Article pubs.acs.org/Macromolecules
Composite Poly(vinylidene fluoride)/Polystyrene Latex Particles for Confined Crystallization in 180 nm Nanospheres via Emulsifier-Free Batch Seeded Emulsion Polymerization Mingwang Pan,*,† Lianyun Yang,‡ Jianchuan Wang,‡,§ Saide Tang,‡ Ganji Zhong,§ Run Su,‡,§ Mani K. Sen,∥ Maya K. Endoh,∥ Tadanori Koga,∥ and Lei Zhu*,‡ †
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 § College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, P. R. China ∥ Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794-2275, United States ‡
S Supporting Information *
ABSTRACT: Recently, nanoconfined poly(vinylidene fluoride) (PVDF) and its random copolymers have attracted substantial attention in research. In addition to the drastic change in crystallization kinetics, major interest lies in crystal orientation and polymorphism in order to understand whether enhanced piezoelectric and ferroelectric properties can be achieved. For example, PVDF has been two-dimensionally (2D) confined in cylindrical nanopores of anodic aluminum oxide (AAO) with various pore diameters. The crystal c-axis becomes perpendicular to the cylinder axes, which favors dipole switching in the impregnated AAO membrane. However, no polar phases have been obtained from 2D confinement even down to 35 nm pores after melt recrystallization. In this work, we realized three-dimensionally (3D) confined crystallization of PVDF in 180 nm nanospheres by employing a facile emulsifier-free batch seeded emulsion polymerization to prepare PVDF@polystyrene (PS) core−shell particles. Influences of polymerization temperature, PVDF/styrene feed ratio, and polymerization time were systematically investigated to achieve completely wrapping of PS onto PVDF particles and avoid the formation of Janus particles. Exclusive confined PVDF crystallization was observed in these core−shell composite particles. Intriguingly, after melt recrystallization, polar β/γ phases, instead of the kinetically favored α phase, were resulted from 3D confinement in 180 nm nanospheres. We attributed this to the ultrafast crystallization rate during homogeneously nucleated PVDF crystallization. For the first time, we reported that 3D confinement was more effective than 2D confinement in producing polar crystalline phases for PVDF.
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(2D), one-dimensional (1D), and finally to zero-dimensional (0D), and the corresponding confined space decreases from the bulk to nanometer scales. The mechanism is explained as follows. When the number of droplets per unit volume is much higher than the number density of heterogeneous nuclei (i.e., no existing nuclei in most of the droplets), homogeneous nucleation has to take place for crystallization, which usually occurs at a large supercooling (ca. 60−100 °C).9 For example, homogeneous nucleation of poly(ethylene oxide) (PEO) is observed at ca. −23 °C in microdroplets,10−14 which is much lower than that of heterogeneous nucleation in the bulk PEO (ca. 40 °C). The critical size of the droplets for the appearance of homogeneous nucleation depends on the density of heterogeneous nuclei in the specific polymer and can vary
INTRODUCTION
Confining crystalline polymers in nanospaces can lead to new or enhanced physical properties that are different from those of the bulk 1−4 and thus has good potentials for novel applications.5,6 Several strategies have been employed to achieve confined crystallization of polymers, including finely dispersed immiscible polymer blends,7 polymers in nanopores,8 microphase-separated crystalline block copolymers,1,2 and multilayer polymer films.3,5,6 After confinement, crystallization behavior, crystal orientation, polymorphism, phase transformation (including melting), and crystallinity of semicrystalline polymers can be drastically changed.1−4 Among these effects for nanoconfined polymer crystallization, the most pronounced change is observed for the crystallization behavior, including both crystallization temperature (Tc) and kinetics.1 Homogeneous nucleation, which is rarely observed in bulk polymers, can take place when the confinement geometry changes from three-dimensional (3D) to two-dimensional © 2014 American Chemical Society
Received: February 1, 2014 Revised: April 4, 2014 Published: April 11, 2014 2632
