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Processing temperature dependent mechanical response of a thermoplastic elastomer with low hard segment
Polymer 53 (2012) 4310e4317
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Polymer journal homepage: www.elsevier.com/locate/polymer
Processing temperature dependent mechanical response of a thermoplastic elastomer with low hard segment Yongsheng Zhao, Nanying Ning, Xin Hu, Yuhan Li, Feng Chen*, Qiang Fu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2012 Received in revised form 17 May 2012 Accepted 8 July 2012 Available online 24 July 2012
The mechanical responses including monotonic and cyclic tensile responses have been investigated on a microphase-separated poly (styrene-isoprene-styrene) triblock copolymer (SIS). The specimens were injection-molded by using different melt temperatures to acquire different microphase structures. As a result of temperature-dependent segregation driving force, the specimens with reduced microphase separation can be obtained by increasing processing melt temperature from 180 C to 240 C. On the basis of stress-strain behavior, Young’s modulus was found to increase with increasing PS domain continuity in the order of disorder state to disordered spheres to body-cubic-centered (BCC) spheres to oriented cylinders morphology. Meanwhile, cyclic hysteresis decreases with reduced microphase separation and with decreasing the applied predetermined maximum tensile strain. In addition, the MooneyeRivlin phenomenological approach was used to evaluate and explore the relationship between the polymer topological networks and the rubber elasticity of thermoplastic elastomers. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Microphase separation Mechanical response Rubber elasticity
1. Introduction Thermoplastic elastomers (TPE) are unique polymer materials possessing both elastomeric and thermoplastic feature [1e5]. Representative commercialized thermoplastic elastomers like styrenic block copolymers [6e8], thermoplastic polyurethanes [9], polyolefin elastomers are mainly block copolymers [10e13]. These materials are consisted of distinct homopolymers that are covalently linked and have been used extensively as thermoplastic elastomers due to their elastic attribute [14e16], shape memory feature [17,18] and good processability. Generally, block copolymers undergo microphase separation and spontaneous self-assemble into a variety of nanoscale periodic structures such as spheres, hexagonally packed cylinders, interpenetrating double gyroid morphologies and alternating lamellae [19,20]. Moreover, the resultant phase morphology of some TPEs is profoundly influenced by the processing methods and thermal history. As is reported in some previous studies [21,22], Hashimoto et al. conducted the Tdrop experiments for solution-casted films and observed different morphologies under different temperatures. Therefore, studies on the unique morphologies and corresponding mechanical behaviors of TPEs are undoubtedly important for fundamental understanding and practical application. * Corresponding authors. Fax: þ86 28 85405401. E-mail addresses: [email protected] (F. Chen), [email protected] (Q. Fu). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.07.016
Attempts to interpret the mechanical response of TPEs with various morphologies in terms of their microdomain structure and phase behavior thus become a hot topic [23e25]. C. Honeker et al. [26] have examined the mechanical properties of SIS with the double-gyroid cubic phase which exhibited necking and drawing and higher strength than other morphologies. Takahashi and Matsushita [27] have reported the elongation behavior of sphereformed triblock copolymer with different bridge fractions and they found that the bridge fraction of the middle block had an effect on the restoring force of lattice. Hotta et al. [19] have investigated the phase morphology and the mechanical properties of SIS triblock copolymers and provided a discussion on the relationship between properties and microstructures based on the stress-strain tests and time-temperature experiments. Several attempts [28e30] have also been made to develop a molecular description on the molecular mechanism of rubber elasticity such as slip-tube model [31] and MooneyeRivlin model [32]. From the point of industrial view, it is of fatal importance to establish mechanisms of regulating microphase structure in thermoplastic elastomers under melt processing strategies like injection molding which possesses ultrahigh production efficiency and are especially suitable for abundant manufacture. To the best of our knowledge, non-equilibrium processing conditions including temperature and external stress field must involve various structural arrangements on nanoscale in TPEs, which is yet poorly understood. In our work, we use a commercial SIS block copolymer
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with low polystyrene weight fraction as a model material to investigate the mechanical response during systematic deformation process. We focus on the effect of processing temperature on the phase morphology, how the morphology impacts the mechanical performance and exploring the relationship between topological network structure and the rubber elasticity of SIS triblock copolymer.
a fixed crosshead speed of 50 mm/min and five specimens were tested for each group. The relaxation behavior of these samples was investigated via tensile stress relaxation tests on this tensile testing machine. The relaxation of the stress was recorded after the samples were rapidly stretched to 300% deformation and fixed.
2. Experimental section
Pseudo-cyclic tests were also performed on an SANS Universal tensile testing machine under the program of the tensile loadingeunloading cycles in which the samples were cycled for three times after being initially stretched to various predetermined strains and immediately unloaded at a fixed tensile and unloading velocity of 500 mm/min.
2.1. Materials and sample preparation The material used in this study is a commercial SIS triblock copolymer under the trade name Vector 4111 (Dexco Polymers Co.). Vector 4111 is a linear, symmetric, pure SIS triblock copolymer which has a weight-averaged molecular weight, Mw, equaling to 1.4 105 g/mol, and the polydispersity index, Mw/Mn, is 1.11. The weight fraction and volume fraction of PS are 0.183 and 0.164, respectively. It is well-suited for use in elastomeric film compounds and in formulating pressure-sensitive adhesives. The granules were injection-molded into dumbbell-shaped specimens via an SZ 100 g injection-molding machine under different barrel temperature which is selected as 180 C, 200 C, 220 C and 240 C respectively. So the samples are nominated as SIS-180, SIS-200, SIS-220 and SIS-240 for simplicity. In addition, the sample size is 100 10 4.3 mm3. 2.2. Structure analysis Ultrathin sections of ca 80 nm thickness were obtained by cryomicrotoming along the flow direction of the samples using a Leica EMUC6/FC6 microtome at 100 C. These sections were then collected and stained for 20 min in a w2% aqueous solution of osmium tetroxide (OsO4) that selectively stained the polyisoprene (PI) microdomains. Transmission Electron Microscopy (TEM) was performed with an FEI-Tecnai G2 F20 S-TWIN type transmission microscope operating at 200 kV. Small angle X-ray scattering (SAXS) measurements were performed to confirm the initial microphase-separated structures of these samples. The samples were observed with the X-ray beam along three different directions to gain through, end and edge view of 2d-SAXS profiles. The X-ray wavelength was 0.154 nm, and a CCD detector was employed to collect two-dimensional (2D) SAXS patterns. The sample-to-detector distance was 2300 mm Fit2D software from European Synchrotron Radiation Facility was used to analyze SAXS patterns in term of the scattering vector q ¼ 4p sin q/l with 2q as the scattering angle and as the X-ray wavelength. 2DSAXS data were integrated about the azimuthal angle and the background scattering was subtracted to achieve the scattering intensity as a function of scattering vector.
