PMMA multilayered materials: Experiments and finite element modeling

Polymer 182 (2019) 121829

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Scratch damage behaviors of PVDF/PMMA multilayered materials: Experiments and finite element modeling Yang Xu a, Dun Li a, Jiabin Shen a, *, Shaoyun Guo a, **, Hung-Jue Sue b a b

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu, 610065, China Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M University, College Station, TX, 77843-3123, United States

A R T I C L E I N F O

A B S T R A C T

Keywords: Scratch resistance Multilayer structure Polyvinylidene fluoride

In this work, polyvinylidene fluoride (PVDF)/polymethyl methacrylate (PMMA) alternating multilayered ma­ terials were fabricated. Firstly, the scratch damage behaviors of PVDF side was investigated experimentally. The results presented that the maximum of critical normal load of scratch damage (Fcr) was obtained by 32-layer material, which was 40% higher than that of pure PVDF and very close to that of pure PMMA. This indicated that the scratch resistance of PVDF could be significantly improved via multilayer assembly with PMMA. Furthermore, numerical modeling revealed that with the multiplication of layers, the decrease of tensile stress imposed on the first PVDF layer led to the delay of scratch damage, whereas the decrease of the distance between the first PMMA layer and the scratch tip promoted its early damage. Therefore, the optimum scratch performance was determined by the critical thickness of PVDF and PMMA layers, which was significant for designing scratchresistant materials.

1. Introduction Due to high electronegativity of fluorine atom, high bond energy of C–F bond and shielding effect of fluorine atom to C–C main chain, flu­ oropolymers are endowed with superior corrosion, aging and solvent resistance. They are extensively used as protective layer for metals in the field of construction and are frequently subjected to harsh conditions during longtime service. On the other hand, fluoropolymers are quite vulnerable to damage by scratching owing to their poor mechanical properties, which can result in severe degradation of their durability, thus scratch resistance of fluoropolymers is required. Scratch is defined as damage made by a hard instrument (scratch tip) when moved across a test specimen surface [1], which is small scale deformation localized on surface involving complex triaxial stress [2] and high strain rate [3]. Surface of most polymers is susceptible to scratch damage even under low contact loads owing to their low hard­ ness. Scratch damage either generates unfavorable changes on surface aesthetic of polymer products, or leads to degradation of surface integ­ rity and durability, which is the case of fluoropolymer coatings. To design polymers with enhanced scratch resistance and meet the market demands, it is necessary to comprehensively understand the relationship

between their scratch behaviors and their structures and morphologies. With the establishment of standardized ASTM/ISO scratch test method [1,4], efforts have been devoted to gain fundamental understanding on scratch of polymers, and great progresses have been achieved on how the intrinsic properties and structures of polymers affect their scratch behaviors [5–14]. It is found that constitutive behaviors, especially ductility, compressive yield stress and strain hardening, and surface coefficient of friction (COF) of polymers have the most dramatic in­ fluences on their scratch behaviors. Strategies to fabricate scratch resistance polymers are developed accordingly, such as using slip agent to decreasing COF of polymers [15,16], and enhancing their yield stress by incorporation of well-dispersed nano-particles [17,18], by blending with strong and compatible polymers [19–21], or by crystallizing modulation [22–24]. Scratch resistant polymers can also be prepared through coating a hard layer on surface [25,26], or endowing polymers with self-healing capacity [27,28]. However, certain drawbacks exist among the current strategies developed for scratch resistance modification of polymers, such as migration of slip agent, aggregation of nanoparticles, complex proced­ ure and treatment required for synthesis of self-healing polymer. Coating a hard layer on surface is the most effective and practical

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Shen), [email protected] (S. Guo). https://doi.org/10.1016/j.polymer.2019.121829 Received 11 July 2019; Received in revised form 16 September 2019; Accepted 21 September 2019 Available online 21 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic of layer-multiplying co-extrusion technology.

Fig. 2. Dimensions, boundary conditions and meshing method of FEM model.

method to enhance scratch resistance of polymers. Nevertheless, in the case of fluoropolymers, they are constantly required to locate at outer surface of products and in direct contact with external environment, hence surface coating is not feasible. To date and to the best of our knowledge, there has been no scratch resistance modification strategy developed for fluoropolymers. It is of significance and urgent to exploit new avenues to design polymers with enhanced scratch resistance which serve under special circumstances. The key to achieving scratch resistance polymer is to realize a bal­ ance between rigidity and ductility. An emblematic example of balanced

mechanical performance is that exhibited by shell nacre via the unique multilayered microstructure consists of a strong but brittle calcium carbonate phase and a soft and ductile organic phase. Inspired by the multilayered biomaterial in nature, the multilayer assembly of two polymers with complementary performance is believed to be useful for obtaining polymers with balanced mechanical properties. Construction of a similar structure to nacre, i.e. alternating multilayered structure, can be readily realized in polymers through an advanced meltprocessing technology called layer-multiplying coextrusion, and has been proved very effective in toughening of polymers [29,30] and 2

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Fig. 3. (a) POM photographs of cross section for 32L sample, and (b) theoretical thickness of individual layer in PVDF/PMMA multilayered materials with different layer number.

to polyolefin layers, and found the film exhibiting superior scratch performance also possesses significantly better stress distribution through its layers during the scratch test, as well as better layer adhesion during severe deformation. Hamdi et al. [36] investigated the scratch behaviors of PP/PA and PA/PP layered films, and demonstrated that PP/PA laminate has considerably higher scratch resistance than PA/PP, which was explained by its elastic, plastic, and density-graded structure. These studies suggest that the scratch resistance of layered polymer systems can be altered by modifying the layer thicknesses, layer stacking order, and material properties, and provide useful guidelines and a promising opportunity for designing scratch resistant fluoropolymers. To probe the potential of alternating multilayered structure to improve scratch resistance of fluoropolymers, PVDF/PMMA layered material is a perfect model. PVDF has excellent intrinsic resistance to natural aging and remarkable stability against solar radiation when exposed to direct sunlight, and has been widely used as protective coating for solar panel. In terms of mechanical properties, these two polymers have great disparity and mismatch, i.e., PMMA is a stronger and more scratch resistant polymer than PVDF. Meanwhile, interfacial delamination in PVDF/PMMA layered material is unlikely to happen owing to perfect compatibility between PVDF and PMMA down to molecular level [37]. Since sharp interface and abrupt transition in mechanical properties exists in PVDF/PMMA layered material, stress distribution is bound to change during scratching. Therefore, through imbedding PMMA layers between PVDF layers, one can expect dramatic changes in scratch behavior and resistance of surface PVDF layer to occur. Emphasis of this study is to acquire a fundamental understanding on scratch damage behaviors of pure PVDF and PVDF side of PVDF/PMMA multilayered materials, and to provide a scratch resistance modification strategy for polymers served under special circumstances. In this work, alternating multilayered PVDF/PMMA sheets with different number of layers (2L, 4L, 8L, 16L, 32L, 64L, 128L) were fabricated, and their scratch damage behaviors of PVDF side were investigated and compared with pure PVDF under standardized progressive load scratch test. 3-D finite element modeling (FEM) on scratch test process was combined

