Bioinspired Polylactide Based on the Multilayer Assembly of Shish

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Bio-inspired 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02738 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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ACS Sustainable Chemistry & Engineering

Bio-inspired Polylactide Based on the Multilayer Assembly of Shish-kebab Structure: A Strategy for Achieving Balanced Performances Longfei Yi1, Shanshan Luo1, Jiabin Shen1*, Shaoyun Guo1*, Hung-Jue Sue2 1

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of

Sichuan University, Chengdu 610065, China 2

Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M

University, College Station, TX 77843-3123, USA

AUTHOR INFORMATION Present Address Longfei Yi, Shanshan Luo, Jiabin Shen, Shaoyun Guo State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu 610065, China. Hung-Jue Sue Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M University, 575 Ross Street, College Station, Texas 77843, USA

Corresponding Authors ∗ E-mail: [email protected] (Jiabin Shen). * E-mail: [email protected] (Shaoyun Guo).

ACS Paragon Plus Environment


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Abstract:

Achieving balanced mechanical performances in a polymer material has long been attractive, but it is still a big challenge. Herein, a non-additive 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 obtained 50% increase in tensile strength, 4-fold improvement in elongation at break, as well as largely enhanced resistance to heat distortion. For comprehensively understanding the origin of the balanced performances, the tailored crystalline structure was carefully observed and analyzed. It was demonstrated that the multilayer-assembled shish-kebab structure was fabricated due to the iterative extensional and laminating effects occurring in the layer-multiplying process. Inspired by that 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 for achieving high ductility. More significantly, since no external additives are required, the layer-multiplied PLA is capable of maintaining a high transparency as well as good biodegradability and biocompatibility, which makes it be of competitive advantages in packaging, biomedical and tissue engineering applications.

Keywords:

polylactide

(PLA);

shish-kebab

structure; multilayer assembly;

mechanical performances.


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INTRODUCTION

As the front runner of biodegradable polymers derived from renewable resources, polylactide (PLA) has received increasing attentions thanks to its fantastic merits such as high strength, 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 big challenge has to be faced to achieve balanced performances for practical applications.

Traditional approaches toward modifying PLA are commonly associating PLA with flexible components, such as polycaprolactone6, 7, poly(butylene succinate) (PBS)8, 9, thermoplastic polyurethane10 and poly(butylene succinate-co-adipate)11, etc. However, the improvements in toughness and ductility are usually accompanied by substantial sacrifice of the resistance ability to mechanical and thermal deformation. Therefore, complex recipes or procedures are usually required in order to balance these

performances. For example, Zhang K et al.12 blended PBS and

poly(3-hydroxybutyrate-co-hydroxyvalerate) (PHBV) together with PLA through conventional melt-blending method. By combining with flexible chain mobility of PBS and high crystallization of PHBV, the ternary blend exhibited over five-fold improvement in elongation at break and 30% increment in heat deflection temperature



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but still had about 36% decrease in tensile modulus, compared to neat PLA. Zhang N et al.13 introduced a random terpolymer (refered as “T-GMA”) as a reactive compatibilizer

in

the

melt

compounding

of

PLA

and

poly(butylene

adipate-co-terephthalate) (PBAT). Results demonstrated that both ultimate strain and notched impact strength of the binary blend were increased without decreasing tensile strength compared to the uncompatibilized system. Furthermore, Zhou S 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 in tensile strength and elongation at break. Although aforementioned methods are widely regarded as a helpful strategy in 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, transparent appearance would be more attractive and a simplified recipe could substantially reduce the risk in biomedical applications. Accordingly, an interesting question comes out: is it possible to fabricate a high-performance PLA without introducing any other components? Unfortunately, rare reports could be found in this field from exiting literatures, to the authors’ best knowledge.

With regard to semi-crystalline PLA, the regulation of its crystalline morphology in conventional polymer processing is recognized as a non-additive route to tailor performances. Pioneering explorations have substantiated that intensive extensional


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and shearing effects occurring in the melt-flowing process could promote the extension of polymer chains along the force-field direction leading to the formation of highly-oriented crystalline architecture, which contributed to the reinforcement of the polymer.16, 17 On the other side, a dilemma had to be faced inevitably that the crystal orientation might cause a high brittleness due to the limitation of molecular mobility.14 Thus, another philosophy is urgently required to overcome the deficiency of present situation.

