Enhanced Nucleation and Crystallization of Poly(l-lactic acid) by

Feb 5, 2014 - Polymorphic Crystallization and Crystalline Reorganization of Poly(l-lactic acid)/Poly(d-lactic acid) Racemic Mixture Influenced by Blen...
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Enhanced Nucleation and Crystallization of Poly(L‑lactic acid) by Immiscible Blending with Poly(vinylidene fluoride) Pengju Pan,* Guorong Shan, and Yongzhong Bao State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ABSTRACT: Effects of poly(vinylidene fluoride) (PVDF) on the non-isothermal and isothermal crystallization kinetics, crystalline structure, and spherulitic morphology of poly(L-lactic acid) (PLLA) in their immiscible blends were investigated. Both the cold and melt crystallizations of PLLA are accelerated after blending with PVDF. The crystallization rate of PLLA further increases as the PVDF content increases. PLLA does not crystallize upon cooling (10 °C/min) from the melt, while it can almost crystallize sufficiently under the same conditions after blending with 10% PVDF. PLLA crystallizes after the crystallization of PVDF in the melt crystallization process. The isothermal crystallization kinetics of PLLA in the blends was analyzed by the Avrami model. Crystallization half-time decreases and crystallization rate constant of PLLA increases after blending with PVDF. With the presence of PVDF, the spherulitic size of PLLA decreases and spherulitic number increases. Nucleation density of PLLA increases by 2−3 orders of magnitude by blending with 10 or 25% PVDF. It is found that PLLA spherulites can epitaxially grow on the PVDF crystalline domains, forming a transcrystalline structure in the PLLA/PVDF interface. It is proposed that the PVDF-promoted crystallization of PLLA in their immiscible blends is ascribed to the heterogeneously epitaxial and interfaceassisted nucleation mechanism.

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INTRODUCTION Polymer blending has been extensively investigated because it is a simple and efficient method to produce new materials with the desired properties. For the polymer blends in which one of the components is crystallizable, the presence of another component strongly influences the crystallization behavior. Both the crystallization kinetics and crystalline morphology in the blends of miscible as well as immiscible polymers are different from those of the neat crystallizable component. Because of the small mixing entropy contribution, the polymer pairs are generally not miscible in each other and thus almost all of the commercial blends are immiscible.1 The crystallization becomes much more complicated in the immiscible blends due to the effects of phase separation and interface.2 In most cases, the immiscible blending has a retardant effect on the crystallization rate of the crystallizable component.3,4 It has also been reported that the immiscible blending can promote the nucleation and crystallization of the crystallizable component in some blend pairs.5−14 Krasnikova et al. have found that the isotactic polypropylene (iPP) could preferentially nucleate on the surface of atactic polystyrene (aPS) in their immiscible blends.5 Wenig and Asresahegn have reported that the heterogeneous nucleation of iPP was promoted after blending with the amorphous or crystalline ethylene− propylene−diene terpolymers (EPDM), due to the decreasing of surface free energy.6 Tsuburaya and Saito have reported that the crystallization of polycarbonate was accelerated by blending with poly(ethylene oxide) (PEO), as a cause of the liquid− liquid phase separation (LLPS) induced by spinodal decomposition.7 Recently, Han and co-workers have systematically investigated the interplay between nucleation, crystallization, and LLPS of poly(ethylene-co-butene) (PEB)/poly(ethyleneco-hexene) (PEH)8,9 and iPP/poly(methyl methacrylate) (PMMA) blends10 and claimed that the nucleation preferen© 2014 American Chemical Society

