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14 Jun 2012 - Nanjing Dingfeng Plastic Limited Company, Nanjing 211134, P. R. China. •S Supporting Information. ABSTRACT: The partially sulfonated ...
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Improvement of the Compatibilization of High-Impact Polystyrene/Magnesium Hydroxide Composites with Partially Sulfonated Polystyrene as Macromolecular Compatibilizers Zhen Yang,† Chengang Zhou,† Hu Yang,*,† Tao Cai,† Jun Cai,† Haibo Li,† Dao Zhou,‡ Bifeng Chen,‡ Aimin Li,† and Rongshi Cheng† †

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China ‡ Nanjing Dingfeng Plastic Limited Company, Nanjing 211134, P. R. China S Supporting Information *

ABSTRACT: The partially sulfonated polystyrene (SPS) is a very simple and easily prepared material. In this work, SPS was used for the first time as a macromolecular compatibilizer to improve the compatibilization of high-impact polystyrene (HIPS)/ magnesium hydroxide (MH) composites by self-compatibilization technology. The compatibilization effects of SPS were systematically studied by mechanical performance tests, limiting oxygen index (LOI) measurements, thermal stability analyses, and scanning electron microscopy (SEM) observation. On the basis of these experimental results, SPS was proven efficient to enhance the compatibilization of the HIPS/MH composites due to the coupling effects. The sulfonic groups of SPS could anchor onto the surface of MH particles through interaction with hydroxyl groups of MH. Meanwhile, the long alkyl chains of SPS readily entwisted with HIPS matrix. Besides, the effects of both the sulfonation degree and the content of SPS on the performance of HIPS/MH composites were investigated also. In order to obtain the best overall final performance, both of the aforementioned parameters should be controlled in a suitable range. The optimal condition in this study was 3−4.5 wt % of SPS with sulfonation degree of 24.8−35.5% in the composites. Furthermore, compared with styrene-butadiene-styrene block copolymer (SBS), a commercial compatibilizer widely used in HIPS materials, SPS exhibited better compatibilization effects but lower cost. Therefore, it could be concluded that SPS was applicable as a cost-effective compatibilizer in HIPS/MH composites.

1. INTRODUCTION Plastics have been found a great deal of uses in many fields of modern day life. However, most polymer materials are flammable due to their high carbon and hydrogen contents.1,2 Therefore, the major problem of their poor fire retardancy arises and largely limits their practical application. Among various polymer materials, high-impact polystyrene (HIPS), which is a commodity plastic manufactured on a large scale and widely applied in electricity, automobiles, packaging, and so on,3−5 also encounters the disadvantage of flammability. To solve this problem, the most commonly used method is to feed flame retardants (FRs) to the polymer matrix.6−8 Traditional FRs are usually halogen based ones (HBFRs),3,9,10 which have been proven effective in flame retardance. Nevertheless, polymers containing HBFRs will be accompanied by negative effects such as corrosiveness, smoke emission, and toxic combustion products in fire.11,12 With the enhancement of public awareness of environmental protection and under the pressure of new laws and regulations,12,13 there is a growing demand for halogen free FRs (HFFRs)14 instead of HBFRs, in which magnesium hydroxide (MH)15−17 and alumina trihydrate (ATH)18 have received considerable attention from both academia and industry. MH, due to its higher decomposition temperature,15,16 can be applied in a wider range than ATH. However, when applying MH as FR, there are still several obvious shortcomings. On the one hand, high loading of MH19 is usually required for expected flame resistance, which may © 2012 American Chemical Society

give rise to serious deterioration of mechanical properties, due to poor interfacial adhesion16,20,21 between polar inorganic particles and nonpolar polymer matrix. On the other hand, selfagglomeration of MH particles also spoils their homogeneous dispersion into the polymer matrix. In order to overcome these drawbacks, it is essential to modify the surface of MH particles using surface modifiers,2,22 which include macromolecular compatibilizers and low-molecularweight ones.2,23−25 Although low-molecular compatibilizers, such as surface active agents,23 silane coupling agents,24,25 and titanate coupling agents,2 are inexpensive and have shown availability to some degree, their hydrophobic chains are too short to anchor to the polymer matrix tightly.13,16,26 On the contrary, macromolecular compatibilizers, in comparison with low-molecular ones, are more beneficial to bridging the inorganic and organic phases through physical entanglements and van der Waals interactions in virtue of their longer flexible chains. Hence, in recent years, an increasing number of studies have been focused on macromolecular compatibilizers.4,13,22 In the past decades, many kinds of macromolecular compatibilizers4,13,22 based on various functional groups at various hydrophobic chains have been reported to enhance the Received: Revised: Accepted: Published: 9204

