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(vinylidene fluoride) (PVDF),9 poly(methyl meth- acrylate) (PMMA)/PVDF,9 poly(ethylene glycol)/. PS,10 polysulfone/polyimide,11 and PS/PS ion- omer.12.
Foaming Behavior of Polypropylene/Polystyrene Blends Enhanced by Improved Interfacial Compatibility WENTAO ZHAI,1,2 HONGYING WANG,1,2 JIAN YU,1 JINYONG DONG,1 JIASONG HE1 1

Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China 2

Graduate School, Chinese Academy of Sciences, Beijing 100039, China

Received 11 January 2008; revised 7 May 2008; accepted 8 May 2008 DOI: 10.1002/polb.21498 Published online in Wiley InterScience (www.interscience.wiley.com).

A series of polypropylene (PP)/polystyrene (PS) blends were prepared by solvent blending with PS-grafted PP copolymers (PP-g-PS) having different PS graft chain length as compatibilizers. The interfacial compatibility was significantly improved with increasing PS graft chain length until the interface was saturated at PS graft chain length being 3.29 3 103 g/mol. The blends were foamed by using pressure-quenching process and supercritical CO2 as the blowing agent. The cell preferentially formed at compatibilized interface because of low energy barrier for nucleation. Combining with the increased interfacial area, the compatibilized interface lead to the foams with increased cell density compared to the uncompatibilized one. The increase in interfacial compatibility also decreased the escape of gas, held more gas for cell growth, and facilitated the increase in expansion ratio of PP/PS blend foams. ABSTRACT:

C 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1641–1651, 2008 V

Keywords: CO2

blends; microcellular foam; poly(propylene); polystyrene; supercritical

INTRODUCTION Because of the ability to combine the properties of each phase, polymer blends have gained an increasing popularity in the field of polymer science and industry. In the recent years, substantial researches have concentrated on the preparation of microcellular foam of polymer blends. A typical polymer blend is polyethylene (PE)/ polypropylene (PP).1–6 Other microcellular foaming systems are PE/polystyrene (PS),7,8 PS/poly (vinylidene fluoride) (PVDF),9 poly(methyl methacrylate) (PMMA)/PVDF,9 poly(ethylene glycol)/ PS,10 polysulfone/polyimide,11 and PS/PS ionomer.12 Ternary polymer systems, such as Correspondence to: J. Yu (E-mail: [email protected]) or J. Y. Dong (E-mail: [email protected]) Journal of Polymer Science: Part B: Polymer Physics, Vol. 46, 1641–1651 (2008) C 2008 Wiley Periodicals, Inc. V

HDPE/PP/wood fiber3 and PS/PMMA/clay,13,14 also have been used to prepare the microcellular foams. It is widely accepted that for the multiphase polymer systems, such as the semicrystalline polymers,15–18 polymer nanocomposites,19–22 and polymer blends,23,24 the low energy barrier for cell nucleation at interface accounts for the increase in cell density according to the classical heterogeneous nucleation theory.25–27 However, to the best of our knowledge, there is no direct evidence for the preferential cell nucleation at the low-energy barrier sites. Most of polymer blends are immiscible, and the weak interaction between the dispersed phase and matrix limits their broad usage. An effective method with the aim to strengthen the interfacial adhesion is the introduction of the interfacial active block or graft copolymers as compatibilizers into blends.28–31 Subsequently, the foaming behavior of polymer 1641

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Table 1. The Physical Characteristics of PP-g-PS Copolymers

Sample

Graft Content (mol %)

4

Mn (10 g/mol)

Nba

Ngb

Lsc (10 g/mol)

