacent PS bulbs. Another reason is the increased viscosity of the polymerization loci upon increasing the St conversion. Consequently, the PS/St bulges could not merge to form perfect core-shell particles.


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Figure 2. (A) SEM and (B) TEM micrographs of PtBA/PS composite particles at different polymerization times: i) 0.5, ii) 1, iii) 3, iv) 4, and v) 5 h, when the St monomer/seed feed ratio was 4.0 g/1.0 g. The polymerization temperature was 70 °C and the initial monomer swelling time was 0.5 h.

To further confirm that the bulges emerged on the composite particles were PS, XPS was utilized to identify the surface composition at different polymerization times. The XPS spectra are shown in Figure 3. The O1s signal of PtBA on the surface of composite particles was clearly detected. The strong peaks at 532 and 284 eV belong to O1s and C1s, respectively. As shown in


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Figure 3A, at the polymerization time of 1 h, the content of oxygen on the PtBA/PS particle surface was 18.58%. When the polymerization time increased to 5 h, the detected oxygen content decreased to 14.41% (Figure 3B). In contrast, the content of carbon in the composite particle surface increased from 81.42% to 85.59%. These results indicate that the oxygen content in the surface layer of PtBA/PS composite particles continually decreased with increasing the polymerization time, indicating that the PtBA seeds were gradually wrapped by PS as St polymerized. Note that the oxygen content of final particles obtained at 5 h of polymerization did not decrease to zero, consistent with the fact that the composite particles did not have a uniform perfect core-shell structure.

Figure 3. XPS spectra of the PtBA/PS composite particles at the polymerization time of (A) 1 and (B) 5 h, when the St/PtBA feed ratio was 4.0 g/1.0 g and the soak time was 0.5 h.

To further understand the morphology evolution of PtBA/PS composite particles, we also studied the case when the St/PtBA feed ratio was 5.0 g/1.0 g. The corresponding SEM and TEM results are shown in Figure 4. In the initial stage (0.5 h), Janus particles were seen. On the uncovered PtBA surface, some tiny PS bulges nucleated, as shown in Figures 4 A-i and 4B-i. Different from the situation of the feed ratio being 4.0 g/1.0 g, at 1 h of polymerization,


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mushroom-like particles were observed with the PS bulge forming the cap (or pileus) and PtBA seed forming the short stipe. On the exposed PtBA surface, several small PS bulges were nucleated. On most PS caps, a small hole appeared. We speculated that at a higher feed ratio of St/PtBA, the PS bulges were highly swollen by the St monomers.51 During freeze-drying at -45 °C, some St monomers might crystallize to form a separated domain (melting point of -30 °C). After drying, a hole was formed and the size of the hole depended upon how much St monomers crystallized. Similarly, holes were also observed when the St/PtBA feed ratio increased to 7.0 g/1.0 g (data not shown). With extension of the reaction time, the size of the hole first increased, then decreased, and finally disappeared. This appeared obvious especially in the TEM images (Figure 4B). Furthermore, the single hole always appeared in the largest PS bulge, because its large volume could be swollen by more St monomers. Note that some particles in the SEM image of Figure 4 (A-ii) looked like without a hole. This could be attributed to the random orientation of the anisotropic particles under SEM observation. When the reaction time reached 2 h, full bowl-like particles with an open hole (Figure 4A-iii) were formed and the diameter of the hole became larger (~157 nm) (see arrows in Figure 4B-iii). After reaction for 3 h, from the SEM images (Figure 4A-iv), the hole seemed to disappear. However, from the TEM observation, the hole inside the composite particles became bigger (~185 nm) rather than disappeared. As the reaction time increased to 4 h, most of holes in the composite particles disappeared with only a few exceptions (see arrow in Figure 4B-v). In Figure 4A-v, we found two particular particles indicated by dark arrows. In the cavity of the bowl, there was one or two newly formed PS bulges. This indicates that decreasing of the hole size was caused by the continuous polymerization and depletion of St monomers in the particles.


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Figure 4. (A) SEM and (B) TEM micrographs of PtBA/PS composite particles at different polymerization times: i) 0.5, ii) 1, iii) 2, iv) 3, v) 4, and vi) 5 h, when the St/PtBA feed ratio was 5.0 g/1.0 g. The polymerization temperature was 70 °C and the swelling time was 0.5 h.

