Coexistence of Transcrystallinity and Stereocomplex Crystals Induced

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Materials and Interfaces

Coexistence of Transcrystallinity and Stereocomplex Crystals Induced by the Multilayered Assembly of Poly (L-lactide) and Poly (Dlactide): A Strategy for Achieving Balanced Mechanical Performances Longfei Yi, Dun Li, Yang Xu, Jiabin Shen, Shaoyun Guo, and Zhuo Zheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05148 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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Industrial & Engineering Chemistry Research

Coexistence of Transcrystallinity and Stereocomplex Crystals Induced by the Multilayered Assembly of Poly (L-lactide) and Poly (D-lactide):

A

Strategy

for

Achieving

Balanced

Mechanical

Performances Longfei Yi, Dun Li, Yang Xu, Jiabin Shen*, Shaoyun Guo*, and Zhuo Zheng

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

Abstract: The multilayer architecture consisting of alternating poly (L-lactide) (PLLA) and PLLA/poly(D-lactide) (PDLA) blend layers was fabricated through layer-multiplying coextrusion. With the multiplication of layers, the spherulites tended to disappear and the whole specimen exhibited a transition from isotropic to highly-oriented crystalline morphology. Finally, a specific TC/SC alternating multilayer structure was created for the first time when the layer numbers reached 1024. Resulting from such unique structure, the tensile strength and elongation at break of the 1024-layer specimen were respectively increased by 17% and 1528%, in comparison to those of the conventional blend specimen containing the same content of PDLA (~2.5wt%). Besides, largely enhanced resistance to scratch destruction which is critically related to the mechanical strength and ductility was also achieved. Thus, this research opens up a new horizon for achieving balanced mechanical performances of polylactide by tailoring its crystalline structures.

Key

words:

polylactide;

multilayer

structure,

transcrystallinity; mechanical performances.

ACS Paragon Plus Environment

stereocomplex

crystal;

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents use only


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1. Introduction Due to the ever-increasing concern over the green and sustainable issues associated with traditional petroleum-based polymers, significant importance has been attached to the development of biodegradable polymers originating from natural resources. Taking poly (L-lactide) (PLLA), the frontrunner of biodegradable polymers as an example, the combination of intrinsic properties such as transparency, renewability, and thermal processability has made it a promising substitute to traditional plastics.1-3 Meanwhile, accompanying the expanded usage of PLLA in structural and packaging applications, there is a growing demand for maintaining its esthetics, integrity and durability, which has greatly necessitated the PLLA materials with competitive scratch resistance. It has been revealed from previous researches that scratch, defined as indentation of an asperity followed by friction induced sliding on material surface, is highly dependent on the bulk mechanical property, especially the strength and toughness of polymers.4-6 There is a growing awareness that the key to improve the scratch performance of polymers lies in simultaneous increment of both the mechanical strength and toughness. So far, various strategies have been proposed to meet the requirement. In particular, incorporation of rigid fillers7-9 or toughening components10-12 has been widely regarded as the most versatile and economic route to solve the dilemma. However, the employment of fillers remains unsatisfying because it usually fails to offer balanced performance. Moreover, the incorporation of foreign components would inevitably exert negative impact on the transparency, processability, and biocompatibility of PLLA.13 Therefore, it remains a significant challenge to prepare fully bio-based and biodegradable PLLA materials with superior and balanced performances. In 1990s, Ikada divulged that an equimolar physical blend of PLLA with its enantiomer, namely poly (D-lactide) (PDLA), would create a new structure: stereocomplex crystal (SC)14. The existence of dense and strong intermolecular hydrogen bonding in SC would greatly benefit the mechanical properties of



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polylactide (PLA), thereby providing an amazing alternative to the modification of PLLA products15-17. Unfortunately, such improvement can only be achieved when comparatively large amount of PDLA was incorporated,18,

