Morphology, rheological and crystallization behavior in thermoplastic polyurethane toughed poly(l -lactide) with stereocomplex crystallites

Morphology, rheological and crystallization behavior in thermoplastic polyurethane toughed poly(l -lactide) with stereocomplex crystallites

Polymer Testing 62 (2017) 1e12 Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest Materia...

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Polymer Testing 62 (2017) 1e12

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material Properties

Morphology, rheological and crystallization behavior in thermoplastic polyurethane toughed poly(L-lactide) with stereocomplex crystallites Yu-Dong Shi, Yue-Hong Cheng, Yi-Fu Chen, Kai Zhang, Jian-Bing Zeng, Ming Wang* Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2017 Received in revised form 14 June 2017 Accepted 15 June 2017

Melt-blending poly(L-lactide) (PLLA) with elastomers has been well demonstrated to improve toughness of PLLA. Here, we show a poly(D-lactide) (PDLA) grafted thermoplastic polyurethane (TPU) (TPU-g-PDLA) toughed PLLA with simultaneous formation of few amount stereocomplex crystallites (SCs) which exhibited higher efficient toughening than that of PLLA with TPU. The TPU-g-PDLA was prepared by the in-situ melt-reaction of TPU and PDLA with 4, 4’-diphenylmethane diisocyanate (MDI). A comparative study on morphology, rheological and crystallization behavior was also carried in PLLA/TPU, PLLA/TPU-gPDLA and PLLA/TPU/PDLA samples. The PLLA/TPU-g-PDLA samples show the highest crystallization rate, complex viscosity, impact strength and tensile strength among PLLA/TPU, PLLA/TPU-g-PDLA and PLLA/ TPU/PDLA samples, indicating that the higher interfacial interaction between TPU-g-PDLA and PLLA. Furthermore, TPU chains in TPU-g-PDLA were thought to break the intermolecular interaction of PLLA and rapid its crystallization and increase crystallinity. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Thermoplastic polyurethane Poly(L-lactide) Stereocomplex crystallites Morphology Crystallization

1. Introduction Poly(L-lactide) (PLLA) has been attracted much attention in both industry and academia due to its attractive sustainability, biocompatibility, biodegradability, mechanical strength and easy processability [1e3]. However, its poor impact resistance and tensile toughness significantly limits its use in large-scale commercial applications [4e6]. Therefore, it is very necessary to toughen PLLA to widen its applications. Melt-blending with elastomers or flexible polymers, such as natural rubber [7e10], ethylene-co-vinyl acetate [11], poly(ethylene glycol) [12], ethylene-acrylic ester-glycidyl methacrylate terpolymer [13], poly (butylene succinate) [14], poly(ε-caprolactone) (PCL) [15,16] and thermoplastic polyurethane (TPU) elastomer [17e20] was found to be an effective way to toughen PLLA. Among those elastomers or flexible polymers, TPU was thought to be the most elegant and powerful one. TPU elastomers have both aromatic or aliphatic polyurethane “hard segments” and aliphatic polyester or polyether “soft segments”, which endows TPU has a unique toughness, durability and processability [21e23]. It has been reported that impact strengths and

* Corresponding author. E-mail address: [email protected] (M. Wang). http://dx.doi.org/10.1016/j.polymertesting.2017.06.013 0142-9418/© 2017 Elsevier Ltd. All rights reserved.

elongations at break of PLLA can remarkable increase by the incorporation of TPU. For example, a 20 wt% thermoplastic polyurethane (TPU) elastomer toughed PLLA can reach 8-fold and 70fold higher impact strength and elongation at break than pure PLLA, respectively [24]. Normally, the direct blending PLLA with TPU usually leads to low toughening efficiency, due to the phase separation and weak interfacial adhesion between them [5]. Some compatibilization strategies, including adding pre-synthesized block or graft copolymers and in-situ reactive compatibilization, have been used to enhance interfacial adhesion between PLLA and TPU [25e27]. Among those strategies, the in-situ reactive compatibilization has been found to be a powerful technique to control morphology, enhance interfacial adhesion and improve final properties of PLLA/ TPU blends [28e31]. For example, the TPU prepolymer with isocyanate (-NCO) group can successfully react with the -OH groups at both sides of the PLLA, then in-situ compatibilization between PLLA and TPU. The final PLLA/TPU blends exhibited super toughness [32]. On the other hand, stereocomplex crystallites (SCs) which formed between enantiomeric PLLA and poly(D-lactide) (PDLA) can act as efficient rheological modifier to tune morphology of PLLA based materials [33e35]. In addition, the melting point of SCs is ~230  C, which make SCs can be reserved at the normal processing temperature of PLLA (~190  C). The reserved SCs were found to

