Article pubs.acs.org/IECR
Co-toughened Polystyrene by Submicrometer-Sized Core−Shell Rubber Particles and Micrometer-Sized Salami Rubber Particles Yunjiao Deng,† Guanghui Gao,† Zhenguo Liu,§ Chunlei Cao,† and Huixuan Zhang*,†,‡ †
Engineering Research Centre of Synthetic Resin and Special Fiber, Ministry of Education, and School of Chemical Engineering, Changchun University of Technology, Changchun 130012, China ‡ Changchun Institute of Applied Chemistry, Graduate School, Chinese Academy of Science, Changchun 130022, China § Research Institute of Jilin Petrochemical Co. Ltd., Petro China, Jilin 132021, China ABSTRACT: A series of submicrometer-sized polybutadiene (PB) rubber particles with different diameters ranging from 100 to 450 nm were prepared via an emulsion polymerization. Subsequently, core−shell polybutadiene-graft-polystyrene (PB-g-PS) particles were synthesized using a redox initiator system (cumene hydroperoxide and ferrous sulfate) by an emulsion grafting polymerization. Then the submicrometer sized PB-g-PS impact modifiers were utilized to toughen polystyrene (PS) by blending PS and high-impact polystyrene. From measuring mechanical properties, it was found that the notched impact strength of toughened PS was significantly influenced by core−shell PB-g-PS particle size. When the rubber particle size was above 300 nm, the notched impact strength was 208 J/m, nearly 2 times than that of toughened PS with particle size 100 nm. Moreover, the stress whitening zones were observed by transmission electron microscopy, and the result showed that multiple crazing is its dominative toughening mechanism.
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INTRODUCTION Polystyrene (PS) is widely used as thermoplastics because of its easy process, steady dimension, and transparence properties. Nevertheless, the applications of polystyrene are significantly confined due to its brittleness. To improve its toughness, highimpact polystyrene (HIPS) was obtained consisting of polystyrene continuous phase and rubber dispersive phase.1−3 Some viewpoints insist that the dominant deformation mechanism is multiple crazing4−8 and crazes are associated with rubber particles.9 In particular, Donald and Kramer have investigated the crazing initiation, growth, and fracture mechanisms of HIPS with micrometer-sized salami rubber particles in detail.10 They found that the effective toughness of HIPS was significantly dependent on the rubber particle size and crazes cannot be initiated from the rubber particles with a diameter less than 1 μm. Moreover, Bucknall and co-workers pointed out that the crazes could be initiated and terminated effectively when the rubber particle size was large enough at a micrometer-size level.11 It is well-known that the introduction of rubber particles with bimodal distribution can improve comprehensive performance significantly in rubber-toughened plastics.12−15 Chen and Jan have investigated the toughening of epoxy resins with bimodal distribution of the rubber particles and pointed out that the fracture toughness of epoxy resin increased significantly with the synergy of the rubber particles.16 Previously, we merely introduced PB-toughened PS with a bimodal system between micrometer-sized salami rubber particles and PB-g-PS with a particle size of 300 nm.17 However, the most interesting thing is co-toughened PS by micrometer-sized salami rubber particles and submicrometersized rubber particles with large-scale diameters. Few published reports have mentioned this point in recent years. But it is also very important in the field of toughening polystyrene. © 2013 American Chemical Society
In this work, the principal purpose is to investigate the influence of submicrometer-sized rubber particles with different diameters and micrometer-sized salami rubber particles on the mechanical properties and deformation mechanism of toughening polystyrene. First, a series of submicrometer-sized PB rubber particles with different diameters ranging from 100 to 450 nm were prepared via an emulsion polymerization. Subsequently, the core−shell PB-g-PS particles were synthesized using a redox initiator system of cumene hydroperoxide and ferrous sulfate by an emulsion grafting polymerization. Then the HIPS/PS/PB-g-PS blends were prepared by blending submicrometer-sized PB-g-PS impact modifiers, PS and HIPS. Then the Izod notched impact test and the tensile test were employed to characterize the mechanical properties of samples. Subsequently, the stress whitening zones for HIPS/PS/PB-g-PS blends under impact conditions were also investigated. Finally, the corresponding deformation mechanisms were extensively discussed based on mechanical properties and deformation morphologies.
