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Enhancing Nanofiller Dispersion Through Prefoaming and Its Effect on the Microstructure of Microcellular Injection Molded Polylactic Acid/Clay Nanocomposites Haibin Zhao,†,§,∥ Guoqun Zhao,*,† Lih-Sheng Turng,*,‡ and Xiangfang Peng§ †

Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong, China ‡ Department of Mechanical Engineering and Wisconsin Institute for Discovery, University of Wisconsin−Madison, Madison, Wisconsin, United States § Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou, China ∥ State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai, China ABSTRACT: Promoting better dispersion of nanofillers is crucial in producing high performance nanocomposite foams. For the effects of nanofillers on controlling foam structures and mechanical properties to be enhanced, prefoamed pellets were produced via supercritical fluid (SCF) extrusion foaming and subsequently microcellular injection molded. Structural, thermal, and mechanical properties of the prefoamed pellets and resultant polylactic acid (PLA)/clay nanocomposite foams were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy, and tensile testing. The diffraction peaks in the XRD plots of extruded prefoamed pellets were observed to shift to lower angles, indicating that prefoaming improved the intercalation of clay platelets. The driving forces behind the intercalation and dispersion of clay were (i) enhanced diffusion via the plasticizing effect of SCF and (ii) the phase change of SCF from the supercritical to the gaseous state within the polymer melt. In the nanocomposites made with prefoamed pellets, clay was present mostly in separate layers, and the distance between the clay platelets was usually greater than the effective radius of gyration of the polymer molecules. With the much improved nucleating effect of the dispersed clay platelets, the microcellular injection molded nanocomposite foams exhibited the smallest cell size and the highest cell density. They also displayed a desirable microstructure in which cells were well-separated, closed, and round in shape. DSC analyses showed that PLA was able to crystallize upon heating and that the prefoaming step promoted PLA crystallization. TGA analyses showed that prefoaming did not cause the molecular weight of PLA to decrease or change significantly.

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as lightweight structural materials.2−4 Thus, it is essential to further develop the microcellular injection molding process to produce lightweight parts with high strength. The mechanical properties of polymer foams are dependent on both the intrinsic properties of the polymer and their final microstructure. For the properties of the polymer matrix to be enhanced while keeping the relative density constant, nanofillers are commonly used for matrix enhancement and microstructure refinement. The physical properties, including electrical, thermal, mechanical, and barrier properties of polymers, can be improved if nanofillers can be dispersed and distributed within the polymer matrix effectively.5−7 This method of using nanofillers has been widely explored by many researchers. Han and Lee8 produced polystyrene nanocomposite foams with a small amount of nanoclay using supercritical fluid extrusion foaming. They showed that when the clay platelets were fully exfoliated, the cell size could be reduced to as small as 4.9 μm with the cell density as high as 1.5

INTRODUCTION With increasing concerns about the environment and the cost of manufacturing, lightweight performance polymers are increasingly in demand in an effort to reduce fuel consumption. Microcellular injection molding (commercially known as the MuCell Process) is capable of mass-producing plastic parts with complex geometries and excellent dimensional stability and is widely used in a variety of industries, including automotive, electronics, consumer goods, food service, and consumer structural parts. In this process, supercritical fluid (SCF), such as nitrogen (N2) or carbon dioxide (CO2), is used as a plasticizer and physical blowing agent to create evenly distributed and uniformly sized microscopic cells throughout the thermoplastic polymer.1 These result in benefits such as reductions in cycle time, material consumption, melt and mold temperatures, injection pressure, and clamp tonnage. Postproduction, these parts have a higher mechanical strength-toweight ratio than conventional injection molded plastic parts at equivalent densities. However, the mechanical properties of neat polymer foams are usually inferior to the properties of their solid counterparts due to the significant density reduction, uneven cell distribution on core and skin layers, and structural defects, all of which significantly limit their potential application © XXXX American Chemical Society

