Undergraduate · 15 min

Chain architecture: from thermoplastics to thermosets

Dial the cross-link density on a cartoon polymer chain and connect the topology to rubber elasticity, thermoplastics and hard networks.

Goal

Relate chain length distribution ( $M_n$ , $M_w$ , Đ) and cross-linking to mechanical behaviour of plastics, fibres and rubbers.

Apparatus and reagents

The chain viewer below; a table of $M_n$ , $M_w$ , Đ = $M_w/M_n$ for a few real samples.

Procedure

Set cross-links = 0: the chain is free to slide — the picture of a thermoplastic like PE or PVC that softens on heating.
Add 3–6 cross-links: segments between bridges can still move, but the network snaps back — a rubber.
Push to 15–20 cross-links: the chain is locked into a rigid 3D network — thermosets like Bakelite or epoxies.
Connect to molar mass: longer chains entangle more, raising viscosity and strength; Đ tells how uniform the chain lengths are.

What to observe

Zero bridges → chains can reptate past each other (thermoplastic); a few → entropy-elastic network (rubber); many → glassy thermoset.
A polymer "sample" is a distribution: two batches with the same $M_n$ can behave differently if Đ differs.

Explanation

Polymer properties are topology plus statistics. Un-cross-linked chains slide under stress (thermoplastics: PE, PP, PVC); sparse sulphur or peroxide bridges store deformation elastically (vulcanised rubber); dense bridges forbid flow entirely (phenolics, epoxies). Meanwhile every real sample mixes chain lengths: $M_n$ counts molecules, $M_w$ weighs mass, and the dispersity Đ = $M_w/M_n$ ≥ 1 measures the spread — controlling viscosity, toughness and crystallisation into fibres.

Chemists behind it

Hermann Staudinger Paul J. Flory