1993 рік

The toughening effects of chromium and cobalt acetylacetonate (Cr (acac)3 and Co (acac)3 ) additives on diglycidyl ether of bisphenol A/diethylenetriamine (DGEBA/DETA) and tetraglycidyldiaminodiphenylmethane/ diaminodiphenylsulphone (TGDDM/DDS) epoxy resins were investigated and correlated with the changes of their fracture surface morphology. According to fracture tests of their compact tension specimens (CTS), Cr(acac)3 had much higher toughening effects than Co(acac)3. Scanning and transmission electron microscope investigation of their fracture surfaces implied that the epoxy resins incorporated with Cr (acac)3 had more cohesive structure than those incorporated with Co (acac)3 or neat resins. The results have been further supported by gel permeation chromatography and wide-angle X-ray scattering analyses, which revealed that Cr (acac) 3 could interact with hydroxy groups in the epoxy networks and reinforced the structure

Детальніше...  

It has been found that electron beam irradiation of physically aged, glassy polymeric films results in an apparent reversal of physical ageing (i.e. 'deageing') as determined by differential scanning calorimetry and yield stress behaviour. This is the first reported observation of 'deageing' in glassy polymers that does not involve heating above the glass transition or an imposed deformation process. Effects of dose level and dose rate on the physically aged state of polystyrene, poly(methyl methacrylate) and l~Olycarbonate have been determined. The specific deageing behaviour has been found to be material-dependent and the extent of deageing is proportional to the total absorbed dose. The experimental results support a proposed hypothesis that this deageing phenomenon is the result of radiation-induced internal gas evolution, which produces an increase in free volume and corresponding reversal of physical ageing.

Детальніше...  

The influence of the type of chain length regulator on the molecular weight, mechanical properties and solution viscosity of poly(urethane ureas) (PUUs) was investigated. As a model, a PUU based on poly(oxytetramethylene)glycol, 4,4'-diphenylmethane diisocyanate, a mixture of ethylenediamine as chain extender and propylene-l,2-diamine as co-chain extender was used. The monoamines diethanolamine (DEA), ethanolamine (EA) and 3(5)-methylpyrazole (3-MP) were compared as chain length regulators. Additionally, model reactions between phenyl isocyanate, n-butylamine and the three monoamines were performed to obtain data about the reactivity and possible side reactions of this mixture. These investigations were carried out by quantitative h.p.l.c, analysis. The lower reactivity of 3-MP led to a minor increase in the molecular weights of the PUUs due to further NCO-conversion, whereas all the mechanical properties of the different polymers were nearly identical. In contrast to 3-MP, however, the application of DEA or EA gave significantly lower solution viscosities, obviously due to the hydroxyl groups of these agents.

Детальніше...  

An epoxy resin system based on a diglycidyl ether of bisphenol-A type difunctional epoxy resin containing different amounts of a tetrafunctional curing agent, namely metaphenylene diamine, was used to prepare two types of specimen. First, an aluminium strip was coated with a thin layer of the epoxy-amine mixture and then subjected to two curing-cooling cycles. Second, bulk specimens were cast in the form of sheets which were cut into rectangles and their dynamic torsional spectra obtained from -100 to 180°C. In the resin-coated metal plate, the internal stress was measured by the bending beam technique and was found to be highest in samples with high glass transition temperatures (Tgs). This is apparently because the internal stress arises due to the difference in the thermal contraction coefficients between the resin and the metal and develops as the sample cools from the Tg to room temperature. The rate of stress relaxation in these samples is also high, apparently because they are the farthest from their equilibrium state and so have higher free volume and a more open structure. In the case of cast sheets, the internal stresses are again the highest in samples with high Tg but arise from non-uniform cooling. An intermediate relaxation peak, the ~' peak, observed in these samples has been shown to be related to the frozen-in internal stresses which relax on sub-Tg ageing due to free volume collapse.

Детальніше...  