PP-g-PS1 PP-g-PS2 PP-g-PS3 PP-g-PS4

0.54 5.10 12.68 20.39

ND ND 3.92 ND

1.8 3.2 3.6 3.6

0.4 1.8 2.3 2.3

0.61 1.56 3.29 5.80

3

ND, not determined. The average number of branch/1000 C of backbone. The average number of graft/1000 C of backbone. c The average length of graft chain. a b

blend would be changed by improving the interfacial compatibility. In this study, a series of well-defined PP-g-PS copolymers with different PS chain lengths were synthesized by atom transfer radical polymerization (ATRP) according to our previous studies.32,33 By taking these grafted copolymers as the compatibilizer, blends composed of PP, PS, and PP-g-PS with 49/49/2 of weight ratio and, for comparison purpose, a 50/50 blend of PP and PS was prepared by solvent blending. Phase morphology and dynamic rheological properties of blends were investigated to show the influence of PS graft chain length of PP-g-PS copolymers on the interfacial compatibility of PP/PS blends. By using a pressure-quenching process, these blends were foamed with supercritical CO2 as physical blowing agent. The effects of phase morphology and gas diffusion, which was influenced by interfacial compatibility, of unfoamed blends on the foaming behavior were investigated from the aspects of cell nucleation and growth.

EXPERIMENTAL Materials and Characterization The basic materials used here were a commercial grade PP with molecule weight (Mn) of 8.7 3 104 g/mol and melting point of 162.8 8C, supplied by Lanhua Petrochemical Co., China; a commercial grade PS with Mn of 3.6 3 104 g/ mol, supplied by Beijing Yanshan Petrochemical Co., China. PP-g-PS copolymers were synthesized by copolymerizing p-(3-butenyl)styrene and propylene and then grafted with PS by ATRP using the formed copolymer as the poly-

merization precursor. The details of polymerization were shown elsewhere.32 The grafted copolymers with different PS contents were coded as PP-g-PS1 (0.54 mol %), PP-g-PS2 (5.10 mol %), PP-g-PS3 (12.68 mol %), and PP-g-PS4 (20.39 mol %), respectively. The characteristics of grafted copolymers are shown in Table 1. It is noted that PS graft chain length increased with increasing PS graft content. CO2 with a purity of 99.95% was supplied by Beijing Analytical Gas Factory, China.

Sample Preparation In a typical experiment, a 250-mL round-bottomed flask was added with 5.00 g of pure PP, 5.00 g of PS, 0.21 g of PP-g-PS, and then 180 mL of dimethyl benzene was added under stirring. After dissolved completely at 140 8C, the blend was poured into ethanol, filtered, and washed with ethanol several times. A series of blends with the same content of compatibilizers, having different PS contents (PP-g-PS1, PP-gPS2, PP-g-PS3, and PP-g-PS4), were hereafter coded as PP/PS1, PP/PS2, PP/PS3, and PP/PS4, respectively. For comparison, a PP/PS blend without PP-g-PS (PP/PS0) was also prepared under the same conditions. After being dried at 60 8C under vacuum for 24 h, the prepared samples were molded by compression at 200 8C into sheets of 1-mm and 50-lm thickness, respectively. The sheets of 1 mm were cut into specimens with dimensions of 5 3 25 mm for batch foaming and into round disks with diameter of 25 mm for dynamic rheological measurement. The membranes of 50 lm were used for polarized optical microscope (POM) observation. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

FOAMING BEHAVIOR OF POLYPROPYLENE/POLYSTYRENE

Desorption Kinetics Measurement The polymer sheets were enclosed in a highpressure vessel preheated to 50 8C. The vessel was flushed with low pressure CO2 for about 3 min, followed by increasing the pressure to 12 MPa and maintained for 10 h to ensure the saturation absorption of CO2 in samples. Then, the samples were removed out following a rapid venting of the vessel and transferred within a 1-min interval to a digital balance (sensitivity of 0.1 mg) to record mass loss as a function of time.

where n is the number of cells in the SEM micrograph, M the magnification factor, A the area of the micrograph (cm2), and Vf the void fraction of the foamed sample, which can be estimated as Vf ¼ 1 

