科研之友
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摘要
Forming robust associative interactions has been an effective strategy for the design of tough hydrogels. However, the role of associative interactions in the dynamics of hydrogels still remains elusive. Here, we report a series of poly(acrylamide-co-methacrylic acid) hydrogels with moderate water contents and excellent mechanical properties that are facilely synthesized by free-radical copolymerization. The mechanical properties of these hydrogels vary with the feeding molar fraction of acrylamide (f(am)). The gels with f(am) of 0.2-0.35 exhibit high toughness and good stability in water, which is related to the dense hydrogen bonds and relatively high segment rigidity of the matrix. Dynamic modulus spectra extended by time-temperature superposition and relaxation measurements indicate that the gels undergo glassy-to-rubbery transition with decreased frequency, and the robust hydrogen bonds, whose density is 1-3 times that of entanglements, retard chain disentanglement and contribute to the plateau modulus of the gels at low frequencies. The activation energy for the dissociation of the robust hydrogen bonds is similar to 46 kJ mol(-1). Furthermore, a decrease in water content results in the shift of dynamic modulus spectra to low frequencies and an increase in transition temperature due to the reduced segment relaxation. To further examine the structure of gel networks, the tensile behaviors of the gels are analyzed using a viscoelastic model. It is found that each partial chain includes 20-30 Kuhn segments, which are stretched after the fracture of intrachain hydrogen bonds to release the hidden length, dissipate energy, and thus toughen the gels. This understanding of the dynamics of the network at different timescales and the contribution of associative interactions to the mechanical properties should be informative for the design of other tough hydrogels.
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