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In this work, three different polyolefin blends (FCC, FCS1 and FCS2) have been used during coextrusion experiments.

Uniaxial extensional viscosity at low extensional rates was measured using ARES equipped by the SER Universal Testing Platform (SER-HV-A01 model) from Xpansion Instruments [13-14] whereas twin bore Rosand RH7-2 control speed capillary rheometer has been used to determine shear and uniaxial extensional viscosity at high deformation rates by using Cogswell model. The measured rheological data for corresponding samples are provided in Figures 1-4.

Experimental work has been done on two different feedblock geometries (see Figures 5-6) at different processing conditions which are summarized in Table 1.

where τ is the stress tensor and W denotes elastic potential depends on the invariant I

where G is the linear elastic modulus, β and n are nonlinear parameters. In this work, both non-linear parameters β and n were adjusted to be zero. Dissipation term, b, is included in irreversible rate of strain equation, e

where δ stands for the unit tensor. The Elastic strain is related to the deformation history as

Here, D denotes the rate of deformation tensor. Modified dissipation term, b is defined by

where ξ(λ) and ν(λ) are adjustable parameters which it is possible to change with the relaxation time, λ.

Modified Leonov model has been used to fit all rheological data by using relaxation spectrum, flow activation energy and particular non-linear model parameters which are summarized in Tables 2-4. As it can been seen in Figures 1-3, modified Leonov model has excellent capability to describe rheological data for all tested polymer samples which fully justify the use this model in the theoretical flow analysis. The Figure 4 compares the differences between FCS1 and FCS2 sample. Clearly, even if the flow behavior in the shear flow is very similar, the extensional viscosity of both samples differs significantly. In more detail, the sample FCS2 is more extensional strain hardening than the FCS1 sample.

Where τ

Based on Table 5, it can also be concluded that the use of the FCS2 in the multilayer flow leads to much more unstable flow from the interfacial instabilities point of view than the use of the FCS1. This can be explained by the significantly higher extensional viscosity of FCS2 sample in contrary to FCS1 sample as shown in Figure 4.

- It has been found that the onset of the interfacial instabilities can be captured by the quantification of the stretching discontinuity occurring at the interface between different polymers through the sign change of the FDNSD.
- It has been revealed that the FCS2/FCC interface is much more sensitive to the interfacial instability onset than FCS1/FCC interface because the much higher FCS2 extensional viscosity in comparison to FCS1.

2. M. Zatloukal, C. Tzoganakis, J. Vlcek, P. Saha, International Polymer Processing, 16: (2), 198 (2001).

3. M. Zatloukal, J. Vlcek, C. Tzoganakis, P. Saha, Polymer Engineering and Science, 42: (7), 1520 (2002).

4. M. Zatloukal, W. Kopytko, A. Lengalova, J. Vlcek, Journal of Applied Polymer Science, 98, 153 (2005).

5. M. Martyn, R. Spares, P. Coates, M. Zatloukal, Journal of Non-Newtonian Fluid Mechanics, 156(3), 150 (2009).

6. M.T. Martyn, R. Spares, P.D Coates and M. Zatloukal, Plast. Rubber Comp., 33(1), 27 (2004).

7. M. Zatloukal., J. De Witte, Plastics Rubber and Composites 35(4), 149 (2006).

8. M. Zatloukal, W. Kopytko, P. Saha, M. Martyn., Plastics Rubber and Composites 34(9), 403 (2005).

9. M. Zatloukal, M.T. Martyn, P.D. Coates, J. Vlcek, Plastics Rubber and Composites 33(7), 305, (2004).

10. M. Zatloukal, J. Vlček, C. Tzoganakis, P.Sáha, SPE Antec papers, 44, (2001).

11. M. Zatloukal, J. Vlček, C. Tzoganakis, P.Sáha., TAPPI, San Diego, Canada (2001).

12. M. Zatloukal, M.T. Martyn, P.D. Coates, J. Vlcek, Plastics Rubber and Composites 33 (7): 305-309, (2004).

13. Sentmanat M.L., Rheol. Acta 43, 657 (2004).

14. Sentmanat M.L., Wang B.N., MCKinley G.H., J. Rheol. 49, 585 (2005).

15. Leonov A.I., Rheol. Acta 15, 85 (1976).

16. Zatloukal M., J. Non-Newtonian Fluid Mech. 113, 209 (2003).

Key Words: Coextrusion, Interfacial instabilities, Feedblock Coextrusion die.

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