Tailoring Crystalline Morphology by High-Efficiency Nucleating Fiber

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Applications of Polymer, Composite, and Coating Materials

Tailoring crystalline morphology by high-efficiency nucleating fiber: Towards high-performance poly(L-lactide) biocomposites Tao Gao, Zheng-Min Zhang, Le Li, Rui-Ying Bao, Zhengying Liu, Bang-Hu Xie, Ming-Bo Yang, and Wei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04907 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Applied Materials & Interfaces

Tailoring crystalline morphology by high-efficiency nucleating fiber: Towards high-performance poly(L-lactide) biocomposites Tao Gao, Zheng-Min Zhang, Le Li, Rui-Ying Bao*, Zheng-Ying Liu, Bang-Hu Xie, Ming-Bo Yang, Wei Yang*

College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, 610065, Sichuan, China

Abstract In this work, high melting point poly(L-lactide) fiber (hPLLA fiber) with high-efficiency nucleation activity was prepared and introduced into PLLA matrix to prepare fully biodegradable PLLA biocomposites. The highly active nucleating surfaces of hPLLA fiber induced chain ordering and lamellar organization, leading to preferable formation of well-organized PLLA transcrystallinity at the surface of hPLLA fiber in quiescent conditions. The construction of such compact transcrystallinity increased the crystallinity and enhanced the interfacial adhesion, which largely promoted the heat resistance, tensile strength, and barrier property of PLLA biocomposites at a low content of hPLLA fiber. With the addition of 1 wt% hPLLA fiber, the storage modulus of PLLA biocomposite was enhanced by 82 times from 4 to 330 MPa at 80 ºC, and the oxygen permeability coefficient and water permeability coefficient were decreased by 52% and 51% to be 5.9×10-15 cm3·cm/cm2·s·Pa and 4.5×10-14 g·cm/cm2·s·Pa, compared with pure PLLA. Moreover, the transparency of PLLA was maintained with the incorporation of hPLLA fiber. Thus, this strategy paved a new way to prepare high performance and fully biodegradable biocomposites. Key words: poly(L-lactide), fully biodegradable composite, high-efficiency nucleating fiber, heat resistance performance, barrier property

Introduction Poly(L-lactide) (PLLA), one of the most promising ecofriendly biopolymers, has attracted much attention due to its excellent performance in transparency, renewability, biodegradability and biocompatibility in recently years and it holds great potential in the application of packaging, biomedical devices and automotive industries.1-7 However, the crystallization ability of PLLA is poor due to the relatively short chain lengths and semi-rigid molecular backbones and the crystallinity of PLLA is usually *

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

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very low under conventional processing conditions. Additionally, the glass transition temperature (Tg) of PLLA is relatively low, leading to poor heat resistance of PLLA products, which limits the applications of PLLA as engineering or structural materials.8-11 In order to improve the heat resistance performance of PLLA, enhancing the crystallinity of PLLA is regarded as one of the simplest and most practical approaches, and the performance such as barrier property can also be improved simultaneously.12-15 Various chemical/organic/mineral components and carbon fillers have been employed to enhance the nucleation by providing heterogeneous nucleation sites, and thus to promote the crystallization kinetics of PLLA.16-19 The fillers with larger aspect ratio are more attractive considering the effect of filler morphology and dimensions on the property enhancement of PLLA. It is interesting to notice that fibrous fillers show better reinforcement effect than sphere fillers even if sphere fillers have better nucleation effect at the same content.20 Fibrous-filler can also act as nucleating agent and be used to induce polymer crystallization. Crystallization of polymer on the surface of fillers i.e., interfacial crystallization has a potential to improve the interfacial interaction, which is a key for high performance polymer/filler composites.21-22 For polymer/fibrous-filler composites, it is widely reported that the transcrystallinity or hybrid shish–kebab morphology can be obtained. Many factors, such as surface chemical and physical characteristics of the fillers, molecular weight, chain conformation and functional groups of polymer matrix, and extensional/shearing fields, affect the formation and growth of interfacial crystalline structures.23-26 Till now, numerous studies are trying to develop special interfacial crystalline structures in PLLA composites. Bai et al.27 induced hybrid shish-kebab superstructures in PLLA/nucleating agent (tetramethylene-dicarboxylic dibenzoyl-hydrazide, TMC-306) through TMC-306 self-organizing into fibrils to function as a template. To improve nucleation ability of glass fiber for PLLA crystallization, Ning et al.28 obtained reduced graphene oxide coated glass fiber (GF-RGO), and found that transcrystallinity structure can be successfully induced at the PLLA/GF-RGO interface. However, the incorporation of nonbiodegradable small molecular organic nucleating agents and fillers is contrary to the concept of ACS Paragon Plus Environment