Fig. 4. Critical normal load of scratch damage for pure PVDF, PMMA and PVDF side of PVDF/PMMA multilayered materials with different layer number.

endowing polymers with remarkable anisotropic functionality [31,32]. This unique structure may also provide a promising strategy for enhancing scratch resistance of polymers. Scratch behaviors of polymers with alternating multilayered struc­ tured have not been studied, while that of polymer laminates with two layers have been reported. Hossain et al. [33] studied the effect of layer thicknesses on scratch behavior of multilayered brittle acrylic coating systems with variation in soft base layer thicknesses. Their experimental results and numerical analysis suggest that increase in soft base layer thickness delayed the onset of crack formation during scratching by redistributing the stress and reduce the magnitude of tensile stress behind the scratch tip. Hare et al. [34,35] investigated scratch-induced damages on multilayered polymeric packaging films consisting of a metalized oriented polyethylene terephthalate (oPET) layer laminated

Fig. 5. Microscope photographs of top view for (a) pure PVDF at normal load of 72 N, (b) PVDF side of 32L sample at a normal load of 98 N. 3

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Fig. 6. Scanned images and microscope photographs of scratch induced deformation and damage along scratch path for (a) pure PVDF, (b) PVDF side of 8L sample, (c) PVDF side of 32L sample, (d) PVDF side of 128L sample, and (e) pure PMMA.

utilized and an in-depth analysis on evolution of stress field during scratching was performed to understand the observed experimental phenomenons. Differences in scratch damage behaviors and scratch resistance between pure PVDF and PVDF/PMMA multilayered materials were explained from the perspective of mechanics. The present research paves a new way for designing and manufacturing scratch resistant fluoropolymer.

2.2. Specimen preparation PVDF and PMMA were dried at 60 � C for 24 h in a vacuum oven prior to melt processing. Pure PVDF and PMMA sheets with a width of 3.0 mm and a thickness of 1.5 mm were fabricated through single screw extru­ sion. The temperature of the extruder was maintained at 170 � C, 200 � C and 220 � C from hopper to die. To prepare PVDF/PMMA multilayered materials with different number of layers and individual layer thickness, layer-multiplying co-extrusion technology was employed by applying different number of layer-multiplying elements (LMEs) as illustrated in Fig. 1, the mechanism of which has been described previously [38–40] and is also presented in supporting information. By adjusting the relative speed of screw and roll rotation, the width and total thickness of each PVDF/PMMA multilayered extrudate was kept at 3.0 mm and 1.5 mm respectively with a thickness ratio of around 1:1. All extrudates were annealed at 200 � C for 5 min to eliminate thermal history before scratch tests.

2. Experimental section 2.1. Materials A commercially available PVDF resin, Solef® 6010, with a melt flow rate of 6.8 g/10 min (230 � C/5 kg) and a density of 1.78 g/cm3 was purchased from Solvay Solexis (Belgium). PMMA resin (CM207) with a melt flow rate of 8 g/10 min (230 � C/3.8 kg) and a density of 1.19 g/cm3 was purchased from Chimei Chemicals (China). Both polymers are extrusion grade and used without further purification.

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Fig. 7. Scanned images and microscope photographs of scratch induced deformation and damage along scratch path for (a) first PVDF layer, (b) second PVDF layer, (c) third PVDF layer, and (d) fourth PVDF layer of 32L PVDF/PMMA multilayered materials.

Fig. 8. Comparison of cross-sectional profiles of scratch groove between experimental and simulation results. (a) PVDF at a normal load of 70 N and (b) PMMA at a normal load of 100 N.

2.3. Scratch test

were specified for all scratch tests. A stainless-steel spherical scratch tip of 1 mm diameter was used to conduct the tests. Five scratch tests were performed on the same plaque and average values were reported.

Scratch tests were carried out on a Scratch 3.5 Machine (Surface Machine Systems, USA) according to the ASTM D7027-05/ISO 19252:08 standard [1,4]. During scratch test, linearly progressive normal loads from 1 N to 130 N were applied on the surface of specimen by scratch tip, and the corresponding tangential load imposed on tip and distance of tip from origin were recorded. Afterwards, the scratch groove formed on specimen were observed by optical light microscope, and the onset point of scratch damage was determined and its distance from origin was measured, thus the corresponding normal load at onset location of scratch damage can be identified. This normal load is termed as critical normal load of scratch damage (Fcr), and can be used to quantify the scratch resistance of polymers. The ratio of tangential load to normal load applied on scratch tip is defined as scratch coefficient of friction (SCOF). A constant scratch speed of 25 mm/s and a scratch length of 100 mm

2.4. Uniaxial compression test The uniaxial compression behavior of PVDF was determined using a CMT4104 type compressive tester (MTS Systems Corporation, USA) equipped with a 30 kN capacity load cell and a laser extensometer at 23 � C with a relative humidity of 50%. The cylinder-shaped specimens (6 mm � 6 mm � 1.5 mm), prepared from extruded sheet, were com­ pressed along the thickness direction at a speed of 1 mm/min and a nominal strain rate of 0.01/s according to ASTM D695. All the samples were kept at the test condition for 24 h before test. Grease was used between the sample end faces and the platens to minimize friction under compression.

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Fig. 9. Comparison of SCOF as a function of normal load for (a) PVDF and (b) PMMA between experimental and simulation results.

