In an embodiment, a method of processing a polymer comprises: mixing polymer and solid carbon dioxide; introducing the polymer and solid carbon dioxide mixture to a hopper of a vented extruder; plasticizing the mixture to form a polymer melt with dissolved carbon dioxide; venting carbon dioxide as a gas before the die exit; and extruding the polymer melt through a die. In an embodiment, a method of making an article comprises: introducing a polymer powder to an extruder; introducing solid carbon dioxide powder to the extruder at the hopper; plasticizing and extruding the polymer and solid carbon dioxide mixture; venting the solid carbon dioxide before the die exit; extruding the polymer melt through a die to form pellets; and injection molding the polymer pellets without solid carbon dioxide to form molded articles.