Can I submit published articles to arXiv as well?
I suggest you first of all check 


the copyright transfers you signed
the publisher's FAQ on rights you retain as author
Many publishers nowadays allow you to self-archive the version of the manuscript that passed the review. Some do not allow self-archiving on public repositories (but e.g. Elsevier makes an exception explicitly for arXiv). For a quick overview have a look at the SHERPA/RoMEO site.
and your local copyright legislation.
E.g. the German UrhG now allows secondary publication (e.g. to arXiv) of your manuscript (including the version with exactly the content of the published paper) for journal contributions that were financed mainly by public grants.