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Two epoxy networks, differing significantly in their glass transition temperatures (Tss), were subjected to uniaxial compression tests in the glassy state (20°C). One of the systems was based on the diglycidyl ether of bisphenol-A (DGEBA) cured.with ethylenediamine (EDA). The other one was based on an epoxidized novolac (EPN) cured with 4,4'-diaminodiphenyl sulfone (DDS). Both systems were physically aged by specific thermal treatments. The endothermic enthalpy relaxation peak, characteristic of aged glasses, could be erased at temperatures well below T~ by large mechanical deformations, i.e. close to the incipient strain hardening level in uniaxial compression tests. This phenomenon, which was accompanied by an increase in specific volume, constitutes a clear manifestation of rejuvenation produced by large mechanical stimuli. The validity of statements presenting an opposite point of view is also discussed.

Детальніше...  

The study of the phase behaviour of syndiotactic polystyrene (sPS) in o-xylene is reported. Differential scanning calorimetry, microcalorimetry, infra-red spectroscopy, wide angle X-ray scattering, transmission electron microscopy and thermogravimetric analysis have been used. The polymer chains can adopt a T2G2 helix or a planar T4 zig-zag conformation, depending on the experimental conditions. The initial concentration, the cooling rate and the thermal history strongly influence the formation and the relative contribution of the different conformations, leading to a complex phase behaviour. A decrease of the polymer concentration, and/or an increase of the scanning rate, tends to favour the formation of the helical structure. This helical conformation is always formed when a vitrified, concentrated solution is heated. The transformation of the helical conformation into the planar zig-zag conformation proceeds by a two-step mechanism of melting and recrystallization. Solvent-induced crystallization of amorphous, glassy sPS results in the formation of the helical conformation. The zig-zag conformation is always formed by crystallization from the pure melt. The formation of an incongruent polymer-solvent compound is proposed to explain the complex phase behaviour.

Детальніше...  

The location of the wetting transition in symmetric polymer blends is calculated as a function of the degree of incompatibility and the difference in surface energy between the pure components, using mean-field (square-gradient) theory, coupled with a lattice model for the bare surface energy. The conditions for second-order wetting are discussed.

Детальніше...  

Monte Carlo simulations of isolated ring polymers and pairs of catenated ring polymers were done on a simple cubic lattice. Differences in static and dynamic properties of these systems are presented and discussed

Детальніше...  

An epoxy resin based on diglycidyl ether of bisphenol A has been modified with poly(ether sulfone) (PES) and poly(acrylonitrile-co-butadiene) rubber (NBR) and cured with 4,4'-diaminodiphenylsulfone. The fracture behaviour of the ternary blend was examined using a fracture mechanics approach. Depending on the combination of PES and NBR, two distinct morphological states were shown, and the fracture behaviour varied from brittle to ductile with changing morphology. From fracture surface observations by scanning electron microscopy, the ternary blends that showed ductile fracture behaviour have a phase-inverted morphology in which the NBR-rich phase formed a continuous matrix, the epoxy-rich phase formed particles and the PES-rich phase also formed particles around the epoxy particles. Dynamic mechanical thermal analysis revealed that the ternary blends exhibited high Tg, as high as the T B of the unmodified epoxy resin

Детальніше...  

In the work described in this paper the dielectric properties of solid solutions of p-nitroaniline in poly(methyl methacrylate), poly(ethyl methacrylate) and poly(n-butyl methacrylate) were measured in the frequency range from 20 to 106 Hz and the results obtained were compared with those of the pure polymers. It is shown that the presence of the solute has a strong influence on the relaxation process of poly(methyl methacrylate), and this is ascribed to the formation of hydrogen bonds between the amino group of the solute and the side groups of the polymer. In poly(ethyl methacrylate) this effect is less pronounced and it is absent in the case of poly(n-butyl methacrylate), suggesting that the increasing size of the n-alkyl group prevents hydrogen bond formation between the solute and the polymer.

Детальніше...  

Interpenetrating hydrogel networks based on poly(vinyl alcohol) (PVA) and 2-hydroxyethyl methacrylate have been synthesized. Water absorption and desorption properties of these hydrogels were analysed extensively. The influence of solutes (urea, potassium iodide, D-glucose, sodium chloride) on equilibrium water content (EWC) of these gels was also studied. Incorporation of PVA into poly(2-hydroxyethyl methacrylate) in the form of an interpenetrating polymer network (IPN) increases the EWC. The existence of different classes of water in this IPN system is shown by water melting curves using differential scanning calorimetry. The morphological features of these films were studied by scanning electron microscopy

Детальніше...  

The isothermal degradation of ethylene-propylene elastomers, ethylene-propylene rubber and ethylenepropylene- diene terpolymer, in four NaCI aqueous solutions (0%, 0.1%, 1.0% and 10%) was investigated. The film specimens were examined by i.r. spectroscopy to detect the evolution of oxygenated group content. It was shown that the lower the salt concentration, the more oxidative damage takes place in the polymer. There is a large discrepancy between the carbonyl and hydroxyl formation rates of these two materials. The ethylene-propylene copolymer is more oxidatively stable than its corresponding terpolymer

Детальніше...  