Fig. 10. Damage feature and stress state of PVDF at onset of scratch damage. (a) Microscope photographs of top view at onset of scratch damage for pure PVDF at a normal load of 72 N, (b) Stress triaxiality contour of PVDF at a normal load of 70 N, (c) Location of maximum tensile stress of PVDF at a normal load of 70 N, (d) Tensile strain contour of PVDF at a normal load of 70 N.

2.5. Optical light microscopy

2.7. Laser scanning confocal microscope (LSCM)

Transmission optical light and polarized light microscopy was per­ formed on an Olympus BX51 polarizing microscope equipped with equipped with a crossed polarizer and a video camera. For observing the layered structure of PVDF/PMMA multilayered materials, a thin slice of approximately 30 μm in thickness was cut vertically to the flow direction of extrusion from each sample by using a microtome.

The size and cross-sectional profile at different locations on the scratch path of PVDF and PMMA were obtained by using a laser scanning confocal microscope (LSCM, VK-X250, Keyence, Japan). The microscope is equipped with a 408 nm wavelength violet laser and has a height resolution of ~0.5 nm and width resolution of ~1.0 nm. LSCM analysis was carried out 24 h after the completion of scratch tests to allow for viscoelastic recovery.

2.6. Scanned image

2.8. Attenuated total reflection fourier transform infrared (ATR-FTIR) spectroscopy

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 di­ rection was aligned perpendicular to the scanner light source movement.

The ATR-FTIR spectra were obtained on a IS10 (Thermo Fisher Sci­ entific, USA) Fourier transform Infrared Spectrometer (FTIR) in the wavenumber range of 600–4000 cm 1. At room temperature, each spectrum was recorded at a resolution of 4 cm 1 with a total of 32 scans.

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Fig. 11. (a)FTIR-ATR and (b) Raman spectra of α and β-phase characteristics region of PVDF at different locations along scratch path of the first PVDF layer of 32L sample.

for maintaining a good level of accuracy.

Table 1 β-Phase fraction in first PVDF layer of 32L sample at different locations along scratch path as determined by ATR-FTIR and Raman spectra. Location along scratch path

β-phase fraction determined by FTIR (%)

β-phase fraction determined by Raman (%)

Point A: Non-scratched, normal load ¼ 0 N Point B: Fish-scale, normal load ¼ 50 N Point C: Before Scratch damage, normal load ¼ 95 N

37

40

42

59

52

77

3.2. Determination of material properties and coefficient of friction (COF) The fundamental principles on quantitative modeling of scratchinduced deformation in polymers have been established [41,44]. Since the strain rate can reach up to hundreds of s 1, while the contact pres­ sure can reach up to hundreds of MPa during scratch deformation of polymer, it is suggested by Hossain et al. [41] that the rate dependent constitutive behavior and pressure dependent frictional behavior of polymer need to be taken into account in FEM to quantitatively predict the scratch behavior of polymer. Following their pioneering work, the rate dependent constitutive behavior and pressure dependent frictional behavior of PVDF and PMMA were also incorporated in the current study. Although significant temperature rise of polymers’ surface at high normal load during scratching has been observed due to frictional heating and plastic deformation [45], which might result in thermal softening of polymers [46], the temperature dependency of constitutive behavior was not considered. More detailed information about FEM modeling can be found in supporting information.

2.9. Raman imaging microscope The Raman spectra were obtained with a Raman system (DXRxi, Thermo Fisher Scientific, USA) in a range of 50–3400 cm 1. The exci­ tation source is 532-nm laser with a laser power below 1 mW on the sample to avoid laser-induced heating. The spectra were analyzed, and Raman images were then constructed using a parameter (peak fre­ quency, peak intensity, integrated peak intensity, or peak width) by using DXRxi Project software.

4. Results and discussion 4.1. Structure of PVDF/PMMA multilayered materials Through layer-multiplying coextrusion, PVDF/PMMA multilayered materials with different layer number were obtained. The cross-section POM image of 32L sample is shown in Fig. 3(a), where the bright and dark layers correspond to the semi-crystalline PVDF and amorphous PMMA component, respectively. Parallelly aligned layer interfaces can be clearly recognized, denoting well-defined layer structures. By varying the total number of layers from 2 to 128, individual layer thickness was readily regulated between ~11.7 μm and ~750 μm as shown in Fig. 3 (b). Two surfaces existed in PVDF/PMMA multilayered material. One surface was PVDF side while the other was PMMA side. Since emphasis of the current study is to probe the potential of alternating multilayered structure to improve scratch resistance of PVDF, scratch tests were only performed on the PVDF side for multilayered materials and compared with pure PVDF.

3. FEM modeling To better understand the scratch damages in multilayered PVDF/ PMMA systems and reveal the mechanics behind, numerical modeling of scratch tests was conducted using the commercial finite element pack­ age ABAQUS/Explicit. The meshing method, model dimensions and boundary conditions for scratch modeling in accordance with ASTM/ ISO standard have been well established [8,9,33,41–43]. Similar modeling technique was adopted in the present study. 3.1. FEM model Dimensions (14 mm � 2.461 mm � 1.536 mm), boundary conditions and meshing method for all models were the same. Here, 32L PVDF/ PMMA model is taken as an example as shown in Fig. 2. Due to the width symmetry along the scratch path, the computational domain was reduced by half to significantly reduce the computational time. Gradient mesh was employed to reduce the computational time and mesh was refined near the contact area as shown in the magnified region of Fig. 2

4.2. Scratch-induced deformation and damage Critical normal load of scratch damage (Fcr) of different materials is 7

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Fig. 12. Damage feature and stress state of PMMA at onset of scratch damage. (a) von Mises stress contour of PMMA at a normal load of 100 N. (b) Stress triaxiality contour of PMMA at a normal load of 100 N, (c) Microscope photographs of top view at onset of scratch damage for pure PMMA at a normal load of 100 N, (d) Top view of scratch damage features obtained through FEM modeling by using maximum tensile stress and maximum von Mises stress as scratch damage criterion for PMMA.

