Article pubs.acs.org/IECR
Solubility and Diffusivity of CO2 in Isotactic Polypropylene/ Nanomontmorillonite Composites in Melt and Solid States Dongdong Hu, Jie Chen, Shaojun Sun, Tao Liu,* and Ling Zhao State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *
ABSTRACT: The solubility and diffusivity of CO2 in polymer matrix play crucial roles in controlling the nucleation and growth of bubbles in the polymer/CO2 foaming process. In this work, the effects of polymeric aggregation states (solid and melt states) and layered filler with one-dimensional nanostructures on the solubility and diffusivity of CO2 in isotactic polypropylene (iPP)/ nanomontmorillonite (nano-MMT) composites were investigated. The results show that the solubility in the iPP composites has little change with an increase of nano-MMT concentration while the diffusivity of CO2 decreases with increasing nano-MMT content. Considering the effects of a one-dimensional nanostructure filler and a polymeric crystalline region on the free volume and the diffusion path, two models on the basis of the free volume theory were established and used to well correlate the experimental diffusion coefficients of CO2 in melt-state iPP composites and predict the CO2 diffusivity in solid-state iPP composites. have focused on swelling in the polymer/CO2 system,26−28 and the solubility and diffusion coefficient of CO2 in different pure polymers.22,29−34 However, very few research results have been reported on CO2 solubility and diffusivity in polymer composites.35 Thus it is meaningful to survey the thermodynamic properties including swelling, solubility, and diffusivity in the polymer composite/CO2 solution. Nanoclay is a common reinforced filler for polymer composites. The intercalation and peeling of polymer chains have been proven to be a successful approach to achieve nanoscale dispersion of the filler in one dimension.36,37 Nanomontmorillonite (nano-MMT), a common nanoclay, is a kind of layered alumino silicate clay with a wide distribution and abundant storage in nature as well as low cost. Jiang et al.21 used two types of montmorillonite as a filler to explore their effects on the iPP composites foaming. The results showed that the better the nano-MMT dispersed, the more nucleating sites could be provided, leading to a more uniform cell structure. Meanwhile, the cell density increased and the average cell size decreased noticeably. Some other polymer/nanoclay composites were also used for the preparation of polymer composite foams including PS/nanoclay,38 HDPE/nanoclay,39 PLA/nanoclay,40 PC/nanoclay,41 and PU/nanoclay42 composites, etc. Therefore, fundamental research on the solubility and diffusion coefficient of CO2 in the polymer/nano-MMT composite is very meaningful to understand the effect of one-dimensional nanoscale layered filler on the foaming mechanism and foams morphology. On the basis of the aggregation state, semicrystalline polymer foaming can be generally classified as solid-state or melt-state. The “solid state” in this work refers to merely an operating
1. INTRODUCTION The microcellular foaming process proposed by Suh et al.1 in the early 1980s is aimed at producing novel high-performance polymeric foams. It is an approach that reduces materials consumption while retaining mechanical properties2−5 and has extensive application prospects in thermal insulation, cushioning, and construction, etc.6,7 However, how to maintain the performance of foams is a great challenge in developing the microcellular foaming process. Polymer composites incorporated with inorganic fillers (e.g., talcum, CaCO3, silica and clay) have been frequently used to improve the mechanical and thermal properties of a polymer at a low cost.8−13 Therefore, in recent years, the foams of polymer/inorganic filler composites have drawn extensive attention in the academic and industrial fields due to their better thermal stability, enhanced mechanical properties, and improved flammability over the pure polymer foam.14−16 As a physical foaming agent, CO2 has been employed in many foaming processes17−21 because of its