2.5. Cyclic tests
3. Results and discussion 3.1. Monotonic tension response of SIS The uniaxial deformation behavior of Vector 4111 obtained under different temperature has been investigated theoretically. Fig. 1 displays the nominal stress-strain curves for SIS triblock copolymer processed at a series of non-equilibrium processing temperatures. A notable feature in monotonic tension is that the tensile strength and Young’s modulus significantly depend on the processing temperature. Owing to the measuring range limitation of tensile testing machine, these injection-molded specimens were stretched to a large strain (600%) instead of fracture. The detailed values of each sample’s elastic modulus and stress at 300% strain are listed in Table 1. As can be seen from the data in Table 1, sample SIS-180 has the highest Young’s modulus about 8.8 MPa, which means an almost 44% increase compared to the sample SIS-200. With further increasing the processing temperature, Young’s modulus decreases and SIS-240 owns the lowest modulus. In addition, the stress at 300% strain (MPa) can be used to characterize the strength of all these specimens and SIS-180 has the highest value as expected. In general, higher processing temperature goes against excellent mechanical properties for styrenic block copolymers. 3.2. Viscoelastic response and relaxation behavior Fig. 2 illustrates the evolution of the loss tan d for four samples as a function of temperature from 100 C to 120 C. The dynamic
2.3. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was performed with a TA Q-800 type machine to assess the glass transition behavior and the viscoelastic properties of different samples. The measurements were performed under single cantilever mode with a frequency of 1 Hz from 100 C to 120 C at a heating rate of 3 C/min. The dimensions of the tested samples are 17.5 10 4.3 mm3. 2.4. Tensile tests Monotonic tensile tests were performed on a SANS Universal tensile testing machine according to the GB/T 528-2009 standard. All the tests were conducted at ambient temperature (23 C) at
Fig. 1. Tensile stress-strain relations of the injection-molded specimens at ambient temperature obtained under different processing temperatures.
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Table 1 Young’s modulus and stress at 300% strain for different specimens. Samples
Young’s modulus (MPa)
SIS-180 SIS-200 SIS-220 SIS-240
8.8 6.1 3.6 2.0
0.2 0.3 0.1 0.1
Stress at 300% strain (MPa) 1.9 1.7 1.4 1.2
0.1 0.1 0.1 0.1
Possible structure
Orientation factor (F)
Oriented cylinders BCC spheres Disordered spheres Disorder state
0.957 0.954 0.948 0.878
mechanical behavior is typical of SIS and shows two glass transitions and a rubbery plateau. The lower glass transition temperature is that of the PI block (TgPI z 40 C) while the higher glass transition temperature at about 100 C is that of the PS block. It is interesting to note that the tan d peak intensity of PI block increases with increasing the processing temperature while that of PS block is opposite, which can be attributed to the segregation power (c) and the amount of PS incorporated in the PI block. Meanwhile, there is a slight increase in the glass transition temperature of the soft phase and a decrease of the whole relaxation time of the elastomers in theory when the short PS chains or chain segments intermix into the PI phase [33]. The more PS segments mixing into the PI phase, the larger the interface between them can be and the more influence on the extent and efficiency of the stress transfer. On the other hand, the plateau region of the storage modulus vs. temperature is wide and flat when the two phases strongly segregated from each other. A significant feature of sample SIS-240 is that the peak at PS glass transition diminished, which indicates PS and PI blocks turn into a near-homogenous state. Above all, low segregation power and fast quench introduced by injection molding go against fully separation into distinct phase microdomains for the two chemical linked blocks and thus result in a wider interface [34]. The interface width between PS and PI phase dramatically improves with increasing processing temperature. Generally, styrene segments in the vicinity of the interface have higher degree of freedom and greater mobility than those in the interior. As a consequence, the ductility of PS phase induced by good mobility weakens the tensile strength of SIS block copolymers obtained under high temperatures. Tensile relaxation tests were conducted as a complementary experiment so as to investigate the elastic and recovery behavior [35]. Fig. 3 shows the evolution of the normalized stress after
Fig. 3. Normalized stress measured after stretching to a fixed strain ε ¼ 300% at room temperature.
a 300% tensile deformation characterized via tensile stress relaxation tests. It is found that SIS-180 has the highest rate of stress relaxation among these injection-molded styrenic block copolymers while this material relaxes more slowly when processed under higher temperature. Stress relaxation of styrenic block copolymers is mainly caused by the physical flow of the elastomeric phase and trapped entanglements, the conformational change of phenyl group in hard domain and possible PS pullout from microdomains under stress [36]. At room temperature, the glassy PS holds the soft PI segments and thus acts as an effective crosslink for the PI network and a stress bearing media. In consideration of the situation of PS chains being pulled out of the glassy domains, one can easily understand the relaxation mechanism of stress reduction due to decreased fraction of bridging type of PI block. It has been demonstrated that the physical flow is the fastest relaxation process while the relaxation of PS hard domains is the slowest. It also means that the stress relaxation at room temperature may be on a time scale much shorter than that of a PS chain pullout from glassy crosslinks. So the difference in relaxation behavior can be attributed to the difference in entanglement density and numbers of effective PI chains. 3.3. Pseudo-cyclic responses and hysteresis during cycles
Fig. 2. Temperature dependence of loss tan d of SIS samples processed under different temperatures: (1) SIS-180, (2) SIS-200, (3) SIS-220, (4) SIS-240.
A typical feature of this TPE is the pronounced stress softening during quasi-static deformation, which is also termed as the Mullins effect. Under cyclic deformation, the stress monotonically decreases with numbers of cycles. Mullins phenomenon is always accompanied by stress relaxation, material anisotropy and formation of microvoids [37]. Most of the softening occurs in the first deformation and approaches a steady state with a constant cyclic stress-strain behavior after a few deformation cycles. It can be seen from Fig. 4 that all these samples consistently show obvious irreversibility during the cycle between the loading and subsequent unloading pathway. Moreover, the tensile modulus of each loading cycle becomes significantly lower with increasing number of cycles and a residual elongation is induced after each cycle which is due to the deformation of PS domains and preserved localized heterogeneous regions in PI phase. The sample SIS-180 which has the uppermost strength also shows the largest Mullins effect while SIS240 shows the least Mullins effect. This indicates that the sample prepared under higher temperature always has a softer elastic response under deformation. Besides, all the samples show a strain hardening phenomenon in large deformation after several cycles.
