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Fabrication of Poly(lactic acid)/Graphene Oxide Foams with Highly Oriented and Elongated Cell Structure via Unidirectional Foaming Using Supercritical Carbon Dioxide Tai-Rong Kuang,†,‡ Hao-Yang Mi,† Da-Jiong Fu,† Xin Jing,† Bin-yi Chen,‡ Wen-Jie Mou,*,‡ and Xiang-Fang Peng*,† †

National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Wushan Street 381, Guangzhou 510640, China ‡ The School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China ABSTRACT: The significant influence of graphene oxide (GO) on the unidirectional foaming of poly(lactic acid) (PLA) using supercritical CO2 as blowing agent was investigated in this study for the first time. Highly oriented and elongated cell structures were obtained from the PLA/GO nanocomposites foams. The thermal, rheological, and CO2 absorption properties of the PLA/ GO nanocomposites were studied to investigate the effect of GO on PLA unidirectional foaming. It was found that the incorporation of GO improved the storage modulus, loss modulus, and complex viscosity of the PLA/GO nanocomposites significantly. The addition of GO improved the CO2 absorption ability of the nanocomposites, which caused high expansion ratio and increased average cell size during foaming process. The high expansion force by enhanced CO2 absorption, high matrix viscosity of PLA/GO nanocomposites, and restriction of the mold in three directions together caused the formation of the highly elongated cell structure during foaming.

1. INTRODUCTION Microcellular foaming technology has attracted special attention since the 1980s due to the advantages of its light weight, heat insulation ability, high impact strength, and low cost. Compared with traditional foaming that employs chemical foaming agent, microcellular foaming uses an environmentally friendly physical blowing agent such as carbon dioxide (CO2) and nitrogen (N2). The cell size of the microcellular foamed samples normally ranges from 0.1 to 100 μm, and the cell density is usually higher than 109cells/cm3.1 As it is well-known that the properties and applications of polymeric foams are highly dependent on the cell morphologies,2−5 efforts have been taken to investigate the relationship between the properties of foamed parts and cell structures, especially oriented cell structure. In general, orientated cell structure could be obtained by using highly oriented elastomer domains in the polymer matrix for foaming and moldconstrained foaming in the polymer sheets. Tomoyuki Nemoto et al.6 created highly oriented cell structure in PP/rubber system using nanoscale-order dispersed rubber domains as bubble nucleation sites. David J. Frankowski et al.7 indicated that cell morphology can be suitably controlled by the surfaceconstrained foam. Moreover, the oriented cell structure offers unique properties over traditional structure. Jin-Biao Bao et al.8 found that the oriented cells could significantly improve the impact strength of polystyrene (PS) foams. Kelyn A. Arora et al.9 demonstrated PS foams with oriented cell structures show a proportional relationship between compressive strength and bimodal oriented cell morphologies. However, little research has been reported on the biodegradable polymers such as polylactide and polymer/additive system in the microcellular foaming. Considering the advantages of the orientated cell © 2014 American Chemical Society

structure of polymeric foams, the related research on the biodegradable polymers is worthwhile to broaden its potential biomedical applications. Poly(lactic acid) (PLA), a biodegradable thermoplastic aliphatic polyester, has been recognized as the most commonly used biomaterial due to its outstanding attributes, such as process ability, biocompatibility, high strength and modulus, etc.10−12 However, in practice, PLA needs to be modified for improving properties, such as melt strength, stiffness, crystallization ability, and heat deflection temperature.13,14 It has been widely proven that the addition of nanofillers could act as nucleation sites in polymer foaming process to improve their foaming ability in recent years. A variety of nanofillers, such as nanoclay, nanosilica, carbon nanotubes, and carbon nanofibers, have been used as nucleation agents in PLA foaming. Laurent M. Matuana et al.15 reported that the presence of the nanoclay increased the heterogeneous nucleation of PLA during the foaming process. Ting He et al.16 also investigated the effects of the CNTs on the PLA foaming in order to broaden its application. By far, the research on preparation of PLA or PLA nanocomposite foams with oriented cell structure was barely found. Graphene, a monolayer of sp2-bonded carbon atoms in a two-dimensional honeycomb lattice, has attracted much attention because of their remarkable electronic, thermal, and mechanical properties.17−19 Among the common graphene derivatives is graphene oxide (GO) that has been extensively Received: Revised: Accepted: Published: 758

