Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes

Three-dimensional networks can be hydrophilic and/or hygroscopic. Optical, mechanical, and electrical properties of these materials encompass many fields of technology. Composites of carbon nanotubes (CNTs) in polymeric materials have attracted considerable attention in the research and industrial communities due to their unique optical, mechanical, and electrical properties. CNT polymer nanocomposites possess high stiffness, high strength, and good electrical conductivity at relatively low concentrations of CNT filler. Here, in this chapter, we survey the optical, mechanical, and electrical mechanisms for various polymeric systems. Composite gels and films were prepared with various molar percentages of multiwalled carbon nanotubes (MWCNTs). The optical, mechanical, and electrical behaviors of various composite gels doped by MWCNT are also discussed in the each sections of the chapter. The optical behaviors of the composites were performed by the UV-Vis spectroscopy and fluorescence spectroscopy in the first part of the chapter. On the other hand, compressive testing technique and rheological measurements were employed to determine the variations of mechanical properties of the composites in the second part of the chapter. Lastly, we review the electrical properties of the composites improved significantly by addition of MWCNTs researches

Reversible film formation from nano-sized PNIPAM particles below glass transition

Reversible film formation process from nano-sized Poly(N-isopropylacrylamide) (PNIPAM) microgel particles were studied during heating-cooling cycles at various rates. Photon transmission technique was used and transmitted photon intensity I (tr) was monitored during heating-cooling cycles. The increase and decrease in I (tr) during heating and cooling was explained with the void closure and void reconstruction processes, and the corresponding activation energies were measured. It was observed that PNIPAM microgels required less energy during reconstruction of voids than their closure.Publisher's Versio

Swelling of iota-carrageenan gels prepared with various CaCl2 content: A fluorescence study

Iota carrageenan gels prepared with various CaCl2 content were completely dried and then swelled in water vapor. Steady-state fluorescence ( SSF) technique was used to monitor the swelling process of each iota carrageenan gels at various temperatures. Pyranine was used as a fluorescence probe. Apparent fluorescence intensity, I increased as swelling time increased for all gel samples. The increase in I was modelled using Li-Tanaka equation from which the swelling time constants, tau(1) and cooperative diffusion coefficients, D-c were determined. It was observed that D-c increased as the swelling temperature was increased. On the other hand at each temperature, it was seen that D-c decreased as CaCl2 content was increased. Activation energies for swelling were obtained and found to be 60.5, 61.0, 61.5 and 62.8 kJmol(-1) for the gels prepared with increasing amount of CaCl2 content.Publisher's Versio

Dissolution of percolation cluster on UV-cured polymeric films; a fluorescence study

UV-cured polimerik filmlerin çözünmesini çalışmak için kararlı durum floresans teknik (SSF)kullanıldı. Bu filmler piren(P) ile etiketli poli(metil metakrilat) (PMMA) zincirlerinden elde edildi. Burada P çözünme işlemini takip etmek için floresans madde olarak tanımlandı. Çeşitli oranlarda etilen-glikol dimetakrilat (EGDM) çapraz bağlayıcı içeren, 3 ve 25 saat UV radyasyonu ile ışınlanmış PMMA filmler %(80-20) klorofrom-heptan karışımında çözdürüldüler. 3 saat ışınlanmış filmler için bulunan perkolasyon eşiği (pc=0.35) nokta perkolasyon modeline uyarken, 25 saat ışınlanmış film için ölçülen perkolasyon eşiği (pc=0.25) bağ perkolasyon modeline uyar. Anahtar Kelimeler: Floresans, çapraz-bağlanma, perkolasyon, çözünme.Steady state fluorescence (SSF) technique was used for studying dissolution of UV-induced polymer films. These films are formed from poly(methyl methacrylate) (PMMA) chains labeled with pyrene (P) which was introduced as a fluorescence probe to monitor the dissolution processes. PMMA films in various ethylene glycol dimethacrylate (EGDM) content, cured by UV radiation for 3 and 25 hours, were dissolved in chloroform-heptane (80%-20%) mixtures. Dissolution of the films were monitored in real-time by the pyrene fluorescence intensity, IP change in the solvent mixture. It was observed that maximum P intensity, IP¥ increased and decreased  above a critical EGDM content for 3h and 25h irradiated films, respectively. It was atributed to the formation of a percolation cluster. The percolation treshold (pc=0.35) found for 3h irradiated films obeys site percolation model, however the measured percolation threshold (pc=0.25) for 25h irradiated films obeys the bond percolation model. Desorption rates, Dd were measured for films prepared with various EGDM content was observed that Dd values are much lower above pc than below pc for both films. These UV cured films dissolve much faster below the percolation threshold (pc) where dissolution coefficients were found to be around 10-9 cm2s-1. However above pc highly crosslinked films dissolve at least 4 times slower than low crosslinked films i. e. UV cured films hardly dissolve above critical EGDM content (pc) in the film.  Keywords: Fluorescence, cross-linking, percolation, dissolution