Received: March 26, 2015 Revised: June 25, 2015 Accepted: June 29, 2015

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DOI: 10.1021/acs.iecr.5b01130 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research × 109 cells/cm3. Furthermore, the nanocomposite foams exhibited higher moduli and better barrier properties. Goren and Ozisik9 studied poly(methyl methacrylate) (PMMA)/silica nanocomposite systems with supercritical carbon dioxide by batch foaming. They found that SCF led to drastic improvements in the dispersion and distribution of silica nanoparticles in PMMA. Furthermore, increasing the saturation pressure led to a better dispersion of silica nanoparticles. However, microcellular injection molding is a semicontinuous foaming process, thus making it a challenge for fabricating polymer nanocomposite foams with adequate homogeneous nanofiller dispersion in a comparably short cycle time. Because of low loading concentrations and high surface energies, nanofillers tend to aggregate, and it is not easy to disperse these nanofillers into the polymer matrix evenly. As such, it limits their ability to act as reinforcing agents and leads to defects that affect the overall mechanical and transport performances of the resulting nanocomposite foams.10 For the nanofillers’ potential reinforcement and cell nucleating effects to be fully materialized, extensive studies and techniques have been conducted to develop nanocomposite foams, including surface functionalization, coating, high-shear mixing, and the use of co-blowing agents.11−14 Saha and colleagues15 examined the properties of polyurethane foam by in situ polymerization with ultrasonic treatment to disperse three types of nanofillers. However, only small volumes around the horn region were effectively improved by the ultrasonic horn, which limits its wide application. Lee and Sun et al.16 produced foamed injection molded parts with extruded gasladen pellets. Injection molded foams with better morphologies and cell structures were finally produced using an optimal content ratio of co-blowing agents (N2 and CO2) as well as the proper sequence of introducing gases (N2 + CO2 or CO2 + N2). Another important way to improve the nanofiller dispersion and physical properties of polymer foams is by tuning the processing conditions or parameters.17,18 On the basis of the traditional theory of cell nucleation and growth, optimizing critical processing parameters like temperature, pressure, and gas content can reduce the energy barrier for cell nucleation and lead to smaller cell sizes. Furthermore, adjusting the viscosity or melt strength of the polymer melt to prevent cell coalescence is important as well as it will further decrease the average cell size.19,20 Because the cycle time of microcellular injection molding is relatively short, process conditions that improve the nanofiller dispersion, such as longer mixing times in the barrel and higher mold temperatures, are not desirable as they may degrade the material and increase production costs. In this work, a new foaming method was developed for producing microcellular PLA nanocomposites using extruded SCF prefoamed pellets followed by microcellular injection molding. Using pellets prefoamed with supercritical fluid to disperse nanoparticles provides a new technique for the design and control of the cell structure in the fabrication of high performance microcellular polymer foams. PLA is typically a challenging material to foam. Thus, this paper is dedicated to the structure and properties of PLA foams with emphasis on the improvement of clay dispersion and cell morphology of PLA and PLA/clay foams.

2.16 kg), respectively. Organomodified montmorillonite (MMT) Cloisite 30B (C-30B) containing a methyl bis-2hydroxyethyl ammonium quaternary salt with a cation exchange capacity (CEC) of 90 mequiv/100 g was supplied by Southern Clay Products, Inc. Both materials were used as received. Prior to processing, PLA pellets were predried at 60 °C for 24 h in a vacuum oven and the clays at 90 °C under vacuum for 6 h. Processing. Preparation of PLA/Clay Nanocomposites. Solid PLA/clay nanocomposites at a 4 wt % clay loading level were melt mixed in a counter-rotating twin-screw extruder with a screw diameter of 27 mm and an L/D ratio of 42. Melt compounding was conducted at an average temperature of 180 °C along the barrel of the extruder through a decreasing temperature profile. The screw speed was 40 rpm. The torque displayed on the control board of the extruder was first stabilized, and then the rotor speed was gradually increased to 100 rpm. After extrusion, the extruded strands were cut into pellets. Pre-Foaming with Supercritical Fluid (SCF) Extrusion. The prefoamed pellets were produced using a single-screw extruder (Extrudex EDN 45−30) equipped with a high-precision syringe pump (Teledyne ISCO 260D). The gas flow rate and gas content could be calculated by precisely maintaining gas injection at either a constant pressure or a constant volumetric flow rate. Upon injection into the barrel, the supercritical CO2 was mixed with the polymer melt by screw rotation to form a single-phase solution. Nucleation occurred due to the rapid and large pressure drop that ensued as the material passed through the narrow capillary nozzle at the die. The water in the cooling bath cooled the polymer strands quickly. Figure 1 shows a

