Swelling dynamics and thermomechanical properties of multifunctional hybrid systems based on N-alkyl methacrylate esters

Abstract

Increasing demand for functional polymers in applications based on smart materials has led to the need for modified synthetic tools and new ways to design advanced materials containing specific functional groups. Improved material design is possible using specific monomers with functional groups that remain proactive under polymerization conditions and can be selectively converted to other functional groups in subsequent steps. N-alkyl methacrylate-based monomers, which can be easily polymerized under mild conditions, are widely used in design of various precisely defined functional materials. Thanks to these extraordinary features of smart gels; hydrogels have many applications in processes ranging from industrial to biological. In particular, it have many uses in many areas from food industry, pharmaceutical applications, drug delivery systems, agriculture, contact lenses, sensors, tissue engineering, controlled drug release and water purification, etc. In the first part of this study, a synthesis group was designed for the preparation of n-alkyl methacrylate ester-based multifunctional hybrid systems. It was aimed to perform the synthesis of a series of n-alkyl-methacrylate estrer-based terpolymer hydrogels consisting 2-(dimethylamino)ethyl-methacrylate (DMAEMA), hydroxypropyl-methacrylate (HPMA), and glycidyl-methacrylate (GMA) using different monomer feed compositions at a fixed crosslinker ratio. The aim of the second part of this study was to successfully synthesize terpolymer cryogels using the monomers and crosslinkers in equal proportions to those synthesized in the first part and to investigate the effect of tertiary amine groups of DMAEMA and epoxy functional groups of GMA on the gel properties. In the third part of the study; the effect of the gel preparation temperature on the gel properties was investigated. For this purpose, the results obtained from the first and second part were interpreted and the terpolymer with 20 mol% HPMA, 70 mol% DMAEMA and 10 mol% GMA structure was selected, and In this part, the terpolymer gel prepared by free radical crosslinking in water at constant monomer and crosslinker concentrations was successfully synthesized by changing the polymerization temperature between -18 and 60 oC. The effect of the change in DMAEMA/GMA mol percent ratio from 80/0 to 0/80 in the terpolymer matrix on the gel properties was investigated and it was observed that the gel properties are controlled by the tertiary amino groups in the structure of DMAEMA as well as the GMA monomer. In order to evaluate the macroscopic properties of terpolymer gels, the specific structural properties of the groups in the terpolymer matrix and their physico-chemical properties that vary depending on the monomer feed composition and gel preparation temperature were investigated. In the first part of the experimental work, it is aimed to develop a new polycationic ternary gel system with different functional groups and to evaluate the scaling relationships between the network parameters and the elastic properties. Multifunctional ternary-gels based on 2-(dimethylamino)ethyl methacrylate (DMAEMA), hydroxypropyl methacrylate (HPMA) and glycidyl methacrylate (GMA) were prepared in a wide gel preparation concentration range. The resulting gels denoted as terpolymeric poly(hydroxypropyl methacrylate-co-glycidyl methacrylate-co-2-(dimethylamino)ethyl methacrylate), PHDm/Gn-Hgs, were characterized by ATR-FTIR, and TGA measurements. The thermal stability of the terpolymer hydrogels was observed to be improved with the increase in GMA content. Swelling of ternary-gels in different solvents showed that cyclohexanol was a good solvent for the present terpolymer gel system. Swelling is controlled by DMAEMA portion of terpolymers and DMAEMA-rich gels tended to swell more than GMA-rich gels and increase in GMA content increased the polymer-solvent interaction parameter. It was determined that the addition of hydrophobic GMA to the terpolymer matrix led to a significant increase in the stiffness. The increase in elastic modulus as-prepared state is not monotonic and can be thoroughly controlled by adjusting DMAEMA/GMA ratio since tertiary amine groups triggers cascading epoxy ring-opening reaction. Dependence of swelling on the gel preparation concentration as a function of solvent quality was examined. A good solvent prediction indicated a crossover between entanglement-dominated case and strongly cross-linked case corresponding to low and moderate degrees of swelling. It has been observed that the reduced modulus decreased with increasing degree of swelling. The scaling relationship between the modulus and degree of swelling which indicates that ternary-hydrogels, limited to weak stretching regime, obey Gaussian statistics. In the second part of the experimental work, pH/thermo-responsive cationic terpolymer cryogels were successfully prepared by varying the feeding DMAEMA/GMA mol ratio. Terpolymer cryogels with different DMAEMA/GMA ratio have been extensively characterized with ATR-FTIR, X-ray diffraction analysis (XRD) and TGA measurements. The presence of GMA imparts hydrophobic character that reduces the swelling and provides additional crosslinks leading to a significant increase in the flexibility. Reducing the polymerization temperature below the freezing point of the solvent resulted in harder and non-brittle cryogels. With the increase in the amount of DMAEMA in the terpolymer matrix, the pH-dependent transition point was found to be 7.7. The water transport was strongly affected by incorporation of hydrophobic component GMA and protonation of amine functional groups. The swelling mechanism was shifted from anomalous to diffusion controlled with increasing GMA and overall swelling processes followed Schott second order dynamic equation. It was observed that lower equilibrium volume swelling at constant temperature was achieved by increasing GMA content of the terpolymers. In low temperature range, while DMAEMA-rich terpolymer gels were swollen, swelling decreased significantly when the gels were enriched in GMA. A significant difference was observed in salt-sensitive swelling in dilute salt solution, which is controlled by the amount of DMAEMA in terpolymer structure. This part of the work is important in that it provides new ideas for the design of (meth)acrylate ester-based cationic terpolymer cryogels and explains the relationship between the gel matrix and their specific swelling behavior. From the first and second part of the experimental study, the terpolymer PHDG structure containing 10% mol of GMA, 70% mol% of DMAEMA and 20 mol% of HPMA was selected and an attempt was made to establish a relationship between the the gel preparation temperature and the elasticity as well as pH/temperature sensitivity of n-alkyl methacrylate ester-based cationic gels. In the third part of the experimental study, the gel preparation temperature Tprep was changed and terpolymer gels were prepared at -18, 1, 5, 8, 24, and 60 oC. The structure and physical properties of ternary gels was fully characterized using TGA, ATR-FTIR and XRD. Tprep has been shown to be an effective independent variable to adjust both macroscopic and microscopic properties of ternary gels as desired. The dependence of the swelling and compressive elasticity on the gelation temperature was investigated and it was determined that terpolymer gels synthesized at 5 °C showed maximum swelling capacity. A significant change in the elastic modulus was observed as Tprep decreased from 60°C to -18°C. Terpolymer gels showed pH-sensitive swelling characteristic of cationic DMAEMA monomer and exhibited high swelling ratio in acidic solutions induced by the electrostatic repulsion between quaternary amine groups. It was observed that the swelling ratio decreased due to hydrophobic effect of the alkyls when environmental pH is higher than 7.7. Regardless of Tprep, the gels gradually contracted as the swelling temperature increased from 25°C to 75°C. The effectiveness of terpolymer gels to remove the anionic dye Methyl Orange from aqueous solution was tested and the results showed that the terpolymer gels prepared under low temperature conditions were promising for removing anionic dyes from wastewater. The adsorption was spontaneous and found to be a multistep process with the surface adsorption followed by the intraparticle diffusion. The results obtained will assist in the design of n-alkyl methacrylate ester-based ternary gels as an effective adsorbent for dye removal from wastewater and pharmaceutical preparations.