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Yüklenici firmaların rekabeti: İş çevresi bağlamı
Get PDFCompetitiveness is a multi defined term which can be applied to firms, industries, and nations. Different definitions of competitiveness can be found in literature due to the fact that the term fulfils different meanings at different levels. More comprehensively, D'Cruz and Rugman (1992) define competitiveness at firm level as "the ability of a firm to design, to produce and or market products superior to those offered by competitors, considering the price and non price qualities". A quick review of the management literature reveals that there exist various understandings when dealing with an intricate concept like competitiveness. Consequently a number of theories and theoretical frameworks have been developed to investigate the competitiveness of firms. These theories can be classified into two groups according to their focus points: (i) theories associated with the internal environment of the firm such as resource based, competence based and knowledge based competitiveness theories, and (ii) theories focusing on the external environment of the firm such as industrial organization competitiveness theory. Firms operating in the construction industry are facing more and more risks due to the worldwide globalization trends and rapidly changing, unpredictable, -sometimes- unstable, and complex environments. Considering the intensity of the competitiveness in the construction industry, today it is clear that focusing on low cost production alone might be inadequate to survive in highly competitive markets. This paradigm shift within the construction industry has revealed the need for a comprehensive understanding of the environments of the construction firms, especially for the firms who seek to gain competitive advantage. Moreover understanding their competitive behavior is of particular importance to create an environment with common sustainable practices. This change caused a rapid increase in the number of research studies focusing on construction competitiveness in the Construction and Project Management (CPM) field. However the research studies in the literature focusing on construction competitiveness are mostly adopted from management field, therefore reflecting the dynamics of the manufacturing industry. Although there is a substantial amount of research demonstrating the importance of competitiveness issues in the construction industry, the special product and process characteristics of the construction industry (such as project based production, fragmented nature of the industry and uniqueness of the construction product) and the competitive environment of the contracting firms are yet to be explored. Furthermore, the research studies focus on the competitiveness in the Project and Construction Management field, the majority of which tends to follow the positivist methods relying on the quantitative evidences, fail to explain the competitive environment and also behavior of the contacting firms. Since to adapt to the rapidly changing environment is a key determinant of competitive advantage, this paper attempts to address the above mentioned gaps and presents the findings of a doctoral research which examines the competitive environment of contracting firms operating in Turkish Construction Industry using qualitative design strategy, as well as a multiple case study research. In this study, a widely accepted tool for analyzing the competitive environment of a firm the "Five Competitive Forces Model" introduced by Porter was used to investigate the competitive environment of contracting firms. Data collected from the owner managers or executives of nine the contracting firms through face to face in-depth interviews were analyzed using content analysis. The findings of the study revealed that the characteristics of the competitive environment of the contracting firms are fourfold: (i) The contracting firms are facing an intensive competition due to the ease of the market entry, fluctuating demand, the existence of too many competitors, the role of informal relations on their business environment and unfair competition in the market. (ii) The competitive environment of the contracting firms is mostly dominated by resource based competition. (iii) Contracting firms prefer cost focus and differentiation strategies to gain competitive advantage, which means competing on the basis of low cost production and a low level of profit margin. On the other hand the gaining expertise at some project types, building technologies or services firms to differentiate from their competitors in the market. (iv) The competitive environment of the contracting firms differs in terms of contextual factors such as client type or organizational size. Keywords: Construction competitiveness, contracting firms, business environment. Rekabet konusunda geniş bir zaman içinde oluşan büyük bir bilgi birikimi bulunmaktadır. Bugüne kadar yönetim bilimlerinde firma düzeyinde rekabeti inceleyen çok sayıda çalışma yapılmış ve