Dissociation And Characterization Of Reversible Hydrogels Formed By Hydrophobic Interactions

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2013Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2013Hidrojeller, üç boyutlu polimer ağ yapısı içinde su ihtiva eden iki ya da çok bileşenli sistemlerdir. Bu özelliklere sahip malzemeler; aralarında tarım, elektronik, ilaç salınım sistemleri, doku mühendisliği, kaplama, kozmetik, biyosensör, kontakt lens gibi kullanım alanlarını içeren çok geniş uygulamalara sahiptir. Ancak kırılgan olmaları, kendi kendilerini iyileştirme yeteneklerinin olmaması gibi eksiklikler nedeniyle hidrojellerin kullanım alanları kısıtlanmaktadır. Öte yandan dayanıklı, kendi kendini iyileştirebilen ve oldukça kısa sürede dıştan gelen uyarılara cevap veren akıllı malzemelerin üretilmesi mümkündür. Söz konusu akıllı malzemeler, mevcut hidrojellerin modifikasyonu ile elde edilebilmektedir. Bu amaçla akademik ve endüstriyel laboratuvarlarda çok geniş kapsamda çalışmalar yapılmaktadır. Kendi kendini iyileştirme ve yüksek seviyede tokluk gibi gelişmiş mekanik özelliklere sahip jeller sentezlemek amacıyla hidrofilik polimer zinciri boyunca rastgele asılı hidrofobik bloklar oluşturularak hidrofobik olarak modifiye edilmiş hidrojellerin sentezlenmesi yapılan araştırmalar arasındadır. Bununla birlikte, akıllı jellerin sentezlenmesinden sonraki en önemli aşamalardan biri modifiye edilmiş polimer ağ yapısının karakterizasyonudur. Bu çalışmanın amacı; farklı hidrofobik blok uzunluğuna sahip hidrofobik modifiye poliakrilamid (HMPAAm) jellerinin sentezlenip saflaştırılmasından sonra uygun koşullar sağlanarak çözünür hale getirilmesi ve bu sayede hem hidrofobların polimer ağ yapısına girdiğinin ispatlanması hem de bloklu yapının varlığının kanıtlanmasıdır. Bu çalışma kapsamında hidrofobik modifiye hidrojeller, akrilamid (AAm) ve stearil metakrilatın (C18) NaCl-sodyum dodesil sülfat (SDS) çözeltisindeki miseller kopolimerizasyonu ile sentezlenmiştir. Bu tekniğe göre, SDS miselleri içinde çözünmüş olan hidrofobik komonomer C18, sulu ortamda çözünmüş hidrofilik AAm monomeri ile kopolimerleştirilmiştir. NaCl, küresel SDS misellerinin boru tipi misellere dönüşerek büyümesine yol açmaktadır. Büyüyen SDS miselleri içerisinde çözünen C18 hidrofobu, miseller kopolimerizasyon yöntemi ile hidrojel ağ yapısına katılmaktadır. HMPAAm jellerinde ağ yapı zincirleri üzerindeki bloklu yapının gösterilmesi amacıyla farklı hidrofobik blok uzunluğuna sahip jellerin sentezi için reaksiyon hacmi, başlatıcı sistemi, NaCl ve SDS konsantrasyonu sabit tutulmuştur. Hidrofobik komonomerin toplam başlangıç monomer konsantrasyonuna oranı değiştirilmezken, toplam monomer konsantrasyonu % 5-30 (ağ./hac.) arasında değiştirilmiştir. Bu sayede, HMPAAm jellerinin sentezinde misel sayısı değişmezken, toplam monomer konsantrasyonunun artmasıyla miseller içerisinde çözünen hidrofobik monomer miktarı da artmıştır. Sonuç olarak, jellerin şişme, mekanik ve reolojik özelliklerinin değişimi, misel başına düşen hidrofobik monomer miktarı (NH) değerinin yani polimer ağ yapısı üzerindeki hidrofobik blok uzunluğunun değişimine bağlı olarak incelenmiştir. Sentezlenen hidrojellerin ağ yapı zincirlerine C18’in katıldığını ispatlamak için spektroskopik analizlerin yapılması amaçlanmıştır. Ancak bunun öncesinde şişme ve diyaliz yöntemleri ile hidrojel ağ yapısından SDS’nin uzaklaştırılması ile saflaştırma çalışmaları gerçekleştirilmiş ve ardından ağ yapı zincirlerinin çözünürlük çalışmaları yürütülmüştür. Ağ yapı zincirleri arasındaki SDS’nin tümüyle uzaklaştırıldığı metilen mavisi deneyi ile takip edilmiştir. HMPAAm jellerinin, yüksek sıcaklığın etkisiyle dimetil sülfoksit (DMSO) içerisinde çözündüğü bulunmuştur. Saflaştırılıp, donarak kurutulan HMPAAm jelleri, döteryumlanmış dimetil sülfoksit (DMSO-d6) içerisinde homojen bir şekilde çözüldükten sonra proton nükleer manyetik rezonans (1H-NMR) analizi gerçekleştirilmiştir. Bu sayede hidrofilik polimer omurgasında yer almayan ve hidrofobik monomer C18’in yapısında bulunan metil hidrojenlerinin oluşturduğu kimyasal kaymaların gözlenmesi mümkün hale gelmiştir. 