- Email: [email protected]
Morphology and mechanical properties of polyolefinic thermoplastic elastomer I. Characterization of deformation process
Polymer 45 (2004) 5301–5306 www.elsevier.com/locate/polymer
Morphology and mechanical properties of polyolefinic thermoplastic elastomer I. Characterization of deformation process Takuo Asamia,b, Koh-hei Nittab,c,* b
a Material Science Laboratory, Mitsui Chemicals Inc., 580-32 Nagaura, Sodegaura, 299-0265 Chiba, Japan School of Material Science, Japan Advanced Institute of Science and Technology (JAIST), 2-1 Asahidai, Tatsunokuchi, 923-1292 Ishikawa, Japan c Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, 920-8667 Kanazawa, Japan
Received 13 January 2004; received in revised form 6 May 2004; accepted 10 May 2004 Available online 11 June 2004
Abstract The structural origin of rubber elasticity in the polyolefinic thermoplastic elastomers composed of isotactic polypropylene (iPP) matrix and ethylene– propylene – diene rubber (EPDM) domains was investigated using scanning and transmission electron microscopes under uniaxial deformation and the computational analysis by a three dimensional finite element method. The rubber domains were dominantly deformed and elongated by accompanying localized yielding in iPP region between neighboring EPDM domains perpendicular to the stretching direction. The iPP region between adjacent EPDM domains in the stretching direction remained undeformed, suggesting that the undeformed iPP region plays the role in connecting rubber domains. q 2004 Elsevier Ltd. All rights reserved. Keywords: Polyolefinic thermoplastic elastomer; Morphology; Mechanical properties
1. Introduction Thermoplastic elastomers (TPEs) have been widely used as the materials in the automobile, industrial, and electrical products etc. These TPE materials, which are prepared by the dynamic vulcanization method [1], display unique mechanical behavior: TPE behaves like the rubbers around room temperature and thermoplastic resins in the higher temperatures. The TPEs are composite elastomeric materials consisting of a relatively high volume fraction of fully elastomeric domains in a continuous thermoplastic matrix. This morphological feature is produced by the process of vulcanizing a rubber component during its melt mixing with a thermoplastic resin. This work focuses on the polyolefinic TPE composed of isotactic polypropylene (iPP) matrix and ethylene – propylene – diene rubber (EPDM) domains. It is important morphological feature that the rubber phase is dispersed in the thermoplastic matrix although the content of rubber * Corresponding author. Address: Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920-8667, Japan. Tel.: þ81-76-234-4818; fax: þ 81-76-2344829. E-mail address: [email protected] (K. Nitta). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.05.017
usually exceeds or comparable with that of the thermoplastic resin. Therefore, the question arises as to why the TPE materials with ductile thermoplastic matrix exhibit rubber elasticity. A great deal of effort has been made to clarify the origins of the overall elastomeric stress –strain behavior of iPP/EPDM systems. However, the origins of the elasticity have not been clarified yet and several models have been proposed. The elastic – plastic analysis using FEM (finite element method) by Kikuchi et al. [2] indicated that the matrix between rubber domains in the stretching direction acts as an interconnector between adjacent rubber domains. Okamoto et al. [3,4] demonstrated that the origin of the rubber elasticity of TPE materials is caused by the difference in the Poisson ratio between the thermoplastic matrix resin and the rubber domains. Wright et al. [5,6] discussed the elastic mechanical response of iPP/EPDM thermoplastics with the microcellular modeling in which three types deformation such as elastic and plastic deformation of iPP, elastic deformation of EPDM, and localized elastic and plastic rotation about iPP junction points are considered. Boyce et al. [7 –9] simulated the loadextension curves under the compression and recovery deformation using one dimensional constitutive model and FEM, indicating that a pseudo-continues rubber phase
5302
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
develops as a result of the stretching of iPP ligaments and shear of rubber phases. The present study focuses on the morphological changes of an iPP/EPDM system during the uniaxial drawing process to explore the underlying mechanisms of elastic mechanical response of the immiscible thermoplastic elastomers. The morphological changes induced by the drawing process were directly viewed by electron microscopic methods. In addition, we conduct a three-dimensional FEM (finite element method) analysis on the tensile deformation of two-phase system. Comparing the computational results with the deformation behavior of the iPP/EPDM thermoplastic elastomers, we discuss the origin of their elasticity.
