Enhancing the Oxygen-Barrier Properties of Polylactide by Tailoring

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Enhancing the Oxygen Barrier Properties of Polylactide by Tailoring the Arrangement of Crystalline Lamellae Chunhai Li, Ting Jiang, Jianfeng Wang, Shuangjuan Peng, Hong Wu, Jiabin Shen, Shaoyun Guo, Xi Zhang, and Eileen Harkin-Jones ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00026 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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ACS Sustainable Chemistry & Engineering

Enhancing the Oxygen Barrier Properties of Polylactide by Tailoring the Arrangement of Crystalline Lamellae

Chunhai Lia, Ting Jiangb, Jianfeng Wang a, Shuangjuan Peng a, Hong Wu a*, Jiabin Shena* Shaoyun Guoa, Xi Zhang a, Eileen Harkin-Jones a aThe

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

University, Chengdu 610065, China bSchool

of Chemistry, State Key Laboratory of Biotherapy of Sichuan University, Chengdu 610065, China

Full mailing address of all the authors are listed as follows: Chunhai Lia: E-mail: [email protected], South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN Ting Jiangb: E-mail: [email protected], South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN

Shuangjuan Peng a: E-mail: [email protected], South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN Hong Wu a: E-mail: [email protected], South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN Shaoyun Guoa: E-mail: [email protected], South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN Xi Zhang a: E-mail: [email protected] South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN Eileen Harkin-Jones a: E-mail:[email protected], South Section 1, Yihuan Road Chengdu, Sichuan, 610065, CN * Corresponding author. Tel: 028-85466077. E-mail: [email protected]. (Hong Wu) * Corresponding author. Tel: 028-85466077. E-mail: [email protected] (Jiabin Shen)

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ABSTRACT: The gas barrier properties of semicrystalline polymers can be significantly adjusted by tailoring the arrangement of their impermeable crystalline lamellae. In particular, the highest barrier efficiency is achieved when the arrangement of the lamellae stacks are perpendicular to the direction of gas diffusion. The work reported on in this paper provides a strategy to achieve such lamellar arrangement with the aid of a self-assembly nucleator and a two-dimensional interface. PT (PLA+TMC-300) and PTG (PLA+TMC-300+Graphene) were coextruded to form alternating PT/PTG multilayers with different layer numbers. During isothermal treatment at 140 °C, the dissolved TMC-300 first self-assembles into solidstate fibrils which are perpendicular to the two-dimensional PT/PTG layered interface due to the induced effects of graphene. Subsequently, these TMC-300 fibrils induce the epitaxial growth of PLA lamellae with a normal parallel to the fibrillar direction of the TMC-300. In this way, a designed arrangement where the PLA lamellae stack perpendicular to the direction of gas diffusion is achieved. As expected, the resulting PLA exhibits impressively enhanced gas barrier properties: a decrease of 85.4 % in oxygen permeability coefficient (𝑃𝑂2 ) was observed for the 16-layer sample (0.7 ×10-19 m3·m/m2·s·Pa) compared with the sample without layer structure. Through the construction of “lamellae-barrier walls” by tailoring the arrangement of lamellae, this work provides a route to fabricate semicrystalline polymers with superior gas barrier properties with great potential for use in high barrier applications such as food packing, beverage bottles and fuel tanks. KEYWORDS：Crystallization, Self-Assemble, Multilayers, Polylactide, Graphene, Gas Barrier


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INTRODUCTION Today, more than 40% of synthetic plastics are utilized as packaging materials in applications ranging from food to cosmetics and pharmaceutics.1 However, almost all the currently used plastics are petroleumbased and do not biodegradable in landfills, thereby making a significant contribution to natural resources crisis and causing “white pollution”.2 Consequently, growing attention has been paid to developing biodegradable biopolymers and innovative processing technologies that not only reduce the dependence on fossil fuels but also move to a sustainable material basis.3-7 Numerous types of biopolymers have been introduced for various packaging applications. Among them, polylactide (PLA) holds the biggest production share (38%) due to its excellent transparency, easy processability, good mechanical strength, and relatively low cost.8 However, PLA has poor gas barrier properties, especially for oxygen barrier 9-10 and this deficiency needs to be overcome if PLA is to be successfully used in packaging or other applications requiring good barrier performance. The gas barrier properties of PLA can be improved by inclusion of impermeable nanosheets such as clay and graphene because the gas penetrant molecules have to adopt a tortuous diffusion path around the impermeable nanosheets.11-15 Theoretical models and empirical studies show that the barrier properties of nanosheet/polymer composites are strongly affected by factors such as the aspect ratio, dispersion and orientation of the nanosheets, the nanosheet/polymer interface, and the crystallinity of the polymer matrix.11,