firma içi veya firma dışı faktörlere odaklanan rekabet teorileri geliştirilmiştir. Konu inşaat sektörü özelinde ele alındığında, yönetim bilimlerindeki ilgiye paralel olarak özellikle 1990’lardan sonra konuya olan ilginin arttığı görülmektedir. Bununla birlikte yapılan çalışmaların (i) imalât sektörü için geliştirilen modelleri uyarlamada sektörün özelliklerini yeterince dikkate almaması, (ii) genellikle uluslararası pazarlarda faaliyet gösteren büyük firmalara odaklanması ve (iii) rekabeti kantitatif/nicel yaklaşımla sayısal göstergeler üzerinden incelemesi sebebiyle sektördeki rekabetin nasıl şekillendiğini ve nelerden etkilendiğini açıklamaktan uzaktır. Yukarıda tarif edilen bilgi boşluğunu doldurmak üzere yüklenici firmaların rekabetini iş çevresindeki ilişkiler ışığında incelemek ve firma büyüklüğü, hizmet verilen müşteri tipi ve faaliyet gösterilen pazar gibi bağlamsal faktörlerin rekabeti nasıl etkilediğini ortaya koymak üzere nitel bir araştırma yapılmıştır. Araştırmanın sonuçları (i) iş çevresinde etkili olan ilişkilerin yüklenici frimaların rekabetlerini doğrudan etkilediğini ortaya koymaktadır. Bu ilişkilerde özellikle sektörün ana aktörlerinden olan ve talebi yaratan müşteriler ile üretimin gerçekleşmesinin vaz geçilmez parçası olan tedarikçiler ön plana çıkmaktadır, (ii) rekabet ortamının firma büyüklüğü, hizmet verilen müşteri ve faaliyet gösterilen pazardan etkilen-diğini göstermektedir, (iii) proje bazlı üretim, parçalı yapı, enformel ilişkiler ve talebin süreksizliği gibi inşaat sektörünün kendine has özelliklerinin rekabetle ilgili yapılacak çalışmalara dahil edilmesi gerektiğini ortaya koymaktadır. Anahtar Kelimeler: İnşaat sektöründe rekabet, yüklenici firma, iş çevresi
Characterization of historical mortars in Roman, Byzantine, Seljuk and Ottoman buildings located in Kayseri and the design of restoration mortars
Get PDFGeleneksel malzemeler ve onlardan üretilmiş mimari elemanlar, gerek özgün yapıları, gerekse bozulma süreçleri karşısında geçirdiği değişimler nedeniyle kendilerine özgü problemlere sahip olan ve korunması gereken tarihi belgelerdir. Koruma biliminde, mevcut özgün malzemeler hakkında yapılan değerlendirme sonucu elde edilen bilgiler, özgününe uygun onarım malzemelerinin hazırlanabilmesi açısından önemli bir yere sahiptir. Ancak genel karakterizasyonu yapılan bu malzemeler için onarımlarda kullanılmak üzere yeni üretimler yapılması ve yeni üretilen bu malzemelerin orijinal malzemeler ile uygunluğunun koruma biliminin ölçütlerine göre değerlendirilmesi gerekmektedir. Bu çalışma kapsamında Kayseri’de Roma, Bizans, Selçuklu ve Osmanlı Dönemleri’nde inşa edilmiş 15 adet tarihi yapı belirlenerek bu yapılardan duvar örgü harcı ve iç mekân sıva harcı örnekleri alınmıştır. Örneklerin malzeme içerikleri ve oranlarının belirlenebilmesi amacıyla fiziksel, kimyasal, mekanik ve petrografik analizler gerçekleştirilerek benzer özelliklere sahip örnekler gruplandırılmıştır. Bu değerlendirmelere dayanarak, yapıların restorasyonunda kullanılabilecek onarım harcı önerileri geliştirilmiştir. Öncelikle mevcut hammadde kaynakları araştırılmış, özgününe en yakın malzemelerle laboratuar ortamında onarım harçları üretilmiştir. Üretilen 3 çeşit duvar örgü harcı ve 2 çeşit sıva harcının 6 aylık süreç içerisindeki fiziksel ve mekanik özellikleri saptanarak özgün örnekler ile uyumu karşılaştırılmıştır. Anahtar Kelimeler: Harç, sıva, konservasyon, onarım harçları, restorasyon, Kayseri, Roma, Bizans, Selçuklu, Osmanlı. Historical buildings, which have survived throughout history to the present, are exposed to a rapid corrosion process due to natural effects, misuse and conscious destruction. The only way to ensure their survival without losing their characteristics is to adopt the right measures which will result in conscientious restorations. The main problem with restoration of buildings that have suffered from different effects is to find building materials to repair the structural damage and material decay. The repair techniques must match the original materials used in the building. Information determined about the original materials in buildings is very important in order to choosing the restoration materials. After general evaluation, it is necessary to determine what restoration materials can match the original mortars in both form and structure. General principles of restoration, include conserving the original construction materials and techniques as much as possible and preserving a building?s authenticity by choosing techniques which will not harm the building's essential historical structure were used during initial construction. For this reason, it is very important to carefully determine what original materials and to prepare a matching restoration material that will not alter the building's originality. Many mistakes have been seen especially with materials and building techniques used in most of the restorations in Turkey. The recent restorations, in particular with a few exceptions, have damage buildings because conscientious approaches, where not employed. These hasty restorations