This paper describes the effect of physically trapped polymer chains (PTPCs) on the mechanical behavior of pseudo-semi-interpenetrating (pseudo-SIPN) and pseudo-interpenetrating (pseudo-IPN) double-network (DN) hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS') and acrylamide (AAm). DN hydrogel containing a low concentration of very high molecular weight poly(ethylene oxide) (PEO) had markedly higher tensile strength and elongation at break than the neat DN hydrogel. DN hydrogels with trapped ex situ prepared polyacrylamide (PAAm) chains also exhibited toughness enhancement. This, effect was attributed to the role of PTPCs as a molecular release agent that reduced internal friction among polymer chains during large deformations and therefore increased energy dissipation and elongation at break. The reduction of mechanical properties of a DN hydrogel aged for 12 Months was, consistent with that conclusion in that the PTPC diffused, out of the pseudo-SIPN during aging. Increasing temperature also accelerated the diffusion and reduced the mechanical properties improvements due to the PTPCs. The effect of PTPCs on the toughness enhancement of pseudo-SIPNs was more efficient than their effect on pseudo-IPNs.