Physical gels with remarkable properties were obtained by copolymerization of acrylamide with the hydrophobic monomer stearyl methacrylate (C18) in a micellar solution of cetyltrimethylammonium bromide (CTAB) containing up to 15 mol% sodium dodecyl sulfate (SDS). The addition of SDS causes the CTAB micelles to grow and thus enables solubilization of C18. The gels exhibit time-dependent dynamic moduli, high elongation ratios at break (1800-5000%), and self-healing, as evidenced by rheological and mechanical measurements and substantiated by dynamic light scattering. As the size of the micelles in the gelation solution increases, both the degree of temporary spatial inhomogeneity and the lifetime of hydrophobic associations in the gels increase while the elongation ratio at break decreases. Although the physical gels were insoluble in water due to strong hydrophobic interactions, they could be solubilized in surfactant solutions thus providing a means of characterization of the network chains.Viscometric and rheological behaviors of polymer solutions show a substantial increase in the associativity of the network chains with rising micelle size, which results in prolonged lifetime of hydrophobic associations acting as physical cross-links in gels. The internal dynamics of self-healing gels could thus be controlled by the associativity of the network chains which in turn depends on the size of CTAB micelles.

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Strain hardening observed in many biological gels is nature’s defense against the external forces to protect the tissue integrity. Here, we show that double-stranded (ds) DNA gels also stiffen as they are strained. Chemical DNA gels were prepared by solution cross-linking of ds-DNA (about 2000 base pairs long) using the cross-linker ethylene glycol diglycidyl ether (EGDE), while physical DNA gels were prepared by the heating−cooling cycles. Stress relaxation experiments show that strain hardening in both chemical and physical gels starts to appear at 40% deformation, the extent of which increases when the amplitude of the deformation is increased up to the yield strain amplitude. The degree of strain hardening greatly depends on the contour length L c of DNA network strands as well as on the time scale of the measurements; the gel exhibits strong strain hardening at short time scales and soften at long time scales. The maximum degree of hardening appears if the contour length of the network chains approaches 100 nm, which is the Kuhn length of ds-DNA. DNA gels exhibit universal scaled stiffening behavior that can be reproduced by a wormlike chain model taking into account the entropic elasticity of DNA strands. The results of our experiments also show that chemical DNA gels exhibit liquidlike response at strain amplitudes above 1000%, but reversibly, if the force is removed, the solution turns back to the gel state. The partial recovery of the initial microstructure of gels suggests stress-induced denaturation of ds-DNA network strands.

The effect of the initiator system used in the gel preparation on the spatial inhomogeneity in poly(acrylamide) (PAAm) and poly(N,N‐dimethylacrylamide) (PDMA) hydrogels was investigated by static light scattering and elasticity measurements. The hydrogels were prepared by free‐radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N‐dimethylacrylamide (DMA) with N,N′‐methylenebisacrylamide as a crosslinker. Two different redox‐initiator systems, ammonium persulfate (APS)–N,N,N′,N′‐tetramethylethylenediamine (TEMED) and APS–sodium metabisulfite (SPS), were used to initiate the gelation reactions. Compared to the APS–TEMED redox pair, no significant scattered light intensity rise was observed during the crosslinking polymerization reactions initiated by the APS–SPS system. It was found that both PAAm and PDMA gels are much more homogeneous when the APS–SPS redox pair was used as the initiator. The results are explained by the formation of shorter primary chains as well as the delay of the gel point in APS–SPS initiated gel formation reactions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3228–3237, 2007