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Fig. 4. Pseudo-cyclic tests for all SIS specimens: (a) SIS-180, (b) SIS-200, (c) SIS-220, (d) SIS-240.
In consideration of the different well-organized structure of hard segment (PS), the varying degrees of Mullins effect can be attributed to the stiffness and the connectivity of the PS domain. Previous literatures [37,38] have demonstrated that the extent of Mullins effect increases with the domain diameter and the dspacing. At a higher processing temperature, the block chains are not easily segregated away from the interface and thus results in a thicker domain boundary and a smaller domain size. The feature of domain boundary determines the stress transfer efficiency between the rubbery phase and the hard phase while PS domain size is of fatal importance for the total stiffness or reinforcing effect. In this section, we apply a more quantitative way to analyze the hysteresis via the following equation:
even microvoids in the soft phase (PI) which cannot immediately generate self-healing and also indicates some degree of heterogeneity in the topological network. In a word, the samples obtained under lower processing temperatures show higher modulus, faster relaxation and larger hysteresis. To better understand the origin of different mechanical responses of these samples, we conduct TEM and SAXS tests to dig detailed information on microphase structures and orientation. 3.4. Effect of processing temperature on phase morphologies in SIS
(1)
Fig. 6 displays TEM images of injection-molded samples under different temperatures. TEM images indicate that there are significant differences in the microphase structures for these samples. SIS-180 owns a highly oriented cylindrical structure while SIS-200
In other words, hysteresis can be calculated by subtracting the area of unloading curve at a fixed preconditioned strain from that of loading curve. Fig. 5 displays the quantized hysteresis as a function of numbers of cycles and quench depth which is equivalent to the gap between the processing temperature and the glass transition temperature of PS block. Quench depth is also a quantitative representation of the thermodynamic driving force for phase separation. It can be concluded that deep quench is in favor of decreasing the hysteresis and the hysteresis augments with increasing the preconditioned strain and number of cycles. As a matter of fact, hysteresis or dissipated energy for filler-rubber composites under cyclic loading is adequately elaborated by considering breakage and restoration of aggregates of fillers. Similarly, PS domains in SIS triblock copolymers exactly play the role of reinforcing rubbery matrix as well as physical junctions. Nevertheless, there are some key factors for the energy loss including rupture in PS domains and structural breakup such as fracture of polymer chains occurring when the chains are fully stretched and stress relaxation during entire tensile stretching. To our knowledge, deformation also can cause density fluctuation and
Fig. 5. 3d plot of hysteresis as a function of pre-strain and quench depth for all SIS samples.
Z Hysteresis ¼
Z
sloading dε
sunloading dε
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Fig. 6. TEM images of injection-molded samples obtained under different processing temperatures. The dark region represents PI domains as OsO4 selectively stains isoprene phase.
has spheres packed on a BCC lattice. Disordered spheres form in SIS-220 when the temperature is higher than lattice disordering transition temperature (TLDT). SIS-240 shows wormlike structure or even disorder zone where polystyrene (PS) blocks cannot separate from polyisoprene (PI) domains. In this study, we choose 240 C to obtain a transit structure between order and disorder phase instead of a temperature higher than TODT (TODT ¼ w275 C for the material in our work) in consideration of possible degradation and chemical crosslinking caused by ultrahigh processing temperature. Fig. 7 shows the SAXS intensities, I, as a function of scattering vector q for injection-molded styrenic thermoplastic elastomers. Comparing with other samples processed under higher temperatures, SIS-180 has high-order scattering peaks at q ¼ O3qm and q ¼ O4qm relative to that of the first-order maxima qm. This indicates that a hexagonally packed cylindrical morphology had formed in SIS-180 on the basis of the classical researches done by Norihiro Sota et al. [23]. Red circles in Fig. 7 represents the qspacing profile of SIS-200 which suggests that spherical microdomains with a cubic lattice exist according to the appearing highorder peaks at q ¼ O2qm and q ¼ O3qm. The intensity peak at q ¼ O3qm still exists in sample SIS-220, which means it has disordered spheres while this intensity peak weakens obviously in SIS240 which suggests a near-disorder state when processed under a high temperature close to the material’s temperature of order-todisorder transition (TODT). So these samples finally have microphase structures far away from the themodynamic equilibrium state because of fast quench introduced by injection molding. As a whole, we achieve a series of phase morphologies including oriented
cylinders, BCC spheres, disordered spheres and disorder state by expediently tuning the injection molding temperature. It is important to note that the bulk phase morphology of triblock copolymers at equilibrium state depends upon thermodynamic driving force (cN) and block composition (f), especially in the weakly segregation limit. The FloryeHuggins interaction parameter c is known to have an inverse proportional relationship with temperature (c f 1/T) [39]. Fig. 7 also presents different phase morphologies at equilibrium state and phase transitions of SIS triblock copolymer. When the temperature reaches a narrow range of about 183 Ce190 C, there is an ordereorder phase transition (OOT), that is, hexagonally packed cylinders transforms into spheres with a BCC lattice. Further increasing temperature can result in different arrangements of spherical domains. For vector 4111 studied in our work, processing temperature as high as about 215 C is another critical point which is in correspondence with the lattice disordering transition (LDT). When the temperature increases above TLDT, it becomes disordered spheres, namely spherical microdomains with a short-range liquid like order. When the temperature reaches near the temperature of order-to-disorder transition (TODT), the system is inclined to form disordered phase or homogeneous state so as to maximize entropy [40]. 3.5. Shear induced orientation in samples processed under different temperatures Injection molding can impart extreme high order and orientation in these samples, which means that the samples can acquire higher
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Fig. 7. SAXS profiles for SIS triblock copolymers obtained at different temperatures and schematic representation of their phase morphology at equilibrium state. The profiles have been vertically shifted for clarity.
Young’s modulus than those prepared under quiescent processing conditions because of flow-induced chain alignment [41e43]. As shown in Fig. 8, 2d-SAXS patterns of samples from three different directions were recorded as through, end and edge view. Shear flow introduced by screws during injection molding can induce orientation of samples. Orientation is also of great importance to influence the final mechanical response of TPEs except above-mentioned difference in the microphase structures. Fig. 9
Fig. 8. Schematic diagram of X-ray beam directions relative to samples.
lists all the patterns of samples obtained under different processing temperature and shows obvious differences. From the through and edge view, the pattern nearly evolves from bright two-point pattern to isotropic rings when increasing processing temperature. From the end view, the pattern evolves from degenerated six-
Fig. 9. 2d-SAXS profiles for samples processed under different temperatures from three different directions.