Swelling of heterogels in good solvents; A fast transient fluorescence study

Disk şekline sahip kopolimer jellerin şişme işlemini incelemek için zaman ayrımlı floresans teknik (FTRF) kullanılmıştır. Metilmetakrilat (MMA) ve stiren (S) monomerlerinin karışımı ile hazırlanan kopolimer jellerde, etilen-glikol-etilmetakrilat (EGDM) çapraz bağlayıcı ve 2,2'-azobisizobutironitril başlatıcı olarak kullanılmıştır. Floresans molekül piren(P) polimerleşme sırasında jel içerisine tuzaklanmıştır. P moleküllerinin yaşam zamanları, difüzyon ve kullanılan organik çözücü kalitesi arasındaki ilişkiyi saptamak için iyi çözücü içerisindeki kopolimer jellerin şişmesi sırasında gözlenmiştir. P yaşam zamanının davranışı Stern-Volmer yasasıyla yorumlanmıştır. Şişme zaman sabiti (tc) ve kollektif difüzyon katsayısını (Dc) bulmak için Li-Tanaka denklemi kullanılmış ve tc ile Dc’nin kullanılan organik çözücü ve kopolimeri oluşturan polimerik malzemelere bağımlı olduğu görülmüştür. Anahtar Kelimeler: Şişme, zaman ayrımlı floresans teknik, çözücü kalitesi.The swelling of disc-shaped heterogels were studied using Fast Transient Fluorescence Technique (FTRF). In this technique the photo multiplier tube (PMT) is gated or strobed by a voltage pulse that is synchronized with the pulsed light source. Disc-shaped heterogels were prepared by combination of methyl-methacrylate (MMA) and styrene (S) with ethylene glycol dimethacrylate (EGDM) as a crosslinker agent in the presence of 2,2-azobisisobutyronitrile (AIBN). Pyrene (P) was introduced as an extrinsic floroprobe during polymerisation. Seven different gels were prepared at 70oC temperature in constant (1.5 % v/v) EGDM content with various combinations of MMA and S contents. Lifetimes of P were measured in-situ swelling processes of heterogels in good solvents to determine the relationship between the diffusion and solvent quality. Chloroform and toluene were chosen as swelling agent which have the solubility parameter, d of 19 and 18.2 (MPa)1/2 respectively. These values of d of the solvents are sufficient to make the chloroform and toluene good solvent for PMMA (dp=19 (MPa)1/2) and PS (dp=18.5 (MPa)1/2), respectively. An equation is derived for low-quenching efficiencies to interpret the behaviour of the lifetime of P during swelling. Li-Tanaka equation was employed to determine the swelling rate constant, tc and the cooperative diffusion coefficients, Dc in heterogel and they were found to be strongly correlated with the chosen solvent and the polymeric material in the heterogel system. Keywords: Swelling, fast transient fluorescence technique, solvent quality

