Polyamide-6 Nanocomposites

May 21, 2015 - Relationship between structure and properties in high-performance PA6/SEBS- g -MA/(PPO/PS) blends: The role of PPO and PS. Yijian Wu , ...
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Preparation of Poly(phenylene oxide)/Polyamide‑6 Nanocomposites with High Tensile Strength and Excellent Impact Performance Yijian Wu, Hui Zhang, Baoqing Shentu,* and Zhixue Weng State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ABSTRACT: Organo-montmorillonite (OMMT) was introduced into SEBS-g-MA toughened poly(phenylene oxide)/ polyamide-6 (PPO/PA6) blends simply by melt extrusion. The effects of OMMT on morphology, rheology, and mechanical behaviors of toughened PPO/PA6 nanocomposites were studied by scanning electron microscope, transmission electron microscope, capillary rheometer and mechanical tests. The addition of OMMT could decrease the size of dispersed PPO domains in PA6 matrix and melt viscosity of the toughened PPO/PA6 nanocomposites. Besides, exfoliated OMMT also brought significant improvement of tensile properties and heat deflection temperature (HDT), simultaneously. For nanocomposites with exfoliated OMMT, OMMT induced more core−shell toughening particles with the optimized size, and thus, elastomer-OMMT synergistic toughening was achieved in the PPO/PA6 nanocomposites. Consequently, PPO/PA6 nanocomposites with high tensile strength, excellent impact performance, improved HDT, and good processability were obtained.

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INTRODUCTION Poly(phenylene oxide) (PPO) exhibits good moisture resistance and thermal properties, but the poor processability limits its application.1 Meanwhile, polyamide-6 (PA6) is a kind of engineering plastic with wide applications for its good processability and excellent solvent resistance. However, its heat deflection temperature (HDT) is low and its moisture resistance is poor.1−3 When properly compatibilized, PPO/PA6 blend can potentially offer a material with complementary properties of incompatible PPO and PA6. Nevertheless, the toughness of compatibilized PPO/PA6 blend is rather poor. To toughen PPO/PA6 blend, impact modifiers were introduced into the blend. Styrene-ethylene− butadiene-styrene block copolymer grafted with maleic anhydride (SEBS-g-MA), ethylene-propylene-diene elastomer grafted with maleic anhydride (EPDM-g-MA) and poly(ethylene-1-octene) grafted with maleic anhydride (POE-gMA) were the mostly reported impact modifiers.4−11 However, elastomer-toughening of plastics usually brings some deficiencies, such as low strength,7,12 modulus,8 and HDT,13−15 which are detrimental for engineering application. For several years, organo-montmorillonite (OMMT) has been attracting great interest in both industry and academia due to its ability to improve HDT and tensile performance of materials at low loadings.16−22 Recently, elastomer-OMMT synergistic toughening has been found in various polymer matrix, such as PA6,16,23,24 polypropylene (PP),25−27 poly(ethylene terephthalate) (PET),28 and polystyrene (PS).29 Paul et al. concluded that the toughening effect was induced by the optimized value of elastomer particle size,23,25,26 while Kelnar et al. ascribed the toughening effect to the formation of the core− shell structure.24,28 These studies provided some references for achieving balanced mechanical properties. However, previous researches about elastomer-OMMT synergistic toughening were restricted within homopolymer. To the best of our knowledge, there were no reports about elastomer-OMMT synergistic toughening in polymer blends. © 2015 American Chemical Society

Therefore, we applied elastomer and OMMT simultaneously to modify PPO/PA6 blends. Our main objective was to achieve a convenient methodology to obtain PPO/PA6 nanocomposites with excellent impact strength, high tensile properties, improved HDT, and good processability. The effect of OMMT on microstructure, rheology and mechanical behavior was explored.