1H-NMR analizi ile hidrofobik monomer C18’in, polimer yapısına katıldığı ispatlanmıştır. Ayrıca infrared absorbsiyon spektroskopisi (FT-IR); C18’in, hidrofilik polimer yapısına katıldığını destekler niteliktedir. FT-IR analizinden alınan sonuçlara göre metil grubuna ait hidrojen pikinin, lineer PAAm spektrumundaki pike oranla kuvvetlendiği gözlenmektedir. Mekanik ölçümler ile yüksek başlangıç monomer konsantrasyonuna sahip jellerin, daha düşük başlangıç monomer konsantrasyonunda sentezlenen jellere göre daha yüksek elastik modüle (G’) sahip oldukları bulunmuştur. Elde edilen HMPAAm jellerinin saf su içerisindeki şişme davranışları incelendiğinde başlangıç monomer konsantrasyonunun artışıyla maksimum şişme derecesinin azaldığı görülmüştür. HMPAAm jellerinin saf su içerisinde çözünmemesine karşın yüzey aktif madde içeren sulu veya NaCl çözeltilerinde çözünebilmesi, jel yapısındaki çapraz bağların fiziksel etkileşimlerle oluştuğunu ispatlar niteliktedir. Bununla birlikte, jellerin NaCl çözeltisi içerisinde çözünmesinden sonra gerçekleştirilen viskozite ölçümleri, HMPAAm jellerinde başlangıç monomer konsantrasyonunun artmasıyla birlikte hidrofobik etkileşimlerin kuvvetlenmesinden dolayı çözelti viskozitesinin arttığı göstermiştir. Tüm bu sonuçlar, başlangıç monomer konsantrasyonunun artışıyla polimer ağ yapısındaki çapraz bağ yoğunluğunun yani hidrofobik etkileşimlerin arttığını göstermektedir. Bir başka deyişle, hidrofobik blok uzunluğunun arttığına işaret etmektedir.Hydrogels are two or multicomponent systems containing water in a three-dimensional polymer network. Such materials have a wide variety of applications such as in agriculture, electronics, drug delivery systems, tissue engineering, breast implants, coatings, cosmetics, biosensors and contact lenses. However, hydrogels that are highly swollen in water are normally very brittle. They suffer from the lack of mechanical stability and self-healing ability. In addition, they exhibit a slow rate of response against the external stimuli. These drawbacks limit the use of hydrogels in many potential application areas. Recently, it was shown that hydrogels formed by hydrophobic associations exhibit a high degree of toughness due to the mobility of the junction zones within the gel network, which contributes to the dissipation of the crack energy along the hydrogel sample. Hydrophobically modified hydrogels were prepared by copolymerization of the hydrophilic monomer acrylamide (AAm) in the presence of a small amount of a hydrophobic comonomer via micellar polymerization technique. In this technique, a water insoluble hydrophobic comonomer solubilized within the micelles is copolymerized with a hydrophilic monomer in aqueous solutions by free-radical addition polymerization. Because of high local concentration of the hydrophobe within the micelles, the hydrophobic monomers are distributed as random blocks along the hydrophilic polymer backbone. Our research group has shown that that n-alkylacrylamides or n-alkyl methacrylates with an alkyl chains length between 4 and 12 carbon atoms can easily be copolymerized with AAm to obtain tough hydrogels. However, larger hydrophobes such as stearyl methacrylate (C18) or dococyl acrylate (C22) cannot be copolymerized with AAm simply in a micellar solution of sodium dodecyl sulfate (SDS). This is due to the very low water solubility of these monomers (e.g., 10-9 mL/mL for C18), which restricts the monomer transport through the continuous aqueous phase into the micelles. Our research group has recently shown that large hydrophobes can be solubilized in a micellar solution of sodium dodecyl sulfate (SDS) provided that an electrolyte, such as NaCl, has been added in sufficient amount. Salt leads to micellar growth and, hence, solubilization of the hydrophobes within the grown SDS micelles. After solubilization of the large hydrophobes C18 within the wormlike SDS micelles of salt solutions, they could be copolymerized