Morphology and Properties of Injection Molded Microcellular Poly

Article pubs.acs.org/IECR

Morphology and Properties of Injection Molded Microcellular Poly(ether imide) (PEI)/Polypropylene (PP) Foams Tao Liu,† Shiyi Zhou,*,‡ Yajie Lei,† Zhenglun Chen,† Xianzhong Wang,† Jingli Li,† and Shikai Luo† †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, People’s Republic of China College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, People’s Republic of China

‡

ABSTRACT: A series of PEI/PP blends were injection molded by conventional and microcellular methods. Polypropylene (PP) was selected as the disperse phase in order to improve the cell nucleation of poly(ether imide) (PEI)/PP blends. The effect of the PP content on the morphology of the PEI/PP blends or foams, the mechanical, thermal and dielectric properties was investigated. In the immiscible PEI/PP blends, the cell density of microcellular PEI/PP foams increased greatly with the increasing of the content of PP because the surface tension effect and interface effect significantly promoted the cell nucleation in the PEI/PP blends. Meanwhile, a large amount of gaps appeared in the interface because the processing temperature of the blends was close to the starting decomposition temperature of PP, which significantly improved the PEI/PP blends toughness because of tiny spherical cells that acted as crack arrestors by blunting the crack tip.

1. INTRODUCTION Microcellular foam is defined as having average cell diameters in the range of 1−10 μm and cell densities on the order of 109− 1015 cells/cm3. Microcellular foam was invented at the Massachusetts Institute of Technology (MIT) under the direction of Professor Nam P. Suh1,2 and has been studied extensively for the past 20 years.3−5 Because of its high strength-to-weight ratio, excellent heat and sound insulations, high energy or mass absorption and materials saving properties, microcellular foams have attracted significant attention.6−14 However, microcellular injection molding has become the fastest developing technology in all microcellular processes because of attractive savings of material, cycle time and energy.11 Generally, microcellular foaming requires a very high pressure drop and high nucleation ability within a very short time, in which the nucleation rate is greatly higher than the cell growth rate and all the bubbles nucleate simultaneously.12 However, it is a great challenge to effectviely enhance the nucleation and further decrease the cell diameter in the microcellular injection molding process because the rheological properties of melts and pressure drops are not as conveniently controlled as those of batch processing. Meanwhile, when N2 acts as a blowing agent in the MuCell process, both the pressure drop and the solubility of N2 in the polymer melt are low, which makes it difficult to generate efficient nuclei especially for high surface tension and neat polymer melts. To improve the cell density, the nucleating agents are introduced into polymers to make nucleation much easier.13−16 However, these agents coalesced easily and eventually affected the growth and distribution of cells. On the other hand, some polymers also have many shortcomings for microcelluar foaming. For example, the low viscosity of polymer cannot support growth of the cells, resulting in cell coalescence.17 The low gas solubility and diffusivity cannot provide sufficient blowing gas to the foam. © 2014 American Chemical Society

Thus, methods to improve cell properties has been attracting more attention. The blends can achieve multiple characteristics such as higher gas solubility, improved viscosity and appropriate melt index. So far, there are a few investigations on microcellular foams based on binary blends18−25 such as PS/ SBM,18 PMMA/PVDF,19 PP/TPS,20 PS/LCP,21 PEG/PS22 blends and so on. In the blends, the interface provides a lower nucleating energy barrier region, resulting in high nucleation density. Therefore, it is more flexible to obtain the excellent cell structure for the blends. Poly(ether imide) (PEI) is one of the most important highperformance engineering thermoplastics that is extensively used in commercial applications due to its excellent thermal stability, remarkable tensile strength, electrical insulating property, wearresisting property, dimensional stability, flame resistance, etc. However, there are a few investigations on microcellular foams based on PEI26−33 and, according to such research efforts, most microcellular foams based on PEI were prepared by a batch foaming method but there remains a lack of papers on the research of microcellular injection molding methods.31,33 In our previous works, the effects of processing parameters on cell morphology and material properties were studied.31 No matter how hard we tried, the cell size just reached to about 20 μm in the pure microcellular PEI foam. To further decrease the cell size, PP was selected as the disperse phase to induce a higher ability of nucleation. Then, a series of PEI/PP blends were prepared by a twin-screw extruder. The PEI/PP blends and foams were injection molded by conventional and microcellular methods, respectively, and the cell morphologies, mechanical properties, thermal and electric properties of microcelluar PEI/PP foams were investigated in detail. Received: Revised: Accepted: Published: 19934