have caused both a loss of information about past technologies and materials used in construction, and they have destroyed the historical value of these buildings. One of the most damaging effects of these above-named restorations is the use of materials incompatible with the historical features of the building without having performed necessary analyses. Many studies have analyzed the corrosion process and the structure of mortars used to connect natural stones which were preferred due to their aesthetics, variety and strength in traditional buildings all along the history of monumental constructions. However, when we consider that every district and every period has its own particular of construction materials, it is clearly seen that these studies have not yet become widespread, especially in Anatolia. Although some studies have been conducted on puzzolanas (volcanic ashes which are used to improve hydraulic features), it is necessary to expand these studies in order to be certain that these materials are appropriate for different districts and different time periods. The purpose of this thesis is to design the recipes of repair mortars and plasters for the conservation works in Kayseri which has a great potential of volcanic materials and tradition of hydraulic mortars. In this context, fifteen buildings in Kayseri belonging to the Roman, Byzantine (4th, 6th, 11th century), Seljuk (13th, 14th century) and Ottoman (15th, 16th, 18th, 19th century) periods have been selected for sampling. The original mix proportions and properties of the samples have been identified by physical, chemical, mechanical and petrographical analysis and classified according to their similar properties. Evaluating these analyses, plaster samples which are appropriate for restorations of the overmentioned buildings were designed. In this process, the existing sources for raw materials have been researched; samples from the reactive aggregates, lime kilns and the stone quarries of the region have been searched and evaluated to determine the material that has the similar properties with the original. Conservation mortars have been produced in laboratory medium using the same ratio of mixture determined through the characterization study. Three types of conservation mortars and two types of plasters were tested for their mechanical and physical qualities during six months and these values were compared with original samples. Keywords: Mortar, plaster, conservation, conservation mortars, restoration, Kayseri, Roman, Byzantine, Seljuk, Ottoman
Avrupa Para Birliği ve Türk ekonomisine etkileri
Get PDFThis paper is a revised and up-dated summary of the PhD thesis concerning the European Monetary Union (EMU) and its impact on the Turkish economy. The paper begins with an overview of the Economic Monetary Union and continues with the evaluation of the Euro, its single currency. The launching of the Euro on January 1999 created the world's second largest single currency area after the USA in terms of economic size. The international attributes of the Euro and its role as an international currency are also briefly discussed. Within this context, the causes of the orderly depreciation of the Euro against major currencies in its initial years and the prospects of the single currency are analyzed in summary. Turkey is a candidate for full membership to the European Union (EU). Therefore, it is a major concern to evaluate the possible effects of the EMU on the Turkish economy. Hence, the paper concludes with the evaluation of the possible effects of the European Monetary Union and its currency, the Euro, on Turkey. As Turkey will be preparing herself to be a full member of the EU, and in the distant future, to the EMU, the flexibility of the exchange rate policy and to have Euro as a nominal anchor will be of particular importance. Keywords: European Monetary Union, USA, European Union, Euro.Bu makale orijinal adı "European Monetary Union and its Impact on the Turkish Economy" adlı doktora çalışmasından yararlanılarak hazırlanmıştır. Makalede tezden yararlanılmakla beraber bazı bilgiler eklenmiş ve yenilenmiştir. Çalışmada Avrupa Para Birliği'nin (APB) kuruluşundan bu yana meydana gelen gelişmeler kısaca anlatılmakta, bu birliğin bugüne kadar sergilediği performans, başarılı olduğu ya da başarısız olduğu hususlar gözden geçirilmektedir. Öte yandan, APB ile birlikte yürürlüğe giren Avrupa tek para birimi Euro'nun durumu, diğer para birimlerine karşı olan değeri ve uluslararası bir para olmanın gerektirdiği koşulları karşılayıp karşılamadığı yine kısaca ele alınmaktadır. Bu çerçevede, Türkiye'nin AB (Avrupa Birliği) ile olan özel ilişkileri nedeniyle APB'den etkilenmesi kuşkusuzdur. Çalışmanın sonunda APB'nin ve Euro'nun Türk ekonomisine olası etkileri konusunda bir değerlendirme yer almaktadır.Anahtar Kelimeler: Avrupa Para Birliği, ABD, Avrupa Birliği, Euro
Mühendislikte etik: afet etiği ve ötesi
