Ramie Fiber

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Promoting Interfacial Transcrystallization in Polylactide/Ramie Fiber Composites by Utilizing Interfacial Stereocomplex Crystals Yuan-Ying Liang, Jia-Zhuang Xu, Yang Li, Gan-Ji Zhong, Ruyin Wang, and Zhong-Ming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01317 • Publication Date (Web): 24 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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ACS Sustainable Chemistry & Engineering

Promoting Interfacial Transcrystallization in Polylactide/Ramie Fiber Composites by Utilizing Stereocomplex Crystals

Yuan-Ying Liang,1,# Jia-Zhuang Xu,*,1,# Yang Li,1 Gan-Ji Zhong,1 Ruyin Wang,2 and ZhongMing Li1

1 College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, People’s Republic of China 2 Total Corbion, Unit 08-09, 30F, No. 6088 Humin Road, Minhang District, Shanghai 201100, People’s Republic of China

* Corresponding author: Jia-Zhuang Xu Tel.: +86-28-8540-6866; Fax: +86-28-8540-6866. E-mail: [email protected] (J.-Z. X.).

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ABSTRACT: Tailoring interfacial crystallization has been widely recognized as a promising pathway to enhance interfacial adhesion between a polymeric matrix and a reinforcing fiber. However, the induction ability of natural fiber is usually disturbed by an uneven internal structure. In this work, to encourage interfacial crystallization between the ramie fiber (RF) and the poly(L-lactic acid) (PLLA), stereocomplex (sc) crystallites were physically decorated on the surface of RF as nucleation promoters. Compared to the sparse spherulites dispersed around the raw RF, sc-coated RF fostered a well-defined transcrystallinity (TC). The nucleation sites on the surface of the decorated RF increased with the thickness of sc coating substantially, thus changing the contour of TC from a fan shape to a brush shape, while the growth rate of TC was dependent on the crystallization temperature. Given the primary credit to nucleation effect of sc coating, the TC generated in the PLLA/sc-coated RF composite showed notably higher overall crystallinity than the common spherulites in the PLLA/RF counterpart, especially at a high crystallization temperature. The proposed strategy provides a smart yet flexible method to regulate the interfacial crystalline structure of PLLA/natural fiber composites. Keywords: Polylactic acid; Transcrystallization; Stereocomplex; Interfacial crystallization; Natural fiber.


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INTRODUCTION Fiber-reinforced polymer composites, by dint of their high performance, light weight and cost effectiveness, are the most important categories of composite materials and have been extensively applied in infrastructure engineering fields as well as in automotive industries for decades.1, 2 With the grave concerns regarding oil depletion, global warming, and environmental pollution, there is a pressing need to flourish the fully bio-based and renewable polymer/fiber composites to replace petroleum-based plastics and man-made fibers. For example, poly(L-lactic acid) (PLLA), the most popular biodegradable polyester, is utilized to form composites with different natural fibers, including ramie fiber (RF),3 kenaf fiber,4 hemp fiber,5 hydroxyapatite fiber,6 and flax fiber,7 to enhance the mechanical performance and heat resistance. One of our previous works showed that the tensile strength of PLLA was increased by 40% up to 91.3 MPa on adding 30 wt% RF.3,8 Strong interfacial bonding has been widely established as a vital prerequisite for the mechanical enhancement of the polymer/fiber composites. Unfortunately, the hydrophilic nature of natural fiber makes it incompatible with the hydrophobic polymeric matrix, resulting in poor interfacial interaction.9 To overcome this hurdle, plenty of efforts have been devoted, such as corona/plasma treatment, introducing a compatibilizer, and chemical surface modification.10 Among these, interfacial crystallization is regarded as a promising means to attaining interfacial enhancement.11 Originating from heterogeneous surface-induced crystallization, polymer lamellae growth on the surface of a high-aspect-ratio filler could serve as a mechanical interlock to effectively transfer stress from the bulk to the fiber. For instance, the prevailing transcrystallinity (TC) of poly(butylene succinate) (PBS) was induced by coating polydopamine on the surface of RF. A 95.6% increase in the interfacial shear stress between the PBS and the



