Viscoelastic and swelling properties of acrylamide copolymeric hydrogels

The viscoelastic and swelling properties of polyacrylamide-based hydrogels containing ionic moiety were investigated as a function of anionic comonomer structure. The hydrogels were synthesized from free-radical crosslinking copolymerization of acrylamide and ionic comonomer; i.e. monoprotic acid: acrylic acid, crotonic acid and diprotic acid: maleic acid, itaconic acid. Initiation by a redox initiator of ammonium persulfate (APS) and N,N,N',N'– tetramethylethylenediamine (TEMED). The N,N'-methylenebisacrylamide (MBA) crosslinking agent was used in the copolymerization. The copolymerization was carried out at 250 rpm, 45 degrees celsius under nitrogen atmosphere for 30 minutes. Feeding composition of all components, i.e. monomer, comonomer, initiator, co-initiator, and crosslinker was fixed to the same value for all copolymeric systems. The synthesized polymers were characterized by RAMAN Spectroscopy and Scanning Electron Microscopy. The mechanical strength of the swollen hydrogels was measured by a rheological method. The copolymers have a higher storage modulus ([storate modulus]) which agrees with the lower water absorption. The average molecular weight between crosslinks ([average molecular weight between crosslinks]) and polymer-solvent interaction parameter ([interaction parameter of polymer with solvent]) was determined from swelling data using equations based on Flory-Rehner swelling theory, and the approaches of Brannon-Peppas and Peppas on monoprotic acid-containing hydrogels. The equation modified by Şen and Güven was used for determining M[subscript c] and chi for diprotic acid-containing hydrogels. Equilibrium elastic modulus was used to determine M[subscript c] based on the polymer network theory. Large difference between [average molecular weight between crosslinks] from both measurements was observed. This difference on the basis of different theories indicates an existence of an intrinsic heterogeneous structure within the hydrogels.