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point pattern nearly to the final isotropic rings. For cylinders formed in SIS-180, bright two-point pattern from through view and six-point pattern from end view mean hexagonally packed cylinders align well along the flow direction. The difference between Fig. 5b2 and c2 indicates well ordered spheres formed in SIS-200 relative to SIS-220. All these information are consistent with the information provided by TEM imaging. The degree of orientation can be calculated by Hemans’ orientation factor:
F ¼
E . D 3 cos2 f 1 2
(2)
Zp=2 E D cos2 f ¼
IðfÞsinfcos2 fdf 0
(3)
Zp=2 IðfÞsinfdf
Fig. 10. MooneyeRivlin plot of SIS triblock copolymers.
0
Where 4 is the azimuthal angle and denotes the average of cos2 4, and I(4) represents the scattered intensity. The orientation factor F ranges between 0 and 1. In addition, F ¼ 0 means isotropic while F ¼ 1 means perfectly parallel oriented structure. The calculated orientation factors of samples were also listed in Table 1. It shows a slight decrease in the degree of orientation when the processing temperature is below 220 C and an obvious decrease for SIS-240. Orientation changes a little from 180 C to 220 C which means microphase structure dominates the final mechanical performance at lower temperatures. Investigations on isolating the influence of orientation and microphase structure need more systematic researches and will be carried out in our future work. 3.6. Rubber elasticity of SIS topological networks To better investigate detailed structure differences and rubber elasticity of polymer networks, the stress-strain data of monotonic tension for all samples are evaluated in terms of the MooneyeRivlin equation [32], which quantifies the deviation of the stress-strain relation relative to the prediction from Gaussian rubber elasticity in a semi-empirical approach. The nominal stress (sN ) at different stretch ratio (l) can be predicted with the following relationship:
sN ¼ 2ðC1 þ C2 =lÞ l 1=l2
(4)
Where C1 and C2 are two numerical coefficients corresponding with material features. In addition, the Mooney stress or reduced stress, defined by
sM ¼ sN = l 1=l2
(5)
For an incompressible neo-Hookean rubber, the Mooney stress is constant as a function of the inverse of stretch ratio (1/l) and is simply equal to the shear modulus Gr [44]. Taking entropy origin of rubber elasticity into account, the affine network model proposed by Flory describes rubber elasticity with topological interactions between network chains regardless of topological entanglements. As a result, the shear modulus which is related to the elastic modulus (E ¼ 3Gr) is proportional to the effective elastic chain number density (Gr ¼ yekT) at a constant temperature in this model. Fig. 10 shows the MooneyeRivlin plots of the samples with different morphologies. All curves nearly show an incompressible neo-Hookean material behavior in the range at 0 < 1/l < 0.6, which
is corresponding to the large deformation region, while the four samples show great difference when 1/l > 0.6. The sample with disorder phase behaves as an ideal elastomer in the entire deformation range compared to other samples which demonstrates that the material becomes softer under high non-equilibrium processing temperature. With decreasing the inverse of stretch ratio, Mooney stress drops sharply at small strains and then decreases slowly nearly on a plateau in large deformation which means stress softening. It should be noted that entanglements always dominate the elasticity of the material at small strains while PS crosslinks play a key role under large deformation [45]. The sample with high degree of order derivates apparently from the prediction of ideal polymer network because of the existence of entanglements. Indeed, the existence of trapped entanglements and stiffness of crosslinks do play a dominant role in improving the elastic modulus in triblock thermoplastic elastomers. In small strain region, the total deformation is always associated with a uniform distribution of internal stress on account of stress transfer efficiency of the PI chains anchored to PS domains on both ends. The stiffness and continuity of hard phase also play a dominant role in the initial modulus like reinforcing filler effect. In the case of the cylindrical sample, namely SIS-180 upon stretching, PS cylinders always deforms into quasi-spheres which additionally enhance the PI phase in large deformation. It is reasonable to conclude that the physical crosslinks of PS hard segments act more effectively when polystyrene domains close-packed into cylinders than spherical domains with lower continuity. Considering SIS composed of spherical domains of styrene component dispersed in the rubber matrix of isoprene segments, no matter on a BCC lattice or arranged disordered, external stress inevitably causes deformed spheres and a decrease in the segment density of isoprene matrix between the spherical domains along the stretching direction at large strains. The irregularity of PS microdomain arrangement in soft phase tends to localize the applied deformation and then appears cavitations of rubbery PI domains. So the disordered spheres have poor mechanical performance in comparison with PS spheres arranged regularly on a BCC lattice. For linear SIS triblock copolymer, the entangled PI chains are easily fixed by the glassy PS microdomain at both ends including bridging and looping configuration. It can be concluded that high processing temperature results in low physical entanglement density. As a result, cylindrical morphology possess higher fraction of physical entanglements than other morphologies in consideration of pronounced reinforcing effect of physical entanglements
Y. Zhao et al. / Polymer 53 (2012) 4310e4317
on tensile strength at small strains. At the same time, destruction of PS crosslinks under large deformation makes some elastic ineffective loops become bridges which are in favor of stress transfer. 4. Conclusions The influence of the non-equilibrium processing conditions on the mechanical properties and the stress-strain behavior of a model physically associating polymer networks during systematic deformation process were investigated in this work. Our results demonstrate an unambiguous relationship between temperaturesensitive phase morphology and the mechanical responses of styrenic thermoplastic elastomers. The external processing temperatures can be tuned to acquire anticipated phase morphologies and orientation. Samples obtained at lower processing temperature show higher tensile strength and Young’s modulus than those obtained at higher processing temperatures. This could be understood as due to a change of internal structure from cylindrical morphology with low hard segment to spheres and other morphologies with lower segregation power. Meanwhile, the samples with hexagonally packed cylinders also show the largest amount of Mullins effect and hysteresis during cyclic tests. Nevertheless, the origin of rubber elasticity in styrenic thermoplastic elastomers can be attributed to entropic elastic PI bridges anchored between glassy PS microdomains from the analysis of MooneyeRivlin approach. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no.51173112 and grant no.51121001) and the Special Funds for Major State Basic Research Projects of China (2011CB606006). We would like to thank Prof. Liangbin Li for his help in SAXS experiments and Dr. Chen Chen for her help in TEM tests. References [1] Baeurle SA, Hotta A, Gusev AA. Polymer 2005;46(12):4344e54. [2] Meng Y, Zhang X-H, Du B-Y, Zhou B-X, Zhou X, Qi G-R. Polymer 2011;52(2): 391e9. [3] Huang Y-F, Chian Y-W, Ruan J, Jin S, Jeong K-U, Tang H-Y, et al. Polymer 2011; 52(18):4114e22. [4] Yu H, Kobayashi T, Hu G-H. Polymer 2011;52(7):1554e61. [5] Stasiak J, Zaffora A, Costantino ML, Moggridge GD. Soft Matter 2011;7(24): 11475e82. [6] Pedemonte E, Dondero G, Alfonso GC, Candia Fd. Polymer 1975;16(7):531e8.