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performance of HIPS based flame retardant composites, for example, styrene-butadiene-styrene block copolymer (SBS),4 ethylene-propylene-diene monomer rubber (EPDM),4 ethylene-vinyl acetate copolymer (EVA),4 poly[styrene-b-(ethyleneco-butylene)-b-styrene] (SEBS),22 poly[styrene-b-(ethylene-cobutylene)-b-styrene]-g-maleic anhydride (SEBS-g-MAH),13,22 etc. All of them have improved the mechanical and/or thermal properties more or less. However, most of the compatibilizers previously reported lack the remarkable cost-effectiveness. They are often too expensive for real industrial application and/or too complicated for preparation. In this work, a few polystyrenes (PS) were treated with concentrated sulfuric acid, and the partially sulfonated PS (denoted as SPS), which contained both the polar sulfonic groups and the nonpolar alkyl chain of PS, was used as a macromolecular compatibilizer for the first time to improve the compatibilization of the HIPS/MH composites by selfcompatibilization technology. The influences of both the sulfonation degree and the content of the compatibilizer to the performance of HIPS/MH composites including their compatibilization mechanisms were studied, respectively. At last, the compatibilization effects of SPS were also brought into comparison with those of the traditional compatibilizer SBS.

Figure S1 of Supporting Information. The intense resonances around 1.4 and 1.8 ppm in Figure S1a (Supporting Information) are attributed to −CH2− and −CH− of the main chain of PS.28 The signals of protons on phenyl groups are represented at 6.5 and 7.1 ppm.28 Although the spectrum of SPS as shown in Figure S1b (Supporting Information) is similar to that of PS, its characteristic peak area ratio has changed, indicating that sulfonic groups have partially replaced the protons on phenyl groups. Then, the sulfonation degree can be calculated according to eq 1: sulfonation degree = (5 − 3 × A phenyl /A main chain ) × 100% (1)

where Aphenyl and Amain chain are the peak areas corresponding to the protons on phenyl groups and the main chain of SPS, respectively. The sulfonation degree of the products prepared at 25, 50, and 60 °C are 12.7, 24.8, and 35.5%, respectively. 2.3. Preparation of HIPS/MH Composites. According to experimental formulations described in Table 1,27 the surface of Table 1. Compositions of Various Composites compositions (wt %)

2. EXPERIMENTAL SECTION 2.1. Materials. HIPS (type PH88) and PS (type PG33) pellets were purchased from Zhenjiang Chimei Chemical Co., Ltd. (Zhenjiang, China). MH (BET surface area, 6.79 m2/g) was purchased from Chuangye Co., Ltd. (Yixing, China). Polyphenylene Oxide (PPO) and tricresyl phosphate (TCP) were purchased from Nanjing Huarun Rubber and Plastic Co., Ltd. (Nanjing, China). Resorcinol bis (diphenyl phosphate) (RDP) was purchased from Jiangsu Yoke Technology Co., Ltd. (Yixing, China). Ethylene bis stearamide (EBS) was purchased from Bocai Plastic Materials Co., Ltd. (Lianyungang, China). PPO, TCP, RDP, and EBS were all applied as synergists here. Styrene-butadiene-styrene block copolymer (SBS) was purchased from Sinopec Baling Petrochemical Co., Ltd. (Yueyang, China). All other chemicals were purchased from Nanjing Chemical Reagent Co., Ltd. and used without further purification. 2.2. Preparation of SPS. SPS was used as a macromolecular compatibilizer in HIPS/MH composites. The preparation process was described in brief as follows. The desired amount of PS pellets was added into concentrated sulfuric acid (98 wt %) with continuous mechanical stirring. After 10 min for sulfonation reaction at certain temperature, the pellets were leached apart from the solution. The reaction was heterogeneous and only took place at the surface of the pellets. Then the pellets were washed with water and acetone alternately for three times and dried at 80 °C for 48 h. The final product was SPS.27 In addition, the reaction temperature was changed from 25 to 60 °C for each synthesizing experiment in order to prepare a series of SPS samples with various sulfonation degrees. However, for the limitation of current characterization methods, overall average sulfonation degrees of various SPS samples in the modified PS pellets were detected and estimated using 1H nuclear magnetic resonance spectroscopy (1H NMR). 1 H NMR spectra of PS and SPS were recorded on a Bruker AVANCE Model DRX-500 NMR spectrometer. An entire SPS pellet was totally dissolved in CD3Cl homogeneously. 1 H NMR spectra of PS and SPS prepared at 50 °C are shown in

a

sample code

sulfonation degree of SPS (%)

