Bernd Stühn

Direct Observation of Structure Formation at the Temperature-Driven Order-To-Disorder Transition in Diblock Copolymers

The order-to-disorder transition in a nearly symmetric diblock copolymer of polystyrene/poly(cis-1,4)isoprene (Mw = 15 700) was reproducibly observed in heating and cooling runs using small-angle scattering and rheological measurements. Small-angle x-ray scattering characterizes the ordered structure as consisting of stacks of about 20 lamellae. The transition temperature is TMST = 362 K and about 30 K below the spinodal temperature Ts which is extrapolated from the high-temperature phase. Measurements of the dynamic shear modulus reveal a drastic change of the spectrum of relaxation times at TMST. The transition is shown to be discontinuous and accompanied by strong concentration fluctuations in accordance with theoretical expectations.

Redox- and mechano-chromic response of metallopolymer-based elastomeric colloidal crystal films

A novel and convenient route for the preparation of monodisperse ferrocene-containing core-interlayer-shell particles using emulsion polymerisation protocols is described. These metallopolymer-based particles can be used to produce highly ordered elastomeric opal films with fascinating distinct reflection colours. The present work additionally describes the combined addressability of both stimuli, redox-chemistry and mechanical stress. The obtained materials are interesting since application of the melt-shear process followed by crosslinking of the matrix provides access to large-area, mechano-responsive elastomeric opal films featuring an additional redox response. Both, basic synthesis aspects and first steps towards application in the field of stimuli-responsive sensing of respective materials are discussed.

Utilizing Stretch‐Tunable Thermochromic Elastomeric Opal Films as Novel Reversible Switchable Photonic Materials

In this work, the preparation of highly thermoresponsive and fully reversible stretch-tunable elastomeric opal films featuring switchable structural colors is reported. Novel particle architectures based on poly(diethylene glycol methylether methacrylate-co-ethyl acrylate) (PDEGMEMA-co-PEA) as shell polymer are synthesized via seeded and stepwise emulsion polymerization protocols. The use of DEGMEMA as comonomer and herein established synthetic strategies leads to monodisperse soft shell particles, which can be directly processed to opal films by using the feasible melt-shear organization technique. Subsequent UV crosslinking strategies open access to mechanically stable and homogeneous elastomeric opal films. The structural colors of the opal films feature mechano- and thermoresponsiveness, which is found to be fully reversible. Optical characterization shows that the combination of both stimuli provokes a photonic bandgap shift of more than 50 nm from 560 nm in the stretched state to 611 nm in the fully swollen state. In addition, versatile colorful patterns onto the colloidal crystal structure are produced by spatial UV-induced crosslinking by using a photomask. This facile approach enables the generation of spatially cross-linked switchable opal films with fascinating optical properties. Herein described strategies for the preparation of PDEGMEMA-containing colloidal architectures, application of the melt-shear ordering technique, and patterned crosslinking of the final opal films open access to novel stimuli-responsive colloidal crystal films, which are expected to be promising materials in the field of security and sensing applications.

Dielectric study of the crystal-amorphous interphase in poly(vinylidene fluoride)/poly(methyl methacrylate) blends

Abstract We investigated the dielectric properties of crystalline poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blends in a wide range of frequency f at various temperatures T . A large peak of the dielectric loss e ″( f ) due to the interfacial polarization was found in the blend below the frequency region of the α c process. Such a peak was not observed in neat PVDF. The difference may be ascribed to the existence of a large amount of PMMA at the crystal-amorphous interphase in the blend. We also demonstrated that a low T peak in e ″( T ), which was thought of as an interfacial peak, is not related to the α a process of PVDF in the interphase, but to the β process of PVDF.

Dielectric studies of specific interaction and molecular motion in single-phase mixture of poly(methyl methacrylate) and poly(vinylidene fluoride)

Abstract It is well known that the miscibility of dissimilar polymers with high molecular weights arises from specific interaction, such as hydrogen bonding. To discuss the influence of the interaction on a side group rotation and main chain motion of poly(methyl methacrylate) (PMMA) in PMMA/poly(vinylidene fluoride) (PVDF) blends, we investigated the dielectric loss e″ as a function of frequency f. e″(f) showed a peak at around 104 Hz, which is assigned to the β process characteristic of the side group rotation of PMMA below the glass transition temperature (Tg) and the αβ process characteristic of the side group rotation cooperative with the main chain motion above Tg. Above Tg, the peak frequency fmax increased and then decreased with increasing PVDF content (/gf); fmax(/gf) showed a peak at /gf ≈ 10 wt.%. The increase may be interpreted by the acceleration of main chain motion by lowering of Tg with increasing the content of the lower Tg component (PVDF). Increasing further the PVDF content, the specific interaction seems to restrict the main chain motion and prevails over the Tg effect so that the peak appears in the fmax(/gf) curve. fmax decreased with increasing annealing time ta, suggesting the increase in restriction with ta. This may imply that the association of dissimilar polymers by the specific interaction is a very slow rate process on a time scale of thousand minutes. Also observed after annealing were broadening of the relaxation time distribution and the appearance of interfacial polarization, suggesting a change in chain conformation from random coil to a locally stretched state to form a nematic domain.