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Processing temperature dependent mechanical response of a thermoplastic elastomer with low hard segment Yongsheng Zhao, Nanying Ning, Xin Hu, Yuhan Li, Feng Chen*, Qiang Fu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2012 Received in revised form 17 May 2012 Accepted 8 July 2012 Available online 24 July 2012
The mechanical responses including monotonic and cyclic tensile responses have been investigated on a microphase-separated poly (styrene-isoprene-styrene) triblock copolymer (SIS). The specimens were injection-molded by using different melt temperatures to acquire different microphase structures. As a result of temperature-dependent segregation driving force, the specimens with reduced microphase separation can be obtained by increasing processing melt temperature from 180 C to 240 C. On the basis of stress-strain behavior, Young’s modulus was found to increase with increasing PS domain continuity in the order of disorder state to disordered spheres to body-cubic-centered (BCC) spheres to oriented cylinders morphology. Meanwhile, cyclic hysteresis decreases with reduced microphase separation and with decreasing the applied predetermined maximum tensile strain. In addition, the MooneyeRivlin phenomenological approach was used to evaluate and explore the relationship between the polymer topological networks and the rubber elasticity of thermoplastic elastomers. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Microphase separation Mechanical response Rubber elasticity
1. Introduction Thermoplastic elastomers (TPE) are unique polymer materials possessing both elastomeric and thermoplastic feature [1e5]. Representative commercialized thermoplastic elastomers like styrenic block copolymers [6e8], thermoplastic polyurethanes [9], polyolefin elastomers are mainly block copolymers [10e13]. These materials are consisted of distinct homopolymers that are covalently linked and have been used extensively as thermoplastic elastomers due to their elastic attribute [14e16], shape memory feature [17,18] and good processability. Generally, block copolymers undergo microphase separation and spontaneous self-assemble into a variety of nanoscale periodic structures such as spheres, hexagonally packed cylinders, interpenetrating double gyroid morphologies and alternating lamellae [19,20]. Moreover, the resultant phase morphology of some TPEs is profoundly influenced by the processing methods and thermal history. As is reported in some previous studies [21,22], Hashimoto et al. conducted the Tdrop experiments for solution-casted films and observed different morphologies under different temperatures. Therefore, studies on the unique morphologies and corresponding mechanical behaviors of TPEs are undoubtedly important for fundamental understanding and practical application. * Corresponding authors. Fax: þ86 28 85405401. E-mail addresses: [email protected] (F. Chen), [email protected] (Q. Fu). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.07.016
Attempts to interpret the mechanical response of TPEs with various morphologies in terms of their microdomain structure and phase behavior thus become a hot topic [23e25]. C. Honeker et al. [26] have examined the mechanical properties of SIS with the double-gyroid cubic phase which exhibited necking and drawing and higher strength than other morphologies. Takahashi and Matsushita [27] have reported the elongation behavior of sphereformed triblock copolymer with different bridge fractions and they found that the bridge fraction of the middle block had an effect on the restoring force of lattice. Hotta et al. [19] have investigated the phase morphology and the mechanical properties of SIS triblock copolymers and provided a discussion on the relationship between properties and microstructures based on the stress-strain tests and time-temperature experiments. Several attempts [28e30] have also been made to develop a molecular description on the molecular mechanism of rubber elasticity such as slip-tube model [31] and MooneyeRivlin model [32]. From the point of industrial view, it is of fatal importance to establish mechanisms of regulating microphase structure in thermoplastic elastomers under melt processing strategies like injection molding which possesses ultrahigh production efficiency and are especially suitable for abundant manufacture. To the best of our knowledge, non-equilibrium processing conditions including temperature and external stress field must involve various structural arrangements on nanoscale in TPEs, which is yet poorly understood. In our work, we use a commercial SIS block copolymer
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with low polystyrene weight fraction as a model material to investigate the mechanical response during systematic deformation process. We focus on the effect of processing temperature on the phase morphology, how the morphology impacts the mechanical performance and exploring the relationship between topological network structure and the rubber elasticity of SIS triblock copolymer.
a fixed crosshead speed of 50 mm/min and five specimens were tested for each group. The relaxation behavior of these samples was investigated via tensile stress relaxation tests on this tensile testing machine. The relaxation of the stress was recorded after the samples were rapidly stretched to 300% deformation and fixed.
2. Experimental section
Pseudo-cyclic tests were also performed on an SANS Universal tensile testing machine under the program of the tensile loadingeunloading cycles in which the samples were cycled for three times after being initially stretched to various predetermined strains and immediately unloaded at a fixed tensile and unloading velocity of 500 mm/min.
2.1. Materials and sample preparation The material used in this study is a commercial SIS triblock copolymer under the trade name Vector 4111 (Dexco Polymers Co.). Vector 4111 is a linear, symmetric, pure SIS triblock copolymer which has a weight-averaged molecular weight, Mw, equaling to 1.4 105 g/mol, and the polydispersity index, Mw/Mn, is 1.11. The weight fraction and volume fraction of PS are 0.183 and 0.164, respectively. It is well-suited for use in elastomeric film compounds and in formulating pressure-sensitive adhesives. The granules were injection-molded into dumbbell-shaped specimens via an SZ 100 g injection-molding machine under different barrel temperature which is selected as 180 C, 200 C, 220 C and 240 C respectively. So the samples are nominated as SIS-180, SIS-200, SIS-220 and SIS-240 for simplicity. In addition, the sample size is 100 10 4.3 mm3. 2.2. Structure analysis Ultrathin sections of ca 80 nm thickness were obtained by cryomicrotoming along the flow direction of the samples using a Leica EMUC6/FC6 microtome at 100 C. These sections were then collected and stained for 20 min in a w2% aqueous solution of osmium tetroxide (OsO4) that selectively stained the polyisoprene (PI) microdomains. Transmission Electron Microscopy (TEM) was performed with an FEI-Tecnai G2 F20 S-TWIN type transmission microscope operating at 200 kV. Small angle X-ray scattering (SAXS) measurements were performed to confirm the initial microphase-separated structures of these samples. The samples were observed with the X-ray beam along three different directions to gain through, end and edge view of 2d-SAXS profiles. The X-ray wavelength was 0.154 nm, and a CCD detector was employed to collect two-dimensional (2D) SAXS patterns. The sample-to-detector distance was 2300 mm Fit2D software from European Synchrotron Radiation Facility was used to analyze SAXS patterns in term of the scattering vector q ¼ 4p sin q/l with 2q as the scattering angle and as the X-ray wavelength. 2DSAXS data were integrated about the azimuthal angle and the background scattering was subtracted to achieve the scattering intensity as a function of scattering vector.
