Swelling-Driven Microphase Separation and Strengthening of Polyampholyte Hydrogels to Afford Room-Temperature Phosphorescence

We report a tough polyampholyte hydrogel of poly(acrylic acid-co-4-vinylbenzyl-(tris(hydroxymethyl)phosphine chloride), which exhibits mechanical strengthening and room-temperature phosphorescence (RTP) after a non-monotonic swelling–deswelling pathway driven by free-ion diffusion and formation of ionic and hydrogen bonds. Combined spectroscopic, X-ray scattering, and microscopic analyses reveal that early-stage swelling dilutes the network, leading to reduced stiffness and strength, while no RTP is observed. During deswelling, amplified density fluctuations develop into finite polymer-rich heterogeneities through nucleation and coarsening, which increases mesoscale phase contrast and drives a transparent-to-whitish transition. Once the domain size of these heterogeneities (Rg) reaches ∼12.4 nm, they provide sufficient local confinement to activate RTP, whereas the mechanical properties increase only modestly because the domains remain largely isolated. Pronounced mechanical reinforcement occurs as continued coarsening establishes an effective load-bearing network, characterized by domain spacing (d0) of ∼90 nm and Rg of ∼25.2 nm, while RTP is further enhanced. Upon equilibration, reduced mesoscale phase contrast weakens RTP and restores transparency, whereas continued network densification maximizes mechanical performance. Furthermore, RTP can be reversibly regulated through pH by tuning ionic associations and microphase separation. These findings identify swelling-driven phase separation as the structural origin of RTP and mechanical strengthening in polyampholyte hydrogels, providing a mechanistic framework for designing multifunctional soft materials.