June 18, 2014 August 28, 2014 November 4, 2014 November 4, 2014 dx.doi.org/10.1021/ie5023145 | Ind. Eng. Chem. Res. 2014, 53, 19934−19942

Industrial & Engineering Chemistry Research

Article

Meanwhile, a new assumption based on the above observation was proposed to explain the mechanism of the formation of the cell.

Vf = 1 −

Figure 1. Schematic of the ENGEL MuCell system and foaming processing.

Table 1. Processing Parameters of Microcellular Injection Molding rear

270

290

injection temperature (°C) shot size (mm) injection speed (mm/s) content of SC-N2 (wt %) mold temperature (°C)

middle

front

nozzle

310

315

320

50 100 0.37 80

2.4. Characterization. 2.4.1. Scanning Electron Microscopy (SEM) Observations. The microcellular structure of the molded parts was examined using a CamScan Apollo 300 scanning electron microscope. The SEM specimens were taken from the middle of the molded dog-bone bar. The SEM specimens were characterized, as shown in Figure 2. The cell density (Nf), number of cells per cubic centimeter of the foams, was calculated by eq 1:34 Nf =

⎛ n ⎞3/2 ⎛ 1 ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ A ⎠ ⎝ 1 − Vf ⎠

(2)

3. RESULTS AND DISCUSSION 3.1. Blend Morphology. Understanding the relationship between the properties and the phase morphology is necessary for material design. As shown in Figure 2, the interface between the disperse phase PP and the PEI matrix is very obvious and a sea-island morphology is observed. The results indicate that there is an awful compatibility between PP and PEI. Meanwhile, the number of interfaces obviously increases with the growth of the content of PP. There are lots of gaps in the interface between the disperse phase PP and the PEI matrix. This phenomenon is due to the low decomposition temperature of PP. It is well-known that the starting decomposition temperature of PP is about 350 °C,35 and the processing temperature of PEI/PP blends is about 330 °C. A small amount of decomposing gas appears in the interface because the processing temperature of PEI/PP blends is close to the starting decomposition temperature of PP. The thermal conductivity of PEI/PP blends is carried out in order to prove the presence of gas. As shown in Figure 3, one can see that the thermal conductivity of PEI/PP blends decreases with increasing of the content of PP. When the contenf of PP reaches to 10 wt %, the thermal conductivity of PEI/PP blend decreases by 29.4%. However, the thermal conductivity of the pure PP resin is equal to that of the pure PEI resin (0.25 W/ (m·K)). In the general case, the thermal conductvity of

value hopper

ρ

where ρ and ρf were the mass densities of solid and foamed samples, respectively. The void fraction of the foamed sample is shown in Table 2. 2.4.2. Mechanical Properties. The tensile and flexural properties of samples were carried out with a CMT 7015 material test instrument (SUNS, China) according to ISO 5275:1997 and ISO 178:1993, respectively. The crosshead speed for both tensile and flexural measurements was 5 mm/min. The testing of notched Charpy impact of samples was carried out with a PTM 1100 impact testing machine (SUNS, China) according to ISO 179-1:2000. All tests were carried out in an air-conditioned room (25 °C). 2.4.3. Electrical Properties. The dielectric constant of samples was carried out with a 4292A precision impedance analyzer (Agilent, USA), equipped with 16451B dielectric test fixture. A B-type electrode was used for the contacting electrode method. The frequency range was 50 Hz−30 MHz and the environment temperature was 25 °C. 2.4.4. Thermal Properties. The thermal conductivity of samples was carried out with an LFA447 laser thermal analyzer (Netzsch, Germany) following the ASTM E1461-2001 standard. The size of samples was Φ 12.7 × 2.0 mm, and the environment temperature was 25 °C. The HDT (heat detection temperature) of the PEI blends was determined according to the standard ISO 75-1:2004. A load of 1.80 MPa was placed on each specimen, and the temperature was increased at a rate of 2 oC min‑1 until the specimen deflected 0.32 mm. 2.4.5. Contact Angle Measurements. The contact angles were measured in a sessile drop mold with a DSA100 (Krüss, German). PEI and EAGMA samples were injected to test the samples with a Minijet 2 microscale injection molding machine (Thermfisher, USA). The contact angles were measured on 3 μL of wetting solvent at 25 °C.

