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Ind. Eng. Chem. Res. 2008, 47, 9636–9643
CO2 Permeability of Polystyrene Nanocomposites and Nanocomposite Foams† Zhihua Guo, L. James Lee, and David L. Tomasko* Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210
Steady state permeability coefficients P of carbon dioxide (CO2) in polystyrene (PS) and PS nanocomposites and their corresponding foams at 0.10 MPa (gauge pressure) and three different temperatures were measured. Permeability coefficients of foams are about 1 order of magnitude higher than those of corresponding plates. Three PS nanocomposites were used: PS + 5% 20A (treated nanoclay provided by Southern Clay), PS + 5% CNFs (carbon nanofibers), and PS + 5% MHABS (nanoclay treated with 2-methacryloyloxyethylhexadecyldimethylammonium bromide). For both plates and foams, the permeability coefficient decreases considerably with addition of nanoparticles. Foam structure is shown to play a more important role than nanoparticles in determining the permeability coefficient values for nanocomposite foams. Introduction Polymer Nanocomposites and Foams. Polymer nanocomposites represent an alternative to conventional filled polymers. In this new class of material, a small quantity (usually 99.9%) was provided by Praxair. Two types of organically modified clays, a commercially available Cloisite 20A (20A for short) and custom modified MHABS, were used to fabricate nanocomposites. Vapor grown CNFs (PR-24-PS, Applied Science Inc.) were pyrolytically stripped to remove the surface organic contamination, claimed by the provider. No further purification was pursued. The average diameter of these CNFs was 100 nm, and the original lengths ranged from 30 to 100 µm. The PS + 20A nanocomposite and PS + CNFs nanocomposite were mechanically blended at 200 °C using a twin-screw extruder (Leistritz ZSE-
PS PS PS PS PS PS PS PS
+ 5% MHABS + 5% 20A + 5% CNFs + 5% MHABS + 5% 20A + 5% CNFs
Ep (kJ/mol) 2.7 3.5 3.1 2.5 1.6 3.0 2.5 1.9
27; L/D ) 40; D ) 27 mm) and pelletized. The exfoliated nanocomposite PS + MHABS (with weight percent of about 20% MHABS) was synthesized from an in situ polymerization and then diluted using pure PS using the same twin-screw extruder at 200 °C and pelletized. The temperatures were chosen for compounding PS + 20A, PS + CNFs, and PS + MHABS such that they were good for processing and minimizing degradation. The transmission electron microscopic (TEM) images of these three nanocomposites were shown in previous publications.2,30 Sample Preparation. All PS, PS + 20A, PS + MHABS, and PS + CNFs were in pellet form (from manufacturer or by pelletizing). The plates were compression molded according to the following procedure. Polymer pellets were placed in a square mold in the hydraulic unit (Model No. 3925, Carver) at 180 °C (for PS, 200 °C for PS + 20A, PS + MHABS, and PS + CNFs; again, the temperatures were chosen to be high enough for smooth surface and minimize degradation). After 30 min of melting, the press (about 10 tons) was applied to compress the pellets into a square shape with thickness of about 0.7 mm. The thickness of the plates (and also foam plates) was measured by a caliper (Mitutoyo Corp.) with an accuracy of 0.01 mm. For foams, polymer plates of about 2 mm thick were compression molded and then sandwiched between steel plates in a high pressure vessel at 13.8 MPa and 100 °C (83 °C was also used to get PS foam with different cell size). The samples were allowed to equilibrate at pressure for 24 h; then the pressure was released quickly (about 15 s) to atmospheric pressure and the bubble growth was stopped by putting the sample in ice-water 60 s after pressure release (the amount of time it took to open the high pressure vessel and take out the samples). Characterization of Foam Structure. A scanning electron microscope (SEM, HITACHI S-4300) was used to observe the cell morphologies of foam samples. Details about sample preparation and image analysis can be found elsewhere.30 Three SEM images (at different spots and with over 50 cells on
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Figure 5. Representative SEM images of PS foams under conditions of (a) 13.8 MPa, 100 °C (cell diameter 82.6 ( 18.4 µm and cell wall thickness 7.0 ( 5.6 µm) and (b) 13.8 MPa, 83 °C (cell diameter 41.5 ( 12.4 µm and cell wall thickness: 13.0 ( 10.9 µm).
