Morphology and Properties of Injection Molded Solid and Microcellular

Article pubs.acs.org/IECR

Morphology and Properties of Injection Molded Solid and Microcellular Polylactic Acid/Polyhydroxybutyrate-Valerate (PLA/ PHBV) Blends Haibin Zhao,† Zhixiang Cui,‡ Xiaofei Sun,§ Lih-Sheng Turng,*,§ and Xiangfang Peng*,† †

National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering Ministry of Education, South China University of Technology, Guangzhou, China ‡ School of Materials Science and Engineering, Fujian University of Technology, Fuzhou, China § Polymer Engineering Center, University of Wisconsin−Madison, Wisconsin, United States ABSTRACT: Blending poly(lactic acid) (PLA) with polyhydroxybutyrate-valerate (PHBV) presents a practical approach to producing fully biobased blends with tailored material properties and improved foam morphologies. This study investigated the effects of the PLA/PHBV blend composition on the morphology, as well as the thermal and mechanical properties, of both solid and microcellular PLA/PHBV injection molded components. Nitrogen (N2) in the supercritical state was used as the physical blowing agent for the microcellular injection molding experiments. Thermal analysis results showed no difference in the thermal properties of solid and microcellular injection molded specimens. It was also found that the Tg of the PLA phase in the PLA/ PHBV blends decreased with increasing PHBV content for both solid and microcellular specimens. In addition, PHBV content exceeding 45% significantly increased the crystallinity of PHBV in the PLA/PHBV blends and improved the storage modulus of both solid and microcellular components. PLA/PHBV blends were immiscible when the content of PHBV exceeded 30%; PLA/ PHBV blends were only miscible with a low weight ratio of PHBV. The increase of PHBV content significantly decreased the cell size and increased the cell density in the microcellular specimens and resulted in some interesting bimodal microcellular structures within the PLA/PHBV (70:30) blend. Additionally, adding PHBV decreased the tensile strength slightly for both solid and microcellular specimens. Furthermore, adding PHBV did not cause any significant changes in the modulus of the solid or microcellular specimens.

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INTRODUCTION Poly(lactic acid) (PLA) can be synthesized by either ringopening polymerization of lactide or condensation polymerization of lactic acid monomers that are produced from renewable resources via a fermentation process. Lactic acid (LA) has two optically active configurations known as D-LA and L-LA. The L form is the most common in nature and therefore is the main constituent of commercially available PLA. Minor amounts of D-LA are typically used to control the crystallinity of PLA since the D-LA units will disrupt the crystallization of L-LA chains.1 PLA has a high strength (50−70 MPa), high modulus (3 GPa), and high thermal plasticity, all of which are comparable to many petroleum-based plastics.2 Furthermore, it is biodegradable and biocompatible since it can be naturally recycled by biological processes. Due to these unique properties, as well as its affordable cost as compared to other biodegradable polymers, PLA represents an interesting alternative to nonrenewable polymers for applications with short life spans like packaging. Medical grade PLA is also suitable for biomedical applications such as sutures, bone screws, and scaffolding for tissue engineering.3−5 However, PLA exhibits poor melt properties,6 mechanical brittleness,7,8 low heat resistance,9,10 and a slow crystallization rate (thermoformed and injection molded PLA parts appear transparent suggesting a lack of crystallinity). These properties need to be improved in order to widen its range of applications in medical devices and consumer products. © 2013 American Chemical Society

The blending of polymers is an effective, practical, and economic way of obtaining materials with desirable properties. Blends exhibit superior physical and mechanical properties over individual polymers. Blending PLA with other polymers can substantially modify its mechanical and thermal properties, degradation rate, and permeability.11 Polyhydroxyalkanoates (PHAs) are a family of polyesters that are synthesized and intracellularly accumulated as a carbon and energy storage material by various microorganisms.12 PHAs are widely used for various biomedical applications, including drug delivery and tissue engineering scaffolds, due to their excellent biocompatibility and biodegradability.13−17 The first and most prevalent PHA is poly(β-hydroxybutyrate) (PHB). Polyhydroxybutyratevalerate (PHBV) is a copolymer of PHB with randomly arranged 3-hydroxybutyrate (HB) groups and 3-hydroxyvalarate (HV) groups. PHB exhibits high stiffness and crystallinity. In order to increase its flexibility and processing capabilities, PHB is often copolymerized with PHV to form PHBV. Increasing PHV content in PHB also reduces the stiffness, melting point, and crystallinity of PHB.18 Both PLA and PHBV are biodegradable polymers. Blending PLA with PHBV provides a practical way of improving or Received: Revised: Accepted: Published: 2569

June 15, 2012 January 16, 2013 January 22, 2013 January 23, 2013 dx.doi.org/10.1021/ie301573y | Ind. Eng. Chem. Res. 2013, 52, 2569−2581