19

posing challenge to

fabricate PLLA materials with high performance under a low content of PDLA. Thus, another approach is urgently required to overcome the deficiency of this situation. For semi-crystalline PLA, the regulation of crystalline morphology has been proven a facile method to optimize its resultant mechanical property from existing literatures.20-22 In particular, with distinctive morphology, the transcrystallinity (TC) induced by oriented interfaces can remarkably contribute to the tensile strength.23-25 Moreover, during uniaxial stretching, the deflection of TC was found to be capable of continuously absorbing destructive energy, leading to highly improved toughness.26 Inspired by such result, if a concentration gap is introduced through local accumulation of PDLA, the interfaces between PDLA-rich and PDLA-poor phases may induce the formation of regularly arranged TC. Herein, the synergistic effect between PDLA and such interfacial TC may provide a potential avenue to realize the high performance of PLA materials through incorporating as few PDLA as possible. To fulfill such goal, the key challenge lies in finding a satisfactory solution to constructing multiple interfaces in PLLA matrix through efficient regulation of PDLA distribution. As an advanced melt-processing technology, layer-multiplying co-extrusion has been widely adopted to prepare the [AB···AB]-like multilayer polymer materials with alternating A and B layers.27-30 The number of layers can be easily multiplied by applying different numbers of layer-multiplying elements (LMEs) without changing layer thickness. In addition to the alternating layered structure, such technology could also impart great impact on the distribution of fillers according to our previous studies.

Gao

et

al.31

found

that

the

layer

multiplication

of

PP/PPCB

(Polypropylene/Polypropylene filled with carbon black) system would lead to the accumulation of conductive CB particles toward the interfaces, giving rise to improved microwave absorbing capacity following the increase of layer numbers. Zhu et al.32 demonstrated that the confined distribution of CB in 256-layer polyvinylidene


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fluoride-based multilayered system would promote the interfacial polarization and greatly enhance dielectric constant as well as breakdown strength compared with traditional blends. Recently, the layer-multiplying technology was also proven a facile and efficient method to in-situ fabricate highly oriented PLLA with TC structure. 5, 33 However, such structure is unstable after experiencing second melting process, since its formation is totally determined by the stretching force in the layer-multiplying process. Therefore, exploring another way to achieve high performance PLA material with more stable architecture is of great significance. In this work, multilayered specimens containing alternating PLLA and PLLA/PDLA blend layers were fabricated through a layer-multiplying co-extrusion process. The processing temperature was chosen between the melting temperature of homogeneous crystal (HC) and SC to ensure the nucleating effect of SC. The resultant evolution of crystalline morphology and physical properties following the change of layer numbers were investigated, and the mechanisms behind the phenomenon were carefully analyzed. This study intends to demonstrate the effectiveness of achieving high performance at low PDLA content by virtue of layer-multiplying co-extrusion technology. Moreover, the easy processability and tunability would also lay solid foundation for industrial fabrication of PLLA materials with balanced performance in the future.

2. Experimental Section 2.1. Materials. Commercially available PLLA (4032D) was provided by Nature Works. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were 2.1 × 105 and 1.2 × 105 g/mol, respectively. Poly (D-lactide) (PDLA) with a Mw of 1.2 × 105 g/mol was kindly supplied by Professor Xuesi Chen’s group in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. The content of D-lactide in PDLA was 99.5%. 2.2. Sample Preparation. Both PLLA and PDLA were dried in vacuum over night before melt processing. Prior to the layer-multiplying co-extrusion,



PLLA/PDLA pellets (denoted as PLA-SC) containing 5 wt% PDLA were prepared using a SHJ-20 twin-screw extruder (screw diameter is 21.7 mm, L/D = 40:1, Nanjing Giant Co. Ltd., China) with an extrusion temperature of 195 ℃ and extrusion rate of 150 rpm. Subsequently, the PLLA/PLA-SC alternating multilayered composites were prepared by using the layer-multiplying co-extrusion system. As schematically illustrated in Figure S1, PLA-SC and PLLA were simultaneously extruded from two extruders (a, b), and merged into a 2-layer PLLA/PLA-SC melt in the co-extrusion block (c), followed by flowing through a series of LMEs (d). As described in the authors’ previous work27, the extrudate with 2(n+1) layers can be eventually obtained when n LMEs were applied. In this work, 2-, 4- , 64- and 1024-layer materials (denoted as 2L, 4L, 64L and 1024L) were fabricated by employing 0, 1, 5 and 9 LMEs, respectively. The thickness ratio of PLA-SC and PLLA layers was 1:1, thus the total content of PDLA in the whole multilayered composite was about 2.5 wt%, regardless of the layer numbers. By controlling the coextruding speed, the total thickness of each as-extruded product was maintained at ∼1 mm. For comparison, PLLA/PDLA blend sample was also prepared through melt extrusion. The content of PDLA in both the multilayer and blend specimens was 2.5wt%. 2.3. Morphological Analysis. The microstructure analysis of layer structure was conducted on a polarizing light microscope (PLM, BX51, Olympus) equipped with a camera. Prior to observation, a thin slice (15 µm in thickness) was obtained by a microtome along the extruding direction. Crystalline morphology was observed through a field emission scanning electron microscope (SEM, JEOL JSM- 5900LV). Before observation, the cryo-fractured surface (parallel to extrusion direction) was put into a water-methanol (1:2 by volume) solution containing 0.025mol/L of sodium hydroxide for 20 hrs to selectively etch the amorphous phase. 2.4. Differential Scanning Calorimetry (DSC). Non-isothermal crystallization behaviors of PLLA/PLA-SC multilayer materials were studied through DSC instrument (Q20, TA instrument) under a nitrogen atmosphere. Each specimen was heated from 20 to 240 °C at a rate of 10 °C/min, and held at 240 °C for 5 min, then cooled down to 20 °C at a cooling rate of 10 °C/min, followed by a 2nd heating


