Retarded stress and morphology relaxation of deformed polymer blends in the presence of a triblock copolymer

Abstract

The effect of linear triblock copolymer compatibilizer (styrene–ethylene/butylene–styrene, SEBS) with strong viscoelasticity on the stress relaxation behavior of polypropylene (PP)/polystyrene (PS) (20/80) blends under various shear deformations has been investigated. With the addition of SEBS, the initial deformation of dispersed droplets under step shear strains was suppressed, and the following stress relaxation was found to be continuously retarded. The strain sensitivity of the stress relaxation modulus became weaker with the addition of SEBS possibly due to the improved viscoelasticity and interfacial adhesion. But the increase of strain led to more pronounced retardation in the stress relaxation of compatibilized blends. These phenomena were discussed in terms of the competitive effect of morphology refinement and the changes in interfacial and viscoelastic properties brought by compatibilization. The dominant factors determining the relaxation behavior were suggested to rely on the SEBS loading.

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1. Introduction

The final properties of immiscible polymer blends depend not only on their melt processing histories but also to a large extent on the temporal morphology and stress relaxation behavior after melt flow.^1–4 For example, oriented morphology frozen from shear deformed structure during melt mixing usually generates anisotropic conductive and mechanical properties,⁵ which is highly desirable in many functional applications.⁶ Therefore, revealing the morphological relaxation mechanism and improving the anisotropy of oriented polymer blends are of great significance.

Stress relaxation is a superior method to deduce the pathway and kinetics of structure recovery in deformed polymer blends.^7,8 The stress relaxation of blends usually proceeds via two consecutive steps,⁸ including a fast step due to the relaxation of matrix phase and a shower one corresponding to the interfacial relaxation of deformed droplets.^1,9 In practice, many immiscible polymer blends with industrial interest need compatibilization, which can be physical or reactive, to relieve the deterioration in mechanical properties caused by the poor interface between thermodynamically immiscible phases.^10–12 Physical compatibilization is usually made by adding tailor-made copolymers,^13–15 while reactive compatibilization often involves in situ formation of copolymers at the blend interface.^10,16,17 In both cases, interfacially located copolymer molecules can interact with two phases through chain entanglement,^18–20 and thus improve the interfacial adhesion, enhance interfacial adhesion and refine the morphology. Undoubtedly, the stress relaxation behavior of compatibilized blends will be complicated by the changes in the interfacial, rheological and morphological properties caused by the addition of copolymers.^21–23 Some studies found that compatibilization can retard the stress relaxation of immiscible polymer blends,¹ while others^24,25 found that the stress relaxation was accelerated with a small amount of copolymer. The difference in the stress relaxation behavior can be attributed to a series of competitive and corporative factors brought by compatibilization. The improved interfacial adhesion and decreased interfacial tension²⁵ can retard the stress relaxation behavior. While the morphology refinement²⁴ and the Marangoni stress,²⁶ which is caused by the steady flow-induced concentration gradient of compatibilizer copolymers along the surface of droplets, are suggested to accelerate the stress relaxation. It is noteworthy that these effects of compatibilizer on a blend may vary with its concentration. However, few efforts have been dedicated to revealing the compatibilizer concentration dependence of the cooperative/competitive effects of compatibilizer on the stress relaxation behavior of polymer blends.

In terms of conformation, a diblock copolymer chain crosses the interface only once, while a triblock copolymer can theoretically cross the interface twice.²⁷ In this sense, triblock copolymers are expected to lead to faster stress relaxation of blends due to their higher compatibilization efficiency than diblock ones.¹² However, some widely used triblock copolymers, such as styrene–butadiene–styrene (SBS) and styrene–ethylene/butylene–styrene (SEBS), possess much stronger viscoelasticity than homopolymers due to microphase separation.²⁸ The competition between the changes in morphology refinement, interfacial properties and viscoelasticity would undoubtedly complicate the stress relaxation behavior of blends compatibilized by triblock copolymers with strong viscoelasticity.²⁴ To our best knowledge, relevant efforts are relatively scarce.

