Enhanced Crystallization Rate of Poly(l-lactic acid) (PLLA) by

Article pubs.acs.org/JPCB

Enhanced Crystallization Rate of Poly(L‑lactic acid) (PLLA) by Polyoxymethylene (POM) Fragment Crystals in the PLLA/POM Blends with a Small Amount of POM Jishan Qiu,† Jipeng Guan,† Hengti Wang,† Shanshan Zhu,† Xiaojun Cao,† Quan-lin Ye,‡ and Yongjin Li*,† †

College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China Department of Physics, Hangzhou Normal University, Hangzhou 310036, China

‡

S Supporting Information *

ABSTRACT: Phase diagrams and glass transition behaviors of poly(L-lactic acid)/ polyoxymethylene (PLLA/POM) blends have been investigated in our previous work (Macromolecules 2013, 46, 5806−5814). In this work, the crystallization behaviors and physical properties of the PLLA/POM blends with the PLLA as the major component have been systematically studied. POM was crystallized into the fragment crystals that were finely dispersed in the PLLA matrix when cooling down from the melt of the blends. It was found that the POM fragment crystals accelerated the crystallization process of PLLA matrix and increased the final crystallinity of PLLA significantly in the blends. At the same time, the PLLA spherulites nucleated by POM fragment crystals were much smaller than those obtained from neat PLLA. It was further found that the crystallization rate of PLLA was quite dependent upon the POM loadings and the highest crystallization rate was observed at POM loadings of 7 wt %. It is considered that the POM fragment crystals take the nuclei role to initiate the crystallization of PLLA at low POM loadings, while a high content of POM in the blends leads to the large POM spherulites that cannot nucleate PLLA crystallization effectively. The obtained PLLA/POM blends at low POM loadings with small PLLA spherulites exhibited excellent optical transmittance and good mechanical performance.

1. INTRODUCTION Among various modifications, polymer blending has been investigated as the most versatile and economical method to develop new high performance materials.1 The blending to disperse one polymer (modifier) in another polymer (matrix) is typically achieved by mechanical shearing in a mixer at high temperature. The physical and mechanical properties of polymeric materials depend largely on the morphology and crystallinity degree. Polymer blends with different physical properties often exhibit the possibility of enhancing the overall properties of a material through a synergistic combination of the desirable properties of the original polymers. Nevertheless, a large amount of modifier is usually necessary in order to achieve the desired properties. Therefore, most of the investigations up to now pay mainly attention to the polymer blending by addition of a pretty large amount of dispersed polymer, and extensive research was conducted by adding the modifier with the interval of 10 wt % or even 20 wt %. For example, to prepare a rubbertoughened plastic material, the rubber content used is usually more than 10 wt % for achieving sufficient toughening effects. Unfortunately, polymer blends by addition of a large amount of rubber are inevitably accompanied by a significant drop in the modulus and tensile strength. We consider that the polymer blends with several percent of modifier in polymer matrix are also very interesting because the small amount of modifier loadings (impurities for the matrix) may significantly change the nature of © 2014 American Chemical Society

the matrix, for example, the crystallization behaviors of semicrystalline polymers. One of the authors reported that several percent of acrylic rubber enhances the impact strength of PVDF significantly with very limited sacrificing of the modulus and strength of PVDF.2 It was proposed that the very small acrylic rubber domains take the role as nucleation sites for the crystallization of PVDF and decreasing the PVDF spherulite size drastically. Poly(L-lactide) (PLLA) has attracted great interest in the past decade because it can be derived from renewable resources and is biodegradable.3−7 Especially during recent years, PLLA has been utilized for various purposes, including industrial and commodity applications.8−14 However, PLLA still has some drawbacks, such as inherent brittleness, relatively slow crystallization rate, and low heat distortion temperature.15,16 Since the slow crystallization of PLLA is a major industrial problem, numerous studies have been carried out for enhancing the crystallization rate of PLLA. The incorporation of heterogeneous nucleating agents into the PLLA matrix is one of the most viable strategies. Various kinds of inorganic nucleating agents, such as clay,17−20 talc,21−25 and nanostructured carbon,26−29 have been used for the purpose. However, inorganic nucleating agents usually tend to aggregate Received: December 20, 2013 Revised: May 16, 2014 Published: June 2, 2014 7167