Industrial & Engineering Chemistry Research

Article

screw diameter of 27 mm and an L/D ratio of 42. Generally, melt mixing must be done above the glass transition temperature of amorphous polymer components or above the melting point of semicrystalline polymer components. The extrusion temperature was independently controlled on eight zones along the extruder barrel and a stand die was used to achieve a temperature profile ranging from 165 to 180 °C. This temperature range was selected to be just high enough to process the materials but low enough not to degrade them. The screw speed was maintained at 100 rpm for all of the specimens. After compounding, the extrudate was cooled in a water bath, dried under dry air, and then cut into pellets. The reason to run pure resins, namely, PLA/PHBV (100:0) and PLA/PHBV (0:100), through the twin-screw extruder is to ensure that all specimens undergo the same thermal-mechanical treatment and history. Standard tensile test bars (ASTM D638-03, type I) were injection molded using an Arburg Allrounder 320S (Lossbrug, Germany) with a 25 mm diameter screw and equipped with microcellular injection molding capability (Trexel, Inc., Wilmington, MA). The processing conditions for both solid and microcellular injection molding are listed in Table 1. The pack/hold stage was absent in microcellular injection molding due to the homogeneous packing pressure that results from the nucleation and growth of micro cells.

tailoring the structure and properties of the blends, especially their foam morphologies, without compromising the biodegradability. A number of studies have reported preparing PLA/ PHBV blends by melting mixing or solvent casting process.19,20 Nanda et al. studied the effect of process parameters on the properties of PLA/PHBV blends and found that the elongation of the optimized blends was significantly improved and the two polymers were immiscible in the blends. The heat distortion properties of PLA could be significantly improved by the incorporation of PHBV into the PLA.21 Zhang et al. studied the blends of PLA and PHB with different weight ratios and found that the biodegradability of PLA improved with increasing PHB content at room temperature.22 Richards et al. prepared the foams of PLA and PHBV by a batch foaming method and found that although PLA and PHBV were immiscible, the presence of small quantities of PHBV (25 wt %) led to low density foams with finer, more uniform cells.18 However, no study was found in the public domain that had investigated the structure and properties of microcellular injection molded PLA/PHBV blends. The resulting biobased and biodegradable microcellular injection molded PLA/PHBV may find many potential applications in the fields of structural components, packaging, and biomedical devices. In this study, solid and microcellular components made of PLA/PHBV blends were prepared using conventional and microcellular injection molding processes, respectively. Microcellular injection molding produces components with excellent dimensional stability while using lower injection pressures, shorter cycle times, and less material.23,24 During this process, supercritical fluid (SCF) N2 was used as the physical blowing agent. This process blended SCFs with the polymer melt in the machine barrel to create a single-phase polymer−gas solution. Cell nucleation first occurred as material was being injected into the cavity through the nozzle due to the sudden pressure drop, thus inducing a thermodynamic instability in the polymer−gas solution.25,26 Cell growth then followed during the mold filling stage but mostly within the mold cavity to help fill and pack the mold cavity. This study investigated the effects of the PLA/PHBV blend composition on the morphology as well as thermal and mechanical properties of both solid and microcellular PLA/PHBV injection molded components. The overall goal was to produce PLA and PHBV blends with suitable properties and good foam morphologies while maintaining biodegradability. The specific objectives of this work were to assess the miscibility, phase morphology, and thermal and mechanical properties of PLA/PHBV blends.

Table 1. Experimental Conditions for the Solid and Microcellular Injection Molding Processes nozzle temperature (°C) mold temperature (°C) injection speed (cm3/s) cooling time (s) back pressure (MPa) SCF flow rate (kg/h) SCF injection pressure (MPa) SCF dosage time (s) pack/hold pressure (MPa) pack/hold time (s)

solid

microcellular

180 10 20 60 60 n/a n/a n/a 100 1

180 10 20 60 60 0.11 30 1 n/a n/a

Rheological Measurements. The samples for oscillatory rheological experiments were prepared by cutting the injectionmolding bars into 25 mm diameter circles which were then dried overnight prior to analysis. The measurements were performed using a strain-controlled AR 2000 Rheometer (TA Instruments, New Castle, DE) in parallel-plate oscillatory mode at 180 °C under a nitrogen atmosphere. The strain amplitude was set at 2% to ensure that all dynamic measurements were within a linear domain according to strain sweep measurements. The measurements were performed at a range frequency of 0.1 to 100 rad/s. The gap was set at 1 mm. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (TA Instruments, Q20) was used to study the thermal properties of the blends. The weight of the specimens was approximately 7−9 mg. All specimens were placed in hermetically sealed aluminum pans. The specimens were first heated from 25 to 200 °C (to remove any prior thermal history resulting from extrusion and injection molding) and then kept isothermal for 5 min, cooled down to 25 °C, and reheated to 200 °C. The ramp rates for all of the heating and cooling cycles were 2.5, 5, 10, and 20 °C/min. The crystallization temperature (Tc), melting temperature (Tm),