Figure 6. Permeability coefficients of CO2 for two PS foams ((a) and (b) prepared at different conditions) at different temperatures (1/T).
material and is picked up by the carrier gas (N2). The gas mixture passes through the sensor (a pressure modulated infrared detection system), which determines the transmission rate. All the samples were big enough to cover the permeation area (a circle with area of 50 cm2). Permeant CO2 (>99.99%) and carrier gas N2 (>99.998%) were provided by Praxair. The pressure for carrier gas N2 was 0.310 MPa (gauge pressure, 45 psig) controlled by a regulator. Its flow rate was 100 sccm (standard cubic centimeters per minute). The pressure of CO2 was 0.10 MPa (gauge pressure, 14.5 psig) controlled by a regulator (also measured accurately by a pressure sensor (Sensotec, Model AG 300)), and the flow rate was 100 sccm. Usually about 2 days was taken to ensure steady state permeation data values. The data have been corrected to standard pressure (0.101 MPa). Results and Discussion
Figure 7. Permeability coefficients with cell diameter/cell wall thickness ratios of PS plates and foams. The PS plate is assumed to be foams with cell diameters of zero; the straight line is the trend line.
the image) were analyzed to get the average cell diameter. The cell number on three SEM images (at different spots and with over 200 cells on the image) were counted to get the average cell number density. Over 100 different spots were chosen (including walls and struts) to get the average cell wall thickness. Because of nonuniform cell morphology, the standard deviations of these evaluations were comparatively large, especially for cell wall thickness. Permeation Measurement. A commercial apparatus (PERMATRAN-C Model 4/41) was used to measure steady state permeation data of CO2. The test gas (CO2) permeates the
To study the effect of nanoparticles on permeation of the polymer matrix, both volume fraction and weight fraction of the nanoparticles have been used. The former is more common because the volume of the filler is directly related to the permeation behavior. In the foam industry, however, the weight fraction of the filler is more frequently used. In this study, the polymer nanocomposites were fabricated based on the same weight fraction (5 wt %) to relate permeation to other properties. The volume fraction of the particles can be calculated based on the density of the nanocomposites and nanoparticles. These results are summarized in Table 2. Here MHABS is assumed to have the same density as 20A, both of which are treated nanoclays. For most literature data on polymer permeation, films with thickness about 0.25 mm (10 mil) or less were used. In our study, the thickness is about 0.7 mm for polymer plates and about 3 mm for foam samples; hence we refer to our samples as “plates” rather than “films”. The sample selection in our study is of more practical interest for our applications as PS is rarely applied in film form. It is well-known that, for most systems, the permeability coefficient is independent of thickness;9 therefore, it is used to compare permeation properties between samples with different thicknesses. Permeability measurements were validated by checking for repeatability and comparing with the available literature data. Table 3 shows the repeatability for PS + 5% 20A plate. Relative errors for foam samples were less than 3%. For the systems we studied, only the permeability coefficient of CO2 in PS is
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Figure 8. Representative SEM images of four foams under the same foaming condition (13.8 MPa and 100 °C): (a) PS, (b) PS + 5% CNFs, (c) PS + 5% 20A, and (d) PS + 5% MHABS. Table 5. SEM Analysis Results of Four Different Foams under Conditions of 13.8 MPa and 100 °C system
sample thickness (mm)
average cell diameter (µm)
average cell wall thickness (µm)
average cell density (cm-3)
PS PS + 5% MHABS PS + 5% 20A PS + 5% CNFs
3.20 ( 0.14 3.50 ( 0.20 3.38 ( 0.25 3.11 ( 0.12
82.6 ( 18.4 41.4 ( 9.9 46.9 ( 12.8 25.1 ( 6.4
7.0 ( 5.6 4.7 ( 2.5 7.7 ( 7.4 3.4 ( 2.4
(1.7 ( 0.2) × 106 (1.1 ( 0.2) × 107 (6.2 ( 1.4) × 106 (1.3 ( 0.7) × 107
available in the literature. Brubaker10 et al. reported 3.7 × 10-8 cm3(STP) · mm/cm2 · s · cmHg (2.8 × 10-16 m3(STP) · m/ m2 · s · Pa) at 30 °C, and Paine24 et al. reported 0.88 × 10-8 cm3(STP) · mm/cm2 · s · cmHg (6.6 × 10-17 m3(STP) · m/ m2 · s · Pa) at 30 °C. Our result, 0.65 × 10-8 cm3(STP) · mm/ cm2 · s · cmHg (4.9 × 10-17 m3(STP) · m/m2 · s · Pa) at 30 °C, agrees reasonably well with Paine’s datum, which is the most recent and widely cited. Permeability Coefficient Measurement Results. Polymer and Polymer Nanocomposite Plates. Transmission rate data can be obtained directly from the apparatus. Figure 1 shows CO2 transmission rates for four different plates at three temperatures.
Figure 9. Permeability coefficients with cell diameter/cell wall thickness ratios of PS and nanocomposite plates and foams at 25 °C. The PS and nanocomposite plates are assumed to be foams with cell diameters of zero.