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process from 20 to 240°C (10 °C/min). The isothermal crystallization kinetics was also investigated using the same instrument. Each specimen was first held at 200 °C for 5 min to erase the thermal history, then rapidly cooled down to different temperatures (145 and 136 ℃) for isothermally crystallization. 2.5. Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). 2D-SAXS measurements were performed on the beamline BL15U1 of SSRF (Shanghai Synchrotron Radiation Facility, China) to examine the lamellar structure of PLA. A monochromated X-ray beam with a wavelength of 0.124 nm was used, and the sample-to-detector distance was fixed at 1866 mm during the experiments. SAXS images were recorded with an X-ray CCD detector (Model Mar165, a resolution of 2048 × 2048 pixels). Each SAXS pattern was collected after 10s exposure, which was then background corrected and normalized using the standard procedure. 2.6.

Two-Dimensional

Wide-Angle

X-ray

Diffraction

(2D-WAXD).

2D-WAXD was conducted on the beamline (BL15U1, SSRF, China). X-ray beam with a wavelength of 0.124nm was applied. The sample-to-detector distance was set as 148mm and the exposure time was 60s. The 2D-WAXD patterns were collected with an X-ray CCD detector (Model SX165, Rayonix Co. Ltd., USA). 2.8. Tensile Tests. Tensile tests were performed on an Instron 4302 tension machine (Canton, MA, USA) at room temperature with a crosshead speed of 10 mm/min following the ASTM D638 standard. At least five specimens for each sample were tested, and the average value was calculated. 2.7. Scratch Tests. A commercially available scratch machine, whose accuracy and repeatability have been verified in precious researches 5, 34, was utilized here. The scratch test was conducted according to the ASTM D7027-05 standard under a linear progression of normal load from 0.5–80 N at 25 mm/s. A SiC spherical tip (1 mm in diameter) was employed, and the scratch distance was set to be 80 mm. At least five scratch tests were performed on each sheet along the extruding direction, and average values were calculated. It is noted that all scratch properties were obtained on the largest surface of each extrudate parallel to the extrusion. We failed to conduct scratch experiments on other surfaces because it is indeed very hard to provide a regular



sample for testing. Any cutting operation would lead to severe deformation of samples. 2.8. Groove Morphology Analysis. To avoid interference from viscoelastic recovery, scratch damage analysis was carried out 24 h after the scratch tests. The optical microscopy images and the topographical information were obtained by using optic microscope (OM, BX51, Olympus) and laser scanning confocal microscope (LSCM, VK-X250, Keyence), respectively. The scanned images for specimens were obtained on a commercial PC scanner (Perfection V330, Epson) by employing “16-bit gray level” mode.

3. Results and Discussion The microstructure of 4L and 64L PLLA/PLA-SC materials was observed through PLM, and that of the 1024L specimen was examined by employing SEM due to inadequate magnification of PLM. As exhibited in the PLM images of Figure 1, the bright and dark layers, corresponding to the PLA-SC and PLLA components, are assembled alternately along the thickness direction, forming a well-defined multilayer structure. It is noted that higher brightness was obtained in PLA-SC layers, mainly due to the different crystalline morphologies caused by the addition of PDLA. Moreover, each continuous layer space is separated by the interfaces between PLA-SC and PLLA layers, and its thickness decreases proportionally with increasing the number of layers because the total thickness of each specimen and the thickness ratio of two components both remained unchanged during the layer-multiplying process. Thus, such multilayer-assembled specimens with a controllable and co-continuous structure are believed to be ideal candidates for further investigation of the relationship between microstructure and macroscopic performances.


