Andreas Schönhals

Broadband Dielectric Spectroscopy

A. Schoenhals, F. Kremer: Theory of Dielectric Relaxation.- F. Kremer, A. Schoenhals: Broadband Dielectric Measurement Techniques.- A. Schoenhals, F. Kremer: Analysis of Dielectric Spectra.- F. Kremer, A. Schoenhals: The Scaling of the Dynamics of Glasses and Supercooled Liquids.- P. Lunkenheimer, A. Loidl:Glassy Dynamics Beyond the a-Relaxation.- F. Kremer, A. Huwe, A. Schoenhals, S. Rozanski: Molecular Dynamics in Confining Space.- A. Schoenhals: Molecular Dynamics in Polymer Model Systems.- G. Floudas: Effect of Pressure on the Dielectric Spectra of Polymeric Systems.- J. Mijovich: Dielectric Spectroscopy of Reactive Polymeric Systems.- F. Kremer, A. Schoenhals: Collective and Molecular Dynamics of (Polymeric) Liquid Crystals.- L. Hartmann, K. Fukao, F. Kremer: Molecular Dynamics in thin Polymer Layers.- F. Kremer, S. Rozanski: The Dielectric Poperties of Semiconducting Disordered Solids.- P.A.M. Steeman, J. v. Turnhout: The Dielectric Properties of Inhomogeneous Media.- R. Boehmer, G. Diezemann: Principles and Applications of Pulsed Dielectric Spectroscopy and Nonresonant Dielectric Hole Burning.- R. Richert: Local Dielectric Relaxation by Solvation Dynamics.- T. Pakula: Dielectric and Dynamic Mechanical Spectroscopy-A Comparison.- R. Boehmer, F. Kremer: Dielectric and (Multidimensional) NMR Spectroscopy-A Comparison.- A. Arbe, J. Colmenero, D. Richter: Polymer Dynamics by Dielectric Spectroscopy and Neutron Scattering-A Comparison

Analysis of Dielectric Spectra

The complex dielectric function ɛ*(ω) in its dependence on angular frequency (\( \omega = 2\pi v \) (v-frequency of the outer electrical field) and temperature originates from different processes: (i) microscopic fluctuations of molecular dipoles [1] (rotational diffusion1), (ii) the propagation of mobile charge carriers (translational diffusion of electrons, holes or ions), and (iii) the separation of charges at interfaces which gives rise to an additional polarization. The latter can take place at inner dielectric boundary layers (Maxwell/Wagner/Sillars-polarization [2, 3] ) on a mesoscopic scale and/or at the external electrodes contacting the sample (electrode polarization) on a macroscopic scale. Its contribution to the dielectric loss can be orders of magnitude larger than the dielectric response due to molecular fluctuations.

Molecular Motions in Amorphous Ibuprofen As Studied by Broadband Dielectric Spectroscopy

The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as alpha, beta, gamma, and D, were detected and characterized, and a complete relaxation map was given for the first time. The gamma-process has activation energy E a = 31 kJ.mol (-1), typical for local mobility. The weak beta-relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the alpha-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, T B = 265 K. From the low temperature VFTH regime, a T g (diel) (tau =100 s) = 226 K was estimated, and a fragility or steepness index m = 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the alpha-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.

T g depression and invariant segmental dynamics in polystyrene thin films

We investigate the segmental dynamics and glass transition temperature (Tg) of polystyrene (PS) thin films. The former is investigated by alternating current (AC) calorimetry and dielectric spectroscopy (BDS). The Tg, underlying the equilibrium to out-of-equilibrium crossover from the supercooled liquid to the glass, is obtained by differential scanning calorimetry (DSC) and capacitive dilatometry (CD). We show that the intrinsic molecular dynamics of PS are independent of the film thickness both for the freestanding and supported films, whereas Tg decreases with film thickness from several microns down to 15 nm. This result is found for complementary methods and in a simultaneous measurement in BDS and CD. This questions the widespread notion that segmental mobility and the equilibrium to out-of-equilibrium transition are, under any experimental conditions, fully interrelated. For thin films, it appears that the molecular mobility and Tg are affected differently by geometrical factors.

Segmental and chain dynamics of polymers: from the bulk to the confined state

Dielectric spectroscopy and temperature modulated DSC are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover the results obtained for the polymer are compared with the corresponding monomer. For large pore sizes an acceleration of the segmental dynamics compared to the bulk state is observed which is already known for low molecular-weight glass forming liquids. For smaller pore sizes the molecular dynamics is slower than in the bulk. The observed behavior is nearly independent of the molar mass of the polymer and of the treatment of internal glass surfaces. The experimental results are discussed in the frame of an interplay of confinement and adsorption effects. Moreover a length scale of about 1.6 nm is estimated as a minimal length scale for the cooperativity for the glass transition. In addition to the α-relaxation the whole chain dynamics (normal mode relaxation) can be measured by dielectric spectroscopy because PPG has a dipole component parallel to the chain. For virgin internal surfaces the relaxation rate of the normal mode relaxation is shifted dramatically to lower relaxation rates. That can be explained by adsorption effects. For treated surfaces this effect is strongly reduced and it is concluded that also in this case the chain dynamics are influenced by geometric (confinement) effects.

