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  • 1
    ISSN: 1040-0397
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An analysis scheme is presented that separates the individual effects of complexing agents and surfactants on the deposition and stripping steps in trace metal determination by differential pulse anodic stripping voltammetry (DPASV) at a hanging mercury drop electrode. The method involves DPASV measurements of total and electroactive (ASV-labile) metal in pH 7.0 phosphate buffer before and after medium exchange, with and without acidification of the sample solution. Copper, lead, and cadmium were determined in the presence of Triton X-100, a nonionic detergent, and the complexing agents fulvic and humic acids. Both the complexing agents and the surfactant affected deposition more than stripping, although Triton X-100 had a significant effect on the stripping process. Triton X-100 also acted as a complexing agent towards copper. Application of the analysis scheme to pristine and polluted freshwater samples showed that the decrease in the electroactivity of copper and lead was principally caused by the effect of surfactants on deposition. This study underlined the limitations of high-frequency ASV techniques (e.g., DPASV) at a hanging mercury drop electrode for the determination of the toxic fraction of metals in water samples and raised questions about the usefulness of ASV copper complexing capacity titrations for water toxicity investigations.
    Additional Material: 5 Tab.
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  • 2
    ISSN: 0022-3832
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The slow rates of polymerization and the low molecular weights of polymer obtained by the free radical-initiated polymerization of vinyl benzoate is noted. The kinetics have been investigated and indicate that addition of the growing chain to the aromatic nucleus to form a stable, nonpropagating radical is the probable reason. The chain length and extent of branching are consistent with the observed rates and also with the methyl affinities of similar compounds.
    Additional Material: 3 Ill.
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  • 3
    ISSN: 0173-2803
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Additional Material: 1 Tab.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 33 (1987), S. 1585-1600 
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Linear viscoelastic properties are found to be a sensitive measure of flow-induced structural changes in a block copolymer. Styrene-butadiene-styrene block copolymer (SBS) with 26% polystyrene (PS) forms a macrostructure in the quiescent state with grains of the order of 1-10 μm. Within each grain, phase separation gives rise to a regular two-phase microstructure with cylindrical PS domains with radius of the order of 200 Å. Large-amplitude oscillatory shear (γ = 4.5) at temperatures between 139 and 181°C was applied to after the grain structure with the objectives of removing the discontinuities at the grain boundaries and of aligning the domains into a continuous ultrastructure. The SBS behaved like a solid (tan δ 〈 1 at low ω) before and like a liquid (tan δ 〉 1) after shear modification. This change expressed itself in the removal of the long relaxation times from the linear viscoelastic spectrum; the intermediate and low relaxation times were not affected by the shear modification. The viscoelastic spectrum slowly recovered during annealing with recovery times of the order of the longest relaxation time of the quiescent structure. Birefringence studies showed that the SBS did not recover into its original grain structure but into a highly oriented domain structure. The discontinuities at the grain boundaries could not be removed completely.
    Additional Material: 14 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    Journal of Polymer Science 62 (1962), S. S19 
    ISSN: 0022-3832
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 6
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 193 (1992), S. 303-313 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The model of Maxwell et al. (Macromolecules 24, 1629 (1991)) for initiator efficiencies in emulsion polymerization has been extended to take into account the effect of added chain transfer agent (CTA). The model supposes that the rate-determining step for radical entry into latex particles is aqueous-phase propagation of the primary free radicals to a critical chain length z required for entry (forming species such as .MzSO-4, where M is a monomer entity and peroxodisulfate S2O2-8 is the initiator). The effect of CTA on the entry rate is assumed to occur by facilitating the production of aqueous-phase free-radical species (CTA.) by transfer between species such as .MzSO-4 (where n 〈 z) and CTA in the aqueous phase. The CTA. will be formed at a reasonable rate provided the CTA is not too water-insoluble (e.g. C12H25SH); it should also be capable of entering the latex particles rapidly because of its relative insolubility. If the monomer-derived .MnSO-4 tend to suffer aqueous-phase termination rather than entry, the overall rate of entry (and hence initiator efficiency) will be increased. This can explain the accelerating effect of intermediate molecular weight CTA's on emulsion polymerization of monomers such as butadiene, where z is large and hence initiator efficiency is very low in the absence of CTA, because most .MnSO-4 undergo termination rather than entry into the latex particles.
