In Situ Nanofibrillar Networks Composed of Densely Oriented

Research Article pubs.acs.org/journal/ascecg

In Situ Nanofibrillar Networks Composed of Densely Oriented Polylactide Crystals as Efficient Reinforcement and Promising Barrier Wall for Fully Biodegradable Poly(butylene succinate) Composite Films Sheng-Yang Zhou,† Hua-Dong Huang,† Ling Xu,† Zheng 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, No.24 South Section 1, Yihuan Road, Chengdu 610065, China ‡ Department of Chemistry, Stony Brook University, 100 Nicolls Road, Stony Brook, New York 11794, United States S Supporting Information *

ABSTRACT: Developing a sustainable and environmently friendly scheme to fabricate fully degradable barrier films with robust mechanical properties is still a great challenge. Here, we first put forward a methodology that through taking advantage of an elongational flow field followed by woven hot compaction, in situ nanofibrillar networks of polylactide (PLA) are creatively constructed within a poly(butylene succinate) (PBS) matrix serving as an efficient “barrier ball” and reinforcement. The in situ PLA nanofibrils tend to overlap to constitute into a kind of interwoven network, in which highly oriented PLA lamellae are regularly arranged. Simultaneously, this network produces a spatial confinement effect on the crystallization of PBS, resulting in a confined environment around the nanofibrillar networks. This unparalleled hierarchical structure can availably attribute to an exceptional gas barrier and mechanical properties of the composite films. Ultimately, the oxygen permeability coefficient of the composite films can be reduced more than 60%, and the tensile strength increases nearly twice compared with that of pure PBS film. Meanwhile, the ductility certainly does not deteriorate. Of more practicable significance is that this processing method provides a new route to manufacture multiphase biopolymers with high performance and multifunctional sustainability. KEYWORDS: Biodegradable films, Barrier property, Mechanical property, Oriented crystal, In situ nanofibrillar network

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INTRODUCTION

mechanical performances resulting from relatively low strength likewise restricts its wide range of use.17−19 Therefore, there is still great necessity for improvement to obtain a superior gas barrier property, simultaneously achieving a desirable balance regarding mechanical performance. With regard to the improvement of the gas barrier property, it appears from the existing literatures that numerous schemes could be adopted to modify PBS to offer adequate barrier to gas permeation such as the incorporation of impermeable nano-

With the extremely growing awareness of sustainability, the focus has shifted from conventional plastic materials to more environmentally friendly alternatives in different fields of application such as food wrapping, drugs, and biomedical packaging.1−7 Poly(butylene succinate) (PBS) being a fully biodegradable polyester, which could be derived completely from renewable resources, has received numerous attention in recent years because of its good thermomechanical property and excellent processability.8−16 However, it can hardly meet the requirements for various situations such as packing due to its insufficient capability for the resistance to gas, which is quite important for packaging applications. Meanwhile, undesirable © 2016 American Chemical Society