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6 naphthalene dicarboxylate). I. Amorphous films

Abstract The dielectric and dynamic mechanical behaviour of amorphous poly(ethylene 2,6 naphthalene dicarboxylate) (PEN) and of thermally annealed samples is reported as a function of the morphology. During the treatment at temperatures close to the melting temperatures the samples become semi-crystalline. differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment. However the degree of crystallinity as well as the melting temperatures increase with the annealing temperature. Both dielectric (DEA) and dynamic mechanical (DMA) analysis display three relaxation processes. In order of decreasing temperature the α-relaxation due to the glass transition, the β ∗ -process assigned to the out of plane movements of the naphthalene rings or aggregates of it and the β-relaxation due to local molecular motions of carbonyl groups. The α-relaxation process shifts to higher temperatures for the semi-crystalline samples compared to the amorphous one. On the contrary, at a fixed frequency the temperature associated to β ∗ -relaxation is higher for the amorphous sample than for the semi-crystalline ones. The associated apparent activation energies are rather high and depend on the thermal treatment and also surprisingly on the method of measurement. It is concluded that the β ∗ -relaxation is probably due to cooperative molecular motions of the naphthalene groups which aggregate in the amorphous state and that these aggregates are prevented from forming when the degree of crystallinity changes due to the thermal treatment. Finally, the activation energy for the β-process is nearly independent of the thermal treatment and the value agrees with that found for poly(ethylene terephthalate) (PET).

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). II. Semicrystalline oriented films

The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed semicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the a-relaxation due to the glass transition, the b p -process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the b-relaxation due to local fluctuations of the carbonyl groups. The a-relaxation process shifts to higher temperatures for the 250 and 260 8C treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 8C according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to b p -relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the b-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the b p -a nda-relaxation.

Dielectric normal mode relaxation of poly(propylene glycol) melts in confining geometries

Abstract Dielectric spectroscopy is employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (MW=1200, 2000 and 4000 g/mol) confined to porous glasses (pore dimensions 2.5, 5.0 and 7.5 nm). In addition to the α-relaxation the whole chain dynamics can be measured by that method because PPG has a dipole component parallel to the chain. For the α-relaxation it was found that near the glass transition temperature its relaxation rate is lower within the pores than in the bulk. This finding was analyzed from several points of view including density effects, surface effects, and the concept of hindered glass transition. Besides the α-relaxation a further relaxation process (N1-process) was found for the PPG molecules confined to the pores. Its relaxation rate is drastic shifted to lower frequencies compared to that of the normal mode process of the bulk material and has a strong pore size dependence. This N1 process was assigned to the dynamics of whole chain or larger parts of it. We argue that the slowing down of its relaxation rate compared to that of the bulk material was mainly controlled by adsorption effects.

Dynamics of linear poly(N-isopropylacrylamide) in water around the phase transition investigated by dielectric relaxation spectroscopy.

The molecular dynamics of linear poly(N-isopropylacrylamide) (pNIPAM) in aqueous media at temperatures below and above the lower critical solution temperature (LCST) are investigated using broadband dielectric relaxation spectroscopy in a frequency range from 10(-1) to 10(11) Hz. Below the LCST, two relaxation processes are observed in the megahertz and gigahertz region assigned to the reorientation of dipoles of the solvated polymer segments (p-process) and water molecules (w-process), respectively. Both relaxation processes are analyzed using the Havriliak-Negami (HN) function, taking special attention to the w-process. Above the LCST, the dielectric spectra of the pNIPAM solutions resemble that of pure water, showing only the high frequency relaxation process of the water molecules with a more or less Debye-type behavior. The non-Debye behavior of the w-process below the LCST is mainly induced by the interactions between water and pNIPAM chains via hydrogen bonding. The relaxation time and strength of the w-process is studied with dependence on the concentration, temperature, and the polymer chain length (molecular weight). The information obtained is useful for a deeper understanding of the dehydration behavior at the phase transition. The suggestion of dehydration of the pNIPAM chains at the LCST is confirmed by calculating a dehydration number.

Comparison of the molecular dynamics of a liquid crystalline side group polymer revealed from temperature modulated DSC and dielectric experiments in the glass transition region

Temperature modulated DSC (TMDSC) in the frequency range from 10–3 Hz to 3.4 10–2 Hz and dielectric spectroscopy in the frequency range from 10–2 Hz to 106 Hz is employed to analyze the temperature dependence of the relaxation rates of the α- and of the δ-relaxation of a liquid crystalline polymethacrylate having a derivative of (p-alkoxy-phenyl)-benzoate as mesogenic unit in the side group. Especially the molecular assignment of the α-relaxation was discussed controversial in the literature up to now. By applying a temperature derivative method it was found that the temperature dependencies of the relaxation rates of the dielectric α-relaxation and of the dynamic glass transition measured by TMDSC can be described by Vogel/Fulcher/Tammann laws with the same Vogel temperature. Therefore it is argued that the dielectric α-relaxation of liquid crystalline polymers is rather due to the dynamic glass transition related to the segmental dynamics of the backbone than to the transverse dipole component of the mesogenic unit.