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modified RF was observed.12 Xu et al., found that the interfacial shear stress between PLLA and RF showed a steep increase with maturation of the TC by control over the crystallization condition.13 However, the nucleation effect of natural fiber is limited due to the inherent nonuniform structure resulted from the culture environment. More importantly, the nucleation density of PLLA on the RF surface was far below the saturation level and varied for different manufacturing batches. Thereby, in a bid to pursue robust interfacial crystallization and thus firm interfacial adhesion, improving and controlling the nucleation ability of RF of primary importance. Stereocomplex (sc) crystallization between PLLA and poly(D-lactic acid) (PDLA) is distinguished by its extremely high melting point, 50 °C higher than that of either PLLA and PDLA homocrystallization.14 Such a high melting temperature stems from the strong intermolecular interplay between the enantiomers, i.e., intermolecular hydrogen bonds.15 It has been demonstrated that sc crystallites could remain in the PLLA melt and act as powerful nucleation agents to promote the homocrystallization in the form of epitaxial crystallization.16, 17 The crystallization peak temperature of PLLA blended with 3 wt% PDLA increased by 30 °C and this accelerating effect was further boosted with the rise of PDLA concentration.16 The halfcrystallization time of PLLA was reduced by about 6 times on adding as little as 0.25 wt% PDLA into PLLA at the crystallization temperature (Tc) of 140 °C.18 The nuclei density showed a prominent increase for the sample containing 3 wt% PDLA. In addition, sc fibers prepared by melt-spinning were proven to facilitate the formation of sector-like PLLA crystals along the fiber axis.19 Inspired by the above findings, we hypothesized that a thin coating of sc crystallites on the surface of RF could considerably improve the nucleation ability of RF towards PLLA. Therein,


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raw RF was physically decorated with a fine sc layer by a dip-coating method. The interfacial crystallization between the resultant sc-coated RF and the PLLA was scrutinized by polarized optical microscopy (POM), scanning electron microscopy (SEM), and scanning microbeam twodimensional wide angle X-ray diffraction (2D-WAXD). Our results demonstrated that the sc coating on the surface of RF remarkably enhanced the interfacial crystallization, offering a practical implication for fabricating fully degradable fiber-reinforced biocomposites with high performance.

EXPERIMENTAL SECTION Materials. Commercially available PLLA (trademark L130, Mw = 17.3 × 104 g/mol and Mn = 8.9 × 104 g/mol) and PDLA (trademark D1010, Mw = 8.5 × 104 g/mol and Mn = 4.7 × 104 g/mol) were kindly provided by Total-Corbion PLA B.V., the Netherlands. Ramie fiber (RF) with an average diameter of ~20 µm in the range of 15 - 50 µm was purchased from Yuzhu Plant Fiber Industrial Co. Ltd., China. Ethanol and dichloromethane (CH2Cl2) were purchased from Kelong Chemical Reagent Company, China, and were used without further purification. Sample Preparation. PLLA and RF were first dried at 80 °C under vacuum overnight to avoid hydrolysis degradation during isothermal crystallization. For preparation of sc-RFs, PLLA and PDLA (1: 1 by weight) were fully dissolved in CH2Cl2 at room temperature to generate a transparent solution with a concentration of 0.1 g/mL. Raw RFs were immersed into the mixed solution for different time periods (3, 5, and 10 min) to obtain the coating layers with varied thickness. The coated RFs were then dried in an air-circulating oven at 60 °C overnight to remove any trace of CH2Cl2. According to the coating time, the obtained RFs above were termed as sc-RF3, sc-RF5, and sc-RF10, respectively. 20 mL PLA/CH2Cl2 solution (0.1g/mL) was carefully dripped onto a glass slide to obtain a ∼50 µm thin film. All films were dried at 60 °C



