Remarkably Enhanced Impact Toughness and Heat Resistance of

Research Article pubs.acs.org/journal/ascecg

Remarkably Enhanced Impact Toughness and Heat Resistance of poly(L‑Lactide)/Thermoplastic Polyurethane Blends by Constructing Stereocomplex Crystallites in the Matrix Zhenwei Liu, Yuanlin Luo, Hongwei Bai,* Qin Zhang, and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China S Supporting Information *

ABSTRACT: As an eco-friendly polymer with tremendous potential to replace traditional petroleum-based and nonbiodegradable polymers, the current use of poly(L-lactide) (PLLA) in large-scale commercial applications still faces some barriers mostly associated with its inherent brittleness and poor heat resistance. In this work, we propose a novel and facile strategy to simultaneously address these obstacles by introducing small amounts of poly(D-lactide) (PDLA) into thermoplastic polyurethane (TPU) toughened PLLA blends through melt-blending. The results manifest that the introduced PDLA chains can readily interact with PLLA matrix chains and rapidly cocrystallize into stereocomplex (sc) crystallites capable of acting as an efficient rheology modifier to dramatically improve melt viscoelasticity of the PLLA matrix and subsequently induce the morphological change of the dispersed TPU phase from a typical sea−island structure to a unique networklike structure, thus endowing PLLA/TPU/PDLA blends with remarkably improved impact toughness as compared to its PLLA/TPU counterparts. Moreover, the formed sc crystallites can also serve as a highly efficient nucleating agents to substantially accelerate matrix crystallization, which makes it possible to prepare PLLA/TPU blends with a highly crystalline matrix using conventional injection molding technology. More interestingly, the improvement in the matrix crystallization can significantly enhance the heat resistance of the blends without evidently weakening the contribution of the tailored phase morphology to the toughness improvement. These inspiring findings suggest that the construction of sc crystallites in the matrix could be a promising avenue toward fabricating high-performance PLLA/elastomer blends via simultaneously tuning phase morphology and matrix crystallization. KEYWORDS: poly(L-Lactide), Polyurethane, Stereocomplex, Toughness, Heat resistance

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INTRODUCTION With increasing awareness on sustainability and environmental issues related to conventional petroleum-derived and nonbiodegradable polymers, bioderived and biodegradable polymers have attracted growing attention in recent years.1,2 One such attractive environment-friendly polymer is poly(L-lactide) (PLLA), which exhibits tremendous application value and market potential as a sustainable alternative to petroleumderived polymers due to its complete renewability in nature and biocompatibility in soil, favorable biocompatibility, extraordinary transparency, impressive mechanical strength and elastic modulus, easy processability, and competitive price.3−5 Nevertheless, the inherent brittleness of PLLA makes it very sensitive to impact loading and even environment stress, as evidenced by the poor impact resistance and tensile toughness.6,7 Moreover, even though PLLA is a typical crystallizable polymer, only amorphous articles can usually be obtained using commonly used melt-processing technologies such as injection molding because the low crystallization rate makes it hard to crystallize © XXXX American Chemical Society

during melt processing, thus inevitably leading to a poor heatresistance.8−13 The highest heat distortion temperature (HDT) for amorphous PLLA is as low as its relatively low glass transition temperature of ca. 50−65 °C, while HDT for highly crystalline PLLA is over 100−120 °C.9,10,12 Nowadays, the practical incorporation of PLLA into large-scale commercial applications has been greatly restricted by these obstacles. Therefore, a great deal of effort is urgently needed to simultaneously improve fracture toughness and heat resistance of PLLA so as to fully realize its application potential in various fields where both excellent toughness and high heat resistance are required. Up to now, a variety of strategies have been adopted to improve fracture toughness of PLLA.7,14−26 In particular, meltblending with various elastomers including thermoplastic Received: August 5, 2015 Revised: November 14, 2015

