КОЛЛОИДНЫЙ ЖУРНАЛ, 2014, том 76, № 1, с. 111-119
УДК 541.18
SHAPE OF THE PARTICLES PRODUCED BY SEEDED DISPERSION
POLYMERIZATION OF STYRENE
© 2014 г. Soleyman Hosseinzadeh*, Younes Saadat**1, Samira Abdolbaghi**, Faramarz Afshar-Taromi**, Alireza Hosseinzadeh*
*Department of Chemistry, Payame Noor University PO Box 19395-3697, Tehran, Iran **Department of Polymer Engineering and Color Technology, Amirkabir University of Technology
15914, Tehran, Iran Поступила в редакцию 08.05.2013 г.
In this work, seeded dispersion polymerization of styrene was carried out in the presence of various types of seed particles. We found that in the case of polystyrene and poly(methyl methacrylate) seeds, the shape of the resulting particles remained spherical. For styrene/poly(n-butyl methacrylate) (Pn-BMA) and sty-rene/poly(lauryl methacrylate) seeding particles, raspberry-like particles were produced along with those of occluded morphology. We studied the effects of various polymerization factors such as concentrations of a stabilizer, an initiator, and a monomer, a weight ratio of methanol to water, a type of initiator, weight ratio of styrene to Pn-BMA seed particles, and polymerization temperature on the formation of these raspberry-like particles. The experimental results showed that the increase of concentrations of the initiator and the stabilizer as well as that of methanol favors the formation of such particles by increasing their surface roughness. An increase of the temperature of polymerization had the same effect on the morphology of resulting product. We hypothesized that the nucleation and growth of specifically fine-structured polystyrene domains on the surface of the Pn-BMA particles guides the formation of non-linear morphology during seeded polymerization in colloidal solution.
DOI: 10.7868/S002329121401008X
INTRODUCTION
In recent years, composite polymeric particles with non-spherical morphologies and the methods of their preparation have attracted mush attention of scientific community because of the interesting properties of such objects [1—7]. Minimization of the interfacial energy between the growing particles and the reaction medium resulted in the spherical shape of polymer particles, which the polymerization in emulsion, suspension, and dispersions normally yields [8]. For preparation micron-size non-spherical particles, several techniques have been developed such as seeded emulsion polymerization [2—5], seeded polymerization with monomer swollen particles [6, 7], solvent evaporation method [9, 10], solvent absorbing-releasing method [11], deformation of spherical particles with tension or pressure [12], seeded dispersion polymerization (SDP) [13—18], and so forth. Among those listed, seeded dispersion polymerization is the most advantageous approach [13—18]. By using this method, one can prepare the particles with widely varied shapes and morphologies like egg-like [13], disklike [14], golf-ball like [15], rugby ball-like [16], confetti-like [17], polyhedral particles [18] and etc. Such
1 Corresponding author. E-mail: younes.saadat.gh@gmail.com.
structural diversity makes these objects promising for a range of applications, e.g., light scattering [19], biomedical field, microelectronics, and so forth.
We recently used dual SDP in the presence of saturated hydrocarbon droplets to fabricate non-spherical particles exhibiting a variety of shapes with predominantly asymmetric almond-shell morphology [20, 21]. We proposed a novel technique, which we called "micromolding polymerization", for one-step preparation of such particles [22]. We showed that SDP of styrene initiated by ammonium persulfate in the presence of seed particles with a glass transition temperature lower than the temperature ofpolymerization media also yielded various nonspherical particles [23]. In this work, we discuss the formation of nonspherical raspberry-like and occluded composite particles via SDP of styrene in the polar media. The effect of various polymerization conditions on the formation of raspberry-like composite particles of styrene/P«-BMA is reported.
EXPERIMENTAL
Materials
Styrene, methyl methacrylate (MMA), and «-butyl methacrylate (b-BMA) (Merck Chemical, Germany) were puried by distillation under vaccum. All of these
Table 1. Preparation of various seed particles by dispersion polymerization
(1) and (2) by counting at least 100 individual particles from the microscopic images.
Ingredients,weight, g Methacrylatea Styrene
Monomer 1.35 1.95
AIBN 0.027 0.05
PVP 0.27 0.4
Methanol 8.4 -
Ethanol - 12
Water 3.6 1.35
Prepared in sealed glass bottles: N2, 65°C, 24 h, tumbling speed: 24 rpm.
a MMA, b-BMA or LMA.