with AAm to obtain physical hydrogels. The surfactant-containing gels formed using C18 blocks as physical cross-links exhibit unique characteristics such as insolubility in water but solubility in SDS solutions, nonergodicity, self-healing, and a high degree of toughness. A recent issue in supramolecular chemistry of smart materials is organization of these type of metarials into desired superstructures in attemt to obtain high degree of toughness, good mechanical performance etc. Characterization of these metarials through crosslinks formed by covalent bonds or physical interactions is a key factor in view of this point. It should be noted that, an indispensable factor to design tailored materials with good controllable rheological properties is the better understanding of the modified polymer structure. The objective of this study is to characterize the microstructure of the hydrophobically modified polyacrylamide networks having different hydrophobic block lengths. The first aim of the present study is to demonstrate the presence of hydrophobic (C18) blocks in the hydrophobically modified polyacrylamide backbone and the second aim is to investigate the variations of the hydrophobic block length at a constant hydrophobe and surfactant level depending on the initial monomer concentration. For this purpose, hydrophobically modified polyacrylamide (HMPAAm) hydrogels with different hydrophobic block lengths were synthesized and then, purified by extraction with water. Next, solubilization of HMPAAm gels under suitable conditions was investigated to achieve characterization of isolated network chains. In this study, large hydrophobic monomer stearyl methacrylate (C18) was copolymerized with the hydrophilic monomer acrylamide (AAm) in a micellar solution of NaCl-sodium dodecyl sulfate (SDS) via micellar copolymerization technique. In this technique, hydrophobic monomer C18 was solubilized within surfactant micelles whereas the hydrophilic monomer AAm was dissolved in aqueous continuous medium. Solubilization of C18 was achieved by the addition of salt (NaCl) into the reaction solution. Salt leads to micellar growth and hence, solubilization of the hydrophobes within the SDS micelles. Copolymerization of acrylamide and C18 was carried out at a fixed salt and surfactant concentration. The total initial monomer concentration (C0) was varied between 5 to 30 % (w/v), while the ratio of hydrophobic comonomer to the hydrophilic monomer was fixed. Thus, although the number of the micelles was constant in all HMPAAm gels, the amount of C18 increased with increasing initial monomer concentration leading to increased number of hydrophobic monomers per micelle (NH). On that account, increasing NH, in other words increasing hydrophobe/surfactant ratio during the synthesis resulted in polymers with different hydrophobic block lengths. The effect of the copolymer microstructure controlled by varying hydrophobe/surfactant ratio on the properties of the network chains was investigated by means of swelling tests, mechanical and reological measurements. Spectroscopic characterization of the network chains was carried out to reveal the incorporation of the hydrophobes to hydrophilic polymer backbone. However, SDS and C18 have the same 1H NMR and FT-IR peaks preventing the spectroscopic characterization of the network chains. In this respect, to obtain physical gels free of SDS micelles, the gel samples were extracted in water by means of swelling tests and dialysis. Methylene blue active substances assay (MBAS) was conducted in parallel to the swelling tests. The results indicate that SDS content of gels dropped below the detection limit of the MBAS after nearly ten days and the cumulative release of SDS is approximately equal to the SDS amount used in the course of the HMPAAm gel synthesis. After the swelling tests, physical gels free of SDS micelles were freeze dried. Then, solubilization tests were carried out by immersing the physical gels in several solvents and solutions. Although the gels were insoluble in water due to the strong hydrophobic interactions, they could be dissolved in dimethyl sulfoxide (DMSO) at 80 oC, providing microstructural characterization of the network chains. 