Critical regimes of shear in linear polymers

Abstract

The state of the problem of elastic turbulence is critically discussed. It is shown that the development of elastic turbulence is associated with the transition of polymer systems to the high elastic state. Model experiments with a narrow‐MWD polybutadiene have been carried out. The method of vizualization of the flow in a flat slit with the aid of circular‐polarized light has been used. The results of polarization‐optical investigation are compared with the results of capillary viscometry and dynamic measurements at small amplitudes. It is shown, in accordance with theoretical predictions, that when a polymer passes from the fluid to the high elastic state with an increase in the shear rate, it ceases to behave as a fluid under shear. This is accompanied by various forms of flow perturbation at the duct exit and entrance and inside the duct. Perturbations at the duct entrance and exit are due to stress concentration in these zones. The transition of a polymer to the high elastic state near the walls inside the duct gives rise to the “stick‐slip” process or to continuous slippage along the duct walls, depending on the velocity of polymer movement. This can be clearly traced by the changes in the interference band pattern. Still higher velocities result in an intensive process of continuity break (rupture) in the polymer and in its chaotic movement in the duct as an aggregate of irregularly shaped lumps, which, relaxing, may form a continuous body again. Attention is drawn to the significance of these observations for a quantitative description of the viscosity anomaly of polymer systems and of the relationship between the viscosity anomaly with various flow irregularities and perturbations in such systems.

Wall slip of molten high density polyethylene. I. Sliding plate rheometer studies

“…For example, hysteresis phenomena have been observed in dynamic experiments when elastomers slide over metal surfaces, and various dynamic models have been proposed to predict the transient slip behavior of these materials (Moore and Geyer, 1974). Vinogradov et al ( 1972) studied slip in melts and interpreted their results in terms of a "forced high elastic state" that facilitates slip at high shear stresses. The behavior of melts at high rates of deformation thus may resemble that of elastomers, and a dynamic slip model might then account for hysteresis effects.…”

The rheological behavior of flexible-chain polymers in the region of high shear rates and stresses, the critical process of spurting, and supercritical conditions of their movement at T > Tg

“…Subsequent experiments were carried out in polarized light with filming of the polymer (PB) flow through a capillary made in the form of a flattened channel [11,12]. As soon as the critical stress is attained, the capillary ejectes a jet corresponding, in cross-sectional area and length, to the channelparameters.…”

“…Weill and co-workers [23][24][25][26] have not taken into account the criticism of Huseby's work made in an earlier work [12]. Moreover, in Weill's last paper no mention is made whatsoever of Uhland's work [22] in which the spurting of LPE was studied extensively.…”

Flow property at capillary extrusion for ethylene–tetrafluoroethylene copolymer

“…Moreover, it should be notified that the strand extruded in the shear rate region from 1120 to 2800 s -1 has smooth surface as shown in Figure 3(d), although the shear rate is much higher than those in the shark-skin and slip-stick regions. The phenomenon is known as "super-extrusion" that is previously reported for linear polymers, such as LLDPE, PTFE, FEP, polybutadiene, and others [20,27,28].…”

“…However, it is known that the velocity profile in the capillary is rather plug-type flow because of the significant slippage [28]. According to the previous researches [4], the following relationship was detected in the super-extrusion region,…”