Plasticization of [C12MIM][PF6] Ionic Liquid

Sep 5, 2012 - ABSTRACT: A series of poly(methyl methacrylate) (PMMA)/1-dodecyl-3-methylimidazolium hexafluorophosphate ionic liquid ([C12MIM][PF6]) ...
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Plasticization of [C12MIM][PF6] Ionic Liquid on Foaming Performance of Poly(methyl methacrylate) in Supercritical CO2 Fang-fang Tong,† Hong Xu,‡ Jian Yu,*,‡ Li-xiong Wen,*,† Jun Zhang,‡ and Jia-song He‡ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Beijing National Laboratory for Molecular Science (BNLMS), Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

ABSTRACT: A series of poly(methyl methacrylate) (PMMA)/1-dodecyl-3-methylimidazolium hexafluorophosphate ionic liquid ([C12MIM][PF6]) blends were prepared by melt blending. The compatibility of blends was verified by scanning electron microscopy and differential scanning calorimetry. The introduction of [C12MIM][PF6] decreased the glass transition temperature of the blends due to its plasticization effect and increased the diffusivity of CO2 without significantly affecting the CO2 solubility. These blends were foamed with supercritical CO2 as the blowing agent via a pressure-quenching process. The foaming conditions are divided into three regions according to the main morphological difference of foams. At relatively low temperatures and pressures, neat PMMA could not be foamed, while the addition of [C12MIM][PF6] facilitated the formation of cells. By increasing temperature or pressure, neat PMMA foams were obtained with wide cell size distribution, while the blend foams were formed with narrowed cell size distribution and increased cell size. At high temperatures and pressures, neat PMMA was foamed with narrow cell size distribution and high cell density, while the major effect of [C12MIM][PF6] was to increase the cell size. These results indicated that the plasticization effect and the low surface energy of [C12MIM][PF6] accounted for the differences on the cell morphology between neat PMMA and the blend foams.

1. INTRODUCTION The foamed plastics have found applications in extensive fields because of their excellent performances, such as lightweight, high specific strength, low thermal conductivity, and superior sound insulating properties. As an environmentally benign physical blowing agent, supercritical carbon dioxide (scCO2) has drawn great attention in polymer foaming,1−12 due to its unique properties of mild critical point (Tc = 31.1 °C, Pc = 7.37 MPa),13 zero surface tension, low cost, nonflammability, chemical friendly, and substantial solubility in many polymers. Recently, ionic liquids (ILs) have attracted growing interest as a new type of green solvents,14−18 which are completely composed of anions and cations. ILs possess the advantages of extremely low volatility, tunable solvation, nonflammability, high conductivity, and excellent thermal and chemical stability by comparison with traditional organic solvents. Many studies have focused on the interesting topic to combine advantages of scCO2 and ILs, both of which are environmentally friendly. The results have shown that CO2 is significantly soluble in ILs, compared to other gases, e.g. H2, N2, CH4, and CO.19−21 The CO2 solubility in IL is not only related to its pressure and temperature but also to the chemical structure of cation and anion of ILs. Imidazolium-based ILs have higher CO2 solubility than corresponding guanidium-based and ammonium-based ILs,22,23 because of the activity of 2-H in imidazolium ring,24 and the larger alkyl group in the imidazolium-based ILs leads to lower bubble-point pressures, hence higher solubility of CO2.25 In addition, the anion also has a larger impact on CO2 solubility in imidazolium-based ILs.24,26 It has been reported that anions containing fluoroalkyl groups have a larger CO2 solubility27 because of the Lewis acid−base interaction between the fluorinated anion and CO2.28 © 2012 American Chemical Society

