Search

Cataloged from PDF version of article. Thesis (Ph.D.): Bilkent University, Graduate Program in Materials Science and Nanotechnology, İhsan Doğramacı Bilkent University, 2023. Includes bibliographical references (leaves 92-108). The development of load-carrier reinforced composites is crucial in terms of a wide range of applications, such as aerospace, automotive, sports industry and so on. When these fields are taken into consideration, reducing the excessive weight of structural materials without any sacrifice in the performance is required. Thus, using reinforcement materials (e.g. carbon fibers) for polymeric matrices in composites is the most convenient way to follow. In this study, carbon fiber (CF) was used as a reinforcement material for thermoplastic based composites. Since bare CFs are too fragile to process they must be coated / sized such that the brittleness of CFs can be avoided during industrial applications. Therefore, sizing of carbon fibers is crucial for guiding them into service by protecting the CF’s surface. Yet, the traditional sizing agent (i.e. epoxy) is not suitable for handling continuous CF reinforced thermoplastic composites (CFRTPs) with high processing temperatures above 300 ℃. In this study a novel sizing agent was developed for this purpose. The effects of this sizing on the CFs’ physicochemical as well as surface properties were investigated. As a result, the impact on fiber-matrix interphase behavior can be analyzed. Moreover, the main problem for thermoplastic based composites which is the delamination between the reinforced fiber and thermoplastic matrix can be solved throughout the development of novel coating material so that inert carbon fibers can be made compatible with the matrix. In this thesis, the activation of carbon fiber’s surface, then providing a functional sizing agent and method in order to expel the present voids because of incompatibility between CF and thermoplastic matrix (i.e. Polyetherether ketone) were carried out by enhancing the adhesion. Hence, the wettability of CF by polyetherether ketone (PEEK) matrix was enhanced by altering the surface free energy of CF resulting in optimized adhesion. Thus, the delamination problem in thermoplastic based composites was solved throughout the sizing of CFs. The first part of this work consists of the elimination of current epoxy coating on the aerospace grade commercial carbon fibers. Then, the application of surface activation method was performed by acidic modification to make CFs ready for sizing process. The formation of functional groups (-COOH, -OH) on CF’s surface was achieved after degrading of present epoxy coating throughout CFs. As a result, the developed sizing agents could be binded easily onto CF’s activated surface through the hydrogen bonding. In the second part, four different polymeric sizing agents were prepared by taking the chemical compatibility with the matrix into consideration. The sizing process was performed via dip-coating method for the surface-activated CFs. The chemical and physical analyses for neat and treated CFs were carried out via microscopic and spectroscopic techniques. As a result of sizing process, the enhanced compatibility between the matrix and reinforcement material was proved by the Contact Angle Analysis and surface free energy calculations according to Young’s equation. by Zelal Yavuz Ph.D.

Polimerlerin gündelik hayatta dokunma ve sürtünme sonucu elektriksel yük ile yüklenmeleri sonucu açığa çıkan statik elektriklenme olayı oldukça sık karşılaşılan bir durum olmakla birlikte, bu durum polimerlerin kullanıldığı birçok alanda, örneğin elektronik cihazlarda, tekstil sanayisinde ve uzay endüstrisinde sorun yaratabilmektedir. Bu sebeple, ani elektriksel yük birikmesi ve yük boşalması sonucu ortaya çıkan statik elektriklenmenin mekanizmasını anlamak ve kontrol altına alabilmek uygulamalar açısından son derece önemli bir konudur. Yalıtkan ve büyük ölçekli malzemelerin statik elektriklenmesini anlamaya çalışırken, fiziksel ve morfolojik özellikleri incelemek kritik bir rol oynamaktadır. Öte yandan, fiziksel özellikleri ele alındığında polimerlerin yapısının önem arz ettiği görülmekte olup bu yapısal özellikler ile triboelektriklenme arasındaki ilişki hakkında literatürde çok sınırlı miktarda bilgi yer almaktadır. Bunun nedeni olarak sürtünme elektriklenmesinin aydınlatılması konusunda şu ana kadar önerilen mekanizmaların yapı-özellik ilişkisini genellikle göz ardı etmiş olması gösterilebilir. Bu tez çalışmasında, polimerlerin fiziksel özellikleri ele alındığında önemli bir faktör olan kristallenme üzerinde çalışılarak, farklı muamele teknikleriyle (örneğin, mikrodalga ışıması, mekanik