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Constructing Sandwich-Architectured Poly(L-lactide)/High-MeltingPoint Poly(L-lactide) Non-Woven Fabrics: Towards Heat Resistant Poly(L-lactide) Barrier Biocomposites with Full Biodegradability Tao Gao, Shun-Jie Zhao, Rui-Ying Bao, Gan-Ji Zhong, Zhong-Ming Li, Ming-Bo Yang, and Wei Yang ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.9b00056 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Bio Materials

Constructing Sandwich-Architectured Poly(L-lactide)/High-Melting-Point Poly(L-lactide) Non-Woven Fabrics: Towards Heat Resistant Poly(L-lactide) Barrier Biocomposites with Full Biodegradability Tao Gao, Shun-Jie Zhao, Rui-Ying Bao*, Gan-Ji Zhong, Zhong-Ming Li, Ming-Bo Yang, Wei Yang*

College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, 610065, Sichuan, People’s Republic of China.

Abstract Heat resistant poly(L-lactide) (PLLA) barrier biocomposites with full biodegradability was realized through the construction of local oriented and compact transcrystallinity supernetworks in the network of high-melting-point poly(L-lactide) (hPLLA) non-woven fabrics composed of high-efficiency nucleating hPLLA fiber through design of two kinds of sandwich-architectures for PLLA/hPLLA non-woven fabrics, where single or double hPLLA non-woven fabrics were introduced at the core or two sides of PLLA matrix film, respectively. The hPLLA fiber induced dense and oriented PLLA transcrystallinity in networks of hPLLA non-woven fabrics and impermeable crystalline layers were formed with well-interlinked lamellae, which served as impermeable barriers to oxygen and water vapor molecules. Moreover, hPLLA non-woven fabrics involving the compact transcrystallinity behaved as framework to support the PLLA matrix and resist the thermal deformation. The sandwich-architectured PLLA with double hPLLA non-woven fabrics exhibited better barrier properties and heat resistance than that with single hPLLA non-woven fabrics. Compared with neat PLLA, the oxygen permeability coefficient and water permeability coefficient of PLLA/double hPLLA non-woven fabric biocomposites significantly decreased by 61.7% and 58.7%, and the storage modulus increased by 160 times at 80 ºC. This work provides a novel method to fabricate heat resistant PLLA barrier film with full biodegradability for packaging application. Key words: poly(L-lactide), full biodegradability, high-melting-point poly(L-lactide) non-woven fabrics, barrier properties, heat resistance performance.

*

Corresponding authors. Tel.: + 86 28 8546 0130; fax: + 86 28 8546 0130. E-mail addresses: [email protected] (RY Bao) and and [email protected] (W Yang)

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Introduction Biopolymers have attracted much attention to deal with the increasing environmental issues caused by conventional petrochemical based polymers in different application fields such as packaging, biomedical usage and agricultural mulching film

1-8

Among ecofriendly biopolymers, poly(L-lactide) (PLLA) is

regarded to be with great potential in packaging applications owing to its outstanding performance with biocompatibility, biodegradability, and good processability.9-17 Unfortunately, the unsatisfactory barrier properties and heat resistance hinder the packaging application of PLLA owing to its insufficient resistance to oxygen and water vapor molecules and thermal deformation.18-21 With the purpose of enhancing barrier properties, two-dimensional nanosheets including graphene oxide and nanoclay are introduced into PLLA as impermeable barriers to improve the barrier performances by forming lengthened and more tortuous pathway for the oxygen and water vapor molecules to diffuse.22-25 Sufficient amount of nanosheets should be well dispersed and arranged in the polymer to observably enhance the barrier performances of the nanocomposites.26-28 Nevertheless, it is difficult to obtain uniform dispersion and an ordered alignment of nanosheets under the conventional processing condition.29 Layer-by-layer (LBL) deposition technique is regarded as a feasible route to achieve the oriented arrangement of the nanosheets in polymer matrix and significantly enhanced barrier properties can be obtained.30-33 It is obvious that the assembly process is complex and the production efficiency is relatively low, which limits the practical applications. Besides, although the utilization of nonbiodegradable nanosheets can largely improve the barrier properties of PLLA, it sacrifices the full biodegradability and the effect of exposed nanosheets at the surface of packaging materials on food security is still unascertained. Recently, it is interesting that sandwich-structured PLLA/graphene composite film comprising impermeable graphene as core barrier, PLLA films as outer encapsulation, and polyvinylpyrrolidone (PVP) as a binder can achieve excellent barrier properties owing to the longer diffusion path as well as the denser lamellar structure in comparison with the bulk of polymer/nanosheet composites.34 ACS Paragon Plus Environment