August 30, 2014 December 22, 2014 December 30, 2014 December 30, 2014 DOI: 10.1021/ie503434q Ind. Eng. Chem. Res. 2015, 54, 758−768

Article

Industrial & Engineering Chemistry Research

Figure 1. Diagram of the temperature and pressure evolution versus the time in the PLA/GO nanocomposite microcellular foaming process.

2.2. Graphene Oxide (GO) Synthesis. Graphite oxide (GO) was prepared using a modified Hummers’ method.23−25 Briefly, 5 g of graphite, 5 g of NaNO3, and 230 mL of H2SO4 were stirred together in a round-bottom flask in an ice bath. Then, 30 g of KMnO4 was slowly added into the solution to prevent the temperature from exceeding 5 °C. Next, the suspension was transferred to an oil bath and maintained at 35 °C for 1 h. After forming a thick paste, 400 mL of water was slowly added into the mixture. The solution was then transferred to the oil bath (temperature at 90 °C), where it stayed for 30 min. Finally, the suspension was further diluted with 1000 mL of water, and then 30 mL H2O2 was added slowly, turning the color from dark brown to bright yellow. After being cooled, the products were filtered and subsequently washed in succession with a 3% HCl aqueous solution and ethanol (2×). Multiple washes with DI-water were then performed until the pH of the product was ∼7. The solid paste was obtained using a freeze drier (Freezone 4.5, Labconco) drying for 5 days. 2.3. Preparation of PLA/GO Nanocomposites. The PLA/GO nanocomposites were prepared using N,N-dimethylformamide (DMF) as a mutual solvent. Briefly, in order to form a transparent and homogeneous solution, PLA was completely dissolved in DMF at a concentration of 100 mg/mL with the assistance of stirring and ultrasound at 80 °C for 4 h. The desired amounts of GO were suspended into DMF of 1 mg/mL for the same method. Then, the PLA solution was added to the GO suspension and stirred for 4 h at 60 °C to obtain homogeneous PLA/GO solution. After been mixed uniformly, the solvent was evaporated at 105 °C for 24 h in a vacuum oven (DZF-6050, China), and the PLA/GO nanocomposite film was further dried at 80 °C in the vacuum oven for a week to remove the solvent completely. In this way, PLA/GO nanocomposites with various GO content were prepared, and the samples were named according to the weight concentration of GO in PLA matrix as PLA, PLA/GO0.2%, PLA/GO0.4%, PLA/GO0.6%, PLA/GO0.8%, and PLA/GO1 wt %.

studied as reinforcement additive to improve the properties of polymer.20 Graphene and graphene oxide have also been extensively studied as reinforcement additive to improve the properties of polymer. Studies on their effect on the polymer microcellular foaming have been made as well. Hao-bin Zhang et al.21 found that the incorporation of graphene to PMMA foams greatly improved its ductility and tensile toughness. Jintao Yang et al.22 studied the effect of GO in microcellular PS foams, and found higher cell density and smaller size could be achieved compared with the neat PS foam. Nevertheless, GO has not been applied in PLA microcellular foaming yet, and the investigation of the unidirectional foaming behavior of PLA/ GO system has not been done. In this study, PLA/GO nanocomposites were prepared by solution blending, and its microcellular foams were fabricated by a batch foaming process using supercritical CO2 (scCO2) as a physical blow agent in a three dimensional limited mold (unidirectional foaming). The thermal, rheological, CO2 absorption properties of the nanocomposites were characterized, and the foaming behavior of the PLA/GO nanocomposites was investigated comprehensively. The effects of GO on the unidirectional foaming process were discussed as well.

