Antonios K. Doufas

Quiescent and shear-induced crystallization of polyprophylenes

In this paper, the effect of shear on the flow-induced crystallization (FIC) of several polypropylenes of various macrostructures was studied using rheometry combined with polarized microscopy. Generally, an increase in strain and strain rate or decrease of temperature is found to decrease the thermodynamic barrier for crystal formation and thus enhancing crystallization kinetics at temperatures between the melting and crystallization points. Secondly, popular models based on suspension theory which are used to relate the degree of crystallinity to normalized rheological functions (such as viscosity) are validated experimentally. For this purpose, the space filling of crystals in the polarized micrographs determined from image processing was plotted as a function of normalized viscosity under various shear rates. It is found that the constant(s) of various suspension models should be dependent on the flow parameters in order for the suspension models to describe the effect of shear on FIC, particularly at higher shear rates.

A microstructural flow-induced crystallization model for film blowing: validation with experimental data

The two-phase microstructural/constitutive model for film blowing of Doufas and McHugh (D-M) (J Rheol 45:1085–1104, 2001a) is validated against online film data of a linear low-density polyethylene (LLDPE) at a variety of processing conditions. The D-M model includes the effects of thermal and flow-induced (enhanced) crystallization (FIC) coupled with the rheological response of both the melt and semicrystalline phases under fabrication conditions. The model predictions of bubble radius, velocity, and crystallinity profiles are in quantitative agreement with available experimental data over a wide range of blow-up ratios (BUR), take-up ratios (TUR), and bubble cooling rates using the same set of material/model parameters. The model naturally predicts the location of the frost line as a consequence of system stiffening due to crystallization overcoming the pitfalls of traditional modeling approaches that impose it as an artificial boundary condition. For a wide range of processing conditions, it is found that key film mechanical properties including elongation to break, yield stress, tensile modulus, and tear strength correlate well with predicted locked-in extensional stresses and molecular orientation at the frost line enabling development of quantitative structure-process-properties relationships that are useful in product and process development. The D-M model for film blowing is physics-based including elements of molecular rheology (polymer kinetic theory), suspension, and nucleation theories as well as irreversible thermodynamics principles, yet being tractable for continuum-based numerical simulations with practical industrial applicability. The FIC enhancement factor of the model is shown to be proportional to expλeff,w2−1

Flow-induced crystallization of polypropylenes in capillary flow

\exp \left (\lambda _{\text {eff},\textnormal {w}}^{2} -1\right )

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

, where λeff,w is a molecular chain stretch ratio of the whole chain and proportional to exp (λ2 − 1), where λ is the stretch ratio of the remaining (uncrystallized) amorphous chain, consistent with fundamental kinetic Monte Carlo simulations of flow-induced nucleation of Graham and Olmsted (Phys Rev Lett 103:115702-1–115702-4, 2009).

POLYPROPYLENE FIBERS AND FABRICS

In this paper, the flow-induced crystallization (FIC) behavior of various polypropylenes with different molecular characteristics was investigated using a capillary rheometer. The Cogswell analysis was applied on the capillary data to obtain the apparent extensional strain rate and strain as well as the apparent extensional viscosity. The extensional viscosity obtained using this method was in good agreement with the zero shear viscosity obtained using a cone-and-plate rotational rheometer (Anton Paar MCR-502). Extensional flow parameters did not influence crystallization kinetics in the capillary die. FIC was found to depend strongly on the length-to-diameter (L/D) ratio of the capillary die that is directly related to the residence time. It was also found that the crystallization kinetics were enhanced with increasing molecular weight, indicating the importance of high-end tail of molecular weight distribution (MWD) on FIC. Finally, temperature impacted the FIC behavior significantly since it alters the activation energy needed for the formation of macroscopic structures.

Characterization of the primary and secondary crystallization kinetics of a linear low-density polyethylene in quiescent- and flow-conditions

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.