Katanyonik surfaktan çözeltilerinde sentezlenen kendini onarabilen hidrojeller
Self-healing hydrogels formed in catanionic surfactant solutions
Yazar: GİZEM AKAY
Danışman: PROF. OĞUZ OKAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı / Fizikokimya Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Kimya = Chemistry ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology

Özet:
Bu tezin amacı hem yüksek mekanik dayanıma sahip hem de kendini iyileştirme yeteneği olan hidrofobik modifiye poliakrilamid hidrojellerini katanyonik yüzey aktif madde çözeltilerinde, miseller kopolimerizasyon tekniği ile sentezlemektir. Hidrofobik modifiye hidrojelleri elde etmek için kullanılan uzun alkil zincirli hidrofoblar, miseller içerisinde çok az çözünmekte veya çözünmemektedirler. Bu nedenle, uzun alkil zincirli hidrofobların çözünmesine olanak sağlayan daha büyük miselleri elde etmek amacıyla katanyonik surfaktan çözeltileri kullanılmıştır. Ayrıca çalışmada katanyonik surfaktan çözelti konsantrasyonlarının, bu çözeltilerde sentezlenen hidrojellerin mekanik özelliklerine etkisinin araştırılması amaçlanmıştır. Hidrofobik modifiye hidrojeller ile ilgili özellikle son yıllarda yoğun çalışmalar yayınlanmasına rağmen, bu jellerin katanyonik surfaktan çözeltilerinde sentezine dair hiçbir çalışma yapılmamıştır.Bu tez kapsamında, öncelikle katanyonik surfaktan çözeltilerinin viskoziteleri ve hidrofobik monomer çözme kapasiteleri tayin edilerek hidrojellerin sentezi için gerekli ön koşullar belirlenmiştir. Katanyonik miselleri oluşturmak için anyonik yüzey aktif madde sodyum dodesil sülfat (SDS) ile katyonik setiltrimetilamonyum bromür (CTAB) kullanılmıştır. Ardından miseller kopolimerizasyon tekniği uygulanarak katanyonik surfaktan çözeltilerinde kendini iyileştirme yeteneği olan ve mekanik dayanıklılığı yüksek hidrojeller sentezlenmiştir. Miseller kopolimerizasyonu ile suda çözünmeyen hidrofobik monomerler ile hidrofilik monomerler polimerleştirilebilmektedir. Bu amaçla yüzey aktif madde molekülleri kullanılarak oluşturulan misellerde hidrofobik monomerin çözünmesi sağlanır. Hidrofilik monomer akrilamid (AAm) ile hidrofobik stearil metakrilatın (C18) kopolimerizasyonu, CTAB? ın Krafft sıcaklığının üzerinde olan 35 °C? de gerçekleştirilmiştir. Redoks başlatıcı sistemi olarak amonyum persülfat (APS)/N,N,N?,N?- tetrametiletilendiamin (TEMED) kullanılmıştır. Toplam yüzey aktif madde konsantrasyonu 0,24 M olarak sabit tutulurken SDS içeriği molce % 0 ile 15 arasında değiştirilmiştir. Hidrojelleri elde etmek için hazırlanan reaksiyon çözeltilerinin jelleşme reaksiyonları ve hidrojellerin reolojik özellikleri reometre yardımıyla izlenmiştir. Reaksiyon çözeltilerinin bir kısmı şırıngalara aktarılmış ve elde edilen slindirik numuneler ile jel ağ yapı karakterizasyonu, şişme ve mekanik testleri yapılmıştır. Tüm sonuçlar SDS içeriğine bağlı olarak mukayese edilmiştir.Katanyonik surfaktan çözeltilerinin karakterizasyonu sonucu çözeltilerde SDS miktarı artışına bağlı olarak misel boyutunda önemli ölçüde artış meydana geldiği gözlemlenmiştir. Misel boyutunda meydana gelen bu artış sayesinde çözeltilerin hidrofobik monomer çözme kapasitesinin de arttığı sonucuna varılmıştır. Ayrıca miseller kopolimerizasyonunda kullanılacak uygun bileşimdeki katanyonik surfaktan çözeltileri ve hidrofobik monomer konsantrasyonu belirlenmiştir. Sertbest radikal mekanizması ile gerçekleşen miseller kopolimerizasyonu sonunda kendini iyileştiren ve üstün mekanik özelliklere sahip hidrojeller elde edilmiştir. Reolojik ölçümlerden elde edilen sonuçlar doğrultusunda; SDS içeriğinin artmasına bağlı olarak meydana gelen misel boyut artışının, hidrofobik blok uzunluğunun artmasına, hidrofilik zincir başına düşen çapraz bağ yoğunluğunun azalmasına yol açmıştır. Jel ağ yapı karakterizasyonu ile elde edilen sonuçlar, şişme ve çekme test sonuçları da reolojik ölçüm sonuçlarını desteklemektedir. Ayrıca jel yapısındaki hidrofobik bloklar arasında oluşan hidrofobik etkileşimlerin fiziksel çapraz bağ işlevi görmesi dinamik modül değerlerine yansımış ve zamana bağlı değişen dinamik modül değerleri elde edilmiştir. Şişme test sonuçları ise, bu fiziksel çapraz bağların suda parçalanamayacak kadar kuvvetli olduğunu ortaya koymuştur. Çekme testleri ile jellerin yüksek uzama kapasiteleri ve kendini iyileştirme davranışları gözlemlenmiştir. Sonuç olarak, yaklaşık % 98 kendini iyileştirme etkinliğine ve yüksek mekanik dayanıma sahip hidrofobik modifiye hidrojeller, katanyonik çözeltilerinde sentezlenmiştir.

