Polymer melts or solutions are viscoelastic, that is, there will be viscous loss during deformation, and elastic memory effects will also occur during flow. Conceptually, this viscoelasticity can be divided into linear viscoelasticity and nonlinear viscoelasticity. Among them, nonlinear viscoelasticity is also main research content of polymer material rheology. It is worth noting that elasticity of polymer melts or solutions is somewhat different from bulk elasticity of polymers in our conventional sense. For example, after cross-linking, rubber-like materials have high elasticity at room temperature. This high elasticity comes from motion of polymer segments in high elastic state, and because of cross-linked network, deformation can be completely recovered. Elasticity of polymer melt or solution, or elasticity of polymer in viscous flow state, is always accompanied by irreversible viscous flow, so it is also called viscoelasticity; its principle is not related to formation of polymer entanglement. It is related to perfect network structure, which is also different from cross-linked rubber network.
So-called linear viscoelasticity refers to rheological behavior of polymers under small deformation. For example, using a rotational rheometer to test dynamic viscoelasticity (alternating stress and strain) of a polymer is to test its linear viscoelasticity under small amplitude and small deformation. Dynamic viscoelasticity test mentioned here is different from rheological test in steady state shear flow field. Measurement of dynamic viscoelasticity usually uses a rotor-type rheometer, such as a cone-plate rheometer, a coaxial cylinder rheometer, etc. Test adopts an oscillation mode, that is, a strain is set, and material is dynamically frequency scanned at different oscillation frequencies, where different oscillation frequencies are similar to shear rate during steady-state scanning. Determination of this strain value is usually obtained by strain scanning material after fixing scanning frequency. Strain value taken should be in linear viscoelastic region, that is, region where melt structure is not damaged. Measurement of dynamic viscoelasticity can obtain viscous behavior parameters and elastic behavior parameters at the same time, including storage modulus, loss modulus, complex viscosity and dynamic viscosity, etc. In addition, use of time-temperature equivalence principle can expand frequency range of measurement . Shear rate range tested by rotor-type rheometer is about 10-3~10-2 s-1, while shear rate range that capillary rheometer can test is generally between 10-1~104 s-1 .
Nonlinear viscoelasticity studies rheological behavior of polymer liquids under action of large deformation and long-term stress. For example, shear viscosity is measured by a capillary rheometer, which studies viscous behavior in nonlinear viscoelasticity, and belongs to stable viscoelasticity. Most polymer liquids are pseudoplastic fluids, and shear thinning occurs during their flow. Another important component of nonlinear viscoelasticity of polymer liquids, elastic effect, has more diverse manifestations and more complicated principles. Elastic effect of polymer liquids can be quantitatively described by some specific physical quantities, such as extrusion expansion ratio, die outlet pressure drop, inlet pressure drop, normal stress difference, extensional viscosity, etc. In addition, polymer melt flow instability is also closely related to melt elastic effects, such as inlet pressure oscillation or capillary pressure oscillation during capillary extrusion, and extrudate surface distortion.

Viscosity of polymer liquids is different from that of Newtonian fluids, which varies with shear rate. They are called non-Newtonian fluids, and can be divided into Bingham plastics, pseudoplastic fluids, and dilatant fluids. Most polymer liquids are pseudoplastic fluids. Before shear thinning occurs, shear viscosity is constant and is called zero shear viscosity. Zero shear viscosity is a very important constant of polymer materials, which is related to average molecular weight of material, and viscous flow activation energy of material can be obtained from zero shear viscosity. In general, the better chain flexibility, the smaller activation energy of viscous flow, the smaller sensitivity of polymer viscosity changes to temperature, and the higher sensitivity to shearing. Materials such as PE and POM belong to this type of material. Zero shear viscosity is often difficult to obtain experimentally, especially for very low shear rates that are often difficult to measure by capillary rheometers. At this point, it can be tested by a rotor rheometer. Pseudoplastic fluid not only has an important parameter, value of zero shear viscosity, but also value of critical shear rate on viscosity curve from Newtonian region to nonlinear Newtonian region. This critical value reflects two properties of polymer liquids: non-Newtonian strength and relaxation time. There are different theories for explanation of shear thinning phenomenon in non-Newtonian flow region, such as theory of polymer conformational change and theory of rubber-like liquids. In short, it is change in conformation (orientation) or disentanglement of polymer that results in a decrease in viscosity when flowing under high shear.

As mentioned earlier, capillary rheometer is a powerful means to test shear viscosity of polymer materials. In material laboratory of Suzhou Chengmo Precision Technology Co., Ltd., capillary rheological testing has become the most commonly used method to characterize fluidity of materials. Capillary rheometer can measure change of plastic viscosity at different shear rates and temperatures, can obtain flow properties of plastic melts at temperatures and shear rates close to plastic injection molding. Test standards are ISO 11443 and ASTM D3835. Test principle of capillary rheometer is to measure pressure when plastic melt is extruded through a capillary die of a fixed size at a set shear rate. Shear stress of plastic melt at shear rate, and shear stress divided by shear rate gives viscosity. Viscosity obtained at this time is apparent shear viscosity rather than real viscosity, because capillary rheometer measures inlet pressure when plastic melt passes through capillary die, we need to use Bagley correction to get plastic melt passing through capillary die. Therefore, outlet pressure of plastic melt is obtained, and true shear stress of plastic melt is calculated through outlet pressure and die size. Since plastic is a non-Newtonian fluid, shear rate at pipe wall is higher than that of a Newtonian fluid, so we need to use Rabinowitsch-Weissenberg correction to correct shear rate to get true shear rate. Corrected shear stress is divided by corrected shear rate to obtain true shear viscosity of plastic melt.
In addition to characterizing flow characteristics of different materials, shear viscosity is also one of important data in material data package used by Moldex3D mold flow simulation software. In Chengmo Precision's Material Application Research Center, we have studied rheological properties of a large number of PCR materials. Figure 1 shows shear viscosity curves of ABS materials with different PCR contents.
 
Figure 1: Shear viscosity curves of ABS with different PCR contents
As can be seen from curves, although melt flow rate values for three grades are same, shear viscosity curves are different. With increase of PCR content, shear viscosity showed a decreasing trend. Therefore, in order to more accurately predict filling process of polymer materials containing PCR in mold cavity, material laboratory of Chengmo Precision will test shear viscosity of each batch of incoming PCR materials (different batches of same brand) to ensure that mtr packets formed are accurate.
In addition to PCR-containing materials, we also test their shear viscosity separately if they are different colors of same brand.
 
Figure 2: Shear Viscosity Curves of EXL1414 in Different Colors
As shown in Figure 2, three materials are all EXL1414, which are primary color material, black material and white material respectively, and their shear viscosity also shows different changes. ■