2. EXPERIMENTAL SECTION 2.1. Materials. PEI (Ultem 1000) was supplied by Sabic, Saudi Arabia. PP (KF2682) was supplied by Andrea Basel, Holland. N2 with a purity of 99.9% was used as the physical blowing agent in microcellular injection molding experiments. 2.2. Blends Preparation. The PEI resin was first dried at 140 °C for 6 h to remove residual moisture. The PEI composites were processed in a PTW252 twin-screw extruder (HAAKE, Germany) to give the samples. The rotational speed of the extruder was 120 rpm, and the temperatures of its eight sections, from the charging hole to the ram head, were 310, 320, 320, 320, 325, 325, 325 and 325 °C. The samples were dried at 140 °C for 6 h to remove moisture and then conventionally injected to standard testing samples. 2.3. Microcellular Foams Preparation. Microcellular foams were prepared by A VC 330H/80L injection molding machine (Engel, Austria) using supercritical nitrogen (SC-N2) as the foaming agent, as shown in Figure 1. The supercritical fluid supply system was the SII-TR-10 model (Trexel, America). The parameters of microcellular injection are shown in Table 1.

parameters

ρf

(1)

where n is the number of cells on the SEM micrograph, A the area of the micrograph (cm2) and Vf the void fraction of the foamed sample, which can be estimated as 19935

dx.doi.org/10.1021/ie5023145 | Ind. Eng. Chem. Res. 2014, 53, 19934−19942


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Figure 2. SEM micrographs of PEI/PP blends. (A) PEI/PP = 98/2; (B) PEI/PP = 96/4; (C) PEI/PP = 94/6; (D) PEI/PP = 92/8; (E) PEI/PP = 90/10.

that the cell walls of the microcelluar PEI/PP foams are also much thinner than that of microcellular PEI foam, suggesting that the microcellular PEI/PP foams have higher cell densities and expansion ratios of foams. As shown in Figure 5, as the content of PP increases from 0 to 10 wt %, the cell density of microcellular PEI/PP foams increases greatly from 7.4 × 107 to 5.6 × 108 cells/cm3, and the cell diameter of microcellular PEI/ PP foams decreases from 23.3 to 14.8 μm. This is because the large numbers of interface provide more nucleation sites for gas. The situation is also observed in the foaming of PMMA/PS blends.36 In the previous work,31 we knew that the outer layer of microcellular PEI foams was the solid skin layer, and the inner layer of microcellular PEI foams was the foam structure. Meanwhile, the parameters of microcellular injection molding have an important effect on the solid skin layer. In this paper, the effect of the nucleation rate on addtion of PP is investigated by means of skin thickness. The number of cells is counted in the different zones in order to investigate the effect of the disperse phase PP on the nucleation of the microcellular PEI/ PP foams. As shown in Figure 6, the microcellular PEI foam and PEI/PP foam are divided into small zones, and the cell density of each zone is counted. From Figures 6 and 7, one can see that the number of cells in the surface zone is obviously smaller than that of cells in the center zone. Moreover, in the microcellular PEI foams, as the distance reaches 0.4 mm (thickness of outer layer), the cells begin to appear. Compared with the microcellular PEI/PP foams, the distance of appearing cells is just 0.1 mm (thickness of outer layer). The distance of the appearing cells is related to two factors: the diffusion rate of gas and the nucleation rate of a cell. When the diffusion rate is higher than the nucleation rate, the blowing agent reduces because a large amount of the gas diffuses from the surface, indicating that the distance of the appearing cells increases. When the nucleation rate is higher than the diffusion rate, the distance of appearing cells decreases becasue the gas does not have enough time to diffuse. Therefore, with the addition of PP, the nucleation rate of PEI/PP blends greatly increases, leading to the adjustment of the thickness of solid skin layer. Meanwhile, not only the parameters of microcellular injection

