Using Supercritical Carbon Dioxide as Foaming Agent

Jan 20, 2014 - Foaming Agent: Influence of Crystallinity and Spherulite Size on Cell ... Large spherulites evolve into entities surrounded by elongate...
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Foaming of Poly(lactic acid) Using Supercritical Carbon Dioxide as Foaming Agent: Influence of Crystallinity and Spherulite Size on Cell Structure and Expansion Ratio Lin-Qiong Xu and Han-Xiong Huang* Lab for Micro Molding and Polymer Rheology, The Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: Using supercritical carbon dioxide (Sc-CO2) as physical foaming agent, foamed poly(lactic acid) (PLA) samples were prepared in a batch process via constant- and varying-temperature modes (CTM and VTM). Their crystallinity, cellular structure, and expansion ratio were investigated. In the CTM, the samples foamed at low saturation temperatures present three regions (skin, inner, and core regions). The uniformity of cellular structure is much improved with increasing saturation temperature. In the VTM, saturation temperature exerts a significant impact on the size of spherulites formed in gas saturation stage. Large spherulites evolve into entities surrounded by elongated cells or submicro-sized cells in interlamellar regions after foaming at 140 °C and into small cells (mean diameter of 0.6 μm) at 160 °C, whereas small spherulites generally evolve into stamen-like cell structure at 140 °C. Interestingly, uniform cellular structure with high expansion ratio (49.8) or bimodal cellular structure can be obtained by tuning saturation and foaming temperatures.

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INTRODUCTION Biobased and biodegradable polymers have attracted great attention because of increasing concerns over the environmental influence and sustainability of petroleum-based polymer materials.1 Poly(lactic acid) (PLA), a biobased and biodegradable aliphatic polyester, displays high rigidity and is thought as one of the most promising replacement materials for petroleum-based polymer. The brittleness of PLA is a major defect for several applications, such as packaging applications.1,2 Microcellular foaming is an effective method to improve the toughness of PLA.2 In addition, PLA foams are used in biological and medical applications, such as tissue engineering, because of the good biocompatibility of PLA.3−6 PLA is a semicrystalline polymer and so has more complicated microcellular foaming behavior compared with amorphous polymers owing to the presence of crystals.7−16 The crystals not only depress the solubility of carbon dioxide (CO2) in the polymer14 but also dramatically affect the cell nucleation and growth. In the gas saturation stage, the CO2 exposure significantly enhances the crystallization of semicrystalline polymer,7−9,14 and the growing crystals exclude CO2 to the interface of the crystals and amorphous phase.11,14 The excluded CO2 accumulates at the interface, which results in less Gibbs free energy necessary for nucleating a stable cell at the interface than that required for homogeneous nucleation in the amorphous phase,17,18 leading to the preferential nucleation of cells at the interface.11 In the bubble growth stage, crystallization of the polymer facilitates to stabilize the cell structure, thus expanding the foamability window.7,15 High crystallinity, however, is not favorable to the expansion of semicrystalline polymer owing to the high stiffness.8,9,12,16 The previous researches focused on the effects of the dissolved CO2 on the crystallization behavior and of the resultant crystallinity on the foaming behavior of PLA.7−9,14−16 For example, Zhai et al.8 investigated the crystallization and foaming behaviors of the © 2014 American Chemical Society

PLA using CO2 as the physical foaming agent. The results showed that the PLA exhibits very low crystallization kinetics under atmospheric pressure, whereas CO2 exposure significantly increases its crystallization rate. The expansion ratio of the PLA foams significantly increases with increasing the saturation time within 10 min and then quickly decreases with further increasing saturation time, which is attributed to the fact that the PLA exhibits high crystallinity when the saturation time is longer than 10 min. Liao et al.9 investigated the effect of crystallinity on the foaming behavior of poly(L-lactic acid) (PLLA) samples prepared by saturating with CO2 at gas pressures ranging from 0.1 to 5.5 MPa. Higher pressure leads to a higher crystallinity, resulting in higher cell density and smaller cell diameter. Various cellular structures, such as skin-core, layered, and interconnected structures, are obtained by tuning the crystallinity in PLLA. The presence of crystals in semicrystalline polymer results in the heterogeneous structure and concomitantly nonuniform dispersion of CO2 in the polymer. Jiang et al.19 investigated the effect of crystal structure on the foaming behaviors of an isotactic polypropylene (iPP) during foaming using supercritical CO2 (Sc-CO2) as a foaming agent. The results showed that microcells appear at the centers of spherulites, in the amorphous domains located in between spherulites, and in the interlamellar regions of spherulites of iPP. As for foamed PLA samples, Zhai et al.8 observed some spherical particles with a diameter of about 30−40 μm in the junction of cells. In addition, peculiar cell rupture features such as highly cavitated cell walls were found in the semicrystalline PLA by several researchers.7,13 However, to the best knowledge of the authors, Received: Revised: Accepted: Published: 2277

