Fabrication of Scratch Resistant Polylactide with Multilayered Shish

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Materials and Interfaces

Fabrication of Scratch Resistant Polylactide with Multilayered Shish-Kabab Structure Through Layer-Multiplying Coextrusion Longfei Yi, Yang Xu, Dun Li, Jiabin Shen, Shaoyun Guo, and Hung-Jue Sue Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00221 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Industrial & Engineering Chemistry Research

Fabrication of Scratch Resistant Polylactide with Multilayered Shish-Kabab Structure Through Layer-Multiplying Coextrusion

Longfei Yi 1, Yang Xu 1, Dun Li 1, Jiabin Shen 1,*, Shaoyun Guo 1,*, Hung-Jue Sue 2

1

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

Sichuan University, Chengdu 610065, China; 2

Polymer Technology Center, Department of Materials Science and Engineering, Texas

A&M University, College Station, TX 77843-3123, USA.

ACS Paragon Plus Environment

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

Abstract: Recent demands for preserving esthetics of polymer parts over the duration of their service life have necessitated a favorable scratch resistance of polymer surfaces. Herein, a layer-multiplying coextrusion process was chosen to prepare a type of scratch resistant polylactide (PLA) materials containing multilayer-assembled shish-kebab structure. Their scratch behavior and potential mechanisms were investigated. It was demonstrated that the greatly improved mechanical strength and toughness following the formation of ordered and oriented crystalline structure in neat PLA played a significant role in improving the scratch performance, particularly the resistance to the formation of cracking. Since no foreign additives were required, the present work provided an economical and environmental-friendly strategy for fabricating scratch resistant polymers.

Keywords: polylactide, multilayer structure, shish-kebab, scratch resistance


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INTRODUCTION

The scratch performance of polymers has caught significant attention in the past few years because of their greatly expanded usage in the electronic, optical, household, and automotive applications.1,

2

Due to the rising concern over the aesthetic

appearance, the demand for scratch resistant polymers has tremendously increased in recent years.3-5 In the past years, many scratch testing methods and evaluation systems have been proposed to probe a deeper insight into the scratch behavior of polymers. A noticeable progress in this field is the recently established ASTM/ISO scratch test standard,6,

7

which has paved the way for more systematical and quantitative

evaluation of the scratch resistance of polymers. Based on such methodology, great efforts have been made to gain in-depth understanding of scratch behavior.8-10 It has been found that many bulk mechanical parameters, such as the strength, toughness, and Young’s modulus have significant influences on the scratch resistance of polymers and composites.3, 11, 12 Liang et al.10, 13 have revealed the positive association between tensile strength and scratch performance of acrylonitrile-butadine-styrene (ASA), namely the decrease of tensile strength (by increasing the rubber content) in ASA led to the earlier onset of surface cracking and plowing. Similar conclusion has also been drawn by Browning14, who found that with enhanced tensile strength, styrene-acrylonitrile copolymer (SAN) resins would display improved scratch resistance. More meaningful work was done by Jiang et al.15, who carried out a series of scratch tests on four categories of polymers: I) ductile and strong, II) ductile and weak, III) brittle and weak, and IV) brittle and



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strong; and a scratch evolution map has been constructed. In addition to the higher scratch resistance displayed by strong polymers, Jiang concluded that for ductile polymers, the scratch tip usually caused periodic plastic drawing and formed fish-scale pattern. In contrast, brittle polymers generally show periodic cracking with higher scratch visibility. Therefore, from the perspective of achieving better aesthetic appearance, strong and tough properties are more favorable in designing scratch resistant polymers. Though past research efforts have provided valuable knowledge within their own merits, it must be pointed out that all those studies were conducted on unrecyclable petroleum-based polymers, rare attention has been paid to biodegradable polymers such as polylactide (PLA) to the best of authors’ knowledge. As an important representative of biodegradable polymers that derived from renewable resources, PLA has never failed to catch attention due to its fantastic merits such as high transparency, easy processability, together with excellent biodegradability and biocompatibility.16-19 The combination of these characteristics has made PLA a promising

alternative

in

tissue

engineering,

biomedical

and

packaging

applications.20-23 Given its promising prospects as an alternative to traditional polymers, research efforts are therefore needed to conduct more systematic and quantitative research into the scratch resistance of PLA, not only because of its high academic value, but also due to the necessity to meet the requirement of sustainable development in the future.