The aim of this study was to investigate the comprehensive influence of triblock copolymers with strong viscoelasticity on the stress relaxation behavior of immiscible polymer blends. For this purpose, two widely used but immiscible commodity polymers, namely polypropylene (PP) and polystyrene (PS), with good comprehensive properties and low costs were employed as the components of blends. And the triblock copolymer SEBS with strong viscoelasticity was selected as the compatibilizer. Then, the stress relaxation of PP/PS blends compatibilized with various contents of SEBS after a step shear strain in the linear and nonlinear region was studied and correlated with the morphological evolution.

2. Experimental

2.1 Materials and sample preparation

Commercial PP (T30S) was provided by Lanzhou Petrochem Co. and has a weight average molecular weight (M̄_w) of 5.87 × 10⁵ g mol⁻¹, a melt flow index (MFI) of 2.6 g/10 min (230 °C, 2.16 kg). PS (GP5250) was supplied by Taihua Plastic (Ningbo) Co. Ltd. and has a M̄_w of 2.33 × 10⁵ g mol⁻¹ and a MFI of 7.0 g/10 min (190 °C, 2.16 kg). The compatibilizer was SEBS triblock copolymer (Kraton G1650, Shell) with M̄_n = 70 000 g mol⁻¹, M̄_w = 74 000 g mol⁻¹ and 29 wt% PS.²⁹ Before melt mixing, all the materials were dried under vacuum at 60 °C for 8 h. PP/PS(20/80) blends compatibilized with 0–8 wt% SEBS (based on the weight of blends) were prepared via a two-step method by using an internal mixer (XSS-300) operated at 200 °C. The rotation rate was set at 50 rpm. Firstly, desired amount of SEBS compatibilizer was mixed with the minority phase (PP) for 5 min. Then the PP/SEBS mixtures were blended with the matrix phase (PS) for another 5 min. Uncompatibilized PP/PS (20/80) blend also experienced the same compounding protocol to assure a same thermal history. Table 1 shows the compositions and designations of blends prepared. The as-prepared PP/PS/SEBS blends were compressed into disk-like plates with 25 mm in diameter and 1.5 mm in thickness at 200 °C and 5 MPa. PS disk-like plates with the same diameter but 1 mm in thickness were compressed at the same condition for the measurement of interfacial tension.

Table 1 Sample and composition for PP/SEBS blends and PP/PS/SEBS blends

Sample	Component (wt%)
	PP	PS	SEBS
PP/SEBS	100	0	0, 2.5, 5, 10, 20, 40
PP/PS/SEBS	20	80	0, 0.5, 1, 2, 4, 8 (B0, B0.5, B1, B2, B4, B8)

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2.2 Rheological measurements

All rheological measurements were carried out on a strain-controlled rotational rheometer (ARES, TA instruments, USA) with a 25 mm parallel plate geometry. The gap was set at 1.3 mm and the temperature was 200 °C. Firstly, dynamic strain sweep was performed on pure components and their blends at 6.28 rad s⁻¹ in a strain range of γ = 0.1–100% to obtain their linear viscoelastic regions. Then dynamic frequency sweep tests were performed from 100 to 0.01 rad s⁻¹ to obtain the viscosity of samples in the linear viscoelastic region. Stress relaxation behaviors of these samples after different step shear strains were examined.

