Alkylacrylamide-based semi-interpenetrating networks for temperature-sensitive smart systems

Abstract

Hybrid gels containing nanoparticles offer opportunities to selectively tailor the final properties as desired. Novel approaches in design of hybrid gel systems have led to the design of high-performance complex materials. These alternative preparation approaches focus on tuning the required functional and structural properties with a new material structure. Within the scope of this thesis, semi-IPN structures containing linear polymer polyacrylamide (PAAm) chains in different amounts were firstly prepared. The temperature sensitive monomer N-isopropylacrylamide (NIPA) was used as the main monomer, and a series of temperature and pH sensitive gels in the form of semi-interpenetrating polymer network (semi-IPN) structures were prepared by adding Methacrylic acid (MA) comonomer. In the second part of the thesis, the semi-IPN gel composition with the optimum structure obtained from the first part was selected and a semi-IPN structure hybrid gel synthesis was carried out by incorporation of different amounts of silica particles. In the first part of the experimental work of this thesis, a series of thermoresponsive semi-IPN gels based on linear polymer PAAm, NIPA and MA were synthesized by varying the linear polymer content via free radical polymerization. An interpenetrated network of P(NIPA-MA)/PAAm was designed to modulate elasticity by varying inner composition and to improve rate of swelling-deswelling phenomenon. The correlation between swelling and compression elasticity was demonstrated. The inclusion of linear PAAm chains into P(NIPA-MA) network induced physical entanglements, increased apparent crosslinking density and enlarged the compressive elasticity. The dependence of apparent crosslinking density on the linear polymer content was expressed as a cubic polynomial function. The extent of swelling of semi-IPN P(NIPA-MA)/PAAm gels was sensitive to presence of linear PAAm chains. The existence of linear polymer decreased apparent ionic group density and increased the crosslinking density compared to that of copolymer P(NIPA-MA) network which in turn decreased the equilibrium swelling. A significant increase in the swelling/shrinking rate was observed in the presence of linear PAAm. Due to ionization of carboxylic acid groups in P(NIPA-MA) network, semi-IPN P(NIPA-MA)/PAAm gels showed different degrees of swelling depending on linear PAAm content and temperature of swelling medium. Semi-IPNs exhibited phase transition temperatures shifted higher temperature, suggesting physical entanglements between P(NIPA-MA) network and linear PAAm. An increase in the swelling temperature resulted in an increase in Flory-Huggins interaction parameter. The entropic contribution increased and the enthalpic contribution decreased with PAAm content. In the second part of the experimental work of this thesis, a series of semi-IPN P(NIPA-MA)/PAAm hybrid gels reinforced with silica particles (SiP) were designed by incorporation of linear PAAm chains. Formation of temperature-sensitive hybrid semi-IPN gels was evaluated by simultaneous radical polymerization under warm and cold conditions, and the role of polymer/particle interfaces in the elasticity of hybrid gels was explained. Nanoparticle-mediated enhancements were studied to understand the effect of SiP added to anionically modified semi-IPNs. The formation of hybrid network was confirmed by FTIR and an increase in the amount of SiP led to enhanced Si-O-Si absorption peak in the hybrid samples. P(NIPA-MA)/PAAm/SiP gels displayed a decrease of swelling with increasing amount of SiP. Flory-Huggins parameter of interaction of SiP-loaded semi-IPN hybrid-solvent were estimated using the extended equation. The compressive testing results showed an improvement in the stiffness and modulus attributed to transference of stress from the hybrid to the nanoparticles. The swelling processes of SiP-loaded semi-IPN hybrids obtained by cold polymerization are anomalous diffusion owing to polymer relaxation, while Fickian behavior was observed for the gels obtained by warm polymerization. During oscillation shrinking-swelling of SiP-loaded semi-IPN hybrids upon ionic-strength switching in NaCl solutions, the gels retained their shape and integrity for 10 cycles of measurement. The effect of changes in solution environment on the swelling and elastic properties of SiP-loaded semi-IPN hybrids was investigated. Sodium salts of CH3COO−(Ac), NO3−, and SCN− were employed to understand specific ion effects and to gain an insight into the influence of kosmotrope and chaotrope solutes on the swelling and elasticity at various ionic strength and temperature. The identity of counterion had a strong influence on swelling and compression modulus and was found to follow the reverse order of classical Hofmeister series (HS). The phase transition temperature decreased with increasing salt concentration, and transition changed from discontiuous to continuous. At low salt concentrations, the gels showed discontinuous phase transition at 50 oC, while the transition temperature shifts towards 35-45 oC with increasing salt concentration. Dilution of the solution to 10-3 M resulted in a 2.0-fold reduction in the compressive modulus of hybrid gels in NaAc solutions while a 3.0-fold reduction in NaSCN solutions. The compression modulus increased with shrinkage of hybrids due to increasing salt concentration. The effect of pH change of swelling medium on the swelling, phase transition profile and elasticity was evaluated. The continuity of pH-dependent swelling curve depended on the swelling temperature. pH-triggered oscillation kinetics have been investigated to understand whether they can be used as a pH oscillator since the phase transition pH is close to 5.7. The swelling of SiP-loaded hybrids followed a Fickian type diffusion. To evaluate the adsorption characteristics of SiP-loaded semi-IPN hybrids, methyl violet (MV) was chosen as model cationic dye. The effects of contact time, silica content and initial dye concentration were studied. The time-dependent adsorption data was fitted with six kinetic models. MV uptake capacity of SiP-loaded semi-IPN hybrids increased with an increase in initial MV concentration as well as with silica content. The adsorption process followed pseudo-second order type adsorption kinetics and the mechanism of process was better described by intraparticle difusion. Different adsorption isotherm models were investigated to describe interfacial adsorption of MV which was strongly dependent on compositions of hybrids. Langmuir isotherm model was found to be most appropriate to describe adsorption process. The superior properties imparted to the present system prepared with anionic modification, semi-IPN structure and SiP-loading make semi-IPN hybrid gels suitable candidates for controlled release systems and for adsorbents of cationic dyes.