Table 2. Polymerization conditions for SDP
Components Weight, g
Seed particlesa 0.4
Styrene 0.4
Initiatorb 0.008
PVP 0.08
Methanol 2.8
Water 1.2
In sealed glass bottles: N2, 65°C, 24 h, tumbling speed: 24 rpm. a PS, PMMA, Pn-BMA or PLMA.
b
idi
Yjnd Yani
¿nnidi
Dn = PS =
(1)
(2)
AIBN, BPO, APS or KPS.
monomers were stored in the refrigerator before use. A portion of 2,2'-azobis(isobutyronitrile) (AIBN) (Merck Chemical, Germany) of reagent grade was puried by recrystallization. All other materials were used without further purication, including lauryl methacrylate (LMA), PVP (weight-average molecular weight 36000 g/mol; Merck Chemical, Germany), benzoyl peroxide (BPO), ammonium persulfate (APS), potassium persulfate (KPS) and ethanol and methanol of guaranteed reagent grade.
Dispersion Polymerization Method
Dispersion polymerization of styrene, MMA, n-BMA, and LMA was carried out under the conditions listed in Table 1. The morphology of produced particles was observed directly by Olympus optical microscope and Philips XL30 scanning electron microscope (SEM).
The number- and weight-average diameters (Dw and Dn, respectively) and polydispersity index (PDI = = Dw/Dn) of seed particles were calculated using Eqs.
Where nt is the number of particles with diameter d.
All of the obtained seed particles except polystyrene (PS) ones were used as seeds in the SDP without any washing.
Typical Seeded Dispersion Polymerization
The SDP of styrene was carried out in sealed glass bottles under the conditions described in Table 2. The polymerization was performed for 24 h under nitrogen atmosphere by tumbling at 24 rpm in a thermostatic bath at 65°C. The obtained composite particles were investigated with optical microscope and SEM. Fourier transform infrared (FTIR) spectroscopy was performed with a Thermo Nicolet NEXUS 670 FTIR spectrophotometer using the KBr standard to characterize the obtained particles.
RESULTS AND DISCUSSION
The SEM micrographs ofPS, PMMA, and Pn-BMA seed particles and optical micrograph of PLMA seed particles produced by dispersion polymerization under the conditions described in Table 1 are shown in Fig. 1. For PMMA and Pn-BMA particles, the values of Dn were 3 and 1.7 ^m and those of PDI were 1.004 and 1.012, respectively. Because of the adhesion of PLMA particles to the support due to the low glass transition temperature (Tg) of PLMA, we studied these seeds using optical microscopy to determine Dn = 2.1 ^m and PDI = 1.018. In case of PS, seed particles obtained by dispersion polymerization had bimodal size distribution (2.4 and 1.6 ^m) with PDI = = 1.008 for each size.
The SEM micrographs of the particles produced in styrene—PS, styrene—PMMA, and styrene—Pn-BMA systems and the optical micrograph of the particles prepared in styrene—PLMA mixture are shown in Fig. 2. In the case of styrene—PS and styrene—PMMA systems, the size of the obtained particles increased because of polymerization of styrene but their shape remained almost spherical. For styrene-Pn-BMA system, the particles exhibiting raspberry-like morphology were formed.
Okubo et al. applied SDP of styrene in the presence of Pn-BMA seed particles and produced Pn-BMA/PS composite particles with oil-in-oil structure, in which multiple fine PS domains formed inside and near the surface of the Pn-BMA particles [24]. In our study, it seems that the size of the PS domains became larger
Fig. 1. SEM micrographs of (a) PS, (b) PMMA and (c) P«-BMA seed particles; (d) optical micrograph of PEHMA particles produced by dispersion polymerization under the conditions listed in Table 1.
upon changing the polymerization conditions that results in raspberry-like morphology of P«-BMA/PS composite.
The polymerization of styrene in the presence of PLMA seeds under similar conditions gave particles with occluded morphology and with a size larger than that of the seeds. It is noteworthy that, after drying, all of the PLMA particles, which are visible in optical micrographs, are coagulated (because of the particle adhesion due to the low Tg of PLMA phase). The darker spots in these particles are the polymerized styrene domains. The presence of styrene seems to decrease the stability of the PLMA seeds that resulted in floccula-tion increasing the size of the PLMA domains. The polymerization of styrene in flocculated structure guided the formation of the particles with occluded morphology.
The raspberry-like particles described herein are attractive due to their unique surface morphology promising for the formation of light scattering coatings, those of controlled surface roughness at the micrometer scale and so forth. Most studies reported in
the literature have been focused on the preparation of organic/inorganic and inorganic/inorganic raspberrylike composite particles [25—27]; to the best of our knowledge, only a few articles reported the preparation of such particles with organic/organic composition [28, 29]. The method we suggested allows for successful preparation of such particles.
To develop our approach further, we studied the influence of polymerization conditions on the formation of such particles.
FTIR analysis was performed to characterize the obtained raspberry-like particle (Fig. 3). The FTIR spectrum show
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