Bioinspired Polylactide Based on the Multilayer Assembly of Shish

Research Article pubs.acs.org/journal/ascecg

Bioinspired Polylactide Based on the Multilayer Assembly of ShishKebab Structure: A Strategy for Achieving Balanced Performances Longfei Yi,† Shanshan Luo,† Jiabin Shen,*,† Shaoyun Guo,*,† and Hung-Jue Sue‡ †

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State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China ‡ Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M University, 575 Ross Street, College Station, Texas 77843-3123, United States S Supporting Information *

ABSTRACT: Achieving balanced mechanical performances in a polymer material has long been attractive, but it is still a significant challenge. Herein, a nonadditive strategy was proposed by tailoring the crystalline structure of neat polylactide (PLA) through a layer-multiplying extrusion. Compared with normal PLA, the layer-multiplied material exhibited a 50% increase in tensile strength, a 4-fold improvement in elongation at break, and greatly enhanced resistance to heat distortion. To comprehensively understand the origin of the balanced performances, we carefully observed and analyzed the tailored crystalline structure. It was demonstrated that the multilayer-assembled shish-kebab structure was fabricated because of the iterative extensional and laminating effects occurring in the layer-multiplying process. Inspired by that process in nacres, the layer-packed shish-kebab skeleton was regarded as the strong phase possessing the ability to offer sufficient strength for resisting mechanical and thermal deformation. Meanwhile, the tenacious interfaces played a significant role in crack deflection and termination in achieving high ductility. More significantly, because no external additives are required, the layer-multiplied PLA is capable of maintaining high transparency as well as good biodegradability and biocompatibility, which gives it competitive advantages in packaging, biomedical, and tissue engineering applications. KEYWORDS: Polylactide (PLA), Shish-kebab structure, Multilayer assembly, Mechanical performance

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INTRODUCTION

Traditional approaches to modifying PLA are commonly associating PLA with flexible components, such as polycaprolactone,6,7 poly(butylene succinate) (PBS),8,9 thermoplastic polyurethane,10 poly(butylene succinate-co-adipate),11 etc. However, the improvements in toughness and ductility are usually accompanied by a substantial sacrifice of the resistance to mechanical and thermal deformation. Therefore, complex recipes or procedures are usually required to balance these performances. For example, Zhang et al.12 blended PBS and poly(3-hydroxybutyrate-co-hydroxyvalerate) (PHBV) together with PLA through a conventional melt-blending method. With the combination of the flexible chain mobility of PBS and the

As the best example of a biodegradable polymer derived from renewable resources, polylactide (PLA) has received an increasing amount of attention because of its fantastic merits such as its high strength and easy processability, together with excellent biodegradability and biocompatibility. The combination of these characteristics has made PLA a promising alternative in tissue engineering, biomedical, and packaging applications.1−3 However, neat PLA commonly fails to offer adequate toughness and ductility and also exhibits poor resistance to thermal deformation.4,5 These inherent deficiencies have become the paramount bottleneck of its usage in structural materials. Hence, a significant challenge has to be faced to achieve balanced performances for practical applications. © 2017 American Chemical Society

Received: November 14, 2016 Revised: March 10, 2017 Published: March 15, 2017 3063

DOI: 10.1021/acssuschemeng.6b02738 ACS Sustainable Chem. Eng. 2017, 5, 3063−3073

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ACS Sustainable Chemistry & Engineering

Figure 1. (A) Schematic of the layer-multiplying coextrusion system: (a and b) single-screw extruders, (c) coextrusion block, (d) layer-multiplying elements (LMEs), (e) exiting block, (f) rolling and cooling block, and (g) as-extruded PLA. (B) Schematic of the layer-multiplying process occurring in LMEs.

