Morphological, Mechanical, and Crystallization Behavior of Polylactide

Sep 8, 2015 - ABSTRACT: L-Lactide/caprolactone copolymer (LACL) was employed as a compatibilizer in the immiscible blends of polylactide (PLA) and ...
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Morphological, Mechanical, and Crystallization Behavior of Polylactide/Polycaprolactone Blends Compatibilized by L‑Lactide/ Caprolactone Copolymer Chunmei Zhang,*,†,‡,§ Tianliang Zhai,§ Lih-Sheng Turng,*,‡ and Yi Dan§ †

College of Chemistry and Materials Engineering, Guiyang University, Guiyang 550005, China Wisconsin Institute for Discovery, University of WisconsinMadison, Madison, Wisconsin 53715, United States § State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡

ABSTRACT: L-Lactide/caprolactone copolymer (LACL) was employed as a compatibilizer in the immiscible blends of polylactide (PLA) and polycaprolactone (PCL). Films of neat PLA, PLA/PCL blend, and PLA/PCL/LACL blends were prepared by solution casting and conditioning compression. The effects of LACL on the morphology, mechanical properties, and crystallization behaviors of the blends were investigated. The addition of LACL decreased the dimensions of the dispersed PCL domains in the ternary blends. The ultimate strain of the blend with 5% LACL was significantly improved and was accompanied by a small drop in tensile strength and modulus. Both the nonisothermal and isothermal cold crystallization of PLA were enhanced by PCL, and further improved by incorporating LACL. Polarized optical microscopy observation showed that the size of crystals decreased with the addition of PCL and LACL.

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INTRODUCTION The promising characteristics of polylactide (PLA) include good mechanical performance, thermal stability, processability, and environmental friendliness because it is renewable and fully biodegradable.1−3 However, PLA is generally considered to be a brittle polymer due to its stiff backbone chain, which limits its use in applications where mechanical toughness or high elongation is required (e.g., screws and fracture fixation plates).4 Moreover, PLA exhibits a rather slow crystallization rate and low crystallinity, which also greatly limits its practical applications.5 To improve the toughness of the PLA materials, many strategies have been developed, including grafting, polymer blending, and the use of plasticizers.6 Among these strategies, blending PLA with other polymers to obtain desirable mechanical properties is one of the most inexpensive methods and has found wide utility, especially in industrial settings.7 In order to improve the toughness of the PLA matrix without compromising its biodegradability, degradable and/or partially renewable polymers with superb flexibility, such as polycaprolactone (PCL),8 poly(butylene adipate-co-terephthalate) (PBAT),9 poly(butylene succinate) (PBS),10 and polyhydroxyalkanoates (PHAs),11 have been employed as the second component in the PLA matrix. Modified natural rubber12−14 has also been fabricated in our group as a toughening agent for PLA. PCL is likely the most widely studied toughening agent for PLA.15 PCL exhibits high flexibility with an elongation-at-break of about 600% due to its low glass transition temperature.16 However, these two biodegradable polymers have been reported to be immiscible, thus leading to insignificant improvements in mechanical properties.17 Hence, compatibilization is necessary. Generally, a copolymer whose chemical structure is identical to that of the polymer blend can be used as © 2015 American Chemical Society