On the other hand, ILs are widely used in polymer science as polymerization media, polymer solvents, and functional additives.29−39 Brazel et al.37−39 found that the plasticization effect of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) in poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) is better than that of the traditional plasticizer dioctyl phthalate, and it was indicated by the similar depression of glass transition temperature (Tg), widened compatible composition range, and improved thermal stability of polymers. The usage of ILs as potential foaming additives by using scCO2 as blowing agents has been explored in our previous studies.40,41 In the incompatible polypropylene (PP)/1tetradecyl-3-methylimidazolium bromide system, the growth of cells was facilitated to obtain lower unfoamed region and bulk density by comparison with pure PP foamed under the same conditions.40 As for compatible PMMA/1-dodecyl-3methylimidazolium hexafluorophosphate ([C12MIM][PF6]) blends, it was found that the presence of IL also displayed significant effects on the foaming behavior of polymer.41 In the present study, more systematic experiments have been carried out to arrive a better understanding of foaming performance of the compatible polymer/IL system via a pressure-quenching process, and the influence of cell morphology by [C12MIM][PF6] was investigated in detail by taking into account the temperature, pressure, and IL content. The results were further discussed based on the classical nucleation theory. Received: Revised: Accepted: Published: 12329

May 29, 2012 September 3, 2012 September 5, 2012 September 5, 2012 dx.doi.org/10.1021/ie301409h | Ind. Eng. Chem. Res. 2012, 51, 12329−12336

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Figure 1. SEM images of the fractured surfaces of the (a) neat-PMMA, (b) PMMA/3 phr [C12MIM][PF6], (c) PMMA/5 phr [C12MIM][PF6], and (d) PMMA/10 phr [C12MIM][PF6] samples.

2. EXPERIMENTAL SECTION 2.1. Materials. Granular PMMA was supplied by Aladdin Chemistry Co., Ltd., Shanghai, China and used as received. [C12MIM][PF6] with its melting point at 58 °C was purchased from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. CO2 with a purity of 99.95% was provided by Beijing Gas Factory. 2.2. Sample Preparation. After drying at 80 °C under vacuum for 24 h, PMMA and [C12MIM][PF6] with the mass ratio of 100/0, 100/3, 100/5, and 100/10, respectively, were melt blended in a mixer (Haake Rheocord RC90) at 200 °C and 50 rpm for 5 min. The obtained blends were compression molded into sheets with 1 mm thickness at 200 °C. The sheets were cut into specimens with dimensions of 1 cm × 1.5 cm for batch foaming. 2.3. Gas Sorption Measurement. The neat PMMA and blend samples were placed in a high-pressure vessel preheated to 35 °C. The vessel was flushed with low pressure CO2 for about 2 min, followed by increasing the pressure to 8 MPa. After saturation for different lengths of time under this condition, these samples were removed out following a rapid quenching of pressure and transferred within a 1 min interval to an electronic balance with sensitivity of 0.1 mg to record mass loss as a function of time. The plots of CO2 content that remains in the sample (Md) against the square root of desorption time (td)1/2 for all samples are linear in the initial stage of desorption, which can be described by eq 1 Md Dt = −4 2d d Ms l π

The morphology of unfoamed and foamed samples was observed with a scanning electron microscope (SEM, JEOL JSM-6700). These samples were freeze-fractured in liquid nitrogen and sputter-coated with platinum. The cell size (D) and cell density (N) of foamed samples were obtained from SEM images. D was calculated by averaging diameters of at least 100 cells in the SEM images, and the cell size was measured as the average of two orthogonal cell dimensions.42 N, the number of cells per cubic centimeter of solid samples, was calculated by the method suggested by Kumar and Suh4

⎛ nM2 ⎞3/2 N=⎜ ⎟ VE ⎝ A ⎠

where n is the number of cells in the SEM image, M is the magnification factor, and A is the area of image.