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Tailoring the crystalline structure of semicrystalline polymers is regarded as a promising green method as the crystalline lamellae can serve as impermeable barriers to oxygen and water vapor molecules similar to nanosheets.35-37 Two important factors i.e., crystallinity and crystalline morphology govern the barrier properties of semicrystalline polymers. However, a complex relationship is found between crystallinity and the barrier performances of PLLA, that the increase of the crystallinity does not always enhance the barrier performances.38-40 Tailoring the crystalline morphology of PLLA by constructing oriented crystals interlinked with each other is reported to be efficient to enhance the oxygen barrier property of PLLA.41-43 Bai et al.41 constructed parallel-aligned shish kebab-like crystals with the assist of a nonbiodegradable nucleating agent (N,N′,N″-Tricyclohexyl-1,3,5-benzene-tricarboxylamide), which can serve as template to change the crystalline morphology of PLLA when it self-organizes into fibrils at a certain temperature. Then the oxygen permeability ( PO2 ) of PLLA is reduced. Zhou et al.42 constructed the dense and well-aligned crystals networks by the formation of in situ nanofibers of poly(butylene adipate-co-terephthalate) in PLLA matrix through the processing methods of “slit die extrusion-stretching-thermal treatment” to achieve a reduction of PO2 . Though the constitution of plenty oriented crystals in PLLA can effectively improve the gas barrier performances, it is hard to achieve by using conventional processing technologies, especially for PLLA with poor ability of crystallization. The reason lies in both constructing a three-dimensional network of nucleated fibrils in PLLA matrix and pursuing the fibrils with high-efficiency nucleation activity can be really not an effortless project in the conventional processing of PLLA. Inspired by sandwich-architectured PLLA-graphene composite aforementioned,34 it seems preferable to construct oriented lamella networks induced by pre-customized networks composed of high-efficiency nucleating fiber in the partial layer of PLLA matrix, where lamella are more likely to interconnect with each other to construct a coherent network imposed by the pre-customized networks of nucleated fiber, providing an impermeable crystalline layer as outstanding barrier to the permeation of gas molecules like the graphene film. ACS Paragon Plus Environment


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As we previously reported, in quiescent condition, the crystallization of PLLA can be effectively tailored by high-melting-point PLLA (hPLLA) fiber. Specifically, the crystallization rate of PLLA is remarkably accelerated. In addition, the crystalline morphology is transformed from spherulite to transcrystallinity due to the high-efficiency nucleation activity of hPLLA fiber.44 However, the spatial distribution of transcrystallinity layers is uncontrollable, and the well-interlinked lamellae of inter-transcrystallinity need to achieved at high hPLLA fiber contents for further increased barrier properties. The introduction of a large amount of hPLLA fiber leads to the non-uniform dispersion of hPLLA fiber in PLLA matrix. More challenging, the sharp increase in melt viscosity induced by the introduction of a large amount of hPLLA fiber makes the melting processing impossible. So in this work, network structured hPLLA non-woven fabrics composed of hPLLA fiber were introduced into PLLA matrix, to construct two different kinds of fully biodegradable sandwich-architectures with single or double hPLLA non-woven fabrics at the core or the two sides of PLLA matrix. Well-organized lamellae are expected to be formed in the network of hPLLA non-woven fabrics to construct surpernetworks interconnected with each other and served as impermeable layers to oxygen and water vapor molecules. The hPLLA non-woven fabrics fully filled with compact transcrystallinity are also expected to support the PLLA matrix and resist the thermal deformation.

Experimental section Materials PLLA (2003D) was from Nature Work (United States), with about 4.3% D-units and a melting point (Tm) of 153 °C. The weight-averaged molecular weight and polydispersity index were 1.70 × 105 g·mol−1 and 1.7, respectively. The hPLLA non-woven fabrics with the thickness of 60 μm and hPLLA fiber were from Zhejiang Hisun Biomaterial Co, Ltd (China) and used as received. The Tm of hPLLA non-woven fabrics and hPLLA fiber was 176 ºC. Sample Preparation ACS Paragon Plus Environment

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Neat PLLA films with thicknesses of 80 μm and 100 μm were firstly fabricated, respectively. For PLLA/single hPLLA non-woven fabric biocomposites, a piece of hPLLA non-woven fabrics was assembled between two pieces of PLLA films with a thickness of 80 μm. For PLLA/double hPLLA non-woven fabrics composites, a piece of PLLA film in thickness of 100 μm was assembled between two hPLLA non-woven fabrics. The samples were shaped into films by compression molding at 165 °C with a pressure of 10 MPa, after that, the samples were cooled at 120 ºC with a pressure of 10 MPa for different time to allow the growth of transcrystallinity in the pores of hPLLA non-woven fabrics. Scheme 1 schematically presents the preparation approach of PLLA/hPLLA non-woven fabrics composites. The diameter of final samples is 100 mm and the thickness is approximately 220 μm. For comparison, neat PLLA films in thickness of approximately 220 μm were fabricated as well and isothermal crystallized at 120 ºC for different time. For convenience, the obtained PLLA/single hPLLA non-woven fabric biocomposites, PLLA/double hPLLA non-woven fabric biocomposites and neat PLLA were marked as single-x, double-x and PLLA-x (x=0, 10, 20, 40, 60 min), where x stands for the isothermal crystallization time.