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there is a lack of research on the cellular structure for the PLA with different sizes of spherulites.20 Therefore, in this work, the PLA foams were prepared using Sc-CO2 as blowing agent in a batch process via depressurization. The emphasis was put on the foaming behaviors for the PLA with different sizes of spherulites formed at different saturation temperatures using constant- and varying-temperature modes (CTM and VTM).

(Quanta 200, FEI, Eindhoven, Holland) at an accelerating voltage of 15 kV. The mean cell diameter (D) and cell density (ρc) of the foamed samples were estimated by ∑ dini ∑ ni

(1)

EXPERIMENTAL SECTION Materials. PLA (grade 2002D, NatureWorks LLC) with Dlactic acid monomer content of about 4%,7 melt flow index of 8.8 g/10 min (2.16 kg, 210 °C), and density of 1.24 g/cm3 was used in this work. Industrial CO2 with a purity of 99.5% was used as a foaming agent. Sample Preparation. PLA pellets were dried at 90 °C for 2 h in a vacuum oven. Then, they were compression molded into sheets with a thickness of 4 mm at 180 °C and 15 MPa for 10 min, followed by cooling the mold by water. These sheets were cut into cubes with a length of about 10 mm and a width of about 4 mm. The cubic PLA samples were foamed with a batch foaming apparatus via a pressure quenching manner by using Sc-CO2 as the physical foaming agent. The batch foaming apparatus consists of a high-pressure vessel and a syringe pump (ISCO 500D).21 The foaming procedure was as follows: (1) Heat the vessel to saturation temperature within about 15 min. (2) Place the aforementioned samples in the vessel. (3) Slowly flush the vessel using CO2 gas. (4) Pressurize the vessel to foaming pressure (14 MPa) using high pressure CO2. (5) Use two different temperature modes, that is, CTM and VTM (as shown in Figure 1). (6) Depressurize the vessel to atmospheric

⎛ NM2 ⎞3/2 ρc = ⎜ ⎟ ⎝ A ⎠

(2)

D=

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where di is the single cell diameter, ni is the number of cells, A is the area of SEM micrograph, N is the number of cells with di, in area A, and M is the magnification factor of SEM micrograph. The volume expansion ratio (Vr) of the foamed PLA sample was defined as the ratio of the density of the unfoamed PLA (ρ0) to that of the foamed one (ρf) ρ Vr = 0 ρf (3) The ρf was measured using a specific gravity bottle with a volume of 25 mL by the water displacement method in accordance with ASTM D792-0022 and calculated by ρf =

W0 ρ W0 + W1 − W2 w

(4)

where W0 is the weight of the foamed sample, W1 is the weight of the specific gravity bottle filled with water, W2 is the weight of the specific gravity bottle containing both water and sample, and ρw is the density of water, which is 1 g/cm3. Wide angle X-ray diffraction (WAXD) measurements were carried out using a D8 Advance X-ray diffractometer (Bruker, Germany; Cu Kα, λ = 0.154 nm) operating at a scanning rate of 12°/min and a scanning step of 0.04° in the diffraction angle (2θ) range of 5−40°. A deconvolution procedure was applied to the WAXD spectra. Then the crystallinity (Xc) was determined by the following equation: Xc =

Ac × 100% Ac + A a

(5)

where Ac and Aa are the areas under the crystalline peaks and of the amorphous phase, respectively.