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confinement in reducing the ferroelectric domain size in short rods. Finally, the effect of nanoconfinement on polymorphism of crystalline polymers has not been well-understood. There are several reasons. First, most common crystalline polymers, such as polyethylene (PE), PEO, and polycaprolactone (PCL), do not exhibit polymorphism under normal crystallization conditions. Second, even though some crystalline polymers, such as PP32,33 and nylon-6,34−37 exhibit rich polymorphism, only mesomorphic phases with poorly organized crystalline structures are obtained upon nanoconfinement. As we know, rich polymorphism exists for PVDF due to the flexibility in chain conformation: (1) the nonpolar TGTG′ α phase (or form II), (2) polar all-trans β phase (or form I), (3) polar TTTGTTTG′ γ phase (or form III), and (4) polar TGTG′ δ phase (or form IIp).30,38,39 When crystallized from the melt under moderate cooling rates, the nonpolar α phase, rather than the polar β phase, will be obtained because the α phase is kinetically favored. Furthermore, under suitable conditions, various polymorphism and phase transformations have been observed.40 Therefore, PVDF can be ideal for the study of confinement effect on polymorphism. Note that PVDF is different from P(VDF-TrFE), which only has a low-temperature ferroelectric phase and a high-temperature paraelectric phase, separated by the Curie temperature. For PVDF confined in AAO nanopores, contradictory results have been reported so far. In some reports,8,17−20 the nonpolar α phase is observed, whereas in other reports22,23,41,42 the polar γ or β phase is observed on the basis of WAXD study, nearly regardless of the pore diameter (ranging from 5 to 400 nm) and the confinement substrate (Al2O3 or SiO2). Careful inspection of the methods to infiltrate PVDF homopolymer into the nanopores reveals that the polar γ or β phase is caused by the solution method using N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and N,N-dimethylformamide (DMF), whereas the nonpolar α phase is obtained by the melt infiltration method. It is well-known that polar solvents such as DMAc, NMP, and DMF can induce polar phases for PVDF.40 Therefore, we consider that other than the solvent effect, 2D confinement cannot induce polar phases for PVDF when crystallized from the melt, at least down to 35 nm.8,17−20 From the above discussion, it is still not clear whether 3D nanoconfinement will lead to any change in polymorphism for melt-crystallized PVDF or not. In this work, we will focus on the effect of 3D nanoconfinement on PVDF polymorphism. However, it is not easy to confine PVDF in well-defined nanospaces and achieve completely confined crystallization. First, PVDF-based block copolymers with well-defined chemical structures are difficult to synthesize.43 Second, submicrometer droplets have been obtained from immiscible PVDF blends when PVDF is a minor component.9 However, only fractionated PVDF crystallization is observed with a minor crystallization peak at 110 °C, which is lower than the major crystallization peak at 135 °C for unconfined crystallization. Recently, taking advantage of the Plateau−Rayleigh instability in the electrospun cocontinuous PVDF/polysulfone (PSF) blend fibers (PVDF content less than 30 wt %), nanosized (mostly 100−300 nm) PVDF droplets were successfully prepared via thermal annealing at an appropriate temperature above the glass transition temperature (Tg) of the PSF matrix.44 Fractionated crystallization for PVDF droplets is observed with nonisothermal crystallization peaks at ca. 110 and 60 °C, respectively. The lower temperature crystallization peak at 60
from sample to sample. It was reported that exclusive homogeneous nucleation was observed when isotactic polypropylene (PP) was finely dispersed at a density of ca. 1012 droplets/cm3.15 The second most significant effect of nanoconfinement is on crystal orientation. Uniform crystal orientation can be achieved by confining crystalline polymers in nanoscale lamellae and cylinders.1−4 The reason is attributed to the direction of preferred crystal growth conforming to the long axes of the nanoconfinement geometry under normal crystallization conditions. Namely, under 1D confinement, the preferred crystal growth direction will be parallel to the lamellar surface. Under 2D confinement, the preferred crystal growth direction will be parallel to the cylinder axis. Poly(vinylidene fluoride) (PVDF) is a semicrystalline functional polymer, which is of substantial interest for transducers, sensors, and actuators because of its piezoelectric and ferroelectric properties as well as outstanding mechanical strength and chemical resistivity.16 When PVDF is confined in nanopores of ordered anodic aluminum oxide (AAO, pore diameter of 20−400 nm) by the melt-wetting method, the preferred crystal growth along the baxes of the nonpolar α phase is deduced to be parallel to the cylinder axis, as evidenced by the fact that the reflection-mode wide-angle X-ray diffraction (WAXD) shows only the (020)α reflection.8,17−20 When the P(VDF-co-trifluoroethylene) [P(VDF-TrFE)] random copolymer is confined in AAO pores (diameter of 15−400 nm), the c-axis of the low-temperature ferroelectric (FE) phase is deduced to be perpendicular to the cylinder axis because only the (110/200)FE reflection is observed in the reflection-mode WAXD profile.21−25 However, it cannot be