a compatibilizer to improve the phase morphology and interfacial adhesion of the two components. Maglio et al.18 found that the addition of a small amount of PLA−PCL−PLA triblock copolymer (∼4 wt %) to the PLA/PCL (70/30, w/w) blend improved the dispersion of the PCL in the PLA matrix and enhanced the ductility of the resultant ternary blend. The dimension of the dispersed PCL domains decreased from 10 to 15 μm down to 3 to 4 μm with the addition of the triblock copolymer, resulting in an increase in the elongation-at-break from 2% for the PLA/PCL blend to 53% for the ternary blend. Tsuji et al.19 saw a similar improvement in the elongation-atbreak when a PLA−PCL diblock copolymer was added to their binary PLA/PCL blend. For example, in the PLA/PCL (80/20, w/w) blend, the elongation-at-break increased from 175% to 300% when 10 wt % of the block copolymer was added. It is well-known that the mechanical properties of a semicrystalline polymer are greatly dependent on its solid state morphology and crystallinity. Hence, the study of crystallization behavior of semicrystalline polymers has received great interest in the past few decades, especially for PLA.20−22 The crystalline structure and morphology of semicrystalline polymers are greatly affected by their thermal histories. Therefore, it is important to research their crystallization kinetics because control of the crystallization factors allows for the design of materials with desirable properties. Previous works on binary and ternary PLA/PCL blends have been mainly focused on mechanical and morphological study. In addition, the effect of copolymer compatibilizers on crystallization, especially the isothermal and nonisothermal Received: Revised: Accepted: Published: 9505

June 12, 2015 August 12, 2015 September 8, 2015 September 8, 2015 DOI: 10.1021/acs.iecr.5b02134 Ind. Eng. Chem. Res. 2015, 54, 9505−9511

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Industrial & Engineering Chemistry Research

experiments were performed at room temperature (25 °C) and atmospheric conditions (relative humidity of 20 ± 5%), with a crosshead speed of 10 mm/min. The samples were sliced into 10 mm wide strips. The distance between the jaws was 20 mm. Five samples were tested for each material and the average values and standard deviations were reported. The crystallizations of the samples were characterized by a Thermal Analysis Instruments Q20 DSC with Universal Analysis 2000 software. The samples were first heated from room temperature to 200 °C at a rate of 20 °C/min, and then held for 3 min to eliminate any prior thermal history. For nonisothermal cold crystallization, the samples were cooled from 200 to 20 °C, and then heated to 200 °C, both at 10 °C/ min. For isothermal cold crystallization, the samples were cooled from 200 to 20 °C and then heated to a predetermined temperature at a rate of 60 °C/min and held for a period of time until isothermal crystallization was complete. The evolution of heat flow with crystallization time was recorded during the isothermal cold crystallization process for later data analysis. After isothermal crystallization at various temperatures, the samples were heated up again to 200 °C at 10 °C/min to study their melting behaviors. All experiments were performed under nitrogen atmosphere. The crystalline morphology of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) after isothermal cold crystallization at 95, 105, and 115 °C was studied using a POM (Olympus BX51, Olympus Corp., Tokyo, Japan) equipped with a CCD camera. The samples were sandwiched between two thin glass slides to a thickness of around 50 μm, heated to 200 °C on a hot stage, and then held for 5 min to eliminate any residual thermal history. Then, these molten films were promptly quenched in ice water to freeze their amorphous states. The samples were wiped with tissue and vacuum-dried at room temperature for 3 h before isothermal crystallization at 95, 105, and 115 °C for 3 h in a vacuum oven. The crystalline morphologies of the samples were recorded by the CCD camera.

crystallization behavior of PLA in the ternary blend, have yet to be detailed. Through this kind of study, the interrelation of toughening, compatibilizing, and crystallization behavior of the blends can be obtained and better tailored according to different environmental or physiological requirements. In this study, a commercially available L-lactide/caprolactone copolymer (LACL) was incorporated into the PLA/PCL blends as a compatibilization agent. The unary PLA, binary PLA/PCL blend, and ternary PLA/PCL blends with different amounts of LACL were prepared by solution mixing, casting, and conditioning compression. The morphological and mechanical properties of these blends were investigated by scanning electron microscopy (SEM) and tensile testing. Comprehensive and detailed studies concerning the effects of the added LACL on the isothermal and nonisothermal cold crystallization behaviors of PLA in the ternary blends were also measured by differential scanning calorimetry (DSC). Furthermore, the crystal growth behavior of PLA and the blends was also studied by polarized optical microscopy (POM).

