Morphology, mechanical and dielectric properties, and rheological behavior of EAGMA toughened microcellular PEI–EAGMA foam

Abstract

A series of toughened polyetherimide (PEI)–ethylene-acrylate-glycidyl methacrylate (EAGMA) copolymer blends were prepared using a twin screw extruder and then injection molded by conventional and microcellular methods. The effect of the EAGMA content on the mechanical, thermal and dielectric properties, rheological behavior and morphology of the PEI–EAGMA blends was investigated. The results showed that the PEI–EAGMA blends exhibited a notched Charpy impact strength of 27.9 kJ m⁻². This is 4–5 times higher than that of pure PEI and indicates that EAGMA had an excellent toughening effect on the PEI–EAGMA system. The interfacial tension calculation and SEM micrographs indicated that the PEI matrix was highly compatible with the EAGMA elastomer. The rheological results showed the restriction in the molecular mobility of PEI was more significant as the content of EAGMA increased. Moreover, the dielectric constant and loss of the materials can be effectively reduced through the technology of micro-foaming. EAGMA plays an important role in the microcellular structure formation and final dielectric properties of the PEI–EAGMA foam.

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1. Introduction

A microcellular foam is defined as having average cell sizes in the range of 1–10 μm, and cell densities on the order of 10⁹–10¹⁵ cells per cm³. The microcellular foam was invented at the Massachusetts Institute of Technology (MIT) under the direction of Professor Nam P. Suh^1,2 and has been studied extensively over the past 20 years.^3–5 Because of their high strength-to-weight ratio, excellent heat and sound insulation ability, high energy or mass absorption and materials saving properties, microcellular foams have attracted significant attention.^6–14 Polyetherimide (PEI) is one of the most important high-performance engineering thermoplastics and is extensively used in commercial applications due to its excellent thermal stability, remarkable tensile strength, electrical insulating properties, wear-resisting properties, dimensional stability and flame resistance etc. However, so far there are few investigations on microcellular foams based on PEI.^15–18 According to research, most microcellular foams based on PEI were prepared by the batch foaming method but there is a lack of papers on the research of the microcellular injection molding method. Nemoto et al.¹⁸ prepared microcellular poly(ether ether ketone) (PEEK)–para-diamine poly(ether imide) (p-PEI) and PEEK–meta-diamine poly(ether imide) (m-PEI) foams. They found that the difference in chemical configuration between m- and p-PEI gave rise to a prominent change in the cell structure of the respective foams. Miller et al.^16,17 identified that the micro-scale cells or nano-scale cells of PEI can be created by means of controlling the CO₂ pressure and the glass transition temperature of PEI. At 5 MPa CO₂ pressure the glass transition temperature was lowered to 95 °C from 215 °C at normal atmospheric pressure. Meanwhile, they found that the nanofoams showed a significantly higher strain to failure, resulting in an improvement in the modulus of toughness of up to 350% compared to microcellular foams.

Moreover, the rigid structure of the PEI molecular chain leads to poor resistance to the notched impact strength. For this reason, polymer blending has been applied as a facile method to solve this problem. Thus, a variety of polymers have been fully miscible with the PEI matrix, such as poly(ether ether ketone),^19–29 poly(ethylene naphthalate),^22,30–32 poly(butylene terephthalate),^33–35 poly(ethylene terephthalate)^36,37 polyarylate,^38,39 poly(ether sulfone)⁴⁰ and polyamide 66⁴¹ etc. Although a lot of literature reported the mechanical behavior of various PEI–polymer unfoamed blends in detail, very little attention was paid to the elastomer toughened PEI microcellular foams.

In this article, a series of toughened PEI–EAGMA blends were prepared using a twin screw extruder. The PEI–EAGMA blends and foams were injection molded by conventional and microcellular methods, respectively. The effect of the EAGMA content on the mechanical, thermal, dielectric and cell properties, rheological behavior and morphology of PEI–EAGMA blends and foams was investigated.

2. Materials and experimental

2.1. Materials

PEI (Ultem 1100) was purchased from Sabic, America. It is an amorphous thermoplastic with a measured density of 1.24 g cm⁻³. EAGMA (AX 8900) was supplied by Arkema Inc, France. The molecular weight of PEI and EAGMA is 3.2 × 10⁴ g mol⁻¹ and 8 × 10⁴ g mol⁻¹, respectively.

