Super-Robust Polylactide Barrier Films by Building Densely Oriented

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Super-Robust Polylactide Barrier Films by Building Densely Oriented Lamellae Incorporated with Ductile in Situ Nanofibrils of Poly(butylene adipate-co-terephthalate) Sheng-Yang Zhou,† Hua-Dong Huang,† Xu Ji,‡ Ding-Xiang Yan,† Gan-Ji Zhong,*,† Benjamin S. Hsiao,§ and Zhong-Ming Li*,† †

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

ABSTRACT: Remarkable combination of excellent gas barrier performance, high strength, and toughness was realized in polylactide (PLA) composite films by constructing the supernetworks of oriented and pyknotic crystals with the assistance of ductile in situ nanofibrils of poly(butylene adipate-co-terephthalate) (PBAT). On the basis that the permeation of gas molecules through polymer materials with anisotropic structure would be more frustrated, we believe that oriented crystalline textures cooperating with inerratic amorphism can be favorable for the enhancement of gas barrier property. By taking full advantage of intensively elongational flow field, the dispersed phase of PBAT in situ forms into nanofibrils, and simultaneously sufficient row-nuclei for PLA are induced. After appropriate thermal treatment with the acceleration effect of PBAT on PLA crystallization, oriented lamellae of PLA tend to be more perfect in a preferential direction and constitute into a kind of network interconnecting with each other. At the same time, the molecular chains between lamellae tend to be more extended. This unique structure manifests superior ability in ameliorating the performance of PLA film. The oxygen permeability coefficient can be achieved as low as 2 × 10−15 cm3 cm cm−2 s−1 Pa−1, combining with the high strength, modulus, and ductility (104.5 MPa, 3484 MPa, and 110.6%, respectively). The methodology proposed in this work presents an industrially scalable processing method to fabricate super-robust PLA barrier films. It would indeed push the usability of biopolymers forward, and certainly prompt wider application of biodegradable polymers in the fields of environmental protection such as food packaging, medical packaging, and biodegradable mulch. KEYWORDS: polylactide films, gas barrier property, elongational flow filed, in situ nanofibrils, network of oriented lamellae

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properties.22−24 Simultaneously, we must envisage the reality that the existence of complex processing conditions and undesirable dispersion of fillers in polymeric matrix indeed imposes prodigious restrictions on its commercialization. Moreover, its limited ability to ameliorate the ductility is usually manifested in the literature.25,26 Hence, it is still a great challenge to develop a strategy that could achieve robust properties with excellent barrier properties, effective reinforcement, and ductility, as well as the satisfaction of being environmentally friendly. Simultaneously it could enable large scale production with industrially feasible processing.

INTRODUCTION

As an unparalleled and most promising frontrunner among the territory of emerging biodegradable polymers, polylactide (PLA) has been extensively investigated ranging from controlling microstructure to modifying bulk performance,1−4 leading to enhanced mechanical properties and new functions combined with its inherent advantages of biodegradability,5−10 relatively low cost, and good processability.11−17 Unfortunately, PLA still fails to provide sufficient resistance to gas, which hinders its further application for packaging.18,19 The improvement of gas barrier capability for PLA has stimulated numerous endeavors so as to make it competitive with petroleum-based barrier polymers.20,21 Thereinto the compounding of nonpermeable barriers such as nanoclay and graphene oxide undeniably has made headway in the enhancement of barrier © 2016 American Chemical Society

Received: January 13, 2016 Accepted: March 9, 2016 Published: March 9, 2016 8096

DOI: 10.1021/acsami.6b00451 ACS Appl. Mater. Interfaces 2016, 8, 8096−8109

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crystallization of PLA with perfect lamellar structure and to achieve promising combination of barrier and mechanical properties. Moreover, in our previous works, we demonstrated that extensional flow could effectively tailor dispersed phase to in situ nanofibrils, leading to a promising combination of strength, modulus, and ductility in PLA/poly(butylene succinate) (PBS) system.54 In this work, because of the poor barrier property of PBAT and scarified stiffness of PLA reported previously,53 it is necessary to make the phase size of PBAT toward a small scale, so the efficiency of toughening is preferable due to remarkable suppression of crazing, meanwhile the barrier capability would not suffer much loss. That is, on the one hand, the synergetic function of elongational flow field and “chain mobility promoter” could efficaciously promote the motion of PLA molecular chain to develop oriented crystalline architecture; on the other hand, elongational flow field could deform the dispersed phase PBAT into nanofibrils featuring large specific surface area to perfect the phase interface, which will improve crystallization and block gas diffusion more effectively. The results based on this idea indicate that the dispersed phase of PBAT in situ takes shape into nanofibrils, which can induce PLA to form into a kind of unique superstructure analogous to the network of abundant and oriented lamellae coordinating with well-organized amorphous region. Ultimately, the fully degradable films exhibit striking reduction of gas permeability. Even more beyond expectation is its excellent mechanical properties, featuring high strength and excellent ductility. Such type of superstructure is remarkably beneficial to ameliorate the barrier and mechanical property due to plenty of oriented lamellae of PLA and diminished size of the PBAT phase. This simple and feasible methodology of structural manipulation during processing exhibited here would certainly show its abilities in development of multifunctional or high-performance green polymer products.

