Foto-`click` tepkimesiyle hazırlanan gümüş nanopartikül içeren termoset polimerlerin endüstriyel uygulamaları

Abstract

Termoset polimerler üç ya da daha çok fonskiyonel gruplara sahip monomerlerin polimerizasyonu sonucu elde edilen yüksek miktarda kovalent çapraz bağ içeren polimerlerdir. Bu çapraz bağların yarattığı molekül ağı erimez ve çözünmez özelliklere sahiptir. Bu malzemeler boyutsal bir kararlılığa sahiptir ve darbe dirençleri yüksektir. Termoset polimerleri oluşturmak için ısı ve/ve ya basınca ihtiyaç duyulabildiği gibi tamamen ısıl ya da basınç işlemleri gerektirmeyen yöntemler de mevcuttur. Fotopolimerizasyon, fotoaktif moleküllerin aydınlatılmasıyla oluşan reaktif türlerin vasıtasıyla gerçekleşen çevreye duyarlı bir polimerizasyon tekniğidir. Termal polimerizasyona göre fotopolimerizasyon tekniğinin, oda sıcaklığında yüksek hızda tepkime gerçekleştirilebilme, daha az enerji tüketilmesi ve çözücüsüz formülasyonların hazırlanabilmesi gibi birçok üstün özellikleri vardır. Tepkimeye girecek formülasyondaki monomer ve diğer kimyasalları düzenleyerek tepkime sonrasındaki sertlik, renk, çözünürlük, geçirgenlik, yapışkanlık, elektriksel iletkenlik gibi malzemenin performansına etki eden özellikler kolaylıkla ayarlanabilmektedir. Gümüş antibakteriyal özelliği sayesinde bakteri ve mantar oluşumunu önleyen ve varsa öldüren bir metaldir. Hijyen gerektiren uygulama alanlarında antibakteriyel özellik sağlamasından dolayı nano-seviyede gümüş veya gümüş tuzu içeren nanokompozit malzemelerin kullanımı son yıllarda gittikçe artmaktadır. Bu noktadan hareketle, fotokimyasal yöntemlerle başlatılmış `Click` tepkimesi kullanımıyla gümüş nanopartikülleri içeren termoset malzemeler hazırlanmıştır.Bu yöntemde birbirleriyle yarışan iki farklı fotokimyasal indirgenme tepkimesi rol oynamaktadır. UV ışığıyla uyarılan fotobaşlatıcının parçalanmasıyla oluşan radikaller bakır(II)'yi bakır(I)'e indirgerken hem de Ag(I)NO3'ı metalik gümüşe (Ag(0)) indirgemektedir. Üç ya da dört fonksiyonlu azit-alkin molekülleri üretilen bakır(I) iyonlarıyla katalizlenerek başarılı bir şekilde çapraz-bağlı yapılara dönüştürülmüştür. Diğer taraftan indirgenme tepkimeleri moleküler boyutta gerçekleştiği için elde edilen gümüş partikülleri de nanometre boyutundadır. Bu tepkimeler sonucunda gümüş nanopartiküller içeren çapraz-bağ oranı oldukça yüksek termoset polimerler başarılı bir şekilde elde edilmiştir. Elde edilen malzemelerin yapıları FT-IR spektroskopiyle aydınlatılmıştır. Termal özellikleri incelendiğinde gümüş nanopartikülleri içeren polimerlerin içermeyen polimerlere göre daha kararlı oldukları gözlemlenmiştir. Antibakteriyel aktiviteleri incelendiğinde gümüş nanopartiküller içeren malzemeler gram-pozitif (Staphylococcus Aureus) ve gram-negatif (Escherichia Coli) bakterilerine karşı yüksek etkinlik göstermişlerdir.