Two Glass Transitions and Secondary Relaxations of Methyltetrahydrofuran in a Binary Mixture

We investigate the molecular dynamics in the binary glass forming system methyltetrahydrofuran (M-THF) and tristyrene. Although the components are miscible in the full concentration and temperature range, two glass transitions can clearly be distinguished in differential scanning calorimetry. We selectively probe the reorientational dynamics of M-THF and tristyrene by means of dielectric spectroscopy and depolarized dynamic light scattering, respectively. While, apart from the observed plasticizer effect, the motion of the larger molecules remains almost unchanged, it is shown that the smaller M-THF molecules take part in both glass transitions. Moreover, below the upper T(g) of the mixture, the remaining mobile M-THF molecules clearly show confinement effects in their relaxation behavior. In order to elucidate the nature of the observed secondary relaxation processes, we first characterize the influence of the methyl group of M-THF on the dynamics in the mixtures by comparing the results obtained so far with the relaxation behavior observed in blends of THF and tristyrene. Finally, we employ (2)H NMR spectroscopy to clarify the nature of the secondary relaxations of THF-d(8) in the latter mixtures and conclude on the basis of the NMR and dielectric results that the high-frequency wing observed in neat M-THF appears as a genuine Johari-Goldstein β-relaxation in the mixtures, whereas the faster secondary process is due to internal degrees of freedom of the nonrigid THF ring.

Supercooling of water confined in reverse micelles

We report on the temperature dependence of the nanosecond-timescale dynamics of the ternary mixture water/AOT/oil with deuterated heptane, toluene or decane as the oil. Water-swollen reverse micelles as formed in such microemulsions allow us to investigate the freezing behaviour of water confined in a soft environment. We report here on the first neutron scattering studies in which the freezing of the confined water and of the oil is followed down to temperatures at which the whole system is frozen. We focus on studies of water confined in three different droplet sizes: by means of small-angle neutron scattering we have determined the radii to be 46, 18, and 7 A for water to surfactant ratios ω = 40, 12, and 3. From elastic temperature scans by neutron backscattering we deduce a strong supercooling of water confined in the reverse swollen micelles which increases with decreasing droplet size. For the smallest droplets we find a supercooling of more than 45 K compared to bulk water.

Relaxation phenomena and development of structure in a physically crosslinked nonionic microemulsion studied by photon correlation spectroscopy and small angle x-ray scattering

We performed small angle x-ray scattering (SAXS), photon correlation spectroscopy (PCS), and oscillatory frequency sweep experiments to investigate the static and dynamic properties of a transient network. The system under investigation is an oil in water microemulsion containing the nonionic surfactant C12E5 (penta ethylene glycol mono dodecyl ether). The microemulsion consists of micelles (spherical oil droplets covered by the surfactant) with radius Rm≈10 nm, which are dispersed in water. Adding a triblock copolymer with a hydrophilic midblock and hydrophobic end blocks (stearate-poly ethylene oxide-stearate) to the microemulsion leads to the interconnection of the micelles and thus to the formation of a transient network. We describe the structural behavior under addition of the triblock copolymer with a hard sphere model based on the Percus–Yevick pair correlation function. In the dynamic light scattering experiment we observe three relaxational processes. The fast and the slow relaxation time vary w...

Dynamics of water confined to reverse AOT micelles

We use quasi-elastic neutron scattering (QENS) to study the dynamics of water confined inside reverse micelles. As a model system we use a water-in-oil droplet microemulsion based on the anionic surfactant AOT (sodium bis[2-ethylhexyl] sulfosuccinate), that forms spherical water droplets coated by a monolayer of AOT dispersed in the continuous oil matrix. Combining neutron time-of-flight (TOF) and backscattering (BS) spectroscopy, we access the dynamical behaviour of water over three decades in time from pico- to nanoseconds. We investigate the influence of reverse micelle size on the water dynamics by comparing two sample systems with bigger and smaller water core radii of about Rc ≈ 12 A and 7 A. The temperature is varied over a range where both microemulsion systems are stable, from room temperature down to the region where the confined water is supercooled: 260 K ≤ T ≤ 300 K. Taking explicitly into account the previously measured diffusion of entire reverse micelles in the oil matrix we find the average mobility of the confined water to be considerably slowed with respect to bulk water. The translational diffusion decreases with decreasing reverse micelle size. Dependent on the reverse micelle size we can interpret our data by assuming two dynamically separated water fractions. We identify the faster one with bulk-like water in the middle of the core while the slower one seems to be surfactant bound water. We find that 4 molecules of water per AOT molecule are immobilized on the timescale of QENS, i.e. shorter than nanoseconds.

Low-frequency relaxation modes in ferroelectric liquid crystal/gold nanoparticle dispersion: impact of nanoparticle shape

ABSTRACT Low-frequency (1 mHz–100 Hz) dielectric relaxation modes were experimentally studied in ferroelectric liquid crystal (FLC)/gold nanoparticles (nanospheres and nanorods) dispersion. It was demonstrated that the dielectric spectra of nanodispersion are strongly influenced by the shape of nanoparticles. Using different formalisms of the impedance spectroscopy, three possible low-frequency relaxation processes were found in the dispersions and the pure FLC. Due to the electrical double layers (EDLs) near nanoparticles and the alignment layers, one can observe the relaxation of the EDL polarisation around the nanoparticles (Schwarz’s relaxation) and near the driving indium tin oxide (ITO) electrodes (electrode polarisation). The other possible relaxation process is interfacial polarisation (Maxwell–Wagner mode) in which the frequency is unaffected by the nanoparticles. It was shown that Schwarz’s relaxation frequency strongly depended on the shape and size of the nanoparticles. Moreover, dispersion of nanoparticles significantly reduced direct current conductivity of the FLC mixture. GRAPHICAL ABSTRACT