3 Polymer Molecular Structures Linear structure – chain-like structureCharacteristic of thermoplastic polymers (strength of the polymer is because of more crystallization)Branched structure – chain-like but with side branchesAlso found in thermoplastic polymers (strength is because of entanglements)Cross-linked structureLoosely cross-linked, characteristic of elastomersTightly 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‑linkingElastomers are elastic and resilientFigure 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‑linkingThermosets are hard and brittleFigure 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 setThe reaction cannot be reversedThe polymer structure is permanently changed; if heated, it degrades or burns rather than melt