Synthesis and characterization of size-tunable core-shell structural polyacrylate-graft-poly(acrylonitrile-ran-styrene) (ASA) by pre-emulsion semi-continuous polymerization

European Polymer Journal 120 (2019) 109247

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Synthesis and characterization of size-tunable core-shell structural polyacrylate-graft-poly(acrylonitrile-ran-styrene) (ASA) by pre-emulsion semi-continuous polymerization Wenxin Huanga, Zepeng Maoa,b, Zhiren Xua, Bo Xianga, Jun Zhanga,b, a b

T

⁎

Department of Polymer Science and Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211816, China Jiangsu Collaborative Innovation Centre for Advanced Inorganic Function Composites, Nanjing 211816, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Core-shell structure Butyl acrylate Acrylonitrile-styrene copolymer Seed emulsion polymerization Nucleation mechanism

The core-shell polymeric polyacrylate-graft-poly(acrylonitrile-ran-styrene) (ASA) has been explored for the polymer industry due to its excellent properties. However, it has been a longstanding challenge to synthesize large size-tunable particles by the conventional batch polymerization. In this paper, an accurate method, i.e. preemulsion semi-continuous polymerization, is applied to prepare ASA polymeric particles, which makes a series of different particle sizes (100–450 nm) of the crosslinked poly(butyl acrylate) (PBA) inner core coated with a hard shell using the poly(acrylonitrile-ran-styrene) (AS) copolymer. Specifically, the feed modes and proportion of seed are investigated to tune the particle size. The results show that the PBA seed microsphere in pre-emulsion semi-continuous polymerization is enlarged by accurate feed rate, but not in batch polymerization. A possible mechanism for the growth of the PBA core is proposed that secondary particles from homogeneous nucleation in semi-continuous feed mode is less than that of batch mode, which attenuate the impact on the growth of core particle. The size and morphology of resulting particles are characterized using various analytical techniques including the transmission electron microscopy (TEM), the scanning electron microscopy (SEM) and the dynamic light scattering (DLS). Core-shell separates phase corresponding to different glass transition temperatures is evaluated using differential scanning calorimeter (DSC) analyses indirectly. Moreover, the mechanical properties including impacts and tensile strengths are also analyzed. This study thus highlights a detailed strategy to tune polymeric particles related to different properties and the nucleating mechanism of seed emulsion polymerization that governs structure, particle size and distribution of ASA polymer with a bespoke structure for various application.

1. Introduction Plenty of efforts have been made to enhance the characteristics of products for the functional requirements in many different materials industries. Micro-structure particle design is a particularly attractive method to customize the molecular structure of the polymer, with its superior performance over the conventional blending materials, which has been developed rapidly in the past few decades in the scientific and industrial field [1,2]. The idea of core-shell structure is inspired by the egg-shaped object in nature, where the soft core phase is protected by hard shell layers. Many studies have been investigated to synthesize a variety of ideal core-shell particles including organic or composite for various material modification with suspension, dispersion, emulsion, etc [2]. Emulsion polymerization has strong reaction heat release capacity in aqueous phase and spherical forming ability. The emulsion is ⁎

used to stabilize the particles, remain and complicate the polymerization system. Owing to the water-based dispersion medium environmental friendly, emulsion polymerization has been established a complementary theory applied in industrial technique [3]. Emulsion polymerization is a feasible method in preparation of multi-monomer phase-separated core/shell morphology particles. It avoids the the implosion phenomena in bulk polymerization due to low viscosity and water phase dispersion of emulsion. The differential microemulsion polymerization method is used to synthesize the PS/PMMA nanoparticles which are less than 20 nm, reducing the amount of surfactants significantly compared with the conventional microemulsion method [4]. Hassouna et al. [5] highlighted that cellulose nanocrystals (CNC) and poly(ethylene glycol) (PEG) were utilized in copolymer core and nanocomposites shell as the biobased materials by semi-continuous emulsion polymerization, which overcomes the coalescence

Corresponding author at: No.30 Puzhu Road, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211816, China. E-mail address: [email protected] (J. Zhang).

https://doi.org/10.1016/j.eurpolymj.2019.109247 Received 1 July 2019; Received in revised form 6 September 2019; Accepted 8 September 2019 Available online 16 September 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