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EXPERIMENTAL SECTION Materials. Polystyrene was supplied as GPPS 525 by Panjin Petrochemistry Company, China. Commercial HIPS were supplied as POLYREXPH-88 by CHI MEI CORPORATION, China. Butadiene was supplied by Jilin Chemical Industry Group synthetic resin factory, China, and distilled directly from the storage vessel into a cooled steel recipient. Abietic acid sodium (AAS), potassium persulfate, and tert-dodecylmercaptan (TDDM) were supplied by Jilin Chemical Industry Group Received: Revised: Accepted: Published: 5079
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in a vacuum oven at 60 °C for 24 h. Then injection molding was carried out to prepare notched Izod impact specimens and tensile specimens. Grafting Degree of PB-g-PS Copolymers. The degree of grafting was determined by extracting ungrafted or free PS of the dried PB-g-PS copolymers (5.0 g) with 5 mL of methyl ethyl ketone (a solvent for PS but not for PB). After the methyl ethyl ketone solutions of the dried PB-g-PS impact modifiers were shaken for 24 h at room temperature, the solutions were centrifuged at 10 000 rpm in a GL-21 M centrifugal machine for 40 min with a temperature of −5 °C. After centrifugation the clear solution was poured out, and the insoluble residue was dried at 50 °C in the oven for 24 h. The degree of grafting was calculated from the following equation:
synthetic resin factory, China. Styrene was purified by washing with 5% sodium hydroxide solution to remove the inhibitor before use Cumene hydroperoxide (CHP), ferrous sulfate (FES), dextrose (DX), potassium hydroxide (KPS), and sodium pyrophosphate (SPP) were purchased from Aldrich Chemical Co. Preparation of PB Latex. The PB rubber particles with different diameters were prepared by an emulsion polymerization. First, AAS, KPS, and TDDM were weighed and added into a 3 L cylindrical steel bottle, and then the bottle was capped and sealed. Butadiene was injected into the bottle under high pressure. The bottle was then placed in a safety reactor supported on a rotor which was rotated at a speed of 30 rpm in a water bath kept at 65 °C. PB rubber particles with different sizes ranging from 100 to 450 nm were synthesized under different reaction times. Particle size of PB latex was measured by dynamic light scattering (DLS) using Brookhaven 90 Plu Laser Particle Analyzer. The morphology of PB was observed using a JEOL 2000 transmission electron microscope with an accelerating voltage of 100 KV. All samples were prepared by dispersing diluted latex on a 230 mesh copper grid coated with a thin layer of Formvar. After about 5 min, the grid was placed in the 2% phosphotungstic acid (PTA) solution for 3 min at room temperature followed by drying in a dust-free environment before observation. Preparation of PB-g-PS Copolymers. PB-g-PS copolymers were achieved using a redox initiator system (cumene hydroperoxide and ferrous sulfate) via an emulsion polymerization. The emulsion polymerization was performed with a reflux condenser, a sampling device, a nitrogen inlet, and a stirrer coated with PTFE (stirrer speed: 300 rpm). First, the DI water, PB latex, initiator, and KOH were added to the glass reactor. After degassing with N2 for 10 min to replace O2, the St was added in a continuous feeding way to the glass reactor. After 3 h, antioxidant solution was added, the reactor temperature was decreased to 60 °C, and the reaction was ended. The polymers were isolated from the emulsion by coagulation and dried in a vacuum oven at 60 °C for 24 h before being used. The recipe for the preparation of PB-g-PS is given in Table 1.
weights (g)
water SPP DX FES EMS PBL CHP KOH St
1000 2.0 2.4 40 12 484 0.92 4 120
Weight of grafted PS × 100% Weight of PB
(1)
GE =
Weight of grafted PS × 100% Total weight of polymerized PS
(2)
Dynamic Mechanical Analysis. The dynamic mechanical (DMA) measurement for the PB-g-PS copolymers and HIPS/ PB-g-PS/PS blends were made in the single cantilever mode by the Netzsch DMA 242 (Germany) at 1 Hz. The specimen bar was sized 30 × 10 × 1 mm3 and the temperature varied from −150 to 150 °C at a constant heating rate of 3 °C/min. Izod Impact Test and Tensile Test. According to ASTM D256, notched impact strength was determined with a XJU-22 impact tester at the maximum speed of 3.5 m/s. The tensile test is measured according to ASTM D638 using Instron AGS-H tensile tester at a cross-head speed of 50 mm/min. The test was performed at 23 ± 2 °C, and at least six specimens were tested for each average value given. Morphology Observation. The microstructure morphologies of the samples were observed by using JEM-2000EX transmission electron microscope (TEM). The specimens were cut to 60 nm in thickness using a microtome at −100 °C, and the samples were stained with OsO4 solution for 8 h before observation.