excellent properties (e.g., nontoxic, nonflammable, and easily obtained).22 The solubility and diffusion coefficient of CO2 in a polymer matrix is vitally important to the nucleation and growth of bubbles in the foaming process, optimizing the CO2 microcellular foaming process and regulating the structure of the foam that ultimately affect the product performance.23−25 The gas solubility in the foaming process will affect the nucleation rate of bubbles and the cell density of the end product to a great extent while the gas diffusivity will control the growth rate of bubbles and affect the cell size. Thus, the investigation of the impact of fillers on gas solubility and diffusion coefficient in polymer is of vital practical significance for understanding and controlling the foaming process. The pressure−volume−temperature (PVT) relation is very important for obtaining solubility and diffusion coefficient data.26 When CO2 diffuses into a polymer and causes it expansion, the amount of swelling can be characterized indirectly by its PVT property. In the past decade, many studies © 2014 American Chemical Society
Received: Revised: Accepted: Published: 2673
October 23, 2013 January 22, 2014 January 30, 2014 January 30, 2014 dx.doi.org/10.1021/ie403580x | Ind. Eng. Chem. Res. 2014, 53, 2673−2683
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Table 1. Formulation of Various iPP/nanoMMT Samples sample
substance
iPP (wt%)
iPP-g-MA (wt%)
nano-MMT (wt%)
1 2 3
iPP/iPP-g-MA (9:1) iPP/iPP-g-MA/2% nano-MMT iPP/iPP-g-MA/4% nano-MMT
90.0 88.2 86.6
10.0 9.8 9.4
2 4
Figure 1. TEM images of iPP and iPP/nano-MMT composites: (a) sample 1; (b) sample 2; (c) sample 3.
2. EXPERIMENTAL SECTION 2.1. Materials. Isotactic polypropylene (iPP) was supplied by Sinopec Shanghai Petrochemical Co. (Item No. Y1600, Mw = 197 kg/mol, and melt flow index = 1.6 g/min). Nano-MMT (I.44P) with the density of 1.7 g/cm3 was provided by Nanocor Chemical Co., America. CO2 with a purity of 99.99 wt % was purchased from Chenggong Gas Co., China. iPP graft maleic anhydride (iPP-g-MA, the MA content = 0.6 wt %), prepared by scCO2-assisted solid-phase grafting modification, was used as a compatibilizer in the preparation of composites. The content of MA in iPP-g-MA was estimated by determining acid anhydride content using a neutralization titration method. In the measurement, the anhydride groups were hydrolyzed completely and then the acid content was measured by titration. The detailed determination procedures of the MA content have been presented in our previous report.48 All the materials were used in the as-received condition. 2.2. Preparation and Characterization of iPP/nanoMMT Composites. iPP and nano-MMT filler were meltcompounded to prepare the iPP/nano-MMT composites by using HAAKE MiniLab (Thermo Fisher Scientific Inc., America). Simultaneously, iPP-g-MA was added into the mixtures to improve the interface compatibility and eliminate voids between the iPP matrix and nano-MMT filler. For comparison, the blend of iPP and iPP-g-MA (the mass ratio of 9:1) was also prepared under the same preparation conditions. The detailed formulations of the samples used in this work are listed in Table 1. iPP, iPP-g-MA pellets, and nano-MMT powder, preliminarily vacuum dried 24 h at 80 °C for anhydration, were premixed sufficiently and fed simultaneously to the inlet of a HAAKE MiniLab for thorough mixing. The operation parameters of the MiniLab were set as follows: screw speed of 50 rpm, mixing time of 10 min, mixing temperature of 190 °C, and nitrogen atmosphere at 0.6 MPa. The products were collected at the discharge port and then thermopressed at 190 °C subsequently in a cylindrical mold with a height of 10 mm and a radius of 3 mm for the swelling measurement and in a sheet mold with a radius of 12 mm and a thickness of 2 mm for the solubility and diffusivity measurements. The morphologies of iPP/nano-MMT samples were characterized by transmission electron microscopy (TEM) (JEM-2100, Japan) to survey the dispersion of nano-MMT in the mixtures. The TEM images shown in Figure 1 show that