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EXPERIMENTAL SECTION Materials. Polylactide (PLA, 2002D), a commercial product of NatureWorks Co. Ltd., U.S.A., had a D content of 4.25 wt %, a residual monomer content of 0.3 wt %, and a density of 1.24 g/cm3. The polycaprolactone (PCL, CAPA6500) used in this study was purchased from Solvay Co. Ltd., Belgium. Its melt flow index (MFI) was about 7 g/10 min (160 °C/2.16 kg, ASTMD1238), and its −OH value was lower than 2 mg KOH/ g. The L-lactide/caprolactone copolymer (LACL) was kindly donated by Purac Biochem, Netherlands. The molar ratio of the L-lactide/caprolactone was 70/30. Sample Preparation. PLA and PCL, in the form of pellets, and LACL in the form of powder, were separately dried in a vacuum oven at 80, 60, and 60 °C for 8 h, respectively. The neat PLA, PLA/PCL (80/20, w/w) blend, and the blends with 5, 10, and 20 wt % LACL, labeled as PLA/PCL (5% LACL), PLA/PCL (10% LACL), and PLA/PCL (20% LACL), were separately dissolved in chloroform to form 0.1 g/mL solutions. These solutions were stirred at room temperature for 12 h and cast in Petri dishes, followed by solvent evaporation. However, the films obtained in this way were not flat. They were further formed into sheets through a conditioning compression process in which samples were placed in a template frame in a hydraulic press at 180 °C. A poly(ethylene terephthalate) film was used to cover the sample to prevent adherence to the press plates. The materials were placed between the press plates for 3 min of preheating, and then the applied pressure was increased to 10 MPa for an additional 3 min to make sure that the material fully filled the mold cavity. The samples were removed from the press plates and cooled in air until they reached ambient temperature. The molded sheets were about 0.67 mm thick and void-free. Characterization. The morphologies of the PLA/PCL (80/20), PLA/PCL (5% LACL), PLA/PCL (10% LACL), and PLA/PCL (20% LACL) blends were examined using SEM (JEOL, Tokyo, Japan) with an accelerating voltage of 10 kV. Before SEM observation, the sheets were submerged in liquid nitrogen and broken to expose their internal structure. The fractured surfaces were sputter coated with gold. The tensile behaviors of the PLA, PLA/PCL (80/20), PLA/ PCL (5% LACL), PLA/PCL (10% LACL), and PLA/PCL (20% LACL) blends were analyzed using an Instron 5967 universal testing machine with a load cell of 30 kN. The

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RESULTS AND DISCUSSION Morphology. Figure 1 shows the micrographs of (a) PLA/ PCL (80/20), (b) PLA/PCL (5% LACL), (c) PLA/PCL (10% LACL), and (d) PLA/PCL (20% LACL) blends taken directly on the surface of samples fractured in liquid nitrogen. The PLA/PCL (80/20) blend showed a sea−island morphology with spherical PCL particles dispersed in the PLA matrix. The size of the PCL particles is in the micron scale, ranging from 1 to 5 μm. Generally, when two immiscible polymers are blended together, phase separation will take place among the polymer interfaces, leading to the minority polymer phase being dispersed in the majority polymer matrix as in the sea−island morphology. The distinct particle interfaces between PCL particles and the PLA matrix in Figure 1 (a) indicated poor compatibility between the two materials. However, with 5% LACL added, the dimensions of the dispersed PCL particles in the blend decreased to less than 1 um, which was much smaller than that of the PLA/PCL (80/20) blend. The decreased PCL particle size indicates an improved compatibility between the dispersed PCL phase and the PLA matrix. It is hard to identify the phase boundaries in the blends with 10% and 20% LACL, which means that the added LACL effectively emulsified the blend components and significantly increased the compatibility between the PLA and PCL phases. Mechanical Properties. The tensile mechanical behaviors of the PLA, PLA/PCL (80/20), PLA/PCL (5% LACL), PLA/