With further extending polymerization time to 5 h, it is interesting to note that the morphology of all the particles looked like a snowman (see Figures 4A-vi and 4B-vi). All the snowman-like particles still possessed a narrow seam, as indicated by the arrows in Figures 4Avi and 4B-vi, and also highlighted in the inset of the TEM image. In contrast to the


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aforementioned particles obtained at the St/PtBA feed ratio of 4.0 g/1.0 g at 5 h, the seam position shifted from close to the equator to one side of the composite particle. Judging from the SEM images in Figures 4A-iv and 4A-v, this was formed because of the pathway of mushroomlike particles formed earlier. Further nucleation and polymerization on the minor exposed surface of the PtBA seeds resulted in the rough small bulges (note that on the small head of most particles, there were a few small bulges; however, the surface of the large PS bulge appeared to be smooth). To identify the chemical constituents in the composite particles, FTIR spectra of the PtBA seeds and PtBA/PS composite particles are shown in Figure 5A. The absorption peaks at 2935 and 2979 cm-1 were assigned to the vibration of saturated C-H bond (-CH3 and -CH2), as shown in spectrum (a). The strong absorption peak at 1730 cm-1 was attributed to the stretching vibration of carbonyl group (C=O) in PtBA. The stretching vibrations caused by -CH3 bending in tert-butyl groups were observed at 1392 and 1367 cm-1, while the sharp absorption peaks at 1255 and 1151 cm-1 represented C-C-O and C-O stretching vibrations in PtBA, respectively.52 Compared with spectrum (a), spectrum (b) presented typical absorption peaks of PS. The absorption bands at 1600, 1492, and 1452 cm-1 belonged to stretching vibrations of the benzene ring skeleton. The peaks at 756 and 698 cm-1 were attributed to flexural vibrations of single substituted benzene ring. The characteristic peak at 3025 cm-1 was attributed to stretching vibrations of unsaturated C-H bonds in benzene ring. Therefore, we determined that the components of the composite particles are PtBA and PS, respectively. To further investigate the internal microstructure and formation mechanism of the PtBA/PS composite particles with different morphologies, it is necessary to reveal their internal structures. The PtBA/PS composite particles (obtained at the St/PtBA feed ratio of 5.0 g/1.0 g


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and at 70 °C for 5 h) were microtomed into ultrathin sections using an ultramicrotome, and the microstructure was studied by TEM. A typical result is shown in Figure 5B. The bright regions inside the particles were PtBA seeds (or the core) and the dark regions were PS bulges. Note that the sample was not stained, and we speculate that the contrast could originate from electron density difference between PS and PtBA and was further enhanced by under-focusing in TEM. The cross-sections of the PtBA/PS composite particles showed complicated morphologies (e.g., a, b, c, and d), with different sizes and arrangements of the bright (PtBA) and dark (PS) domains. This is because the obtained cross-sections originated from different cutting positions and directions during ultramicrotoming. For better understanding, different cutting positions and directions are proposed in the inset of Figure 5B. The c1 and c2 cross-sections were viewed from opposite directions. It is worth noting that there were quite a few darker spots (ca. 40-50 nm in size) dispersed within the bright PtBA seeds (see c1 and c2 in the TEM image), which could be attributed to phase separated PS domains polymerized from swollen St monomers in PtBA seeds. At the same time, the a and b regions should be mainly PS bulges because of the uniform black contrast. We consider that polymerization of St was the driving force for the microphase separation of PS from PtBA seeds.


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Figure 5. (A) FTIR spectra of (a) PtBA seed particles and (b) PtBA/PS composite particles. (B) TEM mocrograph of an ultrathin cross-section of PtBA/PS composite particles embedded in standard epoxy resin. The inset image in (B) shows different cutting directions of the PtBA/PS composite particles. The St/PtBA feed ratio was 5.0 g/1.0 g, and the polymerization time was 5 h at 70 °C.