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Figure 1. Polarized optical micrographs of 4L and 64L PLLA/PLA-SC multilayered samples and the SEM photographs of 1024L multilayered sample (below).

Figure 2A shows the first heating curves of the PLLA/PLA-SC multilayered samples recorded by a DSC instrument. Four transitions were identified, including the glass transition of PLLA (Tg), cold crystallization of PLLA (Tcc), melting of homogeneous crystal (HC) of PLLA (Tm-hc) and melting of SC (Tm-sc) in blend layers, which are basically consistent with those reported previously

17, 18.

For comparison,

the curves of the PLLA/PDLA blended specimen possessing the same PDLA content (2.5wt%) and the neat PLLA were exhibited as well. The Tm-sc around 219 ℃ is visible for all multilayered and blended specimens, indicating the maintenance of SC after experiencing the melt-processing process. To clarify the observed DSC behavior, the enthalpies of cold crystallization, melting of HC, melting of SC were integrated and listed in Table S1. As presented, the cold crystallization enthalpies obviously decreased from 28.53 (L/D-4L) to 16.82 J/g (L/D-1024L) with the increase of layer numbers, and a slight increment in melting enthalpies (from 32.79 to 34.01 J/g) was also observed. Herein, the crystallinity of HC remarkably increased accompanying the layer multiplication process. Such significant change clearly indicates that the



increase of layer number led to the suppression of cold crystallization and accelerated the crystallization process, mainly due to the more uniform distribution of SC particles. Since the cooling behavior has significant importance in analyzing the crystallization behavior of PLA, the cooling curves of neat PLLA, PLLA/PDLA blend, and 4L, 64L, 1024L PLLA/PLA-SC multilayered specimens recorded by DSC instrument have been collected as well. As indicated in Figure S2, following the more uniform distribution of SC along thickness direction by increasing layer numbers, SC particles could exhibit stronger heterogeneous nucleating effect and accelerate the crystallization of HC, as manifested by sharper crystallization peak and increased integral area (see Table S2) in cooling curves. Figure 2B records the DSC curves of the second heating process. Gradual disappearance of the cold crystallization peak was observed following the multiplication of layers, which suggested that the SC particles distributed in PLA-SC layers might act as heterogeneous nucleating agents and accelerate the crystallization process of adjacent PLLA layers accompanied with the increase of layer interfaces. To provide more quantitative analysis, isothermal crystallization experiment was also conducted. The half-crystallization time, t0.5, often regarded as a key parameter to evaluate the overall crystallization kinetics, was obtained from the curves of Xc (t) at Xc (t) = 50% and displayed in Figure S3. A similar phenomenon of an initially rapid decrease of t0.5 following the increase of layer numbers at each isothermal crystallization temperature is observed, confirming the distribution dependence of SC in multilayered structure. These results attested the remarkable promoting effect on the overall crystallization rate of PLLA through regulation of SC distribution.


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Figure 2. The (A) first and (B) second heating curves of neat PLLA, PLLA/PDLA blend, and 4L, 64L, 1024L PLLA/PLA-SC multilayered specimens recorded by DSC instrument.

The crystalline morphology influenced by the multiplication of layers was further observed through SEM by selectively etching the amorphous phase of each specimen. As shown in Figure 3, diverse crystalline morphology was seen in different layers for 4L specimen. Isotropic big spherulites appeared in PLLA layer, whereas densely-packed smaller spherulites were distinctly observed in PLA-SC layer. Additionally, few oriented lamellae perpendicular to the layer interface could be seen between PLLA and PLA-SC layers, indicating the formation of TC induced by interfacial nucleation. Similar results were obtained in 64L sample, the difference is the reduced crystal numbers in PLLA layer, because following the decrement of layer thickness, there would be inadequate space for the growth of spherulites. By further increasing the layers to 1024, no contrast of crystalline morphology between adjacent layers could be observed, and only regularly arranged lamellae perpendicular to the layer interfaces appeared instead. PLLA/PDLA blend with the same PDLA concentration (2.5 wt%) was also extruded through 9 LMEs for comparison (see Figure S4). As clearly illustrated, small and isotropic spherulites was exclusively observed without the trace of any TC. Therefore, it could be regarded that the formation of oriented lamellaes in PLLA layers of the multilayered samples was originated from heterogeneous nucleating effect of adjacent PLA-SC layers through



the layer interfaces.

