Comparative gelation of acrylic acid and acrylamide in diacrylate and dimethacrylate crosslinked matrices

The radiation-induced gelation of two monomers, acrylic acid and acrylamide, crosslinked using four model compounds was investigated using Fourier transform mechanical spectroscopy. The gel point characteristics of the resulting hydrogels were compared. The gelation characteristics of the two monomers were very similar. The fractal dimensions of the critical gels ranged between 1.77 and 1.82 when the reactions were controlled. The patterns of viscoelastic events of the products at the gel point were also similar, with the matching reactivity of the monomers and the comparable molecular weight of the crosslinkable species, providing a foundation for such events. All of the reactions exhibited low mutation numbers, indicating that the samples tested were quasi-stable during the measurements. The extent of the UV irradiance during crosslinking reactions is an important parameter in the gelation process, with an increase in UV irradiance decreasing the gel times, decreasing the network relaxation exponents (increased fractal dimensions), leading to stiffer critical gels. The evolution of δ with G* , undertaken as van Gurp–Palmen plots, shows that in the fully formed gels, however, the monomer choice and the crosslinker variation can dictate the viscoelasticity of the materials. In particular, the potential for greater steric effects/greater stabilities of the radical active centres of the dimethacrylate species, in comparison with the diacrylate species, resulting in lesser viscoelasticities in the fully formed gels. Graphic abstract