Analysis POM was used to show the distribution of crystal regions in polymer blends. Before observation, the samples of 50 lm were melted at 180 8C for 5 min and then isothermally crystallized at 138 8C for 30 min. Dynamic rheological measurement was carried out on a Rheometrics SR 200 dynamic stress rheometer with a 25-mm parallel plate geometry and a 1-mm sample gap. The dynamic viscoelastic properties were determined with frequencies from 0.1 to 100 rad/s, by using strain values determined with a stress sweep to lie within the linear viscoelastic region. All measurements were carried out under nitrogen atmosphere at 190 8C. The morphology of unfoamed [before and after etching by tetrahydrofuran (THF)] and foamed samples was observed with a Hitachi S-530 scanning electron microscope (SEM). The samples were freezefractured in liquid nitrogen and sputter-coated with platinum. The cell density (N0), the number of cells per cubic centimeter of solid polymer, was calculated from eq 1 according to the SEM micrographs, and more than 100 cells per SEM picture were used to determine the cell density. 

nM 2 N0 ¼ A

3=2 

1 1  Vf

 ð1Þ

Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

qf q

ð2Þ

where q and qf were the mass densities of samples before and after foaming treatment, respectively, which were measured via water displacement method according to ISO 1183-1987. And the expansion ratio, VE, of polymer foam can be calculated according to eq 3.

Batch Foaming The basic process of polymer quench foaming was similar to that of gas absorption as mentioned earlier. After saturating at saturation pressure and foaming temperature, the pressure was quenched to atmospheric pressure within 5 s. Then the foam structure was allowed to fully grow at the foaming temperature for 15 min.

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VE ¼

q qf

ð3Þ

RESULTS AND DISCUSSION Interfacial Compatibility of PP/PS Blends Improved by PS Grafted Copolymer It is known that PP/PS blend is immiscible. PPg-PS copolymer shows physicochemical affinity to both PP and PS phases, which tend to locate at the interface and interact with these phases by chain entanglement. The PS-grafted copolymers with different PS chain lengths, as shown in Table 1, were added into PP/PS blends as the compatibilizer. Influence of PS chain length on the interfacial compatibility of blends was investigated by phase morphology and dynamic rheological property. The morphology of the PP/PS blends before and after etching by THF was studied by SEM, and low magnification pictures were supplied here to show the phase size on a large scale. As shown in Figure 1(a), PP/PS0 blend exhibits the typical immiscible morphology, such as big phase domains with average size of 65 lm, smooth phase surface, and holes formed during fracture. An obvious cocontinuous structure is observed in the PP/PS0 sample after PS phase was etched by THF [Fig. 1(f)], because of the similar volume fraction of PP and PS. By adding PP-g-PS with PS chain length as short as 0.61 3 103 g/mol, PP/PS1 blend [Fig. 1(b,g)] shows a significantly decreased phase size, indicating that the interfacial compatibility was improved between PP and PS. With further increasing the PS chain length of PP-g-PS, an increased

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Figure 1. Morphology of PP/PS blends before and after extracted by THF. PP/PS0 (a,f), PP/PS1 (b,g), PP/PS2 (c,h), PP/PS3 (d,i), and PP/PS4 (e,j). Before extraction: a–e; after extraction: f–j. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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Figure 2. Optical micrographs of PP/PS0 (a), PP/PS1 (b), PP/PS2 (c), PP/PS3 (d), and PP/PS4 (e).

improvement of adhesion is clearly observed, that is, the phase size decreases gradually and the interface becomes almost indiscernible at the present magnification. These results indicate that the compatibilizing effect of PP-g-PS copolymers was gradually enhanced with increasing the length of PS sequences. Similar phenomenon has been observed in the same blend system29 and others.34 As PS chain length being longer than 3.29 3 103 g/mol, no further improvement in phase morphology is observed in Figure 1(i,j), indicating that the interface was saturated by the compatibilizer and the interfacial adhesion reached its maximum improvement.35 It is noted that PS phase still keeps the irregular shape and penetration in PP phase to some degree even for well-compatibilized cases. Influence of PP-g-PS on the phase morphology of PP/PS blends was further investigated with POM observation, and the results are shown in Figure 2. In these micrographs, the bright regions are associated with the crystalline PP phase and the darker regions with the amorphous PS phase. For PP/PS0 sample, the phase separation of PP and PS is very clear, and the shape of PP phase is irregular. With introducing a small amount of PP-g-PS, this obvious phase separation disappears quickly and PP phase with small phase size tends to distribute uniJournal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

formly in PP/PS blends, especially when the interface is saturated. Uniform distribution of PP phase in the blends verifies the increase of compatibility. However, the introduction of PP-gPS copolymer did not obviously affect the melting point and crystallinity of PP/PS blends (results not shown). Figure 3 shows the complex viscosity of PP/ PS blends at various frequencies. It is seen that the compatibilized blends exhibit much higher

Figure 3. Complex viscosity versus angle frequency for PP/PS blends at 190 8C.