As a relatively new but rapidly expanding field in material science, bionics has attracted considerable interest in recent years due to 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, following structural features are considered to grant it with unbelievable balanced performances: (a) mineralized strong phase shows very high stiffness but limited toughness; (b) non-mineralized tenacious phase closely connecting adjacent mineralized layers contributes to the crack termination or deflection. Inspired by these structural origins, the multilayer assembly of highly-oriented crystals is believed to be a promising strategy to obtain high ductility. The key challenge is constructing this kind of nacre-like 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



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by combining different number of layer-multiplying elements (LMEs) with two extruders. By far, all previous work only focused on multicomponent systems which had well-defined interfaces between layers, for example, the plastic/elastomer assembled systems25-27, and the particle-filled polymer/neat polymer assembled systems28-30, few efforts have been made to fabricate the alternating multilayer structure in a single component system. However, it has been revealed that the convergent flowing behavior in LMEs could induce intensive extensional effect on a melt, which effectively led to in-situ fibrillation of dispersed phase31 and the orientation of high-aspect-ratio particles.26 Hence, the present work attempted 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 layer-multiplying effects occurring in LMEs. The oriented crystallization was regulated through extensional flow and the layer-packed structure was constructed through iterative layer multiplication. Since no external additives were required, such structure was expected to provide high transparency and excellent combination of mechanical performances for packaging and tissue engineering applications.


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EXPERIMENTAL SECTION Materials. Commercially available PLA comprising around 4% D-LA, was purchased from Nature Works (trade name 2003D) with a density of 1.24g/cm3 and melt index of 6.7g/10min at 210°C. The weight-average molecular weight and number-average molecular weight were 2.53 × 105g/mol and 1.46 × 105g/mol, respectively. Sample Preparation. Prior to melt processing, PLA was dried under 80°C for over 12hrs in a vacuum oven. As schematically illustrated in Fig. 1(A), pure PLA of the same kind was co-extruded from two extruders (a and b) and combined together as two-layer melt in the coextrusion block (c), then flowed through n LMEs (n=0, 1, 5, 9). In a LME, the melt was sliced into two left and right sections by a divider, and then recombined vertically, leading to the doubling of layer numbers as shown in Fig. 1(B). The temperatures of two extruders were both 185℃, and the temperature of LMEs was set at 182℃ for more stable melt flow during the layer multiplying process. For convenience, the as-extruded samples were named as PLA-n. By controlling the coextruding speed, the total thickness of each as-extruded product was fixed at about 1.5mm.



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

Scanning Electron Microscope (SEM). Crystal morphology of PLA was observed through a field emission SEM (JEOL JSM- 5900LV, Japan) at an accelerating voltage of 5kV. Prior to observation, the cryo-fractured surface was etched in a water-methanol (1:2 by volume) solution containing 0.025mol/L of sodium hydroxide for 26hrs at 20°C and subsequently sputter-coated with a thin layer of gold.


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Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). 2D-SAXS measurements were performed on a Xeuss 2.0 (, France) laboratory beamline to examine the lamellar structure of PLA. A multilayer mirror focused Cu Kα X-ray source (GeniX3D Cu ULD, Xenocs SA, France, λ = 0.154nm) was used and the sample-to-detector distance was fixed at 2500mm during the experiments. Each SAXS pattern was collected within 10min exposure.

Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). 2D-WAXD was conducted on the beamline (BL15U1, SSRF, China). X-ray beam with a wavelength of 0.124nm was applied. The sample-to-detector distance was set as 148mm and the exposure time was 60s. The 2D-WAXD patterns were collected with an X-ray CCD detector (Model SX165, Rayonix Co. Ltd., USA).