9506

DOI: 10.1021/acs.iecr.5b02134 Ind. Eng. Chem. Res. 2015, 54, 9505−9511

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seen that neat PLA fractured shortly after yielding with a very low elongation-at-break of 14%. When 20% PCL was added, the PLA/PCL (80/20) blend exhibited neck propagation after yielding, and the ultimate strain increased dramatically to 284%, which is characteristic of a ductile material. Meanwhile, the tensile strength and modulus decreased to 24.7 and 1117 MPa, respectively. Interestingly, with the addition of 5% LACL, the elongation-at-break of the PLA/PCL (5% LACL) blend improved to 580%, which was an increment of about 104% compared to that of the PLA/PCL (80/20) blend. This improvement in ductility was accompanied by a slight decrease of tensile strength and modulus to 22.9 and 1074 MPa, which was only a 7.3% and 3.8% drop, respectively. Moreover, with increasing LACL content, the elongation-at-break of PLA/PCL (10% LACL) and PLA/PCL (20% LACL) blends was further enhanced, but also resulted in lower tensile strength and modulus. It is obvious that the compatibilization effect took place between PLA and PCL components upon the addition of LACL. Non-Isothermal Cold Crystallization and Melting Behaviors. To investigate the effect of the copolymer on the nonisothermal cold crystallization of PLA, the DSC second heating scans of the samples at a rate of 10 °C/min were recorded and are presented in Figure 3. In the thermogram of

Figure 1. SEM images of the cross sections of (a) PLA/PCL (80/20), (b) PLA/PCL (5% LACL), (c) PLA/PCL (10% LACL), and (d) PLA/PCL (20% LACL).

PCL (10% LACL), and PLA/PCL (20% LACL) blends were characterized at room temperature. Typical stress versus strain curves for these materials are shown in Figure 2. Table 1 lists

Figure 3. DSC thermograms of PLA, PLA/PCL (80/20), and the blends with different amounts of LACL at a heating rate of 10 °C/min.

Figure 2. Stress−strain curves of PLA, PLA/PCL (80/20), and the blends with different amounts of LACL.

PLA, the heat jump occurred at about 61 °C, corresponding to the glass transition temperature of PLA. As the melting temperature of PCL was about 60 °C, the sharp endothermic peak at around 59 °C appeared in the thermograms of the PLA/PCL (80/20) blend and blends with LACL, which was attributed to the overlapping of the glass transition temperature of PLA and the melting peak of PCL. Hence, it is hard to study the influence of the copolymer on the glass transition temperature of PLA due to this overlap. However, the cold crystallization behavior of PLA was influenced by the presence of PCL and LACL. It can be seen that neat PLA did not develop any crystallization or melting during the heating scan, which means PLA is hard to crystallize during this nonisothermal heating process at 10 °C/min. However, the PLA/ PCL (80/20) blend showed a broad exothermal band with a peak crystallization temperature of 123 °C, indicating that PCL increased PLA’s chain mobility and improved the crystallization capacity of PLA. The followed endothermic peak at 153 °C was caused by the melting of the PLA crystalline phase, which was developed during the cold crystallization process. By adding 5%

Table 1. Tensile Properties of PLA, PLA/PCL (80/20), and the Blends with Different Amounts of LACL samples PLA PLA/PCL PLA/PCL PLA/PCL PLA/PCL

(80/20) (5% LACL) (10% LACL) (20% LACL)

tensile strength (MPa) 45.7 24.7 22.9 20.5 16.0

± ± ± ± ±

2.8 1.0 0.7 2.1 1.8

tensile modulus (MPa) 1775 1117 1074 990 779

± ± ± ± ±

73 52 43 79 31

elongation-atbreak (%) 14 284 580 627 652

± ± ± ± ±

1 18 28 24 27

the corresponding data deduced from the curves; i.e., the tensile strength, tensile modulus, and elongation-at-break. The standard deviations from these mean values are also given in the table. As seen, the standard deviations are confined to a narrow range and are relatively small as compared to that of the corresponding mean values, indicating that the data are representative of the properties of the materials. It can be 9507