Scheme 1. Schematic representation for the preparation approach of PLLA/hPLLA non-woven fabric biocomposites. Differential Scanning Calorimetry (DSC). hPLLA non-woven fabrics and compression molded samples were heated from 40 to 210 °C at a scanning rate of 10 °C/min using DSC (Q20, TA Instruments, USA) under protection of nitrogen atmosphere. Wide-Angle X-ray Diffraction (WAXD). WAXD measurement of the hPLLA non-woven fabrics was ACS Paragon Plus Environment


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tested using a X-ray diffractometer (DX-1000, Dandong Fanyuan Instrument Co. LTD, China) and equipped with a Cu Kα radiation source (40 kV, 25 mA). Scans were made between angles of 2θ = 5-50 º and the scan speed is 3 º/min. For hPLLA non-woven fabrics, the degree of crystallinity (Xc) was estimated by equation (1):

Xc 

A A A cryst

cryst

where

A

cryst

and

(1)

amorp

A

amorp

represent the fitted areas of crystal and amorphous region.

Polarizing Optical Microscopy (POM). An Olympus BX51A POM (Tokyo, Japan) equipped with heating stage (LINKAM THMS 600) was used to observe the morphological evolution of PLLA in the neighboring of hPLLA fiber. Specifically, two parallel hPLLA fiber and four hPLLA fiber joined at nearly right angles were putted upon a piece of PLLA film, of which the thickness is 10 μm, respectively. After thermal history was erased at 168 ºC for 3 min, samples were cooled to the selected isothermal crystallization temperatures at a rate of 30 °C/min Scanning Electronic Microscopy (SEM). The morphology of hPLLA non-woven fabrics and crystalline morphologies of PLLA biocomposites were observed using a JEOL JSM-5900LV SEM (Japan) at an accelerating voltage of 5 kV. The surface and cryo-fractured cross section of hPLLA non-woven fabrics were used for observation. Moreover, to reveal the crystalline morphology, the surface of the samples experienced the POM observation, the surface and cryo-fractured cross section of compression molded samples were etched by a mixed solution composing of methanol-water (2:1, v/v) and 0.025 mol/L of sodium hydroxide for 12 h at 30 ºC to remove the amorphous area. Before SEM observation, all treated surfaces were cleaned by distilled water and ultrasonication. The samples were sputter-coated with gold before observation Synchrotron 2D-WAXD measurements. Crystalline structures generated in the neighboring of hPLLA fiber were measured by scanning microfocus 2D-WAXD on the beamline BL15U1 of Synchrotron Radiation Facility (China) equipped with an SX165 detector (Rayonix Co. Ltd., USA). Samples of PLLA ACS Paragon Plus Environment

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with one hPLLA fiber and PLLA with two parallel hPLLA fiber were used for test. Specifically, one fiber and two parallel hPLLA fiber were putted upon a piece of PLLA film, of which the thickness is 10 μm, respectively. Samples were isothermally crystallized at 120 ºC for 3 hours after eliminating thermal history at 168 ºC for 3 min. For PLLA with one hPLLA fiber, the sample was scanned from the center of the fiber with a step of 10 μm. For PLLA with two parallel hPLLA fiber, the sample was scanned from the center of one hPLLA fiber to the other with a step of 20 μm, and the detailed process is shown in Scheme 2. The wavelength of synchrotron X-ray beam was 0.124 nm, and the beam dimension was 3.0 × 2.7 μm2. The distance from sample to detector was 165.5 mm.

Scheme 2. Schematic description of the scanning method for local crystalline structure in the vicinity of hPLLA fiber for PLLA with one hPLLA fiber (a) and PLLA with two parallel hPLLA fiber (b). The selected positions for 2D-WAXD analysis were 0, 10, 20, 30 and 40 μm from the center of hPLLA fiber for (a) and 0, 20, 40, 60, 80, 100 μm from the center of one hPLLA fiber for (b). The crystal orientation was estimated with Herman’s orientation parameter (  ) by 2D-WAXD pattern using equations (2) and (3): f 

3 cos 2   1

(2)

2

cos  2

 

 /2

0

I ( ) sin  cos 2 d



 /2

0

(3)

I ( ) sin d

where φ and I represent the angle between the normal of a given crystal plane and the fiber direction, and the corresponding integral intensity, respectively. Performance Evaluation. Oxygen permeability coefficient ( PO2 ) of PLLA/hPLLA non-woven ACS Paragon Plus Environment


