Porous Structural Transformation from Closed Microcellular to

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Porous Structural Transformation from Closed Microcellular to Bicontinuous Nanoporous Based on Poly(phthalazinone ether sulfone ketone) Containing Biphenyl Moieties by Carbon Dioxide Foaming Na Wen,†,‡ Yajie Lei,*,† and Shikai Luo*,†,‡ †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, China, 621900 Material Science and Engineering College, Southwest University of Science and Technology, Mianyang, China, 621010

‡

ABSTRACT: Foams from poly(phthalazinone ether sulfone ketone) containing biphenyl moieties (PPBESK) are prepared by a two-stage batch process using CO2 as foaming agent. Closed microcellular and bicontinuous nanoporous cell morphologies are observed. The effects of saturation pressure, the concentration of CO2 dissolved in PPBESK, and foaming temperature on the porous structure were studied. The results indicate that the key factor for the structure transformation is the concentration of CO2 dissolved in PPBESK. When the CO2 concentration is above the critical value (2.80 g CO2/100 g polymer), the closed microcellular structure changes to bicontinuous nanoporous. So controllable preparation of closed microcellular and bicontinuous nanoporous foams from PPBESK can be achieved by controlling the concentration of CO2 which dissolved in the polymer. Moreover, the tensile properties of the foams were investigated. It is found that the tensile strength of the foams is closely related to its cell structure especially the cell type.

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INTRODUCTION Compared with unfoamed polymers, porous polymers possess higher performance, such as better mechanical strength, more excellent thermal property, better insulation, and so on. They are currently applied in the fields of separators,1 packaging,2−5 automobile,6−9 insulations,3−5,8−10 bone substitute materials,1−3,11 and transportation5,6 as well as controlled release systems.1,2,12 The use of porous materials is determined not only by the properties of the polymer matrix used but also by the foam structure, such as cell type (closed or bicontinuous). Microcellular thermoplastic foam with a closed-cell structure, as one type of the porous polymeric material, has good comprehensive performance. So they are widely used in various fields such as aerospace,4,6,7 sporting equipment, and so on. Lots of studies were focused on the cell structure−activity relationships. For example, Cafiero et al.13 prepared microcellular foams based on PEEK/PEI blends. The influences of blend composition, solubilization pressure, and crystallinity on density and morphology parameters of the foams were investigated. Okolieocha et al.4 prepared microcellular foams © XXXX American Chemical Society

of PMMA nanocomposite, and the influence of additive type on the cell morphology was investigated. With the development of nanotechnology and microcellular thermoplastic foams with closed-cell structure,14,15 the thermoplastic foams with cells on the of order 10 nm have aroused great interest in recent years. Nanocellular polymeric foams with bicontinuous cell structure are a new generation of materials with cell size in the nanometric range and have more unique properties than traditional unfoamed materials or microcellular foams because of their more unique structure.16,17 So they are commonly used in separations,1,2,11,18,19 catalysis,19 drug delivery,3,20 and so on. During the past few years, they have become of great interest in scientific research. Polysulfone as an engineered polymer has been tested by Guo et al.21 and microcellular and nanocellular foams were obtained. Notario et Received: Revised: Accepted: Published: A

January 5, 2018 February 25, 2018 March 13, 2018 March 13, 2018 DOI: 10.1021/acs.iecr.8b00057 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research al.22 prepared PMMA nanocellular foams, a new generation of polymeric foams that possesses enhanced physical properties. However, most of the studies have focused on the influence of processing conditions and the nature of general-purpose plastics on the porous structure. Few works have investigated the transformation from closed microcellular to open nanoporous based on one polymer matrix especially high performance engineering plastics (for example PPBESK). Poly(phthalazinone ether sulfone ketone) containing biphenyl moieties (PPBESK), a novel high-performance engineering polymer, possesses excellent comprehensive performance.23,24 In the past few years, the research covering the synthesis and modification of PPBESK has been numerous.25,26 However, to our knowledge, there are rare systematic studies on the preparation and performance of foamed PPBESK materials. To further expand the application of the PPBESK and obtain high performance foams, studying the preparation and characterization of microcellular foams based on PPBESK is necessary. In this paper, we prepared microcellular foams based on PPBESK by CO2 foaming. The foaming behavior of PPBESK films was studied. In particular, we discussed the influence of the saturation pressure, the concentration of CO2 dissolved in the polymer matrix and foaming temperature on the resulting porous structure. The key factor of which causes the microstructure change from closed microcellular to bicontinuous nanoporous was confirmed. An explanation for the formation of open cell structure when the CO2 saturation level is above the critical threshold was given. Moreover, the relationship between the cell structures and the tensile property of the foams was discussed.