2. MATERIALS AND METHODS 2.1. Materials. PLA (2003D, Mw = 98 000g/mol, Mw/Mn = 1.85, 4 mol % D-lactide, density 1.24 g/cm3) was purchased from Natureworks LLC (Minnetonka). Graphite powders were purchased from Sigma-Aldrich (Shanghai, China). N,NDimethylformamide (DMF), sodium nitrate (NaNO3), concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), and hydrochloric acid (HCL) (37%) were supplied by the Yuhuashaobo Co. Ltd. Reagent (Guangzhou, China). All chemicals were used as received. Commercial grade CO2 with a purity of 99.9% was obtained from Shengtong Co. Ltd. (Guangzhou, China) and used as the physical blowing agent. 759

DOI: 10.1021/ie503434q Ind. Eng. Chem. Res. 2015, 54, 758−768

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Figure 2. (a) AFM image of synthesized GO. (b) The height profile of the AFM image. (c) TEM image of synthesized GO. (d) FTIR spectrum of synthesized GO.

absorbance mode in the range 600−4000 cm−1 with a resolution of 4 cm−1. 2.5.2. Graphene Oxide (GO) Morphology. A Philips CM100 transmission electron microscope (TEM) (Philips/FEI Corperation, Holland) was used to study the microstructures of the GO at an accelerating voltage of 100 kV. The samples were prepared by dipping a Formvar-Carbon coated grid into a 1% GO solution in DI-water. Atomic force microscopy (AFM) was used to measure the thickness of the synthesized GO by a Nanoscope MultiMode SPM (Veeco, American). The measurement was carried out in dry nitrogen atmosphere, operated in uncontact mode, with a glass substrate. 2.5.3. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC) tests were carried out under nitrogen atmosphere at a heating and cooling rate of 10 °C/ min to investigate the thermal properties of neat PLA and PLA/GO nanocomposites with a DSC 204C instrument (Netzsch, Germany). All samples were first heated to 200 °C and kept isothermal for 5 min to eliminate thermal history, and then they were cooled to 30 °C and subsequently scanned to 200 °C. 2.5.4. Rheological Measurement. The influence of GO on the rheological properties of the PLA matrix was investigated by a Bohlin Gemini 200 Rheometer (U.K.) equipped with a parallel-plate fixture. The thickness and diameter of the sample

2.4. Unidirectional Microcellular Foaming Process. The unidirectional microcellular foaming process was performed on a homemade batch foaming device with scCO2 as the blowing agent; the detailed procedures of the foaming process are shown in Figure 1. (a) The prepared PLA/GO films (1 × 1 mm2, diameter × thickness) were first placed into a tailor-made cylindrical mold (three directions with limited space) and then put into a high-pressure vessel. (b) The vessel was heated to the saturation temperature (160 °C) within 30 min, and the time at the moment was recorded as to. (c) When the temperature reached 160 °C, the vessel was slowly filled with scCO2 in order to eliminate air and followed by compressing scCO2 to desired saturation pressure (20 MPa). (d) The sample was kept in equilibrium at 160 °C and 20 MPa for 0.5 h. (e) Then, the temperature was reduced to the foaming temperature (100 °C), the saturation pressure remained in the mean time, and the samples maintained under this stage for 5 min. (f) The high-pressure vessel was instantaneously depressurized to atmospheric pressure to induce cell nucleation and growth. (g) The foamed specimens were solidified by a water-cooling system for 10 min, and the samples were quickly removed from the vessel. 2.5. Characterizations. 2.5.1. Fourier Transform Infrared Spectroscopy (FTIR). Fourier transform infrared spectroscopy (FTIR) measurements were carried out using a Bruker Tensor 27 instrument (Germany). The samples were analyzed in 760


