2012 Annual Report - Jesus College - University of Cambridge

Complex Fluids: Size Matters
Stuart Clarke
MATERIALS SCIENCE I Jesus College Annual Report 2012 23
Experimenting with bouncing putty
At the Easter term President’s Evening I introduced the Society to polymers and how
their properties and behaviour are a direct result of the fact they are very long
molecules, rather like string. Like string, polymers are prone to getting entangled,
resulting in slow molecular motion, often making polymer fluids rather thick (‘viscous’).
The exploitation of this behaviour in a range of products was outlined from toys, to food
thickeners to enhanced oil recovery. The President himself helped to demonstrate that he
could successfully race the polymer molecules, by stretching the material faster than the
molecules can respond, leading to the polymers snapping/breaking.
The connection of individual polymers by cross-links to make a single, super large
molecule, or network, was outlined and illustrated with hot and cold jellies. Hot jellies are
networks that are permanent. However, cold jellies have crosslinks that can make and
break reversibly on changing the temperature. Materials similar to cold jellies have a
number of topical applications exploiting the triggered breaking of the networks and
releasing agents such as drugs or other actives. Dr Ian Wilson demonstrated a ‘taste
explosion’ from a collapsing cold jelly network.
Polymers can also be cooled to low temperatures. However, unlike most small
molecules, many polymers don’t form crystals, where the molecules are in perfect
alignment, but they remain highly disordered and form a glass. The contrasting soft
character of a warm polymer (‘melt’) and the brittle nature of a cold polymer glass was