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environmental sustainability. Natural fiber have attracted much more attention as green yet high efficiency reinforcing elements for polymers due to the low cost, excellent performances, biodegradability and derivation from renewable resources.29 However, the sparse and less-ordered transcrystallinity are induced by pristine ramie due to the inferior nucleation activity. To obtain well organized transcrystallinity, Xu et al. tried to generate rich nuclei of PLLA at the fiber surface through surface modification or very strong shear field such as oscillation shear injection molding,30-31 and facilitated the transcrystallization kinetics by plasticizer to accelerate the chain mobility of PLLA.32 Fillers with high nucleation activity are feasible for tailoring the crystalline morphology at the interface and accelerating the crystallization rate of PLLA matrix. However, there is still no report about a highly efficient biodegradable fibrous filler which can both accelerate the crystallization rate and tailor the crystalline morphology in quiescent condition as far as we know. In our previous work, it has been proved that high-melting-point PLLA (hPLLA) powders exhibit an excellent nucleation effect on PLLA and the crystallization accelerating effect of hPLLA crystallites can also be efficient at a high cooling rate due to the similarity of crystal structure and complete miscibility induced by their identical chemical composition.33 It arouses our interest to design a fibrous biodegradable nucleating agent to tailor the crystallization rate and morphology of PLLA for high performance PLLA biocomposites simultaneously. In this work, hPLLA fiber with high efficiency nucleation activity was prepared and introduced into PLLA to prepare fully biodegradable PLLA biocomposites. The tailored crystalline morphology by hPLLA fiber and improved crystallinity are expected to contribute to the heat resistance, tensile strength, and barrier property of the biocomposites.

Experimental section Materials A commercial PLLA (trade name 2003D, NatureWorks, USA) comprising about 4.3% D-units was ACS Paragon Plus Environment


used as the matrix and the weight-averaged molecular weight (Mw) determined by gel permeation chromatography (GPC) is 1.70 × 105 g·mol−1 and polydispersity (PDI) was 1.7. High-melting-point PLLA (trade name REVODE190) with a melting point of 178 ºC, Mw of 2.05 × 105 g·mol−1 and PDI of 2.10 was purchased from Zhejiang Hisun Biomaterial Co, Ltd (China). Preparation of hPLLA fiber. hPLLA fiber was prepared through melt spinning. The hPLLA pellets were melt extruded through a spinneret (18 holes with a diameter of 0.4 mm) at 195 °C, followed by collecting with a take-up speed of 500 m/min at room temperature, and then the as-spun fiber underwent hot-drawing with a draw ratio of around 2 at 100 ºC. The prepared hPLLA fiber was characterized in detail according to the methods described in supporting information. Preparation of PLLA/hPLLA fiber biocomposites. Prior to mixing, hPLLA fiber was cut into 2 cm of length, then PLLA and hPLLA fiber were vacuum dried at 60 ºC for 12 h. The PLLA/hPLLA fiber biocomposites were prepared by melt mixing in an internal mixer (XSS-300, Shanghai Kechuang Rubber Plastics Machinery Set Ltd, China) at 150 ºC and 40 rpm for 5 min. To disperse hPLLA fiber homogeneously in PLLA matrix, the biocomposites with 5 wt% hPLLA fiber was firstly prepared and then diluted to desired contents. For convenience, the obtained PLLA/hPLLA fiber biocomposites were marked as hPLLA fiber-x, where x stands for the weight fraction of hPLLA fiber in the PLLA/hPLLA fiber biocomposites. After compounding, the prepared samples were compression molded into sheet on a hot-press at 165 ºC and 10 MPa and then cooled on a cold-press at 110 ºC and 10 MPa for 5 min. A schematic representation describing the processing process for the PLLA/hPLLA fiber biocomposite is shown in Scheme 1.


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Scheme 1. The processing process of the PLLA/hPLLA fiber biocomposites. Differential Scanning Calorimetry (DSC). The thermal behaviors of pure PLLA and PLLA/hPLLA fiber biocomposites were investigated using a DSC Q20 (TA Instruments, USA) under nitrogen atmosphere. Samples of around 4-8 mg were first heated to 168 ºC at a rate of 100 ºC /min and held at 168 ºC for 3 min, then cooled to 0 ºC at a rate of 5 ºC/min to record the nonisothermal crystallization behaviors. The subsequent melting behaviors were recorded at a heating rate of 10 ºC/min. For isothermal crystallization, samples were also first held at 168 ºC for 3 min and then cooled to the desired crystallization temperatures at 40 ºC/min and an isothermal crystallization period of 120 min was monitored. The melting behaviors for the first scan of raw hPLLA, hPLLA fiber, and compression molded samples were recorded at a heating rate of 10 ºC/min. Wide-Angle X-ray Diffraction (WAXD). One-dimensional WAXD measurements of the samples were carried out with a DX-1000 X-ray diffractometer (Dandong Fanyuan Instrument Co. LTD, China) using a Cu Kα radiation source (λ = 0.154 056 nm, 40 kV, 25 mA) in the scanning angle range of 2θ = 5−50 º at a scan speed of 3 º/min. The degree of crystallinity (Xc) was calculated according to the following equation: Xc =

where

∑ Acryst ∑ Acryst + ∑ Aamorp ∑ Acryst and

(1) ∑ Aamorp are the fitted areas of crystal and amorphous region, respectively.