16

For example, to significantly improve the barrier properties in such a nanocomposite, a

sufficient quantity of uniformly dispersed nanosheets with a high degree of alignment is required.17-18 However, as the loading of nanosheets increases so also does the tendency to agglomerate which in turn



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reduces the barrier (and mechanical) properties. The incorporation of impermeable nanosheets into a polymer does not therefore always lead to a satisfactory improvement in barrier properties.17, 19 For semicrystalline polymers with crystalline and amorphous phases, the crystalline lamellae can also be regarded as impermeable to gas molecules similar to the inorganic nanosheet and their gas barrier properties can be adjusted by manipulating their degree of crystallinity.

20-22

Increasing the crystallinity

usually gives rise to an enhancement of the gas barrier properties of most semicrystalline polymers like polypropylene (PP) and polyethylene (PE) because crystalline phases are excluded volume through which the gas cannot penetrate.23-24 The importance of the arrangement and orientation of the crystalline lamellae to the gas barrier properties have also been demonstrated by Eric Baer’s group in their work on alternating polymer multilayer structures. Here, a two-dimensional confined space is provided for the confined crystallization of poly(ethylene oxide) (PEO),25 poly(ε-caprolactone) (PCL),26 thereby leading to highly oriented lamellae and an unexpected reduction of 99.99 % in oxygen permeability (𝑃𝑂2 ) compared with the random bulk spherulitic structure. Using similar methods, Stephane Marais’s group also demonstrated a reduction of 74.0 % in 𝑃𝑂2 for poly(butylene succinate-co-butylene adipate) (PBSA).27 Unlike common semicrystalline polymers, the barrier properties of PLA exhibit a complicated dependence on crystallinity and increasing the crystallinity does not always improve the barrier properties.28-29 Colomines et al. even found similar barrier properties with an amorphous PLA and a semicrystalline PLA.21 However, the formation of perfect crystals with an oriented, closely packed arrangement is favorable for enhancing the barrier properties of PLA. For example, decorating poly(butylene adipate-co-terephthalate) (PBAT) nanofibrils with oriented and closely packed PLA


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lamellae causes a reduction of 89 % in 𝑃𝑂2 .30 Filling in the space between graphene oxide sheets (GO) with PLA lamellae produces robust GO/lamellae “barrier walls”, leading to a reduction of 78 % in 𝑃𝑂2 compared with a GO/PLA control sample.30-31 More interestingly, by constructing parallel-aligned shish kebab-like crystals with well-interlocked boundaries where the lamellae (kebab) packs densely to form an impermeable nanobrick wall, Bai et al. unprecedentedly achieved a reduction of 97.4% in 𝑃𝑂2 compared with spherulitic PLA.32 All this work serves to support the fact that the barrier properties of PLA can be significantly improved by constructing impermeable “lamellae-barrier walls”, in which the PLA lamellae are packed closely and regularly to resist gas penetration. From a geometrical perspective, “lamellae-barrier walls” where the lamellae are stacked perpendicular to the direction of gas diffusion are the most efficient structure for resisting gas penetration.16, 18 However, to the best of our knowledge, only Baer’s group has achieved such lamellar arrangement so far.25-26 In our previous work, we demonstrated that octamethylenedicarboxylic dibenzoylhydrazide (TMC-300), a highly active nucleator for PLA, can be totally dissolved in a PLA melt subsequently self-assemble into fibrils upon cooling and then induce the growth of PLA lamellae.33 Based on this mechanism we propose a strategy to construct the “lamellae-barrier walls” with the optimum lamellar arrangement.17,

