The entanglement concept in polymer rheology

Viscoelastic instabilities are of practical importance, and are the subject of growing interest. Reviewed here are the fresh developments as well as earlier work in this area, organized into the following categories: instabilities in Taylor-Couette flows, instabilities in cone-and-plate and plate-and-plate flows, instabilities in parallel shear flows, extrudate distortions and fracture, instabilities in shear flows with interfaces, instabilities in extensional flows, instabilities in multidimensional flows, and thermohydrodynamic instabilities. Emphasized in the review are comparisons between theory and experiment and suggested directions for future work.

Instabilities in viscoelastic flows

Wall slip of molten polymers

We report on a continuous method for controlled electrospinning of polymeric nanofibers on two-dimensional (2D) and three dimensional (3D) substrates using low voltage near-field electrospinning (LV NFES). The method overcomes some of the drawbacks in more conventional near-field electrospinning by using a superelastic polymer ink formulation. The viscoelastic nature of our polymer ink enables continuous electrospinning at a very low voltage of 200 V, almost an order of magnitude lower than conventional NFES, thereby reducing bending instabilities and increasing control of the resulting polymer jet. In one application, polymeric nanofibers are freely suspended between microstructures of 3D carbon on Si substrates to illustrate wiring together 3D components in any desired pattern.

Controlled continuous patterning of polymeric nanofibers on three-dimensional substrates using low-voltage near-field electrospinning.

Using a new surface force apparatus, static and dynamic measurements have been conducted to investigate the squeeze of a liquid from the contact between a relatively smooth sphere and a plane. Simple liquids and polymer melts (polyisoprene) have been studied, with different solid surfaces (mica, cobalt, gold, platinum, and steel). It has been found that an ‘‘immobile’’ layer of fluid is present on each solid surface which does not participate in the hydrodynamic flow of the liquid. The thickness of this film is relatively independent of the roughness of the solid, corresponds to a small number of molecular layers (1 to 5), and is influenced by the structure of the molecule. These fluids shows two different behaviors: for small molecules the thickness of the ‘‘immobile’’ layer is proportional to the viscosity; for larger molecules the thickness is proportional to the one‐tenth power of the viscosity. During the squeeze process, when the solid surfaces are close, a ‘‘confined’’ layer occurs due to molecule entanglement. This phenomenon is described for a semirigid hydrocarbon.

Drainage of thin liquid films between relatively smooth surfaces

The dynamic shear modulus and the flow rate through capillaries under constant pressure and under constant velocity of the piston, have been measured for polybutadienes and polyisoprenes of narrow molecular weight distribution with molecular weights ranging, respectively, from 3.8 × 104 to 5.8 × 105 and from 1.06 × 105 to 6.02 × 105. The phenomena of the discontinuous increase of volume flow rate and self-oscillatory flow regime at critical rates of deformation have been considered in detail. It is proposed that these phenomena are due to the induced transition of the polymer from the fluid to the high-elastic state at higher deformation rates. As a result, an inference has been made that polybutadienes and polyisoprenes with a narrow molecular weight distribution in the high-elastic state, behave in certain respects as crosslinked polymers incapable of displaying fluidity. The quantitative relationships among the viscoelastic characteristics measured under dynamic regimes, the parameters determining the critical flow regimes, and the molecular weights of polybutadienes and polyisoprenes have been worked out.

Viscoelastic properties and flow of narrow distribution polybutadienes and polyisoprenes

Viscoelastic properties and flow of polybutadienes and polyisoprenes

MOST investigations of the rheological properties of concentrated polymer solutions have beech restricted to the study of viscosity of solutions [1], although during the flow of polymer systems numerous specific effects arise, which can only be understood and explained when the solutions are regarded as viscoelastic liquids. The problem therefore is how to generalize the experimental data from investigations of various characteristics of solutions and the relations between them so that an indiection of the role of concentration and mode of deformation in the parameters measured could be achieved. This study, which was carried out using solutions of polystyrene in decalin at different concentrations, reports experimental determinations of three main characteristics of :heological behaviour of flowing polymer systems: shear stress z, first difference of normal stresses a and high elastic deformation ~e. Generalizations of these characteristics are proposed in order to plot eoncentration-invariant characteristics of the rheological properties of solutions.

Experimental investigation of the rheological properties of concentrated polymer solutions

The superelastic strain is represented in universal form as a function of the product shear rate times maximum relaxation time in the relaxation spectrum for a polymer system. A relation is established between the superelastic strain and the relaxation spectrum as well as the change of the latter due to the shear rate.

Universal superelasticity characteristic for polymer systems

It is shown that there is a critical concentration corresponding to the formation of a compact, fluctuating network, which results in a sharp change in the concentration dependence of the viscosity, first difference of normal stresses and high-elasticity deformations. It is found that at concentrations below the critical concentration high-elasticity deformations do not develop, although normal stresses exist. In consequence, the relationship whereby the modulus of high elasticity is equivalent to the ratio between the square of the viscosity and the coefficient of normal stress is fulfilled only for concentrations higher than the critical.

G. V. Berezhnaya

Papers

Viscoelastic properties and flow of narrow distribution polybutadienes and polyisoprenes

Viscoelastic properties and flow of polybutadienes and polyisoprenes

Experimental investigation of the rheological properties of concentrated polymer solutions

Universal superelasticity characteristic for polymer systems

Viscoelastic and high‐elastic properties of solutions of “monodisperse” polybutadienes in the region of critical concentrations