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biocomposites was tested by a Labthink VAC-V2 film permeability tester (China) at room temperature with 50% relative humidity, according to ISO Standard 15105-1: 2007 based on the differential pressure method. Water permeability coefficient ( PH 2O ) was tested gravimetrically by a Labthink W3-031 water permeability tester (China) at room temperature with 98% relative humidity, according to ISO Standard 15105-1: 2007. Dynamic mechanical analysis (DMA) was measured in tensile mode with a TA Q800 instrument (USA). The storage modulus of PLLA/hPLLA non-woven biocomposites were measured from 20 ºC to 150 ºC at a heating rate of 3 ºC/min and frequency of 1 Hz. A heat resistance measurement was conducted in an oven at selected temperatures with a step of 5 °C to visualized demonstrate the heat resistance of PLLA/hPLLA fiber composites. Rectangle samples with a size of 0.22 mm × 50 mm × 10 mm were used in this test. To be specific, an oven was firstly rised to the selected temperature, afterwards, a standard weight of 5g was put on the samples for 5 min, and two metal blocks were used to fix both ends of the samples. The distances from the horizontal line of the test samples to the point with maximum deformation were recorded to evaluate the heat distortion.

Results and discussion Crystal structure and morphology of hPLLA non-woven fabrics. DSC melting curve of hPLLA non-woven fabrics is given in Figure 1a. The Tm of hPLLA non-woven fabrics is around 176 °C and the crystallinity of hPLLA non-woven fabrics calculated from WAXD is 61.9% (Figure 1b), which shows a high crystallinity. Figure 1c and d show SEM micrograph for the surface and cross section of hPLLA non-woven fabrics. A network structure is formed through the overlapping of fiber and the non-woven fabrics are composed of several fiber layers. The pores with different sizes are clearly observed. As shown in Figure 1e, the sizes of majority pores are 20 and 30 μm, and the average pore size is calculated to be 25.4 μm.


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Figure 1. The characteristics of hPLLA non-woven fabrics. DSC melting curve (a) and WAXD pattern (b) for hPLLA non-woven fabrics. SEM micrograph for the surface (c) and the cryo-fractured cross section (d) of hPLLA non-woven fabrics. Relative frequency versus pore size of hPLLA non-woven fabrics (e). Crystallization morphology tailoring of PLLA/hPLLA non-woven fabric biocomposites

Figure 2. POM micrographs showing the crystalline morphology for PLLA with two parallel hPLLA fiber (a) four hPLLA fiber joined at nearly right angles (b) at Tc of 120 ºC. As we previously demonstrated, in quiescent condition, hPLLA fiber with high-efficiency nucleation activity can induce the generation of dense and well-organized PLLA transcrystalline layers closely wrapping the hPLLA fiber surface.44 It is expected that well-organized PLLA transcrystallinity region can be formed in the networks of hPLLA non-woven fabrics composed by hPLLA fiber, forming impermeable barriers to reduce the diffusion path of oxygen and water vapor molecules. For exploration of the ACS Paragon Plus Environment


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development of crystalline during the network of fabrics in situ, the region composed by a few hPLLA fiber was used for tracing the evolution of crystalline morphology by POM. POM micrographs of PLLA in the region composed by two parallel hPLLA fiber and four hPLLA fiber joined at nearly right angles are showed in Figure 2. It can be seen that hPLLA fiber enables the generation of compact transcrystalline layers around the fiber surface due to the high-efficiency nucleation activity of hPLLA fiber. With development of crystallization, the compact transcrystallinity layers induced by different fiber impinge and fully fill the space between the hPLLA fiber. It is exhibited in Figure 2b that the transcrystallinity layers induced by two fiber perpendicular to each other show two distinct colors maybe due to opposite orientation of lamellae induced by the perpendicular fiber, and the interface between the transcrystallinity layers is more clearly observed in the square region. These results reveal that sufficient spatial hindrance created by the incredibly high nucleation density of hPLLA fiber gives the oriented alignment of PLLA lamellae and lamellae grow perpendicularly to the hPLLA fiber axis to fully fill the region composed by hPLLA fiber.45-46 The values of transcrystallinity radius were gathered as a function of crystallization time in PLLA with two parallel hPLLA fiber during isothermal crystallization and the linear growth rate of transcrystallinity was estimated (Figure S1). As shown in Figure S1b, The growth rate for transcrystallinity at 120 ºC (0.34 μm/min) is faster in comparison with that of 110 ºC and 130 ºC, and it is estimated that the average pore size of hPLLA non-woven fabrics (25.4 μm) can be fully filled at about 37.4 min at 120 ºC .