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3. RESULTS AND DISCUSSION 3.1. Morphology and FTIR Characterization of GO. The microstructure image of synthesized GO was examined using AFM and TEM as shown in Figure 2a,c. It indicated the typical ultralarge GO sheet morphology, which was around 1 or 2 μm in lateral size. Similar results have also been observed in our previous works.26 The thickness was measured from the height profile of the AFM image, as shown in Figure 2b, which is around 1 nm consistent with other research, indicating the monolayer GO sheet was successfully prepared. Figure 2d shows the IR spectra for GO in order to confirm the GO chemical structural feature. In the spectrum of GO, the broad bands at ∼3420 cm−1 are attributed to the stretching vibration of the O−H groups in water molecules. Other characterization peaks appeared at ∼1720, 1373, and 1074 cm−1 corresponding to the CO, CO, and COC stretching vibrations, respectively, which were similar to the results reported by Mingwei Tian et al.27 Apart from these sharp features, the peak at 1620 cm−1 (C−C stretching vibration) was possibly due to the addition of oxidant, which resulted in destruction of the former sp2 hybrid structure. FTIR results clearly suggested that different types of oxygen-containing functional groups were presented on the GO. 3.2. Thermal Properties of PLA and PLA/GO Nanocomposites. DSC curves of the second heating of neat PLA and PLA/GO nanocomposites are shown in Figure 3, and the

disk are 1 mm and 25 mm, respectively. The dynamic complex viscosity (η*), storage modulus (G′), and loss modulus (G″) were measured at 175 °C as functions of angular frequency (ω) ranging from 0.1 to 100 rad/s. The measurements were carried out under nitrogen atmosphere. 2.5.5. CO2 Absorption Measurement. The main purpose of the CO2 absorption experiments was to investigate the amount of CO2 absorbed in the PLA and PLA/GO nanocomposites. The original weights of these samples were measured using an electronic balance readable to 0.0001 g (FA2104, Shanghai). Absorption of CO2 was facilitated by placing the specimens in a high-pressure vessel under a CO2 gas pressure of 20 MPa at 100 °C for 0.5 h (the same as the foaming condition). Afterward, the vessel was slowly depressurized (to avoid foaming), and the CO2-absorbed samples were removed from the pressure vessel and weighted using a digital balance. Furthermore, the desorption weight loss over time at atmosphere pressure was measured during the first hour. Five specimens of each group were measured to obtain the average value. In order to further analyze the effect of GO on the CO2 absorption in PLA and PLA/GO nanocomposites, another absorption experiment of CO2 was conducted by the similar method as mentioned above. The samples were placed under a CO2 gas pressure of 1000 psi (6.894 MPa) at 100 °C for 0.5, 1.5, 3, and 4 h, respectively. Afterward, the vessel was very quickly depressurized (under this condition, the sample could not be foamed), and the CO2absorbed samples were removed from the pressure vessel and weighted using a digital balance. 2.5.6. Scanning Electron Microscopy (SEM). The foamed structure of PLA and PLA/GO nanocomposites was observed via a S-3700N scanning electronic microscopy (SEM, Hitachi, Japan) operated at 10 kV. Samples were immersed in liquid nitrogen for 1 h to maintain the cell structure before brittle fracture. The fractured surfaces were sputter-coated with gold for 5 min prior to examination. Cell size and cell density were measured from SEM micrographs by the Image-Pro Plus software (Image-Pro Plus 6.0). The average cell diameter (D) of all of the cells in the SEM micrograph was calculated with eq 1: n

D=

∑i = 1 di n

(1)

Here, n is the number of cells in the SEM micrograph, and di is the equivalent diameter of one cell. The cell density (No) of the foamed samples was calculated by eq 2: ⎡ N ⎤3/2 No = ⎢ ⎥ ⎣A⎦

Figure 3. DSC thermograms of neat PLA and PLA/GO nanocomposites with various GO contents.

statistical results are summarized in Table 1. Compared with neat PLA, the PLA/GO nanocomposites showed higher glass transition temperature (Tg), which was due to the structure of GO having restricted the molecular mobility of PLA associating with filler−matrix interaction. It is noted that no obvious cold crystallization peak was detected for neat PLA, and its melting peak was weak. This phenomenon indicated the poor crystallization ability of neat PLA. When the samples were loaded with GO, the cold crystallization and melting periods became obvious as shown in Figure 3. The cold crystallization temperature (Tc) decreased as the GO content increased in PLA matrix indicating the dispersed GO platelets promoted the cold crystallization of PLA during the second heating process

(2)