direction leading to the formation of highly oriented crystalline architecture, which contributed to the reinforcement of the polymer.16,17 On the other side, the dilemma that the crystal orientation might cause high brittleness because of the limitation of molecular mobility inevitably had to be faced.14 Thus, another approach is urgently required to overcome the deficiency of this situation. As a relatively new but rapidly expanding field in material science, bionics has attracted considerable interest in recent years because of the noteworthy mechanical behaviors exhibited by biological materials.18,19 The most famous example is probably the imitation of the hierarchical structure in nacres.20 As a classic model, the following structural features are thought to provide a material with unbelievable balanced performances: (a) a mineralized strong phase that shows very high stiffness but limited toughness and (b) a nonmineralized tenacious phase closely connecting adjacent mineralized layers that contributes to crack termination or deflection. Inspired by these structural origins, the multilayer assembly of highly oriented crystals is believed to be a promising strategy for obtaining high ductility. The key challenge is constructing this kind of nacrelike structure in a neat polymer without incorporating any additives. In recent years, an advanced melt-processing technology, layer-multiplying coextrusion, has been developed to prepare the [AB···AB]-like multilayer polymer materials with alternating A and B layers.21−24 The number of layers can be multiplied by combining different numbers of layer-multiplying elements (LMEs) with two extruders. By far, all previous work focused on only multicomponent systems that had well-defined interfaces between layers, for example, the plastic/elastomer assembled systems25−27 and the particle-filled polymer/neat polymer assembled systems,28−30 and little effort has been made to fabricate the alternating multilayer structure in a single-component system. However, it has been revealed that

high crystallinity of PHBV, the ternary blend exhibited >5-fold improvement in elongation at break and a 30% increase in heat deflection temperature but still exhibited an ∼36% decrease in tensile modulus, compared to those of neat PLA. Zhang et al.13 introduced a random terpolymer (“T-GMA”) as a reactive compatibilizer in the melt compounding of PLA and poly(butylene adipate-co-terephthalate) (PBAT). The results demonstrated that both the ultimate strain and the notched impact strength of the binary blend were increased without a decrease in tensile strength compared to that of the uncompatibilized system. Furthermore, Zhou et al.14 fabricated a highly oriented crystalline structure in PLA with the assistance of in situ fibrillation of PBAT by applying the uniaxial stretching force in an extrusion process. It was reported that such unique superstructure finally induced simultaneous enhancement of tensile strength and elongation at break. Although the aforementioned methods are widely regarded as a helpful strategy for achieving balanced performances, it must be pointed out that the addition of foreign components may lead to negative effects on originally good transparency, processability, biodegradability, or biocompatibility of neat PLA.15 From the perspective of material designs, a transparent appearance would be more attractive and a simplified recipe could substantially reduce the risk in biomedical applications. Accordingly, an interesting question of whether it is possible to fabricate a high-performance PLA without introducing any other components arises. Unfortunately, to the best of our knowledge, reports about this topic are rare in the existing literature. With regard to semicrystalline PLA, the regulation of its crystalline morphology in conventional polymer processing is recognized as a nonadditive route to tailor performance. Pioneering explorations have revealed that intensive extensional and shearing effects occurring in the melt-flowing process could promote the extension of polymer chains along the force-field 3064


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Figure 2. (A) Stress−strain curves recorded by the stretching machine with a crosshead speed of 10 mm/min (the inset shows the enlarged linear portion of each curve). (B) Comparison of tensile strength and elongation at break among PLA-0, PLA-1, PLA-5, and PLA-9. (C) Comparison of fractured surfaces between PLA-0 and PLA-9 observed through a SEM for evaluating the brittle-to-ductile transition in the damage mechanism with the evolution of the crystalline structure. (D) Comparison of the increased ratios of tensile strength and elongation at break among some previously reported PLA-based materials blended with (a) polyoxymethylene,33 (b) tetramethylene-dicarboxylic dibenzoyl-hydrazide,34 (c) graphene oxide,35 (d) cotton fiber,5 (e) poly(butylene adipate-co-terephthalate),14 (f) hemp fiber,5 (g) ZnO whisker,36 (h) acetyl triethyl citrate,5 (i) polyethylene glycol,37 (j) polybutylene succinate,8 (k) polypropylene glycol,38 (l) triethyl citrate,5 (m) polybutylene succinate/poly(3-hydroxybutyratecohydroxyvalerate),12 (n) glass fiber,39 (o) carbon nanofiber,40 (p) microfibrillated cellulose,5 (q) multiwalled carbon nanotube,41 (r) jute fiber,5 (s) man-made cellulose,42 and (t) abaca fiber.5 The star represents the result of our work.

the convergent flowing behavior in LMEs could induce an intense extensional effect on a melt, which effectively led to in situ fibrillation of the dispersed phase31 and the orientation of high-aspect ratio particles.26 Hence, the work presented here attempts to fabricate a type of unprecedented superstructure in neat PLA with the multilayer assembly of highly oriented crystals, by utilizing the combination of extensional and layermultiplying effects occurring in LMEs. The oriented crystallization was regulated through extensional flow, and the layerpacked structure was constructed through iterative layer multiplication. Because no external additives were required, such a structure was expected to provide high transparency and an excellent combination of mechanical performances for packaging and tissue engineering applications.