Received: March 24, 2016 Revised: April 1, 2016 Published: April 6, 2016 2887

DOI: 10.1021/acssuschemeng.6b00590 ACS Sustainable Chem. Eng. 2016, 4, 2887−2897

Research Article

ACS Sustainable Chemistry & Engineering platelets.20−24 It could serve as a “barrier wall”, compelling diffusing gas molecules to follow a longer and more tortuous pathway. Nonetheless, at present, the consumption of time or energy and sacrifice of biodegradability and ductility coupled with a complex processing technique destines this technology to be a challenge to realize it as an industrially feasible manufacturing process. Therefore, looking for a more environmental and convenient approach to improve the gas barrier property is extremely urgent for us. It has been well established that polymer blends are a simple and economical method for generating novel structures and materials, providing performance beyond the properties of any neat polymers.25−29 Hence, replacing inorganic nanofiller with a kind of polymer as “barrier elements” becomes an alternative approach to ameliorate the barrier property. However, it is far from effortless to enhance the barrier property by traditional direct blending of PBS with another biodegradable polymer. First, loose chain packing of the polymer disperse phase has a limited ability to hinder gas permeation, far below the gas barrier capability of nanoplatelets such as graphene oxide (GO).23 Second, an existing interfacial flaw resulting from phase separation would further destroy the resistance to gas permeation. Therefore, the feasible alternative pathway to acquire the desirable gas barrier and mechanical performance would be tailoring the construction of a particular phase morphology and interaction between each phase of polymeric blends toward an optimized microstructure (or nanostructure); that is, primarily, the indispensable procedure is required to improve the gas barrier capability of the disperse phase. Furthermore, the peculiar design of the interface appears to be necessary for overall enhancement of the gas barrier property. As a matter of fact, the gas barrier property has great dependence on the macromolecular superstructure, e.g., the crystalline structure.30−37 An array of elaborate work has been launched to reveal that anisotropy of a polymer chain has a positive improvement on the gas barrier property. Because the orientation leads molecular chains to densely compact with each other along a specific direction, the gas molecule would experience less free volume and a more frustrated permeation path.38−40 Naturally, it enlightens us to strive for an oriented hierarchical structure of a dispersed phase for improving the PBS barrier properties. Specifically, the disperse phase has the characteristic of a high orientation serving as an effective barrier wall. In addition, for the sake of increasing the strength of PBS, introducing a kind of polymer with a higher strength would be taken for granted. It has been widely recognized that the fibrilform disperse phase due to preferential orientation and larger specific surface area would substantially increase the efficiency of reinforcement.41−44 Therefore, we conceive a superstructure featuring a network of highly oriented polymer fibrils, which could act as a promising barrier wall and effective reinforcement for the PBS matrix. On the one hand, the fibril-form networks with highly oriented aggregation could act as a “barrier wall” to enhance the gas barrier property. On the other hand, the formation of well-aligned micro/nanofibrils permits the construction of a strong dispersed phase and interfacial interactions for the amelioration of mechanical performance. Certainly an appropriate technique should be used to realize the formation of the aforementioned superstructure. Pioneering exploration has been launched by Fakirov et al. revealing the feasibility of formation of in situ fibrils by virtue of such an external elongational flow.45−48 According to the basic requirements of in situ fibrillation, at the same time without

sacrifice of biodegradability, there would be nothing better than polylactide (PLA) to serve as the optimum candidate for the fibrillation phase. First, being the same as a biodegradable polymer,49−51 it is well known that PLA has a higher melting point and comparative viscosity in comparison to PBS that is propitious to deform into a fibril-form morphology within the matrix of PBS.52 Moreover, the higher strength and well processability of PLA would be just right for us to construct a fibrous network to strengthen PBS.53 Here, we utilized the modified technology of “slit die extrusion−stretching−woven compression molding”.54,55 Through taking full advantage of the elongational flow field and orthotropic woven molding, we ultimately obtained fully biodegradable composite films with a preferable gas barrier property and superior combination of strength and ductility. The results from structural analysis manifest that the disperse phase PLA in situ forms into nanofibrils interlinking with each other into a kind of network, which is composed of highly oriented lamellae. Moreover, the different contents of PLA have a significant influence on the structure of the nanofibrillar networks including accumulation degree, orientation degree, and crystal morphology of the PLA nanofibrils, affecting the ultimate properties of the composite films. The hierarchical superstructure showed here displays its efficient ability to improve the gas barrier property and mechanical performance, which would serve to promote its practical application in food packaging and agricultural mulch in large-scale production and in a sustainable way.

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EXPERIMENTAL SECTION

Materials. PBS (Bionolle #1001MD) is a commercial product from SHOWA Highpolymer Co., Ltd. (Japan). It has a number-average molecular weight of ∼6.0 × 104 g/mol, dispersity index of 2.33, and melt flow index of 1.5 g/10 min (190 °C/2.16 kg, ASTM D1238). Commercially available PLA was supplied by NatureWorks (4032D) with around 2% D-lactic acid. The weight-average molecular weight and number-average molecular weight were 2.23 × 105 and 1.06 × 105 g/mol, respectively. Preparation of PBS/PLA Composite Films. PBS/PLA composite films were made by “slit die extrusion−stretching−woven compression molding”. The detailed procedures are described as follows: To avoid the degradation resulting from hydrolysis and thus formation of voids during processing, PBS, PLA, and their blends were dried at 80 °C under vacuum overnight before extrusion or compression molding. The melt blending of PBS and PLA was carried out in a co-rotating twin screw extruder (Nanjing Rubber & Plastics Machinery Plant Co., Ltd., China) with a ratio of screw length to diameter (L/D) of 40. The mass fractions of PLA were set at 10, 30, and 40 wt %. The temperature profiles were set at 160, 175, 190, 190, 175, and 160 °C from hopper to die, and the screw speed was constantly held at 150 rpm in order to achieve stable melt flow. Then, the extruded pellets after drying were charged in a single-screw extruder with a slit die which has a width of 15 cm and a height of 1 mm. The temperature profiles were 175, 180, 190, and 170 °C from hopper to die. The ribbon of extrudate was hot stretched by a take-up device with two countermovement rolls to provide a steady elongational flow field, wherein the rolling speed was fixed at 60 rpm and the stretch ratio was estimated to be approximately 12.7, which is the maximum value permitting stable flow. The roll temperature was controlled at 30 °C by the flux of cooling water. Finally, the ribbons of stretched composites with a thickness of about 800 μm were obtained. After being dried, ribbons of composites were woven into platelets, and then compression molding was carried out at 135 °C, which is above the Tm of PBS (about 115 °C) but below the Tm of PLA (about 167 °C) for 10 min with a pressure of 10 MPa after preheating for 5 min. Then, all the films were cooled to room temperature by cold compression molding at a pressure of 10 MPa for 2888


