on Poly(lactic acid) - ACS Publications - American Chemical Society

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Effect of Poly(butylenes succinate) on Poly(lactic acid) Foaming Behavior: Formation of Open Cell Structure Peng Yu,†,‡ Hao-Yang Mi,† An Huang,† Li-Hong Geng,† Bin-Yi Chen,† Tai-Rong Kuang,† Wen-Jie Mou,*,‡ and Xiang-Fang Peng*,† †

National Engineer Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou, 510640, P. R. China ‡ The School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, P. R. China S Supporting Information *

ABSTRACT: Biodegradable poly(lactic acid) (PLA) based poly(butylenes succinate) (PBS) foams with open cell structure were prepared via batch foaming method using supercritical carbon dioxide as blowing agent. It was found that PLA was immiscible with PBS, and PBS phase was dispersed as tiny spheres or large domains at various concentrations. The addition of PBS reduced the viscosity of the blends. During the foaming process, the PLA/PBS interfaces acted as cell nucleation sites and the low melt strength PBS contributed to the formation of cell connection channels, which resulted in open cell structure. The investigation of PBS content and foaming temperature found that PLA/PBS (80/20) foamed at 100 °C, obtaining the highest cell opening rate (96%). Two-step depressurization foaming experiments proved that bimodal cell structure PLA/PBS foams with high cell opening rate (97%) were able to be fabricated.

1. INTRODUCTION Polymer foams exhibit high impact strength, stiffness-to-weight ratio and a low thermal conductivity, compared with unfoamed polymer materials. These excellent properties broaden the application of polymer foams in wide range of areas, such as automobile industry, food packaging, and thermal insulation.1−3 There are several methods to fabricate polymeric foams, including gas foaming, emulsion freeze-drying, thermally induced phase separation, particulate leaching/solvent casting, and 3D printing technique.4−8 Among them, gas foaming is the simplest and most commonly used method to prepare polymeric porous materials because it avoids organic solvent usage and expensive molding equipment. In particular, supercritical carbon dioxide (sc-CO2) as a physical foaming agent has attracted considerable attention because of its environmental friendly nature and relatively mild critical conditions (Tc = 31.1 °C and Pc = 7.37 MPa).9 For foamed materials, the cell structure can be classified into two categories: (1) open cell structure, whose cells are interconnected with each other via microchannels; (2) closed cell structure, which possesses isolated cells with continuous cell wall. For particular applications, different cell structure is preferred. For instance, open cell structure is required for bioscaffold and acoustic absorption applications. An ideal scaffold must possess highly interconnected pores to allow cell infiltration, attachment and colonization, and the transportation of nutrients and metabolites.10 It has been reported that better sound absorption could be achieved for porous materials with highly interconnected pores throughout the entire foam body.11 Because of the performance of opening foams, they have been widely used in applications including acoustic and thermal insulation, high energy or mass absorption, microfiltration, and tissue engineering, etc.12−15 However, it is difficult to prepare foams with open cell structure via gas foaming method, since © XXXX American Chemical Society

the expansion of gas is not easy to break the cell wall without causing cell collapse. Efforts have been made to create interconnected cell structure via batch foaming method, such as making high temperature differences between the surface and core of an extrudate,16 using mixed blowing agents to induce secondary nucleation and changes in cell densities,17 and blending of two polymers with different crystallization temperatures.18 Park et al.19 prepared LDPE/PS blended foams with open cell structure via extrusion foaming. The presence of soft and hard components in the blends assisted the formation of cell connection channels and prevented cell collapse. Similar results were reported in some other studies of immiscible polymer foams as well.20−22 However, to the best of our knowledge, there is no study about fabrication of biodegradable polymeric foams with open cell structure using blended polymer system. In recent years, biodegradable polymer foams have been attracting great attention from both industrial and academic areas. The reasons are the increasing concerns over the environmental influence and sustainability of petroleum-based polymer materials.23−25 Poly(lactic acid) (PLA) is one of the most widely used biodegradable polymers, which is typically produced from renewable resource-based materials (usually starch-rich products like corn, wheat, and so on). It has been used in various areas of people’s daily lives because of its good biocompatibility and biodegradability, such as food packages, medical devices, daily supplies, tissue engineering, etc.26,27 PLA foams prepared via sc-CO2 batch foaming have been studied extensively. However, it has been found that preparing PLA Received: February 3, 2015 Revised: May 7, 2015 Accepted: May 26, 2015

