Miseller Polimerizasyonu Tekniği İle Şekil Hafızalı Hidrojellerin Sentezi Ve Karakterizasyonu

Abstract

Suda şişen hidrojeller doğada genellikle amorf yapıda bulunurlar ve düzenli bir yapıya sahip değillerdir. Yakın geçmişteki çalışmalarda, hidrojellere uzun yan zincirler eklenerek düzenli - kristalin bir yapı kazandırılmıştır. Ana zincirleri arasında oluşan bu düzenli yapılara sahip hidrojeller, sıcaklık değişimlerinde amor- kristalin faz geçişinden dolayı fiziksel ve kimyasal özellikleri değişebilmektedir. Literatürde bilinen ilk kristalin bölgeler içeren hidrojeller, organik ortamda 18 karbonlu oktadesil akrilat (C18A) ve akrilik asidin (AAc) rastgele kopolimerizasyonu ile elde edilmiştir. Bu hidrojellerin kristalin yapısı, hidrofobların AAc ana zincire rastgele dağılmasıyla oluşmaktadır. Bu tezin amacı, miseller polimerizasyonu tekniği ile hidrofobik grupların bloklar halinde hidrojel yapısına sokulması sonucu daha düzenli kristalin bölgeler içeren ve şekil hafıza özellikli hidrojeller elde etmektir. Tez çalışmasında, C18A ve AAc monomerleri sulu sodyum dodesil sülfat (SDS)-NaCl misel çözeltisi içerisinde kopolimerleştirilerek C18A-AAc jeli sentezlenmiştir. SDS moleküllerinden oluşan misel çözeltisine NaCl eklenmesi ile misellerin boyutu artmakta ve uzun alkil zincirli hidrofobların misel içerisinde çözünmesi sağlanmaktadır. Ancak SDS molekülleri, hidrofoblar arasındaki etkileşimleri zayıflatarak kristalin yapının oluşmasını engellemektedir. Bu nedenle sentez sonrası SDS molekülleri jellerden uzaklaştırılmıştır. Hidrofob miktarı %20 - %50 mol aralığında sentezlenen C18A-AAc jellerinin yapısında % 61-84 su bulunduğu saptanmıştır. DSC (diferansiyel taramalı kalorimetre) ölçümlerinde erime ve kristallendirme sıcaklıkları tespit edilmiş ve bu sıcaklıkların hidrofob miktarına bağlı olmaksızın sırasıyla 48 2oC ve 43 2oC olduğu belirlenmiştir. C18A-AAc hidrojelleri ısıtma – soğutma çevrimine tabi tutularak sıcaklığa bağlı jelin elastik ve viskoz modüllerindeki değişimler incelenmiştir. Kristalin bölgelerin erime sıcaklığı altında ve üstünde elastik modül, yaklaşık 1000 misli tersinir değişim göstermektedir. Ayrıca erime sıcaklığının üzerinde deforme edilen C18A-AAc jelinin soğutulması ile geçici bir şekil kazandığı ve tekrar sıcaklığın yükseltilmesiyle hafızasındaki ilk şekline geri döndüğü gözlenmiştir. Misel polimerizasyonu tekniği yardımıyla ana zincir üzerine bloklar halinde yerleşen hidrofoblar, jellerin hem mekanik özelliklerinden hem de şekil hafıza özelliğinden sorumludur.