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accelerate the crystallization of homocrystallites (HCs) for PLLA [36e39]. As a result, both the crystallinity of PLLA matrix and the morphology of PLLA/TPU blends can be tuned by the construction of SCs in the blends. In fact, the final mechanical properties of PLLA/ TPU are mainly dependent on the crystallinity and morphology. Therefore, it is still very valuable to study the TPU toughed PLLA with SCs, although some sporadic work has been reported [40e43]. First, the crystallization kinetics of PLLA chains with PDLA chains is well demonstrated [36], but the crystallization kinetics of PLLA chains with both TPU and PDLA chains, especially with TPU-g-PDLA chains is still covered. Second, the location of TPU or TPU-g-PDLA in PLLA matrix, especially the crystallized PLLA with SCs, which is believed to affect the toughness of PLLA/TPU blends, is also unclear. Third, the toughening mechanism based on the interfacial interaction, SCs and matrix crystallinity still needs to further discuss. In this work, the effect of TPU and SCs on the crystallization behavior of PLLA was first investigated. In order to get the optimally synergistic effects of TPU and SCs, the TPU-g-PDLA chains were produced by MDI linking. The morphology, especially the location of TPU in PLLA matrix, was also evaluated with the absence or presence of SCs. The interfacial interaction of the TPU chains with PLLA chains with SCs was also discussed by the rheological analysis. The results indicated that the PLLA/TPU-g-PDLA samples exhibited the higher interfacial interaction, crystallization rate and as well as higher toughness than that of the PLLA/TPU/PDLA directly mixing samples.

products after extracting were also confirmed by Fourier Transform Infrared (FT-IR) spectroscopy. The absorption peak at 1751 cm1 which are attributed to the C¼O stretching vibration in PDLA was found in products after extraction (Fig. S1), also indicating the success in grafting PDLA molecules onto TPU to form TPU-g-PDLA grafting copolymers. After that, PLLA and TPU-g-PDLA were directly mixed together at 190 С and 80 rpm for 5 min to obtain the PLLA/TPU-g-PDLA samples. Standard tensile and notched Izod impact samples were injection molded by a Xinshuo MiniJet (Shanghai, China) with a barrel and mold temperatures of 200 and 50 С, respectively. All materials were dried overnight in a vacuum oven at 60  C before melt-blending and injection molding. The sample codes and their compositions were also given in Table 1.

2. Experimental section

Xc ¼

2.1. Materials Poly(L-lactide) (PLLA, trade name 4032D) with 98.7 mol% Lisomeric content, a weight-average molecular weight (Mw) of 2.10  105 g/mol and a density of 1.24 g/cm3was purchased from Nature Works LLC (USA). Poly(D-lactide) (PDLA) with the Mw of 1.34  105 g/mol was obtained from Jinan Dai Gang Biological Engineering Co. Ltd. (China). Polyester-based thermoplastic polyurethane (TPU, trade name U-85A10) with a density of 1.21 g/cm3, a Shore Hardness A of 90 and a glass transition temperature of about 40  C was obtained from Bayer Material Science and Technology (China) Co., Ltd. 4, 4’-diphenylmethane diisocyanate (MDI) was bought from Aladdin Industrial Corporation, China. 2.2. Sample preparation For the PLLA/TPU samples, PLLA and TPU were directly meltmixed in a Hapro rotational mixer (RM-200A, Harbin, China) at 190 С and 80 rpm for 5 min. In order to investigate the effect of SCs on the properties of PLLA, the PLLA/SCs and PLLA/TPU/PDLA samples were fabricated. For the PLLA/SCs samples, the SCs were first prepared by melt-mixing isometric PLLA and PDLA at 190 С and 80 rpm for 5 min. The resulting SCs were then mixed with PLLA to get the PLLA/SCs samples at 190 С and 80 rpm for 5 min. For the PLLA/TPU/PDLA samples, TPU and PDLA (90/10) were first mixed at 190 С and 80 rpm for 5 min, and then further mixed with PLLA for 5 min at the same rotation speed and temperature. In order to enhance the interfacial interaction between TPU and PDLA, the grafting reaction was carried out by melt-blending of TPU (45 g) and PDLA (5 g) in the presence of small amount (0.3 wt%) of MDI using the Hapro rotational mixer at 190 С and 80 rpm for 5 min to obtain TPU-g-PDLA master batch. In order to confirm the PDLA chains grating with TPU chains, the soxhlet extraction was used with chloroform as the solvent. It is found that there is only 2.0 wt% reduction after 48 h' extraction, suggesting MDI molecules have successfully chemical connection of TPU and PDLA. The

