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Earlier estimates for the Young's modulus of polyethylene based on the AM1 semi-empirical molecular orbital method were 15% larger than the largest experimental value. Neglect of electron correlation in the single determinant wavefunction for polyethylene leads to a Hessian matrix which is too stiff. Here we apply the biased Hessian approach of Dasgupta and Goddard 1 to the polyethylene chain at equilibrium and under tensile and compressive strain. The chain modulus is 340GPa with the biased Hessian, in agreement with extant experimental results. Although the variation of the vibrational spectrum with strain is reduced with biasing, the stress-dependence of vibrational frequencies is unchanged.

Детальніше...  

Polymer blends of various compositions were analysed by liquid chromatography at the critical point of adsorption. By operating at the critical point of one blend component, it is possible to separate the blend regardless of the chemical structure and the molar mass of the second blend component. In addition, the molar mass and the molar-mass distribution of the second blend component may be determined via a conventional size exclusion chromatography procedure. It is shown that, in addition to homopolymer blends, blends comprising a copolymer as one component may be separated.

Детальніше...  

Fourier transform infra-red (FTi.r.) spectroscopy was used to study the thermal degradation that occurs in composite materials consisting of poly(ether ether ketone) (PEEK) reinforced with carbon fibres. Spectra were measured by diffuse reflectance for samples of prepreg and moulded laminate exposed to temperatures in the range 400-485°C for different periods of time in both air and nitrogen atmospheres. In spite of some spectral distortion arising from front-surface reflection, useful information can be obtained concerning the reactions that occur and their relative rates under different conditions. The reactions and their activation energies are found to be similar to those observed for neat PEEK film, described in a previous publication. In an inert atmosphere, the degradation involves a pyrolytic-type mechanism which produces a new carbonyl species absorbing at 1711 cm- 1, probably a fluorenone-type structure; the activation energy for this process is around 240 kJ mol- 1. In an oxidizing atmosphere, the same species is produced but at a faster rate, with an activation energy of around 200kJmol-1. In addition there is a second reaction which requires the participation of oxygen and produces a species absorbing at 1739 cm- 1, probably ester groups; the activation energy for this reaction is about 130kJmol-L Some samples were also analysed by photoacoustic spectroscopy. Within the limits of the technique, no significant difference could be discerned between the degradation behaviour of PEEK-carbon composites and that of neat PEEK films.

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Natural jute fibres mercerized by 20% NaOH treatment and by liquid ammonia reverted to the natural morphology on washing with water. Mercerization treatments produce more amorphous content than cellulose I to cellulose II conversion. Equatorial X-ray diffraction patterns obtained from these specimens were analysed by a function that best fits the profile giving appropriate weights to the Cauchy and Gaussian components in the peaks. Particle sizes were calculated after applying the Stokes deconvolution method to correct instrumental broadening. The difficulty in mercerizing this natural fibre is attributed to the higher lignin and hemicellulose contents that surround the cellulose I, thus preventing swelling, which is the starting point for mercerization.

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Poly(styrene-b-methyl methacrylate)s were analysed by liquid chromatography at the critical point of adsorption. By operating at chromatographic conditions corresponding to the critical mode for poly(methyl methacrylate) and the size exclusion mode for polystyrene, the molar mass and the polydispersity of the polystyrene block in the block copolymers may be determined. The data were found to be in excellent agreement with the corresponding parameters of the polystyrene precursor homopolymers

Детальніше...  

The transport of liquids into semicrystalline thermoplastic resins such as poly(aryl ether ether ketone) (PEEK) is an extremely complex process, markedly dependent upon the temperature, crystallinity and concentration of the penetrant at the surface. Similarities and differences in the sorption, desorption, resorption, swelling and solvent-induced crystallinity of toluene and carbon disulfide into amorphous and semicrystalline PEEK are reported. The transport of these two liquids into PEEK illustrates many of the complexities associated with non-Fickian diffusion: sharp penetration front, slow relaxation processes, solvent-induced crystallinity and anisotropic swelling and deswelling. Desorption is a two-step process in which 30-50% of the penetrant is rapidly desorbed in a few hours. The rate of CS 2 sorption and desorption is much greater than that of toluene. The rate of resorption is significantly less than that of the initial sorption but the desorption process is essentially unchanged.