dx.doi.org/10.1021/jp412519g | J. Phys. Chem. B 2014, 118, 7167−7176

The Journal of Physical Chemistry B

Article

2.2. Structural Characterization. The morphology of the blends was observed by a field-emission scanning electron microscope (FESEM). A Hitachi S-4800 SEM system was used for SEM measurements at an accelerating voltage of 5 kV. All of the samples were fractured after immersion in liquid nitrogen for about 15 min. The phase structure of the blends was also observed directly using a transmission electron microscope (TEM) (Hitachi HT7700) operating at an acceleration voltage of 80 kV. The blend samples were ultramicrotomed at −120 °C to a section with a thickness of about 70 nm. The sections were then stained by ruthenium tetroxide (RuO4) for one night. Differential scanning calorimetry was carried out under nitrogen flow at a heating rate of 10 °C/min. It was measured with a differential scanning calorimeter (DSC) system (TA Instruments DSC Q2000). The heating and cooling DSC traces were recorded. Isothermal melt crystallization was performed after melting at 190 °C for 10 min and rapid cooling to the desired crystallization temperature at a rate of 100 °C/min. Before sample scan, the heat flow and temperature of the instrument were calibrated with sapphires and pure indium, respectively. The crystallinity (χ) of the PLLA in the composite blends was calculated from the following formula:

in the PLLA matrix and very limited enhancement in the physical performance could be achieved in the blends. Low molecular weight aliphatic polyesters have also been used to enhance the crystallization rate of PLLA,30,31 but the low molecular weight aliphatic polyesters are very easy to migrate to the surface with long-term use. At the same time, the addition of the components with low molecular weight leads to the decrease in both mechanical and thermal properties. Very recently, the stereocomplex of PLLA/poly(D-lactic acid) (PDLA) has been applied as the nucleating agent for PLLA.32−39 However, the efficient fabrication of the PLLA/PDLA stereocomplex poses technical challenges up to now. Therefore, the high efficient PLLA nucleation agents that are both environmentally friendly and compatible with PLLA are still highly desired. Polyoxymethylene (POM), as a crystalline thermoplastic polymer, has been widely used in industry due to its excellent chemical resistance, mechanical properties, abrasion resistance, fatigue resistance, and mold ability.40−42 In our previous work,43 we have reported that the PLLA/POM blends exhibit typical lower critical solution temperature (LCST) behaviors. PLLA and POM are miscible in the melt state at low temperature and become phase-separated at elevated temperatures. It was found that the weak interactions between the carboxyl groups of PLLA and methylene groups of POM account for the miscibility of the two components. We consider that enhancement of the overall crystallization rate of PLLA by the addition of POM in PLLA is also of both scientific and technological interest. In this work, we focus on the crystallization behaviors of PLLA in its blends with less than 10% POM. It was found that POM forms fragment crystals first in the PLLA matrix and these fragment POM crystals take the role as the nucleation site for the crystallization of PLLA. Therefore, the fragmented POM crystals accelerate the crystallization of PLLA significantly. Moreover, the obtained PLLA/POM blends exhibit excellent optical transmittance and good mechanical performance with high yield strength as compared with the neat PLLA. During the revision of this manuscript, we found a master thesis on the crystallization behaviors of PLLA/POM blends and the crystallization kinetics of PLLA in the presence of POM has been reported.44