2. Experimental 2.1. Materials The thermoplastic elastomers studied in this work are iPP/EPDM systems in which a commercial grade of iPP was used as the matrix polymer and ethylene propylene based EPDM crosslinked by peroxide was used as the dispersed rubber domains. Hydrocarbon paraffin oil was added for a processing aid. For comparison, we examined the deformation behavior of pure iPP, oil extended iPP and chemical crosslinked EPDM materials. These materials were provided by Mitsui Chemicals, Inc. and the compositions of the samples used in this study are shown in Table 1. The molecular weight of iPP in TPE was Mw ¼ 130 K and Mn ¼ 44 K which were measured by the gel permission chromatography (Waters150C) after iPP was extracted from TPE by n-decane. 2.2. Tensile properties Load-extension curves were obtained by uniaxially stretching at 23 8C and at constant strain rate of 200 mm/min. Samples were prepared by comp-molding at 190 8C for 4 min and at 11 MPa pressure between two aluminum plates with 1 mm thickness spacer and then quenching in room temperature under 11 MPa. Tensile specimens are cut to the dumbbell shape No. 3 (1 mm thickness) of ISO 37 from the quenched sheets and tested on a tensile instrument (Toyo Seiki; Strograph AR).
Table 1 Sample formation of TPE and iPP/Oil samples
TPE-PP40 TPE-PP60 iPP/Oil
iPP (wt%)
EPDM (wt%)
Oil (wt%)
40 60 91
43 29
17 11 9
2.3. Scanning and transmission electron microscopies The microstructure of the uniaxially deformed TPE samples was directly observed by scanning electron microscope (JEOL JSM-5410) and transmission electron microscope (JEOL JEM-100). The sample specimens were stained by ruthenium(IV)oxide and were sliced by the ultra microtome on room temperature after drawn to strain ¼ 1.7, 5.5, and breaking point by the tensile instrument. The feature of drawn samples was photographed by a digital camera. 2.4. Finite element method analysis (FEM) The stress concentration state of iPP/EPDM TPE system was analyzed by FEM technique. The computer program ABAQUSe was used in this work. For calculation of iPP/EPDM TPE systems the dispersed rubber phase was assumed to be sphere shape, and one eighth model including two sphere particles in cubic was used for saving the calculation time. The tetra pod mesh form was made using ABAQUSe and the number of meshes was about 4300. The load-extension curves of oil extended iPP and of chemically crosslinked EPDM were used for the input data of the FEM analysis.
3. Results and discussion Fig. 1 shows the electron micrographs of the blend films, in which the dark regions correspond to the stained EPDM phase and bright regions to the semi-crystalline iPP. The EPDM rubber domains were found to be dispersed as several mm in the iPP matrix phase. Furthermore, we observed the morphology of several pieces sliced from one sample in order to three-dimensionally characterize the dispersability of EPDM domains, resulting in that EPDM domains were confirmed to be completely isolated in the iPP matrix for all TPE samples. Load-extension curves of TPE-PP40, TPE-PP60, and iPP, iPP/Oil ¼ 91/9, chemical crosslinked EPDM were shown in Fig. 2. The yield region was shown in the loadextension curves of iPP and iPP/Oil ¼ 91/9. The elongation at break of the iPP and the iPP/Oil ¼ 91/9 was relatively low and they broken in the brittle fashion. This is because the molecular weight of iPP used in this study was lower than that of iPP showing a high extensinability. The crosslinked EPDM exhibited high drawability but the stress level was much lower than that of iPP and a series of TPE samples. On the other hand, no clear yield point was observed but an inflection point was shown in the low strain region on the load-extension curves of the TPE samples. The stress level of the TPE samples was higher than that of crosslinked EPDM. It should be noted here that the TPE specimens drawn at a strain 1 ¼ 2:0 were completely recovered to the original shape after removing the tensile
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
Fig. 1. Transmission electron microscope pictures of (a) TPE-PP40 and (b) TPE-PP60 after staining the samples. The white portion denotes the iPP matrix, and the dark one denotes the chemically crosslinked EPDM domains.
machine, indicating that both TPE samples show considerably high drawability and elasticity in spite of brittle iPP matrix. The deformation-induced morphological change of TPEs was examined with SEM observation for drawn TPE
Fig. 2. Load-elongation curves of TPE-PP40, TPE-PP60, iPP, iPP/Oil ¼ 91/9, and chemically crosslinked EPDM. The tensile tests were performed at 23 8C and at the elongation speed of 200 mm/min.
