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Головна
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Журнал Polymer
- 1994 рік
1994 рік
When butadiene is polymerized in dichloroethane using dibutyl magnesium plus neodymium versatate as catalyst, high-trans polymer is formed during the first few hours of polymerization, followed by slow polymerization of the remaining butadiene to high-cis polymer after 7-8 h. This change probably occurs as a result of slow reaction between catalyst and solvent.
Procedures for the preparation of poly(vinylidene chloride)/poly(butyl methacrylate) (PVDC/PBMA) composites based on concentrated emulsions are described. In one of them, two partially polymerized paste-like concentrated emulsions in water of two different systems were blended and subjected to complete polymerization. One of the systems was butyl methacrylate monomer (BMA), while the other was either vinylidene chloride monomer (VDC) or a dilute solution of PBMA in VDC. In the other procedure, a partially polymerized concentrated emulsion of VDC in water was blended with BMA monomer, and the paste-like system obtained was further polymerized. The composites thus obtained contained PVDC, PBMA and also some copolymer. The glass transition temperatures and the melting points measured by differential scanning calorimetry allowed qualitative identification of the presence of the above compounds. VDC-co- BMA copolymers of various compositions were also prepared via the concentrated emulsion method for comparison purposes. The effects of the preparation procedure, of the molar ratio of the two monomers and of the concentration of surfactant in the continuous water phase on the tensile properties were investigated. The toughness of the composites is much better than that of the copolymers.
The 'in situ" compatibilization of poly(ethylene terephthalate) (PET) copolymer (PETG) with ethylene vinyl acetate (EVA) copolymer via catalysed transesterification reactions was examined through rheological, morphological and mechanical studies. The addition of dibutyltin oxide catalyst was found to diminish the size of the dispersed phase, enhance its adhesion with the matrix and lead to an increase in the dynamic elastic modulus (G') with temperature. This improvement in adhesion also results in a considerable increase in the elongation at break, from 20 to 140% when 2.4 wt% of catalyst is added to a 70/30 PETG/EVA blend. The comparative behaviour of blends with and without catalyst is explained via copolymer formation at the interface and co-crosslinking of EVA and PETG chains.
Emeraldine base films cast from N-methylpyrrolidinone solutions are not readily doped by acids with large anions. Structural modifications can facilitate the access of these anions into the film matrix. The problems in determining the protonation levels at the surface and in the bulk are highlighted.
A new processing route for poly(2,6-dimethyl-l,4-phenylene ether) (PPE), an intractable polymer on account of its thermal and oxidative sensitivity, was explored. PPE can be dissolved at elevated temperatures in epoxy resin and these solutions can then be processed at temperatures as low as 175°C. For solutions of PPE with a molecular weight of 10, 20 and 30kg tool-1, the phase diagram and the flow curves in the homogeneous region were determined. The upper critical solution temperature (UCST) cloud point curves intersect the glass transition--composition lines at a PPE content of ~ 70wt%. Below this composition, thermoreversible gelation is observed upon cooling which prevents complete phase separation. Curing of the homogeneous solutions, using diethyltoluene diamine, resulted in virtually complete phase separation. In the composition range that was studied (30-70wt% PPE), the chemically induced phase separation is accompanied by phase inversion, yielding a final morphology of epoxy spheres dispersed in a PPE matrix. Thus, after processing, the (reactive) solvent is converted into a dispersed phase. The mechanical and thermal properties of the final materials, such as toughness and glass transition temperature, are dominated by the continuous PPE matrix.
The problem of using a polystyrene-standard-based calibration curve for other polymers has been overcome by developing an algorithm based on the concept of parameter estimation and optimization. The algorithm modifies the regression coefficients of the polystyrene-standard-based calibration curve by interactively using the chromatographic output and the absolute number-average molecular weight of the experimental polymer. Analytical expressions have been derived for calculating the directional derivatives, which ensure rapid convergence of the objective function. The algorithm, applied to commercial poly(vinyl chloride), offered a number-average molecular weight of 32 037 whereas that measured by membrane osmometry is 32 000. The polymer characterization parameters calculated from the optimized calibration curve closely matched those obtained from the universal calibration curve and Q-factor values. The algorithm needs no narrow standard of the experimental polymer, and holds for homopolymers, copolymers and polymer blends. It can be easily incorporated into commercial size exclusion chromatography data-reduction software packages.
In this communication, we report that aniline can be conveniently polymerized directly into conducting material using an emulsion polymerization technique. The polymerization is carried out in non-polar or weakly polar solvents in the presence of a functionalized protonic acid that acts simultaneously as a surfactant (emulsifier) and as a protonating agent (dopant) for the resulting electrically conducting polyaniline. It was found that the present direct, one-step preparation ofpolyaniline-protonic acid complexes in organic solvents may lead to polyaniline with significantly higher molecular weight than that prepared in acidic aqueous solutions. In addition, the present polyaniline-functionalized acid complexes displayed higher solubilities in non-polar or weakly polar organic solvents than corresponding complexes obtained by mechanically mixing the emeraldine base form of polyaniline with the protonic acid. Films cast from concentrated solutions routinely exhibit conductivities in excess of 100 S cm- '. Additionally, it was found that, using the emulsion polymerization route, polyaniline-functionalized protonic acid complexes offibrillar morphology can be produced, which have an exceptional degree of crystalline order and orientation.
An enhancement in the elastic response of two bacterial thermoplastic elastomers, poly(/~-hydroxyoctanoate), a saturated copolymer, and poly(/~-hydroxyoctanoic-co-undecylenic acid), an unsaturated copolymer, was attempted using peroxide crosslinking both with and without multifunctional co-agents. Sol-gel analysis verified that crosslinking had occurred and that crosslinking varied with peroxide type and concentration. A peroxide efficiency model suggested that the olefin group improved the peroxide efficiency and reduced the probability of chain scission. Differential scanning calorimetry showed that crosslinking could eliminate all crystallinity. The elastic response was improved as indicated by a reduced tensile set. In general, the crosslinked materials exhibited a decrease in tensile modulus, and a very low tensile strength and tear resistance
We have studied the crystallization of poly(ethylene terephthalate) during drawing at temperatures of 83, 90 and 96°C and at strain rates in the range 0.01-2.1s -1. Decreasing strain rate shifts the onset of crystallization to higher draw ratios and reduces the rate at which crystallinity increases with draw ratio, an effect that becomes more pronounced as draw temperature increases. When the degree of crystallinity is plotted against draw time, it becomes apparent that the effect of changing strain rate is simply to shift the curves along the log-time axis. The shift factor and the strain rate are related by a power law, and the value of the exponent, which reflects the rate of molecular relaxation at the draw temperature, rises with temperature at an increasing rate. Due to strain-rate/draw-time superposition, empirical equations permit prediction of the degree of crystallinity and the crystallization rate at any strain rate or temperature in the range studied from knowledge of draw time or draw ratio. The study also reveals that increasing draw temperature does not necessarily increase the draw ratio for onset of crystallization 2? as previously supposed: at sufficiently high strain rates, 2 c decreases with increasing draw temperature.
A catalytic amount of chlorodiphenylphosphine induced the polymerization of ~,fl-unsaturated esters. The polymerization was greatly accelerated by addition of a catalytic amount of epoxide
Корисні статті
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.
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.
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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.
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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.