With regard to semicrystalline PLA, good barrier properties could inherently be imparted by the ability of polymer chains packing into crystalline phase, which can serve as a natively impermeable parclose to small gas molecules with its compact molecular packing.27,28 Deservedly, tailoring crystalline morphology seems to be among the easiest ways to regulate its barrier properties. In terms of crystalline architecture, it is pertinent to get the verdict that, for spherulites, random and unconsolidated arrangement of lamellae would have limited capability to be a barrier for the permeation of gas molecules, because previously reported results also indicate that barrier property would be negligibly affected by crystallinity for PLA.29−34 Fortunately, the formation of highly oriented construction plays a pivotal role in the enhancement of barrier property due to the diminution of efficient free volume for the permeation of gas molecules, as reported by the pioneering work of Hiltner et al. in other semicrystalline polymers such as poly(ethylene terephthalate) (PET).35−38 The water barrier property of PLA was likewise improved by stretching-induced crystals reported very recently.39 Therefore, integrating the above points, it is highly expected that the construction of abundant and oriented crystals of PLA could obtain drastic enhancement of gas barrier properties. The construction of plentifully oriented crystals within PLA would be really not an effortless project owing to its weaker ability of crystallization.40−42 Among various different strategies of tailoring PLA crystallization, imposing intensive flow field seems to be a straightforward pathway to trigger dramatically enhanced kinetics of crystallization. Polymer chains tend to extend along flow direction, leading to the formation of highly oriented row-nuclei which are essential for well-aligned crystalline architecture.43−47 Nonetheless, for the inferior regularity and semirigidity of PLA molecular chain, flow field could not substantially meet our requirements of highly oriented crystallization that is prerequisite for high gas barrier. In addition, an inevitable predicament is that both crystal orientation and high crystallinity are detrimental for the ductility of PLA. Hence, of desirable need is a suitable scheme which could achieve a balance between well-organized lamellae with high crystallinity and proper ductility to the satisfaction of actual utility. Incorporation with various flexible biopolymers, such as poly(butylene succinate) (PBS),48,49 poly[(butylene succinate)co-adipate] (PBSA),50 and poly(butylene terephthalate) (PBT),51 has been verified to be introduced into PLA with the goal of enhancing the ductility. Taking into consideration of feasibility and implementation, it is necessary to find a suitable candidate which not only would toughen the matrix phase but also could facilitate the crystallization of PLA. Very recently, a series of elaborate work has been launched by Favis et al. in an endeavor to explore the morphology and interaction of PLA/ PBAT blends.52 Significant finding reveals that PBAT possesses high ductility (about 800%) and good degradability. Moreover, it has been acknowledged that the addition of PBAT can accelerate the crystallization ability of PLA.53 It is conceivable based on pioneering work that PBAT seems to be suitable to serve as the “structure modifier” on account of its impressive ductility and satisfactory promotion to crystallization behavior of PLA. Taking the aforementioned issues into considerations, a protocol of the “slit die extrusion−stretching−thermal treatment” process is proposed to impose intensively elongational flow to the PLA/PBAT system, aiming to regulate the oriented

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EXPERIMENTAL SECTION

Materials. Commercially available PLA was supplied by NatureWorks (4032D) with around 2% of D-lactic acid. The weight-average molecular weight and number-average molecular weight are 2.23 × 105 and 1.06 × 105 g/mol, respectively. The dispersity index is 2.10. Commercial PBAT (TH801) was kindly supplied by Blue Bridge Tunhe Polyester Co., XinJiang, China. The weight-average molecular weight and number-average molecular weight are 7.50 × 104 and 4.02 × 104 g/mol, respectively. The dispersity index is 1.87. The content of the butylene terephthalate unit of this sample was determined to be 47%. The rheological behavior with regard to the frequency dependency of complex viscosity of PLA and PBAT is shown in Figure S2. Preparation of PLA/PBAT Composite Films. PLA composite films containing various contents of PBAT (5, 10, 15, 20, 40 wt %) were fabricated by melt extrusion compounding followed by “slit die extrusion−stretching”. To avoid the degradation due to hydrolysis and prevent the formation of pinhole during processing, PLA and PBAT were dried at 80 °C under vacuum for 24 h before extrusion. Melt blending of PLA and PBAT was carried out in a corotating twin screw extruder with a ratio of screw length to its diameter (L/D) of 40. The barrel temperatures were 160, 180, 190, 180, and 170 °C from hopper to metering section, respectively, the die temperature was 165 °C, the screw speed was held constantly at 130 rpm, and the feeding rate was about 70 g/min. The as-extruded pellets after drying were charged in a single-screw extruder with a slit die, and the temperature profile was 175, 180, 190, 170 °C from hopper to die, respectively. The slit die has a width of 15 cm and a height of 1 mm. The ribbon of extrudate was hot stretched by a take-up device with two heterodromous rolls to provide the steady elongational flow field, wherein the rolling speed was fixed at 80 rpm from which the linear velocity is calculated to be 8097