2.2. PEI–EAGMA blend preparation

The PEI resin was firstly dried at 140 °C for 6 h to remove residual moisture. It was mixed with the EAGMA elastomer and processed in a PTW252 twin-screw extruder (HAAKE, Germany) to produce 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, 330, 330, 335, 330, 320, and 325 °C. The samples were dried at 120 °C for 8 h to remove moisture and were then conventionally injected to standard testing samples.

2.3. Microcellular PEI–EAGMA foam preparation

Microcellular PEI–EAGMA foams were prepared using an A VC 330H/80L injection molding machine (Engel, Austria) using supercritical nitrogen (SC-N₂) as the foaming agent. The supercritical fluid supply system was a SII-TR-10 model (Trexel, America). The parameters of the microcellular injection are shown in Table 1.

Table 1 Processing parameters of the microcellular injection molding

Parameters	Value
Injection temperature (°C)	330
Shot size (mm)	49
Content of SC-N2 (wt%)	0.4
Injection speed (mm s^−1)	20
Mold temperature (°C)	120

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2.4. Characterization

2.4.1 Mechanical properties. The tensile and flexural properties of the samples were determined using a CMT 7015 Material Test Instrument (SUNS, China) according to ISO 527-5: 1997, ISO 178: 1993, respectively. The crosshead speed for both tensile and flexural measurements was 5 mm min⁻¹. The notched Charpy impact testing of the samples was carried out with a PTM 1100 Material Test Instrument (SUNS, China) according to ISO 179-1:2000. All tests were carried out in an air-conditioned room (25 °C).

2.4.2 Rheological characterization. Viscoelastic characterization of the blends was conducted with an RS600 rotational rheometer (HAAKE, Germany). The linear viscoelastic range of deformation was obtained by a strain sweep test. The response to applied oscillatory deformation at 330 °C was evaluated in the frequency range of 0.01 to 100 Hz.

2.4.3 Thermal properties. The HDT (heat deflection temperature) of the PEI–EAGMA blends was determined according to the standard of 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 °C min⁻¹ until the specimen deflects 0.32 mm. The dynamic thermomechanical (DMA) behavior of the PEI–EAGMA blends, with various EAGMA contents, was studied using a Thermomechanical Analyzer, RSA3. The experiments were carried out using the three-point bending mode. PEI, EAGMA and PEI–EAMGA blends were measured at a temperature range from −50 to 300 °C with a heating rate of 3 °C min⁻¹. All the tests were carried out at a frequency of 1 Hz and a strain level of 0.1%. TGA thermograms of the PEI–EAGMA blends, with various EAGMA contents, were determined by a TA Instruments TGA-Q50 thermogravimetric analyzer with a heating rate of 20 °C min⁻¹, from room temperature to 800 °C, under a nitrogen atmosphere.

2.4.4 SEM analysis. The morphology of the solid blends and foams was observed by scanning electron microscopy (SEM) using a CamScan Apollo 300 scanning electron microscope. The cell density (N_f) – number of cells per cubic centimeter of the foams – was calculated by eqn (1):^42,43where n is the number of cells on the SEM micrograph, A is the area of the micrograph (cm²), and V_f is the void fraction of the foamed sample. This can be estimated as:where ρ and ρ_f are the mass densities of the solid and foamed samples, respectively.

2.4.5 Dielectric constant measurements. The dielectric constant of both the unfoamed and foamed samples was measured with a 4292A precision impedance analyzer (Agilent, America). All the tests were carried out in the frequency range of 50 Hz to 30 MHz at room temperature.

2.4.6 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 (Thermofisher, America). The contact angles were measured on 3 μl wetting solvent at 25 °C.

3. Results and discussion

3.1. Blend morphology

To understand the relationship between the properties and the phase morphology is necessary for material design. As shown in Fig. 1, the interface between the EAGMA elastomer and the PEI matrix is very obscure and no sea–island morphology is observed. The results indicate that there is a good compatibility between PEI and EAGMA. Moreover, the fracture surfaces of the PEI–EAGMA blends reflected different degrees of plastic deformation and tearing behavior. These are compelling proof of the increase in toughness with the increase of EAGMA content.