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Figure 3. SEM micrographs of crystalline morphology of PLLA with two parallel hPLLA fiber isothermally crystallized at 120 ºC for 80 min (a). Enlarged views of (a) are given in (b-e). (c-e) represent the lamellar morphology of hPLLA fiber, lamellar morphology near the hPLLA fiber and lamellar morphology at the interface of transcrystallinity, respectively. Crystalline morphology of PLLA with four hPLLA fiber joined at nearly right angles after isothermal crystallization at 120 ºC for 100 min (f), (g) is the local amplifications of (f). The growth direction of lamellae in (d) is represented by dashed lines. The boundary of hPLLA fiber and transcrystallinity in (b) and (d) is represented by dotted lines, and the boundary of transcrystallinity in (g) is represented by dashed lines. Figure 3 elucidates the lamellar structure of PLLA developed at hPLLA fiber surface. It is revealed in Figure 3a and b that well-organized PLLA transcrystallinity is formed in the region between two hPLLA fiber. As exhibited in Figure 3c, the hPLLA fiber is composed of oriented crystals, and the lamellae align in only one direction, i.e., parallel to each other and the lamellae of hPLLA fiber are perpendicular to fiber axis. The lamellae of hPLLA fiber induce the growth of new lamellae of PLLA, and the new lamellae remains the oriented alignment of hPLLA fiber observed in Figure 3d. At the interface formed by impinged transcrystallinity layers induced by different fiber, the lamellae still keep the perpendicular growth direction to the hPLLA axis and are connected to each other, indicating the formation of dense transcrystallinity region between two parallel hPLLA fiber (Figure 3e). As for the PLLA with four hPLLA fiber joined at nearly right angles, the lamellae induced by hPLLA fiber grow perpendicular to the hPLLA axis (Figure 3f), which is consistent with that of PLLA with two parallel hPLLA fiber. At the interface of transcrystallinity layers, the lamellae grown from two opposite directions restrict the growth of each other and generate a diagonal boundary at the interface (Figure 3g), which contributes to the formation of dense transcrystallinity region in the square composed by four hPLLA fiber. The lamellae alignment of transcrytallinity induced by hPLLA fiber was measured by microfocus 2D-WAXD. Figure 4a exhibits the evolution of the crystalline structure induced by one hPLLA fiber from ACS Paragon Plus Environment


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the center of the hPLLA fiber to the boundary of transcrystallinity at a step of 10 μm. At the position of 0 μm, diffraction arcs exhibit in the hPLLA fiber, which is attributed to lattice planes (200)/(110) and (203) of PLLA in α-form, indicating that the hPLLA fiber is highly oriented, confirming the orientation structure of hPLLA fiber that has been illustrated by SEM. With the increasing distance from the hPLLA fiber, the signals are still arc-like diffraction instead of homogeneous diffraction circles though the intensities of arc-like diffraction are relatively weakened, demonstrating the oriented lamellae induced by hPLLA fiber forms in a large scale. Figure 4b shows the evolution of the crystalline structures in the region between two parallel hPLLA fiber from the center of one hPLLA fiber to the other one at a step of 20 μm. At the position of 0 μm and 100 μm, diffraction arcs exhibit in the hPLLA fiber. With the increasing distance from the hPLLA fiber, the intensities of arc-like diffraction are weakened, and all the signals in the whole region keep arc-like diffraction, indicating the oriented lamellae forms in the whole region between two parallel hPLLA fiber.

Figure 4. 2D-WAXD patterns of transcrystallinity induced by one hPLLA fiber (a) and two parallel hPLLA fiber (b). 1D-WAXD profiles of PLLA with one hPLLA fiber and two parallel hPLLA fiber at different location are obtained by 2D-WAXD patterns and displayed in Figure S2, and the results indicate that hPLLA fiber has the same crystal structure with transcrystallinity. Moreover, for illustrating the orientation of lamellae, ACS Paragon Plus Environment

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intensity distribution of α(200)/(100) along azimuthal angle are obtained and Herman’s orientation parameters are calculated and given in Figure 5. In Figure 5a and c, the strong equatorial peak and two off-axis peaks can be observed for hPLLA fiber, indicating a high orientation degree, and the calculated Herman’s orientation parameters are above 0.50. With the increasing distance from the hPLLA fiber, the three-peak azimuthal profile still keeps, although the intensity of the equatorial peak decreases due to the reduced orientation induced by the decreasing spatial hindrance and the radially growth of lamellae away from hPLLA fiber. The Herman’s orientation parameters are above 0.23 though decreasing gradually away from hPLLA fiber (Figure 5b, d), demonstrating the lamellae can keep oriented in a large scale, and the transcrystallinity shows well-organized lamellae alignment.