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Figure 1. Schematic representation describing the processing approach and structure evolution for the PLA/PBAT composite films. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). To probe further crystal structure, we used a WAXD at the beamline BL15U1 of SSRF, Shanghai, China (the monochromated X-ray beam with a wavelength of 0.124 nm), and the sample-to-detector distance was 172 mm. Then the 2D-WAXD images were collected with an Xray CCD detector (model SX165, Rayonix Co. Ltd., U.S.A.). The WAXD intensity profiles as a function of q were obtained by integration in the azimuthal angular range of a whole circle (0−360°) from the sample patterns employing the Polar package after background scattering was subtracted. Additionally, the azimuthalscan profiles were acquired through integrating the diffraction intensities azimuthally at a q value of specific crystal refection of PLA. The mean size of crystal domain (Lhkl) of composite films before and after thermal treatment was calculated by Scherrer equation L = Kλ/(B cos θ), where K = 0.89 and B is peak width at half-height of specific crystal planes of PLA. Differential Scanning Calorimeter (DSC). Cold crystallization behavior of pure PLA and PLA/PBAT composite films was investigated by DSC on a TA Q2000 instrument (U.S.A.). The experiments were carried out in nitrogen atmosphere using about a 5 mg sample obtained from the as-stretched films, and thermally treated samples were sealed in aluminum pans. The samples were heated from 40 to 200 °C at a heating rate of 10 °C/min. The crystallinity (Xc) can be calculated by subtracting the enthalpy of cold crystallization from the enthalpy of melting by using eq 1.

31.3 mm/s, and the stretch ratio was estimated to be approximately 13.6, which is the largest draw ratio available before unstable flow. The roll temperature was controlled at 30 °C by the flux of cooling water. At the same time, samples of neat PLA subjected to the same thermal condition were also prepared. Detailed information on the “slit die extrusion−stretching−thermal treatment” process is described by a schematic diagram in Figure S1. Finally, the thickness of as-stretched composite films was about 400 μm. After drying, the perfection of oriented structure was carried out at 150 °C, which is below the the Tm of PLA (about 167 °C) but above the Tm of PBAT (about 115 °C) for 10 min with a pressure of 10 MPa after preheating for 5 min. Then all the films were cooled to room temperature at a pressure of 10 MPa for 3 min; consequently, a series of composite films with the thickness about 150 μm were obtained for barrier and mechanical measurements. Schematic representation describing the structure evolution for the PLA/PBAT composite films is shown in Figure 1. Scanning Electronic Microscopy (SEM). SEM observation was performed to provide intuitional revelation of phase morphology and crystalline morphology. The composite films were placed in liquid nitrogen for 50 min, and then the samples were cryogenically fractured along the stretching direction. The smooth cryofractured surfaces were taken for direct SEM observation. Preferential dissolution was also carried out at 25 °C for 1 min to reveal two phase morphology more clearly by taking advantage of tetrahydrofuran (THF), in which the dissolution rate of PLA was faster than that of PBAT. Besides, with the purpose of showing crystalline morphology, we used selective etching to get rid of the amorphous PLA as well as PBAT by a mixture solution compounding of water−methanol (1:1 by volume) and 0.05 mol/L of sodium hydroxide for 24 h at 25 °C. Before SEM observation, all treated surfaces were cleaned by distilled water and ultrasonication. A field-emission SEM (Inspect F, FEI, Finland) was utilized with the accelerated voltage of 5 kV; all composite samples were sputter-coated with gold prior to observation. Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). 2D-SAXS measurements were performed at the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) to examine lamellar structure. The SAXS images were collected with an X-ray CCD detector (Mar165, a resolution of 2048 × 2048 pixels). The monochromated X-ray beam operated at a wavelength of 0.124 nm, and the sample-to-detector distance was held at 1895 mm. The intensities I(q) (q = 4π sin θ/λ) were obtained through integration in the azimuthal angular range of whole circle of diffraction pattern, where 2θ is the scattering angle and λ represents the X-ray wavelength (0.124 nm).The long period between the adjacent oriented lamellae was calculated using the Bragg equation L = 2π/q, where L is long period and q represents the peak position in scattering curves.

Xc =

ΔHm − ΔHcc φ × ΔHm0

× 100%

(1)

where ΔHm is the enthalpy of melting for PLA, ΔHcc is the enthalpy of cold crystallization, φ is mass fraction of PLA, and ΔH0m is the enthalpy of melting for a 100% crystalline of PLA (93.7 J/g).55 Polarized Fourier Transform Infrared Spectroscopy (P-FTIR). The oriented properties both in crystalline region and amorphous phase were analyzed by FTIR equipped with Polaroid (Nioclent 6700, Thermal Scientific, USA). We got the absorption spectra of composite films over a wavenumber range of 900−1000 cm−1. To give a more comprehensive demonstration of structure, we did a multiangle scan from 0° to 180° per 5° through precise controlling of the polarizers’ angle. Gas Barrier Measurements. Oxygen permeation analysis of neat PLA and its composite films tailored into a disk with 100 mm diameter was conducted using a VAC-V2 film permeability testing machine (Labthink Instruments, Jinan, China) at room temperature with 50% relative humidity according to ISO2556:1974 based on the differential pressure method. A gas permeation cell was separated into two compartments by film specimen, air in both compartments was evacuated to ensure that the static vacuum pressure changes in the 8098


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ACS Applied Materials & Interfaces downstream compartment were smaller than the pressure changes due to the gas diffusion, then the gases were filled in the upstream compartment at a pressure of about 1.01 × 105 Pa, and the pressure variations in the downstream compartment were recorded as a function of time with pressure sensors. In order to facilitate the comparison of gas permeability coefficients that fluctuate due to various thicknesses, the time (t) was normalized to the thickness of the sample (L). As a result, the curves of the pressure and the normalized reduced time (t/L) were obtained. The permeability coefficient of oxygen (PO2) was determined by the slope of the steady-state permeating line. The diffusion coefficient (D) was estimated using the time-lag method. The time lag t0 is defined as the time that is required to reach a steady state which could be determined in permeating line on the time axis. The relationship between D and t0 is D = d2/6t0, where d is the specimen thickness. The solubility coefficient (S) was subsequently calculated from the equation P = D × S. 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, USA) with a crosshead speed of 20 mm/min and a gauge length of 20 mm. Our stress−strain testing was obtained in the stretching direction. The results of stress−strain testing of our composite films in the direction perpendicular to the stretching direction are also provided in Supporting Information. We tested a minimum of 6 bar for each sample at the same condition, and the average values are presented with standard deviations.