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by a divider and then recombined vertically, leading to the doubling of layer numbers as shown in Figure 1B. The temperatures of two extruders were both 185 °C, and the temperature of LMEs was set at 182 °C for a more stable melt flow during the layer-multiplying process. For the sake of convenience, the as-extruded samples were named as PLA-n. By controlling the coextruding speed, we fixed the total thickness of each as-extruded product at ∼1.5 mm. Scanning Electron Microscope (SEM). The crystal morphology of PLA was observed through a field emission SEM (JEOL JSM5900LV) at an accelerating voltage of 5 kV. Prior to observation, the cryo-fractured surface was etched in a water/methanol (1:2 by volume) solution containing 0.025 mol of sodium hydroxide/L for 26 h at 20 °C and subsequently sputter-coated with a thin layer of gold. Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). 2D-SAXS measurements were performed on a Xeuss 2.0 laboratory beamline to examine the lamellar structure of PLA. A multilayer mirror-focused Cu Kα X-ray source (GeniX3D Cu ULD, Xenocs SA; λ = 0.154 nm) was used, and the sample-to-detector distance was fixed at 2500 mm during the experiments. Each SAXS pattern was collected within a 10 min exposure. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). 2D-WAXD was conducted on beamline BL15U1 (SSRF, China). An X-ray beam with a wavelength of 0.124 nm was applied. The sampleto-detector distance was set to 148 mm, and the exposure time was 60 s. The 2D-WAXD patterns were collected with an X-ray CCD detector (model SX165, Rayonix Co. Ltd.). Polarized Fourier Transform Infrared Spectroscopy (P-FTIR). A Fourier transform infrared microscope (iS10, Thermo Nicolet) equipped with a polaroid was used to detect the orientation of

EXPERIMENTAL SECTION

Materials. Commercially available PLA comprising around 4% DLA was purchased from Nature Works (trade name 2003D) with a density of 1.24 g/cm3 and a melt index of 6.7 g/10 min at 210 °C. The weight-average molecular weight and number-average molecular weight were 2.53 × 105 and 1.46 × 105 g/mol, respectively. Sample Preparation. Prior to melt processing, PLA was dried at 80 °C for >12 h in a vacuum oven. As schematically illustrated in Figure 1A, pure PLA of the same kind was coextruded from two extruders (a and b) and combined together as two-layer melt in the coextrusion block (c) and then passed through n LMEs (n = 0, 1, 5, and 9). In a LME, the melt was sliced into two left and right sections 3065


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Figure 3. (A) Temperature dependence of the storage modulus recorded by the DMA instrument using a stretching mode (the inset shows enlarged curves around 20 °C). (B) Storage moduli of PLA-0 and PLA-9 at 20 and 80 °C. (C) Comparison of thermal deformation ability by simultaneously placing PLA-0 and PLA-9 on a heated plate maintained at 80 °C. The photographs were taken after 60 min. (D) Comparison of the degree of thermal deformation between PLA-0 and PLA-9 by moving them to a piece of paper. molecular chains in the as-extruded PLA. A thin slice with a thickness of ∼20 μm was obtained by a microtome from each sample along the extruding direction. A multiangle scan from 0° to 180° (by rotating a wire-grid polarizer) was collected with 32 scans at a resolution of 4 cm−1. All scan curves were managed with the commercially available software OMNIC to obtain the three-dimensional (3D) field visualization. The orientation function of the crystalline region ( fc) was calculated on the basis of the following equation:32

fc = R − 1/R + 2

transparency of the materials was compared visually by placing them on a white paper printed with a red badge, and the photos were taken with a digital camera.