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Since the scratch resistance is highly dependent on the bulk mechanical property, it is essential to improve the mechanical performance of polymers in order to obtain high scratch resistance. With regard to semi-crystalline PLA, the regulation of crystal morphology in conventional polymer processing remains an advancing front in optimization of mechanical properties.24, 25 With striking features, the shish-kebabs turn out be the most representative one, which can impart polymer articles with significantly improved mechanical performance compared with spherulites.26-28 However, the formation of shish-kebab structure is highly dependent on complex recipes or procedures, which may lead to deteriorated biodegradability, and transparency in neat PLA.29, 30 Accordingly, an interesting question comes out: is it possible to fabricate shish-kebab structure in neat PLA without introducing any other components? Unfortunately, rare reports could be found in this field from exiting literatures, to the authors’ best knowledge. Thus, another philosophy is urgently required to overcome the deficiency of present situation.

In recent years, an advanced melt-processing technology, layer-multiplying coextrusion, has been developed to prepare the [AB…AB]-like multilayer polymer materials with alternating A and B layers.31-36 By combining an assembly of layer-multiplying elements (LMEs) with two extruders, the number of layers can be simply multiplied by changing the number of LMEs. It has been revealed that the convergent flowing behavior in LMEs could induce intensive extensional effect on a melt, which effectively led to in-situ fibrillation of dispersed phase37,

38

and the

orientation of high-aspect-ratio particles.39 However, less attention has been paid to



the transition of crystalline morphology during the layer multiplying process in neat polymer. According to our previous researches,40 the strong shearing and elongating forces in each LME have been proven effective in generating highly oriented microfibrillar structure, thereby providing a potential in fabricating oriented shish-kebab crystals. Inspired by that in nacres, the layer-packed shish-kebab skeleton possessed the ability to resist mechanical deformation, and the tenacious interfaces promoted crack deflection and termination. Herein, PLA with multilayer-assembled crystalline morphology was attempted to be fabricated by virtue of the combination of extensional and layer-multiplying effects occurring in LMEs. The resultant mechanical behavior and scratch resistance following the evolution of crystalline morphology were investigated.

EXPERIMENTAL SECTION

Materials. Commercially available PLA containing around 4% D-LA, was provided by Nature Works (trade name 2003D) with a density of 1.24 g/cm3 and melt index of 6.7 g/10 min at 210 °C. The weight-average molecular weight and number-average molecular weight were 2.53 × 105 and 1.46 × 105 g/mol, respectively.

Sample Preparation. In this work, multilayer-assembled PLA was prepared through layer-multiplying coextrusion technology. Prior to melt processing, PLA was dried under 80 °C for over 12 hrs in a vacuum oven, then pure PLA of the same kind was co-extruded from two extruders and combined together in the coextrusion block, then flowed through n LMEs (n=0, 1, 5, 9). The schematic of the layer-multiplying


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process occurring in LMEs has been described previously in detail.31,

32, 41

For

convenience, the as-extruded samples were named as PLA-n, where n represents n LMEs were applied. By controlling the coextruding speed, the total thickness of each as-extruded sheet was fixed at about 1.5 mm.

Scanning Electron Microscope (SEM). A field emission SEM (SEM, JEOL JSM- 5900LV) was adopted to conduct the crystalline morphology observation of PLA at an accelerating voltage of 5 kV. Prior to observation, the cryo-fractured (along the flow direction) surface was etched in a water-methanol (1:2 by volume) solution containing 0.025 mol/L of sodium hydroxide for 28 hrs at 20 °C and subsequently sputter-coated with a thin layer of gold.

Differential Scanning Calorimetry (DSC). The melting enthalpy (Δ‫ܪ‬௠ ) of each sample was measured by utilizing a DSC machine (Q20, TA Instruments, New Castle, DE) under a nitrogen atmosphere. Each sample, with a weight of 6−8 mg, was heated from 20 to 200 °C at a heating rate of 10 °C /min. The crystallinity (Xc) was calculated by using following equation:

ܺ௖ =

Δ‫ܪ‬௠ − Δ‫ܪ‬௖௖ × 100% ௢ Δ‫ܪ‬௠

௢ where, Δ‫ܪ‬௖௖ is the enthalpy of cold crystallization, and Δ‫ܪ‬௠ is the melting enthalpy

of PLLA with 100% crystallinity (93.7 J/g).