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ACS Sustainable Chemistry & Engineering

Figure 1. (A) Oxygen permeability coefficient (PO2) of PBS and composite films with different contents of PLA. (B) Comparison of PO2 and tensile strength with a wide range of packaging film materials. 3 min. The detailed process of “slit die extrusion−stretching−woven compression molding” is described in Figure S1 of the Supporting Information. At the same time, pure PBS subjected to the same processing conditions was also prepared. Ultimately, a series of composite films with a thickness of about 300 μm were obtained for barrier and mechanical property measurements. Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS) Measurements. 2D-SAXS measurements were conducted to examine the crystalline superstructure of PBS and PLA at the beamline BL16B1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). An X-ray CCD detector (Model Mar165, 2048 pixels × 2048 pixels of 80 μm × 80 μm) collected the 2D-SAXS images. The distance from sample to detector was held at ∼1895 mm, while the monochromated X-ray beam operated at a constant wavelength of 0.124 nm. The radically integrated intensities I(q) (q = 4π sin θ/λ) are obtained for integration in the azimuthal angular range of a whole scattering ring, where 2θ represents the scattering angle, and λ represents the wavelength of the X-ray. The technique of one-dimensional electron density correlation function analysis has been also used to give detailed structural information on the films.56,57 The electron density correlation function K(z) can be derived from the inverse Fourier transformation of the experimentally intensity distribution I(q) as

Here, cos2 φ is the orientation factor defined as 2

(cos φ) =

∞

(1)

Xc =

where z denotes the location measured along a trajectory normal to the lamellar surfaces. For systems with a structure of stacks of lamellae, the correlation function shows characteristic features that allow the long spacing defined as the average thickness of a lamella together with one interlamellar amorphous layer measured along the lamellar normal to be determined. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD) Measurements. 2D-WAXD experiments were employed to determine orientation and crystalline structure at the beamline BL15U1 of SSRF The wavelength of the X-ray was 0.124 nm. The distance from sample to detector was ∼173 mm, and 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 as a function of q (q = 4π sin θ/λ) 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. The azimuthal scan profiles were acquired through integrating the diffraction intensities azimuthally at a q value of specific crystal refection. To evaluate the molecular orientation, the orientation parameter can be estimated from the value of Herman’s orientation factor58

fH =

3(cos2φ)hkl − 1 2

I(φ)cos2 φ sin φ dφ π /2

I(φ)sin φ dφ

(3)

where φ is the azimuthal angle, and I(φ) is the scattered intensity along the angle φ. The azimuthal intensity distribution I(φ) was analyzed at q = 11.8 nm−1, where the peak represents the (200/110) reflection of PLA. Scanning Electronic Microscopy (SEM) Observation. SEM was adopted to get a clear observation of the phase morphology of the PBS/PLA system. First, the composite films were placed in liquid nitrogen for ∼50 min, and the samples were cryogenically fractured. The smoothly fractured surfaces were then sputter-coated with gold for observation. A field-emission SEM (Inspect F, FEI, Finland) was utilized for all samples; the accelerated voltage was held at 5 kV. Differential Scanning Calorimeter (DSC) Characterization. The thermal behaviors such as melting and crystallization behaviors for PBS and PBS/PLA composite films were probed by DSC in a DSC Q200 (TA Instruments, U.S.A.). Around 5−6 mg of PBS or PBS/PLA composite films were heated from 30° to 180 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. The crystallinity (Xc) can be calculated by subtracting the enthalpy of cold crystallization and premelt crystallization from the enthalpy of melting by using