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EXPERIMENTAL SECTION Materials. Commercial PLA (2002D) in pellet form was purchased from NatureWorks LLC (Minnetonka, MN, USA). Its specific gravity is 1.24, and its melt flow index is around 7.0 g/10 min (210 °C/2.16 kg). PHBV (Tm = 158 °C, 6L600N19) was purchased from Bopol Monsanto. It has a reported density of 1.25 g/cm3. Both materials were used as received and in pellet form. All specimens were dried in an oven at 65 °C overnight to remove any excess moisture. N2 with a purity of 99.9% was used as the physical blowing agent in the microcellular injection molding experiments. Processing. This study intended to use PLA as the primary material and PHBV as the property modifier at 15% by weight increments. Hence, PLA/PHBV blends with mixture ratios of 100/0, 85/15, 70/30, 55/45, and 0/100 wt % were compounded using a corotating twin-screw extruder with a 2570

dx.doi.org/10.1021/ie301573y | Ind. Eng. Chem. Res. 2013, 52, 2569−2581


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apparent melting enthalpy (ΔHf), and enthalpy of cold crystallization (Hcc) were determined from the DSC curves. Parameters Tm and ΔHf were taken as the peak temperature and the area of the melting endotherm, respectively. Crystallinity of the PLA and PHBV phase was calculated by χc (% crystallinity) =

ΔHf − ΔHcc ΔH

0

×

100 W

Article

RESULTS AND DISCUSSION

Linear Viscoelastic Properties. As shown in Figures 1a−c, PLA and PLA/PHBV melts exhibited viscoelastic behaviors which are a combination of reversible elastic deformations due

(1)

where ΔH0(PLA) and ΔH0(PHBV) are the enthalpies of melting per gram of 100% crystallinity (perfect crystal) of PLA and PHBV (93.7 and 109 J/g), respectively, and W is the weight fraction of PLA and PHBV in the blend.27 Thermogravimetric Analysis (TGA). All TGA analyses were performed on a TA Thermogravimetric Analyzer Q200. Specimens of 20 mg were heated from room temperature to 500 °C at a rate of 10 °C/min under a nitrogen atmosphere. All TGA results are the average of a minimum of three measurements, and the temperatures are reproducible to ±1 °C. Dynamic Mechanical Analysis (DMA). Dynamic mechanical analysis was carried out using a TA Q800 Dynamic Mechanical Analysis (DMA) instrument. Rectangular specimens with dimensions of 17.6 by 12.7 by 3.2 mm were cut from injection molded parts and tested in single cantilever mode. The characterization of the glass transition temperature was carried out by tests performed in the temperature range of −20 to 80 °C at a heating rate 3 °C/min with a 1 Hz frequency and a 0.02% prestrain, which was in the linear viscoelastic region as determined by a strain sweep. Scanning Electron Microscopy (SEM). The morphology of the microcellular injection molded specimens was examined using a scanning electron microscope (SEM LEO 1530) with an accelerating voltage of 5 kV. The SEM specimens were taken from the cross-section at the middle of the molded tensile bar, which was fractured in liquid nitrogen. The surfaces of the fractured specimens were sputter coated with gold prior to observation. A comparison between the SEM images of different specimens was taken at the same magnification. The cell size was analyzed using an image analysis tool (UTHSCSA Image Tool), and the cell density was calculated using the following formula: cell density =

⎛ N ⎞3/2 ⎜ ⎟ M ⎝ L2 ⎠

(2)

where N is the number of cells, L is the linear length of the area, and M is a unit conversion resulting in the number of cell per cubic centimeter. To be consistent, the data was obtained using SEM micrographs taken from the center portion of the crosssection of the tensile bars. In order to avoid skewing of the data, a few abnormally large voids that were observed in some specimens were excluded from the calculation of the average cell size and cell density. Tensile Tests. The injection molded tensile bars were tested on a screw-driven universal testing instrument (MTS, Sintech 10/GL) per ASTM D638-03 at room conditions. Using a constant crosshead speed of 10 mm/min, at least eight tensile bars were tested for each material and the mean and range of the modulus of elasticity (Young’s modulus), ultimate tensile strength, and strain-at-break for each material were calculated.