1H NMR spectra of the HMPAAm gels in DMSO- d6 exhibits characteristic protons emerging from C18 units. Peak at 0.9 ppm arises due to the protons of α-methyl backbone and of the terminal methyl of the alkyl chain, while the peak at 1.2 ppm was caused by the protons attached to carbon atoms on the side alkyl chain of C18 units. Although NMR technique was not sensitive enough for the determination of copolymer microstructure due to the low hydrophobe content, increasing peak intensities with increasing NH indicates blockiness of the polymers. Apart from 1H NMR, FT-IR spectra of the network chains isolated from physical gels shows the characteristic bands at 2920 cm-1 and 2850 cm-1 due to the stretching of the methylene groups of C18 units which are absent in PAAm chains. Mechanical measurements show that the degree of crosslink densities of hydrogels increases by introduction of C18 blocks into the hydrophilic polymer chains. Also, elastic modulus, G,’ of gels increases with increasing initial monomer concentration. This indicates that degree of toughness increases with increasing hydrophobic block length. Swelling behavior of HMPAAm gels shows that self-healing hydrogels exhibit unusual swelling kinetics due to the osmotic pressure of SDS counterions inside the gel network. Thus, the gel initially behaves like an ionic gel and thus exhibits a large swelling ratio. However, as SDS is extracted, the gel gradually converts into a nonionic gel having a reduced swelling ratio. Maximum swelling ratio of gels increases with decreasing initial monomer concentration. Swelling tests also show that the hydrophobic associations are too strong to be destroyed in water during the expansion of the gel network. In addition, viscometric measurements show a strong enhancement of the viscosities of the network chain solutions as compared to PAAm solution. This demonstrates the existence of hydrophobic blocks in the network chains. The solutions of the network chains exhibit a Newtonian plateau at low shear rates followed by an abrupt shear thickening region before the onset of shear thinning. The shear thickening region is typical for associative flexible polymers and, is a result of the formation of transient intermolecular hydrophobic associations. These associations are favorable at a certain degree of coil deformation while they become disrupted at higher shear rates. Furthermore, viscometric and rheological behavior of polymer solutions also showed a substantial increase in the associativity of the network chains with increasing NH, i.e., with increasing length of the hydrophobic blocks. effects and they don t have self-healing properties. Fortunately, it is possible to produce high tough and self-healing hydrogels. One method to synthesize such smart materials is incorporation of a small amount of hydrophobic groups into the hydrophilic polymer chain. In addition, an indispensable factor to design tailored materials with good controllable rheological properties is the better understanding of the modified polymer structure. The aim of this study is characterization of hydrophobically modified polyacrylamide hydrogels (HMPAAm) which have different hydrophobic block lengths. HMPAAm hydrogels were prepared by micellar copolymerization technique. In this technique, hydrophobic monomer was solubilized within surfactant micelles whereas the hydrophilic monomer was dissolved in aqueous continuous medium. The number of micelles was kept constant and the amount of C18 was increased by C0. HMPAAm gels were characterized with 1H-NMR and FTIR-ATR analysis, as well as rheological measurements and swelling tests. FTIR and 1H-NMR techniques revealed the incorporation of hydrophobic monomers to the hydrophilic polymer backbone. Rheological measurements showed that hydrophobic block length was increased by increasing C0.Yüksek LisansM.Sc