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What is rheology?

Polymer rheology is the study of flow behavior or the deformation behavior of a polymer. Studying the flow behavior is important because when products are manufactured from raw polymers, they should be melted. Raw polymer is in a solid state and the final product is also in a solid state. But in the intermediate state (in the process where the raw polymer is converted into products) polymer is in a liquid state.

During plastic product manufacturing, solid polymer granules are taken through a heating process and allowed to melt. Then the melted polymer is extruded or molded into a specific shape.

Melt flow changes its flow dynamics as we apply shear stress and change its viscosity during the process. As we improve the shear stress, viscosity varies as a function of corresponding shear strain, time, and external environmental parameters.

Why it is important to study polymer rheology?

  • To understand the processability of polymers
  • To determine the processing techniques for polymers based on their rheological properties
  • To determine the most suitable polymer for the product.
  • To understand the structure-property relationship of polymers.

Factors affecting the rheology

There is a direct correlation between chemical properties and rheological properties. Chemical properties that are affecting to the rheology are,

Molecular mass

Increasing molecular weight increases the weight of individual molecules and the length of the molecule. When the length of the molecule increases, the number of entanglements and the number of molecular interactions will increase. Because of these reasons, resistance to flow is also increased.

Flow behavior of short and long polymer chains
Figure 01: Flow behavior of short and long polymer chains

Molecular weight distribution

Due to the synthetic challenges, it is very hard to make polymers with a single molecular weight. Generally, polymers have molecular weight distribution. Therefore, the same polymer has different molecules with different molecular weights and different chain lengths. Hence different rheological behaviors can be obtained.

Chemical structure/ Molecular architecture

The chemical structure is the three-dimensional arrangement of a molecule in space. The same polymer can have different chemical structures.

Different types of polymer architecture

If we take two polymer samples with the same chemistry, and the same molecular weight but with different architecture, less complex architecture has a higher viscosity. As an example, linear polymers have a higher viscosity than ring polymers. Because linear polymer has a higher ability to form entanglements.

Intermolecular interactions

The strength of the intermolecular interactions among neighboring polymer chains determines how easily they flow at the melt stage. If the interactions are high, that means the resistance to flow at the melt stage is high.

For example, if we take polyethylene and ionomer with almost the same molecular weight, polyethylene has short-range intermolecular interactions. Because it only has Wander Waal’s interactions. Ionomers (iron-containing polymers) have strong intermolecular interactions due to dipole-dipole interactions. Therefore, ionomers are harder than PE.

Material behavior to outer actions

Polymers are known as viscoelastic materials. Because they have viscose components as well as elastic components. Viscoelastic materials have intermediate properties between ideal solids and ideal liquids.

Ideal solidsIdeal liquids
Follows Hook’s lawFollow Newton’s law
Stress is proportional to strainStress is inversely proportional to strain
Stores energyDissipates energy
Deformations occurFlow
Table 01: behavior of ideal solids and ideal liquids

Flow behavior of liquids

The flow behavior of a liquid can be explained by the parallel plate model. This is the simplest model that explains the behavior of a liquid. (Especially for ideal liquids)

The parallel plate model
Figure 03: The parallel plate model

The flowing material between two moving plates is divided into layers depending on their relative speed with respect to their adjacent layer. Here, we assume that the molecular flow is laminar. That means the flow trajectories are parallel to each other and have no flow perpendicular to the main flow.

Polymer rheology eq 01

Flow and viscosity curves of polymer melts
Figure 04: Flow and viscosity curves of polymer melts

Factors affecting the viscosity

  • Chemical and physical properties of the material (Nature of the intermolecular interactions, crosslinking, molecular weight, number of entanglements, etc)
  • Temperature
  • Shear rate or applied load
  • Pressure

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References and Attributes

Figures:

The cover image was created using an image by Rudy and Peter Skitterians from Pixabay


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