Remarkably Improved Impact Fracture Toughness of Isotactic

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General Research

Remarkably improved impact fracture toughness of isotactic polypropylene via combining the effects of shear layerspherulites layer alternated structure and thermal annealing Mingjin Liu, Rui Hong, Xuanbo Gu, Qiang Fu, and Jie Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02858 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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Industrial & Engineering Chemistry Research

Remarkably improved impact fracture toughness of isotactic polypropylene via combining the effects of shear layerspherulites layer alternated structure and thermal annealing Mingjin Liu, Rui Hong, Xuanbo Gu, Qiang Fu, Jie Zhang* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China [email protected]

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ABSTRACT The effect of thermal annealing on the mechanical properties and microstructure of polymers has been studied intensively. Nevertheless, sparse work investigated the influence of annealing on the mechanical properties of materials with alternated structure. In this work, the evolution of properties and microstructure of iPP samples with different structure after annealing were discussed. Note that annealing plays a little role on microstructure of all the specimens, whereas its effect on the variation of impact strength of different samples does not show the similar trend. For sample CIM and V1, impact fracture toughness does not improve significantly after annealing within the range of temperature used. But the promotion of the impact strength of sample V2 is extremely remarkable (from 29 KJ/m2 to 90.5KJ/m2), which can be attributed to better interfacial adhesion, resulting in violent crack deflection and plastic deformation, the formation of microfibers and the better ability of load transfer. Keywords: iPP; alternated structure; thermal annealing; impact strength

INTRODUCTION Isotactic polypropylene(iPP), one of the most important polymeric materials, exhibits excellent performances, and has been widely used in many fields. However, its poor impact fracture toughness restricts the range of application. Hence, how to improve the impact strength of iPP has been attracted interest of academia and industry in the past decades. To achieve desired fracture toughness, several methods were applied to modify the iPP properties, e.g. the addition of β nucleating agent1-6, elastomer

7-8and

fiber9-12, and the formation of shish-kebab13-18. It is well known that iPP can crystallize into two distinctly different crystal morphologies under different conditions, namely spherulites and shish-kebab superstructures. The formation of spherulites is under quiescent crystallization conditions, while shish-kebabs under intensive shear or elongational flow condition1922.

Shish-kebab, which is regarded as a kind of self-reinforced superstructure, can

pronouncedly improve impact strength of the materials without sacrificing stiffness and modulus, unlike the addition of elastomer23-26.


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Materials with multilayer alternated structure were firstly found in natural shells. Inspired by such unique structure, some work paid attention to studying the effect of multilayer alternated structure on the comprehensive properties to provide guidance for the preparation of novel polymer materials27-31. The results showed that materials with such unique structure not only manifest high strength, modulus, but also excellent impact toughness. It is also found that the interfacial adhesion strength of multilayer alternated materials plays an important role on the enhancement of its mechanical properties. However, most materials with alternated structure were fabricated by thermodynamically immiscible components with different properties up to now, resulting in the inferior interfacial adhesion strength. Such poor interfacial strength of materials makes impact strength improve predominantly only when the layer number is very high32-33. An ideal approach to elevate obviously fracture toughness is that the materials with multilayer alternated structure are constituted by same components but different structure, leading to a suitable interfacial adhesion. Injection molding is one of the most common polymer processing methods. However, it is hardly used to fabricate products with multilayer alternated structure. Luckily, the limitation has been broken by our group using a technology named multiflow vibration injection molding (MFVIM)34. The product was of shear layer-spherulites layer alternated structure, and exhibited excellent properties. Thermal annealing, between the glass transition temperature (𝑇𝑔) and melting temperature (𝑇𝑚), is one of the common and effective ways to improve mechanical properties of polymer35-38. Until now, a large quantity of work has been carried out to study the influence of annealing on the evolution of microstructure and mechanical properties of iPP39-42. It is widely accepted that the motion ability of molecular chain would be more active with increasing annealing temperature, which can affect significantly the development of microstructure and properties of iPP. Some researchers investigated the influence of thermal annealing on the adhesion of partial miscible blends 43.The results indicated that molecules could interdiffuse through the interface and entangle with each other owing to the improved motion ability of the molecular chains or segments, leading to the enhancement of mechanical adhesion that