HIPS

MH

SPS

SBS

other synergistsa

A0 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E

12.7 12.7 12.7 12.7 24.8 24.8 24.8 24.8 35.5 35.5 35.5 35.5

50 49 48 47 45.5 49 48 47 45.5 49 48 47 45.5 47

31 31 31 31 31 31 31 31 31 31 31 31 31 31

0 1 2 3 4.5 1 2 3 4.5 1 2 3 4.5 0

0 0 0 0 0 0 0 0 0 0 0 0 0 3

19 19 19 19 19 19 19 19 19 19 19 19 19 19

Mass ratio of PPO/RDP/TCP/EBS = 8/4/6/1.

the MH particles was modified first. The MH powder and SPS were mixed in a high-speed mixer (Type FW100; Taisite Instrument Co., Ltd.; Tianjin, China) with a rotor speed of 24 000 rpm. After 6 min for the modification, the SPS was bound to the surface of MH particles to form a core−shell structure.26,29 Then, the modified MH, HIPS, and other synergists8 were mixed, extruded, and palletized in a corotating twin-screw extruder (Type SHJ35B; Guangda Chemical Equipment Co., Ltd., Nanjing, China) with an L/D ratio of 40 and a screw diameter of 35.6 mm. The temperature profiles from hopper to die were 180, 190, 200, 210, 215, 220, 215, and 210 °C at eight different zones, respectively. The screw speed was 400 rpm. At last, the pellets were injection molded at 175 °C into various specimens for further tests and characterization using an injection molding machine (Type 80-A, Grand Machinery Group Ltd., Wuxi, China). The sample codes of various HIPS/ MH composites (A−E) were described in Table 1 also. 2.4. Particle Size and Particle Size Distribution Analyses. Particle size and particle size distribution analyses of various MH samples were carried out by particle size and particle size distribution analyzer (Type MasterSizer 2000; 9205

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size and particle size distribution analyses of MH and various SPS modified MH samples are first carried out. Particle size distribution curves of MH samples with and without modification are shown in Figure 1. The various SPS samples with different sulfonation degree and various content have been tried in the MH modification process, respectively. It is manifested from Figure 1a,b that, in comparison with MH

Malvern Instruments Co., Ltd., Worcestershire, UK). The volume weighted mean diameter (Dv) was calculated by data processing software from the manufacturer, according to eq 2 as shown below:

Dv =

∑ D4 ∑ D3

(2)

where D was the diameter of equivalent sphere. 2.5. Attenuated Total Reflectance/Fourier Transform Infrared Spectroscopy (ATR-FTIR). ATR-FTIR spectra of unmodified MH, SPS, and modified MH samples were recorded by FTIR spectrometer (Type IFS 66/S; Bruker Co.; Germany) with Pike MIRacle ATR accessory. Before this test, the modified MH sample was packed with filter paper and extracted by CHCl3 in Soxhlet apparatus for 48 h in order to remove the SPS residue that was not chemically bounded on the MH surface, and then fully dried. 2.6. Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) Spectroscopy. After being sputter coated with gold, the SEM-EDX images of various MH particles were recorded on a SEM (Type SSX-550; Shimadzu Co., Kyoto, Japan) with an acceleration voltage of 15.0 kV. The morphologies of the fracture surfaces of the composites were also observed directly with the same SEM with the acceleration voltage of 15.0 kV. The SEM samples were prepared as follows. The composites were broken after cooling below their glass transition temperature by liquid nitrogen for keeping the original morphologies, and then, the fracture surfaces were coated with gold. 2.7. Mechanical Performance Tests. Tensile strength, elongation at break, and flexural strength of HIPS/MH composite samples were measured at a tensile speed of 50 mm/min according to Chinese standards of GB/T 1040 and GB/T 9341 on an electronic tensile testing machine (Type JDL-5000N, Zhenwei Machinery Co., Ltd., Jiangdu, China). Notched impact strength tests were performed according to Chinese standard of GB/T 1043 on a pendulum impact tester (Type MZ-2054, Mingzhu Tester Machinery Co., Ltd., Jiangdu, China). All measurements were carried out at room temperature. Five specimens were tested for each sample. 2.8. Limiting Oxygen Index (LOI) Measurements. LOI measurements were carried out according to Chinese standard of GB/T 2406 by LOI instrument (Type HC900-2, Shangyuan Analysis Instrument Co., Ltd., Nanjing, China). The LOI values were calculated according to the equation as shown below: LOI = [O2 ]/([O2 ] + [N2]) × 100%