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DOI: 10.1021/acssuschemeng.5b00816 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

effective toughening of elastomer on highly crystalline PLLA matrix as that on the amorphous one. If such was the case, it should then be possible to obtain elastomer-toughened PLLA blends exhibiting both excellent impact strength and high heat resistance by introducing relatively small amounts of elastomers. Unfortunately, to the best of our knowledge, little attention has been paid to this issue until now probably because it is still a challenge to simultaneously tailor phase structure and matrix crystallization of such blends. In this work, taking PLLA/TPU blends with a typical sea− island morphology as an example, we aimed at developing a facile and promising strategy to address this challenge and then to investigate the combined role of phase structure and matrix crystallization in the toughening of immiscible PLLA/elastomer blends. Inspired by the results that stereocomplex (sc) crystallites formed between enantiomeric PLLA and poly(Dlactide) (PDLA) can act as both efficient rheological modifier and nucleating agent for PLLA because the much higher (ca. 50 °C) melting temperature relative to homocrystallites makes them able to reserve in the melts of asymmetric PLLA/PDLA blends and form network structure,44−50 small amounts of PDLA was introduced into the PLLA/TPU blends through directly melt-blending with PLLA and TPU at 190 °C, which is a favorable temperature for facilitating the cocrystallization between the introduced PDLA chains and their surrounding PLLA matrix chains.51,52 It was expected that the introduced PDLA chains could readily collaborate with PLLA matrix chains in a side-by-side manner in the early stage of the melt-blending process and rapidly cocrystallize into sc crystallites capable of serving as a rheological modifier to effectively tailor meltviscoelasticity of PLLA matrix and then induce the morphological change of dispersed TPU phase from a typical sea− island structure to a unique network-like structure in the blends, thus causing a remarkably enhanced impact toughness. On the other hand, the formed sc crystallites were also expected to act as a highly efficient nucleating agent to substantially accelerate matrix crystallization during subsequent injection molding. In addition, the sc crystallite network formed in the amorphous matrix could impart the blends with a greatly enhanced heat resistance over the glass transition temperature. The combined role of the sc crystallites tailored phase morphology and matrix crystallization in the toughening is also highlighted and the applicability of such a strategy to design supertough and heat-resistant PLLA materials via the construction of sc crystallites in the matrix has been demonstrated for the first time.

polyurethane (TPU) has been widely used as the most versatile and economic route to produce supertough PLLA materials.19−24,27 Unfortunately, a pretty large amount (15−25 wt %) of elastomers is required to achieve a remarkably improved impact toughness of PLLA, which inevitably gives rise to a substantial deterioration in strength, modulus, and even heat resistance.7,15,16,20−23 In order to minimize such deficiencies, optimizing toughening efficiency (i.e., obtaining sufficient toughening effect with less elastomers) becomes a matter of great concern.7,15,16,20,21,24,27 According to classical toughening theories,28,29 toughening is efficient only if elastomer particles can effectively initiate plastic deformation of their adjacent matrix and bring average interparticle distance (T) below the critical value (Tc), when the stress fields around the elastomer particles overlap and the plastic deformation percolates through the whole matrix, finally giving rise to an effective energy dissipation. Tailoring of phase morphology such as shape and spatial distribution of the dispersed elastomer droplets within a polymer matrix provides an interesting way to substantially reduce the critical elastomer content that is necessary for achieving a desired toughening effect.15,20,21,30−34 Compared with the most common sea−island morphology with spherical elastomer particles well-dispersed in a polymer matrix, a unique pseudonetwork or networklike morphology composed of discrete particles unevenly distributed in the matrix has been demonstrated to exhibit a much higher toughening efficiency because the networklike distribution of the elastomer particles can facilitate the percolation of stress filed as well as resulting matrix plastic deformation around them at lower volume fractions.20,21,30−34 The morphological transition of immiscible polymer blends from the sea−island to the networklike or even cocontinuous structure can be readily realized by increasing melt-viscosity ratio between the matrix and dispersed phases33−37 or adding some inorganic nanoparticles (e.g., SiO2) with strong self-networking capability in polymer melts.21,38−40 Undoubtedly, increasing toughening efficiency via tailoring phase morphology can endow the toughened PLLA blends with a good stiffness−toughness balance but still at the cost of the deterioration in originally poor heat resistance of PLLA because all the blending cannot effectively improve matrix crystallization kinetics. This is undesirable for the fabrication of high-performance PLLA materials with both super toughness and high heat resistance. Although extensive efforts have been devoted to improving fracture toughness of PLLA by adding various elastomers, much less attention has been paid to simultaneously enhance the toughness and heat resistance until now. Interestingly, enhancing matrix crystallization has been reported to be an effective strategy toward supertoughed and heat-resistant PLLA/elastomer blends recently.23,41,42 Both thermal annealing and nucleating agent induced matrix crystallization could significantly enhance heat resistance of the blends while maintaining or further increasing toughening efficiency.41,42 However, increasing matrix crystallinity alone cannot guarantee the toughness improvement in most cases because suitable morphological parameters (e.g., elastomer particle size) must be satisfied for PLLA matrix crystallization to work effectively in the toughening.23,43 Specially, a substantial decrease in the optimum elastomer particle size for toughening PLLA has been identified with the change of matrix crystalline state from amorphous to high crystalline.43 In this context, it is natural to think whether the morphological transition from the sea−island structure to the network-like structure is beneficial for realizing