2.5. Cyclic tests
3. Results and discussion 3.1. Monotonic tension response of SIS The uniaxial deformation behavior of Vector 4111 obtained under different temperature has been investigated theoretically. Fig. 1 displays the nominal stress-strain curves for SIS triblock copolymer processed at a series of non-equilibrium processing temperatures. A notable feature in monotonic tension is that the tensile strength and Young’s modulus significantly depend on the processing temperature. Owing to the measuring range limitation of tensile testing machine, these injection-molded specimens were stretched to a large strain (600%) instead of fracture. The detailed values of each sample’s elastic modulus and stress at 300% strain are listed in Table 1. As can be seen from the data in Table 1, sample SIS-180 has the highest Young’s modulus about 8.8 MPa, which means an almost 44% increase compared to the sample SIS-200. With further increasing the processing temperature, Young’s modulus decreases and SIS-240 owns the lowest modulus. In addition, the stress at 300% strain (MPa) can be used to characterize the strength of all these specimens and SIS-180 has the highest value as expected. In general, higher processing temperature goes against excellent mechanical properties for styrenic block copolymers. 3.2. Viscoelastic response and relaxation behavior Fig. 2 illustrates the evolution of the loss tan d for four samples as a function of temperature from 100 C to 120 C. The dynamic
2.3. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was performed with a TA Q-800 type machine to assess the glass transition behavior and the viscoelastic properties of different samples. The measurements were performed under single cantilever mode with a frequency of 1 Hz from 100 C to 120 C at a heating rate of 3 C/min. The dimensions of the tested samples are 17.5 10 4.3 mm3. 2.4. Tensile tests Monotonic tensile tests were performed on a SANS Universal tensile testing machine according to the GB/T 528-2009 standard. All the tests were conducted at ambient temperature (23 C) at
Fig. 1. Tensile stress-strain relations of the injection-molded specimens at ambient temperature obtained under different processing temperatures.
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Table 1 Young’s modulus and stress at 300% strain for different specimens. Samples
Young’s modulus (MPa)
SIS-180 SIS-200 SIS-220 SIS-240
8.8 6.1 3.6 2.0
0.2 0.3 0.1 0.1
Stress at 300% strain (MPa) 1.9 1.7 1.4 1.2
0.1 0.1 0.1 0.1
Possible structure
Orientation factor (F)
Oriented cylinders BCC spheres Disordered spheres Disorder state
0.957 0.954 0.948 0.878
mechanical behavior is typical of SIS and shows two glass transitions and a rubbery plateau. The lower glass transition temperature is that of the PI block (TgPI z 40 C) while the higher glass transition temperature at about 100 C is that of the PS block. It is interesting to note that the tan d peak intensity of PI block increases with increasing the processing temperature while that of PS block is opposite, which can be attributed to the segregation power (c) and the amount of PS incorporated in the PI block. Meanwhile, there is a slight increase in the glass transition temperature of the soft phase and a decrease of the whole relaxation time of the elastomers in theory when the short PS chains or chain segments intermix into the PI phase [33]. The more PS segments mixing into the PI phase, the larger the interface between them can be and the more influence on the extent and efficiency of the stress transfer. On the other hand, the plateau region of the storage modulus vs. temperature is wide and flat when the two phases strongly segregated from each other. A significant feature of sample SIS-240 is that the peak at PS glass transition diminished, which indicates PS and PI blocks turn into a near-homogenous state. Above all, low segregation power and fast quench introduced by injection molding go against fully separation into distinct phase microdomains for the two chemical linked blocks and thus result in a wider interface [34]. The interface width between PS and PI phase dramatically improves with increasing processing temperature. Generally, styrene segments in the vicinity of the interface have higher degree of freedom and greater mobility than those in the interior. As a consequence, the ductility of PS phase induced by good mobility weakens the tensile strength of SIS block copolymers obtained under high temperatures. Tensile relaxation tests were conducted as a complementary experiment so as to investigate the elastic and recovery behavior [35]. Fig. 3 shows the evolution of the normalized stress after
Fig. 3. Normalized stress measured after stretching to a fixed strain ε ¼ 300% at room temperature.
a 300% tensile deformation characterized via tensile stress relaxation tests. It is found that SIS-180 has the highest rate of stress relaxation among these injection-molded styrenic block copolymers while this material relaxes more slowly when processed under higher temperature. Stress relaxation of styrenic block copolymers is mainly caused by the physical flow of the elastomeric phase and trapped entanglements, the conformational change of phenyl group in hard domain and possible PS pullout from microdomains under stress [36]. At room temperature, the glassy PS holds the soft PI segments and thus acts as an effective crosslink for the PI network and a stress bearing media. In consideration of the situation of PS chains being pulled out of the glassy domains, one can easily understand the relaxation mechanism of stress reduction due to decreased fraction of bridging type of PI block. It has been demonstrated that the physical flow is the fastest relaxation process while the relaxation of PS hard domains is the slowest. It also means that the stress relaxation at room temperature may be on a time scale much shorter than that of a PS chain pullout from glassy crosslinks. So the difference in relaxation behavior can be attributed to the difference in entanglement density and numbers of effective PI chains. 3.3. Pseudo-cyclic responses and hysteresis during cycles
Fig. 2. Temperature dependence of loss tan d of SIS samples processed under different temperatures: (1) SIS-180, (2) SIS-200, (3) SIS-220, (4) SIS-240.