Figure 5. Intensity distribution of α(200)/(100) along azimuthal angle of transcrystallinity located at different regions induced by one hPLLA fiber (a) and two parallel hPLLA fiber (c), and the corresponding Herman’s orientation parameter (b and d). Based on the high-efficiency nucleation activity of hPLLA fiber on PLLA and the dense and oriented transcrystallinity generated in the region composed by hPLLA fiber, the hPLLA non-woven fabrics are expected to facilitate the well-organized transcrystallinity growing in the pores composed by hPLLA fiber and form impermeable crystalline layers with well-interlinked lamellae under traditional processing conditions, thus serving as the impermeable barriers to enhance the barrier properties. The ACS Paragon Plus Environment


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sandwich-architectured PLLA biocomposites with introduction of single and double hPLLA non-woven fabrics were prepared by compression molding. Figure 6 shows SEM observations of crystalline morphology for the cross sections of etched PLLA/single hPLLA non-woven fabric biocomposites. As shown in Figure 6a, no crystalline structure of PLLA matrix can be observed either around or apart from the hPLLA non-woven fabrics for the sample of single-0. Contrast to that the sparse spherulites spontaneously formed in PLLA matrix appear beyond the isothermal crystallization time of 40min, the hPLLA fiber can enable transcrystallinity growing, and the clear transcrystallinity layers are closely wrapped hPLLA non-woven fabrics at core of the samples beyond 20min. It is noted from the enlarged view of Figure 6b-d that the oriented alignment of PLLA lamellae is perpendicular to the axis of hPLLA fiber. With the increasing isothermal crystallization time, the thickness of transcrystallinity layers increase. When the sample is isothermally crystallized for 20 min, the thickness of transcrystallinity layers are 10 μm and the thickness increases to 20 and 30 μm when samples are isothermally crystallized for 40 min and 60 min, respectively.

Figure 6. SEM micrographs of the crystalline morphology for the cross sections of etched single-0 (a), single-20 (b), single-40 (c) and single-60 (d). The local amplifications of (a-d) are shown in inserts. The boundary of transcrystallinity layers surrounding the hPLLA non-woven fabrics is represented by dotted ACS Paragon Plus Environment

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lines and the growth direction of lamellae is represented by arrows. As for PLLA/double hPLLA non-woven fabric biocomposites, the morphologies of surface and cross section are observed and given in Figure 7. Figure 7a and g exhibit that no crystalline structure of PLLA matrix can be observed form the surface and cross section of the sample of double-0. In Figure 7b-d, compact and well-organized transcrystallinity is formed in the pores of hPLLA non-woven fabrics, indicating that the hPLLA non-woven fabrics can be infiltrated well by PLLA matrix due to their identical chemical composition and then the hPLLA fiber can facilitate transcrystallinity growing. Figure 7e and f give the clearly observation of lamellae growth of transcrystallinity. The lamellae of hPLLA fiber induce the growth of new lamellae of PLLA, and the new lamellae remains the oriented alignment of hPLLA fiber, which is well consistent with the result of Figure 3d. The morphologies observation of the cross section in Figure 7h-j show the transcrystallinity layers closely wrap the lower and upper surface of two hPLLA non-woven fabrics at the surface of the biocomposites, and the thickness of transcrystallinity layers are consistent with the results of PLLA/single hPLLA non-woven fabric biocomposites.



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Figure 7. SEM micrographs of crystalline morphology for the surface of etched double-0 (a), double-20 (b), double-40 (c) and double-60 (d). (g-j) are the cross section morphology of double-0, double-20, double-40 and double-60 respectively. (e), (f), (k) and (l) are the local enlarged views of (b), (d), (h) and (i). The growth direction of lamellae is represented by dashed lines in (e) and (f), and the boundary of transcrystallinity region surrounding the hPLLA non-woven fabrics is represented by dotted lines in (h-j). 3. Barrier properties and heat resistance of PLLA/hPLLA non-woven fabric biocomposites

Figure 8. PO2 (a) and PH 2O (b) of neat PLLA, PLLA/single hPLLA non-woven fabric biocomposites, and PLLA/double hPLLA non-woven fabric biocomposites at different isothermal crystallization time. Comparison of the percentage decline of PO2 and PH 2O in this work with other fully biodegradable PLLA biocomposites (c); the percentage decline is defined as the decrease of PO2 or PH 2O to the initial value of PLLA, (black square) PLLA/cellulose nanocrystals grafted hydrocinnamic acid (CNC-g-BzAA),47 (orange circle) PLLA/hydrochloricacid derived CNC/coconut oil (DCNC-S/Coco),48 (blue triangle) PLLA/ bacterial cellulose nanowhiskers (BCNW),49 (blue rhombus) PLLA/poly(butylene 2,5-furan dicarboxylate) (PBF),50 (purple triangle) PLLA/hPLLA fiber.44 Figure 8 shows

PO2

and

PH 2O of neat PLLA, PLLA/single hPLLA non-woven fabrics and

PLLA/double hPLLA non-woven fabric biocomposites at different isothermal crystallization time. With the increase in isothermal crystallization time, PO2 and PH 2O of the neat PLLA and the biocomposites are reduced, due to the enhanced crystallization. The remarkable decreased PO2 and PH 2O of PLLA/hPLLA non-woven fabric biocomposites compared with neat PLLA can be observed, and PLLA/double hPLLA ACS Paragon Plus Environment