gerilme) değiştirilen kristallik derecesi ile triboelektrik yüklenme (yada yük oluşumu) arasındaki ilişki çalışılmıştır. Yarıkristal özelliğe sahip olan polipropilen (PP) hem ekonomik değerinden dolayı hem de kristallenme derecesinin ısısal işlem sayesinde oldukça farklı değerler alabilmesi nedeni ile seçilmiş ve bu tezde farklı spektroskopik ve mikroskopik teknikler kullanılarak PP in fiziksel özellikleri ve triboelektriklenme özellikleri arasındaki ilişki detaylı bir şekilde ortaya konulmaya çalışılmıştır. Muamele edilmiş ve edilmemiş polipropilende meydana gelen fiziksel ve kimyasal özelliklerindeki değişimleri anlayabilmek için X-ışınlı Fotoelektron Spektroskopisi (XPS), Raman Spektroskopisi, X-ışını Saçılma Difraktometresi (XRD), Atomik Kuvvet Mikroskobu (AFM), Diferansiyel Tarama Kalorimetresi (DSC) tekniklerinden yararlanılmıştır.Bu çalışmanın ilk bölümünde öncelikle statik elektriklenmenin mekanizması göz önünde bulundurularak elektriklenme olayı ile polimerlerin fiziksel özelliklerini belirleyen en önemli faktörlerden biri olan kristallenme derecesi arasında var olduğu düşünülen ilişkiler belirlenmiştir. Çalışmanın ikinci bölümünde ise, mekanik gerilime maruz kalan bir polipropilen film yüzeyinde elektrik yükü oluştuğu da ilk defa olarak bu çalışmada gözlemlenmiştir. Bu sonuçlar, yalıtkan malzemelere fiziksel kuvvetler uygulayarak, etkili bir şekilde mekanik enerjiyi elektrik enerjisine dönüştürmenin cisimler arasında dokunma olmadan da mümkün olduğunu göstermiştir. Bu olayın sebepleri açıklanmaya çalışılmıştır. Bu çalışmadan elde edilen bilgiler ışığında mekanik enerjiden elektrik enerjisi elde edebilen daha verimli triboelektrik jeneratörlerin tasarlanabileceği düşünülmektedir. Anahtar Kelimeler: fiziksel özellikler, kristallik, polipropilen, statik elektriklenme, triboelektrik yük, triboelektriklenme. The electrostatic charging of polymers due to friction is such a common phenomenon in daily usage of polymers that can be a problematic issue for various applications, such as in electronic devices, textile, space industry and so on. Hence, understanding and controlling of the mechanism behind the static electrification, which is basically because of the charge accumulation on the material, is an important subject in terms of the applications.In the way to understand static electrification of bulk materials, examining the physical and morphological properties is crucial. On the other hand, when the physical properties are considered, the structure of polymer plays a significant role, yet there is a lack of knowledge in the literature about the relation between these structural properties and triboelectricity. As a reason of this, it can be pointed out that in the proposed mechanisms about the frictional electrification the structure-property relation could not get sufficient attention so far. In this thesis, the crystalline structure of polymer, which plays a crucial role in the determination of physical properties of polymeric materials, was studied and by using different treatment techniques, such as microwave radiation and mechanical stress, and the relation between the degree of crystallinity and triboelectric charging was investigated. Due to its economical cost and heat-sensitive degree of crystallinity that can be changed in a significant way polypropylene (PP) which is a semi-crystalline polymer was used in this study. Hence, by utilizing different spectroscopic and microscopic techniques the relation between physical properties and triboelectrification of polypropylene was investigated in detail. In order to understand the physical and chemical changes taking place in untreated and treated polypropylene X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Differential Scanning Calorimetry (DSC) are the techniques that were employed. In this study, by considering the mechanism behind static electrification the potential link between electrification and degree of crystallinity was designated. Furthermore, the generation of charge on the surface of mechanically treated polypropylene film was observed for the first time by this current work. The results lead to the fact that it is possible to convert mechanical energy into electrical energy without any contact between the objects by introducing physical forces onto the insulating materials and the reasons behind non-contact electrification was investigated. Therefore, in the light of the results obtained from this study, more efficient triboelectric generators can be designed to harvest electrical energy from mechanical energy.Keywords: physical properties, crystallinity, polypropylene, static electrification, triboelectric charging, triboelectrification. 124