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EXPERIMENTAL SECTION Materials. PA6 (1030B, ρ = 1.14 g/cm3), neat PPO (S201A, ρ = 1.08 g/cm3), SEBS-g-MA (FG1901, ρ = 0.91 g/ cm3) and styrene−maleic anhydride copolymer (SMA) with 8 wt % of maleic anhydride were purchased from UBE Engineering Plastics, Asahi Kasei, Kraton Polymers and International Lab, respectively. Analytically pure trimethyloctadecylammonium bromide (TMODAB) was bought from Aladdin. Analytically pure chloroform, n-hexane and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co. Na-montmorillonite (Na-MMT) was friendly supplied by Zhejiang Fenghong New Material Co., Ltd. Sample Preparation. Na-MMT was modified with TMODAB according to the literature.20,30,31 Na-MMT (20 g, 110 mequiv/100 g) was dispersed into 1.8 L of hot water (80 °C) under vigorous stirring. After Na-MMT was uniformly dispersed, 0.6 L TMODAB (9 g, 0.23 mol) solution was poured into the Na-MMT suspension under vigorous stirring at 80 °C for 30 min to yield white precipitate. Then, the precipitate was filtered and washed with distilled water several times, until no bromide could be detected in the filtrate when 0.1 mol/L AgNO3 solution was added. The dried product was grinded with a mortar and pestle, and then screened with a 200 mesh Received: Revised: Accepted: Published: 5870

March 18, 2015 May 17, 2015 May 21, 2015 May 21, 2015 DOI: 10.1021/acs.iecr.5b01041 Ind. Eng. Chem. Res. 2015, 54, 5870−5875

Industrial & Engineering Chemistry Research

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RESULTS AND DISSCUSION Exfoliation and Dispersion of OMMT in PPO/PA6 Nanocomposites. Exfoliation and dispersion of OMMT were the key points to improve mechanical properties of the nanocomposites. WAXD and TEM were used to characterize the exfoliation and dispersion of OMMT in nanocomposites. WAXD profiles of nanocomposites N-2, N-4, and N-6 were exhibited in Figure 1. The marked interlayer spacing of OMMT

sieve to yield powders, which were denoted as OMMT in this study. All the materials were dried in vacuum at 80 °C overnight before extrusion and injection. All the blends and nanocomposites were extruded twice via a twin-screw extruder (HAAKE Polylab OS, Thermo Electron Erlangen GmbH, Germany). The screw speed was maintained at 200 rpm, and the barrel temperature was 260−270 °C. Then, the extruded pellets were molded into standard specimens via an injection molding machine (PNX40III-2A, Nissei Plastic Industril Co. Ltd., Japan). The barrel temperature was 255/260/270/270/ 270 °C and the mold temperature was kept at 80 °C. To make a comparison, we prepared the binary blend (PPO/PA6 = 30/ 70, denoted as B), uncompatibilized ternary blend (PPO/PA6/ SEBS-g-MA = 30/70/25, denoted as T1), SMA-compatibilized blend (PPO/PA6/SEBS-g-MA/SMA = 30/70/25/2, denoted as T2), and nanocomposites with various OMMT loadings (PPO/PA6/SEBS-g-MA/SMA/OMMT = 30/70/25/2/x, denoted as N-x). Characterization. The interlayer distance of OMMT in N2, N-4, and N-6 nanocomposites was measured by wide-angle X-ray diffraction (WAXD) (RIGAKU D/MAX 2550/PC, Japan). The scanning speed was 5°/min with a step width of 0.02°, and the scanning range was 1−30°. The impact fracture surface was observed using transmission electron microscopy (TEM) (JEM-1230, JEOL, Japan) operating at an accelerating voltage of 90 kV. The impact fracture surface was ultramicrotomed using a cryoultramicrotome with a thickness of about 100 nm. The section was stained by osmium tetroxide (OsO4). Before being observed by scanning electron microscope (SEM) (Utral 55, CorlzeisD, Germany), the injected samples were disposed by two different methods. The first approach: the injected samples were initially kept in liquid nitrogen for some time, brittle fractured, and then etched with chloroform for 8 h at room temperature to remove the PPO domains. The second approach: the brittle-fractured surface was etched with boiled n-hexane for 3 h to remove SEBS-g-MA domains. Then the etched surface was preserved in vacuum at 80 °C overnight. After gold coating, the morphology was observed with SEM. The weight-average diameter (dw) of dispersed domains was analyzed with an image analyzer (Image-Pro PLUS) and calculated according to eq 1:26 dw =

∑ n id i 2/∑ n id i

Article

Figure 1. WAXD patterns of nanocomposites N-2, N-4, and N-6.

