Hello Readers, welcome to your own website to understand each and every topic related to the manufacturing process where we transform complex content into simpler ones. In this article, we are focused to cover the viscoelastic behavior of polymers, advantages, disadvantages, and applications of viscoelastic property.
VISCOELASTIC BEHAVIOR OF POLYMERS:
Another property that is characteristic of polymers is viscoelasticity.
Viscoelasticity is the property of a material that determines the strain it experiences when subjected to combinations of stress and temperature over time.
As the name suggests, it is a combination of viscosity and elasticity.
Viscoelasticity can be explained with reference to below figure.
The two parts of the figure show the typical response of two materials to an applied stress below the yield point during some time period.
The material in (a) exhibits perfect elasticity; when the stress is removed, the material returns to its original shape.
By contrast, the material in (b) shows viscoelastic behavior.
The amount of strain gradually increases over time under the applied stress.
When stress is removed, the material does not immediately return to its original shape; instead, the strain decays gradually.
If the stress had been applied and then immediately removed, the material would have returned immediately to its starting shape.
However, time has entered the picture and played a role in affecting the behavior of the material.
A simple model of viscoelasticity can be developed using the definition of elasticity
as a starting point.
Elasticity is concisely expressed by Hooke’s law, s = Ee, which simply relates stress to strain through a constant of proportionality.
In a viscoelastic solid, the relationship between stress and strain is time dependent; it can be expressed as,
The time function f(t) can be conceptualized as a modulus of elasticity that depends on time.
It might be written E(t) and referred to as a viscoelastic modulus.
The form of this time function can be complex, sometimes including strain as a factor.
Without getting into the mathematical expressions for it, nevertheless the effect of the time dependency can be explored.
One common effect can be seen which shows the stress–strain behavior of a thermoplastic polymer under different strain rates.
At low strain rate, the material exhibits significant viscous flow.
At high strain rate, it behaves in a much more brittle fashion.
Temperature is a factor in viscoelasticity.
As temperature increases, the viscous behavior becomes more and more prominent relative to elastic behavior.
The material becomes more like a fluid.
At low temperatures, the polymer shows elastic behavior.
As T increases above the glass transition temperature Tg, the polymer becomes viscoelastic.
As temperature increases further, it becomes soft and rubbery.
At still higher temperatures, it exhibits viscous characteristics.
The temperatures at which the modes of behavior are observed vary, depending on the
Also, the shapes of the modulus versus temperature curve differ according to the proportions of crystalline and amorphous structures in the thermoplastic.
Thermosetting polymers and elastomers behave differently; after curing, these polymers do not soften as thermoplastics do at elevated temperatures.
Instead, they degrade at high temperatures.
Viscoelastic behavior manifests itself in polymer melts in the form of shape memory.
As the thick polymer dissolve is transformed during processing from one shape to another, it ‘‘remembers’’ its previous shape and attempts to return to that geometry.
For example, a problem in the extrusion of polymers is died swell, in which the profile of the expelled material grows in size, mirroring its tendency to return to its bigger cross-section in the extruder barrel immediately before being squeezed through the smaller die opening.
All cycles utilized in polymer production lead to chains of change lengths and hence with various molecular weights.
Accordingly, there isn’t a single molecular weight in a given polymer but a distribution of molecular weights that is more or less broad relies upon uniformity in length of chains that composed its macromolecules.
Consequently, molecular weights of polymers are addressed as moderate molecular weights.
Ideal molecular weight for a polymer relies on the structure and its end use.
It must be said that polymers with very high molecular weights are get hard to process.
Typically, molecular weights in industrial polymers are in the period of 50.000-300.000.
Then again adaptable macromolecules may receive enormous number of conformations that are controlled by the position taken in the space by their atoms since this area changes by straight forward turn about single bonds, as represented in three carbon atoms.
Furthermore, a limited number of conformations is accessible in polymers with rigid chains.
Additionally, flexible polymer in the crystalline state adopt fixed conformations whereas in solution or in the liquid state they exhibit a wide range of conformations.
The various macromolecules made of an enormous number of identical building blocks interact with each other.
These inter and intramolecular interactions gives strength that will be smaller or bigger elying on the sort of interaction.
There are four different types in polymeric materials.
These three mentioned characteristics: a large and heterogeneous molecular weight, the chance of adopting a great number of conformations, and the existence of intra and intermolecular interactions are variables in polymers that advance their peculiar and versatile mechanical behavior which is very important from a practical point of
Traditional theories of elasticity and viscosity of a body expect steady-state stress, strain, and strain rate.
Thusly, the time it takes to arrive at equilibrium conditions is considered.
Spectator over one hundred of years before found that most materials failed to arrive equilibrium in an observable time period.
Moreover, the constant coefficients of viscosity and elasticity were reliant on pre-treatments and loading history.
Such effects, which were not accounted for the classical theories were called “elastic after effects”.
These were more pronounced in certain materials such a food dough, wet clays, pitch and unvulcanized rubbers.
With the improvement of synthetic polymers, these impact turned out to be even more common and dominated equilibrium properties.
Viscoelasticity provide plastic the capacity to absorb energy, flex and spring back without cracking.
That property gets through molecular movement and rearrangement when a stress is applied and needs to be accounted for during the design process to avoid creep.
Since viscoelasticity includes molecular rotation and rearrangement when a stress is applied, this needs to be accounted for in the design.
This movement is referred to as creep, and results in the plastic part being irreversibly deformed over time—think of the movement of a damper.
APPLICATIONS OF VISCOELASTIC PROPERTY:
Viscoelastic materials are utilized in automobile bumpers, on computer drives to protect from mechanical shock, in helmets (the foam padding inside), in wrestling mats, etc.
Viscoelastic materials are also used in shoe insoles to reduce impact transmitted to a person’s skeleton.
We have covered all the important concepts related to the viscoelastic behavior of polymers. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!