Get PDFFracture Professional engineering societies and colleges of engineering are increasingly recognizing that they should take new steps to educate engineers in the ethics and social implications of their work. From my own experience and my preference for STS methods, I think STS scholars are well positioned to bring their knowledge and research skills to bear on such a vital subject. By providing a more in-depth picture of engineering practice and its wider ties with politics, economics, and other occupations, STS scholars can complement the traditional approach by moral philosophy and practicing engineers. Although engineering accidents have historically played a role in improving the design of artifacts (Petroski, 1985) and have a conspicuous presence in the current pedagogy of engineering ethics, I think the STS approach helps us move beyond "disaster ethics." By using STS analytical tools, including Vaughan's study of an accident, we can bring the everyday practice of engineering into classrooms to enrich the teaching of engineering ethics. Studyingon some projects, more so than spectacular accidents, may be more persuasive in convincing engineering students and academic colleagues alike that they need better methods in how to deal with every day issues of tremendous significance regarding the ethical and social implications of engineering. Keywords: Ethics, engineering ethics, disaster ethics.Mühendislik üretimlerinden kaynaklanan işlerin tasarım, yapım, denetim hataları günlük yaşamda kazalara yol açabiliyor veya deprem benzeri ciddi büyük ölçekli afetler sonucunda onarılamaz yıkımlar yaratabiliyor. Mühendislik mesleğindeki etik ve eğitim sorunları, sosyal yaşamda meydana gelen kazalar ve deprem gibi afetler söz konusu olduğunda çok önem kazanıyor. Bu nedenle mühendisliğe yönelik etik eğitiminin geliştirilmesi kaçınılmazdır. Geleneksel etik felsefesi ve mühendislik pratiğinin buluştuğu bir eğitim yönteminin geliştirilmesi ve afet etiğinin ötesine geçilmesini kolaylaştıracaktır. Bu yazı, böyle bir eğitim yönteminin neden gerekli olduğu nasıl geliştirilebileceği ve önemini ele almakta; dahası mühendislik hatalarının sosyal yaşamda yarattığı kaza ve afetler örneklendirilmektedir. Anahtar Kelimeler: Etik, mühendislik etiği, afet etiği
Mimesis in Ottoman paintings: An evaluation in regards the paintings of Şeker Ahmed Paşa
Get PDFOsmanlı resminde mimesis kendi iç dinamikleri sonucu ulaştığı bir yöntem değildir. Batıdan ithal edilen biçim dilinin Osmanlı sanatçıları tarafından uygulanmasından ibarettir. Osmanlı resminde bu kavram 17. yüzyıldan itibaren geleneksel biçim dilinin çözülmeye başlamasıyla gündeme gelir, ancak programlı olarak 19. yüzyıl resminde uygulanmaya başlanır. Bu makale, mimesis kavramını Şeker Ahmed Paşa’nın resimleri bağlamında incelemektedir. Felsefi anlamda mimesis, akılcı, nesnel ve bilimsel bir gerçekliğin resim düzleminde ifadesidir. Osmanlı-İslam kültürü içinde yetişen, tasavvufun ontolojik görüşü ve geleneksel ahlak sistemine büyük ölçüde bağlı olan bir sanatçının resimlerinde mimesisin kendini özgü yönü sorgulanmaktadır. Anahtar Kelimeler: Mimesis, Osmanlı resmi, Şeker Ahmed Paşa.Mimesis was not a method that was attained as a result of internal dynamics in the Ottoman paintings. It consisted of the Ottoman artists implementation of a style that was imported from the Western world. This concept came into being in the Ottoman paintings, starting from 17th Century, when the style of language used became disintegrated; however, it’s programmed implementation in paintings dates back to the 19th Century. The image in painting is not only physical and technical result but also mental and cultural expression. This article presents a epistemological approach to the subject. Ottoman mentality that is based on a priory and idealist epistemology is in contrast with Western mentality that is based on empirical and positivist epistemology. These two different ways of thinking and seeing were conjoined and that constituted peculiar structure in Westernization period. Philosophically speaking, mimesis is the expression that is projected as painting as it is realized on a rationalistic, objective and a scientific scale. This paper examines the concept of mimesis in relation to paintings of Şeker Ahmed Paşa. The interrogation, in question, here is about the style of use of mimesis in paintings of Şeker Ahmed Paşa who was raised in an Ottoman-Islamic culture, who was bound by the views of Islamic mysticism and who was also tied to a traditional moral system. Keywords: Mimesis, Ottoman painting, Şeker Ahmed Paşa
Satisfaction with a performance appraisal system and affecting factors