χ=

ΔHmc × 100% WPLLA ·ΔHm°

where ΔHmc is the enthalpy of crystallization of the PLLA, WPLLA is the weight percent of PLLA in the blends, and ΔH°m is the melting enthalpy for 100% crystallized PLLA (93 J/g).46 Dynamic mechanical analysis (DMA) was carried out with a TA Instruments Model Q800 apparatus in the tensile mode. All of the measurements were performed in the linear region with a strain of 0.03%. Dynamic loss (tan δ) was determined at a frequency of 5 Hz and a heating rate of 3 °C/min, as a function of temperature (from 30 to 170 °C). Polarized optical microscopy (POM) observation was performed on an Olympus BX51 polarizing optical microscope equipped with a digital camera. The samples were placed between two glass slides, and the temperature was controlled by a Linkam LTS 350 hot stage. The samples were prepared by heating at 190 °C and held for 10 min at this temperature. The samples were pressed and spread into a thin film before being quenched to the desired crystallization temperature. The wide-angle X-ray diffraction (WAXD) data was collected from 2θ = 5−50° at a scanning rate of 2° min−1. The instrument was operated at a 35 kV voltage and 30 mA current. The instrument was operated at a 35 kV voltage and 30 mA current. The samples were hot-pressed to a film with a thickness of 300 μm, followed by being cooled at a rate of 10 °C/min to room temperature. 2.3. Physical Property Measurements. Tensile tests were carried out according to the ASTM D 412-80 test method, using dumbbell-shaped samples punched out from the molded sheets. The tests were performed using a tensile testing machine (Instron, Model 5966) at a crosshead speed of 10 mm/min at 20 °C and 50% relative humidity. At least five specimens were tested for each sample. The transmittance spectra of the blends were evaluated by a UV−vis−NIR scanning spectrophotometer (PerkinElmer, Lambda 900). The samples were hot-pressed to a film with a thickness of 300 μm, followed by being cooled at a rate of 10 °C/ min to room temperature. Note that the neat PLLA was melt crystallized at 120 °C for 1 h.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The PLLA sample used was purchased from Nature Works Co. LLC (USA), under the trade name of 3001 D. The Mn and Mw/Mn are reported to be 89 300 ± 1000 g/mol and 1.77 ± 0.02, respectively.45 The sample included 1.6% of D-lactide content. The POM (MC 90) samples used in this work were kindly provided by Shenhua Co., Ltd., China. The melt flow index is 9.23 g/10 min. The weight-average molar mass and molecular polydispersity of the POM sample are Mw = 174 300 g/mol and Mw/Mn = 2.19. PLLA and POM were dried in a vacuum oven at 80 °C for 12 h prior to use. The blends with PLLA/POM weight composition varying from 80/20 to 99/1 were prepared using a batch mixer (Haake Polylab QC) with a twin screw at a rotation rate of 20 rpm at 190 °C for 1 min, and then, the rotation rate was raised to 50 rpm for 5 min. After blending, all the samples were then hotpressed at 190 °C under a 14 MPa pressure for 3 min to a film with a thickness of 300 μm, followed by quenching (quickly move to another cold press machine under same pressure) to room temperature. The cooling rate is higher than 100 °C/min for the sample during this process. The obtained films were used for the following characterization. 7168



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Figure 1. SEM images for the fractured surface: (a) PLLA/POM = 90/10 and (b) PLLA/POM = 95/5.

Figure 2. TEM images of (a) PLLA/POM = 93/7 quenched blends with low magnification, (b) PLLA/POM = 93/7 quenched blends with high magnification, and (c) PLLA/POM = 80/20 quenched blends.

3. RESULTS 3.1. POM Fragment Crystals in PLLA/POM Blends with a Small Amount of POM. The PLLA/POM blends exhibit typical LCST phase behaviors. In this work, all the samples investigated are in the homogeneous state and no phase separation occurs in the melt. Figure 1 shows the typical SEM images of the fracture surface of the quenched PLLA/POM blends from 190 °C (where all the blends are in the homogeneous state) with the indicated compositions. It is

clear that no any detectable structure was observed, indicating that no phase separation occurs for all the blends during cooling. POM is a high crystalline polymer, and it has a very high crystallization rate from the melt during cooling.47−49 We have shown that POM can be well crystallized in its blends with PLLA in our previous work.43 To detect the crystal morphology of POM in the PLLA/POM blends with a small amount of POM, TEM experiments were carried out for the PLLA/POM (93/7) blend quenched very quickly from the melt. The TEM images of 7169