Polarizing Optical Microscopy (POM). The crystalline morphologies of the PLLA/hPLLA fiber composites were observed using an Olympus BX51 polarizing optical microscopy (Olympus Co., Tokyo, ACS Paragon Plus Environment


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Japan) equipped with a hot-stage (LINKAM THMS 600). Specifically, A single hPLLA fiber was placed on a PLLA film with a thickness 10 µm and the sample were melted at 168 ºC for 3 min and then quickly cooled at 30 ºC/min to the desired isothermal crystallization temperatures. Scanning Electronic Microscopy (SEM). The crystalline morphologies were also examined using a JEOL JSM-5900LV scanning electron microscope (SEM, Japan) operating at an accelerating voltage of 5 kV. Samples used for POM observation, and the fracture surface of compression molded samples cryo-fractured in liquid nitrogen were etched by a water-methanol (1:2 by volume) solution containing 0.025 mol/L of sodium hydroxide at 30 ºC for 12 h to remove the amorphous region. Subsequently, the etched samples were repeatedly cleaned using distilled water under ultrasonication, dried in vacuum oven overnight at 40 °C, and then coated with a thin layer of gold prior to being observed. Performance Evaluation. Dynamic mechanical properties of PLLA/hPLLA fiber biocomposites were measured using a TA Q800 instrument (USA) in tensile mode at a frequency of 1 Hz and a heating rate of 3 º

C/min from 20 ºC to 150 ºC. To illustrate the heat resistance of PLLA/hPLLA fiber biocomposites directly,

the rectangle specimens with a dimension of 0.5 mm × 60 mm × 10 mm were used for the test. The heat resistance measurement was carried out in the oven at the desired temperature for 5 min, and the temperature step was 5 °C. Specifically, the temperature of oven was heated to the desired temperature at first, then the specimen with a standard weight of 10g on was hold for 5 min, and the each ends of the specimen was fixed by two metal blocks. The distance from the horizontal line of the test specimen to the maximum deformation point was used to quantitatively judge the deformation. In accordance with ASTM standard D638-10, the tensile test was performed on a universal tensile instrument (Model 5967, Instron Instrument, USA) at a crosshead speed of 5.0 mm/min at room temperature (23 ± 2 ºC). At least five specimens with a thickness of 0.5 mm were tested for each sample and the average results were reported. According to ISO Standard 15105-1: 2007, oxygen permeability coefficient ( PO2 ) of the composite films was measured on a VAC-V2 film permeability testing machine (Labthink instrument, Jinan, China) at room temperature with 50% ACS Paragon Plus Environment

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relative humidity. Also according to ISO 15105-1: 2007, the water permeability coefficient ( PH 2O ) of the composite films was measured on a W3-031 water permeability testing apparatus (Labthink Instruments, Jinan, P. R. China) at room temperature and 98% relative humidity. Samples with a dimension of 100 mm in diameter and 0.2 mm in thickness were used for the barrier tests. The transmittance spectra of PLLA/hPLLA fiber biocomposites films were recorded on a UV-3600 spectrophotometer (Shimadzu Co., Japan), and the samples with a thickness of about 0.2 mm were used for test.

Results and discussion Characterization of hPLLA fiber.

Figure 1. Characterization of hPLLA fiber. SEM micrograph of hPLLA fiber (a), DSC heating curve for the first scan of raw hPLLA and hPLLA fiber (b), 2D-SAXS pattern of hPLLA fiber (c), 2D-WAXD pattern of hPLLA fiber (d). Figure 1a shows the surface morphology of the melt-spun hPLLA fiber, which has a smooth surface with a diameter of approximately 28 µm. Figure 1b displays the DSC melting curve of hPLLA fiber. It can be seen that the melting point of hPLLA fiber is higher than that of the raw hPLLA, resulting from the formation of oriented crystals during the hot drawing process. The formation of oriented crystals in the fiber can be demonstrated by two-dimensional small-angle x-ray scattering (2D-SAXS) and two-dimensional wide-angle X-ray diffraction (2D-WAXD). As shown in Figure 1c, a strong signal appears at the equator ACS Paragon Plus Environment


direction of 2D-SAXS pattern, which is resulted from the formation of oriented crystals perpendicular to the fiber axis. From 2D-WAXD pattern in Figure 1d, remarkable arc-like diffraction with high intensities assigning to the lattice planes (200)/(110), (203) and (015) of α-form PLLA are observed,34-35 which also reveals the preferred orientation of PLLA lamellae. The Herman’s orientation parameter calculated from (200) and/or (110) plane is 0.55, showing a high degree of lamellar orientation. The crystallinity of hPLLA fiber calculated from 2D-WAXD is 63.2%. As it have been proved that both the crystallinity and lamellae orientation play a critical role in enhancing the performance of polymer fibers,36-37 the strength of hPLLA fiber is expected to be high. Figure 2 shows the diagram of the specific strength versus elongation of hPLLA fiber, comparing with typical natural fibers, and the detailed values are given in Table S1. The specific strength of hPLLA fiber is up to 472.8 kN·m/kg taking the density of 1.25 g/cm3. Even more intriguing, the specific strength of hPLLA fiber is comparable with other typical natural fibers, such as jute, sisal, flax, lotus, cotton, and coir, and the elongation is higher than these natural fibers, indicating a potential reinforcing effect of hPLLA fiber on PLLA matrix as it has been well demonstrated that the natural fibers can reinforce polymers effectively.30