Table 2. Void Fraction of the Foamed Sample content of PP (wt %)

ρ (g/cm3)

ρf (g/cm3)

weight reduction (%)

Vf (%)

0 2 4 6 8 10

1.240 1.225 1.212 1.194 1.180 1.170

1.180 1.168 1.157 1.140 1.127 1.118

6.0 5.7 5.5 5.4 5.3 5.2

4.84 4.65 4.54 4.52 4.45 4.44

Figure 3. Curves of the thermal conductivity versus the content of PP for PEI/PP blends.

immiscible polymers is slightly lower than that of matrix because of the interfacial thermal resistance. In our previous work, when the reduce weight of PEI matrix reached to 5.4 wt %, the thermal conductivity of microcellular PEI foam decreased by 30.0%.31 So, the result of thermal conductivity of PEI/PP blends proves that there are lots of gas in the blends. 3.2. Foam Morphology. PP is selected as the disperse phase, to provide sufficient sites for cells growth in the processing conditions. Figures 4 and 5 show the cell properties of the microcellular PEI/PP foams. From Figure 4, one can see 19936



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Figure 4. SEM micrographs of microcellular PEI/PP foams. (A) PEI/PP = 100/0; (B) PEI/PP = 98/2; (C) PEI/PP = 96/4; (D) PEI/PP = 94/6; (E) PEI/PP = 92/8; (F) PEI/PP = 90/10.

PEI matrix. The homogeneous nucleation theory indicates that the surface tension of polymer plays an important role in the cell nucleation when the processing parameters remain constant. According to the theory, the lower the surface tension γ is, the easier the nucleation appears. The contact angles of the different materials with water and diiodomethane are listed in Table 3 and the surface tension, dispersion and polar components of the materials are calculated by eqs 3 and 4.37

molding but also the improvement of nucleation rate can adjust the thickness of the solid skin layer. On the other hand, the number of cells in the microcellular PEI/PP foams is greatly higher than that of cells in the microcellular PEI foams. The result is attributed to the amount of nucleation because of the presence of PP. 3.3. Mechanism of Nucleation. The change of those morphologies is due to the improvement of cell nucleation. The disperse phase PP enhances the cell density significantly because of two main effects: surface tension effect and interface effect. 3.3.1. Surface Tension Effect. Cell nucleations can occur in three positions of PEI/PP blends: disperse phase PP domain, PEI matrix and interface of PEI/PP blends. Nucleation can be explained by classical homogeneous nucleation theory when the cell nucleations appear in the disperse phase PP domain and

⎛ γd γd γHp Oγ p ⎞ H 2O ⎜ ⎟ + p 2 (1 + cos θH2O)γH O = 4⎜ d p⎟ d 2 γ + γ γ + γ H 2O ⎝ H 2O ⎠ 19937

(3)



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Figure 5. Curves of the cell properties versus the content of PP for microcellular PEI/PP foams.

Figure 7. Curves of the cell numbers versus distance for microcellular PEI and PEI/PP foams.

p ⎛ γd γd γCH γp ⎞ CH I I (1 + cos θCH2I2)γCH I = 4⎜⎜ d 2 2 d + p 2 2 p ⎟⎟ 2 2 γCH I + γ ⎠ ⎝ γCH2I2 + γ 2 2

Table 3. Contact Angle and Surface Tension of the Polymers contact angle (deg)

(4)

⎛ γ dγ d γ pγ p ⎞ γ12 = γ1 + γ2 − 4⎜⎜ d 1 2 d + p 1 2 d ⎟⎟ γ1 + γ2 ⎠ ⎝ γ1 + γ2

(5)

surface tension (mN/m)

sample

water

diiodomethane

total (γ)