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the quenched sample are shown in Figure 2a and b with different magnifications. Many discrete crystal lamellae were observed, and they are homogeneously dispersed in the PLLA matrix. These crystals are not well organized, and no spherulites were found. We considered that these short lamellae are POM fragment crystals because POM has a much higher crystallization rate than PLLA. In the quickly quenched sample with the cooling rate higher than 100 °C/min, PLLA is in the amorphous state in all the PLLA/POM blends, which can be confirmed by the WAXD results in Figure S1 (Supporting Information). For comparison, the TEM experiment was also carried out for the quickly quenched PLLA/POM = 80/20 sample, as shown in Figure 2c. It is clear that POM forms the large spherulites and PLLA molecules are expelled out from the POM spherulites. We will show that the large POM spherulites cannot nucleate the crystallization of PLLA effectively in the next section. 3.2. Melt and Cold Crystallization of PLLA Nucleated by POM Fragment Crystals during Cooling and Heating. Crystallization behaviors of PLLA/POM blends were investigated by DSC under nonisothermal and isothermal conditions, respectively. Parts a and b of Figure 3 show the DSC curves of PLLA/POM blends recorded during the nonisothermal melt crystallization (cooling at 10 °C/min) from 190 °C and subsequent heating scans at 10 °C/min, respectively. As one can see clearly from Figure 3a, no obvious crystallization peak is detected for neat PLLA and PLLA/POM (99/1). The PLLA/ POM (97/3) blend showed a very small crystallization exotherm at a peak temperature of about 97 °C. For the blend with the composition ranging from 95/5 to 90/10, two discrete crystallization peaks were observed for each sample during cooling. Obviously, the crystallization peak at high temperature corresponds to the melt crystallization of POM in the blends, while the peak at low temperature corresponds to the melt crystallization of PLLA. For the PLLA/POM (80/20) sample, only one exotherm peak was found, which originates from the crystallization of POM, and no PLLA crystallization peak was observed. It is clear from Figure 3a that several percent of POM accelerates the crystallization of PLLA drastically. Few POM crystals were formed in the PLLA/POM blends with extremely low content of POM (PLLA/POM = 99/1 and 97/3 blends), which means that few nuclei formed in the cooling process and no drastically enhanced crystallization rate can be achieved as compared with neat PLLA. With increasing POM loading in the blends, POM crystallizes into the sparse fragment crystals in the PLLA matrix during cooling (Figure 2a). These POM crystals take the role as the nucleation sites for PLLA and PLLA subsequently crystallized after the crystallization of POM. Therefore, we observed a very sharp PLLA crystallization peak. However, too much POM in the blends results in the very high concentration of POM crystals or forms the compact POM spherulites (Figure 2c). These POM spherulites cannot induce the crystallization of PLLA effectively. It was seen from Figure 3a that the PLLA melt crystallization during cooling from the melt in the PLLA/POM blends is quite dependent on the POM loadings. It is interesting to investigate the subsequent cold crystallization of all the blends during heating. The heating curves of all the samples are shown in Figure 3b with a heating rate of 10 °C/min after cooling from the melt to room temperature, which indicates the PLLA cold crystallization behaviors. For neat PLLA, no crystallization occurs during cooling at the cooling rate to 10 °C/min (Figure 3a) and the heating DSC curve exhibits a small crystallization peak at 111.5

Figure 3. (a) DSC curves of nonisothermal melt crystallization at a cooling rate of 10 °C/min, (b) DSC curves of the subsequent heating scans at 10 °C/min for neat PLLA and PLLA/POM blends, and (c) the crystallization exothermic peak temperature of PLLA as a function of the PLLA/POM composition.

°C. This means that the crystallization rate of neat PLLA is very slow and neat PLLA does not well crystallize during the cooling and the heating at 10 °C/min, which can also be confirmed by the small melting peak in the heating DSC curve. Although no clear melt crystallization occurs for the PLLA/POM = 99/1 and 97/3 7170