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RESULTS AND DISCUSSION Effect of Saturation Temperature on Crystallization and Foaming Behaviors in Constant-Temperature Mode. Using the CTM, the foamed PLA samples were prepared at six saturation temperatures ranging from 60 to 160 °C. The SEM micrographs of their fractured surfaces are showed in Figure 2. The samples that were foamed at saturation temperatures of 60, 80, and 100 °C exhibit low expansion ratios (1.1−3.9, as shown in Figure 3), and so the SEM micrographs were taken across their overall fractured surfaces (Figure 2a−c); however, the samples that were foamed at 120, 140, and 160 °C display higher expansion ratios (8.1− 25.8, as shown in Figure 3) and more uniform cellular structure across their fractured surfaces, and the micrographs were taken from the region close to the surfaces (Figure 2d−f). As can be directly observed in Figure 2a and b, three regionsskin, inner, and core regionscan be distinguished from the surface to the center of the samples foamed at 60 and 80 °C saturation temperatures. Interestingly, the inner region exhibits an annual shape. With increasing the saturation

Figure 1. Schematic of temperature evolution versus time in batch foaming process. TF: foaming temperature; TS: saturation temperature.

pressure in less than 0.5 s. (7) Inject the foamed samples out of the vessel and cool them to the room temperature in the air. In the aforementioned CTM, the foaming temperature was the same as the saturation temperature, and the saturation time was 40 min; in the VTM, after saturating the samples in the vessel for 40 min (the first saturation), the temperature of the vessel was raised or decreased to the foaming temperature within about 15 min and then kept at this temperature for 25 min (the second saturation). Characterization. The foamed PLA samples were immersed in liquid nitrogen for about 10 min and fractured. Then they were gold sputtered and their cryofractured surfaces were examined using scanning electron microscope (SEM) 2278

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Figure 2. continued

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Figure 2. SEM micrographs of foamed PLA samples prepared at saturation temperatures of (a) 60 °C, (b) 80 °C, (c) 100 °C, (d) 120 °C, (e) 140 °C, and (f) 160 °C using CTM. Micrographs in (a−c) were taken across overall fractured surfaces and (d−f) were taken close to surfaces of foamed PLA samples.

can be clearly observed in Figure 2d−f, the samples foamed at 120, 140, and 160 °C saturation temperatures present similar cellular structure with more uniform cellular shape and size distribution. With increasing the saturation temperature from 120 to 160 °C, the mean cell diameter decreases from about 55 to 43 μm. A higher magnification allows observing the stamenlike cell structure with the entity. The size and number of the entity decrease with increasing saturation temperature. Further observing Figure 2f clearly shows that most cells tend to collapse, thereby promoting the cell opening. The aforementioned entities are supposed to be the corresponding spherulites in the PLA foams. The foamed samples prepared at 100 and 140 °C saturation temperatures are analyzed to verify this inference. As mentioned above, for the former sample, not only the stamen-like cell structure with small entity is observed in the core region but also a number of large entities are formed in the skin region; for the latter sample, only the stamen-like cell structure with small entity is observed. It is suggested that the former sample exhibits higher crystallinity than the latter one. This is verified by the WAXD test results of the two samples, as shown in Figure 4a, in which the result of the compression molded PLA sample is also given for comparison. As for the foamed sample obtained at 100 °C, a strong diffraction peak at 2θ = 16.7°, and four relatively weak diffraction peaks at 2θ = 12.4, 14.7, 18.9, and 22.2° are observed at the WAXD curve. Among them, the diffraction peaks at 2θ = 14.7, 16.7, 18.9, and 22.2° belong to α form,8,23,24 and the diffraction peak at 2θ = 12.4° belongs to α′ form.24 However, only two diffraction peaks (2θ = 16.7 and 18.9°) appear at the WAXD curve for the sample foamed at 140 °C. The crystallinities of the samples foamed at 100 and 140 °C are calculated using eq 5 and their values are 34.3% and 15.0%, respectively. It is also clearly visible from Figure 4a that the WAXD curve of the compression molded PLA sample exhibits a broad diffraction peak at 2θ = 16.7°, which indicates that its crystallinity is too low to be detected by the WAXD measurement.8 In addition, when comparing the WAXD curve of the compression molded PLA sample with those of

Figure 3. Expansion ratio of foamed PLA samples prepared at different saturation temperatures using CTM.