tially occurred near the phase interface. They proposed that the concentration fluctuation associated with LLPS led to the alignment and/or orientation of polymer chains, thereby resulting in the promotion of nucleation.8−11 Sakai et al. have proposed that the locally depressed glass transition temperature (Tg) at the interface of poly(ε-caprolactone (PCL) domains accelerated the nucleation of poly(L-lactic acid) (PLLA) during its cold crystallization in PLLA/PCL blends.12 However, the mechanism associated with the interface-assisted nucleation has not been sufficiently clarified. Poly(L-lactic acid) is a promising biobased and biodegradable semicrystalline thermoplastic and has attracted considerable attentions in recent years. PLLA has been widely used for biomedical applications such as implant material, surgical suture, and controlled drug delivery systems. Meanwhile, PLLA has been an alternative to the conventional petroleumbased thermoplastics for the everyday commodity applications.15 Unfortunately, PLLA has several drawbacks, in terms of the processability and mechanical property. The crystallization rate of PLLA is very slow, resulting in the long processing cycle time and low production efficiency of products in the melt processing and molding.15,16 A lot of nucleating agents (NAs) have been reported to promote the crystallization rate of PLLA, most of which are low molecular weight substances.16−20 Iinspired by the interface-assisted nucleation, we envision that the immiscible blending would be an efficient approach to promote the crystallization rate of PLLA. Besides, blending Received: Revised: Accepted: Published: 3148

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isothermal melt crystallization, the blends were cooled from 190 °C to the desired temperature (80−140 °C) at a rate of 100 °C/min and then held at this temperature for crystallization. After the completion of crystallization, the sample was reheated to 190 °C to detect the melting behavior. Polarized Optical Microscopy (POM). Spherulitic morphology was measured on an optical microscopy (Eclipse E600 POL, Nikon Co., Tokyo, Japan) under crossed polarizers. The microscopy was equipped with a Linkam THMS600 temperature controller (Linkam Scientific Instrument, Surrey, UK). To study the spherulite morphology of PLLA/PVDF blends, the samples, sandwiched between two glass sheets, were heated to 190 °C for 2 min to erase the thermal history and then cooled to 150 °C at a rate of 100 °C/min for crystallization. The spherulitic morphology was recorded after the completion of crystallization. To investigate the growth of PLLA spherulites near the PLLA/PVDF boundary, neat PLLA and PVDF films were heated to 190 °C for 2 min between two glass sheets. The glass sheets were slightly pressed, making the PLLA and PVDF melts converge and form a boundary. The sample was cooled to 150 °C at a rate of 100 °C/min and held at this temperature for 8 min for the crystallization of the PVDF layer. It was then cooled to 140 °C for the crystallization of the PLLA layer. To study the spherulite growth of PLLA near PVDF domain, the PLLA-rich blends were heated to 190 °C for 2 min and then cooled to 140 °C at a rate of 100 °C/min. Spherulitic morphology was recorded after the different crystallization periods. Wide-Angle X-ray Diffraction (WAXD). WAXD patterns were recorded on a Rigaku RU-200 (Rigaku Co., Tokyo, Japan) instrument with a Ni-filtered Cu Kα radiation (λ = 0.154 nm), working at 40 kV and 200 mA. WAXD patterns of blends meltcrystallized at different temperatures were recorded in the 2θ range of 5°−50° at a scan rate of 1°/min. All the WAXD measurements were performed at room temperature. Dynamic Mechanical Thermal Analysis (DMTA). DMTA was performed on a DMS210 (Seiko Instruments, Tokyo, Japan) instrument equipped with a SSC5300 controller, working at a frequency of 5 Hz and a heating rate of 2 °C/ min. The sample for measurement was a thin rectangular strip and had a dimension of 30 × 10 × 0.4 mm3.

