Polymer Rheology

Polymer Rheology

Polymers are viscoelastic in nature i.e they act as liquid ( visco portion) as well as solid ( elastic portion). The description of viscoelastic properties of materials is given in a field called Rheology.

Rheology is the branch of science which deals with the study of deformation and flow of materials. The term ' Rheology' is given by Bingham ( known as father of modern Rheology). The prefix ' Rheo' is derived from Greek word 'Rheos' which means current or flow. Thus Rheology deals with study of two different branch of mechanics, fluid and solid mechanics.

The elastic component is dominant in solids hence their mechanical properties are described by Hooke's law which states that the applied stress( s) is directly proportional to the strain(𝛾) of material.

S=E𝛾 ----------(1)

For isotropic solids ( i.e solids having same properties in all directions) the strain is defined by poison's ratio.

When there is no change in volume after stretching the poison's ratio is 0.5,  this ratio decreases with increase in glass transition temperature and approaches to 0.3 for PVC and ebonite.
For simplicity polymers are considered to be viscoelastic solids with poison's ratio 0.5 and the deformation in shear and tension are only considered.
Replacing Young's modulus of elasticity (E) with shear modulus (G) in equation (1) we get shear stress as
S=G𝛾
=> dS= Gd𝛾 -----------(2)
The viscous component is dominant in liquids hence their flow properties are described by Newton's law which states that the applied stress (s) is directly proportional to the rate of strain d𝛾/dt and is independent of strain (𝛾) i.e
S=η d𝛾 /dt

Where η is the coefficient of viscosity.