Influences of the Viscosity Ratio and Processing Conditions on the

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Influences of viscosity ratio and processing conditions on the formation of highly-oriented ribbons in polymer blends by tape extrusion Long Wang, Lianfang Feng, Xueping Gu, and Cailiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03240 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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Influences of viscosity ratio and processing conditions on the formation of highly-oriented ribbons in polymer blends by tape extrusion

Long WANG, Lian-Fang FENG, Xue-Ping GU, Cai-Liang ZHANG* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, PR China

Abstract: The deformation of the dispersed phase in polymer blends by a tape extrusion was studied systematically under different processing conditions and viscosity ratios of the dispersed phase to the matrix. During the tape extrusion, the spherical dispersed phases were first elongated into rotational ellipsoidal or rod-like shapes, and then were merged to form longer and even endless microfibrils by coalescence, which were eventually squeezed into highly-oriented ribbons. The speed of two running rollers has a large effect on the morphology development of dispersed phase. With the increase of roller surface speed, the stretching and squeezing forces increased dramatically. As a result, the dispersed phase gradually deformed from spherical to ellipsoidal, fibrous and ribbon-like shapes. More interestingly, the tensile strength of the obtained blend film with highly-oriented ribbons increased significantly as well. The viscosity ratio of the dispersed phase to the matrix is another important factor to influence the formation of highly-oriented polymer blend. It is found that a low viscosity ratio facilitates the deformation and coalescence of dispersed phase to form highly-oriented ribbons. Therefore, in order to prepare a well-controlled polymer blend with highly-oriented ribbons, the combination of high viscosity matrix and low viscosity dispersed phase is preferred. Key words: polymer blend, tape extrusion, highly-oriented ribbon, draw ratio, viscosity ratio

*

Author to whom all correspondence should be addressed: [email protected] (C. L. Zhang) 1


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1. INTRODUCTION Blending of two or more existing polymers has become a very attractive approach to develop new polymer materials. The final properties of those polymer blends are strongly related to the shapes and size scales of their microstructures. Comparing with a conventional sphere-matrix blend, a microfibril-matrix blend counterpart can dramatically improve the mechanical properties1-6. For this reason, the dispersed phase of polymer blend often needs to be processed into different shapes and sizes to meet specific end-use requirements. Nowadays, polymer blend with a hierarchical structure such as ribbon-matrix or lamella-matrix has attracted more attention because it can significantly widen the application range from mechanical enhancement7,8 to electrical conductive composite9-11, gas barrier12-15 and excellent optical performance16-18. For example, thin and long lamellas of highly-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) in poly(lactic acid) (PLA) matrix improved the elongation at break and gas barrier property by a factor of 3 and 2 over neat PLA, respectively12. Currently, those hierarchical structural blends are mainly produced by using a co-extrusion process in which polymers are extruded and joined together in a feedblock or die to form a single structure with multiple layers19-22. This technology, however, requires a high cost and complex control, and presents a recyclability limitation. It is well known that a related shear or tension flow field in the process of mixing can dramatically influence the final morphology of polymer blend. Various morphologies such as ellipsoidal, rod-like and microfibrillar shapes have been generated successfully by drawing extrudate along the extrusion direction to deform the spherical dispersed phase in the process of post-extrusion. Moreover, the deformation degree of dispersed phase is determined by various parameters such as draw ratio23,

24

, viscosity ratio25,

26

, and interfacial tension27. As the draw ratio

increases, the dispersed phase evolves gradually from sphere to ellipsoid and rod-like shape, and finally to microfibril24. Yi et al.28 found that a lower viscosity ratio of the dispersed phase to the matrix resulted in smaller and more uniform dispersed phase 2



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particles, which could deform into finer microfibrils with a narrower diameter distribution. Therefore, this may provide us with numerous opportunities for tuning the phase behavior to obtain hierarchical structural polymer blends. Our previous work had successfully prepared a polymer blend with highly-oriented ribbons to achieve a color effect by a simple approach of tape extrusion29. In the post-extrusion process of this method, a squeezing force perpendicular to extrusion direction was exerted to deform the dispersed phase into ribbons besides a stretching force along the extrusion direction. However, the formation mechanism of highly-oriented ribbons and its influencing factors were not fully understood. Therefore, in this work, the effects of the roller surface speed producing stretching and squeezing forces, and the viscosity ratio of the dispersed phase to the matrix on the morphology development of polymer blend in the process of tape extrusion were studied systematically to illuminate the formation mechanism of highly-oriented ribbons. Moreover, the mechanical properties of the obtained polymer blends with highly-oriented ribbons were also investigated.