Fig. 13. Evolution of the (a) maximum tensile stress and (b) maximum tensile strain as a function of normal load for first PVDF layer.

presented in Fig. 4. Fcr of PVDF and PMMA is 70 N and 100 N respec­ tively, indicating PMMA is a more scratch resistant polymer than PVDF. In multilayered materials, as layer number increased, Fcr of PVDF side gradually increased, and reached up to a maximum value at layer number of 32. Upon further increasing layer number to above 32, Fcr of PVDF side decreased to be even lower than that of pure PVDF. In particular, Fcr of PVDF side of 32L sample was 100 N, which was the same as that of pure PMMA and 40% higher than that of pure PVDF, indicating the scratch resistance of PVDF material can be significantly improved via multilayer assembly with PMMA. A more straightforward comparison between scratch resistance of pure PVDF and PVDF side of 32L sample is shown in Fig. 5. The maximum normal load during scratch test was set to be 72 N for pure PVDF sample and 98 N for 32L sample. Conspicuous plowing occurred

on surface of pure PVDF at a normal load of 72 N as shown in Fig. 5(a), whereas the PVDF side of 32L sample still remained intact at a normal load of 98 N as shown in Fig. 5(b), which further demonstrates the enhancement in scratch resistance of PVDF side of 32L sample compared with pure PVDF. The scratch-induced deformation and damage features of pure PVDF, PVDF side of 8L, 32L and 128L sample and pure PMMA along scratch path are demonstrated in Fig. 6. At the initial stage of scratch defor­ mation, fish-scale pattern was observed for pure PVDF and PVDF side of 8L and 32L samples (Fig. 6(a,b,c)). Fish-scale deformation is a widely observed phenomena for ductile semi-crystalline polymers, which is believed to be caused by ‘stick-slip’ phenomenon [2]. It is also shown that only one type of scratch damage, i,e, material removal (or plowing), existed in pure PVDF (Fig. 6(a)) and 8L sample (Fig. 6(b)), which was 8

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Fig. 14. Evolution of the (a) maximum tensile stress and (b) maximum von Mises stress as a function of normal load for first PMMA layer.

scratch damage, deformation of the first PVDF layer was considerably larger than that of the second PVDF layer before onset of scratch dam­ age. These results indicate that before onset of scratch damage, the load distribution and deformation range of multilayered materials along depth was mainly constrained on the surface within a very limited dis­ tance from scratch tip during scratching. Mechanism for variation of Fcr and shift of onset location of scratch damage in PVDF/PMMA alternating multilayered materials with different layer number will be discussed in detail and revealed by FEM modeling in the next part. 4.3. FEM modeling and mechanism 4.3.1. Validation of material properties and COF By using the determined material property data and COF values, the scratch coefficient of friction (SCOF) curves and cross-sectional profiles were obtained by simulation, and comparison between experimental and simulation results were made as shown in Fig. 8 and Fig. 9. The fluctuations at the beginning of the experimentally obtained SCOF curve were due to the inertia effect of the scratch tip movement. When scratch test started, the scratch tip had to be accelerated from static state to a velocity of 25 mm/s within a very short time, resulting in the fluctuation of the initial portion of the experimental data. For PVDF, simulation result matched well with experimental result in terms of both SCOF curve and cross-sectional profiles of scratch groove as shown in Figs. 8 (a) and Fig. 9(a). SCOF value obtained by simulation became larger than experimental result as normal load increased, but the discrepancy was no more than 20%. For PMMA, simulation result matched well with experimental result regarding SCOF curve as shown in Fig. 9(b), while the discrepancy of cross-sectional profile was relatively large as shown in Fig. 8(b). In particular, shoulder height of scratch groove obtained through simulation was only half that of experimental result. This might be resulted from the aforementioned thermal softening of PMMA due to temperature rise during scratching [45,46], which was not included in the current modeling. Giving the fact that scratch deformation of polymer is a process with high complexity, fully quantitative modeling on scratch of polymer is exceedingly challenging and difficult. On the whole, our simulation results on scratch deformation of PVDF and PMMA can be considered as quantitative, and the consistency between experimental results and simulation can be regarded as satisfactory. Thus, it was possible to provide deep insights into the scratch deformation mechanism and scratch induced damage of PVDF, PMMA and PVDF/PMMA multilay­ ered materials by our simulation results.

Fig. 15. Critical normal load of scratch damage for PVDF side of PVDF/PMMA multilayered materials determined by FEM modeling and scratch test.

also the case for PVDF side of 2L, 4L and 16L samples. While the pres­ ence of two types of scratch damage, i.e., micro-crack and material removal, were observed for PVDF side of 32L sample, 128L sample and pure PMMA as shown in Fig. 6(c), (d) and (e), respectively. These two types of scratch damage also occurred on PVDF side of 64L sample with an onset location between that of 32L and 128L sample. Micro-crack is a characteristic scratch damage feature of brittle polymers. Its formation is believed to be triggered by tensile stress built up at the rear end of scratch tip [2,5,44]. Since PVDF is a ductile material with much higher elongation at break than PMMA, the periodic micro-crack observed at scratch damage onset location in 32L sample can only be initiated from PMMA layer located beneath PVDF layer. These results manifest that in PVDF/PMMA multilayered materials, as layer number increased, the onset location of scratch damage shifted from PVDF layer to PMMA layer. Since PVDF side of 32L sample exhibited highest scratch resistance, deformation of this particular material was further analyzed after scratch test in a layer by layer manner. This procedure was realized through dissolving all PMMA layers in dichloromethane, thus remaining 16 integrated PVDF layers. Scanned images and microscope photo­ graphs of the top four PVDF layers are demonstrated in Fig. 7. As PVDF layer moved further from scratch tip, plastic deformation extenuated layer by layer and ultimately terminated by the fourth PVDF layer. It is shown in Fig. 7(a) and (b) that although both the first and second PVDF layer were penetrated by scratch tip in the last region denoting severe 9

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Fig. 16. Schematic of mechanism of location shift and delay of scratch damage onset on PVDF side of PVDF/PMMA multilayered materials with different layer number.

