Jörg Adams

Self-Stratification During Film Formation from Latex Blends Driven by Differences in Collective Diffusivity

Coatings with vertical gradients in composition were produced by drying an aqueous polymer dispersion containing both charged and neutral particles. After drying, the neutral component was enriched at the film/air interface. The spontaneous vertical segregation between the two types of particles goes back to a difference in collective diffusivity. As the film dries, a layer enriched in polymer develops at the top. Due to their mutual repulsion, charged spheres escape from this layer more quickly than their neutral counterparts. Provided that the total time of drying is between the times of diffusion for the two types of particles (approximately H(0)(2)/D(c) with H(0) the initial film thickness and D(c) the collective diffusivity of the respective species), a concentration gradient persists after the film has turned dry. This effect can be used to create a functionally graded material (FGM) in a single coating step.

Addition of Halloysite Nanotubes Prevents Cracking in Drying Latex Films

Investigating the process of film drying from aqueous dispersions containing a polymer latex as well as halloysite nanotubes (HNTs), we found that composite films could be formed without cracking under conditions where films of the pure polymer would always crack. Scanning electron micrographs showed that the HNTs were well dispersed and, further, that the distribution of fiber orientations was close to isotropic. The pendulum hardness of films formed from acrylate dispersions strongly increased upon addition of the inorganic phase. The pencil hardness, on the other hand, was poor, which presumably goes back to insufficient coupling between the organic and the inorganic phase. All films were white in appearance. For fiber concentrations higher than 10 vol %, the final films were porous.

Fabrication and fluorescence properties of perylene bisimide dye aggregates bound to gold surfaces and nanopatterns

Perylene bisimide dyes with two different imide substituents have been synthesized by sequential imidization reactions to give the disulfide 8 bearing two perylene bisimide dyes. Aggregation properties of this bis-perylene dye were studied by UV/Vis absorption and steady-state polarization-dependent as well as time-resolved fluorescence spectroscopy and the results were compared to those of the symmetrical perylene bisimide dye 4. In both cases, aggregation is expressed in a strong bathochromic shift of the excitation and the emission spectra and increasing concentration results in an increase in lifetime from 5 to 8 ns. By means of the thiol linker, self-assembly of 8 on gold surfaces was accomplished leading to surface-bound dye aggregates. Intense fluorescence from these dye aggregates was observed on surfaces decorated with hexagonal gold patterns, whereas the fluorescence is only weak on plain gold substrates.

Pyridinium salts and ylides as partial structures of photoresponsive Merrifield resins

Merrifield resin was treated with 4,4′-bipyridine and 2,4′-bipyridine, respectively, to give photochromic materials. On exposure to light, reversible color changes are observable. These resins also serve as indicators because reversible color changes are observable on addition of bases, which deprotonate the benzylic position of the N-benzylpyridinium salts to ylides. These can be detected spectroscopically and by trapping reactions on monomeric model compounds. EPR and HYSCORE measurements in combination with DFT calculations prove that radical species are formed in either case. Evidence is found that the carbanionic centers of the ylide partial structures serve as electron donators in these reversible color changes, thus setting them apart from known photochromic materials.

Ferroelectricity in a Langmuir–Blodgett multilayer film of a liquid crystalline side‐chain polymer

The observation of ferroelectric behavior in a Langmuir–Blodgett (LB) film of a liquid crystalline polymer is reported. It is established that a 30‐layer film exhibits spontaneous polarization and electro‐optic switching. The magnitude of the polarization of the LB film is found to be similar to that of the bulk material. The polarization current in the LB film follows the applied triangular wave field to higher frequencies as compared to the same material in a surface stabilized (sandwich) cell.

Solution behavior of two liquid crystalline polymers of different architectures

Solutions of two different liquid crystalline polymers of high molecular weight are investigated by static and dynamic light scattering (LS), membrane osmometry and size-exclusion chromatography (SEC). Measurements in dilute solution in different solvents showed no specific behavior as formation of aggregates or chain stiffening. Large discrepancies between the LS results and the results from osmometry and SEC show that the latter methods are in the present cases not suitable for molecular weight determination. In semi-dilute solution the osmotic modulus and the time correlation function were studied. Behavior of flexible chains was observed. In one system a slight aggregation of small molecules onto longer chains was found causing less interpenetration of the chains in that solvent. At moderately high concentrations cluster formation was observed from i) a small angle excess scattering, ii) a downturn of the osmotic modulus, and iii) the appearance of a slow motion in the time-correlation function.