Figure 1. Schematic of gas-laden SCF extrusion during the prefoaming process.

schematic of cell nucleation and growth during the prefoaming process. After the extruded strands were cooled by passing through the chilled water in the bath, moisture on the surface of the strands was removed by a blowing fan. Then, the strands were pelletized to produce prefoamed pellets for injection molding using an Extrudex SGS 100-E pelletizer. Foaming with Microcellular Injection Molding. Once the prefoamed pellets were produced, solid and microcellular standard tensile test bars (ASTM D638-03, Type I) were prepared with an Arburg injection molding machine (Allrounder 320S, Lossburg, Germany) equipped with MuCell technology (Trexel Inc., Wilmington, MA) under the following process conditions (Table 1). Supercritical CO2 and N2 (99.99% purity) were used as the physical blowing agents in the microcellular extrusion and injection molding experiments,

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EXPERIMENTAL WORK Materials. PLA (2002D) used in this study was purchased from NatureWorks LLC, Minnetonka, MN, USA. Its density and melt flow index were 1.24 g/cm3 and 7 g/10 min (210 °C/ B


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sections using a transmission electron microscope (TEM, JEOL 1200 EX) with an 80 kV accelerating voltage. The cellular structures of the microcellular injection molded specimens were characterized using a scanning electron microscope (SEM, LEO 1530) with an accelerating voltage of 5 kV. The injection molded tensile test bars were frozen in liquid nitrogen and fractured at the middle section of the bars to obtain a clean cross section for SEM (see Figure 3). Then, they were sputter coated with gold for observation. Micrographs were subsequently analyzed using an image analysis tool (UTHSCSA Image Tool) to determine the average cell size and cell density. Tensile Tests. Tensile tests were performed in accordance with ASTM D638-03 on a universal testing instrument (MTS, Sintech 10/GL). The values of the tensile strength, modulus, and strain-at-break represent the average of eight specimen measurements at a constant crosshead speed of 5 mm/min. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (TA Instruments, Q20) was used to determine the thermal properties of the nanocomposites with/ without prefoaming by measuring their melting and crystallization temperature and corresponding enthalpy changes. The weight of the samples was in the range of 7−9 mg. The cooling and heating was applied at a rate of 10 °C/min. The curves of the second heating were used to measure the exothermic peak resulting from the crystallization and endothermic peaks resulting from crystal dissolution, whereby the crystallinity of the PLA in the composites was calculated as follows

Table 1. Processing Parameters of the Solid and Microcellular Foam Injection Molding Trials solid melt temperature (°C) mold temperature (°C) injection speed (cm3/s) cooling time (sec) back pressure (bar) SCF flow rate (kg/h) SCF Injection pressure (MPa) SCF dosage time (sec) pack/hold pressure (MPa) pack/hold time (sec)

microcellular 180 10 20 60 60

n/a n/a n/a 100 1

0.11 30 1 n/a n/a

respectively. Note that with microcellular injection molding, the pack/hold stage was absent due to the homogeneous packing pressure that resulted from the nucleation and growth of microcells. Figure 2 shows the overall procedure and schematic of prefoaming and microcellular injection molding for this study. Characterization. Wide Angle X-ray Diffraction (WAXD). WAXD analyses were performed for the PLA/clay composites using a STOE high resolution X-ray diffractometer (Cu Kα radiation, λ = 1.5405 Å). The specimens were cut from the injection molded tensile test bars as shown in Figure 3 together with orientation of the examined cross sections. Samples were scanned at room temperature in fixed time mode with a counting time of 2 s under a diffraction angle of 2θ in the range of 1−15°. With Bragg’s law, λ = 2d sin θ, the basal spacing (dspacing) of the clay in the PLA/clay composites can be estimated from the (001) diffraction peak. Microstucture Characterization. The transmission electron micrographs were taken from 100 nm thick, microtimed

χc (%crystallinity) =

ΔHf − ΔHcc ΔH 0

×

100 W

(1)

where ΔHcc was the enthalpy of cold crystallization, ΔHf was the melting enthalpy, ΔH0 was the enthalpy of melting per gram of 100% crystallinity (perfect crystal) of PLA (93.7 J/g),

Figure 2. Schematic of microcellular injection molding combined with the prefoaming process. C


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Figure 3. Schematic drawing of the sample zone for 2D-WAXD and SEM measurements.