2.3 Morphological and interfacial characterization

The morphologies of as-prepared PP/PS (20/80) blends were characterized by using scanning electron microscopy (SEM; JEOL SJM-5900VL) at a 20 kV accelerating voltage. The samples were fractured in liquid nitrogen and coated with a thin gold layer before measurements. The morphologies were quantified from SEM images by using the free ImageTool software. About 200 droplets were measured per sample. The number and volume averaged diameter of PP droplets were calculated according towhere N_i represents the number of droplets with a diameter of D_i. Transmission electron microscopy (TEM, Tecnai G2F20) was employed to determine the location of SEBS in the PP/PS/SEBS (20/80/4) blends. The sample was first sliced into ultrathin sections of about 60 nm thick using a microtome at −30 °C and then stained by OsO₄ vapor for 24 h. The observation was then carried out at 200 kV. The morphology relaxation behavior of PP/PS (20/80) blends compatibilized with 0, 2 and 8 wt% SEBS after a shear strain of γ = 100% was studied at 200 °C using a microscope (Olympus BX51, Japan), which is equipped with a double-side heated shearing stage (CSS450 from Linkam Scientific, UK) and a liquid nitrogen cooling system. After relaxed for 0, 200 and 600 s after a step shear, samples were quenched rapidly to 90 °C (below the glass transition temperature of PS, T_g,PS = 102 °C) by liquid nitrogen to freeze the morphology.² After that, all samples were fractured in liquid nitrogen at the observation position of the samples (about 7.5 mm from the center) along the shearing direction and characterized by SEM. The interfacial tension between PS and PP (or PP/SEBS) at 200 °C was determined by using the deformed drop retraction method (DDRM).³⁰ Firstly, a small amount of PP or PP/SEBS sample was placed between two PS disks, then this sandwiched sample was tested on the optical-shear system. The retraction behavior of a slightly deformed PP or PP/SEBS droplet in the PS matrix was recorded by a Linksys32 DV image acquisition system and analyzed by a home-developed digital image analysis software package. All the interfacial tension tests were repeated for three times.

3. Results and discussion

3.1 Compatibilization effect of SEBS

Fig. 1 presents the emulsification curve of PP/PS/SEBS blends. The addition of SEBS effectively decreases the size of dispersed phase up to 2 wt% loading, which can be defined as the critical micelle concentration (CMC) suggesting the saturation of copolymers at the interface of blend.³¹ This morphology refinement effect in the copolymer compatibilized blends is commonly ascribed to the suppressed coalescence of dispersed droplets due to steric hindrance³² and promoted breakup due to a decline in interfacial tension.³³ The TEM images (Fig. 2) indicate that SEBS tends to migrate towards the interface of PP and PS during mixing, showing the good chemical affinity of SEBS both with PP and PS component. The TEM images also illustrate that some PP droplets form clusters without coalescence in the compatibilized blend, which may be due to the bridging effect of SEBS interfacial layer.³⁴ With the content of SEBS further increasing from 2 wt% to 8 wt%, the emulsification curve displays a slight increase instead of leveling off as commonly reported.¹⁹

Fig. 1 Emulsification curve of PP/PS (20/80) blends compatibilized with SEBS triblock copolymer. The insets show the SEM images of pure blend (B0) and blend with 8 wt% SEBS (B8).

Fig. 2 TEM images of PP/PS (20/80) blend with 4 wt% SEBS (B4). The scale bar in (a) and (b) is 1 μm and 0.5 μm, respectively.

This extraordinary phenomenon can be explained by the increase of viscosity ratio (p = η_d/η_m, in which η_d and η_m represents the viscosity of dispersed phase and matrix phase, respectively) caused by the SEBS remaining in the PP phase which can suppress the deformation and breakup of droplets during melt mixing.¹² Fig. 1 also shows that the fractured surface in compatibilized blends has less concave holes and convex particles as that found in the pure blend, indicating that the interfacial adhesion between PP and PS has been enhanced greatly with the addition of SEBS.

3.2 Stress relaxation behavior

The determination of linear viscoelastic (LVE) region for compatibilized PP/PS blends is important for the assessment of their stress relaxation behavior. Fig. 3 illustrates the dynamic strain sweep curves of PP, PS, SEBS and PP/PS/SEBS blends in terms of normalized storage modulus (G′/G^′₀). The critical strain (γ_c) in this study was selected as the strain at which the complex viscosity decreased by 10% of the linear plateau value.³⁵ The γ_c of PP and PS is ∼70%, which is much larger than that of SEBS (∼35%) and uncompatibilized blend (∼35%). The addition of SEBS copolymer significant reduces the γ_c of blend and the γ_c of compatibilized blends decreases gradually to ∼4% for the B8 blend. This finding is reasonable because it has been reported that the droplet clusters (as shown in Fig. 2) can enhance the nonlinearity of blends.³⁴

Fig. 3 Normalized storage modulus (G′/G^′₀) vs. strain (γ) curves of PP, PS, SEBS and PP/PS blends compatibilized with different contents of SEBS.