with other polymers is also a potential way to modify the mechanical properties (especially toughness) of PLLA. Herein, we select poly(vinylidene fluoride) (PVDF) as a model semicrystalline polymer and prepared the binary crystalline/crystalline blends of PLLA and PVDF. Even though PVDF is miscible with some aliphatic polyesters,21,22 PLLA and PVDF are reported to be immiscible in the entire composition range.23−25 Previous studies have found that PLLA can facilitate the α- to β-phase transition of PVDF under eletrospinning24 and uniaxial stretching.25 However, the effects of PVDF on the crystallization kinetics and polymorphic crystalline structure of PLLA have not been explored. PVDF has higher crystallization temperature and faster crystallization rate than PLLA, in spite of their similar melting points. Therefore, PVDF will separate from the melt and crystallize first in the cooling process of the blends. Considering the effects of interface, the phase separation and crystallization of PVDF would change the crystallization kinetics and crystalline morphology of PLLA. Additionally, PVDF is flexible and can modify the brittleness of PLLA.24 In this study, the melt- and cold-crystallization kinetics, crystalline structure, and spherulite morphology of PLLA in the PLLA/PVDF blends were investigated. It is found that the crystallization and nucleation of PLLA are highly enhanced by blending with PVDF. The mechanism for the enhanced nucleation of PLLA induced by the immiscible blending was discussed.

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EXPERIMENTAL SECTION Materials. PLLA (trade name 9020, Mn = 121 kg/mol, Mw/ Mn = 1.54, D-isomer lactide < 2.0%) was kindly supplied by Shimadzu (Kyoto, Japan). PVDF (Mn = 71 kg/mol, Mw = 180 kg/mol, analytical grade) was purchased from Sigma-Aldrich Co. The chemical structures of PLLA and PVDF are shown in Scheme 1. PLLA/PVDF blends were prepared by solution Scheme 1. Chemical Structure of PLLA and PVDF

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RESULTS AND DISCUSSION Non-Isothermal Crystallization Kinetics. The effect of PVDF on the crystallization kinetics of PLLA was first investigated by DSC. Figure 1 shows the DSC heating curves of quenched PLLA/PVDF blends. In the quenched blends, PLLA remains nearly amorphous while PVDF is crystalline, due to the relatively slow crystallization of PLLA. As shown in Figure 1, neat PLLA and the blends exhibit the glass transition of PLLA, the exothermic cold crystallization peak of PLLA, and the joint melting peak of PLLA and PVDF upon increasing the temperature. The glass transition temperature (Tg), cold crystallization temperature (Tcc), crystallization enthalpy (ΔHcc) of PLLA, and total melting enthalpy (ΔHm,total) of PLLA and PVDF are summarized in Table 1. Although the Tg of PLLA is slightly decreased after blending with PVDF (Table 1), two individual Tg values are observed in the DMTA result, as shown in the following part. Measurement of optical microscopy also indicates that PLLA and PVDF are phaseseparated in the melt state (data not shown). These results demonstrate that PLLA and PVDF are immiscible. The Tcc of PLLA decreases dramatically after blending with PVDF,

casting using N,N-dimethylformamide (DMF; >99.5%, analytical grade) as a common solvent. Predetermined amounts of PLLA and PVDF were dissolved in DMF with a total concentration of 0.05 g/mL. The mixed solution was then cast on a Teflon dish. After the evaporation of solvent at 80 °C for 1 day, the cast films were further dried in vacuo at 60 °C for 12 h. The PLLA/PVDF blends are labeled as 90/10, 75/25, 50/ 50, and 25/75, where the numerals denote weight percentages of PLLA and PVDF, respectively. Measurements. Differential Scanning Calorimetry (DSC). DSC measurements were performed on a TA Q200 DSC (TA Instruments, New Castle, DE) under the flow of nitrogen gas. The blends were first heated to 190 °C and held at the same temperature for 2 min to erase the thermal history. The thermal procedures of non-isothermal and isothermal crystallizations are as follows: (1) In the non-isothermal cold crystallization, the blends were quenched from 190 to 0 °C by a fast cooling at 100 °C/min and then reheated to 190 °C at 10 °C/min. (2) For the non-isothermal melt crystallization, the blend was cooled from 190 to 0 °C and then reheated to 190 °C. Both the heating and cooling rates were 10 °C/min. (3) For the 3149

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Figure 1. DSC curves of quenched PLLA/PVDF blends recorded in the heating process.

