kpilogo shields

Crystallization processes of poly(ethylene terephthalate) (PET) when annealed at 80°C, or 5"C above the glass transition temperature, Tg, have been investigated by a small-angle X-ray scattering (SAXS) technique. At a very early stage of annealing, a scattering maximum appears at around Q = 0.04 ]~ 1, where Q is the length of the scattering vector (Q = 47r sin 0/2). During the so-called induction period of crystallization, this maximum intensity increases with annealing time and the maximum position shifts towards the low Q side. These results confirm the previously reported new finding that the long-range ordered structure is formed in the induction period before crystallization. These ordering processes can be divided into two stages: an early and a late stage. The scattering behaviour in the early stage is in accordance with the prediction of Cahn's linearized theory for spinodal decomposition. The scattering profiles in the late stage can be described in terms of Furukawa's scaling theory for the cluster growth regime. From these experimental results, we conclude that the growth process of the density fluctuation occurs in the induction period and this process is very similar to the spinodal decomposition type of phase separation process. It is considered that after this dense domain grows to a certain size, crystallization begins

Детальніше...  

The crosslinking mechanisms and kinetics of carbon fibre (four samples) and glass fibre (five samples) reinforced epoxy formulations subjected to various treatments were studied by 1H and 13C nuclear magnetic resonance spectroscopy (liquid and solid cross-polarization/magic-angle spinning) and Fourier-transform infrared spectroscopy, differential scanning calorimetry, high-performance liquid chromatography and Calvet microcalorimetry. By comparison with the results obtained in previous work on pure matrices, it was shown that the presence of fibres does not change the reaction mechanisms or the network structure. On the other hand, it was possible to show that the nature of the fibre reinforcement, the various treatments undergone, the formulation and heat cycles led to notable differences in the reaction rate, particularly at low temperature.

Детальніше...  

Racemic and meso D,L-lactide were polymerized at 90 or 120°C in xylylene or at 120, 150 and 180°C in bulk. Furthermore, copolymerizations of racemic D,L-lactide and L,L-lactide were conducted at 180°C in bulk. Tributyltin methoxide (Bu3SnOMe), dibutyltin dimethoxide (Bu2Sn(OMe)2) and Sn(II) octoate were used as initiators. Despite high yields only low molecular weights were obtained with both tin methoxides. Sn(II) octoate gave significantly higher molecular weights. The stereosequences were analysed by 1H and 13C n.m.r, spectroscopy on the basis of tetrad effects. The signal assignments are discussed. Bu3SnOMe and Bu2Sn(OMe)z are effective transesterification catalysts and cause 'back-biting' degradation even at 90°C. In all series of polymerizations initiated with tin methoxides two tendencies are detectable: increasing randomization of the stereosequence with increasing reaction time and with higher reaction temperatures. In contrast, Sn(II) octoate does not cause transesterification at ~< 120°C and even at 180°C randomization of the stereosequences is slow.

Детальніше...  

Dielectric relaxation measurements are reported over a frequency range of 0.01 -50 k Hz and from 100-490 K for a series of poly (~-dialkyl-fl-propiolactones) : poly (~-dimethyl-fl-propiolactone) (PPL), poly (a-methyl- ~t-ethyl-fl-propiolactone) (PMEPL) and poly (~-methyl-~-n-propyl-fl-propiolactone) (PMPPL). First of all, the three polyesters investigated show a high temperature fl-relaxation peak which is due to their glass transitions. Secondly, PPL and PMEPL exhibit a 7 relaxation peak, at 275 and 210 K, respectively, which is related to the crystalline fraction of the polyester since its intensity increases with the degree of crystallinity. PMPPL does not show such a relaxation due to its low degree of crystallinity. Thirdly, a weak 6 relaxation peak is observed at low temperatures (~ 130 K) for PMPPL and PMEPL. This peak is absent in PPL. This observation suggests that the 6 peak is due to ethyl and propyl group rotations around the main chain of the linear polyester in its amorphous fraction; PPL does not show this relaxation since it has only methyl substituents and a high degree of crystallinity. Finally, the fl, 7 and 6 relaxation data of these three polyesters were analysed using a method based on the frequency-temperature superposition principle and using the Cole-Cole representations

Детальніше...  

Isotactic polypropylene (iPP) melt-blown webs have been studied in terms of the relationships among the processing conditions, structure and mechanical properties. The melt-blowing process conditions investigated in this study were die and air temperatures, die-to-collector distance (DCD) and attenuation air flow rate at the die. The macroscopic web structure was characterized in terms of interfibrous bonding and fibre diameter using scanning electron microscopy. The degree of fibre entanglements was found to increase with decreasing DCD, with increasing die temperature, or with increasing air flow rate. Using wide-angle X-ray diffraction and differential scanning calorimetry (d.s.c.), the microscopic molecular structure of iPP has been investigated. The fibres in a web consist of the a-form crystal, smectic phase and amorphous regions. The crystallinity of the iPP webs obtained from d.s.c, was found to be similar (39-44%) since the smectic phase transforms to crystals on heating in a d.s.c. A wide range of tensile properties and flexural rigidity values in the machine direction and cross direction were obtained at various processing conditions. The results have been analysed in terms of the degree of fibre entanglements, fibre orientation in the machine direction and the fraction of smectic phase

Детальніше...  