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DOI: 10.1021/acs.iecr.5b00477 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram of the pressure and temperature evolution versus the time in the foaming strategy: (a) one-step depressurization and (b) two-step depressurization. Ps is the saturation pressure. Pi is the intermediate pressure. Ts is the saturation temperature. Tf is the foaming temperature, and ts is the saturation time.

zip-lock bag and subsequently fed into the hopper of the extruder for melt compounding. The temperature profile in the extruder was set at 160 °C in the feeding zone, sequentially at 170−180−180−185−180−175 °C in the mixing zone and 170 °C at the die. The raw pellets feeding rate and screw rotation speed were set at 9 and 60 rpm, respectively. The extrudate was cooled in a water bath and subsequently granulated by a pelletizer and sufficiently dried. Sheet specimens with a thickness of 1 mm for rheological measurement and batch foaming process were prepared by compression molding at 180 °C and 10 MPa. Neat PLA and PBS samples were also prepared under the same processing conditions for comparison. 2.3. One-Step and Two-Step Depressurization Foaming. The sheet samples were cut into small specimens with dimensions of 10 mm × 10 mm × 1 mm and then placed into a high-pressure vessel to foam. Figure 1 shows the batch foaming procedures. The vessel was heated to a saturation temperature (TS, 150 °C) within 5 min. Then the vessel was flushed with low pressure CO2 for 3 min via a syringe pump (ISCO-260D, USA) followed by compressing CO2 to desired saturation pressure (Ps, 16 or 20 MPa). The system was kept in equilibrium at Ts and Ps for desired saturation time (ts, 1 h) to ensure CO2 adsorption equilibrium. Thereafter, the vessel temperature was cooled to foaming temperature (Tf, 90, 100, or 110 °C) with the Ps unchanged during the temperature decrease and followed by equilibrium for additional 10 min. Finally, the vessel was depressurized to atmosphere pressure by two kinds of depressurization procedures as illustrated in Figure 1a and Figure 1 b, respectively. One-step depressurization was achieved by direct pressure release using a bullet valve. Twostep depressurization was employed to create bimodal cell structure. The vessel was depressurized to an intermediate pressure (Pi, 16 MPa) within 3 s and maintained for 10 min. Then the pressure was quickly released to ambient pressure as the one-step depressurization. Thereafter, the vessel was cooled to room temperature rapidly by circulating water to maintain the foamed structure. 2.4. Characterization. Rheological measurements of PLA/ PBS blends were performed on a rheometer (TA R2000EX, USA) using a parallel-plate geometry with a diameter of 25 mm. Tests were performed at 180 °C under a nitrogen atmosphere to avoid any degradation. All samples were dried in a vacuum oven before the test, and a fresh sample was loaded for each type of rheological test. First of all, dynamic strain sweep tests were carried out to confirm the linearity of viscoelastic region up to 100% strain at 10 rad/s frequency and 180 °C. It was found that the linear viscoelastic limits for all samples extend to a strain of about 25%. In this linear viscoelastic region microstructure of polymer would not be

foams with open cell structure via batch foaming is relatively difficult.23 In this work, we propose to fabricate open-cell biobased polymeric foams using PLA as major matrix and poly(butylene succinate) (PBS) as minor phase. PBS is a biodegradable polymer that has many interesting properties such as excellent toughness and relatively low melt temperature.28 The properties of the bulk material could be altered by combining PLA with PBS, and the change of material property may be influential for the formation of foamed structure. In this study, sc-CO2 was used as foaming agent to fabricate PLA/PBS foams via batch foaming technique. The basic concept is using a hard/soft nonhomogenous system to promote cell opening during foaming to produce PLA-based open-cell biodegradable foams, and the cell opening mechanism will be discussed in detail. Foams that contain both large cells and small cells are referred to as bimodal cell structure foams. It is attracting more and more attention recently because the large cells could reduce the bulk weight and the small cells could ensure sufficient mechanical strength. These foams were found also to possess good thermal insulation properties.29,30 Moreover, there is demand of such bimodal cell structure foams with high cell opening rate for tissue engineering scaffold applications, in which the large pores allow cells to attach and proliferate and the small pores to facilitate effective nutrient supply, gas diffusion, and metabolic waste removal.31 Therefore, we also conducted two-step depressurizing batch foaming for PLA/PBS blends to fabricate bimodal cell structure foams with highly opened cells.