Figure 1. Particle size distribution curves of MH samples without and with modification of (a) 3% content of various SPS and (b) different contents of SPS with sulfonation degree of 24.8%.

without modification, modified MH samples have remarkable larger particle size and wider particle size distribution. It may be due to two facts. One is that some parts of MH particles have aggregated to form larger ones during the modification process. On the other hand, given the molecular structure of both SPS and MH including the results of our previous work,24−26 it can be also inferred that MH particles are encapsulated by SPS to form a core−shell structure,26,29 through interaction between sulfonic groups of SPS and hydroxyl groups on the surface of MH. This viewpoint will be further confirmed by ATR-FTIR and SEM-EDX analyses. Besides, for modified MH particles in the tested range, there are no significant effect of the sulfonation degree and the content of compatibilizers on the particle size and particle size distribution for the tight linkage between SPS and MH particles. Figure 2 gives the ATR-FTIR spectra of unmodified MH, SPS, and modified MH by 3% content of SPS with sulfonation degree of 24.8%. As shown in Figure 2a, the sharp and intense peak at 3693 cm−1 is attributed to the O−H vibration of MH, and the peak at 966 cm−1 corresponds to the primary Mg−OH stretch. Figure 2b represents the characteristic peaks of SPS. The C−H

(3)

where [O2] and [N2] were the concentration of O2 and N2, respectively. Five specimens were tested for each sample. 2.9. Thermal Analyses. Thermogravimetric (TG) analyses of composites (about 10 mg) were carried out under air atmosphere purge (30 mL/min) from 20 to 800 °C at a scanning rate of 10 °C/min by an integrative thermal analysis instrument (Type STA-449C; NETZSCH Instrument Co., Ltd., Selb, Germany).

3. RESULTS AND DISCUSSTION 3.1. Characterization of MH Particles. It has been reported19,24,25 that the particle size and particle size distribution of the inorganic fillers could influence the structures and morphologies of composites. Therefore, particle 9206

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Waals forces. Therefore, the coupling effects24,26 evidently raise the compatibility between inorganic fillers and polymer matrix. Moreover, the effects of the sulfonation degree and the content of SPS are also discussed. From Table 2 and Figure S2 (Supporting Information), when the sulfonation degree of SPS is low (12.7%, composites of B1 to B4), the poorest mechanical properties of composites are always obtained among the samples with modification. As the sulfonation degree increases, the mechanical properties are improved. However, when the sulfonation degree continues increasing, the tensile strength and notched impact strength reduces whereas the flexural strength still increases. As is known, the sulfonation degree reflects the quantity of the polar groups on SPS, i.e., higher sulfonation degree means more sulfonic groups and stronger polarity of SPS. In addition, SPS plays a very important role in the connection of MH and HIPS as a compatibilizer. At lower sulfonation degree, sulfonation groups are not enough to combine the SPS with MH entirely. However, if the sulfonation degree is too high, it may cause the reduction of the binding ability between SPS and HIPS matrix for higher polarity of SPS. Hence, the sulfonation degree of SPS should be kept in a suitable range for better compatibility, which is around 24.8− 35.5% here on the basis of the experimental facts. Besides the sulfonation degree, the content of SPS also has similar effect on the mechanical performance of the composites. Neither too low nor too high content of SPS is beneficial for the improvement of the mechanical properties of the composites. Although the highest tensile strength values appear at the highest content in the test range, both the notched impact strength and flexural strengths show up-climax-down variation trends. At lower content of the compatibilizer, SPS cannot cover the surface of MH particles completely, but excessive SPS will deteriorate the combination between MH and HIPS due to the high polarity of SPS. In the current measured range, the optimal range of the content of SPS is 3−4.5 wt %. 3.3. Flame Retardancy. Then, the flame retardancy of HIPS/MH composites is investigated. Since the LOI2 is widely used to evaluate the flame retardancy of polymer materials, the LOI values of different HIPS/MH composites are measured and shown in Table 2, and their variation is illustrated in Figure S3 of Supporting Information. It is found from Table 2 and Figure S3 (Supporting Information) that the flame retardancy is evidently improved when MH is filled into HIPS matrix. The LOI value increased from 16.0 to 20.5%. Moreover, the LOI rises further after compatibilizers are added into the composites. The enhancement of LOI is due to the better dispersion of MH particles in HIPS matrix after modification. However, Table 2 also demonstrates that, in order to obtain the better flame retardancy, both the sulfonation degree and the content of SPS should be controlled in suitable ranges. This phenomenon is quite consistent with the effects of the aforementioned two factors on the mechanical properties and can be explained in the similar way. In addition, from Table 2 and Figure S3 (Supporting Information), C3 sample has the highest LOI value and best flame retardancy for good compatibility, in which the sulfonation degree and the content of SPS are 24.8% and 3 wt %, respectively. 3.4. Morphology Analyses of Fracture Surface of the Composites. It is well-known that the morphology of composites is one of the crucial factors24,26 to the final performance of the materials. Therefore, micromorphologies of HIPS/MH composites are investigated by SEM.