temperature between melting temperature (Tm) and glass transition temperature (Tg) of the polymer.43 In this case, polymer chains may be partially moved but the bubble growth would be restricted by the crystalline region and the hard layer in the amorphous region, resulting in small foam size and expansion ratio of the product.44,45 However, in the melt-state foaming, the movement of polymer chains is free and the polymer crystalline regions disappear so that the bubble growth is free, leading to bigger foam size and expansion ratio compared with the solid-state method. Each foaming method has its own advantages, disadvantages, and different application purposes for foam products.46,47 Polymers with low melt strength in the melt state have a weak ability to hold a bubble, so that the bubbles are easily broken. Therefore, the solid-state foaming is suitable for these polymers, while the melt-state foaming is only suitable for the high melt strength ones. Meanwhile, the appropriate foaming method could be selected according to the expansion ratio of the product needed. Thus, it is meaningful to survey the effect of aggregation state on the solubility and diffusivity of CO2, and thereby it is helpful to understand the different foaming processes. This work aims at investigating the effects of layered fillers with one-dimensional nanostructure and polymer aggregation state on the solubility and diffusion coefficient of CO2 in iPP composites containing 2% and 4% nano-MMT in solid state (120 °C) and melt state (200 °C). The swelling ratios of iPP/ nano-MMT samples were measured by using a high-pressure visible device under the conditions of 120 and 200 °C and CO2 pressures from 5 to 22 MPa. Subsequently, combined with the swelling data, the solubilities of CO2 were accurately calculated by the apparent data obtained from magnetic suspension balance (MSB, Rubotherm Prazisionsmesstechnik GmbH, Germany) tests under the corresponding conditions. Meanwhile, the diffusion coefficients of CO2 at 5−10 MPa were calculated by the curves of the sorption process in the iPP/ nano-MMT samples. The free volume models, considering the filler shape and the aggregation state of polymer matrix, were proposed to correlate the CO2 diffusion coefficients in meltstate iPP composites and predict that in the solid-state iPP and iPP/nano-MMT samples, respectively. The diffusion coefficients obtained by experiment and prediction were compared to verify the reliability of the model. 2674
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nano-MMT fillers are dispersed in the iPP matrix uniformly with one-dimensional nanostructure. CO2 is insoluble in crystalline regions of semicrystalline polymers as demonstrated in Michaels and Bixler’s work.49 The solubility of CO2 in the amorphous regions of iPP in the solid state could be calculated after determining the crystallinity of the sample. The melting behaviors of iPP and iPP/nano-MMT composites were studied at CO2 pressures from 0.1 to 10 MPa by using a high-pressure differential scanning calorimeter (DSC, NETZSCH 204 HP, Germany) to make sure if the samples remained in the solid state at 120 °C under compressed CO2 atmosphere. The crystallinities of iPP and iPP/nano-MMT composites before and after CO2 treatments were also measured under atmospheric nitrogen by using the DSC to inspect the effect of CO2 treatment and the filler on the crystallinities of samples. Before the DSC measurement, the samples were dried in a vacuum oven for 24 h at 80 °C. To determine the CO2-induced depression in melting temperature (Tm) in each DSC measurement, about 10 mg of the sample was placed into the high-pressure chamber which would be pressurized by CO2 up to the test pressure. The sample was kept in the chamber for 5 h at 120 °C to sufficiently absorb CO2, and subsequently heated to 200 °C at a heating rate of 10 °C/min. The melting onset temperature (Tonset), Tm and melting enthalpy (ΔHm) of the sample were measured by the start point, maximum, and area of the melting curve, respectively. The crystallinity (wcry, weight fraction) of iPP sample can be calculated by using the following equation: wcry =