ambiguously determined whether the growth direction for the confined P(VDF-TrFE) crystals along the cylinder axes of AAO nanopores is along the (200)FE or the (110)FE direction. This question is resolved by employing selected area electron diffraction of individual nanorods after dissolving away the AAO template because the (200)FE is seen to be exclusively along the rod long axis, indicating that the crystal growth direction of (200)FE conforms to the cylinder axes of AAO nanopores.25 Third, phase transition, including crystal melting, can also be altered by nanoconfinement, although this effect is usually less significant than the above two effects. For example, when nylon-6 is confined in the form of nanofibers (200 nm diameter), the Brill transition (i.e., from the monoclinic α-form to the high-temperature pseudohexagonal form) is observed at 180−190 °C, which is about 20 °C higher than that (160 °C) in the bulk sample.26 When P(VDF-TrFE) random copolymer is confined in long (>10 μm) AAO nanopores, the melting temperature decreases as the pore diameter reduces to below 60 nm.21,24 This could be attributed to decreased crystal thickness or crystallite size as the confining pore diameter decreases to below a critical value. Different from the melting temperature changes, the Curie (ferroelectric-to-paraelectric transition) temperature of P(VDF-TrFE) upon heating does not show much change as the pore diameter decreases down to even 15 nm.21,24,27 Nonetheless, when the length of P(VDFTrFE) nanorods (40 nm diameter) reduces to only ∼35 nm, an obvious change in Curie temperature is observed.28 This is attributed to the fact that the Curie temperature of P(VDFTrFE) is most sensitive to the ferroelectric domain size, not much to the crystallite size.29−31 We consider that 2D confinement in long nanorods is less effective than 3D 2633
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°C/min. A 5 mL aqueous solution containing 0.022 g of K2S2O8 (i.e., 0.45 wt % of the styrene monomer) was added into the mixture, and polymerization reaction was kept at 65 °C for 13.5 h. Sample aliquots were taken from the reacting mixture at preset time intervals throughout the polymerization process for morphology observation of PVDF@PS composite particles as well as for determination of styrene conversion gravimetrically. Briefly, a sample aliquot of 1.3−1.5 g total weight was taken from the reaction mixture and weighed accurately in a 20 mL vial. The polymerization was stopped by adding 1.0 mg of an inhibitor, hydroquinone. After drying in a vacuum desiccator for 2 days and then in a vacuum oven for 3 days, a constant solid weight was obtained. Because the PVDF weight percentage in the original reaction mixture was known, the increased weight percentage was taken as PS and used to calculate the styrene conversion. As we fixed 5.1 g of the St feed and varied the PVDF feed, different weight ratios of PVDF/PS were prepared. Finally, the product was dried by the same method mentioned above to obtain PVDF@PS particle powder for various characterizations except for transmission electron microscopy (TEM). Instrumentation and Characterization. Morphology of PVDF@PS colloidal particles was studied by TEM. A drop of the PVDF@PS latex sampled at different time intervals was diluted into 2.5 mL of deionized water to obtain a translucent suspension. After ultrasonication for 10 min, a drop of the suspension was cast onto a 400-mesh carbon-coated copper grid and was dried at room temperature under a reduced pressure overnight. A Zeiss Libra 200FE TEM at an accelerating voltage of 300 kV was used for the particle morphology study. A charge-coupled device (CCD) camera was used for digital image record. Here, an electron energy filter technique was used to obtain energy-filtered TEM images. When the energy loss for the transmitted electrons was chosen at 20 eV, there was a good contrast between fluorinated and hydrocarbon polymers. Therefore, PVDF and PS phases in the PVDF@PS latex particles could be clearly distinguished in energy filtered TEM images. In addition, some PVDF@PS latex particles with larger sizes were stained by RuO4 vapor for 10 min to enhance the contrast between PVDF and PS phases in the TEM study. Field-emission scanning electron microscopy (FE-SEM) images were obtained on a Hitachi S4500 FESEM at an accelerating voltage of 5.0 kV. Samples were sputter-coated with a thin layer (∼5 nm) of gold before SEM observation. Thermogravimetric analysis (TGA) was performed using a TGA Q500 instrument (TA Instruments, Inc., New Castle, DE) at a scanning rate of 10 °C/min from room temperature up to 700 °C under N2 flow. 1H NMR spectra were performed using an Oxford AS600 NMR instrument (Varian, Inc., Palo Alto, CA) to calculate the PVDF content and PVDF/PS ratio. Typically, a small amount of PVDF@PS particle sample was dissolved in 0.5 mL of