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Polymers - Hydrogels - Structure - Sodium Polyacrylate - Hydrogen Bonding.
Hydrogels
are cross
linked polymers which 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 is poly(sodium propenoate).
The repeat
unit for the structure is shown
below.
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.
Water molecules are attracted to the negative charges by 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 next page).
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