Polimerik membranlarda kimyasal ve makromoleküler yapının membranın oksijen ve iyonik geçirgenliğine etkileri

Abstract

ÖZET Sentetik pölimerik numb rani ar r birbirinden ayırdıkları ortamlar arasındaki gaz, sıvı, kati, iyon, ısı, ışık, radyoaktivite taşınmasını düzenleyen, kalınlıkları eni ve boylarına oranla çok düşük olan yüzeylerdir. Pölimerik membranların kimyasal ve makromoleküler yapıları bu membranların özelliklerini etkilemekte ve buna bağlı olarak da kullanıldığı alanları belirlemektedir. Membranların en belirgin işlevi seçimli geçirgenlik özelliği göstermeleridir. Bu çalışmanın amacı, sentetik pölimerik membranlarda, membranın kimyasal ve makromoleküler yapısının geçirgenlik özelliklerine etkisinin incelenmesidir. Bu çalışma kapsamında, tıpta kullanılan membranların karakterizasyonu için gerekli oksijen geçirgenliği ve endüstride kullanılan iyon değiştirici membranların karakter izasyonu için gerekli iyon geçirgenliği incelenecek özellikler olarak seçilmiştir. Çalışmanın birinci bölümünde, yüksek oksijen geçirgenliğine sahip, tıp alanında kullanılabilecek membran yapımına el verişli olan silikon monomerleri çıkış maddesi olarak alınmış tır. Bu tür bileşikler, Rj_ - Si - (OR2)3, genel formülü ile gösterilebilir. Bunada R1 vinil veya metakriloksipropil gibi doymamış bir fonksiyonlu grubu, R. ise metil veya etil gibi doymuş bir grubu belirtir., Bunlara örnek olarak alınan Y-metakriloksipropil trimetoksisilan ve vinil- trietoksisilan gibi bileşiklerdeki doymuş alkoksi grupların transesterifikasyon reaksiyonuna sokulup, dallanmış alkoller, yüksek monohidrik alkoller, eter alkoller, polihidrik alkoller, a-hidroksi asitlerle 7er değiştirilmesiyle, yeni monomerler üretilmiştir. Bu siloksan monomerlerin, metakrilik asitIV esteri monomerlerle kopo iimer izasyonundan membranlar hazırlan mıştır. Elde edilen kopolimer membranların oksijen geçirgenliği, kritik yüzey gerilimleri,, protein abborpsiyon yeteneği ölçülerek tıp uygulamalarındaki kullanılabilirliği araştırılmış tır. Tıpta kullanılan polimer membranların `çözünmüş oksijen geçirgeni iğ i kat sayısı` ölçüsünde kullanılan polarografik yön temle tam sağlıklı ölçüde yapılamamaktadır. Bunun nedeni kullanılan küçük ve karıştırmasız hücrelerde membran yüzeyinde bir sınır filmi oluşması, bunun da oksijen geçirgenliğini etkilemesidir, ayrıca hücre elektrodları zaman zaman polarize olmak` ta ölçüm esnasında elektro kimyasal yan reaksiyonlar oluşmaktadır, ölçümleri daha sağlıklı hale getirmek için cihaz sürekli kalibre edilmektedir, bu da zaman gerektirmektedir. Bu çalışma kapsamında, `çözünmüş oksijen geçirgenliği katsayısı` ölçümün de oluşan hataların önüne geçen ve ölçüm süresini kısaltan kolorimetrik bir yöntem de. geliştirilmiştir. Çalışmanın ikici;, bölümünde, 70-75 C sıcaklıktaki asit ve baza dayanıklı katyer. değiştirici membran yapımı ve iyon geçirgenliğinin incelemesi gerçekleştirilmiştir. Bu membranın Türkiye'de bol miktarda bulunan boraks bileşiğinden, bir dış satım ürünü olan borik asit ile yurt gereksiniminin yarısı dış alım yolu ile karşılandı sodyum hidroksitin beraberce üretildiği membranlı elektroliz prosesinde kullanılabilmesi amaçlanmış tır. Yurt dışında, klor-alkali prosesinde kullanılmak üzere üretilen teflon esaslı xatyon değiştirici membranların fiyatları pahalıdır ve çok korrozif ortamlara dayanmak üzere yapıl mıştır. Boraks elektrolizi prosesinde klor gazı çıkmadığından ortam daha az korroziftir, bu nedenle teflon esaslı olmayan katyon değiştirici membranların da kullanılabilme olasılığı vardır. Çalışmada, önce polikondensasyonla katyon değiştirici membranlar hazırlanmış,, bunlardan anisolsülfonik asit-formaldehit esaslı meabranın proses koşullarına dayanıklı olduğu, ancak elektroliz perf ormi:V.s:.n:.?. 3'jşÜk olduğu saptanmıştır. Daha sonra yapılan sülfolanmış polistiren-polistiren-divinilbenzeninterpolimer katyon değiştirici membranların uygun divinilbenzen oranı için gerek proses koşullarına dayanıklılık, gerekse membranlı elektroliz performansı açısından başarılı olduğu görülmüştür. Bu interpolimer membranların, karşılaştırma için aynı proseste denen teflon esaslı (Nafion 324) ve polistiren- divinilbenzen esaslı <Permutit C-20) membranlarına yakın per formans gösterdiği bulunmuştur. Katyon değiştirici membranların karakterizasyonu için iyon değiştirme kapasiteleri, su tutma yüzdeleri; membranlı elektroliz esnasındaki- sodyum katyonu dinamik taşıma sayısı, elektroosmotik su geçirgenlikleri ve hücredeki akım verimi ölçümleri yapılmıştır.