2.3. Characterization For the nonisothermal crystallization behaviors, the samples with ~5 mg were tested by a NETZSCH DSC-214 differential scanning calorimeter in a dry nitrogen atmosphere. The samples were first heated from 20 to 200  C at a heating rate of 10  C/min and hold for 5 min to remove thermal history, then cooled to 20  C at a cooling rate of 10  C/min, and finally reheated to 260  C at a heating rate of 10  C/min. The degree of crystallinity (Xc) was calculated by equation (1) from the second heating curve [44].

DН m  DН c wf DН om

(1)

where DН m , DН c , wf and DН om are the measured enthalpies of melting peaks, the measured enthalpies of cold crystallization, the weight percent of PLLA matrix and the melting enthalpies of 100% crystalline PLLA of 93.7 J/g [45], respectively. For the isothermal crystallization behaviors, the samples with ~5 mg were also tested by a NETZSCH DSC-214 differential scanning calorimeter in a dry nitrogen atmosphere. The samples were melted at 190  C for 5 min to remove thermal history, and then cooled to 132  C at a cooling rate of 50  C/min, and isothermally recrystallized at 132  C. The crystallization exothermal curves were recorded for analysis. The isothermally recrystallized at 130, 128, 126, 124 and 122  C was also evaluated at the same procedure. In order to exploit the crystalline types of the samples, a wideangle X-ray diffraction (WAXD, Shimadzu XRD-7000) was

Table 1 The sample codes and compositions of the PLLA/TPU, PLLA/SCs, PLLA/TPU/PDLA and PLLA/TPU-g-PDLA samples. Samples

PLLA (wt%)

TPU (wt%)

PDLA (wt%)

SCs (wt%)

PLLA PLLA/TPU-5 PLLA/TPU-10 PLLA/TPU-15 PLLA/TPU-25 PLLA/SCs-0.3 PLLA/SCs-0.5 PLLA/SCs-1.0 PLLA/SCs-3.0 PLLA/SCs-5.0 PLLA/TPU/PDLA-5 PLLA/TPU/PDLA-10 PLLA/TPU/PDLA-15 PLLA/TPU/PDLA-25 PLLA/TPU-g-PDLA-5 PLLA/TPU-g-PDLA-10 PLLA/TPU-g-PDLA-15 PLLA/TPU-g-PDLA-25