Here, n is the number of cells in the micrograph, and A is the area of the micrograph (cm2). The expansion ratio (Rv) of the foamed samples was calculated by eq 3: Rv =

ρp ρf

(3)

Here, ρp is the density of neat PLA and its nanocomposites, and ρf is the density after foaming. 761


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of GO. Moreover, it should be noted that when the GO content in the PLA matrix was at 1%, the crystallinity reduced, which may be due to the effect of GO agglomeration. As we concluded from the nonisothermal crystallization behavior, the effectiveness of GO was sufficient to promote crystallization of PLA in agreement with the results of Defeng Wu and coworkers.32 3.3. CO2 Absorption of PLA and PLA/GO Nanocomposites. To investigate the CO2 absorption ability of neat PLA and PLA/GO nanocomposites under high pressure, they were originally weighed by electronic balance and then placed into the pressure vessel filled with CO2 (20 MPa, 100 °C) for 0.5 h. The amount of CO2 absorbed was determined right after depressurizing by weighing the total weight and subtracting the original weight. As a summary of the whole result in Figure 4a, it was found that the weight ratio of CO2 absorbed by neat PLA was about 2.7 mg (the value may be lower than actual due to the slow depressurization process and the transfer procedure during the test). With the addition of 1% GO, the weight ratio of CO2 absorbed increased by as much as 120.7% higher than that of neat PLA. It is believed that the higher CO2 solubility of the PLA/GO nanocomposites was mainly due to the addition of GO in PLA matrix, which could provide more sorption sites for CO2 on their surfaces as reported in a number of studies.33,34 Therefore, the absorbed CO2 weight ratio in the nanocomposites increased as the GO content increased. Moreover, the desorption curves of CO2 in the pure PLA and PLA/GO nanocomposites during the first hour are shown in Figure 4b. During the period of time, the curves became relatively flat as the GO content increased indicating the reduction of CO2 diffuse speed. This was because

Table 1. Thermal Properties of PLA and PLA/GO Composites sample PLA PLA/ GO0.2% PLA/ GO0.4% PLA/ GO0.6% PLA/ GO0.8% PLA/ GO1.0%

Tg (°C)

Tc (°C)

Tm1 (°C)

Tm2 (°C)

ΔHm (J/g)

ΔHc (J/g)

χc (%)

59.1 59.4

0.0 133.8

153.4 154.0

0.0 157.7

0.6 6.3

0 −4.6

0.6 1.8

59.5

132.9

154.2

157.6

7.3

−5.4

2.0

59.4

131.2

153.9

157.8

13.7

−8.6

5.4

59.4

130.0

154.1

157.3

19.8

−12.5

7.8

60.1

132.2

154.0

157.5

8.5

−5.3

3.4

due to the heterogeneous nucleation effect of GO. The differences in melting temperatures (Tm) of the PLA/GO nanocomposites were not obvious, and double endothermic melting peaks were observed for all PLA/GO nanocomposites which were attributed to the melt-recrystallization behavior.28,29 The degree of crystallinity (χc) of PLA and PLA/GO nanocomposites was determined by eq 430 χc =

ΔHm − ΔHc φ × ΔHm0

× 100% (4)

where ΔHm is the melting enthalpy and ΔHc is the crystallization enthalpy obtained by the DSC curves. ΔH0m is the enthalpy of 100% crystalline PLA sample, which is 93 J/g for the PLA used in this study.31 Table 1 results indicated that the crystallinity of PLA matrix was promoted with the presence

Figure 4. (a) Absorbed CO2 weight ratio in PLA and PLA/GO nanocomposites with 0, 0.2, 0.4, 0.6, 0.8, and 1.0 wt % GO at 100 °C, 20 MPa for 0.5 h. (b) Desorption curves for CO2 weight ratio in PLA and PLA/GO nanocomposites during the first hour: Mt is the mass by the sample at time t, and Mi is the mass by the sample at initial time. (c) Absorbed CO2 weight in PLA and PLA/GO nanocomposites with 0, 0.4, and 1.0 wt % GO at 100 °C, 6.894 MPa for 0.5, 1.5, 3, and 4 h, respectively. 762