Figure 4. (a) TEM image of prefoamed PLA/clay composites and (b) XRD graphs for PLA/clay nanocomposites foamed with and without prefoaming and Cloisite 30B.

and W was the weight fraction of PLA in the PLA/clay nanocomposites.21 Thermogravimetric Analysis (TGA). To examine thermodegradability and clay composition in the composite after melt mixing, TGA analyses were performed on a TA Thermogravimetric Analyzer Q200. Twenty milligrams of each specimen, taken from the middle portion of the solid and microcellular injection molded tensile bars, were heated from room temperature to 500 °C at a constant rate of 10 °C/min under a nitrogen atmosphere. The normalized sample weight loss was recorded as a function of the temperature.

indicating that the prefoaming process played an important role in the clay intercalation within PLA. It was also noticed that the diffraction peaks corresponding to the PLA/clay foam without prefoaming exhibited stronger and sharper peaks than those of PLA/clay foams with prefoaming. However, with the prefoaming process, the peak of the samples was less sharp, implying that the spacing of the silicate layers continually increased. These phenomena indicated that the prefoaming process caused the clay platelets to deviate from an ordered structure to a more dispersed status. Moreover, for both of the specimens, a small weak diffraction peak appeared at 2θ = 6.8° due to the second registry (d002) of the clay. Moreover, compared with regular microcellular injection molded samples, the intensity of the peaks decreased for the prefoamed specimens. From the WAXD analysis, we can conclude that prefoaming helped clay intercalation and exfoliation in the PLA matrix after melt mixing. A stronger interaction of hydrogen bonding between the carbonyl group in the main chain of PLA molecules and the hydroxyl group in the organic modifier of 30B could have had a profound influence on the foam structure and properties of the microcellular injection molding parts. A more detail discussion and explanation will be given in the following section on morphology. Cell Morphology of PLA and PLA/Clay Nanocomposites. If the clay could be well-intercalated and exfoliated in the polymer matrix, enhancement of the properties and cellular structure of the PLA composites foam could be expected due to their large aspect ratio and surface area. The cellular structures of prefoaming PLA/clay nanocomposites were further exam-

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RESULTS AND DISCUSSION Dispersion of Clay in PLA Nanocomposites. Figure 4 shows a transmission electron microscopy image of prefoamed PLA/clay composites and the XRD patterns of PLA/clay nanocomposites with and without prefoaming. The primary silicate reflection at 2θ = 4.7° of pure nanoclay 30B corresponds to the interlayer spacing of 1.8 nm. The diffraction patterns of the PLA/clay nanocomposite foams with or without prefoaming in Figure 4(b) shows that the main characteristic diffraction peak of Cloisite 30B shifted to a lower angle at 2θ = 2.5° (from 4.7°), indicating that insertion of polymer into the clay galleries forced the clay platelets apart and melt intercalation of the PLA polymer chain occurred. These observations showed that the clay nanoparticles were welldispersed within the PLA by microcellular foaming either with prefoaming or without prefoaming in this study. However, the primary diffraction peak of prefoaming PLA/clay composites moved to a lower degree compared to that without prefoaming, D


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Figure 5. Cellular structures of PLA and PLA nanocomposites (100× magnification): (a) neat PLA foam without prefoaming, (b) PLA/clay foam without prefoaming, (c) neat PLA foam with prefoaming, and (d) PLA/clay foam with prefoaming.