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RESULTS AND DISCUSSION Barrier and Mechanical Performance of Composite Films. Figure 2 and Figure 3 demonstrate the remarkable combination of excellent gas barrier and outstanding Figure 3. (A) Typical stress−strain curves of various composite films. (B) Detailed mechanical results regarding yield strength, Young’s modulus, and elongation at break.

mechanical performance. From Figure 2A, we can see that unexpected decrease of oxygen permeability is evidently obtained in PLA/PBAT composite films. Specifically, compared to the initial value of 1.8 × 10−14 cm3 cm cm−2 s−1 Pa−1 of pure PLA films, the composite films with 5, 10, and 15 wt % of PBAT are much more impermeable to oxygen, resulting in wondrously declined PO2 of 1.3, 0.4, and 0.2 × 10−14 cm3 cm cm−2 s−1 Pa−1, respectively. In particular, there is almost 1 order of magnitude reduction in PLA/PBAT (85/15 w/w), which is a considerable achievement of PO2 in PLA films. Note that, instead of deteriorating the gas barrier of PLA by common blending of PBAT (see Figure S5), the incorporation of PBAT with inferior barrier property improves the resistance capability of PLA to gas permeation amazingly in this work. Moreover, improved barrier property of composite films originates from the simultaneous decline of solubility and diffusivity coefficients (Figure 2B), indicating more circuitous pathway and remarkably lessened quantity of gas dissolution in the composite films. The mechanical performance, especially strength and ductility, are likewise crucial for practical application. Figure 3A describes the representative stress−strain curves of various samples in stretching direction. The composite films demonstrate absolutely different tensile behavior from pure PLA film, while the stress−strain curves of composite films not only evidently tower over that of the pure PLA film but also show distinct yielding and stable necking featuring representatively ductile fracture. Detailed tensile properties regarding yield strength, Young’s modulus, and elongation at break are

Figure 2. Performance evaluation to demonstrate the excellent resistance to gas permeation. (A) Oxygen permeability of PLA film and various composite films. (B) The curves of solubility and diffusivity coefficient belonging to various composite films. 8099


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packing films. This unparalleled result actually provides an innovative methodology to availably regulate and control the gas barrier property of polymer films. In order to better understand the correlation between the structure and performance, we are also curious to investigate the hierarchical microstructure of composite films including multiphase morphology, lamellar superstructure, crystal structure, and molecular orientation both in crystals and amorphous, which has been shown in the following sections. Phase Morphology of PLA/PBAT Composite Films. The phase morphology of PLA/PBAT composite film is a basic factor influencing the aforementioned barrier and mechanical performances. As shown in Figure 5, under the elongational flow field, the nanofribrillar morphology of PLA/PBAT composite films is remarkably different from the case of PLA/PBAT common blends (see Figure S3). Figures 5a−5e present the direct cryofractured surfaces of films along the stretching direction. Visually, one can only see oriented fibrillar embossment, apparently distinct from typical sea−island phase morphology, leading to favorable interfacial interaction between PLA and PBAT. In addition, because of the almost identical estimated solubility parameters of PLA (21.9 MPa1/2) and PBAT (22.2 MPa1/2),52 there is not a suitable solvent to etch any one exclusively. However, we find that PLA has larger dissolution rate in THF than PBAT. The consequence of preferentially dissolving away PLA is exhibited in Figures 5a′−5e′, the exposed nanofibrils of PBAT are clearly observed for the composite films, and the quantitative analyses in terms of the diameter distribution of PBAT nanofibrils are shown in Figures 5a″−5e″. The content of PBAT has a great influence on its ultimate shape, and the diameter of nanofibrils increases with the mass fraction of PBAT, rising from the minimal value of ∼60 nm for PLA/PBAT (95/5) to ∼340 nm for the case of 40 wt % PBAT. This can be attributed to deformation, combination, and agglomeration of dispersive phase PBAT when it undergoes the intensive elongational flow provided by “slit die extrusion−stretching” process. The nanoscale dispersion of nanofibrils possibly results from the smaller interfacial tension (0.60 ± 0.15 mN/m),52 larger viscosity ratio of PLA/PBAT (about 15), and intensively elongational flow, which definitively shows ultrahigh specific area stemming from high ratio of length to diameter. Note that, the decrease of PBAT phase size to nanoscale would be advantageous for overcoming the drawback of poor barrier property of PBAT. On the other hand, it could alter the crystallization behavior of PLA; herein, it has been reported that PBAT could facilitate the crystallization kinetics of PLA. Crystalline Superstructure of PLA in Composite Films. The crystalline morphology was observed in the etched composite films, as clearly manifested in Figure 6. The amorphous region of PLA and PBAT would be etched away due to the loose chain packing and thus easy hydrolysis in the etching solution. It is impressive from SEM micrographs that substantial difference of oriented lamellae between pure PLA film and composite films is presented. The isolated shishkebabs with short and imperfect lamellae are intuitively revealed in pure PLA films. Intriguingly, well-regulated lamellae arrange periodically in one direction and concatenate each other constituting a sort of pyknotic network. Beyond the aforementioned features, the formation of a network has particular difference in each sample, the elevation of lamellar length results in the networks of lamellae becoming more and more perfect along with increased incorporation of PBAT.