PEI PP

76.02 99.29

23.98 50.86

57.56 37.95

dispersion component (γd)

polar component (γp)

46.66 34.98

10.90 2.97

according to the above analysis, lots of nucleating sites first form in the disperse phase PP domains because of pressure unloading (Figure 9b). Subsequently, the cells grow quickly in the PP domains, but the low melt strength of PP cannot support these cells, leading them to coalesce together. At the same time, the cell nucleation and cell growth appear in the PEI matrix (Figure 9c). Finally, disperse phase PP will coalesce together because of the surface tension, and is finally embraced by the cells (Figure 9d). 3.3.2. Interface Effect. In addition, blends can introduce a great deal of interface into the melt, and provide much lower energy barrier regions for nucleation. The cell nucleation is related to the interfacial tensions of the blends. As described by

In the formula above, γ is the surface tension, γ is the dispersion component, γp is the polar component and θ is the contact angle with water or diiodomethane. γ12 is the interfacial tension between materials 1 and 2, and γ1 and γ2 are the surface tensions of the two contacting components in the blends. As shown in Table 3, the surface tension of PP is obviously lower than that of PEI. When the pressure drops quickly in the processing, the cell nucleation can first form in the disperse phase PP domains. From Figure 8, it can be clearly observed that the spherical PP domains are embraced by the cells. The phenomenon can be described in Figure 9. Initially, the SC-N2 and melt of PEI/ PP blends form the homogeneous system (Figure 9a), and then d

Figure 6. SEM morphologies of the microcellular PEI and PEI/PP foam. (A) microcellular PEI foams and (B) microcellular PEI/PP foams. 19938



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Figure 8. SEM morphologies of spherical PP domains in microcellular PEI/PP foams. The the PP concentration is 6 wt %. (A) magnification, 500×; (B) magnification, 5000×.

Figure 9. Schematic diagram of the formation mechanism of cells embracing the PP globes: (A) initial state, (B) cell nucleation, (C) cell growth and coalescence and (D) PP globes formation.

Sharudin,36 the larger the interfacial tension of two polymers is, the smaller the contact angle become, resulting in fast nucleating at interface. The interfacial tension of the PEI/PP blend calculated by the equation from Wu (eq 5)37 is 6.42 mN m−1, which is 22−23 times higher than that of the compatible PEI/EAGMA blends.33 According to the rule that the bigger the interfacial tension of blends is, the worse the compatibility of blends is, in the PEI/PP system, the PEI and PP are not compatible which is consistent with the morphology of the PEI/PP blends, as seen in Figure 2. So, the interface of PEI/PP blends can provide a great deal of much easier nucleation zones, as shown in Figure 10. 3.4. Mechanical Properties. Figure 11 shows the mechanical properties of the PEI/PP blends and microcellular foams with various PP content. The tensile strength and flexural strength of PEI/PP blends decrease from 102.5 to 51.9 MPa, and 146.3 to 99.6 MPa as the content of PP increases from 0 to

Figure 10. Schematic diagram of the interface inducing cell nucleation.

10 wt %, respectively. The reduction of tensile strength and flexural strength of PEI/PP blends can be attributed to two main effects: degradation of PP and an incompatible reason. First, the tensile strength and flexural strength of PP is obviously lower than that of PEI, and the degradation results in the further decreasing of mechanical properties in the 19939



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blends obviously enhances with the content of PP increasing. It is probably that the presence of the gaps between the disperse phase PP and the PEI matrix can be regarded as microcells. Meanwhile, the cell diameter is about 2 μm, which significantly promotes the PEI/PP blends toughness because of tiny spherical cells that act as crack arrestors by blunting the crack tip. The mechanical properties of the microcellular PEI/PP foams exhibit a similar trend to the PEI/PP blends. The tensile strength and fexural strength of the microcellular PEI/PP foams decrease from 79.9 to 45.1 MPa, and from 131.9 to 96.7 MPa as the content of PP increases from 0 to 10 wt %, respectively. The notched Charpy impact strength of the microcellular PEI/ PP foams reaches 16.2 KJ/m2. The mechanical properties of the microcellular PEI/PP foams are obviously lower than that of the PEI/PP blends. It is probable that the cell diameter is still too large to prevent the expansion of the crack during the impact test. 3.5. Electrical and Thermal Properties. The dielectric constant of microcellular PEI foams has direct relationships with the morphology and accumulation mode of cells. As shown in Figure 12, the dielectric constant of samples declines

