Another Unusual Water Soluble LCST System

A family of block copolymers known as poloxomers (or by the BASF tradename Pluronics) can also act "funny". These polymers are made of two blocks of polyethylene oxide (which is readily water soluble at all temperatures) and a mid-block of polypropylene oxide (which isn't water soluble). You can get the blocks in varying lengths and the relative amounts, so it really is a family of copolymers. There's even a scheme to numbering and lettering the various grades, but I've never worried about it too much. In water, the propylene oxide blocks try to isolate themselves from teh water by hooking up with other PPG blocks from other molecules. As a result, weak crosslinks form.

F127 is a great example of some of the unusual behavior you can find. A 20 wt% solution makes a great gel at room temperature, but cool it down just a few degrees and it thins down to a very thin liquid. (You can also get the same viscosity if you heat it up to about 80 ^oC. Note that this is an example of LCST or UCST as the polymer is soluble in all cases. It is instead a matter of balancing the interaction energies of the two blocks and the water.