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of 10 °C/min due to the nucleation effect of POM fragment crystals. The main crystallization parameters during the melt and subsequent cold crystallization for all the samples have been shown in Table 1. It is reported that talc has very good nucleation

blends due to few POM crystals when cooling down from the melt state at 10 °C/min, the two samples exhibit sharper and lower crystallization peak temperatures during heating. By comparing the cold crystallization peak temperatures for the neat PLLA and the PLLA/POM blends with 1 and 3 wt % POM, it is concluded that the small amount of POM increases the PLLA crystallization rate. This can also be attributed to the nucleation effects of POM fragment crystals during the PLLA cold crystallization process. More POM fragment crystals were formed in the PLLA/POM = 97/3 blend than in the PLLA/ POM = 99/1 blend, so PLLA has lower cold crystallization temperatures in the 97/3 blend than in the 99/1 blend. For the PLLA/POM = 95/5 and PLLA/POM = 93/7 blends, no cold crystallization peak can be observed during the heating. This can be attributed to the fully crystallized PLLA during the cooling from the melt. The fact that no cold crystallization peaks were observed for the 95/5 and 93/7 blends indicates again the very fast crystallization rate for the two samples during cooling. With further increase of the POM loadings (90/10 and 80/20), the cold crystallization peaks appear again. This means that the PLLA in the blends is not fully crystallized during the cooling from the melt and some part of PLLA crystallizes during the subsequent heating process. Note that all of the samples show the multiple melting behaviors in Figure 3b, which can be attributed to overlapping of melting of POM crystals and PLLA crystals. The melt crystallization peak from the melt during cooling has been used to evaluate the crystallization rate for semicrystalline polymers. The samples with high crystallization rate present higher melt crystallization peak temperature. Figure 3c shows the melt crystallization peak temperature as a function of the POM loadings in the blends. It is clear that the crystallization rate of PLLA has a maximum value in the PLLA/POM = 93/7 blends. It is considered that the 7 wt % POM in the sample crystallizes into many fragment crystals dispersed in the PLLA matrix, which nucleate the crystallization of PLLA. Figure 4 shows the TEM image of the PLLA/POM = 93/7 blends cooled from 190 °C to room temperature with a cooling rate of 10 °C/min. By comparing the image with Figure 2, it is clear that PLLA lamellae were well crystallized with a cooling rate

Table 1. Thermal Properties of Neat PLLA, PLLA/POM Blends, and the PLLA/Talc Tmca (°C)

sample neat PLLA PLLA/POM = 80/20 PLLA/POM = 90/10 PLLA/POM = 93/7 PLLA/POM = 95/5 PLLA/POM = 97/3 PLLA/POM = 99/1 PLLA/Talc = 95/5 a

99.2 103.0 99.7 93.6 93.5 110.4

Tccb (°C)

ΔHcc (J/g)

135.0 96.4 96.3

8.6 19.4 9.1

98.1 107.0

24.0 26.9

Peak temperature during melt crystallization. during cold crystallization.

b

ΔHmc (J/g)

χ (%)

0.6 12.9 20.0 20.2 3.0 0.7 27.1

15.4 23.1 22.9 3.3 0.8 30.6

Peak temperature

effects for the crystallization of PLLA.24,25 We have also investigated the crystallization behaviors of PLLA nucleated by talc for the comparison. The crystallization parameters of the PLLA/talc composite are also listed in Table 1. It is shown that POM crystals in the PLLA/POM = 93/7 blends have very similar nucleation agent effects for PLLA to the inorganic talc particles. 3.3. Isothermal Crystallization of PLLA in the PLLA/ POM Blends. The DSC curves have been recorded at 135 °C with time, as shown in Figure 5a. It can be seen from Figures 5a that almost all the blends show two exothermic peaks. POM has a much higher crystallization rate than PLLA, so the first peak originates from the crystallization of POM and the second peak is the subsequent PLLA crystallization. In other words, the crystallization of PLLA occurs after the crystallization of POM. Therefore, the POM crystals can be seen as the nuclei for the crystallization of PLLA. In other words, the fragment lamellae crystals of POM first formed from melt and these crystals acted as nucleation sites of PLLA to induce PLLA crystallization. By using the peak fitting program, we have separated the isothermal crystallization processes of POM and PLLA. The plots of PLLA relative crystallinity against crystallization time at 135 °C in the PLLA/POM blends are shown in Figure 5b. It is clear from Figure 5b that all of these curves have a similar sigmoid shape; moreover, the corresponding crystallization time for PLLA in the blends is much shorter than the neat PLLA. For instance, it takes neat PLLA about 85 min to finish crystallization at 135 °C, but for the PLLA/POM = 93/7 sample, the time to finish crystallization became only around 7 min. It is obvious that the incorporation of the POM enhances the isothermal melt crystallization of PLLA remarkably when compared with neat PLLA. It can be further confirmed by the in situ optical microscopic observation shown in Figures S2 and S3 (Supporting Information). The half crystallization time (t1/2) derived from the DSC isothermally crystallized at 135 °C as a function of the POM loading is displayed in Figure 5c. Similar to the nonisothermal crystallization, the highest crystallization rate occurs for the sample at a POM content of 7 wt %. 3.4. Crystal Morphologies of PLLA in Neat PLLA and PLLA/POM Blends. The polarized optical microphotographs for neat PLLA and the typical PLLA/POM (=93/7) blends are