After the sample aliquots at 1, 3, and 5 h of SSEP (St/PtBA = 5.0 g/1.0 g) were thoroughly dried, TGA was used to determine the PtBA/PS weight ratios (Figure 6). The peak weight losses at 209 and 439 °C in the derivative curves were denoted as the fastest degradations of PtBA and PS, respectively. From the weight loss curves, pure PS almost completely decomposed in nitrogen atmosphere above 500 °C, whereas pure PtBA left about 10 wt.% carbon residue. Using this information, the PtBA/PS weight ratios at 1, 3, and 5 h of SSEP could be calculated to be ca. 1:1.5, 1:3.9, and 1:4.7, respectively. The calculated results were generally consistent with the St conversion results measured by weighing the solid content.


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Figure 6. TGA weight loss (solid lines) and derivative (dashed lines) curves for the PtBA/PS composite particles prepared at 70 °C for 1, 3 and 5 h, respectively. The St/PtBA feed ratio was kept at 5.0 g/1.0 g in the SSEP. The TGA weight loss curves of pure PtBA and PS are included for comparison.

Comparing the morphological results obtained from the above two feed ratios, it is clear that the relative amount of St monomers has a great influence on the intermediate morphology of St/PtBA composite particles. We further investigated the influence of a low St/PtBA feed ratio of 3.0 g/1.0 g on the morphology development for PtBA/PS composite particles (Figure 7). In the early polymerization stage (0.5 h), a number of non-uniform bulges grew on the composite particles (~339 nm), as shown in Figures 7A-i and 7B-i, indicating microphase separation in the composite particles. Compared with the situation at St/PtBA = 4.0 g/1.0 g (Figure 2A-i), there was not enough swelling of the PS bulges by St monomers, and then the viscosity of the PS/St bulges was high, leading to a lower mobility for inter-bulge fusion. Therefore, multiple bulges nucleated and grew independently. The small bulge size was about 76 nm and the large one was about 123 nm (measured along the widest width, the same below). At 1 h (Figures 7A-ii and 7Bii), about four to six large PS bulges (~171 nm) appeared on the surface of the composite particles (~351 nm). When the polymerization time reached 2 h (Figures 7A-iii and 7B-iii), most composite particles (~362 nm) contained two or three large PS bulges (~247 nm). After reaction for 3 h, the number of PS bulges mainly reduced to one or two (Figures 7A-iv and 7B-iv) owing


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to the continued growth and fusion between adjacent PS/St bulges. Thus, we called those anisotropic particles with two PS protrusions as hamburger-like particles. At this time, the composite particle size reached ~381 nm. With further increasing the polymerization time to 4 h (Figures 7A-v and 7B-v), it is interesting to see that at one side of these composite particles appeared a big bulge, while a lot of small bulges were on the other side with one or two seams formed among them. These composite particles had an overall ellipsoidal shape with the length being ~427 nm and the width being ~380 nm. Prolonged reaction time would not eliminate the seams. From the SEM and TEM images in Figures 7A-vi and 7B-vi, it was found that the seams still existed, just the size of composite particles at 5 h (length ~462 nm and width ~410 nm) became a little larger compared with that at 4 h.


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Figure 7. (A) SEM and (B) TEM images of PtBA/PS composite particles at polymerization time of i) 0.5, ii) 1, iii) 2, iv) 3, v) 4, and vi) 5 h when the feed ratio of St/PtBA being 3.0 g/1.0 g. The polymerization temperature was 70 °C and the swelling time was 0.5 h. Scale bar in insets: 200 nm.

The average sizes and size distributions of the PtBA seed and the PtBA/PS composite particles obtained at different St/PtBA feed ratios and different polymerization times were determined by DLS (see Figure S1 in the Supporting Information). From Figure S1B, a single


