1999 рік

A new approach to self-assembly of polymer blends in solution was studied by dynamic light scattering and transmission electron microscopy. Stable isolated particles and their clusters were obtained by dropping sulfonated polystyrene/THF solution into poly(4-vinyl pyridine)/methanol solution. The particles and clusters are stabilized by soluble poly(4-vinyl pyridine). The concentration of poly(4-vinyl pyridine) in methanol plays an important role in the self-assembly of polymer blends. Only in a suitable concentration range, stable particles can be prepared. Gelation appears at high poly(4-vinyl pyridine) concentration, while precipitation of sulfonated polystyrene occurs at low poly(4-vinyl pyridine) concentration.

Детальніше...  

Investigations based on static and dynamic light-scattering studies were conducted to assess the influence of amylose content (0.0–65.8%) and microwave dispersion treatment (MDT) on the macromolecular features of starches, including fractal behaviour. Values for apparent  W †; gyration radius R G † and hydrodynamic radius R H † were obtained using common extrapolation procedures to the average molar mass M zero scattering vector. The general trend showed a decrease in these values when amylose level or the intensity of MDT was increased. However, unexpected results were obtained, mainly for the starch with the highest amylose content and for long MDT times. The high qR G range [where q is the scattering vector; q ˆ 4pn=l†sin u=2†] allowed the internal behaviour of starch samples to be studied as well. Kratky plots, fractal dimensions, and relaxation rate dependence on q revealed significant differences in internal structures and the influence of amylose content. At low amylose contents (0.0–28.5%), light-scattering behaviour was dominated by the amylopectin contribution, in agreement with Galinski and Burchard (Macromolecules 1995;28:2363), whereas at high amylose contents (52.5–65.8%), light-scattering behaviour was influenced by amylose despite the high dimensions of amylopectin.

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The photoreactivity of new homo- and co-photosensitive polymers containing pendant chalcone moieties attached through hexamethylene spacer lengths between the photosensitive group and the backbone were studied. The rates of crosslinking of the prepared polymers were investigated under different conditions in solutions such as: backbone structures, copolymerization, different solvents, different concentrations and different temperatures. The structures of the prepared monomer and polymers were confirmed by different spectroscopic methods and elemental microanalysis. The average molecular weights of the polymers were determined by gel permeation chromatography. The thermal stability of the polymers was measured by thermogravimetric analysis under a nitrogen atmosphere.

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Turbulent drag reduction (DR) in a rotating disk apparatus was examined for a homologous series of polyisobutylene (PIB) with two different solvent systems to investigate the effect of solvent on universal DR characteristics. The concentration dependence of DR for these systems obeys an empirical universal DR equation. A linear correlation between polymer concentration (C) and C/DR for different molecular weights of PIB was also obtained. The intrinsic concentration was found to be an extremely useful quantity in normalizing the DR data for a homologous series of PIB. The characteristic value, K, was found to depend upon the solvent system.

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Copolymers of methyl methacrylate (MMA), and the methacrylic monomers derived from salicylic acid, 2-hydroxy-4-methacrylamidobenzoic acid (4-HMA), and 2-hydroxy-5-methacrylamidobenzoic acid (5-HMA), were prepared by free radical polymerization in N, Ndimethylformamide, DMF, solution at 508C using 2, 2 0 -azobisisobutyronitrile (AIBN), as initiator. Fineman–Ross and Kelen–Tüdos linearization methods, as well as the Tidwell and Mortimer non-linear least-squares treatment were used to calculate the monomers reactivity ratios, giving the Tidwell and Mortimer values from the 95% confidence diagrams, the most probable ones r4 ˆ 0.83, rM ˆ 1.52 and r5 ˆ 0.29 rM ˆ 2.15. The microstructural analysis of the prepared copolymers was performed by 13C (75.5 MHz) nuclear magnetic resonance (NMR) spectroscopy, in terms of sequence distribution and stereochemistry of monomeric units along the copolymer chains. The stereochemical parameters s * ˆ s 4M ˆ s M4 ˆ 0.21 and s * ˆ s 5M ˆ s M5 ˆ 0.19 were determined from the 13C NMR spectra of the copolymers 4-HMA– MMA and 5-HMA–MMA, respectively. Also the Tg of the corresponding homopolymers and copolymers, were analysed by differential scanning calorimetry (DSC) in terms of the Fox equation. Hydration degree of the prepared copolymers was studied in comparison to poly (4HMA), poly (5-HMA) and poly (MMA).

