A liquid polymer was a fluidic substance with a repeating molecular structure.
Starfleet officer Edward Hagler was one of several officers abducted from the USS Enterprise-D by the solanogen-based lifeforms and subjected to bizarre medical experimentation in 2369. One of the procedures performed on Hagler caused his blood to transform into a liquid polymer, causing his death. (TNG: "Schisms")
Later that year, while tasting some pasta al fiorella he had been looking forward to eating at the Replimat on Deep Space 9, the disgusted Geordi La Forge referred to it as tasting like liquid polymer. Worf, however, thought it was delicious. (TNG: "Birthright, Part I")