3. RESULTS AND DISCUSSION 3.1. The Compatibility of [PMMA/C 12 MIM][PF 6 ] Blends. The compatibility of PMMA/[C12MIM][PF6] blends was investigated by SEM and DSC. Figure 1 shows the SEM images of PMMA/[C12MIM][PF6] blends before foaming. The blends exhibit homogeneous morphology of fractured surface similar to that of neat PMMA, and no dispersed phase is observed up to IL content of 10 phr. Figure 2 shows the Tg of

(1)

where l is the thickness of sample. The diffusivity coefficient (Dd) was calculated from the slope of desorption curve, while the sorption of CO2 (Ms) was obtained by extrapolating the desorption curve of CO2 to time zero. 2.4. Batch Foaming. The basic process of polymer foaming was similar to that of gas sorption measurement. The samples were saturated at certain temperature and pressure for 6 h before the pressure was quenched quickly, and then the samples were kept at the temperature for 2 min to allow the full growth of the cell structure. The foamed samples were cooled by immersing in ice water. 2.5. Analysis. DSC analysis was carried out on a differential scanning calorimeter (DSC, Perkin-Elmer DSC-7) from −80 to 150 °C at a heating rate of 20 °C/min under a nitrogen atmosphere. The Tg was obtained from the midpoint of glass transition step. The mass density of samples before and after foaming (ρ and ρf) were measured by the water displacement method according to ISO 1183-1987, and the volume expansion ratio (VE) of foamed samples can be calculated as

VE =

ρ ρf

(3)

Figure 2. Tg of various samples against [C12MIM][PF6] content in PMMA.

various samples obtained from the second heating curves in DSC tests after eliminating thermal history. All blends exhibit only one glass transition as neat PMMA does, and the Tg decreases with increasing IL content. In addition, the DSC curves of blends display no melting endotherm of pure [C12MIM][PF6] at about 58 °C. Therefore, both morphology and thermal behavior indicate that the PMMA/[C12MIM][PF6] blends have high compatibility up to 10 phr IL content, which is consistent with their high optical transparency by

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Figure 3. (a) Sorption of CO2 in neat PMMA and PMMA/10 phr [C12MIM][PF6] as a function of time and (b) diffusion coefficient of CO2 in various samples after saturated at 8 MPa/35 °C against normal Tg.

Figure 4. SEM images of various samples obtained by foaming at 12 MPa/40 °C: (a) neat PMMA, (b) PMMA/3 phr [C12MIM][PF6], (c) PMMA/ 5 phr [C12MIM][PF6], and (d) PMMA/10 phr [C12MIM][PF6].

with good linear relationship were obtained (results not shown). The sorption and diffusion coefficient of CO2 were calculated from their intercept and slope, respectively. Figure 3a shows the CO2 sorption in neat PMMA and PMMA/10 phr [C12MIM][PF6] blend as a function of the treatment time in scCO2. It can be seen that the sorption increases over time and levels off when the time is above 4 h. Therefore, the saturation time of 6 h was used in the following foaming experiments to ensure the equilibrium sorption of CO2 in the samples before pressure-quenching. Although the fluorinated group of [PF6] anion facilitates the solution of CO2 in imidazolium-based IL,24,27 the addition of [C12MIM][PF6] in PMMA only leads to insignificantly increased CO2 solubility. It is due to the relative high solubility of CO2 in PMMA, in which the carbonyl groups have favorable interaction with CO2. As mentioned above, the presence of IL decreases the Tg of PMMA and the stiffness of polymer matrix. Consequently, the plasticization effect reduces the diffusive resistance to CO2, which accelerates the escape of CO2 from the sample. As shown in Figure 3b, the CO2 diffusion coefficient in saturated samples increases continuously and gradually with increasing plasticization effect. 3.3. Cell Morphology of Foamed Samples. After saturation under different scCO2 conditions for 6 h, all samples were foamed via pressure-quenching process. The pressure and temperature were used in the range of 8−25 MPa and 40−120 °C (only 8−12 MPa for 120 °C), respectively. The cell morphology was investigated by using SEM. Figure 4 shows the fractured surface of various samples after treatment at 12 MPa/ 40 °C. The treated neat PMMA exhibits solid morphology without any observable cells, indicating that the foaming did not occur. As for the blends, the closed cells are observed with broad size distributions, and the number of cells increases with increasing IL content. It indicates that the addition of IL

