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Sep 23, 2014 - ... In Situ Nanofibrillar Poly(butylene succinate) for Green Packaging ... Department of Chemistry, Stony Brook University, Stony Brook...
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Unprecedented Access to Strong and Ductile Poly(lactic acid) by Introducing In Situ Nanofibrillar Poly(butylene succinate) for Green Packaging Lan Xie,† Huan Xu,† Ben Niu,† Xu Ji,*,†,‡ Jun Chen,† Zhong-Ming Li,*,† Benjamin S. Hsiao,§ and Gan-Ji Zhong*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China ‡ College of Chemical Engineering, Sichuan University, Chengdu, 610065, Sichuan People’s Republic of China § Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States S Supporting Information *

ABSTRACT: The notion of toughening poly(lactic acid) (PLA) by adding flexible biopolymers has generated enormous interest but has yielded few desirable advances, mainly blocked by the sacrifice of strength and stiffness due to uncontrollable phase morphology and poor interfacial interactions. Here the phase control methodology, that is, intense extrusion compounding followed by “slit die extrusion-hot stretchingquenching” technique, was proposed to construct well-aligned, stiff poly(butylene succinate) (PBS) nanofibrils in the PLA matrix for the first time. We show that generating nanosized discrete droplets of PBS phase during extrusion compounding is key to enable the development of in situ nanofibrillar PBS assisted by the shearing/stretching field. The size of PBS nanofibrils strongly dependent on the PBS content, showing an increased average diameter from 83 to 116 and 236 nm for the composites containing 10, 20, and 40 wt % nanofibrils, respectively. More importantly, hybrid shish-kebab superstructure anchoring ordered PLA kebabs were induced by the PBS nanofibrils serving as the central shish, conferring the creation of tenacious interfacial crystalline ligaments. The exceptional combination of strength, modulus, and ductility for the composites loaded 40 wt % PBS nanofibrils were demonstrated, outperforming pure PLA with the increments of 31, 51, and 72% in strength, modulus, and elongation at break (56.4 MPa, 1702 MPa, and 92.4%), respectively. The high strength, modulus, and ductility are unprecedented for PLA and are in great potential need for packaging applications.

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which greatly blocks its range of applications.32 Ductile PLA materials are thus in great demand, especially for packaging beverage, food and drugs and biomedical applications.33 The past decades have witnessed extensive developments in modifying PLA to offer adequate ductility, which is extremely attractive both academically and commercially.34 It is pertinent to point out that the traditional approaches toward modifying PLA are normally characterized by formulating and associating with flexible biopolymers and plasticizers.35,36 It appears from the existing literature that without specifically designed morphology these blending and plasticizing systems, however, are found insufficient to realize the satisfactory combination of strength, stiffness, and ductility.36 This can be exemplified by the largely sacrificed strength and stiffness after plasticization with poly(ethylene glycol)37 or poly(propylene glycol).38 More importantly, the incorporated plasticizers were found to immigrate

INTRODUCTION Polymer science has been one of the first beneficiaries of bioscience and biotechnology under the biorefinery concept, contributing to the revolutionary developments of biodegradable polymers (biopolymers) to catch the interest of both the technical community and the public.1−13 As the front runner in the emerging bioplastics market, poly(lactic acid) (PLA) has long been attractive as a versatile model thanks to the renowned merits of high strength and modulus, coupled with excellent biodegradability, biocompatibility, and renewability.14−18 This property combination predestined PLA to be the appealing candidate for tissue engineering, biomedical, and packaging applications.19−25 However, PLA found limited usages in packaging primarily due to the low heat resistance, poor gas (or water vapor) barrier, and intrinsic brittleness.26−28 The relatively short chains and rigid chain backbone of PLA, in essence, limit most common plasticity mechanisms and therefore prevent large ductile behavior.29−31 This lack of plasticity is often the paramount bottleneck for the use of PLA-based materials, resulting frequently in catastrophic and unpredictable failure, © XXXX American Chemical Society

Received: July 27, 2014 Revised: September 23, 2014

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are built, the achievement of high performance is highly expected, mainly derived from stiff extended nanofibrils and enhanced interfacial interactions.55,56 Here we show the feasibility to create nanofibrillar PBS phase in the PLA matrix, for the first time, by developing a nanosized dispersed PBS phase during the intense melt compounding, followed by the applied shearing/stretching flow through “slit die extrusion−hot stretching−quenching” process. Evolution of the structure and morphology of PBS phase was traced by direct observation, molecular and lamellar analysis and thermal behavior measurements. The nanofibrillar PLA/PBS composites exhibited outstanding mechanical properties, featuring a well-tailored balance by altering the PBS loadings. The morphology control methodology shown here should find uses in the design and processing of low-cost commercial biopolymers for transportation and packaging-related applications.

