1999 рік

Isotactic polypropylene (iPP) resin was doped with 1% of pimelic acid/calcium stearate (composition ratio ¼ 1/1.5). It was crystallized in a two-step isothermal process, first at 130°C and then at 95°C, and finally cooled at a rate of 0.5°C min ¹1 to room temperature. The specimens were basically free of a-crystals and could be considered as pure b-PP. The crystallinity of the specimens was determined on the basis of their density. The heat of fusion of the b-crystal was found to be 168.5 J g ¹1. In comparison, the heat of fusion of the a-crystal in the undoped resin was slightly higher at 177.0 J g ¹1.

Детальніше...  

In this study of the inverse-emulsion homopolymerization of acrylamide, surfactant blends of traditional fatty acid esters, ethoxylated fatty acid esters and ABA-type block copolymeric stabilizers were employed. Stable and transparent inverse latices with shelf lives of over one year were generated. Particle sizes were determined using quasi-elastic light scattering and small-angle neutron scattering and were found to be close to the threshold traditionally associated with microemulsions (100 nm). The turbidity of the polymer latices and the changes in viscosity during the polymerization are also similar to that observed in inverse-microemulsions. The radii of gyration calculated from SANS data were observed to be consistent with the hydrodynamic radius determined by QELS for the final polymerized products. SANS measurements of the inverse-emulsions at different reaction coordinates revealed a decrease in droplet diameters with conversion, with particles best described using a polydisperse spherical core þ shell model. The decrease in droplet diameter with conversion has been attributed to a surfactant rearrangement in the interfacial sheath due to the consumption of surface active acrylamide.

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Blends of ethylene–propylene rubber (EPM) and nylon 6 are immiscible and highly incompatible. These blends are characterised by a two-phase morphology, narrow interface, and poor physical and chemical interactions across the phase boundaries. Therefore a reactive route was employed to compatibilise these blends by the addition of maleic anhydride grafted EPM (EPM-g-MA). In this reactive route, the maleic anhydride group of EPM reacts with the amino end group of nylon forming a graft copolymer of nylon and EPM (nylon-g-EPM) at the blend interface which decreases the interfacial tension and reduces the coalescence. The influence of the concentration of EPM-g-MA, blend composition, molecular weight of nylon 6, mode of addition of EPM-g-MA and mixing time on the phase morphology of the blends was studied quantitatively by scanning electron microscopy and image analysis. It was found that the addition of EPM-g-MA reduces the domain size of the dispersed phase followed by a levelling off at high concentrations; the levelling off is an indication of interfacial saturation. The optimum amount of the compatibiliser required to saturate unit volume of the interfacial zone was estimated from the emulsification curves. This optimum concentration of the compatibiliser can be considered as the so-called critical micelle concentration (CMC) above which micelles of the copolymer are formed in the bulk phase, which is highly undesirable. Emulsification master curves were obtained by plotting the reduced domain size as a function of EPM-g-MA for the different blend compositions. This was explained based on the interfacial area occupied by the compatibiliser molecule at the blend interface. The experimental results were compared with the current compatibilisation theories. The phase morphology development was studied as a function of the mixing time in the presence and absence of compatibiliser. The influence of reactive compatibilisation on phase inversion and the co-continuous nature of the blends was also investigated. Finally the stability of the blend morphology was analysed by high temperature isothermal annealing in the presence and absence of compatibiliser

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After isothermal crystallization of the amorphous poly(ether ether ketone), double endothermic behaviour can be found through differential scanning calorimetry experiments. During the heating scan of semicrystalline PEEK, a metastable melt, which comes from the melt of the thinner lamellar crystal populations, can be obtained between these two endotherms. The metastable melt can recrystallize immediately just above the lower melting temperature and form slightly thicker lamellae than the original ones. The thickness and the perfection depend upon the crystallization time and the crystallization temperature. By comparing the TEM morphological observations of the samples before and after partial melting, it can be shown that lamellar crystals, having different thermodynamic stability, form during isothermal crystallization. After partial melting, only the type of lamellar crystal exhibiting the higher thermodynamic stability remains. Wide angle X-ray diffraction measurements shows a slightly change in the crystallinity of the samples before and after the partial melting. Small angle X-ray scattering results exhibit a change in the long period of the lamellar crystals before and after the partial melting process. The crystallization kinetics of the metastable melt can be determined by means of differential scanning calorimetry. The kinetic analysis showed that the isothermal crystallization of the metastable PEEK melt proceeds with an Avrami exponent of n ¼ 1.0⬃1.4, reflecting that probably onedimensional or an irregular line growth of the crystal occurred between the existing main lamellae with heterogeneous nucleation.

