Introduction

Block copolymer refers to the copolymer formed by two or more block chains with different chemical properties connected together in different ways [1, 2]. Thermoplastic elastomer is a special block copolymer which combined the high elasticity of rubber at room temperature and the plasticity of plastic at high temperature [3]. They can be processed on traditional plastic equipment, such as injection molding, blow molding and profile extruders, etc [4, 5]. Thermoplastic elastomer exhibits a great industrial application value. Due to the thermodynamic incompatibility between different blocks, complex microscopic phase separation exists in the inner structure of block copolymers [6, 7]. Many particular properties can exhibit, such as microphase separation structure [8, 9], order-order transition (OOT) [10], order-disorder transition (ODT) [11, 12], complex thermal rheology, etc. Microscopic phase separation and transition:the OOT and ODT had been carried out on star block copolymers [13], salt-doped deblock copolymers [14], linear triblock copolymers [15, 16], cross-linked block copolymers [17], graft block copolymers [18], copolymer/homopolymer blends [19] etc.

Linear A-B-A type thermoplastic elastomers are composed of a long elastomeric B block with low Tg and two relatively short thermoplastic blocks with high Tg. The presence of two Tg indicates that there is a microscopic phase separation between component A and B [20]. This special structure leads to the self-assembly of different blocks into a microphase structure from lamellar to hexagonally-stacked cylindrical and finally to spherical microdomains of cubic lattice [21]. Ivy Mathew et al. [22] investigated the viscous and elastic behavior of the saturated triblock copolymer polystyrene-block-poly (ethylene-co-butylene) -polystyrene (SEBS) through a capillary viscometer. The flow curve of SEBS showed obvious two-section straight line characteristics, which was due to the microphase separation leading to two different flow behaviors. Styrene and isoprene as monomer, triblock copolymers polystyrene-block-polyisoprene-block-polystyrene (SIS) were successfully synthesized by anionic polymerization using a difunctional t-BuLi initiator [23]. SIS is an excellent thermoplastic elastomer, which has been widely studied due to its complex structure. Naoki Sakamoto et al. [24] investigated the OOT and ODT of SIS in a solution dioctyl phthalate via SAXS and rheology. OOT took place at 130 ℃ ~ 140 ℃ and ODT at about 205 ℃. Chang Dae Han et al. [25] synthesized three SIS block copolymers with spherical, cylindrical and lamellar structures respectively. They observed that the morphological state of microdomain structure in the SIS block copolymers had a pronounced effect on the OOT. The Han curve was drawn to analyze the ODT of three SIS samples, which occurred at 180 ℃, 90 ℃ and 105 ℃, respectively. Naoki Sakamoto et al. [26] investigated OOT and ODT of a SIS copolymer by SAXS, TEM and rheological study. These results showed that the SIS formed hexagonally-packed cylindrical and spherical microdomains. Dino Ferri et al. [27] investigated the steady rheological behavior and the temperature dependence of phase behavior of three SIS with different Mw (molecular weight), narrow molecular weight distribution, and different PS (polystyrene) contents. The loss tangent obtained from the DMA showed that the three SIS all had a significant peak at about -53 ℃ and 100 ℃, corresponding to the glass transition (Tg) of the PI block and PS block. A distinct broad peak was observed around the temperature greater than 100 ℃. It was assigned to OOT or ODT depending on the weight content of PS in the compositions.

When studying the wall slip phenomenon of SIS (SIS1209), Hui Han et al. [28] found an unknown rheological response peak near 168.6 ℃ on the dynamic viscoelastic spectrum. The SAXS showed that there was no OOT or ODT at this temperature. It was proposed to be a viscous flow transition peak. In order to fully study this transition, another SIS5562 was investigated systematically. The molecular structure of SIS1209 and SIS5562 were listed in Table 1. More rheology analysis was introduced to explain this phenomenon, besides the DMA and SAXS in reference 28. The results verified the viscous flow theory further.

Table 1 Molecular characteristics of SIS1209 and SIS5562
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Experimental section

Materials

The linear SIS5562 was supplied by LCY Chemical COR Company (TaiWan, China). The linear SIS1209 was supplied by Baling Petrochemical Company (Hunan, China). The detailed molecular structure information of SIS5562 and SIS1209 were shown in Table 1.