Two linear low density polyethylene (LLDPE) samples containing n-hexyl and iso-butyl branches have been etched with fuming nitric acid and the products of the etching analysed by 13C n.m.r, spectroscopy to determine the concentration of branches. Different weight loss behaviour was observed with the two LLDPEs; the weight of material lost during etching plateaued at 50% with a constant low crystallinity for the iso-butyl material while the n-hexyl continued to lose weight and the residue improve in crystallinity. The residual concentration of branches was 10 and 20% of the initial concentration and clearly the two types of branches were incorporated into the crystalline lattice to different extents. The parent copolymers were fractionated by degree of branching and a comparison made between the two sets of fractions to determine their melting points, degree of crystallinity and unit cell volume. In general it was concluded that more of the hexyl branches are incorporated than the iso-butyl at the same branch content, and that chain flexibility is an important parameter in determining the extent of exclusion of the branches

Детальніше...  

The present paper describes the periodate oxidation of pullulan. Special attention is paid to the structures and characteristics of the activated products. Pullulan contains three different anhydroglucoside moieties in the repeating unit. Therefore, periodate oxidation of pullulan results in different types of dialdehyde structures. From titrimetric analysis of the formic acid generated during the oxidation of pullulan, the numbers of singly and doubly oxidized anhydroglucoside units can be calculated. The aldehyde content in completely periodate-oxidized pullulan as determined by reaction with hydroxylamine hydrochloride is only 67%. A plausible explanation is the presence of a stable, six-membered hemiacetal. The presence of aldehyde groups with different reactivities is also evidenced by the data collected from reduction experiments and by ~3C n.m.r. Finally, the hydrolytic stability of the polysaccharide backbone was studied.

Детальніше...  

The effects of temperature and initiator on the curing kinetics of unsaturated polyester resins at 30-50°C were studied by d.s.c, and i.r. spectroscopy over the whole conversion range. It was found that at the beginning of reaction, the kinetic mechanism was dominated by near-azeotropic copolymerization, while styrene conversion became much more favourable than that of polyester C=C bonds later in the reaction. The transition point of the near-azeotropic copolymerization and the crosslink length of styrene varied with the concentrations of initiator and promoter used. The experimental results have been explained by a kinetic model relating to microgels.

Детальніше...  

Diffusion and sorption of eight aliphatic alcohols -- namely methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-l-propanol, 1-pentanol and 3-methyl-l-butanol -- into three engineering polymer membranes has been studied in the temperature range 25-60°C to investigate the kinetics of the transport mechanism. The results have been analysed in terms of the Fickian diffusion equation. The anomalous transport behaviour of rubber-alcohol systems is attributed to a leaching out of the polymer systems during solvent immersion. The dependence of transport coefficients and activation parameters, as obtained from a temperature dependence of these coefficients, have been discussed in terms of the polymer-solvent interactions and their effect on the size, shape and type of penetrant molecules. Furthermore, the transport results have been interpreted in terms of enthalpy and entropy contributions. Dependence of the transport parameter on penetrant concentration has also been investigated

Детальніше...  

The present paper describes the chloroformate activation of pullulan by reaction with 4-nitrophenyl chloroformate. N.m.r. analysis indicated that the ester formation takes place preferentially at the C6 hydroxy groups of pullulan. It was further demonstrated that activation gives linear carbonates, aliphatic carbonates including cyclic five-membered carbonates, and non-strained aliphatic carbonates. All of these carbonates react with amines to yield the corresponding urethane derivatives. The 4-nitrophenyl chloroformate activation of pullulan is an easy method for obtaining amine-containing pullulan derivatives.

Детальніше...  

Adaptation of sample preparation methods used previously for amorphous glassy polymers has also allowed us to examine tensile deformation of thin films of poly(ether ether ketone) (PEEK), which is of particular interest given the commercial importance of this polymer. As the preliminary results presented here suggest, this appears to be a promising technique, particularly in view of the possibility of examining such films using transmission electron microscopy. In amorphous PEEK films, the dominant mode of deformation is simple shear at all temperatures below Tg. In semicrystalline PEEK, however, deformation at room temperature is characterized by both shear and crazing. Shear becomes gradually more dominant as the temperature is raised, and replaces crazing altogether above T~.

Детальніше...