100 95 90 85 75 99.7 99.5 99 97 95 95 90 85 75 95 90 85 75

0 5 10 15 25 0 0 0 0 0 4.5 9 13.5 22.5 4.5 9 13.5 22.5

0 0 0 0 0 0 0 0 0 0 0.5 1.0 1.5 2.5 0.5 1.0 1.5 2.5

0 0 0 0 0 0.3 0.5 1.0 3.0 5.0 0 0 0 0 0 0 0 0

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performed by using a Cu Ka radiation with a wavelength of 1.54 Å and an angle range 2q ¼ 5e40 at 40 kV and 40 mA. WAXD patterns of the samples were obtained. The crystalline morphologies of the sample were first observed by a polarized optical microscope (POM) (Olympus BX51) equipped with a Huozi RT600 (Shanghai, China) hot stage. Films with a thickness of 20 mm were first microtomed from the injection molded samples. Before the observation, the films were first melted at 190  C for 5 min to erase thermal history and then rapidly cooled down to 122  C for isothermal crystallization. The phase morphologies for the cryo-fractured surfaces of the samples and The impact-fractured surfaces of the samples were observed using field-emission scanning electron microscope (SEM, JSM-7800F; JEOL) with an accelerating voltage of 5 kV. The surfaces were coated with a layer of platinum in a vacuum chamber before observation. To clearly observe the location of TPU and crystallites, the cryo-fractured surface was also etched in a water-methanol (1/ 2 v/v) solution containing 0.025 mol/L sodium hydroxide to remove the amorphous regions of the PLLA matrix, and finally coated with a layer of platinum in a vacuum chamber before SEM observation. Tensile tests were performed on a Sansi CMT6503 Universal Testing Machine (Shenzhen, China) at a crosshead speed of 5 mm/ min at room temperature. At least five specimens were tested for each sample, and the average results were evaluated. The notched Izod impact strength was measured on a Sansi ZBC7000 (Shenzhen, China) impact tester at room temperature in general accordance with ASTM D256. Five measurements have been carried out for each sample and the averaged result was reported. The rheological behavior of the samples was measured on a rotational rheometer (TA AR200ex) with two parallel plates. The dynamic frequency sweep mode was used, with a strain of 1% from 0.01 to 100 Hz at 190  C. 3. Results and discussion 3.1. Effect of TPU and SCs on crystallization behavior of PLLA The crystallinity of PLLA with TPU and SCs was first evaluated by the nonisothermal crystallization. Fig. 1 shows the melting behavior of pure PLLA and the blends with different mixing ratio. Both pure PLLA and the PLLA/TPU samples exhibit multiple transitions upon heating: a glass transition temperature (Tg) of ~59  C, a cold crystallization peak (Pcc) of ~106  C and a melting peak (Pm) of ~170  C, as shown in Fig. 1a. Meanwhile, the crystallinity of pure PLLA and the PLLA/TPU samples is very low (<3.0%), suggesting that few PLLA chains were crystallized at the cooling rate of 10  C/min. The results also indicate that the only addition of TPU chains does not affect the melting and crystallization behavior of PLLA. However, the crystallinity of PLLA increases with the addition of SCs. For example, the crystallinity of PLLA can reach 17.97% by the incorporation of 5.0 wt% SCs, as shown in Fig. 1b. The intensity of Pcc is also reduced by SCs, and even disappears at loading of SCs above 3.0 wt%. The results indicate that the SCs can act as nucleating agents for PLLA crystallization, which is consistent with the reported work [37,38]. Furthermore, the melting peak of SCs at ~220  C was also found in the PLLA/SCs-3.0 and PLLA/SCs-5.0 samples. Interestingly, the crystallinity of PLLA can further increase by the incorporation of TPU and simultaneously constructing SCs, as shown in Fig. 1c. Supposing all the PDLA chains to form SCs, the percent of SCs in PLLA/TPU/PDLA-5, PLLA/TPU/PDLA-10, PLLA/TPU/ PDLA-15 and PLLA/TPU/PDLA-25 are approximate 1.0, 2.0, 3.0 and 5.0 with 0.5, 1.0, 1.5 and 2.5 wt% PDLA, respectively. The crystallinities of PLLA/TPU/PDLA-15 and PLLA/TPU/PDLA-25 are 23.63 and

