Question

In your own words describe the mechanisms for the elastic deformation of thermoplastics and for the plastic deformation of thermoplastics. please write clearly

Answer 1

Thermoplastic Polymer

These plastics soften when heated and harden when cooled. The processes are totally reversible and can be repeated. These materials are normally fabricated by the simultaneous application of heat and pressure. They are linear polymers without any cross-linking in structure where long molecular chains are bonded to each other by secondary bonds and/or by van der waals bonds. They have the property of increasing plasticity with increasing temperature which breaks the secondary bonds between individual chains.

Every thermoplastic polymer follows stress and strain curve when it is loaded under any applied load.

By close refering to stress and strain diagram, we can understand the mechanical behaviour of thermoplastic polymers. As we apply load on a polymer, the young's modulus given by starts i

ncreasing which is a ratio of stress and strain given by

1. As the load is applied on the polymer, the polymer starts elongating till the point A which is the propotionality limit and it is defined as the limit that enables the polymer to regain its original shape without deformation.
2. Further elongation of polymer leads polymer to the elastic limit notified by point B, uptill which elastic polymer usually gets elongated and on the removal of load, the polymer regains its original shape with negligible deformation.
3. After point B, point C and point C' came into existence which stands for upper and lower yeild point which acts as a division between elastic and plastic deformation. Stress applied lower than than yeild point leads to elastic deformation of polymer and stress applied above yeild point leads to palstic deformation of polymer.
4. Point D is the point of ultimate stress like the point of maximum possible stress we can apply on a polymer lies under plastic deformation region and point E is the point of rapture or fracture where polymers breaks due to load applied.

Mechanism of deformations

Despite the similarities in ductile and brittle behavior with to metals, elastic and plastic deformation mechanisms in polymers are quite different. This is mainly due to:-

• difference in structure they made of
• size of the entities responsible for deformation

Deformation in plastic polymers

Plastic polymers deform elastically by elongation of the chain molecules from their stable state in the direction of the applied stress by the bending and stretching of the strong covalent bonds. In addition, there is a possibility for slight displacement of adjacent molecules, which is resisted by weak secondary / van der Waals bonds. Plastic deformation in polymers is not a consequence of dislocation movement as in metals. Instead, chains rotate, stretch, slide and disentangle under load to cause permanent deformation. This permanent deformation in polymers might occur in several stages of interaction between lamellar and intervening amorphous regions. Initial stages involve elongation of amorphous tie chains, and eventual alignment in the loading direction. Continues deformation in second stage occurs by the tilting of the lamellar blocks. Next, crystalline block segments separate before blocks and tie chains become orientated in the direction of tensile axis in final stage. This leads to highly orientated structure in deformed polymers. The plastic deformation occurs after the yeild point reached in any thermoplastic polymer according to stress and strain diagram.

Deformation of elastic polymers

Elastic polymers or elastomers, deform elastically by simple uncoiling, and straightening of molecular chains that are highly twisted, kinked, and coiled in unstressed state. The driving force for elastic deformation is change in entropy, which is a measure of degree of disorder in a system. When an elastomer is stretched, the system’s order increases. If elastomer is released from the applied load, its entropy increases. This increased entropy will help the streached polymer to regain its actual shape under elastic limits according to stress and strain diagram.