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non-woven fabric biocomposites show more effective effect on decreasing oxygen permeability than PLLA/single hPLLA non-woven fabric biocomposites. It is noted a significant drop in the PO2 occurs for single-40 and double-40 compared with that of PLLA-40. To be specific, in comparison with 10.1 × 10-15 cm3·cm/cm2·s·Pa of PLLA-40, single-40 and double-40 are much more impermeable to oxygen, leading to remarkable reduced PO2 of 6.4 and 6.8 × 10-14 cm3·cm/cm2·s·Pa in comparison with that of neat PLLA respectively, which is 36.6% and 40.6% lower than that of PLLA-40. This may be attributed to that the majority pores of hPLLA non-woven fabrics can be fully filled by the transcrystallinity when biocomposites are isothermally crystallized for 40 min. When the sample is isothermally crystallized for 60 min, the crystallization of PLLA matrix tends to be completed and the values for single-60 and double-60 decrease to 5.5 and 4.9 cm3·cm/cm2·s·Pa. The values decline 57.0% and 61.7% in comparison with neat PLLA. Similar to oxygen permeability, the water vapor permeability for PLLA/hPLLA non-woven fabric biocomposites are much lower than neat PLLA after isothermal crystallization. PH 2O for single-60 and double-60 are 4.6 and 3.8 g·cm/cm2·s·Pa, and the values decrease 50.0% and 58.7% in comparison with neat PLLA, respectively. It has been reported that tailoring the crystalline morphology is effective to improve the gas barrier property. Unfortunately, the PLLA matrix is needed to with a relatively higher crystallinity in the reported literatures. For example, Bai et. al found that the oxygen permeability coefficient was not significantly decreased until the crystallinity reaches to 50.9% in the PLLA/TMC-328 composites.41 Li et al. also found that the oxygen permeability coefficient was not significantly decreased until the crystallinity reaches to 42.1% in the 16-layed sample by multilayer coextrusion.43 Figure S3 gives the DSC heating curves of neat PLLA and PLLA/hPLLA non-woven fabrics at different isothermal crystallization time. It is found that cold crystalline peaks are existed in all samples even after isothermal crystallization for 60 min, and it is also noted that the PLLA matrix utilized in this work has limited crystallinity, which is no more than 40% considering the impact of optical purity, but a dramatic decrease is found in the samples of single-40 and double-40. These results indicate that compact transcrystallinity structure can contribute to the barrier ACS Paragon Plus Environment


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performances of PLLA even if the crystallinity of PLLA matrix is not very high, and the interconnecting lamellae and the perfect arrangement of lamellae are much more important than crystallinity, even just located at local layer of PLLA matrix. When the pores of hPLLA non-woven fabrics are fully filled by the transcrystallinity, the impermeable crystalline layers can serve as impermeable barriers to reduce the diffusion path of oxygen and water vapor molecules . Compared with PLLA/single hPLLA non-woven fabric biocomposites with single impermeable crystalline layers only, the PLLA/double hPLLA non-woven fabric biocomposites with two impermeable crystalline layers behave better barrier properties owing to much more impermeable area. Figure 8c exhibits the superiority of this work to other fully biodegradable PLLA composites with improved oxygen and water barrier performance. It is observed that both the oxygen and water barrier performance are improved in comparison with other fully biodegradable PLLA-based biocomposites, especially for double-60. As for PLLA/PBF composites in Figure 8c, though the oxygen barrier performance is higher than double-60, the water barrier performance is relatively poor than double-60 duo to the hydrophilia of PBF chain.50 Then, the PLLA/double hPLLA non-woven fabric biocomposites shows a great potential to be used as packaging materials.

Scheme 3. Schematic representation for the oxygen and water penetration through neat PLLA (a), the impermeable crystalline layers composed of hPLLA non-woven fabrics (b) and PLLA/double hPLLA non-woven fabric biocomposites (c). The diffusion path of oxygen and water are represented by dotted lines. The mechanism of oxygen and water penetrating through the films is described in Scheme 3. The ACS Paragon Plus Environment