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under vacuum overnight to remove the residual solvents. SEM Observation. Field-emission SEM (Inspect F50, FEI, USA) was utilized to carefully examine the surface morphology of sc-RF and the crystalline morphology formed around sc-RF at an accelerated voltage of 5 kV. Before the observation, RF and sc-RF were sputter-coated a thin golden layer. To observe the crystalline morphology, isothermally crystallized PLLA/sc-RF and PLLA/RF composites were etched using a water−methanol (1:2 by volume) solution containing 0.025 mol/L of sodium hydroxide at 25 °C for 11 h. Then, the etched samples were cleaned by distilled water. POM observation. RF and sc-RF were sandwiched between two pieces of PLLA film and melted on a hot stage (CSS450, Linkam Scientific Instruments, UK) at 200 °C for 5 min to eliminate any thermal history. Then, the sample was cooled at a rate of 30 °C/min to the preset crystallization temperatures (Tc), i.e., 135, 140, and 145 °C, and held at these temperatures for 30 min. Micrographs were captured by using a BX51 POM (Olympus Co., Tokyo, Japan) equipped with a Micro Publisher 3.3 RTV CCD. Scanning Microbeam 2D-WAXD Measurement. High spatial-resolution scanning microbeam 2D-WAXD measurement was performed at the beamline BL15U1, Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China), to determine the lamellar orientation and crystalline modification of PLLA formed around the sc-RF. The crystallized specimen was scanned from the central axis of sc-RF along the radial direction in 5 µm steps. The selected analysis locations were 0, 20, 50, and 100 µm away from the sc-RF axis as shown in Figure 1. The monochromated X-ray beam with a wavenumber of 0.124 nm was focused on an area of 3 × 2.7 µm2 (length × width). The distance between the sample and the detector was maintained at 147.5 mm. A 2D-CCD detector (Model SX165, Rayonix Co. Ltd., USA) with a resolution of


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2048 × 2048 pixels was used to acquire the 2D-WAXD images. Additionally, multi-peak Gaussian fitting was used to obtain the areas of the crystalline and amorphous peaks by integrating the 1D-WACD curves. The overall crystallinity (Xc) was calculated according to Equation (1):

Xc = ∑ Acryst /(∑ Acryst + ∑ Aamorp)

(1)

where Acryst and Aamorp represent the fitting areas of the crystalline and amorphous phases, respectively.

Figure 1. Schematic representation for characterizing the local crystalline structure of TC. The selected spots for 2D-WAXD analysis were 0, 20, 50, and 100 µm from the central axis of scRF10. This POM image shows the crystalline morphology of PLLA/sc-RF10 after isothermal crystallization at 145 °C for 60 min.

RESULTS AND DISCUSSION Surface Morphology and Structure of sc-RF. To better understand the contribution of



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sc coating to the nucleation ability of RF, SEM observation was first performed to examine the surface morphology of sc-RF. Compared to the raw RF with a diameter of ~20 µm (Figure 2a), sc-RF shows an increased diameter owing to the presence of the sc coating (Figure 2b), which could be tuned by control over the dipping time, from ~30 µm for sc-RF3 to ~100 µm for scRF10 (Figure S1). Moreover, raw RF is observed to have a relatively smooth surface (Figure 2a), while the honeycomb-like and coarse textures are found on the surface of sc-RF10 (Figure 2b). It has been documented that the increase in the surface roughness could ameliorate heterogeneous nucleation of a foreign substrate.20 To ascertain the crystal modification of the decorated layer, microfocus 2D-WAXD measurement was conducted. As indicated in Figure 2c, the crystal reflection corresponding to the lattice plane (200) of cellulose I crystal is observed for raw RF. Apart from the diffraction arcs of cellulose I crystal, sc-RF10 shows multiple diffraction rings, which are representative of the lattice planes (110)sc, (200)/(110)α, (300)/(030)sc, (015)α, and (220)sc from the inner to outer circles (Figure 2d). Despite the coexistence of sc and α crystals in the coating layer, the relative fraction of sc crystallites extracted form 2D-WAXD image is 84.8%, suggesting that sc crystallites are dominant in the coating layer. This evidence is also amenable to the DSC results (Figure S2). The above results verify that the RF is successfully wrapped by the sc coating, which offers an opportunity to evaluate its influence on the interfacial crystallization.