Figure 2. ATR-FTIR spectra of unmodified MH (a), SPS (b), and modified MH (c) by 3% of SPS with sulfonation degree of 24.8%.

vibration on the polymer backbone is at 2850−2920 cm−1; the C−H vibration on the phenyl rings is at 1452, 1492, 1601, and 3000−3090 cm−1. The SO vibration is at 1028, 1154, and 1181 cm−1,30,31 and the vibration of para-substituted phenyl rings is at 841 and 1003 cm−1,31 respectively. Since the nonbounded SPS has been already removed through Soxhlet extraction, the presence of the characteristic peaks of C−H in Figure 2c indicates that there is SPS chemically adsorbed on the surface of MH particles. Moreover, the intense new peak at 1018 cm−1 provides clear evidence of the chemical bonds of S−O−Mg,32 which confirms the interaction between SPS and MH. SEM-EDX images of various MH particles are further shown in Figure 3 for investigation of the encapsulated core−shell structure of the modified MH. As shown in Figure 3b, the distribution of element carbon in unmodified MH was uniform, which is in contrast to the distribution of magnesium in Figure 3c and oxygen element in Figure 3d. However, Figure 3f demonstrates that carbon element enriches on the surface of the modified MH particle. This phenomenon is fully coincident with the ATR-FTIR spectra that SPS concentrates on the surface of MH particles and confirms the core−shell structure.26 Furthermore, the encapsulated MH particles are also expected to have better compatibilization than unmodified MH to HIPS matrix, which will be further confirmed in the following sections. 3.2. Mechanical Properties. In order to assess the actual effects of compatibilizers, the mechanical properties of various HIPS/MH composites are measured. The results are shown in Table 2. On the basis of Table 2, the variations of tensile strength, notched impact strength, and flexural strength of various samples are summarized in Figure S2 of Supporting Information. From Table 2 and Figure S2 (Supporting Information), it is found that, in contrast to neat HIPS, the mechanical properties of HIPS/ MH composite (A0 sample) decrease greatly after addition of MH but without modification, which are quite similar to our previous findings.24,26 This is ascribed to the poor compatibility between MH and polymer matrix. Therein, the rigid particles contributed remarkably to the sharp reduction in notched impact strength of the materials. However, on the basis of Table 2, the mechanical properties of HIPS/MH composites are all improved efficiently when SPS is incorporated into the composites, which implies the enhanced compatibility between SPS modified MH and HIPS. On the one hand, as mentioned above, interaction between sulfonic groups of SPS and hydroxyl groups of MH results in the core−shell structure of modified MH particles. On the other hand, the long chains of SPS on the shell can readily entwist with HIPS polymer chains through van der 9207

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Figure 3. SEM-EDX images of particles of unmodified MH26 (a−d) and modified MH by 3% of SPS with sulfonation degree of 24.8% (e−h). Scanning electron micrographs (a, e); distribution of element: carbon (b, f)/magnesium (c, g)/oxygen (d, h) on the MH particle surface.

