1999 рік

Optical birefringence induced during steady start-up shear flow of linear and long chain-branched metallocene linear low density polyethylene (LLDPE) melts, and their blends, were investigated at 200°C. Experiments were performed on a high temperature shearing cell, with a pair of parallel quartz plate windows, mounted on a rheometrics optical analyser which employs a rapid polarisation modulation technique for optical anisotropy measurements. The influence of long chain branching (LCB) on chain orientation and stretch were examined at a constant molecular weight (Mw) and molecular weight distribution (MWD), in terms of the amount of non-linear transient response (stress overshoots) and the steady-state value of the flow-induced birefringence of these polymers. Stress overshoots (hence molecular stretch) were found to occur at lower rates in polymers containing LCB than in linear polymers.

Детальніше...  

The solid structure and biodegradation behavior of the compositionally fractionated bacterial poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)s [P(3HB-co-3HP)] were examined. Utilizing the extracellular poly(3-hydroxybutyric acid) [P(3HB)]depolymerase purified from the bacterium strain Alcaligenes faecalis T1, enzymatic degradation behavior was investigated for P(3HB), poly(3-hydroxypropionic acid) [P(3HP)]and the fractionated P(3HB-co-3HP)s. It was revealed that P(3HB) and P(3HP) showed the distinguishable depolymerase concentration dependence of enzymatic degradation feature. For the fractionated copolyesters, the results indicated that P(3HB-co-3HP)s with higher 3HB contents which form P(3HB)-type of crystallites exhibited much higher enzymatic degradation rates than those of copolyesters with higher 3HP contents which form P(3HP)-type of crystallites, and that the amorphous copolyester could hardly be degraded. The 1H NMR analyses of the enzymatic degradation products revealed that P(3HB) was first degraded into dimers and then into monomers. In contrast, the presence of oligomers higher than dimers was suggested in the degradation products of P(3HP), and the 3HP content dependence of degradation products was confirmed for the fractionated P(3HB-co-3HP)s. In addition, environmental degradabilities were assessed by BOD method in the river water. The results confirmed the environmental degradabilities of P(3HB) and copolyesters except for P(3HP), indicating the inducing effect of 3HB comonomers on the biodegradation of 3HP units in copolyesters.

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The structural behaviour of the orthorhombic multi-n-alkane crystalline b' phase, observed in mixtures consisting of 23 (19

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Relationships between the properties of ternary blends of nylon 6, a thermotropic liquid crystalline polymer (TLCP, poly(ester amide), 20 wt%) and a compatibilizer (maleic-anhydride-grafted polypropylene, MA-PP, 2 wt%) are studied under various processing conditions. The immiscible binary blend can be compatibilized by the addition of MA-PP. The compatibilizer provides extrudate surface stability, thus enabling high drawing. Strong elongational flow, which was not possible when maleic-anhydride-grafted ethylene-propylene-diene terpolymer was used as a compatibilizer, favors fibril development. The shear viscosity of the ternary blend is slightly lower than that of the binary blend. The morphology of the dispersed TLCP phase varies between droplets and oriented fibrils when the drawing is weak and is highly correlated with changes in tensile properties. The optimum die exit temperature (280°C) provides a more uniform morphology and more fibril shapes which are associated with the increased tensile properties. The die exit temperature effect still exists when the draw ratio is high. In all cases, it is manifest that, whenever applicable, extension is a decisive factor in the deformation of the dispersed TLCP droplets, which is necessary for significant improvements in the mechanical properties.

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A random copolymer of methyl methacrylate and 5.7 mol.% of acrylic acid has been neutralized by zinc cation to different extents in order to study the effect of zinc carboxylate pendant groups on the miscibility of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF). The interaction parameter (x ) has been calculated from the experimental depression of the PVDF melting point. x is minimum at zinc carboxylate content of 2.8 mol.%. The dynamic mechanical analysis of the PVDF–PMMA ionomer binary blends does not agree with the additivity rule of the properties, consistently with the phase morphology that changes with composition. Blends are indeed amorphous as long as the PVDF content does not exceed 40 wt.%, otherwise, semicrystalline PVDF coexists with an amorphous mixed PVDF/copolymer phase.