Figure 4. TEM image of PLLA/POM = 93/7 blends cooling down from 190 °C at the cooling rate of 10 °C/min (PLLA in the sample was well crystallized). 7171



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Figure 6. Polarized optical microscopy images of (a) PLLA and (b) PLLA/POM = 93/7 showing the isothermal crystallization morphologies obtained after isothermal crystallization at 135 °C for 3 h.

density of PLLA spherulites increases significantly in the presence of POM. WAXD experiments were performed to investigate the effect of the addition of POM on the crystallization of PLLA in the PLLA/POM blends, as shown in Figure 7. All of the samples in the figure have the same thermal history with first melting at 190 °C for 10 min followed by cooling down to room temperature at a cooling rate of 10 °C/min. Neat PLLA exhibits only an amorphous hole, indicating the amorphous state of the PLLA. The diffraction peak at 2θ = 22.7° originates the (100) diffraction of POM crystals.50−52 The characteristic diffraction peak of POM at 22.7° appears in the PLLA/POM (93/7) and PLLA/POM (95/5) blends, suggesting that POM are well crystallized when they are dispersed in the PLLA matrix. In addition, two strong sharp diffraction peaks are shown at 16.2 and 18.5° for the PLLA/POM (93/7) and PLLA/POM (95/5) blends, which is characteristic of the reflection of the PLLA α-form.53−55 This suggests that POM fragment lamellae could serve as heterogeneous nucleation agents and significantly enhanced the level of the crystallization of PLLA. 3.5. Dynamic Mechanical Properties of the Highly Quenched PLLA/POM Blends with a Small Amount of POM. Figure 8 shows plots of the storage modulus E′ by DMA as a function of temperature for neat PLLA, neat POM, and PLLA/ POM blends. Note that all samples used for the DMA

Figure 5. (a) Heat flow as a function of isothermal crystallization time of the PLLA/POM blends, (b) the relative degree of crystallization of PLLA/POM blends as a function of the isothermal crystallization time from part a, and (c) the half-times of isothermal crystallization of PLLA as a function of the neat PLLA and PLLA/POM composition from part b.

shown in Figure 6. The samples are fully crystallized at 135 °C. It can be seen from Figure 6a that the neat PLLA forms distinct crystalline spherulites with a size of more than 200 μm. In contrast, the size of PLLA spherulites becomes much smaller with addition of several percent of POM, as shown in Figure 6b. Moreover, the spherulite boundaries become obscure in the PLLA/POM (93/7) blend. Obviously, the POM fragment crystals nucleate the crystallization of PLLA and the nucleation 7172



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better transmittance. Figure 9 shows the quantitative UV−vis transmittance spectra of the neat crystallized PLLA and the

Figure 7. Wide-angle X-ray diffraction curves of samples cooled from melt temperature to room temperature at a rate of 10 °C/min. Figure 9. UV−vis transmittance spectra of samples cooled from melt temperature to room temperature at a rate of 10 °C/min.