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EXPERIMENTAL SECTION

Materials. Commercially available PLLA (trade name 4032D) was purchased from NatureWorks LLC, USA. It has an optical purity of 98.6%, a weight-averaged molecular weight (Mw) of 1.7 × 105 g/mol and a polydispersity index of 1.74. PDLA with an optical purity of 99.5% and an Mw of 1.2 × 105 g/mol was kindly supplied by Zhenjiang Hisun Biomaterial Co. Ltd., China. Both PLLA and PDLA share the same density of 1.25 g/cm3. Polyester-based TPU (trade name WHT1570) with a poly(1,4-butylene adipate) block (69 wt %) as the soft segment and a 1,4-butylene glycol extended 4,4′-diphenyl-methanediisocyanate (31 wt %) block as the hard segment was obtained from Yantai Wanhua Polyurethanes Co. Ltd., China. It has a density of 1.19 g/cm3, a glass transition temperature of about −40 °C, an Mw of 1.3 × 105 g/mol, and a polydispersity index of 1.70. The chemical structure of the TPU is given in Figure S1. Sample Preparation. PLLA/TPU blends without and with various amounts of PDLA (0−25 wt % on the basis of actual weight B


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specimen temperature with an accuracy of 0.1 °C and the POM micrographs were recorded automatically with the camera during isothermal crystallization at 135 °C after melting and erasing thermal history at 200 °C. Mechanical Testing. Notched Izod impact testing was conducted on a VJ-40 impact tester (China) according to ISO180/179 standard and tensile properties were measured using a SANS tensile testing machine (China) at a crosshead speed of 5.0 mm/min in accordance with ISO 527-3 standard. The measurements were carried out at room temperature (23 °C), and the reported value was obtained from at least five independent specimens for each sample. Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties were measured using a TA Q800 instrument (USA) in the single-cantilever mod from 0 to 150 °C at a heating rate of 3 °C/min. The sinusoidal oscillating strain and the frequency were set as 10 μm and 1 Hz, respectively.