2. EXPERIMENTAL SECTION 2.1. Materials. Polylactic acid used in the study is of commercial grade (PLA 4032D), obtained from NatureWorks Co. Ltd., USA. It has a D-lactic acid monomer content of about 1.5%; weight-average molecular weight, Mw, of 2.1 × 105 g/ mol; melt flow index around 2.57 g/10 min (210 °C, 2.16 kg); specific gravity of 1.25 g/cm3. Poly(butylenes succinate) (PBS Bionolle no. 1903) was supplied from Showa Highpolymer Co. Ltd., Japan. It has melt flow index of around 4.5 g/10 min (190 °C, 2.16 kg), specific gravity of 1.26 g/cm3. Commercial purity grade CO2 (purity 99%, Air Liquide) was used as physical blowing agent. 2.2. Preparation of Compounds. Prior to melt mixing, PLA and PBS were dried at 80 °C for 8 h in a vacuum oven. Then PLA/PBS blends with PBS weight ratios of 10, 20, and 30 wt % were prepared by twin screw extruder (TSE25/42, Brabender, Germany), respectively. The mixtures of PLA and PBS pellets were manually premixed by tumbling in a plastic B


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Industrial & Engineering Chemistry Research affected by shear alignment during the experiment.32 Dynamic frequency sweep was then performed over a frequency range of 0.1−100 rad/s at 180 °C. A fixed strain of 1% was used to ensure that the measurements were carried out within the linear viscoelastic range of the samples invested. Scanning electron microscope (SEM, Nano 430, FEI, U.S.) was used to observe the phase morphology of the PLA/PBS blends and the cell morphology of the foamed samples. The blended and foamed samples were freeze-fractured in liquid nitrogen and gold-sputtered. Both cell size and density were determined from the SEM micrographs using Image Pro Plus software. The cell density N0 (cells/cm3), the number of cells per cubic centimeter of foamed polymer, was determined from eq 133 N0 =

6[1 − (ρf /ρp )] πD 3

× 1012

(1)

where D (μm) is the average cell diameter and ρp (g/cm3) and ρf (g/cm3) are the mass densities of the sample before and after foaming, respectively, which were measured by the water displacement method in accordance with ASTM D792. The volume expansion ratio (Φ) of the foamed sample was defined as the ratio of the density of the unfoamed sample (ρp) to that of the foamed one (ρf), ρp Φ= ρf (2)

Figure 2. SEM micrographs of PLA, PBS, and PLA/PBS blends with various blending ratios: (a) PLA; (b) PLA/PBS (90/10); (c) PLA/ PBS (80/20); (d) PLA/PBS (70/30).

to a decrease in the number of particles and an increase in the domain size. 3.2. Viscoelastic Properties of the PLA/PBS Blends. Generally, rheology is one of the most effective approaches to study the phase behaviors of immiscible blends. A dynamic spectrum can be used to understand the structures and properties of polymers. Figure 3a and Figure 3b exhibit the G′ and G″ versus ω curves, from which it can be seen that the G′(ω) curves of the blends were located between those of neat PLA and PBS in the high frequency region, whereas the values of G′(ω) of the blends were larger than both pure PLA and PBS, and the improvement increased with addition of PBS content in the low frequency region. The G″(ω) of the samples showed similar tendency, while the variations were insignificant. For the immiscible blends, the enhanced elastic indices exhibited in the storage modulus curves in low frequency regions can contribute to the shape relaxation of the dispersed droplets during oscillatory shear flow.35,36 In the high frequency region, the dispersed droplets do not have enough time to relax, and the energy supplied by deformation is completely dissipated in the bulk of the matrix. Therefore, the G′(ω) curves of the blends lie between those of the pure components in the high frequency region.36 The complex viscosity η* results (Figure 3c) indicated that PBS has a much lower viscosity than PLA and the viscosity curves of the blends were located between PLA and PBS and reduced as the PBS content increased. In the low frequency region, all samples showed a Newtonian plateau followed by the power-law-like behavior at higher frequency region. Moreover, the shear thinning behavior of PLA/PBS blends started at a lower frequency than that of neat PLA and PBS, suggesting that the complex viscosities of PLA/PBS melts possess stronger shear-thinning tendency and it becomes stronger with the increasing of PBS content. This phenomenon may be attributed to the relaxation of the deformed droplets and interfacial slip between two phases and was also observed for other blends.35,37,38 3.3. Foaming Behavior of PLA/PBS Blends at Various Temperature. Foaming temperature is a crucial factor in batch