was calculated according to Bragg equation. According to our previous research, the d spacing of pristine OMMT was 2.21 nm.20 For nanocomposite N-6, there was a salient peak around 2θ = 2.44°, which corresponded to d spacing of 3.62 nm. The increased d spacing indicated that intercalated structure was formed in nanocomposite N-6. For nanocomposites N-2 and N-4, the peaks around 2θ = 2.44° disappeared, which suggested the formation of the exfoliated nanostructure. To further investigate the dispersion of OMMT, the morphology of nanocomposites was investigated using TEM, and the results are shown in Figure 2. The dark lines represent the OMMT

(1)

where ni was the number of domains with diameter di. The total number of domains was 250−400 in the analysis. Rheological properties were determined with a capillary rheometer (RH7, Bohlin Instrument, UK) in the shear rate region of 20−1000 s−1 at 270 °C. The capillary had a diameter of 1 mm and length of 16 mm. Tensile properties were characterized with a universal testing machine (Zwick/Roell Z020, Zwick, Ulm, Germany) according to GB 16421−1996 (75 × 4 × 2 mm) at 23 °C with a tensile speed of 10 mm/min. The stress at 10% strain (σ10%), tensile strength (σt), elongation at break (εb), and Young’s modulus (E) were evaluated. HDT was determined in an HDT-VICAT tester (HDT-3, CEAST, Italy) according to GB 1634.1−2004 (ISO 75−1) and using a load of 1.8 MPa. Notched Izod impact strength was measured with Ceast Impactor according to GB 1843−2008 (primary samples 80 × 10 × 4 mm, V-type notch depth 2 mm) at 23 °C.

Figure 2. TEM micrographs of nanocomposites N-2, N-4, and N-6.

layers. For nanocomposites N-2 and N-4, OMMT showed welldispersed exfoliated structure. However, for nanocomposite N6, the dispersion of OMMT was inhomogeneous and many stacks were observed. Morphology of PPO/PA6 Nanocomposites. Morphology is an intuitional way to evaluate the compatibility of immiscible polymeric materials. Figure 3 exhibited the SEM images of different samples. All the samples were brittle fractured in liquid nitrogen and etched with chloroform to remove the PPO domains. In all the blends and nanocomposites, PPO formed dispersed domains in PA6 matrix. For blend T1, the particle size of the dispersed PPO domains was 1.38 μm. As SMA was an effective compatibilizer, the particle size of PPO domains in blend T2 (0.96 μm) became much 5871

DOI: 10.1021/acs.iecr.5b01041 Ind. Eng. Chem. Res. 2015, 54, 5870−5875

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Industrial & Engineering Chemistry Research

due to the barrier effect of OMMT, which resulted in the decrease of viscosity. Thus, the viscosity of the nanocomposites decreased after the introduction of OMMT and consequently OMMT improved the processability of the nanocomposites. Tensile Properties and HDT of PPO/PA6 Nanocomposites. It is of great importance to balance toughness and strength of the polymeric materials. In this study, we tried to prepare PPO/PA6 nanocomposites with both high tensile strength and excellent impact performance. Figure 5 showed the typical stress−strain curves of different samples, and the corresponding tensile properties and HDT Figure 3. SEM micrographs of different samples. All the samples were brittle fractured in liquid nitrogen and etched with chloroform.

smaller than that in blend T1. For nanocomposites N-2, N-4, and N-6, the particle size of PPO domains decreased gradually with the increase of OMMT loadings (0.90, 0.72, and 0.48 μm for N-2, N-4, and N-6, respectively), which was consistent with the literature and our previous research.20,32,33 In the immiscible polymer blend, the formation of two-phase structure is based on dynamic equilibrium between particle breakup and coalescence.34 Because OMMT selectively located in PA6 matrix,20,32 OMMT platelets could effectively prevent the coalescence of dispersed PPO domains due to its barrier effect, resulting in the decrease of the dispersed PPO domain size. Furthermore, the presence of MMT inside PA6 phase altered the viscosity ratio of PA6 matrix to the dispersed PPO phase, which was also beneficial for the dispersion of PPO phase. Rheological Properties of PPO/PA6 Nanocomposites. The viscosity curves of different samples were presented in Figure 4. All polymer melts showed shear-thinning behavior.

Figure 5. Typical stress−strain curves of different samples.