Table 1. Thermal Parameters of PLLA/PVDF Blends in Non-Isothermal Cold Crystallization PLLA/PVDF

Tg,PLLA (°C)

Tcc,PLLA (°C)

ΔHcc,PLLA (J/g)

ΔHm,total (J/g)

100/0 90/10 75/25 50/50 25/75 0/100

58.6 57.5 56.2 57.4 57.8

113.3 94.7 91.0 92.3 92.3

−40.5 −33.8 −34.2 −33.0 −32.6

46.0 35.3 37.8 43.6 45.3 46.2

indicating that PVDF accelerates the cold crystallization of PLLA. The Tcc of PLLA decreases by ca. 20 °C after blending with 10% PVDF, but no significant change in Tcc is observed with a further increase of PVDF content. It is notable that the ΔHm,total values of blends are smaller than those of neat PLLA and PVDF, which is more distinct for the PLLA-rich blends. This is ascribed to the effects of PLLA crystallization temperature, because the less ordered crystals with smaller ΔHm are formed at a lower crystallization temperature. Figure 2 shows the DSC curves of non-isothermal melt crystallization and subsequent heating scans for the PLLA/ PVDF blends with different compositions. The thermal parameters such as melt crystallization temperature (Tmc), crystallization enthalpy (ΔHmc), and total melting enthalpy (ΔHm,total) were evaluated from the DSC results, as summarized in Table 2. In the cooling process, the generated degree of crystallinity (Xc) of PLLA and PVDF was estimated by comparing the crystallization enthalpy (ΔHmc) with the melting enthalpy of an infinitely large crystal (ΔHm0), i.e., Xc = ΔHmc/ ΔHm0 × 100%, in which the ΔHm0 values of PLLA26 and PVDF27 are 93.4 and 102.5 J/g, respectively. For neat PLLA, no crystallization peak is detected in the cooling process at 10 °C/min, and an exothermic peak (near 110 °C) corresponding to the cold crystallization is observed in the following heating. This indicates that the crystallization of neat PLLA is very slow and it almost does not happen during cooling. Interestingly, all the blends exhibit two crystallization exotherms in cooling, of which the ones at higher and lower temperatures correspond to the crystallizations of PVDF and PLLA, respectively. PVDF crystallizes prior to PLLA during the cooling process of the blends. For the blends, PLLA crystallizes nearly sufficiently in the cooling process and almost no cold crystallization is present in the following heating. The Tmc of PVDF (Tmc,PVDF) changes little with varying the blend composition, while the Tmc of PLLA (Tmc,PLLA) increases as

Figure 2. DSC curves of PLLA/PVDF blends recorded in the (a) cooling and (b) subsequent heating processes.

the PVDF content increases, indicating the increase of PLLA crystallization rate with the PVDF content. It is notable that the melting behavior of PVDF depends also on the thermal procedure. Because the less perfect crystallites of PVDF are formed under quenching, the quenched PVDF undergoes the melt−recrystallization−remelt process and shows two melting peaks upon heating (Figure 1). However, for the PVDF crystallized at slow cooling (10 °C/min), it has enough time to crystallize into the perfect α-form crystallites and melts directly in the following heating, without the occurrence of recrystallization (Figure 2b). Isothermal Crystallization Kinetics. Isothermal melt crystallizations of PLLA/PLLA blends were also investigated by DSC. Figure 3 show the DSC curves of PLLA/PVDF blends recorded in the isothermal crystallization at 120 and 140 °C. 3150

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Table 2. Thermal Parameters of PLLA/PVDF Blends in Non-Isothermal Melt Crystallization and Subsequent Heating PLLA/PVDF

Tmc,PLLA (°C)

ΔHmc,PLLA (J/g)

100/0 90/10 75/25 50/50 25/75 0/100

96.7 108.3 119.6 122.7 126.6

−4.5 −35.9 −47.3 −48.0 −49.2

Tmc,PVDF (°C) 138.5 138.6 139.6 139.6 139.4

ΔHmc,PVDF (J/g)

ΔHm,total (J/g)