Polarized Fourier Transform Infrared Spectroscopy (P-FTIR). Fourier transform infrared microscope (iS10，Thermo Nicolet, USA) equipped with a polaroid was used to detect the orientation of molecular chains in the as-extruded PLA. A thin slice about 20µm in thickness 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 4cm−1. All scan curves were managed with the commercially available software OMNIC to obtain the 3D field visualization. The orientation function of crystalline region (fc) was calculated based on the following equation32,

fc=R−1/R+2

(1)



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where R is the dichroic ratio of the absorbance of a crystalline absorption band collected at 0 and 90°.

Differential Scanning Calorimetry (DSC). Melting point temperature of each sample was recorded by a DSC machine (Q20, TA Instrument, USA) under a nitrogen atmosphere. Each sample, with a weight of 6−8mg, 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 the change of temperature were measured through a DMA instrument (Q800, TA Instrument, USA). Each sample with dimensions of 10mm (length) × 4mm (width) × 1.5mm (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 10µm and 1Hz, respectively.

Tensile Tests. Tensile tests were performed on an Instron 4302 tension machine (Canton, MA, USA) at room temperature with a crosshead speed of 10mm/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 UV-VIS machine (UV-3600, Shimadzu, Japan) equipped with an integrating sphere accessory. The wavelength range was from 400 to 800nm. Moreover, the transparency of the materials was compared visually by placing them on a white paper printed with a red badge and the photos were taken by a digital


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camera.

RESULTS

Tensile Properties. Fig. 2(A) 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 increasing the LME numbers, a larger deformation was obtained accompanied with 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 (Fig. 2(B)), that of the PLA-9 is extremely coarse and bumpy, which is indicative of a brittle-to-ductile transition in damage mechanism when the neat polymer experienced iterative layer-multiplying process in LMEs. The average values of the measured tensile strength and elongation at break are included in Fig. 2(C). Compared to the original values of 56.5 MPa for PLA-0, the PLA-9 obtains a 53% increment in tensile strength, reaching 85.9 MPa. Of particular importance is the largely enhanced ductility with about 4-fold improvement in elongation at break from 10.2% (PLA-0) to 50.1% (PLA-9). It is worth noting that such integrate increment of strength and ductility is indeed a remarkable phenomenon that has never been reported before in neat PLA. Even in multi-component systems, it is still a big challenge to trade-off the both sides. Herein, some of representative PLA-based



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systems and corresponding results measured through a similar stretching procedure are compared in Fig. 2(D) (The star represents the result of this work). Considering that each work may choose different material types, compositions or processing methods, the comparison is performed by calculating the increase ratios of tensile properties of the modified system and corresponding neat PLA. Undoubtedly, the ideal target of introducing foreign components is the simultaneous increment of 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 grey area, in other words, getting

one

side

usually

has

to

lose

another

side.

Fortunately,

the

multilayer-assembled PLA fabricated in present work appears to be a good solution. The comparison clearly indicates that the excellent strength-ductility balance could be achieved through the non-additive processing method.


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Figure 2. (A) Stress-strain curves recorded by the stretching machine with a crosshead speed of 10 mm/min (the inserted image enlarges the linearly 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 SEM for evaluating the brittle-to-ductile transition in damage mechanism with the evolution of crystalline structure. (D) Comparison of the increase ratios of tensile strength and elongation at break among some previously reported PLA-based materials blended with (a) polyoxymethylene33; (b) tetramethylene-dicarboxylic dibenzoyl-hydrazide34; (c) graphene oxide35; (d) cotton fiber5; (e) poly(butylene adipate-co-terephthalate)14; (f) hemp fiber5; (g) ZnO whisker36; (h) acetyl triethyl citrate5; (i) polyethylene glycol37; (j) polybutylene succinate8; (k) polypropylene glycol38; (l) triethyl citrate5; (m) polybutylene succinate/poly(3-hydroxybutyrateco-hydroxyvalerate)12; (n) glass fiber39; (o) carbon nanofiber40; (p) microfibrillated cellulose5; (q) multi-walled carbon nanotube41; (r) jute fiber5; (s) man-made cellulose42; (t) abaca fiber5. The star represents the result of present work.