is responsible for the interfacial adhesion. As aforementioned, the effect of thermal annealing or alternated structure on the polymer properties has been studied at different levels. However, the impact of thermal annealing on the properties of materials with multilayer alternated structure has rarely been reported, especially for different layers constituted by identical components. In the present work, the preparation of iPP injection molded parts with different hierarchic structure was realized by using conventional injection molding (CIM) and selfdeveloped multiflow vibration injection molding. The iPP samples prepared by MFVIM own dramatically different microstructure compared to sample CIM showing typical skin-core structure. On the basis of our previous work44, we try to fabricate super toughened iPP specimen via combining the effects of shear layer-spherulites layer alternated structure and thermal annealing. Here, the evolution of mechanical properties and microstructure were detected by mechanical properties test, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and two-dimensional small angle X-ray scattering (2D-SAXS). The results demonstrate that thermal annealing has a key impact on mechanical properties of iPP sample with alternated structure, whereas there is little influence on crystalline morphology. The mechanisms of super toughness induced by thermal annealing and alternated structure are proposed. It can provide an effective and simple way to improve substantially the impact fracture toughness of iPP, which may also be suitable to other polymer materials with same structure. EXPERIMENTAL SECTION Material and Sample Preparation Isotactic polypropylene (trade name T30S), with the density of 0.910g/cm3 and melt flow index (MFI) of 2.90g/min (230℃, 2.16kg), was a commercial product of Lanzhou Petrochemical Company. Samples used in this study were made by multiflow vibration injection molding and conventional injection molding. More detail information about MFVIM can be found in our previous work34. The injection temperature was 160-200℃ from hopper to nozzle, and the mold temperature was fixed at 50℃. Two kinds of specimens, which were


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marked as V1, V2 respectively, were manufactured by tuning the MFVIM processing parameters, including pressure and interval time. Where V1 represents the specimen with simply increasing thickness of shear layer and V2 is the specimen with shear layerspherulites layer alternated structure. It should be noted that both samples have the same thickness of shear layer. The sample prepared by conventional injection molding was marked as CIM. The thermal annealing was conducted on an air-circulating oven at moderate temperatures (80-140℃) for 4h. After being annealed at different temperatures, the specimens were cooled down to ambient temperature in the oven. Polarized Optical Microscopy (POM) The microstructure of the samples was observed by a polarization optical microscope (DX-1, Jiang-Xi Phoenix Optical Company) equipped with a Canon 500D camera. Mechanical Testing The specimens for mechanical testing were cut from the samples (70mm 59mm3mm) along the flow direction. Notched Izod impact strength was measured using a XJUD5.5 Izod machine (Chengde Jinjian Testing Instrument Co. Ltd., Heibei, China) at ambient temperature. The specimen dimension was 60mm10mm3mm. Before the test, a 45 V-shaped notch (depth of 2mm and root radius of 0.25mm) was made. Tensile testing was conducted on a dumbbell-shaped specimen using an Instron 5567 tensile testing machine (USA). The crosshead speed was 50mm/min. The specimen was cut off according to ASTM D638-14, whose dimension was 3.18mm and 9.53mm for the width and length of narrow section, respectively. For each sample, the mean value reported was derived from at least five specimens. Scanning Electron Microscopy (SEM) To observe crystalline morphology clearly, the specimens were chemically etched in mixing acids at 60℃ for 7h to eliminate amorphous phase of iPP. For the exploration of toughening mechanism, the impact fracture surface was prepared by notched Izod impact strength test. After gold sputtering treatment, the etched surface and impact fracture surface were characterized by using a FEI (Nova Nano SEM450) SEM device with an acceleration voltage of 20 kV. Differential Scanning Calorimetry (DSC)



