G. Meier

Density fluctuations around the glass-transition of low molecular weight glass-forming liquids

Abstract The Rayleigh-Brillouin spectra of ortho-terphenyl (OTP) samples with and without ‘clusters’ were measured in the temperature range from Tg to Tg + 200 K. The values of the Landau-Placzek ratio for cluster-free OTP were low, in agreement with the theoretical predictions, while for OTP with clusters much higher values were obtained. The isotropic part of the Rayleigh intensity for OTP with clusters contained an additional angular-dependent component that could be reversibly reduced after annealing the samples at temperatures > Tm. Rotational relaxation times of OTP samples with and without clusters were measured by means of depolarized Rayleigh spectroscopy. In addition to the usual rotational relaxation time, an additional fast component was found. The relaxation time of the fast process was practically constant in the entire temperature range amounting to 3 ps and the intensity decreased to zero around TO. The temperature dependence of the rotational relaxation time cannot be described by the Wohlrausch-Landel-Ferry formula over the entire relaxation time range from 4 × 10−11 to 10 s, but only down to 10−10 s.

Depolarized light scattering studies of the boson peak and the transversal phonons in a glass-forming liquid: OTP☆

A lineshape analysis of the Rayleigh wing of ortho-terphenyl obtained by low frequency Raman spectroscopy has been performed. The so-called boson peak was studied from 100 to 300 K, from far below to above the glass transition at Tg = 244 K. A model of disorder-induced scattering was used to interpret the features of the boson peak. A complementary measurement of the transversal phonons of OTP was done in the temperature range from 250 to 360 K. It was found that the shear waves are only present up to 320 K.

Mode-coupling and phenomenological analysis of the depolarized light scattering spectra of ortho-terphenyl

The depolarized dynamic light scattering spectra of ortho-terphenyl covering a broad frequency range from 10−2 to 6 × 1012 Hz have been analyzed using both the mode coupling theory (MCT) and the phenomenological approach. Similarities and differences of these two approaches as well as the frequency range and the quality of the fits to the experimental data are discussed. The main differences between these two models occur only in the intermediate frequency range (0.1–100 GHz) where either the MCT β-relaxation with a scaling time diverging on both sides of Tc or a ‘fast’ process of a constant relaxation time can be used to fit the data.

Phase behaviour of mixtures of polyethylene glycol and polypropylene glycol: Influence of hydrogen bond clusters on the phase diagram

The phase behaviour of a binary polymer blend of polyethylene glycol (PEG600) and polypropylene glycol (PPG1000) of low polydispersity was investigated. The coexistence curves of five different compositions, including the critical composition, were measured by differential refractometry. From the coexistence curves, the critical exponents of the order parameter could be determined. The order parameter of the coexistence curve of the critical composition was βu2006=u20060.357, near to the Ising value. The measured coexistence curves for different compositions were different to each other and to the cloud point curve measured by small angle light scattering. These differences are explained by clusters built of intermolecular hydrogen bonds which lead to a broad effective molar mass distribution of the mixture. Two of the investigated mixtures, the mixture of critical composition (yPPGu2006=u20060.46, yPPG: nmass fraction of polypropylene glycol) and a mixture of non-critical composition (yPPGu2006=u20060.365) were also measured by static and dynamic light scattering. The measurements were carried out in the homogeneous one-phase region (above the critical temperature TC) and in the two-phase region in both coexisting phases. The light scattering measurements showed, additionally to the composition fluctuations due to the phase transition of demixing, a contribution of larger objects, which could be attributed to hydrogen bonded clusters. The size of these clusters was estimated to be 250 nm to 550 nm.

Phase behaviour of mixtures of polyethylene glycol and polypropylene glycol: Influence of hydrogen bond clusters on critical composition fluctuations

The critical behaviour of a binary polymer blend of polyethylene glycol 600 (PEG600) and polypropylene glycol 1000 (PPG1000) was investigated by static and dynamic light scattering. The measurements were carried out in the homogeneous one-phase region (above the critical temperature Tc) and in the two-phase region in both coexisting phases. Additionally to the critical composition (yPPGu2006=u20060.46, yPPG: mass fraction of PPG), a mixture of non-critical composition (yPPGu2006=u20060.365) was investigated in the one- and two-phase region. From the light scattering measurements, the critical exponents and the amplitudes of the correlation length ξ and the mutual diffusion coefficient D were determined. For the critical mixture the results for the critical exponents and amplitudes of ξ in the one- and two-phase regions are in accordance with the predictions of the 3d-Ising model. The measurements of D in the two-phase region in a wider temperature range gave an indication for a crossover from Ising to mean field behaviour. Since the low molar mass of the PEG and PEG oligomers would lead to Ising behaviour in the composition and temperature range studied, this crossover has to be attributed to an increase of the effective molar mass caused by clusters built by intermolecular hydrogen bonds. The assumption of these clusters is supported by static light scattering and the phase diagram. The data of the non-critical mixture could be described in the frame of the pseudospinodal concept. The results for the non-critical mixture support the interpretation of the data of the critical mixture.

Depolarized light scattering spectroscopic study of OTP dynamics above Tg : a mode coupling analysis approach

The dynamics of ortho-terphenyl (OTP) was studied using a Tandem-Fabry-Perot interferometer (FPI) and a grating monochromator. The results were analyzed using the mode coupling approach. The scaling laws predicted in this theory were partly confirmed.

Composition and molecular weight dependence of the interdiffusion coefficient and critical behavior in a binary polymer blend as measured by quasi-elastic light scattering

Abstract Quasi-elastic light scattering experiments by means of photon correlation spectroscopy have been employed for the study of diffusional dynamics in the compatibble polymer blend poly(dimenthylsiloxane)PDMS/poly(ethylmethylsiloxane)PEMS at various temperatures (−20 to 120°C) and over the entire composition (θ) range. From the relaxation time of the concentration fluctuations the mutual diffusion coefficient, D , could be determined for a series of molecular weights ranging from 5000 D up to 30 000 D below the entanglement value. In addition the static structure factor, S(q), was measured. It was found to be indepents of the waveve ctor, q, at temperatures far above the spinodal. The Onsager transport coefficient Ω = D S(q = 0 ) was found to be independent from chain length implying ‘Rouse’-like behavior. Further, the composition dependence of D in the present blend, which possesses a virtually θ-independent glass transition temperature, Tg, in order to distinguish between the Binder-Brochard-de Gennes and the Kramer-Sillescu approaches which connect the mutual diffusion D with the tracer diffusivities differently, was studied. The experiments support the Kramer-Sillescu approach, which means that there is a linear dependence between W0 and the θ-weighted tracer diffusivities if one assumes θ-independent monomeric friction coefficients for the mixture components. Further, it was possible to measure that the interdiffusion coefficient, D , undergoes a critical slowing down for the critical composition as the temperature is lowered towards the critical tempperature, Tc. Up to very close Tc, we observe a mean field type behavior.