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The 1 3C-enriched human hair was prepared by labeling human hair with 1 3C-enriched methyl iodide for the thiol groups formed by reduction of the disulfide bonds. In the 1 3C CP/MAS n.m.r. spectrum, the Cys S- 1 3CH 3 peak split into two peaks with a chemical shift difference of 1.7 ppm. With increasing reduction time in the process of the sample preparation, the relative intensity of the down field peak is increased from 0.17 (20 min reduction time) to 0.53 (20 h reduction time), although that of the up field peak stays almost constant. Here the main peak of the carbonyl carbon was used for the reference of the peak intensity ( ¼ 1.00). On the other hand, the residual ratio of the disulfide bond is decreased from 15% at the reduction time of 20 min to approximately 0% at that of 20 h. These results indicate that the down field peak is assigned to the Cys S- 13CH 3 carbons in the domain with the ordered structure where the disulfide bond is relatively difficult to reduce during the sample preparation. Besides, the up field peak is assigned to the Cys S- 1 3CH 3 carbons in the amorphous domain where the reduction occurs completely for at least 20 min. These two Cys S- 1 3CH 3 peaks are remarkably different in the half-height width. From the determination of the 1 3C spin-lattice relaxation times of two peaks, this difference is due to a difference in the chemical shift distribution rather than the difference in the molecular motion.

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By using X-ray photoelectron spectroscopy (XPS) and Fourier transform Raman (FT-Raman) spectroscopy, the worn specimens of polyetheretherketone (PEEK) tested under unlubricated sliding friction and wear conditions at a constant sliding speed were investigated in order to reveal mechanochemically and mechanophysically induced structural changes of polymer as well as wear mechanisms on a molecular scale. Chain scission was found on the worn surface layer. The results suggest that oxidation was the major mechanochemical reaction that followed the chain scission on the top surface. Evidence for chain branching or even crosslinking in bulk materials was also presented. Moreover, two-stage loading dependencies were found for both surface and subsurface in the bulk as revealed through wear rate measurement and wear debris analysis. It was proved that a thermo-activation of polymer segments may be responsible for the transition in the dependence of structure on load. The results of the present work also provide a method (based on spectral analyses) that can be used for studying micromechanisms accounting for shear deformation and failure.

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The isoionic dilution method was used to measure the intrinsic viscosity, [h], and the Huggins coefficient, KH, of two poly(acrylic acid) samples, 50% neutralized with NaOH, with molecular weights 5.0 × 10 3 (Na50PA5) and 4.8 × 10 4 (Na50PA48) at different ionic strengths. For Na50PA5 at low ionic strength, [h] is higher than the value predicted by a rigid-rod model. Assuming that the chain has reached its maximum extension, we have combined the rigid-rod model with the idea of effective dimensions that depend linearly on the Debye–Hückel screening length, k ⫺1, to account for a primary electroviscous effect, as a means to describe the observed behavior. As a consistent approach, we have also combined the effective dimensions with the wormlike chain model to calculate the persistence length of the higher molecular weight sample, Na50PA48. The result is that the electrostatic persistence length, le, is proportional to k ⫺1, in the region of high to moderate ionic strength, as is usually observed experimentally for flexible polyelectrolytes, while at the lowest ionic strengths, le tends to level off, which was predicted theoretically. Both the samples show a considerable increase in the Huggins coefficient as the ionic strength is lowered, with the sample having the lowest molecular weight giving the highest value at any particular ionic strength. This is attributed to the increasing intermolecular electrostatic interactions, which have a relatively larger importance in the case of Na50PA5, because the dimensions of the chain are smaller when compared with the range of the electrostatic interactions, as measured by k ⫺1, than for Na50PA48.

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Vinyl monomers having a Schiff-base structure were synthesized and their thermal and photopolymerization behavior was explored for different phases of the monomers. It was found that conversion of the monomers increased with temperature in the nematic phase, while it decreased in the isotropic phase. This result suggests that the polymerization behavior may be affected by the molecular alignment. Arrhenius plots for the initial rates of photopolymerization showed a straight line, while those of thermal polymerization changed in slope at the nematic–isotropic phase transition temperature. For MBEA, the number-average molecular weight of the polymer produced by thermal polymerization in the nematic phase was higher than that in the isotropic phase. For ABBA, it was observed that birefringence partially remained in the polymer produced in the nematic phase, although polymer produced by solution polymerization showed no liquid-crystalline phase. In addition, the orientation of the crosslinked polymer obtained by photopolymerization was better than that of the polymer produced by thermal polymerization.

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The temperature dependence of the reactivity ratios of the system styrene/butyl acrylate is studied. Copolymers of styrene and butyl acrylate are therefore synthesised in bulk at 50, 90, 100 and 120°C. Copolymer composition is determined by 1H NMR, the resulting data are evaluated, and Arrhenius parameters are obtained for rBA. In order to investigate the molecular-weight dependence of the copolymer composition, copolymerisations of styrene and butyl acrylate are carried out at 50 and 90°C in the presence of mercapto ethanol. No molecular-weight dependence is found for the investigated system.

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