of both PLLA and PDLA in the blends) were prepared by melt blending using a Haake Rheomix 600 internal mixer (Germany) at a temperature of 190 °C and a rotor speed of 60 rpm for 5 min. For convenience, the obtained PLLA/TPU/PDLA blends were abbreviated as PLLA/xTPU/yPDLA, where x and y represent the contents of TPU and PDLA, respectively. Considering that, despite small amounts of sc crystallites (e.g., 0.75 wt %) can greatly enhance crystallization rate of PLLA as efficient nucleating agent,45,46,48 at least 2.6−5.0 wt % sc crystallites are necessary to significantly improve viscoelasticity of PLLA melt due to the existence of a critical concentration required for the formation of sc crystallite network structure in the melt,48,50,53 5−25 wt % PDLA were incorporated into the PLLA/TPU blends. After melt-blending, standard specimens for mechanical testing were injection molded using a HAAKE MiniJet (Germany) at a barrel temperature of 200 °C and a mold temperature of 130 °C. Specially, to clarify the influence of PLLA matrix crystallization on the impact toughness of the blends, two isothermal annealing time of the blend melts (i.e., 0.1 and 3 min) in the preheated mold were applied to control the crystalline state of PLLA matrix (i.e., amorphous and highly crystalline). All materials were dried overnight in a vacuum oven at 60 °C before melt-blending and injectionmolding. Wide-Angle X-ray Diffraction (WAXD). WAXD patterns of injection molded blends were recorded on a PANalytical X’Pert pro MPD X-ray diffractometer (Holland) equipped with Cu Kα radiation (λ = 0.154 nm) in the diffraction angle range of 5°−40° at 40 kV and 40 mA. To demonstrate the formation of stereocomplex crystallites in PLLA matrix, the disk-shaped specimens with a thickness of 1 mm were prepared by compression molding on a KT-0701 hot press (China) at 200 °C, followed by quenching to room temperature. Dynamic Rheological Analysis. Melt rheological properties were analyzed using a Bohlin Gemini 2000 rotational rheometer (Malvern Instruments Limited, UK) with two parallel plates (25 mm in diameter and 1 mm in gap) under a nitrogen atmosphere. The dynamic frequency sweep was performed at 190 °C in the range of 0.01−100 rad/s, and the strain was fixed at 1%. The disk-shaped specimens with a diameter of 25 mm and a thickness of 1.5 mm were prepared by compression molding at 190 °C. Scanning Electron Microscope (SEM). Phase morphology was investigated with a FEI Inspect F field emission scanning electron microscope (FE-SEM, USA) operating at an accelerating voltage of 5 kV. Specimens used for the morphology observation were prepared by cryo-fracturing the injection molded bars in liquid nitrogen. Before SEM imaging, all the fractured surfaces were sputter-coated with a thin layer of gold. Differential Scanning Calorimetry (DSC). Thermal analysis was performed using a PerkinElmer pyris-1 DSC (USA) under a dry nitrogen atmosphere. For the analysis of isothermal crystallization kinetics, about 5 mg of specimen was rapidly cooled down (100 °C/ min) to a designated crystallization temperature (130, 135, and 140 °C) after being melted at 200 °C for 3 min to erase any thermal history and then maintained at this temperature until the crystallization is completed. The crystallization kinetics was evaluated by the parameter of half-crystallization time (t1/2) obtained from the curves of relative crystallinity vs crystallization time. The crystallinity of PLLA matrix (Xc) was determined by first DSC heating runs at a rate of 10 °C/min from 50 to 200 °C using the following equation:

Xc =

ΔHm − ΔHcc wf ΔHmo

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RESULTS AND DISCUSSION Formation of Stereocomplex Crystallites in PLLA Matrix. PLLA/TPU/PDLA blends with various amounts of PDLA were prepared by melt-blending at 190 °C, which is a favorable melt-processing temperature for facilitating the collaboration of the introduced PDLA chains with their surrounding PLLA matrix chains and subsequent formation of sc crystallites through cocrystallization.51,52 WAXD analysis provides a clear-cut evidence for the in situ formation of sc crystallites in the blend melts during the melt-blending process. As shown in Figure 1, although PLLA matrix cannot crystallize

Figure 1. WAXD profiles of melt-quenched PLLA/TPU(85/15) blends with various amounts of PDLA after being melted at 200 °C.

upon quenching from melt (evidenced by the absence of characteristic diffractions from usually observed α-form PLLA homocrystallites in all WAXD patterns at 2θ values of around 14.8°, 16.9°, 19.0°, and 22.5°47,55), three visible characteristic peaks are noticed in the patterns of the melt-quenched PLLA/ TPU/PDLA blends at about 12.0°, 20.9°, and 24.0°, assigned to the (110), (300)/(030), and (220) planes of sc crystallites.47,55 Moreover, these peaks become stronger with increasing PDLA concentration from 5 wt % to 25 wt %, indicating greatly increased contents of sc crystallites as the same to that observed in the asymmetric PLLA/PDLA blends.48,53 Stereocomplex Crystallites Tailored Phase Morphology As Rheological Modifier. It has been confirmed that sc crystallites embedded in the melt of PLLA can serve as an efficient rheological modifier to sharply enhance viscosity and elastic response of PLLA melt due to the filler effect and crosslinking effect of the sc crystallite network.48,50,53 In this work, such sc crystallites are selectively formed in the melt of PLLA

(1)

where ΔHm and ΔHcc are the melting enthalpy and cold crystallization enthalpy, respectively; wf is the weight fraction of PLLA matrix in the sample, and ΔHom is the melting enthalpy of fully crystalline PLLA (selected as 93.6 J/mg54). Polarized Optical Microscopy (POM). Crystal morphology of PLLA matrix was observed using a Leica DMLP polarized optical microscopy equipped with a Linkam THMS-600 hot stage and a Cannon digital camera. The hot stage was used to control the C