Figure 3. SEM images of 4L, 64L and 1024L PLLA/PLA-SC multilayered specimens by selectively etching the amorphous phase. The figures in the left contain both layers, while the images in the middle and right represent the local magnifications of PLLA and PLA-SC layers.

In order to obtain more information on the unique crystalline structure, 2D-WAXD and 2D-SAXS were conducted, respectively. Figure 4A exhibits representative 2D-WAXD patterns of 4L and 1024L. Distinctly, isotropic diffraction rings can be observed in the pattern of low-layer specimen, demonstrating a low orientation induced in the extruding process. When 9 LMEs were applied, strong arc-like diffraction patterns corresponding to highly oriented lattice plane (200/110) and (203) of α-form PLA were traced, elucidating a transition from isotropic to anisotropic state with the increase of layers. Furthermore, the integrated 1-D intensity profile included in Figure S5 verified that the layer multiplication only strengthened


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the orientation of lattice plane in HC, but yielded little influence on those in SC. In addition, the representative 2D-SAXS patterns compared in Figure 4A displays that only isotropic ring could be distinctively observed for low-layer specimen, but symmetrical bulb-shape lobes appeared in the pattern of the high-layer specimen, which evidences the presence of strong anisotropic crystalline morphology perpendicular to the flow direction. Azimuthal angle results were further integrated for more quantitative analysis. As illustrated Figure S6A, following the increase of layer numbers, not only the increase of orientation function, but also the shift of max azimuthal angle could be seen, indicating the change of orientation direction. Such result yields good consistency with the SEM observation, namely the structural evolution from isotropic state to highly oriented kebab lamellae perpendicular to layer interfaces was achieved. The 1D intensity profiles in Figure S6B manifest that 1024L displays a long period of 20.1nm, slightly lower than those of 4L and 64L, indicating that the confined space generated between adjacent layers by adopting 9 LMEs would lead to more compact arrangement of SC lamellae, thereby giving rise to the decrement of long period.35-37

Figure 4. Representative patterns of (A) 2D-WAXD and (B) 2D-SAXS of 4L and 1024L PLLA/PLA-SC multilayered specimens.



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Based on the structural observation and analysis, a probable formation mechanism was proposed. As schematically illustrated in Figure 5, dominant presence of spherulites could be seen in PLLA layers of low-layer specimen (4L) and only small quantities of oriented lamellaes nucleated from SC particles were generated around the layer interfaces. With the multiplication of layers, the space of each layer was proportionally reduced and the number of layer interfaces was simultaneously increased. Therefore, more and more TC structure was created and gradually occupied the space of big spherulites in PLLA layers. When the layer numbers reached 1024, almost all spherulites were suppressed by the growth of TC. Consequently, a unique TC/SC alternating multilayered architecture was generated as presented in SEM observation.

Figure 5. Schematic of the multilayer-assembled crystalline structure in PLLA/PLA-SC multilayered specimens.

Stereocomplexation has been proved as one of efficient ways to improve the mechanical properties of PLA according to previous researches.18,