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shows the desorption curves and diffusion coefficient of PP/PS blends, pure PP and PS. This low temperature was selected here to prevent polymer foaming, which would affect the accuracy of the experimental data.38 As shown in Figure 4(a), the desorption curve of all blends are not linear, which are very different from those of pure PS and PP samples. This result indicates that the desorption kinetics of blends did not meet the Fickian law in the present desorption time scope. In this study, the desorption curves of blends can be divided into three parts, that is, the beginning of curves (Part I), the end of curves (Part II), and the middle of them. The linear fitting of Parts I and II was carried out to calculate the corresponding apparent diffusivity coefficients (Dd), which would be deduced to set the limits of real diffusivity coefficient, according to the eqs 4 and 5. MdðIÞ 4 ¼ l MðIÞ

MdðIIÞ 4 ¼ l MðIIÞ

Figure 4. Desorption kinetics of PP/PS blends, pure PP, and PS saturated at 12 MPa and 50 8C (a), and corresponding diffusion coefficient (b).

viscosity at all frequency than uncompatibilized one as a result of the formation of transition layer, due to the increase in interfacial adhesion.36,37 With increasing PS graft chain length, the complex viscosity of PP/PS blends gradually increases and keeps constant when PS chain length is longer than 3.29 3 103 g/mol. This result indicates that the improvement of compatibility between phases occurs when the graft chain length is shorter,37 and the interface has been saturated by PP-g-PS copolymer when the graft chain length is long enough, which is consistent with SEM micrographs. CO2 Escape Restricted by Improved Interfacial Compatibility in PP/PS Blends Influence of interfacial compatibility on the gas diffusion was investigated with the PP/PS samples saturated at 12 MPa and 50 8C. Figure 4

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DdðIÞ tdðIÞ p

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DdðIIÞ tdðIIÞ p

ðPart IÞ

ð4Þ

ðPart IIÞ

ð5Þ

where Md is the measured percentage weight loss at desorption time (td). M(I) and M(II) are obtained by linearly extrapolating Parts I and II to zero desorption time, respectively. The Dd was thus determined by plotting Md/M against ((td)1/2/l). For comparison, the Dd of pure PP and PS is also calculated. The results are shown in Figure 4(b). It is seen that the Dd(I) of blends gradually decreases from 1.39 3 109 m2/s for PP/PS0 to 9.25 3 1010 m2/s for PP/PS1, 6.54 3 1010 m2/s for PP/PS2, 5.65 3 1010 m2/s for PP/PS3, and 5.59 3 1010 m2/s for PP/PS4; that Dd(II) from 1.22 3 1010 m2/s for PP/PS0 to 1.44 3 1010 m2/s for PP/PS1, 1.62 3 1010 m2/s for PP/PS2, 1.89 3 1010 m2/s for PP/PS3, and 1.90 3 1010 m2/s for PP/PS4, respectively, indicating the Dd of blends tended to level off when the interface was saturated by PP-g-PS copolymer. At the same saturation condition, the Dd of pure PP and PS is 4.67 3 1011 and 1.06 3 1010 m2/s, respectively, which were similar to other reports.17,38 According to the similar compositions and crystallinity for all PP/PS blends, no obvious change in CO2 solubility in blends should be occurred. However, the PP/PS blend with higher Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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Figure 5. SEM micrographs of foamed PP/PS0 (a–c), PP/PS1 (d–f), and PP/PS4 (g–i) samples obtained by foaming at 12 MPa and different temperatures.