visual inspection. It was reported that [BMIM][PF6] is compatible with PMMA up to 50 wt % IL content.39 The longer alkyl chain in [C12MIM][PF6] would endow it with higher hydrophobicity, thus increasing attractive interaction with hydrophobic PMMA. Additionally, the solubility parameter (δH) of hexafluorophosphate-based IL was reported to have a decreasing trend with increasing alkyl chain length in imidazolium cation,43 i.e., from 29.8 MPa1/2 for [BMIM][PF6] to 27.8 MPa1/2 for 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6]). Therefore, [C12MIM][PF6] would have δH closer to PMMA (∼22.0 MPa1/2)44 than [BMIM][PF6], which agrees with the high compatibility between PMMA and [C12MIM][PF6]. On the other hand, the Tg decreases significantly from 116.4 to 100.3 °C with increasing IL content from 0 to 10 phr, indicating that [C12MIM][PF6] can plasticize PMMA effectively. The relatively small molecules of IL diffuse into PMMA matrix, resulting in the increase of the fraction of terminal groups and free volumes. Meanwhile, the motion of anions and cations in IL weakens the adhesion forces between polymer chains and hence decreases the Tg of polymer in blends due to the increasing mobility of polymer segments. Therefore, [C12MIM][PF6] has high compatibility with PMMA and displays considerable plasticization effect similar to [BMIM][PF6], which induces about 80 °C depression in Tg of PMMA with 50 wt % IL as plasticizer.38 3.2. Sorption and Diffusion of CO2 in PMMA/ [C12MIM][PF6] Blends. The effect of IL on the sorption and diffusion of CO2 in the PMMA/[C12MIM][PF6] blends was investigated at 8 MPa/35 °C by using the commonly accepted gravimetric method. These conditions were used to avoid foaming of the sample, which would affect the accuracy of the experimental data. By plotting the percentage mass uptake against the square root of desorption time, desorption curves 12331

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Figure 5. SEM images of (a, c) neat PMMA (b, d) and PMMA/10 phr [C12MIM][PF6] foams obtained by foaming at (a, b) 20 MPa/40 °C and (c, d) 12 MPa/100 °C.

Figure 6. SEM images of various foams obtained by foaming at 25 MPa and (a-d) 40 °C, (e-h) 80 °C, and (i-l) 100 °C.

induced the formation of cells by facilitating the cell nucleation and growth. Figure 5 shows the SEM images of neat PMMA and PMMA/ 10 phr [C12MIM][PF6] foams obtained at 20 MPa/40 °C and 12 MPa/100 °C, respectively. Neat PMMA was foamed with broad cell size distribution under both conditions. By comparison, the addition of IL not only increases the cell size but also narrows the cell size distribution. On the other hand, neat PMMA foams exhibit different morphology after foaming at 25 MPa and 40−100 °C, as shown in Figure 6. It is seen that the cells in these foams display smaller size and narrower size distribution than those obtained at 20 MPa/40 °C and 12 MPa/100 °C. Meanwhile, the cell size increases with increasing IL content and increasing foaming temperature, with the similar cell size distribution. By putting all the results into one plot, the foaming conditions used in the present study are divided into three regions relating with different cell morphology, as shown in Figure 7. Region I includes all the conditions (i.e., 8−15 MPa/ 40 °C) under which neat PMMA was not foamed by scCO2 due to the relatively low temperature and pressure. The treated PMMA had VE close to 1 and the morphology similar to that shown in Figure 4a. The polymer matrix was stiff enough to restrict the formation of cells. Under other foaming conditions, except for the mildest condition of 8 MPa/40 °C, cells with broad size distribution were formed in all blends to attain the VE up to 1.5. As for 8 MPa/40 °C, only the PMMA/10 phr

Figure 7. Illustration of three regions taking into account the morphological difference between neat PMMA and PMMA/ [C12MIM][PF6] blends. These three regions represent the conditions for neat PMMA (I) could not be foamed, (II) foamed with broad cell size distribution, and (III) foamed with narrow size distribution, respectively.