g/cm3, which was measured via water displacement method according to ISO 1183−1987. 2.3. Gas Saturation and Desorption. The absorption and desorption of CO2 dissolved in PPBESK film were investigated. Saturation pressure and saturation time were controlled to vary the amount of gas delivered to the polymer matrices. The saturation pressure was set to 1−12 MPa, respectively, and controlled by an accurate pressure pump controller (model SFT-10) provided by Supercritical Fluid Technologies Inc. to an accuracy of ±0.2 MPa. The saturation temperature was 40 °C. During the saturation, samples were periodically removed from the pressure vessel and weighed on an electronic balance (Mettler Toledo XS105) with an accuracy ±10 μg to measure the content of gas uptake. Samples were then promptly returned to the pressure vessel and repressurized. The amount of time needed for weighing the samples was extremely small compared to the amount of time needed to achieve full saturation, so the error introduced in gas uptake measurements is not significant. Desorption experiments of the PPBESK samples were conducted at room temperature and atmospheric pressure. Periodic mass measurements were taken to record the amount of carbon dioxide dissolved in the samples. The diffusivity (Dd) of CO2 in PPBESK matrices can be calculated by Fickian diffusion eq 1:27 Md = M∞ −

4M∞ I

Dd td π

(1)

where Md denotes the amount of gas dissolved in the samples at desorption time td, expressed as the weight percentage of CO2 per unit weight of composites. M∞ denotes the total sorption amount of gas that has saturated for a sufficiently long time and simultaneously the amount of gas at the beginning of the desorption process. M∞ was obtained by extrapolate back the linear part of the desorption curves to td = 0. I is the thickness of the film. 2.4. Foams Preparation. The foaming experiments were performed in a two-stage batch process. First, the dense PPBESK films were cut into 2 cm × 4 cm pieces and placed in a pressure vessel connected to a carbon dioxide cylinder at 40 °C and elevated pressure (1−12 MPa). After the equilibrium amount of gas has been absorbed, the pressure was quickly released and the samples were removed from the pressure vessel and weighed to determine the amount of gas absorbed. Second, the samples were immersed in a silicon oil bath maintained at the desired temperature for 30 s (foaming time). After foaming, the samples were quenched in cold water. This sudden quench locked in the microstructures of the resulting foams. The foamed samples were next washed using ethanol until the silicon oil covered on the surface of samples was removed completely. Then the samples were kept in air at room temperature to remove traces of ethanol. The porous films always exhibit dense skin parts due to the CO2 loss from the surface regions prior to the foaming step. 2.5. Foam Characterization. The foamed polymer films were characterized to determine their mass density, cell morphology (cell diameter, open or close et al.), and tensile strength. The mass densities of foamed samples (ρf) without removing the solid skin were measured by the water displacement method according to ISO 1183−1987. The microcellular morphologies of the foamed samples were observed using a ZEISS scanning electron microscope (ZEISS

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EXPERIMENTAL SECTION 2.1. Materials. PPBESK resin (grade 2080) with a numberaverage molecular weight of 25300 g/mol, melting point of 310 °C, Tg of 233 °C, Young’s modulus of 3801 MPa, and tensile strength of 59 MPa, was obtained from Dalian Polymer Materials Corp (Liaoning, China). Figure 1 shows the

Figure 1. Molecular structure of PPBESK.

molecular structure of the resin. N-Methyl-2-pyrrolidone (NMP, purity 99%) was supplied by Kermel Chemical Reagent Company (Tianjin, China). CO2 (purity 99.999%) was purchased from Sichuan Run Tai Special Gas Co. Ltd. 2.2. Film Preparation. Solutions of PPBESK were prepared by dissolving 10 g of polymer in 90 mL of NMP. Thin films of around 200 μm thickness were prepared by solution-casting on a glass plate and dried at 200 °C for at least 5 h to remove the solvent sufficiently. Subsequently, the homogeneous dense films were removed from the glass plate with the help of a small amount of water and then dried for several hours at 100 °C. The film showed a mass density of 1.25 B

DOI: 10.1021/acs.iecr.8b00057 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

g polymer as a function of saturation time at different CO2 saturation pressure and at 40 °C. It can be seen that the CO2 concentration dissolved in the polymer matrices increases quickly in the early stage of the sorption. After 120 min, the sorption reaches equilibrium. Meanwhile, the slope at the early stage of the CO2 sorption curves obviously increases with the increase of the saturation pressure, which indicates higher gas diffusivity. Moreover, on the basis of the results above, the CO2 sorption isotherms for the PPBESK films at 40 °C was obtained. As Figure 3B shows, symbols are experimental values and lines represents a dual-mode sorption model fit. According to Figure 3B, the saturation of CO2 in PPBESK matrix under definite pressure can be easily obtained. CO2 desorption experiments were also carried out to determine the CO 2 content in the polymer matrices corresponding to certain desorption times and to calculate the desorption diffusivities of CO2 in PPBESK matrices under different pressure. The CO2 desorption curves of PPBESK at different pressure are presented in Figure 4, and the inset