The stress relaxation behavior of PP/PS/SEBS blends after a step strain of 2% (all blends are in the linear region), 8% (only B0–B2 are in the linear region) and 100% (all blends are in the nonlinear region) was investigated in this study. As shown in Fig. 4(a–c), PS and PP present a fast one-step relaxation behavior after a step shear strain of 2%, 8% or 100%. On the contrary, SEBS displays a much higher relaxation modulus and possesses a rather slow relaxation process which cannot finish within the experimental timescale. The uncompatibilized and compatibilized PP/PS blends relax in two consecutive steps, with the first step corresponding to the relaxation of matrix phase and the second slower step due to the relaxation of droplets and/or interfaces.^1,8 With increasing SEBS loading, the relaxation modulus of PP/PS blends after a step strain of 2% and 8% rises gradually but the relaxation rate of the second step becomes slower even for the B0.5 blend with the lowest SEBS content. The accelerated relaxation behavior in compatibilized blends as reported previously^24,25 is not observed here. When the content of SEBS increases to 8 wt%, the relaxation modulus is hard to relax fully at a strain of 2% and 8% (Fig. 4(a and b)). It is noteworthy that after a step strain of 100%, the relaxation modulus curves of SEBS and B2–B8 blends present a short plateau at the beginning period in Fig. 4(c), which may be caused by the slippage between the sample with strong viscoelasticity and the solid plate fixtures.³⁶ As a result, the relaxation modulus data of SEBS and B2–B8 blends are not reliable, and are supposed to be smaller than the real values. For B0 and B1 blends after a step strain of 100%, the enhancement in modulus and retardation in relaxation are also found with increasing SEBS loading. Interestingly, the relaxation modulus of B4 blend, which can relax fully after a step strain of 2% and 8%, becomes hard to relax fully at a strain of 100% (Fig. 4(c)). This means that the strain suffered by the samples is still large enough to enhance the retardation in the relaxation even in the presence of slippage.

Fig. 4 Relaxation moduli of PS, PP, SEBS and PP/PS (20/80) blends compatibilized with various contents of SEBS subjected to a step shear strain of (a) 2%, (b) 8% and (c) 100% at 200 °C. (a′–c′) show the calculated contribution of interface. The inset of (a′) shows the corresponding relaxation moduli of PP/SEBS blends after a step strain of 2%. The arrows in (a) denote the onset of the second step relaxation. The dashed lines denote the unreliable data due to the slippage between sample and fixture.

Usually, the interfacial contribution ΔG(t,γ) in immiscible polymer blends can be calculated by assuming a linear additivity of the component contribution since the components relax almost independently,⁸

ΔG(t,γ) = G(t,γ) − [(1 − f)G_m(t,γ) + fG_d(t,γ)] (2)

where f is the volume fraction of dispersed phase, G(t,γ), G_m(t,γ) and G_d(t,γ) represents the relaxation modulus of blend, matrix phase and dispersed phase, respectively. This equation has been used by Yee et al. to calculate the contribution of interface in the compatibilized PP/poly(methyl methacrylate) (PMMA) blend.¹ While as shown in the inset of Fig. 4(a′), the relaxation moduli of PP/SEBS after a step strain of 2% increase with SEBS loading. It is because compatibilizer used in this study (SEBS) owns a strong viscoelasticity. Therefore, the relaxation modulus of compatibilizer cannot be ignored and the G_d(t,γ) in eqn (2) was treated as the relaxation moduli of corresponding PP/SEBS blends since a two-step compounding process was used. As shown in Fig. 4(a′–c′), the interfacial modulus displays a retarded initial growth due to interfacial slippage³⁷ and a subsequent decline due to the shape recovery of droplets. The peak in ΔG(t,γ) appears at earlier times with the addition of SEBS due to the suppression of interfacial slippage. The interfacial modulus of blends after a step strain of 2% and 8% increases with increasing SEBS content. Despite the fact that the modulus data of B2–B8 blends after a strain of 100% are not reliable, the interface contribution of B0–B1 blends still shows a positive correlation with the SEBS loading. The increase of interfacial modulus with SEBS loading can be interpreted with the refined morphology and increased interfacial area as shown in Fig. 1.