4.3.2. Determination of scratch damage criterion for PVDF and PMMA To uncover the cause of differences of critical normal load and onset location of scratch damage in PVDF/PMMA multilayered materials with different layer number, establishment of scratch damage criterion for both PVDF and PMMA is necessary. Damage feature and stress state of pure PVDF at onset of scratch damage is shown in Fig. 10. Scratch tip was removed for better visual­ ization. Focus on the detail of Fig. 10(a), it can be seen that PVDF ma­ terial piled up in front of scratch tip remained intact except for the two flanks of scratch groove where disconnection of PVDF materials occurred, indicating the location of scratch damage initiation of PVDF. Stress triaxiality contour (TRIAX, Fig. 10(b)) shows that the value of stress triaxiality at the two flanks of PVDF material piled up in front of scratch tip is positive, which means PVDF material at this location is under tensile stress. The maximum tensile stress (Fig. 10(c)) and maximum tensile strain (Fig. 10(d)) also located at the same position. The FEM modeling results strongly suggest that the onset of scratch damage of PVDF might be triggered by tensile tearing from the two flanks of PVDF material piled up in front of scratch tip. To further confirm the existence of tensile deformation in PVDF material during scratch, three different locations (denoted as A, B and C in Fig. 7(a)) along scratch path of the first PVDF layer of 32L sample after scratch test was analyzed by ATR-FTIR and Raman spectroscopy. It has been widely verified and recognized that crystals of PVDF can transform form non-polar α-phase to β-phase during stretching below certain temperature [47–49]. FTIR and Raman results are commonly

used to quantify the electroactive β-phase content of PVDF. The obtained ATR-FTIR spectra of α and β-phase characteristics region of PVDF is shown in Fig. 11(a). 766 cm 1 represents a characteristic band of the α-phase of PVDF, whereas 838 cm 1 is a strong band just for β-phase. According to Gregorio et al. [50], the relative fraction of the β-phase in a PVDF sample is: � � Aβ F β ¼ (1) ðKβ =Kα ÞAα þ Aβ where F(β) represents the β-phase content; Aα and Aβ the absorbance at 763 cm 1 and 838 cm 1; Kα and Kβ are the absorption coefficients at the respective wavenumber, which values are 6.1 � 104 and 7.7 � 104 cm2 mol 1 respectively. The obtained Raman spectra of α and β-phase characteristics region of PVDF is shown in Fig. 11(b). 795 cm 1 and 838 cm 1 represents a characteristic peak of the α and β-phase of PVDF, respectively. Accord­ ing to Riosbaas et al. [51], the relative fraction of the β-phase in a PVDF sample is: � � Iβ F β ¼ (2) Iα þ Iβ where F(β) represents the β-phase content; Iα and Iβ the intensity at 795 cm 1 and 838 cm 1 respectively. The relative fraction of the β-phase in the PVDF layer of 32L sample at different locations along scratch path was calculated by using formula (1) and (2), and the results is presented in Table 1. Both ATR-FTIR and 10

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Raman spectra results indicate an increase of β-phase fraction in first PVDF layer of 32L sample as normal load and deformation increased, which can only be attributed to tensile deformation of PVDF during scratch test. Thus, based on experimental test and simulation results, it can be speculated that the onset of scratch damage of PVDF is triggered by tensile tearing from the two flanks of PVDF material piled up in front of scratch tip, and maximum tensile stress and tensile strain can be utilized as scratch damage criterion for PVDF. Damage feature and stress state of PMMA at onset of scratch damage is shown in Fig. 12. Scratch tip was removed and the location of maximum von Mise stress was encircled by a white dotted line for better visualization. As mentioned before, the presence of two types of scratch damage, i.e., periodic micro-crack and material removal, was observed for PMMA. Periodic micro-crack is believed to be triggered by tensile stress built up at the rear end of scratch tip [2,5,42]. As shown in Fig. 12 (b), the value of stress triaxiality for PMMA material at the rear end of scratch tip is positive, which means PMMA material at this location is under tensile stress. The direction of the double arrow in Fig. 12(a) and (b) indicates the direction of the maximum tensile stress, and the midpoint of double arrow indicates the location of the maximum tensile stress, which confirms that this maximum tensile stress is responsible for the formation of periodic micro-crack of PMMA during scratching. Also shown in Fig. 12(b), the value of stress triaxiality for PMMA material carrying the maximum von Mises stress is negative. The magnitude of von Mises stress determines the degree of deformation, and hence PMMA material undergoing maximum deformation is under compres­ sive stress. Unlike PVDF, PMMA is a brittle material which can fail in compression under large strain [52,53]. Therefore, it can be inferred that material removal of PMMA during scratching is caused by the maximum von Mises stress right beneath scratch tip, and the maximum tensile stress and maximum von Mises stress are employed as scratch damage criterion for periodic micro-crack and material removal of PMMA respectively. By defining appropriate values of tensile stress and von Mises stress as damage criterion for PMMA in FEM modeling, scratch damage features of PMMA can be generated as shown in Fig. 12 (d), which mimics the major characteristics of actual scratch damage features of PMMA as shown in Fig. 12(c).

models. The variation tendency of maximum tensile stress and maximum tensile strain as a function of normal load for the first PMMA layer is shown in Fig. 14. At the same normal load, both the maximum tensile stress and maximum von Mises stress becomes larger as layer number increases, which is caused by the decreased distance of the first PMMA layer to the contact surface. Fcr of pure PMMA is 100 N according to experimental test result, thus the values of maximum tensile stress and maximum von Mises stress corresponding to a normal load of 100 N in PMMA model were employed as critical scratch damage criterion for PMMA material, and Fcr of the first PMMA layer in multilayered models were obtained accordingly as indicated by black dotted lines in Fig. 14. By using maximum tensile stress as scratch damage criterion, Fcr of the first PMMA layer in 32L, 64L and 128L models were determined to be 104 N, 78 N and 55 N, respectively. By using maximum von Mises stress as scratch damage criterion, Fcr of the first PMMA layer in 32L, 64L and 128L models were determined to be 112 N, 88 N and 63 N, respectively. Both the maximum tensile stress and von Mises stress were too small to meet scratch damage criterion in the first PMMA layer of 2L, 4L, 8L and 16L models. The Fcr of PVDF/PMMA multilayered models obtained via FEM modeling are summarized in Fig. 15. Experimental results are also presented to make a comparison. As shown in Fig. 15, scratch damage criterion of PVDF material is met first for 2L, 4L, 8L and 16L models, while scratch damage criterion of PMMA material is met first for 64L and 128L models, which correlates well with experimental results. The case of 32L model is a little bit more complicated. Scratch damage criterion of which material is met first depends on the scratch damage criterion. To be more specific, scratch damage criterion of PMMA material is met first by using maximum tensile stress as damage criterion for PVDF material, whereas the contrary is the case by using maximum tensile strain as damage criterion for PVDF material. Given the fact that scratch damage of 32L material was initiated from the first PMMA layer as shown in Fig. 6(c), maximum tensile stress seems to serve as a more appropriate and accurate scratch damage criterion for PVDF material. As shown in Fig. 15, by using maximum tensile stress as scratch damage criterion for PVDF material, good agreement is achieved between FEM modeling and experimental test results of scratch damage onset of PVDF/PMMA multilayered materials in terms of both the magnitude of Fcr and onset location.