gradually from the PLA matrix, a disadvantage caused by the cold crystallization of PLA during the long-term usage.39 Furthermore, various flexible biopolymers, such as poly(butylene succinate) (PBS),40 poly[(butylene succinate)-co-adipate] (PBSA),41 and poly(butylene terephthalate),42 have been introduced into PLA with the goal of enhancing the ductility and impact resistance, only to find limited promotion of ductility and impact toughness but greatly reduced strength and stiffness. It arises from, in principle, the excessive phase separation caused by inherent immiscibility between the PLA matrix and the incorporated biopolymers.36 It has been established that the performance of an immiscible polymer blend not only depends on the physical characteristics of its components, but also highly on the phase interface and phase morphology, both of which are closely associated with the viscoelasticity of the polymer components and the flow histories during the manufacturing process.43 Hence, there is a clear scientific imperative requiring the control of phase morphology of the immiscible blends, aiming at the desirable balance of mechanical properties. Very recently, an array of elaborate work has been launched by Ray et al. in an endeavor to tailor the phase morphology and properties of PLA/PBSA blends.36,41,44 It was of great significance to find that the specific interfacial area exposed by the PBSA phase that allowed extensive chain interactions profoundly affected the mechanical performance.41 They further proposed in situ reactive compatibilization by adding triphenylphosphite during the melt-blending to decrease the dispersed-phase size and create fibrillated links.44 Upon the intraphase chain extension in the PLA matrix and interphase compatibilization, a demonstration of largely improved impact strength and ductility was presented.44 The tensile strength and modulus, unfortunately, showed a significant decrease of over 27 and 29% for the blends loaded 30 wt % PBSA compared to those of pure PLA, respectively. It seems still a practical challenge to fabricate strong and ductile PLA introducing great potential in packaging applications. Naturally, it stimulates us to directly control the phase morphology of a dispersed flexible polymer during the practical processing to develop green-packaging-use PLA featuring simultaneously enhanced strength and ductility. Pioneering exploration toward converting polymer blends into in situ composites instead of adding concept has been launched by Fakirov et al., revealing that intense shearing/stretching flow emerged as an especially useful strategy to control the phase morphology.45−50 They have demonstrated the formation of well-aligned micro/nanofibrils by virtue of such an external field, permitting the construction of strong dispersed phase and interfacial interactions. These discoveries actually stretched the endurance boundary of composite materials and pushed the performance limit of the composite structure. Is it possible to extend the concept of developing in situ fibrillar composites to the scope of fully biodegradable PLA blends? The history of developing fully biodegradable, in situ fibrillar composites, however, is poor, and almost none of the obvious opportunities to establish ordered nanofibrils have been examined. PLA/PBS blends have been perceived as a typical system penetrating full biodegradability and have been investigated for more than two decades without exhausting their advantages and challenges, normally accompanied by uncontrollable morphology and undesirable properties. Taking into consideration the enormous interest in this effort, PBS has been chosen to serve as the fibrillar phase in terms of its impressive ductility and toughness, as well as excellent processability.51−54 Once the ordered PBS nanofibrils

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EXPERIMENTAL SECTION

Materials. PLA under a trade name of 4032D comprising ∼2% D-LA was commercially purchased from NatureWorks (U.S.A.), presenting a weight-average molecular weight and number-average molecular weight of 2.23 × 105 and 1.06 × 105 g/mol, respectively. PBS (Bionolle #1001MD) with a number-average molecular weight of ∼6.0 × 104 g/ mol and a dispersive index of 2.33 is a commercial product from SHOWA Highpolymer Co. Ltd. (Japan). It shows a melting point of 114 °C and a melt flow index of 1.5 g/10 min (190 °C/2.16 kg, ASTM D1238). Sample Preparation. To avoid the degradation due to hydrolysis and prevent the formation of voids during processing, PLA and PBS were dried at 100 °C under vacuum overnight before both extrusion and injection molding. Melt blending of PLA and PBS was carried out in a corotating twin screw extruder with a ratio of screw length to its diameter (L/D) of 40. The contents of PBS were set at 10 to 20 and 40 wt %. Temperatures in seven zones were 80, 120, 150, 160, 170, 170, and 165 °C from feed section to die, respectively. Particularly, two crucial extrusion parameters are to achieve nanosized PBS particles: first, a high feeding rate (fixed at ∼100 g/min) was applied to minimize the staying in the extruder, and a high screw speed (200 rpm) was employed to strengthen the mechanical stress and shearing action imposed on the blends and at the same time to reduce the residence time. The extruded compound pellets after drying were prepared into in situ fibrillar composites by a “slit die extrusion−hot stretching−quenching” process in a single-screw extruder at the processing temperature of PLA. And the temperature profiles were set at 130, 170, 170, 170, and 165 °C from hopper to nozzle, respectively. The slit die has a width of 20 mm and a height of 2 mm. The extrudate was hot stretched by a take-up device with two pinching rolls to generate in situ fibrils for PBS phase, wherein the rolling speed was fixed at 50 rpm and the stretch ratio was calculated to be approximately 6.7. The roll temperature was controlled at 30 °C by the flux of cooling water. After stretching, the extrudate was immediately quenched in the cold water bath (20 °C) to freeze the formed fibrils in the composites, and finally, the ribbons of in situ fibrillar composites with a thickness of 0.3 mm were obtained. It is worth noting that the control samples of neat PLA subjected to the same thermal conditions were also prepared. Detailed information on the “slit die extrusion−hot stretching−quenching” process is available in our previous work.57 Scanning Electronic Microscopy (SEM). SEM was employed to observe the phase morphology, fibrillar structure, and crystalline morphology. Cryogenic fracture and selective etching were applied to obtain the specific surfaces. For the cryogenic fracture, the extrudate blend pellets or composite sheets were placed in liquid nitrogen for 0.5 h, finally the samples were cryogenically fractured along or perpendicular to the shearing/stretching direction. The smooth fractured surfaces then can be taken for direct SEM observation or the further etching process. We herein applied two methods to selectively etch the specific phase: (1) PLA matrix was etched by immersing in dichloromethane (CH2Cl2) at 5 °C for 40 s, ensuring the dissolution of PLA but the preservation of PBS phase; (2) amorphous PLA was etched by a water−methanol (1:2 by volume) solution B