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based on ABS, the core-shell structure has been known as ASA that made the replacement of nuclear with PBA. The core phase PBA whose the main chain contains saturated bonds greatly improves the weatherability including the UV resistance, sunlight yellowing and antioxidant capacity. The crucial factors appearing to high toughness of core-shell structure has been explored, such as core/shell ratio [19,23] and particle size [23]. It has been proved that a high toughening efficiency was determined by large-size particles and high rubber content [23]. The interesting properties of the core-shell heterogeneous polymer have been focused on the nucleation process of particles, especially the core phase size [24,25]. The aim is to prepare latex particles with narrow distribution and good stability of colloidal particles, which includes a sequential feed strategy to achieve nanoscale organization [26]. The core phase size of the core-shell structure material needs to locate in a reasonable range to optimize the properties. Generally, the small particle size latex particles are easily obtained in the emulsion polymerization process. PB as a core phase is easily coated by the shell layer because of its poor hydrophilicity, which makes the ABS with large nuclear phase easier to prepare. However, the PBA synthesis process is perfectly hard to control because its hydrophilicity is greater than PB, which is the key difficulty of ASA in the synthesis process. A small amount of a crosslinker used in formulations can decrease the degree of particle swelling and reduce the rate of particle growth [27]. If polymer particles are starved with a semibatch emulsion polymerization process, the feed rate is the key of determination the regime of particle formation [27–29]. Sajjadi et al. [29] proposed a basic theory about the fraction of emulsifier charge and feed of butyl acrylate system with the particle size floating in the 100–250 nm. The monomer with different water solubility were controlled to establish the model of nucleation mechanism [30,31]. The lower water solubility revealed the larger particles and lower particle number density with the increase feed rate [30]. Therefore, the feed strategy of semi-continuous mode is designed and controlled precisely, including the emulsifier and monomer, and compared with the conventional batch polymerization. Furthermore, the number density of particles is quantified and the mechanism of emulsion polymerization by different feed method is concretized to be comprehensible. The main research in this paper focuses on tailored core-shell latex particles with a crosslinked soft core and hard shell to design new material that can realize the use of controllable materials applied in different domains. Therefore, the present study will contribute to a better understand in nucleation mechanism of particles growth and the relationship between specific core-shell structure size or morphology and the final properties of the material.

nanoparticles, producing the nanocomposite material with renewability, biodegradability and exceptional mechanical properties. Core-shell heterogeneous polymers have attracted much attention for their superior performance in general blending and monomer copolymerization by microstructure designs. Polybutadiene-graft-poly (acrylonitrile-ran-styrene) (ABS) is a typical core-shell polymer whose core polybutadiene (PB) rubber phase provides a good resistance to impact, which coated with random poly(acrylonitrile-ran-styrene) (AS) perform the strength, rigidity and high gloss levels. The material property is controlled by multiple parameters such as rubber content [6], particle size [6–8], size distribution [8], shape and morphology [6] and even various polymerization conditions. In general, as the rubber content increases, the strength, hardness, heat resistance and rigidity of the ABS decrease [8]. The increment of the rubber content can be in the control of the mass ratio and core phase size. In 1960s, commercial success of ABS aroused the exploration interest of researchers. After this, the core phase PB is replaced by poly(butyl acrylate) (PBA) to improve its poor weatherability and poor heat-resistant oxygen aging. Polyacrylate-graft-poly(acrylonitrile-ran-styrene) (ASA) with a saturated structure on its main chain has good resistance to yellowing and to light, dimensional stability and easy processability, which is more suitable as an impact modifier for building and outdoor materials [9]. Based on the well-defined core-shell structure shape, various heterogeneous structures are derived due to the nonequilibrium conditions during the actual process by the phase separation, such as raspberryshaped [10–12], mushroom-shaped [13], confetti-shaped [14] and sandwich-shaped [15]. Tolue et al. [10] synthesized a range of ASA particle with raspberry structure by semi-continuous polymerization, which has obtained the large-size particle (over 400 nm) only using non-ionic surfactant. The aforementioned synthesized 476 nm-ASA shows excellent performance in toughening SAN copolymer as impact modifier [11]. Qiao et al. [12] synthesized raspberry-like hybrid particles through a biphasic sol-gel methods with a Pickering process, which highlighted the core-shell structure that determined by the surface hydrophilicity of the seeds and the monomer swell process. Tanaka et al. [13] prepared mushroom-like Janus particle with the pH-responsive property by internal phase separation from solvent. This method plays a critical role in the acquisition of spherical particles, and also has a great connection with the grafted shell. Okubo et al. [14] prepared the PS/poly(n-butyl methacrylate) (PnBMA) composite particles whose shape changed from spherical to confetti-like in 80/20 of ethanol/water medium by the seeded dispersion polymerization. Kirsch et al. [16] focused on the comprehensive prediction of particle morphology for all core-shell structures based on the computational calculation of thermodynamic and kinetic aspects. In our previous work, commercial ASA with 60 wt% rubber core content exhibited a good impact strength and elongation at break in balance with loss of the toughness and rigidity [17]. Moreover, it showed excellent compatibility with AS and PVC, which enhanced the heat resistance and processability of the blend system [18,19]. Therefore, we synthesize and characterize ASA latexes by seeded emulsion batch polymerization in which the maximum diameter of PBA is only 245 nm [20]. We also report that the chlorinated polyethylene (CPE)/ ASA/AS ternary blend improves the toughness and impact of ASA/AS binary blends without sacrifice of heat resistance at different temperatures [21,22]. In order to reconfirm the formation mechanism of the core/shell content and morphology, the ASA particle would be prepared by seeded emulsion polymerization during the work.

3. Experimental 3.1. Materials Styrene (St) from Lingfeng Chemical Co., Ltd., Shanghai, China, and acrylonitrile (AN) from Lilai Chemical Reagent Co., Ltd., Yixing, China, were distillated under vacuum to remove their inhibitor. Butyl acrylate (BA) from Jurong Chemical Co., Ltd., Yixing was analytical grade. Sodium dodecyl sulfonate (SDS), from Lingfeng Chemical Co., Ltd., Shanghai, China, as anionic surfactant was used directly without any further purification. Ammonium persulphate (APS, AR) and sodium bicarbonate (NaHCO3, AR) from Lingfeng Chemical Co., Ltd., Shanghai, China, were purchased as initiators and electrolyte agent, respectively. Allyl methacrylate (AMA, CP) from Aladdin Chemistry Co., Ltd., Shanghai, China, and tertiary dodecyl mercaptan (TDM, CP) from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, were purchased as cross-linking agent and chain-transfer agent, respectively. Hydroquinone (AR) from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, was purchased for demulsification experiments. Distilled deionized water (DDI) was purchased from Wanqing Chemical

