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SUMMARY:Thick & thin: physics of polymers with sticky ends. - Prof. Martie
 n Cohen Stuart\, Laboratory of Physical Chemistry and Colloid Science\, Wa
 geningen University\, The Netherlands
DTSTART:20090515T131500Z
DTEND:20090515T141500Z
UID:TALK16783@talks.cam.ac.uk
CONTACT:Jurij Kotar
DESCRIPTION:Associative thickeners are linear water soluble polymers with 
 sticky\n(associating) parts\; they are used frequently in formulated dispe
 rsions\nto regulate rheological properties. In their simplest form\, they 
 have\njust two sticky ends\; these are the so-called telechelic polymers t
 hat\nwe discuss in this lecture. Telechelics are interesting because they 
 can\nform networks\, the behaviour of which (in bulk and at interfaces) is
 \nremarkably rich and tunable. We have studied solutions of polyeethylene\
 npolymers with alkyl groups at eithet end: a widely studied\, almost\nclas
 sical system. Above a certain thershold concentration these\nsolutions bec
 ome strongly shear thinning visco-elastic fluids\; we show\nthat the shear
  thinning property also leads to some sort of spontaneous\nself-organisati
 on of the sheared material\, called shear banding.\nMoreover\, the self-or
 ganisation leads to stress fluctuations with a\ncharacteristic statistical
  pattern reminiscent of that of earthquakes. A\nsimple physical picture of
  chain pull-out leads to a constitutive\nequation that can quite well acco
 unt for the data. At phase boundaries\,\nthe self-assembling behaviour lea
 ds to adsorbed layers with strong\ninternal cohesion and structure\, and w
 here such layers form between two\nsurfaces\, surface forces show up with 
 rather unusual features. The\nphysical picture used for the bulk flow prop
 erties can be used again to\nundertand these surface properties. \nWe fina
 lly discuss new telechelics with entirely different association\nmechanism
 s\, either based on charge\, or on triple helix formation. These\nnew syst
 ems offer rather interesting options for hydrogels with unusual\ntuning ch
 aracteristics. 
LOCATION:Small Lecture Theatre\, Cavendish Laboratory
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