VI SUMMARY `The dependence of the oxygen and ionic permeabilities of some polymeric membranes on their chemical and macromolecu» lar structures.` Synthetic polymeric membranes are thin semipermeable barriers which control the gaseous, liquid, solid, ion, heat, light nnd radioactivity transport between two mediums that are separated by this trembrane. The chemical and mac romo 1 ecular structures of the jolymeric membranes determine their proper ties as well as their application fields'. The most important property of the membranes is their selective permeability to chosen materials. The aim of this work is,to examine the effect of the chemical and the macromolecular structure of the polymeric membranes on their oxygen permeabilities in the field of bio medical membranes and on their ionic permeabilities in the field of ion-exchange membranes. In the first part of this research, f -methacryloxy - propyl trimethoxysilane and vinyltr iethoxysilane which are organof unctional silicone compounds having the general formuîai i Rx - Si - (PR2)3 R. «unsaturated group (vinyi or methacryloxy) R2 = satura -ed group (methyl ör ethyl) were transformed into new polymep izable monomers by substuting the saturated group with branched alcohols, high monohydric alcohols, ether alcohols, polyhydric alcohols, o-hydroxy acids and other similar organic compounds by catalyzed tran- ses terif icat ion reactions. These reactions were accompaniedVI 1 by the r.emoval of irethanol (or ethanpl) formed by the replaced methoxy (or ethoxy) groups. The reactions proceeded with the application of hea; at atmospheric conditions or at room tem perature under vacuum that was suitable to strip-off the for med alcohol. ; Gas chromatographic, infra-red and elemental analysis examinations were carried out for the reaction products. The substitution ratios were calculated from the weight increase of the produced siloxanes after the reaction completed and the removal of the alcohol ceased. The transes ter if ication reactions were carried out by using 6 different methods: 1 - Non catalyzed with heating, 2 - Metallic K'a catalyzed with heating, 3 J Tetra butyl titanate catalyzed with heating, A - Ethyl îul-phuric acid catalyzed with cooling, 5 - Metali Lc Ha catalyzed under vacuum at room tempe rature, 6 - Tetra jutyl titanate catalyzed under vacuum at room temperature. Metallic sodium catalyzed transes ter if icat ions at rocm temperature were found to be most suitable to obtain the moc i- fied silanes. Tha weight percentage of the used metallic so dium uns between 0,1 - 0,2 % on the total reactnnts. The t r anse s t er i f iea t ion reactions which wn.t carried out at high temperatures usually resulted in polymerized prn- ducts as expectei due to the thermal polymer izah i I i ty of th>? methacryloxy gro.ps present. The analy< ical examination of the reaction products showed that the substitution reaction was a stepwise process. ( At the begining, the mono subs t itu ted siloxane molecules for- i med in the reaction mixture and after the completion of the monosubs t itut ion, disubs titu'ted siloxane molecules began to form in the expense of mono subs t itut ed molecules. The process repeated for further substitution, e.g. tr isubst i tuted mole cules increased, in the expense of disubstituted molecules. Some organic compounds such as ethyl. cellosolve and n-butanol having less steric hinderance produced tr isubst i :utedviii I siloxanes during trans es ter if icat ion reactions, however some others such as iso-butanol,. sec-butanol having brancned groups produced only mono and disubs t itutcd siloxanes. These new polymer izable silt xane monomers having long and branched pendant groups were copolymer ized with the esters of methacrylic acid to prepare mem>ranes having large free vo-lumes and high oxygen permeabilit i *s suitable to be used in biomedical applications. The siloxane monomers, esters of methacrylic 'acid (met hyl methacrylate or 2-hydroxyethyl meihacrylate), cross-linking agents (ethylene glycol dimethacrylat? or