∫0 I(q)q2cos(qz)dq ∫0 I(q)q2dq

π /2

∫0

∞

K (z) =

∫0

ΔHm + ΔHpc − ΔHcc ΔHm0 × ϕ

× 100%

(4)

where ΔHm is the enthalpy of melting for PBS or PLA, ΔHcc is the enthalpy of cold crystallization, ΔHpc is the enthalpy of premelt crystallization (imperfect crystals), ΔH0m is the enthalpy of melting for a 100% crystalline of PBS (120 J/g) or PLA (93.7 J/g),59,60 and ϕ is the mass fraction of the PBS or PLA component. Gas Barrier Property Testing. The gas permeability measurements of neat PBS and its composite films were conducted using a VAC-V2 film permeability testing machine (Labthink instruments, Jinan, China) at room temperature with 50% of relative humidity according to ISO2556:1974. The gas permeation cell was separated into two compartments for film specimens, 100 mm in diameter and about 300 μm in thickness. Air in both compartments was continuously evacuated for 12 h prior to testing, ensuring that the pressure changes due to the gas diffusion were greater than the static vacuum pressure changes in the downstream compartment. Subsequently, the gases were discharged into the upstream compartment at a pressure of about 1.01 × 105 Pa. The pressure changes in the downstream compartment were recorded as a function of time by pressure sensors. The permeability coefficient (P) 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 a permeating line on the time axis. The relationship

(2) 2889


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of the pure PBS film, illustrating enhanced yield strength and Young’s modulus. Detailed tensile properties regarding yield strength, Young’s modulus, and elongation at break are summarized in Table 1. In terms of yield strength and Young’s

between D and t0 is D = d2/6t0, where d is the specimen thickness. The solubility coefficient (S) was subsequently calculated from the equation (5) P=D×S Mechanical Property Testing. The tensile testing was carried out at room temperature using 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 (ASTM standard D638). Before testing, the films were tailored into the shape of dog bone according to ASTM standard D638. Ultimately, the average values were presented with standard deviations.

Table 1. Summarized Data of Tensile Properties, Including Tensile Strength, Young’s Modulus, and Elongation at Break

PBS/PLA 100/0 PBS/PLA 90/10 PBS/PLA 70/30 PBS/PLA 60/40

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RESULTS AND DISCUSSION Gas Barrier Property and Mechanical Performance of PBS/PLA Composite Films. Figure 1A illustrates the oxygen permeability coefficient (PO2) of PBS and various composite films. It is clearly visible that the PO2 of the PBS/PLA composite films is remarkably decreased by the incorporation of PLA. To be specific, a more than 63% reduction in PO2 from 5.8 × 10−15 to 2.1 × 10−15 cm3 cm cm−2 s−1 Pa−1 is achieved by the incorporation of 40 wt % PLA. According to our previous works and the data reported in literatures,61−63 PO2 of pure PLA with conventional processing is about 2.0 × 10−14 cm3 cm cm−2 s−1 Pa−1 which is almost 1 magnitude of order larger than PBS. However, in this work, instead of PO2 increasing, the composite films show noteworthy reduction in comparison to either PBS or PLA. These results are different from the films by common blending, which has obvious deterioration of gas barrier properties with the introduction of PLA (Figure S2). Apparently, this fully illustrates that the conceived structure constructed by the protocol of “slit die extrusion−stretching− woven compression molding” realizes the function of itself on the enhancement of the gas barrier property of PBS. Next, we also examined the mechanical performance of the composite films. Figure 2 shows the typical stress−strain curves

yield strength (MPa)

Young’s modulus (MPa)

elongation at break (%)

37.6 ± 1.1

587 ± 8

419 ± 12

50.1 ± 0.9

896 ± 5

281 ± 9

56.8 ± 1.4

924 ± 6

212 ± 11

66.4 ± 1.0

982 ± 11

168 ± 6

modulus, compared to the initial values of 37.6 and 587 MPa of the pure PBS film, the composite films loaded with different contents of PLA obtain an unexpected promotion at different extents. Especially in composite films constituting 40 wt % PLA, remarkable improvement arises up to 66.4 and 982 MPa in terms of yield strength and Young’s modulus, respectively, which has prominent enhancement compared with the composite films by common blending (Figure S3). More importantly, no big sacrifice of ductility is obtained; when the content of PLA is increased to 40 wt %, the elongation at break is still maintained at 168%. It is clearly shown that the desirable combination of high strength and ductility is realized in these composite films. As we know, petroleum-based polymers such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) have accounted for a huge share of the packing films market for a long time.64−67 However, these traditional films normally fall into the category of having either an inferior gas barrier property or poor mechanical performance. Specifically, as demonstrated by the tensile strength, PP and PE are usually in a moderate level of 30−40 MPa; with regard to PS and poly(vinyl chloride) (PVC), it is reported that in addition to the poor strength, ductility is often less than satisfactory for practical application.68,69 Apparently, a more superior performance of our composite films in offering high resistance to oxygen permeation and the combination of high mechanical properties can even challenge the traditional materials existing in the market as illustrated in Figure 1B. It could even bear comparison with the excellent gas barrier property of aluminum foil and poly(ethylene terephthalate) (PET). Beyond that, with the growing urgency to pursue sustainable development, we believe that the fully biodegradable composite films are bound to be promising candidates to substitute for petroleum-based packaging materials. In order for better establishment of the relationship between structural details and properties, we investigated the specific construction of composite films including phase morphology, crystal structure, and crystallization behavior. Phase Morphology and Crystalline Superstructure of PBS/PLA Composite Films. It is well established that the phase morphology, including the size of the dispersed phase and specific surface area, is a crucial factor that determines the macroscopic properties of polymer blends. Figure 3 clearly manifests a special phase morphology constituted by the disperse phase of PLA. The significant difference from the