Figure 2. Diagram of specific strength versus elongation of hPLLA fiber, comparing with typical natural fibers including jute, sisal, flax, lotus, cotton, and coir fibers.38-42 Crystallization tailoring of PLLA/hPLLA fiber biocomposites Using the processing temperature window created by the difference between the melting points of PLLA and hPLLA, the unmelted high-melting-point PLLA crystallites turn out to be the most efficient ACS Paragon Plus Environment

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nucleating agent for PLLA and can significantly accelerate the crystallization rate of PLLA as has been revealed in our previous work.33 This is actually self-seeding crystallization achieved in conventional manufacturing processes, and the crystallinity and crystalline morphology of PLLA matrix are expected to be tailored easily by the introduction of hPLLA species.

Figure 3. DSC cooling curves for PLLA with various contents of hPLLA fiber at 5 ºC min−1 from 168 ºC (a) and the subsequent melting curves at 10 ºC min−1 (b). Figure 3 shows the DSC cooling curves for PLLA with various contents of hPLLA fiber and the subsequent melting curves. The detailed data of crystallization temperature (Tc) and crystallinity obtained from DSC are given in Table S2. As shown in Figure 3a, with the incorporation of 0.05 wt% hPLLA fiber, the crystallization peak of PLLA appears at 101.1 ºC while the crystallization peak of pure PLLA does not appear in the cooling process. With the increasing content of hPLLA fiber, Tc shifts to higher temperature and the Tc is 120.8 ºC with the addition of 2 wt% hPLLA fiber. Figure 3b shows the subsequent melting curves of pure PLLA and PLLA/hPLLA fiber biocomposites. It is found that the area of cold crystallization peak of PLLA decreases evidently with the addition of 0.05 wt% hPLLA fiber and the cold crystallization disappears with only 0.1 wt% hPLLA fiber. All the biocomposites reach to a high degree of crystallinity above 30%, which is almost the maximum crystallinity of the used PLLA matrix taking the influence of optical purity into account. It is also noted that, a small exothermic peak at about 180 ºC can be obviously observed when the content of hPLLA fiber is higher than 1 wt%, which refers to the melting peak of hPLLA fiber. These results manifest evidently that a small amount of hPLLA fiber can largely enhance the ACS Paragon Plus Environment


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crystallization rate of PLLA. The excellent crystallization promoting effect of hPLLA fiber on PLLA is further confirmed by isothermal crystallization. Figure 4a displays the isothermal crystallization curves for pure PLLA and PLLA/hPLLA fiber biocomposites at 130 ºC. Extremely weak and broad exothermal peak is observed for pure PLLA due to its extremely slow crystallization rate. In comparison, the exothermal peaks of PLLA/hPLLA fiber biocomposites are sharp and the peak position shift to a shorter time with an increase in content of hPLLA fiber, indicating the more remarkable crystallization accelerating effect with an increased hPLLA fiber content.

Figure 4. DSC heat flow as a function of time (a) and the relative crystallinity as a function of time (b) for the samples isothermally crystallized at 130 ºC and the insert is the enlarged view for PLLA/hPLLA fiber biocomposites. Crystallization half-time (t1/2) at different Tc obtained from X(t) = 50% for the PLLA/hPLLA fiber biocomposites with different contents of hPLLA fiber (c). Figure 4b gives the relative crystallinity as a function of time. The crystallization half-time (t1/2), which is utilized to characterize the overall crystallization kinetics, was obtained from the X(t) = 50% and is shown in Figure 4c. The t1/2 of pure PLLA is above 20 min in the investigated isothermal crystallization temperature (Tc). With the introduction of hPLLA fiber, t1/2 is sharply decreased. Taking the case of Tc=130 ºC for example, the t1/2 of hPLLA fiber-2 is only 2.1 min while the t1/2 of pure PLLA is 33.4, showing the overall crystallization rate (referring to 1/t1/2) increase of 16 times than that of pure PLLA.