2. EXPERIMENTAL 2.1 Materials Polymers used in this study include polystyrene (PS), polyamide 6 (PA6), and polypropylene (PP). Each polymer has two types, which are marked as “H” corresponding to the high viscosity one and “L” corresponding to the low viscosity one, respectively. The selected information of those polymers is gathered in Table 1. Tetrahydrofuran (THF) and formic acid were used as received from Sinopharm Chemical Reagent Co., Ltd.

3


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Table 1. Selected information of polymer materials Polymer

Supplier

Mode

Melt index (g/10min)

PS-H

Yangzi petrochemical

158k

4.2 (200 ºC, 5 kg)

PS-L

Chimei Group

PG-33

8.0 (200 ºC, 5 kg)

PA6-H

UBE Nylon Ltd., Thailand

1030B

--

PA6-L

UBE Nylon Ltd., Thailand

1013B

--

PP-H

Sinopec Corp

F401

2.4 (230 ºC, 2.16 kg)

PP-L

Huahai Refinery and Petrochemical Co. Ltd

085

8.5 (230 ºC, 2.16 kg)

2.2 Preparation of ribbon-matrix blend Prior to the tape extrusion, PS/PA6 and PS/PP blends with different compositions were compounded and extruded into chips at 230 ºC and 215 ºC, respectively, in a twin-screw extruder (HAAKE Polylab OS) with a diameter of 16 mm and a length-to-diameter ratio of 48. The screw speed and feeding rate were set as 100 rpm and 1.3 kg/h, respectively. After going through an aperture die, the extrudate was cooled and solidified on a conveyor, and then was cut into chips using a chopping machine. After drying at 100 ºC for 12 h in a vacuum oven, the pre-blended chips of PS/PA6 or PS/PP blend were used for tape extrusion in a single-screw extruder (Brabender Measurement & Control System) with a diameter of 19 mm and length-to-diameter ratio of 25. Barrel temperatures were set at 230 ºC and 215 ºC for PS/PA6 and PS/PP blend systems, respectively. After the barrel temperatures were stable, those pre-blended chips were fed into the single-screw extruder, and the screw started to run at a speed of 100 rpm. The extrudate was drawn from the outlet of slit die with 15 mm in width and 1 mm in thickness by two water-cooled steel rollers having a polished chrome surface. The temperatures of the squeezing roller were controlled at 25 ºC by circulating water. The distance between the outlet of slit die and rollers was about 5 cm. Different combinations of stretching and squeezing forces

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were obtained by adjusting the running speed of rollers. After being cooled to room temperature, the extruded films were collected.

2.3 Characterizations Thermo Scientific HAAKE MARS rotational rheometer (RS6000) with a dynamic mode was used to analyze the rheological behaviors of PS, PA6 and PP in a frequency ranged from 0.1 Hz to 100 Hz at different temperatures. Prior to the measurement, the samples were made into disks of 20 mm in diameter and about 1 mm in thickness. Scanning electron microscopy (SEM, Spura 55Vp) was used to characterize the morphologies of the obtained blends. PS/PA6 and PS/PP blends were fractured in liquid nitrogen. For some samples, they were etched first by solvents for 24 h at room temperature to selectively remove one phase, and then they were dried in the vacuum oven at 80 ºC for 12 h. After sputtering the gold, the samples were observed by SEM at an acceleration voltage of 5.0 KV. A semiautomatic image analysis method was used to determine the diameter of the dispersed phase domain, which was characterized by the volume average particle diameter, dv, defined as Eq (1):

dv =

∑n d ∑n d

4

i

i

i

i

3

(1)

Tension properties of the obtained blends were tested by Zwick/Roell Z020 at a crosshead rate of 1 mm/min at 25 °C. At least five measurements for each sample were performed, and then the average value was reported.