5303
samples. Fig. 3 shows the SEM pictures of the central part of the drawn TPE-PP40 specimens, which were uniaxially stretched at strains of 1.7, 5.5 and 7.4. The feature of the drawn samples photographed by digital camera is shown in Fig. 4. The sample specimen drawn at strain 1 ¼ 1:7 was completely recovered to the original shape after removing from the tensile machine but the shape of the sample specimen drawn above strain ¼ 5.5 was not completely recovered. The drawn sample specimen after an applied strain of 5.5 and 7.4 was recovered to 1 ¼ 1:9; 5:3; respectively. The offset in deformations is a consequence of the permanent set imposed by the yield process of iPP matrix whereas the dispersed EPDM domains were found to be highly elongated as shown in Fig. 3(b) and (c). According to rough estimation of the extension of EPDM domains, the localized strain in the EPDM domains was found to be much greater than the applied overall strain. This means that the strain concentrated on the rubber domains. The microscopic morphological feature of drawn TPE samples was investigated with the TEM analysis. The thin sections cut from the central part of the drawn TPE samples, which were uniaxially extended at strains of 5.5, were used for the TEM observations. Fig. 5 compares the TEM pictures of undrawn TPE and drawn TPE-PP40 samples. The crystalline lamellar organization of iPP was clearly observed in the matrix phase of the undrawn TPE-PP40. The TEM picture of the drawn TPE-PP40 sample indicated that the dispersed EPDM domains (dark portion in TEM picture) are highly elongated in the extension direction and the drawn process promotes a rather heterogeneous deformation in the iPP matrix (see Fig. 5(b) and (c)). It should be noted here that the iPP matrix phase and the EPDM domains were completely bonded and the interfacial separation between iPP and EPDM phases were not observed in the drawn TPEPP. It is very importance to note that the micro craze-like fracture was clearly observed in the equatorial iPP region between neighboring elongated rubber domains and running in the perpendicular to the extension direction; moreover, the lamellar morphology of iPP was not clear in this area. This crazing process is very similar to the radial crazing which were observed in the yielding deformation of pure iPP spherulites [10]. On the other hand, the ligament portion of iPP matrix located between adjacent rubber domains in the stretching direction still remained undeformed. According to theoretical consideration for yielding process of spherulitic materials by Nitta-Takayanagi [11], a stack of interleaved crystalline lamellae and amorphous layers, i.e. the lamellar cluster, act as one unit during deformation and the intercluster links supporting external force play a role in the destruction of lamellar clusters at the yield point. These morphological features were directly confirmed from TEM observation of two-dimensional iPP spherulites [12]. The lamellar cluster thickness of the present iPP sample was estimated to be 30– 40 nm from single chain statistics using the molecular weight data of the
5304
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
Fig. 3. Scanning electron microscope pictures of the drawn TPE-PP40. (a) Load –elongation curves of TPE-PP40 measured at 23 8C and at the elongation speed of 200 mm/min are included. The SEM pictures of TPE-PP40 drawn at (b) strain ¼ 1.7, (c) strain ¼ 5.5, (d) strain ¼ 7.4.
iPP. Our understanding of the craze-like fracture observed in the iPP region between neighboring elongated rubber domains is that the stress is concentrated perpendicular to the cluster planes located between elongated rubber domains and the crazing progressed along the intercluster regions, resulting in that the intervals between micro crazes was in the same order as the lamellar cluster thickness. On the other hand, iPP lamellar morphology was clearly observed in the polar ligament region between adjacent rubber domains along the extension direction, suggesting that no failure occurred in this region as seen in Fig. 5(c).
Fig. 4. TPE-PP40 samples after drawing at (a) strain ¼ 1.7, (b) strain ¼ 5.5, (c) strain ¼ 7.4. The elongation was performed at 23 8C and the elongation speed of 200 mm/min.