[C12MIM][PF6] blend was foamed to attain a small VE of 1.1. It indicates that the real Tg of PMMA/[C12MIM][PF6] blend was decreased to below 40 °C in the presence of 10 phr IL during scCO2 treatment. Therefore, it was the plasticization effect of IL that induced the cell formation, and the effect was enhanced by the increase of IL content. 12332

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Figure 8. The cell size (a) and cell density (b) of neat PMMA and PMMA/[C12MIM][PF6] foams obtained by foaming at 100 °C and different pressures.

Figure 9. VE of neat PMMA and PMMA/[C12MIM][PF6] foams obtained by foaming at (a) 10 MPa and different temperatures and (b) 100 °C and different pressures.

Region II shows the conditions for neat PMMA foamed with broad cell size distribution, including 8−12 MPa/100 °C, 8−15 MPa/80 °C, and 20 MPa/40 °C. The two conditions of these were demonstrated in Figure 5. Under these conditions, the cell nuclei could not be formed simultaneously due to the low driving force for nucleation, because of low sorption of CO2 at relatively low pressure and high temperature (e.g., 12 MPa/100 °C) or high nucleation energy barrier from the restriction of matrix at relatively low temperature (e.g., 20 MPa/40 °C). However, with the addition of IL, narrower cell size distribution was obtained. Moreover, the VE increased from 1.2 to 2.0 for neat PMMA foams, to 1.3−2.4 for PMMA/3 phr [C12MIM][PF6], 1.3−2.9 for PMMA/5 phr [C12MIM][PF6], and 2.5−3.9 for PMMA/10 phr [C12MIM][PF6] blend foams, respectively. Therefore due to the presence of IL, the slightly increased sorption of CO2 and the plasticization effect favored the cell nucleation, and the decrease of restriction from the matrix facilitated the diffusion of CO2 into the cells to expand the foams remarkably. Region III represents the conditions for neat PMMA foamed with narrow cell size distribution. It is attributed to the fact that the driving force of cell nucleation was large enough to nucleate most of the cells simultaneously at relatively high temperature and/or pressure. The VE was obtained in the range of 1.2−6.4 for neat PMMA and increased with the increase of IL content,

i.e., 1.3−7.2 for PMMA/3 phr [C12MIM][PF6], 1.4−9.5 for PMMA/5 phr [C12MIM][PF6], and 1.8−18.0 for PMMA/10 phr [C12MIM][PF6] blend foams, respectively. Under most conditions, the impinged polyhedral cells were formed instead of the isolated spherical ones. Therefore, the addition of IL facilitated the expansion of cell and kept the cell size distribution. On the other hand, the cell structure was affected by the foaming conditions and IL content in similar trends for all these regions. The cell size increased with increasing temperature and IL content, and with decreasing pressure, while the cell density was influenced in the way contrary to the cell size. Figure 8 shows the influence of pressure and IL content on the cell size and cell density of foams obtained at 100 °C. At 10−25 MPa, all the foams have the cell size below 50 μm and cell density above 107 cells/cm3, which can be regarded as typical microcellular polymer foams.1 3.4. VE of Foamed Samples. The influence of foaming conditions on VE is shown in Figure 9. At 10 MPa (Figure 9a), the VE increases slowly with increasing foaming temperature in the range from 40 to 100 °C, i.e., from 1.0 to 1.8 for neat PMMA and from 1.3 to 3.9 for PMMA/10 phr [C12MIM][PF6] blend foams, respectively. With the temperature increased to 120 °C, the VE increases abruptly to 3.6 for the former and to 12.0 for the latter, respectively. It is known that the decrease in 12333

