1992 рік

A new method to observe a pressure-glassification of polymers by using a high pressure d.t.a, is proposed. Pressure scanning, instead of the usual temperature scanning, gave a d.t.a, curve typical of the glass transition. The glass transition pressure at a constant temperature for polystyrene was determined and analysed.

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In a simple and direct manner, residual strengths of edge-notched gelatin and cellulose acetate (CA) films under tension have been related to a single stress intensity factor. The effective stress intensity factors, or the effective fracture toughnesses, of the 0.012mm gelatin and 0.13 mm CA films are 2.97 and 3.30 MN m-3/2, respectively. Plasticity and finite width effects of edge-notched films are accommodated based on the Fedderson analysis. Stable crack growth before the fracture instability was confirmed using a high-speed video camera. Since stable crack extension guarantees the generation of naturally sharp cracks, sharp notches by fatigue cracking are not necessary for the fracture toughness measurement of thin films. However, with all the specimen widths employed (15-100 mm ) edge-notched poly (ethylene terephthalate) (PET) films of 0.10 mm thickness showed net section yielding during fracture testing. This indicates that the fracture behaviour of PET films with widths up to 100 mm have to be analysed on the basis of the elastic-plastic fracture mechanisms, and the fracture toughness of PET film is much greater than 25.3 MN m-3/2.

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Poly(1,3-dioxolane) can be cleanly depolymerized to the monomer. As such, the 1,3-dioxolane system is a good one to study to learn about microscopic reversibility. Besides describing how to perform the reversible polymerization reaction, this paper also presents data on the pyrolysis-field ionization mass spectrometry (Py-f.i.m.s.) and g.p.c, of the polymer and n.m.r, characterization of the monomer and polymer. The Py-f.i.m.s. data indicate that unhydrolysed polymer fragments in a manner similar to the reverse of the polymerization sequence, i.e. the carbonium ion portion of the oxymethylene end group bites back on oxygen atoms in the backbone. Base-terminated polymer, on the other hand, appears to undergo mid-chain cleavage. A dominant peak in the Py-f.i.m.s. spectrum of either, however, corresponds to the protonated cyclic dimer, a 10-crown-4 ether. A sequestered proton structure is proposed for this protonated species.

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I.r. spectroscopy, d.s.c., WAXD and optical microscopy were employed to investigate the differences between hydrogen bonding and crystallization behaviour depending on hard segment concentration in polyether- and polyester-based polyurethaneureas (PUUs). It has been found that the hard segment crystallization behaviour of polyether-based PUU is higher than polyester-based PUU for the same hard segment content. Hydrogen bonding in polyether-based PUU is formed mainly between hard segments, which promotes crystallization of hard segments. Hydrogen bonding in polyester-based PUU is formed mainly between soft and hard segments, which hinders the crystallization of hard segments.

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Dynamic mechanical properties and internal stress were investigated using several different kinds of epoxide resins with different chemical structures. Resin containing a tetramethyl biphenol structure showed a very high rubbery modulus, a low glassy modulus and a low internal stress, in addition to exhibiting a high glass transition temperature. Furthermore, the effect of the introduction of methyl branches into the biphenol skeleton on viscoelastic properties of cured epoxide resin was investigated by comparing with that of bisphenol-A type resin in detail. In the case of bisphenol-A type resin, the introduction of methyl branches slightly affected the modulus in the rubbery and the glassy regions and the glass transition temperature. These results show that the introduction of methyl branches into the resin does not significantly affect the mobility of network chains or the free volume of cured resins. In contrast, the introduction of methyl branches into biphenol type resin resulted in a decrease in modulus in the rubbery region and an increase in the peak height of tan 6. These results show that the introduction of methyl branches into biphenol type resin increases the mobility of network chains in the rubbery region. On the other hand, the modulus in the glassy region increased according to the introduction of methyl branches. This is due to the decrease in free volume in the glassy region with an increase in the mobility of network chains in the rubbery region