Figure 1. Frequency dependence of storage moduli G′ (a), loss moduli G″ (b), and complex viscosity η* (c) of pure PLA and PLA/PHBV melts. 2571



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Figure 2. (a) Cooling and heating curves of PLA, PHBV, and PLA/PHBV blends of different mixtures at different heating rates: (b) 2.5, (c) 5, (d) 10, and (e) 20 °C/min.

plateau. The slope of log G′ versus log ω decreased with increasing PHBV content, suggesting that PLA/PHBV blends had much larger elasticity with increasing PHBV content. The higher values of dynamic moduli indicated the formation of entanglement structures in the PLA/PHBV melts. The entanglement effect led to the highly reversible elastic deformation of melts and partially prevented the relaxation of the melt structure. On the other hand, the immiscible nature of

to molecular entanglements and irreversible viscous flows due to polymer chain slippage, represented by storage modulus (G′) and loss modulus (G″), respectively. At higher frequencies, samples experienced a shorter thermal history, resulting in overlapping curves. This indicates that the incorporation of PHBV had less effect on the storage moduli (G′) at higher frequencies. However, at low frequencies, G′ increased with PHBV content and resulted in a more profound 2572



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Table 2. Thermal Characteristics of the PLA/PHBV Blends Obtained from the Second Heating Run at Different Heating Rates cold crystallization PLA/PHBV specimens 2.5 °C/min (100:0) (85:15) (70:30) (55:45) (0:100) 5 °C/min (100:0) (85:15) (70:30) (55:45) (0:100) 10 °C/min (100:0) (85:15) (70:30) (55:45) (0:100) 20 °C/min (100:0) (85:15) (70:30) (55:45) (0:100)

melting

degree of crystallinity (%)

temp (°C)

enthalpy (J/g)

temp 1 (°C)

temp 2 (°C)

enthalpy (J/g)

PLA

120.74 105.45 96.59 99.43

28.98 22.71 17.00 14.55

151.56 147.89 145.92 142.01 149.30

157.91 156.71 156.48 156.84 157.36

34.76 27.90 25.25 37.93 59.63

6.17

115.90 107.69 111.99

25.07 22.43 14.04

149.60 147.71 140.15 147.88

156.67 156.49 156.81 156.91

29.82 24.73 40.66 56.81

136.01 122.72 123.51

2.65 20.49 5.80

152.10 138.98 146.14

154.57 156.38 153.28 155.91

4.12 22.49 31.78 62.09

135.81 145.72

154.92 153.71 151.83 154.96

2.21 3.62 26.53 58.53

PHBV

31.74 25.23 47.67 54.71 0 29.05 7.03 54.27 52.12 0 8.99 6.12 52.97 56.96 0

PLA/PHBV blends might play a role in the G′ curves at low frequencies. There was not much difference in the loss modulus (G″) of melts for pure PLA and PLA/PHBV blends, as can be seen in Figure 1b. The master curves of dynamic complex viscosity, η*, for PLA and PLA/PHBV melts are presented in Figure 1c. The complex viscosity of the pure PLA melt displayed a Newtonian fluid behavior at a frequency range smaller than 10 rad/s. It was constant for frequencies up to 10 rad/s, beyond which shearthinning behavior started to appear. Compared to pure PLA melts, the complex viscosity of PLA/PHBV melts showed a stronger shear-thinning tendency at all frequencies; this tendency became stronger as the PHBV content increased. Thermal Properties. Figure 2a shows the representative thermograms obtained from the first cooling scans with a cooling rate of 10 °C/min. Figure 2b−e shows the second heating scans at heating rates of 2.5, 5, 10, and 20 °C/min, respectively. Table 2 tabulates the numerical values of temperature and enthalpy obtained from the second heating scan, as well as the degree of crystallinity for the PLA and PHBV phase, at the different heating rates. Although the DSC analysis was done on both solid and microcellular components, no difference was observed in their thermal properties, especially in the cooling and second heating cycles. This shows that the processing conditions did not have any effect on the thermal properties of the materials. Hence, for simplicity, only the results from solid components are discussed below. Figure 2b−e shows the glass transition shoulders and melting peaks of pure PLA, pure PHBV, and their blends. As shown in the figures, the glass transition shoulder was prominent in pure PLA but not in pure PHBV. Similarly, pure PLA did not exhibit any melting peak (except at the slowest heating rate of 2.5 °C/ min), while pure PHBV exhibited two peaks. The biomodal peak distribution of PHBV might be due to some less-than-