2. Design ideas Material upgrades are accepted as a general concept and an updated unit. As a widely used engineering plastic with core phase PB and shell layer AS random copolymer, ABS is required to be modified to acquire a wider range of applications. One direction of the modifications is weather resistance, and the other is the impact strength. Interestingly, 2

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3.3. Characterization

Table 1 Recipes for the preparation of PBA latexes (g). Ingredients

C0

C25

C50

C100

C200

C400

C0 DDI BA SDS APS AMA NaHCO3

400 100 1.0 0.25 0.5 1.0

4.8 120 40 0.12 0.06 0.2 0.2

2.4 120 40 0.12 0.06 0.2 0.2

1.2 120 40 0.12 0.06 0.2 0.2

0.6 120 40 0.12 0.06 0.2 0.2

0.3 120 40 0.12 0.06 0.2 0.2

3.3.1. Monomer conversions The core phase PBA monomer conversion is determined. The core latex (5.0 g) is added 2 drops 1% hydroquinone solution to prevent the further polymerization. Then the samples are dried in an electric thermostatic drying oven at 80 °C. In addition, all experiments are performed with three repetitions. Monomer conversions (xn) can be obtained from the given Eq. (1), where W0 represents the total mass of latex, Wm is the mass of monomer, W1 is the mass of sample latex and W2 is the mass of dried sample:

Glassware Instrument Co., Ltd., Nanjing, China.

⎛ W2 ⎞ xn = ⎜ × 100% W ⎟ W1 × Wm 0 ⎝ ⎠

(1)

3.2. Core-shell structure synthesis via emulsion polymerization 3.3.2. Gel content of the core phase The gel content, measured by means of solvent extraction method, reflects cross-linking degree to some extent. The dried PBA samples (0.3 g) are added to 25 mL Tetrahydrofuran (THF), and soaks at room temperature for 48 h. The sediment gel phase is separated from the sol phase and dried in an electric thermostatic drying oven at 105 °C. Gel contents (Gel) are calculated by Eq. (2), where M0 represents the mass of dried PBA gel and M1 is the gel after soaking THF:

The reaction is undergone in a 500 mL four-neck glass reactor equipped with a reflux, inlet nitrogen, and a mechanical stirrer. The reactor is held in an oil bath with thermostatic control. The polymerization is carried out at 80 °C and 120 rpm. The PBA seed was prepared previously by batch polymerization according to the reactant recipes listed in Table 1 (C0), and then the ASA core-shell latex is prepared via two-stage seed-emulsion polymerization. In the first stage, the pre-emulsion mixture, including the BA monomer, surfactant, initiator and water, is stirred for 1 h with a magnetic stirrer. Pre-emulsification at room temperature is vital to dissolve and disperse evenly in this stage. The required seed latex combined with10 g-DDI is added into the reactor. Under the same aforementioned reaction condition, the pre-emulsion mixture is added to the seed latex in a reactor by different addition modes: batch feed and semi-continuous feed. The mixture is one-time poured to the reactor at the beginning polymerization for batch feed mode, while the mixture is added to the seed latex with a feed rate of 0.5 mL min−1 during the reaction for semi-continuous feed mode. In second stage, the shell formed monomer with mass ratio of St and AN (3:1) was pre-emulsified as BA monomer, and the shell preemulsion mixture is added in a semi-continuous mode with the same rate. The rubber core PBA particles are named with Cn (‘n’ represented the mass ratio of DDI and seed, ‘0’ represented the seed latex) in Table 1. All seed latex is used in a wet basis and diluted by DDI. The recipes of shell preparation of ASA latexes are given in Table 2. Synthesized ASA particles are named after CnS. The core latex is used in a wet basis. All experiments are conducted with the same seed emulsion, i.e. C0 (100-nm-diameter) at 80 °C. The pre-emulsion mixture including SDS, APS, AMA, NaHCO3, and BA (25 wt% in distilled water) is charged under magnetic stirring. The emulsifier (SDS) is used in a ratio of 0.3 wt% (percentage of monomer as follows), while the ratio of initiator (APS) was 0.15 wt% to prevent the new latex particles from generation. The seed latex feed ranges from 0.375 wt% to 12 wt% of the monomer. The two kinds of particles by different feed modes are compared in monomer conversions and particle size and distribution.

