Nermin Orakdogen

Self-healing hydrogels formed in catanionic surfactant solutions†‡

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.

pH-responsive poly(N,N-dimethylaminoethyl methacrylate-co-2-acrylamido-2-methyl-propanosulfonic acid) cryogels: swelling, elasticity and diffusive properties

A series of ionic poly(N,N-dimethylaminoethyl methacrylate-co-2-acrylamido-2-methyl-propanosulfonic acid) (P(DMAEMA-co-AMPS)) cryogels were prepared by free-radical crosslinking copolymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA) and 2-acrylamido-2-methyl-propanosulfonic acid (AMPS) in aqueous solution. The swelling properties and the elastic behavior of P(DMAEMA-co-AMPS) cryogels drastically change at a gel preparation temperature, −18 °C, below the bulk freezing temperature of the polymerization solvent, water. The equilibrium and dynamic swelling/deswelling properties of the prepared cryogels responding to pH as well as in aqueous solutions of NaCl, KCl, MgC12, Na2S04, K2S04, and MgS04 were investigated. The influence of the relative content of ionic comonomer AMPS on the swelling properties was examined in water and the obtained results were compared with P(DMAEMA-co-AMPS) hydrogels synthesized at usual polymerization temperature. The ionic gels prepared with higher DMAEMA contents exhibited a highly pH-dependent equilibrium swelling behavior. The extent of transition from the swollen state to the shrunken state was strongly related to the DMAEMA content of the network. The results indicated that P(DMAEMA-co-AMPS) cryogels not only had considerable swelling ratio as well as rapid swelling/deswelling kinetics, but also exhibited improved mechanical properties.

Design and synthesis of dual-responsive hydrogels based on N,N-dimethylaminoethyl methacrylate by copolymerization with N-isopropylacrylamide

AbstractCopolymeric hydrogels of N,N-dimethylaminoethyl methacrylate (DMAEMA) and N-isopropylacrylamide (NIPA) of various monomer ratios were evaluated as thermo-responsive and pH-responsive systems for the development of controlled-release and targeted-delivery devices. The swelling properties were investigated with different temperature, pH, and monomer feed ratios. The results show that the temperature-dependent and pH-dependent phase transition of poly(N,N-dimethylaminoethyl methacrylate-N-isopropylacrylamide) (P(DMAEMA-NIPA)) copolymeric hydrogels can be controlled by changing the amount of NIPA units in the network chains. In experiments to determine the temperature-dependent swelling of copolymeric hydrogels in water, it was found that the swelling ratio rapidly decreases as the temperature increases between 35 and 70°C. To characterize the network structure of the copolymeric hydrogels corresponding to effective cross-linking density and average network chain length, uniaxial compressive mechanical testing was carried both after the preparation of hydrogels and after their equilibrium swelling in water. The data obtained demonstrates that the resulting copolymeric hydrogels are promising as materials with tunable hydrophilicity-hydrophobicity and swelling behavior responsive to temperature and pH.

Ion-stimuli responsive dimethylaminoethyl methacrylate/hydroxyethyl methacrylate copolymeric hydrogels: mutual influence of reaction parameters on the swelling and mechanical strength

Novel ion-stimulus-responsive copolymeric hydrogels were synthesized by free radical crosslinking copolymerization from the monomers N,N-dimethylaminoethyl methacrylate (DMAEMA) and 2-Hydroxyethyl methacrylate (HEMA) in the presence of a crosslinker, diethyleneglycol dimethacrylate (DEGDMA). The influences of the reaction parameters, the comonomer composition and the ionic strength of salt solutions on the swelling behavior of P(DMAEMA-co-HEMA) hydrogels were examined. The ion-stimulus-responsive swelling behavior of the prepared copolymers was studied in water as well as in aqueous solutions of NaCl, KCl, KBr, KI, CaCl2, BaCl2 and MgCl2. It was found that, starting from some characteristic concentration of a salt, a further increase of the salt concentration results in the shrinking of copolymeric P(DMAEMA-co-HEMA) hydrogels. The Flory-Rehner theory correctly predicts the swelling behavior of the hydrogels in salt solutions if the variation of the comonomer HEMA content is taken into account. The calculation of the interaction parameter χ between P(DMAEMA-co-HEMA) network and water showed that the specific interactions between cations and side groups of polymeric network affect the mixing term of the free energy. The extent and kinetics of water absorption were studied to determine their relationship with the reaction parameters. The kinetics of the hydrogel collapse is strongly dependent on the kind of salt used. The swelling results will be useful in designing and developing novel controlled delivery systems.