EIGMA). Cell nucleation density (Nf), which refers to the number of cells per cubic centimeter of foam, was determined with a software based on ImageJ and was calculated by applying eq 2:28 Nf =

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

(2)

where n is the number of cells observed in the micrograph of the SEM, A is the area of the micrograph (in cm 2), and Vf is the void fraction of the foamed sample. This can been estimated using eq 3:28 ρ Vf = 1 − f ρ (3) where ρ and ρf are the mass densities of the solid and foamed samples, respectively. Average cell diameter was determined from the SEM images of a foam cross-section, by measuring the distance between two opposite cell walls (at least 100 representative cells). 2.6. Tensile Strength Characterization. After foaming, the original transparent polymer films became opaque and white, and have smooth surfaces as integral skins (as shown in Figure 2). The foamed samples were cut into 2.0 mm × 30.0

Figure 4. Plots of measured desorption weight fraction against the square root of desorption time (td).

Figure 2. Image of foamed and unfoamed samples.

mm pieces and then taken for tensile test by using RSA-G2 solids analyzer with a crosshead rate of 50 mm/min at room temperature. The final results were the average values of six replicate measurements.

curves in Figure 4 show the early stage of CO2 mass vs the square root of desorption time. It can be seen that the relationship between the CO2 mass and the square root of desorption time is almost linear in the early stage, which indicates that the desorption process accords with the Fickian diffusion mechanism.27 The diffusivity (Dd) of CO2 in PPBESK matrices which were calculated by Fickian diffusion eq 1 were listed in Table 1. The results show that Dd increased with the increase of saturated pressure. The phenomenon is due to a bigger pressure drop

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RESULTS AND DISCUSSION 3.1. Gas Uptake and Diffusivity. The first step in the microcellular foaming process involves the absorption of an inert gas into the polymer matrix to form the gas/polymer solution. Figure 3A shows plots of CO2 sorption in g CO2/100

Figure 3. (A) CO2 sorption curves at different saturation pressure and at 40 °C; (B) sorption isotherm of CO2 in PPBESK at 40 °C. Symbols are experimental values, lines represents a dual-mode sorption model fit. C


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transparent into opaque. In addition, SEM micrographs of the samples are shown to confirm the formation of cells. As an example, the micrographs of PPBESK films saturated with 5 MPa CO2 at 40 °C and different foaming temperature are given in Figure 5. Comparing Figure 5panels A and B, it can be seen that the change is obvious. When the foaming temperature was 100 °C, no cells existed in the film. When the foaming temperature increased above 110 °C, typical microcellular morphology existed. Moreover, from Figure 5C−F, it can be seen that at low temperatures, well-separated spherical cells were formed. But as the temperature increases, the cells expanded and gradually connected with neighboring cells to give the polyhedral structure. The result indicates that foaming temperature is a very important influence in cell size and shape of the porous structures. Krause and co-workers31 adopted a technique that only requires the knowledge of one sorption isotherm to obtain the relation between the glass transition temperature and the dissolved concentration of carbon dioxide. In this study, the same technique was used. The glass transition temperature of PPBESK as a function of the dissolved amount of carbon dioxide was presented in Figure 6. The carbon dioxide concentrations were obtained from the sorption isotherms shown in Figure 3B. One linear relationship between the glass transition temperature (Tg) and the dissolved amount of carbon dioxide in the polymer matrix exists, and a least-squares fit was performed. The value of the axis intercepts in Figure 6 at zero of dissolved CO2 concentration is well coincide with the Tg value of pure PPBESK obtained by DSC. 3.3. Foaming Window. The “foaming window” of gas saturated polymers is fixed by two respects. On the one hand, the lower bound temperature, Tlower, determines the minimum foaming temperature, and equals the glass transition temperature, Tg(c), of the polymer/gas mixture at CO2 concentration c. On the other hand, the upper bound temperature, Tupper, is

Table 1. CO2 Sorption Amounts and Desorption Diffusivities of PPBESK under Different Pressure condition

M∞ (g CO2/100 g polymer)