Gel =

C0S

C25S

C50S

C100S

C200S

C400S

Core phase latex DDI AN St SDS APS TDM NaHCO3

175 70 5.83 17.50 0.070 0.035 0.023 0.1

140 70 5.83 17.50 0.070 0.035 0.023 0.1

140 70 5.83 17.50 0.070 0.035 0.023 0.1

140 70 5.83 17.50 0.070 0.035 0.023 0.1

140 70 5.83 17.50 0.070 0.035 0.023 0.1

140 70 5.83 17.50 0.070 0.035 0.023 0.1

(2)

3.3.3. Grafting degree The ASA powder is prepared by demulsification experiment, and placed in a vacuum oven at 80 °C for 6 h in order to remove the residual moisture and monomer. The the dried ASA samples (m0) are soaked in 25 mL acetone at room temperature for 72 h. The sediment (m1) is separated and dried in an electric thermostatic drying oven at 105 °C. The grafting degree (Gd) and grafting efficiency (Ge) are the weight ratio of grafted AS to rubber core particles and to shell phase particles, respectively. They are measured gravimetrically and calculated as given formulas (3) and (4):

Gd =

m1 − m2 × 100% 0.6m 0

(3)

Ge =

m1 − m2 × 100% 0.4m 0

(4)

3.3.4. Latex particle morphology and particle size distribution The morphology of latex particle is recorded by transmission electron microscopy (TEM JEM1400plus, JEOL, Japan) and scanning electron microscope (SEM JSM-5900, JEOL, Japan). The mean particle size of latex is characterized by dynamic light scattering particle size analyzer (DLS 90plus Brookhaven Instruments Corporation, America). The sample is diluted 1000 times by distilled water in glass cuvette to determine the particle size, and then diluted latex is dropped on the copper grids, after which the copper grids are dried in the air to determine the particle size, the particle polydispersity index (PDI) and the morphology.

Table 2 Recipes for the preparation of ASA latexes (g). Ingredients

M1 × 100% M0

3.3.5. The interaction of groups Fourier transform infrared spectrum (FTIR Nicolet IS5, Thermo Fisher, America) is obtained by scanning in transmissive mode over the scanning range of 4000–400 cm−1 after the grinding broken residual samples mixed with potassium bromide powder and pressed into sheets. 3.3.6. Differential scanning calorimetry analysis The glass transition temperature (Tg) of the core phase PBA and the copolymers ASA are measured by differential scanning calorimetry 3

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Fig. 1. Particle size distribution histograms of PBA latex particles synthesized by batch polymerization: (a) C0; (b) C25; (c) C50; (d) C100; (e) C200; (f) C400.

(4) the effect of volatilization is not considered in this work; (5) follow the assumptions made by Smith-Ewart equations. The ideal nucleation of the emulsion polymerization is supposed that the concentration of the seed latex and monomer in aqueous phase is constant in initial and middle reaction without secondary particle formation (new nucleation), and the initiator has no chance to enter the monomer droplet to initiate polymerization. Virtually non-ideal emulsion polymerization nucleation can be explained through mechanistic hypotheses that mainly sort into four types: (i) homogeneous nucleation; (ii) latex particle polymerization; (iii) monomer droplet nucleation; and (iv) micelle nucleation. In this study, the amounts of SDS in pre-emulsion mixture are less than the critical micelle concentration (CMC(SDS) = 8–9 mmol L−1), thus the new micelles hardly exist in the system and the micelle nucleation is impossible [32,33]. Moreover, droplet nucleation has been proved that occurs very barely in conventional free radical polymerization system [32]. The BA monomer with low water solubility tends to diffuse in the aqueous continuous phase and be trapped by other dominate discrete hydrophobic phases easily, and the free radicals are generated by the decomposition of the initiator and diffused through aqueous phase, which is the outset of polymerization. Therefore, two principle nucleations may occur in the first stage for the synthesis of PBA core in batch method and semi-continuous polymerization in this study. The two polymerization reactions: (i) the monomer dissolved in the aqueous phase is initiated by a free radical, which is called the homogeneous nucleation [32]; (ii) in the presence of seed latex particles, the free radicals combined with monomer molecules become surface-active and then enter the particles to initiate internal polymerization resulting in enlarged the seed particles, which is called seed latex particle polymerization [34]. The probability of free radical initiated polymerization is estimated by calculating the number of particles per unit volume which is called number density. Herein, the number density of seed particles (Ns), latex particles in termination of the polymerization (Nl), and monomers dissolved in aqueous phase (Nm) are calculated, respectively.

(DSC Q20, TA, America). The experiments are carried out under a nitrogen atmosphere. After eliminating the thermal history, the samples are scanned from −90 to 160 °C at a heating rate of 10 °C min−1, and the glass transition temperature is obtained in DSC curves. 3.3.7. Mechanical testing The ASA pieces are cut into standard strip specimens (80 × 10 × 4 mm3 ) with a 2 mm-deep and 0.25-mm-radius notch. The specimens are measured by Izod impact testing machine (UJ-4, Chengde Testing, Chengde, China) at ambient temperature (23 ± 2 °C) according to ISO 180. The tensile properties are determined by microcomputer control electronic universal testing machine (CMT-5254, Shenzhen SANS Testing Machine Co., Ltd., Shenzhen, China), using a strain rate of 50 mm min−1 according to ISO 527. 4. Results and discussion 4.1. Effect of pre-emulsion mixture addition mode The particle sizes of the seed and core latex synthesized by the batch method are shown in Fig. 1. The mean particle size of PBA seed latex is around 95 nm shown in Fig. 1(a), while the enlarged core latex particle is limited within the range of 100–220 nm. A phenomenon is that the size of core particle are all small and tends to be consistent to the seed. When the proportion of the seed latex decreases, the growth rate of core size is lowered. The particle sizes of the seed and core latex synthesized by preemulsion semi-continuous polymerization with the same seed latex are shown in Fig. 2. On the contrary, as the proportion of the seed latex decreases, the core particle shows the larger size. Different from the batch method, the latex particles obtained by semi-continuous polymerization have particles size enlarged more than four times up to 426 nm. On the basis of mass balance, the following assumptions are proposed firstly to simplify the mathematical treatment: (1) the polymer particles are spherical; (2) the particle volume is equal to the monomer volume plus the polymer volume; (3) coagulation of particles may exist; 4

