Butil kauçuk esaslı kriyojellerin sentezi ve özelliklerinin incelenmesi
Preparation of cryogels based on butyl rubber and investigation of their properties
Yazar: SEVIL MUSLUMOVA
Danışman: PROF. OĞUZ OKAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Kimya = Chemistry

Özet:
Polimerik jeller,  polimer ağyapı ve bu ağyapı içinde bulunan çözücüden oluşan  yumuşak veya katı benzeri malzemelerdir. Polimerik ağ yapısı fiziksel veya kimyasal çapraz bağlanmalarla oluşur ve bunun sayesinde jeller iyi çözücüde bile çözünmez; ancak şişerler. Kimyasal olarak çapraz bağlanma polimer zincirlerinin kovalent bağlanması sonucu oluşur. Fiziksel çapraz bağlı polimerik jeller ise van der Waals, hidrojen bağları ve hidrofobik etkileşmeler gibi fiziksel etkileşimlerle oluşmaktadırlar. Fiziksel jeller sıcaklık, çözücü veya pH'taki değişimlerle tersine çevrilebilir (tersinir) jeller oluştururlar ve bu davranışlarından ötürü de şekil hafızalı jeller gibi akıllı sistemlerin oluşmasına imkan sağlarlar. 

Diğer yandan kimyasal çapraz bağlı olanlar ise geri dönüşümsüz (tersinmez) jeller oluştururlar. Polimerik jeller kimya, malzeme bilimi, biyomühendislik, biyomedikal mühendisliği gibi birçok farklı alanda kullanılmaktadırlar. Kullanım alanlarına bağlı olarak jellerin dış uyarılara hızlı cevap verebilmesi ve mekanik özelliklerinin iyi olması istenmektedir. Bu nedenle farklı yöntemler geliştirilmiştir.  Makro gözenekli jeller uyarılara hızlı cevap verebilen jel sistemlerinin elde edilmesi için yapılan çalışmalar sonucu ortaya çıkmıştır. Genel olarak makro gözenekli jel hazırlanması için bilinen yöntemler faz ayrımı ve kriyojelleşme yöntemidir. Faz ayrımı yönteminde inert bir seyreltici kullanılır ve reaksiyon sonrasında seyrelticinin ortamdan uzaklaştırılması sonucu makro gözenekli yapıların oluşumu sağlanır. Kriyojelleşme ise çözücünün donma noktasının altındaki sıcaklıklarda gerçekleşen jel oluşum reaksiyonlarıdır. Tepkenleri içeren reaksiyon çözeltisinin dondurulması sonucunda donmamış kalan mikro bölgelerde çapraz bağlanma reaksiyonları gerçekleşir. Jelleşme reaksiyonları sonrasında oda sıcaklığına getirilen sistemin çözücü kristallerinin erimesi ile kriyojel sentezlenmiş olur. Polimerik jellerin kullanım alanlarını kısıtlayan diğer bir dezavantajı ise mekanik özelliklerinin iyi olmamasıdır. Mekanik özellikleri iyileştirme yönünde yapılan çalışmalar sonucu çift ağ yapılı, nanokompozit, topolojik ve kriyojeller üretilmiştir.

Polimerik jel sistemleri üstün özellikleri sayesinde uzun yıllardır araştırmacıların odak noktası haline gelmişlerdir. Jelleri çeşitli özellikleri bakımdan sınıflandırmak mümkündür. En yaygın olarak kullanılan sınıflandırma jellerin çapraz bağlanma şekline, polimer yapısına, düzenleniş biçimine ve çözücü türüne göre yapılmaktadır.  Çözücü türüne göre jeller hidrojeller ve organojeller olmak üzere iki ana başlıkta toplanmaktadırlar. Hidrojellerde çözücü olarak su, organojeller de ise organik çözücü bulunur.

Bu tez çalışmasında siklohekzan çözeltilerinde butil kauçuk esaslı jeller üretilmiştir. Reaksiyonlarda çapraz bağlayıcı olarak kükürt monoklorür (S2Cl2) kullanılmıştır. Jeller farklı koşullarda sentezlenerek özellikleri incelenmiş ve karşılaştırılmıştır.
Jel sentezi için kriyojelleşme tekniğinin kullanılması makro gözenekli ve yüksek şişme/büzülme hızına sahip jellerin oluşumunu sağlamıştır. 

Tez çalışması kapsamında sentezlenen kriyojel özelliklerinin sentez şartlarına bağlı olarak değişimleri ayrıntılı olarak incelenmiştir. Bu amaçla şişme davranışları, mekanik ve termal özellikleri ile morfolojik yapıları belirlenmiştir. Yapılan analizler sonucunda, sentezlerde kullanılan çapraz bağlayıcı miktarının değiştirilmesi ile kriyojel özelliklerinin çok fazla değişmediği; buna karşılık butil kauçuk miktarı ve sentez sıcaklığının kriyo jel özelliklerinde belirgin farklara sebep olduğu görülmüştür.

Abstract:
Polymeric gels are soft or solid like materials composed of the polymer network and the solvent present in this network. The polymeric network is formed by physical or chemical cross-linking. It is able to swell in the case of a good solvent. Chemical crosslinking occurs as a result of covalent bonding of polymer chains. Physical crosslinked polymer gels are formed by physical interactions such as van der Waals, hydrogen bonds and hydrophobic interactions. Physical gels form reversible gels with changes in temperature, solvent and pH, and because of this behavior they allow smart systems such as shape memory gels.

