Luckily, the paper is accessible through SciHub it seems.
From the paper:
Funding: Financial support from The Dow Chemical Company and DuPont is acknowledged. T.E.C. and E.D.G. acknowledge financial support from the National Science Foundation under awards DMR-1609417 and DMR-1905550. K.P.B., A.L.Z., and E.D.G. also acknowledge support from the Center for Membrane Science, Engineering, and Technology (MAST) and the National Science Foundation under award IIP-1841474. M.K. acknowledges support from the National Science Foundation under award CBET-1946392. B.G. and B.K. are funded in part by the National Science Foundation under award CMMI-1906194. B.G. and B.K. also acknowledge computing support from XSEDE TG-CTS110007
Paper directly written by government employees (like national labs) are usually accessible for free (like on https://www.osti.gov/).
I'm pretty sure all of them are required to be in public domain
Also, there are some pretty prestigious journals out there. I'm sure there are academics who'd want a whole lot more money if the government asked them to stop publishing in the journals that their peers read.
> why aren’t these publicly accessible?
And you say: it’s sponsored by Dupont. So let me rephrase the question incorporating the new bit of information you provided:
Why would a paper be hidden behind a paywall if it’s sponsored by a commercial company?
It would make sense if it was completely off limits. But paying a few hundred dollars (?) means it is still semi public. It’s no secret, but it’s not public either. Why?