Roles of the molecular weight of n-ethylene glycol diacrylates and UV irradiance on the mechanical properties at the gel point of acrylic acid based hydrogels

We investigated sol-gel transitions in acrylic acid based hydrogels with real time in situ photocrosslinking and Fourier transform mechanical spectroscopy. Crosslinkable n-ethylene glycol diacrylate (EGDA) compounds with molecular weights (MWs) in the range 171-575 g/mol were used in standard solutions of known polyacrylamide. The mutation number (N-mu) was monitored in all of the experiments, such that N-mu was 0.3 or lower at all of the experimental frequencies. During gelation, a frequency independence of tan delta was observed in all cases, such that the storage and loss moduli were scaled as about omega(n); this indicated the establishment of a sample spanning network in the hydrogels with the Winter-Chambon criteria. The relaxation exponents (ns) ranged from 0.35 to 0.81 when they were affected by both the MWs of the EGDAs and the UV irradiance. The stiffness of the critical gels steadily increased with increases in both the MWs of the EGDAs and the UV irradiance. The fractal dimensions decreased at the gel point with increasing MWs of the EGDAs, whereas it increased with an increase in the UV irradiance. An optimization tool (Design-Expert) was used to compare the effects and interactions of the reaction parameters on the gel-time characteristics of the critical gels and those of the fully formed hydrogels. A map (description) of the reaction parameters that can be used in the design of specific networks, particularly with values of n that are close to or equal to 0.5, is proposed. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47606.