DSC scanning of the specimens was conducted on a TA DSC Q20 differential scanning calorimeter (TA Corporation, USA) in a dry nitrogen atmosphere. Specimens about 5−10 mg were heated from 40 to 200 °C at a heating rate of 10°C/min. The crystallinity of each sample was calculated by the following equation: 𝑋𝑐 =

∆𝐻𝑚 ∆𝐻𝑜𝑚

Where ∆𝐻𝑚represents the measured value of the enthalpy of fusion, ∆𝐻𝑜𝑚 means the fusion enthalpy of completely crystallized iPP. Here, the value of ∆𝐻𝑜𝑚 was selected as 207 J/g. X-ray Measurements The synchrotron X-ray experiment was conducted on the BL16B1 beam line in Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. The wavelength was 0.124 nm, and the rectangle-shape beam had dimensions of 0.5mm × 0.8mm. The sample-to-detector distance was 104 mm for WAXD and 1900mm for SAXS, respectively. When only α and β crystals exist in one sample, the relative amount of the β phase (Kβ) can be calculated with the following equation: 𝐾𝛽 =

𝐼𝛽(300) 𝐼𝛽(300) + 𝐼𝛼(110) + 𝐼𝛼(040) + 𝐼𝛼(130)

Where Iα (110), Iα (040), and Iα (130) are the intensities of the α-form peaks (110), (040), and (130) respectively, while Iβ (300) is the intensity of the β-form peak (300). Herman’s method is employed to evaluate the crystals orientation, which is defined as f=

3𝑐𝑜𝑠2 ― 1 2

In this equation, cos2θ is the orientation factor defined as 𝜋/2

𝑐𝑜𝑠2 =

∫0 𝐼(𝜃)𝑐𝑜𝑠2𝑠𝑖𝑛 𝑑 𝜋/2

∫0 𝐼(𝜃)sin 𝜃𝑑𝜃

where  is the angle between the molecular chain direction and the melt flow direction and I () is the scattering intensity at angle . When the c axes of all crystals are


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perfectly parallel or perpendicular to the flow direction, the value of the orientation parameter f is 1.0 or−0.5. A value of 0 means the orientation is completely random. The long period (𝐿𝑝) of all samples could be derived from one-dimensional electron density correlation function36. RESULTS AND DISCUSSION Crystalline structure Figure 1 shows the POM images of specimens fabricated by CIM and MFVIM respectively. The sample prepared by CIM has a typical skin-core structure, and the skin (shear layer) is thin (the ratio R of the thickness of shear layer to the whole sample is about 20%). The reasons for such structure are as following: the fast melt cooling and high shear rate simultaneously exist in the shear layer, while the center region suffers slow melt cooling and weak shear rate. Compared with CIM, sample V1 owns a largely increased thickness of shear layer, the R is about 56%. Specifically, although V2 has a similar R (54%) to V1, the distribution of shear layer is quite different. That is, V2 shows a unique shear layer-spherulites layer alternated structure.

Figure 1. POM photographs of different specimens. L1 and L3 represent the shear layer, L2 and L4 are spherulites layer. To further investigate the crystalline morphology, SEM was applied to characterize the microstructure of specimens. As Fig 2 shows, numerous closely packed shish-kebab aligned along the flow direction can be well observed in all the shear layers (L1, L3), while spherulites microstructure in other layers (L2, L4). As discussed above, it could be safely concluded that we successfully manufactured the specimens with the enhancement of the thickness of shear layer and shear layer-spherulites layer alternated structure by MFVIM.