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particle size distribution peak was observed, and the average particle sizes increased with the extension of polymerization time (Figure S1A). Influence of the St/PtBA Feed Ratio on Morphology of PtBA/PS Composite Particles. On the basis of the above study, the St/PtBA feed ratio seems to be an important factor for the morphology of PtBA/PS composite particles in SSEP. Here, we continued to investigate the influence of the St/PtBA ratio. First, we fixed the amount of the PtBA seed as 1.0 g, polymerization temperature at 70 °C, and polymerization time of 5 h, while the amount of St was changed from 1.0 to 2.0, 3.0, 4.0, 5.0 or 7.0 g. The corresponding SEM images are shown in Figure 8. When the St/PtBA feed ratio was low (i.e., 1.0 g/1.0 g), many small bulges were grown on PtBA seed surface (Figure 8A). This was primarily because the St supply was not enough to swell into the PS bulges and lower their viscosity. As a result, the polymerizing PS/St bulges could not merge together on the seed surface. When the feed ratio increased to 2.0 g/1.0 g, uniform composite particles with four to six bigger PS bulges were observed (Figure 8B). Besides these bigger PS bulges, we could also see some newly nucleated tiny bulges. When the St/PtBA feed ratio was 3.0 g/1.0 g, hamburger-like particles with a distinct seam between two PS poles were seen in Figure 8C. In this circumstance, the amount of St monomer was sufficient. As a result, the multiple PS/St bulges fused into two opposite poles on the PtBA seeds. Note that no further fusion of these two bulges was observed. It was possible that St monomers were mostly consumed and the two PS bulges could not merge anymore. When the St/PtBA feed ratio was 4.0 g/1.0 g (Figure 8D), many tiny PS bulges filled in the seam between the two large bulges on the seeds. Further increasing the feed ratio to 5.0 g/1.0 g, snowman-like composite particles were achieved (Figure 8E), but a distinct seam deviate from equator still existed. Compared with the morphology in Figure 8C and 8D, it can be found that the seam became narrower and the PS


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bulges at two sides appeared much smooth. This was largely due to the swelling of abundant St monomer and further fusion of the small PS/St bulges. According to the above observation, we conjecture that the seam might disappear upon further increasing the amount of St monomer (e.g. 7.0 g/1.0 g feed ratio). In other words, the PtBA seeds should become uniformly covered by a PS shell. However, this did not happen. The newly formed PS bulges was squeezed to deformation at the seams, rather than fusion into one uniform shell (see Figure 8F).

Figure 8. SEM micrographs of PtBA/PS composite particles at the St/PtBA feed ratio of (A) 1.0 g/1.0 g, (B) 2.0 g/1.0 g, (C) 3.0 g/1.0 g, (D) 4.0 g/1.0 g, (E) 5.0 g/1.0 g, and (F) 7.0 g/1.0 g, when the polymerization time was 5 h. The polymerization temperature was 70 °C, and the initial swelling time was 0.5 h.

Influence of Polymerization Temperature on Morphology of Composite Particles. Polymerization temperature is another controlling factor. When the polymerization temperature varied from 60 to 75 °C, other experimental conditions were kept the same: St/PtBA feed ratio =


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4.0 g/1.0 g, the swelling time = 0.5 h, and polymerization time = 5 h. As shown in Figure 9, the morphology of PtBA/PS composite particles changed from hamburger-like to snowman-like. At a relatively low temperature (60 °C), the sizes of two bulges in the hamburger-like particles were nearly the same (Figure 9A). At 65 °C, one of the two bulges became bigger than the other (Figure 9B). In Figure 9 A and B, an obvious seam existed on each particle. When the temperature reached 70 °C, many small PS bulges emerged in the seam (Figure 9C). When the temperature rose to 75 °C, this interstitial seam became fuzzy. On the one hand, raising the polymerization temperature could increase the polymerization rate of St, leading to more rapid nucleation and growth on the PtBA seed surface. On the other hand, faster consumption of St monomers at high temperatures decreased the swelling of the PS bulges, leading to more difficult merging. As a result, tiny PS protrusions formed in the seams at high polymerization temperatures.


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Figure 9. SEM micrographs of PtBA/PS composite particles at the polymerization temperature of (A) 60, (B) 65, (C) 70, and (D) 75 °C, when the polymerization time was 5 h. The St/PtBA feed ratio was 4.0 g/1.0 g and the initial swelling time was 0.5 h.

