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Complex Fluids

Glass crossover in polymers

 Glass crossover in polymers

 Credit: Eugene Terentjev

Polymers in poor solvent collapse into compact globules, with internal structure and dynamics the same as in 'polymer melt' (an assembly of chains with no solvent at all). When the strength of pair attraction between monomers is high - the local density increases, and when the number of monomers in contact exceeds Z*=4 (for linear chains) - the melt turns into a glass. The glassy dynamics is established because each particle is confined by a 'cage' of its neighbours and is prevented from exploring its phase space.

The image shows two examples of polymer globules, one above and the other below this glass crossover. The 'molten' globule is spherical in shape, and the sequence of images at long times shows that the arbitrary chosen monomer (red) and its initial cage of near neighbours (green) are able to diffuse all around the volume. In contrast, the 'glassy' globule retains its awkward shape established at the moment when Z*=4 threshold was crossed, and the confining cage remains essentially intact (even though there is a lot of small thermal motion and re-arrangement inside the cage.