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Figure 2. Variation of complex viscosity (η*) and storage modulus (G′) as a function of frequency for TPU and PLLA/PDLA blends with various amounts of PDLA. The measurements were carried out with a strain of 0.1% at 190 °C.

the sc crystallites induced enhancement in the viscosity of PLLA matrix could promote the continuity of TPU phase at much lower critical TPU contents. Specially, the calculated critical weight fraction of the TPU phase for PLLA/TPU binary blends is 65 wt %, whereas that for the blends of PLLA/TPU with 15 wt % PDLA is only 11 wt %. To demonstrate the effectiveness of the sc crystallites enhanced melt-viscosity ratio for tailoring phase morphology of the PLLA/TPU blends, the microstructures of the blends with various PDLA concentrations were examined using SEM and some representative micrographs are presented in Figures 3 and 4. As expected, the incorporation of 15 wt % PDLA has no apparent influence on the phase morphology of PLLA/5TPU blend (Figure 3a and a′) because the content of the incorporated TPU is much lower than the critical content (11 wt %, Table 1) that is necessary for achieving phase inversion in the PLLA/TPU/15PDLA blends. Both the blends share the same sea−island structure, where spherical TPU particles are well dispersed in the PLLA matrix. However, it is surprising that no trace of the perfect cocontinuous structure, where TPU phase is completely continuous, can be observed in the blends when the TPU content exceeds the calculated critical content (Figure 3b′ and c′). To further verify this result, the PLLA matrix was first etched away from the cryo-fractured surface of the blend with a methanol−water solution containing 0.025 mol/L of sodium hydroxide before the SEM observation. From Figure S2, one can only see the typical networklike structure composed of discrete TPU particles with irregular shapes. Even for PLLA/15TPU/20PDLA blend with an extremely low phase inversion composition of 97/3, it still fails to exhibit a cocontinuous structure (Figure 4). This disagreement suggests that, besides the melt-viscosity ratio, other factors (e.g., interfacial tension59 and melt elasticity21,60) which are not taken into account in the Paul−Barlow theory might also play an important role in the formation of networklike TPU structure. Herein, one such key factor could be the change of melt elasticity because the sc crystallites selectively embedded in the PLLA matrix is expected to have no apparent influence on the interface between PLLA matrix and dispersed TPU phases. Unlike the melt viscosity, the higher the melt elasticity of one component, the stronger the tendency of forming a continuous phase.60 For PLLA/TPU/PDLA blends, the sc crystallites embedded in the melts can not only significantly enhance the melt viscosity of PLLA matrix but also remarkably increase its melt elasticity, as evidenced by the much higher storage modulus (G′) of PLLA/PDLA blends

matrix during melt-blending of PLLA/TPU with PDLA. In this condition, the formation of sc crystallites could remarkably change the viscoelasticity of PLLA matrix without apparently affecting the intrinsic nature of TPU phase dispersed within the matrix. Thus, the influence of the formed sc crystallites on melt rheological behaviors of PLLA/TPU/PDLA blends was investigated with special attention on the PLLA matrix, and only the dynamic viscoelasticity of TPU and PLLA/PDLA blends with various PDLA concentrations was analyzed at 190 °C. Figure 2a illustrates the variation of complex viscosity (η*) with frequency for these samples. Expectedly, the addition of 5 wt % PDLA induces an evident enhancement in η* of PLLA. With further increasing PDLA concentration up to 20 wt %, this enhancement becomes much pronounced especially at low frequencies, indicating that the sc crystallite can significantly reinforces the melt of the PLLA matrix as highly efficient rheological modifier. According to the classical Paul−Barlow theory,56 the phase morphology of a dual-phase polymer blend is not only dependent on the composition but also strongly dependent on the melt-viscosity ratio between the two components. The composition at phase inversion point can be predicted using the following equation:57 ϕA ϕB

=

ηA ηB

(2)

where ϕi and ηi represent the volume fraction and melt viscosity of component i, respectively. In general, decreasing melt viscosity would enhance the continuity of the minor phase at low volume fraction, thus inducing the morphological transition from a sea−island structure to a cocontinuous one. Based on the melt viscosity at a shear rate of 49.1 Hz, which is the average shear rate estimated for our melt-processing in the internal mixer using the model proposed by Bousmina et al.,58 the phase inversion compositions of the PLLA/TPU blends with various PDLA concentrations were calculated from eq 2, and the results are listed in Table 1. It is interesting to find that Table 1. List of the Calculated Phase Inversion Compositions for PLLA/TPU Blends without and with Various Amounts of PDLA PDLA concentration (wt %) phase inversion composition (w/w)