13.52 11.07 54.09 53.70

perfect crystals which had enough time to melt and reorganize into crystals with higher structural perfection that were subsequently remelted at a higher temperature.28 In either case, i.e., glass transition shoulder or melting peaks, the blends exhibited a mixed behavior corresponding to their blend ratio. As shown in Figure 2b, for pure PLA and pure PHBV specimens, the double peaks were evident in the second heating curve at 2.5 °C/min. The double peaks still existed for PLA/ PHBV blends, with the high temperature melting peak being the dominant one. Numerous researchers have reported multiple melting behaviors for PLA, PHBV, and their copolymers under isothermal crystallization using step-scan DSC. Multiple melting peaks were observed due to the following reasons: (1) melting, recrystallization, and remelting during heating, (2) isodimorphism or polymorphism, which is the presence of multiple crystal forms, (3) different lamellar thickness, distribution, and morphology, and (4) species with different molecular weight. The results of the PLA/PHBV blends showed one large melting peak and a residual peak from the remaining crystals for all concentrations of PHBV. For the polymer blends, the polymer with the higher melting point crystallized first causing spherulites to fill in the available space. The polymer with the lower melting point was spatially limited inside of the pre-existing spherulites. When polymers have similar melting temperatures, both polymers have the ability to cocrystallize. The melting temperatures of PLA and PHBV used in this study were around 160 °C. The cocrystallization behavior made another phase by acting as a heterogeneous nucleation agent, thereby inducing a higher crystallization rate and improving crystal perfection during the melting process. Thus, the double peak DSC curves of the PLA/PHBV blends exhibited a dominating melting peak at a higher temperature in the second heating scan. Meanwhile, the enhancement of 2573



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crystallization had a positive effect on the foam morphology of the PLA/PHBV blends. Table 2 shows the degrees of crystallinity of pure PLA, pure PHBV, and those of PLA and PHBV in the PLA/PHBV blends. The degree of crystallinity of pure PLA was found to be approximately 6.17% at a heating rate of 2.5 °C/min. PLA was totally amorphous at the heating rates of 5, 10, and 20 °C/min. The crystallinity of PLA was calculated with the same formula as that of PHBV; i.e., eq 1, where the ΔH0(PLA) is 93.7 J/g.29 This result suggests that PLA was not able to recrystallize with a heating rate higher that 5 °C/min. This is because the PLA used in this study has slow nucleation and crystallization rates so that the specimen would be nearly 100% amorphous after being rapidly cooled during injection molding. Therefore, the PLA in the blends was assumed to be totally amorphous when calculating the crystallinity of PHBV. As shown in the table, the degree of crystallinity of PHBV did not change with increasing PHBV content up to 30%. It is noted that the crystallinity of PHBV in PLA/PHBV (70:30) is always a little lower than that in PLA/PHBV (85:15). But when the PHBV content exceeded 45%, the degree of crystallinity of PHBV sharply increased, even higher than that of pure PHBV. This is mainly because the PLA in the blend acted as a crystal nucleating agent for the PHBV phase.22 Furthermore, the high crystallinity of PHBV in PLA/PHBV blends indicated that the PLA and PHBV are immiscible when the content of PHBV was up to 45%. The significant increase in the degree of crystallinity in the PHBV phase had an obvious effect on the mechanical properties of the blends, which will be discussed later in the DMA analysis. Moreover, comparing curves in Figures 2b−d, the cold crystallization temperature upon heating decreased and the peak width narrowed with increasing PHBV content, indicating enhanced crystallization kinetics of the PLA/PHBV blends. However, the cold crystallization peak was more profound for the PLA/PHBV (70:30) specimen, which exhibited very little crystallization behavior during the first cooling scan. Thermal Stability. The thermal degradation of PLA, PHBV, and PLA/PHBV blends was investigated in terms of weight loss by TGA carried out under a nitrogen atmosphere. As shown in Figure 3a, pure PLA and pure PHBV exhibited single−stage thermal degradation at around 320 and 260 °C, respectively. Thus, PLA possessed a higher thermal stability than PHBV. On the other hand, PLA/PHBV blends showed multiple stages of degradation and onset temperatures between those of PLA and PHBV. That is, the blends exhibited lower thermal stability than PLA, but higher thermal stability than PHBV. Also, the two-stage degradation profile of the blends may have been correlated to the degradation of the PHBV and PLA, respectively. The derivative thermogravimetric (DTG) curves were a fair indication of the temperature at which the maximum weight loss was triggered.30 As can be seen in Figure 3b, the blends showed two well-defined decomposition peaks (in the DTG curves), that were attributed to the thermal degradations of the PLA and PHBV components by comparing them with the decomposition of neat polymers. The decomposition temperatures of the PLA component in the blend decreased with an increase of PHBV content, whereas the decomposition temperature of the PHBV component did not change. Dynamic Mechanical Properties (DMA). The viscoelastic properties of solid and microcellular PLA, PHBV, and PLA/ PHBV blend components were studied using DMA to track the temperature dependence of the storage modulus at different

Figure 3. TGA curves of PLA, PHBV, and PLA/PHBV blends of different mixtures at a heating rate of 10 °C/min: (a) TGA curves and (b) DTG curves.

