Marzieh Ebrahimi

Surface fractionation effects on slip of polydisperse polymer melts

The slip behavior of several high-density polyethylenes with broad range of molecular weight (MW) including bimodals is studied as a function of molecular weight (MW) and its distribution. A formulation similar to the double reptation theory is used to predict the slip velocity of the studied polymers as a function of MWD coupled with a model of surface molecular weight fractionation. While surface fractionation has a minor effect on slip of narrow to moderate MWD polymers (particularly unimodal), its role is significant for broad bimodal MWD polymers. The entropy driven migration of short chains toward the die wall has a profound effect and should be considered in order to calculate the effective MWD on the boundary layer and thus the correct magnitude of wall slip.

Slip of polymer melts over micro/nano-patterned metallic surfaces

The slip behavior of high-density polyethylenes (HDPEs) is studied over surfaces of different topology and surface energy. Laser ablation has been used to micro/nano-pattern the surface of dies in order to examine the effect of surface roughness on slip. In addition, fluoroalkyl silane-based coatings on smooth and patterned substrates were used to understand the effect of surface energy on slip. Surface roughness and surface energy effects were incorporated into the double reptation slip model (Ebrahimi et al., J. Rheol., 2015, 59, 885-901) in order to predict the slip velocity of studied polymers on different substrates. It was found that for dies with rough surfaces, polymer melt penetrates into the cavities of the substrate (depending on the depth and the distance between the asperities), thus decreasing wall slip. On the other hand, silanization of the surface increases the slip velocity of polymers in the case of smooth die, although it has a negligible effect on rough dies. Interestingly, the slip velocity of the studied polymers on various substrates of different degrees of roughness and surface energy, were brought into a mastercurve by modifying the double reptation slip velocity model.

Non-isothermal extrudate swell

The non-isothermal extrudate swell of a high molecular weight high-density polyethylene (HDPE) in long capillary and slit dies is studied numerically (ANSYS POLYFLOW®) using an integral K-BKZ constitutive model including crystallization kinetics, determined experimentally. The Nakamura model is used for crystallization of the HDPE, where the crystallization rate parameter is evaluated by using the well-known Ziabicki equation. This non-isothermal extrudate swell phenomenon is simulated using the pseudo-time integral K-BKZ model with the Wagner damping function along with the differential form of the Nakamura model to account for the crystallization of the extrudate. The swell measurements were carried out under non-isothermal conditions by extruding the polymer melt at 200 °C through long capillary and slit dies to ambient air at 25 °C, 110 °C, and 200 °C. The numerical results are found to be in excellent agreement with experimental observations.

Dynamic slip of polydisperse linear polymers using partitioned plate

The slip velocity of an industrial grade high molecular weight high-density polyethylene (HDPE) is studied in steady and dynamic shear experiments using a stress/strain controlled rotational rheometer equipped with a parallel partitioned plate geometry. Moreover, fluoroalkyl silane-based coating is used to understand the effect of surface energy on slip in steady and dynamic conditions. The multimode integral Kaye-Bernstein-Kearsley-Zapas constitutive model is applied to predict the transient shear response of the HDPE melt obtained from rotational rheometer. It is found that a dynamic slip model with a slip relaxation time is needed to adequately predict the experimental data at large shear deformations. Comparison of the results before and after coating shows that the slip velocity is largely affected by surface energy. Decreasing surface energy by coating increases slip velocity and decreases the slip relaxation time.

On the molecular weight dependence of slip velocity of polymer melts

Theoretical models regarding the slip of polymer melts are compared with the experimental results of several researchers on the basis of the molecular weight (Mw) dependence of the slip velocity (Vs). Using simple assumptions, it is shown that agreement between theory and experiment can only be achieved if the models are adjusted to address the random adsorption density of polymer chains on high energy surfaces and the stiffness of the adsorbed chains as assessed by the molecular weight of entanglements. With respect to adsorption density, the transition from the mushroom to the overlap regime results in the development of interactions between adsorbed chain segments which changes the Vs-Mw scaling. As these interactions involve mutual entanglements, their development is hindered by the stiffness of the adsorbed chains. Accordingly, a novel regime within the overlap regime is proposed to exist, observed when the segments of the adsorbed chains are not flexible enough to interact with the neighboring chains.Theoretical models regarding the slip of polymer melts are compared with the experimental results of several researchers on the basis of the molecular weight (Mw) dependence of the slip velocity (Vs). Using simple assumptions, it is shown that agreement between theory and experiment can only be achieved if the models are adjusted to address the random adsorption density of polymer chains on high energy surfaces and the stiffness of the adsorbed chains as assessed by the molecular weight of entanglements. With respect to adsorption density, the transition from the mushroom to the overlap regime results in the development of interactions between adsorbed chain segments which changes the Vs-Mw scaling. As these interactions involve mutual entanglements, their development is hindered by the stiffness of the adsorbed chains. Accordingly, a novel regime within the overlap regime is proposed to exist, observed when the segments of the adsorbed chains are not flexible enough to interact with the neighboring chains.

Melt fracture of linear low-density polyethylenes: Die geometry and molecular weight characteristics

The melt fracture phenomena of three linear low-density polyethylenes are investigated as a function of die geometry (capillary, slit, and annular) and molecular weight and its distribution. The onset of melt fracture instabilities is determined by using capillary rheometry, mainly studying the extrudate appearance using optical microscopy. It is found that the onset of flow instabilities (melt fracture phenomena) is significantly affected by die geometry and molecular weight characteristics of the polymers. Use of annular die eliminates the stick-slip transition (oscillating melt fracture) and delays the onset of sharkskin to higher values of shear rate and shear stress. Moreover, it is shown that the molecular weight characteristics of the polymers are well correlated with critical conditions for the onset of flow instabilities based on a criterion proposed in the literature [A. Allal et al., “Relationships between molecular structure and sharkskin defect for linear polymers,” J. Non-Newtonian Fluid Mech. 134, 127–135 (2006) and A. Allal and B. Vergnes, “Molecular design to eliminate sharkskin defect for linear polymers,” J. Non-Newtonian Fluid Mech. 146, 45–50 (2007)].The melt fracture phenomena of three linear low-density polyethylenes are investigated as a function of die geometry (capillary, slit, and annular) and molecular weight and its distribution. The onset of melt fracture instabilities is determined by using capillary rheometry, mainly studying the extrudate appearance using optical microscopy. It is found that the onset of flow instabilities (melt fracture phenomena) is significantly affected by die geometry and molecular weight characteristics of the polymers. Use of annular die eliminates the stick-slip transition (oscillating melt fracture) and delays the onset of sharkskin to higher values of shear rate and shear stress. Moreover, it is shown that the molecular weight characteristics of the polymers are well correlated with critical conditions for the onset of flow instabilities based on a criterion proposed in the literature [A. Allal et al., “Relationships between molecular structure and sharkskin defect for linear polymers,” J. Non-Newtonian Fluid Mec...