ined. In Figure 5, SEM images of microcellular injection molded (MIM) PLA and PLA/clay with and without prefoaming are shown. Compared to neat PLA foams, it is obvious that the foams of PLA/clay nanocomposites exhibited better cell morphologies. In Figure 5(a), the cell sizes of the neat PLA foam were very large (∼100 μm), with cells only appearing in the core layer, whereas the skin layer was almost solid. However, in Figure 5(b), which shows the microstructure of the PLA/clay foam without prefoaming, smaller cells were observed near the skin layer, whereas larger pores could be found toward the center. The cell sizes in the skin layer were ∼1 order of magnitude smaller than those in the core layer. A possible explanation for this kind of sandwich structure is that the clay platelets tended to disperse and agglomerate parallel to the skin layer due to the shear and fountain flow effects of injection molding. Most clay platelets existed as stacks, layers, or tactoids and served as nucleation agents. The cell density also increased significantly due to the shear stress along the mold cavity.22 As soon as the nucleation sites were created, the cells were frozen owing to rapid cooling near the mold surface, whereas cells at the center of the cavity continued to grow and coalesce due to a much slower cooling rate. The cell density corresponded directly to the number of cell nucleation sites and the available surface area. If the nanofillers were better dispersed in the matrix, and therefore exposed more surface area, more heterogeneous nucleation sites would be created. Thus, the nanocomposite foams had the smallest cell sizes and highest cell densities and showed a desirable

microstructure in which the cells were round in shape, closed, and well-separated (see Figure 5(d)). Compared to Figure 5(b), the cell sizes of the samples with prefoaming were smaller and better distributed than those of the samples without prefoaming. The cell density (obtained from the center portion of the cross section of the microcellular tensile bars) was calculated using the formula cell density =

⎛ N ⎞3/2 ⎜ ⎟ M ⎝ L2 ⎠

(2)

where N was the number of cells, L was the linear length of the area, and M was a unit conversion factor resulting in the number of cells/cm3. The calculated average cell size and cell density were 5 μm and 1.5 × 109 cells/cm3, respectively. Moreover, even for pure PLA, the cell morphology improved when prefoamed extrusion pellets were used for microcellular injection molding due to the coblowing agent effect of employing both CO2 and N217 (see Figure 5(c)). Mechanism of Enhanced Nanoparticle Dispersion and Cellular Structure Due to Prefoaming. With the establishment of favorable interactions between the polymer and the clay surface and subsequent system energy reductions, polymer chains penetrated into the interlayer region of the clay platelet layers to induce layer separation and exfoliation. Thus, nanocomposites with good clay dispersion were fabricated. SCF foaming promoted better dispersion of the nanoparticles. The high diffusivity resulting from the plasticizing effect of SCF E


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Figure 6. Mechanism of clay dispersion and exfoliation during prefoaming and microcellular injection molding.

Figure 7. (a) Tensile strength, (b) modulus, and (c) strain-at-break of PLA and PLA/clay solid specimens and foams with and without prefoaming.

F


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with a high modulus but reduced toughness. However, with the incorporation of prefoaming pellets, the strain-at-break of microcellular specimens was comparable with that of the solid specimens, especially for the PLA/clay nanocomposite samples (see Figure 7(c)). If the nanofillers could not be effectively dispersed and instead behaved as stress concentrators and crack initiators, this would lead to an overall reduction in mechanical properties. Thus, the improvement of the strain-at-break indicated the good dispersion of clay in the polymer matrix. Typical tensile stress−strain behaviors of the solid and microcellular specimens are shown in Figure 8. The foams