DMF-d7, and the 1H NMR spectra were recorded at room temperature. Confined crystallization of PVDF in the PVDF@PS composites was investigated using a TA Q100 DSC under a dry nitrogen atmosphere. Approximately, 3 mg of sample was heated to 200 °C (or 180 °C sometimes), above the melting temperature of PVDF (∼175 °C), at a rate of 10 °C/min, and isothermal for 5 min before cooling down to room temperature at a cooling rate of −10 °C/min. WAXD experiments were carried out at the synchrotron beamline X27C, National Synchrotron Light Source (NSLS), at Brookhaven National Laboratory (BNL). The wavelength (λ) of the incident X-ray was 0.1806 nm. The d-spacing was calibrated by using silver behenate with the first-order reflection at a scattering vector (q) of 1.076 nm−1, where q = (4π sin θ)/λ and θ is the half scattering angle. A MAR charge-coupled device (CCD) was used as detector, and the typical data acquisition time was 60 s. One-dimensional (1D) XRD curves were obtained by integrating the corresponding 2D patterns radially. In some cases, 1D WAXD was also performed on a Rigaku 1.6 kW Xray diffractometer in the reflection mode with an angular scanning rate of 0.2°/min. FTIR study was conducted on a Bomem Michaelson MB 110 FTIR spectrometer in the transmission mode. The scanning range was from 4000 to 400 cm−1. A total of 32 scans were collected at a resolution of 4 cm−1.
°C represents the homogeneously nucleated confined crystallization, whereas the higher temperature crystallization around 110 °C can be attributed to partially confined crystallization. Nonetheless, due to the coexistence of partially and fully confined crystallization, the polymorphism of PVDF in nanoconfinement is difficult to determine. Here, we take advantage of nanosized (180 nm) PVDF latex particles from emulsion polymerization to achieve confined crystallization and study PVDF polymorphism constrained in 180 nm nanospheres. However, direct blending of PVDF latex particles in an immiscible polymer matrix, such as polystyrene (PS) or PSF, always leads to significant agglomeration of PVDF latex particles and no fully confined crystallization is observed. Alternatively, seeded emulsion polymerization using PVDF latex particles as seeds can be carried out to achieve core−shell composite particles, similar to the method reported for polytetrafluoroethylene (PTFE)/PS core−shell particles.45,46 In a first attempt, instead of forming core−shell particles, PVDF@PS Janus particles are obtained when the polymerization temperature is 75 °C.47 Even at late polymerization stage before particle flocculation, PVDF cores still partly protrude from the composite particles and only partially confined crystallization is seen. In this work, we change the seeded emulsion polymerization temperature to 65 °C and core−shell PVDF@PS composite particles can be obtained under appropriate conditions. Exclusive confined PVDF crystallization is confirmed by differential scanning calorimetry (DSC), and polymorphism is investigated by wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy. Intriguingly, polar β/γ phases are observed as a result of homogeneously nucleated confined crystallization of PVDF in 180 nm nanospheres.
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EXPERIMENTAL SECTION
Materials. Styrene (St, Acros Organics) was distilled under reduced pressure to remove inhibitors and stored in a freezer prior to use. Potassium persulfate (K2S2O8, Acros Organics, 99+%) was used without further purification. PVDF latex, Kynar Latex 32, was kindly provided by Arkema Inc. (King of Prussia, PA) and had an average particle size of ca. 180 nm.47 The PVDF latex was thoroughly dialyzed for 2 weeks using a dialysis tube with a molecular weight cutoff (MWCO) of 3500 Da (Spectrum Laboratories, Inc., Rancho Dominguez, CA). The aqueous solution in the dialysis bath was replaced by fresh deionized water every 24 h to remove free surfactants as much as possible. Finally, the solid content (ca. 10.8 wt %) of the dialyzed PVDF latex was determined gravimetrically. Preparation of Composite PVDF@PS Colloidal Particles. PVDF@PS composite particles were synthesized by emulsifier-free batch-seeded emulsion polymerization with the dialyzed PVDF seeds, as described in a previous report.47 Polymerization reactions were carried out at 60−75 °C in a 250 mL three-necked round-bottom flask equipped with a condenser, a magnetic stirrer, and a nitrogen gas inlet. As a typical example, 10.5 g of dialyzed PVDF latex dispersion with a solid content of 10.8 wt % was added into a beaker containing 60.0 g of deionized water at room temperature, and the mixture suspension was sonicated in an ultrasonic cleaner for 1 h to avoid any colloidal particle agglomeration. The ultrasonicated PVDF latex dispersion, containing 1.1 g of PVDF colloidal particles, was introduced into a three-necked round-bottom flask at a mild stirring speed of 300 rpm. Dry nitrogen was purged into the dispersion for 15 min to remove O2 before polymerization, and it maintained purging during the entire polymerization process. Then, 5.1 g of purified styrene monomer was added in one batch into the flask containing the PVDF dispersion. The mixture was agitated for 30 min for the styrene monomer to swell the anchored surfactant-stabilized PVDF colloidal particles at room temperature, followed by heating to 65 °C at a heating rate of 2.5 2634