Sample preparation

SIS particles were dried at 60 ℃ for 12 h in a vacuum drying oven. The dried SIS particles were hot-pressed, at a temperature of 150 ℃ and lasted for 15 minutes. Some SIS were tailored into disk specimens (the diameter was 25 mm and the thickness was about 1.5 mm) for the parallel plate tests. Some SIS were cut into rectangle (the length, width and thickness were 10.5 mm, 3.8 mm and 1.9 mm respectively) specimens for the torsion fixture tests.

Experimental characterization

Rheological measurement

The steady-state rheological properties of SIS were measured on RH2000 a constant-velocity double tube capillary rheometer. The diameter of the two barrels was 15 mm. The length-to-diameter ratio of the zero-length capillary (used to measure the entrance pressure drop) was 0.25/1 (mm/mm). The length-to-diameter ratio of the other normal capillary was 16/1 (mm/mm). The apparent shear rate varied from 20 s-1 to 2500 s-1. The characterization temperature was from 140 ℃ to 200 ℃.

The dynamic mechanical properties of SIS were performed by Advanced Rheological Expansion System (ARES-G2, TA). There were two kinds of test modes:

  1. 1.

    Dynamic frequency sweep: Dynamic frequency sweep experiments were performed from 5 \(\times\) 10-3 Hz to 100 Hz at different temperatures (130 ℃ ~ 200 ℃) with the parallel plate fixture test mode. All the experimental specimens were tested within the linear viscoelastic range. The experiment test adopted the sweep method from high frequency to low frequency.

  1. 2.

    Dynamic temperature sweep: Torsion fixture test mode were carried out at low temperature (-130 ℃ ~ 100 ℃, rectangle specimens). Parallel plate fixture test mode at high temperature (100 ℃ ~ 220 ℃, disk specimens) under nitrogen atmosphere. The temperature raising rate was 1.5 ℃/min, the strain was 1%, and the angular frequency was 1 Hz.

Small-angle X-ray scattering (SAXS)

The SAXS experiments were conducted using a GANESHA 300XL+ system. The instrument was equipped with a Pilatus 300K detector with pixel size of 172 μm \(\times\) 172 μm. The wavelength was 1.54 Å. The detector was kept in a vacuum chamber. The distance between the specimen and the detector varied between 0.115 m and 1.47 m, depending on the configuration used. Silver behenate was used as the calibration standard. The range of q varied from 0.003 to 3 Å-1. The specimen was first placed between Aluminum foils with temperature control using a temperature stage (Linkam. U.K.). The vacuum chamber was evacuated before temperature increase. Then the temperature was firstly increased from room temperature (25 ℃) to 200 ℃ at a rate of 10 ℃/min. The SAXS measurement was conducted during cooling at the same rate (10 ℃/min). After the specimen reached the set temperature (i.e. 200 ℃, 180 ℃, 160 ℃, 140 ℃, 120 ℃), it was equilibrated for 5 min before the measurement. The accumulation time was 30 min. In this paper, microstructure of SIS in the capillary extrusion process under various temperatures was studied. The time required for SIS melt to flow through the capillary is very short (less than 5 min). That is reason that the annealing time for SAXS measurement was selected as 5 min. The SAXS data was processed via SAXSGUI program.

Transmission electron microscope (TEM)

The structure of microphase separation was observed by transmission electron microscope (TEM, JEM-2100). Because the electron density contrasts were small, the SIS needed to be dyed before measurement. Firstly, the SIS sample was immersed and stained in an aqueous solution of osmium tetroxide (w = 4 wt%) for 20 min. Then ultra-thin sectioning was performed at a temperature below Tg.