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RESULTS Tensile Properties. Figure 2A illustrates typical stress− strain curves of as-extruded PLA by applying 0, 1, 5, and 9 LMEs, and different stretching behaviors could be observed. For PLA-0, the fracture happened just after the yielding point. With an increase in the number of LMEs, a larger deformation was obtained accompanied by the appearance of a distinct strain hardening phenomenon. When PLA-9 was stretched to the ultimate strain, the stress was even larger than that at the yielding point. Furthermore, in deep contrast to the smooth and flat fractured cross-sectional surface of PLA-0 (Figure 2B), that of the PLA-9 is extremely coarse and bumpy, which is indicative of a brittle-to-ductile transition in the damage mechanism when the neat polymer experienced the iterative layer-multiplying process in LMEs. The average values of the measured tensile strength and elongation at break are included in Figure 2C. Compared to the original values of 56.5 MPa for PLA-0, the tensile strength of PLA-9 increases by 53%, reaching 85.9 MPa. Of particular importance is the largely enhanced ductility with an ∼4-fold improvement in elongation at break from 10.2% (PLA-0) to 50.1% (PLA-9). It is worth noting that such an integrated increase in strength and ductility is indeed a remarkable phenomenon that has never been reported before in neat PLA. Even in multicomponent systems, it is still a significant challenge to trade off both sides. Herein, some representative PLA-based systems and corresponding results

(1)

where R is the dichroic ratio of the absorbance of a crystalline absorption band collected at 0° and 90°. Differential Scanning Calorimetry (DSC). The melting point temperature of each sample was recorded by a DSC machine (Q20, TA Instruments, New Castle, DE) under a nitrogen atmosphere. Each sample, with a weight of 6−8 mg, was heated from 20 to 200 °C at a heating rate of 10 °C/min. Dynamic Mechanical Analysis (DMA). The storage and loss moduli with a change in temperature were measured through a DMA instrument (Q800, TA Instruments). Each sample with dimensions of 10 mm (length) × 4 mm (width) × 1.5 mm (thickness) was heated from 0 to 120 °C at a heating rate of 3 °C/min using a stretching mode. The sinusoidal oscillating strain and frequency were 10 μm and 1 Hz, respectively. Tensile Tests. Tensile tests were performed on an Instron (Canton, MA) 4302 tension machine at room temperature with a crosshead speed of 10 mm/min following the ASTM D638 standard. At least five specimens for each sample were tested, and the average value was calculated. Light Transmittance. The light transmittance of each specimen was measured by using an ultraviolet−visible (UV−vis) machine (UV3600, Shimadzu) equipped with an integrating sphere accessory. The wavelength range was from 400 to 800 nm. Moreover, the 3066


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Figure 4. (A) Photos of PLA-0 and PLA-9 coverings on a red badge taken by a digital camera. (B) Light transmittance of PLA-0 and PLA-9 as a function of wavelength measured through a UV−vis spectrometer.

Figure 5. SEM images of the crystalline structure in as-extruded PLA-0, PLA-5, and PLA-9 observed at different magnifications: 20000× (top) and 80000× (bottom). Prior to the observation, the amorphous phase has been selectively etched by immersing the as-extruded specimen in a water/ methanol solution.

Thermal Deformation Behaviors. DMA was performed to acquire information about the resistance to thermal deformation. As shown in Figure 3A, each as-extruded specimen was heated from 0 to 120 °C and the storage modulus started to sharply decrease around 60 °C because of the glass transition of PLA, which caused thermal deformation. Thus, the modulus was compared at low- and high-temperature sides (Figure 3B). At 20 °C, a 21.5% increase from 3475 to 4222 MPa was obtained by increasing the number of LMEs from 0 to 9. Such a stiffening effect was more obvious at 80 °C. The modulus was remarkably increased by 75-fold from 5.6 MPa (PLA-0) to 426.5 MPa (PLA-9). To visually illustrate the distinct improvement in the resistance to thermal deformation at high temperatures, PLA-0 and PLA-9 were simultaneously placed on a heated plate maintained at 80 °C, and the photographs were taken after 60 min. It could be clearly observed in panels C and D of Figure 3 that PLA-0 experienced

measured through a similar stretching procedure are compared in Figure 2D (the star represents the result of this work). Considering that each work used different types of material, compositions, or processing methods, the comparison is performed by calculating the increased ratios of the tensile properties of the modified system to those of the corresponding neat PLA. Undoubtedly, the ideal target for introducing foreign components is the simultaneous increase in tensile strength and elongation at break, as indicated in the blue area. However, the fact is most systems are mainly located in the purple or gray area; in other words, acquiring one side usually requires the loss of another. Fortunately, the multilayer-assembled PLA fabricated in this work appears to be a good solution. The comparison clearly indicates that the excellent strength− ductility balance could be achieved through the nonadditive processing method. 3067