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30.83%, which are 70 and 72% higher than that of PLLA/SCs-3.0 and PLLA/SCs-5.0, respectively. The results indicate that TPU and PDLA have synergetic effect on the crystallization of PLLA. More interestingly, this synergetic effect will become more obviously when PDLA and TPU are chemical connected. Fig. 1d shows the melting curves of the PLLA/TPU-g-PDLA samples. The PLLA/TPU-g-PDLA samples also exhibit multiple transitions upon heating. However, the Pcc disappears at high content of TPU-g-PDLA. The crystallinity of PLLA/TPU-g-PDLA samples is higher than that of the PLLA/TPU/ PDLA samples at the same content. For example, the crystallinity of PLLA/TPU-g-PDLA-15 is 52% higher than that of PLLA/TPU/PDLA-15. The results indicate that TPU-g-PDLA is more efficient for improving PLLA crystallization than that of directly adding TPU and PDLA. In order to get direct evidence for the formation of SC crystallites, XRD patterns of the above samples were recorded as show in Fig. 2. The pure PLLA, PLLA/TPU-15 and PLLA/TPU-25 samples show amorphous feature without characteristic sharp peak, while PLLA/ TPU/PDLA and PLLA/TPU-g-PDLA blends show three distinct characteristic peaks at 2q values of 11.9 , 21.4 and 23.7, corresponding to the (110), (300)/(030), and (220) planes of SCs, respectively [34]. The results indicate that SCs are successfully formed during melt processing. Surprisingly, it can be found that the crystallinity values from WAXD are lower than the values obtained by DSC for the samples with the same components. The reason for this inconsistency was that the samples were suffered the different cooling rate. For the WAXD testing, the samples were cooled from 200 to 50 С for 5 min. The cooling rate is ca. 30  C/min. However, the DSC crystallinity was obtained at the cooling rate of 10  C/min. The PLLA based samples with low cooling rate give the higher crystallinity than the samples suffering the high cooling rate [40]. The synergetic effect of TPU and PDLA on the crystallization of PLLA was also demonstrated by the isothermal crystallization. Fig. 3 shows the relative crystallinity (Xt) versus crystallization time at various crystallization temperatures for pure PLLA, PLLA/TPU-15, PLLA/TPU/PDLA-15 and PLLA/TPU-g-PDLA-15. All the curves exhibit a similar sigmoid shape, and the crystallization time is extended by increasing in crystallization temperature for all samples because of the increased nucleation barrier. The crystallization of PLLA finishes in shorter time for PLLA/TPU-15 (Fig. 3b) as compared to pure PLLA (Fig. 3a) at the same crystallization temperature. It costs ~135.5 min for pure PLLA to finish crystallization at 132  C, while the time obviously reduced to ~60.5 min for PLLA/ TPU-15. The TPU chains probably act as chain mobility promoters for PLLA crystallization, like poly(ethylene glycol) [46e49]. The crystallization of PLLA can be further accelerated by simultaneously adding TPU and PDLA. For example, the crystallization finishes ~13.5 min for PLLA/TPU/PDLA-15 at 132  C (Fig. 3c), which reduces 78% crystallization time in comparison with PLLA/ TPU-15 Furthermore, the crystallization finishes ~8.0 min at 132  C in PLLA/TPU-g-PDLA-15 (Fig. 3d), indicating the highest crystallization rate is achieved by the chemical connection between TPU and PDLA. Normally, the crystallization rate can be described by the reciprocal of the crystallization half-life time (t1/2), when the time required to achieve 50% of the final crystallinity [50]. Fig. 4 shows the 1/t1/2 of pure PLLA and its blends at different crystallization temperature. It can be seen that the PLLA/TPU-15 samples have slightly higher than that of pure PLLA at the same crystallization temperature, while the PLLA/SCs-3.0 samples show the obvious enhancement on the crystallization rate. For instance, the 1/t1/2 values of the PLLA/TPU-15 samples and the PLLA/SCs-3.0 samples at 124  C are 0.07 and 0.32 min1, which are about 2.3- and 14.2-fold higher than that of pure PLLA at 124  C (0.021 min1), respectively. The large enhancement in the PLLA/SCs-3.0 samples are ascribed to

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Fig. 1. DSC melting curves of the PLLA/TPU samples (a), the PLLA/SCs samples (b), the PLLA/TPU/PDLA samples (c) and the PLLA/TPU-g-PDLA samples (d). The values of PLLA matrix crystallinity (Xc) are given in the profiles.