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Figure 2. Surface morphology and 2D-WAXD images of (a, c) RF and (b, d) sc-RF10.

Interfacial Crystalline Morphology of PLLA Induced by sc-RF. As shown in Figures 3a, 4a, and 5a, the raw RF exhibits almost no nucleation ability towards PLLA, resulting in the sparse dispersion of spherulites in the bulk. In clear contrast, plentiful nuclei appear at the PLLA/sc-RF interface, proving that the introduction of sc coating on the surface of RF notably improves the nucleation. On account of the dense nuclei, the lateral extension of lamellae is hindered and then gradually impinges with each other, forcing the growth of crystals perpendicular to the sc-RF axis.13,

19, 21

Ultimately, the TC superstructure arising from this

restriction is fostered in the PLLA/sc-RF system (Figure 3c, 4c, and 5c). In particular, this unique interfacial crystalline morphology could be tuned by the thickness of the sc layer, in compliance with a nucleation-dominated mechanism. Nucleation of a semicrystalline polymer on the substrate is influenced by mutiple factors, such as roughness and surface free energy.22 The



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increased thickness of the coating layer enhances nucleation density on the modified RF on the one hand and endows RF with an uneven texture on the other hand (Figure 2b), to encourage the dislocation nucleation on the coating layer. Instead of the sector-like TC generated on the surface of sc-RF3 and sc-RF5 (Figure 3b and 3c), the well–defined TC superstructure is cultured on the surface of sc-RF10 (Figure 3d). It gives an attractive hint of the superior nucleation efficiency of sc coating. The identical chemical composition of sc coating and the matrix not only benefits lattice matching,23,

24

but also enhances the interfacial interactions,25,

26

which also provide

another favorable condition for transcrystallization. The effect of Tc on the morphology of sc-RF induced TC was also investigated, as depicted in Figure 4 and 5. With the increase of Tc to 140 °C and 145 °C, the density of spherulites in the bulk decreases significantly. It is consistent with the classic crystallization theory that more energy is required for the formation of a nucleus with critical size at lower supercooling degree.27 By virtue of the highly thermostable sc crystallites, the nucleation effect of sc-RF, especially for sc-RF10, is preserved to effectively ameliorate interfacial crystallization, where the robust TC is developed in the same manner. Our results suggest that the temperature window for the formation of TC broadens because of sc-RF, offering the practical pathways for structural manipulation in the PLLA products.


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Figure 3. Evolution of crystalline morphology of (a) PLLA/RF, (b) PLLA/sc-RF3, (c) PLLA/ scRF5, and (d) PLLA/sc-RF10 at Tc of 135 °C.




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To acquire quantitative information regarding the growth of TC, the growth rate of TC and spherulites is summarized in Figure 6. It is found that the growth rate of TC is accordant with that of PLLA spherulites and is not pertinent to the thickness of the sc layer (Figure S3). Furthermore, at Tc of 135 °C, the sc-RF induced TC grows at a rate of ∼3.6 µm/min. As the Tc increases to 140 and 145 °C, the growth rate of TC decreases to ~3.1 and ~2.3 µm/min, respectively. On the basis of the classical Hoffman−Lauritzen theory, the crystal growth is intrinsically a compromise between the formation of folded chain lamellae and relaxation of polymer chains.28 At a specific Tc, the segment mobility in the melt is at an identical level. Therefore, the growth rate of the PLLA TC superstructure is temperature-dependent.


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Figure 6. Linear growth rates of TC in PLLA/sc-RF composites at different Tc. The grow rate of the spherulites in PLLA/RF composites was compiled for comparison.



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Figure 7. Representative 2D-WAXD patterns of (a, c, e) PLLA/RF and (b, d, f) PLLA/sc-RF10 samples. These samples were obtained after isothermal crystallization at Tc of (a, b) 135 °C, (c, d) 140 °C, and (e, f) 145 °C for 30 min.

