1994 рік
The morphology of melt-crystallized poly(vinylidene fluoride) (PVF2) fractions and whole polymers is presented for a wide range of crystallization temperatures (To). In both cases, at each isothermal T~ an increase in size of ~t-spherulites with increase in head-to-head (H-H) defects of the chain was observed. However, at the same T~, for some fractions with a higher concentration of defects (> 5.27% defects) the spherulitic radius decreased with increasing defect concentration, and in some cases a disordered morphology was also observed. Possible explanations are offered for these exceptions. When quenching from the melt to room temperature, a decrease in spherulitic size with increasing H-H defect content was found for both the fractions and the whole polymers. When comparing isothermally crystallized samples under the same undercooling, similar deterioration of spherulitic size and morphology with increasing defect structure in the chain was found. A general explanation involving the inclusion of H-H defects into the lamellae is suggested. The spherulites of whole polymers are larger than those of the fractions and a possible explanation for this is offered.
High-quality natural-abundance ~SN cross-polarization/magic-angle spinning (c.p.-m.a.s.) spectra of urea-formaldehyde (UF) resins were obtained by employing a large-volume (2.5 cm 3) magic-angle spinning (m.a.s.) rotor. 15N c.p.-m.a.s, nuclear magnetic resonance (n.m.r.) spectroscopy can clearly distinguish dimethylene ether linkages from methylol groups for UF resins prepared under neutral or basic conditions, a weak point in ~3C c.p.-m.a.s, strategies because of 14N broadening effects, and can detect the existence of tertiary amides and unreacted primary amides in UF resins. 'SN c.p.-m.a.s.n.m.r, also establishes the fact that there are methylol aggregates that are spatially separated from some other structural moieties in UF resins and that the domain size of methylol aggregates diminishes with the concentration of methyloi groups in UF resins. 15N c.p.-m.a.s, results also indicate that the formation of dimethylene ether linkages is least favourable at pH 9 and most favourable at pH 12 among the three pH values 7, 9 and 12.
Air- and ammonia-plasma treatments markedly improve the adhesion of gel-spun high-modulus polyethylene (PE) tapes to epoxy resin, as measured by pull-out and evidenced by the change in failure mode from interface controlled to shear failure within the PE. By monitoring the changes in wetting, surface topography and surface composition, the factors responsible for the adhesion could be identified. These results suggest that for air-plasma-treated PE tapes the adhesion depends on (1) the physicochemical interaction between the various carbon-oxygen functionalities and the epoxy resin, (2) mechanical interlocking, and (3) non-polar dispersion forces. The pull-out strength is the sum of these three terms and their contributions in this specific system are about 76%, 12% and 12%, respectively. Physicochemical interactions of the amine groups, in addition to non-polar dispersion forces, determine the adhesion of ammonia-plasma-treated PE tapes to epoxy resin. Their contributions are 87% and 13%, respectively.
Sodium 2-[~o-methacryloyl oligo(oxyethylene)] ethylsulfonate has been synthesized and polymerized to prepare a polymeric solid electrolyte with Na ? single-ionic conductivity. The length of oligo(oxyethylene) side chain determines the glass transition temperature and salt concentration of the homopolymers, and consequently influences the conductivity. The highest conductivity of 3.5 x I0- 6 S cm- 1 at 25°C is obtained when the number of CH2CH20 repeat units in the side chain is equal to 14. The temperature dependence of the ionic conductivity shows a curved Arrhenius plot, suggesting that ionic conduction is closely associated with segmental motion of the polymer host. The Williams-Landel-Ferry (WLF) empirical equation, which is based on the free-volume theory, is found to be valid for describing the conductivity data, and the obtained WLF parameters are comparable with the universal values.
We report optical scattering and birefringence observations on molten ultra-high-molecular-mass polyethylene (UHMMPE) and correlate these observations with a structure in the melt relating to the original reactor powder. In the quiescent state, pressure annealing can cause the observed light scattering to disappear; however, subsequent flow of the material causes the scattering to develop again. A simple microrheological model is proposed to explain some of the structure, pressure annealing and flow effects that have been observed.