Results and discussion

Temperature scan spectrum of SIS

Figure 1 showed the curve of the dynamic mechanical loss factor by temperature scanning of SIS5562. It could be seen in Fig. 1 that SIS5562 had three rheological response peaks. The peak around -47 ℃ corresponded to the glass transition temperature of the soft PI block (Tg, PI) in the SIS system. The peak around 100 ℃ corresponded to the glass transition temperature of the hard PS block (Tg, PS). The existence of two obvious glass transition peaks demonstrated that PS and PI blocks had microscopic phase separation [29]. The third obvious rheological response peak was at around 152.5 ℃. Compared to the Tg, PI or Tg, PS, this peak was much broader. Obviously, it was not the glass transition peak. Many literatures had reported the similar rheological response peaks. Hashimoto T. et al. [26] attributed it to the loss peak caused by the transformation of SIS melt from one ordered structure to another, OOT. The SAXS showed the transition from one order to another. Canetti M. et al. [27] assigned it to the ODT because they found that the abrupt drop of G′ was taken as a signature of the complete mixing of the PS and PI blocks. In the previous paper [28], we proposed that it was a viscous flow transition, because the SAXS showed the structure did no change. We would discuss systematically about this transition in the following sections through rheology and other analysis. First, the micro structure was investigated through TEM and SAXS.

Fig. 1
figure 1

Temperature dependence of dynamic-mechanical loss factor of SIS5562

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Microstructure characterization of SIS5562

Because of thermodynamic incompatibility, the PS blocks and the PI blocks in SIS would undergo microphase separation [28]. The microstructure of SIS5562 at room temperature was shown in Fig. 2a. The dark (black) areas represented the PI microdomains stained by osmium tetroxide. The light (white) areas represented the PS microdomains, which was not stained. The dark areas and the light areas were distributed alternately, showing a regular lamellar micro structure.

Fig. 2
figure 2

TEM photomicrograph at room temperature (a) and SAXS profiles during cooling at different temperatures (b)

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The SAXS profiles of SIS5562 obtained at different temperature during cooling was given in Fig. 2b. The curves under a series of temperatures had been moved vertically to make the results more clearly. The SAXS profiles of SIS5562 showed multiple interparticle scattering peaks caused by the long-range order periodic microdomain structure. According to the ratio of each scattering vector q of the corresponding scattering peak, the relative positional relationship of the four scattering peaks was 1:2:3:4, no matter the test temperature. It indicated that the microphase structure of SIS5562 did not change. The lamellar microdomain phase structure invariably maintained within the range of test temperature (120 ℃ ~ 200 ℃), indicated that the SIS5562 had good regularity and uniformity. The scattering intensity gradually enhanced with the decrease of the temperature, implied that the microphase separation became coarser with the decrease of temperature. In summary, during the temperature range of 120 ℃ to 200 ℃, SIS5562 had always maintained lamellar structure. There was no OOT or ODT within the test temperature range (i.e. 120 ℃ ~ 200 ℃).

Generally, the rheological properties were related to the structure of polymer melts. Therefore, the following rheology analysis was carried out to verify this result further.

Time-temperature superposition of SIS5562

The time-temperature superposition (TTS) principle was first proposed by Williams et al. [30, 31]. The TTS theory could be applied in experiments investigating time and frequency dependency of materials′ behavior to predict their long-term viscoelastic properties [32]. TTS asserted that for a thermal rheological simple material, a change in temperature would have the same effect on the viscoelastic properties as a corresponding change in time. It could be used to define the relationship between time and temperature based on the deformation and relaxation response of a viscoelastic material under constant stress or strain conditions [33,34,35]. In other words, the TTS implies that time or frequency is equivalent to temperature [36]. The data obtained from the tests were manipulated to produce rheological master curves [31, 37]. The modulus measured at different temperatures could be shifted horizontally and vertically with shift factors \({\alpha }_{T}\) and \({b}_{T}\), respectively [38]. The TTS principle function is [38,39,40,41]:

$${G}^{^{\prime}}\left(\omega ,{T}_{ref}\right)={b}_{T}\cdot {G}^{^{\prime}}\left({\alpha }_{T}\cdot \omega ,T\right)$$
(1)

where \(G{^{\prime}}\) is the storage modulus;\(\omega\) is the frequency; \({\alpha }_{T}\) and \({b}_{T}\) are horizontal and vertical shift factors, respectively; Tref is the reference temperature. The vertical shift factor \({b}_{T}\) should, generally, be small [42]. \({b}_{T}\) results the relationship between temperature and modulus, is defined as [42, 43]:

$${b}_{T}=\frac{{G}^{\infty }\left({T}_{ref}\right)}{{G}^{\infty }\left(T\right)}$$
(2)

where \({G}^{\infty }\) is the plateau value of G′ (at high frequencies). \({G}^{\infty }\) is proportional to \({k}_{B}\cdot T\) for a purely entropic network and it is also proportional to the number density of active elastic chains. \({k}_{B}\) is the Boltzmann factor [42].