38-40

In order to

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probe a deeper insight into the influence of the structural evolution during the layer multiplication on the mechanical performances of PLLA/PLA-SC multilayered samples, tensile tests were performed. It is clearly manifested from Figure 6A that the stress-strain curve of 1024L is far beyond the rest ones including the blend and the specimens with low layer numbers. Specifically, compared to the value of 56.6 MPa for the blend, the 1024L obtained a slight increment in tensile strength, achieving 66.3 MPa. Of particular importance is the largely enhanced ductility and toughness following the increase of layers. As illustrated in Figure 6B, an elongation at break (εb) of 166.1% for 1024L was achieved, which is 15 times higher than that of the blend and the 2L. What’s more, the blend, 2L and 4L specimens broke as soon as they passed the yielding point, whereas 64L and 1024L exhibited strain-softening followed by a stable neck-growth and strain-hardening. It has been revealed from previous reports that the toughening effect of SC on PLLA matrix would only occur when comparatively large amount of PDLA (over 10 wt%) was incorporated.18, 41 Herein, we also prepared a series of PLLA/PDLA blends and reached a similar conclusion that only comparatively great loading of PDLA can lead to the increase of εb (see Figure S7). However, in present work, through the construction of multiple and co-continuous PLA-SC and PLLA layers, simultaneous increment of both strength and toughness of PLA at extremely low PDLA fraction (~2.5 wt%) was successfully achieved. Such remarkable improvement could mainly be ascribed to following reasons: (i) Due to the iteratively layer-multiplying effect occurring in LMEs, multiple interfaces would be formed between PLA-SC and PLLA layers, which may play a significant role in crack deflection and termination for achieving high ductility;18,

41

(ii) During the uniaxial stretching process, the deflection of TC was

found been capable of continuous absorbing destructive energy, leading to highly improved toughness.26 To clarify the advantage of layer-multiplying approach over other systems, some representative PLA-SC materials with various PDLA content and corresponding results are compared in Figure 6C. Usually, limited improvement was obtained through incorporation of PDLA due to lack of morphology design, while it is clearly suggested that significantly optimized elongation at break has been achieved



under a relatively low content of PDLA. Thus, such comparison clearly indicated that the layer-multiplying co-extrusion has set a good example to achieve high performance in PLA products toward higher efficiency and low cost.

Figure 6. The tensile performance of PLLA/PLA-SC multilayered samples: (A) Typical stress-strain curves with the magnification of yielding regions being inserted; (B) Comparison of tensile strength and elongation at break; (C) Comparison of the increase ratio of elongation at break among some previously reported PLA-SC materials.

Furthermore, given the great potential of PLLA/PLA-SC multilayered products in packaging or biomedical fields, their scratch resistance as a function of layer numbers was investigated. The images of scratch groove for 2L, 4L, 64L and 1024L samples were scanned and compared in Figure S8. A slight to severe scratch damage could be observed in all samples by applying a linear increment of normal load from 0.5 to 80N. For the first three samples, there are two critical positions marked by triangle and solid line represent the onset of groove formation and the onset of material removal, which is consistent with classic scratch damage 5, 42. The former one indicates a kind of observable but trivial plastic deformation resulting from comparatively low normal load at the early stage of scratch process, whereas the latter one occurs when the total stress applied to the system surpasses the ultimate strength


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of the material and appears as a plowing fashion. However, it is noteworthy that no plowing damage appeared on the 1024L sample, where only a smooth groove was observed across the scratch path even when the normal load approached to 80N. Such phenomenon remarkably emphasized the efficiency in enhancing the scratch resistance of PLA via construction of multilayered architecture. Quantitively, the normal forces applied at above-mentioned two positions are defined as NG and NR, respectively. As compared in Figure 7A, both NG and NR values exhibit a similarly incremental tendency following the increase of layer numbers, which further reveals that the structural evolution has led to the strengthened ability to resist the scratch deformation on the surface of PLA. As another critical parameter, scratch coefficient of friction (SCOF), which is capable of considering the whole contact area between the scratch tip and the deformed material surface, is widely applied to evaluate the scratch-induced damage.43,

44

At any given scratch position, SCOF is defined as the ratio of the

tangential and normal forces throughout the scratch. Figure 7B offers the SCOF curves for 4L, 64L and 1024L PLLA/PLA-SC composites as a function of normal load. It is clearly shown that 1024L has much lower SCOF than the others across the whole scratch path. In addition to its higher strength that has been demonstrated in tensile tests, such diverse contrast could mainly be attributed to the formation of multi-interfaces during the layer-multiplying process. The shearing effect occurred on the 1023 interface contributed to the dissipation of fracture energy from surface layers to sub-surface layers, leading to the elimination of stress concentration on material surface. This finding is consistent with previous numerical studies which showed that reduced SCOF was associated with slighter stress concentration on material surface during the scratch process.45 Therefore, it is not surprising to see the postponed groove formation and trivial damage for 1024L sample.