Figure 2. SEM images of crystalline morphologies for sample CIM (a-b), V1(c-d) and V2 (e-h) respectively. Mechanical properties Figure 3(a) manifests the notched Izod impact strength of specimens with different microstructure as function of annealing temperature. The influence of thermal annealing on the impact strength of samples with different structure is not similar. The impact strength of CIM and V1 changes only slightly within the whole annealing temperature range. It is obviously, compared to other two samples, the promotion trend of impact strength of sample V2 is distinctly different when they were annealed at suitable temperature. Its impact strength improves gradually when the annealing temperature is below 100℃. Unexpectedly, the strength elevates predominantly with increasing annealing temperature when the temperature is between 100 and 130℃, after that it remains basically constant. Specifically, the impact strength of sample V2 annealed at 130℃ reaches up to 90.5 KJ/m2, which is 3.2, 7.7and 21.5 folds compared to virgin sample V2, V1 and CIM respectively. Even at the same annealing temperature, its impact strength is also 14.8 and 4.1 times of that of CIM and V1 respectively. It should be noted that such high impact strength has rarely been reported in the past. Figure 3(b) shows the variation of tensile strength with the promotion of annealing


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temperature. We can see that the tensile strength of all the specimens show only slight change with the elevation of annealing temperature. Interestingly, under any conditions, the tensile strength of sample V2 is always higher than that of sample V1. We, therefore, can know well from the above discussion that remarkable improvement of impact strength of iPP without the sacrifice of tensile strength could be achieved through combining the effects of shear layer-spherulites layer alternated structure and thermal annealing. It provides an easy and effective way to fabricate super toughened materials to meet the requirements of industry.

Figure 3.Mechanical properties of different samples with the variation of annealing temperature: (a)impact strength and (b) tensile strength It is widely accepted that the development of microstructure plays a key role on affecting mechanical properties of specimens. Since the change of impact strength of sample V2 is pronounced at 130℃, the effect of thermal annealing on the evolution of microstructure is subsequently ascertained at this temperature. Structural evolution upon annealing The DSC melting curves of iPP samples before and after annealing are presented in Figure 4, and the melting temperature (Tm) and the crystallinity (Xc) are listed in Table1. No prominent changes in Tm and Xc can be found for all specimens. The melting point is located between 164 and 166 ℃, and the variation of crystallinity is only within 1.5%, which implies that the differences of both crystallinity and melting point of all samples are limited. After annealing at 130℃, a relatively small shoulder peak exhibits in the DSC curves just a few degrees above the annealing temperature. It is originated from



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the melting of imperfect crystals or thin lamellae induced by the initial lamellas during annealing. In conclusion, thermal annealing has no distinct influence on the development of crystal structure.

Figure 4.DSC melting curves of the samples before and after annealing, CIM-A, V1-A and V2-A are the samples after annealing. Table 1. Variation of melting temperature (𝑇𝑚)and the degree of crystallinity (𝑋𝐶) of specimens before and after annealing. CIM

V1

V2

CIM-A

V1-A

V2-A

𝑇𝑚(℃)

164

166

165.8

165.3

165

165.4

𝑋𝑐(%)

43.5

43

44

42.5

43.5

44


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Figure 5.2D-WAXD patterns for specimens before and after annealing at different zones from surface to core. To verify the morphology of the samples before and after annealing, 2D-WAXD patterns, which can provide more detail information about the orientation degree and crystal form, was applied to investigate the evolution of microstructure of specimens at different zones. It is well known that the arc-like diffraction in 2D-WAXD patterns means the formation of highly oriented structure, e.g. shish-kebab or cylindrical morphology, while the isotropic diffraction ring belongs to the isotropic microstructure (spherulites). Fig 5 gives the 2D-WAXD patterns of each sample at different positions. More detail about sample structure could be drawn from 2D-WAXD patterns, Fig 6 and Fig 7 present the orientation degree and crystal modification of each sample at different regions from surface to core, respectively. From Figure 6, it can be well observed that