The horizontal shift factor \({\alpha }_{T}\) represents the relationship between temperature and frequency (relaxation time) [43], defined as [44]:

$${\alpha }_{T}=\frac{{\tau }_{T}}{{\tau }_{ref}}$$
(3)

where \({\tau }_{ref}\) is the relaxation time at Tref, and \({\tau }_{T}\) is the time required to give the same response at the test temperature T.

Some study [45, 46] of TTS master curve of dynamic storage modulus of block copolymers explored that the character of TTS in low frequency region could be used to explain the microphase structure of block copolymers. This is because that the viscoelastic response in the low frequency region reflects the relaxation behavior of the nano-scale micro-domain structure. Once the OOT or ODT happened for block copolymers, their microphase structures will change. The TTS curve would be much complex than the curve of a homopolymer without the structure change [47, 48].

Due to the complex rheological behavior of SIS, horizontal and vertical shifts were performed via TRIOS software (the software from the TA). The data of the storage modulus at different temperatures on the TTS master curve was shown in Fig. 3. Since the special rheological response peak appeared near 150 ℃ in Fig. 1, 150 ℃ and 160 ℃ were selected as the reference temperature. Table 2 provided detailed information about \({\alpha }_{T}\) and \({b}_{T}\) at each temperature.

Fig. 3
figure 3

TTS master curve of SIS5562 at different temperatures (a) Tref = 150 ℃ (b) Tref = 160 ℃

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Table 2 Details of \({{\alpha }}_{{{T}}}\) and \({{{b}}}_{{{T}}}\) at different temperatures (\({{{T}}}_{{{r}}{{e}}{{f}}}=\) 150 ℃ and 160 ℃)
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In Fig. 3a, 150 ℃ was taken as the reference temperature, the TTS master curve was almost smooth in the whole frequency range without significant temperature dependence. Especially, there was no obvious temperature dependence in the low-frequency region (8 \(\times\) 10-5 ~ 0.02 rad/s). It indicated that the microstructure of SIS5562 hardly changed within the experimental temperature range [46]. When 160 ℃ was taken as the reference temperature, as shown in Fig. 3b, there was also no significant temperature dependence across the experimental range. The master curve was nearly smooth, which further proved that the microstructure of SIS5562 did not change in the experimental temperature range. In this case, the TTS results supported that there was no OOT or ODT occurred around 150 ℃, which was consistent with the SAXS results.

log G′ vs log G′′ (Han plots)

The Han plot [25, 49,50,51] was originally proposed for homogeneous system. It was believed that the order-disorder transition of materials could be determined through the temperature dependence of rheology. According to the common stress relaxation phenomenon of viscoelastic materials, the stress-strain response of the materials in the linear viscoelastic region satisfied the Blotzmann superposition principle. Han curve was the relation curve of log G′(ω) vs log G′′(ω) deriving from the modulus data. The relationship was obtained through the following function [52, 53]:

$$lg{G}^{{^{\prime}}}=x \cdot lg{G}^{{^{\prime}}{^{\prime}}}+\left(1-x\right)\cdot lg\left(8{G}_{N}^{0}/{\pi }^{2}\right)$$
(4)

where, 1<x<2, is the slope of log Han plot. It is less than 2 for polydisperse polymers in the terminal region [52]. \({G}_{N}^{0}\) is the plateau modulus [52, 54].

Figure 4 was log G′(ω) versus log G′′(ω) plots drawn based on frequency sweep data at different temperature, which was manipulated from data in Fig. 8. When studying the OOT of the triblock copolymer SIS (Vector 4111), Naoki Sakamoto et al. [26] firstly observed that a negative slope in the sharp change in Han plots in the terminal region at temperatures ranging from 185 ℃ to 210 ℃. The threshold temperature was ca.185 ℃, which corresponded to the onset of OOT [26]. The negative slope disappeared when the temperature was above 210 ℃, which was the end of the OOT transition [26]. Therefore, the negative slop represented the OOT transition in the system. In Fig. 4, the Han curve of SIS5562 did not show any negative slope at the terminal region (low values of G′′) in the whole range of experimental temperature. It indicated that no OOT occurred during the experimental test.