DOI: 10.1021/acs.iecr.5b02134 Ind. Eng. Chem. Res. 2015, 54, 9505−9511

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Figure 4. (a) DSC thermograms and (b) relative crystallinity of PLA isothermally cold crystallized at various temperatures.

where Qt and Q∞ are the amounts of heat generated at time t and at infinite time, respectively, and dH/dt is the rate of heat evolution. Figure 4(b) presents the plots of the relative crystallinity versus the crystallization time for neat PLA, which was obtained from Figure 4(a) according to eq 1. The wellknown Avrami equation23,24 was employed to analyze the isothermal crystallization kinetics of the materials, assuming the relative crystallinity (Xt) develops with crystallization time (t), as follows:

LACL, the blend crystallized at an even lower temperature which revealed that the addition of LACL further increased the chain mobility of PLA and enabled PLA to begin to crystallize at a lower temperature. It should be noted that there were two melting peaks in the thermogram of PLA/PCL (5% LACL) blend. The two melting peaks corresponded to two types of crystals. The peak at the lower temperature was ascribed to the melting of the crystalline phase, with lower crystallinity forming at low temperatures, while the one at the higher temperature was due to the melting of a more perfect crystalline phase which developed at a high temperature. With more LACL added, the crystallization temperature of the blend did not decrease further, but moved to a higher temperature instead. This was because the blend consisted of more L-lactide segments when a higher amount of LACL was added, as the L-lactide/caprolactone molar ratio of LACL used in the experiment is 70/30. The two melting peaks of PLA/PCL (10% LACL) blend were not very distinct and only one melting peak was observed in the thermogram of PLA/PCL (20% LACL) blend, which once again proves that less perfect crystalline phase was developed at lower temperature and resulted with lower melting temperature. It is concluded that the crystallization capacity of PLA can be accelerated by PCL, and it can be further improved by incorporating 5% LACL due to the compatibilization effect of LACL between PLA and PCL. With a higher content of LACL, the blend showed more characteristic properties of PLA; hence, the crystallization temperature of the blend decreased with more than 5% LACL. Isothermal Cold Crystallization Kinetics. The isothermal cold crystallization of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) at various temperatures was studied with DSC. Figure 4 (a) shows the thermograms of PLA annealed at a temperature range of 90 to 130 °C. The crystallization time (t) decreased with increasing temperature (Texp) up to 120 °C due to the enhanced mobility of the PLA chain segments. It was hard for PLA chain segments to align into crystalline regions at high temperatures since the chain mobility was too active, which resulted in a longer crystallization time for PLA annealed at 130 °C than that at 120 °C. The relative crystallinity (Xt) at a particular crystallization time (t) can be calculated according to the following eq 1, Xt =

Qt Q∞

1 − X t = exp( −kt n)

The linear form of eq 2 can be expressed as follows: log[− ln(1 − X t )] = log k + n log t

Figure 5. Avrami plots of PLA isothermal cold crystallization at various temperatures.

obtained from the slopes and interceptions, respectively. The Avrami parameters for the PLA, PLA/PCL (80/20), and PLA/ PCL (5%LACL) are summarized in Table 2. The index n represents the dimension of crystal growth. It was found for all materials that the n value adopted fractional numbers due to secondary crystallization. The values of n for neat PLA were found to be between 1.99 and 2.18, which may be because most of the crystals grow in two directions. The PLA/PCL (80/20) and PLA/PCL (5% LACL) blends showed higher n values between 2.04 and 3.00, indicating that some of the crystals grew in three dimensions. The k value for PLA first increased with