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Figure 1. SEM images of cryofracture surfaces of (a1) PLA/PBS (90/10), (b1) PLA/PBS (80/20), and (c1) PLA/PBS (60/40) blend samples. (a2), (b2), and (c2) present the dispersed-phase morphology of PBS after etching the PLA matrix of (a1), (b1), and (c1), which produce the quantitative analyses in terms of the distribution of PBS phase size as shown in (a3), (b3), and (c3), respectively. The scale bar represents 5 μm for all images, and the average diameter (D) is marked in the right up corner of (a3), (b3), and (c3). containing 0.025 mol/L of sodium hydroxide for 14 h at 15 °C. Note that all etched surfaces were cleaned by using distilled water and ultrasonication prior to SEM observation. A field-emission SEM (Inspect F, FEI, Finland) was utilized to explore the phase and crystalline morphology of the blend and composite samples sputtercoated with gold, while the accelerated voltage was held at 5 kV. Rheological Measurements. A rheometer (Haake RS600, Thermo Electron Co., U.S.A.) was performed to determine the rheological behavior of extruded PLA/PBS blends, taking advantage of a parallel-plate geometry with a diameter of 20 mm. Prior to rheological measurements, the PLA/PBS extrudates were compression molded (175 °C, 10 MPa) into disc-like sheets with a diameter of 20 mm and a thickness of 5 mm. The small amplitude oscillatory shear (SAOS) was applied in all dynamic measurements. The sheet samples were melted at 190 °C for 3 min to eliminate the residual thermal histories, and the dynamic frequency sweep was then carried out immediately. A common strain level fixed at 10% was predetermined by the dynamic strain sweep. Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). 2D-SAXS measurements performed at the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) were applied to examine the lamellar structure of PLA and PBS in fibrillar composite sheets. The 2D-SAXS images were collected with an X-ray CCD detector (Model Mar165, a resolution of 2048 × 2048 pixels). The monochromated X-ray beam operated at a wavelength of 0.124 nm with a beam size of 80 × 80 μm2 (length × width), and the sample-to-detector distance was fixed at 1900 mm. The radically integrated intensities I(q) (q = 4π sin θ/λ) are obtained for integration in

the azimuthal angular range of a whole circle, where 2θ stands for the scattering angle and λ represents the X-ray wavelength. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). 2D-WAXD determination was employed to evaluate molecular orientation and crystalline morphology of fibrillar composites at the beamline BL15U1 of SSRF, Shanghai. The monochromated X-ray beam with a wavelength of 0.124 nm was focused to an area of 3 × 2.7 μm2 (length × width), and the sample-to-detector distance was set as 185 mm. Then the 2D-WAXD images were collected with an X-ray CCD detector (Model SX165, Rayonix Co. Ltd., U.S.A.). Additionally, the WAXD intensity profiles for each 2θ were obtained by integration in the azimuthal angular range of a whole circle (0−360°) from the sample patterns employing the Fit2D package, while background scattering was subtracted from the sample patterns. Differential Scanning Calorimeter (DSC). A DSC Q200 (TA Instruments, U.S.A.) was used to probe the thermal features such as glass transition, melting and crystallization behaviors for pure PLA and PLA/ PBS fibrillar composites. Samples of the neat PLA and the composites (around 5−6 mg), obtained from the stretched sheets, were heated from 40 to 180 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Mechanical Property Testing. According to ASTM standard D638, the tensile properties were measured at room temperature on an Instron universal test instrument (Model 5576, Instron Instruments, U.S.A.) with a crosshead speed of 20 mm/min and a gauge length of 20 mm. A minimum of 6 bar for each sample were tested at the same conditions, and the average values were presented with standard deviation. C

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Figure 2. Viscoelastic behaviors determined at 190 °C for PLA/PBS blends. Frequency dependency of (A) complex viscocity (η), (B) dynamic storage modulus (G′), and (C) dynamic loss modulus (G″) for (a) pure PLA, (b) PLA/PBS (90/10), (c) PLA/PBS (80/20), and (d) PLA/PBS (60/40) blend samples.

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RESULTS AND DISCUSSION The phase morphology of the extruded blends is the key affecting the formation and morphology of in situ PBS nanofibrils. As a thermodynamically imcompatible system, the extruded PLA/ PBS blends normally present a typical two-phase structure incorporated with microsized PBS particles.35 The large size of dispersed phase (i.e., inadequate interfacial interactions with the matrix) may be the main challenge for creating in situ micro/ nanofibrils during the subsequent fibrillation processing. It is therefore of great importance to optimize the processing conditions and therewith phase morphology of the PLA/PBS blends. Figure 1 reveals the phase morphology by direct SEM observation and quantitative size analyses. The classic “sea− island” structure and poor interactions, in which discrete droplets of PBS phase are dispersed in the PLA matrix without obvious interfacial bonding, are evidently presented for the PLA/PBS blends (Figure 1a1−a3). It essentially arises from the thermodynamic incompatibility40 and high interfacial tension (∼3.7 mN/m)58 between PLA and PBS. It is of great interest that these PBS particles show nanosized diameter which is several orders of magnitude lower than the common PLA/PBS systems.35 Figure 1a2−c2 clearly demonstrates the presence of nanosized PBS droplets by etching the PLA matrix, yielding the quantitative analyses regarding the distribution of the droplet size (Figure 1a3−c3). The average diameter (D) dramatically increases from 168 nm for PLA/PBS (90/10) blend to around a double value for the blends containing 20 and 40 wt % PBS (320 and 355 nm), accompanied by the significantly widened diameter distribution (Figure 1a3−c3). The enhanced agglomeration possibility of PBS melt droplets should be responsible for the rise of particle size and distribution index, resulting from the increased concentrations of PBS melt droplets. We establish the first instance to directly generate nanosized PBS phase in extruded PLA blends. In the immiscible blends, the