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where C is the concentration of CO2 in polymer, f is the collision frequency of CO2 molecules, kB is the Boltzmann constant, and T is absolute temperature. The energy barrier for homogeneous nucleation (ΔGhom*) is related to the interface tension of polymer matrix/CO2 mixture (γmix) by

CO2 sorption with increasing temperature is unfavorable for the cell expansion. However, the stiffness of polymer matrix decreases significantly at the same time to compensate the loss of CO2, which increases the VE especially at temperatures higher than the normal Tg of PMMA at ambient pressure. Due to its plasticization effect, the presence of [C12MIM][PF6] further facilitates the cell expansion to reach high VE, compared to neat PMMA. For the relatively low foaming temperatures of 40 and 80 °C, the VE generally increases with the increase of saturation pressure (results not shown). As for higher temperature of 120 °C, a good linear relationship is displayed between the VE and the pressure in the range from 8 to 12 MPa. At higher pressures, the samples expanded dramatically to touch the wall of high-pressure vessel. For the foaming temperature of 100 °C, the results are shown in Figure 9b. It is seen that the VE increases linearly with the pressures between 8 and 20 MPa, followed by a slow decrease up to 25 MPa. The CO2 solubility increases with increasing pressure, which is favorable for high expansion. However, high CO2 content in polymer matrix also decreases the modulus of the samples in combination with high temperature, leading to a significant gas loss during foaming.9 Consequently, the expansion ratio of the foamed samples is suppressed due to the decreased amount of CO2 available for the cell growth with further increasing pressure to 25 MPa. Figure 9 also shows that the VE increases obviously with increasing IL content at the same temperature and pressure. It is attributed to the additional plasticization effect of [C12MIM][PF6] further decreasing the sample stiffness, which also leads to the severe depression of VE at 25 MPa/100 °C. On the other hand, for most foaming conditions used in this study, the foamed blend samples have cell size smaller than 50 μm. Therefore, the addition of compatible IL favors the formation of microcellular PMMA foams with high expansion ratio. Accordingly, the expansion behavior of foamed samples is explained by considering two opposing effects. One is the combination of CO2 and IL in decreasing the stiffness of polymer matrix, which favors the expansion of samples in the foaming progress. The other one is the significant increase of gas loss, resulting in severe collapse of initially expanded samples especially at high temperatures. 3.5. Plasticization Effect of IL Affecting Cell Morphology. As mentioned above, the introduction of compatible [C12MIM][PF6] in the PMMA matrix has the effects of inducing the cell formation, increasing the cell size, and decreasing both the cell density and cell size distribution according to the foaming conditions. It is known that the cell morphology is affected primarily by the cell nucleation and growth, which are two major steps in the foaming process of polymers. In general, the lower limit of foamable temperature is determined by the Tg of the polymer/CO2 mixture because of its high stiffness at glassy state restricting the expansion of polymer by CO2. In the present study, the depression of Tg by the plasticization effect of [C12MIM][PF6] means that the cells can be formed in PMMA/IL blends at lower temperatures by comparison with neat PMMA. According to the classical nucleation theory, only homogeneous nucleation is considered in both neat PMMA and compatible PMMA/[C12MIM][PF6] blends. The nucleation rate of cells per unit volume (N0) during foaming can be written as3 N0 = Cf exp( −ΔG hom*/kBT )

ΔG hom* =

16πγmix 3 3ΔP 2

(5)

where ΔP is the magnitude of the pressure quench during foaming. γmix is evaluated by Goel and Beckman’s method,2 where the surface tension of pure scCO2 was defined as zero. γmix

⎛ ρ ⎞4 = γmatrix ⎜⎜ mix ⎟⎟ (1 − wgas)4 ⎝ ρmatrix ⎠

(6)