∫0 (dH /dt )dt ∞

∫0 (dH /dt )dt

(3)

where n is the Avrami exponent, which is dependent on the nature of nucleation and growth geometry of the crystals, and k is the overall rate constant associated with both nucleation and growth contributions. Figure 5 shows the corresponding Avrami plots of PLA crystallized at different Texp, in which the n and k can be

t

=

(2)

(1) 9508

DOI: 10.1021/acs.iecr.5b02134 Ind. Eng. Chem. Res. 2015, 54, 9505−9511

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It can be seen that the 1/t1/2 value first increased and then decreased with increasing Texp for all of the materials. Neat PLA showed the fastest crystallization at 120 °C, while PLA/PCL (80/20) and PLA/PCL (5% LACL) exhibited the highest crystallization rate at 110 and 115 °C, respectively. At a given Texp, PLA/PCL (5% LACL) showed the highest 1/t1/2 value. This result is consistent with the nonisothermal cold crystallization result in that the addition of LACL further improved the crystallization rate of PLA in the ternary blend. Melting Behaviors after Isothermal Cold Crystallization. Figure 7 presents the DSC heating scans of PLA, PLA/ PCL (80/20), and PLA/PCL (5% LACL) after isothermal crystallization at various temperatures (Texp). The three samples all experienced a double melting behavior after isothermal crystallization at lower temperatures, and a single melting peak after isothermal crystallization at higher temperatures. As discussed above, the two melting peaks corresponded to two types of crystals with different crystallinities. The crystallinity of the crystals formed during the cold crystallization process increased with increasing Texp, which caused the melting peak at the lower temperature to gradually move to a higher temperature and merge with the second peak after isothermal crystallization at a higher temperature. This was because, at low Texp such as 90 °C, the PLA chain mobility was limited, and the crystalline phase formed was low in crystallinity, which led to the melting of the crystals at a low temperature. Meanwhile, at high Texp like 110 °C or above, the polymer chain mobility was very active and was able to align easily and develop crystals with high crystallinity that melted at a high temperature. It can be seen that PLA experienced only one melting peak after isothermal crystallization at 110 °C and above. However, there were still two melting peaks for PLA/ PCL (80/20) and PLA/PCL (5% LACL) after isothermal crystallization at 110 °C, which were related to the enhanced chain mobility of PLA influenced by PCL and LACL. It can also be seen that the gradually moving melting peaks to higher temperatures for PLA/PCL (5% LACL) were less sharp and exhibited larger endothermic areas than those for PLA/PCL (80/20) after isothermal crystallization at various Texp, indicating that the PLA chain mobility in PLA/PCL (5% LACL) was more active than that in PLA/PCL (80/20), which was attributed to the compatibilizing effect of LACL between PLA and PCL. The POM images of the final morphology of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) after isothermal crystallization at 95, 105, and 115 °C for 3 h are shown in Figure 8. The crystals of the samples formed were all very small in size because it was difficult to fully incorporate the polymer chains into growing crystalline lamellae from the glassy state. In other words, it was hard to form large crystalline domains during cold crystallization due to limited chain mobility and restrictions influenced by adjacent polymer chain segments. This also explains why the size of the crystals increased with increasing crystallization temperature for PLA. Obviously, it was hard to further assess the impact of PCL and LACL on the crystal growth mechanism of PLA using POM because the crystal size was beyond the range of the polarized optical microscopy observations. However, it can be seen that at the same Texp, the size of the crystals was reduced with the addition of PCL, and was further degraded by adding 5% LACL. This was indicative of the heterogeneous nucleating effect of PCL and LACL.