The influence of compatibilization on the strain dependence of the stress relaxation behavior of PP/PS blends subjected to different magnitudes of strain are compared in Fig. 5. PP/PS blend compatibilized with 2 wt% SEBS is chosen here, because the morphology refinement in this blend is more significant than that in B0.5 and B1 blends and the slippage on the fixture after a step strain of 100% is not as obvious as that of B4 and B8 blends. For the pure PP/PS blend, the relaxation modulus declines with increasing strain magnitude as shown in Fig. 5(a). The interfacial contribution exhibits a more evident sensibility on the strain (Fig. 5(a′)). The decrease of relaxation modulus with increasing strain could be attributed to the larger deformation and orientation of droplets along the flow direction.⁹ And the peak in ΔG(t,γ) moves to longer relaxation times with increasing strain, reflecting the aggravation of interfacial slippage. In the presence of 2 wt% SEBS, the relaxation modulus of blend (Fig. 5(b)) and the contribution of interface (Fig. 5(b′)) after a step strain of 2% become more similar to that after 8% strain. The decreased strain sensitivity of interfacial modulus may be related with the suppressed deformation of droplets in the compatibilized blends caused by the enhanced interfacial adhesion and refined morphology as will be discussed later. However, the two-step relaxation behavior in the compatibilized blend is more obvious than that in the pure blend, and displays more noticeable retardation with increasing strain magnitude.

Fig. 5 Relaxation moduli (a and b) and the interfacial contribution (a′ and b′) of B0 and B2 blends after different step strains. The dashed lines are not reliable for the slippage between samples and fixture.

Typically, the driving force for droplet recovery is the interfacial restoring stress, i.e., the ratio of interfacial tension to the radius of droplets.³⁸ The deformation ability of droplets can be suppressed in blends with high viscosity ratios. Upon the addition of copolymer compatibilizers, the increase of viscosity ratio and the decline in interfacial tension are both expected to retard the stress relaxation, while the refinement in morphology will accelerate it.²⁵ Additionally, the improvement in interfacial adhesion, as shown in Fig. 1, will retard the stress relaxation because it can suppress the recovery of deformed droplets by reducing interfacial slippage.³⁷ To provide a more comprehensive insight, abovementioned factors which might be responsible for the retarded stress relaxation behavior are plotted together against the content of SEBS in Fig. 6. The interfacial tension of PP/PS blends was determined according to the deformed droplet retraction method (DDRM) except for the PP/PS/SEBS (20/80/8) blend because the corresponding viscosity ratio (p = 8.5) exceeds the limitation of DDRM method (p ≤ 4).³⁰

Fig. 6 Droplet size, interfacial tension and viscosity ratio of PP/PS blends as a function of SEBS content.

With the SEBS content increasing gradually to 1 wt% (Regime I in Fig. 6), the viscosity ratio stays almost constant. At the same time, the droplet size and interfacial tension decrease sharply, reflecting the successful migration of SEBS towards the PP/PS interface and the improvement of interfacial adhesion. Therefore, the retarded stress relaxation behavior as shown in Fig. 4 indicates that the enhancement in interfacial adhesion and the decline in interfacial tension should play a dominant role over the morphological refinement effect in controlling the stress relaxation behavior when the SEBS content is within 1 wt%. As the content of SEBS increasing from the critical concentration for interfacial tension (1 wt%) to the critical micelle concentration CMC (2 wt%) (Regime II in Fig. 6), both the interfacial tension and viscosity ratio keep almost invariant. However, the stress relaxation is retarded continuously regardless of the accelerating effect caused by the more obvious morphology refinement,²⁴ implying that enhanced interfacial adhesion is expected to be dominate in this regime. After the content of SEBS exceeds the CMC (2 wt%) (Regime III in Fig. 6), both the interfacial tension and droplet size stay nearly constant. The further retardation in the stress relaxation of compatibilized blends (Fig. 4(a–c)) embodies the critical contribution from the significantly increased viscosity ratio as shown in Fig. 6. Above results demonstrate that the dominant factors for the retarded stress relaxation change with the SEBS loading.