ρmix and ρmatrix are the mass densities of PMMA/CO2 and neat PMMA, respectively, in the case of neat PMMA; while they are the mass densities of PMMA/IL/CO2 and PMMA/IL, respectively, for the blends, and wgas is the mass uptake of CO2 by the samples. As shown in Figure 3, the solubility of CO2 in PMMA is not changed obviously by adding IL. Therefore, γmatrix is the crucial factor for the γmix. According to the interfacial tension of PMMA (42.4 mN/m45) and [C12MIM][PF6] (23.6 mN/m at 336 k46), it is reliable to predict that the interfacial tension of compatible PMMA/IL blend is much smaller than γPMMA. Consequently, the addition of [C12MIM][PF6] would decrease energy barrier of nucleation for PMMA, resulting in a high initial nucleation rate. However, PMMA/IL blends have smaller final cell density than neat PMMA, as shown in Figure 8. It is well-known that the cell size and cell density are further changed during cell growth process, which is mainly controlled by both the diffusivity of CO2 in the matrix and the stiffness of the matrix. As a consequence of the plasticization effect of IL, the modulus of PMMA matrix decreased significantly. Therefore, the restriction of CO2 diffusion was weakened obviously and cell coalescence took place intensively during the cell growth. As shown in Figures 2 and 3b, the addition of 10 phr IL increased the diffusion coefficient of CO2 from 2.2 × 10−11 m2/s for neat PMMA to 4.4 × 10−11 m2/s and decreased the Tg significantly from 116.4 to 100.3 °C. These indicate that the cells had higher growth rate in the samples with higher IL contents to obtain foams with big cell sizes, attributed to the quicker diffusion of CO2 into the cells. On the other hand, the high diffusivity of CO2 accelerated the consumption of the foaming agent. The amount of gas available in the matrix decreased quickly, resulting in the decrease of driving force for further cell nucleation. Consequently, the nucleation in PMMA/IL blends proceeded in a short period of time and hence a small total nuclei density would be obtained in the presence of IL, despite the fact that the addition of IL increases the initial nucleation rate. The combination of the small amount of nuclei and the serious cell coalescence results in small final cell density. In conclusion, the presence of IL would account for those differences on foam morphology for the neat PMMA and compatible PMMA/IL blend samples. Its plasticization effect increases the diffusivity of CO2 and decreases the stiffness of the matrix, respectively. In addition, the low surface energy of IL decreases the energy barrier for the initial cell nucleation. Therefore, the combination of the short nucleation time and the quick cell growth results in lower cell density, bigger cell

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size, narrower cell size distribution, and larger expansion ratio in PMMA/IL blend foams than in neat PMMA one.

4. CONCLUSION In this work, PMMA/[C12MIM][PF6] blends with IL content up to 10 phr were prepared by melt blending. The compatibility of PMMA and [C12MIM][PF6] has been verified by SEM and DSC results. The Tg decreased with increasing IL content, i.e., from 116.4 °C of neat PMMA to 100.3 °C of PMMA/10 phr IL blend, due to the plasticization effect of IL. Although the addition of IL did not significantly influence the CO2 sorption measured at 8 MPa and 35 °C, it did increase the diffusivity of CO2. By using scCO2 as the physical foaming agent, PMMA/ [C12MIM][PF6] blends were foamed via a pressure-quenching process. The results showed that the effects of IL on the main morphological difference of foams depended on the foaming conditions. The Tg depression by IL extended the low limit of foamable temperature in the PMMA/IL blends and induced the formation of cells under those conditions where neat PMMA was not foamed. With increasing pressure or temperature, neat PMMA was foamed with broad cell size distribution. Meanwhile, the addition of IL resulted in narrow cell size distribution and large cell size due to short time interval of nucleation, high diffusivity of CO2, and low stiffness of the matrix. At relative high pressures and temperatures, all the foams had narrow cell size distribution, and their cell size increased with increasing IL content. The plasticization effect and the low surface energy of IL could account for these morphological differences.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-6261 3251. E-mail: [email protected] (J.Y.). Email: [email protected] (L.-x.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China, Grant Nos. 50873113 and 20990224. REFERENCES

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