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A binary copolymer chain is treated theoretically in the approximation of a first order Markov process. The microstructure of the chain is described in terms of the mole fraction of the dilute monomer and an intermonomer correlation coefficient. The resulting equation for the sequence distribution shows some symmetry in both parameters. It thus offers the possibility to separate influences of copolymer composition on one hand and the distribution of the monomer run lengths along the chain on the other hand. The connection of this formalism to n.m.r, dyad analysis and the classical copolymerization parameters is shown.

Детальніше...  

A novel thermoforming processing route has been developed that produces a microporous form of ultra high molecular weight polyethylene that demonstrates large negative Poisson's ratios. The microstructure consists of nodules interconnected by fibrils. Poisson's ratios as low as - 1.2 have been obtained, depending on the degree of anisotropy in the material.

Детальніше...  

The structure of adsorbed silane films formed by the adsorption of trichlorosilane molecules containing long alkyl chains from solution onto aluminium substrates has been investigated. The structure of the adsorbed films is dependent on the alkyl chain length. When the chain length is equal to or greater than 18, the molecules form a film in which the hydrocarbon chains are densely packed together and orientated away from the substrate. As the chain length is reduced the films become progressively more disorientated. When a terminal vinylic group is present on the molecules, the films can be activated after adsorption to yield a hydroxyl group that is available for further reaction with a polyurethane resin. The ability of 10-undecenyltrichlorosilane and 18-nonadecenyltrichlorosilane to act as adhesion promoters has been investigated using the blister test. Both these silanes yielded adhesive fracture energies of 18 J m -2 under dry conditions. When water was present, only the joints pretreated with 18-nonadecenyltrichlorosilane exhibited a high resistance to moisture attack. In these joints the dense packing of the hydrocarbon chains impeded the diffusion of water to the interface. In the case of the adsorbed 10-undecenyltrichlorosilane films, the disorientated nature of these films allowed the water to reach the interface more easily, resulting in rapid joint failure.

Детальніше...  

Functional polystyrenes fitted with hydroxyethyl groups at chain ends or as statistically distributed pendent functions along the polymer backbone were prepared, using anionic methods. Functionalization of the polystyrene chain was achieved by metallation followed by the reaction of the metallated sites with ethylene oxide and subsequent protonation. Fractionation of the resulting polymer was required to eliminate branched species formed in a side reaction. The functionality of the polymers was accurately determined by u.v. spectrometry on the labelled samples.

Детальніше...  

The crystal structure of the low temperature phase of poly(di-n-hexyl silane) (PdnHS) was determined by X-ray diffraction techniques. The backbone chain conformation was found to be all-trans and the unit cell to be orthorhombic with dimensions a = 1.376, b = 2.386 and c = 0.399 nm. There are two molecules in a unit cell belonging to the Pna21 space group. Intermolecular interactions between side chains appear to be predominantly responsible for the all-trans backbone. Intramolecular crystallization between the side chains of a molecule does not seem feasible.

Детальніше...  

Ellipsometry was used to estimate the interface thickness 2 between two bulk layers of dissimilar polymers, poly (methyl methacrylate) (PMMA) and poly (styrene-co-acrylonitrile) (SAN), as a function of annealing time t above the glass transition temperature Tg. The value of 2 was estimated to be around 20 nm at an early stage of annealing and increased with time, e.g. up to 60 nm at 130°C after 12 h for a miscible pair such as PMMA and SAN-25 (AN content = 25 wt%), while it remained constant for an immiscible system such as PMMA/SAN-5, even after long annealing for more than 12 h. For miscible systems, the mutual diffusion coefficient D was obtained from the slope of the 2 vs. t 1/2 plot according to the Brochard theory: 2 = 2(Dr) 1/2. The D value was found to be roughly proportional to the thermodynamic driving force IZ - gel, as has also been predicted by Brochard (Z and Zc being the Flory interaction parameter at the annealing temperature and at the critical point, respectively). The molecular-weight dependence of D was found to follow the 'slow theory'. Further, to analyse the adhesion development, the results for 2(t) were combined with the time variation of adhesive strength a(t) observed by Fowler et al. It was found that the late stage could be described well by the reptation theory, tr oc 21/2, while the adhesive strength tr of a thin interface at an early stage was much higher than the theoretical value

Детальніше...  

The preparation of graft copolymers of styrene, acrylonitrile and n-butyl acrylate on to a polybutadiene rubber by emulsion polymerization and up to high degrees of conversion are described and investigated theoretically. The resulting graft copolymers have a higher styrene content and a lower acrylonitrile content than predicted by theory, whereas the n-butyl acrylate has the expected content. Impact and notched impact resistances, bending and tensile strengths, ball-puncture resistance, Vicat softening temperature and glass transition temperatures of the graft copolymers have been measured and are discussed.