made it possible for the polymer molecules to penetrate between the clay platelet layers.23 In addition, nanoscaledispersed clay platelets may have acted as nucleation agents at the cell nucleation stage. The cells tended to nucleate and grow around the platelet surface of the clay platelets. Once the distance between the layer platelets was greater than the effective radius of gyration of the polymer molecules, many tiny cells appeared in between the platelets. Then, during phase transition, the expanding nitrogen or carbon dioxide between the layers could separate individual platelets in the polymer matrix. Thus, the other driving force for the destruction of agglomerates and the dispersion of nanofillers was the phase transformation of nitrogen and carbon dioxide from a supercritical to a gaseous state within the polymer melt.9 Thus, many more interfacial areas were provided for gas adsorption and cell nucleation. After prefoaming and microcellular injection molding, the tactoids may have been completely delaminated and uniformly dispersed in the polymer matrix. As a result, the microcellular injection molded nanocomposite with prefoamed pellets exhibited better cell morphology with the highest cell density and the smallest cell size, as shown in Figure 5. Also, the multiaxial extensional force and flow during cell growth induced the alignment of clay particles along the cell boundary. Such an alignment may limit gas escape during foaming due to an improved gas barrier effect or delay the gas diffusion of the samples.24 Figure 6 shows the overall mechanism of clay dispersion and exfoliation with the prefoaming process. Mechanical Behavior and Properties of PLA and PLA Nanocomposites. Tensile strength, modulus, and strain-atbreak of microcellular injection molded PLA and PLA/clay nanocomposites were measured and are shown in Figure 7. The properties of microcellular specimens reported here were values directly read from the test machine without taking into account their weight reduction or expansion ratio. As shown in Figure 7(a) for solid PLA and PLA/clay specimens, their tensile strength decreased from 54.99 to 51.48 MPa with the addition of nanoclay. The reduction in tensile strength of the solid PLA/clay was probably due to the weak interface between the PLA and clay agglomerates. It was also noticed that the tensile strengths for both microcellular PLA and microcellular PLA/clay nanocomposites were less than that of the solid specimens but still more than 30 MPa. For both microcellular neat PLA and PLA/clay nanocomposite foams, the prefoaming process had a positive effect on their tensile strengths; that is, the PLA and PLA/clay foam samples with prefoamed pellets exhibited higher strengths than samples without prefoaming due to better clay dispersion and finer microcellular structure formation. As shown in Figure 7(b), the addition of nanoclay increased the moduli for both solid and microcellular PLA by 20.82 and 24.66%, respectively. It was noticed that Young’s moduli of microcellular PLA/clay foamed samples were higher than those of neat solid PLA, even with reduced part weight and a small shot volume. Compared to the specimens without prefoaming, the samples processed with prefoaming pellets showed comparable moduli. The strain-at-break for all of the samples posed the same trend as the tensile strength (i.e., it decreased with the addition of nanoclay and the foaming process). It was reported that if the clay platelets were exceedingly well bonded with polymer chains, with increasing stress, fracture would occur but without significant development of strain, thereby producing materials

Figure 8. Strain−stress curves of PLA and PLA/clay foams with and without prefoaming.

exhibited a reduction in both tensile strength and strain-atbreak compared to solid specimens. Moreover, PLA/clay foams with prefoamed pellets exhibited a strain-at-break that was higher than that of the regular microcellular injection molded specimens. Thus, the mechanical properties of PLA and PLA/ clay foams were influenced by either the cellular structure or the present status of the nanoclay. Thermal Properties. For any prior thermal history during microcellular extrusion and injection molding foaming to be removed, all of the specimens were first heated from 25 to 200 °C, kept isothermal for 5 min, and cooled to 25 °C, and then a second heating scan was performed to 200 °C. DSC plots for the second heating scan of neat PLA and PLA/clay foams are presented in Figure 9. Table 2 tabulates the numerical values of the temperatures and enthalpies obtained from the second heating scan as well as the degree of crystallinity for the PLA matrix. The cold crystallization temperature (Tcc) and melting temperature (Tm) were taken from the peak temperatures of the cold crystallization exothermic and melting endothermic peaks, respectively. DSC tests were conducted on all of the solid and microcellular neat PLA and PLA/clay composites as well as the PLA/clay foam with prefoaming pellets. By comparing the thermograms and calorimetric parameters of the curves of the second heating scan, it was found that there was not much difference between solid and injection molding foam for neat PLA and PLA/clay composites, indicating that the SCF foaming process had no significant effect on the thermal properties of the PLA specimens. Thus, the effect of the addition of nanoclay and its dispersion in the polymer matrix can be studied without taking into account SCF foaming and G


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Figure 10. TGA mass loss curves for neat PLA and PLA/clay foams with and without the prefoaming process.

Figure 9. Second heating curves of neat PLA and PLA/clay foams with and without prefoaming.

