Nino Grizzuti

Effects of coalescence and breakup on the steady-state morphology of an immiscible polymer blend in shear flow

The steady-state morphology of an immiscible polymer blend in shear flow has been investigated by optical microscopy techniques. The blend is composed by poly-isobutylene (PIB) and poly-dimethylsiloxane (PDMS) of comparable viscosity. Experiments were performed by means of a home-made transparent parallel plate device. The two plates can be independently counterrotated, so that sheared droplets of the dispersed phase can be kept fixed with respect to the microscope point of view, and observed for long times. The distribution of drops and their average size were measured directly during flow at different shear rates and for different blend compositions. It was found that the average drop size in steady-state conditions is a decreasing function of the applied shear rate, and does not depend on blend composition for volume fractions up to 10%. Experiments have proved that, in the shear rate range which could be investigated, the stationary morphology is controlled only by coalescence phenomena, droplet breakup playing no role in determining the size of the dispersed phase. More generally, it has been shown that the steady-state morphology is a function not only of the physical parameters of the blend and of the shear rate, but also of the initial conditions applied to the blend. The steady-state results reported in this paper constitute the first direct experimental confirmation of theoretical models which describe the mechanisms of shear-induced drop coalescence.

Transient and steady‐state rheology of a liquid crystalline hydroxypropylcellulose solution

Cone and plate rheological measurements have been performed on aqueous solutions of hydroxypropylcellulose in the liquid crystalline phase. The time‐ dependent experiments (start‐up of shear and oscillatory shear flow) suggest the presence of a supramolecular structure which is modified by the flow. The characteristic time of structure evolution is estimated, and a tentative explanation of such a process is suggested in terms of polydomain rearrangement. Steady‐state measurements confirm some peculiar properties of liquid crystalline polymers, such as the presence of a shear thinning regime at the lowest shear rates and the occurrence of negative values of the first normal stress difference. Finally, an anomalous, yet reproducible, behavior of the dynamical properties is reported and discussed.

Measurements of the rheological behavior of a crystallizing polymer by an “inverse quenching” technique

In this paper we show that a suitable thermal history can be used to produce a polymer melt in which a fixed amount of crystalline phase has been frozen. We call this novel method “inverse quenching,” since a stable amorphous/crystalline system is obtained by heating up the sample rather than cooling it down. If the inverse quenching temperature is suitably chosen, the polymer can remain stable for a long time, thus allowing different types of experimental measurements. Here we first prove the validity of the inverse quenching method in quiescent crystallization conditions, and then we use the inverse quenching method to perform rheological measurements on an isotactic polypropylene at a constant degree of crystallization. In particular, steady-state viscosity measurements in the early crystallization stages are reported for the first time, showing that the viscosity at low shear rates is much larger than that of the purely amorphous melt even for small values of crystallinity. The technique is also used to study the liquid-to-solid transitional behavior of the crystallizing polymer, which can be seen as a gelation process, at temperatures that are forbidden to traditional techniques. Such measurements are shown to provide further, robust validation of this novel method.In this paper we show that a suitable thermal history can be used to produce a polymer melt in which a fixed amount of crystalline phase has been frozen. We call this novel method “inverse quenching,” since a stable amorphous/crystalline system is obtained by heating up the sample rather than cooling it down. If the inverse quenching temperature is suitably chosen, the polymer can remain stable for a long time, thus allowing different types of experimental measurements. Here we first prove the validity of the inverse quenching method in quiescent crystallization conditions, and then we use the inverse quenching method to perform rheological measurements on an isotactic polypropylene at a constant degree of crystallization. In particular, steady-state viscosity measurements in the early crystallization stages are reported for the first time, showing that the viscosity at low shear rates is much larger than that of the purely amorphous melt even for small values of crystallinity. The technique is also used ...

Viscous behavior and mixing rules for an immiscible model polymer blend

In this work the viscosity of a model blend, composed of poly-isobutene and polydimethyilsiloxane, has been measured at two different temperatures and for a wide range of blend compositions and shear rates. Cone and plate and capillary measurements showed a good overlap in the shear rate window accessible to both instruments. The experimental results have been compared to the predictions of two emulsion models [Frankel and Acrivos (1970), Choi and Schowalter (1975)], and of some simple, empirical mixing rules. Good quantitative agreement was found between the data and the Frankel–Acrivos model, once the correct dependency of the droplet radius upon shear rate is accounted for. To this end, droplet radii were independently determined by viscoelastic measurements upon cessation of steady shear flow. The Frankel–Acrivos model is also able to predict the change from a positive to a negative deviation from the ideal mixing rule for increasing shear rates, which is a typical feature of polymer blends.

Phase separation effects in the rheology of aqueous solutions of hydroxypropylcellulose

Aqueous solutions of hydroxypropylcellulose (HPC) have been widely used as a model system to study liquid crystalline behavior in polymers. The HPC limiting concentration for mesophase formation in water is about 40% wt, quite independent of molecular weight. Most rheo-optical investigations have been carried out in the concentration range from 50% to 65% wt, on the assumption that only the liquid crystalline phase was present. In this study, by using video-enhanced contrast light microscopy, we show that an isotropic phase in form of tiny droplets is also present at concentrations up to 60% wt, both in quiescent and in sheared samples at room temperature. The isotropic phase can be made to disappear by lowering the temperature. The effects of phase separation on the rheology of the HPC/water system are studied by measuring viscosity as a function of temperature and concentration. A slope of Region I of the viscosity curve close to −0.5 is found only at low temperatures, when the sample is fully anisotropic, whereas an anomalous dependence of the viscosity on temperature is observed when phase separation is significant. This study shows that special care is needed when interpreting experimental results from the HPC/water system in terms of theories for liquid crystalline polymers.