Fig. 1 SEM micrographs of PEI–EAGMA blends. (A) PEI/EAGMA = 100/0; (B) PEI/EAGMA = 95/5; (C) PEI/EAGMA = 90/10; (D) PEI/EAGMA = 85/15; (E) PEI/EAGMA = 80/20.

3.2. Interfacial tension

The interfacial tension is a key factor in the study of the formation of the phase structure between the EAGMA elastomer and PEI matrix. The processing parameters of the microcellular injection molding process are listed in Table 1. The contact angles of the different materials with water and diiodomethane are listed in Table 2 and the surface tension, dispersion and polar components of the materials were calculated by eqn (3) and (4).⁴⁴

Table 2 The contact angle and surface tension of the polymers

Sample	Contact angle (°)		Surface tension (mN m^−1)
	Water	Diiodomethane	Total (γ)	Dispersion component (γ^d)	Polar component (γ^p)
PEI	76.02	23.98	57.56	46.66	10.90
EAGMA	81.07	32.2	52.70	43.58	9.12

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In the formula above, γ is the surface tension, γ^d is the dispersion component, γ^p is the polar component and θ is the contact angle with water or diiodomethane. γ₁₂ is the interfacial tension between materials 1 and 2, γ₁ and γ₂ are the surface tensions of the two contacting components in the blends.

The interfacial tension of the PEI–EAGMA blend, calculated by the equation from Wu (eqn (5))⁴⁴ is very small, only 0.27 mN m⁻¹. According to the rule that the smaller the interfacial tension of blends is, the higher the compatibility of blends is, in the PEI–EAGMA system, the PEI matrix and the EAGMA elastomer are highly compatible which is consistent with the morphology of the PEI–EAGMA blends, as seen in Fig. 1.

3.3. Rheological behavior

Dynamic strain sweeps at 330 °C are obtained in order to determine the linear viscoelastic region (LVR), and the data are displayed in Fig. 2. According to the LVR results, the response to the applied oscillatory deformation at 330 °C is evaluated in the frequency range of 0.01 to 100 Hz at a strain of 100%.

Fig. 2 Comparison of the linear viscoelastic region of the different samples at 330 °C.

Fig. 3 shows the relationship between the dynamic viscosity and the frequency of the PEI matrix and PEI–EAGMA blends. It is seen that the dynamic viscosity of the PEI–EAGMA blends decreases with the increase in the frequency which indicates that the PEI–EAGMA blends are obviously pseudoplastic fluids. Meanwhile, the dynamic viscosity of the PEI–EAGMA blends increases remarkably with the increase of EAGMA in the low frequency region. This phenomenon was due to the higher dynamic viscosity of EAGMA. The EAGMA elastomer is elastic, so the motion of the PEI molecular chain will be hindered in the PEI–EAGMA blends.

Fig. 3 Comparison of the dynamic viscosity of the different samples at 330 °C.

Fig. 4 and 5 show the storage modulus G′ and loss modulus G′′ against the frequency for the PEI–EAGMA blends. The G′ and G′′ exhibit a similar trend. At the low frequency region, the G′ of the PEI–EAGMA blends increases with the increase in EAGMA content which is related to the melt elasticity enhancement. The stronger the hydrodynamic effect in the melt, the larger the gradient of the modulus, as shown in the relevant graphs in Fig. 4.⁴⁵ Whereas, at the high frequency region, G′ of the PEI–EAGMA blends decreases to a value even lower than the PEI matrix.

Fig. 4 Comparison of the storage modulus curves of the different samples at 330 °C.

Fig. 5 Comparison of the loss modulus curves of the different samples at 330 °C.

The restricted molecular mobility can also be traced using the crossover point where the values of G′ and G′′ are equal. By increasing the angular frequencies, the crossover point indicates a transition from a viscous deformation to an elastic behavior. The shift in the crossover frequency represents the changes in the molecular mobility and relaxation time behavior. As seen in Fig. 6, the crossover frequency of the blends decreases with the increase in the EAGMA content. This may be due to the influence of EAGMA on the molecular mobility of the PEI matrix. EAGMA is a kind of elastomer with a high viscosity, the molecular chain of EAGMA is more difficult to move than PEI. Therefore, the mobility of the PEI molecular chain will be restricted by the incorporation of EAGMA. Meanwhile the restriction in the molecular mobility is more significant with the increase in the EAGMA content.^45,46

Fig. 6 Curves of the crossover frequency versus the content of EAGMA for the PEI–EAGMA blends.