On the other hand, chemically cross-linked ones form irreversible gels. Polymer gels are used in many different fields such as chemistry, materials science, bioengineering and biomedical engineering. Depending on the application area, gels should be able to respond quickly to external stimuli and have good mechanical properties. 
Therefore, different methods have been developed. Macroporous gels are the result of studies to obtain gel systems that can respond quickly to stimuli. In general, known methods for the preparation of macroporous gels are the phase separation and cryogelation method. In the phase separation method, an inert diluent is used and after the reaction it is removed from the reaction medium and macroporous structures is achieved. 

Cryogelation is the low temperature gelation technique, known as cryotropic gelation or cryogelation.  Since the pioneering works of Lozinsky and coworkers, cryogelation technique has been widely used to produce macroporous gels (cryogels) of high toughness and superfast responsivity.  By this technique, cross-linking reactions are conducted in apparently frozen reaction solutions.  During freezing of an aqueous polymer solution containing a chemical cross-linker, the polymer chains and the cross-linker molecules expelled from the ice concentrate within the liquid channels between the ice crystals, so that the cross-linking reactions only proceed in these unfrozen domains.  After cross-linking and, after thawing of ice, a macroporous material is produced whose microstructure is a negative replica of the ice formed.  In contrast to the mechanically weak macroporous gels prepared by phase separation technique, cryogels are very tough and withstand very large strains without permanent deformation or fracture. 

Polymer gel systems have become the focus of researchers for many years thanks to their outstanding properties. It is possible to classify gels in terms of various properties. The most commonly used classification methods are classified according to the shape of the polymer, according to the form of the cross-linking, according to the form of regulation and the type of used solvent. Depending on the type of solvent, gels are grouped under two main headings: hydrogels and organogels. In hydrogels, water is used as solvent and organogels are organic solvents.

In this thesis, macroporous gels with aligned and regular porous structures were prepared by cryogelation of BR in cyclohexane using sulfur monochloride (S2Cl2) as a cross-linking agent.  Gels were synthesized under different conditions and their properties were examined and compared.
In the first part of the study, cryogel synthesis was performed at different crosslinker (S2Cl2) concentrations. The reactions were carried out at a constant butyl rubber concentration (5% w/v) and at -18°C. In order to determine the properties of cryogels produced at five different crosslinker (S2Cl2) concentrations, swell tests,  pore volume and total porosity calculations were performed, and using the Zwick-Roell Z 0.05 testing machine tensile tests were performed.  SEM images were taken at different magnification rates by a Jeol JSM 6335F Field Emission scanning electron microscope.

 Gravimetric swelling tests were performed using two different solvents. The results of swelling measurements of cryogels in solutions of cyclohexane and toluene showed that a change in the amount of a crosslinking agent does not affect the properties of swelling, and the results are close to each other. Investigating the effect of the solvent on swelling rates, showed that the results of swelling tests in cyclohexane were higher than in toluene. This is due to the fact that the solubility of cyclohexane is closer to the degree of solubility of rubber. Results from the pore volume and total porosity of the gels, viewed that an increase in the amount of crosslinking agent did not change the total amount of porosity caused by the decrease in pore volume. An increased amount of S2Cl2 in the reaction solution caused an increase in crosslinking points and, therefore, decreased the pore size of the material obtained, which led to a decrease in the pore volume of the material, but did not affect the overall porosity.
SEM images were taken at different rates of magnification to study the morphological features of cryogels. It was noted that the materials were internally interconnected macroporous. Pore size calculation from SEM images also show that pore size decrease with increasing the amount of S2Cl2. The amount of S2Cl2 in the reaction solution increases the intensity of crosslinking reactions, which leads to a decrease in pore sizes.

In the second part of this thesis study, BR concentration was chosen as an independent variable in cryogel production. The reactions performed at four different (5% -20% w/v) BR concentrations were synthesized for the first time at -18oC. At this stage, the crosslinker concentration was fixed at 12% (v/w). In the third stage of the study, the gels with independent concentrations of BR concentration were synthesized at -2oC and the effect of synthesis temperature on the properties of cryogels was investigated. Swelling tests, uniaxial tensile tests, pore volume calculation, DSC measurements were made and SEM images were taken to examine the properties of the gels.

In this study, the effect of butyl rubber concentration on the properties of BR cryogels for the first time was investigated. It was seen that increasing the amount of BR from the results of swelling tests did not affect the mrel and vrel values of the cryogels, but qw decreasing with increasing BR concentration. The volume swelling ratio qv is not affected by increasing BR concentration. As is known, qw shows the amount of solvent in both the polymer region and pores of the gel. The decrease in this value with increasing BR concentration indicates that pore size is reduced by increasing BR concentration. 

Uniaxial tensile tests were performed to investigate the mechanical properties of cryogels.

For all of gel samples it was observed that the increase of the butyl rubber concentration increased the nominal stress value and the deformation rate decreased. In addition, the nominal stress and deformation ratio of the cryogels synthesized at -18oC were lower than those synthesized at -2oC. When Young module values were considered viewed that Young modulus increased linearly for gels synthesized at -2oC and this value was not changed for gels synthesized at -18oC. It was observed that the diffraction stress increased with increasing BR concentration for the gels synthesized at -2oC. From the SEM images it has been observed that the increasing BK concentration, decrease the pore size and thickening of the pore walls. For example, the pore size of the gel produced at -18oC and 5% (w/v) BR concentration was calculated as 12 ± 2 μm for 48 ± 8 μm 20% (w/v) BR gel. 

 From DSC measurements observed that the freezing point of the solution decreased by increasing the concentration of BR.

It was also found that the gels also exhibit superfast swelling and deswelling properties as well as reversible swelling–deswelling cycles in toluene and methanol, respectively.
As it is known from previous studies in our laboratory  BR gels are reusable, i.e., they can be used again after simple squeezing.

All gels prepared in this work, except synthesized with CBR=%15 and CBR=%20 at 
-2oC, can be squeezed completely to give up the pollutants without any crack developments.