Considering the interconnection between morphology and rheology, the morphological relaxation in B0, B2 and B8 blends after a step strain of γ = 100% is illustrated in Fig. 7 and the statistic results are compared in Fig. 8. The degree of deformation was evaluated by the deformation parameter, D = (L − B)/(L + B), where L and B represents the major and minor length of the droplet, respectively. After the application of 100% step strain, the dispersed droplets in the B0 blend acquired an ellipsoidal shape with an average initial D = ∼23% which relax rapidly to ∼2% after 200 s. With the addition of 2 wt% SEBS, the droplets possess a much refined morphology and a slightly decreased initial D = ∼21%. However, the droplets in the B2 blend need longer time (600 s) to retract to D = 2% than that in B0 blend. The droplets in the B8 blend have the smallest size and the lowest initial D of 12% at 0 s. But these droplets are hard to retract into spheres and still hold a deformation of ∼7% after relaxation for 600 s. This suppressed recovery of deformation observed in compatibilized blends is coincident with the retarded stress relaxation behavior displayed in Fig. 4, suggesting the retention of anisotropy in structure.

Fig. 7 Comparison of morphological relaxation for PP/PS blends after been subjected to a step shear strain of γ = 100% and then relaxed at 200 °C for 0, 200 and 600 s: (a–c) B0 blend; (d–f) B2 blend; and (g–i) B8 blend. The images in the fourth column show the enlarged detail of (e), (f) and (i).

Fig. 8 The deformation parameter D = (L − B)/(L + B) as a function of relaxation time for B0, B2 and B8 blend determined from the morphological evolution shown in Fig. 7.

4. Conclusions

The addition of SEBS triblock copolymer with strong viscoelasticity always retarded the stress relaxation of PP/PS (20/80) blends after a step shear strain both in the linear and nonlinear region. Compared with the pure blend, compatibilized blends exhibited weaker strain sensitivity of relaxation modulus but more profound retardation in stress relaxation with increasing strain. The qualitative analysis based on the rheology–morphology correlation suggested that the dominant factors for the retarded stress relaxation changed with the SEBS loading. With the SEBS loading increasing from the critical concentration for interfacial tension to the critical micelle concentration (CMC), the dominant factors for the retarded stress relaxation behavior changed from the decline in interfacial tension to the enhancement in interfacial adhesion. Above the CMC, the increase of viscosity ratio due to the incorporation of SEBS with strong viscoelasticity showed a prominent retarding effect on the stress relaxation. Although the incorporation of SEBS refined the blend morphology, it resulted in structure with higher anisotropy after long-time relaxation.

Acknowledgements

We are grateful to the financial support from the National Natural Science Foundation of China (51373109, 51121001), the Fundamental Research Funds for the Central Universities (2013SCU04A02), the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant 2013TD0013) and State Key Laboratory of Polymer Materials Engineering (Grant no. sklpme2014-3-07). We thank Dr Yong Luo from Analytical Testing Center of Sichuan University for his help in TEM measurements.