3. RESULTS AND DISCUSSION 3.1 Rheology behaviors of PS, PA6 and PP Figure 1A and 1B show the complex viscosities (η*) of PS and PA6 at 230 ºC, and PS and PP at 215 ºC as a function of frequency, respectively. Obviously, the complex viscosities of PS-H and PS-L are higher than those of PA6-H and PA6-L at 230 ºC. At 215 ºC, the complex viscosity of PS-H is close to that of PP-H, and the latter is much higher than that of PP-L. The difference can be further clearly seen 5


from the complex viscosity ratios of PS/PA6 at 230 ºC and PS/PP at 215 ºC as a function of frequency, as shown in Figure 2.

η * (Pa•s)

A

PS-H PS-L PA6-H PA6-L

3

10

2

10

0.1

1

10

100

Frequency (Hz)

PS-H PP-H PP-L

B

η * (Pa•s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

4

10

3

10

2

10

1

0.1

1

10

100

Frequency (Hz)

Figure 1. Complex viscosities of polymer materials at 230 °C (A) and 215 °C (B) as a function of frequency

10

2

PS-H/PP-L PS-H/PA6-L PS-L/PA6-H PS-H/PP-H

Viscosity ratio

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10

1

10

0

0.1

1

10

100

Frequency (Hz)

Figure 2. Complex viscosity ratios of PS/PA6 at 230 ºC and PS/PP at 215 ºC as a function of frequency. 6



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3.2 Morphology evolution of polymer blend in the process of tape extrusion Taking the tape extrusion of PS-H/PA6-L (80/20 by mass) blend system at a roller surface speed of 4.5 m/min as an example, its morphology evolution was examined by longitudinal section images of the samples taken at different stages shown in Figure 3. To avoid any deformation of phase morphology, those taken samples were frozen immediately in liquid nitrogen. Before extrusion, the dominating morphology was the spherical domain of PA6-L in the continuous PS-H matrix, as shown in Figure 3A. Upon being extruded out of the slit die, some spherical particles of PA6-L started to deform into rod-like shapes (Figure 3B). With the progress of the drawing, the quantity of such rods increased. Moreover, they were gradually evolved into long microfibrils along the drawing direction (Figure 3C). Further approaching the running rollers, microfibrils became thinner and longer (Figure 3D). After being squeezed by the running rollers, those microfibrils were evolved into ribbons (Figure 3E). The morphologies of dispersed phase PA6-L after PS-H matrix was removed by THF, as shown in the inserted images in Figure 3, more clearly verify the morphological evolution from spheres into ribbons with a very high aspect ratio and more than 1000 µm in length in the process of tape extrusion. This is in line with a previous result for PP/PS blend system29.

7


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Figure 3. Morphology evolution of PS-H/PA6-L (80/20 by mass) blend along the tape extrusion line at a roller surface speed of 4.5 m/min. The inserted images are the morphologies of dispersed PA6-L phase after PS-H matrix was removed by THF.

This interesting morphology evolution in the process of tape extrusion results essentially from two driving forces: one is the stretching force to produce a 8