temperature from 60 to 80 °C, the thickness of the annual inner region increases from about 0.3 to 0.6 mm, and the diameter of the circular core region decreases from about 2.4 to 1.7 mm. Cells are formed in the inner region, whereas no visible cells are observed in the skin and core regions. In the inner region, the cell diameter increases with the distance from the surface of the foamed sample. Some entities are formed at the interface between the skin and inner regions (as indicated by the dashed lines in Figure 2a and b). The entities are surrounded by elongated cells. As shown in Figure 2c, two regions (i.e., the skin and circular core regions) exist on the fractured surface of the sample foamed at 100 °C saturation temperature. In the skin region, a number of large entities are nonuniformly distributed and some of them cluster together locally. The entities are surrounded by some elongated cells. There also exist domains with more uniform cellular structure, in which the mean cell diameter is about 23 μm. In the core region, relatively large cells (mean diameter of about 39 μm) are formed. There also exist some small entities. These entities are surrounded by several fan-shaped cells, which is similar to a stamen and so is referred to as “stamen-like cell structure”. As 2280

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Figure 5. Schematic of cellular structure evolution of foamed PLA samples prepared using CTM with increasing saturation temperature. Typical cellular structure of foamed PLA samples prepared at saturation temperatures of (a) 60−80 °C, (b) 100 °C, and (c) 120−160 °C.

of CO2 from the crystal growth front. In the inner region, because the CO2 diffusion front moves ahead of the crystal growth front in the gas saturation stage, no crystallization takes place, resulting in relatively low stiffness and the formation of cells. Moreover, the concentration of CO2 decreases and then the cell diameter increases with the distance from the surface of the foamed sample. No cells in the core region are attributed to the fact that no CO2 diffuses into it at short saturation time. At 100 °C saturation temperature, CO2 may diffuse to the core region of the sample owing to the relatively high diffusion rate of CO2 in the PLA sample. The CO2 diffusion front reaches the skin region earlier than the core region, and so the saturation time of CO2 in the skin region is longer. In addition, 100 °C is close to the maximum crystallization temperature of the PLA at high pressure.8 Consequently, large spherulites are formed and some of them cluster together locally in the skin region, whereas small spherulites are formed in the core region. The large spherulites evolve into entities surrounded by elongated cells, whereas small spherulites evolve into the stamen-like cell structure after foaming. The elongated cells and stamen-like cell structure result from the aforementioned exclusion effect of the CO2 from the crystal growth front. Relatively low stiffness in the amorphous phase leads to more uniform cellular structure. Moreover, compared with the skin region, the core region has lower CO2 concentration, resulting in a lower number of nucleation sites and then in larger cells. High saturation temperatures (120−160 °C) lead to high diffusion rate of CO2 and then to relatively uniform CO2 distribution and cellular structure across the overall fractured surface for the foamed samples. Some relatively small spherulites are formed due to the high saturation temperature. These small spherulites evolve into the stamen-like cell structure after foaming because of the aforementioned exclusion effect of CO2. At 160 °C, low melt strength leads to the collapse of the cells. Effect of Saturation Temperature on Crystallization and Foaming Behaviors in Varying-Temperature Mode. The above results manifest that the saturation temperature in the CTM obviously affects the crystallization and cellular structure of the foamed samples. However, in the CTM, the foaming temperature is the same as the saturation temperature, implying that the aforementioned samples are prepared at different foaming temperatures, which affect the cell nucleation and growth.27 In this section, the effect of saturation temperature on the PLA foaming behavior is investigated at constant foaming temperature using the VTM.

Figure 4. WAXD curves of (a) foamed PLA samples prepared at 100 and 140 °C saturation temperatures using CTM and compression PLA sample and (b) PLA samples prepared at 100 and 140 °C saturation temperatures using the rapid cooling method.