Formation Mechanism of Various PtBA/PS Composite Particles. To elucidate formation process of the PtBA/PS composite particles with various intriguing morphologies, we propose a formation mechanism, as shown in Figure 10. Before the swelling stage, PtBA seeds are stabilized by the polar functional groups on the seed surface, including the anionic sulfate groups from decomposed KPS and the ester groups. In the St swelling stage, the loci of St monomers may include following three scenarios: (a) a part of St monomers diffuse into the PtBA seeds; (b) some St monomers form a thin swelling layer between the PtBA seed surface and water; (c) certain St monomers are distributed around the PtBA seeds. Here, we use a green


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droplet and a purple sphere respectively to represent St monomer and swollen PtBA seed. The longer the swelling time, the deeper the PtBA domains are swollen by the St monomers.

Figure 10. Schematic representation of morphology evolution of the PtBA/PS composite particles during SSEP.

When the temperature of the system is elevated, free radicals produced by thermal decomposition of water soluble KPS mostly migrate onto the seed surface to initiate St monomers. Although PtBA can be swollen by St, it is thermodynamically immiscible with PS. As St polymerizes into PS, the St-swollen PS bulges are nucleated on the seed surface. If the degree of St-swelling is high and the viscosity of the PS/St bulges is low, the subsequent growth and polymerization processes of the PS/St bulges will lead to fusion of many small bulges into one or two large bulges on the PtBA seed surface. For the St/PtBA feed ratio being 3.0 g/ 1.0 g and 4.0 g/ 1.0 g, the composite particles in the system take the growing path 1 and 2, respectively (see Figure 10). Two processes compete with each other in forming various composite particle morphologies. First, polymerization of St monomers generates new PS/St bulges on the seed surface (Figure 2A-i and Figure 7A-i). Second, small PS/St bulges can merge into fewer big PS/St bulges as they grow. With the extension of the reaction time, the PS/St domains grow bigger by absorbing the St monomers from both the seed and the surrounding. Meanwhile,


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adjacent PS/St bulges continually merge together. As a result of the growing kinetics, a number of interesting intermediate morphologies, such as strawberry-like and hamburger-like particles, can be observed for the composite particles (Figures 2A ii-iv and Figures 7A ii-iv). From the thermodynamic point of view, this behavior can reduce the interfacial tension/energy between particles and water and make the latexes stable in water. Continuing to extend the polymerization time, the particles formed do not have a uniform core-shell structure, but there is a seam on the surface of composite particles (Figures 2A-v, 4A-vi, 7A-vi and 8F). Some reasons can account for the seam formation. First, the longer the polymerization time or the higher the St conversion, the lower the degree of swelling of the PS bulges by St monomers. The high viscosity decreases the liquidity of PS/St bulges and thus PS bulges can’t merge together. Second, the hydrophilic groups on the seed surface may prevent further fusion of PS/St bulges due to their poor wettability on PtBA surface. When a relatively high feed ratio (i.e., ≥5.0 g/ 1.0 g of St/PtBA) is used, composite particle will grow following path 3 (Figure 10). In this case, there is a large number of St monomers to swell into the seeds. Due to polymerization-induced phase separation, PS/St can form a cap on one side of the seed in the early polymerization stage (Figure 4A-i). Because of the high St swelling from sufficient St supply, a small hole is formed possibly due to the crystallization of excess St monomers during freeze-drying at -45 °C (Figures 4A ii-iv). Further nucleation and growth in the later stage of polymerization result in not only continued growth of the PS/St cap, but also small PS/St bulges on the other side of the seed due to decreased St swelling. In the final polymerization stage, the small bulges on the minor surface merge together. However, the former cap-like large bulge and the later small bulge cannot fuse into a uniform