PLLA/POM blends (all the samples are fully crystallized at 135 °C). The PLLA/talc composite was also shown in the figure for comparison. All samples have a thickness of about 300 μm. Neat PLLA exhibits a transmittance of ∼88% above a wavelength of 400 nm. The PLLA/POM blends exhibit a tremendously increased transmittance over the entire spectrum region, compared with the neat PLLA. The transmittance in the visible region is above 97% in the visible region, indicating the high clearance of the prepared blends. The increased transmittance in the blends can be attributed to the smaller size of the spherulites in PLLA.56 In contrast, the PLLA/talc composites show much lower transmittance even though talc decreases the size of PLLA spherulites, when compared with the PLLA/POM blends. This may be due to the bad dispersion of talc in the PLLA matrix (as shown in Figure S4, Supporting Information). Stress versus strain curves of neat PLLA, neat POM, and PLLA/POM blends are shown in Figure 10, and the

Figure 8. Dynamic loss for the quenched PLLA/POM blends as a function of temperature: (a) neat PLLA, (b) PLLA/POM = 99/1, (c) PLLA/POM = 97/3, (d) PLLA/POM = 95/5, (e) PLLA/POM = 93/7, (f) PLLA/POM = 90/10, and (g) neat POM.

measurements are very quickly quenched from the melt miscible region. The PLLA in all the samples is amorphous with no discerned crystals. It is seen that the storage modulus of neat POM decreases gradually. The storage modulus of neat PLLA exhibits a long plateau up to the glass transition followed by a dramatic drop at about 50−60 °C and then rises at temperatures ranging from 80 to 140 °C, because of the cold crystallization of PLLA.56,57 All the PLLA/POM blends show similar storage modulus behaviors with increasing temperature. However, it is seen that the temperature at which E′ starts to increase, due to the cold crystallization of the PLLA component, shifts to a lower temperature with the addition of POM. The largest shift occurs for the PLLA/POM (93/7) and (95/5) samples, which again indicates the highest crystallization rate for the samples induced by the POM fragment crystals. 3.6. Physical Properties of PLLA/POM Blends with a Small Amount of POM. The PLLA was well crystallized in the PLLA/POM blends with POM contents of 5 and 7 wt %. It is therefore interesting to investigate the physical properties of the PLLA/POM with a small amount of POM. As an important environmental friendly polymer, PLLA has been widely used as a film for packaging and agriculture industry. The optical transmittance is an important property for this application. We considered that the samples with smaller PLLA spherulites show

Figure 10. Stress−strain curves for neat PLLA, neat POM, and PLLA/ POM blends. 7173



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Table 2. Mechanical Properties of PLLA/POM Blends (Quenched) sample

tensile modulus (GPa)

tensile strength at yielding (MPa)

tensile strength at break (MPa)

elongation at break (%)

PLLA POM PLLA/POM = 99/1 PLLA/POM = 97/3 PLLA/POM = 95/5 PLLA/POM = 93/7 PLLA/POM = 90/10

2.3 ± 0.1 1.7 ± 0.3 2.7 ± 0.1 2.1 ± 0.2 2.1 ± 0.2 2.6 ± 0.3 2.3 ± 0.2

64.8 ± 0.9 68.0 ± 1.5 77.6 ± 0.8 79.7 ± 0.4 78.2 ± 1.0 74.8 ± 1.1 81.9 ± 1.2

51.9 ± 0.8 61.9 ± 1.7 60.3 ± 0.9 64.1 ± 0.5 62.8 ± 1.1 52.3 ± 1.5 60.5 ± 1.2

7.3 ± 2.8 21.3 ± 8.5 8.0 ± 2.3 7.6 ± 2.1 7.5 ± 1.5 8.2 ± 1.4 10.3 ± 1.5

the blends limits the development of the crystal. No compact POM spherulites can be obtained and POM crystallizes into the sparse (fragment) crystals. Obviously, these fragment crystals are well dispersed in the PLLA matrix, as schematically shown in Figure 11b. The fragment lamellae crystals of POM then acted as nucleation sites of the crystallization of PLLA. In other words, PLLA nucleated from the POM fragment crystals and grew from these fragment crystals. Therefore, we observed the following PLLA crystallization after the crystallization of POM (Figure 11c and d). It should be noted that very few fragment crystals formed with less than 3 wt % POM content and no clear nucleation effects for PLLA can be observed during cooling from the melt. At the same time, too much POM loading in the blends leads to the compact and fully developed spherulites (see Figure 2c). In this case, the PLLA molecular chains are mainly expelled out from the POM spherulites, so the crystallization rate of PLLA is similar to the neat PLLA. For the PLLA/POM blends with POM ranging from 3 to 10%, we observed the drastically enhanced PLLA crystallization rate and increased PLLA crystallinity induced by the POM fragment crystals. Emphasis should be made that such nucleated PLLA by POM fragment crystals in the PLLA/POM blends exhibit obvious advantages over the conventional nucleation strategy for PLLA, such as the inorganic fillers or the rubber domains. First, a small amount of POM is enough to accelerate the crystallization of PLLA, which maintains the high biodegradable PLLA content in the final products. Second, the POM fragment crystals are formed from the miscible stable of POM and PLLA, so there is no phase separation or crystal dispersion problems in the PLLA matrix. Finally, the PLLA/POM blends exhibit higher modulus, higher tensile strength, and higher optical transmittance than neat PLLA due to the crystallized PLLA matrix in the blends. In fact, we consider that this novel strategy is also applied to fabricate other high performance semicrystalline polymer blends. Although POM is not a biodegradable component, the POM fragment crystals are very small (in the size of nanoscale), so the impact on the environment of the POM component remaining after the degradation of PLLA could be ignored.