1 MPa, 40 °C 3 MPa, 40 °C 4 MPa, 40 °C 5 MPa, 40 °C 6 MPa, 40 °C 7 MPa, 40 °C 8 MPa, 40 °C 10 MPa, 40 °C 12 MPa, 40 °C

0.78 2.18 2.80 3.07 3.26 3.59 3.92 4.42 4.88

Dd (m2/sec) 1.83 1.99 2.41 2.51 3.31 3.38 4.25 5.28 6.68

× × × × × × × × ×

10−13 10−13 10−13 10−13 10−13 10−13 10−13 10−13 10−13

that gives a higher driving force to the diffusion of CO2 which is adsorbed inside the polymer matrix. Obtaining the value of Dd is a benefit for us to know how long it will take the concentration of CO2 dissolved in the polymer matrix to reach a certain content. 3.2. Dependence of the Dissolved CO2 Concentration on Glass Transition Temperature. It is well-known that polymers can be plasticized by the dissolved gas.29,30 Because of the high contents of CO2 dissolved in the polymer matrices, the glass transition temperatures (Tg) of the CO2 saturated polymers were significantly lower than that of the pure polymer. The dependence of the glass transition temperature of the PPBESK on the CO2 concentration was determined by saturating the polymer samples at the same temperature (40 °C) and different CO2 pressure (1−12 MPa). Then subsequent foaming of the saturated samples at different foaming temperature (from 100 °C to the Tg of the pure polymer (233 °C)) for the same foaming time (30 s). The temperature at which foaming of the sample just became visible could be determined by systematically increasing the temperature of the heating bath. At this transformation, the sample turns from

Figure 5. SEM micrographs of PPBESK films saturated with 5 MPa carbon dioxide at 40 °C and foamed at (A) 100 °C; (B) 110 °C; (C) 130 °C; (D) 140 °C; (E) 150 °C; (F) 160 °C; (G) 170 °C; (H) 190 °C; (I) 210 °C; (J) 230 °C; (K) 250 °C; (L) 260 °C; (M) 270 °C. D


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foaming window of each sample became wider and moved to the lower temperature zone with the increase of CO2 saturation pressure. Moreover, Tmax (the temperature corresponding to the point of minimum mass density) decreases with increasing CO2 concentration, which indicates higher cell density as Figure 5 panels F and G show. The saturation pressure determines the CO2 saturation concentration dissolved in the polymer, so the root cause of the influence of saturation pressure on the foaming window is the different CO 2 concentrations. 3.4. Cellular Morphology. The saturation pressure (which determines the CO2 concentration dissolved in the polymer) and foaming temperature play important roles in this work. Thus, a broad range of foaming temperatures and saturation pressure were used in experiments to study the effect of the two variables on the porous structure. However, we can see from Figure 7 that the foaming regions of the samples saturated at different pressure are different. So it is impractical to choose one specific foaming temperature to study the influence of the broad saturation pressure range on the porous structure of the foams. In this case, we compared the cell morphology of the samples which saturated at 40 °C and 1−12 MPa, respectively, and then subsequent foaming at the temperature equal to the Tmax which corresponded to each saturation pressure. As shown in Figure 8, two types of cell morphologies are observed: closed microcellular and bicontinuous (open) nanoporous morphologies. From Figure 8 panels A to C, it can be seen that when the saturation pressure is no more than 4 MPa, the microcellular foams exhibit typical closed cell structure. However, when the saturation pressure increases to 5 and 6 MPa, obviously different morphologies are observed. Figure 8 panels D to G reveal a kind of clearly bimodal cell structure, A large number of nanoporous exist in the wall of the large cells. These nanoporous connected with each other to form the open-cell structure. When the saturation pressure goes up further, a typical bicontinuous porous structure is observed. As shown in Figure 8 panels H to O, only bicontinuous nanopores (less than 100 nm) exist in the foamed samples. In this case, the porous structure shown in Figure 8D−G can be deemed as a transition region of the transformation from closed microcellular to bicontinuous porous. For further investigation, the average cell sizes and cell densities of foams which prepared at different foaming temperatures and pressures were calculated from a lot of SEM micrographs, and are presented in Figure 9. Figure 9A reveals the average cell diameters as a function of the foaming temperature and saturation pressure. As expected, high carbon dioxide saturation pressures result in small cells. When CO2 saturation pressures are between 1 and 4 MPa, PPBESK foams show that the range of average diameters is between 2 and 70 μm. However, when CO2 saturation pressures increased to 5 or 6 MPa at a foaming temperature range of 140−170 °C, a drastic drop of the average cell size was observed. The average diameters in this region were suddenly decreased to the range of 80−100 nm (Figure 8D−G). Significantly, when the foaming temperature is out of the range of 140 to 170 °C, for example, below 130 °C and above 180 °C, closed cell structures are observed and the cell diameters are in the range of 2−5 μm. This phenomenon indicates that the transformation from closed microcellular to open porous can be caused by changing the foaming temperatures in the transition stage. Moreover, when the saturation pressure goes up further, bicontinuous

Figure 6. Glass transition temperature of PPBESK vs the dissolved amount of carbon dioxide. The straight line represents a least-squares fit of the experimental data.