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Figure 5. Rheological properties of PLA and PLA/GO nanocomposites: (a) storage modulus, (b) loss modulus, (c) Cole−Cole plots of storage modulus versus loss modulus, and (d) complex viscosity.

when the CO2 pressure reached 5 MPa.40−42 Moreover, it has been proven by Da-chao Li that PLA could not crystallize at 105 °C and 10 MPa CO2 pressure.43 Therefore, at the saturation condition in our study (100 °C, 20 MPa), PLA could not crystallize. The presence of GO in the PLA/GO nanocomposites helped improve CO2 absorption into the PLA matrix, which would further plasticize the PLA molecular chain. The crystallization of PLA would only happen during depressurizing and post cooling. On the basis of the above analysis and findings, we believe the effect of absorbed CO2 by the materials is the main concern in the current study. 3.4. Rheological Properties of PLA and PLA/GO Nanocomposites. To examine the effects of the incorporation of the GO on the rheological properties, the storage modulus (G′), loss modulus (G″), and complex viscosity (η*) were measured and analyzed. The G′ and G″ of the neat PLA and PLA/GO nanocomposite results are shown in Figure 5a,b. It can be seen that the G′ and G″ of PLA/GO nanocomposites were higher than pure PLA and increased with increasing GO content. The differences were more pronounced at low angular frequency due to the reinforcement effect of GO nanofiller, which formed interconnected networks with a polymer matrix resulting in higher elasticity of PLA/GO nanocomposites than neat PLA. The analogous Cole−Cole plots of log G′ versus log G″ are shown in Figure 5c. It has been reported that the Cole−Cole plots slope value approximately indicates the isotropic and homogeneous properties of polymer melt, and the shift and change of the slope imply that a significant microstructure changes in the composites.44 Therefore, the slope change is used to judge heterogeneity of a polymer system. One can

of the gas barrier properties of GO nanocomposites, as reported by others.35,36 Therefore, this demonstrated that the addition of GO can increased the CO2 absorption ability and decreased desorption diffusivity at the same time. In order to further analyze the effect of GO on the CO2 absorption in PLA and PLA/GO nanocomposites under fast depressurization, the measurements were performed under lower pressure (e.g., 6.894 MPa), and the results are shown in Figure 4c. It was observed that the weight of CO2 absorbed increased with increasing saturation time, and the solubility of CO2 in nanocomposites was higher than that of neat PLA, and the weight gain improved as the GO content increased. A similar observation was also reported by L. M. Matuana et al.37 Moreover, we can see that, with the increase of GO content, the sample reached saturation faster. These results also suggested that the solubility of CO2 in PLA/GO nanocomposites was enhanced with the addition of GO. For batch foaming of PLA materials, the potential crystallization of PLA should be considered during CO2 saturation, since it may affect the subsequent foaming process. The solubility of CO2 in semicrystalline and crystalline polymers becomes more complex when crystallization occurs. In general, as the crystallization occurs, the solubility of a gas in a polymer should decrease.38,39 Relevant research on PLA crystallization at high CO2 pressure has been reported extensively so far. We found that many publications have reported the disappearance of PLA cold crystallization peak when CO2 pressure exceeded 3 MPa, and the decreased PLA melt crystallization temperature as the CO2 pressure increased. The melt crystallization temperature was below 100 °C when CO2 pressure reached 3 MPa; it would further reduce to 95 °C 763


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Figure 6. SEM micrographs of foamed neat PLA and PLA/GO nanocomposites and their cell size distributions on the long axis of orientated cells: (a) neat PLA; (b) PLA/GO0.2%; (c) PLA/GO0.4%; (d) PLA/GO0.6%; (e) PLA/GO0.8%; (f) PLA/GO1.0%.