0

5

10

15

20

35/65

46/54

73/27

89/11

97/3

D


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viscosity can promote the continuity of the dispersed TPU particles on one hand, but the increased matrix elasticity could prevent the colliding TPU particles from aggregating and coalescing during the melt-blending process. Consequently, the networklike structure rather than the perfect cocontinuous one is exclusively formed in such blends. Stereocomplex Crystallites Tailored Matrix Crystallization As Nucleating Agent. As is known, sc crystallites can be used as efficient nucleating agent to greatly accelerate crystallization of PLLA by depressing nucleation barrier and increasing nucleation density. To check the effectiveness of the formed sc crystallites for nucleating PLLA matrix crystallization, isothermal crystallization behaviors of PLLA/TPU blends without and with various amounts of PDLA were comparatively studied using DSC and POM. Figure 5 shows the time evolution of DSC heat flow and relative crystallinity (Xc(t)) of the PLLA matrix at different isothermal crystallization temperatures. As expected, the crystallization of PLLA/15TPU blend is very slow, but the introduction of only 5 wt % PDLA into the blend leads to a significant shift of the crystallization peak to a much shorter time, indicating a strong accelerating effect of sc crystallites on the crystallization of PLLA matrix. For comparing the crystallization kinetics, the half-crystallization time (t1/2), a key parameter to evaluate overall crystallization kinetics of semicrystalline polymers, was obtained from the curves of Xc(t) at Xc(t) = 50% (Figure 5f) and presented in Figure 6. Obviously, a dramatically accelerated matrix crystallization can be obtained with the addition of PDLA. Specially, the t1/2 values of PLLA/TPU/PDLA blends (e.g., 1.27−1.35 min at 130 °C) are found to be almost the same as those observed in PLLA containing a highly active nucleating agent, such as N,N′,N″-tricyclohexyl-1,3,5-benzene-tricarboxylamide61 and tetramethylene-dicarboxylic dibenzoyl-hydrazide,11 at the same crystallization temperatures, distinctly demonstrating that the formed sc crystallites in the blend melts can acts as efficient nucleating agent for PLLA matrix crystallization. POM observations provide a more direct evidence for the high nucleating efficiency of the sc crystallites on the PLLA matrix crystallization. As shown in Figure 7a, PLLA/15TPU blend exhibits not only an extremely long crystallization period (85

Figure 3. SEM micrographs of cryo-fractured surfaces of the blends: (a) PLLA/5TPU, (a′) PLLA/5TPU/15PDLA, (b) PLLA/15TPU, (b′) PLLA/15TPU/15PDLA, (c) PLLA/25TPU, and (c′) PLLA/ 25TPU/15PDLA.

relative to neat PLLA (Figure 2b). In this case, there is a competition effect between the enhanced melt viscosity and the increased melt elasticity when they tailor the phase morphology of the PLLA/TPU/PDLA blends. The enhanced matrix

Figure 4. SEM micrographs of cryo-fractured surfaces of the blends: (a) PLLA/15TPU, (b) PLLA/15TPU/5PDLA, (c) PLLA/15TPU/10PDLA, (d) PLLA/15TPU/15PDLA, (e) PLLA/15TPU/20PDLA, and (f) PLLA/15TPU/25PDLA. E


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Figure 5. (a−e) DSC curves of PLLA/TPU(85/15) blends with various amounts of PDLA recorded at different isothermal crystallization temperature and (f) the variation of relative crystallinity vs. time for the blends isothermally crystallized at the same temperature of 135 °C.

