(327a) pH-Dependent and Photoresponsive Swelling of Hydrogels Containing PAMAM Dendrimers

End-linked hydrogels containing high mass fractions of amine-terminated poly(amidoamine) (PAMAM) dendrimers are prepared by reaction of the dendrimers with monodisperse, epoxide-terminated linear poly(ethylene glycol) chains. PAMAM dendrimers impart pH-dependent swelling characteristics to the gels, which absorb large amounts of water due to protonation of the amine groups under neutral or weakly acidic conditions.¹ The equilibrium swelling of the gels passes through a maximum at pH of approximately 4.5, due to protonation of the amine groups, which imparts ionic character to the gels.² The equilibrium swelling ratio decreases markedly at either high external pH or low external pH. We model the swelling behavior using the Donnan equilibrium theory, by treating the gels as phantom networks that contain a high concentration of Lewis bases having pK_b=3.5. The model captures the maximum in swelling near pH=4.5, though equilibrium swelling ratio is overpredicted in some cases. The collapse of the gels at both high and low external pH is explained in terms of the differential between the concentrations of mobile ions inside and outside the gel. The dendrimer-gels exhibit photoresponsive swelling when loaded with a photoacid-generator (PAG) compound during crosslinking, due to generation of an internal acid-base equilibrium upon exposure to ultraviolet light.

References

1. B. Unal and R.C. Hedden. "Gelation and Swelling Behavior of End-linked Hydrogels Prepared from Linear Poly(ethylene glycol) and Poly(amidoamine) Dendrimers." Polymer 2006, 8173-8182.

2. B. Unal and R. C. Hedden. ?pH-Dependent Swelling of Hydrogels Containing Highly Branched Polyamine Macromonomers." Polymer 2009, 50(3), 905-912.