the orientation degree of specimens shows no distinct change after annealing compared to corresponding samples before thermal annealing. Furthermore, the values of the orientation degree calculated from 2D-WAXD patterns confirm the results of POM and SEM about the microstructure of specimens. For sample CIM, the orientation degree decreases from surface to core. For specimen V1, the orientation degree retains high level from surface to a certain distance, dropping dramatically at the core region, which indicates that this specimen owns the enhancement of the thickness of shear layer. Note that sample V2 shows shear layer-spherulites layer alternated structure confirmed by the result of the orientation degree. The orientation degree of all shear layers is similar, remaining about 0.9. It is widely accepted that β-crystal could affect the performance (especially the impact strength) of iPP, and can be induced by the addition of β-NA, shear flow and temperature gradient etc. Because there exists shear flow during the process of MFVIM, it is necessary to quantitatively analyze the content of β-crystal. Figure 7 shows the detail about crystal modification of each sample at different positions, and the content of β-crystal is listed in Table 2. From Figure 7 and Table 2, one can observe that the content of β crystal is low for all samples regardless thermal annealing or not. Hence, the influence of the variation of β crystal content before and after annealing on impact strength is limited.

Figure 6. The degree of orientation for each sample at different positions from surface to core.


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Figure 7.1D-WAXD curve of crystal structure of each sample at different positions from surface to core: (a) CIM, (b) V1, (c) V2, (d) CIM-A, (e)V1-A, (f) V2-A. Table 2. The content of β crystal (𝐾𝛽) for each specimen Surface

Core

CIM

2.6%

9.0%

0.5%

0%

0.3%

V1

1.1%

1.2%

2.7%

7.5%

0.1%

V2

0.4%

0.1%

2.7%

3.0%

0.6

CIM-A

1.5%

0.9%

0.5%

4.1%

2.9%

V1-A

0.6%

1.4%

3.9%

1.3%

2.1%

V2-A

0.8%

3.8%

0.1%

0.3%

6.2%



Figure 8.2D-SAXS patterns of specimens before and after annealing at different zones from surface to core.


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Figure 9. The curves of the one-dimensional electron density correlation function along the meridian for selected specimens (sample V1) before and after annealing. To gain further insight into the development of microstructure, some important parameters are given through one-dimensional electron density correlation function. Figure 8 and Figure 9 exhibit 2D-SAXS patterns and 1D-dimensional electron density correlation function along meridian of each specimen respectively. For the sake of simplicity, Fig 9 just presents results about the variation of the long period of selected sample (V1) before and after annealing. The patterns for other two samples are similar to those of V1 and are not shown here. We can clearly know that the long period of shish-kebab at the shear layer hardly changes distinctly, which just varies from 18.13nm to 17.97nm. The reason for the limited variation possibly is that the motion of molecular chains is restrained by shish-kebab during annealing. Although the long period of spherulites change more apparently than shish-kebab, which climbs from 13.86nm to 17.75nm, the influence of spherulites on the impact strength is extremely tiny, resulting from the inferior interactions among adjacent spherulites. Therefore, the effect of the change of the long period before and after annealing on the impact strength is negligible. So far we can get a conclusion that, the variations of crystallinity, orientation degree, the content of β crystal and the long period of samples may be the main reason for the difference of impact strength of sample CIM and V1 before and after annealing, but those cannot explain the reasons why the improvement of impact toughness of sample V2 after annealing is such significant. In order to get insight into understanding the relationship between alternated structure after annealing and mechanical properties, the impact fracture surface was well observed by SEM.



Figure 10.SEM images of impact fracture surface of each sample, the first picture is the whole impact fracture surface, the other pictures are taken from impact fracture surface at higher magnification. (A) CIM, (B) V1, (C) V2, (D)CIM-A, (E) V1-A, (F) V2-A. The impact direction is from left to right. The same column has the same scale bar. Fig10 (A) and (D) reveal that the plastic deformation of sample CIM and CIM-A is rarely intensive, i.e. the impact fracture surface is smooth. It indicates only a little impact energy can be absorbed, resulting in poor toughness of sample CIM with or without annealing. The different fracture surface of sample V1 can be observed clearly. Fig10 (b) illustrates that slight crack deflection and interface debonding occur but no obvious matrix is torn into microfibers at the interface. V1-A (Fig 10(E)) shows more