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Fig. 2. Particle size distribution histograms of PBA latex particles synthesized by pre-emulsion semi-continuous polymerization: (a) C0; (b) C25; (c) C50; (d) C100; (e) C200; (f) C400.

mass of a seed particle (ms = 5.65 × 10−16 g) can be obtained from the density of PBA ( ρPBA = 1.08 g·mL−1) as given Eq. (7) [29]. The number of seed particles, Ns is calculated by Eq. (8) [29]:

(i) Homogeneous nucleation The chance of homogeneous nucleation is evaluated by the concentration of monomer in aqueous phase. For batch polymerization, the concentration in aqueous phase is constant at saturation of monomer with low water solubility [30]. The solubility of BA monomer in aqueous phase is saturated with the constant concentration (1.4 × 10−3 g·mL−1). These monomers in aqueous phase can be induced to polymerization by the initiators, and the monomer droplets will continue to dissolve to maintain the constant monomer concentration of in aqueous phase:

Nm (batch) =

1.4 × 10−3 × NA = 6.576 × 1018mL−1 MBA

ρπD3 4 m = ρ ⎡ π (D /2)3⎤ = 6 ⎣3 ⎦ N=

FRBA × NA ≤ 9. 211 × 1017 mL−1·s−1 Vseed MBA

Mx n mV

(8)

where ρ is polymer density and D refers to the diameter of particle in eps. (7). M is the amount of monomer fed to the reactor in this stage and m is the mass of single particle, plus the monomer conversion, xn in exp. (8). The number of seed particles are calculated by the addition of seed latex in every single experiment, and Ns are showed in the following table (Table 3). For batch polymerization, the number density of seed latex is 5.203 × 1011 ∼ 8.101 × 1012 mL−1. The batch mode diluted the concentration of seed latex, which restricted the seed particles growth and facilitated monomer to nucleate secondary particles in aqueous phase. While the number density of seed latex by semi-continuous mode is 8.344 × 1012 ∼ 9.239 × 1013mL−1 over the batch polymerization with the rise of probability of enlarged process. The latex particles in termination of the polymerization is obtained by the Eq. (6). As shown in Table 4, the number density of latex particles for batch mode is all greater than the aforementioned Ns in Table 3, proving the formation of secondary particle with a

(5)

where the MBA is the BA molecular weight (128.17 g/mol) and NA represents Avogadro constant. For semi-continuous polymerization, the concentration of monomer in aqueous phase depends on the feed rate of monomer in the preemulsion. The adding monomer concentration is lower than the solubility of monomer in aqueous phase, which means the slowly added monomer is dissolved and consumed rapidly in every second without monomer enrichment in the early term of polymerization, as shown:

Nm (semi − continuous ) =

(7)

(6)

where the FRBA is the feed rate of BA monomer in pre-emulsion, and the Vseed is the volume of seed feed. The monomer concentration in the system of semi-continuous mode is lower than that of batch mode, which decreases the chance of homogeneous nucleation in semi-continuous polymerization. Therefore, the core particle size grows in semi-continuous mode polymerization rather than batch mode polymerization.

Table 3 The number density of seed particles (Ns, n, mL−1) for batch polymerization and semi-continuous polymerization. Number density Ns, Ns, Ns, Ns, Ns,

(ii) Latex polymerization The particle size of seed emulsion is about 100 nm (Ds), and the 5

25 50 100 200 400

Batch polymerization 12

8.101 × 10 4.110 × 1012 2.070 × 1012 1.039 × 1012 5.203 × 1011

Semi-continuous polymerization 9.239 × 1013 5.527 × 1013 3.065 × 1013 1.621 × 1013 8.344 × 1012

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Table 4 The monomer conversions, diameters of core PBA and the number density of particles at termination polymerization (Np, n) for batch polymerization and semicontinuous polymerization. Batch polymerization

Semi-continuous polymerization

Code

Conversion (xn, %)

Diameter (Dn, nm)

Number density (Np, n, mL−1)

Code

Conversion (xn, %)

Diameter (Dn, nm)

Number density (Np, n, mL−1)