size of dispersed phase is generally characterized by the micrometer order ranging from several to hundreds of micrometer, either for the common petroleum-based blends or recently developed fully biodegradable blends. Recent attempts to prepare blends starting from PLA and PBS resulted in true microscale phase size. As an example, Yokahara and Yamaguchi59 found the spherical PBS droplets gave an average diameter of 1.04 and 1.96 μm for the PLA/PBS (90/10) and PLA/PBS (80/ 20) blends, respectively. Our case shows the enormous promise in tailoring the phase morphology of immiscible blends based on common practical processing, which signifies great importance but never attracts deserved attention, especially for the biopolymer blends. By coupling various externally applied fields, for example, temperature and stress fields, many morphological features, such as crystalline morphology, orientation, and phase separation behavior, can be prominently altered, resulting in significant variations in the ultimate hierarchical structure. The dispersed-phase morphology of immiscible blends in the shear flow is primarily determined by the dynamic deformation, melt fracture, and phase agglomeration kinetics. From the external field-dominated morphology point of view, two critical processing conditions were customized during the extrusion to achieve the nanosized PBS phase: (1) we applied a very high screw speed (200 rpm) to realize the intensive shear rate, compelling the large deformation and fracture of PBS phase; (2) the agglomeration of shear-split PBS phase was minimized by employing a rapid feeding (∼100 g/min) to reduce the staying in the extruder. Our hypothesis is sufficiently supported by the morphological observation (Figure 1), which is expected to motivate further efforts in tailoring the phase morphology by simply taking advantage of designing the processing features. Viscoelastic properties of the PLA/PBS blends are one of the most important approaches to understand their phase behaviors and melt features, which may provide useful guidance during the “slit die extrusion−hot stretching−quenching” processing. On D

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Figure 3. Morphological observation of in situ fibrils in the nanofibrillar composites. SEM images of cryofracture surfaces of (a1) PLA/PBS (90/10), (b1) PLA/PBS (80/20), and (c1) PLA/PBS (60/40) composite samples. (a2), (b2), and (c2) present the fibrillar morphology of PBS after etching the PLA matrix of (a1), (b1), and (c1), which produce the quantitative analyses in terms of the distribution of PBS nanofibrils as shown in (a3), (b3), and (c3), respectively. The average diameter (D) is marked in the right up corner of (a3), (b3), and (c3).

interfacial tension and highest interfacial energy between the discrete PBS phase and PLA matrix, probably arising from that each phase can be fully interconnected through the extremely high specific surface area. The blend samples penetrated higher PBS contents, on the other hand, show moderately lower viscosity and modulus. This can be explained by the existence of interfacial slip and increased incompatibility, and reduced interphase interactions as a result of the far higher average diameter and wider diameter distribution of PBS droplets. Obviously the structural features are keys to exciting the rheological response and truly different processing characters. Figure 3 evidence the transition of nanosized PBS particles to nanofibrils after the application of in situ fibrillation technique. Fully extended, compact nanofibrils embedded in the matrix are clearly observed for all in situ fibrillar composites, showing an extremely high aspect ratio (over 50) and orientation degree along the stretching direction. After selective etching of the PLA phase, the exposed nanofibrils are clearly observed in Figure 3a2−c2, indicating the fact that the nanofibrils originate from the PBS phase. Of particular interest are the “kebab-like” filaments strung by the nanofibrils, especially for PLA/PBS (80/20) as shown in Figure 3b2, which may stem from the crystalline entities of PLA matrix that show stronger ability to resist the etching agent.57 The formation of interfacial ligaments offers great opportunity in enhancing interactions between the PBS

the steady rheological responses shown in Figure 2, it is seen that the blends generally present similar viscoelastic behaviors with those of pure PLA, displaying gradually decreased viscosity (Figure 2A), rapid growing storage modulus (Figure 2B), and loss modulus (Figure 2C). At the high-frequency zone (above 1 Hz), the shear-thinning behaviors render the almost same viscoelasticity shared by both pure PLA and its blends. Nevertheless, the low-frequency (below 1 Hz) viscosity, storage modulus and loss modulus show a remarkable increase for the PLA/PBS blends compared to those of pure PLA, especially for the loss modulus (Figure 2C). It principally lies in the shape relaxation of the discrete phase in the matrix, that is, during oscillatory shear flow, the blends present a much longer relaxation time than that of the component polymers due to the increased total area of the interface and interfacial energy.35The highest viscosity and modulus are observed in the PLA/PBS (90/10) sample, in sharp contrast to the existing results which demonstrated the highest values were shown in the half−half blends due to the additional elastic contribution of the cocontinuous phase structure at the phase inversion point.35 It can be explained by the unique dispersed-phase morphology of our case. As illustrated in Figure 1, the PBS droplets in the PLA/ PBS (90/10) blend sample are characterized by the lowest average diameter and extremely narrow diameter distribution. The peculiar structural features therefore create the strongest E