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rough impact fracture surface across the whole surface. More severe crack deflection and plastic deformation are formed at the shear layer, and microfibers induced by interface debonding are more obvious compared to sample V1. Furthermore, the plastic deformation at spherulites layer (Fig 10(e)) is more serious than that of sample V1. Interestingly, the distinctly different morphologies of the impact fracture surface of sample V2 (Fig 10(C) and (F)) with or without annealing can be found. For sample V2 (Fig 10(C)), violent crack deflection not only appears at the shear layer and interface but also at the spherulites layer close to sample surface (layer L2 in Fig 1), which can simultaneously facilitate the crack prolongation and plastic deformation. The cause of crack deflection is that the different load-bearing capacity of spherulites and shishkebab makes the direction of stress transfer change. While the impact fracture surface morphologies at the core region is similar to sample CIM. At higher magnification, the SEM images focusing on the morphologies at the interface between shear and spherulites layer reveal that little matrix is torn into microfibers, which indicates crack initiation and propagation occur easily along the interface, resulting from the poor interfacial strength between layers. Surprisingly, drastic crack deflection and plastic deformation appear at the whole impact fracture surface for sample V2-A. What is more, a large amount of matrix is torn into microfibers and interface debonding does not happen but twist in some places, interface debonding is less likely to occur, which demonstrate the good interfacial strength. We can speculate crack propagation process from the impact fracture surface, only interface debonding happened but no matrix is torn into microfibers when the crack extends to the interface for sample V2. For the sample V2-A, impact loading induced microfibers or twisting when the crack reached the interface. Different morphologies at the interface can be well observed in the positions marked by arrow in Figure 10 (C) and (F), the morphology of sample V2 is smooth but that of sample V2-A is rough. It is widely accepted that interface debonding with smooth interface means the weak interfacial adhesion. Hence, sample V2-A has better interfacial adhesion compared to sample V2. Some researchers have confirmed that the interfacial adhesion is related to the mold temperature and the thickness of shish-kebab layer 45. Here, the low mold temperature and the formation of shear layer



(layer L3 in Fig 1) are the reasons why the interfacial adhesion of virgin sample V2 is weak. The alteration of interfacial adhesion after annealing can also change the process of stress transfer, the loading could transfer farther owing to the better interfacial adhesion for sample V2-A. Hence, the reason why the impact strength of sample V2 after annealing climbing up to 90.5KJ/m2 can be explained by the following reasons: (1) Crack deflection at the whole impact fracture surface facilitates the crack prolongation and plastic deformation, leading to dissipate plenty of energy. (2) Much matrix is torn into microfibers at the interface, resulting in amounts of energy can be consumed. (3) The better interfacial interactions between the shear layer and spherulites layer that could transfer the load more efficiently. CONCLUSIONS The samples with distinctly different microstructure have been successfully fabricated by using CIM and MFVIM. The mechanical properties, thermal behavior and crystalline morphology of injection molded parts with or without annealing were investigated. The variation of impact toughness of specimen V2 is more remarkable than that of the other two samples (CIM and V1) after annealing at appropriate temperature. When annealing at low temperature (100℃), the impact strength of all samples increases slightly which could be ascribed to the poor motion ability of molecular chain induced by the low annealing temperature. When annealing at high temperature (100℃), the promotion of impact strength of CIM and V1 are still flat, but the improvement of fracture toughness of sample V2 is striking, resulting from more intensive crack deflection, plastic deformation, the formation of plenty of microfibers at the interface and the better ability of load transfer. This work proves that combining the effects of annealing at appropriate temperature and shear layer-spherulites layer alternated structure, for iPP, is an easy and effective way to promote the impact toughness significantly without sacrificing tensile strength. ACKNOWLEDGEMENTS The authors sincerely acknowledge the financial support of the National Natural Science Foundation of China (No. 0030905401227) and technical support of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) for help with X-ray measurements.


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F.,

Thermal

Annealing-Induced

Superior

Toughness

in

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