where cells are destabilized by (a) diffusion of carbon dioxide out of the materials and (b) a the strong decrease in viscosity of the polymer (Tupper > Tg(0)). The foaming window is the region between Tlower and Tupper. Tlower can be obtained from Figure 6. To determine Tupper, experiments which are similar to the ones to determine Tg(c) were performed. By increasing the foaming temperature to the region of 233 °C (the Tg of the pure PPBESK) and 280 °C to determine a value at which the film remained transparent, while turning opaque at the previous lower temperature. SEM micrographs of the samples were also shown to confirm this change. In contrast to Figure 5 panels A−F, the opposite phenomenon is shown in Figure 5G−M. It can be seen that with the temperature increasing interconnecting neighboring cells are reduced and separate with each other. Compared with Figure 5L,M, it is obvious that microcellular morphology existed when the foaming temperature was 260 °C. However, when the foaming temperature increased to 270 °C, the cells disappeared. Figure 7 shows the relation between the mass density and foaming temperature for samples saturated at 1−12 MPa CO2

Figure 7. Mass density of PPBESK dependent on foaming temperature for samples saturated with different saturation pressure. The dashed horizontal line represents the mass density of the pure PPBESK.

saturation pressure. Each point in Figure 7 corresponds to the foaming temperature during foaming and the mass density of the material after foaming. One can see that the boundary of Tlower and Tupper are defined as the temperature at which the foam density is equal to the density of the pure polymer, which is indicated by the dashed horizontal line in Figure 7. The E


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Figure 8. SEM micrographs of PPBESK films. The foaming conditions are (A) 1 MPa, 240 °C; (B) 3 MPa, 190 °C; (C) 4 MPa, 170 °C; (D,E) 5 MPa, 160 °C; (F,G) 6 MPa, 150 °C; (H,I) 7 MPa, 140 °C; (J,K) 8 MPa, 140 °C; (L,M) 10 MPa, 140 °C; (N,O) 12 MPa, 140 °C. The foaming temperatures are the Tmax corresponding to each saturation pressure.

Figure 9. Average cell diameter (A) and cell densities (B) of porous PPBESK morphologies vs the foaming temperature and the carbon dioxide pressure used to saturate the samples.

microcellular to bicontinuous nanoporous is caused by the increased concentration of CO2 dissolved in the polymer matrix. There is one critical CO2 concentration. When the CO2 concentration is lower than the critical value, the closed microcellular structure is obtained, when the critical value is exceeded, an open nanoporous structure is obtained. So the primary cause of the strong increase in cell density is the transformation of the porous structure. To better visualize the strong increase in cell density, Figure 10 presents the cell density of porous PPBESK versus the CO2 concentration dissolved in the polymer. Cell densities obtained at different foaming temperature are included. At each foaming temperature, the lines show one general trend. It is clearly that when the concentration of CO2 is above 2.8 g CO2/100 g polymer, the cell density begins to increase rapidly. The result indicates

nanopores are observed and the diameters of cells are less than 100 nm. Figure 9B shows the dependence of the cell density on the foaming temperature and saturation pressures for PPBESK foams. One can see, PPBESK foams show that cell density is in the range of 105 cells cm−3 to 1010 cells cm−3 when CO2 saturation pressure is between 1−4 MPa. When the saturated pressure increased to 5 or 6 MPa and at a foaming temperature range of 140−170 °C, open cell structures were observed and the cell density rapidly increased by 3−4 orders of magnitude. Moreover, when the saturation pressure goes up further, bicontinuous nanopores are observed, and the cell densities are greater than 1014 cells cm−3. According to the research of Krause and co-workers research,31 the porous structure transformation from closed F


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samples were shown in Figure 11. One can see that when CO2 concentration is 2.70 g CO2/100 g polymer (critical CO2 concentration 2.80 g CO2/ 100 g polymer), open porous morphology is observed. Moreover, according to the high magnification images (Figure 11E−H), it can be seen that, when CO2 concentration is equal to the critical value of 2.80 g CO2/100 g polymer, both closed and open porous morphologies are observed. This phenomenon indicates that no matter what saturation pressure and foaming temperature was taken in the experiments, the microstructural change from closed microcellular to open porous mainly depends on the concentration of CO2 dissolved in the polymer matrix. The saturation pressure only determines the maximum concentration of CO2 dissolved in the polymer matrix. Meanwhile, in the temperature range of 140 to 170 °C, the foaming temperature has just a trifling impact on the cell size and cell densities and has no effect on the microstructural change. However, by analyzing Figure 9, we can see that the transformation from closed microcellular to open porous can be caused by changing the foaming temperatures in the transition stage. The reason is that the saturated concentrations of CO2 dissolved in the polymer matrix under 5 and 6 MPa pressure are very close to the critical value, just a little higher than 2.80 g CO2/100 g polymer. In other words, they are in the

Figure 10. Cell densities of porous PPBESK morphologies vs the carbon dioxide concentration of the saturated polymers.

