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Bifunctional monomers of new formulations have been synthesized and tested for potential use in dental composite filling materials. The monomers were designed to have lower viscosity than the standard basic monomer, thus requiring less dilution with low molecular weight monomer to achieve useful viscosities. It was hypothesized that the presence of the diluent is a main source of adverse effects on curing shrinkage and water sorption. The new materials were diluted to viscosities equivalent to commercial blends and tested for curing shrinkage, water sorption and wetting ability. Shrinkage was determined from the density differences between monomers and polymers, and polymer water sorption was determined gravimetrically. Wetting ability was measured by contact angle against human dentin and enamel as well as poly(methyl methacrylate) and glass. The newly synthesized monomers did exhibit reduced viscosity and required less dilution. They exihibited less curing shrinkage and water sorption. The contact angles showed little change from current materials.

Детальніше...  

A high-density polyethylene (HDPE) homopolymer was blended with a linear low-density polyethylene (LLDPE), i.e. an ethylene-hexene copolymer, over the composition range of 0-100%. The resistance to slow crack growth was measured by a notched tensile test under a constant load. The times to failure increased from about 102 min to 10 s min as the composition increased from 0 to 100% of the copolymer. In the range of compositions below 50% of the copolymer, the time to failure increased relatively slowly with composition compared to the very rapid rate of increase for compositions greater than 50% of the copolymer. The results were explained in terms of a network formed by the crystals and tie molecules that contained short-chain branches. The network becomes continuous when the copolymer is the major component and consequently the resistance to slow crack growth increases rapidly

Детальніше...  

A series of copolyimines from terephthalaldehyde and 1,8-diaminooctane and 1,10-diaminodecane is synthesized. X-ray diffraction and cross-polarized microscopic studies reveal that all the copolyimines exhibit a liquid crystalline smectic phase as does one of the parent homopolyimines, PIt0. The transition temperatures and the associated enthalpy and entropy change continuously as functions of copolymer composition. The crystalline and liquid crystalline phases of the copolyimine series are isomorphic. The copolyimines are soluble in chloroform and tetrahydrofuran.

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A polymer/ceramic composite grain boundary capacitor has been fabricated using a semiconducting ceramic as the conducting phase (reduced barium titanate) and a thermoplastic polyimide as the insulating polymer phase (LaRC-TPI). The ceramic grains and the composite capacitors were characterized using scanning and transmission electron microscopy. The dielectric properties of the capacitors were also measured using an impedance analyser. A composite grain boundary capacitor with a dielectric loss below 0.02 was successfully manufactured. The room temperature dielectric constant of the capacitors ranged from below 30 to approximately 2000, with the value dependent upon the ratio of the diameter of the semiconducting ceramic grains to the thickness of the insulating polymer layer coating the grains.

Детальніше...  

Poly (phenylene sulfide) (PPS) and its derivatives, poly (phenylene sulfide-co-phenylene ketone) (PPSK), poly(phenylene sulfide-co-phenylene ether) (PPSE), poly (biphenylene sulfide ) (PBPS) and poly (phenylene sulfide-co-phenylene sulfone) (PPSS), were synthesized and their thermal properties were investigated by differential scanning calorimetry and thermogravimetric analysis. PPSK, PBPS and PPS show high crystallinity, in that descending order, while PPSS is totally amorphous. Semi- and non-crystalline PPS/PPSS polymers were melt blended. The blends showed relatively high miscibility and thermal stability. The T~ of PPS/PPSS blends shows 10 and 45°C inward shift; the melt crystallization temperature (Tmc) of PPS decreased to 227°C in the PPS/PPSS blend compared to 244°C for the PPS homopolymer, as determined by differential scanning calorimetry in a cooling scan.

Детальніше...  

The free-volume distribution in various polymers and the effect of hydrogen bonds on the free-volume distribution were examined by using a photoisomerization of azobenzene as a photosensitive probe. Poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) and phenoxy resin films containing molecularly dispersed azobenzene were irradiated over a wide temperature range (77-340 K). The final cis fraction for the trans to cis photoisomerization of azobenzene decreases with decreasing temperature in PMMA, and smaller fractions are produced than in polycarbonate at the same temperature. These differences are due to the fact that the local free-volume fluctuation of the methylene main chains of PMMA is smaller than that of the aromatic chains of polycarbonate. Since the side chains of PMMA compared to the main chains are larger than those of polycarbonate, the local free volume around the main chains is filled up by the side chains. Hence, the final cis fraction of azobenzene in PMMA is small. Furthermore, the final cis fraction in poly(vinyl alcohol) is smaller than that in PMMA. Since the hydrogen bonds among the polymer chains in poly(vinyl alcohol) restrict the local molecular motion at low temperatures, the azobenzene molecule is tightly seized by the matrix polymer chains and the free-volume fluctuation is restricted in poly(vinyl alcohol). The kinetic model considering the local fluctuation of the free volume was also applied to the results for PMMA, poly(vinyl alcohol) and phenoxy resin, and the parameters for the model in these polymers are discussed.