Thus, the deformation concentrated on the equatorial iPP region between neighboring the rubber domains perpendicular to the extension direction and this leads to the localized yielding in this region. Considering that the rigidity of iPP phase is much greater than that of EPDM phase, the undeformed iPP region acts as a linkage with adjacent rubber domains in the stretching direction. As a result, the stress passes through the polar iPP region and the strain concentrates on the rubber domains, leading to a considerably high extension in the rubber domains. We conduct a three dimensional FEM analysis of a twophase model corresponding to TPE-PP40. According to TEM observation, the iPP matrix and the EPDM domains was assumed to be completely bonded for the FEM analysis. The unloading state of the model was shown in Fig. 6(a), in which the dark portion denotes the rubber phase. The loading state at 1 ¼ 1 was shown in Fig. 6(b) in which the rubber regions were eliminated in order to clearly identify the deformed state of matrix phase. The darker color indicates the more strain concentrated state. The FEM analysis demonstrated that the deformation concentrated on the iPP region between adjacent rubber domains as suggested by the two-dimensional FEM results [2]. This
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
5305
Fig. 6. Three dimensional FEM analysis for uniaxial extension of a TPE model corresponding to TPE-PP40. The dark region denotes the dispersed crosslinked EPDM phase, and the bright region the iPP matrix. The arrow denotes the stretching direction. (a) FEM model for undrawn TPE-PP40, (b) FEM model for drawn TPE-PP40 (strain ¼ 1). The darker color indicates the more strained state.
in the perpendicular to the stretching direction. Moreover, these results suggest that the yielding process in the equatorial iPP region between adjacent rubber domains takes place at relatively small overall strains.
4. Conclusion
Fig. 5. Transmission electron microscope pictures of TPE-PP40 after drawing at strain ¼ 5.5 The drawing was performed at 23 8C and the elongation speed of 200 mm/min. The EPDM rubber phases were stained. The arrow denotes the stretching direction. (a) Before extension, (b) equatorial iPP region between neighboring elongated rubber domains in the perpendicular to the extension direction, (c) polar iPP region between adjacent rubber domains in the extension direction.
computational result can explain the experimental results that the deformation state in iPP matrix is heterogeneous and the failure or yielding process takes place in the localized iPP portion between neighboring rubber domains
In this work, the deformation process in polyolefinic thermoplastic elastomers prepared by dynamic vulcanization of iPP/EPDM blends was investigated. The dynamic vulcanization yields an immiscible morphology that the EPDM domains are completely dispersed in the matrix of iPP. The craze-like fracture occurred in the equatorial iPP region neighboring elongated rubber domains in the perpendicular to the stretching direction and the polar ligament iPP region was undeformed as expected from the three dimensional FEM analysis. Consequently, the rubber domains are considered to be linked by the undeformed iPP portions along the strain direction. These results led us to conclude that the polar ligament region plays the role in stress transmission to EPDM rubber domains and the local yielding of the equatorial iPP region between neighboring rubber domains allows the EPDM domains to highly deform. The high drawability of EPDM
5306
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
domains leads to a rubber elasticity of the iPP/EPDM thermoplastic elastomers.
Acknowledgements The authors wish to acknowledge Mitsui Chemicals, Inc. for their financial support and their permission to publish this paper. The authors also thank to Mr T. Ando of JAIST, Messrs Y. Itou, M. Inaba, T. Terada of Mitsui Chemicals Inc., Messrs H. Tamura and H. Kodama of Mitsui Chemical Analysis and Consulting Service Inc. for their experimental support.
References [1] Coran AY, Legge NR, Holden G, Schroeder HE, editors. Thermo-
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
plastic elastomers, a comprehensive review. Munich: Hanser Publishers; 1987. Chapter 7. Kikuchi Y, Fukui T, Okada T, Inoue T. Polym Engng Sci 1991;31:14. Okamoto M, Shiomi K, Angola JC, Inoue T. IRC 95 1995;71. Okamoto M, Shiomi K, Inoue T. Polymer 1994;35:4618. Wright KJ, Lesser AJ. Rubber Chem Tech 2001;74(4):677. Wright KJ, Indukuri K, Lesser AJ. Polym Engng Sci 2003;43(3):531. Boyce MC, Kear K, Socrate S, Shaw K. J Mech Phys Solids 2001;49: 1073. Boyce MC, Socrate S, Kear K, Yeh O, Shaw K. J Mech Phys Solids 2001;49:1323. Boyce MC, Yeh O, Socrate S, Kear K, Shaw K. J Mech Phys Solids 2001;49:1343. Nitta K, Takayanagi M. J Mater Sci 2003;38:4889. Nitta K, Takayanagi M. J Macromol Sci, Phys 2003;B42:107. Takayanagi M, Nitta K, Kojima O. J Macromol Sci, Phys 2003;B42: 1049.