summarized in Figure 3B. Appreciable enhancement of strength and ductility on composite films is quite clear at a glance, with unexpected promotion of Young’s modulus and yield strength up to 3484 and 104.5 MPa in PLA/PBAT (85/15 w/w) compared to the initial values of 1246 and 44.9 MPa of pure PLA film, prominently different from the usual sacrifice of strength after incorporating soft polymers.53,56,57 More laudably, our composite films permit significant perfection on ductility; apparent improvement of elongation at break from 17.2% of pure PLA film to 228.1% of PLA/PBAT (60/40 w/w) has been achieved. Meanwhile, the mechanical property in the direction perpendicular to the stretching direction was also investigated (see Figure S6 and Table S1). Although the strength and ductility manifest slight reduction compared with the stretching direction, it still shows a good performance. Its yield strength can be able to achieve ∼95 MPa in PLA/PBAT (85/15 w/w), which is a fairly high value among PLA materials. In summary, through the proposed methodology of “slit die extrusion−stretching−thermal treatment” processing, fully biodegradable composite films with low permeability, ultrahigh strength, and superior ductility were created successfully. The superiority of the composite films with comprehensive performance is particularly clarified in Figure 4. We construct a

Figure 4. Performance triangle to clarify the superiority of PLA/PBAT composite films in comparison with other packaging film materials in terms of PO2, tensile strength, and ductility. The area size of each triangle represents the extent of comprehensive performance.

performance triangle to evaluate the comprehensive performance based on the literature. Currently on the market, the most commonly used packing film materials are high density polyethylene (HDPE), biaxial-oriented polypropylene (BOPP), polystyrene (PS), poly(ethylene terephthalate) (PET), aluminum foil, and so on.58,59 Compared to these petroleum-based polymers, the PLA/PBAT barrier film shows superiorly comprehensive properties. To be specific, although HDPE and BOPP have outstanding ductility, the gas barrier performance is lower than that of PLA/PBAT barrier film; the brittleness of PS, PET, and aluminum foil is still higher than that of PLA/PBAT barrier film, which is hard to overcome serving as packing films.60−63 Therefore, various advantages, such as the preferable ductility compared to HDPE or BOPP, robustness, and high gas barrier are altogether realized in PLA/ PBAT composite films. More importantly, the full degradability of composite films is more beneficial for it to be a promising candidate for substituting the traditional petroleum-based 8100


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Figure 5. SEM images of cryofractured surfaces of (a) PLA/PBAT (95/5 w/w), (b) PLA/PBAT (90/10 w/w), (c) PLA/PBAT (85/15 w/w), (d) PLA/PBAT (80/20 w/w), (e) PLA/PBAT (60/40 w/w) composite films. Panels a′−e′ present the fibrillar morphology of PBAT after etching the PLA phase of specimens a−e. Quantitative analyses in terms of diameter distribution of PBAT nanofibrils as shown in panels a″−e″. The average diameter (D) is marked in the upper left corner.

shown in Figures 7a−7f respectively. Symmetrical bulb-shape lobes appear in the 2D-SAXS patterns regardless of PBAT contents. This result manifests the presence of oriented PLA lamellae, which is consistent with the results observed by SEM. It should be noted that the scattering signals at a certain extent of obliquity off the meridional direction could be ascribed to the slippage of lamellae caused by the flow of PBAT melts during thermal treatment. In order to better understand the evolution of the oriented lamellar crystalline superstructure, we also detected the as-stretched films before thermal treatment, and the 2D-SAXS patterns are displayed in Figures 7a′−7f′. For PLA film after stretching, obvious streaks appear both in the equatorial direction and in the meridional direction. It evidently indicates the formation of deficient and immature shish-kebabs which possess organized shish and oriented lamellae with small size after stretching.64 After the introduction of PBAT, 2D-

Obviously, the formation of PBAT nanofibrils can availably regulate the morphology of PLA crystals toward a kind of compact network constituted by oriented lamellae. However, when the content of PBAT exceeds 20 wt %, inversely, the extent of lamella arrangement reduces, and a fraction of scattered lamellae forms resulting from flowable PBAT in the process of oriented structure perfection under pressure. On the whole, abundant and oriented PLA lamellae are virtually formed into a kind of interlinked network; meanwhile, it is reasonable to speculate that PBAT nanofibrils are located between arrayed crystals of PLA with favorable interface. For further identifying the characteristics of crystalline morphology regarding lamellar structure, 2D-SAXS was performed to offer an opportunity to analyze the oriented lamellar structure quantitatively as demonstrated in Figure 7. The 2D-SAXS patterns of PLA film and composite films are 8101