Tensile Tests. Tensile tests were performed on a universal testing machine (CMT4104, SANS, China) to investigate the effect of crystalline morphology on



tensile properties according to ASTM D638-91 standard. All dumbbell-shaped specimens were tested at room temperature with a stretching speed of 20 mm/min. At least five specimens for each sample were tested, and the average value was calculated.

Scratch Tests. A home-built scratch machine (Fig. 1A), which has been widely adopted in precious researches,42, 43 was utilized in this work. The scratch test was conducted according to the ASTM D7027-05 standard by applying a linearly increasing normal load from 0.5–80 N (Fig. 1B) at constant speeds of 25, 50 and 100 mm/s, respectively. A stainless steel spherical tip with a diameter of 1 mm was employed, and the scratch distance was set to be 80 mm. At least five scratch tests were performed on each sheet in the direction parallel to the extruding direction at room temperature.

Figure 1. (A) Schematic of the home-built scratch machine and (B) the linearly increasing normal load with scratch distance.


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Groove Morphology Analysis. Scratch damage analysis was carried out 24 hrs after the completion of scratch tests to allow for viscoelastic recovery. The optical microscopy images and the topographical information were obtained by using a laser scanning confocal microscope (LSCM, VK-X250, Keyence, Japan). The scanned scratched images for analysis were obtained by using a PC scanner (EpsonV R Perfection Photo 4870) at 3200 dpi resolution under “16-bit gray level’’ mode. For all samples, a black paper was placed behind the sample to enhance the contrast, and the scratch direction was aligned perpendicular to the scanner light source movement.

RESULTS AND DISCUSSION

Structural Characterization.

The evolution of crystalline morphology during

the layer-multiplying process was tracked through SEM by selectively etching the amorphous phase in as-extruded PLA. As exhibited in Fig. 2A-C, the transition from isotropic to highly oriented crystalline morphology is clearly evidenced with the increase of LME numbers from zero to nine. In contrast to the isotropic morphology in PLA-0, multilayer-assembled shish-kebabs can be observed in PLA-9. More significantly, all shishs and kebabs were closely packed and further arranged in a regular layered pattern, which was entirely different from the traditional injection-molded products where the shish-kebab structure could only be formed in skin layers. Based on the authors’ previous work40, it is considered that the convergent flowing behavior in each LME could create intensive extensional effect on the PLA melt, which benefits for the formation of highly extended shishs along the flowing



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direction and the oriented kebab lamellae perpendicularly growing on the shish surfaces. Accompanied with the iterative layer multiplication effect occurring in LMEs, the unique shish-kebab multilayer structure is gradually generated in the neat polymer as observed in SEM images. In addition to the obvious transition of crystalline morphology, layer-multiplying coextrusion process has also led to the increase of crystallinity. More specifically, by increasing the LME numbers from 0 to 9, a substantial increment from 5.4 % to 28.3 % was achieved in this work. It is worth noting that, unlike those in hybrid systems, both the shishs and kebabs in present system are composed of the same component, hence the theoretically perfect interface would be constructed regardless of the miscibility or phase separation.

Figure 2. SEM images (×40,000) of (A) PLA-0, (B) PLA-5 and (C) PLA-9 (Prior to the observation, the amorphous phase has been selectively etched by immersing the as-extruded specimen in a water/methanol solution.).