Детальніше...  

Microstructural analysis was carried out on five isotactic polypropylenes produced with different heterogeneous Ziegler-Natta catalyst systems and different polymerization processes. The samples were fractionated according to stereostructure using a direct extraction method. Pentad tacticities of unfractionated samples and selected fractions were determined by 13 C n.m.r, spectroscopy. I.r. isotacticities were also measured. Statistical analysis of the polymerization mechanism was carried out on the basis of the two-site model from the pentad tacticities, and average lengths of meso and racemic sequences were determined. In addition to atactic and isotactic materials produced at different catalytic sites, the polypropylene samples contained syndiotactic material as stereoblocks in isotactic chains. On the basis of the analysis carried out on the fractions, slight differences in the microstructure were found to exist between the samples produced by different processes and catalyst types. These differences have a significant effect on average sequence lengths

Детальніше...  

In the present paper we examine the dielectric properties of solid solutions of p-nitroaniline in a poly(vinyl acetate) matrix in the frequency range from 16 to 105 Hz and the results are compared with those for pure poly(vinyl acetate). It is shown that the presence of the solute increases the activation energy of the ~t relaxation process of the polymer and broadens the distribution of relaxation times for this process. It is suggested that these observations can be interpreted in terms of the formation of hydrogen bonds between the solute and the side groups of the polymer

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The majority of existing models for processing of thermoset matrix composites concentrate nearly exclusively on heat and mass transfer problems and account for material-related aspects in a very simplified manner. The objective of this work is to provide that missing element by proposing a clearly defined system which yields all the information of Gillham's T-T-T diagram but can be used for non-isothermal curing cycles. To build the system, three elements are needed: the gelation and vitrification curves, expressed as functions of degree of conversion; the kinetic model of conversion; and a diagram combining all complex information provided by the system. The proposed system for Narmco resins incorporates Cole's recently developed mechanistic model for the kinetics of conversion. The algorithm calculates and graphically presents the progress of thermoset processing using the developed diagrams which reintroduce time as an independent variable. The diagrams and the system may be applied to any processing cycle. Results obtained from the system are positively verified by experiments conducted on the Rheometrics dynamic analyser

Детальніше...  

In the so-called in situ sequential interpenetrating polymer networks (IPNs), the two networks are formed after each other, and the network formed first is thought to impede gross phase separation in the final material. This is contrary to the other type of IPNs (in situ simultaneous), in which the formation of both networks is initiated at once and proceeds to completion more or less simultaneously. In order to verify more accurately this assumption, which is not inconsistent with transmission electron microscopy findings, a solid-state nuclear magnetic resonance lineshape analysis technique has been used to investigate the degree of phase dispersion of IPNs of both types composed of an elastomeric polyurethane (PUR) (25 wt%) and a crosslinked poly(methyl methacrylate). The results confirm that such IPNs, when prepared sequentially, have a higher degree of phase dispersion than those obtained by the simultaneous synthesis method. Furthermore, in the corresponding neat PUR networks, built up from aromatic pluriisocyanate and poly(oxypropylene glycol) (POPG), the rigid crosslink points are not composed of isocyanate only, but include some oxypropylene mers; it appears that the amount of the rigidified part is the same, whatever the molecular weight of POPG.

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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.

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).

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.

Вибір професії

Кожна людина зіштовхується у своєму житті з вибором, який найсильніше вплине на все її подальше життя. Йдеться про вибір професії та вибір вищої освіти. Закінчуючи школу, молоді люди стикаються з величезним вибором професій та спеціальностей: інженер, економіст, юрист, менеджер, маркетолог, логіст, фінансист і т.д. При цьому навколо можна чути безліч стереотипних фраз: "Юристи багато заробляють", "Фінансисти працюють з грошима, тому у них хороші зарплати", "Маркетолог - основний людина в будь-якому бізнесі", а часом і просто без обґрунтування - "Менеджер - це круто ". Часом, такі "поради" впливають на вибір професії.

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.