Fig. 2. WAXD profiles of the samples after injection molding with cooling rate of ca. 30  C/min.

the nucleating effect of SCs on PLLA crystallization. Interestingly, the simultaneous addition of TPU and PDLA show only a little improvement on the crystallization rate, while the chemical bonded TPU-g-PDLA exhibits the obviously improvement in comparing with the PLLA/SCs-3.0 samples. The PLLA/TPU-g-PDLA15 samples show the highest crystallization rate at the same crystallization temperature. The results indicate that the TPU and PDLA chains, especially the chemical bonded TPU-g-PDLA chains have synergetic effect on the crystallization of PLLA. There are two possible reasons for the synergetic effect of TPU and PDLA on the PLLA crystallization, as illustrated in Fig. 5. First, the PDLA can construct SCs with PLLA during the melt-mixing, and the SCs can act as nucleating agents of PLLA. Second, the TPU chains probably can weaken the intermolecular interaction of PLLA chains, and promote the mobility of PLLA chains. It is very interesting that the mobility of PLLA chains seems to be more easily enhanced by the TPU-g-PDLA chains. We believe that the PDLA ends in TPU-gPDLA chains can easily absorb PLLA chains which make the TPU ends in TPU-g-PDLA chains easily destroy the intermolecular interaction of PLLA chains, as illustrated in Fig. 5a. The activated PLLA chains can form more homocrystallites from SCs. Thus, PLLA/

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Fig. 3. Plots of relative crystallinity (Xt) versus crystallization time at various crystallization temperatures for pure PLLA (a), PLLA/TPU-15 (b), PLLA/TPU/PDLA-15 (c) and PLLA/TPU-gPDLA-15 (d).

Fig. 5. Schematic draw of the accelerated crystallization of PLLA with TPU chains and SCs in PLLA/TPU-g-PDLA (left) and PLLA/TPU/PDLA (right). The TPU chains destroy the intermolecular interaction of PLLA chains, while SCs act as nucleating agents for PLLA. Fig. 4. 1/t1/2 of neat PLLA and its blends at various crystallization temperatures.

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TPU-g-PDLA has the higher crystallinity and crystallization rate than that of PLLA/TPU/PDLA at the same blending ratio.

3.2. Effect of TPU and SCs on morphology of PLLA Fig. 6 shows phase morphologies of the melt-blended samples. The spherical TPU particles are dispersed in continuous PLLA matrix for all samples, which is usually defined as “sea-island” morphology. Normally, a small size of “islands” indicates a high interfacial adhesion between two polymers. The PLLA/TPU-g-PDLA samples have the smallest elastomer domains, while the PLLA/TPU samples have the largest elastomer domains at the same blending ratio. The size of elastomer domains in PLLA/TPU/PDLA samples is between the above two samples. For example, the average sizes of elastomer domains are 1.7, 1.2 and 0.8 mm for PLLA/TPU-15, PLLA/ TPU/PDLA-15 and PLLA/TPU-g-PDLA-15, respectively. The results indicate that the PLLA/TPU-g-PDLA samples have the best interfacial adhesion. Interestingly, the larger SCs domains (indicated by red arrow) are found in the PLLA/TPU/PDLA samples (Fig. 6c and d) in comparison with the PLLA/TPU-g-PDLA samples (Fig. 6e and f), suggesting TPU-g-PDLA can prevent PDLA chains separating from TPU domains to aggregate. The crystalline morphology has been found to be important for improving mechanical properties of the samples [50e52]. To evaluate the TPU and SCs effect on crystallization morphology of PLLA, the crystalline morphology of pure PLLA and the corresponding blends after isothermally crystallization were first observed by POM. Pure PLLA displays well-developed spherulites with large size and clear boundaries as shown in Fig. 7a and b. However, the size of spherulites for PLLA/SCs-3.0 is reduced so extensively that integrated spherulites are not observed and the boundaries are not discriminated (Fig. 7c and d), indicating the well nucleating effect of SCs. The PLLA/TPU samples also exhibit some reduction in the size of spherulites, but still have clear boundaries in comparison with pure PLLA (Fig. 7e and f), indicating the little nucleating effect on the crystallization of PLLA. However, many TPU particles are embedded in PLLA spherulites, suggesting the phase separation happens during the crystallization. Both the obvious size reduction of spherulites and the embedding TPU particles are found in the PLLA/ TPU/PDLA (Fig. 7g and h) and PLLA/TPU-g-PDLA (Fig. 7i and j)