Moreover, the Cole–Cole technique developed by Han⁴⁷ in which η′′ is plotted versus η′, is a useful tool to assess the compatibility of blends. As shown in Fig. 7, the PEI–EAGMA blends have only one circular arc in the curve, suggesting a homogeneous phase.^48–50

Fig. 7 Cole–Cole plots for the PEI–EAGMA blends at 330 °C.

3.4. Thermal properties

The dynamic thermomechanical behavior and thermal stability of pure EAGMA, PEI and PEI–EAGMA blends were studied. The glass transition temperature (T_g), thermal degradation temperatures of 5 and 10% weight losses (T_5% and T_10%) and the carbon yields at 800 °C of pure EAGMA, PEI and PEI–EAGMA blends are listed in Table 3. It can be seen, that the T_g of EAGMA and PEI was −25 °C and 215 °C, respectively, the T_g of the PEI–EAGMA blends was a little lower than PEI and decreases with the increase in EAGMA content. Meanwhile, the thermal stability of the PEI–EAGMA blends decreases with increasing EAGMA content as well.

Table 3 Thermal properties of EAGMA, PEI and PEI–EAGMA blends

EAGMA content (wt%)	0	5	10	15	20	100
Tg (°C)	215	210	206	201	198	−25
T5% (°C)	538	488	465	456	451	418
T10% (°C)	547	540	489	484	471	431
Carbon yield at 800 °C (%)	48.46	47.68	46.58	44.74	42.32	0.71

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Moreover, the HDT of neat PEI and PEI–EAGMA blends were investigated. As seen in Fig. 8, the EAGMA exhibits a large effect on the HDT of the PEI–EAGMA blends and the difference between the maximum value and the minimum value was 24 °C. It is well known that the thermal stability of the elastomer is poor, so the introduction of the EAGMA resin will decrease the HDT of PEI. Although the HDT of the PEI–EAGMA blends decreases with the increasing EAGMA content, the minimum HDT is still 165 °C which is much higher than that of conventional plastics such as PP, PA66 and PC etc.

Fig. 8 Curve of the HDT versus the content of EAGMA for the PEI–EAGMA blends.

3.5. Mechanical properties

Fig. 9 shows the mechanical properties of the PEI–EAGMA blends and microcellular foams with various EAGMA contents. The tensile strength and flexural strength of the PEI–EAGMA blends decrease from 102.7 MPa to 56.5 MPa, and 146.6 MPa to 90.7 MPa as the content of EAGMA increased from 0 to 20 wt%, respectively. According to the morphology analyses in section 3.1, the introduction of EAGMA increases the toughness of PEI. The maximum values of the notched Charpy impact strength were observed in the 20 wt% EAGMA-introduced PEI–EAGMA blends. They were about 27.9 kJ m⁻², which is 4–5 times higher than that of pure PEI. The results indicate that the EAGMA has a good toughening effect on the PEI matrix. The mechanical properties of the microcellular PEI–EAGMA foams exhibited a similar trend to the unfoamed PEI–EAGMA blends. The tensile strength and flexural strength of the microcellular PEI–EAGMA foams decreased from 77.9 MPa to 46.3 MPa, and from 136.7 MPa to 72.53 MPa as the content of EAGMA increased from 0 to 20 wt%, respectively. The notched Charpy impact strength of the microcellular PEI–EAGMA foams reached 10.2 kJ m⁻². The mechanical properties, especially the notched Charpy impact strength, of the microcellular PEI–EAGMA foams are obviously lower than that of the unfoamed PEI–EAGMA blends. Huang et al.⁵¹ reported that the smaller the cell size, the better the mechanical properties. It is probable, that the cell size is still too large to prevent the expansion of the crack during the impact test.

Fig. 9 Curves of the mechanical properties versus the content of EAGMA for PEI–EAGMA blends and foams.