30

As shown in Figure 1, this strategy involves (1) fabricating alternating

(PLA+TMC-300)/(PLA+TMC-300+graphene) multilayers (Figure 1(A)); (2) graphene induces the TMC300 to self-assemble into fibrils that are perpendicular to the two-dimensional layered interface (Figure 1(C)); (3) the self-assembled TMC-300 fibrils induce the growth of PLA lamellae that are perpendicular to the fibrillar direction of the TMC-300 fibrils, i.e., the lamellae are stacked perpendicular to the direction



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of gas diffusion (Figure 1(D)). As expected, such lamellae arrangement confers PLA with an impressive enhancement in gas barrier properties, leading to a reduction of 85.4 % in 𝑃𝑂2 compared with the blended control sample.

Figure 1. (A) Scheme for the fabrication of multilayers consisting of PT alternating PTG multilayers; (B~D) thermal treatment achieves the “lamellae-barrier walls”; (E) gas diffusion path in PLA with conventional spherulites.


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EXPERIMENTAL SECTION Materials. PLA comprising 2% DLA, trade name 4032D, was purchased from Nature Works Co. (U.S.A.). It has a density of 1.24 g/cm3, and its MFR is 7 g/10 min at 210 ℃, 2.16Kg. Octamethylenedicarboxylic dibenzoylhydrazide (TMC-300) was provided by Shanxi Provincial Institute of Chemical Industry, China. The chemical structure of TMC-300 is given in Figure S1. Graphene powder, trade name SE1231, was purchased from The Sixth Element Materials Technology Co. Ltd, China. Sample Preparation. PLA, TMC-300 and graphene were dried in a vacuum oven at 80 °C for 12 h before melt processing. Prior to coextrusion, PLA/TMC-300 composite pellets containing 0.5 wt.% TMC300 and PLA/TMC-300/graphene containing 0.5 wt.% TMC-300 and 1 wt.% graphene were prepared using a SHJ-20 twin-screw extruder (screw diameter = 21.7 mm, L/D = 40:1; Nanjing Giant Co. Ltd., China) with an extrusion temperature of 190 °C and extrusion screw speed of 125 rpm. Subsequently, a multilayer sheet consisting of PT (PLA+TMC-300) alternating with PTG (PLA+TMC-300+graphene) was prepared using multilayer coextrusion. As schematically illustrated in Figure 1(A), the PT pellets and PTG pellets were simultaneously extruded from extruder A and extruder B, combined as a two-layer melt in the coextrusion connector, and then flowed through a series of LMEs (Layer Multiplying Elements). In a LME, the melt is first sliced into two left and right sections by a divider, and then each section flows up and down through two channels; before flowing into the next LME, the polymer melt is vertically recombined. When n LMEs are applied, a materials with 2(n+1) layers can be produced. In this work, 4-, 8-, 16-, 32- and 64-layer materials were fabricated by using 1, 2, 3, 4 and 5 LMEs, respectively. By controlling the feed speed, the total thickness



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of each extrudate was maintained at about 1.8 mm, and the thickness ratio of the PT layer and PTG layer was kept at 6.3:1. To prepare the control sample, the multilayer extrudate was first mixed in an internal mixer (HAAKE, Thermo Scientific, U.S.A.) using 45 rpm min-1 for 8min at 190 °C, and then compression molded on a hot press (KT-0701, China) at 190 °C to form a sheet with a thickness of 1.8 mm. Thermal Treatment Achieves the “Lamellae-Barrier Walls”. The temperature protocol for the thermal treatment of PLA is illustrated in Figure 2. The multilayer extrudate was held at 210 °C for 5 min so as to melt the PLA crystallites and dissolve the TMC-300, and then cooled to 140 °C for 40 min for both the self-assembly of TMC-300 and the growth of the PLA lamellae. The samples were then immediately quenched in ice water. For comparison, the control sample was also treated at the same conditions. After thermal treatment, the crystallinity of all samples was about 50% (by DSC).

Figure 2. Temperature protocol for the thermal treatment.