The phase transitions in liquid crystal polymers

Foton geçirgenliği yöntemi kullanılarak sıvı kristal akrilat monomer, (6-4-siyanobifenil-4’-oksi)hekzil akrilat (LC6), homopolimeri (PLC6) ve politetrahidrofuranla aşılanmış aşı kopolimerinin (GLC6) faz dönüşümleri incelendi. Faz dönüşümleri polarizasyon mikroskobuyla doğrulandı. LC6 monomerinde izotropik-nematik-smektik A-smectik C faz dönüşüm serisi gözlenirken PLC6 ve GLC6 polimerlerinde, nematik ve smektik A fazları baskın görünmektedir. LC6 monomerinde nematik-smektik A faz dönüşümü ikinci dereceden olup düzen parametresi için efektif kritik üstel olarak hesaplandı. Gözlenen nematik-smektik A faz dönüşümü PLC6’da birinci derece ve GLC6’da ikinci derece olurken dönüşüm sıcaklığı aynı kalmaktadır. GLC6’daki donmuş rastlantısal bağların etkisnini donmuş rastlantısal etkileşmelerin kuramsal sonuçlarıyla tutarlı olduğu görüldü. GLC6 aşı polimerinde nematik-smektik A faz dönüşümünde kritik üsteli olarak hesaplandı. Anahtar Kelimeler: Foton geçirgenliği yöntemi, sıvı kristal homopolimer, aşı kopolimeri, nematik düzen, donmuş rastlantısal bağlar.The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylate monomer, 6-(4-cyanobiphenyl-4-oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer (GLC6) with polytetrahydrofuran grafts.The structure of the phases was identified by polarizing microscopy. The transition temperatures were also determined by polarizing microscopy, from changes in the characteristic texture of the phases observed under cross polarizers. We observed the phase transition sequence isotropic-nematic-smectic A-smectic C in the LC6 monomer. In LC6 monomer, the nematic-smectic A transition was found to be of second order as observed by Mc Millan ratio. The effective critical exponent of the order parameter was found to be near the nematic-smectic A transition. The value of the exponent was very close to the tricritical value. Nematic-smectic A transition is first order in PLC6 and second order in GLC6, with the transition temperature remaining the same. It has been argued that first-order phase transitions are converted to second-order phase transitions by the introduction of frozen disorder into a system. The effect of quenched random constraints introduced in GLC6 are consistent with the theory of quenched random interactions. Keywords: Photon transmission technique, liquid crystal homopolymer, graft copolymer, nematic order, quenched random constraints

Studying on critical exponents around glass transition; A fast transient fluorescence study

Serbest radikal çaprazbağ kopolimerizasyonu (FCC) ile elde edilen etilmetakrilat (EMA) kopolimerizasyonunda zaman ayrımlı floresans teknik kullanılarak camsı geçiş çalışılmıştır. Floresans molekül olarak Piren (Py) seçilmiştir.  Floresans bozunum eğrilerinden piren molekülünün floresans yaşam zamanı hesaplanmış ve camsı geçişin izlenmesi için kullanılmıştır. Camsı yapı oluşumu ile reaksiyon ortamının yoğunluğundaki değişim piren moleküllerinin floresans verimini etkiler. Bu etki farklı sıcaklık ve çapraz bağlayıcı konsantrasyonlarında EMA’nın camsı geçişinin çalışılmasına olanak sağlamıştır.  Camsı geçiş noktası civarında jel kesrine ve ortalama küme büyüklüğü ait kritik üsler b=0.36±0.002 ve  g=1.68±0.01 hesaplamış ve sonuçlar sızma teorisi ile uyum içerisinde bulunmuştur. Anahtar Kelimeler: Camsı geçiş, kritik üsler, zaman ayrımlı floresans teknik.The bulk free radical crosslinking copolymerization is divided into three different stages; low conversion stage, gel effect stage, and glass effect stage. The glass effect stage occurs as the last stage of polymerization, if the reaction temperature is lower than the glass transition point of the polymer. The glass transition during polymerization was studied in free-radical crosslinking copolymerization (FCC) of ethyl methacrylate, (EMA) using the fast transient fluorescence (FTRF) techniques. Pyrene (Py) was used as a fluorescence probe. The fluorescence lifetimes of Py from its decay traces were measured and used to monitor the glass transition. Changes in the viscosity of the pregel solutions due to glassy formation dramatically enhance the fluorescent yield of Py molecules. The reaction time at which the Py lifetime exhibit sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effcet. This effect is used to study the glassy formation of EMA, as a function of time, at various crosslinker concentrations and different temperatures.  The results were interpreted in the view of percolation theory. The gel fraction, b and weight average degree of polymerization, g exponents were measured near the glass transition point as b =0.36±0.002, g =1.687±0.01 and the results are found in agreement with percolation results. Keywords: Glass transition, critical exponents, time resolved fluorescence technique