Xc,PLLA (%)

−9.3 −44.2 −47.9 −47.8 −48.0

40.4 38.2 45.2 52.1 52.1 50.0

4.8 38.4 50.6 51.4 52.7

Xc,PVDF (%) 9.1 43.1 46.7 46.6 46.8

Figure 3. DSC curves of PLLA/PVDF blends recorded in isothermal melt crystallizations at (a) 120 and (b) 140 °C. Panel c is the image of panel b in the initial stage of crystallization. The DSC curves were vertically shifted for clarity. Figure 4. DSC heating curves of PLLA/PVDF blends after isothermal melt crystallization at (a) 120 and (b) 140 °C.

The crystallization peak of neat PLLA is very broad; it becomes much sharper and shifts to the shorter time side after blending with PVDF. The crystallization time further shortens with the PVDF content increasing. As magnified in Figure 3c, we can observe the isothermal crystallization peak of PVDF before the crystallization of PLLA. Figure 4 shows the DSC melting curves of PLLA/PVDF blends after the isothermal crystallization. Because the melting peaks of PLLA and PVDF converge in the temperature range of 155−175 °C, the effects of blending on the melting behavior of PLLA cannot be identified. To evaluate the isothermal crystallization kinetics, the heat flow of the isothermal DSC curve was integrated to attain the relative degree of crystallinity (Xt) t

Xt =

∫t (dHc/dt )dt 0

Figure 5. Crystallization half-times of PLLA in PLLA/PVDF blends crystallized at different temperatures.

∞

∫t (dHc/dt )dt 0

(1)

where the limits t0, t, and ∞ are the starting point of PLLA crystallization peak, the elapsed time in the course of crystallization, and at the end of the crystallization, respectively. dHc is the measured crystallization enthalpy in an infinitesimal time interval dt. A horizontal line from a point after the crystallization exotherm was used as the baseline for integration. Crystallization half-time (t1/2) of PLLA was evaluated from the time with Xt = 50%. Figure 5 shows the

t1/2 values of PLLA/PVDF blends crystallized at different temperatures. The t1/2 values of PLLA in both the neat PLLA and blends show minimums at Tc = 100−110 °C and they increase with increasing or decreasing Tc. The t1/2 of PLLA dramatically decreases after blending with PVDF. After blending with 10% PVDF, the t1/2 values of PLLA crystallized at 100 and 140 °C decrease from 4.4 and 60.2 min of neat PLLA to 1.4 and 16.7 min, respectively. 3151

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Table 3. Kinetic Parameters of PLLA/PVDF Blends Crystallized at 120 and 140 °C Tc = 120 °C PLLA/PVDF

t1/2 (min)

n

k (min−n)

100/0 90/10 75/25 50/50 25/75

6.24 3.80 2.22 2.24 1.47

2.4 2.9 2.0 2.0 1.8

0.013 0.023 0.21 0.49 0.72

Tc = 140 °C N (mm−3)

t1/2 (min)

n

k (min−n)

N (mm−3)

× × × × ×

60.2 16.7 8.68 8.45 5.78

2.9 2.8 2.6 2.4 2.3

5.4 × 10−6 5.9 × 10−4 6.3 × 10−3 0.010 0.031

1.1 1.2 1.2 2.0 6.1

9.0 1.6 1.5 3.4 5.0

104 105 106 106 106

× × × × ×

102 104 105 105 105

On the basis of DSC data, the isothermal crystallization kinetics of PLLA in the blends crystallized at 120 and 140 °C were analyzed by the Avrami equation.28 Considering the presence of crystallization reduction time t0 for the PLLA component, the Avrami equation can be written as29 1 − X t − t0 = exp[−k(t − t0)n ]

(2)

where n is the Avrami index and k is the overall rate constant. The linear form of eq 2 can be stated as log[− ln(1 − X t − t0)] = log k + n log(t − t0)