Figure 2. Typical stress−strain curves of PBS and composite films. Yield region is amplified for clear identification of yield strength as shown in left.

of the PBS and composite films, and all samples show typical ductile fracture. However, dissimilar tensile behavior is also presented and the composite films show double yield peaks compared with pure PBS film. The first yield occurs in a low strain region, featuring a sharp transition combined with larger yield strength; the second yield exhibits moderate transformation, which has lower yield strength. Beyond that, the tensile curves of the composite films evidently tower over that 2890


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intensive arrangement develops substantially into the formation of an interwoven state. The increased diameter of the nanofibrils can be attributed to the deformation, combination, and agglomeration of the dispersed phase of PLA when it undergoes intensive elongational flow provided by the “slit die extrusion−stretching−woven compression molding” process. As a whole, through the most intuitive approach, we here verify that the specific structure of the composite films, wherein the dispersed phase of PLA in situ forming into nanofibrils, is successfully constructed in the matrix of PBS. X-ray scattering or diffraction is a powerful means for crystallization characterization by virtue of its ability to reveal molecular arrangement and crystal packing at the multiscale. As a complement for SEM observations, 2D-SAXS measurements were further performed to obtain more detailed and quantitative insights into the distribution and orientation of PLA nanofibrils. Representative 2D-SAXS patterns are depicted in Figure 4A−D. Primarily, homogeneous scattering rings appear on all patterns evidencing the isotropic property of the PBS matrix.43 It is certified that in our suitable processing conditions, the matrix phase of PBS could adequately integrate into a homogeneous system. Beyond that, the scattering pattern, as symmetrical bulb-shape lobes close to the beamstop, evidently reveals the generation of oriented PLA lamellae in the nanofibrils for the various composite films. Furthermore, we notice that the symmetrical bulb-shape lobes appear in two orthogonal directions, which reflects that the nanofibrils including a large number of orientated PLA crystals are in the form of overlapping arrangement. Hence, combined with the results of SEM observations and SAXS results, it can be reasonably confirmed that the formation of nanofibrillar networks are composed of abundant PLA nanofibrils with regularly oriented lamellae. The biaxial orientation of the nanofibrillar networks is presumably due to the hot compression of the orthotropic woven ribbons of composites. Therefore, it is authenticated that the orthotropic networks of

Figure 3. SEM images of cryo-fractured surfaces of PBS and PBS/PLA composite films for (a) PBS, (b) PBS/PLA (90/10 w/w), (c) PBS/ PLA (70/30 w/w), and (d) PBS/PLA (60/40 w/w).

traditional “sea island” structure could be intuitively identified as that of fibril-form disperse phase reciprocally overlapping with each other in close resemblance to constituting an array of networks and that the diameter of the fibril is approximately in the nanoscale (50−200 nm). In addition, the content of PLA has an obvious influence on the dimensionality and distribution of the nanofibrils. When the content of PLA is 10 wt %, the diameters of the nanofibrils is in the range of 50−150 nm, and the distribution of the nanofibrils is virtually scattered among the matrix of PBS, not forming into an interlacement of each other. However, once the content of PLA reaches a certain amount (30 and 40 wt %), there is slight increase in terms of the size of the PLA nanofibrils (150−200 nm) and a more

Figure 4. Representative 2D-SAXS patterns of (A) PBS films, (B) PBS/PLA (90/10 w/w), (C) PBS/PLA (70/30 w/w), and (D) PBS/PLA (60/40 w/w). (E) Extracted 1D-SAXS curves show the intensity profiles of the scattering patterns in panels (A)−(D). Peak value q* is marked above the corresponding curve. (F) Evolution of lamellar thickness (dc) for PLA and PBS as a function of PLA content. 2891


