Bernd Ewen

Dynamics of hydrogen‐bonded liquids confined to mesopores: A dielectric and neutron spectroscopy study

In this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 A pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen‐bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low‐ and high‐frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature Tg by up to ΔTg≊+47 K; and (ii) relatively ‘‘free’’ molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in Tg for the α process with ΔTg...

Defect structure and molecular motion in the four modifications of n ‐tritriacontane. II. Study of molecular motion using infrared spectroscopy and wide‐line nuclear magnetic resonance measurements

It was possible to specify the motional mechanisms, which are effective in the high‐temperature modifications (B,C,D) of n ‐C33H68, using NMR and ir measurements in connection with the previously reported x‐ray experiments. Modification B is characterized by the occurrence of coupled rotational jumps with amplitudes of about 180 degrees. In modification C flip‐flop screw motions are superimposed. Passing to modification D, intramolecular defects appear, which diffuse along the chains. At the same time the rotational component of motion changes to a more continuous and less jumplike form.

Neutron spin echo investigations on the segmental dynamics of polymers in melts, networks and solutions

Neutron spin echo (NSE) spectroscopy, an advanced high-resolution quasi-elastic neutron scattering technique, provides the unique opportunity to investigate long-range relaxation processes of macromolecules simultaneously in space and time on nano-scales. In particular, information on the single-chain behavior is not restricted to dilute solutions, but may also be obtained from concentrated solutions and melts, if labelling by proton deuterium exchange is used. Thus, this method facilitates a direct microscopic study of molecular models developed to explain the macroscopic dynamic properties of polymers, e.g. transport and viscoelastic phenomena.

Phase transitions in crystals of chain molecules. Relation between defect structures and molecular motion in the four modifications of n-C33H68

n-Tritriacontane (n-C33H68) exhibits three solid–solid phase transitions before melting. Applying a variety of experimental techniques, including X-ray scattering, Raman spectroscopy, n.m.r., dielectric relaxation and quasielastic neutron scattering, it was possible to specify for all four modifications defect structures and the corresponding molecular motions.Each phase transition is accompanied by a step-like decrease in the degree of order resulting from the successive onset of rotational jumps, translational jumps in chain direction and the creation and diffusion of intrachain defects.

Phase equilibrium of poly(n-butyl acrylate) and E7

The experimental equilibrium phase diagram of mixtures of linear poly(n-butyl acrylate) of molecular mass Mw = 112000 g mol-1 and the low molecular mass LC mixture E7 has been established using polarized optical microscopy and light scattering techniques. The diagram is found to be reminiscent of an upper critical solution temperature system. Two independent series of samples with the same composition were studied, yielding consistent results. A region of nematic and isotropic coexisting phases and a region of a single isotropic phase were identified in the composition-temperature phase diagram. The results were analysed within a theoretical model combining the Flory-Huggins lattice theory for isotropic mixing and the Maier-Saupe theory for nematic ordering. Interestingly, no region of isotropic coexisting phases was observed in our experiments. This is probably due to the fact that the nematic interaction overwhelms the isotropic interaction in the region where (I + I) coexisting phases could appear. A preferential solubility of certain constituents of the LC mixture in the polymer could possibly be a reason for this behaviour.

Geometry of phenylene motion in polycarbonate from NMR spectroscopy and neutron scattering

In view of the importance of molecular dynamics in condensed matter both time scale and geometry of such processes should be determined experimentally. Whereas many techniques are available for the former, only NMR spectroscopy and neutron scattering can provide detailed information on the latter. Because of the different time scales of the dynamics, which the two techniques can detect best, direct comparisons of probing the geometry of the dynamics in the same system are scarce. Here we present such a comparison for the complex rotational motion of the phenylene groups in amorphous polycarbonate based on published (2)H NMR and newly recorded (13)C NMR data covering a wide temperature range, and recent quasielastic neutron scattering (QENS) data. We show that the results of the two techniques are in remarkable agreement, provided the data are consistently analyzed. No evidence is found for additional motions characterized by 90 degrees flips recently deduced from QENS data alone. Instead, the phenylene motion in the glassy state displays a broad heterogeneous distribution of rotational angles, about 80 degrees in width, centered at a flip angle of 180 degrees , which stays essentially constant over a wide temperature range. Thus, the phenylene motion that can consistently be observed in NMR and neutron scattering experiments is sensitive to the local packing.

Light scattering from acrylate-based polymer dispersed liquid crystals: theoretical considerations and experimental examples

A light scattering (LS) study made using acrylate-based polymer dispersed liquid crystals (PDLCs) is presented. The polarized component IVV is measured for blends of a polyacrylate and the liquid crystal (LC) E7 at several compositions. Only the off-state configuration of the droplets with no external fields is considered here. These composites consist commonly of micron-sized nematic LC droplets dispersed in a solid polymer matrix. Theoretical expressions for the scattered intensities in the case of isotropic and anisotropic spherical droplets are given both in the Rayleigh-Gans approximation (RGA) and in the anomalous diffraction approximation (ADA). Series of VV and VH components of the scattering intensities are calculated using the models of Meeten, Stein and coworkers. The model calculations are compared with the light scattering data. This comparison enables us to extract information on the size and the shape of droplets assuming that the size distribution is uniform and that the scattering is due ...

Phase diagrams of poly(dimethylsiloxane)/E7 mixtures

Abstract The phase behavior of blends of poly(dimethylsiloxane) and the eutectic mixture of liquid crystals E7 is investigated. Experimental phase diagrams are established by polarized optical microscopy for two systems with widely different polymer molecular weights. Surprisingly, we find a very slight loss of miscibility of the mixture as a result of an increase of the polymer molecular weight by almost an order of magnitude. This unexpected phase behavior is in contrast with the results reported so far on other systems.

Mechanical Properties of Electron Beam Cured Monomer and Monomer/Liquid Crystal Films

This article reports on mechanical properties of electron beam cured tripropylene glycol diacrylate (TPGDA) and propoxylated glycerol triacrylate (GPTA) films. This study has been motivated by the need to have direct access to those properties for analyzing the thermo-mechanical behavior and electro-optical properties of polymer dispersed liquid crystal systems in general, and systems made either of TPGDA or GPTA and low molecular weight liquid crystals in particular. Representative examples of these systems are considered in this work. The effects of the degree of crosslinking on the mechanical strength of the polymer network are analyzed by considering different doses of the electron beam irradiation. As the radiation dose increases, the mechanical strength of the film is enhanced. Addition of a small amount of liquid crystals leads to remarkable plasticizing effects.

Neutron spin echo investigations on the segmental dynamics in semidilute polymer solutions under Θ- and good solvent conditions

Abstract Semidilute polymer solutions can be considered as transient networks of lifetime, τg, and of average meshsize, ξ. ξ depends on the polymer concentration, c, and the solvent conditions. If the segmental diffusion of such systems is studied on timescales t ⪡ τg using the neutron spin echo technique, different relaxation modes and continuous transitions between them are identified by varying the magnitude of the scattering vector, q. Under good solvent conditions, a crossover from single chain Zimm dynamics, as valid in dilute solutions on the entire intramolecular length scale, to the collective many chain diffusion is found at qξ(c) ≈ 1. The corresponding relaxation rates, Ω(q) , vary from Ω(q) ∼ q 3 to Ω(q) q2. In Θ-solvents, the many chain regime was not accessible. However, due to the existence of self-entanglements, which introduce an additional length scale ξi(c) (Ω(q) ∼ q 3 ) to entangled single chain relaxation (Ω(q) ∼ q) becomes visible.