Get PDFİşe yerleştirme, performans değerleme, ücretlendirme, eğitim ve motivasyon insan kaynakları yönetiminin temel unsurlarıdır. Bunlar arasında performans değerleme, diğer insan kaynakları sistemlerinin kurulmasında üstlendiği yardımcı rol nedeniyle, önem derecesinde en ön planda yer almaktadır. Ayrıca değerleme sayesinde çalışanların verimliliği belirlenir ve sürekli gelişim planlarıyla bireylerin desteklenmesi ve geliştirilmesi sağlanır. Sonuç olarak performans değerleme bir organizasyonun insan kaynaklarının verimliliğini arttırmada temel bir gerekliliktir. Çalışanların organizasyona karşı tutum ve tepkileri, uygulamalı psikoloji dalında en çok araştırılan konu başlıklarından biri olmuştur. Benzer olarak araştırmacı ve uygulamacılar tarafından yoğun bir şekilde araştırılan bir diğer konu da performans değerlemedir. Çalışanların performans değerleme süreci hakkındaki tutumları ve tepkileri, performans değerleme sisteminin faydası değerlendirilirken göz önünde bulundurulması gereken en önemli kriterlerden biridir. Bu çalışmada, konuyla ilgili yazın incelenmiş ve bu doğrultuda çalışanların performans değerleme sistemi memnuniyetlerini etkileyebilecek algı ve tutumlar belirlenmiştir. Teorik beklentiler doğrultusunda oluşturulan araştırma hipotezleri, kamunun sahipliğini yaptığı özel bir işletmede sınanmıştır. Veriler, güncel yazın incelenerek, özgün olarak hazırlanan bir soru formu aracılığıyla toplanmıştır. Soru formunda yer alan ifade grupları, önceki çalışmalarda geçerliliği ve güvenilirliği sınanmış ölçekler temel alınarak geliştirilmiştir. Toplanan veriler, yapılan betimleyici faktör analizlerinden sonra regresyon analizi yapılarak incelenmiştir. Elde edilen bulgular doğrultusunda, memnuniyetle ilişkili unsurlar açığa çıkarılmıştır. Araştırma sonuçları, daha önceden yapılmış çalışmalarla karşılaştırılarak oluşan farklılıklar yorumlanmıştır. Anahtar Kelimeler: Performans değerleme sistemi, algı ve tutum, etkinlik, memnuniyet.Employee selection, performance appraisal, compensation, training, and motivation are the major topics of the human resources management. Among these topics, performance appraisal is the most important one because of the supportive role in forming the other systems. Furthermore, an appraisal is helpful determining effectiveness of the employees, which concludes with the continuous improvement, and support plans of the individuals. Finally, a performance appraisal is a basic necessity to increase the effectiveness of the human resources of an organization. One of the most researched topics of the applied psychology branch is the attitudes and reactions of the employees towards the organization. Research about job satisfaction and the organizational commitment is the main axis of the issue. Another frequently researched topic is the "performance appraisal". The attitudes and the reactions of the employees about the process are one of the most important criteria while evaluating the usefulness of performance appraisal systems. In the last decade, there has been an increase in the number of studies, which have investigated the characteristics of performance appraisal systems. These studies have used questionnaires to assess individuals' reactions to various aspects of the appraisal system. In this research, literature regarding the opinions of employees' has been reviewed, and the perceptions and attitudes that might affect employees' satisfaction level of performance appraisal have been determined. Research hypothesis that was formed on the basis of the theoretical expectations have been tested in a private sector organization which is owned by government. All the data were collected with a questionnaire which was prepared genuinely after reviewing the contemporary literature. The expression patterns of the questionnaire were based on the previous studies in which their validity and the reliability were already approved. The collected data were examined with the regression analysis technique after the descriptive factor analysis. Based on the findings of the analysis, the relationships which forms the satisfaction construct were revealed. As a result of the research, it is observed that, a two-sided interaction exists between the satisfaction and effectiveness variables. This observation points out that an inefficient fraction of the integrated system may cause unsatisfactory feelings among the employees. Due to the obtained results, the satisfaction level of the appraisal is having parallel motives with the major aims of the projected targets of the system, which is the main effectiveness indicator of the entire system. "Justice Perception" is one of the major elements which is influential on the entire perception of the satisfaction issue. On the previous researches, it has been declared that the satisfaction level about the appraisal is directly related with the "Justice Perception" of the employers. The findings of the research, verifies the relation between the two dimensins of justice which are, procedural and distributional justice and the satisfaction figures. This result shows that any lack of justice in the appraisal systems is likely to decrease the satisfaction levels of the employees. In the previous studies, it has been indicated that the knowledge of the appraisees about the appraisal system is related with their satisfaction levels. In the realized research findings, it was seen that this relation was related with the "Personal System Knowledge" perception of the appraisee which is a sub-factor of "Perceived System Knowledge". Personal knowledge perception is a variable figure, which shows the knowledge of the related