Morphology and mechanical properties of polyolefinic thermoplastic elastomer I. Characterization of deformation process Takuo Asamia,b, Koh-hei Nittab,c,* b
a Material Science Laboratory, Mitsui Chemicals Inc., 580-32 Nagaura, Sodegaura, 299-0265 Chiba, Japan School of Material Science, Japan Advanced Institute of Science and Technology (JAIST), 2-1 Asahidai, Tatsunokuchi, 923-1292 Ishikawa, Japan c Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, 920-8667 Kanazawa, Japan
Received 13 January 2004; received in revised form 6 May 2004; accepted 10 May 2004 Available online 11 June 2004
Abstract The structural origin of rubber elasticity in the polyolefinic thermoplastic elastomers composed of isotactic polypropylene (iPP) matrix and ethylene– propylene – diene rubber (EPDM) domains was investigated using scanning and transmission electron microscopes under uniaxial deformation and the computational analysis by a three dimensional finite element method. The rubber domains were dominantly deformed and elongated by accompanying localized yielding in iPP region between neighboring EPDM domains perpendicular to the stretching direction. The iPP region between adjacent EPDM domains in the stretching direction remained undeformed, suggesting that the undeformed iPP region plays the role in connecting rubber domains. q 2004 Elsevier Ltd. All rights reserved. Keywords: Polyolefinic thermoplastic elastomer; Morphology; Mechanical properties
1. Introduction Thermoplastic elastomers (TPEs) have been widely used as the materials in the automobile, industrial, and electrical products etc. These TPE materials, which are prepared by the dynamic vulcanization method [1], display unique mechanical behavior: TPE behaves like the rubbers around room temperature and thermoplastic resins in the higher temperatures. The TPEs are composite elastomeric materials consisting of a relatively high volume fraction of fully elastomeric domains in a continuous thermoplastic matrix. This morphological feature is produced by the process of vulcanizing a rubber component during its melt mixing with a thermoplastic resin. This work focuses on the polyolefinic TPE composed of isotactic polypropylene (iPP) matrix and ethylene – propylene – diene rubber (EPDM) domains. It is important morphological feature that the rubber phase is dispersed in the thermoplastic matrix although the content of rubber * Corresponding author. Address: Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920-8667, Japan. Tel.: þ81-76-234-4818; fax: þ 81-76-2344829. E-mail address: [email protected] (K. Nitta). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.05.017
usually exceeds or comparable with that of the thermoplastic resin. Therefore, the question arises as to why the TPE materials with ductile thermoplastic matrix exhibit rubber elasticity. A great deal of effort has been made to clarify the origins of the overall elastomeric stress –strain behavior of iPP/EPDM systems. However, the origins of the elasticity have not been clarified yet and several models have been proposed. The elastic – plastic analysis using FEM (finite element method) by Kikuchi et al. [2] indicated that the matrix between rubber domains in the stretching direction acts as an interconnector between adjacent rubber domains. Okamoto et al. [3,4] demonstrated that the origin of the rubber elasticity of TPE materials is caused by the difference in the Poisson ratio between the thermoplastic matrix resin and the rubber domains. Wright et al. [5,6] discussed the elastic mechanical response of iPP/EPDM thermoplastics with the microcellular modeling in which three types deformation such as elastic and plastic deformation of iPP, elastic deformation of EPDM, and localized elastic and plastic rotation about iPP junction points are considered. Boyce et al. [7 –9] simulated the loadextension curves under the compression and recovery deformation using one dimensional constitutive model and FEM, indicating that a pseudo-continues rubber phase
5302
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
develops as a result of the stretching of iPP ligaments and shear of rubber phases. The present study focuses on the morphological changes of an iPP/EPDM system during the uniaxial drawing process to explore the underlying mechanisms of elastic mechanical response of the immiscible thermoplastic elastomers. The morphological changes induced by the drawing process were directly viewed by electron microscopic methods. In addition, we conduct a three-dimensional FEM (finite element method) analysis on the tensile deformation of two-phase system. Comparing the computational results with the deformation behavior of the iPP/EPDM thermoplastic elastomers, we discuss the origin of their elasticity.
