Question

1. describe the effects of each of the following characteristics on polymer properties:
a) molecular weight
b) crystallinity
c)crosslinking

2. define the terms:
a) dislocation core
b)edge dislocation
c)screw dislocation

3. describe how dislocations in crystalline materials contribute to strain hardening

Answer 1

Ans 1) Following are the effects of each characterstic on polymer properties :

A) Molecular weight - Effect of molecular weight on properties of polymer are as follows :

a) Higher the molecular weight higher will the material will stretch before rupture

b) Higher molecular weight increases the resistance to impact as high polymer chains results in high polymer bonds so polymer can absorb more energy before failing

c) Higher the molecular weight higher will be chemical resistance

d) Higher the molecular weight higher will be viscosity of material as longer the chain , harder will be to make them flow as they are are more tangeled.

B) Crystallinity - It defines the degree of long range order in material.The more crystalline a polymer more will be the regular aligned chains. Increase in crystallinity of material results in increase in hardness and density of polymer

C) Crosslinking - Cross link is a bond that link one polymer chain to another. Crosslinking of polymer increase the rigidity and decreses the eleasticity. It also make polymer more stronger and harder

Ans 2) Following are the required definations :

a) Dislocation core - A dislocation core is cylinderical core that includes severely distorted material immediately around dislocation line. It is the region where elesticity theory is inapplicable

b) Edge dislocation - It is a type of crystal defect in which the locus of defective points produced in lattice by dislocation lie in a line,therefore this defect is also known as line defect. In this defect the interatomic bonds are severly distorted only in a immediate viscinity of dislocation line.

c) Screw dislocation - It is another type of crystal defect occur due to shear stress in which the defect line movement is perpendicular to the direction of stress and atom displacement ,rather than parallel

Ans 3) When a crystalline material is deformed plastically, dislocation moves and additional dislocations are generated.The more dislocation in material, more they will interact with each other serving as pinned joints.This results in increase in yield strength of material with decrease in ductility,hence,contributing to strain hardening