individual, regarding the concept of the appraisal system. A different sub-factor of this structure is the "General Knowledge Perception Variable" which shows the knowledge level of the appraisees about the appraisal system. This result also proves the importance of the correct "Appraisal Definition" which needs to be explained to the appraisees. The more the appraisees learn about the appraisal system, the higher appraisal satisfaction levels are reached by the appraisees. There are several limitations of the present study which should be noted. Since this study focused on a single appraisal system in one organization, generalizations regarding the results are somewhat limited. Additional research is needed which replicates the present findings using other appraisal systems in other organizations. This would provide additional evidence for understanding the characteristics of effective appraisal systems, which are acceptable to both managers and employees. Keywords: Performance appraisal systems, perceptions and attitudes, effectiveness, satisfaction
Investigation of the spatial inhomogeneity in poly(acrylamide) (PAAm) gels by static light scattering technique
Get PDFPoliakrilamid (PAAm) jellerinde ışık saçınım tekniği ile yapısal inhomojenitenin nasıl değiştiği incelenmiştir. PAAm jellerinde başlangıç monomer konsantrasyonu sabit tutularak, sadece çapraz bağlayıcı konsantrasyonu değiştirilmiştir. Işık saçınım ölçüm sonuçlarından polimer jellerinin, polimer çözeltilerinden daima daha fazla ışığı saçtığı bulunmuştur. Jellerde gözlenen bu fazla saçınım intensitesi çapraz bağlayıcı konsantrasyonu arttıkça belirgin bir şekilde artmaktadır. Bu ölçülen fazla saçınım intensitesi değeri yapısal inhomojenitenin ölçüm parametrisi olduğundan çapraz bağlayıcı konsantrasyonu artıkça jellerdeki inhomojenite derecesinin arttığı sonucuna varılmıştır. Işık saçınım intensitelerinden Debye Bueche teorisi yardımıyla inhomojen bölgelerin boyutları x ve konsantrasyon dalgalanmalarının kareleri ortalaması değerleri hesaplanmıştır. Çalışmanın ikinci bölümünde amonyum persülfat – N,N,N',N'-tetrametilendiamin (TEMED) redoks başlatıcı sistemi ile PAAm jelleri sentezlenmiş ve bekleme süresi ile saçınım intensitelerinin değişimi takip edilmiştir. PAAm jellerinin zamanla saçınım intensitelerinin azalarak daha homojen hale geldiği gözlenmiştir. Çalışmanın üçüncü bölümünde ise çapraz bağlayıcı konsantrasyonu sabit tutularak farklı başlangıç konsantrasyonlarında hazırlanan PAAm jellerinde sentez sonrası ve şişme sonrası dengeye geldikten sonra yapısal inhomojenileri incelenmiş ve inhomojenite derecesinin maksimum olduğu kritik bir konsantrasyonun varlığı tespit edilmiştir. Elde edilen sonuçlar Debye Bueche teorisi yardımı ile açıklanarak nedenleri tartışılmıştır. Anahtar kelimeler: Poliakrilamid, inhomojenite, ışık saçınımı.Hydrogels are important materials from both fundamental and technological points. Swelling properties and the elastic behavior of hydrogels have been intensively studied in the last four decades. However, theories are still unable to predict their physical properties from the synthesis conditions. This is due to the several non-idealities of the gel formation system such as the different and conversion-dependent reactivities of the vinyl groups, cyclization, multiple crosslinking, and diffusion-controlled reactions (Funke et al., 1998). In contrast to ideal gels with a homogeneous distribution of crosslinks, real gels always exhibit an inhomogeneous crosslink density distribution, known as the spatial gel inhomogeneity. The inhomogeneities present in gels are of considerable interest and importance in attempts to characterize such materials physically. From the practical point of view, spatial inhomogeneity is undesirable because it dramatically reduces the optical clarity and strength of gels, which are properties closely connected with many industrial applications such as contact lenses, super absorbents, cosmetics, etc. Since the spatial gel inhomogeneity necessarily produces local concentration fluctuations in gels, scattering methods have been used to investigate the inhomogeneities. The gel inhomogeneity can be manifested by comparing the scattering intensities from the gel and from a semi-dilute solution of the same polymer at the same concentration. The excess scattering over the scattering from polymer solution is related to the degree of the inhomogeneities in gels. In general, the gel inhomogeneity increases with the gel crosslink density due to the simultaneous increase of the extent of network imperfections producing regions more or less rich in crosslinks On the other hand, the inhomogeneity decreases with the ionization degree of gels due to the effects of the mobile counter ions, electrostatic repulsion and the Donnan potential. Degree of swelling of gels subjected to scattering measurements also affects the scattering intensities. Bueche reported in 1970 the enhancement of scattering intensity associated with the swelling of polymer Networks. The scattered light intensity from gels was found to be much