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A light scattering instrument capable of monitoring the scattering from samples under the influence of a simple shear field has been constructed. The apparatus consists of transparent cone-and-plate fixtures and a two-dimensional charge coupled device array detector. The detector unit is also capable of measuring the scattering patterns as a function of time. The phase separation kinetics of an 8% solution of a polystyrene/polybutadiene (50:50) blend in dioctyl phthalate following cessation of a steady shear is monitored with this instrument. The sample is two-phase in the quiescent state and the applied shear is sufficient to suppress all scattering observed from the quiescent sample (shear-induced mixing). The evolution of the scattering profiles following cessation of shear is quite different parallel and perpendicular to the original flow direction. In the normal direction, a spinodal growth and coarsening mechanism similar to that observed in temperature quench experiments is observed. In the parallel direction, a different mechanism is followed. We believe this is the first report of this type of anisotropic behaviour in the phase separation kinetics of a polymer blend.

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The 0 temperature of poly (N-vinyl pyrrolidone) (PVP) in water has been determined accurately by studying the variation of the cloud points of polymer solutions in the presence of an electrolyte (Na2CO 3). The cloud point of PVP solutions decreased linearly with electrolyte concentration. The 0 temperature was determined by extrapolating the cloud points of dilute solutions to polymer volume fractions of 1. The phase diagram of PVP in water was also constructed by the same procedure over wider polymer concentrations.

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The effects of sub-Tg ageing on the impact properties of neat versus impact-modified poly(ethylene terephthalate) (PET), polycarbonate (PC) and a glycol-substituted PET polyester (PETG) were studied. Notched Izod bars of unaged neat PC fail in a ductile manner at room temperature whereas PET and PETG bars fail in a brittle (craze-initiated) mode. When PC is aged or tested at -20°C, however, the PC fails by the same brittle mode as the polyesters. This behaviour is attributed to an increase in yield stress and the subsequent change in the stress state during impact from more of a plane stress to plane strain state. At the strain rates employed, this causes the failure mechanism to switch from shear yielding to crazing. The embrittlement that accompanies ageing is greatly suppressed by the addition of impact modifiers to neat resins. Toughness in the impact-modified materials is believed to derive from both an increase in the number of deformation sites as well as the promotion of shear yielding in the matrix phase even after ageing or during low-temperature impact

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Atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been used to image well-characterized algal cellulose microfibrils. Cross-sections of the microfibrils observed by TEM are square, whereas the AFM topography of these microfibril surfaces shows a rounded profile due to convolution with the shape of the AFM tip. Height and base width measurements taken from cross-sections of these AFM micrographs also show a marked dependence on the scan rate of the AFM tip. AFM images of the surface of the highly crystalline cellulose microfibrils were obtained at atomic resolution under ambient conditions ; the images showed periodicities along the microfibril axis of 1.07 and 0.53 nm that may correspond to the fibre and glucose unit repeat distances, respectively.

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effects of molecular weight, type of neutralizing cation and excess neutralizing agent on the structure and properties of carboxylato-telechelic polyisoprene have been studied. In particular, the effects of cation valence and cation size were studied for group IA and IIA elements, for which the bonding to the carboxylate anion is primarily ionic. It was found that increasing cation valence and decreasing cation size have the general effect of increasing the electrostatic association of the ion pairs, resulting in a more elastic stress-strain response. Neutralization with zinc(n) and nickel(n), elements of the first transition series that form less ionic, more coordinative complexes with the carboxylate ion, resulted in materials with quite different mechanical properties. The zinc-neutralized material displayed rather poor mechanical properties, while the nickel-neutralized material was much stronger. Materials neutralized with aluminium(nI) or titanium(Iv) were observed to display a wide range of properties depending upon the amount of cation incorporated. It was determined that at least four times the stoichiometric amount of titanium(Iv) is necessary to crosslink the material effectively. Materials of higher number-average molecular weight (33 000 vs. 15 000) displayed higher stresses and higher ultimate elongations owing to the development of a more extensive entanglement network. Finally, small-angle X-ray scattering studies of the _~o = 15 000 and 33 000 materials indicate that an ionic peak is observed for the/~n = 15 000 materials but is generally not observed for the M, = 33 000 materials even with 100% excess neutralizing agent. The position of this peak is essentially unaffected by the type of neutralizing agent used. Analysis of the tail region of the scattering curves indicates that the interface between the ionic and non-ionic regions is sharp.

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