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Головна
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Журнал Polymer
- 1993 рік
1993 рік
To explain the characteristics of X-ray scattering by isotactic polystyrene dry gels, a coiled-coil molecular model is proposed instead of the previously reported almost fully extended 12/1 helix having sixfold helical symmetry (per dimer). The coiled-coil chain is assumed to have basically the same 3/1 helical structure of (-TG-)3 conformation as in the crystalline phase and the 3/1 helix axis itself again makes a large helix having a long pitch. The observed chain repeat length of 30.6 ,~ comprises six turns of the 3/1 helix (a total of 18 monomer units) in one turn of the large helix: the 3/1 helix axis itself makes a large helix of 4.08 A radius and tilts by about 40 ° from the direction of stretch. The coiled-coil can be expressed again in terms of a single 6/1 large helix in which the asymmetric unit comprises three monomer units in one turn of the original 3/1 helix. Since the 3/1 helix axis tilts by about 40 °, the axial length of the asymmetric unit becomes 5.1 ~ (6.65 ~ x cos 40 ): the strong 5.1 A meridional reflection comes from the asymmetric unit. Although the detailed structure cannot be defined at this stage, because of uncertainty about the unit cell and the solvent content which contributes coherently with the polymer to the X-ray scattering, the molecular transforms of several coiled-coil models possessing different helical senses seem to explain the characteristics of the X-ray scattering.
The transesterification reaction in the molten state of ester groups of ethylene vinyl acetate copolymers and ethylene acrylic ester copolymers, in the presence of dibutyltin oxide as a catalyst, leads to crosslinking of the copolymer chains. The kinetics of this crosslinking reaction has been assessed by two different experimental methods: viscoelastic measurements of the dynamic storage modulus and thermogravimetric analysis coupled with gas chromatography which allows measurement of the evolved methyl acetate which is a volatile product of the crosslinking reaction. The equilibrium storage modulus calculated from theories of elastic modulus has shown that the kinetics of the crosslinking reaction may be determined through variations of the dynamic storage modulus
Vinyl fluoride-trifluoroethylene copolymers were prepared by free-radical polymerization. The hysterisis loop of electrical displacement D versus electrical field E was obtained for 44 moi% vinyl fluoride copolymer. Values of the remanent polarization ranged from 20 to 50 mC m- 2. The temperature dependence of the dielectric constant indicated a dielectric anomaly with frequency dependence. The reciprocal dielectric constant was thus plotted in conformity with the Curie-Weiss law. The remanent polarization gradually decreased from room temperature and completely disappeared at about 100°C. A discontinuous change in the linear thermal expansion coefficient was observed at 100°C. It was found that this vinyl fluoride copolymer is ferroelectric at the Curie point. It was also found that the copolymer has a phase transition in relaxation that resembles second order.
Blends of poly(p-phenylene sulfide) (PPS) and polyethylene (PE) of various component ratios were prepared via melt mixing in a laboratory-scale internal mixer. Cryogenically fractured surfaces of these blends were examined by means of scanning electron microscopy. General morphological features including the domain size and the phase inversion process were discussed in terms of droplet dispersion during melt mixing and phase coarsening after cessation of flow. Interestingly, the surface of dispersed PE particles showed distinct features of protruding fibrils whereas the surface of dispersed PPS particles was characterized by an orange-peel appearance. These were explained in terms of the sequential crystallization of the two phases and the loose packing of PPS crystallites in the intermediate stage of solidification
The chemical structure of electrically conducting polypyrrole films has been studied for the first time by Fourier transform (FT)-Raman spectroscopy. The technique offers considerable signal-to-noise advantages over dispersive Raman spectroscopy and is useful for the analysis of weak scatterers including conducting polymers. Spectra of polypyrrole doped with p-toluene sulfonate and dodecyl sulfate are presented together with spectra obtained from the corresponding dedoped polymers. Samples were analysed in sealed glass containers and spectra obtained in 25 min. Analysis of the FT Raman spectra shows them to be consistent with polaron and bipolaron descriptions of the electron transport mechanism in polypyrrole.