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RESULTS AND DISCUSSION Morphology and Size of PB Rubber Particles. To investigate the influence of the rubber particles size on the properties of HIPS/PB-g-PS/PS blends, first, a series of submicrometer-sized PB rubber particles were obtained by seeded emulsion polymerization. The morphologies of PB rubber particles by TEM are illustrated in Figure 1 and the images show PB rubber particles with different diameters ranging from 100 to 450 nm on the copper meshes. Clearly, the PB rubber particles were spherical regularly and the size of rubber particle was uniform. Also we measured the PB rubber particle sizes by DLS (shown in Table 2.). The results indicated the particle sizes by DLS were in accordance with the results from TEM. Grafting Degree of PB-g-PS Copolymers. To improve the toughness of PS resins, the soft PB rubber phase as a core should play an important role in the toughening process. However, the PB rubber particle cannot be directly dispersed in PS matrix uniformly. As a result, the PS as a shell should graft onto the PB rubber particles to increase compatibility between PB rubber particles and PS matrix. For PB rubber particles with different diameters, the PS shells could influence the grafting
Table 1. Recipe for PB-g-PS by Emulsion Grafting Polymerization ingredients
GD =
Preparation of HIPS/PB-g-PS/PS Blends. The blends of HIPS/PB-g-PS/PS were carried out in a twin-screw extruder. Constitutes of HIPS, PB-g-PS, and PS were 500/200/300 (w/ w/w). The temperatures along the extruder were 180, 190, 190, 190, 190, 190, and 190 °C, and the rotation speed of the screw was 70 rpm. The melt stripes of blends were cooled in a water bath and then pelletized. HIPS/PB-g-PS/PS blends were dried 5080
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PB-g-PS/PS blends was investigated. As can be shown in Figure 2, it was found that the notched impact strength of HIPS/PB-g-
Figure 2. Influence of the rubber particle size on the notched impact strength of HIPS/PB-g-PS/PS blends.
PS/PS blends first increased significantly with the increase of rubber particle size. When the rubber particle size was 300 nm, the notched impact strength was 208 J/m. However, the notched impact strength influenced insignificantly when the rubber particle size was larger than 300 nm. The influence of particle size on the tensile strength of HIPS/PB-g-PS/PS blends was shown in Figure 3. It can be seen Figure 1. TEM images of PB latex particles with different particle sizes: (a) 100 nm, (b) 180 nm, (c) 230 nm, (d) 300 nm, and (e) 450 nm.
Table 2. Effect of PB Rubber Particle Size on Grafting Degree of PB-g-PS Copolymers sample
particle size of PB (nm)
polydispersity
GD (%)
GE (%)
PB-g-PS1 PB-g-PS2 PB-g-PS3 PB-g-PS4 PB-g-PS5
100 180 230 300 450
0.075 0.072 0.068 0.071 0.065
38.8 36.7 35.9 34.4 33.8
90.5 85.6 83.9 80.3 78.9
degree during reactions, and subsequently, toughness on PS matrix. As a result, the influence of PB rubber particle size on grafting degree of PB-g-PS was investigated and the results are shown in Table 2. For clearly comparing with each sample, in this experiment, PB and St with a fixed ratio of 70/30 were added into the system. From observing the results of the grafting degree and grafting efficiency, it was found that the grafting degree slightly but not much decreased with the increase of the PB rubber particle size. This is mainly because particle surface area decreased with the increase of the rubber particle size. That is, the rubber particle with a small particle size was grafted slightly by polystyrene chains, whereas the rubber particle with a big particle size has a little longer PS grafting chain. Therefore, this was the reason why the rubber particles with 100 nm were dispersed poorly in the matrix, while the rubber particles with over 230 nm were dispersed well in the PS matrix. Mechanical Properties of HIPS/PS/PB-g-PS Blends. The core−shell copolymers PB-g-PS were used as impact modifiers to prepare HIPS/PB-g-PS/PS blends. The influence of rubber particle size on the notched impact strength of HIPS/
Figure 3. Influence of particle size on tensile strength of HIPS/PB-gPS/PS blends.