Thermal Deformation Behaviors. DMA was performed to acquire information on the resistance to thermal deformation. As shown in Fig. 3(A), each as-extruded specimen was heated from 0 to 120°C and the storage modulus started to sharply drop down around 60°C due to the glass transition of PLA, which caused the occurrence of thermal deformation. Thus, the modulus was compared at low and high temperature sides (Fig. 3(B)), respectively. At 20°C, an increment of 21.5% from 3475 to 4222



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MPa was obtained by increasing the LME numbers from 0 to 9. Such stiffening effect was more obvious at 80°C. The modulus was remarkably increased by 75 times from 5.6 MPa (PLA-0) to 426.5 MPa (PLA-9). In order to visually illustrate the distinct improvement on the resistance to thermal deformation at high temperature, PLA-0 and PLA-9 were simultaneously placed on a heated plate maintained at 80 °C and the photographs were taken after 60min. It could be clearly observed in Fig. 3(C) and (D) that PLA-0 experienced serious heat distortion, while the PLA-9 still preserved its original shape after the thermal treatment. Such comparison further emphasizes that the multilayer-assembled PLA is capable of resisting mechanical deformation in a wide temperature range, thus leading to higher durability under complex circumstances.


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Figure 3. (A) Temperature dependence of storage modulus recorded by the DMA instrument using a stretching mode (the inserted image enlarges the 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 and the photographs were taken after 60min; (D) Comparison of thermal deformation degree between PLA-0 and PLA-9 by moving them to a piece of paper.

Transparency. From the perspective of industrial design, the materials having attractive appearance are always welcomed. At least, good transparency usually implies that few additives are incorporated. To present a clear comparison, PLA-0 and PLA-9 were respectively placed on a white paper printed with a red badge and the photos taken by a digital camera were recorded in Fig. 4(A). 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 indistinguishable effect on good transparency of PLA. Furthermore, the light transmittance of each specimen in the range of visible light wavelength (400-800nm) was measured through a UV-Vis spectrometer. As displayed in Fig. 4(B), the measured values of PLA-9 are basically around 80% only having a slight reduction from those of PLA-0, which is remarkably superior to the previously reported binary blend.36 Hence, the present effort finally provides a promising access to fabricate the transparent PLA with competitive mechanical performances.



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Figure 4. (A) Photos of PLA-0 and PLA-9 covering 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.

DISCUSSION

Structural Observations. The unusual combination of aforementioned mechanical performances suggests the formation of superstructure in neat PLA during the layer-multiplying 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 While for the neat PLA system, the same component in each layer may make the layer interfaces hardly be observed due to the molecular diffusion in the melt processing. Herein, the crystalline morphology was first observed through SEM, by selectively etching the amorphous phase in each specimen. In order to carefully


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trace the structural details, the observation was performed in different scales as exhibited in Fig. 5. Even at a low magnification, the transition from isotropic to highly-oriented crystalline structure is clearly evidenced with the increase of LME numbers from zero to nine. Highly-magnified images further suggest the generation of shish-kebab structure (indicated by red arrows) in PLA-9 where plenty of short kebabs decorate on well-aligned shishs, 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 under a magnification of 40,000 are also presented in Fig. S1. More significantly, the shishs 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 only appears in skin layers.37 Besides, unlike those in hybrid systems8, 34, both the shishs and kebabs in present system are composed of the same component. To the authors’ knowledge, such unprecedented crystalline structure has rarely been reported in neat PLA from existing literatures.



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Figure 5. SEM images of the crystalline structure in as-extruded PLA-0, PLA-5 and PLA-9 observed at different magnifications: ×20,000 (above) and ×80,000 (below). Prior to the observation, the amorphous phase has been selectively etched by immersing the as-extruded specimen in a water/methanol solution.