Characterization and Testing. Differential Scanning Calorimeter (DSC). The thermal analysis of the samples was performed under a nitrogen atmosphere using a DSC instrument (Q2000, TA Instruments). For nonisothermal crystallization, samples with weight of 7~8 mg were kept at 210 °C for 5 min to remove


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the thermal history and then cooled to 20 °C. After that, the samples were reheated to 210 °C at a scanning rate of 10 °C/min. For isothermal crystallization, the samples were also kept at 210 °C for 5 min to remove the heat thermal history and then cooled to 140 °C for 40 min at a cooling rate of 80 °C/min. The crystallinity (𝑋𝑐 ) was calculated by using following equation: ∆𝐻

𝑋𝑐 = ∆𝐻𝑚 0 × 100% 𝑚

(1)

0 Where the ∆𝐻𝑚 , is the enthalpy of melting for PLA, and ∆𝐻𝑚 is the enthalpy of melting for a 100%

crystalline PLA (93.7 J/g).34-35 Polarized Optical Microscopy (POM). First, a thin slice of approximately 20 μm was obtained from the multilayer extrudate by microtoming along the extrusion direction. Then both the self-assembly behavior of TMC-300 and the growth of PLA lamellae were observed using a POM (BX51, OLYMPUS) equipped with a crossed polarizer and a video camera. The temperature protocol was programmed according to Figure 2 on a hot-stage (HCS 302, INSTEC). Scanning Electronic Microscopy (SEM). The thermally treated extrudate was placed in liquid nitrogen for 2h and the surface of the fractured sample blocks was chemically etched using a watermethanol (1:2, v:v) solution containing 0.025 mol/L sodium hydroxide for 12h at 30 °C. After that, the etched surface was cleaned with distilled water and ultrasonication. A field-emission SEM (JSM-5900LV, Japan) was utilized with an acceleration voltage of 10 kV. All samples were sputter-coated with gold prior to observation. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). 2D-WAXD was conducted on a Xeuss 2.0 system of Xenocs, with a sample-to-detector distance of 172 mm. A multilayer focused Cu Kα X-ray



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source (GeniX3D Cu ULD, Xenocs SA, France, λ = 0.154 nm) and scatterless collimating slits were used during the experiments. The size of the primary X-ray beam at the sample position was 0.5 × 0.5 mm2. WAXD images were recorded with a Pilatus 300K detector of Dectris, Swiss (680 pixels × 600 pixels, pixel size = 172 μm). A silver behenate standard was used to calibrate the parameters of the scattering geometry (i.e., beam center and sample-to-detector distance). Gas Barrier Measurements. Oxygen permeation analysis of the multilayer extrudate and the control sample was carried out using 50mm diameter discs in a VAC-V1 permeability testing machine (Labthink Instruments, Jinan, China) at 40 °C, 50% relative humidity according to ISO2556:1974 and based on the differential pressure method. The gas permeation cell was separated into two compartments by the extruded sheets. Air in both compartments was evacuated to ensure that the static vacuum pressure changes in the 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 × 10 5 Pa, and the pressure variations in the downstream compartment was recorded as a function of time. 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 can be determined from the permeation 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.


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RESULTS AND DISCUSSION Two-Dimensional Layered Interface Induce TMC-300 Self-Assemble Perpendicular to Layered Interface. TMC-300 is one of the derivatives of benzhydrazide (Figure S1). Its self-assembly behavior is driven by its intermolecular interactions, such as hydrogen bonding among hydrazide moieties, stacking among aromatic groups.