the η* of nanocomposites gradually showed a decreasing trend as the angular frequency increased. These results suggested that the addition of GO could gradually change the rheological behavior of nanocomposites from Newtonian fluid behavior to pseudoplastic fluid behavior. Moreover, it was found that the η* of the PLA/GO nanocomposites increased with increasing GO content significantly at low angular frequency. As reported previously,45 this η* improvement may be attributed to the nanofiller−nanofiller interaction through hydrogen bonding

observe that the slope of log G′ versus log G″ decreased with increasing GO content indicating that the composites became more heterogeneous and also suggesting that a significant microstructure changes in the PLA/GO nanocomposites. Figure 5d shows the variation of complex viscosity (η*) of the neat PLA and PLA/GO nanocomposites. One can observe that neat PLA and PLA/GO0.2% exhibited a typical Newtonian fluid pattern; the η* remained about the same in a wide range of angular frequency. When the GO content further increased, 764


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Figure 7. (a) Mean long axis cell size and minor axis cell size and (b) ratio between long axis and minor axis cell size of PLA and PLA/GO nanocomposites foams.

and the nanofiller−polymer matrix interfacial interaction. The slight difference between PLA/GO nanocomposites with GO content of 0.8% and 1% may be caused by the agglomeration of GO at high loading level. The melt viscosity is a significant factor that affects the cell nucleation, cell growth, and stabilization of cell structure in the foaming process. 3.5. Morphology of PLA and PLA/GO Nanocomposites after Foaming. The morphologies of the PLA and PLA/ GO nanocomposites after foaming were shown as Figure 6, from which it was noticed that the neat PLA exhibited nearly round cell structure, while the cells of PLA/GO nanocomposites were elongated in the vertical direction presenting elliptical shape, especially those with high GO content. To further specify the elongation of the cells, the cell diameters in the long axis were measured, and the size distributions were showed underneath corresponding SEM micrographs. The statistical results showed that neat PLA had a relatively concentrated cell size distribution in vertical direction with cell size range from 5.6 to 19.3 μm, while the distribution became wider and shifted to greater values as the GO content increased in the PLA, indicating the addition of GO assisted the formation of elongated cell structure. The significance of this effect was further investigated by comparing the mean cell diameter in long axis and minor axis (Figure 7a), and the cell diameter ratio of the diameter in long axis and minor axis (Figure 7b). The foamed neat PLA showed slightly larger diameter in the long axis than in the minor axis, and the ratio is lower than 2.5. The foamed PLA/GO nanocomposites had higher cell diameter in the long axis and lower cell diameter in the minor axis than neat PLA, and the difference increased with the increasing of GO content. The diameter ratios of PLA/GO nanocomposites with high GO content were higher as well. These statistical results further emphasized the importance of GO in the formation of orientated cell structure in the unidirectional foaming of PLA. The reasons would be elaborated in the next section by combining the properties of PLA/GO nanocomposites and the foaming results. The average cell size and cell density were measured as well to investigate the overall foamed structures as shown in Figure 8. It was found that the average cell size increased from 16.9 to 26.5 μm while the average cell density decreased from 3.4 × 108 cell/cm3 to 1.1 × 108 cell/cm3 as the GO content increased. This behavior is different from the traditional unrestricted foaming of polymer composites, which typically shows enhanced average cell density and reduced average cell size as

Figure 8. Average cell size and cell density of PLA and PLA/GO nanocomposite foams.

the filler content increased due to the heterogeneous nucleation effect of the fillers.46 However, the foaming behavior of polymers in restricted environment might be different from unrestricted foaming. Similar foaming results were reported by Kelyn A. Arora et al.9 in restricted microcellular foaming of polystyrene (PS). The expansion ratios of PLA and PLA/GO nanocomposites before and after foaming were measured to investigate the causes of the high cell size and low cell density of the foamed PLA/GO nanocomposites. Figure 9 demonstrated that the expansion ratio increased as the GO content increased in PLA matrix when all the samples were foamed under the same condition. This result was different from some literature reports that reported that the addition of nanofillers into the polymer matrix caused lower expansion ratio than neat polymer because of the polymer composites had higher melt strength represented by the composite viscosity.47 In this case, the improvement of the expansion ratio was attributed to the superior CO2 absorption ability of PLA/GO nanocomposites compared with neat PLA. The expansion advantages of additional CO2 overcame the restriction of the increased matrix viscosity during the foaming process. With the assist of GO, the PLA/GO nanocomposites absorbed more CO2 than neat PLA under the same condition leading to higher specimen expansion ratios after foaming, which further caused larger average cell size and smaller cell density due to the reduction of cell quantity in a specified area. 765