On the other hand, the mean cell wall thickness (δ) in μm was estimated by the following equation.34 ⎞ ⎛ 1 ⎜ δ=D − 1⎟ ⎟ ⎜ 1 − ρ /ρ f p ⎠ ⎝

(3)

The open porosity was defined as the ratio of open porous volume to the total volume of the tested sample, measured by a helium pycnometer (Ultrapyc 1200e, Quantachrome Instruments, USA).

3. RESULTS AND DISCUSSION 3.1. Phase Morphology of PLA/PBS Blends. Figure 2 shows the SEM images of fracture surface for the PLA/PBS blends with various blending ratios. Pure PLA (Figure 2a) showed a smooth and flat fracture surface, and a typical twophase morphology was observed in the PLA/PBS blends with PBS content range from 10 to 30 wt % indicated by the uniformly dispersed PBS discrete droplets in the PLA matrix, which suggest the PLA and PBS materials are immiscible. It was also proved by the dynamic mechanical analysis results (Figure S1 and Table S1 in Supporting Information). The size of the spherical PBS phase increased remarkably as the PBS content increased from 10 to 30 wt %. The average size of PBS phase in PLA/PBS (90/10) sample was 1.33 μm, and it was slightly increased to 1.81 μm for PLA/PBS (80/20) sample, while it further increased to 4.32 μm for PLA/PBS (70/30) sample. Meanwhile, it was found that the number density of PBS dispersed droplets increased from 6.84 × 104/mm2 to 7.69 × 104/mm2 when the PBS content increased from 10 to 20 wt %. However, the number density of PBS droplets decreased to 4.49 × 104/mm2 when the PBS content increased to 30 wt % because of the collision probability of the PBS phase during the mixing increased when the PBS content was too high, which led C


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Figure 3. (a) Storage modulus (G′), (b) loss modulus (G″), and (c) complex viscosity (η*) of PLA, PBS, and PLA/PBS blends as a function of frequency at 180 °C.

foaming process, since the melt viscosity and melt strength of polymer and polymer blends were highly dependent on temperature. If the temperature is too low, the melt strength of the cell wall increases and restricts the cell growth, resulting in small cell size and thick cell walls. On the contrary, if the temperature is too high, the low melt strength will cause cell wall rupture and collapse and lead to low expansion ratio. In this study, three foaming temperatures (90, 100, and 110 °C) were used to investigate the foaming behavior of the neat PLA and PLA/PBS blends. Figure 4 shows the structures of the neat PLA and PLA/PBS blends after foaming. The related statistical results are listed in Table 1. It can be seen that neat PLA foams exhibited uniform cell structure with almost closed cell walls except for PLA foaming at 110 °C. Some ruptured cell wall appeared at this foaming temperature; however, its opening rate only reached 38.32%. All three blends formed uniform cell structure under various foaming temperatures and exhibited open-cell structure with interconnection channels between cells. According to the statistical results, it was found that the cell size increased and cell density decreased as the foaming temperature increased. This phenomenon is typical because at low foaming temperature (90 °C), which gives a late nucleation (i.e., a short cell growth time) and a large nucleation rate simultaneously, small cell size and large cell density tended to be obtained.39 However, it was found that the volume expansion ratio, mean cell wall thickness, and cell opening rate showed different tendency. The PLA/PBS (80/20) foams showed the highest expansion ratio and cell opening rate, with the smallest cell wall thickness. Generally, foaming at elevated temperature, it will be easier to achieve thin cell wall and more open cells,18 whereas cell coalescence would occur if the temperature was too high due to the reduced melt strength. In this study, the foaming temperatures used were close to the melt point of PBS phase. When foaming was at 90−110 °C, the melt strength of the polymer matrix was either too high or too