Swelling kinetics of iota carrageenan gels: Fluorescence study

Fiziksel jeller, sıcaklık ve iyon etkisi gibi faktörlerin değiştirilmesi ile sol ve jel durumları arasında tersinir bir dönüşüm meydana getirirler. Gıda sektöründe katkı maddesi olarak, tıp ve ilaç sektöründe yapay organ uygulamalarında ve ilaç denetim sistemlerinde, tekstil sanayinde boyama ve giyim eşyalarında dayanıklılık arttırıcı olarak kullanılmaktadırlar. Birçok uygulama alanına sahip olan fiziksel jellerin yapısında bulunan başlıca biyopolimerlere örnek olarak hayvansal dokulardan elde edilen jelatin, deniz yosunu ve alglerden elde edilen karagenan ve agarose gösterilebilir. Karagenan, kırmızı deniz yosunundan elde edilen sülfatlı polisakkarittir. İçerdikleri 3.6 anhidro-D-galaktoz miktarı ve ester sülfat gruplarının sayısı ve konumunun farklı olması farklı tipte karagenanlar oluşmasına neden olur. Kappa, iota ve lambda olmak üzere üç temel çeşit karagenan vardır ve hepsi farklı jel özelliklerine sahiptir. Bu çalışmada fiziksel jeller sınıfından iota karagenan jelleri kullanılmıştır. Disk şeklinde hazırlanan iota karagenan jellerin şişme kinetiğine sıcaklık etkisini incelemek için floresans teknik kullanılmıştır. Floresans molekül olarak suda çözünebilen piranin kullanılmıştır. Floresans şiddet, I ve saçılan ışık şiddeti, Isc şişme zamanının fonksiyonu olarak izlenmiştir. Floresans şiddet zamanla artarken saçılan ışık ise zamanla azalmıştır. Su buharı altında şişen jellerde bu sonuçları açıklayabilmek için jellerde şişme kinetiğini tasvir eden Li-Tanaka denklemi kullanılmıştır. Sıcaklığın artması şişme zaman sabitinin azalmasına neden olurken kooperatif difüzyon katsayısı buna bağlı olarak artmıştır. Difüzyon katsayısının sıcaklıkla değişiminden Arrhenius bağıntısı kullanılarak şişme enerjileri bulunmuştur. Anahtar Kelimeler: Şişme, floresans, karagenan.Carrageenans are water soluble polysaccharides extracted from red algae. Their basic structure consists of a repeating unit of 3,6-anhydro-a-D-galactose and b-D-galactose. They are differentiated by the number and the position of ester sulphate groups and the amount of 3,6-anhydro-D-galactose which they contain. They come in three major types designated by means of Greek letters as kappa (k-), iota (i-) and lambda (l-). They are well known for their gel forming properties and are used extensively in food and pharmaceutical industry as gelling or thickening agents. Kappa carrageenan and iota-carrageenan undergo a temperature dependent coil (disordered state) to helix (ordered state) transition in aqueous solution. k-carrageenan usually forms firm, brittle gels and it is sensitive to potassium ions. i-carrageenan usually generates soft, elastic gels and it is sensitive to calcium ions. Iota carrageenan is the most highly sulphated of the helix-forming polysaccharides and is also the most fully characterized in the solid state and in solution (Morris etal., 1980). It is well known that iota carrageenan forms a three dimensional network constituted of polysaccharide chains structured as double helices. This gelation is induced by cooling heated suspensions of the polysaccharide in water under appropriate salt conditions resulting in a coil-helix conformational transition (Morris etal., 1980; Tako etal., 1987). It has been known that in solution, iota carrageenan can reversibly transform from an ordered to a disordered conformation. Naturally at high ionic strength and low temperature iota carrageenan forms an ordered state. On heating, the helices dissolve and the -carrageenan forms a random coil conformation (Rees etal., 1982). Intermolecular double helix formation should result in a doubling in the observed molecular weight of the -carrageenan, which has been observed in many groups (Austen etal., 1985; Morris etal., 1980). However some authors have proposed monomolecular single-helix formations (Norton etal., 