the two foamed samples, the spherulites in PLA foams are believed to be formed in the gas saturation or cell growth stage. To determine the stage in which the spherulites are formed, a rapid cooling experiment was carried out. Using the same pressure and saturation time as those used in Figure 2, the cubic PLA samples were saturated at temperatures of 100 and 140 °C, respectively. After saturation, the vessel with PLA samples was quenched in ice water to 0 °C for about 20 min, and then CO2 was released at a very slow rate to avoid PLA foaming. The WAXD curves of the prepared PLA samples are displayed in Figure 4b. It can be clearly seen that the two samples have similar diffraction peaks to those prepared at the corresponding saturation temperatures using the CTM (as shown in Figure 4a). The calculated crystallinities of the samples prepared at 100 and 140 °C are 31.0% and 18.6%, respectively. Therefore, the spherulites in the PLA foams are formed in the gas saturation stage. Figure 2 clearly demonstrates that three saturation temperature ranges can be distinguished, that is, 60−80 °C, 100 °C, and 120−160 °C. The typical cellular structures of the samples foamed at three saturation temperature ranges are schematically displayed in Figure 5. At 60 and 80 °C saturation temperatures, it is believed that the CO2 only diffuses to the inner region in the gas saturation stage due to the low diffusion rate of CO2 in the PLA sample25,26 and the short saturation time (40 min). As for the skin region of the sample, the dissolution of CO2 increases the crystallization rate of PLA,7−9,14 and the CO2induced crystallization leads to high stiffness at low saturation temperature. As a result, no cells are observed in this region. Elongated cells around the spherulites result from the exclusion 2281

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and 36 μm, respectively. A higher magnification allows observing the stamen-like cell structure in the small cell region (as indicated by the circle). The samples foamed at 160 and 180 °C present uniform cellular structure in which the mean cell diameters/cell densities are 40 μm/1.6 × 107 cells/cm3 and 45 μm/1.1 × 107 cells/cm3, respectively. A higher magnification allows observing the stamen-like cell structure with small entity in the sample foamed at 160 °C (as indicated by the circle). The WAXD curves of the samples foamed at 100 and 180 °C saturation temperatures using the VTM are illustrated in Figure 9. It can be clearly seen that the former sample has similar diffraction peaks to that prepared at the same saturation temperature using the CTM (as shown in Figure 4a), and the latter sample has only a weak diffraction peak at 2θ = 16.7°. The calculated crystallinities of the foamed PLA samples prepared at 100 and 180 °C are 42.7% and 6.8%, respectively. As mentioned above, a number of large spherulites formed in the gas saturation stage are observed in the skin region of the former sample, and no spherulites are observed in the latter one. The expansion ratios of the two foamed samples are 1.9 and 49.8, respectively. Therefore, the crystallization for the former sample is mainly due to the CO2-induced crystallization in the gas saturation stage, whereas that for the latter sample mainly results from the strain-induced crystallization in the cell growth stage. Moreover, the CO2-induced crystallization facilitates the generation higher crystallinity than the straininduced crystallization. The cellular structure formation in the aforementioned foamed samples is explained as follows. As for the sample prepared at 100 °C saturation temperature, large spherulites are formed in the skin region and small spherulites are formed in the core region in the first saturation stage. Raising the temperature to foaming temperature and keeping at that in the second gas saturation leads to more perfect crystals, which induces higher crystallinity in the skin region. As a consequence, fewer microcells are observed in the skin region of the foamed sample compared with the sample prepared at the same saturation temperature in the CTM. Entities surrounded by elongated cells are evolved from some large spherulites, and submicro-sized cells are formed in the interlamellar zones of other large spherulites after foaming. In the core region, small spherulites and relatively low crystallinity lead to low stiffness in the amorphous phase and then to relatively uniform cellular structure. Some small spherulites are formed at 120 and 160 °C in the gas saturation stage, and their size has little change during increasing or decreasing temperature and the second gas saturation. These spherulites evolve into the stamen-like cell structure after foaming at 140 °C because of the aforementioned exclusion effect of CO2. The bimodal cellular structure for the sample foamed at 120 °C saturation temperature is due to the synergistic effect of temperature rising and depressurization. When increasing the temperature from 120 °C saturation temperature to 140 °C foaming temperature, the solubility of CO2 in PLA decreases, resulting in gas supersaturation and then to cell nucleation. These cell nuclei grow and evolve into relatively large cells in the second gas saturation stage. A large number of cell nuclei are formed, which evolve into small cells; meanwhile, the aforementioned relatively large cells further grow and evolve into large cells in the foamed sample in the depressurization stage. The sample foamed at 160 °C saturation temperature presents more uniform cellular structure owing to the relatively low stiffness in the amorphous region. No crystals are formed at

Using the VTM, the foamed PLA samples were prepared at four saturation temperatures ranging from 100 to 180 °C and a given foaming temperature (140 °C). The sample foamed at 100 °C saturation temperature exhibits a low expansion ratio (1.9, as shown in Figure 6), and so the micrograph was taken

Figure 6. Expansion ratio of foamed PLA samples prepared at different saturation temperatures using VTM. Foaming temperature: 140 °C.