(Figure 7b). The matrix crystallization can be completed within 4 min, and the tiny PLLA crystals induced by the sc crystallites cannot be easily differentiated one by one. On the basis of the above results, it is very clear that sc crystallites formed in the PLLA/TPU/PDLA blends can act as both efficient rheological modifier and nucleating agent to simultaneously tailor the phase morphology and the matrix crystallization, respectively. Influence of SC Crystallites Tailored Phase Morphology and Matrix Crystallization on the Toughening. In order to explore the combined roles of sc crystallites tailored phase morphology and matrix crystallization in the toughening of PLLA/TPU blends, a series of PLLA/TPU/PDLA blends with different phase morphologies and matrix crystalline states were prepared by injection molding. According to the DSC results obtained from the isothermal crystallization, two isothermal annealing time of the blend melts (i.e., 0.1 and 3 min) in the preheated mold (130 °C) were applied to control the matrix crystalline state of PLLA/TPU/PDLA blends (i.e., amorphous and highly crystalline, respectively). Specially, for the purpose of comparison, the PLLA/TPU blends with a highly crystalline matrix were also prepared by annealing the

Figure 6. Half-crystallization time (t1/2) as a function of crystallization temperature for PLLA/TPU(85/15) blends with various amounts of PDLA.

min) but also a very low nucleation density. Only several large PLLA spherulites (about 100−150 μm in diameter) can be observed. In contrast, with the formation of sc crystallites, both the matrix crystallization rate and the nucleating density of PLLA/15TPU/15PDLA blend are increased significantly F


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Figure 7. POM micrographs showing the time evolution of crystal morphology for (a) PLLA/15TPU and (b) PLLA/15PDLA/15TPU blends during isothermal crystallization at 135 °C. The corresponding crystallization time is given in the profiles.

Even for the binary PLLA/20TPU blend, its impact strength (14.4 kJ/m2) is only about 3 times over that of neat PLLA (3.5 kJ/m2). However, it is very interesting to find that, despite the presence of sc crystallites has no great influence on the impact toughness of PLLA matrix (the impact strength of PLLA/ 15PDLA blend is only 4.9 kJ/m2), PLLA/TPU/15PDLA blends undergo a sharp brittle−ductile transition in impact toughness (from 9.6 kJ/m2 to 63.2 kJ/m2) when TPU content increases from 10 wt % to 15 wt %, where the morphological transition from the sea−island structure to the networklike structure is induced by the sc crystallites (Figure 3b′ and c′). The strong dependence of the toughening efficiency on phase morphology can be further proved by the variation of impact strength of PLLA/15TPU/PDLA blends as a function of PDLA content. As shown in Figure 9, a rapid enhancement in the

injection-molded blends with an amorphous matrix in a conventional oven at 90 °C for 30 min. The DSC melting curves as well as some representative WAXD patterns of the asprepared blends are shown in Figure S3. The thermal parameters and the matrix crystallinity (Xc) obtained from the DSC and WAXD results are summarized in Table S1. Figure 8 gives the notched Izod impact strength of the as-

Figure 8. Notched Izod impact strength of PLLA/TPU and PLLA/ TPU/15PDLA blends with different matrix crystalline states.

prepared PLLA/TPU and PLLA/TPU/15PDLA blends with an almost amorphous PLLA matrix (Xc,WAXD is nearly zero). Evidently, the unique network-like phase structure is far more effective than the typical sea−island one in enhancing toughening efficiency of TPU particles on PLLA matrix. Only very limited enhancement in impact toughness of PLLA can be achieved after introducing large amounts of TPU (5−25 wt %).

Figure 9. Notched Izod impact strength of PLLA/15TPU/PDLA blends with an amorphous PLLA matrix. G


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becomes much smaller due to the cross-linking effect of the sc crystallite network on the amorphous matrix chains. Especially, the value of G′ at 90 °C increases significantly from 8.9 to 102.2 MPa. For PLLA/15TPU/15PDLA blend with a highly crystalline matrix, the heat resistance is further enhanced with the complete crystallization of the matrix. The value of G′ at 90 °C is increased to 277.7 MPa and the high G′ value can be sustained upon >130 °C. Moreover, it is interesting to find that although the heat resistance of PLLA/15TPU/15PDLA blend is weaker than PLLA/15PDLA blend with the same high PLLA matrix crystallinity, it shows a higher G′ relative to the highly crystallized PLLA/15TPU blend in the whole temperature range.