samples because of the nucleating effect of SCs and the phase separation of TPU chains. In order to clearly exploit the crystalline morphology of PLLA with TPU and SCs, the cryo-fractured surfaces of the samples were first etched to remove the amorphous phase of PLLA and then evaluated by SEM. Fig. 8 shows SEM images of pure PLLA and the blends. Pure PLLA shows a spherulite with ~20 mm in diameter and a lot of amorphous domains among lamellae. For the PLLA/TPU samples, TPU particles are found in the amorphous domains among lamellae, indicating TPU and PLLA chains probably have high intermolecular interaction in melting state but are phase separation during PLLA crystallization. However, the TPU particles are found to be located between PLLA spherulites in the PLLA/TPU/ PDLA and PLLA/TPU-g-PDLA samples. The result is ascribed to the nucleating effect of SCs which lead to high crystallization rate and the small size of PLLA spherulites. The high crystallization rate results in quick phase separation for TPU chains, and the small spherulites cannot afford enough space for TPU particles. Fig. 9 gives schematic draw of the different crystalline morphologies for neat PLLA and the blends. The large spherulites, the large spherulites embedding with TPU particles, the TPU particles among small spherulites and the TPU particles among small spherulites with chemical bonding are found in neat PLLA, PLLA/TPU, PLLA/TPU/ PDLA and PLLA/TPU-g-PDLA samples, respectively.

3.3. Effect of TPU and SCs on rheological behavior of PLLA It is well-known that the interfacial interaction between polymers can be evaluated by the variation of rheological parameters, such as complex viscosity, storage modulus, loss modulus, and loss tan d [53]. Here, the frequency sweeps were carried from 0.0628 to 628 rad/s at 190  C to investigate the effect of TPU and SCs on the rheological behaviors of PLLA. Fig. 10 shows the dependence of storage modulus (G0 ), loss modulus (G00 ), complex viscosity (jh*j) and loss tangent (tan d) on angular frequency (u). The values of G0 , G00 and jh*j for pure TPU are higher than that of pure PLLA, suggesting the entanglement density of pure TPU is higher than pure PLLA. For the blends, the values of G0 , G00 and jh*j are obvious inequality, although they have the same PLLA content (85 wt%). The PLLA/TPU-g-PDLA-15 samples have higher G0 , G00 and jh*j than that

Fig. 6. SEM images for the cryo-fractured surfaces of PLLA/TPU-15 (a), PLLA/TPU-25 (b), PLLA/TPU/PDLA-15 (c), PLLA/TPU/PDLA-25 (d), PLLA/TPU-g-PDLA-15 (e) and PLLA/TPU-gPDLA-25 (f).

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Fig. 7. Spherulitic morphologies formed by isothermal crystallization at 122  C for 60 min of pure PLLA (a, b), PLLA/SCs-3.0 (c, d), PLLA/TPU-15 (e, f), PLLA/TPU/PDLA-15 (g, h), and PLLA/TPU-g-PDLA-15 (i, j).

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Fig. 8. SEM images showing the crystalline morphologies of neat PLLA (a), PLLA/TPU-15 (b), PLLA/TPU-25 (c), PLLA/TPU/PDLA-15 (d), PLLA/TPU/PDLA-25 (e) and PLLA/TPU-g-PDLA15 (f). All the samples isothermally crystallized at 122  C for 60 min and were then surface etched in a sodium hydroxide water-methanol solution.

Fig. 9. Schematic draw of the different crystalline morphologies in neat PLLA (a), PLLA/TPU blends (b), PLLA/TPU/PDLA blends (c), and PLLA/TPU-g-PDLA blends (e).

of the PLLA/TPU/PDLA-15 samples at the whole testing frequency. The PLLA/TPU-15 samples have the lowest values of G0 , G00 and jh*j. The results indicate that the PLLA/TPU-g-PDLA-15 samples have the highest interfacial interaction because of the chemical bonded TPUg-PDLA. Tan d is usually performed to describe the damping

characteristics of the samples. It is obvious that the PLLA/TPU-gPDLA-15 samples exhibit the lowest value of tan d among three blends, indicating that the PLLA/TPU-g-PDLA-15 samples show higher interfacial interaction which will retard the interfacial energy dissipation.

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Fig. 10. Storage modulus (a), loss modulus (b), complex viscosity (c) and loss tan d (d) of the samples as a function of angular frequency at 190  C.