larger than that of the polymer network alone. Later, various types of gels have been investigated at different swelling stages.The results show that the scattering intensity at low scattering vectors is enhanced as the swelling degree is increased. This behavior was interpreted as the enhancement of the difference of polymer concentration between the more and the less crosslinked regions. The spatial inhomogeneity in (PAAm) gels has been investigated with the static light scattering technique. In the first section, the gels were prepared using N,N'-methylenebis(acrylamide) (BAAm) as a crosslinker at a fixed initial monomer concentration but at various crosslink densities. Elasticity measurements show that 80 to 90 % of the crosslinker molecules used in the hydrogel preparation were wasted by cyclization and multiple crosslinking reactions during the gel formation process. Light scattering measurements show that the excess scattering from gels increases with increasing BAAm content. Since the excess scattering Rex(q) is a measure of the spatial inhomogeneity in a gel, this indicates that the PAAm gel becomes more inhomogeneous with increasing BAAm concentration. The size of the inhomogeneous regins in gels as well as the mean square fluctuations in the reflactive index were calculated using the light scattering data of gels togetter with the Debye- bueche theory. In the second section ammonium persulfateN,N,N',N'-tetramethylethylenediamine redox initiator system was used to initiate the polymerization reactions and to follow up scattering intensities as a function of time of aging gels in the synthesis reactor. It was shown that the excess scattering decreases, PAAm gel becomes increasingly homogeneous with increasing time of aging. The scattered light intensities from aged gels were interpreted using the Debye-Bueche analysis and a thermodynamic model was developed. In the third section the gels were prepared using N,N?-methylenebis(acrylamide) (BAAm) as a crosslinker at a fixed crosslinker ratio X but at various initial monomer concentrations. The gels, both at the state of preparation and at the equilibrium swollen state in water, exhibit a maximum degree of spatial gel inhomogeneity at a critical monomer concentration. Keyword: Inhomogeneity, polyAcrylamide, light scattering
Preparation of crosslinked and graft copolymers via photoinitiated polymerization
Get PDFYan zincir fonksiyonlu önpolimer kullanılarak fırça tipli blok-aşı kopolimer eldesi incelendi. İlk olarak, kümil potasyum başlatıcısı varlığında, –78oC’de ve 10-6 torr basınç altında gerçekleştirilen anyonik yaşayan polimerizasyonla düşük molekül ağırlığı dağılımlı poli(N,N-dimetil-4-vinilfenetilamin)-blok-polistiren sentezlendi. Daha sonra, N,N-dimetil amino yan zincir fonksiyonlu bu polimerden metil metakrilatın fotokimyasal radikal polimerizasyonu sonucu çapraz bağlı polimer ve daha az oranda fırça tipli blok-aşı kopolimer elde edildi. 350 nm dalgaboylu ışıkla uyarılan benzofenonun yan zincirdeki amino gruplarından hidrojen koparmasıyla makroradikal oluşumu sağlanmıştır. Blok-aşı kopolimerin karakterizasyonu 1H-NMR, ve GPC analizleriyle yapıldı. Anahtar Kelimeler: Yaşayan anyonik polimerizasyon, Fotokimyasal radikal polimerizasyonu, Çapraz bağlı polimer, Fırça blok kopolimer, Blok-aşı kopolimer.Block and graft copolymers are the most demanded advanced materials since they display combined physical and mechanical properties. Because of their diverse macromolecular structures, they may possess novel characteristics required for many high-tech applications. Preparation of block-graft copolymers is described using a side-chain N,N-dimethylamino functional precursor polymer. Since the functional sites for grafting are placed in each repeating unit of a segment, the resulting polymer is a brush type block-graft copolymer. For this purpose, monodisperse poly(N,N-dimethyl-4-vinylphenethylamine-block-styrene), [P(PTA-b-St)] block copolymer was synthesized by sequential addition of N,N-dimethyl-4-vinylphenethylamine (PTA) and styrene (St) by anionic living polymerization, carried out at 78°C under a pressure of 10-6 torr using cumyl potassium as the initiator. The diblock copolymer containing N,N-dimethyl amino groups in one segment was used as a backbone in the photografting experiments for the preparation of polymer brushes by the "grafting from" strategy. Photoinduced radical polymerization of styrene through side chain-functional N,N-dimethyl amino groups yielded both crosslinked polymer and a brush type block-graft copolymer to a lesser extent. Upon irradiation at a wavelength of 350 nm, macroradicals were generated via hydrogen abstraction from amino side-chain groups by the excited benzophenone. The carbon centered radicals, thus generated, initiated the free radical polymerization of methylmethacrylate (MMA). Formation of block-graft copolymer was evidenced by GPC and 1H-NMR analyses. Keywords: Living anionic polymerization, photoinitiated radical polymerization, crosslinked polymer, brush block copolymer, block-graft copolymer