33, 36-37

As shown in Figure 3(B), the dissolved TMC-300 can self-

assemble into long fibrils with several branches. These fibrils serve as shish to induce the epitaxial growth of the PLA lamellae approximately orthogonal to their fibrillar direction, leading to a shish-kebab-like structure unlike the isotropic spherulite observed in neat PLA (Figure 3(A)). We propose that TMC-300 can self-assemble on the surface of graphene sheets due to dual interactions. One is the hydrogen bond between the hydrazide moieties in TMC-300 and the hydroxyl or carbonyl on the surface of graphene sheets, the other is the π-π stacking interaction between the aromatic groups in TMC-300 and the sp2hybridized carbon in graphene. This proposal is verified through the 2-layer PT/PTG sample, in which TMC-300 is totally dissolved in both the PT and PTG layer at 210 °C (Figure 3(C)) and starts to selfassemble into fibrils that are perpendicular to the PT/PTG layered interface upon cooling to 140 °C (Figure 3(D)). As TMC-300 fibrils are a highly active nucleator for the growth of PLA lamellae approximately orthogonal to their fibrillar direction,33 the preferential orientation of TMC-300 fibrils provides an ideal prototype to construct the “lamellae-barrier walls” with the lamellae stacked perpendicular to the direction of gas diffusion, as shown in Figure 1.



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Figure 3. POM micrographs of crystal morphology for (A) neat PLA isothermal crystallization at 130 °C for 1500 s and (B) PLA with 0.5 wt.% TMC-300 after isothermal crystallization at 140 °C for 180 s; OM micrographs of (C) the 2-layer of sample at 140 °C for 0 s and (D) TMC-300 self-assemble perpendicular to the two-dimensional layered interface (POM: polarized field; OM: bright field). Figure 4 shows the OM micrographs of 8-, 16-, 32-, 64-layer PT/PTG multilayers, where the PT and PTG layers align alternately vertical to the interface and continuously parallel to each other. The PT layer of the 8-layer sample is filled up by both oriented and random TMC-300 fibrils. The oriented TMC-300 fibrils (127 μm) induced by the PT/PTG layered interface grew perpendicular to the layered interface while the random TMC-300 fibrils (141 μm) induced by the homogenous nucleation of TMC-300 grew


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randomly. Since the interface induced TMC-300 fibrils (oriented section in Figure 4a) have a limited length up to 127 μm, there is no random TMC-300 fibrils for the 16-, 32- and 64-layer samples because the interface induced fibrils self-assemble prior to the random fibrils (induced by homogenous nucleation of TMC-300 itself) and fill up the whole PT layer as the thickness of the PT layer in these samples is lower than 2×127μm.

Figure 4. OM micrographs of the final crystalline morphology of the PT/PTG multilayer with different layer number. In order to observe the self-assembly process of TMC-300 and the subsequent growth of PLA lamellae, the 16-, 32- and 64-layer samples were selected for isothermal heat treatment using a hot-stage.



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The samples were heated to 210 °C for 3 min and then cooled to 140 °C at a ramp rate of 80 °C/min. As shown in Figure 5(A1~C1), TMC-300 is completely dissolved in the PT and PTG layer when the temperature reduces to 140 °C. TMC-300 self-assembles into fibrils perpendicular to the PT/PTG layered interface after about 60 s (Figure 5(A2~C2)). As the isothermal time progresses, these self-assembling

Figure 5. OM micrographs show that TMC-300 self-assembles into fibrils in PT/PTG multilayer and subsequently induce the growth of PLA lamellae.


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TMC-300 fibrils induce epitaxial growth of the PLA lamellae approximately orthogonal to their fibrillar direction, forming “lamellae-barrier walls” where the lamellae are stacked perpendicular to the direction of gas diffusion (Figure 5(A3~C4)). Interestingly, the TMC-300 tends to branch away from the primary fibrils once the primary fibrils grow to a fixed length (24.8 μm) during self-assembling (Figure 3(B) and Figure 5(A2~C2)), thereby producing space confinement to force the branched fibrils to grow with a higher orientation compared to the primary fibrils. The average length of primary TMC-300 fibrils (LP), branch TMC-300 fibrils (LB), and their corresponding percent content PP, PB, were measured and are listed in Table 1. Since the primary TMC-300 fibrils with a length of 24.8 μm are always located on both sides of the PT layer, the percent content of primary fibrils increases with layer number, from 6.2% in a 4-layer

Table 1. The average thickness of the PT layer (TPT) and the PTG layer (TPTG), the average length of primary TMC-300 fibrils (LP) and branch TMC-300 fibrils (LB), the percent content of primary TMC-300 fibrils (PP =LP/(LP + LB)) and branch TMC-300 fibrils (PB=LB/(LP + LB)), and the oxygen permeability (𝑃𝑂2 ) of the PT/PTG multilayer with different layer number (all samples were isothermal at 140 °C for 40 min). 𝑃𝑂2 (×10-19