PLA/PHBV compositions (cf Figure 4). From the figure, the storage moduli of all solid and microcellular specimens declined with increasing temperature, with the most rapid reduction occurring in the glass transition region of PLA around 60 °C. In the glassy region of PHBV (less than 0 °C), significant enhancement of the storage modulus can be observed with increasing PHBV content (specimens >30% PHBV), thus indicating that PHBV had a strong influence over the elastic properties of the PLA in the low temperature region. However, it is noted from Figure 4a that the storage modulus of solid PLA/PHBV (70:30) is lower than that of PLA/PHBV (85:15). This is because PLA/PHBV (70:30) has a lower degree of crystallinity than PLA/PHBV (85:15) (cf Table 2). Only when the PHBV content in the blend increased to above 45% could a high degree of crystallinity in PHBV be achieved, thus leading to a significant increase in the storage modulus, as in the case of PLA/PHBV (55:45). However, for microcellular PLA/PHBV blend specimens, as shown in Figure 4b, in the temperature range under the glass transition temperature of PHBV (0 °C), it was observed that the storage modulus decreased with increasing PHBV content. This is mainly due to the smaller cell size and higher cell density with the increasing of PHBV content. The increasing content of PHBV made the PLA/ PHBV blend getting to be with higher porosity and weight reduction, which will be shown in the morphology discussion. The storage modulus of microcellular PLA/PHBV (55:45) showed in Figure 4b was lower than those of pure PLA and PHBV across almost the full temperature range. This was 2574



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Figure 4. Temperature dependence of the storage modulus for (a) solid and (b) microcellular PLA, PHBV, and PLA/PHBV blends.

Figure 5. Temperature dependence of tan δ for (a) solid and (b) microcellular PLA, PHBV, and PLA/PHBV blends.

probably due to the fact that a cocontinuous morphology existed in PLA/PHBV (55:45), resulting in a lower storage modulus compared with other blends that had a higher PLA content. On the other hand, as will be shown in the morphology section below, the microcellular PHBV sample exhibited a finer cell structure than those of the PLA/PHBV blends, including the PLA/PHBV (55:45) samples. As a result, the storage modulus of microcellular PHBV was higher than some of the PLA/PHBV blends, especially at higher temperatures. Due to the lack of a clear trend, additional studies are needed to provide better picture of the DMA properties of solid and microcellular PLA, PHBV, and PLA/PHBV blends. Figure 5 shows the tan δ curves of all of the specimens. Tan δ is the ratio between the loss modulus and the storage modulus. In the tan δ curve, a peak is observed at the region where the rate of decrease in the storage modulus is higher than that of the loss modulus with an increase in temperature. The temperature corresponding to the tan δ peak is often considered the glass transition temperature, Tg.31 As can be seen in Figure 5, the peaks around 65 °C correspond to the Tg of PLA. The glass transition temperatures corresponding to the PLA phase are tabulated in Table 3. In general, for both solid and microcellular specimens, an increase in PHBV content shifted the tan δ peaks to a slightly lower temperature. That is, the presence of PHBV decreased the Tg of PLA slightly, which

Table 3. Glass Transition Temperature of Solid and Microcellular PLA and PLA/PHBV Blends Based on tan δ Peaks Tg (°C) of PLA specimens PLA/PHBV PLA/PHBV PLA/PHBV PLA/PHBV PLA/PHBV

(100:0) (85:15) (70:30) (55:45) (0:100)

area under the tan δ curve (cm2)

solid

microcellular

solid

microcellular

68.23 66.33 65.12 65.66

65.97 64.93 64.52 62.43

20.8795 15.1574 11.5491 6.8081 0

20.7505 16.8589 12.8629 7.5825 0

is consistent with results from the DSC. However, no significant difference was observed between the Tg of solid and microcellular specimens. Moreover, the area under the tan δ curve for both the solid and microcellular specimens decreased with increasing PHBV content (cf Table 3). Generally, a large area under the tan δ peak indicates a better damping ability.32 Morphology of the Solid and Microcellular PLA/PHBV Blends. SEM was used to investigate the morphology of solid and microcellular PLA, PHBV, and PLA/PHBV blend components. The SEM images provide information on the microstructure, including the cell morphology in microcellular 2575



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Figure 6. SEM micrographs of the fracture surface of PLA, PHBV, and PLA/PHBV blends with magnification of ×5000: (a) 100:0, (b) 85:15, (c) 70:30, (d) 55:45, and (e) 0:100.