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Figure 5. 2D-WAXD patterns of (A) PBS films, (B) PBS/PLA (90/10 w/w), (C) PBS/PLA (70/30 w/w), and (D) PBS/PLA (60/40 w/w). (E) 1D-WAXD intensity profiles of PBS/PLA composite films; the diffraction peaks and their assignments to the specific lattice planes are indicated on the curves. (F) Intensity distribution of the lattice plane (200/110) along the azimuthal angle; the orientation degree of the PLA nanofibrils is marked above the corresponding curve.

bidirectional arc-like diffraction patterns assigned to lattice planes (200)/(100) and (010) of α-form PLA emerging in all composite films confirm again the generation of oriented lamellae in the PLA nanofibrils in a form of crosswise knitted arrangement. In addition, homogeneous diffraction circles with strong intensity for PBS are traced in all samples, which also indicate the isotropic property of the PBS matrix. Figure 5E gathers the 1D-WAXD intensity profiles integrated circularly from the corresponding 2D-WAXD patterns. For direct comparison, one can see that main diffraction reflections from the lattice planes (200)/(110) (11.8 nm−1) and (203) (13.1 nm−1) belong to typical α-form PLA,70 and (020) (13.5 nm−1), (021) (15.1 nm−1), and (110) (15.7 nm−1) are attributed to the α-form PBS,71 which is indicative of the exclusive crystal form of both phases. Moreover, with the purpose of offering more detailed description of PLA nanofibrils, the orientation degree is calculated based on the lattice plane (200)/(110) of the PLA crystals by integrating the intensity distribution along the azimuthal angle (0°−360°) (Figure 5F). A higher orientation degree appears in both PBS/ PLA (70/30) (f H = 0.87) and PLA/PBS (60/40) (f H = 0.91), while a relative weaker orientation emerges in PLA/PBS (90/ 10) ( f H = 0.73). It infers that when the average diameter of the in situ nanofibrils is relatively small, in the process of melt stretching and subsequently cold crystallization, the rearrangement of molecular chains into an ordered state is suppressed by spatial confinement possibly due to a strongly interfacial interaction, which is also proved in our previous works of confined crystallization behavior within electrospun poly(vinylidene fluoride) nanofibrils.72 Therefore, in PBS/PLA (70/30 w/w) and (60/40 w/w), PLA molecular chains would be apt to pack into an established crystal skeleton with certain orientation, thus resulting in higher orientation in the nanofibrils. The combination of X-ray measurements (SAXS/ WAXD) and SEM observation here not only confirms the generation of PLA nanofibrils in the PBS but also determines

PLA nanofibrils composed of densely oriented PLA crystals are successfully created in the isotropic matrix of PBS. The extracted 1D-SAXS intensity profiles of PBS and composite films are illustrated in Figure 4E to provide more quantitative and subtle information about the oriented lamellae in the PLA nanofibrils. It clearly manifests that the curve of the pure PBS films has only one peak, whereas the curves of the composite films have two different peaks of which the weaker peak is assigned to PLA with a lower value of q1*, and the stronger peak is attributed to PBS with larger value of q2*. Furthermore, the technique of one-dimensional electron density correlation function analysis is used to give more structural information on the PBS matrix and PLA nanofibrils; the detailed computational process is provided in Figure S6 of the Supporting Information. Specific data about the lamellar thickness (dc) of PLA and PBS are summarized in Figure 4F. As for PLA, one can distinctly identify that the lamellae are gradually thickened when the quantity of PLA nanofibrils increases. The reason may be that the size increase of the nanofibrils may provide more space for PLA molecular chains, allowing them to pack toward more perfect lamellae. We consider that it would certainly have a higher efficiency of PLA nanofibrils including oriented lamellae with more perfection for gas barrier and reinforcement. With regard to PBS, the lamellar thickness dc would be slightly improved in PBS/PLA (90/10 w/w) compared to that of pure PBS films; however, it will dramatically attenuate in PBS/PLA (70/30 w/w) and (60/40 w/w). We speculate that this phenomenon may be attributed to the confined crystallization of PBS; once the content of the PLA nanofibrils exceeds a certain degree, it may produce spatial confinement on PBS crystallization, which will obstruct PBS molecular chains to pack into crystals. We give more experimental evidence in the discussion on DSC. Molecular orientation and crystal structure are further explored by comparing the 2D-WAXD patterns of PBS and composite films as shown in Figure 5A−D. Evidently, 2892


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Figure 6. (A) Heating traces of DSC for PBS and composite films. (B) Summarized degree of crystallinity (Xc) of PBS and PLA in various samples.

