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
> 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?
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
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.