Fig. 4
figure 4

Han plots of SIS5562 at different temperatures

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When studying the ODT of block copolymers, Han C. D. et al. [51] found that Han plots for an SIS block copolymer varied gradually with the increasing of temperature up to a certain critical value. Then it became virtually independent of temperature as the temperature increased further. When determining the ODT temperature of SIS, Kim J. et al. [50] found that the Han curve for the SIS block copolymer exhibited strong temperature dependence, especially at low values of G′′. It was attributed to the transition from an ordered microdomain structure to a disordered homogeneous phase (ODT). If ODT did not occurred in SIS melt within the experimental temperature range, the Han plots was temperature independent. In Fig. 4, the Han plots had virtually no temperature dependence in the terminal region (low values of G′′, 367.2 ~ 3550.6 Pa), indicated that no ODT occurred. The terminal slopes (around 0.85) of the Han plots (Fig. 4) of SIS5562 were much less than 2, indicated that SIS5562 maintained microphase separation in the whole test process. There was no ODT below 200 °C. In summary, the TTS and Han plot implied that SIS5562 did not undergo OOT or ODT within the experimental temperature range.

Temperature dependence of G′ and G′′

The microstructure of a material determines its macroscopic properties. Subtle change in the microstructure could be reflected in abrupt changes in its macroscopic properties, such as the modulus of the material. The dynamic mechanical measurements (DMA) was used to characterize the viscoelastic transition of polymer [28].

Figure 5a, b showed the storage modulus (G′) and the loss modulus (G′′) of the SIS5562 versus temperature. It could be seen that, within the measured temperature range, G′ and G′′ all gradually decreased with the increase of temperature. However,the slopes of the G′ curve changed (from -0.039 to -0.013) at around 157.1 ℃ as shown by the red dotted line in the Fig. 5a. The temperature (157.1 ℃) in the intersection point was defined as the critical temperature. This was very close to the temperatures of 152.5 ℃ in Fig. 1, which corresponded to the rheological response peak. Stephan Forster et al. [47] proposed that the OOT and the ODT between the ordered structures of polymers could be determined by the change of storage modulus with temperature. In literature 47, a discontinuous decrease of the storage modulus was attributed to be an ODT. The storage modulus increases considerably within a narrow temperature range indicated that the system undergone a phase transition between two ordered states, that was, OOT. This was an obviously different trend from the curve in Fig. 5a. In Fig. 5a, there was no discontinuous decrease but a turning point. Therefore, SIS5562 did not experience OOT or ODT within the test temperature range. This conclusion was in consistent with the previous analysis results of SAXS, TTS principle and Han plots.

Fig. 5
figure 5

Temperature dependence of G′ (a) and G′′ (b) of SIS5562 (\(f=1 \mathrm{Hz}\))