Figure 7. SEM micrographs across overall fractured surface of foamed PLA sample prepared at 100 °C saturation temperature using VTM. Foaming temperature: 140 °C.

across its overall fractured surface, as shown in Figure 7. It is clearly visible that two regionsskin and core regionscan be distinguished across the overall fractured surface. In the skin region, there exist two different cellular structures: entities surrounded by elongated cells (Figure 7b) and submicro-sized cells in the interlamellar regions of the large spherulites (Figure 7c). In the core region, relatively uniform cellular structure with a mean cell diameter of about 80 μm is formed. At a higher magnification it is possible to observe the stamen-like cell structure (Figure 7d). The samples foamed at 120, 160, and 180 °C show higher expansion ratios (31.4−49.8, Figure 6) and more uniform cellular structure, and the micrographs were taken from the region close to their surfaces, as shown in Figure 8. It is interesting to find from Figure 8a that the foamed sample obtained at 120 °C presents a bimodal cellular structure in which the mean diameters of large and small cells are 1400 2282

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Figure 8. SEM micrographs taken close to surfaces of foamed PLA samples prepared at saturation temperatures of (a) 120 °C, (b) 160 °C, and (c) 180 °C using VTM. Foaming temperature: 140 °C.

Using the saturation temperature of 100 °C, the sample foamed at 160 °C foaming temperature has a low expansion ratio (2.3), and the cellular structure across its fractured surface is similar to that of the sample foamed at a temperature of 140 °C. In the core region, relatively uniform cellular structure (not shown here) with the stamen-like cells is observed. In the skin region, there exist cells with a small diameter (0.6 μm) in the spherulites, and deformed cells are observed between these spherulites, as shown in Figure 10. This may be interpreted as

Figure 9. WAXD curves of foamed PLA samples prepared at 100 and 180 °C saturation temperatures using VTM. Foaming temperature: 140 °C.

180 °C during gas saturation and decreasing temperature,28 so the sample presents more uniform cellular structure after foaming at 140 °C. Foaming Behavior of PLA with Large Spheruluites at High Foaming Temperatures in Varying-Temperature Mode. As mentioned above, large spherulites are formed in the skin region and small spherulites in the core region for the sample foamed at 100 °C saturation temperature, and small spherulites are formed across the samples foamed at 120 and 160 °C saturation temperatures in the first gas saturation stage in the VTM. After foaming at 140 °C, the large spherulites evolve into two cellular structures, circular entities surrounded by elongated cells and submicro-sized cells in the interlamellar regions, whereas the small spherulites evolve into the stamenlike cell structure, as shown in Figures 7 and 8. In the following, the foaming behavior of the PLA with large spherulites is further investigated using higher foaming temperatures when keeping other foaming parameters the same as those used in Figure 7.

Figure 10. SEM micrographs in the skin region of foamed PLA sample prepared at 160 °C foaming temperature using VTM. Saturation temperature: 100 °C.

follows. When increasing the temperature from 100 °C saturation temperature to 160 °C foaming temperature, the large spherulites formed in the first gas saturation stage melt locally. Consequently, cells can be formed. However, their growth is constrained owing to high stiffness in the spherulites. The deformed cells are due to the accumulation of CO2 2283

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Figure 11. SEM micrographs taken close to the surface of foamed PLA sample prepared at 170 °C foaming temperature using VTM. Saturation temperature: 100 °C.