Детальніше...  

This work describes the crystallization behaviour and morphological features of the semicrystalline polyimide known as LARC-CPI. The study considers several features affecting crystallization behaviour such as inherent viscosity, crystallization temperature, melt temperature and time in the melt. Crystallization kinetics data were analysed using an Avrami analysis in conjunction with differential scanning calorimetry. Complications encountered when evaluating the crystallization exotherm for this analysis are also discussed. Morphological features were investigated using several techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS). A permanganic etching technique combined with SEM successfully revealed morphological detail further supported by TEM results. This technique is also used to estimate the isothermal linear crystalline growth rate, G. The SEM and TEM data lend support to the Avrami data obtained by crystallization kinetics. SAXS results reflect the influence of crystallization temperature on the long spacing. SAXS also reveals the presence of a broad second scattering peak for semicrystalline samples which appears in the same position regardless of crystallization temperature or inherent viscosity. Molecular modelling predicts a low-energy helical conformation with a near-periodic repeat distance corresponding to that of the second SAXS peak. The periodic character of this conformation is offered as a hypothesis for that peak. This work also considers the effect of potential nucleating agents on the recrystallizability of a higher inherent viscosity LARC-CPI. One of the several nucleators investigated appears effective in enhancing the bulk crystallization rate.

Детальніше...  

The dynamic viscoelastic properties of carboxymethylcellulose sodium salt (NaCMC) were measured isothermally as a function of relative humidity (r.h.). The dynamic modulus decreased with increasing r.h. and dropped markedly at about 70% r.h. In the tan 6 curve, a small peak appeared at about 20-30% r.h. and a large peak was observed at 80% r.h. To study the mechanism of these relaxations, the dynamic modulus was measured isothermally as a function of time at constant r.h. and the data were analysed in terms of the kinetics. Below about 70% r.h. bound water may be formed, having an activation energy of 15.0-15.6 kJ mol- 1. On the other hand, above 70% r.h., two types of mechanism may occur simultaneously: formation of bound water around hydrophilic groups with activation energy of 15.6 kJ mol- 1, and formation of free water with activation energy of 13.0kJmo1-1. Consequently, the tan 6 peak occurring at about 20-30% r.h. may be caused by the onset of local molecular motion of the hydrophilic groups, and the peak at about 80% r.h. by the onset ofmicro-Brownian motion of chain molecules plasticized with free water

Детальніше...  

Electroinitiated polymerization of bis(2,4,6-trichlorophenoxo)bis(pyridine)copper(II) complex was achieved in dimethylformamide by constant potential electrolysis under air and N2. The polymers obtained at several potentials were characterized by 1H n.m.r., laC n.m.r., FTi.r. and d.s.c., and were found to be branched. The kinetics of the polymerization were followed by in situ cyclic voltammetry measurements at 13.5 and 20°C during electrolysis. As found for the thermal polymerization of this complex, electroinitiation begins after 50 rain at 13.5°C, whereas the induction period is negligible for electrolysis at 20°C

Детальніше...  

A series of high-molar-mass and controlled-molar-mass poly(arylene ether ketone)s were synthesized based on bis(p-fluorobenzoyl)aryl monomers and 4,4'-isopropylidenediphenol. The central aromatic unit of the activated bishalide was varied to include 1,4-phenylene, 2,5-thiophene and 1,3-phenylene, which changed the exocyclic bond angle from 180 ° to 148 ° to 120 °, respectively. The decrease in exocyclic bond angle decreased the glass transition temperature of the uncontrolled-molar-mass polymers from 166°C to 156°C to 154°C, respectively. The T~ of the controlled-molar-mass polymer decreased from 160°C to 147°C to 149°C for the same series, i.e. poly(1,4-BFB-BisA), poly(BFTh-BisA) and poly(1,3-BFB-BisA), respectively. The discrepancy between the two series was interpreted in terms of molar-mass differences. The thermal stability of the three polymers also decreased in the order of decreasing exocyclic bond angles. Young's modulus increased as the exocyclic bond angle was decreased, which is explained in terms of the activation theory of Eyring. These three polymers represent a new approach for the study of structure-property relationships in poly(arylene ether ketone)s.

Детальніше...  

Корисні статті

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.

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.

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.

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.

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.