Figure 4. SEM micrographs of PLA foams and PLA/PBS blend foams under various foaming temperatures. PLA: (a) 90 °C, (b) 100 °C, (c) 110 °C. PLA/PBS (90/10): (d) 90 °C, (e) 100 °C, (f) 110 °C. PLA/ PBS (80/20): (g) 90 °C, (h) 100 °C, (i) 110 °C. PLA/PBS (70/30): (j) 90 °C, (k) 100 °C, (l) 110 °C.

low for cell expansion, which resulted in thick cell wall and cell collapse during foaming. Therefore, it was found that 100 °C D


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Industrial & Engineering Chemistry Research Table 1. Cell Structure Data of PLA/PBS Blend Foams at Various Foaming Temperature composition (wt %/wt %) PLA

PLA/PBS (90/10)

PLA/PBS (80/20)

PLA/PBS (70/30)

temp (°C)

cell size (μm)

90 100 110 90 100 110 90 100 110 90 100 110

16.01 ± 0.5 18.22 ± 0.8 19.20 ± 0.9 12.25 ± 0.8 13.69 ± 0.6 14.45 ± 1.0 9.26 ± 0.6 10.56 ± 1.0 12.69 ± 0.8 14.69 ± 1.0 16.32 ± 1.2 18.30 ± 1.5

cell density (×108 cells/cm3)

vol expansion ratio Φ

± ± ± ± ± ± ± ± ± ± ± ±

9.48 ± 0.1 10.51 ± 0.4 9.27 ± 0.9 8.13 ± 0.3 10.49 ± 0.8 9.07 ± 0.6 6.88 ± 0.2 10.27 ± 0.7 7.37 ± 0.3 5.25 ± 0.4 8.42 ± 0.6 3.81 ± 0.2

4.17 2.86 2.41 9.12 6.80 5.64 20.6 14.6 8.08 4.88 3.87 2.30

0.4 0.8 0.7 0.8 1.1 0.6 1.5 2.0 1.0 1.0 1.3 0.5

mean cell wall (μm)

opening rate (%)

± ± ± ± ± ± ± ± ± ± ± ±

3.67 ± 0.7 12.54 ± 0.8 38.32 ± 1.0 78.28 ± 2.8 89.90 ± 0.9 86.38 ± 1.8 85.41 ± 1.4 96.20 ± 1.2 94.57 ± 1.7 84.73 ± 1.7 89.16 ± 1.6 88.27 ± 2.2

0.91 0.93 1.12 0.83 0.64 0.87 0.76 0.55 0.79 1.07 0.92 0.93

0.03 0.05 0.09 0.06 0.04 0.07 0.08 0.05 0.06 0.12 0.02 0.03

Figure 5. Effect of foaming temperature on the (a) cell size and cell densities and (b) cell wall thickness and cell opening rate of PLA/PBS blend foams with various PBS content.

was the optimal foaming temperature to achieve the highest cell opening rate. The effects of material combination under three temperatures were investigated as well. According to the statistical results (Figure 5), it was found that the cell size of PLA/PBS (70/30) foam was the highest followed by PLA/PBS (90/10) foam while the PLA/PBS (80/20) foam had the smallest cell size, and the cell density showed inverse trends. Furthermore, the cell wall of PLA/PBS (80/20) foam was the thinnest but the cell opening rate was the greatest among all blends at different temperatures. These results indicate that the content of PBS phase in PLA matrix has an optimal level to achieve high cell opening rate, which might be 80/20 in this study. Neat PLA, PBS, and PLA/PBS blends were foamed at 100 °C to investigate the influence of PBS content on the foam structure. The cell morphology images are shown in Figure 6, and related statistical results are shown in Figure 7. It is obvious that neat PLA (Figure 6a) shows a uniform cell structure while the cells are mostly closed, while neat PBS (Figure 6e) could not form regular cells because of its low melt strength. According to the viscosity results, the elastic modulus and complex viscosity were lower for the blends having higher PBS content. It was found in the PLA/PBS (90/10) sample that the cell size was reduced and cell density was increased. This was because the PBS phase formed tiny spheres which were uniformly dispersed in PLA and acted as heterogeneous nucleation sites during foaming.40 As the PBS content increased to 20 wt %, the cell size was further reduced and cell density was increased because of the increase of PBS nucleation sites as its content increased (recall the phase morphology results of PLA/PBS blends). However, when the PBS content was further increased up to 30 wt %, the cell size increased and cell density decreased noticeably. This was because the PBS phase started

