Effect of pH on the Dynamics and Structure of Thermoresponsive Telechelic Polyelectrolyte Networks: Impact on Hydrogel Injectability

Abstract

We report the effect of pH on the dynamics and structure of 3D networks formed by the temperature-dependent hydrophobic association of the end blocks of P(nBuMA18-co-TEGMA15)-b-PDMAEMA159-b-P(nBuMA18-co-TEGMA15) where PDMAEMA and P(TEGMA-co-nBuMA) stand for poly(N,N-dimethylamino ethyl methacrylate) and poly(tri-ethylene glycol methyl ether methacrylate/n-butyl methacrylate respectively) triblock terpolymer in aqueous media. Thanks to the design of the sticky end blocks that can be independently controlled by temperature, it was possible to explore the pH effect, influencing the charge density and chain conformation of the hydrophilic midblock, on the network dynamics and structure by oscillatory rheology and small-angle neutron scattering. Three pH regimes are observed: At low pH (<6), the terminal relaxation time, τ, and the elastic modulus, GN, remain unaffected by pH. At pH > 6, an appreciable increase of τ (about 6-fold) was observed, likely due to the enhancement of the network connectivity arising from the decrease of the charge density and the increase of the chain flexibility of the poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) block that promote crosslinking, as reflected in GN. Finally, at even higher pHs, the network connectivity decreases substantially, owing to bridge-to-loop transitions, leading eventually to abrupt decrease of τ and GN, thus revealing a gel-to-sol transition. These results can be utilized to fine-tune the injectability of the thermoresponsive telechelic polyelectrolyte hydrogels based on their pH responsiveness.