The electrochemical polymerization of indole with thiophene
Get PDFİndolün Tiyofen varlığında elektrokimyasal polimerizasyonu in-situ ve ex-situ spektroelektrokimyasal olarak incelenmiştir. In-situ spektroelektrokimyasal çalışma sonucunda çözünen oligomerik çiftlere ait olabileceği düşünülen 390 nm’de elde edilen absorbans değerleri ile Döngülü Voltamogram sonucunda 600 mV’da elde edilen akım değerleri arasında bir korelasyon gözlenmiştir. Kopolimerizasyonda tiyofen oranı arttırıldıkça iletkenlik değerlerin de artışlar gözlenmiştir. nBINB/nBThB:1/1 olduğu durumda iletkenlik 19 kat artarken nBINB/nBThB:1/10 olduğu durumda ise 60 kat artmıştır. Döngülü Voltamogram ile polimerizasyonun potansiyodinamik incelenmesi sonucunda akımlardaki artışlarda iletkenliğe benzer olmuştur. Böylece polimerin kaplanması sırasında elde edilen akım değerleri oluşan polimerin iletkenliği hakkında bilgi verebilmektedir. Th oranı arttıkça TBgB değerlerinde de artışlar gözlenmiştir. Deneysel sonuçlar hem elektrod yüzeyinde hem de çözeltide kopolimerizasyonun gerçekleştiğini desteklemektedir. Anahtar Kelimeler: Poliindol, İndol-tiyofen kopolimeri.Electropolymerization of indole (IN) in the presence of thiophene (Th) was followed by in situ and ex situ spectroelectrochemical studies.A correlation between absorbance (390 nm) and charge (at 600 mV) values indicated that oligomeric species were formed in solution,and similar results were found with in situ measurements. The copolymer was characterized by FT-IR, UV-Visible Spectroscopy, Cyclic Voltammetry and four-point probe conductymeter. The increase in conductivity by the incorporation of Th into polyindole was about 60 times for a feed ratio nBINB /nBThB=1:10 and 19 times for n BINB /n BThB=1:1. Similar effects were also observed during in situ spectroelectrochemical measurements of copolymer formation.It was also found that the cyclic voltametry peak potentials for the electrogrowth of copolymer films were closely correlated to the conductivities of the corresponding films (measured separately by four-point probe method),thereby allowing us to use the peak potential currents to predict the final copolymer film conductivities during the electrochemical growth process. The ex-situ spectroelectrocopolymerization of indole was also obtained in acetonitril medium. The Tg value of the polymer also increased with the incorporation of Th.The results strongly suggest that IN and Th copolymerize on the electrode surface as well as in solution. Keywords: Polyindole, the copolymer of indol-thiophene
Swelling behavior of strongly anionic hydrogels in polymer solutions
Get PDFAkrilamid (AAm) ve N-izopropilakrilamid’in (NIPA), farklı oranlarda 2-akrilamido-2-metilpropansulfonik asit sodyum tuzu (AMPS) ile serbest radikal çapraz bağlanma kopolimerizasyonundan anyonik hidrojeller sentezlendi. Jellerin %10’dan 100’e kadar değişen hacim oranlarında poli(etilenglikol)(PEG) içeren sulu çözeltiler içersindeki şişme davranışları incelendi. Deneysel sonuçların teori ile açıklanması için jellerin su ve saf PEG eriyiği içersindeki şişme oranlarından yararlanarak polimer ağyapı-lineer polimer-çözücüden meydana gelen ve sistemi karakterize eden parametreler hesaplandı ve Flory-Huggins eşitliği yardımıyla sistemin teorik şişme oranları elde edildi. Teorik ve deneysel şişme oranlarının uyum içersinde olduğu görüldü, jellerin PEG çözeltileri içersindeki şişme davranışları açıklandı. Anahtar Kelimeler: Anyonik hidrojeller, Flory-Huggins eşitliği, serbest radikal çapraz bağlanma kopolimerizasyonu, şişme oranları, şişme davranışları.Hydrogels are materials show phase transition by swelling depending on their chemical structure, solvent, pH, temperature, light and electric field etc. In the literature many studies have been published on the swelling behavior of hydrogels in low molecular weight solvents and solvent mixtures. In contrast, a few number of investigations have been reported on the swelling behavior of hydrogels in polymer solutions. In this study of which purpose is determination of swelling behavior of neutral and ionic hydogels in linear polymer solutions, the neutral and ionic poly(acrylamide) (PAAm) and poly(N-isopropylacrylamide) (PNIPA) gels and poly(ethyleneglycole) (PEG) with different molecular weights were preferred as hydrogels and linear polymer, respectively. The effects of ionic group content, PEG concentration of outer aqueous PEG solution and molecular weight of PEG on the swelling behavior of hydrogels in PEG solutions are investigated. In addition of these effects, for PNIPA gels, the temperature effect on the swelling behavior was also researched. On the other hand, the theoretical calculations were performed to these ternary systems (polymer network-linear polymer-solvent) and tried to predict their swelling behavior by using Flory-Rehner theory. Finding from experiments were compared with the theoretical results and the differences between theory and experimental were interpreted. Keywords: Flory-Rehner theory, hydrogel, swelling behavior
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