Self-healing hydrogels formed in catanionic surfactant solutions

Abstract

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.

Self-Healing Hydrogels Formed via Hydrophobic Interactions

“…The swelling kinetics of hydrophobically modified nonionic hydrogels such as HM PAAm and HM poly(N,N-dimethylacrylamide) (PDMA) prepared in salt solutions of SDS, CTAB, or CTAB-SDS are similar to those illustrated in Fig. 7 [37,41]. In contrast, HM PAAc hydrogels formed in SDS solutions exhibit high swelling ratios in water because of the osmotic pressure of the AAc counterions [39].…”

“…Mixtures of the anionic surfactant SDS and the cationic surfactant CTAB form mixed micelles in both SDS-rich and CTAB-rich solutions, whereas between these compositions, vesicles and formation of a 1:1 precipitate are observed [76][77][78][79]. It was shown that the correlation length of 0.24 M CTAB-SDS increases from 0.4 to 2.5 nm as the SDS content of the surfactant mixture is increased from 0 to 15 mol%, leading to increased solubility of the hydrophobic monomer C17.3M in the micellar solution [37].…”

“…DLS measurements on hydrogels formed by hydrophobic associations have been reported at various preparation steps of the gelation solutions: before and after addition of salt and monomers to the surfactant solution, after micellar copolymerization, and after removal of surfactant micelles from the hydrogels [32,33,37]. Figure 9a, b shows typical time-average intensity correlation functions (ICFs) obtained at a scattering angle (θ) of ¼90 from SDS solutions and HM PAAm hydrogels, respectively [33].…”

“…1a), the length of the hydrophobic blocks of the hydrophilic network chains can be changed by varying the SDS content of the cationic surfactant solution. Figure 11a shows the frequency dependencies of G 0 (filled symbols) and G 00 (open symbols) for HM PAAm hydrogels formed in 0.24 M CTAB-SDS solutions with varying SDS content [37]. Figure 11b shows the relaxation moduli, G(t), of the same gels in the linear regime as a function of time (t) at different strains (γ 0 ).…”

“…This effect was investigated in HM PAAm hydrogels formed in aqueous solutions of CTAB containing 0-15 mol% of SDS [37]. Similarly to the addition of NaCl, addition of SDS-CTAB solution increases the size of the micelles (see Sect.…”