Figure 7. (A) O2 pressure variations in the downstream compartment as a function of the reduced time in the gas barrier measurements. (B) Solubility (SO2) and diffusion (DO2) coefficients of neat PBS and its composite films.

the increase in PLA incorporation a large diameter of nanofibrils provides more unconstrained space for PLA molecular chains to pack into lattice in the process of cold crystallization. As for the PBS phase, a gradual decrease in peak temperature with the introduction of PLA nanofibrils proves the existence of space confinement for PBS produced by intensive PLA nanofibrillar networks. Moreover, with regard to the degree of crystallinity shown in Figure 6B, the overall crystallinity of PBS is distinctly increased in PBS/PLA (90/10 w/w), while in PBS/PLA (70/30 w/w) and (60/40 w/w), the reduction of crystallinity is also clearly recognized. Besides, the quantity of imperfect crystals is certainly gradually reduced with the augment of PLA nanofibrils. The above results adequately support the hypothesis that once the PLA nanofibrils exceed a certain quantity, the intersection between nanofibrils would produce a spatial hindrance on the crystallization of PBS. This imposes restrictions on the growth of PBS crystals, which may be referred to as spatial confinement. Simultaneously, the effect of space confinement may restrict the PBS molecular chains to pack into the crystal lattice easily and brings about a decrease in the average thickness of lamellae (SAXS) and wider distribution. Thus, the endothermic melting peak of the PBS normal crystals in PBS/PLA (60/40 w/w) performances at DSC would become wider, even merging with the adjacent peak of imperfect crystals. The generation and existence of this kind spatial confinement would be beneficial for further enhancement on the barrier property of composite films due to which the gas molecules will suffer a lesser passable volume and experience a more frustrated permeation path in the confined molecular chain system. This fact has also been confirmed in our previous works about investigating the positive role of spatial confinement on the gas barrier

that a highly oriented chain existed in the nanofibrils that would have undoubted influence on the gas barrier and mechanical property. However, in our case, some curiousness is still elicited: How will the presence of the PLA nanofibrils influence the crystallization behavior of PBS matrix? It is also a significant element that cannot be ignored to make a contribution to the enhancement of the overall gas barrier and mechanical property of the composite films. In next section, we would move forward to probe into the crystallization behavior of PBS films through exploring their thermal behaviors. Crystallization Behavior and Thermal Property of Composite Films. DSC was carried out to offer insights into the crystallization behavior of PBS and PLA in composite films. Figure 6A illustrates the DSC heating scans for pure PBS film and its composite films. Apparently, three main transitions are successively displayed on the curves of the composite films with 10 and 30 wt % PLA: (1) A minor melting endotherm at about 101 °C is regarded as the imperfect crystal of PBS.73,74 (2) A more prominent melting endotherm at about 112 °C belongs to the normal crystals of PBS. (3) Relatively unconspicuous endotherm peaks ascribed to the melting of PLA are located at around 168 °C. However, when the content of PLA nanofibrils increases to 40 wt %, an obvious difference is the vanishment of the endothermal peak of PBS imperfect crystals; this is usually deemed as the indication that the crystallization of PBS is confined by the effect of space restriction. In addition, in view of the peak temperatures of the PBS and PLA phases, first, we can find that the PLA exerts a large temperature on PBS/PLA (60/40 w/w) of 170.5 °C compared with the relatively low temperature of 167.1 °C in PBS/PLA (90/10 w/w), which certainly results from a decrease in lamellar thickness. This result verifies the assumption by SAXS and WAXD that with 2893


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could availably serve as a “barrier wall” for gas molecules. In addition, the large specific surface area of nanofibrils can also improve the effectiveness of the gas barrier by virtue of less flaws generated by phase separation. The aforementioned factors work synergistically to reduce the quantity of gas dissolution and compel the diffusion path to be more tortuous. With regard to the mechanical property, the high strength and modulus attribute to sufficiently strengthen the networks consisting of the PLA nanofibrils. The intrinsic rigidity of the PLA molecular chain combined with oriented lamellae endows PLA nanofibrils more strength. Moreover, the interwoven nanofibrils constitute into an integral network, with greater interaction with the PBS matrix further prompting the efficacy of the reinforcement. When stress is encountered, the networks first show a strong retardation of crack propagation, and the applied stress is prone to traverse along the length of them quite easily rather than conventional stress concentration, leading to increased strength and modulus.76 Once the composite films experience yielding, the PBS matrix and PLA networks are destroyed, but the nanofibrils can also construct crack bridging with the dissipation and absorption of much energy.77 Therefore, there is a secondary yield in composite films. In summary, the hierarchical structure conceived in this work exhibits prodigious power in synergistically meliorating the gas barrier and mechanical performance.