Tensile Property. Since the tensile parameters can impart significant influence on the scratch resistance of polymers according to previous researches,13,


14

it’s

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necessary to examine the stretching performance firstly. Thus, the tensile tests were performed on the as-extruded PLA specimens prepared by applying 0, 1, 5 and 9 LMEs, and different stretching behaviors could be observed in the stress-strain curves as presented in Fig. 3A. It is apparent that the construction of multilayered shish-kebab structure in PLA would greatly benefit its mechanical property. For PLA-0, the fracture happened just after the yielding point. While by employing 9 LMEs, a 60% increment in tensile strength from 60.7 to 97.9 MPa was achieved accompanied with the appearance of a distinct strain hardening phenomenon. Of particular importance is the largely enhanced ductility and toughness following the formation of multilayer-assembled shish-kebab structure. As illustrated in Fig. 3B, the elongation at break for PLA-0 is only 6.9%, while the structured PLA-9 shows a value of 45.6%, with a substantial increase of 560% compared with that of PLA-0. Such great increment of ductility is indeed a remarkable phenomenon that has never been reported before in pure PLA or any other plasticizer free system. Generally, traditional approaches associated with the introduction of flexible biopolymers or plasticizers have unfortunately resulted in the unbalanced performance or even the deteriorated properties.44, 45 In most cases, only limited promotion of ductility was achieved at the cost of largely sacrificed strength or stiffness, even in some recently developed systems.46, 47 In this work, simultaneous enhancement of strength and roughness was achieved by employing layer-multiplying coextrusion technology. Given the close relationship between tensile property and scratch performance, such result indicates that the multilayer-assembled PLA with shish-kebab crystals should also be endowed



with superior scratch resistance. Considering the extrusion machine possessed the maximum capacity to extrude the melt, more LMEs were not applied. Although the maximum of 9 LMEs might not be the best choice for achieving the optimum mechanical performances, it is sufficient to explore the relationship between microstructure and properties.

Figure 3. The tensile properties of PLA-0, PLA-1, PLA-5 and PLA-9: (A) Stress-strain curves; (B) Comparison of tensile strength and elongation at break.

Scratch Resistance. From the perspective of industrial designing, the materials used in a product should be both durable and attractive. During the service period, unavoidable scratch forces may act on the product and result in destructive effect on its surface quality and inherent performances. Considering the great potentials in biomedical and packaging applications, the scratch resistance of the as-extruded PLA was evaluated according to a test standard of ASTM D7027-05. As illustrated in Fig. 4, by applying a linearly increasing normal load (from 0.5 to 80 N) at a constant speed


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of 25 mm/s, the scratch damage in each specimen gradually developed from slight to severe features which could be divided into three major stages, namely the invisible damage (Stage I), groove formation (Stage II) and material removal (Stage III). It should be noted that the onset of groove formation (marked as a dash line in Fig. 5A) indicates that a subtle plastic deformation can be observed at early stage of the scratch process, while the onset of material removal (marked as a solid line in Fig. 5A) represents the occurrence of severe damage on the surface of a specimen. The normal loads exerted at these two critical positions (denoted as NG and NR) were also recorded in Fig. 5B. It can be found that both NG and NR are obviously moved toward larger values following the increase of LME numbers, revealing that the structural development in the layer-multiplying process may strengthen the ability to resist the scratch deformation on the surface of PLA.

Figure 4. Schematic of three damage stages obtained from LSCM (above) and high-speed camera (below) by applying a linearly increasing normal load.



Figure 5. The scratch testing results of PLA-0, PLA-1, PLA-5 and PLA-9: (A) Scanned images of scratch damages, where the dash and solid lines represent the onset of groove formation and material removal, respectively; (B) Critical normal load applied at the groove formation and material removal stages.

To better reveal the scratch induced damage mechanism, the detailed topographic profiles of scratched PLA-0 and PLA-9 in Stage II and III were imaged through a LSCM, respectively. The images recorded at the same position in Stage II (Fig. 6) display that the grooves are generated in both specimens. In contrast, the profile of PLA-0 shows a larger width and depth than those of PLA-9. More importantly, accompanying with the largely enhanced strength and toughness, there is discernable change presented in the periodic damage features between PLA-0 and PLA-9. Plenty


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of parabolic cracks around the groove can be observed on the surface of PLA-0, like that reported in other brittle polymers. However, only a smooth groove is left on the surface of PLA-9 where nearly no visible cracks are initiated. It has been reported that the cracks formed perpendicular to the scratch direction are a consequence of the buildup of a stretching-like component parallel to the scratch direction.15, 48 Hence the reduced formation of cracks in PLA-9 clearly reveals the scratch performance, particularly the resistance to the formation of cracking, is significantly enhanced due to the formation of multilayered shish-kebab structure. Similar to that in periodic damage zone, Fig. 7 illustrated the greatly reduced groove depth and shoulder height at an equivalent normal load of 70 N by increasing the LMEs from 0 to 9, which is further underlined the enhanced ability to suppress the scratch-induced damage.