3.4. Effect of TPU and SCs on PLLA toughness The effect of TPU and SCs on mechanical properties of PLLA was evaluated by the notched Izod impact strength and tensile test. A comparative study was carried among the PLLA/TPU, PLLA/TPU/ PDLA and PLLA/TPU-g-PDLA samples. As expected, the PLLA/TPU-gPDLA samples have the highest notched Izod impact strength, elongation at break, tensile strength and Young's modulus at the same content of PLLA, as shown in Fig. 11. For example, the notched Izod impact strength of PLLA/TPU-g-PDLA-15 is 84, 46 and 400% higher than that of PLLA/TPU-15, PLLA/TPU/PDLA-15 and pure PLLA, respectively. The elongation at break, tensile strength and Young's modulus of PLLA/TPU-g-PDLA-15 are 19, 13 and 45% higher than that of PLLA/TPU-15. However, the PLLA/TPU/PDLA-15 samples show only 4 and 34% enhancement tensile strength and Young's modulus of PLLA/TPU-15, respectively. Furthermore, the elongation at break of PLLA/TPU/PDLA-15 is 21% lower than that of PLLA/TPU15. These results indicated that TPU can improve toughness of PLLA and PDLA can improve strength of PLLA/TPU. The chemical bonded TPU-g-PDLA can synergistically improve the toughness of PLLA because of the high crystallinity, special morphology and high interfacial interaction between PLLA and TPU-g-PDLA. However, the improvement on the elongation at break is not very impressing.

Although the TPU-g-PDLA can improve the interfacial interaction and show the best elongation at break, the morphology of the PLLA/ TPU-g-PDLA samples is still sea-island morphology with large TPUg-PDLA island (Fig. 6). The results indicate that the compatibility of PLA and TPU-g-PDLA is still not well enough to enhance the interfacial interaction between two polymers, which the further work needs to overcome it. The impact-fractured surface of the samples was observed by SEM to show some insights into the toughness behavior. The PLLA/ TPU-15 and PLLA/TPU/PDLA-15 samples show typical brittle fracture behavior with a smooth surface (Fig. 12a and b), while the PLLA/TPU-g-PDLA-15 samples exhibit rough fractured surfaces, where massive matrix plastic deformation (Fig. 12c). The results indicating that PLLA/TPU-g-PDLA-15 has the highest interfacial interaction between PLLA and elastomer particles.

4. Conclusions In this work, a comparative study was carried in PLLA/TPU, PLLA/ TPU-g-PDLA and PLLA/TPU/PDLA samples to investigate the effect of TPU and SCs on the morphology, crystallization, rheological and mechanical properties of TPU toughed PLLA. It was found that TPU and PDLA had synergetic effect on PLLA crystallization. This

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Fig. 11. The effect of elastomer content on the notched Izod impact strength (a), Young's modulus (b), tensile strength (c) and elongation at break (d) of the blends.

Fig. 12. SEM images for the impact-fractured surfaces of the samples: PLLA/TPU-15 (a), PLLA/TPU/PDLA-15 (b) and PLLA/TPU-g-PDLA-15 (c).

synergetic effect became more obviously when PDLA and TPU were chemical connected (PLLA/TPU-g-PDLA). For example, the crystallinity of the PLLA/TPU-g-PDLA-15 samples was 52% higher than that of the PLLA/TPU/PDLA-15 samples. The PLLA/TPU-g-PDLA samples also showed the highest crystallization rate at the same crystallization temperature. In addition, the morphologies of large spherulites embedding with TPU particles, TPU particles among small spherulites and TPU particles among small spherulites with chemical bonding were found in the PLLA/TPU, PLLA/TPU/PDLA and PLLA/TPU-g-PDLA samples, respectively. Furthermore, the PLLA/ TPU-g-PDLA samples exhibited the higher G0 , G00 and jh*j than

that of the PLLA/TPU and PLLA/TPU/PDLA samples, suggesting the higher interfacial interaction. Because of high crystallinity, special morphology and high interfacial interaction between PLLA and TPU-g-PDLA, the PLLA/TPU-g-PDLA samples had the highest notched Izod impact strength, elongation at break, tensile strength and Young's modulus. Acknowledgments The authors are grateful to the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University)

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