Correlation between particle deformation kinetics and polymer interdiffusion kinetics in drying latex films.

Using an experimental setup which determines the turbidity of the sample and the efficiency of Förster resonance energy transfer (FRET) at the same time, we have correlated the particle deformation kinetics in a drying latex film, quantified by light scattering with the kinetics of polymer interdiffusion. Interdiffusion was quantified making use of energy transfer (FRET) between donor molecules and acceptor molecules, bound to polymer chains on different particles. When the chains cross the interparticle boundaries, the rate of energy transfer increases. The latex was prepared by miniemulsion polymerization. The amount of emulsifier employed during polymerization had a pronounced effect on the relative timing of interdiffusion and particle deformation. Increasing the amount of emulsifier delayed the onset of interdiffusion relative to the time when the film became transparent. This is mostly the consequence of a size effect, as opposed to surfactant acting as a barrier for transport.

On-line time-resolved fluorescence spectroscopy of emulsion polymerizations to study the local dynamics in the polymerizing particles

An on-line time-resolved fluorescence technique is applied to study local dynamics in polymer particles during an emulsifier-free heterogeneous polymerization. The polymerization of styrene in the aqueous phase, initiated with a polymeric initiator (hydrolyzed propene-maleic acid copolymer with tert-butyl perester groups) was chosen as an illustrative system. To analyze the heterogeneous reaction pyrenyl probes were attached to the backbone of the initiator. Within the end phase of the polymerization an increase of fluorescence lifetime and intensity was observed. Fluorescence quenching experiments of the photo excited pyrene with dibenzoyl peroxide in toluene proved that the perester groups act as quencher. The calculated quenching constant points out that the quenching reaction is diffusion controlled. The also investigated bulk polymerization of styrene with dibenzoyl peroxide as initiator and pyrene as fluorophore showed that a change of the quenching probability at the gel point is responsible for a fluorescence lifetime and intensity enhancement. The emulsion polymerization showed the gel effect, too, although the macro-initiator forms a shell around the polystyrene core. At the gel point the chain mobility is reduced so that the encounter probability of pyrenyl and perester groups is limited. Despite the turbidity of the reaction mixture, fluorescence techniques allow to study the dynamics in the polymerizing particles.

ONLINE OBSERVATION OF EMULSION POLYMERIZATION BY FLUORESCENCE TECHNIQUE

Abstract An online observation of local polarity via fluorescence spectroscopy was used to study the formation and growth of polymer particles during an emulsifier-free heterogeneous polymerization. The reaction mixture consisted of styrene dispersed in water and the polymerization was initiated by a macro-initiator (hydrolyzed propene-maleic acid copolymer with t-butyl perester groups). Pyrenyl probes were attached to the backbone of the initiator to analyze the heterogeneous reaction. The experimental results allow a clear distinction of different time regions during the heterogeneous polymerization. Information about the heating period, the latex formation, the particle growth and the final stage of the polymerization process (gel point) were obtained.

Internal Dynamics in Drying Latex Films: A Study Based on Fluorescence Anisotropy and Acoustic Dissipation

The internal dynamics of drying latex films was investigated with an instrument combining measurements of fluorescence depolarization and of acoustic dissipation. A soft latex film, containing a hydrophilic fluorescent probe, was deposited on the surface of a torsional quartz crystal resonator, thereby shifting frequency and bandwidth of the resonance. The depolarization of the fluorescence was recorded in parallel to the acquisition of the resonance parameters. As the film dried out, the fluorescence depolarization and the bandwidth of the resonator both changed in step‐wise manner. The former effect is related to the arrest of rotation on the molecular scale, whereas the acoustic technique probes the viscous compliance on a macroscopic scale. Latexes containing various amounts of sodium dodecyl sulphate (SDS) were studied. For latexes containing little or no added surfactant, the transition from a soft to hard coating (as reflected in fluorescence anisotropy and acoustic dissipation) is rather abrupt. The decrease of rotational mobility and acoustic dissipation occurs at the same time. Adding excess surfactant to the latex decreases the initial softness of the wet film and makes the transition from soft to hard more gradual.