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RESULTS AND DISCUSSION Effect of Polymerization Temperature on PS-Wrapping of PVDF Latex Particles. On the basis of our previous report, complete PS wrapping of PVDF latex particles could not be achieved even at the late stage (right before latex particle flocculation and precipitation) when the seeded emulsion polymerization was carried out at 75 °C.47 In this work, we first investigated the effect of polymerization temperature on the PS wrapping of PVDF latex particles. Here, the PVDF/St weight ratio was fixed at 0.8 g/5.0 g because it was easy to achieve the core−shell morphology for the PVDF@PS composite particles. The dialyzed PVDF latex seeds had a solid content of 10.8 wt %, and the polymerization time was used to control the St conversion. For example, when polymerization times were chosen as 6, 9, and 12 h for 75, 70, and 65 °C, respectively, the same St conversion of ca. 60% was achieved. Because these polymerization reactions were approaching the late stage, a small amount of particle flocculation was observed, and the aggregates were removed by filtration using a filter paper. After complete drying, TGA was employed to determine the PVDF/PS weight ratios for three PVDF@PS latex particles polymerized at 75, 70, and 65 °C (see Figure 1). The peak
Figure 2. DSC cooling curves for PVDF@PS composite particles prepared at 75 °C for 6 h, 70 °C for 9 h, and 65 °C for 12 h. The PVDF/PS weight ratios are the same, i.e., 15/85 (wt/wt), as determined by TGA. The cooling rate is −10 °C/min.
composite latexes polymerized at 75 and 70 °C, the crystallization of PVDF should be partially confined. For the PVDF@PS composite latex polymerized at 65 °C, a welldefined PVDF crystallization peak was observed at 77 °C, and the partial glass transition could be seen around 93 °C for PS upon cooling. This temperature was slightly higher than the confined PVDF crystallization in nanodroplets observed at 60 °C in a previous report.44 Therefore, we concluded that the PVDF particles were confined by the PS shell polymerized at 65 °C. Naturally, a question stands out: Why the PVDF@PS composite latexes polymerized at 75 and 70 °C had partially confined PVDF crystallization whereas the sample polymerized at 65 °C had confined PVDF crystallization? To answer this question, we used TEM to investigate the core−shell morphology of PVDF@PS composite particles polymerized at different temperatures. First, we recalled our recent report on the evolution of Janus PVDF@PS particles during the emulsifier-free seeded emulsion polymerization at 75 °C (see Scheme 1A).47 In the early to medium stage of polymerization, Janus PVDF@PS particles were obtained because of dewetting and merging of St-swollen PS bulges on the PVDF latex surface (note that the room temperature surface tension values are 40.7 and 30.3 N/m for PS and PVDF, respectively48). With further polymerization of St, the PVDF seed is gradually wrapped by PS. However, this process only led to eccentric core−shell morphology at the late stage of polymerization with PVDF still slightly protruding on one side of the composite particle. Further polymerization of St induced new small PS bulges on the protruded part of the PVDF seed (see Scheme 1A). This was experimentally observed by TEM observations in Figure 6 of ref 47 and Figure 3A. In these images, some small PS bulges, which were formed in the late stage of polymerization, are highlighted by white arrows in Figure 3A. When the polymerization was carried out at 70 °C, similar results were observed. Namely, eccentric PVDF@PS core−shell particles were obtained with small PS bulges located on the slightly protruded PVDF seeds (see Figure 3B, especially those highlighted by white arrows). However, when the polymerization was carried out at 65 °C, the PVDF@PS composite particles appeared as more uniform spheres with fewer small PS bulges on them (see Figure 3C). We consider that the PVDF latex particle must be wrapped (or end-capped) by PS (see Scheme 1B). On the basis of these
Figure 1. TGA weight loss (solid lines) and derivative (dashed lines) curves for the PVDF@PS composite particles prepared at 75 °C for 6 h, 70 °C for 9 h, and 65 °C for 12 h. In the seeded emulsion polymerization, the PVDF/St ratio is kept at 0.8 g/5.0 g, and the St conversion is the same for three reactions (ca. 60%).