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However, we provide first observations for the formation of unique PBS nanofibrils. Generally, some specific blend systems constituting a matrix component with a low processing temperature and a fibrous component offering high strength and modulus are employed to fabricate in situ fibrillar composites.61,66,67 The present attempts fail to provide an available inroad to introduce nanofibrillar phase into the strong but brittle matrix like our proposal. The key element in our methodology is the precise control of phase morphology in the extrusion compounding, rendering the generation of rich PBS droplets in the nanoscale. By virtue of the intensive shear flow field created by the slit die and the strong stretching field during the hot stretching, these peculiar nanosized PBS melt droplets are subjected to fierce deformation and ultimately develop into long nanofibrils by connecting each other. Meanwhile, the nanofibrous structure can be well preserved during the immediate quenching process. One may find it instructive to create well-organized fibrous structure for the flexible phase with a low melting point, providing new understanding on the mechanism of the fibrous evolution and motivating further efforts in widening the available blend systems for the fabrication of fibrillar composites. Our first establishment of developing nanofibrous structure in the PLA/PBS blends may help shape a promising approach to tailoring the phase morphology for PLA blends. The proposed methodology for phase morphology controlling should be broadly applicable to the industrial manufacturing of various biomass-based blends, because uncontrollable structuring of incorporated phase and poor interfacial bonding are commonly encountered problems in high-performance blend preparation. Figure 3a2−c2 reminds us of the probable decoration of crystalline PLA filaments at the surfaces of PBS nanofibrils, and we attempt to reveal the origin of the interfacial filaments. The matrix crystalline morphology is tracked by etching the amorphous PLA phase as manifested in Figure 5. What is immediately noticeable from Figure 5 is that, PBS nanofibrils are wrapped by regularly aligned lamellae perpendicular to the nanofibril axis, serving as the central shish to elicit the ordered arrangement of brush-like PLA crystal lamellae (so-called kebabs). Based on the structural features, the unique superstructure can be termed hybrid shish-kebab consisting of the central polymer hybrid shish and ordered lamellar arrangement. In an earlier case involving the transcrystalline morphology of in situ microfibrillar composites of poly(ethylene terephthalate)/ polypropylene, we evidently revealed the similar hybrid shishkebab superstructure in the atomic force microscopic observation.57 Herein, strictly oriented PLA kebabs are periodically attached to PBS nanofibrils, showing a gradually increased diameter with the addition of PBS (1.6, 2.1, and 3.2 μm for the composites containing 10, 20, and 40 wt % PBS nanofibrils, respectively). The increment of kebab diameter is probably a response to the size dependence of nanofibrils and the introduced flexible PBS chains: (1) more stretched PLA chains are absorbed/adhered to involve in the nanofibril entities due to the increased nanofibril size, holding the chance to trigger the subsequent lamellar growth of adjacent PLA chains; (2) the incorporation of flexible PBS chains allows the enhancement of chain mobility PLA and thus the crystallization kinetics.59 The hybrid shish-kebabs can serve as a favorable self-reinforced element and desirably bridge the nanofibrils and PLA matrix, both of which are predestined to make significant contributions to the mechanical and barrier performances in favor of packaging applications.68,69

nanofibrils and PLA matrix with respect to the interfacial enhancement mechanism that arises from the interfacial crystallization.34,57,60,61 Figure 3a3−c3 shows the strong dependence of nanofibril diameter on the PBS content despite the diameter distribution is slightly varied. The average diameter rapidly grows with increasing PBS addition, rising from the minimum value of 83 nm for PLA/PBS (90/10) to 116 and 236 nm for the nanofibrillar composites penetrated 20 and 40 wt % PBS nanofibrils, respectively. It may be a result of the enhanced collision and agglomeration possibility in the existence of higher concentrations of PBS nanofibrils. SEM observations of the section surfaces of nanofibrillar composites after the selective etching of PLA matrix were further performed to gain more understanding on the fibrillar morphology (Figure 4). Without the support of interlinked

Figure 4. SEM micrographs of section cryofracture surfaces of in situ nanofibrillar composites after etching the PLA matrix for (a1) PLA/PBS (90/10), (b1) PLA/PBS (80/20), and (c1) PLA/PBS (60/40) composite samples, and (a2), (b2), and (c2) present their nanofibrillar morphology in higher magnitude, respectively.

PLA backbone, plenty of nanofibrils tend to lean on the surfaces. Specifically, Figure 4c1,c2 illustrates a large quantity of nanofibril bundles consisting of tens to hundreds of nanofibrils, displaying the unique appearance of seaweed rooted in the sea. The interesting structure probably stems from the integration of separate PBS nanofibrils aroused by the extremely high surface energy at the absence of matrix backbone. The agglomeration of PBS nanofibrils during the in situ fibrillar process, nevertheless, is largely restricted during the quenching stage, permitting the formation of separate nanofibrils. It is fortunate that in situ nanofibrillation has been realized in various blend systems or single-polymer composites.62−65 F

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Figure 5. SEM micrographs showing crystalline filaments of PLA at PBS nanofibrils for (a) PLA/PBS (90/10), (b) PLA/PBS (80/20), and (c) PLA/PBS (60/40) composite samples. Cryofracture surfaces of in situ nanofibrillar composites after etching the amorphous PLA phase are employed for SEM observation. The existence of 40 wt % PBS makes it difficult to clearly expose the lamellar structure in (c) during the same etching processing. The big arrows indicate the stretching direction, while the small brown and yellow arrows refer to the nanofibrils serving as hybrid shish and oriented PLA lamellae, respectively.