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shell. Instead, a seam with tiny bulges is obtained. The final morphology of the composite particles looks like a snowman (Figure 4A-vi). On the basis of the above experimental results, it is promising to fabricate hierarchical micro/nano structures of composite particles for superhydrophobic coating at low St/PtBA feed ratio and low polymerization temperatures. Under these conditions, many tiny PS bulges are expected to nucleate on the PtBA surface and grow independently because of the phase separation between PS and PtBA and the difficult fusion of the PS/St bulges. Hence, we conducted a polymerization at 25 °C. The monomer/seed feed ratio was at 2.0 g/1.0 g and the polymerization time was 0.5 h. The KPS/SBS molar ratio was 2:1. Different from other samples, here we used KPS/SBS instead of KPS because KPS could only decompose effectively above 60 °C. Addition of SBS could markedly lower the activation energy by the redox reaction and initiate the polymerization at a much lower temperature such as 25 °C. As expected, we obtained PtBA/PS composite particles with a vast number of tiny PS bulges dispersed evenly on the seed surface (see the inset SEM image in Figure 11A). The morphology was thus termed as litchi-like. This experimental result supported the above formation mechanism in Figure 10. Contact angle measurement was used to characterize hydrophobic property of a film formed by the litchi-like PtBA/PS particles. It was clearly observed that the contact angle of a water droplet on the litchi-like particle film was 147° (see Figure 11A). Under the same test condition, the contact angle for a film from the PtBA seed particles was only 120° (Figure 11C). Even for the strawberry-like composite particles (see the SEM image in the inset of Figure 11B, obtained at St/PtBA = 4.0 g/1.0 g and 70 °C for 0.5 h), the contact angle was 127.4°. In our previous published work,15 the contact angle of pure PS detected was 130°. Namely, the contact angle of the litchi-like PtBA/PS composite particles was much higher than that of the pure PS


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surface. It indicated that hydrophobicity of the composite particle film was greatly improved due to the hierarchical micro/nano structure of the colloidal particles. Therefore, these litchi-like particles are promising as superhydrophobic coatings. To further increase the contact angle for the composite particle films, we propose to use 4-tert-butyl styrene (4tBSt) as the second monomer, because P4tBS is more hydrophobic than PS. Future investigation is currently under way.

Figure 11. Contact angles (A, B, and C) of water droplets on particle films and SEM micrographs (D, E, and F) of the latex particles. The corresponding samples are: (A,D) litchi-like PtBA/PS composite particles prepared at room temperature when the polymerization time being 0.5 h, St/PtBA feed ratio being 2.0 g/1.0 g and swelling time being 0.5 h; (B,E) strawberry-like PtBA/PS composite particles prepared at 70 °C when the polymerization time was 0.5 h, St/PtBA feed ratio was 4.0 g/1.0 g, and swelling time was 0.5 h; (C,F) PtBA seed particles.

Conclusions A feasible approach is presented for the preparation of PtBA/PS composite particles with controllable morphologies from non-crosslinked PtBA seeds with polar functional groups via SSEP, where the stability of the emulsion mainly relied on the polar functional groups on the


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surface through the polymerization procedure. Intriguing morphology evolution of the PtBA/PS composite particles revealed that the St/PtBA feed ratio played an important role in controlling the shape of the composite particles. For example, at a low St/PtBA feed ratio (3.0 g/1.0 g or 4.0 g/1.0 g), the strawberry-like particles with multiple individual bulges appeared within 1 h of polymerization. With continued polymerization, adjacent PS/St bulges merged together to form the hamburger-like morphology. At a high St/PtBA feed ratio (5.0 g/1.0 g), more St monomers could swell into the seeds and promote easy fusion of PS/St bulges. Therefore, mushroom-like particles were achieved in the early stage. With continued polymerization, snowman-like particles were finally obtained. Besides the St/PtBA feed ratio, the reaction temperature could also influence the morphology of composite particles. Using a redox initiating system at room temperature and at a low St/PtBA feed ratio (2.0 g/1.0 g), litchi-like PtBA/PS composite particles were achieved at a low St conversion. A film from these litchi-like particles demonstrated superhydrophobic property owing to the hierarchical micro/nano structure. At present, we are attempting to further control the micro/nano structures of composite particles for superhydrophobic coatings. The formation mechanism for a variety of anisotropic particles was considered to be the appropriate swelling of the PtBA seeds by the second St monomers and phase separation between PtBA and PS due to the thermodynamic immiscibility. The polymerization method and mechanism reported here could be extendable to other systems. The seed polymer should be amphiphilic at the molecular scale. For example, the ester group in tBA is polar and the t-butyl group is hydrophobic. Other suitable candidates include n-butyl (meth)acrylate, n- or i-propyl (meth)acrylate, and glycidyl (meth)acrylate. The second monomer should be able to swell the seeds, but not dissolve them. Viable candidates are styrenic


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monomers such as p-chloromethyl styrene and 4tBSt. In the future, more research will be carried out to investigate these possibilities.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 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).