    Additional Material: 3 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 193 (1992), S. 2049-2063 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Two methods are described for experimentally determining the concentrations of monomer in both the aqueous phase and the latex particle phase during partial swelling of latex particles, and therefore also during intervall III of an emulsion polymerization. The ratio of the monomer concentrations in the aqueous phase, both below and at saturation, can be related to the volume fraction of polymer in the latex particles via the Vanzo equation. Comparison of theory and experiments for the methyl acrylate and poly(methyl acrylate-co-styrene) system shows that the monomer partitioning is insensitive to temperature, latex particle radius, polymer composition, polymer molecular weight and polymer cross-linking. Thermodynamic treatment of these and previously published partitioning results shows, at higher volume fractions of polymer, that the conformational entropy of mixing of monomer and polymer is the significant term determining the degree of partial latex particle swelling by monomer. Theoretical predictions of experimental results are quite insensitive to values of the Flory-Huggins interaction parameter and to the latex particle-water interfacial tension. A simple model is developed for the estimation of monomer partitioning which requires only the saturation monomer concentrations in the particle and aqueous phases.
    Additional Material: 12 Ill.
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  • 8
    ISSN: 0887-624X
    Keywords: emulsion polymerization ; isotachophoresis ; oligomer ; free radical ; aqueous phase ; styrene ; termination ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The concentrations and probable nature of charged oligomers formed by aqueous-phase termination in the persulfate-initiated emulsion polymerization of styrene were measured by isotachophoresis. Isotachophoresis has some advantages over other techniques (e.g., GPC, UV spectroscopy) in that it separates species according to their molecular weight, geometry, and charge. The charged water-soluble oligomeric species were detected in experiments in which particles were nucleated in a surfactant-free environment. Identification of the moieties present was made by comparison with model compounds. Evidence was found for bimolecular combination as a major mechanism of termination in the aqueous phase, although the possibility of disproportionation could not be ruled out. The species formed in the aqueous phase under saturated monomer conditions were found to be subject to further reaction towards the end of polymerization. The surface adsorption characteristics of the compounds formed were compared with those of known surfactants and showed good agreement with the assumptions in the model of Maxwell et. al. [Macromolecules, 24, 1629 (1991)] for initiator efficiencies in emulsion polymerization. The relatively large concentrations of nonradical aqueous-soluble oligomeric compounds demonstrate conclusively that initiator efficiencies are not 100%, as is often assumed in such systems. © 1993 John Wiley & Sons, Inc.
    Additional Material: 18 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 32 (1994), S. 605-630 
    ISSN: 0887-624X
    Keywords: free radical ; exit ; emulsion ; polymerization ; model ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The exit or desorption of free radicals from latex particles is an important kinetic process in an emulsion polymerization. This article unites a successful theory of radical absorption (i.e., initiator efficiency), based on propagation in the aqueous phase being the rate determining step for entry of charged free radicals, with a detailed model of radical desorption. The result is a kinetic scheme applicable to true “zero-one” systems (i.e., where entry of a radical into a latex particle already containing a radical results in instantaneous termination), which is still, with a number of generally applicable assumptions, relatively simple. Indeed, in many physically reasonable limits, the kinetic representation reduces to a single rate equation. Specific experimental techniques of particular significance and methods of analysis of kinetic data are detailed and discussed. A methodology for both assessing the applicability of the model and its more probable limits, via use of known rate coefficients and theoretical predictions, is outlined and then applied to the representative monomers, styrene and methyl methacrylate. A detailed application of the theory and illustration of the methodology of model discrimination via experiment is contained in the second article of this series. © 1994 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 32 (1994), S. 631-649 
    ISSN: 0887-624X
    Keywords: free radical ; exit ; emulsion ; polymerization ; experiment ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In emulsion polymerizations, desorption (exit) from latex particles of monomeric radical species that arise from transfer can be an important determinant of the overall kinetics. An examination of various methodologies for the testing of postulated free radical exit mechanisms is made. These utilize the model descriptions for the exit process presented in the accompanying article of Casey et al., employing data consisting of conversion as a function of time for the approach to steady state polymerization conditions. Experimental data are presented on the exit rate coefficients as a function of such experimental parameters as: particle size, monomer concentration, and aqueous-phase free-radical concentration for a series of styrene polymerizations at 50°C, where the average number of free radicals per particle (n̄) never exceeds 0.5. It is demonstrated for these systems that while the conversion/time dependence from a single run, under conditions sensitive to exit, is insensitive to mechanistic assumptions as to the fate of desorbed free radicals, the variation of the exit rate coefficient with particle size so obtained suggests a second order dependence on n̄, implying complete re-entry of desorbed free radicals under all conditions studied. Once the monomeric radicals have re-entered, they are more likely to remain inside the particle where they will either propagate or undergo termination rather than re-escape. The article also presents an estimate for the rate coefficient at 50°C of the first propagation step of the monomeric radical subsequent to transfer. The conclusions drawn here for seeded systems should prove useful for study of particle nucleation mechanisms, when exit is particularly likely in small, newly formed, particles. © 1994 John Wiley & Sons, Inc.
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