Crystalline Structure Evolution of PLLA/sc-RF Composite. The enhanced nucleation ability of RF by sc decoration inspires us to further detemine the structure of TC layers by employing microbeam 2D-WAXD. The discrepancy in the interfacial crystalline structure induced by sc-RF10 and RF is compared for brevity. As illustrated in Figure 7, evolution of the crystalline structure from the center of the fiber to the boundary of TC is confirmed. At the position of 0 µm, the strong lattice plane (200) of RF is observed for all the composites, coinciding with the aforementioned observation acquired in Figure 2c. For the PLLA/RF sample, the typical α form of PLLA is recognized, with the crystal reflections of lattice planes (200)/(110)


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and (203) (Figure 7a, 7c, and 7e).13 In contrast, simultaneous existence of sc and α crystals is observed near the sc-RF10 (Figure 7b, 7d, and 7f), which shows a spatial distribution along the radial direction depending on the thickness of the sc layer.

Figure 8. 1D-WAXD profiles of (a, c, e) PLLA/RF, (b, d, f) PLLA/sc-RF10 samples. These samples were obtained after isothermal crystallization at Tc of (a, b) 135 °C, (c, d) 140 °C, and (e, f) 145 °C for 30 min. The scanning step was 5 µm.



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Based on the 2D-WAXD patterns, 1D WAXD profiles as a function of the distance from the sc-RF axis are gathered. At Tc of 135 °C, the diffraction peaks at 14.9°, 16.7°, 19.1°, and 22.3° are indicative of (010)α, (110)/(200)α, (203)α and (015)α reflections of the α crystal (Figure 8a). For the PLLA/RF sample, we note that the lattice planes of (010)α and (015)α disappear at Tc of 140 and 145 °C (Figure 8c and 8e). It implies that at high Tc, the completion or perfection of PLLA crystallization requires a long time scale (> 30 min). However, these peaks appear in the PLLA/sc-RF10 sample (Figure 8d and 8f), confirming the role of sc-RF in accelerating the overall crystallization of PLLA. As the distance from the fiber axis increases, the diffraction rings of sc crystallites, i.e., (110)sc and (300)/(030)sc weaken, while those of α crystals, i.e., (200)/(110)α, heighten markedly. As an efficient heterogeneous nucleation agent, sc crystallites on the surface of RF apparently reduce the nucleation barrier, exerting positive effects on the PLLA crystallization (Figure S4). Additionally, only the α form is initiated by sc-RF10 at Tc above 120 °C, which is in accordance with the results reported by Zhang et al.29 It was found that the α form rather than the α' form was preferred at high annealing temperatures. Consequently, the survival of the α form in TC layer implies that sc decorated on RF might not affect the packing of PLLA chains into lamellae.


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Figure 9. Crystallinity distribution of TC superstructure from the central axis of sc-RF10 to the boundary of TC at Tc of (a) 135 °C, (b) 140 °C, and (c) 145 °C. For comparison, the overall crystallinity of the PLLA/RF composite is shown by the dotted line.

To elucidate the interfacial crystalline structure of the PLLA/sc-RF sample, the crystallinity (Xc) of sc and α crystals as a function of distance from the fiber axis is displayed in Figure 9. It is found that the Xc,sc in the range of 0-60 µm is ~ 15% regardless of the Tc. Then, it shows a steep decrease as the distance increases to 100 µm, which is equivalent to the diameter of sc-RF10. Such a spatial distribution of sc crystallites comes to the attestation that the sc coating on the surface of RF is highly stable within the processing window of PLLA. As is well acknowledged, nucleation is the precondition for crystallization.30 Thanks to the nucleation role of the sc coating,



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the growth of the α crystal is significantly promoted at the interface between the sc-RF and the bulk. As a result, the total Xc of TC for the PLLA/sc-RF10 sample outperforms that of the PLLA/RF sample, especially at high Tc. For example, the average Xc of TC for the PLLA/scRF10 composite reaches 32%, 1.2 times higher than that achieved for the PLLA/RF counterpart (~15%) at Tc of 145 °C (Figure 9c).

Figure 10. (a) SEM micrographs of the crystalline morphology formed in the PLLA/RF composite; (b) is the magnified image of the white square in (a). The sample was obtained after isothermal crystallization at Tc of 140 °C for 30 min.