A rheological study of the phase transition in thermoplastic polyurethanes. Critical gel behavior and microstructure development

The phase transition behavior of a commercial thermoplastic polyurethane (TPU) is investigated by rheological techniques. Oscillatory shear flow experiments are performed during both the isothermal annealing following a rapid cooling from the melt state, and the subsequent heating ramp (remelting). Rheological measurements reveal that the microphase separation between hard and soft segments and the concurrent crystallization of the hard phase domains produce a sol-to-gel type of transition. We show that the microstructure of TPUs at the critical gel point is strictly related to the applied thermal history. In particular, the critical gel properties during the cooling down transition are found to be quantitatively different from those measured during the reverse, heating up process. When coupled to differential scanning calorimetry measurements under the same thermal history, the rheological technique is shown to be a useful tool to determine the mechanisms of microstructure evolution.

Rheology of dilute and semidilute noncolloidal hard sphere suspensions

The dissipative behavior of model suspensions composed of non-Brownian, inertialess, rigid spheres immersed in Newtonian and viscoelastic matrices is investigated in the range of volumetric concentrations up to 10%, thus encompassing both the dilute and semidilute regimes. Polymethylmethacrylate beads are dispersed into polyisobutylene, characterized by a Newtonian rheology, and into two viscoelastic polydimethylsiloxanes. Both steady state viscosity and oscillatory shear loss modulus measurements are performed. As expected, the presence of the filler increases both the viscosity and the loss modulus of all suspensions. Following the hydrodynamic calculations of Batchelor, the concentration dependence is described by a second order polynomial expansion in the volume fraction. For low concentrations, the linear Einstein and Palierne predictions for Newtonian and viscoelastic fluids are found to be well obeyed by both the Newtonian and the viscoelastic suspensions. In the semidilute regime, the experimental...

Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene)

In this paper we analyze the crystallization kinetics under steady shear flow conditions of different samples obtained by blending two isotactic poly(1-butene)s with different average molecular weights. It is observed that the addition of a small amount of high molecular weight (MW) polymer (<6wt%) to a low MW sample does not produce any appreciable effect upon the crystallization kinetics under both quiescent and shear flow conditions. When more elevated amounts of high MW polymer are added, only mild effects upon the crystallization kinetics, under both quiescent and shear conditions, are observed. This behavior can be attributed to constraint release of high MW chains due to the relaxation of the shorter chains. Such a physical phenomenon can be described by the double reptation theory, which, indeed, allows for good quantitative predictions of the experimental results by using the relaxation times of the two blend components as the only fitting parameters.In this paper we analyze the crystallization kinetics under steady shear flow conditions of different samples obtained by blending two isotactic poly(1-butene)s with different average molecular weights. It is observed that the addition of a small amount of high molecular weight (MW) polymer (<6wt%) to a low MW sample does not produce any appreciable effect upon the crystallization kinetics under both quiescent and shear flow conditions. When more elevated amounts of high MW polymer are added, only mild effects upon the crystallization kinetics, under both quiescent and shear conditions, are observed. This behavior can be attributed to constraint release of high MW chains due to the relaxation of the shorter chains. Such a physical phenomenon can be described by the double reptation theory, which, indeed, allows for good quantitative predictions of the experimental results by using the relaxation times of the two blend components as the only fitting parameters.

Rheo-optics of hydroxypropylcellulose solutions in Poiseuille flow

The behavior of an aqueous solution of hydroxypropylcellulose in the liquid crystalline phase is investigated when it is flowing in a rectangular channel. Rheological characterization shows that the viscosity vs. shear rate curve follows the typical “three region” pattern, with the intermediate plateau of region II extending over a relative large range of shear rates (more than one decade). Two complementary rheo-optical determinations are performed. Velocity profiles across the channel thickness are measured by a hydrogen bubble visualization technique. Texture evolution is monitored by in situ optical microscopy. Accurate focusing inside the sample thickness allows observation in real time of the texture at various shear rates, as generated in the Poiseuille type of flow in the channel. It is shown that the velocity profiles can be accurately predicted by assuming that the flow in the channel is purely viscous, and using only the viscosity data described above. It is also shown that the morphology of the texture generated inside the flowing system is a function of the local shear rate. In particular, an elongated structure is observed when the shear rate exceeds the critical value corresponding to the onset of region II in the viscosity curve.

Band formation in HPC solutions by consecutive shears along orthogonal directions

Abstract Experiments of banded texture formation in lyotropic HPC aqueous solutions are reported. A parallel plate apparatus is used, which allows for consecutive shears along mutually orthogonal directions. In the double shear experiment, the effects of the following variables have been explored: first and second deformation shear units, shear rate of the second movement, delay time between the two deformations. The results show that the formation of a banded texture oriented orthogonally to the first shear is strongly enhanced by introducing the second shear motion. The mechanisms of formation of the texture are discussed.