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)

Polymer Molecular Structures

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

(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)

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)

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.

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.

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

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 