Negative spherulites and non-birefringent spherulites (the latter arising in two different temperature regions) and the so-called aggregates of nylon 66 have been reproduced and the conditions of their formation have been redefined. Growth rates were measured and the dependence on crystallization temperature (T~) of both growth rates and spherulite forms were studied. Unusual effects were observed in the T~ range where the growth of negative and positive spherulites compete. The most conspicuous, qualitatively new observations relate to crystallization memory effects arising from the presence of negative (and non-birefringent) spherulites. It is found that the localities of such spherulites retain a memory, i.e. they remain morphologically distinct after melting and recrystallization in specific ways, more precisely dependent on the highest melt temperature to which the sample had been exposed with conspicuous consequences for the final crystalline texture. The sum total of observations suggests a gradation of non-equilibrium melt states beyond the optical melting point (as opposed to residual seed crystals) which influences subsequent crystallization, with some suggestions with respect to this melt structure being made.

Детальніше...  

Chain addition polymerization reactions involve active intermediates (i.e. free radicals) whose total concentration at all times is very small. In copolymerization, in contrast to homopolymerization, one can identify two radical populations, 'P' and 'Q', ending in an M1 or in an M2 monomer unit, respectively. As a result, two distinct quasi-steady-state approximations (QSSAs) can be made. The first QSSA is applied to the total radical concentration and implies that the total rate of radical initiation is approximately equal to the total rate of radical termination. The second QSSA is applied to the separate radical populations P and Q and states that the rate of reaction of a P-type radical with an M2 monomer is equal to the rate of reaction of a Q radical with an M1 monomer. However, whether or not both approximations are applicable to a given copolymerization system is a point of concern. In the present study, the validity of the two QSSAs is examined. The conditions under which both approximations can be applied to a given copolymerization system are identified, through the application of the method of moments to several experimentally investigated free-radical copolymerization systems leading to linear and branched copolymers. It is shown that the validity of the QSSAs in free-radical copolymerization depends on the polymerization conditions, namely, the polymerization process (e.g. bulk versus solution) and the importance of diffusion-controlled reactions at high monomer conversions. However, the effect of both approximations on the final molecular properties is, in general, not significant

Детальніше...  

Корисні статті

Комп'ютер для інженера

У сучасному світі комп'ютери дуже поширені. Складно уявити людину, не знайому з цим поняттям. Багато професій зобов'язані своїм виникненням саме комп'ютеру, вони б просто не з'явилися без створення електронно-обчислювальної техніки.

І хоча відносно недавно, на початку XX століття, комп'ютери були розкішшю і використовувалися лише для самих складних розрахунків, у наш час комп'ютери та комп'ютерна техніка дуже глибоко інтегрувалися у наше життя. Сучасне людство залежить від комп'ютерів, що викликає подиву, якщо розглянути, коли і в яких випадках вони використовуються.

Хто такий інженер

Інженер - професія нелегка, але одночасно з цим дуже цікава і захоплююча. Адже інженер це людина, у якого народжуються в голові нові ідеї і тому він здатний винаходити.

У багатьох виникає питання: хто такі інженери? Інженер (франц. Ingénieur) - фахівець з вищою технічною освітою. Спочатку інженерами називали людей, які керували військовими машинами. Поняття громадський інженер з'явилося в XVI столітті в Голландії, застосовано до сфери будівництва мостів і доріг, потім інженери з'явилися в Англії, а потім в інших країнах.

Полімерні матеріали

Полімер це велика молекула, або макромолекула, котра складається з багатьох субодиниць. Через їх широкий спектр властивостей, синтетичні і природні полімери відіграють найважливішу і всюдисущу роль в повсякденному житті. Полімери в діапазоні від знайомих синтетичних пластмас, таких як полістирол природний біополімер, таких як ДНК і білки, які є основоположними для біологічної структури і функцій. Полімери, як природні і синтетичні, створюються за допомогою полімеризації багатьох малих молекул, відомих як мономери.

Інженер-конструктор

Хто такий інженер-конструктор? Даним питанням задаються багато людей, які бажають пов'язати своє життя з цією професією. Варто відзначити, що ця професія однією з найбільш високооплачуваних на сучасному ринку праці, яка характеризується високим попитом з боку роботодавців. Інженер-конструктор машинобудування повинен володіти аналітичним складом розуму, підвищеною уважністю до деталей і відповідальним підходом до роботи. Дана діяльність пов'язана з прорахунками і різноманітним обладнанням. Першокласний інженер-конструктор механік володіє також такими рисами характеру, як раціональність і ерудованість. Важливу роль відіграє стресостійкість, адже робочий процес є досить трудомістким і при потребі замовника вимагає готовності швидко вносити зміни в готові креслення.

Що таке КПІ?

На сьогоднішній день багатьох випускників, ще недавно – школярів, цікавить наступне питання – куди поступити, куди піти навчатися? В нашій країні є дуже багато ВНЗ, які пропонують свої послуги з підготовки і навчання студентів. Одним з таких ВНЗ є Київський політехнічний інститут (КПІ).