A dynamic sealing structure for pressure vessels used in microwave assisted chemistry is disclosed. The structure includes a cylindrical vessel liner and a removable liner cap, each formed of a microwave transparent material. The liner has a circular mouth with a lip formed of respective first and second beveled edges, with the first beveled edge forming an interior edge of the circular mouth and the second beveled edge forming an exterior edge of the circular mouth. The cap includes respective interior and exterior faces, with a sleeve depending from the interior face and having a circumference that engages the interior surface of the vessel liner for being urged under pressure against the interior surface of the vessel liner. The interior face of the cap comprising a circular channel outward of the sleeve and having a circumference that engages the lip of the vessel liner, and with the channel comprising two beveled edges that respectively engage both beveled edges of the lip of the liner.