°C, indicating that the samples had the same thermal stability. It can also be seen that the weight ratio of the nanoparticles in the composites remained stable at ∼4%, indicating that the melt mixing, microcellular injection molding, and prefoaming process did cause a loss of clay content. As shown in Figure 11, all of the samples showed well-defined decomposition peaks

cell growth. As shown in Figure 9, the addition of nanoclay led to a decrease in the cold crystallization temperature (i.e., the Tcc decreased from 124.13 to 116.26 °C). Furthermore, for the PLA/clay composites, the Tcc of the specimens processed with prefoaming pellets shifted to a much lower temperature, 107.28 °C, indicating a better dispersion of nanoclay in the PLA matrix, thereby further promoting the crystallization of PLA due to the creation of more nucleation sites. The PLA 2002D used in this study had a slow crystalline rate due to the rigid segments in its main chain.25,26 The crystallization enthalpies for each of the samples were comparable. As for the melting peaks of neat PLA and PLA/clay composites, the addition of nanoclay and prefoaming did not change the two melting endotherm phenomena of PLA, which is commonly observed for PLA due to its imperfect ordering and different crystalline sizes. However, it was noted that the addition of nanoclay shifted the main melting peak to the right, which was located at a lower temperature for neat PLA. Moreover, the prefoaming process resulted in a continuous decrease for Tm1. Thus, compared to other processes, the prefoaming process had a significant effect on both the cold crystallization and melting temperatures. By calculating the degree of crystallinity of PLA, it was also found that the Xcc of PLA showed little difference with the incorporation of nanoclay or the prefoaming process due to its low crystallization ability. Thermogravimetric Analysis (TGA). For the real clay content in the PLA/clay composites after melt mixing and microcellular injection molding as well as the effect of one more thermal history with the prefoaming process on the thermal degradation behavior of the PLA/clay composites to be investigated, TGA was conducted under a nitrogen atmosphere. As shown in Figure 10, both the neat PLA and PLA/clay composites, as well as their composites with the prefoaming process, exhibited a single-stage thermal degradation at ∼300

Figure 11. DTG mass loss curves of neat PLA and PLA/clay foams with and without the prefoaming process.

at ∼360 °C, indicating that prefoaming did not cause a macromolecular weight decrease or change for either PLA or PLA/clay samples.

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CONCLUSIONS Prefoaming with supercritical fluid to enhance dispersion of nanofillers and foaming provides a new method for the design

Table 2. Thermal Characteristics of Neat PLA and PLA/Clay Nanocomposite Foams Obtained from the Second Heating Run cold crystallization

melting

sample

temp (°C)

enthalpy (J/g)

temp 1 (°C)

temp 2 (°C)

enthalpy (J/g)

degree of crystallinity (%)