ΔHm ΔHm0
×
100% θ
CCD camera, and the inner cross-sectional area (s) of the cylindrical container. For iPP/nano-MMT samples, the iPP matrix volume V(P, T, wCO2) needs to be calculated from the observed volume in the image deducted by that of the filler. Assuming no swelling in the filler, the space occupied by the filler can be calculated from the mass of iPP/nano-MMT samples, msam, the density of the filler, ρf, and the weight percentage of the filler, wf. V0(P, T) could be obtained by the mass of the composites and the iPP density ρ0(P, T), which was estimated by the Tait equation.26 ρ0 (P , T ) =
(1)
(3)
2
× [Vsam(P , T , wf ) + Vbas] + ρCO (P , T ) × ΔVswell(P , T )]/[mpp] 2
= [Wt(P , T ) − W0(0, T ) + ρCO (P , T ) × [Vsam(P , T ) + Vbas] 2
+ ρCO (P , T ) × [Vsam(P , T ) 2
− msam × wf /ρf × Sswell(P , T )]] /[msam × (1 − wf )]
(4)
where W0(0, T) is the initial reading of MSB with loading of the sample, and Wt(P, T) stands for the reading value after dissolving CO2. ρCO2(P, T) and Vsam(P, T) are CO2 density and the sample volume, respectively. Vbas refers to the sample basket volume with related accessories, which is constant during the measuring. All these data could be measured in a separate blank MSB test under the condition of temperature T and pressure P. The last term in the equation, ρCO2(P, T) × ΔVswell(P, T), stands for the correction term of swelling, which will be calculated from the swelling ratio, Sswell(P, T), the density (ρf), and weight percentage (wf) of filler at the measurement condition. The CO2 diffusivity in iPP/nano-MMT samples were simultaneously investigated from the sorption curves by characterizing the continuous change of the sample mass versus sorption time t. In this work, the diameter of the samples has the same size as the inner diameter of the sample basket. Considering the swelling of sample when CO2 is dissolved,
V (P , T , wCO2) V0(P , T ) s × h − msam × wf /ρf msam × (1 − wf )/ρ0 (P , T )
7.46 × 106 (6.45 × 109) + P
SCO2(P , T , wCO2) = [Wapp + ρCO (P , T )
where stands for the enthalpy of crystallization per gram of 100% crystalline iPP and ΔHm refers to the enthalpy of crystallization per gram of sample. θ is the weight fraction of iPP in the composites. The value of ΔHm0 is 209 J/g.50 2.3. Swelling Measurement. When the CO2 pressure is relatively high, the swelling degree could be considerable due to the sorption of CO2 in iPP, and its effect on the solubility of CO2 is very evident and cannot be neglected. The swelling ratios of the iPP and iPP/nano-MMT samples in compressed CO2 were measured at CO2 pressures up to 22 MPa under the conditions of 120 and 200 °C by using a high-pressure visible device. The detailed determination procedures of the swelling ratios had been presented in our previous publication.31 By measuring the captured change in the samples volume, the swelling ratio will be calculated by the difference before and after CO2 sorption. As the filler of nano-MMT is stuffed and the change in volume can be negligible at the test conditions, the volume is expanded only due to the swelling of the iPP matrix for the iPP/nano-MMT samples. The definition of the swelling ratio is given by the following equation:
=
+
where the units of density, pressure, and temperature are kg/ m3, Pa, and °C, respectively. Considering the very low MA content, the density of iPP/iPP-g-MA would be estimated by using the value of pure iPP.26 2.4. CO2 Solubility and Diffusion Coefficient Measurement. The apparent solubilities of CO2 in iPP/nano-MMT composites were measured directly by using MSB at CO2 pressures from 5 to 22 MPa and at 120 °C for the solid-state samples and 200 °C for the melt-state samples. The details of the MSB apparatus and measurement procedures used have been presented in previous literature.51 The real solubilities of CO2 (unit: g CO2 /g polymer), SCO2(P, T, wCO2), can be evaluated from the measured data of apparent CO2 mass dissolved in the matrix, Wapp, with the correction of swelling, as shown in eq 4:
ΔHm0
Sswell(P , T ) =
1 1.06 × 102T (9.86 × 107) + P
(2)