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longitudinal deformation of spherical dispersed phase into rob-like and/or microfibrillar shapes; the other one is the squeezing force to create a latitudinal deformation causing the formation of ribbons. Before being squeezed by the running rollers, the extrudate is mainly subjected to the stretching force leading to an elongation flow field. Under this flow field, the spherical dispersed phases are first elongated into rotational ellipsoidal or rod-like shapes along the drawing direction. However, it should be noted that some dispersed phase spheres with small sizes have not deformed but maintained their original shapes at the early stage of tape extrusion as shown in Figure 3B, which indicates that small dispersed phase spheres are more difficult to deform than large ones. Nevertheless, they almost disappear completely with the progress of drawing. This may imply that a significant potential exists for coalescence of particle-particle, as reported the other literatures30-32. Moreover, comparing the initial dispersed phase spheres in Figure 3A with the formed microfibrils in Figure 3C, their average diameter is not so different but the length of the latter is much longer than that of the former, which further indicates that the coalescence between the deformed dispersed phase should take place during drawing. However, further drawing can result in a reduction of the coalescence of formed microfibrills while the longer and thinner deformation of those mircrofirills becomes predominant, which can be seen from Figure 3C to Figure 3D. Thus, the long and even endless microfibril formation in an immiscible polymer blend under the stretching force is influenced significantly by the deformation and coalescence of dispersed phase. As the extrudate contacts with two running rollers, the squeezing force started to play a main role in the deformation of dispersed microfibrils into ribbons. In order to further confirm the function of squeezing force for the formation of ribbons, the melt of PS-H/PA6-L (80/20 by mass) blend was stretched without any squeezing force on a single roller at a drawing speed of 4.5 m/min from an aperture die, as shown in Figure 4. The diameter of the aperture die was 4 mm, and the distance between aperture die and rollers was 1 m. The draw ratio maintained at 6.7. Before PS-H matrix was etched by THF, the diameter of obtained fiber was 0.6 mm. After PS-H matrix was removed 9


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by THF, the obtained PA6-L phase does not exhibit any ribbon structure but only microfibrillar structure, as shown in Figure 4A. This indicates that a squeezing force is vital to the morphology evolution from microfibrils to ribbons. Therefore, the stretching and squeezing forces are predominant for the stage of microfibril and ribbon formation, respectively.

Figure 4. Schematics of filament extrusion for PS-H/PA6-L (80/20 by mass) blend by an aperture die and (A) the morphology of dispersed phase PA6-L after PS-matrix was removed by THF. In addition, it should be noted that besides the morphology of dispersed phase, the size of extrudate film has also changed, as shown in Figure 3. Before being squeezed by the rollers, both width and thickness of the extrudate film were continuously reduced due to the stretching force. During squeezing, the thickness of extrudate film further decreased but the width increased. More importantly, draw and compression ratios, reflecting the stretching and squeezing forces quantitatively, can be calculated from the size change of extrudate film as expressed in Eq (2) and Eq (3), respectively. Draw ratio =

D1 × L1 D3 × L3

Compression ratio =

D2 × L2 D3 × L3

(2)

(3)

in which, D1×L1 is the area of the die; D2×L2 is the area of cross-section of the film before being squeezed by the rollers; D3×L3 is the area of cross-section of the ultimate film, as shown in Figure 3. 10



3.3 Effect of the roller surface speed It is generally believed that capillary number (Ca=dηm γ& /σ, where d is the initial radius of dispersed phase, γ& is the shear rate, σ is the interfacial tension, ηm is the viscosity of matrix), considered as the ratio of the viscous stress to the interfacial stress, is one of the main factors influencing the morphology development of the dispersed phase. When the value of Ca is higher than a critical capillary number, the deformation of spherical dispersed phase occurs. In this work, the stretching and squeezing forces producing the viscous stress were controlled by two running rollers coupled to an electric motor with different variables. Figure 5 shows the effect of roller surface speed on the draw and compression ratios. It is obvious that both draw and compression ratios increase linearly and significantly with the increase of the roller surface speed, which can result in a strong elongation flow field before being squeezed and a strong compression flow field during squeezing. As a result, the final morphology of polymer blend can be influenced significantly by the roller surface

9

1.6

8

1.5

7

1.4

6

1.3

5

1.2

4

1.1

3

Compression ratio

speed.

Draw ratio

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1.0 1

2

3

4

5

6

7

8

Roller surface speed (m/min) Figure 5. Effect of the roller surface speed on the draw and compression ratios. Figure 6 presents the effect of roller surface speed on the morphology of PS-H/PA6-L (80/20 by mass) blend. At a roller surface speed of 1.7 m/min, the draw and compression ratios were 3.9 and 1.1, respectively. As a result, only a few PA6 11


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spheres were interconnected via coalescence to form rod-like shapes (Figure 6A), which implied that a low stretching force could not deform small dispersed spheres or interconnect them via coalescent, and the squeezing force could also not change their shapes in the latitudinal direction.