specimens and the fracture characteristics of the specimens. Figure 6 shows representative SEM images of the fractured surface of the solid PLA/PHBV blends. All images were taken at the same magnification of ×5000 (scale bar 2 μm). As can be seen from the images shown in Figure 6, pure PLA showed a typical fracture surface of an amorphous polymer without visible plastic deformation, indicating that the specimen fractured under a brittle mode. The fracture surface of the PLA/PHBV (85:15) and PLA/PHBV (70:30) blend specimens were relatively smooth as well. However, pure PHBV and PLA/ PHBV (55:45) showed an irregular facture surface due to their crystalline structure and more ductile fracture behavior. Between the PLA/PHBV blends, PLA/PHBV (85:15) showed

a single phase, whereas PLA/PHBV (70:30) and PLA/PHBV (55:45) consisted of two obvious phases, which indicates that the PLA/PHBV blends with these two weight ratios are not miscible. Two apparent phase morphologies are distinguishable in Figures 6c and d. The micrographs clearly show the transition from a droplet-matrix type of morphology to a cocontinuous morphology with large interpenetrating structures. That is, the blend PLA/PHBV (70:30) in Figure 6c consists of regions of small PHBV droplets dispersed in the PLA matrix and regions where both phases coexisted in a corallike interconnected network structure due to enhanced coalescence when the concentration of the dispersed phase 2576



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Figure 7. Representative SEM images of the fracture surfaces of microcellular PLA, PHBV, and PLA/PHBV blends with magnification of ×400: (a) 100:0, (b) 85:15, (c) 70:30, (d) 55:45, and (e) 0:100.

increased. On the other hand, the morphology of the PLA/ PHBV (55:45) shown in Figure 6d is clearly fully cocontinuous. Figures 7 and 8 shows the representative SEM images of the tensile fractured surfaces of the microcellular PLA/PHBV blends at different magnifications. All images in Figure 7 were taken at the same magnification of ×400 (scale bar 100 μm). The average cell size (obtained from the center portion of the cross-section of the tensile bars) and cell density of the microcellular specimens were qualitatively analyzed and are presented in Figure 9. The average cell size of PLA/PHBV (85:15) was 160 μm. With an increase in the ratio of PHBV to 30%, the average cell size decreased to 120 μm. Upon increasing the content of PHBV further to 45% for the PLA/

PHBV (55:45) specimen, the average cell size decreased to 100 μm. The cell density increased by about 94% and 160% for the PLA/PHBV (70:30) and PLA/PHBV (55:45) specimens, respectively, compared with PLA/PHBV (85:15). The incorporation of PHBV significantly decreased the cell size and increased the cell density of the microcellular PLA/PHBV blends. The observed difference in cell morphology among these specimens can be attributed to differences of material composition and processing parameters. Furthermore, the addition of PHBV increased the melt strength of PLA/PHBV blends, thus hindering cell growth and coalescence, and reducing cell size.33 2577



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Figure 8. SEM images of the fracture surfaces of microcellular PLA/PHBV (70:30) and PLA/PHBV (55:45) blends: (a) PLA/PHBV (70:30) ×1000, (b) PLA/PHBV (70:30) ×5000, (c) PLA/PHBV (55:45) ×1000, and (d) PLA/PHBV (55:45) ×5000.

zoomed in to 5000 to further investigate the phase structure. It is interesting to find that PLA/PHBV (70:30) displayed bimodal foam morphology; that is, a number of smaller cells about 1 μm in diameter surrounded a bigger cell around 120 μm in diameter. However, this phenomenon did not appear in PLA/PHBV (55:45) and other blends. That mainly occurred because PLA/PHBV (70:30) had a suitable melt strength which helped produce a single polymer−gas phase in the melt. A number of cell nucleation sites could be well kept during the cell growth and cooling stages. Bao et al. also found a bimodal cell structure in PS foams by using a two-step depressurization batch process.34 The bimodal cell structure can be controlled by adjusting the process parameters at the holding stage between the two steps. The bimodal cell structure has been seen in some microcellular injection molded parts but has never been studied extensively or reported. Polystyrene (PS) is known as easy-to-foam due to its suitable melt strength, suggesting that the PLA/PHBV (70:30) may behave similarly to PS. Mechanical Properties. Tensile tests (according to ASTM-D638) were performed on the injection molded solid and microcellular specimens of PLA, PHBV, and PLA/PHBV blends. Properties such as strength, modulus, and strain-atbreak were measured. The properties reported here are the actual readings measured for the solid and microcellular specimens without taking into account the weight reduction of the microcellular specimens. Figure 10a, b, and c shows the

Figure 9. Average cell size and cell density of microcellular PLA, PHBV, and PLA/PHBV blends obtained from the SEM analysis.