Figure 6. Laser confocal optical images (left) and the corresponding 3D topographic information (right) of the scratch damages for PLA-0 and PLA-9 in stage II obtained through LSCM.

Figure 7. Topographic information of the scratch damages for PLA-0 and PLA-9 in stage III obtained through LSCM.

Across the scratch damage, and especially when surface cracking or plowing occurred, the tensile stress developed behind the scratch tip would inevitably induce material pile-up. Therefore, the scratch coefficient of friction (SCOF), which is


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defined as the ratio of the tangential and normal forces throughout the scratch process, is also commonly utilized to evaluate the scratch behavior.49, 50 For instance, when the material removal occurs, the SCOF would change severely since the nature of deformation has shifted from surface sliding to surface penetration followed by sub-surface material removal/plowing. Fig. 8 shows the SCOF curves of multilayer-assembled PLA across the scratch path measured at 25 mm/s. As presented, the SCOF values increase linearly during the early stage of scratch process and start to sharply fluctuate after reaching plowing damage. It is observed that the increase of LME numbers has resulted in lower SCOF values during whole scratch process. Such contrast could mainly be attributed to the evolution of crystalline morphology during the layer-multiplying coextrusion. First, the formation of shish-kebab crystals in PLA-9 has strengthened its ability to resist scratch-induced deformations, thereby lower contact areas between the scratch tip and material would be generated compared with PLA-0. Second, during the repeated layer-multiplying process, multiple micro-phase interfaces would be generated. The shearing effect occurred on the multiple micro-phase interface contributes to the dissipation of fracture energy, leading to the elimination of stress concentration on material surface.51, 52 Herein, it is not unreasonable to see the postponed onset of groove formation and material removal following the employment of more LMEs. This finding also shows good consistency with previous experimental and numerical studies that lower SCOF is associated with better scratch performance.



Figure 8. Scratching coefficient of friction versus scratching distance of PLA-0, PLA-5 and PLA-9 at a scratching speed of 25 mm/s.

In practical applications, the scratch speed would have huge impact on the factual scratch resistance.53, 54 Herein, to illustrate the testing speed effect, the SCOF as a function of scratch distance for aforementioned PLA samples measured at the scratch speed of 50 and 100 mm/s was also calculated. As illustrated in Fig. 9A, at a scratch speed of 50 mm/s, the early portions of the SCOF curves show some slight differentiation between different samples. Once reaching the onset of plowing, the SCOF for all the systems appears to converge. As the speed further increases to 100 mm/s (Fig. 9B), however, almost no contrast in the curves could be observed across the whole scratch length. This is likely due to the fact that polymers behave in an increasingly brittle fashion as the testing speed increases. Hence, three conclusions could be drawn so far. First, the SCOF value is highly dependent on LME numbers at


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comparatively low scratch speed. In other words, the increase of LME numbers would lead to the decline of SCOF for multilayer-assembled PLA. Second, the SCOF related to plowing will be quite high at slow scratch speeds and will decrease with increasing scratch speed. Finally, at a low speed, the material in front of the tip imparts a significant amount of resistive (tangential) force, thus resulting in a high SCOF. While following the increase of speed, the response of the polymer becomes more rigid and brittle, and the ability to withstand the applied stress decreases. Therefore, the resistive force, as well as the SCOF, will be lower.

Figure 9. Scratching coefficient of friction versus scratching distance of PLA-0, PLA-5 and PLA-9 at a scratching speed of (A) 50 mm/s and (B) 100 mm/s.