4.3.3. Damage analysis for the first PVDF layer and the first PMMA layer in PVDF/PMMA multilayered models With the establishment of scratch damage criterion for both PVDF and PMMA, now it is possible to perform scratch damage analysis for PVDF/PMMA multilayered models through FEM modeling. Since the emphasis of this study is the onset of scratch damage which is either initiated from the first PVDF layer or the first PMMA layer, damage analysis is only conducted for these two layers in this part. The variation tendency of maximum tensile stress and maximum tensile strain as a function of normal load for the first PVDF layer is shown in Fig. 13. Evolution trends of both the maximum tensile stress and maximum tensile strain in the first PVDF layer of 2L, 4L, 8L and 16L models are similar to pure PVDF, whereas they increase more slowly in the first PVDF layer of 32L, 64L and 128L models. Fcr of pure PVDF is 70 N according to experimental test result, thus the values of maximum tensile stress and maximum tensile strain corresponding to a normal load of 70 N in PVDF model were employed as critical scratch damage criterion for PVDF material, and Fcr of the first PVDF layer in multi­ layered models were obtained accordingly as indicated by black dotted lines in Fig. 13. By using maximum tensile stress as scratch damage criterion, Fcr of the first PVDF layer in 2L, 4L, 8L and 16L models were determined to be 75 N, 67 N, 70 N, and 84 N, lo. By using maximum tensile strain as scratch damage criterion, Fcr of the first PVDF layer in 2L, 4L, 8L, 16L and 32L models were determined to be 70 N, 67 N, 64 N, 62 N and 103 N, respectively. The maximum tensile stress was too small to meet scratch damage criterion in the first PVDF layer of 32L model, and both the maximum tensile stress and tensile strain were too small to meet scratch damage criterion in the first PVDF layer of 64L and 128L

4.3.4. Mechanism of location shift and delay of scratch damage onset in PVDF/PMMA multilayered models As aforementioned, the onset location of scratch damage in PVDF/ PMMA multilayered materials shifted from the first PVDF layer to the first PMMA layer as layer number increases or thickness of an individual layer decreases, and 32L materials possesses the highest resistance to scratch damage. Since the deformation and failure of a material is directly related to the stress field it experienced, these phenomenons can be explained by the change of stress magnitude and stress state in PVDF/ PMMA multilayered materials with different layer number. The mech­ anism of location shift and delay of scratch damage onset in PVDF/ PMMA multilayered materials is shown in Fig. 16. In materials with low layer number (2L, 4L and 8L), the thickness of the first PVDF layer is larger than 150 μm. Deformation of PVDF during scratching is limited and localized within a small range of thickness on its surface and subsurface. As shown in Fig. 16(a), the depth of scratch groove of PVDF before onset of scratch damage is merely 75 μm. Therefore, thickness of the first PVDF layer in 2L, 4L and 8L materials is large enough to demonstrate the same scratch damage behavior as pure PVDF. In materials with intermediate (16L and 32L, Fig. 16(b)) and high layer number (64L and 128L, Fig. 16(c)), the thickness of the first PVDF layer is below 100 μm, which is no longer large enough to confine the scratch deformation on the first PVDF layer. The first PMMA layer starts to undergo plastic deformation and alters the stress field of the first 11

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PVDF layer by decreasing the tensile stress and tensile strain imposed on it during scratching. Since the scratch damage of PVDF is found to be caused by tensile tearing, decreasing of tensile stress and tensile strains leads to scratch damage delay of the first PVDF layer. On the other hand, the first PMMA layer moves closer to the contact surface as layer number increases, resulting in larger stress built up on it. Meanwhile, thickness of the first PMMA layer also decreases with increasing layer number, which leads to its degradation of rigidity and resistance to deformation, and ultimately results in early damage of the first PMMA layer in ma­ terials with high layer number. Therefore, in such PVDF/PMMA multilayered materials, there is a critical thickness (Tcr) for both PVDF layer and PMMA layer. Scratch damage of the first PVDF layer is delayed once its thickness drops below Tcr, whereas early damage of the first PMMA layer occurs once its thickness drops below Tcr. To prepare PVDF/PMMA multilayered ma­ terials with best scratch resistance on the PVDF side, the thickness of PVDF layer should be smaller than Tcr, and that of PMMA layer should be larger than Tcr, which happens to be the case of 32L material.