Figure 6. SEM images of PS-H/PA6-L (80/20 by mass) blend along the tape extrusion direction at different roller surface speeds. (A) 1.7 m/min; (B) 3.1 m/min; (C) 5.6 m/min; (D) 6.6 m/min; (E) 7.2 m/min. When the roller surface speed increased to 3.1 m/min, the draw ratio was 4.3 and the compression ratio was 1.2. All PA6 spheres were changed to long and slightly flat microfibrils completely (Figure 6B). When the roller surface speed reached 5.6 m/min, the draw and compression ratios continued to increase so that the PA6 microfibrils 12



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were evolved into ribbons to form a hierarchical film (Figure 6C). Nevertheless, further increasing the roller surface speed, the sizes of ribbons became narrower (Figure 6D and 6E). More specifically, comparing to the roller surface speed of 5.6 m/min, the widths of obtained films at the roller surface speed of 6.6 m/min and 7.2 m/min reduced from 2.9 µm to 2.1 µm and 1.3 µm, respectively. In short, at a low roller surface speed, the stretching and squeezing forces are too low to form microfibrils by deformation and coalescence of dispersed phase, but at a high roller surface speed, a big stretching force produced makes the formed microfibrils thin so the resulted ribbons are narrow. It should be mentioned that there may exist a physical breakage of the extruded ribbons at a higher draw ratio despite an experiment could not be carried out at such high draw ratio in this work. It is worthy to investigate the effect of different roller surface speeds on the tensile strength of obtained PS-H/PA6-L (80/20 by mass) films along the drawing direction, as shown in Figure 7. The pure PS-H films extruded in the same way are also shown in Figure 7 for comparison. It can be seen that the roller surface speed has little effect on the tensile strength of pure PS-H film. However, the tensile strength of PS-H/PA6-L (80/20 by mass) film continuously increases with the acceleration of roller surface speed. Without any drawing, the tensile strength of the obtained PS-H/PA6-L film with spherical dispersed phases was 25.0 MPa, which was much lower than 34.8 MPa of pure PS-L film. This may be caused by a poor interfacial bonding between PS-H and PA6-L. Interestingly, when the dispersed phase was evolved from spheres into microfibrils at the roller surface speed of 3.1 m/min, the tensile strength increased dramatically to 39.1 MPa from 25.0 MPa, which was much higher than that of pure PS-L film. This indicates that the presence of in-situ microfibrillar can enhance the tensile strength, as reported in other literatures1-6. When the roller surface speed continued to increase to 5.6 m/min, the tensile strength further increased to 43.1 MPa. At this time, the morphology of dispersed phase changed to ribbons from microfibrils. It seems that the ribbon structure is better to strengthen the polymer material than microfibril. Nevertheless, it is more likely due to (i) a better orientation of dispersed phase including its crystallization and structure in the drawing 13


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direction at a high roller surface speed, and (ii) a smaller but more uniform and denser distribution of dispersed phase in the matrix with the increase of roller surface speed. This can be further verified by a stronger film obtained at a roller surface speed of 6.6 m/min. More specifically, although the width and thickness of ribbon obtained at 6.6 m/min were smaller than those obtained at 5.6 m/min, the tensile strength of the former was above 10% larger than that of the latter. Additionally, comparing to a regular polymer composite with nanoparticles such as nano-fiber or clay as additives, the dispersed polymer itself can be in-situ converted into microfibrils or ribbons by tape extrusion to form the microfibril-matrix or ribbon-matrix blend. Moreover, the synergistic effect of the high alignment and orientation of in-situ formed microfibrils or ribbons and its crystallization may improve the tensile strength dramatically, which is even more effective than nanoparticle in enforcement. 60 PS/PA6 80/20 Pure PS

Tensile strength (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

20

0

5.6

3.1

0

6.6

Roller surface speed (m/min)

Figure 7. Effect of roller surface speed on the tensile strength of obtained PS-H/PA6-L (80/20 by mass) and pure PS-H films along the drawing direction.