from Figure 3 that the tensile strength of HIPS/PB-g-PS/PS blends decreased slightly when increasing rubber particle size. The elongation at break increased significantly with the increase of rubber particle size. Han et al. and Correa and De Sousa have studied the toughening of HIPS systems using salami rubber particles with different diameters.18,19 They pointed out that the yield strength of the blends with small rubber particle size was higher than that of the blends with the large size, which was consistent with our results. Dynamic Mechanical Behavior of PB-g-PS Copolymers. Dynamic mechanical analysis (DMA) is used to measure the relationship of dynamic modulus and mechanical loss to programmed temperature under the vibration load. To obtain the characteristic parameters that are related to the material structure, molecular movement, processing, and application, mechanical loss tangent (tan δ) was measured and is shown in 5081
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Figure 4. It shows the curves of tan δ versus temperature for the PB-g-PS copolymers with different rubber particle sizes. It is
Figure 5. Influence of the rubber particle size on the B of HIPS/PB-gPS/PS blends. Figure 4. Tan δ as a function of temperature for PB-g-PS copolymers with different particle sizes.
well-known that PB-g-PS copolymers have two tan δ peaks: one, in the low-temperature area, belongs to the rubber phase; and the other, in the region of about 100 °C, belongs to the PS glassy phases. In the following, only the glass transition belonging to the rubber phase was discussed. It was observed that the glass transition temperature of the rubber phase also decreased from −75.8 to −77.8 °C when the rubber particle size increased from 100 to 300 nm. Furthermore, the maximum tan δ of PB-g-PS copolymers increased with the increase of rubber particle size. Because of the surface area of the small rubber particles being greater than the larger particles, more internal grafting chains hindered the movement of the rubber particles. This is the reason that the small rubber particles had a higher glass transition temperature. Moreover, brittleness is a significant property in the design of products and the development of toughened plastics. Brostow and his co-workers developed a quantitative definition of brittleness evaluation that is not limited to elastic materials and but also applicable to polymers and composites.20,21 The brittleness B can be calculated from the following eq 3, 1 B= εbE′ (3)
Figure 6. Dispersion of PB-g-PS in the PS/HIPS matrix by TEM: (a) 100 nm, (b) 180 nm, (c) 230 nm, and (d) 450 nm.
Deformation Mechanism. TEM was used to study the toughening mechanism by observing the stress whitening zone under the Izod notched impact fracture surface. Figure 7 shows the stress whitening zone morphology of HIPS/PB-g-PS/PS blends, which are perpendicular to the fracture surface. It should be noted that, in TEM images, there are some crazes isolated from the rubber particles. And this may be because crazing is formed under the condition of three-dimensional growth, while the pictures show only two-dimensional images. Therefore, the slice may not be at the equator of rubber particles. When crazing is not on the slice flat, it only shows some “isolated” crazes in the matrix. In addition, because of the overlapping influence of rubber particles on the stress fields, it would lead to crazes deviated from the maximum principal stress direction. In the brittle PS system, multiple crazing was the main toughening mechanism by the introduction of rubber particles. The craze-initiation stress decreased with the increase of rubber particle size.22−24 As a result, the system including rubber particles with a large size will easily yield crazes under the smaller external stress. Polystyrene is a typical brittle polymer matrix. The synergistic effect was obvious when crazing was the major toughening mechanism.25 There were some reports on synergistic toughening of polystyrene using
where εb is the tensile elongation at break and E′ is the storage modulus determined at 1 Hz by DMA (the temperature was set as 25 °C). As can be seen from Figure 5, the relationship between the rubber particle size and the value of the brittleness B obtained by eq 3 was shown. The result suggested that the B value of HIPS/PB-g-PS/PS decreased with the increase of rubber particle size and the results were consistent with the trend of the notched impact strength. Dispersion and Morphology of HIPS/PB-g-PS/PS Blends. The dispersion and morphology of HIPS/PB-g-PS/ PS blends with different rubber particle sizes ranging from 100 to 450 nm were investigated by TEM and shown in Figure 6. As can be clearly seen from Figure 6a,b, the poor particle dispersion and aggregation state were obtained for blends with a particle size of 100 and 180 nm due to high apparent activation energy of small particles. Nevertheless, the rubber particles disperse in the matrix uniformly when the rubber particle size was above 230 nm. 5082
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the particles including submicrometer-sized rubber particles and micrometer-sized salami rubber particles were involved in the process of crazing in the matrix. As a result, the toughness of the material was greatly improved.
