Preparation of fast responsive macroporous hydrogels from frozen solutions

Bu çalışmada, 2-akrilamido-2-metilpropan sülfonik asit sodyum tuzu’nun (AMPS) sulu ortamlarda N,N-metilenbisakrilamid (BAAm) çapraz bağlayıcısı ile kopolimerizasyonu incelenmiştir. AMPS monomeri ile BAAm çapraz bağlayıcısının sulu çözeltileri başlatıcı amonyum persülfat (APS) ile hızlandırıcı N,N,N’,N’-tetraetilmetilendiamin (TEMED) redoks başlatıcı sistemi ile polimerizasyon sisteminin donma noktasının altında ve üzerinde gerçekleştirilmiştir. Elde edilen hidrojellerin şişmiş durumdaki elastik modülleri, sudaki şişme oranları, aseton ve sudaki büzülme ve şişme hızları ölçülmüştür. Taramalı elektron mikroskobu yardımı ile polimer ağyapılarının morfolojileri incelenmiştir. Hidrojellerin şişme ve elastik özelliklerinin -8ºC gibi bir jel hazırlama sıcaklığında belirgin bir şekilde değiştiği gözlenmiştir. -8ºC’nin altında sentezlenen hidrojelleri oluşturan ağyapıların gözenekli olduğu ve gözenek çaplarının 30-50 mm arasında değiştiği buna karşılık -8ºC ve üzerinde sentezlenen ağyapıların gözenek içermediği görülmüştür. Hazırlanan Poli(AMPS)(PAMPS) hidrojellerinin mikroyapısının, karnıbahara benzeyen ve değişik büyüklüklerde agregatlar içeren, reaksiyona bağlı faz ayrılma mekanizmasıyla oluşan makrogözenekli ağyapılardan belirgin bir şekilde farklı oldukları ortaya çıkarılmıştır. Bununla birlikte, -8ºC’nin altında sentezlenen PAMPS hidrojelleri için, suda ve asetonda tamamen tersinir şişme-büzülme eğrileri elde edilmiştir. -8ºC’nin altında oluşan jeller, denge şişme hacimlerine 30 saniyede ulaşırken, yüksek sıcaklıklarda oluşan jellerin suda denge durumlarına ulaşmaları için 1 saat gerekmiştir. Ayrıca, şişmiş PAMPS jelleri asetona daldırıldığında, -8ºC’nin altında hazırlanan jeller 5 ile 10 dakika arasında büzülerek dengeye ulaşırken, yüksek sıcaklıklarda sentezlenenler ise hacim değişimlerine dayanamayarak parçalanmıştır. Böylelikle, jel hazırlama sıcaklığının -8ºC’nin altına indirilmesi ile hacim değişimlerine karşı dayanıklı süper hızlı şişen PAMPS hidrojelleri elde edilmiştir. Anahtar Kelimeler: Hidrojel, şişme, donma, makrogözenek. Responsive hydrogels are smart materials, capable of changing volume in response to specific external stimuli, such as the temperature, solvent quality, pH, electric field, etc. These properties of hydrogels received considerable interest in last three decades and, a large number of hydrogel based devices have been proposed, including artificial organs, actuators, and on-off switches. Important parameters of hydrogels, which to a large extent determine their performance, are an improved mechanical property as well as a fast swelling-deswelling rate to the external stimuli. However, in some applications, such as artificial organs, the conventional hydrogel with slow response rate cannot satisfy this requirement. Therefore, the slow response rate of the hydrogels limits their technological applications. Increasing the response rate of hydrogels has been one of the challenging problems in the last 25 years. In this regard, several strategies have been proposed. A widely used approach to obtain fast responsive hydrogels is to create voids (pores) inside the hydrogel matrix, so that the response rate becomes a function of the microstructure rather than the size or the shape of the gel samples. A new and contradictory method to produce macroporous hydrogels and thus, increasing their response rate is conducting the polymerization reactions below the freezing point of the reaction system. This low temperature gelation so-called cryogelation was used to create an interconnected pore structure within the polymer networks. In this cryogelation process, the reaction system is partially frozen after the onset of polymerization reaction. The ice crystals formed after partial freezing, act as a template, while the dissolved monomer, cross-linker and initiator are concentrated in a small fraction of an unfrozen liquid phase. Although the frozen monomer system appears homogenous, it is composed of two phases: the solid phase and the unfrozen liquid phase where the polymerization takes place. After polymerization and after melting of ice, many pores are formed in the spaces that were originally occupied by solvent crystal. The morphology of the networks consists of polyhedral pores and the network chains building the pore walls. The gels formed under these conditions were named as cryogels. In this study, by applying the cryogelation technique to the present gelling system, cryogels exhibiting superfast swelling-deswelling kinetics were obtained from 2-acrylamido-2-methylpropane sulfonic acid (AMPS) monomer and N,N-methylenebis (acrylamide) (BAAm) crosslinker in aqueous solution at 5 w/v % initial monomer concentration. The crosslinker BAAm content in the monomer mixture was set to 17 mol %. The gel preparation temperature, Tprep, was varied between -22ºC and 25ºC. The swelling, swelling/deswelling kinetics, the morphologies as well as the elastic properties of the gels formed at various temperatures were studied. Tprep was found to be a significant parameter affecting the properties of the hydrogels. Depending on Tprep, two different regimes were observed from the experiments. At Tprep=-8ºC or above, Poly(AMPS) (PAMPS) gels exhibit relatively high swelling ratios Veq of the order of 101, and low moduli of elasticity G in the range of 102-103 Pa. However, decreasing Tprep below -8ºC results in a tenfold decrease in the swelling ratio and about tenfold increase in the elastic modulus of gels. Thus, the swelling and elastic properties of PAMPS drastically change as Tprep is decreased below -8ºC. SEM images of PAMPS networks formed at various Tprep, indicated that all of the polymer samples formed below -8ºC have a porous structure with pore sizes of 30-50 µm while those formed at or above -8ºC exhibit a continuous morphology. Completely reversible swelling-deswelling cycles were obtained using PAMPS cryogels prepared below -8ºC. Gels formed below -8ºC attain their equilibrium swollen volumes in less than 30 sec, while those formed at higher temperatures require about 1 h to reach their equilibrium state in water. Moreover, if swollen PAMPS gels are immersed in acetone, those prepared below -8ºC attain their equilibrium collapsed state in 5 to 10 min, while the hydrogels formed at higher temperatures are too weak to withstand the volume changes. Thus, decreasing Tprep below -8oC results in the formation of superperfast-responsive PAMPS hydrogels, which are also stable against volume changes. Keywords: Hydrogel, swelling, freezing, macroporous