If it is just neutronic matter - i.e. mostly neutrons with a small fraction of protons and electrons to maintain "$\beta$ equilibrium", then to first order, all that would happen is the particles would expand very quickly (relativistically) to low densities and the free neutrons would decay into protons, electrons and anti-neutrinos over the course of an hour.
A slow expansion would be more interesting, because you would in a sense be working your way up through density regimes occupied by the neutron star crust. If you could arrange such an expansion, whereby nuclear statistical equilibrium was maintained, then you start with nuclear pasta, move on to very heavy, very neutron-rich nuclei, then lighter (but still $A>100$) neutron-rich nuclei and finally end up with "normal" iron-peak nuclei. At the same time, as the material become less neutron-rich it must create lots of anti-neutrinos and electrons.
An intermediate case is represented by colliding neutron stars. There you have a distinctly non-equilibrium process where material is rich in free-neutrons and there are lots of seed heavy nuclei that can be built into heavy neutron-rich nuclei through r-process neutron capture.