C0, B C25, B C50, B C100, B C200, B C400, B

81.4 97.2 98.3 97.3 98.4 96.6

94.9 218.4 173.4 138.0 136.2 117.3

– 3.886 × 1013 7.969 × 1013 1.576 × 1014 1.664 × 1014 2.562 × 1014

C0, SB C25, SB C50, SB C100, SB C200, SB C400, SB

81.4 84.9 98.9 91.2 99.2 93.5

94.9 229.5 253.9 311.9 381.3 445.9

– 2.925 × 1013 2.565 × 1013 1.279 × 1013 7.647 × 1012 4.514 × 1012

considerable amount. However, the semi-continuous mode shows the opposite trend that the number density is lower than feeding Ns but in the same order of magnitude as feeding. Consequently, the secondary particle has little effect on the growth of the core particle in semicontinuous polymerization. This measurement approach proves in a simplified way that the semi-continuous mode could attenuate the impact of homogeneous nucleation forming secondary particle and enhance the effect of enlarged process. The number density of particles in the system is shown in the Table 5. It sums up various types of particles of batch and semi-continuous modes and emphasizes the ratio of monomer in aqueous phase for homogeneous nucleation and the preserved additional seed particle. The numbers of particle in initial and terminal polymerization are compared in Table 6. It shows that the numbers of latex particle are larger than that of feed seed regardless of the feed mode, which proves the secondary particle formation. The particle size as a function of particle number can be matched with the actual particle size. For batch mode, the minimal particle number is consistent with the largest particle size, while as the semi-continuous mode, the less addition of feed seed corresponds to the less particle number with the large size. It is inevitable that secondary particle form in the system, and it causes the theoretical values quite differs from the results shown in Table 7. The synthetic core-shell ASA is obtained with the particle size over 400 nm using anionic surfactant. As shown in Scheme 1, the hypothetical nucleation mechanisms are proposed. Above all, homogeneous nucleation is dominated in batch emulsion polymerization, which leads to the formation of secondary particles resulting in increasing the number of small particles. Although seed particle has a relatively large particle size, the number of them is very few. When the proportion of seed is high, the number of seed latex particles is relatively large, and thus the latex particles are more likely to be polymerized to increase the particle size. In addition, the batch method may cause the local concentration too high to dissipate heat and cause agglomeration. The mechanism of the pre-emulsion semi-continuous polymerization method is approximately the same as that of the batch method, but the Nm/Ns of semi-continuous mode is smaller than that of batch method. The low value of Nm/Ns indicates the possibility of the latex particle polymerization is greatly increased. Since the pre-emulsion is gradually added dropwise, the emulsifier is uniformly combined with the seed, and polymerization is initiated inside the seed continuously,

Table 6 The number of seed particle at initial and latex particle at terminal polymerization of batch mode and semi-continuous mode. The number of latex particle

C25 C50 C100 C200 C400

Batch polymerization

Semi-continuous polymerization

Ns (mL−1) Nl (mL−1) Nm (mL−1) Nm/Ns

5.203 × 1011 ~ 8.101 × 1012 3.886 × 1013 ~ 2.562 × 1014 6.576 × 1018 8.118× 105 ~ 1.264 × 107

8.344 × 1012 ~ 9.239 × 1013 4.514 × 1012 ~ 2.925 × 1013 6.374 × 1017 ~ 9.211× 1017 6.899 × 103 ~ 1.104× 104

Terminal polymerization

Seeds feed

Batch polymerization

Semi-continuous polymerization

1.376 × 1015 6.879 × 1014 3.440 × 1014 1.720 × 1014 8.599 × 1013

6.600 × 1015 1.334 × 1016 2.619 × 1016 2.755 × 1016 4.234 × 1016

4.968 × 1015 4.294 × 1015 2.215 × 1015 1.266 × 1015 7.459 × 1014

Table 7 The characteristics of the synthesized core phase PBA and ASA particles diameters. Type

Theoretical calculation

Batch polymerization Pre-emulsion semicontinuous polymerization

Dn (nm) C0

C25

C50

C100

C200

C400

100 C0S 187.5 C0 94.9 C0 94.9 C0S 115.3

447.3 C25S 754.0 C25 218.4 C25 229.5 C25S 344.5

537.5 C50S 922.1 C50 173.4 C50 253.9 C50S 358.0

661.2 C100S 1144.5 C100 138.0 C100 311.9 C100S 378.6

794.5 C200S 1402.7 C200 136.2 C200 381.3 C200S 426.7

975.1 C400S 1740.9 C400 117.3 C400 445.9 C400S 492.2

thereby improving the stability of the emulsion. The semi-continuous polymerization could enlarge particles effectively. It reveals the potential of repeatably enlarge process by a new feed mode pre-emulsion polymerization.

4.2. ASA and PBA particle morphology The morphologies of ASA latex particles with core-shell structure are examined by SEM (Fig. 3) and TEM (Fig. 4), which observes the size of particle directly corresponded with the above results by DLS (Fig. 2). The SEM micrograph of the ASA shows that spherical shape isolated particles are arranged tightly and the distribution of the particle size is relatively uniform. The particle size of the ASA and PBA could also be visually seen from the TEM image, of which the ASA particle size exceeded the size of PBA core phases. The average diameter characterized by TEM is little smaller than that derived from the DLS resulting from the shrinkage of the hydrated layer of colloidal particles in drying process of TEM sample preparation. The morphology of the two particles confirm the spherical shape seen in the SEM image, and these separated particles bulge on the surface by hydrophobic interaction, which shows the ASA core-shell structure and raspberry shape specifically (Fig. 4(d)–(f)). The

Table 5 The number density of particles and the ratio of Nm and Ns for batch polymerization and pre-emulsion semi-continuous polymerization in system. Number density/Ratio

Initial polymerization

6

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Scheme 1. Nucleation mechanism: (a) batch polymerization at beginning period; (b) batch polymerization at the middle period; (c) batch polymerization at final period; (d) pre-emulsion semi-continuous polymerization at beginning period; (e) pre-emulsion semi-continuous polymerization at the middle period; (f) preemulsion semi-continuous polymerization at final period.

large-diameter particle is surrounded by a small-size covered in SEM and TEM image, which is consistent with the proposed nucleation mechanism: homogeneous nucleation is still the dominant nucleation mode, and the increase of latex particle polymerization possibility effectively enlarges the particle size.