Samples

TPTG (μm)

TPT (μm)

LP (μm)

LB (μm)

PP (%)

PB (%)

LR (μm)

m3·m/m2·s·Pa)

4-Layer

115.8

801.2

2×24.8=49.6

204.4

6.2

19.3

547.2

3.3

8-Layer

63.1

394.7

49.6

204.4

12.6

35.6

140.7

1.2

16-Layer

30.5

194.0

49.6

144.4

25.6

74.4

0

0.7

32-Layer

12.1

72.7

49.6

23.1

68.2

31.8

0

1.6

64-Layer

7.8

46.5

46.5

0

100

0

0

3.2



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sample up to 100% in a 64-layer sample. However, the 16-layer sample exhibits the maximum percent of branch fibrils (74.4 %) because further increasing the layer number makes the thickness of the PT layer lower than 2×127 μm, resulting in the lack of adequate space for growth of branch fibrils. The 16-layer sample also exhibits the optimum gas barrier performance when compared with the other samples. This can be attributed to its maximum percent of branch fibrils which cause a more regular arrangement of PLA lamellae.

Figure 6. (a) DSC cooling curves and (b) melting curves of layered samples at a scanning rate of 10 °C/min; (c) DSC thermographs of layered samples isothermally crystallized at 140 °C and (d) the relative crystallinity as a function of crystallization time at 140 °C (the short dash line represent the region that


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were selected for the calculation of the enthalpy).

As aforementioned, graphene sheets act as heterogeneous nucleating sites to accelerate the selfassemble of TMC-300. This means that the presence of graphene sheets first accelerates the self-assemble of the dissolved TMC-300 and therefore promotes the overall crystallization of PLA. This can be verified by referring to the results shown in Figure 6, in which the crystallization kinetics of the PTG layer containing graphene sheets is obviously faster than the PT layer without graphene sheets. Moreover, the graphene sheets located at the PT/PTG layer interface of the PT/PTG multilayers can also induce the selfassemble of TMC-300 in the PT layer (Figure 3(D) and Figure 5). Consequently, the overall crystallization kinetics of PT/PTG multilayer exhibits an upward trend with increasing layer number (Figure 6(c), (d)) because the layered interface area increases with increasing layer number. Crystalline Morphology and Orientation. Due to the low resolution of POM, the morphology and arrangement of PLA lamellae cannot be adequately observed. In order to get more detailed microscopic information, the 16-layer and 64-layer samples were also characterized using the SEM. Before observation, the samples were etched with a methanol-water solution to selectively remove both the amorphous regions and the TMC-300 fibrils from the crystals. Figure 7A with a low resolution shows two PTG layers and one PT layer of the 16-layer sample. As can be seen, the size of the lamellae in the PT layer is larger than that in PTG layer. This is because the TMC-300 fibrils in PTG layer are much denser and shorter than those in the PT layer thereby producing more nucleation sites in the PTG layer and sharply decreasing the size of the PLA lamellae. Figure 7a~c are the details of Figure 7A at an enlarged scale. Cleary, there are



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Figure 7. Micrographs of 16- and 64-layer sample obtained from SEM under different magnification. The yellow rectangles denote the initial position of micrographs before enlarged; the red circles represent the out-of-plane lamellae, the yellow dash lines represent the PT/PTG interface, the blue circles and lines represent TMC-300 fibrils (romoved by etching solvent). few out-of-plane lamellae (marked by the red circles) and almost all the lamellae exhibit an in-plane arrangement. This is because the oriented TMC-300 fibrils induce epitaxial growth of PLA lamellae