Table 2. Summary of Isothermal Crystallization Kinetic Parameters of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) samples PLA

PLA/PCL (80/20)

PLA/PCL (5% LACL)

T(exp o C)

n

90 95 100 105 110 115 120 130 90 95 100 105 110 115 120 130 90 95 100 105 110 115 120 130

1.99 1.99 2.03 1.99 2.07 2.18 2.07 2.11 2.13 2.24 3.00 2.73 2.98 2.69 2.55 2.72 2.04 2.10 2.71 2.63 2.67 2.69 2.47 2.56

K (min−n) 2.3 9.2 3.3 6.2 9.7 1.1 1.5 6.2 6.8 4.0 1.2 4.1 6.0 7.2 5.7 8.3 5.0 1.6 3.2 7.1 9.8 1.1 8.8 1.5

× × × × × × × × × × × × × × × × × × × × × × × ×

10−05 10−05 10−04 10−04 10−04 10−03 10−03 10−04 10−04 10−03 10−02 10−02 10−02 10−02 10−02 10−03 10−03 10−02 10−02 10−02 10−02 10−01 10−02 10−02

1/t1/2 (min−1) 0.006 0.011 0.023 0.029 0.042 0.051 0.052 0.036 0.039 0.099 0.261 0.353 0.439 0.432 0.376 0.197 0.090 0.165 0.321 0.420 0.480 0.511 0.433 0.226

Texp, and then decreased with Texp, after it reached a maximum value at 120 °C. The highest k value was seen at a Texp of 115 °C for the PLA/PCL (80/20) and PLA/PCL (5% LACL) blends. It is not appropriate to compare the overall crystallization rate directly from the k values, because the unit of k is min−n and n is not constant at different Texp. Thus, the crystallization half time (t1/2), which is the time required to achieve 50% of the final crystallinity of the samples, was introduced. The value of t1/2 was calculated below.

(t1/2) =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(4)

The crystallization rate can be easily described by the reciprocal of t1/2. The variations of 1/t1/2 for PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) isothermal crystallization at different Texp are listed in Table 2 and presented in Figure 6.

Figure 6. 1/t1/2 value of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) isothermally crystallized at various temperatures. 9509

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Figure 7. DSC heating scans of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) after isothermal crystallization at various temperatures.

addition of 5% LACL further enhanced the ductility of the blend with only a slight drop in tensile strength and modulus. The addition of LACL led to PLA crystallization at even lower temperatures in the nonisothermal crystallization procedure. The crystallization rate of PLA was also accelerated by the addition of PCL, and further by LACL, as observed from the isothermal crystallization procedure. The size of the crystals decreased with the addition of PCL, and decreased further by adding 5% LACL, indicating a heterogeneous nucleating effect of PCL and LACL. To further illustrate the compatibilization effect of LACL, its impact on the nonisothermal and isothermal melt crystallization of the PLA/PCL blends should be studied in the future.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 0851 85400760; fax: +86 0851 85400760; e-mail: [email protected] (C.Z.). *Tel: +1 608 262 0586; fax: +1 608 265 2316; e-mail: turng@ engr.wisc.edu (L-S.T.). Notes

Figure 8. Polarized optical microscopy images of the final morphology of PLA, PLA/PCL (80/20), and PLA/PCL (5% LACL) after isothermal crystallization at 95, 105, and 115 °C.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge support from the Wisconsin Institute for Discovery and the China Scholarship Council. The University of Guizhou Province Engineering Research Center Project (No. [2012] 023), Science and Technology Foundation of Guizhou Province (No. [2015] 2006), and the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (No. sklpme2015-4-35) are also acknowledged for their financial support.

CONCLUSIONS The influence of LACL on the morphological, mechanical, and crystallization behaviors of the blends was studied by SEM, tensile tests, DSC, and POM. It can be concluded that the addition of LACL decreased the dimensions of the dispersed PCL domains due to enhanced compatibility between the PLA and PCL phases. With 20% PCL, the elongation-at-break of the blend improved dramatically but was accompanied by a dramatic loss of tensile strength and modulus. However, the 9510

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DOI: 10.1021/acs.iecr.5b02134 Ind. Eng. Chem. Res. 2015, 54, 9505−9511