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Polymers - Smart Materials - Hydrogels.
Sodium Polyacrylate and Hydrogen Bonding.

What is a Hydrogel?

A hydrogel is an example of a smart material. It can change its
structure in response to salt concentration, pH and temperature.

What is the Structure of a Hydrogel?

Hydrogels are cross linked polymers that have hydrophillic groups.
They are often polymers containing carboxylic acid groups. One
common polymer used to make hydrogels is sodium polyacrylate.
The chemical name for this polymer is poly(sodium propenoate).

The repeat unit for the structure is shown below.

Sodium Polyacrylate Repeat Unit

The polymer chains usually exist in the shape of
randomly coiled molecules. In the absence of Na+ ions
(if you remove all the the salt) the negative charges on the
oxide ions along the polymer chain all repel each other
and the chains tend to uncoil as shown in the picture below.

 Hydrogel Uncoiled

Water molecules are
attracted to the negative charges by hydrogen bonding.

Hydrogel showing Hydrogen Bonding

In this state the hydrogel can absorb over five hundred times
its own weight of pure water but less salty water.
This ability to absorb so much water makes the hydrogel useful
for the lining of babies' nappies - see other uses.

 When salt is added to the hydrogel, the chains start to change
their shape and water is lost from the gel (see the next page).

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