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Figure 5. POM images showing the evolution of crystalline morphology of PLLA (a) and PLLA/hPLLA fiber composite (b) at Tc of 120 ºC. PLLA (c) and PLLA/hPLLA fiber composite (d) at Tc of 130 ºC. The inserts are the enlarged view. Single hPLLA fiber was placed on a PLLA film for tracing the promoting effect of hPLLA fiber on the crystallization of PLLA and the crystalline morphology visually by POM. The POM images in Figure 5 show the evolution of crystalline morphology of pure PLLA and PLLA in the vicinity of hPLLA fiber under quiescent condition. As shown in Figure 5a and c, pure PLLA exhibits poor crystallization ability, resulting in sparsely dispersed spherulites. In contrast, hPLLA fiber enable the generation of dense lamellae of PLLA at the fiber surface in Figure 5b and d, demonstrating the excellent nucleation effect of hPLLA fiber. The lateral growth of ordered lamellae perpendicular to the fiber axis results in the formation of prevailing transcrystallinity layers closely wrapping the fiber. The result is in accordance with Wang’s work43, in which a single PLLA fiber was placed on a super-cooled PLLA film to observe the interfacial crystalline morphology between PLLA fiber and PLLA matrix. Similar phenomena were also observed when polypropylene fiber and poly(ω-pentadecalactone) fiber were utilized to induce the transcrystallinity growth ACS Paragon Plus Environment


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of homogeneous matrix,44-45 implying that homogeneous fiber have very strong nucleating ability to homogeneous matrix. For comparison, the micrographs of the crystalline morphology formed in the PLLA/ramie fiber composite is given in Figure S1. The fan-shaped transcrystallinity is cylindrically developed around the ramie fiber, which is distinct from the PLLA/hPLLA fiber composite owing to the poor nucleation ability of ramie fiber. Although the spherulitic nucleation is apparently suppressed with the increase of crystallization temperature due to the much higher nucleation energy barrier at lower supercooling degree,46 compact heterogeneous nuclei were generated to induce the columnar transcrystallinity with weak relation to the temperature due to excellent nucleation effect of hPLLA fiber as shown in Figure 5b and d. Figure S2a shows the spherulite radius in PLLA and transcrystallinity radius in PLLA/hPLLA fiber composites as a function of time. The plot for radius of transcrystallinity is almost overlapping with that of radius of spherulite at the corresponding Tc, indicating that the growth rates of spherulite and transcrystallinity are almost the same (Figure S2b), and the addition of hPLLA fiber can only accelerate the nucleation rate of PLLA other than the crystal growth rate. Direct morphological observation was also performed to provide insights into the evolution of lamellar structure of transcrystallinity induced by PLLA fiber. As shown in Figure 6a, b, c, due to the strong spatial hindrance induced by rich heterogeneous nuclei, the lateral extension of lamellae is hindered and then gradually impinges with each other, forcing the growth of lamellae nearly perpendicular to the hPLLA axis.47-48 In pure PLLA, sparse spherulites are distributed and the lamellae of spherulite grow radially from the center (Figure 6d). For comparison, the micrographs of the crystalline morphology formed in PLLA/ramie fiber composite is shown in Figure 6e. It can be seen that heterogeneous nuclei are sparsely generated at the surface of fiber, and fan-shaped transcrystallinity is cylindrically developed around the fiber. Figure 6f schematically compares the crystallization morphology of PLLA in the vicinity of ramie fiber with inferior nucleation activity and hPLLA fiber with excellent nucleating effect. For ramie fiber, the crystal ACS Paragon Plus Environment

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structure between ramie fiber and PLLA are apparently different and the interfacial interaction between ramie fiber and PLLA chains is weak. Then radial lamellae are generated at the surface of ramie fiber. In contrast, the hPLLA fiber exhibits a strong ability to absorb PLLA chains and dense nucleis are formed at the surface of hPLLA fiber. During this surface induced crystallization process, the complete miscibility i.e., a strong interfacial interaction, between the amorphous chains of the hPLLA fiber and PLLA matrix, and the identical crystal structure between the α-form hPLLA fiber and PLLA matrix sharply reduce the energy barrier for heterogeneous nucleation. Then, spatial hindrance arises from the dense nucleis and highly ordered lamellae impel the lateral growth perpendicular to the hPLLA axis.

Figure 6. SEM micrographs of transcrystallinity induced by hPLLA fiber after isothermal crystallization at 120 ºC for 2h (a). (b) and (c) are the local amplifications of (a). The spherulites in pure PLLA (d), and crystallization morphology of PLLA/ramie fiber after isothermal crystallization at 120 ºC for 2h (e) are given for comparison. The inserts are the enlarged view of (b), (c), (d) and (e). Schematic representation of the crystalline morphology of PLLA on the surface of ramie fiber and hPLLA fiber (f). The arrows represent the growth direction of the lamellae.