Figure 12. Curves of the dielectric constant versus the content of PP for PEI/PP blends and foams.

with the increasing of the content of PP. It is probably because the introduction of cell increases the free volume of internal microstructure and reduced the number of polarization groups within the unit volume, which leads to the reduction of dielectric constant.39 Moreover, the HDT of PEI/PP blends and foams are investigated. As seen in Figure 13, the PP exhibits a large effect on the HDT of the PEI/PP blends and the difference between the maximum value and the minimum value is 15 °C. It is wellknown that the thermal stability of PP is poor, so the introduction of the PP resin will decrease the HDT of PEI. Although the HDT of the PEI/PP blends decreases with the increasing PP content, the minimum HDT is still 172 °C, which is much higher than that of conventional plastics such as PET, PA66, PC, etc. However, the HDT of PEI/PP foams depended on the matrix. So, the effect of the content of PP on the PEI/PP foams exhibited the same tendency.

Figure 11. Curves of the mechanical properties versus the content of PP for PEI/PP blends and foams.

processing temperature. Second, the PP is not incompatible with the PEI matrix, which leads to reduction of tensile strength and flexural strength of PEI/PP blends. However, the maximum values of notched Charpy impact strength is observed in the 10 wt % PP-introduced PEI/PP blends. The value is about 21.8 KJ/m2, which is is 3−4 times higher than that of pure PEI. From Figure 2, there is an awful compatibility between PP and PEI. Generally, the incompatible blends will cause the toughness of material to reduce. In the incompatible PA6/ HDPE blends, as the content of HDPE reaches to 15 wt %, the toughness of blends is lower than that of pure PA6.38 However, in the incompatible PEI/PP blends, the toughness of PEI/PP 19940