Figure 6. (a1−c1) Representative 2D-SAXS patterns and (a2−c2) 1DSAXS intensity profiles of in situ nanofibrillar composites. The peak position (q*) and corresponding long spacing (L) are marked on the intensity curves. (a) PLA/PBS (90/10), (b) PLA/PBS (80/20), and (c) PLA/PBS (60/40).

As a complement for the SEM observation, two-dimensional small-angle X-ray scattering (2D-SAXS) was performed to offer quantitative insights into the hybrid shish-kebabs induced by the PBS nanofibrils, as demonstrated in Figure 6. For PLA/PBS (90/ 10) nanofibrillar composite, Figure 6a1 apparently displays a pair of asymmetrical triangular streaks in the equatorial direction and a pair of symmetrical bulb-shape lobes in the meridional direction, showing high similarity with those of PLA shishkebabs and PLA hybrid shish-kebabs induced by nanowhiskers or natural nanofibers.60,68 It therefore evidence the presence of stretch-aligned shish and oriented lamellae decorated at the shish. Coupled with the crystalline morphology observation (Figure 5a), it can be fairly concluded that the PBS nanofibrils serving as the hybrid shish string the PLA kebabs. Figure 6a2 illustrates the extracted 1D-SAXS intensity curve for PLA/PBS (90/10), in which the maxima (q*) at q = 0.24 nm−1 is observed. The long period (L) regarding the lamellar structure is calculated to be 26.2 nm, using the Bragg equation, L = 2π/q*. This L value is moderately higher than that of pure PLA shish-kebabs (22.8 nm),33 an indicative of the larger lamellar spacing between adjacent regular lamellae. With regard to the nucleation ability, PBS nanofibrils cannot match the extended nanostructured PLA shish in providing favorable geometrical lattice matching and available nucleating sites to absorb the folded-chain lamellae. It should give rise to the increment of L. The similar observations were also observed in shear-induced hybrid shish-kebabs of PLA60 and isotactic polypropylene.70 With the increase of PBS content, the two-point SAXS signal representing the oriented PBS lamellae is observed as suggested in Figure 6b,c (note that the PBS phase cannot be etched during the remove of amorphous PLA). Figure 6b1 shows a pair of symmetrical bulb-shape lobes in the meridional direction with strong scattering intensity, accompanied by the weak scattering presenting a pair of arc-like lobes probably assigned to the

formation of well-aligned PBS lamellae parallel to the PLA kebabs. The incorporation of ordered PBS lamellae pushes the maxima q* higher to 0.30 nm−1, because PBS crystals arouse an intrinsic peak position at around q = 0.75 nm−1.71 With the coexistence of PLA kebabs and PBS lamellae, the long spacing falls to 20.9 nm (Figure 6b2). Figure 6c1 illustrates the weak arclike scattering patterns exclusively for oriented PBS lamellae along the stretching direction (in the extended nanofibrils or epitaxially grown lamellae), and the central homogeneous scattering patterns are probably attributed to the PLA crystals confined by the PBS nanofibrils.60 In contrast to the poor crystallization kinetics of PLA, PBS is characterized by high molecular mobility and favorable crystallization ability, resulting in the scattering patterns dominated by the PBS lamellae.40 The q* mainly associated with the nanofibrillar structure and crystal structure of PBS shifts enormously to q = 0.75 nm−1 for PLA/ PBS (60/40) fibrillar composite, producing the lowest L value at 8.4 nm (Figure 6c2). Molecular orientation and crystalline morphology of in situ nanofibrillar composites are followed utilizing two-dimensional wide-angle X-ray scattering (2D-WAXD) as demonstrated in Figure 7. Arc-like diffraction patterns indicating highly oriented lattice plane (200)/(100) of α-form PLA and lattice plane (020) of α-form PBS are traced for both PLA/PBS (90/10) and PLA/ PBS (80/20) samples (Figure 7A). Moreover, the PLA/PBS (80/20) one is featured slightly stronger diffraction intensity compared to that contains lower PBS contents, resulting from the increased amount of PBS lamellae (intensity curves a and b of Figure 7B). With the incorporation of 40 wt % PBS nanofibrils, well-marked diffraction patterns demonstrating oriented lattice planes (020) and (110) of α-form PBS are depicted in Figure 7A. The diffraction intensity of PLA, however, becomes so weak that G

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Figure 8. Heating traces of the DSC program of in situ nanofibrillar PLA/PBS composites. The glass transition temperature (Tg), the cold crystallization temperature (Tcc), and the melting points (Tm1, Tm2) are marked on the curves. The enthalpies of crystallization and melting are marked below Tcc, Tm1, and Tm2. (a) Pure PLA, (b) PLA/PBS (90/10), (c) PLA/PBS (80/20), and (d) PLA/PBS (60/40). Figure 7. (A) Representative 2D-WAXD patterns and (B) 1D-WAXD intensity profiles of in situ nanofibrillar composites, the diffraction peaks and their assignment to the specific lattice planes are marked (black dashed lines for the diffraction peaks of PLA and red ones for PBS). (a) PLA/PBS (90/10), (b) PLA/PBS (80/20), and (c) PLA/PBS (60/40).