2. Experimental 2.1. Materials The thermoplastic elastomers studied in this work are iPP/EPDM systems in which a commercial grade of iPP was used as the matrix polymer and ethylene propylene based EPDM crosslinked by peroxide was used as the dispersed rubber domains. Hydrocarbon paraffin oil was added for a processing aid. For comparison, we examined the deformation behavior of pure iPP, oil extended iPP and chemical crosslinked EPDM materials. These materials were provided by Mitsui Chemicals, Inc. and the compositions of the samples used in this study are shown in Table 1. The molecular weight of iPP in TPE was Mw ¼ 130 K and Mn ¼ 44 K which were measured by the gel permission chromatography (Waters150C) after iPP was extracted from TPE by n-decane. 2.2. Tensile properties Load-extension curves were obtained by uniaxially stretching at 23 8C and at constant strain rate of 200 mm/min. Samples were prepared by comp-molding at 190 8C for 4 min and at 11 MPa pressure between two aluminum plates with 1 mm thickness spacer and then quenching in room temperature under 11 MPa. Tensile specimens are cut to the dumbbell shape No. 3 (1 mm thickness) of ISO 37 from the quenched sheets and tested on a tensile instrument (Toyo Seiki; Strograph AR).
Table 1 Sample formation of TPE and iPP/Oil samples
TPE-PP40 TPE-PP60 iPP/Oil
iPP (wt%)
EPDM (wt%)
Oil (wt%)
40 60 91
43 29
17 11 9
2.3. Scanning and transmission electron microscopies The microstructure of the uniaxially deformed TPE samples was directly observed by scanning electron microscope (JEOL JSM-5410) and transmission electron microscope (JEOL JEM-100). The sample specimens were stained by ruthenium(IV)oxide and were sliced by the ultra microtome on room temperature after drawn to strain ¼ 1.7, 5.5, and breaking point by the tensile instrument. The feature of drawn samples was photographed by a digital camera. 2.4. Finite element method analysis (FEM) The stress concentration state of iPP/EPDM TPE system was analyzed by FEM technique. The computer program ABAQUSe was used in this work. For calculation of iPP/EPDM TPE systems the dispersed rubber phase was assumed to be sphere shape, and one eighth model including two sphere particles in cubic was used for saving the calculation time. The tetra pod mesh form was made using ABAQUSe and the number of meshes was about 4300. The load-extension curves of oil extended iPP and of chemically crosslinked EPDM were used for the input data of the FEM analysis.
3. Results and discussion Fig. 1 shows the electron micrographs of the blend films, in which the dark regions correspond to the stained EPDM phase and bright regions to the semi-crystalline iPP. The EPDM rubber domains were found to be dispersed as several mm in the iPP matrix phase. Furthermore, we observed the morphology of several pieces sliced from one sample in order to three-dimensionally characterize the dispersability of EPDM domains, resulting in that EPDM domains were confirmed to be completely isolated in the iPP matrix for all TPE samples. Load-extension curves of TPE-PP40, TPE-PP60, and iPP, iPP/Oil ¼ 91/9, chemical crosslinked EPDM were shown in Fig. 2. The yield region was shown in the loadextension curves of iPP and iPP/Oil ¼ 91/9. The elongation at break of the iPP and the iPP/Oil ¼ 91/9 was relatively low and they broken in the brittle fashion. This is because the molecular weight of iPP used in this study was lower than that of iPP showing a high extensinability. The crosslinked EPDM exhibited high drawability but the stress level was much lower than that of iPP and a series of TPE samples. On the other hand, no clear yield point was observed but an inflection point was shown in the low strain region on the load-extension curves of the TPE samples. The stress level of the TPE samples was higher than that of crosslinked EPDM. It should be noted here that the TPE specimens drawn at a strain 1 ¼ 2:0 were completely recovered to the original shape after removing the tensile
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
Fig. 1. Transmission electron microscope pictures of (a) TPE-PP40 and (b) TPE-PP60 after staining the samples. The white portion denotes the iPP matrix, and the dark one denotes the chemically crosslinked EPDM domains.
machine, indicating that both TPE samples show considerably high drawability and elasticity in spite of brittle iPP matrix. The deformation-induced morphological change of TPEs was examined with SEM observation for drawn TPE
Fig. 2. Load-elongation curves of TPE-PP40, TPE-PP60, iPP, iPP/Oil ¼ 91/9, and chemically crosslinked EPDM. The tensile tests were performed at 23 8C and at the elongation speed of 200 mm/min.
