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The effects of gamma irradiation under vacuum on the thermal properties and crystallization behaviour of poly (ether ether ketone) (PEEK-a, amorphous; PEEK-c, crystalline) have been investigated by d.s.c. The glass transition temperature for PEEK-a and PEEK-c increased on gamma irradiation, indicating that crosslinking is the main radiation mechanism for PEEK. The heat and temperature of crystallization for PEEK-a slightly changed on irradiation, indicating that the thermal crystallization process was inhibited by crosslinks because of the restriction in chain mobility. The heat and temperature of recrystallization for PEEK-a and PEEK-c on cooling from the molten state also decreased on irradiation and this is good evidence for crosslink formation in the PEEK structure. The disorder in chemical structure caused by crosslinking resulted in a lowering of the heat and temperature of melting. The effect of crystalline domains on the recrystallization behaviour of PEEK-a and PEEK-c was investigated and the effect of irradiation on the isothermal crystallization of PEEK-a and PEEK-c was also studied

Детальніше...  

The kinetics of bulk free radical polymerization of sytrene is studied with the symmetrical bifunctional initiator 2,5-dimethyl-2,5-bis(2-ethyl hexanoyl peroxy)hexane. The initiator, monomers and live polymers undergo complex initiation, propagation, chain transfer and chain termination reactions due to the presence of two labile peroxide groups of equal thermal stabilities in the initiator. The effects of reaction temperature and initiator concentration on the polymerization rate and polymer molecular weight properties have been investigated. It is shown that the composition of a polymerizing mixture is influenced by the initiator concentration and reaction temperature. The apparent initiator efficiency factor was found to decrease as the initiator concentration increased. A detailed kinetic model is also presented and used to analyse the fate of peroxide groups during the course of polymerization.

Детальніше...  

The aggregation states of a series of comb-shaped polymers with perfluoroalkyi side chains were investigated. The main chains of the comb-shaped polymers were poly (fumarate), poly (acrylate) and poly (methacrylate). D.s.c. and w.a.x.d, measurements and p.o.m, observations over a wide temperature range showed that all the poly(fluoroalkylfumarates) (PFAF) investigated were amorphous polymers, but poly(fluoroalkylacrylate ) (PFAA) and poly (fluoroalkyimethacrylate) (PFAMA) having a heptadecafluorodecyl (C a F 17 ) group in the side chain were in a mesomorphic state at room temperature. The type of mesophases can be most likely classified as an ordered smectic liquid crystalline (LC) structure in the Smectic B (SB) modification for PFAA and an unordered smectic LC structure in the Smectic A (SA) modification for PFAMA. Increasing the main chain flexibility in going from poly(fumarate) to poly(methacrylate) and poly(acrylate) enhances the probability of ordered structure formation

Детальніше...  

Structural changes occurring in the ferroelectric phase transitions of vinylidene fluoride-tetrafluoroethylene (VDF-TFE) copolymers with VDF contents of 81-0 mol% have been investigated by X-ray diffraction and i.r. and Raman spectroscopies. In a VDF 81%-TFE sample, the conformational change between the trans (low temperature) and gauche (high temperature) phases occurs in the temperature region close to the melting point with thermal hysteresis. The VDF 75%-TFE sample shows a similar transition between the low and high temperature phases but via the disordered cooled phase: the behaviour is close to that observed for vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer with VDF 65 mol%. As the VDF content decreases, the transition becomes more diffuse: for the VDF 64%-TFE sample the trans-gauche conformational change occurs gradually over a wide temperature region with almost no detectable hysteresis. The X-ray fibre diagram taken at room temperature is of the cooled phase, which reversibly transfers into the low temperature phase by the application of tensile force. The same phenomenon is observed for VDF-TrFE copolymer with VDF 37%. In this way the transition behaviour of VDF-TFE copolymers with VDF contents of 60-80% corresponds to that of VDF-TrFE copolymers with lower VDF content. As the VDF content decreases, the low temperature phase is stabilized more due to the effect of the TFE monomeric units. The as-drawn VDF 41%-TFE sample shows an X-ray fibre pattern characteristic of the low temperature phase, which transforms irreversibly to the cooled phase by heating above 110°C. Tensile force causes the transition to the original low temperature phase irreversibly. VDF 23%-TFE sample exists in the stable 'untilted' all-trans chain conformation above room temperature. Below -40°C, the X-ray fibre diagram changes into an overlap of the patterns of the poly(vinylidene fluoride) planar zigzag structure and of the poly (tetrafluoroethylene) helical structure. The transitional behaviour has been discussed in comparison with that of VDF-TrFE copolymers and the factors governing these structural phase transitions have been investigated

Детальніше...  