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Figure 6. SEM micrographs showing crystalline morphology of PLA for (a) PLA, (b) PLA/PBAT (95/5 w/w), (c) PLA/PBAT (90/10 w/w), (d) PLA/PBAT (85/15 w/w), (e) PLA/PBAT (80/20 w/w), and (f) PLA/PBAT (60/40 w/w) composite films in which the scale bar is 5 μm. More amplified micrographs of corresponding samples in which the scale bar is 3 μm have been inserted in the left bottom. Blue arrow indicates the direction of extensional flow field.

obvious decreased value of q* illustrates the markedly enhanced lamellar thickness through thermal treatment. In addition, the difference of q* also arises in the intensity curves of as-stretched films. Accordingly, the maximal value of q* = 0.592 nm−1 appears in PLA/PBAT (85/15 w/w), corresponding to the minimum L = 10.61 nm, which indicates more close-knit arrangement. It suggests that the presence of PBAT nanofibrils has significant influence on the nucleation density of PLA that is the primary cause for the intensive packing degree of lamellae. Reasonable speculation could be illustrated as follows: under the action of elongational flow field, PBAT transforms into nanofibrils, and favorable interface interactions with PLA could prompt the perfection of PLA oriented lamellae on its nanofibrillar surface. Meanwhile flexible chains of PBAT could also increase the nucleation density of PLA reflected by the increased intensity at the peak of 1D-SAXS curve.

SAXS patterns of various samples are shown in Figures 7b′−7f′. It just has an enhanced intensity of scattering signal which also reveal that PBAT could facilitate the crystallization of PLA during stretching. Quantitative analysis could provide more details with regard to the feature of PLA oriented lamellar crystalline superstructure through the integrated 1D-SAXS curves illustrated in Figure 8. From Figure 8a, the distinctly different locations referred to as q* values between various samples represent the disparity of PLA lamellar packing density. The packing degree of lamellae increases initially and then reduces with the increase of PBAT content. For example, in PLA/PBAT (85/15 w/w), q* = 0.341 nm−1 corresponding the L = 18.42 nm illustrates more compact stacking of lamellae in comparison to that of pure PLA film (q* = 0.249 nm−1 corresponds to L = 25.22 nm). Moreover, compared with the intensity curves of as-stretched films displayed in Figure 8b, the 8102


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Figure 7. Representative 2D-SAXS patterns of barrier films: (a) PLA, (b) PLA/PBAT (95/5 w/w), (c) PLA/PBAT (90/10 w/w), (d) PLA/PBAT (85/15 w/w), (e) PLA/PBAT (80/20 w/w), and (f) PLA/PBAT (60/40 w/w). Panels a′−f′ present the patterns of corresponding as-stretched films before thermal treatment.

Figure 8. (a) Typical 1D-SAXS intensity profiles of composite films, and (b) representative 1D-SAXS intensity profiles of the corresponding asstretched films before thermal treatment. The values of peak (q*) are marked on the intensity curves.

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Figure 9. Representative 2D-WAXD patterns of neat PLA film and composites films: (a) PLA, (b) PLA/PBAT (95/5 w/w), (c) PLA/PBAT (90/10 w/w), (d) PLA/PBAT (85/15 w/w), (e) PLA/PBAT (80/20 w/w), and (f) PLA/PBAT (60/40 w/w). Panels a′−f′ present the corresponding asstretched films before thermal treatment.

We further investigated the crystal structure and molecular orientation in these uniquely oriented crystals of composite films and as-stretched films by utilizing 2D-WAXD patterns (see Figure 9). Relevant 1D-WAXD intensity profiles integrated from 2D-WAXD patterns are also shown in Figures 7a and 7b. First of all, in Figures 9a−9f, strong diffraction arcs representing high orientation degree and crystallinity are exhibited. While in Figures 9a′−9f′, similar arc-like diffraction is turned up with weaker diffraction intensity elucidating a relatively low crystallinity, meanwhile the absent diffraction ring of other lattice planes of PLA (such as 010) stands for the existence of imperfect crystals. Moreover, for the sake of studying the molecular orientation in the crystalline texture of various composite films, we extracted the intensity distribution of lattice plane (200/110) along the azimuthal angle as presented in Figure 10c. Apparently, the degree of orientation increases with the PBAT content, but once the mass fraction of PBAT exceeds 20%, the orientation declines. In addition, Figure 10a and Figure 10b also reveal typical reflections of α-

form crystal of PLA. Direct comparison between composite films and as-stretched films shows that the intensity peaks at q = 10.6, 11.7, 13.6, and 25.8 nm−1 corresponding to lattice planes (010), (200)/(110), (203), and (015) of PLA respectively in composite films are obviously sharper than those of as-stretched samples.65 It indicates that the perfection of oriented crystalline texture occurs during thermal treatment and thus a distinctly enhanced crystallinity obtains. Furthermore, we also examined the mean size of crystal domain (L200/110) of composite films and as-stretched films calculated by the Scherrer equation on the basis of lattice plane (200/110) reflection. Figure 10d shows the noteworthy results; the mean size of oriented crystals in barrier films after annealing is distinctly larger than that of the as-stretched samples. Meanwhile the composite films with PBAT possess bigger crystal size than that of pure PLA films. Specifically, relative to the 21.45 nm of pure PLA film, the L200/110 in PLA/PBAT (85/ 15 w/w) is 23.42 nm. It also shows that with the promotion of PBAT, proper thermal treatment could facilitate the growth of 8104


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Figure 10. (a) Representative 1D-WAXD intensity profiles of PLA film and composite films and (b) 1D-WAXD intensity profiles of corresponding as-stretched films before thermal treatment. The characteristic diffraction peaks and their assignments to the specific lattice planes are marked. (c) Intensity distribution of lattice plane (200/110) reflection along azimuthal angle of PLA film and composite films. (d) The mean size of crystal domain (Lhkl) of PLA film and composite films calculated by the Scherrer equation on the basis of lattice plane (200/110) reflection.

