Erhard W. Fischer

DYNAMICS OF GLASS-FORMING LIQUIDS. I: TEMPERATURE-DERIVATIVE ANALYSIS OF DIELECTRIC RELAXATION DATA

We have measured the dielectric relaxation times of the α process of phenyl salicylate (salol) covering 11 decades in frequency. Being representative for the class of low molecular weight organic glass forming materials, the highly resolved temperature dependence of the dynamics in salol does not follow a particular function like the Vogel–Fulcher–Tammann (VFT) law over the entire accessible range of temperatures. In order to conduct a detailed and unambiguous analysis of the temperature dependence, we take advantage of the derivatives of the experimental log(fmax) values with respect to temperature, which allow us to either linearize the frequently used temperature laws or to resolve subtle changes in fmax(T) by decreasing the number of free parameters. In this manner we observe that none of the common routes for rationalizing the dynamics like Arrhenius, VFT, Souletie scaling, and idealized mode‐coupling theory account for the experimental findings properly. However, we do observe a VFT behavior within ...

Dynamics of glass‐forming liquids. II. Detailed comparison of dielectric relaxation, dc‐conductivity, and viscosity data

We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency fmax(T) of dielectric loss e″(ω) and the dc‐conductivity σdc(T) of several glass‐forming liquids, covering 12 decades in the peak frequency fmax and 9 decades in σdc. Although dc‐conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of fmax(T). The fmax(T) and σdc(T) are analyzed using the temperature‐derivative method and compared to the viscosity data η−1(T). While most liquids reveal a common Vogel–Fulcher–Tammann (VFT) behavior for fmax, σdc and η−1 in an extended temperature range T≥Tm, some liquids deviate from this behavior by displaying a crossover at T=TA to an Arrhenius regime. In these cases the quantity fmax(T) decouples from the common curves for σdc(T) and η−1(T) and attains activation energies in excess (∼40% for alcohols) of those related to translational processes. For many samples a departure from the VFT behavior occurs at lower tempera...

Dynamics of glass-forming liquids. III. Comparing the dielectric α- and β-relaxation of 1-propanol and o-terphenyl

We have measured the dielectric relaxation of the glass-former 1-propanol for temperatures between 65 and 350 K in the frequency range 10−2 to 2⋅1010 Hz and the photon correlation spectro-scopy decays near Tg. Attributing the strong Debye-type process of 1-propanol to distinct -OH group effects leaves two faster processes related to the structural relaxation which can be identified as α-relaxation and Johari–Goldstein type β-relaxation characteristic of nonhydrogen-bonding supercooled liquids. From the temperature dependent relaxation times τ(T) regarding the three distinct loss peaks, we can specify an α-β-bifurcation temperature Tβ, which coincides with characteristic qualitative changes in the τ(T) behavior, as also observed for ortho-terphenyl and other glass-forming liquids. This assignment is confirmed by the correla-tion times derived from incoherent quasielastic light-scattering data obtained from the simultaneously measured photon-correlation spectroscopy.

Light scattering and dielectric studies on glass forming liquids

The α-relaxation behaviour of polymers and glass forming viscous liquids is well described by the Vogel-Fulcher-Tammann equation, introducing a Vogel temperature T0 at which the relaxation time τα diverges (T0 ≈ Tg - 50 K). With the recent development of the mode coupling theory a new critical temperature Tc is introduced located about 50 to 80 K above Tg. The relevance of the various temperatures is discussed on the basis of dynamic light scattering studies and dielectric relaxation data. The dynamic and static light scattering experiments revealed some unexpected features, which cannot be explained on the basis of conventional liquid state theories: (1) In static light scattering the intensity I(q→0) is no longer proportional to the isothermal compressibility. (2) This excess scattering Iexc shows a strong q-dependence (q = (4πn/λ)sin(θ/2)) corresponding to a correlation length ξ in the range of 20–200 nm. (3) The Landau-Placzek ratio IRayleigh/2IBrillouin is much too high, compared with the results of light scattering theories. (4) In photon correlation spectroscopy a new ultraslow hydrodynamic mode (Γ∼q2) is detected with relaxation rates Γ about 10-4 to 10-7 lower than those of the α-process at a given temperature.

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.