A range of poly(butadiene-stat-acrylonitrile), BAN, copolymers with compositions ranging between 17.8wt% and 66.9wt% acrylonitrile, have been mixed with three chlorine containing (~56wt% CI) polymers: conventional poly(vinyl chloride), PVC, chlorinated poly(1,4-butadiene) and chlorinated polyethylene, CPE. The polybutadiene which has been chlorinated at the double bonds can be regarded as head-to-head PVC, (H-H) PVC, whereas the CPE is a random copolymer of head-to-tail PVC, (H-T) PVC, and (H-H) PVC. Thus the samples which contain the same amount of chlorine will vary only in its sequence distribution along the chain. When these were blended with BAN samples, the extent of the miscibility ranges established were found to increase in the order: (H-T) PVC

Детальніше...  

The diffusion coefficient for CO 2 in both unoriented and highly drawn films of polyethylene has been measured using Fourier transform i.r. spectroscopy and the ratio of the solubilities has also been found. In addition, scanning i.r. spectroscopy has been used to characterize the degree of orientation of the CO 2 molecules in the films.

Детальніше...  

Normal-modes calculation was made for a series of model polymer chains of vinylidene fluoride tetrafluoroethylene (VDF-TFE) copolymers so as to interpret the systematic variation in the i.r. and Raman spectra. The model chains are alternating copolymers of [-(VDF)m(TFE),- ] with m/n = ~/0, 8/2, 5/5, 2/8 and 0/~. Utilizing the normal-modes frequencies and the corresponding eigenvectors calculated for these model chains, the frequency-phase difference dispersion curves and the density of state (or the frequency distribution function) were constructed. Based on these results, the assignments of the intrinsic bands of the VDF sequence, TFE sequence and VDF-TFE boundary were made and the changes in the i.r. and Raman spectra observed at room temperature were interpreted.

Детальніше...  

The partial ozonolysis of three random butadiene/acrylonitrile copolymers I-III, with butadiene contents of 78, 67 and 55 mol%, respectively, has been performed in order to form low-molecular-weight products by cleavage of double bonds along the main chain. The fragments thus obtained have been directly analysed by both positive and negative fast atom bombardment mass spectrometry (f.a.b.-m.s.). According to a random distribution of butadiene (Bt) and acrylonitrile (AN) subunits in the copolymer chains, peaks due to fragments with different Bt/AN ratio in the molecule have been detected in the mass spectra. The relative abundances of the species having the same total number of AN and Bt subunits, detected in the mass spectra of ozonolysed copolymers I-III, have been compared with those calculated according to a theoretical combination of AN and Bt units. Good agreement has been found between the two sets of data. A graphical procedure for rapid determination of Bt/AN copolymer composition by f.a.b.-m.s, data has been also developed

Детальніше...  

End-group 1-cyano-l-methylethyl resonances from copolymers of phenyl methacrylate or methyl methacrylate with styrene have been examined using a phase-sensitive two-dimensional 13C-1H shift-correlation experiment. Proton shift values for the methyl resonances, totally obscured on one-dimensional 1H spectra, could be identified and microstructural information observed.

Детальніше...  

The fine structure of poly (amic acid) (PAA) and the structural changes caused by thermal imidization are investigated by 13 C n.m.r, spectroscopy of fluorinated PAA and polyimides cured under various conditions. Split signals reflecting the isomerism of PAA are assigned by measuring model compounds, and the proportions of isomeric units are estimated. Signals assigned to the end groups of PAA are found in the spectrum of samples cured at 120°C. Depolymerization at amide groups thus occurs at low curing temperatures and the molecular weight of the polymer decreases considerably. This low-molecular-weight polymer is, however, converted into high-molecular-weight polyimide by subsequent curing at 300°C. These depolymerization and polycondensation processes are also supported by measuring the viscosity of PAA and polyimide solutions

Детальніше...  

Корисні статті

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.

ВНЗ України

Вища освіта є невід'ємним елементом перспективного кар'єрного росту, тому перед кожним абітурієнтом виникає проблема, в які інститути подавати документи. Варто відзначити, що в Україні існує велика кількість вузів. Всі навчальні заклади поділяються на державні та приватні, пропонуючи різноманітні освітні програми по різних профілів. Щоб пошук інститутів дав задовільні результати, слід визначитися з найбільш прийнятними спеціальностями. Також підбір університету передбачає вибір підходящої форми навчання, наявність високої акредитації у вузу і рівень його престижності.

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.

Інженер-механік

Інженер-механік (від лат. Ingenium – талант, обдарованість, і mēchanicus – механік) – це технічний чи технологічний фахівець з вищою освітою, який застосовує отримані знання для конструювання, проектування, моделювання та експлуатації машин, апаратів та технічного обладнання в різних галузях сільського господарства та технічного виробництва. Першими з інженерів були саме механіки; вони розробляли і збирали різноманітні машини і механізми, в яких використовували принципи і закони механіки.

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).