surface, which is usually preferred for the better compatibilization between the filler particles and the polymer matrix. It is found from Figure 4a that MH particles without compatibilizer are distinctly separated from HIPS matrix. The interface

Figure 4 shows SEM images of samples A0, B3, C3, and D3 at a relatively lower magnification (×6000). Generally, fewer and smaller holes, lower clarity of interface, and more branchlike fibrils between MH and HIPS refer to the smoother facture 9208

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Table 2. Properties of Various Composites Samples sample code

density (g·cm−3)

HIPS A0 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E

1.05 1.28 1.19 1.30 1.27 1.27 1.27 1.31 1.23 1.23 1.30 1.31 1.26 1.26 1.25

elongation at break (%) 40.0 14.8 16.0 15.8 15.8 15.9 15.9 15.9 15.3 16.3 16.7 16.3 16.7 16.9 15.9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.9 0.3 0.3 0.2 0.3 0.2 0.4 0.3 0.3 0.4 0.5 0.4 0.3 0.1 0.1

tensile strength (MPa) 26.0 23.4 25.7 25.8 26.3 27.6 27.3 28.5 28.4 29.1 26.5 27.7 27.4 28.9 22.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.4 0.4 0.2 0.2 0.1 0.1 0.3 0.2 0.2 0.1 0.2 0.3 0.2 0.2 0.4

flexural strength (MPa) 43.5 42.7 43.2 44.1 44.6 43.8 43.8 44.1 45.2 43.9 44.9 45.6 46.4 45.1 40.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.3 0.2 0.1 0.1 0.2 0.2 0.2 0.1 0.2 0.2 0.1 0.1 0.2 0.2 0.3

notched impact strength (kJ·m−2) 8.03 3.83 4.06 4.18 4.96 4.10 4.31 4.88 5.40 4.22 4.20 4.75 4.80 5.00 4.30

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.20 0.10 0.07 0.09 0.11 0.08 0.09 0.07 0.10 0.12 0.11 0.08 0.07 0.10 0.12

LOI (%) 16.0 20.5 21.8 22.0 22.3 22.1 22.3 22.5 23.0 22.8 21.9 21.8 22.3 22.2 22.5

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.4 0.3 0.2 0.2 0.1 0.2 0.3 0.2 0.1 0.1 0.2 0.3 0.3 0.1 0.2

Figure 4. SEM images of HIPS/MH composite samples (a) A0, (b) B3, (c) C3, and (d) D3 under lower magnification (×6000).

compatibilizer. It is found from Figure 5 that sample C3, with the compatibilizer content of 3%, reveals the most homogeneous fracture morphology. The obtained optimal values of both sulfonation degree and content of SPS are fully consistent with those from mechanical performance tests and LOI measurements, which further confirms that the morphology of materials is the most important factor for their final performance. In addition, for study on the compatibilization effects of SPS in detail, higher magnification (×20 000) SEM images of A0 and C3 are shown in Figure 6. It is evidently found that sample A0 has clear interface and no adhesion exists between MH and HIPS, which reflects the poor compatibility between unmodified MH and HIPS. Nevertheless, due to the improved compatibility, the MH particle in sample C3 has some branchlike HIPS fibrils around as marked with arrows in Figure 6b,

between MH and HIPS is obvious. The unsmooth surface has numbers of large holes, and some of them have been identified with arrows as shown in Figure 4a. Such morphology is the intrinsic reason which triggers off the poor performances of A0. Compared with A0, the composites after modification by SPS have more homogeneous surfaces but fewer holes. Meanwhile, the interface between HIPS and MH becomes unclear. Moreover, for the samples B3, C3, and D3, in which the compatibilizers have the same content but different sulfonation degree, the composite modified by SPS with the sulfonation degree of 24.8% (sample C3) demonstrates the smoothest fracture surface (i.e., the fewest holes and the lowest clarity of interface) as shown in Figure 4c. Then, SEM images of composites with different contents of SPS but the same sulfonation degree of 24.8% are shown in Figure 5 for investigation of the effect of the content of 9209

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Figure 5. SEM images of different HIPS/MH composite samples (a) C1, (b) C2, (c) C3, and (d) C4 under lower magnification (×6000).