For all systems the transmission rate increases with increasing temperature. According to eq 1, the permeability coefficient can be calculated from the transmission rate, plate thickness, and pressure difference across the sample. The upstream CO2 pressure was measured by the pressure gauge and kept constant. The partial pressure of CO2 on the sweep (permeate) side can be reasonably assumed to be 0. Figure 2 shows the relationship between permeability coefficients and the reciprocal of absolute temperature. A log-linear relationship is observed in agreement with eq 4. When comparing the permeability coefficient of PS and three different PS nanocomposites, we found that PS has higher values than PS nanocomposites. However, there was not a significant difference between the three nanocomposites. The reason for the slight difference between them might be the dispersion of nanoparticles in the polymer matrix. In situ polymerization was used to synthesize highly exfoliated PS + MHABS nanocomposite, and melt blending was used to fabricate intercalated PS + 20A and PS + CNFs nanocomposites. For intercalated nanocomposites, polymer chains penetrate into clay layers. However, for exfoliated nanocomposites, the clay is completely delaminated. Exfoliated nanocomposites can achieve better property improvement per unit weight.2 Here, PS + MHABS nanocomposite has a better barrier property than PS + CNFs and PS + 20A. The permeation reduction by nanoparticles can be seen more clearly when the data are plotted as relative permeability, defined as the ratio of permeability coefficients of the PS nanocomposite plate to the pure PS plate. Similarly, the relative permeability of PS nanocomposite foam can be defined as the ratio of permeability coefficients of the PS nanocomposite foam to the pure PS foam. Figure 3 shows relative permeability coefficients at 25 °C for the three PS nanocomposite plates and foams.
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Figure 10. Permeability coefficient values of PS and nanocomposite plates and foams at 25 °C. The effective polymer thickness was used for the foams; the white bars represent plates and gray bars represent foams. The first gray bar for PS is calculated from PS foam a, and the second gray bar for PS is calculated from PS foam b.
Messersmith15 et al. used in situ polymerization to synthesize a nanocomposite with full delamination (exfoliated) and a solvent casting technique to form nanocomposite films with oriented silicate layers. Their results showed that the permeability of the PCL + OMTS (poly(ε-caprolactone) + organically modified mica-type silicate) nanocomposite was reduced 11% and 39% by 3 wt % and 6 wt % of nanoclay, respectively. A roughly linear relationship between relative permeability and fraction of nanoclay was observed. In our work, in situ polymerization was also used to synthesize the exfoliated PS + MHABS nanocomposite. Figure 3 shows an approximate 33% reduction of the permeability coefficient with 5 wt % MHABS, which agrees well with the literature data. In addition, several models33-35 were developed to estimate the reduction in permeability caused by the addition of impermeable particles of different aspect ratios to polymers. The Maxwell model33 is a frequently used one, which is strictly applicable to dilute dispersion of spherical fillers. For impermeable fillers, it has the following expression:36 P ) Pp
1 - φf 1 + φf/2
(6)
where P is permeability coefficient of the composite, Pp is for the polymer, and φf is the filler volume fraction. By this model, the permeability coefficients for PS + 5% CNFs and PS + 5% nanoclay (assuming that 20A and MHABS have the same density for the volume fraction calculation) are 6.18 × 10-9 and 6.26 × 10-9 cm3(STP) · mm/cm2 · s · cmHg at 25 °C, respectively. The former is lower because CNF has a lower density, therefore higher volume percentage in the composite. The Maxwell model cannot estimate the extent of reduction mainly because of the spherical particles assumption for our systems. Actually, other models rely on well-distributed (i.e., patterned) particles and would not be applicable to these irregular distributions formed in our samples. In addition, agglomeration and wide distribution of the nanoparticle dimension also increase the difficulty of modeling. Polymer and Polymer Nanocomposite Foams. Figure 4 shows the permeability coefficients of four different foams at three different temperatures. A trend similar to that of the polymer nanocomposite plates (Figure 2) was observed except that PS + 5% CNFs foam has lower permeability coefficient values than both PS + 5% 20A and PS + 5% MHABS foams.