5303
samples. Fig. 3 shows the SEM pictures of the central part of the drawn TPE-PP40 specimens, which were uniaxially stretched at strains of 1.7, 5.5 and 7.4. The feature of the drawn samples photographed by digital camera is shown in Fig. 4. The sample specimen drawn at strain 1 ¼ 1:7 was completely recovered to the original shape after removing from the tensile machine but the shape of the sample specimen drawn above strain ¼ 5.5 was not completely recovered. The drawn sample specimen after an applied strain of 5.5 and 7.4 was recovered to 1 ¼ 1:9; 5:3; respectively. The offset in deformations is a consequence of the permanent set imposed by the yield process of iPP matrix whereas the dispersed EPDM domains were found to be highly elongated as shown in Fig. 3(b) and (c). According to rough estimation of the extension of EPDM domains, the localized strain in the EPDM domains was found to be much greater than the applied overall strain. This means that the strain concentrated on the rubber domains. The microscopic morphological feature of drawn TPE samples was investigated with the TEM analysis. The thin sections cut from the central part of the drawn TPE samples, which were uniaxially extended at strains of 5.5, were used for the TEM observations. Fig. 5 compares the TEM pictures of undrawn TPE and drawn TPE-PP40 samples. The crystalline lamellar organization of iPP was clearly observed in the matrix phase of the undrawn TPE-PP40. The TEM picture of the drawn TPE-PP40 sample indicated that the dispersed EPDM domains (dark portion in TEM picture) are highly elongated in the extension direction and the drawn process promotes a rather heterogeneous deformation in the iPP matrix (see Fig. 5(b) and (c)). It should be noted here that the iPP matrix phase and the EPDM domains were completely bonded and the interfacial separation between iPP and EPDM phases were not observed in the drawn TPEPP. It is very importance to note that the micro craze-like fracture was clearly observed in the equatorial iPP region between neighboring elongated rubber domains and running in the perpendicular to the extension direction; moreover, the lamellar morphology of iPP was not clear in this area. This crazing process is very similar to the radial crazing which were observed in the yielding deformation of pure iPP spherulites [10]. On the other hand, the ligament portion of iPP matrix located between adjacent rubber domains in the stretching direction still remained undeformed. According to theoretical consideration for yielding process of spherulitic materials by Nitta-Takayanagi [11], a stack of interleaved crystalline lamellae and amorphous layers, i.e. the lamellar cluster, act as one unit during deformation and the intercluster links supporting external force play a role in the destruction of lamellar clusters at the yield point. These morphological features were directly confirmed from TEM observation of two-dimensional iPP spherulites [12]. The lamellar cluster thickness of the present iPP sample was estimated to be 30– 40 nm from single chain statistics using the molecular weight data of the
5304
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
Fig. 3. Scanning electron microscope pictures of the drawn TPE-PP40. (a) Load –elongation curves of TPE-PP40 measured at 23 8C and at the elongation speed of 200 mm/min are included. The SEM pictures of TPE-PP40 drawn at (b) strain ¼ 1.7, (c) strain ¼ 5.5, (d) strain ¼ 7.4.
iPP. Our understanding of the craze-like fracture observed in the iPP region between neighboring elongated rubber domains is that the stress is concentrated perpendicular to the cluster planes located between elongated rubber domains and the crazing progressed along the intercluster regions, resulting in that the intervals between micro crazes was in the same order as the lamellar cluster thickness. On the other hand, iPP lamellar morphology was clearly observed in the polar ligament region between adjacent rubber domains along the extension direction, suggesting that no failure occurred in this region as seen in Fig. 5(c).
Fig. 4. TPE-PP40 samples after drawing at (a) strain ¼ 1.7, (b) strain ¼ 5.5, (c) strain ¼ 7.4. The elongation was performed at 23 8C and the elongation speed of 200 mm/min.