where V0(P, T) stands for the volume occupied by iPP matrix of the polymer composites without CO2 swelling while V(P, T, wCO2) is the volume after CO2 swelling. For iPP/iPP-g-MA, V(P, T, wCO2) can be estimated from the sample height (h) in CO2 atmosphere after swelling in the image captured by the 2675
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that the samples should remain in the solid state at 120 °C under the experimental conditions (5−22 MPa CO2). The samples after treatment at 120 °C and 22 MPa CO2 pressure also observed no deformation, indicating that they remained in the solid state. Not only the nano-MMT fillers but also the crystalline domain is CO2 insoluble. Much of the literature44−46 has shown that the adsorption of CO2 could induce the crystallization in the amorphous region of iPP at the temperature between the glass transition temperature (Tg) and Tm. Before the solubility determination, the crystallinity of iPP and iPP/nano-MMT samples treated at 120 °C and different pressures of CO2 was measured using DSC, and the results are given in Table 2. The crystallinity of the treated samples remained almost unchanged compared to the untreated ones.
there was almost no gap between the sample and the inner wall of the basket. More importantly, the other two dimensions of sample sheet were far larger than that of sample thickness, resulting in that the vast majority of CO2 diffused into iPP matrix through the plane face and hardly through the edge. Therefore, this diffusion can be approximated as one-dimensional, with a constant surface concentration of CO2.12,28,51 Assuming the sample thickness remains unchanged, Fick’s second law of one-dimensional diffusion could be applied to the quantitative characterization of CO2 diffusion process in iPP and iPP/nano-MMT composites.52 The diffusion coefficients were measured at 10 MPa or less. Moreover, a stepwise increase in CO2 pressure was employed at 1 MPa/step to minimize the effect of swelling on the sample. Then, the diffusion process could be described by the following equation:53 Mt 8 =1− 2 M∞ π
∞
∑ n=0
⎡ −(2n + 1)2 π 2Dt ⎤ 1 exp ⎥ ⎢ ⎦ ⎣ (2n + 1)2 4L2
Table 2. Crystallinity of the Solid-State iPP and iPP/nanoMMT Composites Treated in CO2 at 120 °C (5)
sample
where D refers to the diffusion coefficient of CO2. The thickness of the sample, L, is a calculation of the sample volume after the swelling correction divided by the bottom area of sample basket. Mt and M∞ are the amount of CO2 which has dissolved into the sample at time t and t = ∞, respectively. t = ∞ represents a sorption equilibrium in a diffusion measurement with stepwise pressure change.
treated CO2 pressure, MPa
1 5.0 10.0 15.0 22.0 2 5.0 10.0 15.0 22.0
3. RESULTS AND DISCUSSION 3.1. Thermophysical Properties of iPP and iPP/nanoMMT Composites. Although systematic research has been carried out on the sorption of CO2 into melt-state iPP, literature on that into solid-state iPP is very limited.30,54 Considering that the plasticization effect of CO2 on polymers may reduce the melting temperatures of the samples, the melting behaviors of the iPP and iPP/nano-MMT composites under compressed CO2 were characterized by using highpressure DSC to make sure if the samples remained in solid state at 120 °C. Figure 2 shows that the melting temperatures,
3 5.0 10.0 15.0 22.0
wcry, wt % 47.8 47.5 47.4 47.6 47.9 46.4 46.8 46.7 46.3 46.4 46.2 46.5 46.4 46.3 46.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.2 0.3 0.3 0.4 0.2 0.4 0.3 0.4 0.5 0.3 0.4 0.2 0.3 0.4 0.2
3.2. Swelling of iPP/Nano-MMT Composites in Melt and Solid States under CO2. The swelling of the iPP and iPP/nano-MMT samples was experimentally measured at 120 and 200 °C under different CO2 pressure. The results show that the experimental swelling of the iPP composites/CO2 solutions in both states increased with CO2 pressure due to the dissolution of CO2, as illustrated in Figure 3. Meanwhile, the swelling of iPP and iPP/nano-MMT composites in the solid state was obviously lower than that in the melt state at the same
Figure 2. Melting temperature of the samples versus CO2 pressure.
