Articles for capturing or separating a target gas from a gas stream may include a porous substrate such as a flexible sheet or mat, or a rigid ceramic monolith impregnated or coated with a sorbent composition. The sorbent composition may include a polyamine and a coexistent polymer chemically bonded to the polyamine. The polyamine may include a polyethylenimine. The coexistent polymer may include a polyurethane, a polyolefin-acrylic acid copolymer, or a combination thereof. The sorbent composition may be substantially less water-insoluble than compositions containing only a polyamine and may have high durability and good adsorption capacity for acidic target gases such as carbon dioxide. Methods for preparing the articles using aqueous polymer solutions are provided. Methods for capturing or separating target gases using the articles are provided.