A seal assembly for a polymer pipe joint, the seal assembly comprising: an annular flexible seal body configured to be installed about an inner periphery of the polymer pipe joint, such that the seal body defines a flow path therethrough with a flow axis, the seal body comprising: a spigot-side sealing surface; a bell-side sealing surface (28, 29) angled with respect to the spigot-side sealing surface, both the spigot-side sealing surface and the bell-side sealing surface (28, 29) facing radially outwardly from the flow axis; a main sealing lobe extending radially inwardly from the seal body; and a stiffener pocket; and an annular stiffener sized to be received within and occupy the stiffener pocket, the stiffener comprising a central portion, a spigot-end portion and a bell-end portion, the spigot-end portion and the bell-end portion being respectively on the side of the spigot-side sealing surface and of the bell-side sealing surface (28, 29) with respect to the central portion, characterized in that the stiffener pocket extends into the seal body from the bell-side sealing surface and interrupts the bell-side sealing surface of seal body such that first and second sealing surfaces (28, 29) of the bell-side sealing surface are on opposite sides of stiffener pocket, in that the spigot-end portion extends radially inwardly away from the central portion and in that the bell-end portion extends radially outwardly away from the central portion. an annular flexible seal body configured to be installed about an inner periphery of the polymer pipe joint, such that the seal body defines a flow path therethrough with a flow axis, the seal body comprising: a spigot-side sealing surface; a bell-side sealing surface (28, 29) angled with respect to the spigot-side sealing surface, both the spigot-side sealing surface and the bell-side sealing surface (28, 29) facing radially outwardly from the flow axis; a main sealing lobe extending radially inwardly from the seal body; and a stiffener pocket; and a spigot-side sealing surface; a bell-side sealing surface (28, 29) angled with respect to the spigot-side sealing surface, both the spigot-side sealing surface and the bell-side sealing surface (28, 29) facing radially outwardly from the flow axis; a main sealing lobe extending radially inwardly from the seal body; and a stiffener pocket; and an annular stiffener sized to be received within and occupy the stiffener pocket, the stiffener comprising a central portion, a spigot-end portion and a bell-end portion, the spigot-end portion and the bell-end portion being respectively on the side of the spigot-side sealing surface and of the bell-side sealing surface (28, 29) with respect to the central portion, characterized in that the stiffener pocket extends into the seal body from the bell-side sealing surface and interrupts the bell-side sealing surface of seal body such that first and second sealing surfaces (28, 29) of the bell-side sealing surface are on opposite sides of stiffener pocket, in that the spigot-end portion extends radially inwardly away from the central portion and in that the bell-end portion extends radially outwardly away from the central portion.