Figure 11. DSC heating traces of (a) as-stretched PLA and composite samples before thermal treatment and (b) corresponding PLA and PLA/ PBAT composite films. The cold crystallization temperature, the melting points, and the degree of crystallinity are marked on the corresponding curves.

PLA lamellae toward larger size. Additionally, we could not find any diffraction peak belonging to PBAT in Figure 10a of composite films and Figure 10b of as-stretched films, which infers that PBAT could not form any crystals under this processing condition possibly due to unfavorable crystallization and nanoscale size. DSC was employed to offer direct insights into the perfection or crystal growth of immature oriented crystals induced by stretching during thermal treatment. The resultant DSC heating thermograms for composite films are plotted in Figure 11a and Figure 11b. Two transitions are successively displayed on the heating curve of as-stretched composite films before thermal treatment: cold crystallization and melting endotherm. For as-stretched PLA film, its cold crystallization peak is

marked at 101.20 °C, which is much lower than the reported about 120 °C for the unstretched PLA films.52,54 Furthermore, the addition of PBAT remarkably brings down the peak temperature of cold crystallization about 10 °C. The cold crystallization temperature also shows a slight dependence on the composition: compared with the 93.15 °C of PLA/PBAT (95/5 w/w), it declines to 86.40 °C in PLA/PBAT (60/40 w/ w). Moreover, the introduction of PBAT broadens the peak of cold crystallization. The above results adequately testify the fact that the elongational flow field and flexible chain of PBAT can synergistically enhance the crystallization of PLA, enabling it to happen at a lower temperature and in a wider temperature range. In terms of crystallinity from the DSC thermograms, compared with stretched PLA film, obvious improvement 8105


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Figure 12. Polarized FTIR spectroscopy of the PLA film and composite films. The maximum absorbance is marked by red arrow, and minimum absorbance is marked by blue arrow. (a) PLA, (b) PLA/PBAT (95/5 w/w), (c) PLA/PBAT (90/10 w/w), (d) PLA/PBAT (85/15 w/w), (e) PLA/ PBAT (80/20 w/w), and (f) PLA/PBAT (60/40 w/w).

appears in PLA/PBAT (85/15 w/w) from 11.98 to 26.06%, whereas it declines to 12.33% in PLA/PBAT (60/40 w/w) along with the increase of PBAT contents. This conclusion has consistency with the analysis in WAXD. The difference in the heating curve of composite films after thermal treatment is the vanishment of cold crystallization peak and the increased melting temperature, indicating again that the soft molecular chain of PBAT could prompt the PLA crystals to be more perfect. Besides, the decline of melting point with the increase of PBAT signifies that more favorable mobility of PLA crystals (or nuclei) with the assistance of high amount of PBAT would be the disadvantage for stabilization and densification of lamellae. It would lead to the decreased regularity of PLA lamellae as shown in the results of SAXS. Taking the aforementioned results into consideration together, we can draw a detailed formation of oriented lamella networks during “slit die extrusion−stretching−thermal treatment” process. After the process of slit die extrusion− stretching, oriented but imperfect lamellae of PLA are induced by the intense elongation flow field; simultaneously dispersed phase of PBAT in situ turns into nanofibrils, and during this procedure, the incorporation of PBAT produces a prodigious influence on the nucleation density and crystal growth of PLA. When the thermal treatment is below the melting point of PLA (150 °C), and exceeds the melting point of PBAT, the established alignment of crystal nuclei at the interface of PBAT would not be disrupted severely. Meanwhile, the active PBAT molecules facilitate the chain mobility of PLA to accumulate into crystal nuclei. It can prompt the orientated lamellae to be more perfect and pyknotic, interconnecting with each other to constitute a consecutive network of oriented lamellae. This unique superstructure doubtlessly is closely related to the enhancement of barrier and mechanical performance. Amorphous Configuration of PLA and Composite Films. For the barrier property of polymer materials, the configuration of amorphous region would likewise have a significant effect. Gas molecules could dissolve and diffuse through the amorphous region more easily. Hence, we