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approximately orthogonal to their fibrillar direction (parallel to layer plane). As a result, only the landscape characterized by the lateral edge of the lamellae can be observed (Figure 7a~c). This is consistent with our initial design (Figure 1) and the OM results (Figure 5). However, as can be observed, rather than the regular in-plane arrangement where the lateral edge of the lamellae are strictly parallel to the PT/PTG layered interface, it is the moderate in-plane arrangement where the lateral edge of the lamellae show a preferential orientation along the PT/PTG layered interface. This moderate in-plane arrangement can also be supported by the 2D-WAXD pattern as shown in Figure 8 (upper panel), from which the α (200)/ (110) exhibits a diffraction circle with a strong arc at the equator. For the 64-layer sample, there is no longer the branch TMC-300 fibrils in its PT layer (Table 1), leading to a lower regular arrangement of lamellae where the more out-of-plane lamellae can be observed (Figure 7d). Unlike the clear layer structure in the 16layer sample, its layer structure is no longer distinguishable by SEM, and this should also be attributed to its lower lamellae arrangement.

Figure 8. 2D-WAXD patterns for the PT/PTG multilayers performed along the layer plane (upper panel)



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and perpendicular to the layer plane (down panel).

In order to further explore the arrangement of PLA lamellae, a series of 2D-WAXD patterns recorded along the layer plane (upper panel) and perpendicular to the layer plane (down panel) are presented in Figure 8. From inner to outer, the reflection circles or arcs originated from α(010), α(200)/(110), α(203) and α(015) lattice plane of PLA,38 more detailed information can be obtained from the 1D-WAXD intensity profiles extracted from 2D-WAXD (Figure S2). As illustrated by the scheme in the upper-left of Figure 8, since the TMC-300 fibrils lie perpendicular to the PT/PTG layered interface and the TMC-300 fibrils serve as shish to symmetrically induce the growth of PLA lamellae, the lamellae show in-plane arrangement with fiber symmetry. The fiber symmetry means that revolving the samples around the normal of the layer plane (direction perpendicular to the layered interface) does not change the WAXD patterns.39 This inplane arrangement is verified by the 2D-WAXD patters performed along the layer plane (Figure 8, upper panel), where the α(200)/(110) shows diffraction circle with strong arc at the equator. The arc originates from the in-plane arrangement of lamellae and its intensity depends on the degree of the in-plane arrangement. Clearly, the 16-layer sample shows the highest arc intensity, suggesting its highest in-plane arrangement compared with other samples. However, once the 2D-WAXD patterns were performed perpendicular the layer plane (Figure 8, down panel), only the isotropic diffraction circles are presented because of the fiber symmetry.39 Barrier Properties. It is apparent from Figure 9A that the construction of dense and impermeable “lamellae-barrier walls” by the PT/PTG multilayers greatly benefits the barrier properties of PLA. The


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Figure 9. (A) oxygen permeability (𝑃𝑂2 ) of the quenched and 140 °C isothermally treated samples; (B) the curves of the solubility (𝑆𝑂2 ) and diffusivity (𝐷𝑂2 ) coefficient belonging to 140 °C isothermally treated samples; (C) 𝑃𝑂2 of the 16-layer sample with various crystallinity degrees; (D) comparison of the percentage reduction of the 𝑃𝑂2 between this work and other PLA based composites, the percentage reduction is defined as the decrease of 𝑃𝑂2 to the initial value of control sample,