interfacial area because of compatibilizing effect has higher M(I), as shown in Figure 4(a), indicating that the gas exists probably in two forms, that is, locating at polymer phase interface and dissolving in polymer matrix. Therefore, the PP/ PS blends did not exhibit linear gas desorption behavior, which is possessed by pure polymer/ gas solution due to the uniform distribution of gas in polymer. In this study, PP/PS blends had the bicontinuous structure, which possibly provided a channel for CO2 quickly escape out of the blends, verified by the much higher Dd(I) than pure polymers [Fig. 4(b)]. This desorption Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

behavior was similar to those of polymer/wood fiber composites with poor surface adhesion.3,39 With introducing the compatibilizer, the Dd(I) of PP/PS blends decreases gradually and levels off when interface was saturated, attributed to the significantly improved interfacial adhesion. It is noted that the diffusion coefficients for all blends are higher than those of pure polymers. A possible reason is that the presence of bicontinuous structure obviously decreased the real gas diffusion distance, which was much less than the thickness of samples, and increased the apparent diffusion coefficient.

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Figure 6. SEM micrographs of foamed PP/PS0 (a,b) and PP/PS2 (c,d) samples obtained by foaming at 12 MPa and 80 8C. The arrow 1 shown in Figure 6(d) is PP phase, arrow 2 is PS phase, and arrow 3 is the interface.

Foaming Behavior of PP/PS Blends Affected by Improved Interfacial Compatibility The foamability of polymers is affected by the phase morphology, gas solubility, and diffusion, and so on. The improvement of interfacial compatibility significantly decreased the phase size and desorption diffusion coefficient of CO2 in PP/PS blends, which would affect the foaming behavior of PP/PS blends to some extent. Influence of interfacial compatibility on the foaming behavior was examined for PP/PS blends with different compatibilizer concentrations, that is, PP/PS0, PP/PS1, and PP/PS4 at 12 MPa and 100–150 8C. Their corresponding SEM micrographs are shown in Figure 5. PP/ PS0 foam obtained at 100 8C [Fig. 5(a)] has the well-defined unfoamed regions corresponding to PP phase, because the foaming temperature was far below than Tm of PP (162 8C), and the obvious gap was observed at interface. With

further improvement of the compatibility, the unfoamed regions in PP/PS foams decrease gradually, and the cell distribution becomes uniform [Fig. 5(d,g)]. At 140 and 150 8C, the influence of compatibilizer exhibits similar trend with that at lower temperature on the cell morphology of PP/PS foams. Therefore, the increased interfacial compatibility significantly improved the foaming behavior of PP/PS blends. The cell morphology of polymer foam was affected mainly by the cell nucleation, growth, and coalescence occurred in the foaming process. The effect of compatibilizer on the cell nucleation was investigated by foaming PP/PS0 and PP/PS2 at 12 MPa and 80 8C. From SEM micrographs of foams shown in Figure 6, it is seen that PP/PS0 foam exhibits the distinct twophase morphology, that is, the foamed PS phase and unfoamed PP phase. The uniform cell size distribution is observed in PS phase, suggesting Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

FOAMING BEHAVIOR OF POLYPROPYLENE/POLYSTYRENE

Figure 7. Cell density of foamed PP/PS blends obtained by foaming at 12 MPa and different temperatures.