its price is relatively high. In comparison with SBS, the new compatibilizer SPS, prepared in this work through a simple method using PS pellets and concentrated H2SO4 as raw materials, is very inexpensive and positive for further reducing the costs of the final products. In this section, the practical compatibilization effects of SPS and SBS on the HIPS/MH composites are brought into comparison. Sample E has been modified by SBS instead of SPS, in which the optimal dosage of SBS is employed, and its detailed formula are shown in Table 1 also. The mechanical properties and LOI value of sample E are presented in Table 2. Although sample E has higher notched impact strength and LOI than those of sample A0 without modification, the aforementioned properties are both lower than those of sample C3, the optimal composite sample using SPS as compatibilizer. Besides, in contrast to sample A0, the tensile strength and flexural strength of sample E have even decreased, which is ascribed to the original soft segment of SBS.4 Then, in order to compare the performance of SPS and SBS further, TG analyses were carried out to investigate their effects on the thermal stability of HIPS/MH composites. TG curves of various samples are shown in Figure 7. It can be found from Figure 7 that sample A0 without compatibilizer begins to decompose at about 250 °C and decomposes completely at 700 °C. Various stages are observed, in which the first main sharp mass loss4 mostly corresponds to the decomposition of HIPS and MH. The slow mass decrease in the following stages4 is attributed to the decomposition of PPO and the loss of some other unstable residues. In the TG curve of sample C3, the thermal stability of the SPS modified composites before 400 °C is improved, since there is the lower mass loss in the corresponding temperature area. It indicates that SPS can efficiently delay the initial thermal decomposition of the composites, resulting from the better dispersion17 of SPS modified MH particles in HIPS matrix. However, in the

Figure 6. SEM images of various HIPS/MH composite samples (a) A0 and (b) C3 under higher magnification (×20 000).

which results from the coupling effects of SPS as mentioned above. 3.5. Comparison of the Compatibilization Effects between SPS and SBS. As is known, SBS is a commercial compatibilizer that is widely used in HIPS materials.4 However, 9210

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degree is 12.7%, 24.8%, and 35.5% (Figure S2). The variation of LOI values of HIPS, HIPS/MH composites without SPS, and HIPS/MH composites with SPS, of which sulfonation degree is 12.7%, 24.8%, and 35.5% (Figure S3). This information is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: 86-25-83686350. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Supported by the Natural Science Foundation of China (Grant No. 51073077) and the Scientific Research Foundation of Graduate School of Nanjing University (Grant No. 2012CL06).

Figure 7. TG curves of different HIPS/MH composite samples, (a) A0, (b) C3, and (c) E, respectively.

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temperature range from 400 to 700 o C, since the original welldefined structures of the composites have been already broken, SPS have no obvious effect on the thermal stability. Hence, composite samples with and without compatibilizer above 400 °C nearly have the same decomposition process as shown in Figure 7. The thermal stability of sample E is also illustrated in Figure 7. It is found that the thermal stability of composite modified by SBS is much worse than that of sample C3 in the whole measured temperature range, which is due to both the poor dispersion of MH in the polymer matrix and the lowtemperature decomposition of SBS itself. Therefore, on the basis of both the mechanical properties and thermal stability of HIPS/MH composites, SPS always exhibits better compatibilization performance than SBS. In other words, SPS is a high cost-effective compatibilizer for HIPS/MH composites.

4. CONCLUSION In this work, the self-compatibilization technology has been applied to improve the performance of HIPS/MH composites. A fraction of the phenyl rings of polystyrene has been sulfonated simply, and the partially sulfonated PS has been applied as a new macromolecular compatibilizer for the HIPS/ MH composite system first. According to the experimental results as mentioned above, SPS is proven to be applicable as a cost-effective compatibilizer for this system. The better compatibilization of SPS is due to the significant coupling effects. On the one hand, interaction between sulfonic groups of SPS and hydroxyl groups of MH results in the core−shell structure of modified MH particles; On the other hand, the long alkyl chains of SPS on the shell readily entwist with HIPS polymer matrix through van der Waals forces. Moreover, various property tests for the composites demonstrate that the sulfonation degree and the content of SPS have remarkable influences on the final performance of the materials. SPS (3−4.5 wt %) with the sulfonation degree of 24.8−35.5% is the optimal condition for the overall properties of HIPS/MH composites in the current system.

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ASSOCIATED CONTENT

S Supporting Information *

H NMR spectra of PS and SPS prepared at 50 °C (Figure S1). The variations of tensile strength, notched impact strength, and flexural strength of HIPS, HIPS/MH composites without SPS, and HIPS/MH composites with SPS, of which sulfonation 1

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