1. Polymers Chapter 8- Part 3
Manufacturing Processes, MET 1311
Dr Simin Nasseri
Southern Polytechnic State University
(© Fundamentals of Modern Manufacturing; Materials, Processes and Systems,
by M. P. Groover)

2. Polymer Molecular Structures

3. Polymer Molecular Structures
Linear structure – chain-like structure
Characteristic of thermoplastic polymers (strength of the polymer is because of more crystallization)
Branched structure – chain-like but with side branches
Also found in thermoplastic polymers (strength is because of entanglements)
Cross-linked structure
Loosely cross-linked, characteristic of elastomers
Tightly cross-linked, characteristic of thermosets

4. (characteristic of thermoplastics)
Linear Polymer
(characteristic of thermoplastics)
Many polymers are built so their molecules consist of many thousands of atoms arranged into long linear chains.
A linear polymer, eg High Density PolyEthylene (HDPE)

5. Non-linear (Branched) polymer (characteristic of thermoplastics)
Sometimes there are chains attached to the backbone chain which are comparable in length to that backbone chain. This is called a non-linear or branched polymer.
A branched polymer (non-linear polymer), eg. Low Density PolyEthylene (LDPE)

6. Loosely Cross-linked Structure
Elastomers possess a low degree of cross‑linking
Elastomers are elastic and resilient
Figure 8.7 Various structures of polymer molecules: (c) loosely cross‑linked characteristic of an elastomer.

7. Tightly Cross-linked Structure
Thermosets possess a high degree of cross‑linking
Thermosets are hard and brittle
Figure 8.7 Various structures of polymer molecules: (d) tightly cross‑linked or networked structure as in a thermoset.

8. Effect of Cross- Linking on Properties
Cross‑linking causes the polymer to become chemically set
The reaction cannot be reversed
The polymer structure is permanently changed; if heated, it degrades or burns rather than melt

9. Summary
Stronger, crystallized  HDPE (Milk and detergent containers, trash bags, yogurt tubs)
Linear
Thermoplastics
Softer  LDPE (Bread bags, squeezable bottles)
Branched
Polymers
Thermosets 
Tightly crosslinked  switches, plugs
Elastomers
Loosely crosslinked  Rubbers, gloves, tires 