corresponding main tensile properties are listed in Table 2. Note that all the samples used for the property measurements are cooled down from the melt at 10 °C/min. PLLA is very rigid and shows high tensile strength, but it breaks at an elongation value of about 7%. POM exhibits a lower modulus and longer elongation at break than PLLA. All the PLLA/POM blends show a higher tensile modulus and yield strength than both neat PLLA and neat POM, while no big variation in the elongation at break was observed. We considered that the increased crystallinity of PLLA in the blends initiated by POM fragment crystals accounts for the enhancement of the tensile properties.

4. DISCUSSION It is interesting to find the enhanced crystallization rate of PLLA in the presence of several percent of POM. Moreover, such PLLA/POM blends with very minor POM content show improved optical and physical properties. The blends of PLLA and POM exhibit the typical LCST phase diagram. All the experiments in this work were carried out for the samples that are in the miscible state in the melt, as shown in Figure 11a. Although

Figure 11. Schematic diagram of crystallization PLLA nucleated by the fragment lamellae crystals of POM in the PLLA blends with a small amount of POM: (a) amorphous sample at 190 °C; (b) POM crystallization as nuclei at 135 °C; (c) PLLA crystallization nucleated from POM lamellae at 135 °C; (d) complete PLLA crystallization after annealing at 135 °C.

5. CONCLUSION The addition of a small amount of POM accelerated the crystallization rate and increased the final crystallinity of PLLA in the PLLA/POM blends. The maximum crystallization rate was achieved at a POM content of 7 wt %, where a large amount of POM fragment crystals were well dispersed in the PLLA matrix and induced the crystallization of PLLA. The polarized optical microphotographs indicate that POM provides a large number of nucleation sites for the crystallization of PLLA and leads to an apparent reduction of the spherulite size. The obtained PLLA/ POM blends exhibit excellent optical transmittance and good mechanical performance with high yield strength. Consequently, it is considered that POM is a new efficient eco-friendly

most of the crystalline polymers show crystallinity depression upon mixing with another miscible polymer,58,59 POM was well crystallized in its blends with PLLA even with the very low concentration in the blends. This can be attributed to the weak interaction between POM and PLLA molecular chains. POM will crystallize first from the miscible POM/PLLA blends during the cooling from the melt because of the much higher crystallization rate of POM than PLLA. However, the low content of POM in 7174



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nucleating agent for improving the thermo-mechanical properties and processability of PLLA, which may be of use for expanding the application fields of PLLA-based materials. Moreover, the strategy to apply the fragment crystals as the nuclei for the second semicrystalline polymer in a polymer blend may also be applicable to fabricate other high performance polymeric materials.

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ASSOCIATED CONTENT

S Supporting Information *

WAXD diffraction patterns of PLLA/POM blends, crystallization rate of PLLA in the neat PLLA and PLLA/POM (93/7) blend, and SEM image for PLLA/talc (95/5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-571-2886-7206. Fax: 86-571-2886-7899. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (51173036, 21374027, 11104054), PCSIRT (IRT 1231), and Program for New Century Excellent Talents in University.

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REFERENCES

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Enhanced Crystallization Rate of Poly(l-lactic acid) (PLLA) by

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