that the critical CO2 concentration corresponding to the porous structure transformation from closed microcellular to bicontinuous porous of PPBESK is 2.8 g CO2/100 g polymer. To confirm the obtained critical value and illustrate the microstructural change, a series of experiments were performed. The samples were saturated with CO2 at 40 °C and 5, 6, and 7 MPa pressure, respectively. After the CO2 dissolved in the samples achieved a certain concentration (2.70 g CO2/100 g polymer, 2.80 g CO2/100 g polymer, and 2.90 g CO2 /100g polymer), the samples were foamed at 140, 150, 160, and 170 °C, respectively. The cell morphologies of these foamed

Figure 11. SEM micrographs of PPBESK films, which were saturated at (I) 5 MPa, (II) 6 MPa, (III) 7 MPa, and at 40 °C: (A−D) 2.7 g CO2/100 g polymer; (E−H) 2.8 g CO2/100 g polymer; (I−L) 2.9 g CO2/100 g polymer. The foaming temperatures are 140, 150, 160, and 170 °C, respectively. G


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foams,32,33 but most of them are on conventional foams. Until in recent years, very limited data on mechanical properties of the microcellular materials especially obtained by the batch foaming method had been reported. The reason for the lack of data is that there are some difficulties in obtaining samples with big size and smooth, glossy surfaces. In this paper, samples which saturated at various pressures and foamed at Tmax corresponding to each saturation pressures (according to Figure 7), respectively, were used for tensile tests. All the foams obtained are smooth and glossy, as shown in Figure 2. The specific strength of the foams as a function of the CO2 concentration was presented in Figure 13, and the mass

transition stage of the transformation from closed microcellular to bicontinuous porous. In this case, the phenomenon can be explained by two factors: molecular mobility of the polymer and the diffusivity of the absorbed CO2. On the one hand, when the foaming temperature is below 130 °C, the molecular mobility of the polymer is low, so the growth of cells is restricted by the polymer matrix and it is difficult to form open cell structures. On the other hand, when the foaming temperature is above 180 °C, the molecular mobility of the polymer and the diffusivity of CO2 is much higher, and the gas diffuses to the outside of the polymer matrix quickly, decreasing the driving force for cell growth, so the wall between the neighboring cells is also difficult to destroy and closed cell structures are observed. However, when the foaming temperature is between 140 to 170 °C, both molecular mobility of the polymer and CO2 diffusivity are in a suitable range. In this case, the polymer matrix has certain restriction on the CO2 diffusion to prevent CO2 diffused to the outside of the polymer matrix, but the rest CO2 still providing enough driving force to cause cell wall failure and rapid fracture, so open cell structures are observed. Moreover, when the concentration of CO2 dissolved in the polymer matrix is apparently higher than the critical value (≥3.52 g CO2/100 g polymer, the saturation of CO2 under 7 MPa), the diffusion of the absorbed CO2 occupied the leading position, no matter what foaming temperature was taken in the experiments, typical bicontinuous porous structures were observed, as show in Figure 7. Thus, the cell structures of PPBESK corresponding to CO2 concentration can be definitely divided into three stages: closed cell structure, bimodal cell structure, and bicontinuous porous structure (as shown in Figure 12).

Figure 13. Curves of the specific strength (red square) and the mass density (black square) of foams which are foamed at T max corresponding to each saturation pressure depending on the CO2 concentration dissolved in PPBESK samples.

densities of the foams were normalized. One can see that the specific strength of the foams decreased with increasing CO2 concentration and then increased. The inflection point is near the critical CO2 concentration. It is worthwhile to note that the specific strength of the foams which were obtained by saturation at 12 MPa pressure is as high as 7.5 × 104 N·m/ kg, higher than that of unfoamed materials. The reasons are, first of all, the mass density of foams is much lower than that of unfoamed materials. Second, smaller cell size and uniform distribution are a benefit to prevent the crack propagation and give the foam material better mechanical properties. On the basis of the cellular morphology analysis, it can be concluded that the tensile strength of foams are closely related to its cell structure especially the cell type (open or closed). Bicontinuous porous structures can endow both advantages of high strength and low density to foams because of their smaller cell size and uniform distribution.

Figure 12. Schematic diagram of the relationship between the CO2 concentration and the porous structure.