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force during foaming. Meanwhile, the presence of GO particles in the matrix led to stronger surface tension and restrictions in the three restricted directions as indicated by the arrows in Figure 10. Therefore, the synergetic effect of them causes the expansion of CO2 mainly in the vertical direction, which led to the formation of elongated cell structure and the increase of the cell diameter ratio in the long axis and minor axis. The PLA/ GO nanocomposites with higher GO content possessed higher melt viscosity, which should provide more matrix surface tension and cause smaller cell size and higher cell density in general. Instead, we observed the cell size gradually increase, yet the density decreased in this study. This was attributed to the higher CO2 absorption rate of the PLA/GO nanocomposites with high GO content, which caused significantly higher expansion ratio of the polymer matrix (Figure 9), which in return led to larger average cell size. The effect of GO on PLA unidirectionally microcellular foaming is significant, and this highly elongated anisotropic cell structure may provide some unique properties for the PLA/GO foams which would be our future research aspects.

Figure 9. Expansion ratios of PLA and PLA/GO nanocomposites after foaming.

3.6. Demonstration of the PLA/GO Nanocomposites Unidirectional Foaming. It was found in this study that the addition of GO exhibited pronounced influence in the unidirectional foaming of PLA, which caused high expansion ratio and vertically elongated cell structure. This phenomenon could be attributed to the properties of PLA/GO nanocomposites and the unidirectional foaming process as illustrated in Figure 10. Under the same foaming condition, the PLA/GO nanocomposites had the ability to absorb more CO2 which led to the high expansion ratio after foaming as presented in Figures 4 and 9. Moreover, the melt viscosity of PLA/GO nanocomposites was higher than neat PLA as well, which indicates the greater melt strength during foaming process. Researchers have reported a positive correction between the melt strength and surface tension.48 We can infer that the nanocomposites also exhibits larger surface tension during foaming process. Generally, elongated cell structure would form during unidirectional foaming.10 However, the cell diameter ratio in long axis and minor axis of neat PLA was found to be only 2.5, which might be caused by the low viscosity of neat PLA at the foaming temperature as illustrated in Figure 10, top. The surface tension and mold restriction for neat PLA were not strong enough to force the material to expand mainly in the vertical direction. On the contrary, in the case of PLA/GO nanocomposites, as illustrated in Figure 10, bottom, more CO2 was absorbed in the material during saturation which would cause strong expansion

4. CONCLUSIONS In this study, PLA/GO nanocomposites were prepared via a solution mixing method at various GO contents. The thermal and rheological properties, and CO2 absorption ability of the composites, were investigated and compared with neat PLA. It was found that the incorporation of GO improved the crystallinity of PLA matrix. The addition of GO enhanced the storage modulus, loss modulus, and complex viscosity of the PLA matrix as well, and the improvements increased with increase of GO content. The unidirectional microcellular foaming revealed that the PLA/GO nanocomposites had the ability to form highly elongated cell structure. Compared to neat PLA, the relatively high GO content (i.e., >0.6% wt) in PLA/GO nanocomposites showed significantly higher expansion ratio, average cell size, and cell diameter ratio in the long axis and minor axis. These behaviors were caused by the enhanced CO2 absorption rate, PLA/GO matrix viscosity, and the unidirectional foaming process. The PLA/GO nanocomposite foams with oriented and highly elongated cell structure were prepared for the first time in this study. The effect of processing parameters and their mechanical and insulation properties will be investigated to develop their potential applications in future work.

Figure 10. Schematic illustration of the unidirectional foaming process of neat PLA and PLA/GO nanocomposites. 766


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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 Nature Science Foundation of China (No. 51073061, No. 21174044), the Guangdong Nature Science Foundation (No. S2013020013855, No. 9151064101000066), and National Basic Research Development Program 973 (No. 2012CB025902) in China.

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