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The atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate with differently substituted model and macroinitiators was performed, in order to evaluate qualitatively the structural features which affect the efficiency of the initiation step. It was found that substituents which stabilize radicals enhance the dissociation of the halogen atom and lead to high initiation efficiencies of the model initiators and macroinitiators. The phenyl group and the oxycarbonyl group result in the efficient initiators 1a and 3a for the initiation of both styrene and methyl methacrylate. Further, it was shown that the poly(ethylene oxide) moiety in the initiator, because of its low solubility, e.g. in styrene/poly(styrene) mixtures, leads to phase separation that effects the reactivity of the initiating group.

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A conventional rubber elasticity equation based on the inverse Langevin function, combined with a yield stress (Y0) has been used for the purpose of studying published tensile stress–strain curves for thermoplastic elastomers. In order to simplify the calculation a Pade approximation has been employed [Cohen A. Rheol. Acta. 1991;30:270] for the inverse Langevin function which leads to the following equation, relating f, the nominal or engineering stress, to the extension ratio l : f ˆ Y0 =l 1 Cr=3†‰l 3 2 l2 =n†= 1 2 l2 =n† 2 1=l2 † 3 2 1=ln†= 1 2 1=ln†Š: When suitable values of the parameters are selected the equation may be used to model nominal stress–strain curves for the new ultralow density polyethylenes, ethylene vinyl acetate copolymers and SBS block copolymers. The parameters Cr and n selected in this way represent identifiable physical entities; Cr the initial modulus and n 1/2 the limit of extensibility. However Cr does not increase with temperature as with a conventional rubber, but declines as the temperature is raised. With the polyethylenes this may be related to the gradual melting of the crystals which are believed to act as cross links [Bensason S, Stepanov EV, Chum S, Hiltner A, Baer E. Macromols 1997;30:2436]. However, with an SBS block copolymer the reason for the fall in Cr and the rise in n are not clear. Generally, for instance when the temperature is reduced and the materials become stiff, Cr will increase and n decrease. However when it is plotted against crystallinity with the ultralow density polyethylenes, n does not follow Cr but shows a minimum at a crystallinity of 30% after which it appears to increase. With polyethylenes n is more sensitive to molecular weight than Cr and gives a linear Flory plot for n 1/2 against 1/T at 08C. At 258C the values of n obtained are very high and when the molecular weight falls to 32 000 and the stress–strain curve is found to follow a Gaussian equation. This supports the mathematical requirement that the equation reduces to a Gaussian form when n is very large. The same result can be predicted from a series approximation suggested by Treloar.

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X-ray pole figures were obtained for samples of lightly plasticised poly(vinyl chloride) which had been subjected to monoaxial (draw ratio 2) and biaxial (draw ratio 2 х 2) orientation at 70°C. These pole figures confirmed the presence of crystallite orientation and suggested the presence of two distributions of crystallites, having the a- and c-axes of the unit cell, respectively, predominantly in the stretch direction. The relative amount of c-axis orientation was greater in the biaxially stretched sheet, possibly at least partly owing to the presence of mesomorphous structure.

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A fundamental theoretical investigation is conducted for the molecular weight distribution formed in free-radical and living copolymerizations with macromonomers by using the random sampling technique. General analytical expressions for the number- and weight-average molecular weight developments are obtained. The full molecular weight distribution functions are presented for some simpler cases with low mole fractions of macromonomers. The present theoretical analysis provides a great insight into the complex molecular buildup processes, and thus leads to a better control of the graft copolymers.

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The phase diagram of the binary system poly(3-octylthiophene) 1 poly(ethylene-co-vinylacetate), P3OT 1 EVA was studied by Differential Scanning Calorimetry (DSC). The blends of low and high content of P3OT were found to be homogeneous. The system was also studied by a dielectric relaxation technique in the frequency range 20 Hz–100 kHz. The complex electric modulus M* is discussed in terms of the well-known Havriliak–Negami function. We also obtained the spectrum of relaxation times using a regularization technique. The spectra allowed us to calculate the d.c. conductivity. A low frequency contribution to the dielectric loss found in two-phase samples was attributed to interface polarisation effects.

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Using Parsons-type scaling, the Onsager theory for the isotropic–nematic (I–N) transition of rigid-rod lyotropic polymer liquid crystals is combined with the equation of state for hard-sphere-chain fluids of Chapman et al. and that of Hu et al. The equation of Hu et al. gives the I–N transition pressure and density in good agreement with computer simulation by Wilson and Allen for a semi-flexible hard-sphere chain consisting of seven segments. For real semi-flexible polymers, we follow the Khokhlov–Semenov theory of persistent chains that introduces chain flexibility into the Onsager theory. Using a consistent procedure to regress the equation-of-state parameters, the equations of Chapman et al. and Hu et al. are also compared with the theory of DuPré and Yang that uses the equation of Lee for hard spherocylinders. These models are compared with experiment for two binary polymer solutions containing poly(hexyl isocyanate) and another solution containing polysaccharide schizophyllan. The concentration of polymer at the I–N transition is predicted as a function of the molecular weight of polymer. All models perform similarly and show semi-quantitative agreement with experiment.

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