[6] J.L. Bucaille, C. Gauthier, E. Felder, R. Schirrer, The influence of strain hardening of polymers on the piling-up phenomenon in scratch tests: experiments and numerical modelling, Wear 260 (2006) 803–814. [7] M.M. Hossain, H. Jiang, H. Sue, Effect of constitutive behavior on scratch visibility resistance of polymers—a finite element method parametric study, Wear 270 (2011) 751–759. [8] M.M. Hossain, R. Browning, R. Minkwitz, H. Sue, Effect of asymmetric constitutive behavior on scratch-induced deformation of polymers, Tribol. Lett. 47 (2012) 113–122. [9] H. Jiang, G.T. Lim, J.N. Reddy, J.D. Whitcomb, H. Sue, Finite element method parametric study on scratch behavior of polymers, J. Polym. Sci., Polym. Phys. Ed. 45 (2007) 1435–1447. [10] H. Pelletier, J. Krier, C. Gauthier, Influence of local friction coefficient and strain hardening on the scratch resistance of polymeric surfaces investigated by finite element modeling, Procedia Eng. 10 (2011) 1772–1778. [11] M.M. Hossain, R. Minkwitz, H. Sue, Minimization of surface friction effect on scratch-induced deformation in polymers, Polym. Eng. Sci. 53 (2013) 1405–1413. [12] H. Jiang, R. Browning, J. Fincher, A. Gasbarro, S. Jones, H. Sue, Influence of surface roughness and contact load on friction coefficient and scratch behavior of thermoplastic olefins, Appl. Surf. Sci. 254 (2008) 4494–4499. [13] H. Pelletier, C. Gauthier, R. Schirrer, Influence of the friction coefficient on the contact geometry during scratch onto amorphous polymers, Wear 268 (2010) 1157–1169. [14] Y. Xu, D. Li, J. Shen, S. Guo, H. Sue, Research progress in scratch behaviors of polymeric materials, Acta Polym. Sin. 10 (2018) 1262–1278. [15] R. Browning, G.T. Lim, A. Moyse, L.Y. Sun, H.J. Sue, Effects of slip agent and talc surface-treatment on the scratch behavior of thermoplastic olefins, Polym. Eng. Sci. 46 (2006) 601–608. [16] B. Kim, H. Kim, B. Choi, H. Lee, An experimental study of the scratch properties of poly(methyl methacrylate) as a function of the concentration of added slip agent, Tribol. Int. 44 (2011) 2035–2041. [17] F. Lei, M. Hamdi, P. Liu, P. Li, M. Mullins, H. Wang, et al., Scratch behavior of epoxy coating containing self-assembled zirconium phosphate smectic layers, Polymer 112 (2017) 252–263. [18] Q. Zhang, W. Huang, G. Zhong, Towards transparent PMMA/SiO2 nanocomposites with promising scratch-resistance by manipulation of SiO2 aggregation followed byin situ polymerization, J. Appl. Polym. Sci. 134 (2017). [19] J. Song, C.M. Thurber, S. Kobayashi, A.M. Baker, C.W. Macosko, H.C. Silvis, Blends of polyolefin/PMMA for improved scratch resistance, adhesion and compatibility, Polymer 53 (2012) 3636–3641. [20] K. Kojio, Y. Kiyoshima, T. Kajiwara, Y. Higaki, H. Sue, A. Takahara, Effect of blend composition on scratch behavior of polystyrene/poly(2,6-dimethyl-1,4phenyleneoxide) blends, Macromol. Chem. Phys. 220 (2019), 1800371. [21] M. Berrebi, I. Fabre-Francke, B. Lavedrine, O. Fichet, Development of organic glass using Interpenetrating Polymer Networks with enhanced resistance towards scratches and solvents, Eur. Polym. J. 63 (2015) 132–140. [22] L. Yi, Y. Xu, D. Li, J. Shen, S. Guo, H. Sue, Fabrication of scratch resistant polylactide with multilayered shish-kebab structure through layer-multiplying coextrusion, Ind. Eng. Chem. Res. 57 (2018) 4320–4328. [23] L. Yi, D. Li, Y. Xu, J. Shen, S. Guo, Z. Zheng, 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, Ind. Eng. Chem. Res. 58 (2019) 1914–1923. [24] Y. Kobayashi, Y. Otsuki, H. Kanno, Y. Hanamoto, T. Kanai, Relating scratch resistance to injection molding-induced morphology of polypropylene, J. Appl. Polym. Sci. 120 (2011) 141–147. [25] N. Le Bail, K. Lionti, S. Benayoun, S. Pavan, L. Thompson, C. Gervais, et al., Scratch-resistant sol–gel coatings on pristine polycarbonate, New J. Chem. 39 (2015) 8302–8310. [26] H. Yavas, C.D.O. Selcuk, A.E.S. Ozhan, C. Durucan, A parametric study on processing of scratch resistant hybrid sol-gel silica coatings on polycarbonate, Thin Solid Films 556 (2014) 112–119. [27] M. Abend, C. Kunz, S. Stumpf, S. Gr€ af, S. Zechel, F.A. Müller, et al., Femtosecond laser-induced scratch ablation as an efficient new method to evaluate the selfhealing behavior of supramolecular polymers, J. Mater. Chem. 7 (2019) 2148–2155. [28] Y. Li, S. Chen, X. Li, M. Wu, J. Sun, Highly transparent, nanofiller-reinforced scratch-resistant polymeric composite films capable of healing scratches, ACS Nano 9 (2015) 10055–10065. [29] C. Li, S. Yang, J. Wang, J. Guo, H. Wu, S. Guo, Unique impact behavior and toughening mechanism of the polypropylene and poly(ethylene-co-octene) alternating multilayered blends with superior toughness, RSC Adv. 4 (2014) 55119–55132. [30] J. Wang, C. Wang, X. Zhang, H. Wu, S. Guo, Morphological evolution and toughening mechanism of polypropylene and polypropylene/poly(ethylene-cooctene) alternating multilayered materials with enhanced low-temperature toughness, RSC Adv. 4 (2014) 20297–20307. [31] K. Zhao, S. Li, M. Huang, et al., Remarkably anisotropic conductive MWCNTs/ polypropylene nanocomposites with alternating microlayers, Chem. Eng. J. 358 (2019) 924–935. [32] L. Li, H. Shi, Z. Liu, et al., Anisotropic conductive polymer composites based on high density polyethylene/carbon nanotube/polyoxyethylene mixtures for microcircuits interconnection and organic vapor sensor, ACS Appl. Nano Mater. 2 (2019) 3636–3647. [33] M.M. Hossain, S. Xiao, H. Sue, M. Kotaki, Scratch behavior of multilayer polymeric coating systems, Mater. Des. 128 (2017) 143–149.