Table 1. Tensile Properties and HDT of Different Samples sample

σ10% (MPa)

σt (MPa)

E (MPa)

εb (%)

HDT (°C)

B T1 T2 N-2 N-4 N-6

52.9 42.7 43.0 45.4 48.2 51.0

53.0 46.7 63.0 63.2 63.7 58.5

2100 1800 2020 2500 2640 2720

73 110 125 128 127 120

67 67 80 82 84

were exhibited in Table 1. For PPO/PA6 binary blend, the tensile properties were poor due to the poor miscibility between PPO and PA6. As SEBS-g-MA was added, the elongation at break (εb) was over 100%, which could be ascribed to the contribution of the elastomer SEBS-g-MA. No obvious yield points were observed for all the toughened materials. Compared with blend T1, blend T2 had a higher σt due to the better compatibility between PPO and PA6. However, there was little improvement in σ10%, Young’s modulus, and HDT, which was of great significance in engineering. To further improve the properties, OMMT was introduced into the blends to obtain PPO/PA6 nanocomposites. After the addition of OMMT, various properties of the nanocomposites were improved. For PPO/PA6 nanocomposites, we took N-4 as an example. Compared with blend T2, the improvement in σ10%, HDT and Young’s modulus were 12.1%, 22.4% and 30.7%, respectively. These results can be attributed to the exfoliation of high-modulus OMMT layers. For nanocomposite N-6, as observed by TEM (Figure 2),

Figure 4. Plots of shear viscosity vs shear rate at 270 °C of different samples.

Compared with blend T1, blend T2 showed a significant increase in viscosity due to the reaction of MA group in SMA and amino group in PA6. The viscosity of PPO/PA6 nanocomposites was much lower than that of the blend T2, and decreased gradually with the increase of OMMT loading (N-2, N-4, and N-6), which could be ascribed to the following reasons. First, the slip between PA6 matrix and OMMT platelets would reduce the viscosity of nanocomposites, which was also demonstrated by Paul.35,36 Second, after OMMT was introduced into the PPO/PA6 blend, the reaction between MA group in SMA and amino group in PA6 would be suppressed 5872

DOI: 10.1021/acs.iecr.5b01041 Ind. Eng. Chem. Res. 2015, 54, 5870−5875

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Industrial & Engineering Chemistry Research

improved the toughness. Thus, for nanocomposites with exfoliated OMMT, elastomer-OMMT synergistic toughening was achieved. To further explore the elastomer-OMMT synergistic toughening mechanism, we observed morphology of SEBS-gMA toughened PPO/PA6 blends and nanocomposites by SEM. As n-hexane was good solvent for SEBS-g-MA, but poor solvent for PPO and PA6, SEBS-g-MA phase could be removed by nhexane etching. The morphology of n-hexane etched blend T2 and nanocomposite N-4 is shown in Figure 8. It was noticed

OMMT formed some stacks. The stacks would induce some defects during the tensile tests, which resulted in a slight drop in tensile strength. Impact Properties and the Toughening Mechanism. Figure 6 showed the comparison of the notched Izod impact

Figure 8. SEM micrographs of blend T2 and nanocomposite N-4 (The samples were brittle fractured in liquid nitrogen, and etched by boiled n-hexane). Small domain marked by rectangle in image N-4a was the source of the magnified micrograph N-4b. Figure 6. Comparison of notched Izod impact strength of different samples.

that SEBS-g-MA mainly located in PA6 matrix, which could be ascribed to the reaction of MA groups in SEBS-g-MA with amino groups in PA6. Besides, the peripheral regions of some PPO domains were also etched by n-hexane. This suggested that SEBS-g-MA also existed at the interface between PPO and PA6, which was driven by the affinity between PS blocks in SEBS-g-MA and PPO. Thus, core−shell structure particles were formed, in which rigid PPO core was encapsulated by soft SEBS-g-MA shell. Similar core−shell structure particles were also obtained by Yan in PA6/SEBS-g-MA/polystyrene blends, which were beneficial to toughen the blends.38 According to Wu’s theory,39 the interparticle distance (τ) between toughened particles was the key factor in toughening materials. The tough-brittle transition would occur when τ was close to a critical value (τc) and τc was an intrinsic property of the matrix. For PA6 matrix, τc was about 0.27 μm,40 and τ could be calculated according to eq 2.39