PLA/GO,31

PLA/TMC-328,32

PLA/PBS,9

PLA/PBF,42

PLA/GO,17

PLA/GONSs,40

PLA/O-MMT,41

PLA/C16-MMT.43


PLA/PBAT,30


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𝑃𝑂2 of the quenched PT/PTG layer samples show a gradual reduction with increasing layer number, from the 7.2 ×10-19 m3·m/m2·s·Pa in the 4-layer sample to 4.5 ×10-19 m3·m/m2·s·Pa in the 64-layer samples. This may be attributed to the greater orientation of the graphene sheets in the samples with high layer number since the shear field is higher in the high layer samples (Figure 1(A)). The reduction of 𝑃𝑂2 is further promoted by the presence of dense and impermeable “lamellae-barrier walls” which resist oxygen permeation. This can be exemplified by the distinct decrease of 85.4% in 𝑃𝑂2 in the 16-layer sample after crystallizing at 140 °C for 40 min. Additionally, compared with primary TMC-300 fibrils, the branched TMC-300 fibrils exhibit a higher orientation and tend to induce the PLA lamellae growth into a more regular arrangement. Since the 16-layer exhibits the highest content of branch fibrils (Table 1), a more regular in-plane arrangement of PLA lamellae will be obtained (Figure 8). Therefore, the 16-layer sample has the best oxygen barrier properties compared with the other layer samples. The diffusion (𝐷𝑂2 ) and solubility (𝑆𝑂2 ) coefficients of the samples after thermal treatment are also shown in Figure 9B. Interestingly, there is no obvious change of 𝑆𝑂2 for the different samples and this should be attributed to their same component and approximate crystallinity degree. However, the 𝐷𝑂2 displays a similar trend to the 𝑃𝑂2 when increasing the layer number. The 𝐷𝑂2 first decreases by 78.6% from the 3.4 ×10-11 m2/s in the 4-layer sample to 0.7×10-11 m2/s in the 16-layer sample, and then climbs to 3.6 ×10-11 m2/s in the 64-layer sample. Combining the variation of 𝑃𝑂2 , 𝐷𝑂2 and solubility 𝑆𝑂2 values it can be easily speculated that the distinct decrease of 𝑃𝑂2 in PT/PTG multilayers is mainly caused by the large decrease of the 𝐷𝑂2 , which results from the presence of the dense and impermeable “lamellaebarrier walls” that resist the diffusion of oxygen (Figure 1(D). In order further verify this mechanism, the


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variation of 𝑃𝑂2 in the 16-layer sample as a function of crystallinity degree (0-48%) was studied. As shown in Figure 9C, the value of the 𝑃𝑂2 does not decrease greatly as expected when the crystallinity degree increases from the 0% to 28.8%, where only the lower regular lamellae induced by the primary TMC-300 fibrils are present. However, the 𝑃𝑂2 decreases dramatically when further increasing the crystallinity degree from 28.8% to 48.0, suggesting that the higher regular lamellae induced by the branched TMC-300 fibrils are responsible for the improved barrier properties. Figure 9D clearly shows the contribution of this work to the design of PLA with enhanced oxygen barrier performance. CONCLUSION In this work, graphene was selectively distributed in specific layered space of a PLA/TMC-300 matrix through microlayer coextrusion to form PT (PLA+TMC-300) alternating PTG (PLA+TMC300+Graphene) multilayers with different layer numbers. When isothermally treated at 140 °C, the TMC300 first self-assembles perpendicular to the PT/PTG layered interface due to the induced effect of graphene. Subsequently, the self-assembled TMC-300 fibrils induce the growth of PLA lamellae approximately orthogonal to their fibrillar direction. This leads to an in-plane arrangement where the PLA lamellae are densely stacked into impermeable “lamellae-barrier walls” which resist gas permeation. Compared with the control sample with random arrangement of lamellae, a distinct reduction of 85.4% in oxygen permeability coefficient (𝑃𝑂2 ) is obtained for the layered sample. This work therefore shows a superior performance in PLA gas barrier and thus may help PLA compete with other petroleum based polymers such as poly (ethylene terephthalate) (PET) for barrier applications.





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SUPPORTING INFORMATION Molecular structure of TMC-300, 1D-WARD diffraction profiles of layered samples.



AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]. Fax: 86-028-85466077. *E-mail: [email protected]. Fax: 86-028-85405135. Author Contributions Chunhai Lia and Ting Jiangb contributed equally to this work. 

ACKNOWLEDGEMENTS Financial support of the National Natural Science Foundation of China (51573118, 51227802 and

51721091), Program for New Century Excellent Talents in University (NCET-13-0392), Sichuan Province Youth Science Fund (2015JQ0015) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-15R48) are gratefully acknowledged. 

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For Table of Contents Use Only GRAPHIC ABSTRACT: Enhancing the oxygen barrier properties of polylactide for green packing materials by constructing in-plane oriented crystalline lamellae through the TMC-300 fibrils that induced by the two-dimensional layered interface.


Enhancing the Oxygen-Barrier Properties of Polylactide by Tailoring

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