Thus, the deformation concentrated on the equatorial iPP region between neighboring the rubber domains perpendicular to the extension direction and this leads to the localized yielding in this region. Considering that the rigidity of iPP phase is much greater than that of EPDM phase, the undeformed iPP region acts as a linkage with adjacent rubber domains in the stretching direction. As a result, the stress passes through the polar iPP region and the strain concentrates on the rubber domains, leading to a considerably high extension in the rubber domains. We conduct a three dimensional FEM analysis of a twophase model corresponding to TPE-PP40. According to TEM observation, the iPP matrix and the EPDM domains was assumed to be completely bonded for the FEM analysis. The unloading state of the model was shown in Fig. 6(a), in which the dark portion denotes the rubber phase. The loading state at 1 ¼ 1 was shown in Fig. 6(b) in which the rubber regions were eliminated in order to clearly identify the deformed state of matrix phase. The darker color indicates the more strain concentrated state. The FEM analysis demonstrated that the deformation concentrated on the iPP region between adjacent rubber domains as suggested by the two-dimensional FEM results [2]. This
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
5305
Fig. 6. Three dimensional FEM analysis for uniaxial extension of a TPE model corresponding to TPE-PP40. The dark region denotes the dispersed crosslinked EPDM phase, and the bright region the iPP matrix. The arrow denotes the stretching direction. (a) FEM model for undrawn TPE-PP40, (b) FEM model for drawn TPE-PP40 (strain ¼ 1). The darker color indicates the more strained state.
in the perpendicular to the stretching direction. Moreover, these results suggest that the yielding process in the equatorial iPP region between adjacent rubber domains takes place at relatively small overall strains.
4. Conclusion
Fig. 5. Transmission electron microscope pictures of TPE-PP40 after drawing at strain ¼ 5.5 The drawing was performed at 23 8C and the elongation speed of 200 mm/min. The EPDM rubber phases were stained. The arrow denotes the stretching direction. (a) Before extension, (b) equatorial iPP region between neighboring elongated rubber domains in the perpendicular to the extension direction, (c) polar iPP region between adjacent rubber domains in the extension direction.
computational result can explain the experimental results that the deformation state in iPP matrix is heterogeneous and the failure or yielding process takes place in the localized iPP portion between neighboring rubber domains
In this work, the deformation process in polyolefinic thermoplastic elastomers prepared by dynamic vulcanization of iPP/EPDM blends was investigated. The dynamic vulcanization yields an immiscible morphology that the EPDM domains are completely dispersed in the matrix of iPP. The craze-like fracture occurred in the equatorial iPP region neighboring elongated rubber domains in the perpendicular to the stretching direction and the polar ligament iPP region was undeformed as expected from the three dimensional FEM analysis. Consequently, the rubber domains are considered to be linked by the undeformed iPP portions along the strain direction. These results led us to conclude that the polar ligament region plays the role in stress transmission to EPDM rubber domains and the local yielding of the equatorial iPP region between neighboring rubber domains allows the EPDM domains to highly deform. The high drawability of EPDM
5306
T. Asami, K.-h. Nitta / Polymer 45 (2004) 5301–5306
domains leads to a rubber elasticity of the iPP/EPDM thermoplastic elastomers.
Acknowledgements The authors wish to acknowledge Mitsui Chemicals, Inc. for their financial support and their permission to publish this paper. The authors also thank to Mr T. Ando of JAIST, Messrs Y. Itou, M. Inaba, T. Terada of Mitsui Chemicals Inc., Messrs H. Tamura and H. Kodama of Mitsui Chemical Analysis and Consulting Service Inc. for their experimental support.
References [1] Coran AY, Legge NR, Holden G, Schroeder HE, editors. Thermo-
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
plastic elastomers, a comprehensive review. Munich: Hanser Publishers; 1987. Chapter 7. Kikuchi Y, Fukui T, Okada T, Inoue T. Polym Engng Sci 1991;31:14. Okamoto M, Shiomi K, Angola JC, Inoue T. IRC 95 1995;71. Okamoto M, Shiomi K, Inoue T. Polymer 1994;35:4618. Wright KJ, Lesser AJ. Rubber Chem Tech 2001;74(4):677. Wright KJ, Indukuri K, Lesser AJ. Polym Engng Sci 2003;43(3):531. Boyce MC, Kear K, Socrate S, Shaw K. J Mech Phys Solids 2001;49: 1073. Boyce MC, Socrate S, Kear K, Yeh O, Shaw K. J Mech Phys Solids 2001;49:1323. Boyce MC, Yeh O, Socrate S, Kear K, Shaw K. J Mech Phys Solids 2001;49:1343. Nitta K, Takayanagi M. J Mater Sci 2003;38:4889. Nitta K, Takayanagi M. J Macromol Sci, Phys 2003;B42:107. Takayanagi M, Nitta K, Kojima O. J Macromol Sci, Phys 2003;B42: 1049.