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Figure 6. (A) Representative 2D-SAXS patterns and (B) 1D SAXS intensity profiles of PLA-0, PLA-5, and PLA-9. PLA-9 containing layer-packed shish-kebab structure displays a maximal peak around q = 0.3 nm−1, and the long period of the lamellae structure is 21.3 nm based on the Bragg equation.

make the layer interfaces be hardly observed because of the molecular diffusion during the melt processing. Herein, the crystalline morphology was first observed through a SEM, by selectively etching the amorphous phase in each specimen. To carefully trace the structural details, the observation was performed on different scales as shown in Figure 5. Even at a low magnification, the transition from isotropic to highly oriented crystalline structure is clearly evidenced with the increase in the number of LMEs from 0 to 9. Highly magnified images further suggest the generation of shish-kebab structure (indicated by red arrows) in PLA-9 where plenty of short kebabs decorate well-aligned shishes, in contrast to the isotropic morphology in PLA-0. For better observation of such crystalline morphology, SEM images of the shish-kebab structure in PLA-9 at a magnification of 40000× are also presented in Figure S1. More significantly, the shishes and kebabs are closely and alternately assembled, forming a unique layer-packed structure throughout the material, which is entirely different from the injection-molded product in which the shish-kebab structure appears in only skin layers.37 Besides, unlike those in hybrid systems,8,34 both the shishes and kebabs in the system presented here are composed of the same component. To the best of our knowledge, such unprecedented crystalline structure has rarely been reported in neat PLA in the existing literatures. Structural Analysis. To observe morphology, 2D-SAXS and 2D-WAXD were performed to gain a more comprehensive understanding of the unique crystalline structure induced by the layer-multiplying process. Figure 6A exhibits representative 2D-SAXS patterns of PLA-0, PLA-5, and PLA-9. For PLA-0, only a weak scattering ring could be observed because of its relatively low crystallinity. To gain deeper insight into the molecular state of PLA-0, the experiment was performed at a higher temperature. As presented in Figure S2A, a stronger

serious heat distortion while PLA-9 maintained its original shape after the thermal treatment. Such comparison further emphasizes the fact that the multilayer-assembled PLA is capable of resisting mechanical deformation over a wide temperature range, thus leading to higher durability under complex circumstances. Transparency. From the perspective of industrial design, the materials with an attractive appearance are always welcome. At least good transparency usually implies that few additives are incorporated. To present a clear comparison, PLA-0 and PLA-9 were placed on a white paper printed with a red badge and the photos taken by a digital camera are shown in Figure 4A. It should be noted that the thickness of PLA-0 is identical with that of PLA-9 (1.5 mm) to eliminate the influence of film thickness. It can be observed that the layer-multiplying process nearly caused an indistinguishable effect on the good transparency of PLA. Furthermore, the light transmittance of each specimen in the range of visible light wavelengths (400− 800 nm) was measured through a UV−vis spectrometer. As displayed in Figure 4B, the measured values of PLA-9 are basically around 80%, being only slightly reduced compared to those of PLA-0, which is remarkably superior to the previously reported binary blend.36 Hence, this effort finally provides promising access to fabrication of the transparent PLA with competitive mechanical performances.

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DISCUSSION Structural Observations. The unusual combination of the aforementioned mechanical performances suggests the formation of superstructure in neat PLA during the layermultiplying process. It is known from previous work that 1024 layers with well-defined 1023 interfaces should be obtained in a binary system when 9 LMEs are applied.43 For the neat PLA system, the same component in each layer may 3068


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Figure 7. (A) Representative 2D-WAXD patterns and (B) 1D WAXD intensity profiles of PLA-0, PLA-5, and PLA-9.

Figure 8. (A) 3D visualization FTIR spectra of PLA-0, PLA-5, and PLA-9 obtained through a multiangle scan from 0° to 180°. The absorbance bands at 921 and 956 cm−1 are associated with the α-form crystalline phase and amorphous phase of PLA, respectively. (B) Calculated orientation functions of the crystalline region in PLA-0, PLA-5, and PLA-9.

isotropic ring (indicated by white arrows) displaying a long period of 23.9 nm (Figure S2B) could be distinctively observed at 120 °C because of its cold crystallization process, which further indicates only randomly arranged lamellar structures could be formed when no LME was employed. In PLA-5, the appearance of a symmetrical triangle-like scattering pattern suggests the transition of a molecular state from isotropic to

anisotropic with an increase in the number of LMEs. When 9 LMEs are applied, a two-lobe scattering pattern (indicated by white arrows) in the meridional direction can be observed. This typically evidences the presence of shish-kebab structure, which is in good agreement with the SEM observations. Moreover, the one-dimensional (1D) intensity profiles displayed in Figure 6B show that only PLA-9 displays a distinct peak around q = 3069