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(6) Yuan, M.; Turng, L. S.; Caulfield, D. F. Crystallization and thermal behavior of microcellular injection-molded polyamide-6 nanocomposites. Polym. Eng. Sci. 2006, 46, 904. (7) Pilla, S.; Kramschuster, A.; Lee, J.; Auer, G. K.; Gong, S. Q.; Turng, L. S. Microcellular and solid polylactide-flax fiber composites. Compos. Interfaces 2009, 16, 869. (8) Pilla, S.; Kim, S. G.; Auer, G. K.; Gong, S. Q.; Park, C. B. Microcellular extrusion-foaming of polylactide with chain-extender. Polym. Eng. Sci. 2009, 49, 1653. (9) Pilla, S.; Kramschuster, A.; Yang, L.; Lee, J.; Gong, S.; Turng, L. Microcellular injection-molding of polylactide with chain-extender. Mater. Sci. Eng., C 2009, 29, 1258. (10) Chen, L.; Ozisik, R.; Schadler, L. S. The influence of carbon nanotube aspect ratio on the foam morphology of MWNT/PMMA nanocomposite foams. Polymer 2010, 51, 2368. (11) Yang, J.; Huang, L.; Zhang, Y.; Chen, F.; Fan, P.; Zhong, M.; Yeh, S. A new promising nucleating agent for polymer foaming: applications of ordered mesoporous silica particles in polymethyl methacrylate supercritical carbon dioxide microcellular foaming. Ind. Eng. Chem. Res. 2013, 52, 14169. (12) Xu, J. Microcellular Injection Molding; Wiley Press: New York, 2011. (13) Naguib, H. E.; Park, C. B.; Lee, P. C.; Xu, D. A study on the foaming behaviors of PP resins with talc as nucleating agent. J. Polym. Eng. 2006, 26, 565. (14) Ji, G. Y.; Zhai, W. T.; Lin, D. P.; Ren, Q.; Zheng, W. G.; Jung, D. W. Microcellular foaming of poly(lactic acid)/silica nanocomposites in compressed CO2: Critical influence of crystallite size on cell morphology and foam expansion. Ind. Eng. Chem. Res. 2013, 52, 6390. (15) Pilla, S.; Kramschuster, A.; Lee, J.; Clemons, C.; Gong, S. Q.; Turng, L. S. Microcellular processing of polylactide-hyperbranched polyester-nanoclay composites. J. Mater. Sci. 2010, 45, 2732. (16) Hwang, S. S.; Liu, S. P.; Hsu, P. P.; Yeh, J. M.; Yang, J. P.; Chen, L. C. Morphology, mechanical, and rheological behavior of microcellular injection molded EVA-clay nanocomposites. Int. Commun. Heat Mass Transfer 2012, 39, 383. (17) Rachtanapun, P.; Selke, S.; Matuana, L. M. Effect of the highdensity polyethylene melt index on the microcellular foaming of highdensity polyethylene/polypropylene blends. J. Appl. Polym. Sci. 2004, 93, 364. (18) Ruiz, J. A. R.; Marc, J.; Pedros, M.; Dumon, M. Two-step microcellular foaming of amorphous polymers in supercritical CO2. J. Supercrit. Fluids 2011, 57, 87. (19) Siripurapu, S.; Gay, Y. J.; Royer, J. R.; Desimone, J. M.; Spontak, R. J.; Khan, S. A. Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process. Polymer 2002, 43, 5511. (20) Nemoto, T.; Takagi, J.; Ohshima, M. Nanoscale cellular foams from a poly (propylene)-rubber blend. Macromol. Mater. Eng. 2008, 293, 991. (21) Wang, J.; Cheng, X.; Yuan, M.; He, J. An investigation on the microcellular structure of polystyrene/LCP blends prepared by using supercritical carbon dioxide. Polymer 2001, 42, 8265. (22) Taki, K.; Nitta, K.; Kihara, S. I.; Ohshima, M. CO2 foaming of poly(ethylene glycol)/polystyrene blends: Relationship of the blend morphology, CO2 mass transfer, and cellular structure. J. Appl. Polym. Sci. 2005, 97, 1899. (23) Zhao, H. B.; Cui, Z. X.; Sun, X. F.; Turng, L. S.; Peng, X. F. Morphology and properties of injection molded solid and microcellular polylactic acid/polyhydroxybutyrate-valerate (PLA/PHBV) blends. Ind. Eng. Chem. Res. 2013, 52, 2569. (24) Madhuchhanda, M.; Rasksh, V. J. Microcellular foam from ethylene vinyl acetate/polybutadiene rubber (EVA/BR) based thermoplastic elastomers for footwear applications. Ind. Eng. Chem. Res. 2012, 51, 10607. (25) Rahida, W. B. S.; Masahiro, O. Preparation of microcellular thermoplastic elastomer foams from polystyrene-b-ethylene-butyleneb-polystyrene (SEBS) and their blends with polystyrene. J. Appl. Polym. Sci. 2013, 128, 2245.

Figure 13. Curve of the HDT versus the content of PP for the PEI/PP blends and foams.

4. CONCLUSIONS In conclusion, a large number of gaps appear in the interface of PEI/PP blends because the processing temperature is close to the starting decomposition temperature of PP. The gaps can be regarded as microcells, which significantly promote the PEI/PP blends toughness because of tiny spherical cells that act as crack arrestors by blunting the crack tip. So, the maximum value of notched Charpy impact strength is about 21.8 KJ/m2, which is is 3−4 times higher than that of pure PEI. Meanwhile, for the surface tension effect and interface effect, cell diameter of microcellular PEI/PP foams decreases from 23.3 to 14.8 μm as the content of PP increases from 0 to 10 wt %. The PEI/PP foam with a high thoughness, low dielectric constant and high HDT is postulated to have potential applications in many areas such as the electrical industry.

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AUTHOR INFORMATION

Corresponding Author

*S. Y. Zhou. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from Science and Technology Development Foundation of China Academy of Engineering Physics (Grant number: 2013B0302041).

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REFERENCES

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Morphology and Properties of Injection Molded Microcellular Poly

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