1983). The swelling shrinking and drying kinetics of physical gels are important in many technological applications. Especially in pharmaceutical industries, in designing controlled release of drugs and in using cosmetic ingredients, understanding the kinetics of gels is highly desirable. The knowledge of the gel kinetics is an important requirement for producing storable foods in agricultural industry and developing artificial organs in medical applications. Many different experimental techniques have been used to study the kinetics of swelling and shrinking of chemical and physical gels such as neutron scattering (Bastide etal., 1984), quasielastic light-scattering (Peters and Candau, 1988), macroscopic experiments (Zrinyi etal., 1993), in situ interferometric measurements (Wu and Yan, 1987) and photon transmission techniques (Kara and Pekcan ,2000). A different method was employed namely the steady state fluorescence technique, to study the swelling of iota carrageenan gels. The fluorescence technique is efficiently used for analyzing the structural properties of gels. This technique ensures the fluorescence molecules added in a structure to be considerably sensitive to the structural properties of a system and provide information about the medium in which they are located. In this work, we studied the swelling process of i-carrageenan gels at various temperatures by using steady state fluorescence technique. The fluorescence intensity measurements were carried out with Perkin-Elmer LS-50 model spectrometer equipped with a temperature controller. Pyranine was embedded in i-carrageenan and used as a fluorescence probe. i-carrageenan gels were completely dried and then swelled in water vapor. The swelling experiments were performed at 30, 40, 50 and 60oC. At each temperature, the disc-shaped gel samples were placed in the wallof a 1x1x4.5cm3 quartz cell saturated with vapor for the swelling experiments. Scattered light intensities, Isc and fluorescence intensities, I were monitored during the swelling of i-carrageenan gels. The scattered light intensity, Isc decreased, however the fluorescence intensity increased exponentially as swelling time is increased. The increase in I was modeled using Li-Tanaka equation from which selling time constants, tc and cooperative diffusion coefficients, Dc were determined. It was observed that swelling time constants, tc decreased and cooperative diffusion coefficients, Dc increased as the swelling temperature was increased from 30 to 60oC. The swelling energies, DE were obtained by using Arrhenius relation and found to be 63.1kJmol-1. Keywords: Swelling, fluorescence, carrageenan

Modelling of swelling by the fluorescence technique in kappa carrageenan gels

Conference: International Congress on Advances in Applied Physics and Materials Science -- Location: Antalya, (Turkey) -- Conference date: 12–15 May 2011 -- Published in: AIP Conference Proceedings (2011)Kappa (-) carrageenan gels prepared with various carrageenan concentrations in pure water were completely dried and then swelled in water vapor. Steady state fluorescence measurements were performed using a spectrometer equipped with temperature controller. Pyranine was embedded in -carrageenan gels as a fluorescence probe during gel preparation. The fluorescence intensity, I, increased exponentially as swelling time is increased for all gel samples. The increase in I was modelled using Li-Tanaka equation from which swelling time constants, c and cooperative diffusion coefficients, Dc were determined. It was observed that Dc increased as the swelling temperature was increased. On the other hand at each temperature, it was seen that Dc decreased as kappa carrageenan concentration was increased. Activation energies for swelling were obtained and found to be 57.4, 58.3 and 62.73kJ mol-1 for the gels with increasing amount of - K-carrageenan content