weight losses at 410 and 454 °C in the derivative curves were assigned to PS and PVDF degradation, respectively. Pure PS decomposed completely in nitrogen, whereas PVDF decomposition left a significant amount of carbon due to char formation. From this figure, three samples had nearly overlapped weight loss curves. This indicated the same core− shell composition for these samples, i.e., PVDF/PS = 15/85 (wt/wt), assuming that no pure PS or PVDF particles were present. Nonetheless, these three samples exhibited different crystallization behavior in DSC during cooling after melting at 200 °C for 5 min (Figure 2). For samples polymerized at 75 and 70 °C, broad PVDF crystallization peaks were observed around 95−105 °C, overlapping with the glass transition of PS at 100 °C. Note that the unconfined PVDF latex had a peak Tc around 135 °C upon cooling at −10 °C/min (e.g., see Figure 10). For confined PVDF crystallization in nanodroplets (average size of ca. 200 nm, similar to the size of our PVDF latex particles), the peak Tc was observed at 60 °C upon cooling at −10 °C/min. 44 Comparing these with the broad crystallization peaks around 95−105 °C for the PVDF@PS 2635
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Scheme 1. Formation Mechanisms for the PVDF@PS Composite Particles Polymerized at (A) 75 °C and (B) 65 °C
Figure 3. Bright-field TEM micrographs of the PVDF@PS composite latex particles [PVDF/PS = 15/85 (wt/wt)] polymerized at (A) 75 °C for 6 h, (B) 70 °C for 9 h, and (C) 65 °C for 12 h.
Figure 4. Bright-field TEM micrographs of the PVDF@PS composite particles obtained at a PVDF/St feed ratio of 1.1 g/5.1 g at the polymerization temperature of (A) 70 °C for 6.5 h and (B) 75 °C for 4.75 h.
further decreasing the polymerization temperature down to 60 °C and below would not work either. First, the half-life of potassium persulfate was about 20 h at 60 °C.50 The polymerization rate would become so slow that a long time (>20 h) would be needed to completely wrap PVDF seeds. Before the PVDF could eventually be wrapped by PS, significant particle flocculation and precipitation had already taken place around 16 h of polymerization (see Figure S1A in Supporting Information). Consequently, both unconfined and confined PVDF crystallizations were observed by DSC cooling curves (see Figure S1B in Supporting Information). Because of this reason, we were limited to 65 °C as the lowest polymerization temperature to completely wrap PVDF seed particles with PS. Effect of PVDF/St Feed Ratio on Confined PVDF Crystallization. On the basis of the above study, we continued to investigate the effect of the PVDF/St feed ratio. First, we fixed the St feed at 5.1 g, polymerization temperature at 65 °C, and polymerization time of 12 h. Then, the PVDF feed varied
observations, it was clear that the partially confined crystallization of PVDF for the composite latex particles polymerized at 70 and 75 °C was a consequence of incomplete wrapping of PS on the PVDF latex particles before latex particle flocculation and precipitation. After complete wrapping of PS on the PVDF particles at 65 °C, confined PVDF crystallization was successfully achieved. We noted that the maximum polymerization times were limited to 6 h at 75 °C, 9 h at 70 °C, and 12 h at 65 °C. Even before these limits, some composite particles were observed to have more than one PVDF seeds, as shown in Figure 4A,B (highlighted by arrows). These multiple core composite particles must be formed via particle merging or flocculation during polymerization. Further polymerization beyond the time limits at all the temperatures studied would lead to accelerated polymerization rate (i.e., the autoacceleration effect49), eventually leading to significant particle flocculation. In other words, further polymerization resulted in latex precipitation instead of complete PS wrapping of PVDF seeds. In addition, 2636