Some researchers believed that such different dependency of SCOF on scratch speed is mainly attributed to the development of damage from penetrative ploughing to surface drawing. In order to gain more quantitative evidence, the evolution of groove depth with increasing the scratch speed for PLA-0 and PLA-9 was measured through LSCM and shown in Fig. 10. Under a prescribed normal load of 70 N, the increased scratch speed leads to the gradually decreased groove depth for both PLA-0 and PLA-9, indicating less penetration when scratched at higher speeds. In addition, unlike the huge gap existing at 25 mm/s, the difference between PLA-0 and PLA-9 becomes smaller with the increase of scratch speed. Especially at 100 mm/s, the depth of PLA-9 is almost equal to that of PLA-9. So far, good consistency could be found in LSCM and SCOF results, which further implies that multilayer-assembled PLA will appear more rigid and less deformable when measured at higher testing speeds.

Figure 10. The groove depth of PLA-0 and PLA-9 as a function of scratching speed at a prescribed normal load of 70 N.


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The critical normal load for damage as a function of scratch speed was recorded in Fig. 11. For both PLA-0 and PLA-9, the increase of testing speed has led to the earlier occurrence of material removal. Substantial decline from 68.8 to 61.9 N was achieved in PLA-9, and PLA-0 also obtained a slight decrease from 36.5 to 31.9 N when the scratch speed was increased from 25 to 100 mm/s. Due to the growing rigidity and brittleness with the increase of speed, the molecular mobility of PLA would gradually lag behind the change of external force. As a result, the ability to withstand the applied stress decreases.55 With less resistive force, it’s undoubtedly to find that plowing would happen under a lower prescribed normal load. In reality, the scratch damage usually happens instantly. Thus, the higher scratch speed is adopted, the more it approaches the factual situation.56 In our experiment, PLA-9 still exhibits high scratch resistance and would not fail until a normal load of 61.9 N was applied.

So far, remarkably improved scratch resistance has been found in PLA-9 with multilayer-assembled shish-kebab morphology, and such results also show high coincidence with the stretching behavior. Taking this into consideration, it further confirms that higher strength and ductility in PLA-9 plays a crucial role in reducing the surface deformation which thus suppresses the formation of cracks. As a result, when the normal load is increased beyond the critical failure value, comparatively less severe damage could be found on the surface of PLA-9. Such findings once again emphasize the efficiency of layer-multiplying coextrusion in the fabrication of scratch resistant PLA through construction of layered shish-kebab structure. The flexibility in the choice of polymers as well as the easy processability would lay solid foundation



for industrial fabrication of other scratch resistant polymers toward low-cost, high performance and large-scale commodities. However, it is worth noting that like that occurring in conventional extrusion process, the melt along the extruding direction would suffer higher stress, so that the multilayer-assembled shish-kebab structure fabricated in this work is anisotropic. Consequently, the scratch performance vertical to the extruding direction is not comparable to that parallel to this direction. Hence, great effort would be made to reduce this deficiency in future work.

Figure 11. The critical normal load at the material removal stage obtained at 25, 50, and 100 mm/s for PLA-0 and PLA-9.

Conclusions In this work, multilayer-assembled PLA with shish-kebab structure was successfully fabricated through layer-multiplying coextrusion. Resulting from such


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unique crystalline structure, substantial increments of 60 and 560% in tensile strength and elongation at break were achieved, respectively. The scratch testing results revealed that accompanied with the evolution of crystalline morphology, the surficial cracking became less severe when experiencing a scratch process. The critical normal load, a significant parameter in evaluating the scratch-resisting ability, was distinctly enlarged from 36.5 to 66.3 N. Particularly, the resistance to the formation of cracking, is significantly enhanced due to the formation of multilayer-assembled shish-kebab structure. The origin of such greatly improved scratch resistance was further investigated, and it was revealed that the higher strength and ductility in PLA-9 play a crucial role in reducing the surface deformation and suppressing the formation of cracks. Thus, it is believed that the layer-multiplying coextrusion has set a successful model to the fabrication of scratch resistant PLA by tailoring crystalline morphology. The easy processability as well as great potential in industrial production would permit broad application of PLA as packaging and structural materials.

AUTHOR INFORMATION Corresponding Authors

∗ E-mail: [email protected] (Jiabin Shen). * E-mail: [email protected] (Shaoyun Guo). Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51420105004, 51227802, 51673136, 51421061) and the Program of Introducing Talents of Discipline to Universities (B13040) is gratefully acknowledged.

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Fabrication of Scratch Resistant Polylactide with Multilayered Shish

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