Glass transition in Ultrathin Polymer Films : A Thermal Expansion Study

Glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of glass transition temperature (Tg) as the thickness of the polymer film reduces below the radius of gyration but the change in the average Tg was found to be very small. Existence of reversible negative and positive thermal expansion below and above Tg increased the sensitivity of our thickness measurements performed using energy dispersive x-ray reflectivity. A simple model of Tg variation as a function of depth expected from sliding motion could explain the results. We observe clear glass transition even for 4 nm polystyrene film that was predicted to be absent from ellipsometry measurements of thicker films.Comment: 11 pages, 5 figure

A quantitative comparison of different methods to detect cardiorespiratory coordination during night-time sleep

BACKGROUND: The univariate approaches used to analyze heart rate variability have recently been extended by several bivariate approaches with respect to cardiorespiratory coordination. Some approaches are explicitly based on mathematical models which investigate the synchronization between weakly coupled complex systems. Others use an heuristic approach, i.e. characteristic features of both time series, to develop appropriate bivariate methods. OBJECTIVE: In this study six different methods used to analyze cardiorespiratory coordination have been quantitatively compared with respect to their performance (no. of sequences with cardiorespiratory coordination, no. of heart beats coordinated with respiration). Five of these approaches have been suggested in the recent literature whereas one method originates from older studies. RESULTS: The methods were applied to the simultaneous recordings of an electrocardiogram and a respiratory trace of 20 healthy subjects during night-time sleep from 0:00 to 6:00. The best temporal resolution and the highest number of coordinated heart beats were obtained with the analysis of 'Phase Recurrences'. Apart from the oldest method, all methods showed similar qualitative results although the quantities varied between the different approaches. In contrast, the oldest method detected considerably fewer coordinated heart beats since it only used part of the maximum amount of information available in each recording. CONCLUSIONS: The method of 'Phase Recurrences' should be the method of choice for the detection of cardiorespiratory coordination since it offers the best temporal resolution and the highest number of coordinated sequences and heart beats. Excluding the oldest method, the results of the heuristic approaches may also be interpreted in terms of the mathematical models

Liquid crystalline behaviour of photochromic LB multilayer films

Amphotropic copolymers containing rod-like side groups combine the ability to form Langmuir-Blodgett multilayer assemblies with thermotropic liquid crystalline self-organization on temperature changes. Photochromic azobenzene side groups hereby affect the supramo-lecular structure of LB multilayer structures in dependence on irradiation wavelength and polarization of the incident light beam as well as on temperature.A phase transition from the constrained LB film into a liquid crystalline like layer structure can be easily reached upon annealing at a temperature where the bulk polymer performs a smectic liquid crystalline phase and/or different irradiation procedures.Thus, changes in the structure of LB multilayers of the observed copolymers were studied in dependence on temperature within the existence range of different glassy, crystalline and liquid crystalline states by means of energy-dispersive small angle X-ray scattering and X-ray grazing incidence diffraction using synchrotron radiation due to the excellent compatibility with the observed polymer materials and the measuring problems

X-ray scattering from thin organic films and multilayers

The real structure of LB-multilayers prepared with fatty-acid salts is dominated by finite-sized scattering aggregates. Their different length scales become visible using AFM. It shows that not the whole substrate is wetted by the film. The molecular order is restricted into domains. These micrometer domains are not homogeneous. They contain mesoscopic subdomains of different heights which vary in steps of double layers. Finally high-resolution AFM-maps display a nearly hexagonal arrangement of molecules within subgrains with a diameter of several 10 nm. This domain structure has to be taken into account when interpreting X-ray diffraction data. The size of the crystalline aggregates is obtained by means of X-ray grazing incidence diffraction. On the mesoscopic scale the domain size is determined by X-ray diffuse scattering experiments. Because Sinha’s model fails for the present kind of multilayers, we used another approach for data analysis. The lateral correlation length caused by height fluctuations is estimated without knowledge of a definite correlation function. Additionally the mosaicity of the domain orientation can be taken into account

X-ray investigations of the molecular mobility within polymer surface gratings

The physical origin of surface relief patterning on amorphous polymer films containing azobenzene-side chains induced by holographic exposure with visible light of about 450 nm is not yet fully understood. To understand the nature of the induced material transport is of special interest to describe the dynamic processes occurring in thin films below the glass transition temperature TG. Thus, we investigated films made from the polar (poly {4′-[2-(methacryloxy) ethyl]-ethyl}amino-4-nitroazobenzene, TG=129 °C) and less polar {poly[4-(2-methacryloxy)-ethyl] azobenzene, TG=80 °C} azobenzene side-chain homopolymers and performed temperature-resolved coherent x-ray and visible (VIS) light scattering measurements of the thermally induced erasure of the surface gratings. The simultaneous use of x-ray synchrotron light (λ=0.14 nm) and VIS laser light (λ=633 nm) allows the detection of the material flow on different lengths scales. We did not find remarkable differences in the thermal behavior of polar and nonpolar materials. Depending on the time used for inscribing the gratings the VIS signal starts vanishing at a critical temperature TK below the glass temperature TG. Up to TG the x-ray grating peak intensities increase to a maximum even if the VIS signal is almost zero. Probing the grating in a different depth below the surface, the first and second order x-ray Fourier components reach their intensity maxima at different temperatures and rise up in intensity with time constants characterized by an Arrhenius-like activation energy of about 2.6 eV. At T>TG the grating peak intensities go to zero. Our measurements can be interpreted by a model of anisotropic viscosity. At TTG the created lateral density modulation becomes equalized by a lateral material flow quantified by a diffusion coefficient of about D=3×10−13 cm2 s−1