Notes and references

1. M. Yee, A. M. C. Souza, T. S. Valera and N. R. Demarquette, Rheol. Acta, 2009, 48, 527–541.
2. Y. D. Lv, Y. J. Huang, M. Q. Kong, H. Zhu, Q. Yang and G. X. Li, Rheol. Acta, 2013, 52, 355–367.
3. M. Q. Kong, Y. J. Huang, G. L. Chen, Q. Yang and G. X. Li, Polymer, 2011, 52, 5231–5236.
4. W. Liu, X. Dong, F. Zou, J. Yang, D. Wang and C. C. Han, J. Chem. Phys., 2013, 139, 114904.
5. C. H. Arns, M. A. Knackstedt, A. P. Roberts and V. W. Pinczewski, Macromolecules, 1999, 32, 5964–5966.
6. F. Yu, H. Deng, Q. Zhang, K. Wang, C. Zhang, F. Chen and Q. Fu, Polymer, 2013, 54, 6425–6436.
7. I. Vinckier, J. Mewis and P. Moldenaers, Rheol. Acta, 1997, 36, 513–523.
8. M. Takahashi, P. H. Macaúbas, K. Okamoto, H. Jinnai and Y. Nishikawa, Polymer, 2007, 48, 2371–2379.
9. M. Iza and M. Bousmina, J. Rheol., 2000, 44, 1363–1384.
10. C. DeLeo, K. Walsh and S. Velankar, J. Rheol., 2011, 55, 713–731.
11. Z. Starý, T. Pemsel, J. Baldrian and H. Münstedt, Polymer, 2012, 53, 1881–1889.
12. D. Wu, Y. Zhang, L. Yuan, M. Zhang and W. Zhou, J. Polym. Sci., Part B: Polym. Phys., 2010, 48, 756–765.
13. C. W. Macosko, P. Guegan, A. K. Khandpur, A. Nakayama, P. Marechal and T. Inoue, Macromolecules, 1996, 29, 5590–5598.
14. J. N. Fowler, T. Saito, R. Gao, E. S. Fried, T. E. Long and D. L. Green, Langmuir, 2012, 28, 2347–2356.
15. Y. Xu, C. M. Thurber, T. P. Lodge and M. A. Hillmyer, Macromolecules, 2012, 45, 9604–9610.
16. S. T. Xi, Y. J. Huang, Q. Yang and G. X. Li, Ind. Eng. Chem. Res., 2014, 53, 5916–5924.
17. A. Singh, R. Prakash and D. Pandey, RSC Adv., 2012, 2, 10316–10323.
18. C. Sailer and U. Handge, Macromolecules, 2008, 41, 4258–4267.
19. M. F. Díaz, S. E. Barbosa and N. J. Capiati, Polymer, 2007, 48, 1058–1065.
20. C.-A. Dai, B. J. Dair, K. H. Dai, C. K. Ober, E. J. Kramer, C.-Y. Hui and L. W. Jelinski, Phys. Rev. Lett., 1994, 73, 2472–2475.
21. I. Labaume, P. Médéric, J. Huitric and T. Aubry, J. Rheol., 2013, 57, 377.
22. A. Maani, B. Blais, M. C. Heuzey and P. J. Carreau, J. Rheol., 2012, 56, 625–647.
23. J. Silva, A. V. Machado, P. Moldenaers and J. Maia, J. Rheol., 2010, 54, 797–813.
24. J. Silva, A. V. Machado and J. Maia, Rheol. Acta, 2007, 46, 1091–1097.
25. N. Mechbal and M. Bousmina, Macromolecules, 2007, 40, 967–975.
26. Y. Huo, G. Groeninckx and P. Moldenaers, Rheol. Acta, 2007, 46, 507–520.
27. E. Eastwood and M. Dadmun, Macromolecules, 2002, 35, 5069–5077.
28. Y. Li and H. Shimizu, Macromolecules, 2009, 42, 2587–2593.
29. H. Macková and D. Horák, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 968–982.
30. P. Xing, M. Bousmina and D. Rodrigue, Macromolecules, 2000, 33, 8020–8034.
31. C. L. Zhang, L. F. Feng, X. P. Gu, S. Hoppe and G. H. Hu, Polym. Eng. Sci., 2010, 50, 2243–2251.
32. J. Kim, H. Zhou, S. T. Nguyen and J. M. Torkelson, Polymer, 2006, 47, 5799–5809.
33. Y. Gong and L. G. Leal, J. Rheol., 2012, 56, 397–433.
34. C. L. DeLeo and S. S. Velankar, J. Rheol., 2008, 52, 1385–1404.
35. A. Durmus, A. Kasgoz and C. W. Macosko, Polymer, 2007, 48, 4492–4502.
36. H. Veenstra, R. M. Hoogvliet, B. Norder and A. de Boer, J. Polym. Sci., Part B: Polym. Phys., 1998, 36, 1795–1804.
37. R. Zhao and C. W. Macosko, J. Rheol., 2002, 46, 145–167.
38. C. L. Tucker III and P. Moldenaers, Annu. Rev. Fluid Mech., 2002, 34, 177–210.

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