Finn Knut Hansen

Kinetics and Mechanism of Emulsion Polymerization

Abstract The course of emulsion polymerization may be considered as involving three intervals: Interval I, where particle formation takes place. The end of this interval is not dependent upon the degree of conversion, but on the total amount of polymer formed. With usual recipes, it ends at about 1-5% conversion. Interval II lasts from the end of Interval I until monomer disappears as a separate phase. In this interval, the particle number is usually found to be constant, the particle volume increases proportional to conversion, the monomer concentration in the particles is approximately constant, and therefore the termination is also constant within the particles. Interval III starts when the monomer disappears as a separate phase. The transition from Intervals II to III is determined by the degree of conversion and differs for different monomers. In cases where the solubility of monomer in the water phase is low, the monomer present in the aqueous phase may usually be neglected compared to the monomer p...

Surface tension by pendant drop: I. A fast standard instrument using computer image analysis

Abstract An improved method has been developed for the determination of surface and interfacial tensions from primary drop shape data. In addition, wetting angles of sessile drops may be determined. The method has been built around a commercial pendant drop instrument and an IBM-compatible PC with a frame grabber card. In order to differentiate the drop profile, a filter routine using a local threshold and interpolation technique has been developed that is combined with an edge-tracing algorithm. The program for calculation of surface tension is divided into two parts. The first part is based on the traditional optical method and uses inflection of the drop profile. By means of several polynomial interpolations and curve fitting of theoretical profiles, the form factor β and surface tension γ are determined. The second part of the calculation utilizes the above values as a first estimate and then performs a further optimalization of γ by comparison between experimental and theoretical Young—Laplace profiles. With a PC AT with a 80287 mathematical coprocessor the measurements take about 5 s and the reproducibility is typically 0.01–0.03 mN/m for a wide range of known liquids.

A General Approach for Preparation of Polymer-Supported Chiral Organocatalysts via Acrylic Copolymerization

Polymer-supported chiral organocatalysts, as well as most other forms of immobilized catalysts, are traditionally prepared by a postmodification approach where modified catalyst precursors are anchored onto prefabricated polymer beads. Herein, we report an alternative and more scalable approach where polymer-supported chiral enamine and iminium organocatalysts are prepared in a bottom-up fashion where methacrylic functional monomers are prepared in an entirely nonchromatographic manner and subsequently copolymerized with suitable comonomers to give cross-linked polymer beads. All syntheses have been conducted on multigram scale for all intermediates and finished polymer products, and the catalysts have proven successful in reactions taking place in solvents spanning a wide range of solvent polarity. While polymer-supported proline and prolineamides generally demonstrated excellent results and recycling robustness in asymmetric aldol reactions of ketones and benzaldehydes, the simplest type of Jørgensen/Hayashi diarylprolinol TMS-ether showed excellent selectivity, but rather sluggish reactivity in the Enders-type asymmetric cascade. The polymer-supported version of the first-generation MacMillan imidazolidinone had a pattern of reactivity very similar to that of the monomeric catalyst, but is too unstable to allow recycling.

Synthesis of Acrylic Polymer Beads for Solid-Supported Proline-Derived Organocatalysts

A completely non-chromatographic and highly large-scale adaptable synthesis of acrylic polymer beads containing proline and prolineamides has been developed. Novel monomeric proline (meth)acrylates are prepared from hydroxyproline in only one step. Free-radical copolymerization then gives solid-supported proline organocatalysts directly in as little as two steps overall, without using any prefabricated solid supports, by using either droplet or dispersion polymerization. These affordable acrylic beads have highly favorable and adjustable swelling characteristics and are excellent reusable catalysts for organocatalytic reactions.

Effect of surfactant and temperature on the rheological properties of aqueous solutions of unmodified and hydrophobically modified polyacrylamide

Linear and nonlinear viscoelasticity of semidilute aqueous solutions of an unmodified reference polyacrylamide (PAM) and of a hydrophobically modified polyacrylamide (HM-PAM) in the presence of various amounts of sodium dodecyl sulfate (SDS) and at different temperatures have been examined. Oscillatory shear experiments in the linear viscoelastic domain revealed significant polymer-surfactant interaction for the HM-PAM-SDS system at a surfactant concentration of ca. 8 mmol/kg, whereas practically no surfactant effect is observed for the PAM-SDS system. The rheological features for the HM-PAM-SDS system are found to be strongly dependent upon the level of surfactant addition, with first an increase and then a decrease in the values of parameters such as the dynamic viscosity. While the position of the maximum of the viscosity curve, with regard to the surfactant concentration, is independent of polymer concentration and temperature, the strength of the network is promoted by increasing polymer content and decreasing temperature. At higher surfactant concentrations, a breakdown of chain associations occurred. A non-Newtonian shear thinning behavior, as well as a thixotropy, is detected for HM-PAM solutions in the presence of surfactant. These effects are most pronounced at ca. 8 mmol/kg SDS at the lowest temperature. The rheological features reveal that the hydrophobic associations, induced by HM-PAM-SDS interactions, play an important role for the viscoelastic properties of the system.