Dynamics of glass-forming liquids. IV. True activated behavior above 2 GHz in the dielectric α-relaxation of organic liquids

We have measured the dielectric relaxation of butylbenzene and of the glass-former propylbenzene in the frequency range 10−2 Hz to 2×1010 Hz in order to characterize the variation of relaxation times with temperature for these low loss liquids. Additionally, salol has been remeasured above 1 GHz with improved resolution. Using the sensitive data representation [−dlog10(fmaxHz)/d(1/T)]−1/2 vs 1/T we find demarcation temperatures TA, at which the temperature dependence changes from a Vogel–Fulcher type law within the limits TB⩽T⩽TA to Arrhenius behavior for T>TA, corresponding to a position of the loss peak fmax>2 GHz. The activation energies derived from dielectric relaxation data for T>TA are associated with the energy of vaporization, Eη∝ΔEvap. A comparison of dielectric relaxation times τD to viscosity data in this wide range of temperatures suggests the relation τD∝η/T rather than τD∝η.

Depolarized dynamic light scattering studies of ortho‐terphenyl dynamics above Tg

Depolarized Rayleigh spectra of ortho‐terphenyl (OTP) were measured in the temperature range from Tg to Tg+190 K. Two samples prepared with and without ‘‘clusters’’ were used for the measurement. Four different Fabry–Perot interferometers covering the time range from about 0.5 ps to 100 ns were employed. Two relaxation modes were observed: a slow mode with all its characteristics of the α process, and a fast mode with a constant, temperature independent relaxation time of about 3 ps. The fast mode has not been reported heretofore. The intensity of the fast mode vanishes at about the Vogel–Fulcher–Tamman temperature T0. The relaxation times of these two processes are found to be identical for both OTP with and without clusters. The temperature dependence of the relaxation time of the α process is Arrhenius at high temperatures, but shows a Williams–Landel–Ferry (WLF) behavior in the range from Tg to Tg+80 K for both samples with and without the long‐range density fluctuations (i.e., cluster).

Dynamical processes in organic glassforming van der Waals liquids

This report presents measurements of the frequency dependent dielectric loss e‘(ω) and quasielastic light scattering of thermal density fluctuations of simple organic glassforming van der Waals liquids BCDE (bis‐phenol‐C‐dimethylether) and BKDE (bis‐kresol‐C‐dimethylether) having glass transition temperatures of Tg=240 K and Tg=261 K, respectively. The dielectric measurements of BCDE were carried out in a temperature range between 250 and 316 K, those of BKDE between 270 and 384 K covering a frequency range from 10−1 to ∼109 Hz in each case. Quasielastic light scattering data were taken between the glass transition temperatures and T≂Tg+30 K in a correlation time window of 10−6 s up to 102 s. As main results we have found the following: The relaxation process in the BCDE melt can be described using a Kohlrausch–Williams–Watts (KWW) function with a temperature independent β parameter. Its value was found to be identical either from the light scattering result or from the stretched exponential fit to the Fo...

Surface melting in melt-crystallized linear polyethylene

Abstract Morphological changes during the premelting of linear polyethylene were investigated by small-angle X-ray scattering using correlation-function analysis. The thickness of the amorphous regions, 〈 d a 〉, showed a reversible change with temperature, which indicates presence of a local equilibrium state. During heating, the average thickness of lamellae 〈 d c 〉 decreases first for temperatures below the initial crystallization temperature ( T c ). The simultaneous decrease in 〈 d c 〉 and increase in 〈 d a 〉 provide evidence for a surface melting of the lamellae. Above T c an irreversible thickness growth of the crystalline lamellae (increase of 〈 d c 〉) is observed. It does not affect the thickness of the amorphous regions, which remains unchanged.

Critical behavior in a binary polymer blend as studied by static and dynamic light scattering

We report static and dynamic light scattering experiments of an almost symmetric polymer mixture made up from poly(dimethylsiloxane) (PDMS), N=260, and poly(ethylmethylsiloxane) (PEMS), N=340, with N being the degree of polymerization, both below the entanglement molecular weights. The mixture exhibits an upper critical solution temperature Tc≂57 °C. The measurements were performed at the critical composition φc,PEMS = 0.465 in a broad temperature range in the one phase region above the spinodal point. The main results for the static case are: the temperature dependence of the static structure factor S(q=0) can be described by a mean field behavior. For T close to Tc, a crossover to an Ising behavior is observed according to a modified Ginzburg criterion. From the angular dependence of S(q), the static correlation length ξ is determined via an Ornstein–Zernike plot. Our experimentally determined values for limT→∞S(0) and limT→∞ ξ, respectively, are in agreement with theoretical predictions. For the dynami...