Figure 12. Schematics of foaming behaviors for samples (a) with large spherulites, (b) with small spherulites, and (c) without formation of spherulites. TS: saturation temperature; TF: foaming temperature.

excluded from the crystal growth front and lower stiffness in the amorphous region. The sample foamed at 170 °C foaming temperature presents a more uniform cellular structure and higher expansion ratio (42.2), and the micrographs shown in Figure 11 were taken

from the region close to its surface. It is clearly visible that the foamed sample displays a bimodal cell structure in which the size distribution of the cells is nonuniform. The mean diameters of large and small cells are 592 and 16 μm, respectively. This can be explained briefly as follows. When increasing the 2284

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temperature from 100 °C saturation temperature to 170 °C foaming temperature, the spherulites formed in the first gas saturation stage melt, leading to low stiffness. The bimodal cell structure is attributed to the aforementioned synergistic effect of temperature rising and depressurization. Foaming Behavior of PLA with Various Spherulites Sizes in Varying-Temperature Mode. From the foregoing, the saturation temperature plays an important role in the crystallinity and spherulite size of the PLA. The spherulites evolve into various structures after foaming at different foaming temperatures. Figure 12 illustrates the schematics of cellular structures of foamed PLA samples prepared in the VTM. Under high pressure and short saturation time (14 MPa and 40 min used in this work, respectively) in the first gas saturation stage, low temperature (100 °C) is favorable to form large spherulites in the skin region, middle temperatures (120−160 °C) facilitate formation of small spherulites across the samples, and high temperature (180 °C) results in the absence of spherulite across the sample. As for the samples with large spherulites, entities surrounded by elongated cells or submicro-sized cells in the interlamellar regions are observed after foaming at low temperature (140 °C). The elongated cells result from the exclusion effect of CO2 from the crystal growth front. Increasing the foaming temperature results in the formation of small cells in the spherulites and deformed cells around the spherulites because of high stiffness of the spherulites and the aforementioned exclusion effect of CO2, respectively. Further increasing the foaming temperature leads to a bimodal cell structure, as shown in Figure 12a. As for the samples with small spherulites, more uniform cellular structure with the stamenlike cells is observed after foaming at low foaming temperature, which is resulted from the relatively low stiffness in the amorphous phase and the aforementioned exclusion effect of CO2. Interestingly, the foamed sample also displays a bimodal cellular structure when the foaming temperature is higher than 20 °C of the saturation temperature, as shown in Figure 12b. As for the sample without formation of spherulites, more uniform cellular structure with an extraordinarily high expansion ratio is formed when the sample is prepared at low foaming temperature, as described in Figure 12c.

is favorable to form large spherulites in the skin region, middle temperatures facilitate formation of small spherulites across the sample, and high temperature results in the absence of spherulites across the sample. Large spherulites generally evolve into circular entities surrounded by elongated cells or submicro-sized cells in the interlamellar regions after foaming at 140 °C owing to high stiffness and the exclusion effect of CO2 from the crystal growth front. Increasing the foaming temperature results in the formation of small cells in the spherulites and deformed cells around the spherulites. Further increasing the foaming temperature leads to a bimodal cell structure. Small spherulites evolve into the stamen-like cell structure owing to the aforementioned exclusion effect of CO2 after foaming at 140 °C. Bimodal cell structure is also observed for the PLA sample with small spherulites after foaming at 120 °C saturation temperature and 140 °C foaming temperature. The sample without the formation of spherulites displays a more uniform cellular structure after foaming at 140 °C. After foaming at 140 °C, the samples with large spherulites and without formation of spherulites have expansion ratios of 1.9 and 49.8 and crystallinities of about 42.7% and 6.8%, respectively.

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AUTHOR INFORMATION

Corresponding Author

*H.-X. Huang. Tel.: +86 20 22236799. Fax: +86 20 22236799. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China, grant no. 11172105.

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CONCLUSIONS The crystallization and foaming behaviors of the PLA using ScCO2 as the physical foaming agent were investigated in a wide saturation temperature range using the CTM (constanttemperature mode) and VTM (varying-temperature mode). In the CTM case, the samples foamed at 60 and 80 °C saturation temperatures and a given foaming pressure of 14 MPa present three different regions: skin region, angular inner region with cells, and circular core region. Some spherulites are formed at the interface between the skin and inner regions in the gas saturation stage. The sample foamed at 100 °C saturation temperature presents two different regions. A number of large entities are nonuniformly distributed and some of them cluster together locally in the skin region, and relatively uniform cellular structure with the stamen-like cells is observed in the core region. The samples foamed at the saturation temperature range of 120−160 °C present relatively uniform cell structure with the stamen-like cells. In the VTM case, the saturation temperature exerts a significant impact on the crystallinity and spherulite size, and then on the cellular structure and expansion ratio of the foamed PLA samples. In the first gas saturation stage, low temperature 2285

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