the homogeneous cell nucleation. With the improvement of interfacial compatibility, PP/PS2 foam exhibits significant different cell morphology. Most of cells are located at the interface between PP and PS, and only a few of them are in the central part of PS phase. It is known that the compatibilizer tends to locate at the interface, interacts with each phase by chain entanglement, and decreases the interfacial tension.40 According to the classical nucleation theory,25–27 this decrease in interfacial tension was energetically more favorable for cell nucleation at interface, compared to that in PS matrix. This study supplied a direct experimental evidence to verify the nuclei preferentially formed at interface with low energy barrier for nucleation. Influence of interface compatibility on the cell nucleation behavior was further investigated by the cell density of PP/PS blends foamed at 12 MPa and 80–150 8C. As shown in Figure 7, the cell density at 80 8C gradually increases with the introduction of PP-g-PS, that is, from 1.42 3 109 cells/cm3 for PP/PS0 to 1.89 3 109 cells/ cm3 for PP/PS1, 3.10 3 109 cells/cm3 for PP/PS2, 3.84 3 109 cells/cm3 for PP/PS3, and 3.82 3 109 cells/cm3 for PP/PS4, respectively. These results indicate that the improvement of interfacial compatibility enhanced the cell nucleation during foaming process of PP/PS blends. As mentioned earlier, the addition of compatibilizer decreased PS phase size, and hence increased the area of interface in PP/PS blends, which was energetically favorable site for cell nucleation. Another distinct feature of foams for compatibilized PP/PS is the weak dependence of cell Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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density on the temperature, which was very different from that of pure polymer.41,42 In Figure 7, PP/PS0 foams show the significantly decreased cell density with increasing temperature, resulting from the reduced cell nucleation rate due to the quick gas loss and increased cell coalescence, especially at higher temperature of 140–150 8C [also shown in Fig. 5(d,g)]. For compatibilized blends, the cell density keeps almost constant up to 140 8C and then slightly decreases at 150 8C. The possible reasons for this phenomenon were the dominant heterogeneous nucleation process and the suppression effect of crystalline PP region on the cell coalescence during compatibilized PP/PS blends foaming process. It should be noted that the average cell size was hard to calculate because of irregular cell structure due to the obvious cell coalescence of PP/PS foams. In this study, the influence of interfacial compatibility on the expansion ratio was investigated during PP/PS blends foaming process. As shown in Figure 8, PP/PS0 foams exhibit low expansion ratio of 1.51 at 140 8C with a low dependence on temperature. With the addition of compatibilizer, PP/PS1 foams show the obviously increased expansion ratio of 2.35 at 140 8C. With further increasing the interfacial compatibility, the expansion ratio of PP/PS blends increases gradually until the interface was saturated. An expansion ratio of 2.89 was obtained for PP/PS4 with the best interfacial compatibility. The almost twofold increase indicates that the improvement in interfacial compatibility

Figure 8. Expansion ratio of foamed PP/PS blends obtained by foaming at 12 MPa and different temperatures.

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was very effective to increase the expansion ratio of immiscible polymer blend foam. It is known that the increase of foaming temperature increases the cell growth during polymer foaming process due to the increased diffusion rate. PP/PS0 sample exhibited poor interfacial compatibility and most gas at interface tended to diffuse quickly out of blend through the cocontinuous channel as shown in Figure 4(b). Therefore, less amount of gas was left for the cell growth and foam expansion, which was more serious at higher foaming temperatures. Consequently, PP/PS0 blend had the weak dependence of expansion ratio on the foaming temperature. For compatibilized PP/PS blends, the improvement in interfacial adhesion decreased the escape of gas, facilitating the cell growth during foaming process, and resulted in the increase of expansion ratio and high dependence of expansion ratio on the foaming temperature.

CONCLUSIONS In this study, a series of PP/PS/PP-g-PS blends (49/49/2) with the increased PS graft chain length in PP-g-PS and PP/PS (50/50) were prepared by solvent blending. The experimental results indicate that the introduction of graft copolymers significantly improved the fracture morphology, such as the decreased phase size, almost indiscernible interface, and increased the complex viscosity of PP/PS blends, suggesting an increase in interfacial compatibility. With increasing PS graft chain length, the compatibility was gradually improved until the interface was saturated at PS chain length being 3.29 3 103 g/mol. By using supercritical CO2 as the physical foaming agent, PP/PS blends were foamed with a pressure-quenching process. The foaming temperature was selected to be lower than the melting point of PP here with the aim to prevent PP foaming. The observed was the obvious cell nucleation at interface with low energy barrier for nucleation, which directly verified the classical heterogeneous nucleation theory. An increase in interfacial compatibility significantly decreased the escape of gas, and hence more gas was held for cell growth and foam expansion. Because of the obvious increase in cell nucleation and growth and the decrease in cell coalescence, the compatibilized PP/PS blends exhibited high cell densities and expansion ratios compared to the uncompatibilized one. Meanwhile, a

weak dependence of cell density on the foaming temperature was also observed in compatibilized PP/PS blends, indicating the significant influence of improvement in interfacial compatibility on the foaming behavior of PP/PS blends. This work was supported by the Innovation Funds of CAS (no. CMS-CX200414) and the National Natural Science Foundation of China (no. 20734002).

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