dispersed PBS domains are prone to provide a nucleating center and therewith permit the advanced crystallization for PLA.40 It probably gives rise to the formation of smaller crystallites of PLA with defect-ridden lamellae and less-ordered structures, resulting in the far lower Tm2 compared to that of pure PLA.35 The introduction of highly flexible biopolymers, such as PBS, PBSA, and poly(3-hydroxybutyrate-co-hydroxyvalerate), to fabricate ductile PLA blends has been the subject of much recent interest. As yet, it is unfortunate to observe the sacrifice of strength and stiffness.40,41,44 In clear contrast, the creation of PBS nanofibrils enormously benefits the mechanical performances in terms of strength, modulus, and ductility (Figure 9). Figure 9A describes representative stress−strain curves for pure PLA and PLA/PBS nanofibrillar composites. We see from Figure 9A that the stress−strain curves of PLA containing PBS nanofibrils evidently tower over that of pure PLA. Figure 9B gathers the detailed tensile properties regarding yield strength, Young’s modulus, and elongation at break. Compared to the initial values of 43.1 and 1130 MPa of pure PLA, the composites loaded 10 and 20 wt % PBS nanofibrils obtains an unexpected promotion of yield strength and Young’s modulus (58.7 and 2533 MPa, 78.7 and 3171 MPa, respectively), although the ductility drops to some extent. Moreover, all the nanofibrillar composites present higher yield strength, ultimate strength, and Young’s modulus compared to those of pure PBS (17.7 MPa, 33.8 and 340.5 MPa). The unusual combination of strength and stiffness, in principal, lies in the excellent performance of stiff nanofibrils with sufficiently extended chains and substantially strengthened interfacial interactions due to the nanofibril-induced hybrid shish-kebabs. Desirable promotion of ductility is observed for the composite constituting 40 wt % PBS nanofibrils, achieving a nearly 2-fold elongation at break (92.4%) compared to that of pure PLA (53.6%). Notably, its strength and stiffness (56.4 and 1702 MPa) are favorably far above those of pure PLA although 40 wt % PBS is added. It is clearly seen that, the creation of stiff nanofibrils desirably permits the effective transfer of applied stress and external deformation through the greatly enhanced interfacial interactions.

no evident reflection is traced (curve (c) of Figure 7B). The synchrotron X-ray determination (2D-SAXS/WAXD) of nanofibrillar composites not only confirms the generation of oriented lamellae at nanofibril surfaces as suggested in SEM observation but also yields the well-coincident structural evolution with nanofibril content that PLA lamellae are likely to hide behind compact PBS nanofibrils and oriented PBS lamellae. Recent findings concerning the role of PBS particles in accelerating the crystallization ability of PLA have been discovered.35,40,59 In our case, enormous curiousness is elicited: how will the presence of PBS nanofibrils alter the thermal behavior of PLA? Differential calorimeter scanning (DSC) was carried out to offer the direct insights. Figure 8 plots the DSC heating thermograms for pure PLA and PLA/PBS fibrillar composites, in which the enthalpies of crystallization and melting are marked. Three main transitions are successively displayed on the heating curve of pure PLA (Figure 8a): a glass transition at 58.3 °C (Tg), a cold crystallization peak at 102.0 °C (Tcc), and finally, a melting endotherm with a maximum rate at 168.5 °C (Tm). The thermal behaviors of PLA are profoundly altered with the introduction of PBS nanofibrils but showing limited dependence on the composition (curves (b−d) of Figure 8): (1) a moderate decrease of Tg to around 56.7 °C; (2) a broad exothermic peak near 89.5 °C corresponding to the cold crystallization of both PLA and PBS; (3) the first endothermic peak adjacent to 111.0 °C associated with the melting of PBS (Tm1), presenting increased areas with the increase of PBS content; and (4) a large endothermic peak around 165.5 °C caused by the melting of PLA (Tm2), which on the contrary show the gradually decreased peak areas. The decline of Tg, Tcc, and Tm2 for PLA/PBS composites signifies that the existence of either glassy or molten PBS chains allows the sufficiently accelerated molecular mobility for PLA. Furthermore, the surfaces of the H

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of sheath, that may construct crack bridging with the dissipation and absorption of much energy.68 The combination of multiple reinforcement and strong interfacial bonding desirably permits the effective transfer of applied stress and impact load from PLA matrix to PBS nanofibrils, resulting in the plenty of plastic deformation and energy dissipation. In this perspective, we provide the organisms for the PLA/PBS nanofibrillar composite system with peculiar structural configurations that exhibit impressive combinations of mechanical response. The exceptional combination of strength, stiffness, and ductility suggests interesting generalizations concerning the role of nanofibrillar structuring in creating evolutionary innovations and adaptive radiation for the fabrication of highperformance PLA-based packaging materials. Of paramount significance is the optimized phase control method (i.e., in situ nanofibrillar structuring) that will inspire and provide design principles for the rational design and reproducible construction of biomass-based nanofibrillar composites with nanostructured phase morphology for multifunctional integration. We anticipate the proposed structuring methodology for the structural design of blend systems should be broadly applicable to the industrial manufacturing of various immiscible biopolymer blends, because uncontrollable optimization of phase structure and poor interfacial interactions are commonly encountered problems in high-performance blends preparation. Moreover, PBS has a lower gas/water permeability compared to that of PLA, allowing the enhancement in the gas barrier performance for PLA/PBS blend systems, which has not been examined up to date.72,73 Therefore, it is peculiarly instructive to study the gas barrier properties of the nanofibrillar composites with the ultimate goal of broadening the packaging applications of PLA,74 currently under the ongoing investigation in our group.