strength of different samples. Compared with blend B, blend T1 achieved high impact strength due to the addition of SEBS-gMA. After the introduction of SMA, the impact strength of blend T2 further increased due to the improvement of the compatibility between PPO and PA6. The impact strength of PPO/PA6/OMMT nanocomposites first increased with the increase of OMMT loading (N-2 and N-4), and then decreased with OMMT loading (N-6) due to the existence of OMMT stacks. This phenomenon could be explained in detail from both macroscopic and microcosmic angles. On a macroscopic level, the Izod impact value represents the area under a force−displacement curve, and the addition of OMMT made the force levels increase (Table 1).37 Besides, OMMT influenced the fracture surface of the impact specimens and the results were presented in Figure 7. The impact fracture

⎡⎛ π ⎞1/3 ⎤ ⎟ − 1⎥ τ = d w ⎢⎜ ⎢⎣⎝ 6Φ ⎠ ⎥⎦

(2)

where Φ is the volume fraction of the toughened particles, and dw is the weight-average particle size. In this study, Φ was about 0.47 for the toughened nanocomposites. As dw was below 1.4 μm, τ calculated according to eq 2 was less than τc, which indicated that all the nanocomposites were tough from the standpoint of interparticle distance. Paul41,42 reported that the supertoughened PA6 could not be prepared by SEBS-g-MA alone because the SEBS-g-MA particles were too small. The combination of SEBS-g-MA particles and appropriate sized core−shell particles could toughen PA6 efficiently. According to Paul’s research, the size upper limit for supertoughened PA6 was 1 μm. Most of the core−shell particles in nanocomposites were below 1 μm due to the barrier effect of OMMT, which was beneficial for the dissipation of impact energy. Thus, supertoughened nanocomposites (N-2 and N-4) were obtained. However, the OMMT stacks (Figure 2) in nanocomposite N-6 acted as defects during the impact test. The stress concentration accelerated the fracture of N-6 specimen and consequently resulted in the decrease of the impact strength.

Figure 7. Photographs of different impact fracture specimens.

surface of blend B was rather flat, which indicated the typical brittle fracture behavior. For other specimens, the fracture surface was convex and stress whitening phenomena appeared. The decreasing sequence of convexity degree was as follows: N4, N-2, T2, N-6/T1. It could be concluded that OMMT with exfoliated structure resulted in a more convex fracture surface, but OMMT with stacks resulted in a less convex fracture surface. As the fracture surface became more convex, the length of crack path during the impact test increased, which was beneficial to dissipate more impact energy and consequently 5873

DOI: 10.1021/acs.iecr.5b01041 Ind. Eng. Chem. Res. 2015, 54, 5870−5875

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CONCLUSION In summary, the PPO/PA6 nanocomposites with high tensile strength and excellent impact performance were prepared by melt extrusion. Under appropriate loading, OMMT showed exfoliated structure in the nanocomposites. OMMT platelets could effectively prevent the coalescence of dispersed PPO domains due to its barrier effect, resulting in the decrease of the dispersed PPO domain size. The viscosity of the nanocomposites decreased after the addition of OMMT, and consequently, OMMT improved the processability of the nanocomposites. The tensile properties and HDT were improved by the introduction of OMMT, which expanded the application of the nanocomposites in industry and engineering. Many dispersed PPO domains were encapsulated by SEBS-g-MA and core−shell particles were formed. After the introduction of OMMT, more toughening particles with the optimized size were obtained, and the elastomer-OMMT synergistic toughening was achieved. PPO/PA6/OMMT nanocomposites with high tensile properties, excellent impact strength, increased HDT, and good processability were obtained. This work provided a new way to obtain immiscible polymer alloy with comprehensive improved properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 571 87951612. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Natural Science Foundation of China (20974100 and 20674075) and Natural Science Foundation of Zhejiang Province (Y404299).

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REFERENCES

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DOI: 10.1021/acs.iecr.5b01041 Ind. Eng. Chem. Res. 2015, 54, 5870−5875

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DOI: 10.1021/acs.iecr.5b01041 Ind. Eng. Chem. Res. 2015, 54, 5870−5875