The Optimum Dispersion of Carbon Nanotubes for Epoxy Nanocomposites: Evolution of the Particle Size Distribution by Ultrasonic Treatment

The ultrasonic dispersion of multiwalled carbon nanotube (MWCNT) suspensions was assessed by studying the differential sedimentation of the particles in an acid anhydride often employed as a curing agent for epoxy resins. The particle size distributions were characterized by the means of a disc centrifuge, and the effect of dispersion time, power density, and total energy input, for both bath and circulation probe ultrasonic dispersing equipment was investigated. The mass of freely suspended MWCNTs relative to agglomerated MWCNTs was estimated as a measure of the quality of the dispersions, and the results showed that this ratio followed a power law scaling with the energy dissipated in the sonication treatment. If the sonication power level was too high, sonochemical degradation of the curing agent could occur. The mean agglomerate MWCNT size distribution was estimated, and the fragmentation of the agglomerates was modeled by means of fragmentation theory. Indications of both rupture and erosion fragmentation processes for the MWCNT agglomerates were observed.

The association of urethane-polyethyleneoxide (HEUR) thickeners, as studied by NMR self-diffusion measurements

The concentration and molecular weight dependence of the self-diffusion coefficient (Dself) of associative polymers of HEUR-type in aqueous solution have been investigated using FT-PGSE-NMR technique. The idea of three-dimensional network formation as a result of aggregation of the hydrophobic end-groups of the polymer in junctions is supported through the observed dramatic lowering ofDself with increased concentration. The network-formation efficiency depends on the polymer molecular weight as well as the hydrophobicity of the end-groups.A double logarithmic dependence of the self-diffusion coefficient versus concentration (c) has been observed:Dself∝c−a1,a2. The first exponent,a1, is valid at low concentration, <1% polymer per weight solution, and ranges from 0.5 to 1, whereas the second exponent,a2, describing systems of higher concentration, ranges from 2 to 2.7.

Dynamic light scattering and rheological studies of thermoreversible gelation of a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer in aqueous solution

Shear relaxation and dynamic light scattering (DLS) experiments have been performed at various temperatures on the thermoreversible gelling aqueous system of Pluronic F68 (35 wt.%). As the gel zone is approached, both the stress relaxation process and the time correlation function indicate three different relaxation modes: a short-time behaviour (exponential profile) followed by a power law at intermediate time and then a long-time tail (stretched exponential profile). At low temperatures the stress relaxation function is described by only two relaxation processes: an exponential at early times followed by a stretched exponential at long time, while the decays of the correlation functions from DLS display three modes at all temperatures. The stress relaxation results reveal that the extension of the power-law regime is most pronounced in the vicinity of the gel point. The value of the power law exponent αs from stress relaxation measurements (G∞t–αs) is 0.5 at all temperatures, except at the lowest temperature where αs≈ 0.6. In the case of the DLS measurements the power-law exponent is much lower (0.05–0.15) and an angular dependence is observed.

The function of surfactant micelles in latex particle nucleation

Abstract The molecular mechanisms involving the surfactant micelles in the nucletion interval of an emulsion polymerization are described. Quantitative expressions for the rate of radical capture in micelles are given, and from these the conclusion is drawn that for ionic surfactants, the micelles must play an important part in the particle nucletion process, also for more water soluble monomers. Modifications to the nucleation mechanism in the case of nonionic emulsifiers are discussed and a possible decrease in radical adsorption rate due to a dense diffusion barrier on the aqueous side of the surface is proposed.

Adhesion in Solid Propellant Rocket Motors

Plasma treatment of EPDM-based rocket motor insulation materials may change the peel strength between these materials and polyurethane polymers by a factor of 0.1 to 10. The matching of surface energies seems to be important for this adhesion process. The surface tension of the components was measured to between 30 and 50 mNm−1. The total surface energy of the insulation could be increased from 27.2 mNm−1 to ca. 70 mNm−1 by the plasma process. Maximum peel strength could be obtained by a treatment of less than 1 min, whereas in most cases longer times gave lower values. In some cases very long treatment also gave good strength, probably due to a rougher surface structure. The rate of cure of the polymer was important for the adhesion process as lower rates of cure correlated with higher peel strengths, which can be explained by diffusion of the polymers components into the insulation.