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As mentioned in reference 28, this transition was probably a viscous flow transition of the whole polymer chain. The temperature was the viscous flow transition temperature (Tf) [28]. When the viscous flow transition occurred, the polymer underwent a transition from a high elastic state to a viscous flow state [55]. A polymer material in the viscoelastic region may at times be considered as either elastic or viscous, or both [55]. In the SIS system, there were two block parts. The soft PI chains turned into the viscous flow state when temperature was much lower the Tf (around 150 ℃). While PS part kept in elastic state below Tf. The PS phase in SIS melts experienced the change from the high elastic state to the viscous flow state [56] before and after the critical temperature Tf. When the temperature was lower than Tf, the storage modulus of the SIS5562 system was mainly contributed by the PS elastic phase. Because at this temperature the PI phase was in the state of viscous flow, the G′PI was quite lower; PS phase was in the state of high elastic, the G′PS was relatively higher. When the temperature was higher than Tf, the whole polymer chain turned into the viscous flow state with the viscous flow transition of PS microdomain. As a result, the G′ decreased quickly, and the decrease rate of G′ slowed down. The viscous flow transition for the block polymer was different for that of a homopolymer [28]. According to literature 28, the Tf of SIS1209 was about 170 ℃, while Tf of SIS5562 was about 150 ℃. This was because the molecular weight of SIS1209 (1.22 \(\times\) 105 g/mol) was much higher than SIS5562 (8.30 \(\times\) 104 g/mol), as showed in Table 1. Tf represents the temperature at which the entire polymer chains begin to move freely. Therefore, for the SIS system, when the temperature is lower than Tf, the PS phase region is under the state of high elastic. When the temperature is higher than Tf, the PS phase region goes into the viscous flow state. As a result, the whole SIS molecular chain move freely. The larger the molecular weight of polymer chains, the higher the viscous flow temperature (Tf). Although the molecular weight of PS block in SIS5562 (Mw(PS)=1.87 \(\times\) 104 g/mol) is slightly greater than that in SIS1209 (Mw(PS)=1.77 \(\times\) 104 g/mol), the molecular weight of SIS1209 (1.22 \(\times\) 105 g/mol) is much greater than SIS5562 (8.30 \(\times\) 104 g/mol), so the Tf of SIS1209 is higher.

The flow behavior

Figure 6 shows the flow curves of SIS5562 at different temperatures. When the temperature was relatively low (140 ℃ ~ 160 ℃), there were obvious turning points in the flow curve (A, B, and C in Fig. 6). The shear stress and the shear rate corresponding to the turning point was noted as critical shear stress (\({\sigma }_{c}\)) and critical shear rate (\(\dot{{\gamma }_{c}}\)), respectively. With the increase of temperature from 140 ℃ to 160 ℃, the critical shear rate corresponding to the turning point increased gradually from 165.4 s-1 to 1142.9 s-1; while the critical shear stress decreased gradually from 372.3 kPa to 331.2 kPa. The critical shear rate and critical shear stress at different temperatures were detailed in Table 3.

Fig. 6
figure 6

The flow curve of SIS5562 at different temperatures

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Table 3 Critical shear stresses and critical shear rates of SIS5562 at different temperatures
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The shear stress increased sharply with the increased of the shear rate before the turning point. While after the turning point, the shear stress changed slightly with the increased of the shear rate or even kept constant. After the temperature exceeded 160 ℃, the turning point disappeared. This phenomenon could be explained from the viscous flow transition point of view. When the temperature was below the Tf (140 ℃ ~ 160 ℃), the PS phase was in the state of high elastic, the PS microdomain maintained. It played a role of physical crosslinking point (PS microdomain) in the SIS melt [28]. The SIS melt was considered to be a strong entangled system. Under the low shear rate (less than the critical shear rate), the shear stress was not big enough to destroy the physical crosslinking point (PS microdomain), the PS microdomain maintained. The shear stress increased much with the increase of shear strain. When the shear rate was greater than the critical shear rate, the physical crosslinking of the PS phase was destroyed under strong shear action. The flow was much easier, the shear stress almost kept the same level when the shear rate increased. Therefore, the flow curve showed a clear turning point. When the SIS flowed in a temperature higher than the Tf (170 ℃ ~ 200 ℃), well, the whole polymer chain was in the state of viscous flow state. Very low shear stress would destroy the microphase structure of the SIS. There were no microdomains as the crosslinking point in the system. The SIS melt transformed into a weaker entangled system [28]. The SIS flowed just like the normal homopolymers. No turning point appeared in the flow curve.

Figure 7 was the variation curves of shear viscosity with shear rate of SIS5562 at different temperatures. SIS5562 exhibited shear-thinning behaviors [57] at all temperatures. It was obviously that at relatively low shear rates, the shear viscosity varied greatly at high and low temperature. The viscosity curve of SIS5562 exhibited the "First Newtonian or Primary Newtonian plateau" [57, 58] when the temperature was in the range of 160 ℃ ~ 200 ℃. As we know, for pseudoplastic fluids, when the shear rate tends to zero, the fluid viscosity tends to be constant, which is called the first Newtonian plateau. The viscosity is called the zero-shear viscosity. This is the reflection of high viscous properties. The corresponding shear rate range of the "First Newtonian plateau" expanded with the increase of temperature, showed in Table 4.