3.6. Cell properties

Fig. 10 and 11 show the cell properties of the microcellular PEI–EAGMA foams. Evidently, as the content of EAGMA increases, the cell size decreases to a minimum value of 13.3 μm and then increases to a maximum value of 39.3 μm. On the one hand, with the increasing EAGMA content the melt strength of the PEI–EAGMA blends enhances, so cell coalescence reduced and the cell size decreased. On the other hand, the solubility of N₂ gas in acrylic materials, such as EAGMA, is small,⁵² thus, with the increase in EAGMA content, the N₂ solubility in the PEI–EAGMA blends reduced. That is to say the amount of N₂ gas which diffused from the PEI–EAGMA melt increased. The diffusion of gas will lead to cell coalescence, and therefore an increase in the cell size. Moreover, the decreasing solubility of N₂ gas in the PEI–EAGMA blends led to a dramatic reduction in the cell density. As shown in Fig. 12, the mass density of both the foamed and unfoamed PEI–EAGMA decreased with the increasing EAGMA content. Furthermore, the two mass density curves gradually approached one another as the EAGMA content increased. This result indicates that the weight reduction of the microcellular PEI–EAGMA foams decreases with the increase in the EAGMA content. This phenomenon also confirms the analyses stated above, that EAGMA plays an important role in the N₂ solubility in the PEI–EAGMA blends.

Fig. 10 SEM micrographs of microcellular PEI–EAGMA foams. (A) PEI/EAGMA = 100/0; (B) PEI/EAGMA = 95/5; (C) PEI/EAGMA = 90/10; (D) PEI/EAGMA = 85/15; (E) PEI/EAGMA = 80/20.

Fig. 11 Curves of the cell properties versus the content of EAGMA for microcellular PEI–EAGMA foams.

Fig. 12 Curves of the mass density versus the content of EAGMA for PEI–EAGMA blends and foams.

3.7. Dielectric properties

Considering the applications of microcellular foams, such as in the electrical industry, the effect of the EAGMA content on the microcellular structure and the dielectric properties of PEI–EAGMA have been studied. Fig. 13 shows the variation in dielectric constant, ε′, and dielectric loss, tan δ, with frequency in the unfoamed PEI, EAGMA and microcellular PEI–EAGMA foam as a function of the increasing EAGMA content. It can be obviously seen that the dielectric constant and loss of the materials can be effectively reduced through the technology of micro-foaming. As shown in Fig. 13(A), all of the PEI–EAGMA foams have lower dielectric constants than that of the unfoamed PEI and EAGMA. Moreover, it is worth noting that the dielectric constant of the PEI–EAGMA foam increases with the increasing EAGMA content. There are mainly two reasons for this phenomenon. Firstly, the dielectric constant of EAGMA is the highest, the higher the EAGMA content in the PEI–EAGMA foam, the higher the dielectric constant will be. Secondly, the N₂ solubility in the PEI–EAGMA blends reduced with the increase in EAGMA content as the solubility of N₂ gas in EAGMA is small. That is to say that the more EAGMA in the PEI–EAGMA blends, the less gas the PEI–EAGMA foam contains, so the dielectric constant of the microcellular PEI–EAGMA foams is close to the unfoamed PEI and EAGMA when the EAGMA content is increased.

Fig. 13 Dielectric properties of unfoamed (closed symbols) PEI, EAGMA and PEI–EAGMA foams with various EAGMA content.

From Fig. 13(B) it can be seen that EAGMA has the highest dielectric loss but PEI is much lower, the dielectric loss of the pure PEI foam was the lowest. Similarly, the dielectric loss of the PEI–EAGMA foam was between EAGMA and PEI and increased with the increasing EAGMA content. Nevertheless, the dielectric loss of the PEI–EAGMA foam with 20 wt% EAGMA content is a little higher than PEI, but much lower than that of EAGMA. The low dielectric constant and loss of the PEI–EAGMA foams make them candidates for potential applications in the electrical industry.

4. Conclusion

In conclusion, EAGMA exhibits an excellent toughening effect on the PEI–EAGMA system. The notched Charpy impact strength of the PEI–EAGMA blends with 20 wt% EAGMA is 27.9 kJ m⁻² which is 4–5 times higher than that of pure PEI. The compatibility between PEI and EAGMA is high and the molecular mobility of PEI can be restricted by EAGMA. Moreover, EAGMA plays an important role in the microcellular structure of the PEI–EAGMA foam and its final dielectric properties. The dielectric constant and loss of the materials can be effectively reduced through the technology of micro-foaming. The PEI–EAGMA foam with high toughness, low dielectric constant and dielectric loss is believed to have potential applications in many areas, such as the electrical industry.

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