3.4 Effect of the viscosity ratio It is well known that besides capillary number, viscosity ratio (λ=ηd/ηm, where ηd is the viscosity of dispersed phase) is another important dimensionless parameter influencing the deformation of the dispersed phase, which subsequently determines the formation of in-situ ribbons25, 26. In order to investigate the effect of viscosity ratio on the formation of highly-oriented ribbons, different polymer blend systems were 14



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chosen as shown in Table 2. In this work, the width (L1) and height (D1) of the slit die were 15 mm and 1 mm, respectively. The apparent shear rates ( γ& ) in the die could be estimated by Eq (4)28, 33:

γ& =

6q L1 D12

(4)

in which, q, a volumetric output of the single-screw extruder, was 153 mm3/min at 100 rpm. Thus, the apparent shear rate was calculated as 61.6 s-1 and was used in the following work to investigate the influence of viscosity ratio on the formation of highly-oriented ribbons. Assume Cox-Merz rule is valid in this work, the viscosity at apparent shear rate (61.6 s-1) corresponds to that at the frequency of 10 Hz in Figure 2. Thus, viscosity ratios of different blend systems at this frequency can be obtained, as shown in Table 2.

Table 2. Complex viscosity ratio of PS and PA6 at 230°C, and PS and PP at 215°C at the frequency of 10 Hz for those chosen polymer blends with different compositions. Blend code

Matrix

Dispersed phase

Composition

λ

Blend-1

PS-H

PA6-L

80/20

0.45

Blend-2

PS-L

PA6-H

80/20

0.79

Blend-3

PA6-H

PS-L

80/20

1.27

Blend-4

PA6-L

PS-H

80/20

2.22

Blend-5

PS-H

PP-L

90/10

0.13

Blend-6

PS-H

PP-H

90/10

1.00

Blend-7

PP-H

PS-H

90/10

1.00

Blend-8

PP-L

PS-H

90/10

7.69

Figure 8A, 8B, 8C and 8D present SEM images of the cryofractured surfaces of unstrected and unsqueezed (i.e., conventional) Blend-1, Blend-2, Blend-3 and Blend-4 after the dispersed phase were etched by formic acid or THF. Obviously, those blends showed a typical incompatible morphology containing spherical dispersed phase in the continuous matrix. Moreover, the size of dispersed phase was smaller for the 15


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blend system with a lower viscosity ratio. More specifically, for the PS/PA6 (80/20 by mass) blend systems, the diameter of spherical dispersed phase was 1.7 µm for Blend-1 with the viscosity ratio of 0.45, and 3.8 µm for Blend-2 with the viscosity ratio of 0.79. For the PS/PA6 (20/80 by mass) blend systems, the diameter of dispersed sphere was 8.0 µm for Blend-3 with the viscosity ratio of 1.27, and 8.9 µm for Blend-4 with the viscosity ratio of 2.22. The reason for that is because the higher viscosity matrix can transfer shear stress into the lower viscosity dispersed phase more efficiently, which leads to the acceleration of deformation and breakup of dispersed phase. In other words, if the viscosity ratio of polymer blend is lower, the deformation of dispersed phase is easier, which seems to imply that microfibrils or ribbons can be formed more easily for polymer blend with a lower viscosity ratio. However, it must be mentioned that a big spherical dispersed phase can be beneficial for tape extrusion because it can deform more easily due to a high value of capillary number. SEM images of those obtained PS/PA6 blends via tape extrusion before and after the matrix is removed by THF or formic acid are shown in Figure 8. Comparing with the undrawn blends, the dispersed phases of those obtained blends by tape extrusion undergone the elongation deformation, which was more obvious for the blend system with a low viscosity ratio. For example, highly-oriented ribbons can be seen in the Blend-1 with the viscosity ratio of 0.45 (Figure 8E and 8I). However, there was only a little deformation of the dispersed phase in the corresponding phase-inversion Blend-4 with the viscosity ratio of 2.22 (Figure 8H and 8L). The same tendency can also be observed from the comparison of Blend-2 with Blend-3. Thus, a preliminary conclusion can be made that the polymer blend with a low viscosity ratio facilitates the deformation and coalescence of dispersed phase to form highly-oriented ribbons. This can be further confirmed by comparing the dispersed phase morphologies of Blend-3 and Blend-4. In contrast to the little deformed PS particles for Blend-4 with the viscosity ratio of 2.22 (Figure 8H and 8L), microfibrillar and rod-like shapes of dispersed phase were formed at the edge and center for Blend-3 with the viscosity ratio of 1.27 (Figure 8G and 8K), respectively. 16