Fig. 7
figure 7

Temperature dependence of the shear viscosity for SIS5562

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Table 4 Shear rate range of the “First Newtonian plateau” at different temperatures
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When the temperature was lower than 160 ℃, the "First Newtonian plateau" did not exist in the viscosity curve at low shear rates. This peculiar phenomenon could be related to the viscous flow transition of PS microphase in SIS melt. When the temperature was below 160 ℃, the PS microdomain existed in the SIS melt in the high elastic state, acting as a physical cross/linking point. The system showed a strong elastic behavior, with less viscous behavior. The "First Newtonian plateau" did not appear on the viscosity curve, because it was the reflection of viscous behavior. When the temperature was higher than 160 ℃, the PS microdomains in the SIS melt changed from a highly elastic state to a viscous flow state. The entire SIS molecular chain completely entered a viscous flow state. In this case, the entanglement between the molecular chains weakened much. The elasticity receded and the viscous property enhanced. The flow of SIS was just similar to the homopolymer. The "First Newtonian plateau" appeared on the viscosity curve. The higher the temperature is, the broader the "First Newtonian plateau".

Dynamic rheological measurements

Small amplitude oscillation shear methods are usually used in the dynamic rheological test of polymer materials. The testing process will not damage or affect the structure of the polymer materials under the low shear rate and stress. Moreover, the dynamic rheological parameters of polymer materials in the linear viscoelastic region are very sensitive to the morphology and structure. Therefore, dynamic rheological methods can effectively characterize the structural changes of polymer materials [59,60,61]. Figure 8 were the plots of the dynamic storage modulus (G′) and loss modulus (G′′) of SIS5562 as a function of angular frequency (ω) at different temperatures (140 ℃ ~ 200 ℃). In the Fig. 8a, with the increase of temperature, the G′ of the SIS5562 decreased gradually throughout the test range. It meant that the thermal motion ability of the molecular chain increased gradually and the elasticity of the polymer melt decreased gradually. What′s more, within the test range, the series of log G′ vs log ω curves could be divided into two groups by 150 ℃. Below 150 ℃ (that was 140 ℃), the G′ within the test frequency range was significantly higher than that at other temperatures (150 ℃ ~ 200 ℃). This can be attributed to the fact that PS phase in the SIS melt was in the state of elasticity with high modulus. It gradually entered into viscous flow state above Tf (150 ℃) with low modulus. The entire SIS molecular chain started to move freely, so G′ significantly decreased at the same angular frequency.

Fig. 8
figure 8

Dynamic storage modulus G′ (a) and loss modulus G′′ (b) of SIS5562 as a function of ω at different temperatures

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Figure 8b given plots of log G′′ vs. log ω for SIS5562. It could be seen that the log G′′ vs log ω curves could also be divided into two groups by 150 ℃. It was the same reason as discussed above. These peculiar phenomena can be attributed to viscous flow transition of PS microphase in SIS melt. In other words, when the experimental temperature was below Tf, strong entanglement was formed between SIS molecular chains due to the existence of the PS microdomains. The strong entanglement required a greater external stress to destroy. When the experimental temperature was higher than Tf, the PS chains entered a viscous flow state. The PS microdomains was easy to be damaged under the flow stress. The entanglement only formed among SIS molecular chains which could be disentangled under relatively low external force. Therefore, compared with G' and G'' at low temperature (140 ℃), G' and G'' at high temperature (150 ℃ ~ 200 ℃) were significantly lower at the same angular frequency, especially at the terminal region of low frequency.

Conclusion

A broad peak at 152.5 ℃ was shown in the DMA viscoelastic curve of SIS5562. The SAXS, TTS and Han plots demonstrated that this thermodynamic transitions of SIS5562 at/near 152.5 ℃ was not OOT or ODT. It was supposed that this was the viscous flow transition temperature of PS microphase in SIS melt. From this point of view, the viscous flow transition of SIS was systematically investigated through the rheological methods. Based on this viscous flow transition and structure characterization, the special flow behavior under both static and dynamic state could be explained reasonably. It confirmed that the viscous flow transition temperature of SIS5562 was around 150 ℃.