Dynamics of overall swelling profile of multiresponsive ionic dimethylacrylamide-based hydrogels and cryogels: Diffusion characteristics evaluation of salt-dependent swelling

ABSTRACT Due to the importance of ion–polymer interactions in many technological applications, systematic measurements have been investigated on a variety of ionic poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels and cryogels to determine the effect of mono- and multivalent ions on their dynamic properties. Using uniaxial compression, the influence of counter-ion valency on flexibility of charged polymer chains was investigated and the results were discussed as a function of ionic comonomer. To estimate the general swelling features, diffusion process and oscillating swelling–deswelling kinetics of ionic PDMAAm gels in salt solutions have been explained by model equations using early-time approximation as a function of characteristic response time. GRAPHICAL ABSTRACT

Quantitative analysis on compressional behavior and swelling kinetics modeling of weakly ionic poly(N,N-dimethylacrylamide-co-acrylic/methacrylic acid or sodium acrylate) gels

AbstractA quantitative theoretical description of unusual elastic properties of rubbery materials in terms of the physico-chemical structural parameters remains a challenging task. The equilibrium and dynamic swelling properties of acrylamide-based ionic gels have been analyzed to optimize the design flexibility in terms of the molecular response, the structural integrity, and tunable diffusion behavior. A series of weakly ionic hydrogels and cryogels based on N,N-dimethylacrylamide (DMAAm) and the anionic comonomers acrylic acid (AAc), methacrylic acid (MAAc), and sodium acrylate (NaA) have been prepared by the crosslinking copolymerization using ammonium persulfate as initiator and N,N′-methylenebisacrylamide as crosslinker. By combining the swelling and elasticity data, quantitative agreement was demonstrated and the earlier qualitative observations was confirmed that both the effective charge density distribution and pKa value of weak polyelectrolytes can change substantially the responsive diffusion kinetics depending on the explicit account of charge association/dissociation of incorporated ionic comonomers. The swelling degree of ionic PDMAAm gels can sensitively respond to change in pH of the swelling media and the presence of ionic acrylic segments influences the swelling kinetics of the prepared gels evidently. Graphical abstractCompression elasticity, equilibrium, and pulsatile swelling behavior of ionic PDMAAm hydrogels and cryogels performed at different pH values were evaluated considering various concentrations of ionic comonomers and pH of the swelling medium.

Compression Modulus and Equilibrium Swelling Studies of Poly(2-Hydroxyethyl Methacrylate) Hydrogels Formed at Various Polymer Concentrations

The equilibrium swelling degree and modulus of elasticity of poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels prepared at various initial monomer concentrations were investigated to understand their characteristic behavior for development as biomaterials. The resulting hydrogels were characterized in terms of the equilibrium swelling and compression stress-strain measurements. The influences of the monomer feed composition on the swelling and the mechanical properties of PHEMA hydrogels were explored. The swelling capacity and network characteristics of the resulting hydrogels were determined through the observation of swelling behavior in water and compression modulus determination both after equilibrium swelling and after their preparation. The equilibrium swelling of PHEMA hydrogels was analyzed by Flory-Rehner theory and the results were combined by the results of elasticity measurements to calculate the molecular characteristics of hydrogels. The mechanical strength of PHEMA hydrogels increased greatly by increasing the gel preparation concentration. The results showed that the gel strength and the mechanical properties of hydrogels can be controlled by changing the polymer concentration and the amount of water used in the gel preparation.