In this paper, we give one explanation for the formation of the open cell structure: at such high gas concentrations the roles of solvent and solute in the gas/polymer solution can be reversed. That is to say, when the concentration of CO2 dissolved in the polymer matrix is below the critical value, PPBESK acts as solvent and CO2 acts as solute; in this case, CO2 dissolves in the PPBESK matrix to form the dispersed phase, so closed cell structures are observed. However, when the CO2 concentration is above the critical value, CO2 acts as solvent and PPBESK acts as solute. In this case, CO2 swells PPBESK to form a continues phase, so bicontinuous porous structures are observed. 3.5. Tensile Strength Characterization. There is considerable literature on rthe mechanical properties of

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CONCLUSIONS In this study, foams from PPBESK with two types of cell structure (closed and open) have been successfully prepared by batch foaming and using CO2 as foaming agent. We have demonstrated that the saturation pressure and the foaming temperature have certain influences on the cell size and density. However, the concentration of CO2 dissolved in PPBESK matrix is the key factor which causes the structure transformation from closed microcellular to bicontinuous nanoporous. The cell structure of PPBESK foams corresponding to CO2 concentration can be detailedly divided into two stages: H


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(10) Chen, L. M.; Rende, D.; Schadler, L. S.; Ozisik, R. Polymer nanocomposite foams. J. Mater. Chem. A 2013, 1, 3837−3850. (11) Rouholamin, D.; Van Grunsven, W.; Reilly, G. C.; Smith, P. J. Morphological effects of porous poly-d,l-lactic acid/hydroxyapatite scaffolds produced by supercritical CO2 foaming on their mechanical performance. Proc. Inst. Mech. Eng., Part H 2016, 230, 761−774. (12) Wang, J.; Zhang, L.; Bao, J. B. Supercritical CO2 assisted preparation of open-cell foams of linear low-density polyethylene and linear low-density polyethylene/carbon nanotube composites. Chin. J. Polym. Sci. 2016, 34, 889−900. (13) Cafiero, L.; Iannace, S.; Sorrentino, L. Microcellular foams from high performance miscible blends based on PEEK and PEI. Eur. Polym. J. 2016, 78, 116−128. (14) Liu, S.; Duvigneau, J.; Vancso, G. J. Nanocellular polymer foams as promising high performance thermal insulation materials. Eur. Polym. J. 2015, 65, 33−45. (15) Guo, H.; Kumar, V. Solid-state poly(methyl methacrylate) (PMMA) nanofoams. Part I: Low-temperature CO2 sorption, diffusion, and the depression in PMMA glass transition. Polymer 2015, 57, 157−163. (16) Costeux, S. CO2-blown nanocellular foams. J. Appl. Polym. Sci. 2014, 131, 1−16. (17) Notario, B.; Pinto, J.; Rodriguez-perez, M. A. Towards a new generation of polymeric foams: PMMA nanocellular foams with enhanced physical properties. Polymer 2015, 63, 116−126. (18) Merlet, S.; Marestin, C.; Schiets, F.; Romeyer, A. O.; Mercier, R. Preparation and Characterization of Nanocellular Poly(phenylquinoxaline) Foams. A New Approach to Nanoporous HighPerformance Polymers. Macromolecules 2007, 40, 2070−2078. (19) Forest, C.; Chaumont, P.; Cassagnau, P.; Swoboda, B.; Sonntag, P. Polymer nano-foams for insulating applications prepared from CO2 foaming. Prog. Polym. Sci. 2015, 41, 122−145. (20) Jones, B. H.; Lodge, T. P. Nanoporous Materials Derived from Polymeric Bicontinuous Microemulsions. Chem. Mater. 2010, 22, 1279−1281. (21) Guo, H.; Nicolae, A.; Kumar, V. Solid-state microcellular and nanocellular polysulfone foams. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 975−983. (22) Aly, M. S. Effect of pore size on the tensile behavior of open-cell Ti foams: Experimental results. Mater. Lett. 2010, 64, 935−937. (23) Xiao, L. H.; Liu, C.; Liao, G. X.; Zhang, Y.; Jian, X. G. Synthesis and characterization of novel copoly(aryl ether sulfone) containing phthalazinone and biphenyl moieties. High Perform. Polym. 2010, 22, 274−285. (24) Xiao, L. H.; Liu, C.; Jian, X. G. Synthesis and characterization of novel poly(aryl ether sulfone ketone)s containing phthalazinone and biphenyl moieties. Chin. Chem. Lett. 2008, 19, 227−229. (25) Gu, T. S.; Liao, G. X.; Wang, J. Y.; Xu, Y. J.; Jian, X. G. Mechanical Properties of PPBES/PEEK Composites Reinforced by Short Glass Fiber. Polym. Mater. Sci. Eng. 2009, 25, 94−96. (26) Zhang, X.; Liao, G. X.; Jin, Q. F.; Feng, X. B.; Jian, X. G. On dry sliding friction and wear behavior of PPESK filled with PTFE and graphite. Tribol. Int. 2008, 41, 195−201. (27) Shi, X. T.; Zhang, G. C.; Liu, Y.; Ma, Z. L.; Jing, Z. X.; Fan, X. Microcellular foaming of polylactide and poly(butylene adipate-coterphathalate) blends and their CaCO3 reinforced nanocomposites using supercritical carbon dioxide. Polym. Adv. Technol. 2016, 27, 550− 560. (28) Wen, N.; Liu, T.; Xiang, B.; Lei, J. Y.; Luo, S. K. Preparation and characterization of microcellular injection molded foams from highperformance blends based on PPS-modified PPBESK. High Perform. Polym. 2017, 095400831770543. (29) Hachisuka, H.; Sato, T.; Imai, T.; Tsujita, Y.; Takizawa, A.; Kinoshita, T. Glass transition temperature of glassy polymers plasticized by CO2 gas. Polym. J. 1990, 22, 77−79. (30) Yun, D. H.; Sung, W. C. The relationship between gas absorption and the glass transition temperature in a batch microcellular foaming process. Polym. Test. 2002, 21, 269−275.