Tm, and the melting onset temperatures, Tonset, of the iPP and iPP/nano-MMT almost decreases linearly with an increase in CO2 pressure from 0.1 to10 MPa, which is expected from the Flory−Huggins theory. The extrapolated straight line in Figure 2 indicates that the melting onset temperature of the iPP/nanoMMT is higher than 120 °C under 22 MPa CO2, which means
Figure 3. The swelling of iPP and iPP/nano-MMT samples versus the CO2 pressure at 120 and 200 °C. (The solid lines are the results of second-order polynomial fitting.) 2676
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acting as the lubricant and resulting in an increase of free volume and decrease of the shear viscosity.35 However, for the layered nanofiller, that is, nano-MMT in Figure 5b, the rotation of layered filler might lead to the entanglement of the vicinal molecular chains and result in the decrease of free volume and increase of the shear viscosity. 3.3. Solubility of CO2 in Melt- and Solid-State iPP/ nano-MMT Composites with Swelling Correction. The actual solubilities of CO2 in amorphous regions of the iPP and iPP/nano-MMT composites at 120 and 200 °C and CO2 pressures from 5 to 22 MPa were calculated by correcting the apparent solubilities with the swelling data under the corresponding conditions, as illustrated in Figure 6. The CO2
CO2 pressure. On the one hand, CO2 cannot enter the crystalline regions of the solid-state iPP and naturally cannot make the iPP matrix swell, leading to the reduction of the swelling in the solid-state composites compared with the meltstate ones. On the other hand, polymer chains were partly moveable in the solid state while completely moveable in the melt, which also resulted in less free volume as well as the swelling ratio of the solid-state samples. The swelling data of the samples were fitted with second-order polynomials so that the swelling correction under different CO2 pressures could be calculated for the solubility determination. The results also show that the swelling of the samples is weaker than that of pure iPP and decreases with an increase of nano-MMT loading at the same experimental conditions. The rheological behaviors of the samples were investigated at 200 °C with relatively low shear rates from 0.1 to 100 s−1 using the Haake rheology system (Mars III, Thermo Fisher Scientific Inc.). Figure 4 presents the changes in the viscosity with the
Figure 6. Solubility of CO2 in the amorphous regions of iPP and iPP/ nano-MMT composites with experimental swelling correction.
solubilities in iPP and iPP/nano-MMT increase almost linearly with increasing CO2 pressure, which indicates that CO2 only dissolves in the iPP matrix and the sorption process complies with Henry’s law. The results also suggest that the CO2 solubilities in the iPP/nano-MMT composites have little change with an increase of nano-MMT concentration compared with those in pure iPP at the same conditions. Looking into the reason of it, in the polymer matrix of the composites, the free volume divides into two parts, the static one and the dynamic one. The static free volume, the unoccupied volume in iPP, almost does not change, which directly relates to the amount of CO2 dissolved in iPP/nanoMMT.58 Thus, the solubilities of CO2 almost keep unchanged with the increase in filler loading. Meanwhile, as can be seen from Figure 6, the solubilities of CO2 in the amorphous regions of the solid-state iPP and iPP/ nano-MMT composites are higher than those of melt-state ones at the same CO2 pressure, especially at the high CO2
Figure 4. Change in viscosity of iPP and iPP/nano-MMT composites with shear rate at 200 °C.
shear rates. The viscosity of iPP composites increases with the content of nano-MMT. Similar viscosity increases in composites filled with a small content of nanoclay (