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The electrical conductivity of polyanilines doped with HCl was decreased upon exposure to oxygen and was increased reversibly upon evacuation. The line intensity and the line width of the EPR signals were decreased and increased, respectively, upon exposure to oxygen. The former represents the reduction in the concentration of polarons due to the spin–spin interaction of oxygen with paramagnetic polarons, generated by HCl doping process, and the latter the reduction in the mobility of polarons due to the partial localization of delocalized polarons. Therefore, it was concluded that the decrease in conductivity arose from the reduction in both the concentration and the mobility of polarons, the charge carriers for electrical conductivity. However, the decrease in the electrical conductivity was very small whereas the decrease in the polaron concentration was substantial. It was also found that the time scale for the diffusion of oxygen molecules was much longer than that for the spin–spin reaction of oxygen with polarons. Therefore, the small decrease in electrical conductivity may be associated mostly with the reduced mobility of polarons by localizing reaction instead of the reduced polaron concentration.

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Using differential scanning calorimetry (DSC) and X-ray analysis it is shown that binary systems of perfluorinated alkanes form solid solutions or exhibit eutectic phase behaviour, depending on their difference in chain length. A simple model, which successfully correlated experimental results for this aspect of phase behaviour of binary n-alkane systems, is demonstrated to quantitatively describe the behaviour of perfluorinated alkanes as well. From the model parameters and experimental observations, it appears that, at equal number of carbon atoms, binary perfluorinated alkane systems allow for a larger chain difference than their hydrogenated analogues, before eutectic phase behaviour sets in.

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UNIFAC-group contribution method and the Wilson formula were used to study the solvent effects in the terminal model for radical copolymerization. Local monomer concentrations in the neighbourhood of growing polymer radicals were estimated in order to determine the true terminal reactivity ratios for the styrene—methyl methacrylate monomer system in bulk, and in benzene, toluene, benzyl alcohol and phenol. Coefficients of monomer partitioning, K1 and K2, in the vicinity of the growing macroradicals –M1 z and –M2 z were estimated. From the results, it was concluded that the solvent does influence the local monomer concentrations in the proximity of both growing macroradicals, –M1 z and –M2 z, but not their ratios expressed as K1s and K2s , respectively. Constants K1s and K2s for solution copolymerization were equal to those for bulk copolymerization K1b and K2b , respectively. From the graphical analysis of the solvent effect, an increasing effect was suggested when changing the solvent from benzene , toluene , benzyl alcohol , phenol.

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Molecular rearrangements and alterations in supermolecular structure of ultra high molecular weight polyethylene (UHMWPE) due to cross-linking and oxidation-induced chain scission following irradiation and subsequent storage in air at room temperature have been studied over a period of 29 months. The techniques that were used are: equilibrium swelling in decalin, differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), Fourier transform infrared spectroscopy (FTIR) and electron spin resonance (ESR). The experimental results indicate that immediately after irradiation, cross-linking and an increase in crystallinity are the important processes. With time, chain scission induced by oxidation takes place resulting in a new phase of thinner crystallites in the amorphous region. In the absence of oxygen diffusion limitations, the free radicals survive for 30 months in the form of peroxides. The effect produced by higher doses and shorter aging times correspond to those produced by lower doses and longer aging times, thus, suggesting a superposition law between dose and aging time.

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Three two-arm poly(ethylene oxide) (PEO) fractions were prepared via a coupling reaction. These PEOs possess the same number average molecular weight of 2220 g mol ⫺1 for each arm and the coupling agents used are para-(1,4-), meta-(1,3-) and ortho-(1,2-) benzene dicarbonyl dichlorides. The configurations of these two-arm PEO molecules thus exhibit angles of 180, 120 and 60°, respectively, at the coupling agent. These coupling agents may substantially affect the overall molecular conformations (OMCs) in the crystals of the two-arm PEOs. Wide angle X-ray diffraction powder patterns indicate that these fractions have the same crystal structure regardless of the coupling agents (which act as defects) used, and therefore the defects must be excluded from the lamellar crystals. Differential scanning calorimetry results show that two populations of crystals exist when the samples are crystallized at relatively low undercoolings. This is particularly evident in the cases of 1,4- and 1,3-two-arm PEOs. Corresponding small angle X-ray scattering experiments show two different long periods in these fractions. Experimental results reveal the possibility that the crystals which correspond to the thinner long period possess an extended OMC in which one layer of defects exists between two neighboring lamellae. The crystals which exhibit the thicker long period possess a once-folded OMC, with two layers of defects lying between neighboring lamellae. During heating, the extended OMC crystals with the thinner long period melt at lower temperatures than the thicker, once-folded OMC crystals. Detailed studies on the efforts of these configurational defects on the lamellar crystal morphology and crystal stability are discussed in the main body of the article.

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