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Figure 9. (A) Melting curves of PLA-0, PLA-1, PLA-5, and PLA-9 recorded by a DSC instrument at a heating rate of 10 °C/min. The peak and the area of each melting curve correspond to the melting temperature and crystallization degree (Xc), respectively. (B) Temperature dependence of the loss modulus recorded by the DMA instrument using a stretching mode (1 Hz). The peak in each curve corresponds to the glass transition of PLA.

0.3 nm−1, suggesting a regular aligned lamellar structure with cylindrical symmetry in the shish-kebab structure. The long period (L) was calculated with the Bragg equation (eq 2) L = 2π /q

region have an orientation level that is higher than those in the amorphous region. (ii) The opposite fluctuation between the bands at 921 and 956 cm−1 suggests that the molecular chains in the amorphous region are approximately perpendicular to those in the crystalline region. That means the direction of orientation of the amorphous chains is parallel to the chainfolded direction in lamellae. Although there is no more evidence at present, we can speculate that the confined spaces generated by the oriented lamellae may compel the alignment of the molecular chains in the amorphous region. The perfection of the crystalline structure and the confined mobility of amorphous chains were further verified by employing DSC and DMA measurements. Figure 9A presents the heating curves of four kinds of PLA prepared with different numbers of LMEs. The corresponding crystallinity (Xc) and melting temperature (Tm) are marked beside each curve. As illustrated, an extremely low crystallinity (5.4%) is obtained in PLA-0, which is responsible for the weak diffraction pattern shown by 2D-WAXD. Via the application of more LMEs, the crystallinity is gradually increased and reaches 28.3% in PLA-9, which confirms that iterative extension occurring in LMEs promotes the crystallization of PLA. Meanwhile, a substantial increase in Tm from 153.8 to 157.3 °C also shows the improved regularity of oriented lamellae as described in previously reported systems.45−47 Commonly, the confined mobility of amorphous chains is evaluated by measuring the glass transition temperature (Tg).48,49 Figure 9B shows the dependence of the loss modulus (E″) on temperature, where the loss peak reaches the maximum defined as Tg. For PLA-0, this critical temperature of 61.5 °C is basically consistent with that of the commercial PLA product. Like what happens in Tm, increasing the number of LMEs produces a higher Tg, which approaches 69.6 °C when 9 LMEs are applied. These results suggest that the mobility of the molecular chains in the amorphous region tends to be confined and accompanied by the perfection of crystalline structure as we speculated on the basis of FTIR analysis. Mechanisms. On the basis of the aforementioned analysis, the structural schematic of the layer-multiplied PLA is proposed in Figure 10 for comprehensively understanding the origin of the balance between strength and ductility. When the neat PLA melt went through an assembly of LMEs, the intensive extensional flow could promote the elongation of the melt along the extrusion direction. As a result, highly extended

(2)

The L of PLA-9 (21.3 nm) is comparable to those of hybrid systems,8,36,37 which implies that the well-aligned shishes composed of highly extended PLA chains can also facilitate the growth of oriented lamellae on their surfaces. The molecular orientation and crystalline structure were further examined using 2D-WAXD. As presented in Figure 7A, a halo amorphous diffraction can be observed in the diffraction pattern of PLA-0, because of the generation of a very low content of spherulites. For PLA-5, an arc-like diffraction pattern detectable at the meridional position indicates that the extensional effect in the layer-multiplying process induces the molecular orientation. On the other side, the weak diffraction intensity and the absence of a diffraction ring for certain lattice lamellae such as (203) can be ascribed to the formation of imperfect crystals. When 9 LMEs are applied, lattice planes (200/110) and (203) of typical α-form PLA are clearly traced and their corresponding diffraction intensities are remarkably sharper than those of PLA-5 as shown in Figure 7B. Significantly enhanced crystallization and orientation induced by the layer-multiplying process are evidently reflected with a strong arc-like diffraction reflection. To probe the configuration of molecular chains in the amorphous region influenced by the development of crystalline structure, P-FTIR has been conducted and 3D field visualization with a change in polarized angles is presented in Figure 8A. Typically, the absorbance bands at 921 and 956 cm−1 are associated with the vibration of chemical bonds in the α-form crystalline phase and the amorphous phase of PLA, respectively.44 For PLA-0, weak absorption at 921 cm−1 can be noticed, in contrast to the band at 956 cm−1. In layermultiplied systems, the former gradually rises and shows an opposite dependence on polarized angle compared to that of the latter. Accordingly, at least two inferences can be drawn from such a unique phenomenon. (i) Neither the amorphous region nor the crystalline region is isotropic. The difference between the maximal and minimal absorbance at the same wavelength can be adopted to represent the degree of orientation quantitatively. The calculated results included in Figure 8B reveal that the molecular chains in the crystalline 3070