Figure 6. SEM micrographs of PLA/PBS blend foams at Tf = 100 °C and Ps = 16 MPa with different PBS content: (a) PLA, (b) PLA/PBS (90/10), (c) PLA/PBS (80/20), (d) PLA/PBS (70/30), and (e) PBS.

to aggregate into small islands in PLA matrix at this concentration, which resulted in less heterogeneous nucleation sites.41 Furthermore, the excessive PBS content and increased PBS phase domain reduced complex viscosity of the blends, which resulted in cell collapse in the foaming procedure and low expansion ratio. 3.4. Cell Opening Mechanism of PLA/PBS Blends. In this study, it was found that the PLA/PBS blends could easily form open cell structure at various foaming temperatures and E


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Figure 7. Effect of PBS content on the (a) cell size and cell densities and (b) cell wall thickness and cell opening rate of PLA/PBS blend foams at foaming temperature of 100 °C.

Figure 9 illustrated the cell opening mechanism of PLA/PBS blend. The interfaces between PLA and PBS acted as heterogeneous nucleation sites during bubble nucleation, and the cells would grow around the PBS domain. Meanwhile, since the foaming temperature was close to the melt temperature of PBS, the surface tension of PBS phase was much lower than that of PLA phase; hence, the adjacent bubbles tend to expand toward the PBS domain, which would more likely result in cell opening. The presence of PBS in PLA matrix helped cell nucleation to form more cells and assisted cell expansion by reducing bulk surface tension which resulted in thinner cell walls and higher cell opening rate as shown in the above results. Nevertheless, it was found that the concentration of PBS and the foaming temperature have optimal levels to achieve the best open cell structure. Among all experimental groups, the PLA/ PBS (80/20) blend that foamed at 100 °C obtained the highest cell opening rate. When the foaming temperature was too low, the bubble could not expand sufficiently to create open channels. On the contrary, cell collapse would happen when the temperature was too high, which would result in low expansion ratio and cell opening rate. Recall the phase morphology of PLA/PBS blends (Figure 2); as the content of PBS increased from 10 to 20 wt %, the quantity of the dispersed PBS spheres increased. At this condition, more bubbles would nucleate and they were easier to expand and connect to adjacent bubbles, as well as more likely to form interconnection channels. Meanwhile, the hard PLA matrix will hold the overall cellular structure to prevent cell collapse. When the PBS content further increased to 30 wt %, the PBS spheres start to merge into larger domains. This would cause the reduction of cell density and lead to cell collapse due to the exceeded PBS phase, as observed in PLA/PBS (70/30) blend. 3.5. Open Cell Structure Formation in Bimodal PLA/ PBS Foams. Polymeric foams with bimodal porous structure were found to have many advantages in both properties and applications. For instance, in tissue engineering scaffolds, macropores are necessary to promote three-dimensional adhesion, proliferation, and migration of cells, while micropores are required for nutrient and metabolic waste transportation.45,46 To achieve this particular cell architecture, a two-step depressurization batch foaming process was applied to the PLA/PBS blending system to investigate whether the cell opening mechanism is applicable in the two-step foaming process. On the basis of the previous research results, neat PLA and PLA/PBS (80/20) blends were used in the two-step depressurization foaming experiments, and the foaming temperature was 100 °C. The foaming pressure was first released from 20 to 16 MPa to induce the formation of

blending ratios with relatively high cell opening rate (over 78%), and the highest cell opening rate (96%) was achieved by PLA/PBS (80/20) blends at a foaming temperature of 100 °C. However, it is typically very hard to achieve open cell structure for neat PLA.23 The basic strategy in this study is to fabricate PLA-based open-cell biodegradable foams by combing PLA with low melt temperature polymer PBS. Since the PLA/PBS is an immiscible system, each component should present its own bulk behaviors in the blend. The interfaces between PLA and PBS have lower activation energy for bubble nucleation and provided favorable heterogeneous nucleating sites for bubble formation.22 In order to maximize the stiffness contrast between hard and soft regions in the blends, the foaming temperature was selected close to the Tm of PBS. It has been widely reported that the solubility of CO2 in polymer decreases as the temperature increases.42−44 In the current PLA/PBS blend system, the foaming temperature was 100 °C, at which CO2 solubility was relatively low in the PBS phase, since its melting temperature was 103 and 113 °C (Figure S2 and Table S2); hence, the CO2 was mainly dissolved in PLA phase. In this case, the higher is the PBS content in the blends, the less CO2 could be absorbed, which would result in lower expansion ratio after foaming. The volume expansion ratio results shown in Table 1 agreed with this theory. To demonstrate the PBS effect on the cell opening, the typical inner cell wall of neat PLA foam and that of PLA/PBS(80/20) blend foam are shown in Figure 8. It can be seen that the PLA foam inner cell wall exhibited