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CONCLUSIONS In this work, a series of PB rubber particles with different sizes ranging from 100 to 450 nm were prepared by an emulsion polymerization. The PB rubber particles were grafted by PS shells to prepare PB-g-PS copolymers and these copolymers were blended with PS and HIPS to prepare HIPS/PB-g-PS/PS blends. The grafting degree of PB-g-PS copolymers decreased with the increase of rubber particle size. The influence of the rubber particles size on the mechanical properties of HIPS/PBg-PS/PS blends was investigated. Moreover, the notched impact strength of toughened PS was significantly influenced with the increase of core−shell PB-g-PS particle size. When the rubber particle size was above 300 nm, the notched impact strength was 208 J/m. The DMA analysis illustrated that the glass transition temperature of the rubber phase also decreased from −75.8 to −77.8 °C when the rubber particle size increased from 100 to 300 nm. Moreover, the TEM figures of the deformation showed that multiple crazing is its dominative toughening mechanism. These crazes were observed to be initiated by the micrometer-sized salami rubber particles, and passed through submicrometer-sized rubber particles. And the neighboring submicrometer-sized rubber particles played a role of “crazing bridge” during the craze propagation. The crazes were stabilized and would not develop into cracks by the submicrometer-sized rubber particles. Crazes could be terminated after absorbing large amounts of energy. As a result, the toughness of the material was greatly improved.
Figure 7. Morphology of the fracture surfaces of HIPS/PB-g-PS/PS blends: (a) 100 nm, (b) 180 nm, (c) 230 nm, and (d) 300 nm.
rubber particles.26−29 In this work, two kinds of rubber particles were introduced, including submicrometer core−shell particles and micrometer-sized salami rubber particles with PS inclusion. As can be clearly seen from TEM images of the HIPS/PB-g-PS/ PS blends, it was further confirmed that the synergetic toughening mechanism was multiple crazing formed between submicrometer-sized rubber particles and micrometer-sized salami rubber particles. This is due to the overlapped stress fields of the two kinds of rubber particles which would reduce the craze-initiation stress, easily leading to craze. Moreover, the distance among the different rubber particles was near and crazes could be terminated easily. Launey and Ritchie suggested that fracture is the result of a mutual competition of damage mechanisms ahead of the crack tip that promotes cracking and shielding mechanisms mainly behind the tip trying to impede it.30,31 Most brittle materials such as ceramics that are invariably toughened rely on shielding toughening.32 Shielding toughening involves microstructural mechanisms that act primarily behind the crazing tip to effectively reduce the craze-initiation stress. As shown in Figure 7a, craze was not found when the rubber particle was 100 nm. Crazes were found when the rubber particle size was larger than 180 nm (Figure 7b); moreover, it can be clearly seen from Figure 7c,d that the number of crazes also increased with rubber particle size. Craze was observed to be initiated by the micrometer-sized salami rubber particles. And the neighboring submicrometer-sized rubber particles played a role of “crazing bridge” during the craze propagation. When the craze came across a neighboring rubber particle, stress was transferred by fibrillation of rubber particles so that crazes were stabilized and would not develop into cracks. As noted earlier, the fibrillation ability of rubber particles was strongly dependent on rubber particle size.33,34 Hence, the ability of crazing bridge increased with the rubber particle size. When the rubber particle was too small, the stress could not be transferred due to the poor fibrillation ability and crazes were not stabilized. Crazes would quickly degrade to cracks, which was the reason why no craze was found when the particle size of the rubber particles was 100 nm. By contrast, crazes could be terminated after absorbing large amounts of energy when the particle size of the rubber particles was larger than 230 nm. All
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AUTHOR INFORMATION
Corresponding Author
*Engineering Research Centre of Synthetic Resin and Special Fiber, Ministry of Education, Changchun University of Technology, Changchun 30012, China. Tel: +8643185716465. Fax: +86-43185716465. E-mail: zhanghx@mail. ccut.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial supports of the Natural Science Foundation of China (No. 51073027) and Developing Scheme of Jilin Province (No. 20110340) are gratefully acknowledged.
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