attempted to probe the configuration of molecular chain in the amorphous region of composite films by polarized infrared absorption spectrum (P-FTIR). Specifically, the band at 921 cm−1 assigned to the coupling of C−C backbone stretching with the CH3 rocking mode is considered as the exclusively crystal-sensitive band of α crystals, while the band at 956 cm−1 is related to the amorphous phase of PLA.66,67 Figure 12 illustrates the curves of polarized FTIR belonging to PLA film and composite film as the change of polarizing angle. There are obviously two peaks at 921 and 956 cm−1 of the graphs consisting of different absorbance curves along with diverse polarized angles. It confirms that neither crystalline region nor amorphous region is isotropic. For the absorption peak at 956 cm−1 indicating the morphology of amorphism, the maximum absorbance is marked by red arrow and minimum absorbance is marked by blue arrow, and the difference value between the maximum and minimum represents the degree of orientation. Clearly, the orientation of amorphism becomes weak with the increase of PBAT; it may result from easy mobility of PLA chains promoted by PBAT. In addition, in terms of the peaks at 921 cm−1, the degree of fluctuation also reflects the degree of crystal orientation, and its result has good consistency with WAXD results. What is more, the angles of oriented crystals are approximately perpendicular to the angles of oriented amorphous region. In other words, it also adequately illustrates that the extensional direction of molecular chain in amorphism is parallel to the direction of molecular chain packing in lamellae. The underlying reason for the oriented chain in amorphous phase along the stretching direction may be that the oriented chain is confined by PLA oriented lamellae during stretching as well as lamellar growth during thermal treatment. Origin of High Barrier and Outstanding Mechanical Performances of Composite Film with Densely Oriented Hierarchy. From the foregoing, one could anticipate that this unique hierarchical structure, including abundant and compact network composed of oriented and pyknotic lamellae bonded by ductile nanofibrils, can find a promising ability in enhancing resistance to gas permeation and in achieving commendable 8106


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Figure 13. Schematic representation for the penetration through (A) pure PLA films with stretched-thermal treatment following relatively easier paths, (B) general compression molding PLA films following numerous and effortless paths, and (C) composite films with barely few and most tortuous paths.

absorb energy to accommodate the increased deformation and thus prevent further failure. It is just this tailored hierarchical structure making the composite films be of desirable strength and superior tenacity. Incontrovertibly, this type of composite film with the uniquely hierarchical structure can find capacious applications in terms of promising performance. More importantly, the processing method in a versatile and practicable way of melt blending is highly efficient for fabrication.

mechanical properties simultaneously. The remarkable decrease of gas permeation is that the nature of orientation in a holistic system brings about the compact packing of molecular chains promoted by intensive elongational flow. It results in the lesser sufficient free volume through which gas molecules could transit. Therefore, we can see that the solubility coefficient of composite films declines conspicuously (see Figure 2B). Moreover, the existence of plentiful and impermeable oriented lamellae interlinked with each other provides outstanding parclose to the diffusion of gas molecules and thus a more tortuous diffusion path, which can be manifested in the drop of diffusivity (see Figure 2B). Besides, on account of the dimension of PBAT being decreased to be nanoscale, confined effect could availably prevent the permeation of gas molecule. But, when the content of PBAT exceeds a certain amount in the composite films, its intrinsic drawback of poor barrier properties would deteriorate the overall barrier properties of composite films due to the lack of nanoconfined effect. Compared with the common PLA/PBAT films with plenty of spherulites (see Figure S4 and Figure S5), the admirable combination of low solubility and smaller value of diffusivity empowers the composite films’ better barrier properties. Subsequently it achieves an order of magnitude reduction for PO2 in composite films. The different mechanism of gas penetration through different films is illustrated in Figure 13. The gas molecules would experience lesser and more tortuous path through composite films with supernetworks of oriented lamellae, whereas in pure PLA films it takes an easier way. In terms of excellent mechanical performance of composite films, we assimilate such particular structure to be analogous to “nacre-like structure”,68−70 thereinto the nanofibrils of PBAT serve as “organic bonding layer”, and regular lamellae with oriented amorphous phase acts as “inorganic platelet”. When the external force is imposed on the material, interconnected lamellae can offer sufficient strength for resistance to deformation. Meanwhile, highly flexible PBAT nanofibrils can

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CONCLUSIONS The present work demonstrates a facile methodology to obtain polylactide composite films possessing superior combination of outstanding barrier property and robust mechanical property using the technology of “slit die extrusion−stretching−thermal treatment”. Under the action of extensive elongation flow, in situ formation of ductile PBAT nanofibrils not only can retard the gas diffusion at the interface but also can synergistically promote the kinetics of PLA oriented crystallization to form the orderly oriented structure of interlinked networks and outstretched tie chains in amorphous phase. This unique superstructure firmly attributes to its superiority in ameliorating the performance of barrier and mechanical properties, the oxygen permeability coefficient in PLA/PBAT (85/15 w/w) can be as low as 2 × 10−15 cm3 cm cm−2 s−1 Pa−1 combining with extremely high strength, modulus, and ductility (104.5 MPa, 3484 MPa, and 110.6%), which can be comparable to the superior barrier films based on general petroleum-based polymers (e.g., PET). Authentically, the particular structure constituted by oriented lamellae cooperating with the wellorganized amorphism and preferable interface was elaborately constructed. It provides an industrially scalable processing method to construct the unique structure to enhance the gas barrier and mechanical property. We have lots of confidence in it to indeed push the usability of biodegradable polymers forward, and certainly prompt wider applications of environ8107


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mentally friendly materials such as food packaging, medical packaging, and biodegradable mulch.

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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/acsami.6b00451. Experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-J.Z.). *E-mail: [email protected] (Z.-M.L.). 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 of China (Grants 51533004, 51273131, and 51473101), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 51421061), the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant 2014TD0002), and State Key Laboratory of Polymer Materials Engineering (sklpme 2014-3-08). Our work was also supported by the Fundamental Research Funds for the Central Universities (2014SCU04A01). The authors are grateful for the kind help and support of Shanghai Synchrotron Radiation Facility (SSRF) in SAXS and WAXD measurement and the analysis of its results.

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REFERENCES

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