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Figure 7. DSC heating curves (a) and WAXD patterns (b) of the PLLA/hPLLA fiber biocomposites prepared by compression molding. Based on the confirmed excellent crystallization promoting effect of hPLLA fiber on PLLA, it is expected that hPLLA fiber can still accelerate the crystallization of PLLA matrix under conventional processing conditions, and the crystalline morphology of PLLA to achieve high performance PLLA/hPLLA fiber biocomposites. Figure 7 gives DSC heating curves and WAXD patterns of the PLLA/hPLLA fiber biocomposites prepared by compression molding. In Figure 7a, the cold crystallization peak can be observed from the heating curves of pure PLLA, hPLLA fiber-0.05 and hPLLA fiber-0.1. With further increasing content of hPLLA fiber, the cold crystallization peaks disappear, and the melting peak of hPLLA fiber are clearly seen in the hPLLA fiber-1 and hPLLA fiber-2, indicating that hPLLA fiber can be well reserved after the compression molding and effectively accelerate the crystallization rate of PLLA. From WAXD patterns in Figure 7b, no diffraction peaks are exhibited for pure PLLA, revealing that it is almost amorphous due to its extremely slow crystallization rate. The diffraction peaks at 2θ values of 16.7º, 19.0º and 22.3º, which correspond to the (200) and/or (110), (203) and (015) planes of PLLA crystals in α-phase34-35, are observed for PLLA/hPLLA fiber biocomposites. With increasing content of hPLLA fiber, the intensity of these diffraction peaks becomes much stronger, indicating the increasing crystallinity of PLLA. The crystallinity is calculated by WAXD and the results are displayed in Table 1. A relatively high crystallinity of 37.7% is achieved when the content of hPLLA fiber is only 0.5%, revealing that with the incorporation of hPLLA fiber, the PLLA matrix can reach a high crystallinity during such a conventional processing. Here, it should ACS Paragon Plus Environment

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be pointed out that the crystallinity of PLLA/hPLLA fiber biocomposites calculated by WAXD is relatively higher than the actual value of PLLA matrix as the crystallites of hPLLA fiber would also contributes to the overall crystallinity of PLLA in the samples. Table 1. Degree of crystallinity (Xc) of PLLA/hPLLA fiber biocomposites obtained by WAXD results. Content of hPLLA fiber (wt %)

0

0.05

0.1

0.5

1

2

Xc (%)

0.5

6.8

25.0

37.7

37.0

39.3

SEM is used to examine the crystalline morphology PLLA/hPLLA fiber biocomposites. The crystalline morphology of the biocomposites is illustrated with the sample hPLLA fiber-1 in Figure 8a-f. For clearly showing the structure at the interface between hPLLA fiber and PLLA matrix, the etched fracture surface of a hPLLA fiber with the maximum diameter of approximate 21 µm (consisting with the actual diameter of hPLLA fiber in Figure 1a) is given in Figure 8a and b. The transcrystallinity layer closely wraps the fiber, and the lamellae of transcrystallinity grow perpendicularly on the surface of hPLLA fiber, indicating that hPLLA fiber can effectively induce the growth of transcrystallinity in compressing molding process. Figure 8c and d shows the surface morphology of hPLLA fiber in sample hPLLA fiber-1, and it is observed that the highly ordered lamellae rows of hPLLA fiber are parallel to each other and perpendicular to the fiber axis, well consistent with the orientation structure of hPLLA fiber as demonstrated by 2D-SAXS and 2D-WAXD. A deep etching of the surface morphology of hPLLA fiber-1 sample in Figure 8e and f further exposes the crystalline morphology in the vicinity of hPLLA fiber. The hPLLA fiber is almost broken while the well-organized transcrystallinity in the vicinity of hPLLA fiber is quite clear. It is visualized that the lamellae of PLLA matrix can grow along the growth edges of lamellae rows in the hPLLA fiber, and the new lamellae keeps the orientation direction of lamellae in the hPLLA fiber as the dotted line guided in Figure 8f. For pure PLLA, the lamellae of spherulite grow radially from the center (Figure 6d). Figure 8h schematically represents the crystalline morphology of PLLA on the surface of hPLLA fiber. Owing to the identical crystal structure between the α-form crystals of hPLLA fiber and PLLA matrix, the PLLA matrix ACS Paragon Plus Environment


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finds a crystalline substrate that is exactly the one it wants to make. The PLLA chains deposit on the growth front of the hPLLA lamellae, and nucleation is induced by the lateral crystalline, growth edges of the hPLLA lamellae. This is consistent with the proposed nucleation mechanism of hPLLA crystallites to PLLA in our previous work.35

Figure 8. SEM observations of crystalline morphology for etched hPLLA fiber-1 (a-f) and pure PLLA (g). Morphology of cross section, surface, and surface etched for a long time, of hPLLA fiber in hPLLA fiber-1 sample are given in (a), (c) and (e), respectively. (b), (d) and (f) are the local amplification of (a), (c) and (e). Schematic representation of the crystalline morphology of PLLA on the surface of hPLLA fiber (h). The arrows and dotted line represent the growth direction of the transcystallinity. 3. Performance of PLLA/hPLLA fiber biocomposites The heat resistance of PLLA/hPLLA fiber biocomposites was evaluated through the thermomechanical properties from dynamic mechanical analysis. Figure 9a presents the temperature dependence of storage modulus of pure PLLA and PLLA/hPLLA fiber biocomposites. For pure PLLA, the storage modulus keeps constant below Tg (67 ºC, obtained from tan δ curves), and a sharp decrease is observed around Tg. With further increasing temperature, the storage modulus shows a rapid increase at around 100 ºC due to the cold crystallization of PLLA and another rapid decrease at around 140 ºC due to the melting of formed PLLA crystals. For PLLA/hPLLA fiber biocomposites, much less decrease and higher storage modulus are ACS Paragon Plus Environment