divinylbenzene) and trie initiator (pi, a' Azo^-iso-butyrilor.itr ile) were mixed for the desired composition in a flask. Tlıea, N` gas was allowed to pass for 5 minutes to remove the oxygen present in the co - monomer solution. The moulds were prepared by using glass plates, melinex sheets, polyethylene gaskets and metal clips. The co-monomer solution was injected with a syringe and a needle inside the glass plate mould. Then the mould was left in an oven for two days at 60 C ani for one day at 95 C. The thicknesses of the membranes were between 0,01-0,03, cm. The oxygen permeabilities of these membranes were mea sured with the polarographic electrode cell system, the wetta bilities of~the membrane surfaces were determined by measuring the water and methylene iodide.droplet contact angles using a special low power microscope, the critical surface tensions were cnlculnti'd from Fox and 7.ismnnn plot and the protein ab sorption properties were determined by microscopic examination of the pre-impr egna t ed membrane sanples in protein solutions. It was found that, there was an increase in oxygen per meabilities of the methyl methacry'le te copolymers formed with the use of high amounts of unsubst i tuted siloxane ( y -raethacy- loxypropyl trimethoxysilane), however the decrease of the wetta bility also acicompanied due to the hydrophobic nature of the siloxanes. When the cross-linking agent (ethylene glycol di- methacrylate) was used in high molar ratio, it was possible to introduce the siloxane monomers in high molar ratio, however excessive cross-linking decreased the oxygen permeability.IX The substitution of phenol, phenoxyacids and O-hydrox- yacids to siloxane compound produced jrigid and less oxygen per meable copolymers having hydrophobic surfaces. This was due to the lack of the free volume inside the copolymers because of the close packing of aromatic pendant groups and the effect of the intramolecular forces between polar groups. When the compounds having more (C-0) bonds in the mole cule substituted to the siloxane monomer, the high oxygen per meability was expected from the copolymers due to the flexibi lity of the (C-0) bond. However this assumption was not true, the longer the pendant group having (C-0) bonds gave less oxygen p_ermeable copolymers. The methacryloxy group containing siloxanes, after being substituted with ethylene glycol or diethylene glycol were found to copolymerize with hydrophilic monomers such as 2-hydroxyethyl methacrylate to give novel hydrogel copolymers having low water content. The copolymers, produced from the n-butanol substituted siloxane monomers with methyl methacrylate and ethylene glycol dimethacryla te had high oxygen permeabilities, whereas the copolymers produced from iso-butanol, sec-butanol substituted siloxane monomers had low oxygen permeabilities due to their low subs t itution` ratios. However, the assumption of obtaining high oxygen permeable copolymers by the substitution of branc hed alcohols was also valid as seen from the high oxygen per meable copolymers produced by the siloxane monomers having low substitution ratio of branched alcohols. It was required to examine other transesterif icat ion catalysts to obtain high substitution ratios for the reactions of branched alcohols with Y -methacryloxy propyl trimethoxy silane. The polarographic technique which was used in the measu rement of the `dissolved oxygen permeability coefficient` (Pd) of biomedical membranes gave frequently erroneous results, due to the stagnant boundary film formation at the polymer-liquid interface in the unstirred small cell and due to the electrode polarization, electrochemical side reactions during measure ment. A novel colorimetric technique that can overcome theseproblems and capable of measuring (?d) in a short time was de veloped within the scope of this work. The technique was de pended on the colorimetric determir ation of the dissolved oxy gen in the reduced indigo-carmine solution at 700 nm wavelength, The indigo-carmine solution, after being reduced by sodiüm dit- hionite was found very sensitive to oxygen oxidation and the color of the solution turned from transparent to deep blue within a short period of time. A plexiglass cell was constructed, seperating