closed cell structure and open cell structure. And the latter can be further divided into bimodal cell structure and typical bicontinuous porous structure. Accordingly, three stages are obtained. Closed microcellular structure occurs at CO 2 saturation levels below 2.80 g CO2/100 g polymer; bimodal cell structure occurs above this critical threshold but below 3.52 g CO2/100 g polymer; bicontinuous porous structure occurs at CO2 saturation levels above 3.52 g CO2/100 g polymer. The reason for the formation of open cell structures is that the roles of solvent and solute in the gas/polymer solution can be reversed at such high gas concentrations. Moreover, the tensile tests of foams indicate that the tensile strength of foams is closely related to its cell structure especially the cell type (open or closed). A bicontinuous porous structure can endow both advantages of high strength and low density to foams.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 0816 2491421. Fax: +86 0816 2482914. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Na Wen: 0000-0003-0214-0464 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Science and Technology Foundation of National High Technology Research and Development Program of China (863 Program, 2015AA033802).

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REFERENCES

(1) Kong, W. L.; Bao, J. B.; Wang, J.; Hu, G. H.; Xu, Y.; Zhao, L. Preparation of open-cell polymer foams by CO2 assisted foaming of polymer blends. Polymer 2016, 90, 331−341. (2) Krause, B.; Boerrigter, M. E.; Van der Vegt, N. F. A.; Strathmann, H.; Wessling, M. Novel open-cellular polysulfone morphologies produced with trace concentrations of solvents as pore opener. J. Membr. Sci. 2001, 187, 181−192. (3) Frerich, S. C. Biopolymer foaming with supercritical CO2thermodynamics, foaming behaviour and mechanical characteristics. J. Supercrit. Fluids 2015, 96, 349−358. (4) Okolieocha, C.; Beckert, F.; Herling, M.; Breu, J.; Mulhaupt, R. Preparation of microcellular low-density PMMA nanocomposite foams: Influence of different fillers on the mechanical, rheological and cell morphological properties. Compos. Sci. Technol. 2015, 118, 108−116. (5) Erick, O.; Fellah, Z. E. A.; Gery, O. Identification of the mechanical moduli of closed-cell porous foams using transmitted acoustic waves in air and the transfer matrix method. Compos. Struct. 2016, 135, 205−216. (6) Hasan, M. A.; Kim, A.; Lee, H. J. Measuring the cell wall mechanical properties of Al-alloy foams using the nanoidentation method. Compos. Struct. 2008, 83, 180−188. (7) Michel, F. S.; Chazeau, L.; Cavaille, J. Y.; Chabert, E. Mechanical properties of high density polyurethane foams: I. Effect of the density. Compos. Sci. Technol. 2006, 66, 2700−2708. (8) Bao, D. F.; Liao, X.; He, T.; Yang, Q.; Li, G. Preparation of nanocellular foams from polycarbonate/poly(lactic acid) blend by using supercritical carbon dioxide. J. Polym. Res. 2013, 20, 290−300. (9) Song, S. A.; Oh, H. J.; Kim, B. G.; Kim, S. S. Novel foaming methods to fabricate activated carbon reinforced microcellular phenolic foams. Compos. Sci. Technol. 2013, 76, 45−51. I


Article

Industrial & Engineering Chemistry Research (31) Krause, B.; Sijbesma, H. J. P.; Munuklu, P.; van der Vegt, N. F. A.; Wessling, M. Bicontinuous Nanoporous Polymers by Carbon Dioxide Foaming. Macromolecules 2001, 34, 8792−8801. (32) Gomez- Monterde, J.; Schulte, M.; Ilijevic, S.; Hain, J.; Arencon, D.; Sánchez-Soto, M.; Maspoch, M. L. Morphology and Mechanical Characterization of ABS Foamed by Microcellular Injection Molding. Procedia Eng. 2015, 132, 15−22. (33) Bao, J. B.; Nyantakyi, A., Jr.; Weng, G. S.; Wang, J.; Fang, Y. W.; Hu, G. H. Tensile and impact properties of microcellular isotactic polypropylene (PP) foams obtained by supercritical carbon dioxide. J. Supercrit. Fluids 2016, 111, 63−73.

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Porous Structural Transformation from Closed Microcellular to

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