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for the massive production of commercial polymers with outstanding balanced performances.

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CONCLUSIONS In this work, bioinspired PLA with multilayer-assembled shishkebab structure was successfully fabricated through layermultiplying coextrusion. The intensive extensional flow induced the formation of well-aligned shishes and highly oriented kebab lamellae, while iterative layer multiplication occurring in LMEs made the shishes and kebabs be closely and alternately packed, forming the unique multilayer structure. Inspired by that in nacres, the layer-packed shish-kebab skeleton possessed the ability to resist mechanical deformation, and the tenacious interfaces promoted crack deflection and termination. Hence, the layer-multiplied material revealed a 50% increase in tensile strength, a 4-fold improvement in elongation at break, and a greatly enhanced resistance to heat distortion, compared with those of normal PLA. Unlike that in most multicomponent systems, such a nonadditive material could maintain a high transparency as well as good biodegradability and biocompatibility, which exhibited potential applications in packaging, biomedical, and tissue engineering fields.

Figure 10. Structural schematic of the multilayer-assembled shishkebab structure in neat PLA.

molecular chains in each layer were preserved acting as shishes and the flow-induced row nuclei on shish surfaces led to the formation of oriented kebab lamellae. In addition, because of the iterative layer multiplication effect occurring in LMEs, the shishes and kebabs were closely and alternately packed forming a unique multilayer-assembled shish-kebab structure as observed in SEM images. The well-aligned shish-kebab skeleton is regarded as the strong phase possessing the ability to offer sufficient strength for resisting mechanical and thermal deformation. Meanwhile, the tenacious layer interfaces may play a significant role in crack deflection and termination for achieving high ductility.50 The distinct layer delamination displayed in Figure S1 can be recognized as the evidence of the increased level of energy dissipation in the mechanical deformation, like that happening in previously reported natural and artificial materials with hierarchical structure.51,52

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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.6b02738. SEM images of PLA-9, 2D-SAXS patterns and 1D SAXS intensity profiles of PLA-0 after it had been heated, side view of the fracture morphology of PLA-0 and PLA-9, and the scratch performances of PLA-0 and PLA-9 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

PERSPECTIVES The well-organized structure fabricated in this work is widely applicable. The balance between strength and ductility can make the product much stronger and more durable. For instance, during the storing or serving period, unavoidable scratch forces may act on the product and have a destructive effect on its surface quality and inherent performances. The preliminary measurements performed on PLA-0 and PLA-9 (see Figure S2) show that plenty of parabolic cracks around the scratched groove can be observed on the surface of PLA-0, like that reported in other brittle polymers.53,54 However, almost no visible cracks are initiated on PLA-9 by applying the same normal load. It has been reported that the material would experience a stretching process when it is scratched; thus, balanced strength and ductility may play a critical role in obtaining high resistance to scratch deformation.53 More significantly, unlike that in most multicomponent systems, such a structure is capable of maintaining a high transparency as well as good biodegradability and biocompatibility because no foreign additives are required, which makes the layer-multiplied PLA have competitive advantages in packaging, biomedical, and tissue engineering applications. Additionally, it needs to be emphasized that a similar structure can be obtained in other semicrystalline polymers after it experiences layer-multiplying coextrusion (e.g., polyethylene, polypropylene, etc.). Accordingly, this work provides a reproducible and efficient strategy

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiabin Shen: 0000-0002-6942-5686 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51420105004, 51227802, 51673136, and 51421061) and the Program of Introducing Talents of Discipline to Universities (B13040) is gratefully acknowledged.

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REFERENCES

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Bioinspired Polylactide Based on the Multilayer Assembly of Shish

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