Water-swollen hydrogels are generally amorphous in nature and have no ordered structure at the molecular level. Recent works reported in the literature show that crystalline domains within the hydrogels can be created by substitution of side alkyl chains to the hydrogel network. Such hydrophobically modified hydrogels containing crystalline domains exhibit drastic changes in their physical and chemical properties in response to temperature changes due to the transition between amorphous and crystalline states. Hydrogels with crystalline domains have been synthesized previously via random copolymerization of n-octadecyl acrylate (C18A) and acrylic acid (AAc) in organic media. Within the framework of this thesis, hydrogels containing crystalline domains were prepared by use of the micellar polymerization technique. To our knowledge, this technique has not been reported before for the preparation of such hydrogels with shape-memory effect. In free radical micellar polymerization technique, as first described by Candau and co-workers, a water-insoluble hydrophobic monomer solubilized within the micelles is copolymerized with a hydrophilic monomer such as acrylamide (AAm) or acrylic acid (AAc) 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. In the first part of this thesis, we prepared classical hydrogels containing crystalline domains using random copolymerization of various hydrophobic and hydrophilic monomers in an organic medium. Gelation reactions were carried out in the presence of N,N-methylenebis acrylamide (BAAm) as a crosslinker. N-octadecyl acrylate (C18A) and stearyl methacrylate (C17,3M), which consists of 65 % n-octadecyl methacrylate and 35 % n-hexadecyl methacrylate, were used as the hydrophobic monomers in the gel preparation while acrylic acid, acrylamide, n-isopropyl acrylamide and tert-butyl acrylamide were used as the hydrophilic monomers. Viscoelastic and thermal behavior of the hydrogels were compared to understand the effect of the type of the hydrophobic and hydrophobic monomers on the formation of crystalline domains. The thermal behavior of the hydrogels was investigated by rheometry using oscillatory deformation tests, while the degree of crystallization of the hydrogels was estimated by differential scanning calorimeter (DSC). As a result of these measurements, C18A and AAc were found to be the most suitable hydrophobic and hydrophilic monomers, respectively, for the creation of crystalline region. It was also found that the elastic modulus of water-swollen gels is higher than that of dry gels. The reason is that water in gels facilitates the alignment of the hydrophobic segments increasing the flexibility of the hydrophilic polymer matrix. In addition, since the presence of water molecules around the hydrophobic moieties will enhance the hydrophobic interactions at this location, it is likely that the stronger hydrophobic interactions in swollen gels are responsible for the high stability of the crystals in equilibrium swollen gels. In the second part of the thesis, free-radical micellar copolymerization technique was used for the preparation of hydrophobically modified hydrogels. C18A and AAc were used as the hydrophobic and hydrophilic monomers, respectively, while BAAm was the chemical crosslinker. Gelation reactions were carried out in aqueous solutions of sodium dodecyl sulfate (SDS) to solubilize the hydrophobic monomer C18A. Preliminary experiments showed that C18A is insoluble in SDS solutions due to the large size of the side alkyl chain. Therefore, experiments were carried out to create suitable conditions so that large amounts of C18A could be solubilized in micellar solutions. This was achieved by the addition of NaCl into aqueous semi-dilute SDS solutions. Dynamic light scattering measurements showed that the hydrodynamic correlation length of SDS solution increases as the salt concentration is increased. The growth of the micelles is due to the change of the micellar structure of SDS from sphere to rod and then, to large cylindrical aggregates or flexible worm-like micelles by the addition of salts. The growth of SDS micelles is accompanied by enhanced solubilization of C18A. In 1.5 M NaCl, the solubility of C18A increases to 16 w/v %, which suffices to conduct the micellar copolymerization of C18A and AAc in equimolar mixtures at a total concentration of 1 M. After solubilization of the hydrophobe in aqueous SDS – NaCl solutions, the micellar copolymerization of AAc and C18A was carried out using ammonium persulfate (APS)- sodium metabisulfite (SMS) redox initiator system in the presence of BAAm crosslinker at a crosslinker ratio of 1/100 (molar ratio of AAc to C18A). Hydrogels with a gel fraction of unity could be obtained at a hydrophobe level (C18A mol %) between 20 and 50 %. The hydrogels equilibrium swollen in water contained 61 to 84 % water that increased with decreasing C18A % or initial monomer concentration. Thermal behavior of water-swollen hydrogels was investigated by DSC as well as by rheometry using oscillatory deformation tests. The gel samples were subjected to heating and cooling cycles between 5 and 80 oC during which the changes in the heat flux and in the dynamic moduli of gels were monitored as a function of temperature. DSC curves revealed that the swollen hydrogels melt and crystallize with a change in temperature. During cooling of the gel samples from 80 to 5 oC, an exothermic peak appears at 43oC, corresponding to the crystallization temperature Tcry. During