A typical feature of this TPE is the pronounced stress softening during quasi-static deformation, which is also termed as the Mullins effect. Under cyclic deformation, the stress monotonically decreases with numbers of cycles. Mullins phenomenon is always accompanied by stress relaxation, material anisotropy and formation of microvoids [37]. Most of the softening occurs in the first deformation and approaches a steady state with a constant cyclic stress-strain behavior after a few deformation cycles. It can be seen from Fig. 4 that all these samples consistently show obvious irreversibility during the cycle between the loading and subsequent unloading pathway. Moreover, the tensile modulus of each loading cycle becomes significantly lower with increasing number of cycles and a residual elongation is induced after each cycle which is due to the deformation of PS domains and preserved localized heterogeneous regions in PI phase. The sample SIS-180 which has the uppermost strength also shows the largest Mullins effect while SIS240 shows the least Mullins effect. This indicates that the sample prepared under higher temperature always has a softer elastic response under deformation. Besides, all the samples show a strain hardening phenomenon in large deformation after several cycles.
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Fig. 4. Pseudo-cyclic tests for all SIS specimens: (a) SIS-180, (b) SIS-200, (c) SIS-220, (d) SIS-240.
In consideration of the different well-organized structure of hard segment (PS), the varying degrees of Mullins effect can be attributed to the stiffness and the connectivity of the PS domain. Previous literatures [37,38] have demonstrated that the extent of Mullins effect increases with the domain diameter and the dspacing. At a higher processing temperature, the block chains are not easily segregated away from the interface and thus results in a thicker domain boundary and a smaller domain size. The feature of domain boundary determines the stress transfer efficiency between the rubbery phase and the hard phase while PS domain size is of fatal importance for the total stiffness or reinforcing effect. In this section, we apply a more quantitative way to analyze the hysteresis via the following equation:
even microvoids in the soft phase (PI) which cannot immediately generate self-healing and also indicates some degree of heterogeneity in the topological network. In a word, the samples obtained under lower processing temperatures show higher modulus, faster relaxation and larger hysteresis. To better understand the origin of different mechanical responses of these samples, we conduct TEM and SAXS tests to dig detailed information on microphase structures and orientation. 3.4. Effect of processing temperature on phase morphologies in SIS
(1)
Fig. 6 displays TEM images of injection-molded samples under different temperatures. TEM images indicate that there are significant differences in the microphase structures for these samples. SIS-180 owns a highly oriented cylindrical structure while SIS-200
In other words, hysteresis can be calculated by subtracting the area of unloading curve at a fixed preconditioned strain from that of loading curve. Fig. 5 displays the quantized hysteresis as a function of numbers of cycles and quench depth which is equivalent to the gap between the processing temperature and the glass transition temperature of PS block. Quench depth is also a quantitative representation of the thermodynamic driving force for phase separation. It can be concluded that deep quench is in favor of decreasing the hysteresis and the hysteresis augments with increasing the preconditioned strain and number of cycles. As a matter of fact, hysteresis or dissipated energy for filler-rubber composites under cyclic loading is adequately elaborated by considering breakage and restoration of aggregates of fillers. Similarly, PS domains in SIS triblock copolymers exactly play the role of reinforcing rubbery matrix as well as physical junctions. Nevertheless, there are some key factors for the energy loss including rupture in PS domains and structural breakup such as fracture of polymer chains occurring when the chains are fully stretched and stress relaxation during entire tensile stretching. To our knowledge, deformation also can cause density fluctuation and
Fig. 5. 3d plot of hysteresis as a function of pre-strain and quench depth for all SIS samples.
Z Hysteresis ¼
Z
sloading dε
sunloading dε
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Fig. 6. TEM images of injection-molded samples obtained under different processing temperatures. The dark region represents PI domains as OsO4 selectively stains isoprene phase.
has spheres packed on a BCC lattice. Disordered spheres form in SIS-220 when the temperature is higher than lattice disordering transition temperature (TLDT). SIS-240 shows wormlike structure or even disorder zone where polystyrene (PS) blocks cannot separate from polyisoprene (PI) domains. In this study, we choose 240 C to obtain a transit structure between order and disorder phase instead of a temperature higher than TODT (TODT ¼ w275 C for the material in our work) in consideration of possible degradation and chemical crosslinking caused by ultrahigh processing temperature. Fig. 7 shows the SAXS intensities, I, as a function of scattering vector q for injection-molded styrenic thermoplastic elastomers. Comparing with other samples processed under higher temperatures, SIS-180 has high-order scattering peaks at q ¼ O3qm and q ¼ O4qm relative to that of the first-order maxima qm. This indicates that a hexagonally packed cylindrical morphology had formed in SIS-180 on the basis of the classical researches done by Norihiro Sota et al. [23]. Red circles in Fig. 7 represents the qspacing profile of SIS-200 which suggests that spherical microdomains with a cubic lattice exist according to the appearing highorder peaks at q ¼ O2qm and q ¼ O3qm. The intensity peak at q ¼ O3qm still exists in sample SIS-220, which means it has disordered spheres while this intensity peak weakens obviously in SIS240 which suggests a near-disorder state when processed under a high temperature close to the material’s temperature of order-todisorder transition (TODT). So these samples finally have microphase structures far away from the themodynamic equilibrium state because of fast quench introduced by injection molding. As a whole, we achieve a series of phase morphologies including oriented
cylinders, BCC spheres, disordered spheres and disorder state by expediently tuning the injection molding temperature. It is important to note that the bulk phase morphology of triblock copolymers at equilibrium state depends upon thermodynamic driving force (cN) and block composition (f), especially in the weakly segregation limit. The FloryeHuggins interaction parameter c is known to have an inverse proportional relationship with temperature (c f 1/T) [39]. Fig. 7 also presents different phase morphologies at equilibrium state and phase transitions of SIS triblock copolymer. When the temperature reaches a narrow range of about 183 Ce190 C, there is an ordereorder phase transition (OOT), that is, hexagonally packed cylinders transforms into spheres with a BCC lattice. Further increasing temperature can result in different arrangements of spherical domains. For vector 4111 studied in our work, processing temperature as high as about 215 C is another critical point which is in correspondence with the lattice disordering transition (LDT). When the temperature increases above TLDT, it becomes disordered spheres, namely spherical microdomains with a short-range liquid like order. When the temperature reaches near the temperature of order-to-disorder transition (TODT), the system is inclined to form disordered phase or homogeneous state so as to maximize entropy [40]. 3.5. Shear induced orientation in samples processed under different temperatures Injection molding can impart extreme high order and orientation in these samples, which means that the samples can acquire higher
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Fig. 7. SAXS profiles for SIS triblock copolymers obtained at different temperatures and schematic representation of their phase morphology at equilibrium state. The profiles have been vertically shifted for clarity.