(3)

k and n can be estimated from the linear fitting of log[−ln(1 − Xt−t0)] vs log(t − t0). To ensure the accuracy of the Avrami analysis, the data in a limited conversion range (3−30%) was employed for fitting. Excellent fits were attained with the correlation coefficients above 0.99. As shown in Table 3, the n values of PLLA in the neat state and blends are under the range of 1.8−2.9. The n values of blends generally decrease with increasing the PVDF content. This might be ascribed to the heterogeneous nucleation effect of PVDF on PLLA crystallization and the retardation effect of PVDF on the threedimensional spherulitic growth of PLLA induced by the spatial confinement. PLLA crystallized in the blends has larger k values than those crystallized in the neat state. The k values of PLLA crystallized in the blends further increase as the PVDF content increases. All the results of non-isothermal and isothermal crystallizations indicate that PVDF enhances the crystallization rate of PLLA in their immiscible blends. Crystalline Structure. The effects of blend composition and crystallization temperature on the crystalline structure of PLLA/PVDF blends were investigated. Figure 6 shows the WAXD profiles of (a) PLLA/PVDF blends crystallized at 140 °C and (b) PLLA/PVDF 75/25 blend crystallized at different temperatures. As shown in Figure 6a, the neat PVDF and PVDF-rich blends exhibit diffraction peaks at 2θ = 17.9°, 18.8°, 20.0°, and 26.8°, referent to the (100), (020), (110), and (021) diffraction plane, respectively, all characteristic of the PVDF αphase.30,31 The neat PLLA and PLLA-rich blends exhibit the characteristic diffractions of PLLA α-phase. Both the diffraction peaks of the α-form PLLA and PVDF are observed in the WAXD patterns of blends. The crystalline structure of PLLA in its blends is dependent on the crystallization temperature (Figure 6b). Similar to the neat PLLA, the α and α′ (or δ) crystals of PLLA are predominantly formed in the blends at higher and lower crystallization temperatures, respectively.32−35 Due to the transition of crystal polymorphism, the major reflections (110/200 and 203) of PLLA shift to the higher diffraction angles, and some new reflections are present as the crystallization temperature is increased from 80 to 140 °C. Besides, only one reflection (indicated by the arrow) is observed at around 2θ = 24.5° for the blends crystallized at below 100 °C, but two reflections (016 and 206) are present in

Figure 6. WAXD profiles of (a) PLLA/PVDF blends crystallized at 140 °C and (b) PLLA/PVDF 75/25 blends crystallized at different temperatures.

this region as the crystallization temperature is increased to above 120 °C (Figure 6b). Therefore, it is considered that the crystalline structure of PLLA in the blends is mainly influenced by the crystallization temperature, but not the PVDF component. Spherulite Morphology. The spherulite morphology of PLLA/PVDF blends was observed by POM. Figure 7 depicts the POM micrographs of blends after crystallizing at 150 °C. PVDF crystallizes prior to PLLA at this temperature, due to the fast crystallization of the former. Neat PLLA crystallizes into the huge spherulites at 150 °C, whose diameter is around several micrometers. Neat PVDF forms much smaller spherulites than PLLA. After blending with PVDF, the spherulite size of PLLA decreases and the spherulite density increases significantly. The nucleation density (N) of PLLA was evaluated quantitatively. Assuming that the spherulites are three-dimensional and are simultaneously initiated from the active nuclei (n ≈ 3), N can be estimated36 N= 3152

3k 4πG3

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arrows in Figure 8b), similar to that of the iPP/poly(methyl methacrylate) (PMMA) blend.10 To further confirm the epitaxial growth of PLLA on the PVDF domain, we investigated the growth of PLLA spherulites around the PLLA/PVDF interface. Figure 9 shows the POM

Figure 7. POM micrographs of neat PLLA, PVDF, and PLLA/PVDF blends after isothermal melt crystallization at 150 °C.