5. Conclusions In this paper, scratch damage behaviors of the PVDF side of multi­ layered materials were investigated. As layer number increased, the onset normal load of scratch damage (Fcr) of PVDF side gradually increased and reached up to a maximum value at layer number of 32. Upon further increasing layer number to above 32, Fcr of PVDF side decreased to be even lower than that of pure PVDF. In particular, Fcr of PVDF side of 32L sample was the same as that of pure PMMA and 40% higher than that of pure PVDF. Numerical modeling revealed that scratch damage of PVDF material was caused by tensile tearing. With the multiplication of layers, the decrease of tensile stress imposed on the first PVDF layer led to the delay of scratch damage, whereas, the decrease of the thickness of the first PMMA layer and its distance from the scratch tip led to its early damage in materials. Therefore, it could be concluded that the optimum scratch performance was determined by the critical thickness of PVDF and PMMA layers. Notes The authors declare no competing financial interest. Acknowledgements The authors would like to thank Mr. Shuoran Du (Polymer Tech­ nology Center, Department of Materials Science and Engineering, Texas A&M University, USA) for his great help in scratch tests. The authors also appreciate the financial support of National Natural Science Foundation of China (51420105004, 51873132, 51721091) and Sichuan Science and Technology Program (2019JDRC0102) to this work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121829. References [1] Plastics – Determination of Scratch Properties. ISO19252-2008. [2] H. Jiang, R. Browning, H. Sue, Understanding of scratch-induced damage mechanisms in polymers, Polymer 50 (2009) 4056–4065. [3] B.J. Briscoe, E. Pelillo, K.S. Sinha, Scratch hardness and deformation maps for polycarbonate and polyethylene, Polym. Eng. Sci. 36 (1996) 2996–3005. [4] Standard Test Method for Evaluation of Scratch Resistance of Polymeric Coatings and Plastics Using an Instrumented Scratch Machine. ASTM D7027-13. [5] C. Xiang, H.J. Sue, J. Chu, B. Coleman, Scratch behavior and material property relationship in polymers, J. Polym. Sci., Polym. Phys. Ed. 39 (2001) 47–59.

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Y. Xu et al.

Polymer 182 (2019) 121829 [45] J. Zhang, H. Jiang, C. Jiang, Q. Cheng, G. Kang, In-situ observation of temperature rise during scratch testing of poly (methylmethacrylate) and polycarbonate, Tribol. Int. 95 (2016) 1–4. [46] J. Richeton, S. Ahzi, K.S. Vecchio, F.C. Jiang, R.R. Adharapurapu, Influence of temperature and strain rate on the mechanical behavior of three amorphous polymers: characterization and modeling of the compressive yield stress, Int. J. Solids Struct. 43 (2006) 2318–2335. [47] P. Martins, A.C. Lopes, S. Lanceros-Mendez, Electroactive phases of poly (vinylidene fluoride): determination, processing and applications, Prog. Polym. Sci. 39 (2014) 683–706. [48] V. Sencadas, R. Gregorio, S. Lanceros-M� endez, α to β phase transformation and microestructural changes of PVDF films induced by uniaxial stretch, J. Macromol. Sci., Part B 48 (2009) 514–525. [49] V. Sencadas, M.V. Moreira, S. Lanceros-M� endez, et al., α-to β Transformation on PVDF films obtained by uniaxial stretch. Materials science forum, Trans. Tech. Publ. 514 (2006) 872–876. [50] R. Gregorio Jr., M. Cestari, Effect of crystallization temperature on the crystalline phase content and morphology of poly (vinylidene fluoride), J. Polym. Sci. B Polym. Phys. 32 (5) (1994) 859–870. [51] M.T. Riosbaas, K.J. Loh, G. O’Bryan, et al., In situ phase change characterization of PVDF thin films using Raman spectroscopy, in: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2014, vol. 9061, International Society for Optics and Photonics, 2014, 90610Z. [52] P. Forquin, M. Nasraoui, A. Rusinek, L. Siad, Experimental study of the confined behaviour of PMMA under quasi-static and dynamic loadings, Int. J. Impact Eng. 40–41 (2012) 46–57. [53] T.J. Holmquist, J. Bradley, A. Dwivedi, D. Casem, The response of polymethyl methacrylate (PMMA) subjected to large strains, high strain rates, high pressures, a range in temperatures, and variations in the intermediate principal stress, Eur. Phys. J. Spec. Top. 225 (2016) 343–354.

[34] B.A. Hare, A. Moyse, H. Sue, Analysis of scratch-induced damages in multi-layer packaging film systems, J. Mater. Sci. 47 (2012) 1389–1398. [35] B.A. Hare, H. Sue, L.Y. Liang, P. Kinigakis, Scratch behavior of extrusion and adhesive laminated multilayer food packaging films, Polym. Eng. Sci. 54 (2014) 71–77. [36] M. Hamdi, X. Zhang, H. Sue, Fundamental understanding on scratch behavior of polymeric laminates, Wear 380–381 (2017) 203–216. [37] T. Nishi, T.T. Wang, Melting point depression and kinetic effects of cooling on crystallization in poly(vinylidene fluoride)-poly(methyl methacrylate) mixtures, Macromolecules 8 (1975) 909–915. [38] X. Ji, D. Chen, Y. Zheng, J. Shen, S. Guo, E. Harkin-Jones, Multilayered assembly of poly(vinylidene fluoride) and poly(methyl methacrylate) for achieving multi-shape memory effects, Chem. Eng. J. 362 (2019) 190–198. [39] Y. Zheng, R. Dong, J. Shen, S. Guo, Tunable shape memory performances via multilayer assembly of thermoplastic polyurethane and polycaprolactone, ACS Appl. Mater. Interfaces 8 (2016) 1371–1380. [40] X. Ji, D. Chen, J. Shen, S. Guo, Flexible and flame-retarding thermoplastic polyurethane-based electromagnetic interference shielding composites, Chem. Eng. J. 370 (2019) 1341–1349. [41] M.M. Hossain, R. Minkwitz, P. Charoensirisomboon, H. Sue, Quantitative modeling of scratch-induced deformation in amorphous polymers, Polymer 55 (2014) 6152–6166. [42] M. Hamdi, M. Puopolo, H. Pham, H. Sue, Experimental and FEM analysis of scratch behavior on polypropylene thin films: effect of film orientation and ethylene monomer content, Tribol. Int. 103 (2016) 412–422. [43] S. Xiao, H. Wang, F. Hu, H. Sue, Effect of moisture exposure on scratch behavior of model polyurethane elastomers, Polymer 137 (2018) 209–221. [44] H. Jiang, J. Zhang, Z. Yang, C. Jiang, G. Kang, Modeling of competition between shear yielding and crazing in amorphous polymers’ scratch, Int. J. Solids Struct. 124 (2017) 215–228.

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