Not all journals request copyright transfer. Depending on your research area, these days you may find very high profile check journalsthat leave ' websites for their sample copyright in forms. It actually depends more on the authors' handspublisher rather than on the journal. For example, Geometry & Topology, I recently signed a copyright form for one of the journals published by Mathematical Sciences Publishers. (I recently signed their copyright form; and all All my obligations are: (1) I don't sue them for publishing my paper; (2) If I publish it elsewhere later, I must acknowledge the prior publication in G&T. their journal. I don't think it prevents me from placing the paper to public domain later, although I doubt this would be a good idea.)
Check journals' web sites for their sample copyright forms. (It actually depends on the publisheridea.Big commercial publishers unfortunately tend to request copyright transfer - at least those whose forms I could find in my mail archive.)
Having said this, I'd like to note that journals are irrelevant to the issue of disseminating your work. Just place it on arXiv . (And make sure you do this before you submit it to a journal.) journal. Journals refusing to publish arXiv'ed papers (are there any such journals in math?) can be safely ignored and deserve to be ignored. If you care about legal matters, arXiv has an option of publishing under a Creative Commons Attribution license, which is almost the same as public domain for all practical purposes.