Abstract:
Synthetic hydrogels are very similar to biological tissues and therefore, received considerable attention for use in tissue engineering, drug release, soft contact lenses, sensors, and actuators. One disadvantage of such hydrogels is mechanical weakness and lack the ability to self-heal, which hinders their use in many application areas. This poor mechanical performance mainly originates from their very low resistance to crack propagation due to the lack of an efficient energy dissipation mechanism in the chemically cross-linked polymer network. To obtain self-healing gels with a high degree of toughness, one has to increase the overall viscoelastic dissipation by creating a polymer network where cross-linking occurs via reversible breakable cross-links with finite lifetimes, instead of permanent cross-links. Numerous studies have been conducted in recent years to improve the mechanical performance of hydrogels. Recently, our research group presented a simple strategy to generate strong hydrophobic interactions between hydrophilic polymers leading to the production of self-healing hydrogels. Large hydrophobes such as stearyl methacrylate (C18) or docosyl acrylate (C22) could be copolymerized with the hydrophilic monomer acrylamide (AAm) in a micellar solution of sodium dodecyl sulfate (SDS). This was achieved by the addition of salt into the reaction solution. Salt leads to micellar growth and thus enables solubilization of large hydrophobes within the grown wormlike SDS micelles. After their solubilization and, after incorporation of the hydrophobic sequences within the hydrophilic polymer chains via micellar polymerization technique, strong hydrophobic interactions can be generated in synthetic hydrogels. It was shown that the hydrophobic associations between the polymer chains prevent dissolution in water and flow, while the dynamic nature of the junction zones provides homogeneity and self-healing properties together with a high degree of toughness. However, drastic structural changes were observed when SDS micelles are removed from the physical gels. Several dynamic characteristics of the gels including their self-healing behavior completely vanish after extraction of SDS micelles, indicating the loss of the reversible nature of the cross-linkages. These findings suggest that the presence of surfactant is critically important for the unique properties of self-healing hydrogels formed by hydrophobic associations.An alternative route to promote micellar growth is mixing of surfactants of opposite charges. Aqueous mixtures of cationic and anionic surfactants, called catanionic surfactants, exhibit unique properties and several types of microstructures arising from the strong electrostatic interactions between the oppositely charged head groups. Depending on the concentration and composition of surfactant systems, temperature, and salt addition, they form mixed micelles, vesicles, lamellar phases, precipitates, spheres, or rod-like structures. Mixtures of the anionic surfactant SDS and the cationic surfactant cetyltrimethylammonium bromide (CTAB) form mixedxxiimicelles in both SDS-rich and CTAB-rich solutions while between these compositions, vesicles and formation of a 1:1 precipitate are observed.Here, we prepared physical polyacrylamide (PAAm) hydrogels by micellar copolymerization of AAm with 2 mol % C18 in aqueous CTAB/SDS solutions. Since the Krafft point for CTAB in water was reported to be around 20 ? 25oC, both the gelation reactions and the characterization of the physical gels were carried out at 35oC. The surfactant and the initial monomer concentrations during gelation were set to 0.24 M and 5 w/v %, respectively. We first investigated the growth of CTAB micelles upon addition of SDS in the CTAB-rich region up to the onset of turbidity. It was found that the viscosity of CTAB-SDS solutions increases with increasing amount of SDS in the solution indicating the growth of CTAB/SDS micelles. Simultaneously, the solubilization extent of C18 increased with increasing SDS content of the solution. The micellar copolymerization reactions were then carried out and the physical gels formed were characterized by rheological and mechanical measurements. The physical gels formed in CTAB/SDS solutions exhibited frequency- or time-dependent dynamic moduli indicating the temporary nature of the hydrophobic associations having lifetimes of the order of seconds to milliseconds.It was also shown that all the physical gels are insoluble in water as well as in organic solvents. When put in a large excess of water, they swell and eventually attain equilibrium (relative) degrees of swelling around 4. Temporarily, the degree of swelling passes through a distinct maximum due to the large osmotic pressure of the enclosed ionic surfactants that are gradually extracted. Gel fractions (mass of dry, extracted network/mass of the monomers in the comonomer feed) were close to 1 over the whole range of SDS % studied confirming the existence of strong hydrophobic associations that are not destroyed during the expansion of the gel in water. Although the physical gels were insoluble in water, they could easily be solubilized in aqueous 0.1 ? 