property.75 Beyond that, the crystallinity degree of PLA presents an increased tendency for the amount of PLA nanofibrils, keeping consistent with the conclusion of SAXS and WAXD that the crystallization within smaller sizes of nanofibrils may be restricted. Therefore, incorporating the above results, we get a reasonable description of the hierarchical structure between the PLA nanofibrils and PBS matrix. In the vicinity of the PLA nanofibrils networks, the effect of spatial confinement would suppress the crystallinity of PBS. However, the restricted molecular chains of PBS will join together with the PLA nanofibrils to establish a conjoint construction to enlarge the effective barrier area or networks. Mechanism of Simultaneously Enhanced Gas Barrier and Mechanical Property. The superior combination of the gas barrier and mechanical performance for the composite film is certainly attributed to the dense PLA nanofibrillar network composed of oriented lamellae. For the gas barrier property, the drastic enhancement results from the tortuous effect and the effective reduction of free volume, brought about by PLA nanofibrillar networks with densely oriented crystals coupled with a constrained environment caused by PBS crystals. The specific reason for the decline in oxygen permeability is interpreted in Figure 7. As shown in Figure 7A, typical time lag permeation curves in terms of oxygen pressure variations in the downstream compartment as a function of the reduced time for oxygen permeation of neat PBS and its composite films are plotted, where oxygen diffusion establishes a constant flow after a transitory nonsteady state. One can clearly see that with an increase in PLA nanofibrils, the nonlinear region (nonsteady state) becomes increasingly larger, demonstrating more the difficult dissolution course of gas molecules in the films. Meanwhile, the slope of linear region (steady state) gets correspondingly gradually diminished, indicating more laborious diffusion of gas molecules. The detailed data of solubility (SO2) and diffusion (DO2) coefficients have been extracted from permeation curves as plotted in Figure 7B. Obviously, the synchronous decrease in SO2 and DO2 is indeed identified, which verifies the above description of the specific permeation behavior of the gas in the composite films. The schematic representation shown in Figure 8 illustrates the gas barrier mechanism of the nanofibril-reinforced composite films. Considering the microstructure of the PLA nanofibrils in which highly oriented lamellae periodically arrange themselves along a specific direction, the molecular chains of PLA tightly pack with each other providing less free volume for gas permeation. So, the organizational networks of the nanofibrils

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CONCLUSION In this work, we creatively fabricate composite films constructed by nanofibrillar networks with highly molecular orientation, successfully obtaining a preferable gas barrier property combined with superior strength and ductility. On the basis of morphological observations and quantitative analyses, it demonstrates that the dispersed phase of PLA in situ takes shape into nanofibrils within the matrix of PBS under the action of an elongational flow field. Through an orthotropic weaving and compression molding process, the nanofibrils tend to make up a kind of network, wherein the highly oriented lamellae of PLA is regularly arranged. Moreover, by means of studying the crystallization behavior of this heterogeneous system, it is found that PLA nanofibrillar networks have certain spatail confinement on the crystallization of PBS. The enhancement in gas barrier and mechanical properties is achieved by constructing this hierarchical structure. The PO2 can be reduced more than 60% compared with that of pure PBS film, and the tensile strength increases nearly twice. Meanwhile, the ductility certainly does not deteriorate. All results demonstrate that the protocol proposed by this work opens a promising door to industrially manufacturing green composite films with superior properties, and the tailored structure paves a promising way to further create new materials with high performance and multifunction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00590. Schematic diagram of “slit die extrusion−stretching− woven compression molding” process. Oxygen permeability of PBS film and various PBS/PLA films by common blending. Mechanical property of PBS film and various PBS/PLA films by common blending. SEM

Figure 8. Schematic illustration of oxygen molecules following a relatively tortuous path through composite films with nanofibrillar network of densely oriented crystals coupled with confined environment of PBS. 2894


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images of cryo-fractured surfaces of PBS/PLA films by common blending. 2D-WAXD and 2D-SAXS patterns of PBS/PLA films by common blending. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: (+86) 28-8540-0211. Fax: (+86) 28-8540-5402. E-mail: [email protected] (G.-J. Zhong). *Tel: (+86) 28-8540-0211. Fax: (+86) 28-8540-5402. E-mail: [email protected] (Z.-M. Li). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science of China (Grant Nos. 51533004, 21576173, 51403139, and 51473101), Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421061), and State Key Laboratory of Polymer Materials Engineering (Grant sklpme 2014-3-08). Our work was also supported by the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant 2014TD0002). The authors are grateful for the kind help and support of the Shanghai Synchrotron Radiation Facility (SSRF) for SAXS and WAXD measurements and the analysis of the results.

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