H. J. Müller

Initiators based on poly(ethylene glycol) for starting heterophase polymerizations - generation of block copolymers and new particle morphologies.

Radical heterophase polymerizations with poly(ethylene glycol) radicals lead to the formation of block copolymer particles where the block copolymer architecture and the particle morphology depend on the number of radicals per poly(ethylene glycol) chain, the radical termination mode, and the polarity of the monomer, respectively. The thermal decomposition of symmetrical poly(ethylene glycol) azo-initiators following the classical recipes of Heitz results in one radical per poly(ethylene glycol) chain whereas the number of radicals can be adjusted between one or two in the redox system poly(ethylene glycol)/cerium ions. The polymerization of styrene results in latex particles with an almost spherical morphology, consisting of block copolymers, only. In case of a methyl methacrylate polymerization the latex morphology depends on the architecture of the block copolymers formed. Heterophase polymerization with poly(ethylene glycol) azo-initiators in oligo(ethylene glycol) (average molecular weight 200 g mol -1 ) instead of in water results in particles with random shape and a strongly indented interface which is explained by the surface tension between polymers and solvent being close to zero.

Evaluation of heterophase polymerizations by means of reaction calorimetry

Abstract Reaction calorimetry is a powerful tool for systematic investigations of heterophase polymerizations. The heat flow–time or heat flow–conversion profiles clearly reflect any changes of the recipe components. Results of batch heterophase polymerizations are presented proving the dependence of the reaction rate profiles on the water solubility of the monomers, on the presence of a chain transfer agent, on the type and concentration of the stabilizer and the initiator, respectively, and on the polymerization temperature. A complete mechanistic interpretation of this data collection is nowadays still impossible.

On the relation between the molecular mass distribution of gelatin and its ability to stabilize emulsions

The relation between the molecular mass distribution of gelatin and its effectiveness in stabilizing emulsions of dibutyl phthalate and dodecane in water have been investigated. The molecular mass distribution was determined using gel permeation chromatography. The ability of gelatin samples to stabilize emulsions was investigated by observing the coalescence of macroscopic oil droplets in a special device. The results show that all samples with a content of more than 30 wt.-% in the low-molecular mass range are good stabilizers, whereas the stabilizing ability is diminished drastically by decreasing the low molecular mass content below 30 wt.-%. Mechanisms for the stabilization and rupture of the thin water film between the oil droplets are discussed, especially in the case of gelatin adsorption layers at the film interfaces. A model is given for the qualitative explanation of the dependence of the stabilizing ability of gelatins on the molecular mass distribution.

X-ray diffraction and foam film investigations of PC head group interaction in water/ethanol mixtures.

The influence of ethanol on single phospholipid monolayers at the water/air interface and in foam films has been investigated. Grazing incidence X-ray diffraction investigations (GIXD) of Langmuir monolayers from 1,2-distearoyl-phosphatidylcholine (DSPC) spread on water subphases with different amounts of ethanol were performed. The thickness and free specific energy of formation of foam films stabilized by 1,2-dimyristoyl-phosphatidylcholine (DMPC) at different concentrations of ethanol in the film forming dispersions were measured. The GIXD investigations show that the tilt angle of the alkyl chains in the PC lipid monolayer decreases with increasing concentration of ethanol caused by a decrease of the diameter of the head groups. With increasing ethanol content of the solution also the thickness of the aqueous core of PC lipid foam films decreases. We assume that ethanol causes a decreasing probability for the formation of hydrogen bonds of water molecules to the PC head groups. The distinct difference between the effects of ethanol on lipid bilayers as described in the literature and on monolayers and foam films found in this study is discussed. Whereas PC monolayers at the water/air interface become unstable above 25 vol.% ethanol, the PC foam films are stable up to 50 vol.% ethanol. This is related to the decrease of the surface excess energy per lipid molecule by the interaction between the two film surfaces.

Influence of Na+, Ca2+ on the thickness and free energy of dmpc foam films.

Abstract Foam films prepared from dispersions of dimyristoylphophatidylcholine (DMPC) in water and a water–ethanol mixture have been investigated. The thickness of the films and their contact angle with the surrounding meniscus of liquid have been measured as a function on the concentration of NaCl and CaCl2. The measurements show that the preparation of the lipid dispersions influences long and short-range interactions. The binding of Ca2+-ions onto the DMPC headgroups leads to charging of the monolayers and electrical double-layer repulsion in the film. There is also some indication for a weak binding of Na+-ions. The free energy of film formation determined from contact angle measurements provides additional information on the estimation of the different contributions to the interaction in the film.

Influence of ethanol on the thickness and free energy of film formation of DMPC foam films

Foam films prepared from 1,2-dimirystoil-sn-glycero-3-phosphorylcholine (DMPC) dispersions in water–ethanol mixtures were investigated. Their thickness and contact angle (foam film/meniscus) were measured. Experimental results show that an increase of EtOH concentration in the film forming dispersions leads to a decrease in the film thickness. At EtOH concentrations above 40% v/v the foam films have a bilayer structure without any noticeable core of solvent. The film thickness remains constant with the further increasing of the EtOH concentration up until 50% v/v. This behaviour is corroborated by the strong increase in the contact angles (a decrease in the free energy of film formation) with increasing EtOH concentration. Ellipsometric measurements on the thickness of adsorbed DMPC monolayers and surface pressure isotherms of DMPC spread on the water–ethanol subphases show that the effective area per lipid molecule decreases, resulting in a larger monolayer thickness, when the EtOH concentration in the subphase is increased. An increase of the EtOH concentration leads to a dehydration of the DMPC molecules and a reduction in strength and range of the repulsive hydration force between the film monolayers. The film thickness and the free energy of film formation are governed by the balance of the van der Waals attraction and the repulsive hydration force between the foam film surfaces.