are difficult to stabilize when the emulsifier feed amount is less than CMC. Except for the monomer droplet nucleation and micelle nucleation (new nucleation), the synthetic system contained homogeneous nucleation and seed latex polymerization for two dominant polymerization which were shifted and competed mutually. The semi-continuous mode was used to increase the chance of nucleation of the latex particles effectively. In the presence of these seed latex particles, hydrophobic monomers added dropwise that were apt to enter the latex particles rather than aqueous polymerization, thus facilitating the polymerization of monomer inside the seed latex particles and enlarging the core gradually. The hard-shell grafting reaction site was the double bond on the branch of PBA core particles. The morphology of latex particles is related to the interfacial energy between polymer and aqueous phase during emulsion polymerization. In the second stage, the AN and St were a secondary and tertiary monomer in the system, respectively. Scheme 2 depicts that the PBA core is partially covered by AS microbeads adhering to the former. The inhomogeneous AS random copolymer was formed from monomer to single chains and clusters by self-assembly in aqueous phase. AS chain clusters adhere and coagulate into subparticles individually, and collide with the core particles simultaneously. Finally they coalesce together to construct the raspberrylike shell formation.

4.3. Mechanism of the raspberry-shape ASA particle formation

4.4. Gel content, grafting degree and grafting efficiency

On the basis of the stated results, a possible mechanism of the formation of the raspberry-like ASA particles is proposed as shown in Scheme 2. The enlarged process with the pre-emulsion semi-continuous mode in first stage is the essential “core” of the whole polymerization, which makes monomers consume controllably. The monomer droplets

As shown in Table 8, the gel content of PBA particle size of different core phases was around 80%, and the gel content of seed PBA was relatively low. It showed that core phase PBA has a similar degree of cross-linking, which should be due to the fact that the cross-linking agent (AMA) feed are the same except for the seed PBA, so the degrees

Fig. 3. Scanning electron microscope of ASA (C200S) particle.

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Fig. 4. Transmission electron microscope of different PBA core and ASA particles: (a) C50; (b) C200; (c) C400; (d) C50S; (e) C200S; (f) C400S.

of cross-linking were approximately equal. From the degree of grafting, most ASA grafting ratios were greater than 35%, and only the seed ASA grafting rate was relatively low. The surface of the latex particles needed to be covered by emulsifiers. Therefore, the smaller latex particle size required for the less emulsifiers on the surface for a single particle. Then the free radicals generated by the decomposition of the initiator initiated the latex particles relatively less, so the surface had less grafting active sites, which was not conducive to the grafting reaction, and the grafting rate and grafting efficiency were relatively low.

Table 8 Gel content, grafting degree and grafting efficiency of ASA with different core phase PBA diameters. Code

Gel (%)

Gd (%)

Ge (%)

C0S C25S C50S C100S C200S C400S

57.79 77.61 86.77 89.36 89.64 88.35

31.32 35.48 38.00 35.33 37.33 38.33

78.31 88.70 95.00 88.33 93.33 95.83

Scheme 2. Synthesis of the raspberry-like ASA particle. 8

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Table 9 Glass-transition temperature of PBA and ASA (oC). PBA

Tg,

C0 C25 C50 C100 C200 C400

−43.5 −43.9 −43.9 −43.7 −43.4 −42.3

PBA

ASA

Tg,

C0S C25S C50S C100S C200S C400S

−42.2 −41.3 −44.1 −41.6 −43.1 −42.4

1

Tg,

2

112.4 114.3 109.6 110.5 112.3 108.4

with different particle sizes were all around −43 °C, and the Tg value was not affected by the particle size of PBA. The ASA system showed two glass transition temperatures: Tg, 1 was at about −43 °C and Tg, 2 was approximately equal to 110 °C. Obviously, the Tg, 1 corresponded to the glass transition temperature of the PBA core and the Tg, 2 corresponded to the glass transition temperature of the AS shell. The glass transition temperature of PS was recorded as 100 °C generally. It had been reported in literature that Furushima [38] used the flash DSC to test the Tg of PAN, 129 °C, which reduced the influence of PAN crystallization for making the Tg value of PAN more accurate during DSC testing. The theoretical Tg value of the shell AS could be calculated as given in Fox Eq. (9):

Fig. 5. Fourier transform infrared spectrum of ASA with different core phase PBA diameters.

4.5. The group contained in the sample Fourier transform infrared spectroscopy was utilized to analyze various groups within the ASA polymers scanning frequency which ranges from 4000 to 400 cm−1 (Fig. 5). It could be seen from the infrared spectrum that the characteristic peaks of the ASA with different core particle sizes did not observe the formation of new groups or group shift. The absorption bands at 3060 and 3024 cm−1 were the C-H stretching vibration in aromatic phenyl ring, and 1602 and 1493 cm−1 were the conjugated C]C in ring, whereas absorption bands at 2961 correspoonds to CeH asymmetric stretching vibration of -CH3. 2930 and 2875 cm−1 corresponds to CeH symmetric and asymmetric stretching vibration and bending vibration of -CH2- in the chain backbone, respectly. Besides, the presence of peaks at 1457, 842, 760 and 700 cm−1 was the evidence of CeH bending or ring puckering [35]. The appearance of the C^N absorption band at 2237 cm−1 in the spectrum gave supporting evidence that the AS were successfully grafted onto core phase [36]. Similarly, the relatively obvious absorption band at 1734 cm−1 was the C]O stretching vibration, and the bands with peak value at around 1261, 1165 and 1064 cm−1 were assigned to the OeC vibrations of the PBA [37]. It was observed that the absorption peaks all become sharper as the addition of core phase size from 100 nm to 450 nm.

