Question 16.8: Comparing Mechanical Properties of Thermoplastics Compare th......
Comparing Mechanical Properties of Thermoplastics
Compare the mechanical properties of low-density (LD) polyethylene, high-density (HD) polyethylene, polyvinyl chloride, polypropylene, and polystyrene, and explain their differences in terms of their structures.
Let us look at the maximum tensile strength and modulus of elasticity for each polymer.
We can conclude that
1. Branching, which reduces the density and close packing of chains, reduces the strength and elastic modulus of polyethylene.
2. Adding atoms or atom groups other than hydrogen to the chain increases strength and stiffness. The methyl group in polypropylene provides some improvement; the benzene ring in styrene provides higher strength and stiffness; and the chlorine atom in polyvinyl chloride provides a large increase in these properties.
| Polymer | Tensile Strength (psi) |
Modulus of Elasticity (ksi) | Structure |
| LD polyethylene | 3000 | 40 | Highly branched, amorphous structure with symmetrical monomers |
| HD polyethylene | 5500 | 180 | Amorphous structure with symmetrical monomers but little branching |
| Polypropylene | 6000 | 220 | Amorphous structure with small methyl side groups |
| Polystyrene | 8000 | 450 | Amorphous structure with benzene side groups |
| Polyvinyl chloride | 9000 | 600 | Amorphous structure with large chlorine atoms as side groups |