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It should also be noted that the diameters of undrawn Blend-1 and Blend-2 were much smaller than those of Blend-3 and Blend-4, but the uniform ribbons of dispersed phase could only be formed in the former. It seems to be unexpected since the spherical dispersed phase with a bigger size is usually more favorable to deform. However, it must be reminded that a big difference between the former blends and the latter blends is the viscosity ratio. The viscosity ratios of the former blends are much smaller than those of the latter blends. This may further imply that the viscosity ratio of polymer blend is the dominant factor to produce the highly-oriented ribbons.

Figure 8. SEM images of (A-D) PS/PA6 blends before stretched with the dispersed phase being etched; (E-H) longitudinal section of obtained PS/PA6 film by tape extrusion; (I, J) dispersed phase after PS matrix was removed; and (K, L) dispersed phase after PA6 matrix was removed. Blend-1: A, E, I; Blend-2: B, F, J; Blend-3: C, G, K; Blend-4: D, H, L. 17


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The same conclusion can also be obtained from the blend system of PP and PS. This blend system should be more miscible than PS/PA6 blend system because the difference of solubility parameters between PP (16.8 (J/cm3)0.5) and PS (18.6 (J/cm3)0.5) is less than that between PA6 (23.8 (J/cm3)0.5) and PS34, 35. As shown in Figure 9, it is easy to draw and squeeze the small dispersed PP spheres into highly-oriented ribbons for Blend-5 which had a low viscosity ratio of 0.13 (Figure 9E and 9I), but the big dispersed PS spheres for Blend-8 with a high viscosity ratio of 7.69 were only deformed into ellipsoids (Figure 9H). Additionally, for the Blend-6 and Blend-7 with same viscosity ratio of 1.0, their dispersed spheres can be stretched into highly-orientated ribbons (Figure 9F, 9J and 9G).

Figure 9 SEM images of (A-D) PS/PP blends before stretched; (E-H) longitudinal section of obtained PS/PP film by tape extrusion; (I, J) dispersed phase after PS matrix was removed. Blend-5: A, E, I; Blend-6: B, F, J; Blend-7: C, G, K; Blend-8: D, H, L. 18



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4. CONCLUSIONS Tape extrusion of heterogeneous polymer blends offers a simple and scalable method to produce polymer blends with highly-oriented ribbons in the matrix. The formation of highly-oriented ribbons is mainly from the deformation and coalescence of dispersed phase. Under the stretching force, the spherical dispersed phases are first elongated to deform, and then contact with each other to merge to form longer and even endless microfibrils. The microfibrils are further squeezed into ribbons. Moreover, as the roller surface speed increases, both stretching and squeezing forces increase linearly. This is beneficial to form the uniform and highly-oriented ribbons. Meanwhile, the tensile strength of the obtained film increased greatly with the acceleration of the roller surface speed. Additionally, in order to get a polymer blend with highly-oriented ribbons, the combination of high viscosity matrix and low viscosity dispersed phase is preferred.

ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (51203133), the Zhejiang Provincial Natural Science Foundation of China (LY15E030001), and the Fundamental Research Funds for the Central Universities (2015FZA4026) for their financial support.

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Influences of the Viscosity Ratio and Processing Conditions on the

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