The Effect of Agar Concentration on Dynamics of AgarPolyacrylamide Double Network Hydrogel

Abstract

Introduction Hydrogels are special type of soft materials having biomimetic and biocompatibility properties due to their three dimensional porous network structure and large water content. So they have gathered much attention for their applicability especially in the field of tissue engineering and drug delivery. However, most hydrogels are mechanically weak which limits their applications requiring toughness. Recent research shows, some specifically designed double network (DN) hydrogels, show improved mechanical properties as they have two contrasting polymer networks with strong entanglements. Here, a hybrid physically-chemically cross-linked agarpolyacrylamide (Ag-PAAM) DN hydrogel has been prepared using heating-cooling photo-polymerization one pot method and its dynamics is being investigated using dynamic light scattering (DLS) measurements. Method The prepared hydrogels are non-ergodic, so time and ensemble averaged scattered data is not equivalent. To study their dynamics, direct ensemble averaged intensityintensity correlation function (ICF), g2 (q,t) are accumulated using an ALV correlator on a sample rotating with the help of a stepper motor. The ICF measured by the correlator was converted to field correlation function g1(q,t) using Siegert relation g2 (q,t) = 1+ ꞵ| | 2 where ꞵ is the instrument coherence factor. Results The DN hydrogel samples were prepared by varying the agar concentration while keeping the acrylamide concentration constant. The correlation curves acquired from DLS runs, decays faster for lower concentrations of agar compared to the higher concentration. All samples present bimodal relaxation. The field correlation function obtained at a scattering angle of 90 are fitted to double exponential equation of the form g1(q,t) =as*exp(-(t/τs))+af*exp(-(t/τf))+g1(q,∞) where τs and τf are the slow and fast relaxation time respectively with as and af as their relative amplitudes and g1(q,∞) represents the baseline. The fast relaxation is associated with the cooperative diffusion of network structure while the slow mode is related to the cluster relaxation. The fast relaxation time increases with increasing agar concentration. This means lower diffusion in DN gels which conveys higher correlation length in gel with increasing agar concentration. Discussion Dynamics of DN hydrogels of Ag-PAAM are examined using DLS measurements. Significant change in gel dynamics are noted with change in first network agar concentrations.