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observed in the investigated temperature range, comparing with pure PLLA. When the content of hPLLA fiber is higher than 0.1 wt%, no change in storage modulus induced by the cold crystallization of PLLA can be observed. Specially, the elastic modulus at 20 and 80 ºC of pure PLLA and PLLA/hPLLA fiber biocomposites are obtained and compared in Figure 9b. At 20 ºC, the storage modulus of PLLA is enhanced from 2268 to 3675 MPa with the introduction of only 0.5 wt% hPLLA fiber. At 80 ºC, the modulus increases by 82 times from 4 to 330 MPa with the incorporation of 1 wt % hPLLA fiber and it further increases to 382 MPa with 2 wt% hPLLA fiber, displaying an excellent heat resistance performance of PLLA/hPLLA fiber biocomposites owing to the enhanced crystallinity.49

Figure 9. Temperature dependence of storage modulus (a) and the specific values at 20 and 80 ºC (b) for pure PLLA and PLLA/hPLLA fiber biocomposites. Digital photographs taken during the heat resistance measurement in hot air, and the corresponding photos of these samples after the test. To illustrate the heat resistance of PLLA/hPLLA fiber biocomposites directly, each ends of the specimen was fixed by two metal blocks in an oven, and a 10g standard weight was put on the samples. As Figure 9b shown, pure PLLA shows serious deformation at 80 ºC within 2 min and the distance from the horizontal line of the test specimen to the maximum deformation point is measured to be 0.40 cm, while no obvious deformation is observed when hPLLA fiber-1 biocomposite is tested at 120 ºC for 5 min. Until the temperature reaches to 145 ºC, which is very close to the melting point of PLLA matrix, a small deformation is observed for hPLLA fiber-1 biocomposite and the distance is measured to be 0.25cm. It is noted that this ACS Paragon Plus Environment


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deformation degree is still smaller than that of pure PLLA at 80 ºC, visually showing the excellent heat resistance performance of PLLA/hPLLA fiber biocomposites. In order to evaluate the influence of hPLLA fiber on the mechanical properties of PLLA/hPLLA fiber biocomposites, tensile test was performed and the tensile yield strength and elongation at break of PLLA/hPLLA fiber biocomposites are shown in Figure 10a. With the introduction of hPLLA fiber, the elongation at break has not changed or decreased slightly, and the tensile strength has increased slightly. When the content of hPLLA fiber is 1 wt%, the tensile strength increases by about 4.3 MPa compared with that of pure PLLA. The change in the tensile strength with the content of hPLLA fiber is in accordance with the results of storage modulus tested by DMA (Figure 9b). For thermomechanical and tensile properties, the enhancement of storage modulus and tensile strength is mainly resulted from the homogeneous dispersion of fibers (Figure S3) and the increasing rigid crystalline phase in PLLA/hPLLA fiber biocomposites.50-51 Even though hPLLA fiber has a potential reinforcement effect on PLLA (Figure 2), and the growth of transcrystallinity between hPLLA fiber and PLLA matrix can improve the interfacial bonding and effectively transfers stress from the bulk to the fiber, the reinforcement effect of hPLLA fiber is not fully exhibited in this work possibly due to that the orientation of fiber cannot be achieved in compression molding process.

Figure 10. Tensile yield strength and elongation at break (a), oxygen permeability coefficient ( PO2 ) and water permeability coefficient ( PH 2O ) (b), UV–visible light transmittance spectra (c) of pure PLLA and PLLA/hPLLA fiber biocomposites. The digital photographs of pure PLLA, sample hPLLA fiber-1 and sample hPLLA fiber-2. As a biodegradable polymer, PLLA is regard to be a promising alternative for petroleum derived ACS Paragon Plus Environment

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polymers especially in food packaging applications. However, the relatively poor barrier property of PLLA limits its application. Incorporation of impermeable two-dimensional (2D) nanofillers such as exfoliated graphene platelets and clay provides a facile method for enhancing the barrier properties of polymers via the formation of a tortuous path for diffusing gas molecules.52-53 However, the utilization of nonbiodegradable nanofillers does not agree with the concept of green sustainability and the exposure of nanofillers on the surface of the nanocomposite causes a concern on food safety.54 Enhancing the crystallinity and tailoring the crystalline morphology can be thought as an environmental friendly way to improve the barrier property of PLLA.13, 55 The combined dense transcrystallinity and the crystals in the matrix induced by high efficient nucleating hPLLA fiber are expected to contribute to the barrier properties of the biocomposites. The oxygen permeability and water permeability of pure PLLA and PLLA/hPLLA fiber biocomposites are shown in Figure 10b. With increasing content of hPLLA fiber, both the oxygen permeability coefficient ( PO2 ) and water permeability coefficient ( PH 2O ) are decreased. When the content of hPLLA fiber is above 1 wt%, the values of PO2 and PH 2O are tend to be constant because the crystallinity of PLLA has reached to the maximum. With the addition of 1 wt% hPLLA fiber, the oxygen permeability coefficient is 5.9×10-15 cm3·cm/cm2·s·Pa and water permeability coefficient is 4.5×10-14 g·cm/cm2·s·Pa, which declines 52% and 51% compared with that of pure PLLA, respectively. Scheme 2 presents the crystalline morphological features in pure PLLA and PLLA/hPLLA fiber biocomposites governing the oxygen and water permeation. For pure PLLA, the crystallinity is very low and only a few spherulites are randomly distributed in the matrix. Then the oxygen and water molecules can pass through the amorphous phase easily. In contrast, the solubility of oxygen in the matrix is decreased with regard to the hPLLA fiber assisted formation of rich crystalline regions, and the oxygen and water molecules will experience more tortuous path through biocomposite films owing to the increased impenetrable crystalline phase. Furthermore, the construction of compact transcrystallinity layer on the surfaces of hPLLA fiber can serve as a fence to resist diffusing molecules. The construction of compact high-barrier ACS Paragon Plus Environment