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°C in the derivative curves were assigned again to PS and PVDF decomposition, respectively. It was clear that the PS weight loss decreased upon increasing the amount of PVDF seeds in the feed. From these weight loss curves, the PVDF/PS weight ratios could be determined, and the results were consistent with 1H NMR results (see Table 1). Thermal behavior of various core−shell PVDF@PS composite particles in Table 1 was investigated by DSC. The composite samples were subjected to identical thermal history composed of (i) a first heating at 10 °C/min to 180 °C for 5 min to completely remove thermal history, (ii) a first cooling at −10 °C/min to 30 °C for 5 min, (iii) a second heating at 10 °C/min to 200 °C for 5 min, and (iv) a second cooling at −10 °C/min to room temperature. The first and second cooling DSC curves are shown in Figures 6A and 6B, respectively. During the first cooling (Figure 6A), PVDF crystallization peaks were seen between 65 and 80 °C for samples CS1−CS4, and the heat of fusion gradually increased upon increasing the PVDF weight fraction in the composite particles. The observation of a single crystallization exotherm between 65 and 80 °C was a clear indication of confined crystallization for PVDF because most PVDF seeds were wrapped by PS shells. When the PVDF/PS weight ratio increased to 39/61 (by TGA), double crystallization peaks for PVDF were observed with a strong peak at 135 °C and a weak peak at 68 °C. The major crystallization peak at 135 °C was attributed to unconfined heterogeneously nucleated crystallization, and the minor crystallization peak at 68 °C was attributed to confined homogeneously nucleated crystallization. During the second cooling from the melt at 200 °C, similar results were seen except that crystallization peaks became sharper. The DSC results in Figure 6 show that when the PVDF/St feed ratio and PVDF/PS ratio were below ca. 20 wt % (see Table 1), confined PVDF crystallization could be successfully achieved. To understand the effect of PVDF/St feed ratio, TEM was used to characterize the core−shell morphology of PVDF@PS composite particles in Table 1, and results are shown in Figure 7. For the CS1, CS2, and CS3 samples (Figures 7A−C), uniform and spherical composite particles were observed, indicating good core−shell morphology. Note that the light gray “connections” among aggregated particles in Figures 7A,B were formed by trace amount of impurities (e.g., surfactants) in the system during solvent drying process, and this is typical for TEM imaging of aggregated latex particles on the grid. In
in order to investigate the influence of the PVDF/St feed ratio on the morphology of PVDF@PS core−shell latex particles and the subsequent confined crystallization. When we varied the PVDF/St feed ratio, the PVDF/PS composition in the PVDF@ PS composite latexes was determined by both TGA and 1H NMR analyses. Table 1 shows detailed experimental conditions for the preparation of a series of PVDF@PS composite particles with different compositions. Table 1. Compositions of Core−Shell (CS) PVDF@PS Latex Particles Polymerized at 65 °C for 12 h samples
PVDF latex (g)
PVDF weight (g)
St weight (g)
PVDF/PS by TGA (wt/wt)
PVDF/PS by 1H NMR (wt/wt)
CS1 CS2 CS3 CS4 CS5
2.5 4.5 6.1 8.5 20.2
0.26 0.47 0.63 0.88 2.10
5.1 5.1 5.1 5.1 5.1
7/93 11/89 14/86 18/82 39/61
6/94 9/91 12/88 15/85 33/67
Figure 5 presents TGA curves of the PVDF@PS core−shell particles in Table 1. The peak weight losses at ca. 410 and 454
Figure 5. TGA weight loss (solid lines) and derivative (dashed lines) of the PVDF@PS composite particles with different PVDF/PS weight ratios (see Table 1). The polymerization is carried out at 65 °C for 12 h with a fixed St feed of 5.1 g. The TGA weight loss curve of pure PS is included for comparison.
Figure 6. DSC cooling curves of different PVDF@PS composite particles (see Table 1) with different PVDF/PS ratios after melting at (A) 180 °C and (B) 200 °C for 5 min. The cooling rate is −10 °C/min. 2637
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Figure 7. Bright-field TEM micrographs of the PVDF@PS composite particles with different core−shell ratios polymerized at 65 °C: (A) CS1, (B) CS2, (C) CS3, and (D) CS5. The inset in (C) shows the energy-filtered TEM image with the same magnification (the electron energy loss is selected at 10 eV). The St feed is fixed at 5.1 g, and the amount of PVDF seeds is varied as shown in Table 1. The latex particles in (A) and (D) are stained by RuO4 vapor.
Figure 8. (A) Styrene conversion (%) and polymerization rate as a function of polymerization time at 65 °C with a fixed PVDF/St feed ratio of 1.15 g/5.1 g. The polymerization rate is defined as the slope in the St conversion (%) curve with a unit of %/h. (B) Change of the average particle size along the long axes of PVDF@PS composite particles polymerized at 65 °C as a function of polymerization time.