Structural Analysis. As the identification of morphological observation, 2D-SAXS and 2D-WAXD were performed to gain more comprehensive understanding on the unique crystalline structure induced by the layer-multiplying process. Fig. 6(A) exhibits representative 2D-SAXS patterns of PLA-0, PLA-5 and PLA-9. For PLA-0, only a weak scattering ring could be observed due to its relatively low crystallinity. To probe a deeper insight into the molecular state in PLA-0, the experiment was performed at higher temperature. As presented in Fig. S2 (A), a stronger isotropic ring (indicated by white arrows) displaying a long period of 23.9nm


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(Fig. S2 (B)) could be distinctively observed at 120℃ due to 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 symmetrical triangle-like scattering pattern suggests the transition of molecular state from isotropic toward anisotropy with the increase of LME numbers. When 9 LMEs are applied, 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 1D intensity profiles displayed in Fig. 6(B) manifest that only PLA-9 displays a distinct peak around q=0.3nm−1, suggesting a regular aligned lamellar structure with cylindrical symmetry in the shish-kebab structure. The long period (L) was calculated through the Bragg equation (eq 2),

L=2π/q

(2)

The L of PLA-9 (21.3nm) is comparable to those of hybrid systems8, 36, 37, which implies that the well-aligned shishs composed of highly-extended PLA chains can also facilitate the growth of oriented lamellae on their surfaces.



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

The molecular orientation and crystalline structure was further examined using 2D-WAXD. As presented in Fig. 7(A), a halo amorphous diffraction can be observed in the diffraction pattern of PLA-0, because of the generation of very low content of spherulites. For PLA-5, arc-like diffraction pattern detectable at the meridional position indicates that the extensional effect in the layer-multiplying process induces the orientation of molecular orientation. On the other side, the weak diffraction intensity and the absence of diffraction ring for certain lattice lamellae such as (203)


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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 exhibited in Fig. 7(B). Significantly enhanced crystallization and orientation induced by the layer-multiplying process are evidently reflected with a strong arc-like diffraction reflection.

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

In order to probe the configuration of molecular chains in amorphous region influenced by the development of crystalline structure, P-FTIR has been conducted and 3D-field visualization with the change of polarized angles is presented in Fig.



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8(A). Typically, the absorbance bands at 921 and 956cm−1 are associated with the vibration of chemical bonds in α-form crystalline phase and amorphous phase of PLA, respectively.44 For PLA-0, weak absorption at 921cm−1 can be noticed, in contrast to the band at 956cm−1. While in layer-multiplied systems, the former one gradually rises and shows an opposite dependence on polarized angle compared to the latter one. Accordingly, at least two inferences can be drawn from such unique phenomenon: (i) Neither amorphous region nor crystalline region is isotropic. The difference between the maximum and minimum absorbance at the same wavelength can be adopted to represent the degree of orientation quantitatively. The calculated results included in Fig. 8(B) reveals that the molecular chains in crystalline region have a higher orientation level than those in amorphous region. (ii) Opposite fluctuation between the bands at 921 and 956cm−1 suggests that the molecular chains in amorphous region are approximately perpendicular to those in crystalline region. That means the orientation direction of the amorphous chains is parallel to the chain-folded direction in lamellae. Although there are no more evidences at present stage, it can be speculated that the confined spaces generated by the oriented lamellae may compel the alignment of the molecular chains in amorphous region.


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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 956cm−1 are associated with the α-form crystalline phase and amorphous phase of PLA. (B) Calculated orientation functions of crystalline region in PLA-0, PLA-5 and PLA-9.

The perfection of crystalline structure and the confined mobility of amorphous chains were further verified by employing DSC and DMA measurements. Fig. 9(A) presents the heating curves of four kinds of PLA prepared with different number of LMEs. Corresponding crystallinity (Xc) and melting temperature (Tm) are marked beside each curve. As illustrated, extremely low crystallinity (5.4%) is obtained in PLA-0, responsible for weak diffraction pattern shown in 2D-WAXD. By applying more LMEs, the crystallinity is gradually increased and reaches 28.3% in PLA-9, which confirms that iterative extension occurring in LMEs promotes the



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crystallization of PLA. Meanwhile, substantial increment of Tm from 153.8 to 157.3°C also evidences the improved regularity of oriented lamellae as reported in previously reported systems.45-47 Commonly, the confined mobility of amorphous chains is evaluated by measuring the glass transition temperature (Tg).48, 49 Fig. 9(B) exhibits the dependence of loss modulus (E’’) on temperature, where the loss peak reaches the maximum is defined as Tg. For PLA-0, this critical temperature is 61.5°C basically consistent with that of commercial PLA product. Like that happens in Tm, increasing LMEs obtains a higher Tg and approaches to 69.6°C when 9 LMEs are applied. These results suggest that the mobility of the molecular chains in amorphous region tend to be confined accompanied with the perfection of crystalline structure as speculated based on FTIR analysis.