Figure 8. Typical inner cell wall of (a) neat PLA foam and (b) PLA/ PBS (80/20) blend foam.

intact and smooth surface, while the PLA/PBS blend foam ruptured in the inner cell wall, indicating that the PBS droplets were torn during the bubble growing process, leaving the irregular reticular structure. Owing to the PBS droplets rupture, the cell interconnections were created. Moreover, in the high magnification image, the interphase boundary of PLA and PBS was not obvious anymore because of the thin cell wall caused by CO2 expansion. F


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Figure 9. Schematic illustration of cell opening mechanism.

Figure 10. SEM micrographs and cell size distribution with bimodal cell structure through two-step depressurization batch process. The pressure was released from 20 to 16 MPa first, and then there was further depressurization to the ambient pressure: (a, a′) neat PLA foams; (b, b′) PLA/PBS (80/ 20) blend foams.

Figure 11. Cell size and density of (a) macrocells and (b) microcells of PLA/PBS blend bimodal foams. (c) Microcell wall thickness and cell opening rate of PLA/PBS blend bimodal foams.

macrocells and then released to atmosphere to create microcells. Figure 10 shows the SEM micrographs of the foamed samples with bimodal cell size distribution, and the related statistical results were shown as Figure 11. It was observed that both PLA and PLA/PBS blend foams exhibited bimodal cell structure with macrocell size ranging from 80 to 150 μm and microcell size range of 10−20 μm. It was found that the PLA/PBS blends showed smaller cell size and larger cell density for both micro- and macrocells (Figure 11 a and Figure 11b). The cell walls of neat PLA foams (Figure 10a′) presented smooth surface with measured cell opening rate of only 9.2%, whereas the PLA/PBS (80/20) foams (Figure 10b′) showed interconnected cell structure with many connection channels within macrocells, and the measured cell opening rate was as high as 97%, as presented in Figure 11c. Therefore, it was proved that the cell opening mechanism of the PLA/PBS blending system could be applied in the two-step depressuriza-

tion foaming process as well. In the foaming process, the macrocells were first induced by the depressurization from 20 to 16 MPa and then the microcells were generated by the second depressurization, during which the PBS phase present on the cell walls of macrocells acted as heterogeneous nucleation sites and induced the cell growth toward the macrocells and the formation of interconnected channels.

4. CONCLUSIONS Biodegradable PLA-based PLA/PBS foams with open cell structure were fabricated through a temperature soak batch foaming technique using sc-CO2 as the foaming agent. The phase morphology revealed that PLA and PBS are immiscible, PBS phase dispersed as droplets in the PLA matrix, and the viscosity of the blends decreased as the PBS content increased. The difference in the melting temperature of PLA and PBS, and the low melt strength of PBS resulted in the formation of G


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interconnected open cell structure in the PLA/PBS foams. During the foaming process, the interface between PLA and PBS had the potential to become the cell nucleation source, leading to cell density increasing noticeably, and the low melt strength PBS contributed to the formation of cell connection channels. The investigation of PBS content and foaming temperature found that PLA/PBS (80/20) that foamed at 100 °C obtained the highest cell opening rate (96%). Moreover, this blended system was applied to two-step depressurization foaming process, and it was found that this foaming method was eligible for fabricating bimodal cell structure foams with cell opening rate as high as 97%.

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ASSOCIATED CONTENT

S Supporting Information *

Dynamic mechanical properties and thermal behavior of PLA/ PBS blends. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.iecr.5b00477.

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AUTHOR INFORMATION

Corresponding Authors

*X.-F.P.: e-mail, [email protected]. *W.-J.M.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Nature Science Foundation of China (Grants 51073061 and 21174044), the Guangdong Nature Science Foundation (Grants S2013020013855 and 9151064101000066), and National Basic Research Development Program 973 (Grant 2012CB025902) in China.

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