The value of the interaction parameter X in dilute polymer solutions is usually higher than would be expected on the basis of simple theory. This is caused by a large entropic contribution, for which there is, as yet, no theoretical explanation. In this paper the results are presented of a study of the behaviour of the energetic and entropic contributions to Z. It is shown that the two contributions are closely correlated and exhibit qualitatively similar molecular-weight dependences. These observations may guide future theoretical efforts towards the understanding of the thermodynamic phase behaviour of polymer solutions
Acrylonitrile-butyl acrylate (A/B) copolymers of different monomer compositions were prepared by bulk polymerization using a free radical initiator. Copolymer compositions were determined by elemental analyses and comonomer reactivity ratios were determined using a non-linear least squares errors-in-variables model. Terminal and penultimate reactivity ratios were calculated using the observed monomer triad sequence distribution determined from 13C{IH} n.m.r, spectra. The triad sequence distribution was used to calculate diad concentrations, conditional probability parameters, number-average sequence lengths and block character of the copolymers. The observed triad sequence concentrations determined from 13C{1H} n.m.r, spectra agreed well with those calculated from reactivity ratios.
Tetraglycidyl-4,4'-diaminodiphenyl methane based resin with 30 phr diaminodiphenyl sulfone as curing agent was toughened with poly(ether imide) (PEI). The effects of morphology on the fracture toughness of modified epoxy resins were investigated. Morphology was controlled by changing the curing conditions. The co-continuous structure and morphology of the PEI spherical domain dispersed in the epoxy matrix were obtained. Phase-inversed morphology with PEI matrix was also obtained with 30 phr PEI content. The cured resin with phase-inversed morphology showed the highest fracture toughness. The modified epoxy resins with enhanced fracture toughness exhibited other improved mechanical properties such as flexural strength, flexural modulus and strain at break
High impact strength, high transparency cast sheets of polyurethane (PU)-reinforced poly(methyl methacrylate) (PMMA) were obtained by means of IPN techniques. The PU content was kept low (less than 10%). The impact strength is maximum at a critical PU concentration, and for a given morphology. The study of various synthesis parameters led to the conclusion that the size of interstitial PMMA nodules - and therefore the optical and mechanical properties of the final material - are determined by the crosslink density of the swollen PU network at the onset of the methyl methacrylate polymerization.
Three triblock copoly(oxybutylene/oxyethylene)s, EssBIvEss, E71BzsE7a and E132B53E132, with similar composition (75-80 wt% E) but different chain lengths, were prepared by sequential anionic copolymerization, and their micellar and gelation properties in aqueous solution were determined. The methods used were light scattering intensity, photon correlation spectroscopy and gel permeation chromatography, together with observation of gelation. Measurements were made at 30°C over the full concentration range from dilute solution into the gel. The lower and upper temperature boundaries of the gel regions were identified. Phase separation was observed in dilute solutions of the copolymer of longest chain length (E132B53E132 ). Critical micelle and gel concentrations decreased markedly as chain length was increased, and micellar weights and sizes increased
Корисні статті
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.
Як стати інженером?
Кожна людина в процесі свідомого життя стикається з проблемою вибору професії. Найбільш актуальною ця проблема є для учнів старших класів – випускників, які добровільно або примусово здають шкільні іспити та зовнішнє незалежне оцінювання, за результатами чого приймають участь в конкурсному відборі на навчання у ВНЗ. Щоб обрана професія не стала важким випробовуванням, потрібно ще у шкільні роки зважити всі «за» і «проти», оцінити свої здібності, схильності, можливості.
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).
Види та функції сучасної упаковки
Різноманітна упаковка щільно увішла у життя кожної людини. На полицях магазинів, в інтер'єрах помешкань можна побачити десятки пляшочок, коробок, аерозольних болончиків. Термін існування упаковки в нашому житті може продовжуватися від кількох хвилин до кількох років. Що ж таке сучасна упаковка? Чому вона займає стільки місця в нашому житті?
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.
