which is more correct or more frequently used? Nonionic or non-ionic (polymers)? It´s for an academic presentation.
It would appear that nonionic is more common than non-ionic according to Google's ngrams:
Merriam-Webster spells it in the closed-form nonionic.
So, if you go by the dictionaries (or by a style guide that says to use a regional dictionary), use nonionic for a US audience and non-ionic for a UK audience.
Having said that, although the British corpus on Google Books Ngram Viewer shows that non-ionic is slightly more common in print in general, between 1989 and 1998 (for some reason), nonionic was actually used more often.