The structure of poly(2,6-hydroxynaphthoic acid) (PHNA) is studied as a function of temperature using X-ray diffractometry. Similarities are observed between its behaviour and that of poly( 1,4-hydroxybenzoic acid) (PHBA), including a high temperature transition to a pseudo-hexagonal rotator phase, which has been commented on previously. There are, however, striking differences between the two polymers, most notably the presence of a melt phase in PHNA, above 440°C, which displays ordering similar to that observed in smectic liquid crystals. The phase observed in PHNA at a given temperature depends on thermal history so that, for example, the room temperature orthorhombic phase reported by MiJhlebach et al. is seen only after annealing, while the temperature of the transition to the pseudo-hexagonal phase decreases from 395°C, in our as-polymerized samples, to 300°C or lower, after heat treatment. The unit cell determined for the pseudo-hexagonal phase of PHNA has similar a and b parameters to those in the corresponding phase of PHBA. Implications of these findings for the structures of the copolymers of PHBA and PHNA are discusse

Детальніше...  

Curing experiments were carried out at several temperatures on blends of diglycidyl ether of bisphenol A (DGEBA) epoxy resin with 34.4 phr of 4,4'-diamino-3,3'-dimethyldicyclohexylmethane (3DCM) hardener and 15wt% carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) rubber. Simultaneous measurements were made of turbidity and of dielectric properties over the frequency range 0.1 to 20 kHz. Changes in viscosity, gel fraction and enthalpy were determined in separate experiments. The results were compared with those for the unmodified resin-hardener mixture. In CTBN blends, relative permittivity e' at low frequencies rose sharply during phase separation, owing to an interfacial polarization process, which can be described by the Maxwell-Wagner-Sillars model: relaxation times z are in the order of 1 ms. In the neat resin, e' fell continuously throughout the cure over the whole frequency range, the fall being particularly steep as the resin gelled and began to vitrify. It is concluded that dielectric techniques provide an effective method for monitoring phase separation, gelation and vitrification in resin blends.

Детальніше...  

The thermal properties of several families of polystyrene-poly(phenylene oxide), sulphonated polystyrene- poly(phenylene oxide), polystyrene-sulphonated poly(phenylene oxide), and sulphonated polystyrene- sulphonated poly(phenylene oxide) blends were studied by differential scanning calorimetry. The appearance of a single glass transition temperature, i.e. blend miscibility, depended on the level of sulphonation of one or both components of the blend. Even though blends formed from the unfunctionalized components were miscible over the complete composition range, miscibility was reduced as the sulphonation level increased in either component. A substantially broader range of miscibility was observed when both blend components were functionalized. Compared with blends containing one sulphonated component, where immiscibility could occur within a relatively modest sulphonation range (typically 2-4 mol%), blends with two sulphonated components retained miscibility over a substantially broader sulphonation range (typically 10 mol% ). The results are interpreted in terms of the mean field theory involving interaction parameters for the individual monomer segments

Детальніше...  

Poly(ether ether ketone) copolymers (I-V) were prepared by the nucleophilic reaction of 4,4'- difluorobenzophenone with varying mole proportions of hydroquinone and resorcinol using sulpholane as solvent and in the presence of K2CO 3. The polymers were characterized by different physico-chemical techniques. The glass transition temperature and crystallinity of the polymers was found to decrease with increase in concentration of the resorcinol units in the polymers. Thermogravimetric studies showed that all the polymers were stable up to 400°C with a char yield above 50% at 900°C in nitrogen atmosphere. The isomeric polymers (I and V) showed distinct differences in their thermal degradation behaviour and the pyrolysis-g.c, data indicated the random nature of the copolymers (II-IV). The amount of resorcinol evolved during pyrolysis at 800°C was correlated with the sequence distribution of the comonomers in the polymer.

Детальніше...  

An i.r. study has been made of the conformational transitions which accompany the tensile drawing of poly(ethylene terephthalate) (PET) and the non-crystallizing polyesters, poly(ethylene methyl terephthalate) (PEMT) and a 60/40 PEMT/PET copolymer. For a given level of overall orientation, measured by birefringence, the gauche conformers were converted into trans more effectively and the trans conformers were more highly oriented in PET than in PEMT or the copolymer. The gauche~tram conformational change also occurred more effectively in hot drawing compared with cold drawing for PEMT and its copolymer. These differences in drawing behaviour were reflected in the development of elastic modulus with molecular orientation in these materials.

Детальніше...  

Корисні статті

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.

Рейтинг вищих навчальних закладів

На даний час в світі існує маса університетів з дуже великою кількістю кваліфікацій, спеціальностей та спеціалізацій. Одні з них більш престижні університети, інші менш.

Рейтинг вищих навчальних закладів переписується щорічно, в зв'язку з тим, що всі прагнуть стати краще в освіті, вдосконалитися в технологіях і підвищити свій рівень акредитації. Рейтинг навчальних закладів варіюється в залежності від предметної області, це природничі науки і математика, техніка/технологія і інформатика, життя і сільськогосподарська наука, клінічна медицина і фармація, соціальні науки.

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).

Хімічне машинобудування

Хімічне машинобудування багатопрофільна галузь машинобудування, що поєднує в собі природні та експериментальні науки (наприклад, фізика і хімія), разом з науками про життя (наприклад, біологія, мікробіологія та біохімія). Математику та економіку вокористовують для розробки, перетворення, транспортування, управління виробничими процесами, які перетворюють сировину в цінні продукти.