A polymer composition includes a base polymer comprising a hydrocarbon rubber, an acrylic copolymer, and a refined oil in which the acrylic polymer is dispersed. The polymer composition is swellable when treated with an aqueous medium, non-aqueous medium, or both. A sealing element for a flow channel includes an expandable section filled with a swellable composition that swells when treated with a swelling material. The swellable composition includes the polymer composition, and the swelling material may be aqueous, non-aqueous, or both.