PS/St bulges would grow independently on the exposed PVDF surface. Finally, irregular shaped PVDF@PS composite particles with protruded PVDF seeds rather than core−shell particles were obtained. More irregular shaped latex particles with more than one PS bulges were obtained when the PVDF/St feed ratio was even higher, i.e., 2.9 g/5.1 g (see Figure S2 in the Supporting Information), and unconfined PVDF crystallization was observed during DSC cooling (data not shown). From this TEM morphology study, it is clear that samples CS1−CS4 with relatively well-defined core−shell morphology will result in
contrast, when the PVDF/St feed ratio was as high as 2.1 g/5.1 g for the CS5 sample (or PVDF/PS = 33/67 (wt/wt) by 1H NMR in the composite particles), irregular shaped particles rather than the core−shell morphology were observed in TEM (Figure 7D). First, a significant protrusion of the PVDF seeds was seen at one side of the particles. Second, several small PS bulges were observed on the PVDF protrusion. On the basis of our previous report,47 when the St feed ratio was low, the viscosity of St-swollen PS bulges was high and the protruded PVDF surface thus became sticky. As a result, newly nucleated 2638
dx.doi.org/10.1021/ma500249p | Macromolecules 2014, 47, 2632−2644
Macromolecules
Article
Figure 9. Energy-filtered TEM micrographs showing evolution of particle morphology when the seeded emulsion polymerization was carried out at 65 °C with the PVDF/St feed ratio being 1.15 g/5.1 g: (A) 1, (B) 2, (C) 3, (D) 4, (E) 5, (F) 6, (G) 7, (H) 8, (I) 10.5, and (J) 13.5 h. All scale bars are 200 nm.
composite particles during seeded emulsion polymerization was investigated by energy-filtered TEM, and results are shown in Figure 9. At 1 h (Figure 9A), PVDF latex particles with one or two surface-nucleated PS bulges were observed because PVDF was highly immiscible with both St and PS, and the Stswollen PS bulge dewetted on the PVDF latex surface. Here, hydrocarbon polymers generated more electrons with 20 eV of energy loss than fluoropolymers. As a result, PS bulges appeared brighter than PVDF cores. At 2 h (Figure 9B), PVDF latex particles mainly had only one PS bulge and particles with two PS bulges almost disappeared. On some PVDF particles, the second PS bulge was fairly small, suggesting that it was just nucleated on the PVDF latex surface. In this stage I polymerization, the average size along the particle long axes increased from 186 nm for the pristine latex particles to ca. 250 nm (see Figure 8B). When the polymerization time reached 3 h (i.e., stage II polymerization), nearly all PVDF latex particles had only one PS bulge (see Figure 9C). These PVDF@PS composite particles are termed as Janus or snowman-like colloidal particles.51−54 With further increasing the polymerization time to 4−7 h (Figures 9D−G), the PS domain started to wrap around the PVDF particles due to continued polymerization. At 8 h, the PS domain nearly wrapped around the PVDF particles, but still leaving a small bulge of PVDF at one side of some Janus particles (see Figure 9H). During this stage polymerization, the average size of particle long axes gradually reached ca. 380 nm at 8 h (see Figure 8B). During stage II polymerization (3−7 h), exclusive PVDF@PS Janus particles with single PS bulge were observed. Additional TEM morphology study was conducted at even longer polymerization time beyond 8 h. The purpose was to see whether the PVDF cores could be fully wrapped by the growing PS domain in the stage III polymerization or not. Because of a slow polymerization rate at 65 °C, significant particle agglomeration would not be seen until 12 h of polymerization, and the St conversion (%) reached as high as 61%. This was different from the polymerization at 75 °C, where latex particle agglomeration took place around 6 h with a conversion of 80%.47 At 10.5 and 13.5 h, fully covered PVDF core−shell particles were often seen, together with some irregular particles with eccentric PVDF cores in the composite particles. For these particles, the high immiscibility between PVDF and PS/St and
confined PVDF crystallization, whereas samples (e.g., CS5) without the core−shell morphology, especially with protruded PVDF seeds in the composite particles, will result in unconfined PVDF crystallization. Seeded Emulsion Polymerization at 65 °C. From the above study, the optimal experimental conditions could be identified for successful confined PVDF crystallization, i.e., the PVDF/St feed ratio