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oxygen and water vapor molecules experience lesser and more tortuous pathway through biocomposites with well-organized lamellae, however, they take easier way in neat PLLA films (Scheme 3a). The hPLLA non-woven fabrics composed of high efficiency nucleating fiber induced well-aligned transcrystallinity to fill the network of the fabrics, constructing the impermeable crystalline layers with well-interlinked lamellae, then prevent the oxygen and water vapor molecules from going into the biocomposites films greatly as shown in Scheme 3b. The multilayer transcrystallinity structure of hPLLA non-woven fabrics also forms a more tortuous path for diffusing oxygen and water vapor. As shown in Scheme 3c, the impermeable crystalline layers with well-interconnected lamellae construct solid barrier on both sides of samples, serving as the impermeable barrier to reduce the diffusion path of oxygen and water vapor molecules, thus significantly decrease the oxygen and water permeability of PLLA/double hPLLA non-woven fabric biocomposites . The heat resistance performance was investigated considering that the formation of impermeable crystalline layers may also contribute the heat resistance performance of PLLA. Figure 9a-c present the changes of storage modulus with increasing temperature for neat PLLA and PLLA/hPLLA non-woven fabric biocomposites isothermal crystallized for different time. Figure 9d gives the values of storage modulus of PLLA and PLLA/hPLLA non-woven fabric biocomposites under the temperature of 80 °C to make a comparison of the heat resistance performance. With the increasing of isothermal crystallization time, the storage modulus at 80 °C hardly change for neat PLLA below 40 min, due to the poor crystallization ability of neat PLLA during melt processing, reflecting by that serious cold crystallization occurred during heating induces significant drop in storage modulus as shown in Figure 9a. For PLLA/hPLLA non-woven fabric biocomposites, the storage modulus at 80 °C is gradually increased with the increasing of isothermal crystallization time, due to the formation of transcrystallinity induced by hPLLA fiber. For PLLA isothermal crystallized for 60 min, the storage modulus is only 87 MPa, and it is 102 MPa and 214 MPa for single-0 and double-0, demonstrating that the well infiltrated hPLLA non-woven fabrics with oriented crystalline ACS Paragon Plus Environment


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structure and high crystallinity have strong interaction with PLLA matrix, and behave as the framework to support PLLA matrix and resist the thermal deformation. With the increase of isothermal crystallization time, the formation of transcrystallinity enhances the interfacial interaction between PLLA matrix and hPLLA fiber, simultaneously, formation of dense and well-organized lamellae connecting to each other reinforces the framework of hPLLA non-woven fabrics significantly. Then the storage modulus at 80 °C increases to 315 MPa for single-60 and 643 MPa for double-60, which enhanced 78 and 160 times than neat PLLA, showing a dramatic enhancement of heat resistance performance of PLLA. It is also noted that the storage modulus of PLLA/double hPLLA non-woven fabric biocomposites are almost the twice of PLLA/single hPLLA non-woven fabric biocomposites, indicating that the introduction of multilayer impermeable crystalline layers largely improves the heat resistance of PLLA.

Figure 9. Changes of storage modulus with increasing temperature for neat PLLA (a), PLLA/single hPLLA non-woven fabric biocomposites (b) and PLLA/double hPLLA non-woven fabric biocomposites (c). The specific values of storage modulus at 80 ºC (d) for neat PLLA and PLLA/hPLLA non-woven fabric composites. Digital images of neat PLLA and double-60 when samples are tested in oven, and the corresponding images after test. For the purpose of visualized demonstrating the heat resistance of PLLA/hPLLA non-woven fabric bioscomposites, the distances from the horizontal line of the test samples to the point with maximum ACS Paragon Plus Environment

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deformation were recorded to evaluate the heat distortion at different temperatures. As illustrated in Figure 9, the huge distortion can be seen when neat PLLA is tested at 80 ºC in less than 2 min and the distance is tested to be 0.35 cm, and in contrast, there is no trace of distortion for double-60 at 120 ºC during 5 min. Till the temperature was rised near the Tm of PLLA, i.e. 145 ºC, a tiny distortion can be seen for double-60 biocomposite and the distance is 0.20 cm. The lower value compared with that of neat PLLA at 80 ºC directly illustrates that PLLA/hPLLA fiber biocomposites are highly efficient in heat resistance.

Conclusion Dense and oriented PLLA transcrystallinity was induced by hPLLA fiber in the networks of hPLLA non-woven fabrics, and impermeable crystalline layers with well-interlinked lamellae were evolved locally, which serve as impermeable barriers to oxygen and water vapor molecules. The sandwich-architectured PLLA with double hPLLA non-woven fabrics behave better barrier properties and heat resistance than that with single hPLLA non-woven fabrics. When the samples are isothermal crystallized for 60 min, the oxygen permeability coefficient and water permeability coefficient of PLLA/double hPLLA non-woven fabric biocomposites are decreased by 61.7% and 58.7%, in comparison with neat PLLA. Furthermore, the heat resistance of PLLA biocomposites is largely improved and the storage modulus of PLLA/double hPLLA non-woven fabric biocomposites is increased by 160 times at 80 ºC. There is a huge potential for PLLA/hPLLA non-woven fabric biocomposites to be used as packaging materials and this work provides a novel way to prepare biocomposites with full biodegradability.

Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (Grants 51503132, 51422305 and 21374065). We also appreciate the help of Beamlines BL15U in Shanghai Synchrotron Radiation Facility for X-ray measurement.

Supporting Information Transcrystallinity radius evolution with the crystallization time during isothermal crystallization for ACS Paragon Plus Environment


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PLLA with two parallel hPLLA fiber, 1D-WA XD profiles of PLLA with one hPLLA fiber and two parallel hPLLA fiber at different location and DSC heating curves of neat PLLA and PLLA/hPLLA non-woven fabrics biocomposites.

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Graphical Abstract


High-Melting

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