the air oxygen from the reduced indigo car sine ' solut ion by â polymer membrane whose permeability coefficient would be measured; in which the indigo carmine solution was stirred with a propeller to prevent the formation of boundary layers. An a asymptotic curve was obtained by the increase of the optical density with time, having the following relation: X-(X`-X0> d - e`at)+X0 where, X = optical density of the reduced indigo-carmine solution at time = t X = optical density of oxygen saturated indigo-carmine solution at time = 00 ' X = optical density of reduced indigo-carmine solution at time = p a = constant which depends on the oxygen permeability of the poiymeric membrane After the experimental determination of (X») an<J (X») tt (a) was calculated from the slop;,of the logarithmic plot of (Ln) and time. Then, it was possible to calculate the X`-X required time (t ) that the oxygen concentration in the ündigo carmine solution reached to saturation for the measu rement temperature with the equation s Xoo - Xo Ln - °'01 x° maxXI In this treatment, we assume that t=t when, max X__` = °i99 x»> for the nearest approximation, max Since, the amount of maximum dissolved oxygen in water for a given temperature under atmospheric. conditions could be found from the published data, the `dissolved oxygen permeabi lity coefficient` (Pd) was calculated from the oxygen flux by Fick Law : Ap ApA t where, F = Flux of oxygen Ap = oxygen pressure difference across the membrane L = thickness of the membrane Q = volume of transported `xygen A = effective cross-sect ioaal area of the membrane sample The `dissolved oxygen permeability coefficients` of the polymers such as low density polyethylene, poly-4-methyl pent-1-ene, polystyrene, ethylene vir. acetate copolymer, siloxane-acrylate. copolymer were measured and were found in good agreement with the polarograph^c method result s. The co- lorimetric technique resulted in higher (P.) values for less permeable polymers than the poLarograpnic method due to the absence of the boundary layer formation by stirring the solu tion in the cell. There was better agreement for hi,ghly oxygen permeable polymers indicating the decrease of the effect of the' boundary layer formation for this type of polymers. In the second part of this work, the synthesis and the examination of the effect of the chemical and the macromolecu- lar structure of the cation exchange membranes on their per formances in the electrolysis of borax solution to produce boric acid and sodium hydroxide simultaneously was attempted. The borax solution was obtained by simple dissolution and filtration of tincal ore which was found in great reserves in Turkey. The membrane electrolysis cf borax solution should be carried out at 70-80°C for high production rates and thecation-exchange membranes should be resistant to acid and al kali at these temperatures.' The teflon based imported cation-exchang^ membranes which were used in chlor-alkali industry, were too expensive 2. (500-700 $/m ) and specially manufactured to resist the corro sive effect of chlorine gas produced. However as no chlorine gas was produced in the membrane electrolysis of borax, it was- possible to use a cheap and less resistant cation exchange membrane for this process. At the begining, the cation exchange membranes were synthesized by polycondensation reactions with functional group containing monomers and the membranes based oh anisole-. sulphonic acid, formaldehyde were round to be resistant to the process conditions, however they s lowed low performance during membrane electrolysis of borax solution due to their high water contents. The ion-exchange capacities of the polycon'densat ion mem branes increased with the increased amount of' sulphonation and with the decreased amount of formaldehyde molar ratio in the polycondensation composition. Similarly, the water contents of the membranes increased with the increased amount of sulp honation and the decreased amount of formaldehyde molar ratio. The attempts to prepare a cat ion-< xchange membrane having low water content and high ion-exohan:je capacity was unsuccesful because of this contradictory reason. Later, sulphonated polyethylene-polystyrene- diviniyl- benzene interpolymer membranes we