Young’s modulus than those prepared under quiescent processing conditions because of flow-induced chain alignment [41e43]. As shown in Fig. 8, 2d-SAXS patterns of samples from three different directions were recorded as through, end and edge view. Shear flow introduced by screws during injection molding can induce orientation of samples. Orientation is also of great importance to influence the final mechanical response of TPEs except above-mentioned difference in the microphase structures. Fig. 9
Fig. 8. Schematic diagram of X-ray beam directions relative to samples.
lists all the patterns of samples obtained under different processing temperature and shows obvious differences. From the through and edge view, the pattern nearly evolves from bright two-point pattern to isotropic rings when increasing processing temperature. From the end view, the pattern evolves from degenerated six-
Fig. 9. 2d-SAXS profiles for samples processed under different temperatures from three different directions.
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point pattern nearly to the final isotropic rings. For cylinders formed in SIS-180, bright two-point pattern from through view and six-point pattern from end view mean hexagonally packed cylinders align well along the flow direction. The difference between Fig. 5b2 and c2 indicates well ordered spheres formed in SIS-200 relative to SIS-220. All these information are consistent with the information provided by TEM imaging. The degree of orientation can be calculated by Hemans’ orientation factor:
F ¼
E . D 3 cos2 f 1 2
(2)
Zp=2 E D cos2 f ¼
IðfÞsinfcos2 fdf 0
(3)
Zp=2 IðfÞsinfdf
Fig. 10. MooneyeRivlin plot of SIS triblock copolymers.
0
Where 4 is the azimuthal angle and
sN ¼ 2ðC1 þ C2 =lÞ l 1=l2
(4)
Where C1 and C2 are two numerical coefficients corresponding with material features. In addition, the Mooney stress or reduced stress, defined by
sM ¼ sN = l 1=l2
(5)
For an incompressible neo-Hookean rubber, the Mooney stress is constant as a function of the inverse of stretch ratio (1/l) and is simply equal to the shear modulus Gr [44]. Taking entropy origin of rubber elasticity into account, the affine network model proposed by Flory describes rubber elasticity with topological interactions between network chains regardless of topological entanglements. As a result, the shear modulus which is related to the elastic modulus (E ¼ 3Gr) is proportional to the effective elastic chain number density (Gr ¼ yekT) at a constant temperature in this model. Fig. 10 shows the MooneyeRivlin plots of the samples with different morphologies. All curves nearly show an incompressible neo-Hookean material behavior in the range at 0 < 1/l < 0.6, which
is corresponding to the large deformation region, while the four samples show great difference when 1/l > 0.6. The sample with disorder phase behaves as an ideal elastomer in the entire deformation range compared to other samples which demonstrates that the material becomes softer under high non-equilibrium processing temperature. With decreasing the inverse of stretch ratio, Mooney stress drops sharply at small strains and then decreases slowly nearly on a plateau in large deformation which means stress softening. It should be noted that entanglements always dominate the elasticity of the material at small strains while PS crosslinks play a key role under large deformation [45]. The sample with high degree of order derivates apparently from the prediction of ideal polymer network because of the existence of entanglements. Indeed, the existence of trapped entanglements and stiffness of crosslinks do play a dominant role in improving the elastic modulus in triblock thermoplastic elastomers. In small strain region, the total deformation is always associated with a uniform distribution of internal stress on account of stress transfer efficiency of the PI chains anchored to PS domains on both ends. The stiffness and continuity of hard phase also play a dominant role in the initial modulus like reinforcing filler effect. In the case of the cylindrical sample, namely SIS-180 upon stretching, PS cylinders always deforms into quasi-spheres which additionally enhance the PI phase in large deformation. It is reasonable to conclude that the physical crosslinks of PS hard segments act more effectively when polystyrene domains close-packed into cylinders than spherical domains with lower continuity. Considering SIS composed of spherical domains of styrene component dispersed in the rubber matrix of isoprene segments, no matter on a BCC lattice or arranged disordered, external stress inevitably causes deformed spheres and a decrease in the segment density of isoprene matrix between the spherical domains along the stretching direction at large strains. The irregularity of PS microdomain arrangement in soft phase tends to localize the applied deformation and then appears cavitations of rubbery PI domains. So the disordered spheres have poor mechanical performance in comparison with PS spheres arranged regularly on a BCC lattice. For linear SIS triblock copolymer, the entangled PI chains are easily fixed by the glassy PS microdomain at both ends including bridging and looping configuration. It can be concluded that high processing temperature results in low physical entanglement density. As a result, cylindrical morphology possess higher fraction of physical entanglements than other morphologies in consideration of pronounced reinforcing effect of physical entanglements
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on tensile strength at small strains. At the same time, destruction of PS crosslinks under large deformation makes some elastic ineffective loops become bridges which are in favor of stress transfer. 4. Conclusions The influence of the non-equilibrium processing conditions on the mechanical properties and the stress-strain behavior of a model physically associating polymer networks during systematic deformation process were investigated in this work. Our results demonstrate an unambiguous relationship between temperaturesensitive phase morphology and the mechanical responses of styrenic thermoplastic elastomers. The external processing temperatures can be tuned to acquire anticipated phase morphologies and orientation. Samples obtained at lower processing temperature show higher tensile strength and Young’s modulus than those obtained at higher processing temperatures. This could be understood as due to a change of internal structure from cylindrical morphology with low hard segment to spheres and other morphologies with lower segregation power. Meanwhile, the samples with hexagonally packed cylinders also show the largest amount of Mullins effect and hysteresis during cyclic tests. Nevertheless, the origin of rubber elasticity in styrenic thermoplastic elastomers can be attributed to entropic elastic PI bridges anchored between glassy PS microdomains from the analysis of MooneyeRivlin approach. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no.51173112 and grant no.51121001) and the Special Funds for Major State Basic Research Projects of China (2011CB606006). We would like to thank Prof. Liangbin Li for his help in SAXS experiments and Dr. Chen Chen for her help in TEM tests. References [1] Baeurle SA, Hotta A, Gusev AA. Polymer 2005;46(12):4344e54. [2] Meng Y, Zhang X-H, Du B-Y, Zhou B-X, Zhou X, Qi G-R. Polymer 2011;52(2): 391e9. [3] Huang Y-F, Chian Y-W, Ruan J, Jin S, Jeong K-U, Tang H-Y, et al. Polymer 2011; 52(18):4114e22. [4] Yu H, Kobayashi T, Hu G-H. Polymer 2011;52(7):1554e61. [5] Stasiak J, Zaffora A, Costantino ML, Moggridge GD. Soft Matter 2011;7(24): 11475e82. [6] Pedemonte E, Dondero G, Alfonso GC, Candia Fd. Polymer 1975;16(7):531e8.
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