where k is the Avrami rate constant, as shown in eq 2. G is the linear radius growth rate of spherulite, which can be determined from the POM measurements. G values of neat PLLA crystallized at 120 and 140 °C were 3.25 and 2.30 μm/min, respectively. It has been reported that the NA37 and immiscible blending5,38 do not change the spherulite growth rate of polymer matrix, because they just affect the primary nucleation but not the secondary nucleation. Therefore, it is assumed that the G values of PLLA crystallized in the blends are the same as those of neat PLLA. As shown in Table 3, the N value of PLLA increases as the content of PVDF increases and it increases by 2−3 orders of magnitude after blending with 10 or 25% PVDF. This demonstrates that PVDF assists the heterogeneous nucleation of PLLA. It should be noted that the unit of k is min−n and the n values of blends with different compositions are not the same. This may result in errors in the estimation of nucleation density from eq 3. We further investigated the crystal nucleation of PLLA around the PVDF domains. Figure 8 shows the POM

Figure 9. POM micrographs of PLLA and PVDF near their interfaces in the sequential crystallization at 150 and 140 °C. The upper and lower sides are PLLA and PVDF, respectively. The line in the middle is the phase boundary of PLLA and PVDF. The samples were sequentially crystallized at 150 °C for 8 min and 140 °C for 80 min. The arrow indicates PLLA transcrystallites grown from the PLLA/ PVDF interface.

micrographs near the boundary of PLLA and PVDF layers. At 150 °C, the PVDF layer crystallizes first and the boundary of PLLA and PVDF layers deforms due to the pushing of PVDF spherulites. After cooling to 140 °C, the nucleation of PLLA first occurs along the PLLA/PVDF boundary. The number of nuclei in bulk PLLA is much less than that at the interface. As the spherulites grow, an orientated transcrystalline layer is formed along the PLLA/PVDF interface. The growth direction of transcrystals is normal to the boundary. The high density of PLLA active nuclei at the surface of PVDF domain impedes the lateral growth of PLLA spherulites and thus forces the spherulites to grow normal to the surface.39 Away from the boundary, the traditional spherulites of PLLA are produced. This specific transcrystalline structure indicates the high nucleating ability of PVDF toward PLLA. The well-known transcrystalline structure is often observed for the polymer crystallized in the fiber-reinforced systems39−41 and on the surface of NAs,16,42,43 which has been considered as direct evidence for the epitaxial and interface-assisted nucleation. Our result has demonstrated that the transcrystallization of one polymer can take place on the surface of other crystalline polymer. On the basis of the aforementioned results, it is proposed that the PVDF-promoted crystallization of PLLA in the immiscible blends is ascribed to two aspects, i.e., the heterogeneously epitaxial nucleation and interface-assisted nucleation. First, the solidified PVDF crystals can act as the NA of PLLA, leading to the acceleration of heterogeneous nucleation. Polymeric NAs have been applied to enhance the crystallization

Figure 8. POM micrographs of PLLA/PVDF 90/10 and 75/25 blends during the isothermal crystallization at 140 °C. PLLA and PVDF are shown as the continuous and dispersed phases, respectively. The arrows indicate the epitaxial growth of PLLA on the PVDF domain.

micrographs of PLLA-rich blends melt-crystallized at 140 °C. In the crystallization of PLLA/PVDF 90/10 blend, PLLA spherulites are favored to grow surrounding the PVDF crystalline domains that are crystallized prior to the PLLA domains. Most of the PLLA spherulites grow with PVDF domain as the center, as indicated by the arrows in Figure 8a. The epitaxial growth of PLLA on PVDF domains is also observed in the PLLA/PVDF 75/25 blend (indicated by the 3153