1 W W = A + S Tg Tg, A Tg, S

(9)

where WA and WS are the proportion of AN and St in all monomers, respectively. Here, Tg, Tg, A and Tg, S signify the theoretical glass transition temperatures of AS, AN and St, respectively. The calculated result, slightly deviated from the actual test results (Table 9). The theoretical Tg of AS is 106.8 °C, which may be due to the fact that the actual ratio of AN and St deviates from the initial feed ratio in semi-continuous polymerization. The ASA is core-shell grafting polymer synthesized by grafting AS onto a cross-linked PBA core phase. Hence, the AS is only grafted onto the surface of the crosslinked PBA core without affecting the PBA itself. The observed lower Tg, 1 of ASA corresponded to the pure PBA phase Tg, PBA −43 °C, and the single Tg, 2 at high temperature of ASA was similar to the theoretical Tg value of random copolymer, which exhibited a reasonably good fit with the core-shell model. Above all, it was possible to realize polymer toughening modification by taking advantage of the core-shell structure ASA as an impact modifier. 4.7. Mechanical properties

4.6. Differential scanning calorimetry curves of polymer 4.7.1. Impact property The energy absorbed by the impact force is closely correlated to the size of the PBA core phase in the material. The impact strength of ASA

DSC characterized the glass transition temperature of PBA and ASA (Fig. 6). As the Table 6 shown, the glass transition temperatures of PBA

Fig. 6. Differential scanning calorimetry curves of polymer: (a) PBA; (b) ASA. 9

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the core phase from being elastically ineffective. With the larger size of core phase particles over 400 nm, the number of latex particles was small correspondingly, and the ability to cause shear yield was limited, so the impact strength tended to decrease. 4.7.2. Tensile properties The strain-stress curves of the ASA core-shell polymer specimens were shown in Fig. 8 and the values of the tensile stress and elongation at break were displayed in Table 10. In general, the ideal theory of tensile property within a set of variables experiment could not be obtained, but a correlation between tensile strength and core phase size was formed, which clarified the direction of verification. It could be seen that as the particle size of the core phase PBA increased to 380 nm, the tensile strength of the ASA polymer showed a twists and turned upward trend, reaching the maximum value of 13.57 MPa. After the core diameter exceeded 380 nm, both the tensile strength and the elongation at break decrease rapidly. The trend of the deformation stress was similar. It could be found that core-shell polymer with the same rubber contents should also keep the proper soft core size in consideration.

Fig. 7. Notched Izod impact strengths of ASA with different core phase PBA diameters (test strips were not completely broken).

5. Conclusion In summary, the work offered a complete and specific methodology to enlarge ASA polymer particles, in which two parameters including monomer addition modes and the proportion of seed latex were changed, independently. The core particles size up to 220 nm was limited by batch polymerization, while the particles were obtained over 400 nm by pre-emulsion semi-continuous method. This work showed that the dominant latex nucleation mechanism could be estimated by the particle number density of nucleation based on theoretical calculations. The tunable-size particle is determined by the feed strategy. In addition, the special sphere raspberry-shape morphology of the latex exhibited by TEM and SEM. Moreover, DSC analysis indicated two different transition temperatures that the lower temperature point was consistent with the Tg of PBA and the higher one was associated with the glass transition of AS copolymer, which was consistent with the calculated Tg value of ASA polymer theoretically and effectively proved the expected heterogeneous core-shell structure. Ultimately, the separate phase polymeric particles of ASA core-shell structure could be correctly adjusted to tune and optimize the properties.

Fig. 8. Stress-strain curves of ASA with different size of the PBA core particle. Table 10 Tensile strength and elongation at break of ASA. Code

Tensile strength (MPa)

Elongation at break (%)

C0S C25S C50S C100S C200S C400S

10.61 ± 0.62 12.88 ± 0.62 11.80 ± 0.60 11.95 ± 0.87 13.57 ± 0.71 9.19 ± 0.60

191 160 201 188 179 112

± ± ± ± ± ±

8 2 21 15 9 13

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

polymer with a range of core size was obtained in Fig. 7. A rise was observed in notched Izod impact strength with the increase diameter of the core phase in the range of 100 nm to 400 nm, and with further increasing the size of the core phase particle over 400 nm, the impact strength decreased sharply. The general mechanism of impact modifier toughening materials mainly includes shear yielding and cavitation of rubber particles. These observations indicated that the efficiency of the initiation of voiding was directly correlated to the size of the core phase. If the size of the core phase was too small, the silver streaks were terminated too quickly, so that the number of induced silver marks was reduced, resulting in a low impact strength. In contrast, when the size of the core phase was too large, the number of silver streaks increased, but the growth of silver streaks was not well terminated, and the impact strength also decreased. In summary, the core phase size needs to be within a reasonable range. The impact strength increased significantly owing to increasing core particle size, which led to an enhanced ability to initiate voiding. Moreover, the ratio of core/ shell feed was certain, and the thickness of the shell layer was gradually reduced for the increase of core particles diameter, where the shell layer had a ridged effect and did not cause too much rigidity to prevent

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