Figure 9. Performance evaluation to demonstrate the exceptional mechanical properties of in situ nanofibrillar PLA/PBS composites. (A) Typical strain−stress curves of (a) pure PLA, (b) PLA/PBS (90/10), (c) PLA/PBS (80/20), and (d) PLA/PBS (60/40). (B) Detailed mechanical results regarding yield strength, Young’s modulus, and elongation at break.

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CONCLUSIONS The methodology, constituting the precise control of dispersedparticle morphology for PBS phase during extrusion compounding and the shearing and stretching of PBS melt droplets by the “slit die extrusion−hot stretching−quenching” technique, was developed to construct well-aligned, stiff PBS nanofibrils in the PLA matrix. It was revealed that generating nanosized PBS droplets during the extrusion was key to enable the development of PBS nanofibrils. The size of PBS particles gradually increased as rising the PBS concentration, giving rise to the enlarged diameter and size distribution index of PBS nanofibrils. It was of great interest to observe the formation of well-organized PLA kebabs orderly strung by extended PBS nanofibrils, displaying the appearance of hybrid shish-kebabs. The formation of crystalline filaments, that is, nanofibril-induced hybrid shishkebabs, conferred largely strengthened interfacial bonding, which was further enhanced by improved interfacial interactions between the PLA matrix and PBS nanofibrils featured extremely high surface areas. The unprecedented control of phase morphology and interfacial ligaments, consequently, permitted the unexpected combination of far higher strength, modulus and ductility (56.4 MPa, 1702 MPa, and 92.4%) for the composites loaded 40 wt % PBS nanofibrils compared to those of pure PLA. Moreover, the strength, stiffness, and ductility can be sufficiently tailored in a wide range by adjusting PBS contents, representing enormous promise in achieving specific properties and thus expanding the application for PLA. Of particular interest are the common polymer processing techniques invovled in the nanofibrillar composites preparation, desirably allowing the industrially feasible fabrication in the future.

We further attempt to diagram the unique crystalline entities existing in the PLA/PBS (60/40) nanofibrillar composites that give rise to the most unexpected promotion of strength and ductility. Figure 10 depicts the major structural features for the in situ nanofibrillar composites, leading to the functional definition of two unique superstructures: plenty of in situ PBS nanofibrils containing stretched chains and partially folded lamellae, and highly oriented PLA kebabs strung by nanofibrils that act as the hybrid shish. With respect to the function role, the well aligned PLA kebabs attached onto the nanofibrils render a significant self-reinforcement of the PLA matrix, and create strong interfacial ligaments for the immiscible blends.60 The preferred generation of strong interfacial bonding is of crucial importance, bridging the stress transfer from the PLA matrix to the PBS nanofibrils. The PBS nanofibrils possess the unique combination of strength and tenacity: (1) high strength and stiffness attributed to the sufficiently stretched chains and orderly folded lamellae; (2) they are also resilient due to the intrinsic flexibility of amorphous PBS chains. When encountered, the external stress deformation, the tight nanofibrils, and PLA kebabs show strong retardation of crack propagation, and the applied stress is prone to traverse along the length of them quite easily rather than the conventional stress concentration, leading to the unusual observation of increased strength and modulus with the existence of 40 wt % PBS phase (Figure 9B). This mechanism can be substantially assisted by the interfacial superstructures, like a kind I

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Figure 10. Schematic diagram demonstrating the structural features in the PLA/PBS (60/40) nanofibrillar composites that are responsible for the exceptionally high strength and ductility. Extended chains and ordered lamellae of PBS existing in the nanofibrils render high stiffness and strength for the flexible phase, while highly oriented PLA kebabs strung by the PBS nanofibrils serve as the self-reinforcing elements and strong ligaments. The wellaligned PBS nanofibrils consist of flexible amorphous chains and strong oriented lamellae. The PBS nanofibrils, which present large surface energy, may provide pinning points to the surrounding PLA row-nuclei. Such anchoring interactions can contribute to the retention of the molecular stretch and orientation after flow, conferring the formation of PLA row-nuclei attached onto the nanofibrils.60

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ASSOCIATED CONTENT

2014T70868). We acknowledge the National Synchrotron Radiation Laboratory, Shanghai, China, for synchrotron SAXS and WAXD measurements.

S Supporting Information *

Rheological behavior, thermal behavior, and tensile property of pure PBS are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-28-8540-6866. Fax: +86-28-8540-6866. E-mail: [email protected]. *Phone: +86-28-8540-6866. Fax: +86-28-8540-6866. E-mail: [email protected]. *Phone: +86-28-8540-0211. Fax: +86-28-8540-6866. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank the financial support from the National Natural Science Foundation of China (Grant Nos. 51120135002, 51273131, 21276168, and 51121001), the Innovation Team Program of Science and Technology Department of Sichuan Province (Grant No. 2014TD0002), and the Doctoral Program of the Ministry of Education of China (Grant Nos. 20130181130012 and 20120181120101), and Project funded by China Postdoctoral Science Foundation (Grant No. J

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