pure PLA PLA/clay solid PLA/clay foam PF-PLA/clay foam

124.13 116.26 116.65 107.28

25.74 32.22 32.54 32.98

152.45 152.37 151.86 147.52

157.33 159.47 158.88 156.80

27.01 32.91 32.56 33.56

1.36 0.74 0.02 0.62

H


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(9) Goren, K.; Okan, O. B.; Chen, L. M.; Schadler, L. S.; Ozisik, R. Supercritical carbon dioxide assisted dispersion and distribution of silica nanoparticles in polymers. J. Supercrit. Fluids 2012, 67, 108. (10) Sauceau, M.; Fages, J.; Common, A.; Nikitine, C.; Rodier, E. New challenges in polymer foaming: A review of extrusion processes assisted by supercritical carbon dioxide. Prog. Polym. Sci. 2011, 36, 749. (11) Lee, L. J.; Zeng, C.; Cao, X.; Han, X.; Shen, J.; Xu, G. Polymer nanocomposite foams. Compos. Sci. Technol. 2005, 65, 2344. (12) Nofar, M.; Park, C. B. Poly (lactic acid) foaming. Prog. Polym. Sci. 2014, 39, 1721. (13) Rasal, R. M.; Janorkar, A. V.; Hirt, D. E. Poly(lactic acid) modifications. Prog. Polym. Sci. 2010, 35, 338. (14) Tsimpliaraki, A.; Tsivintzelis, I.; Marras, S. I.; Zuburtikudis, I.; Panayiotou, C. The effect of surface chemistry and nanoclay loading on the microcellular structure of porous poly(d,l lactic acid) nanocomposites. J. Supercrit. Fluids 2011, 57, 278. (15) Saha, M. C.; Kabir, M. E.; Jeelani, S. Enhancement in thermal and mechanical properties of polyurethane foam infused with nanoparticles. Mater. Sci. Eng., A 2008, 479, 213. (16) Lee, J.; Turng, L. S.; Dougherty, E.; Gorton, P. Novel Foam Injection Molding Technology Using Carbon Dioxide-Laden Pellets. Polym. Eng. Sci. 2011, 51, 2295. (17) Zhao, H.; Cui, Z.; Sun, X.; Turng, L.-S.; Peng, X. Morphology and Properties of Injection Molded Solid and Microcellular Polylactic Acid/Polyhydroxybutyrate-Valerate (PLA/PHBV) Blends. Ind. Eng. Chem. Res. 2013, 52, 2569. (18) Kramschuster, A.; Turng, L. S. An Injection Molding Process for Manufacturing Highly Porous and Interconnected Biodegradable Polymer Matrices for Use as Tissue Engineering Scaffolds. J. Biomed. Mater. Res., Part B 2009, 92B, 366. (19) Keshtkar, M.; Nofar, M.; Park, C. B.; Carreau, P. J. Extruded PLA/clay nanocomposite foams blown with supercritical CO2. Polymer 2014, 55, 4077. (20) Miller, D.; Chatchaisucha, P.; Kumar, V. Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide I. Processing and structure. Polymer 2009, 50, 5576. (21) Pilla, S.; Kramschuster, A.; Lee, J.; Auer, G. K.; Gong, S.; Turng, L.-S. Microcellular and Solid Polylactide−Flax Fiber Composites. Compos. Interfaces 2009, 16, 869. (22) Chen, L.; Wang, X.; Straff, R.; Blizard, K. Shear stress nucleation in microcellular foaming process. Polym. Eng. Sci. 2002, 42, 1151. (23) Horsch, S.; Serhatkulu, G.; Gulari, E.; Kannan, R. M. Supercritical CO2 dispersion of nano-clays and clay/polymer nanocomposites. Polymer 2006, 47, 7485. (24) Antunes, M.; Velasco, J. I. Multifunctional polymer foams with carbon nanoparticles. Prog. Polym. Sci. 2014, 39, 486. (25) Gupta, B.; Revagade, N.; Hilborn, J. Poly(lactic acid) fiber: An overview. Prog. Polym. Sci. 2007, 32, 455. (26) Karageorgiou, V.; Kaplan, D. Porosity of 3D biomaterial scaffolds and osteogenesis. Biomaterials 2005, 26, 5474.

and control of cell structures in the fabrication of high performance microcellular polymer foams. The XRD peak of the samples moved to a lower degree and became less sharp, implying that the space between the clay platelets increased and intercalated effectively. Thus, the nanocomposite foams had the smallest cell sizes and highest cell densities and displayed a favorable microstructure in which cells were well-separated, closed, and round in shape. The tensile strength and strain-atbreak of the PLA and PLA/clay samples with prefoamed pellets increased compared to samples without prefoaming. DSC analyses showed that PLA was able to crystallize upon heating and that prefoaming promoted PLA crystallization. TGA analyses showed that prefoaming did not cause the macromolecular weight of PLA to decrease or change significantly.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (No. 51403118), China Postdoctoral Science Foundation (2014M561923), the Foundation for the Excellent MiddleAged and Young Scientists of Shandong Province (BS2014ZZ010), the Fundamental Research Funds of Shandong University (No. 2014GN001 and 2014QY003-12), the Opening Project of State Key Laboratory of Molecular Engineering of Polymers (No. K2014-07), the Key Laboratory of Polymer Processing Engineering, the Ministry of Education (No. 2013006), and the Wisconsin Institute for Discovery (WID) at the University of Wisconsin−Madison.

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REFERENCES

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