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Figure 11. (a) SEM micrographs of the TC induced by sc-RF10; (b) and (c) are the magnified images of the white squares in (a); (d) is the magnified image of (c). The sample was obtained after isothermal crystallization at Tc of 140 °C for 30 min.

Interfacial Lamellar Morphology in PLLA/sc-RF Composite. As shown in Figure 10a and 10b, the spherulites with a diameter of ~150 µm are distributed in the PLLA/RF sample randomly, confirming the weak nucleation ability of raw RF again. In comparison, TC develops around sc-RF symmetrically, as shown in Figure 11a. Explicitly, the lamellae grow radially from the center of sc-RF10, where the impingement between the growing fronts is overwhelming (Figure 11b). In addition, abundant nucleation sites provided by the sc coating are identified along sc-RF10 and are perceived as the template for the formation of the compact TC (Figure 11c). Further insight gained from Figure 11d shows that the interface between the sc coating and the TC is tenacious. Essentially, the structural configuration established in the sc coating shares the same chemical composition as that of the bulk. Thus, sc-RF could be regarded as the “homogeneous fiber” and the strong interplay could be expected. We attempted to access the interfacial bonding between the PLLA matrix and the sc-RF by using a single-fiber pull-out test. It is found that raw RF could be divorced form the matrix easily. In clear contrast, sc-RF itself is



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fractured before it is pulled out of the matrix, suggesting that the interfacial adhesion of the PLLA/sc-RF sample is stronger than the strength of RF itself and thus stronger than that of PLLA/RF rivals. The interfacial enhancement is achieved by virtue of the sc coating.

Figure 12. Schematic representation of the crystalline morphology developed on the surface of (a) RF and (b) sc-RF.

Combination of above results allows us to conceive a perspective about the formation of TC structure induced by sc-RF, as schematically portrayed in Figure 12. Due to the weak nucleation ability of RF, the majority of PLLA spherulites grow randomly in the bulk (Figure 12a). Inversely, incorporation of the sc coating virtually improves the surface nucleation ability of


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RF.20 Owing to the high melting point, the sc coating is undisturbed in the PLLA melt. Thereupon, the molecular chains are anchored to the sc layer, resulting in the epitaxial growth of the lamellae (Figure 12b). The crowded nucleation sites aligned at the interface hinder the extension of crystallites and compel the growth of lamellae only in one direction (perpendicular to the long axis of sc-RF), giving rise to the unique TC architecture of PLLA.

CONCLUSIONS We demonstrated that the nucleation ability of RF to PLLA was remarkably enhanced by simply decorating the RF surface with a thin layer of stereocomplex (sc) crystals. The presence of the sc layer furnished abundant nucleation sites and induced the formation of a TC superstructure of PLLA. The TC superstructure, attributed to the nucleation-controlled mechanism, became compact with the thickness of the sc coating, while the growth rate of TC was not obstructed. Quantitative WAXD analysis showed that the robust TC induced by sccoated RF exhibited a significantly higher overall crystallinity than the common spherulites, especially at high crystallization temperatures. The current effort provides a smart yet feasible means to tailor the interfacial crystalline morphology/structure between a biopolymer and a natural fiber, by which the next step toward the high-performance biocomposites will be further.

ASSOCIATED CONTENT Supporting Information OM images of sc-RF3, sc-RF5, and sc-RF10; DSC heat flow curves of RF and sc-RF10; Evolution of TC size as a function of crystallization time at Tc of 135 °C, 140 °C, and 145 °C. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +86-28-8540-0211; fax: +86-28-8540-5402. Author Contributions #

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grants 51573116, 51533004, 51421061), the China Postdoctoral Science Foundation (Grant 2016M590887), and the Total-Corbion (the Netherlands). The authors also acknowledge beamline BL15U1 of the Shanghai Synchrotron Radiation Facility (SSRF) for kindly supporting the WAXD measurements.

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FOR TABLE OF CONTENTS USE ONLY

Synopsis: Interfacial crystallization between poly(L-lactic acid) and ramie fiber is significantly promoted by introducing stereocomplex coating.


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Ramie Fiber

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