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transcrystallinity fence combined with the increased crystallinity and enhanced interfacial adhesion is assumed to decrease the solubility and diffusion of oxygen and water molecules, which endows the elevated resistance to oxygen and water permeation in PLLA/ hPLLA fiber biocomposites.

Scheme 2. Schematic representation showing the crystalline morphological features in pure PLLA (a) and PLLA/hPLLA fiber biocomposites (b) governing the oxygen and water permeation. The lines represent the diffusion path of O2 or H2O. The transparency of films, which is an important factor for package materials, usually decreases sharply owing to the increasing crystallinity.56 The high nucleation ability of hPLLA fiber can reduce the size of crystals, making the highly crystallized film high transparency if the dimension of crystals is comparable to the wavelength of visible light. The light transmittance of pure PLLA and PLLA/hPLLA fiber biocomposites in the wavelength range of 250–700 nm is shown in Figure 10c. The light transmittance at around 700 nm for pure PLLA is about 87%. Even though the transmittance is partly suppressed by the increasing crystallinity of PLLA, appreciable transparency is still maintained and the values are almost maintained at ~67% at around 700 nm when the content of hPLLA fiber is above 0.1 wt%. The digital photographs of PLLA/hPLLA fiber films are shown in Figure 10c. With the addition of 2 wt% hPLLA fiber, the film is almost transparent to the naked eyes and the printed blue mark “PLLA” can be clearly seen, indicating that the transparency of PLLA/hPLLA fiber films is well preserved.

Conclusion The biodegradable and high-efficiency nucleating hPLLA fiber can both accelerate the crystallization ACS Paragon Plus Environment

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rate and tailor the crystalline morphology of PLLA matrix in quiescent condition. The highly active nucleating surfaces of hPLLA fiber induced chain ordering and lamellar organization, leading to preferable formation of well-organized PLLA transcrystallinity at the surface of hPLLA fiber. Moreover, it was visualized that the lamellae of PLLA matrix grew along the growth edges of lamellae rows in hPLLA fiber, and the new lamellae kept the orientation direction of the lamellae in hPLLA fiber. The heat resistance, tensile strength and barrier properties of PLLA/hPLLA fiber biocomposites are largely improved compared with pure PLLA. With the addition of 1 wt% hPLLA fiber, the storage modulus is enhanced by 82 times from 4 to 330 MPa at 80 ºC. For barrier properties, the oxygen permeability coefficient is 5.9×10-15 cm3·cm/cm2·s·Pa and water permeability coefficient is 4.5×10-14 g·cm/cm2·s·Pa, which declines 52% and 51% comparing with that of pure PLLA, respectively. Furthermore, the transparency of PLLA/hPLLA fiber biocomposites is well maintained. When PLLA was highly crystallized, a light transmittance of ~67% at 700 nm was obtained. The PLLA/hPLLA fiber biocomposites hold great potential in the field of packaging, biomedical, and tissue engineering etc. This strategy paves a new way to prepare fully biodegradable biocomposites. Supporting Information Characterization for hPLLA fiber, mechanical properties of natural fibers and hPLLA fiber, the crystallization temperature (Tc) and crystallinity (Xc) for PLLA/hPLLA fiber composites measured by DSC, evolution of crystalline morphology of PLLA/ramie fiber, quantitative analysis on time dependence of spherulite radius in PLLA and transcrystallinity radius in PLLA/hPLLA fiber biocomposites and POM micrographs showing the dispersion of PLLA/hPLLA biocomposites.

Acknowledgements This work was supported by the National Natural Science Foundation of China (NNSFC Grants 51503132, 51422305 and 21374065), Sichuan Provincial Science Fund for Distinguished Young Scholars (2015JQO003). The authors are grateful to the kind help on synchrotron X-ray measurements from the ACS Paragon Plus Environment


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Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China).

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Tailoring Crystalline Morphology by High-Efficiency Nucleating Fiber

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