Thermosets are polymers that do not soften under heat and pressure and cannot be remolded or recycled or dissolved completely in any solvents. Thesethree-dimensional cross-linked materials display long term stability, no creeping and a lot of other outstanding properties. The thermosets polymers can be obtained by two stages: (i) monomers are partially polymerized into linear chains and (ii) cross-linking is completed through the addition of chemicals, heat, and/or electromagnetic radiation. Because of their desirable properties, thermoset polymers form the basis of many important advanced materials, such as computer chip packaging (insulation), protective coatings, adhesives and aerospace composites, based on their great strength, high-temperature stability, good processability, and good chemical resistance. However, thermoset materials may have several limitations in terms of their final properties, notably their brittleness and lack of durability. Therefore, thermoset polymers should be reinforced by using various nanofillers to obtain corresponding nanocomposites with outstanding properties. Generally, the thermoset polymers are shaped by applying heat and pressure. Photopolymerization is an environmentally friendly process that transforms a monomer into a polymer initiated by the reactive species upon irradiation of photoactive species. The advantages of photopolymerization over thermal polymerization are their high reaction rates even at room temperature, consumption of small amount of energy and preparation of solvent-free systems. Moreover, photopolymerization provides the materials with desired properties; by regulating the monomer and other chemicals in the formulation, one can adjust the after-reaction properties like hardness, color, solubility, transparency, adhesion and electrical conductivity. Thus, nanocomposites with desired properties can efficiently be prepared by photopolymerization.Nanotechnology research gives the chance to change structure of materials in molecular scale. The many functional molecules can be combined in a single product. By increasing surface area to volume ratio of substance, whereas at the nano-scale level, alters the mechanical, thermal and catalytic properties of materials. These differences enable us to product many nanomaterials which directly affect our daily life such as, more transparent glass, more lighter and durable cars, cleaner clothes and paint with long life-time. Especially, the use of polymer nanocomposites has begun to increase in recent years with the decrease of materials costs and solution of technical difficulties which used in nanocomposite production. There are three main nanostructured additives in nanocomposite production such as, clays which are one dimension in the nano-scale, nanowires and nanotubes which are two dimensions in the nano-scale and silica, metal particles and quantum dots which are three dimensions in the nano-scale.Silver has germicidal effects and kills many bacteria and microorganisms effectively without harm to human body. Its nanocomposites with polymer matrices have received much attention due to their wide application fields such as catalysts, wound dressings, optical information storage and electrochromic devices. Silver nanoparticles are usually prepared in the presence of a stabilizing agent in order to avoid their aggregation, which could lead to the loss of the unique activities associated with the nanoscale. Generally, polyelectrolyte solutions are used as stabilizing agentsto control the formation and the long-term stability of nanoparticles. There are two different methods for the preparation of polymer networks containing silver nanoparticles with controlled size. In the first method, polymer matrix is firstly synthesized and then formerly prepared silver nanoparticles are incorporated into the polymeric material. In the second approach, previously formed cross-linked polymer is swollen with a solution of silver salts and reducing agent, and then reduction takes place within the polymer. However, this method involves some difficulties, such as diffusion of nanoparticles, solubility of polymer matrices and need for large quantity of solvent. Although polymer networks containing silver nanoparticles have elegant features, the homogeneous dispersion of the nanoparticles is not easy using a simple process because of their high surface free energy, which may cause agglomeration. Therefore, preparation of such complex material with desired properties (i.e., silver nanoparticles with convenient size) is an important issue.In this thesis,the use of photoinduced copper catalyzed azide-alkyne cycloaddition (CuAAC) for the preparation of thermoset polymers containing silver nanoparticles has been reported. For this purpose, the tri-alkyne and tri-azide molecules are independently prepared by etherification of 1,1,1-tris[4-(2-propynyloxy)phenyl]-ethane with propargyl bromide and ring-opening reactions of trimethylolpropane triglycidyl ether with sodium azide. Photoinduced CuAAC click reaction of these molecules performed in the presence of Cu(II)Br2/N,N,N',N'',N''-pentamethyldiethylenetriamine/2-dimethoxy-2-phenyl acetophenone as catalyst by applying UV irradiation in short-time. The successful thermoset formations from multifunctional azides and alkynes via photoinduced CuAAC reactions have been clearly confirmed by the FT-IR spectroscopy and thermogravimetric analyses. Moreover, these polymers have good antibacterial effect against Gram-positive (Staphylococcus Aureus) and Gram-negative (Escherichia Coli) bacteria. All kind of bacteria were killed and inhibition zones were formed due to the antibacterial effect of the silver in the surrounding of the samples. On the other hand, no inhibition zone was detected for the thermoset without silver nanoparticles.