0.3M SDS or CTAB solutions. These solutions exhibited low viscosities (~10-2Pa.s) independent on the type of the gel, which is attributed to the weakening of the hydrophobic interactions due to the surfactant micelles. On the other hand, when swelling tests are conducted in a limited volume of water, e.g 10 mL of water per 1 g of gel sample, complete dissolution of the gel takes place typically within several days. The surfactant concentration then drops to approx. 1/10 of the original one (24 mM), but this is still far above the cmc and the micelles provide for the temporary nature of the associations. These solutions containing 0.5 w/v % polymer also exhibited similar and low viscosities. Thus, the necessity of the presence of the surfactant micelles in the solutions prevents visualization of the blockiness of the network chains.To gain insight into the extent of hydrophobic interactions inside the gels, they were dissolved in CTAB/SDS solutions at 35oC having the same concentration (0.24 M) and composition (0 to 15 % SDS) as those used for gel preparation. 10 mL of surfactant solution was applied per 1 g of gel sample so that the polymer concentration became 0.5 w/v-% In this way, we were able to solubilize the gels while the surfactant environment of the disintegrated network chains remained unchanged. The solutions of polymers formed with 0 and 5 % SDS showed rather low viscosities, those with 10 and 15 % exhibited higher viscosities at low shear rates and marked shear thinning. Since the increase in the viscosity with rising SDS % may be attributed to the micellar growth rather than to the increasing associativity of polymers, we also measured the viscosities of the surfactant solutions without the polymers. It was found that the relative viscosity increase due to the polymer rapidlyxxiiiincreases with rising SDS % indicating increasing associativity of the network chains as the amount of SDS at the gel preparation is increased. From these results, it is clear that the network chains formed in surfactant solutions with 10 and 15 % SDS show remarkable associativity. We may attribute this to the pronounced blockiness of the network chains as well as to the increasing size of wormlike CTAB micelles surrounding the hydrophobic blocks.Tensile tests were also conducted on cylindrical gel samples of various SDS content. Although those formed at 0 and 5 % SDS exhibited very high elongation ratios, they were too slippery to be measured accurately due to the sample slippage from or breakage at the grip. Increasing amount of SDS from 10 to 15 % decreased the elongation ratio at break from 5000% to 1800%, while the ultimate strength increased from 6 to 10 kPa. This finding confirms increasing associativity of the network chains.The reversible dissociation - association of the cross-link zones in the gel network also provides self-healing property to the present hydrogels. When the fracture surfaces of ruptured gel samples are pressed together, the pieces merge into a single piece. The joint reformed withstands very large extension ratios as the original gel sample before its fracture. The healing efficiency of the physical gels could be quantified using the gels formed at 15 % SDS. The samples were cut in the middle and then, the two halfs were merged together within a plastic syringe (of the same diameter as the gel sample) at 35oC by slightly pressing the piston plunger. The healing time was set to 30 min and each experiment was carried out starting from a virgin sample. An average healing efficiency of 98 % was found using several gel samples. The healing behavior of gels was also demonstrated by oscillatory deformation tests. The gel samples were subjected to 4 successive time-sweep tests for 2 min at a strain amplitude of ?o = 0.01 followed by strain-sweeps between ?o = 0.01 and 4. The dynamic moduli of all gels could be recovered after each cycle indicating that the damage done to the gel samples at large deformations is recoverable in nature.In conclusion, the internal dynamics of self-healing gels formed in CTAB/SDS solutions are controlled by the associativity of the network chains, which in turn depends on the size of CTAB micelles. The results also show that replacement of SDS-NaCl with CTAB/SDS mixed micellar system further improves the mechanical performance of self-healing hydrogels.