heating back to 80oC, the swollen gels melt as evidenced from the endothermic peak at 48oC corresponding to the melting temperature Tm. At these transition temperatures, both elastic G’ and viscous moduli G’’ of gels drastically change due to the formation and dissolution of the crystalline domains in the gel sample. Simultaneously, the loss factor tan  changes between below and above 0.1, demonstrating weak-to-strong gel transitions. The change in the moduli of the gels depending on the temperature was fully reversible in several cycles. The extent of the change in the dynamic moduli of gels was strongly dependent on the hydrophobe level. As the hydrophobe level is increased from 20 to 50%, G’ at 5oC also increases while the loss factor attains smaller values. The gel with 50 % C18A undergoes a reversible 3 orders of magnitude change in the elastic modulus between 80 and 5oC, i.e., between 17 kPa and 18 MPa. Such a large difference in the modulus below and above the transition temperature is the most significant factor to induce the shape memory behavior. The melting Tm and crystallization temperatures Tcry, determined by DSC measurements, are 48 2oC and 43 2oC, respectively, independent of the hydrophobe level of the hydrogels. The hysteresis behavior, that is, the melting occurs at a higher temperature than crystallization was also observable in the viscoelastic behavior of the hydrogels. During heating from 5 to 80oC, the drastic changes in the dynamic moduli of gels occur at a higher temperature than during cooling back to 5oC. Although both Tm and Tcry are independent on the amount of C18A, the enthalpy change during melting (Hm) per mole of C18A unit in the hydrogel increased with rising level of the hydrophobe. For the hydrogels with 20, 35, and 50 mol % C18, Hm was calculated as 28, 31, and 43 kJ/mol, respectively. Assuming that Hm is 71.2 kJ for the melting of one mole of crystalline C18A units, this indicates an increase in the degree of crystallinity from 40 to 60 % as the amount of C18A is increased from 20 to 50 mol %. XRD patterns of the swollen gel samples exhibited a crystalline peak at 21.4o corresponding to a Bragg d-spacing of 0.42 nm. This Bragg d-spacing is typical for the paraffin-like hexagonal lattices formed by the packing of n-alkyl chains. Besides this peak, a second order diffraction peak at 5.4o and a sharp peak at a very low angle (1.4o) were apparent. The latter corresponding to lattice spacing of 6.3 nm indicates that C18A side chains form tail-to tail alignment perpendicurly to the main chains. The thermal behavior of the hydrogels formed by micellar polymerization was also compared with that formed by random copolymerization. These experiments were carried out using gel samples prepared at the same hydrophobe level (35 %) and water content (46 %). Temperature dependent variations of the dynamic moduli of the hydrogels show that, in contrast to 120-fold change in G’ of the hydrogels formed by random polymerization, those formed by micellar polymerization exhibit 1000-fold change in G’ in response to temperature changes. This difference mainly occurs due to the lower modulus and higher loss factor of the gels formed by micellar copolymerization in their amorphous states, i.e., at high temperatures. Since hydrophobe level is fixed in both gels, blocky structure of the polymer chains formed by micellar polymerization necessarily reduces the number of blocks per chain. This would decrease the number of hydrophobic associations as compared to the chains containing randomly distributed single hydrophobic units, leading to lower modulus. Indeed, visual observations also showed that, at 80 oC, the gels with blocky structure were too weak as compared to those formed by random copolymerization. It was also found that the hydrogels formed by micellar polymerization technique exhibit self-memory behavior. To illustrate this behavior, a rod-shaped hydrogel sample with 50 % C18A was prepared. Thus, the permanent shape of the sample was rod. After heating to 60oC, the gel became soft and could easily be deformed to a spiral shape. This temporary shape was fixed by cooling the sample to 24oC. Immersing the gel sample in a water bath at 60oC, it returned to its initial shape within 20 s. The covalently crosslinked network structure of the hydrogel determines the permanent (rod) shape while crystalline domains formed by C18A blocks act as switching segments with transition temperature to fix the temporary (spiral) shape. Cylindrical hydrogel samples were also stretched at 60oC to six times their original lengths and then, the temporary shapes were fixed by cooling to room temperature. When immersed in water at 60oC, the samples recovered their permanent shapes within a few seconds. The principle of thermoresponsive shape memory effect of hydrophobically modified hydrogels is that the covalently crosslinked network structure restores its random coil conformation when the temperature is elevated above the melting temperature of crystalline domains. To quantify shape memory properties of the hydrogels, the shape recovery ratio was recorded at various temperatures. All hydrogel samples exhibited shape-recovery ratios of 100 % at or above 57oC. For the hydrogels with 35 and 50 % C18, the recovery ratio equals to zero below 45 and 50oC, respectively, indicating that the shape fixing efficiency is 100 %. This reveals the ability of these gel samples to hold the temporary shape up to temperatures close to the transition temperature.