impact toughness is observed with the increase of PDLA content from 10 to 15 wt % because the phase morphology changes from the sea−island structure to the network-like structure (Figure 4c and d), but the toughening efficiency remains almost constant with further increasing PDLA content up to 25 wt % since there is no obvious morphologic change in this case (Figure 4e and f). Interestingly, although the morphological transition induces an apparent decrease in tensile strength of PLLA/TPU blends, the excellent toughening effect is obtained without greatly scarifying the tensile strength and Young’s modulus (Figure S5) because the networklike phase structure is composed of discrete TPU particles rather than the completely continuous TPU phase. Moreover, it should be highlighted that, with the change of matrix crystalline state from almost amorphous to highly crystalline (Xc,PLLA is about 45%), both the Young’s modulus (Figure S5b) and the heat resistance of PLLA/TPU/15PDLA blends enhance markedly while maintaining the same high toughening efficiency (Figure 8). More interestingly, at the same high matrix crystallinity, the PLLA/TPU binary blends display a much lower toughness level as compared to the PLLA/TPU/ 15PDLA blends, indicating that both phase morphology and matrix crystallinity contribute to the enhanced impact toughness of PLLA/TPU/15PDLA blends with a highly crystalline matrix. Most importantly, taking completely biodegradable PLLA toughened with sustainable poly(butylene succinate) (PBS) as an example, we further demonstrate that the construction of sc crystallites in the matrix is an universal method to substantially improve the impact toughness of PLLA/elastomer blends (Figure S6), suggesting a general applicability of our strategy in creating sustainable PLLA materials with high performance. The heat resistance of injection-molded PLLA/15TPU and PLLA/15TPU/15PDLA blends with different PLLA matrix crystalline states was evaluated by DMA analysis and the curves of storage modulus (G′) vs temperature are presented in Figure 10. Clearly, the G′ of PLLA/15TPU blend with an amorphous

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CONCLUSIONS In summary, we have demonstrated that PLLA/TPU blends with both super toughness and excellent heat resistance can be prepared by incorporating small amounts of PDLA through simple melt blending and subsequent injection molding. The results show that, during the melt-blending process, the incorporated PDLA chains can readily collaborate with PLLA matrix chains and cocrystallize into sc crystallites capable of acting as a rheology modifier to sharply reinforce the PLLA matrix melt and dramatically increase the elastic response of the matrix melt, finally inducing the formation of a unique networklike TPU structure. Compared with the PLLA/TPU blends with a typical sea−island structure, the sc crystallites tailored phase morphology can impart the PLLA/TPU/PDLA blends with a remarkably enhanced toughening efficiency. More importantly, the sc crystallites formed in the blend melts can also work as a highly efficient nucleating agent to substantially accelerate matrix crystallization of the injection-molded blends, and the highly improved matrix crystallinity could significantly enhance the heat resistance of the blends without obviously sacrificing the toughness efficiency of the tailored phase morphology. We believe that this work provides a promising and industrially meaningful strategy for fabricating highperformance PLLA materials with tunable impact toughness and heat resistance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00816. SEM micrographs of cryo-fractured and chem-etched surface of PLLA/5TPU and PLLA/15TPU/15PDLA blends, DSC melting curves and some representative WAXD patterns of PLLA/15TPU/PDLA blends prepared at different conditions, tensile properties of PLLA/ TPU and PLLA/15PDLA/TPU blends, and notched Izod impact toughness of PLLA/15PBS and PLLA/ 15PBS/15PDLA blends (PDF)

Figure 10. PLLA/15TPU and PLLA/15TPU/15PDLA blends with different matrix crystalline states.

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matrix drops sharply across the glass transition temperature (Tg), indicating a very poor heat resistance. The subsequent increase in the G′ at higher temperatures (90−105 °C) is associated with the cold crystallization of the amorphous PLLA matrix occurring during the DMA heating process. It is very interesting that a greatly improved heat resistance can be observed in PLLA/15TPU/15PDLA blend with the same amorphous matrix, where the degree of modulus decrease

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected]. Tel./ Fax: +86 28 8546 0953 (H.B.). *E-mail: [email protected]. Tel./Fax: +86 28 8546 1795 (Q.F.). Notes

The authors declare no competing financial interest. H


Research Article


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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Nos. 51421061 and 21404075), China Postdoctoral Science Foundation (No. 2014T70869), the Project of State Key Laboratory of Polymer Materials Engineering (No. sklpme2015-3-01), Science Foundation for The Excellent Youth Scholars of Sichuan University (No. 2015SCU04A28), and Scientific Research Foundation for Young Teachers of Sichuan University (No. 2015SCU11007).

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Remarkably Enhanced Impact Toughness and Heat Resistance of

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