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rate and control the crystalline polymorphism of polymers such as iPP,44−46 poly(3-hydroxylbutyrate) (P3HB),47,48 and poly(butylene succinate) (PBS).43 Generally, the enhanced nucleation of NA-containing polymer is caused by the epitaxial mechanism. An essential requirement for epitaxial nucleation is at least one-dimension lattice matching between the polymer matrix and NA. The α-form PLLA crystallizes in an orthorhombic unit cell with a dimension of a = 1.034 nm, b = 0.597 nm, and c (fiber axis) = 2.88 nm.49 The α-form (form II) crystal of PVDF has an orthorhombic unit cell with the lattice parameters a = 0.496 nm, b = 0.964 nm, and c (fiber axis) = 0.462 nm.30,50 The c axis length of PLLA α-crystals is 3 times that of the b axis of PVDF α-crystals, with a tiny mismatching of 0.4%. This excellent matching suggests that PLLA may grow on the surface of PVDF crystals by an epitaxial nucleation mechanism. Second, the phase interface plays an important role on the polymer crystallization, as demonstrated by many studies.7−12 The presence of phase interface would decrease the surface free energy to form crystal nuclei via the heterogeneous nucleation.7,8 Also, the phase separation as well as the crystallization of PVDF domain can induce the molecular ordering, alignment, and/or orientation of PLLA chains at the interfaces by active interdiffusion, facilitating the formation of crystallizing embryos.8−11 On the other hand, because of the low Tg of PVDF, the locally depressed Tg and enhanced chain mobility of PLLA near the PVDF domains may enhance the nucleation.12 This interface-assisted nucleation would accelerate the crystallization of PLLA in PLLA/PVDF blends. Dynamic Mechanical Analysis. Dynamic mechanical properties of PLLA/PVDF blends were analyzed by DMTA. Figure 10 depicts the storage modulus (E′) and loss tangent (tan δ) curves as a function of temperature for neat PLLA, PVDF, and their blends. All the samples used for measurements were prepared by quenching from the melt state, in which PLLA is nearly amorphous while PVDF is semicrystalline. The storage moduli of blends drop dramatically at the temperature regions from −70 to −30 and 50 to 70 °C, because of the glass transitions of PVDF and PLLA, respectively, and then slightly rise in the temperature ranging from 80 to 140 °C, because of the cold crystallization of PLLA (Figure 10a). The storage moduli of blends are dependent on the temperature. At a temperature lower than the Tg of PVDF, the storage modulus increases with increasing the PVDF content. At a temperature between the Tg values of PVDF and PLLA, the storage modulus decreases as the PVDF content increases, because PVDF is flexible at this temperature range. From the tan δ curves (Figure 10b), it is observed that all the blends exhibit individual glass transitions of PVDF and PLLA, characteristic of the immiscible blends. However, the Tg value of PLLA slightly decreases and its Tg peak becomes broader after blending with PVDF. This Tg depression may originate from the partial miscibility of PLLA and PVDF at the phase interface. Besides, the presence of PLLA nuclei at the interface may also increase the mobility of PLLA chains.

Figure 10. (a) Storage modulus and (b) loss tangent curves of PLLA/ PVDF blends as a function of temperature.

the crystallization rate constant of PLLA increases remarkably after blending with PVDF. Spherulitic size of PLLA decreases and spherulitic number increases with the presence of PVDF. PLLA spherulites can epitaxially grow on the PVDF domains, forming a transcrystalline structure in the PLLA/PVDF interface. The mechanism of PVDF-promoted crystallization of PLLA in their blends is proposed to be the heterogeneously epitaxial and interface-assisted nucleation. This study has provided a convenient approach to enhance the crystallization rate of PLLA by immiscible blending, which is able to modify the processability of PLLA-based materials. The results shown here also shed light on the understanding of epitaxial and interface-assisted nucleation and crystallization in the polymer blend systems.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87951334. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the Natural Science Foundation of China (51103127, 21274128) and the State Key Laboratory of Chemical Engineering (SKL-ChE-12D06).

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CONCLUSIONS Through investigating the crystallization kinetics of PLLA in PLLA/PVDF blends, we have found a new method to accelerate the crystallization of PLLA by immiscible blending via the epitaxial and interface-assisted nucleation. Both the cold and melt crystallization rates of PLLA are enhanced by blending with PVDF. Crystallization half-time decreases and

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REFERENCES

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