Детальніше...  

A new finding is reported concerning some structural change in the amorphous state of the polymer which occurs during the so-called induction period before the start of crystallization. Annealing of poly (ethylene terephthalate) (PET) at 115°C, or 40°C above the glass transition temperature, Tg, has been investigated by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. At a very early stage of the induction period the SAXS intensity starts to increase in the range of Q = 0.02-0.04 ,~- 1, where Q is the length of the scattering vector (Q = 4~ sin 0/2), and continues to increase until the beginning of crystallization. Corresponding to the initation of crystallization, another strong peak, caused by the usual long-period structure, appears in the vicinity of Q = 0.05 A- 1. It is confirmed from the WAXS measurements that no local ordering takes place during the induction period. To clarify the crystallization process, distance distribution analysis is also carried out

Детальніше...  

Three glycidylether epoxy resins with functionalities of 2, 3 and 4 were purified and extensively characterized. The cure of the resins with diaminodiphenylsulphone, both alone and as blends, was studied using rheometry and differential scanning calorimetry. The effect of resin functionality on the rate of cure was greater than would be expected from simple concentration effects. The kinetics were best described by an autocatalytic model over approximately the first 40% of reaction. Thereafter an nth order expression gave a better fit to the data. The modulus crossover point, tw=w, was found to be a reproducible indication of gelation, and the fractional conversions at this point, obtained by combining d.s.c, and rheology data, were compared to the values predicted by gelation theory. A model was used to predict the viscosity of samples during cure as an aid to processing

Детальніше...  

Ethylene carbonate was bulk-polymerized via a ring-opening mechanism using monoethylene glycol (MEG) as initiator and sodium stannate trihydrate as heterogeneous catalyst. Due to the loss of CO2 from a fraction of the repeat units during polymerization, a copolymer comprised of ethylene oxide and ethylene carbonate repeat units, poly (ethylene ether carbonate ) (PEEC) resulted. The purified polymers were shown to be hydroxy-terminated by acetylation of the chain end and subsequent titration. Molecular weights higher than Mn ~ 4600 g mol- ~ were not attainable due to a prevailing level of MEG present adventitiously in the system. PEEC-based polyurethane networks resulted from the reaction of four different molecular weight PEEC diols with a trifunctional isocyanate, triphenylmethane triisocyanate. Network characterization included equilibrium swelling, determination of sol content by solvent extraction, glass transition (T s) by differential scanning calorimetry, and tensile properties by stress-strain measurements. A network curing schedule of 7 days produced networks with high sol contents (10%o). Extension of the curing schedule to 19 days produced networks with low sol contents (2-5%) with the minimum sol content centred about a stoichiometric ratio, OH/NCO, of approximately one. Tensile strength ranged from 1.3 to 12.3 MPa, modulus ranged from 1.5 to 26 MPa, and strain at break ranged from 60 to 240 for networks from PEECs of -M, in the range of 730-4400 g mol- 1. Tensile properties, Tss and equilibrium swelling measurements indicated that at increased isocyanate content the crosslink density of the network increased, possibly due to side reactions such as isocyanurate or allophanate formation.

Детальніше...  

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.

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?

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.

Інженер-машинобудівник

Ні для кого не секрет, що при сучасних умовах життя, темпах розвитку промисловості, безперервній автоматизації та оптимізації роботи механізмів та виробничих процесів, великою популярністю та попитом на ринку праці користується професія інженера, особливо інженера-машинобудівника.

Щоб відповісти на питання «Хто такий інженер-машинобудівник?», необхідно розуміти , що несе в собі кожне з цих слів окремо. Інженер – це людина, яка отримала освіту з визначеного фаху. Інженер – це творець техніки. Інженер – це особа, що професійно займається інженерією, тобто на основі поєднання прикладних наукових знань, математики та винахідництва знаходить нові рішення технічних проблем. Тобто, виходячи з цих загальновживаних визначень слова «інженер» зрозуміло, що цій професії може присвятити себе лише людина з неабиякими здібностями, які ґрунтуються на знанні точних наук, логічному мисленні, невичерпному терпінні і постійному бажанні вдосконалювати світ інженерії. Від латини ingenium — здатність, винахідливість, що є свідченням того, що інженером перш за все є людина-думаюча, яка знаходиться в безперервному пошуку відповідей на складні технічні завдання.

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.