This work deals with the properties of the temperature-concentration phase diagram of ternary polyacrylonitrile solutions that were mainly observed from the transverse magnetic relaxation of protons , attached to polymer chains, used to disclose the presence of solid-state domains. The binary solvent was a mixture of dimethylformamide (DMF) and water; the molar fraction of monomeric units was varied from 0 to 0.5, while the relative molar fractions of DMF and water were l:l or 2:1. The solvent quality was found to decrease upon addition of water. A curve (liquidus) that relates the temperature of formation of solid-state domains to the initial polymer concentration in solution was drawn from nuclear magnetic resonance (n.m.r.). The demixing part of the liquidus, determined as a plateau from n.m.r., was identified on observing a cloud-point curve. The main feature of the phase diagram concerns the specific behaviour of solutions observed whenever the concentration of disordered segments becomes lower than half the initial polymer concentration.
This study, summarized in two parts, is directed at the investigation of the non-linear performance of Nomex, Kevlar and polypropylene fibres, when undergoing stress relaxation tests. The description of the strain-dependent performance is based on a two-component model, which comprises an elastic and a viscous contribution. A shape-invariant relaxation function is introduced that is based on the cumulative log-normal distribution. This first part is directed at the description of the experimental procedures and of the approach to the analysis of the stress relaxation data.
Semiempirical molecular orbital calculations have been used to investigate the effects of protonation on the behaviour of the structural repeat units of poly(p-phenylene benzobisthiazole), PBZT, and poly(p-phenylene benzobisoxazole), PBO. Single and multiple protonations of the heterocyclic rings at the nitrogen, oxygen and/or sulfur atoms were examined. It was found that protonation at the nitrogen was more favourable than at the oxygen or sulfur atoms. Delocalization of the charge into the phenyl ring resulted in a higher bond order for the inter-ring bond. While the energy barrier for torsional rotation increased with protonation, the resistance to bending the rings in an out-of-plane fashion decreased somewhat. Results of the semiempirical calculations indicated that protonation of the oxygen in PBO might result in opening of the five-membered heterocyclic ring by breaking the carbon-oxygen bond. Ab initio calculations on relevant models, however, did not support this result.
A very simple plastic deformation process was applied to thin films of ultra-high-molecular-weight polyethylene (UHMWPE) prepared from dilute solution and mounted on standard 100 mesh electron microscopy grids in order to study the changes in supermolecular morphology which occurred during necking. Starting from an as-cast spherulitic type of microstructure, processes such as macromolecular reorientation, the destruction of lamellar crystallinity and the initial stage of microfibril formation were directly observed in the transmission electron microscope. Microfibrils were found to be composed of 10 nm thick crystals, in contrast to an initial lamellar thickness of 20 nm for the original microstructure. It is believed that the new crystalline morphology was generated by a process of complete lamellar destruction followed by the stress-induced recrystallization of extended chain segments
Styrene/methyl methacrylate (75/25) monomer mixtures were copolymerized at 323 K in the presence of a polybutadiene seed latex using either a water soluble (potassium persulfate) or an oil soluble (cumene hydroperoxide) initiator. The polybutadiene seed latex was made via an emulsifier free emulsion polymerization process. The graft copolymerizations were performed over a range of emulsifier concentrations. The emulsifier free polymerization with cumene hydroperoxide shows a very high degree of grafting while neither secondary nucleation nor coagulation was observed. Furthermore, the graft polymers have been extensively studied in terms of intra- and intermolecular microstructure. For this purpose, the breaking of the polymer backbone by means of ozonolysis was necessary. The grafted styrene/methyl methacrylate copolymer is less rich in styrene than the free copolymer. The chemical composition distributions of the graft copolymer were broader than those of the free copolymer.
Корисні статті
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.
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?
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.
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.
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.