Provided herein is an electrophoresis separation medium comprising: (a) a non-crosslinked or sparsely cross-linked polymer or copolymer; (b) one or more denaturant compounds, in an amount sufficient to inhibit re-naturation of single stranded polynucleotides; (c) an aqueous solvent; (d) optionally, a wall-coating material suited to inhibition of electroosmotic flow; and (e) optionally, an organic water miscible solvent such as DMSO or acetonitrile, wherein the electrophoresis separation medium exhibits functional stability for at least seven days at 23° C. Also provided herein are sieving compositions, including polymer-based sieving compositions, for molecular sieving as well as related kits, devices and methods of use. Such compositions can be useful for separation of biomolecules such as nucleic acids, proteins, glycoproteins and glycans.