In order to further study the phase morphology of microcellular PLA/PHBV blends, the magnified SEM images of microcellular PLA/PHBV (70:30) and (55:45) with greater magnifications of ×1000 and ×5000 are presented in Figure 8. The area around the cell in Figures 8a and c, with 1000 magnification for PLA/PHBV (70:30) and (55:45), was 2578



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strength, modulus, and strain-at-break of solid and microcellular PLA, PHBV, and PLA/PHBV blends specimens, respectively. As shown in Figure 10a, the incorporation of PHBV decreased the tensile strength of solid PLA; the tensile strength

decreased from 54.99 to 42.23 MPa by increasing the PHBV content from 0% to 45%. The reduction in the tensile strength of solid PLA/PHBV blends may be attributed to the relatively low strength of PHBV and relatively weak polymer interfaces, which was discussed in the morphology analysis. The tensile strength of microcellular specimens was found to be less than that of their solid counterparts; however, it is still more than 30 MPa. Furthermore, there was no significant difference in the tensile strengths of microcellular PLA, PHBV, and PLA/PHBV blends with different weight ratios of PHBV. As shown in Figure 10b, the addition of PHBV did not cause any significant difference in the modulus as well either. Also, similar to the tensile strength, the moduli of the microcellular specimens were found to be less than that of their solid counterparts which can be attributed to the presence of voids in the microcellular specimens. It should be noted that a few large bubbles were found in the microcellular injection molded specimens. These large bubbles may become points of stress concentrations, thereby potentially decreasing the mechanical properties of the specimens.28,35 Figure 10c shows that the strain-at-break of neat PLA and PHBV were only about 3.5%. These materials had no necking in the tensile tests and exhibited a distinct yield point (maximum load) with subsequent failure by neck instability. However the incorporation of PHBV in the PLA/PHBV blend enhanced the strain-at-break as compared to pure PLA and PHBV. The blends showed yielding and stable neck growth though cold drawing. It was also noticed that the strain-at-break of solid PLA/PHBV (85:15) and microcellular PLA/PHBV (70:30) was significantly higher than the other solid and microcellular specimens. This was expected since solid PLA/ PHBV (85:15) has a better polymer interface and shows a single phase (cf Figure 6) in the morphology analysis. Meanwhile, it can be seen from Figure 10c that the strain-atbreak of microcellular PLA/PHBV (85:15) was lower than its solid counterpart. However, microcellular PLA/PHBV (70:30) and PLA/PHBV (55:45) had a larger strain-at-break than their solid counterparts. For microcellular PLA/PHBV (85:15), the decrease in strain-at-break, as compared to solid specimens, may be attributed to the existence of large bubbles that may be have acted as stress concentrators in the specimens. However, by increasing the amount of PHBV, a morphology with a much smaller cell size and higher cell density was produced, especially for the microcellular PLA/PHBV (70:30) specimens. This modified morphology led to a bimodal cell morphology with cells as small as 1 μm. The large bubbles, which could be points of stress concentration, were eliminated (cf Figures 7 and 8). Thus, the combined effects of interfacial adhesion and foam morphology (cell size and cell density) yielded different results for the strain-at-break of the microcellular PLA/PHBV blends.

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CONCLUSIONS Solid and microcellular injection molded tensile test bars of PLA/PHBV blends with different weight ratios (100:0, 85:15, 70:30, 55:45, and 0:100) were produced using both conventional and microcellular injection molding processes. Their morphology, fractured structure, crystallization behavior, thermal, and dynamic mechanical properties were investigated. The results showed that a content of PHBV exceeding 45% could significantly increase the crystallinity of PHBV in the PLA/PHBV blends and improve the storage modulus of both solid and microcellular components. It was also found that the Tg of the PLA phase in the PLA/PHBV decreased with

Figure 10. Mechanical properties of the solid and microcellular PLA, PHBV, and PLA/PHBV blends: (a) tensile strength, (b) tensile modulus, and (c) strain-at-break. 2579



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increasing PHBV content for both solid and microcellular specimens. PLA/PHBV blends were only miscible with low weight ratios of PHBV. Furthermore, when the content of PHBV exceeded 30%, the blends became immiscible. The increase of PHBV content significantly decreased the cell size and increased the cell density in the microcellular specimens. Additionally, adding PHBV decreased the tensile strength slightly for both solid and microcellular specimens. Adding PHBV did not cause significant changes in the moduli of solid and microcellular specimens. The incorporation of a low amount of PHBV improved the strain-at-break significantly for both solid PLA/PHBV (85:15) and microcellular PLA/PHBV (70:30).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.-S.T.); [email protected]. cn (X.P.). 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. 51073061, No. 21174044), the Fundamental Research Funds for the Central Universities (No. 2011ZZ0011), and the 973 Program (2012CB025902). The first two authors would like to acknowledge the China Scholarship Council for the financial support of their visits to UW−Madison and the facility and technical support by the Wisconsin Institute for Discovery and the Polymer Engineering Center at UW-Madison.

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Morphology and Properties of Injection Molded Solid and Microcellular

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