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Figure 1 (A) Size distribution histogram and (B) SEM micrograph of the PtBA seed particles. 36x15mm (300 x 300 DPI)


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Figure 2. (A) SEM and (B) TEM micrographs of PtBA/PS composite particles at different polymerization times: i) 0.5, ii) 1, iii) 3, iv) 4, and v) 5 h, when the St monomer/seed feed ratio was 4.0 g/1.0 g. The polymerization temperature was 70 °C and the initial monomer swelling time was 0.5 h. 160x203mm (300 x 300 DPI)


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Figure 3 XPS spectra of the PtBA/PS CPs at the polymerization time of A) 1, B) 5 h when the St/PtBA feed ratio being 4.0 g/1.0 g. 59x22mm (300 x 300 DPI)


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Figure 4. (A) SEM and (B) TEM micrographs of PtBA/PS composite particles at different polymerization times: i) 0.5, ii) 1, iii) 2, iv) 3, v) 4, and vi) 5 h, when the St/PtBA feed ratio was 5.0 g/1.0 g. The polymerization temperature was 70 °C and the swelling time was 0.5 h. 160x203mm (300 x 300 DPI)


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Figure 5 A) FTIR spectra of (a) PtBA seed particles and (b) PtBA/PS CPs; B) TEM image of ultrathin crosssections of PtBA/PS CPs. The inserted image in Figure 5B shows the different cutting directions of PtBA/PS CPs. The St/PtBA feed ratio was 5.0 g/1.0 g and polymerization time was 5 h. 65x28mm (300 x 300 DPI)


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Figure 6 TGA weight loss (solid lines) and derivative (dashed lines) curves for the PtBA/PS CPs prepared at 70 °C for 1, 3 and 5 h. The St/PtBA feed ratio was kept at 5.0 g/1.0 g in SSEP. The TGA weight loss curves of pure PtBA and PS are included for comparison. 54x37mm (300 x 300 DPI)


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Figure 7. (A) SEM and (B) TEM images of PtBA/PS composite particles at polymerization time of i) 0.5, ii) 1, iii) 2, iv) 3, v) 4, and vi) 5 h when the feed ratio of St/PtBA being 3.0 g/1.0 g. The polymerization temperature was 70 °C and the swelling time was 0.5 h. Scale bar in insets: 200 nm. 160x203mm (300 x 300 DPI)


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Figure 8 SEM images of PtBA/PS CPs at St/PtBA feed ratio of A) 1.0 g/1.0 g, B) 2.0 g/1.0 g, C) 3.0 g/1.0 g, D) 4.0 g/1.0 g, E) 5.0 g/1.0 g, F) 7.0 g/1.0 g when the polymerization time being 5 h. The polymerization temperature was 70 °C and the swelling time was 0.5 h. 80x53mm (300 x 300 DPI)


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Figure 9 SEM images of PtBA/PS CPs at the polymerization temperature of A) 60, B) 65, C) 70, D) 75 °C when the polymerization time being 5 h. The St/PtBA feed ratio was 4.0 g/1.0 g and the swelling time was 0.5 h. 80x80mm (300 x 300 DPI)


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Figure 10. Schematic representation of morphology evolution of the PtBA/PS composite particles during SSEP. 54x18mm (300 x 300 DPI)


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Figure 11. Contact angles (A, B, and C) of water droplets on particle films and SEM micrographs (D, E, and F) of the latex particles. The corresponding samples are: (A,D) litchi-like PtBA/PS composite particles prepared at room temperature when the polymerization time being 0.5 h, St/PtBA feed ratio being 2.0 g/1.0 g and swelling time being 0.5 h; (B,E) strawberry-like PtBA/PS composite particles prepared at 70 °C when the polymerization time was 0.5 h, St/PtBA feed ratio was 4.0 g/1.0 g, and swelling time was 0.5 h; (C,F) PtBA seed particles. 66x29mm (300 x 300 DPI)


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