The trend is also shown by the relative permeability of foams in Figure 3. It should be related to the foam morphology, including the cell size distribution, wall thickness, and strut thickness, as well as nanoparticle dispersion. Comparing Figures 2 and 4, we can conclude that permeability coefficients of foams are about 1 order of magnitude higher than their corresponding plates. The reason is that CO2 needs to permeate much less polymer in the foam than in the plate for the same thickness of sample. Temperature Effect. The results in Figures 2 and 4 demonstrate that log P is linear with reciprocal temperature, which is widely accepted.9 The activation energy for permeation Ep can be calculated from the plot of log P-1/T and is provided in Table 4. According to eq 5, Ep can be represented as the sum of ∆HS and Ed. Ed is always positive because the diffusion coefficient always increases with increasing temperature. The sign of ∆HS can be positive or negative.9 For permanent gases, such as O2 and N2, ∆HS is positive; for condensable vapors such as water, ∆HS is negative. Therefore, Ep is always positive for permanent gases and can be positive, negative, or close to zero for condensable vapors. CO2 can be considered as a permanent gas in our measurement conditions; therefore a positive Ep is expected. PS nanocomposite plates and foams (except PS + 5% CNFs plate) have higher Ep values than PS and PS foams, respectively. Between plates and foams, Ep values are higher for plates than for corresponding foams, indicating a bigger temperature dependency of Ep values in plates. Foam Morphology. As shown in Figure 3, the permeation reduction from PS plate to PS nanocomposite plates is different from the reduction from PS foam to PS nanocomposite foams. This implies that the foam morphology plays an important role and should also be appropriately addressed. To explore the effect of foam morphology on permeation, we studied pure PS foam with different cell sizes (and therefore cell densities and cell wall thicknesses) foamed under different conditions. Foam samples were also prepared under conditions of 13.8 MPa, 83 °C. PS foam samples prepared under this condition are called “foam b”, and PS foam samples prepared under 13.8 MPa, 100 °C, are called “foam a”. Basically, the same initial CO2 pressure was maintained but different temperatures were used. Different temperatures led to different initial CO2 solubilities in the PS matrix and also pressure drop rates due to different initial CO2 densities. Both initial solubility and pressure drop rate affect
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foam morphology directly. Figure 5 gives representative SEM images of PS foam a and foam b. The images show that bubble size increases with increasing temperature. CO2 has lower solubility in PS under higher temperature, resulting in larger cell size. Figure 6 shows the permeability coefficients of CO2 for the two PS foam samples. We assume that when CO2 permeates through the foam sample, only the polymer (i.e., cell walls and struts) needs to be considered as the barrier. The inside of each cell, filled with CO2 or air, contributes negligibly as a barrier. For simplicity, we also assume that CO2 permeates on a straight route through the foam regardless of the nature of the barriers along that route. Then we expect to see a linear relationship in the plot of P vs cell diameter/cell wall thickness ratio. Figure 7 shows a linear trend and verifies these assumptions. R2 ) 0.9893 is reasonably good considering the cell structure of these foams, with a fairly wide distribution of cell size and cell wall thickness. In this plot, the plate was assumed to be a foam with a cell diameter of zero. Figure 8 shows representative SEM images of four different foams under the same foaming condition (13.8 MPa and 100 °C). Table 5 gives the SEM analysis results of cell diameter, cell wall thickness, and cell density. Similar to Figure 7, the PS nanocomposite plates were assumed to be foams with cell diameters of zero. The results of the PS nanocomposite plates and foams were added in Figure 7 and are shown in Figure 9. In general, all these data points follow this correlated line in Figure 7. This result suggests that foam structure (different cell diameter/cell wall thickness ratios) plays a more important role than nanoparticles in determining the permeability coefficient values for nanocomposite foams. Alternately, since we assumed that only the polymer was considered as the barrier for permeation, the effective polymer thickness in the foams can be used to calculate the permeability coefficients. The effective polymer thickness is the thickness of polymer (i.e., cell wall) in the permeability measurement direction. We evaluate this value by using the thickness of the foam sample and cell diameter/cell wall thickness ratio from the SEM image analysis. Figure 10 shows the results at 25 °C, where the white bars represent plates and gray bars represent foams. The first gray bar for PS is calculated from PS foam a, and the second gray bar for PS is calculated from PS foam b. Ideally, the white bars and gray bars should have equal heights for each system, and the discrepancy is likely due to the wide distribution of cell diameter and wall thickness. Conclusions A commercial apparatus was used to measure steady state permeability coefficients (P) of CO2 in PS and PS nanocomposites and their corresponding foams at three different temperatures. Based on the experimental results, the following conclusions can be drawn. For both foams and plates, good linear relationships are shown between log P and 1/T. Permeability coefficients of foams are about 1 order of magnitude higher than those of plates. For both plates and foams, nanoparticles considerably decrease permeation values. However, there is no significant difference in P values between the three nanocomposites. Foam structure is shown to play a more important role than nanoparticles in determining the permeability coefficient values for nanocomposite foams. Acknowledgment The authors would like to thank Owens Corning, Plaskolite, and Southern Clay for material donations. Financial support from
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ReceiVed for reView January 2, 2008 ReVised manuscript receiVed August 30, 2008 Accepted September 19, 2008 IE8000088