Figure 7. The scratch testing results of PLLA/PLA-SC multilayered samples: (A) NG and NR as the function of layer numbers; (B) SCOF as the function of normal load. It has been clearly revealed that the 1024L sample showed considerable improvement in scratch resistance. For better investigating the deformation and damage mechanism of this system, optic microscope was employed to conduct intuitive surface analysis. At a prescribed normal load of 50N, extensive cracking damage was developed on the surface of 2L, as illustrated in Figure S9. This observation suggested that brittle fracture takes place to continuously release the energy built up due to the increasing normal load, leading to the formation of periodic cracks. However, only alleviated micro-cracking damage and surface deformation started appearing on 4L specimen when the normal load was increased to 56 N, indicating the suppression of surface cracking following the increase of layers. This micro-cracking damage pattern (parabolic cracks) in 64L multilayered sample became much lighter and less periodic at higher load of 63 N, which meant that it did not experience significant deformation at this load. By further increasing the layers to 1024, there seemed to be no brittle cracking upon scratching even at an applied normal load as high as 75N, leaving behind a smooth groove across the observation zoom instead. It has been concluded from existing literatures that the formation of periodic cracks perpendicular to scratching direction can be regarded as the consequence of successive release of stored energy around the spherical tip along the scratching direction. Hence, the remarkably alleviated severity in surface cracking for


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1024L sample clearly indicates the scratch performance, especially the ability to resist the brittle fracture on PLA surface is significantly promoted due to the formation of multilayered structure. Such results strongly evidence the effectiveness of layer multiplication in facilitating the load transfer, and thus improving the scratch resistance. To better reveal the scratch induced damage mechanism, the detailed topographic profiles of scratched multilayered samples and blends were imaged through LSCM, respectively. As illustrated in Figure 8, substantial decrement from 393.1 to 229.7 μm, as well as from 283.6 to 152.2 μm in groove width and height were observed, respectively, accompanying the increase of layers from 2 to 1024. Besides, remarkably reduced groove size in both width and height were found in multilayered samples compared with the blend at an equivalent content of PDLA. It has been revealed from previous research46 that the scratch visibility is highly dependent on the resultant specular reflection following the formation of groove upon scratching. In other words, minor specular reflectance and diffusion would be generated from a groove with smaller size, giving rise to notably reduced scratch visibility in 1024L. Herein, from the perspective of both damage and visibility, the fabrication

of

alternating

multilayered

structure

through

employment

of

layer-multiplying co-extrusion has been proved an effective and facile method to improve the scratch performance of plastics. Given the easy processability as well as great potential in industrial production, the controlled properties by the multilayered structure can be applicable to other polymer systems5, attractiveness in a variety of high-performance applications.


47

and support further


Figure 8. (A) The groove width and height of PLLA/PLA-SC multilayered samples and PLLA/PDLA blend; (B) the corresponding 3-D morphologies of the 1024L and blending samples at a prescribed normal load of 80N.

4. Conclusions The multilayered distribution of SC was fabricated in PLA through the alternating assembly of PLLA and PLA-SC layers. Due to the heterogeneous nucleation of SC, the oriented lamellaes grew from the layer interfaces and tended to occupy the PLLA layers with the multiplication of layers forming a TC/SC alternating multilayered architecture. Benefiting from such a unique crystalline structure, the mechanical performances were remarkably enhanced. Both the tensile strength and elongation at break of the 1024L were respectively increased by 17% and 1528%, in comparison to those of the conventional blend specimen containing the same content of PDLA (~2.5wt%). The results of scratching test revealed that the evolution of crystalline morphology played a significant role in suppressing surficial deformation. Nearly no micro-cracks could be observed on the surface of the 1024L by applying


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the same load, implying that a transition from brittle to ductile damage occurred with the multiplication of layers. Thus, the present research paved a high-efficiency and green strategy for fabricating high-performance PLA materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the website. Schematic of layer-multiplying co-extrusion, DSC cooling curves, relative crystallinity of multilayered composites, SEM images of PLLA/PDLA blend, WAXD and SAXS intensity profiles, tensile performance of blending samples, scanned images of scratch groove and micro-cracking observation. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (J. Shen); [email protected] (S. Guo) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (51420105004, 51673136, and 51721091) for providing the financial support of this work. We would also like to express our sincere gratitude to Professor Hung-June Sue (Texas A&M University) for valuable discussion and suggestion on this work.

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Coexistence of Transcrystallinity and Stereocomplex Crystals Induced

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