Figure 9. (A) Melting curves of PLA-0, PLA-1, PLA-5 and PLA-9 recorded by DSC instrument with 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 loss modulus recorded by DMA instrument using a stretching mode (1Hz). The peak in each curve corresponds to the glass transition of PLA.


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Mechanisms. Based on aforementioned analysis, structural schematic of the layer-multiplied PLA is proposed in Fig. 10 for comprehensively understanding the origin of the balance between strength and ductility. When 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 molecular chains in each layer were preserved acting as shishs and the flow induced row-nuclei on shish surfaces led to the formation of oriented-kebab lamellae. Besides, due to the iterative layer multiplication effect occurring in LMEs, the shishs 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 Distinct layer delamination displayed in Fig. S1 can be recognized as the evidence of the increased 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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Figure 10. Structural schematic of the multilayer-assembled shish-kebab structure in neat PLA.

PERSPECTIVES

The well-organized structure fabricated in present 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 result in destructive effect on its surface quality and inherent performances. The preliminary measurements performed on PLA-0 and PLA-9 (see Fig. 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, nearly 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 a high resistance to scratch deformation.53 More significantly, unlike that in most multi-component systems, such structure is capable of maintaining a high


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transparency as well as good biodegradability and biocompatibility since no foreign additives are required, which makes the layer-multiplied PLA be of competitive advantages in packaging, biomedical and tissue engineering applications. Additionally, it needs to be emphasized that a similar structure can be obtained in other semi-crystalline polymers after experiencing the layer-multiplying coextrusion (e.g polyethylene, polypropylene, etc.). Accordingly, this work provides a reproducible and efficient strategy to massively produce commercial polymers with outstanding balanced performances.

CONCLUSIONS

In the present work, bio-inspired PLA with multilayer-assembled shish-kebab structure was successfully fabricated through layer-multiplying coextrusion. The intensive extensional flow induced the formation of well-aligned shishs and highly-oriented kebab lamella, while iterative layer multiplication occurring in LMEs made the shishs 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 the crack deflection and termination. Hence, the layer-multiplied material obtained 50% increase in tensile strength, 4-fold improvement in elongation at break, as well as largely enhanced resistance to heat distortion, compared with normal PLA. Unlike that in most multi-component systems, such non-additive material could



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maintain a high transparency as well as good biodegradability and biocompatibility, which exhibited potential applications in packaging, biomedical and tissue engineering fields.

ASSOCIATED CONTENT Supporting Information SEM images of PLA-9, 2D-SAXS patterns and 1-D SAXS intensity profiles of PLA-0 after being heated, side view of the fracture morphology of PLA-0 and PLA-9, and the scratch performances of PLA-0 and PLA-9. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Authors ∗ E-mail: [email protected] (Jiabin Shen). * E-mail: [email protected] (Shaoyun Guo).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China


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(51420105004, 51227802, 51673136, 51421061) and the Program of Introducing Talents of Discipline to Universities (B13040) are gratefully acknowledged.

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For Table of Contents use only

Bio-inspired Polylactide Based on the Multilayer Assembly of Shish-kebab Structure: A Strategy for Achieving Balanced Performances Longfei Yi1, Shanshan Luo1, Jiabin Shen1*, Shaoyun Guo1*, Hung-Jue Sue2 1

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of

Sichuan University, Chengdu 610065, China 2

Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M

University, College Station, TX 77843-3123, USA

Bio-inspired PLA with layer-packed shish-kebab structure was fabricated, providing a non-additive strategy for achieving balanced performances.


Bioinspired Polylactide Based on the Multilayer Assembly of Shish

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