Soft and smart nanocomposite gels responsive to environmental stimulus

Soft and smart gels represent a versatile and very promising class of materials for biomedical applications, tissue engineering, sensors and muscle-type actuators owing to their resemblance of soft tissues, their autonomous intelligent structures as well as their easy operation in wet environments. In this thesis, new types of soft and smart polymeric gels as well as nanocomposite gels have been designed and the basis of environmentally-responsive properties in polymeric gels has been described. The research described herein presents the preparation of stimuli-responsive gels based on weak cationic polyelectrolyte poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with a broad range of gel preparation concentration. In the first part of the thesis, clay-free homopolymeric PDMAEMA hydrogels and cryogels have been synthesized via aqueous free radical crosslinking copolymerization as well as via cryopolymerization of the main monomer DMAEMA in the presence of the crosslinker diethyleneglycol dimethacrylate (DEGDMA). Ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED) were used as redox-initiator system to initiate the polymerization reactions. The swelling response, mechanical behavior and the effect of polymerization temperature on the structural characteristics of the prepared clay-free PDMAEMA gels were investigated as a function of the gel preparation concentration. The variation in the structural properties; effective-crosslinking density, average molecular weight of polymer chains between crosslinks and thermodynamic parameters from the combined swelling and elasticity results were disscussed as a function of the gel preparation concentration, and the correlation between synthesis conditions and structural parameters of gel networks have been determined. Oscillating swelling-shrinking studies were performed and the effective diffusion coefficient of water within the clay-free PDMAEMA cryogel and hydrogel matrix was estimated using the dynamic swelling profiles. In the second part of the thesis a series of polymer-clay nanocomposites were prepared via cryopolymerization and in situ free-radical crosslinking of the main monomer DMAEMA and 2-acrylamido-2-methyl-propanosulfonic acid (AMPS) between the interlayer spacing of montmorillonite (MMT) layered silicates using APS-TEMED as a redox-initiator system. The formation mechanism, thermal stability and surface morphology of the nanocomposite PDMAEMA/MMT NC-cryogels and NC-hydrogels incorporated with various amount of inorganic clay MMT were characterized by ATR-FTIR, XRD, SEM and TGA. The prepared nanocomposites were also characterized by uniaxial compression testing as well as swelling measurements. The effects of clay content on the equilibrium swelling, dynamic swelling kinetics, mechanical properties, swelling/deswelling behaviors under various pH and salt solutions were investigated and compared. For MMT concentrations ranging from 0.20 to 3.40 w/v%, the nanocomposite PDMAEMA/MMT NC-gels had larger volume swelling ratio and stronger mechanical strength than those for clay-free PDMAEMA gels. The obtained results provided more insights into the pH- and ionic strength- tunable swelling of slightly charged dialkylaminoethyl methacrylate-based polymer-clay nanocomposite gels and supported the development of potential applications combining the swelling and mechanical properties of weakly basic nanocomposite gels as biomedical materials.