Dielectric relaxations in amorphous polymers with complex chain architectures (original) (raw)
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Low-Frequency Chain Dynamics of Poly(n-hexyl methacrylate) by Dielectric Spectroscopies
Macromolecules, 2002
The study of poly(n-hexyl methacrylate) has been performed by both thermo-stimulated currents (TSC) and dynamic dielectric spectroscopy (DDS) in order to better define the nature of molecular mobility in the glass transition temperature range. TSC results reveal the existence of three dipolar relaxation modes: R, ascribed to the glass relaxation; , assigned to a secondary relaxation mode; and R′, associated with the dielectric manifestation of the liquid-liquid transition. DDS allowed us to follow the merging of the R and modes as frequency increases from 10-2 to 10 6 Hz. Then the fractional polarizations protocol permitted us to define at low frequency (10-3-10-2 Hz) the fine structure of these complex relaxations: relaxation times isolated for the and R modes are widely distributed and obey an Arrhenius law while those isolated for the R′ mode are narrowly distributed and are better defined by a Vogel-Tamman-Fulcher law. Finally, the evolution of the activation enthalpy helps us to precise the nature of involved molecular movements.
Macromolecular Symposia, 2009
Broadband Dielectric Spectroscopy (BDS) provides a direct, powerful means of studying the chain dynamics of solid polymers. The present work discusses experimental data, phenomenological theories, molecular theories and recent molecular dynamics simulations of the relaxation functions and average relaxation times for the multiple dielectric relaxations observed for amorphous polymers. Examples of current applications of BDS are given, including studies of the dynamics of ultra‐thin polymer films and real‐time studies of (i) the crystallization behaviour of bulk polymers and (ii) thermo‐polymerizing systems.
Macromolecules, 2008
In this paper, we show that it is possible to predict the dielectric loss spectrum of methoxy groups in amorphous poly(vinyl methyl ether) from their rotational energy profiles obtained by a simple quasistatic technique described in our previous paper. Kramers' transition rate theory is used with a slight modification to account for the nonquadratic profiles encountered and short molecular dynamics runs are used to obtain the moment of inertia and rotational frictional coefficient of a methoxy group. The predicted dielectric loss spectrum and its temperature dependence are in good agreement with experimental observations with no adjustable parameters other than those specified in the force field. It is found that the width of the dielectric loss spectrum is essentially due to the spread of activation energies with a negligible contribution from the spread of relaxation time prefactors.
Polymer, 2007
The molecular relaxation characteristics of rubbery amorphous crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] and poly(propylene glycol) diacrylate [PPGDA] have been investigated using broadband dielectric spectroscopy. Dielectric spectra measured across the sub-glass transition region indicate the emergence of an intermediate ''fast'' relaxation in the highly crosslinked networks that appears to correspond to a subset of segmental motions that are more local and less cooperative as compared to those associated with the glass transition. This process, which is similar to a distinct sub-T g relaxation detected in poly(ethylene oxide) [PEO], may be a general feature in systems with a sufficient level of chemical or physical constraint, as it is observed in the crosslinked networks, crystalline PEO, and PEO-based nanocomposites.
Polymer, 2005
Dielectric relaxation spectroscopy using dielectric probes was applied to study the (glass transition) dynamics in binary blends of isotactic PP, PS and LDPE. The blends were prepared by melt-mixing and doped with 0.5% of the dielectric probe 4,4 0 -(N,N-dibutylamino)-(E)nitrostilbene (DBANS) (van den Berg O, Sengers WGF, Jager WF, Picken SJ, Wübbenhorst M. Macromolecules 2004;37:2460. [17]). Due to the selective amplification of the dielectric relaxation processes related to the dynamic glass transition of the polymers, accurate relaxation data were obtained, even for the minor phases. No substantial influence of the blend composition and the blend morphology on the glass transition dynamics was found, indicating that both blend constituents behave like homogeneous bulk materials. The normalised relaxation strength of glass transition processes remained constant, regardless of the blend type and blend composition. This indicates that the probe molecule, DBANS, was equally distributed over the two blend components in all three polymer combinations PE-PP, PE-PS and PP-PS. q (M. Wübbenhorst).
Macromolecular Chemistry and Physics, 2012
The molecular dynamics of liquid-crystalline polymethacrylates with biphenyl in the side group forming a highly ordered smectic E phase at low temperatures is investigated by dielectric spectroscopy (DS). Although no glass transition is found by DSC, DS detects a relaxation in the smectic E phase resembling to glassy dynamics. The temperature dependence of its relaxation rate is Arrhenius like at low temperatures, which changes to a VFT law at higher temperatures. Such a behavior is found to be characteristic for glassy dynamics in confi ning space. It is discussed, considering the structure of the polymers as nanobiphasic consisting of liquid-crystalline layers while the backbones fi ll the interlayer space. A comparison is made to polymethacrylates having phenyl benzoate as mesogen.
Mössbauer and dielectric spectroscopy of the dynamic glass transition of a block copolymer
Journal of Physics-condensed Matter, 1998
The enlargement of the time window achieved by combining 0953-8984/10/5/006/img9 Mössbauer investigations (0953-8984/10/5/006/img10 to 0953-8984/10/5/006/img11) with conventional dielectric spectroscopy 0953-8984/10/5/006/img12 to 0953-8984/10/5/006/img13 on poly(vinylferrocene-b-propylene sulphide) allows one to characterize three different relaxation processes. The main process of the glass transition is measured by dielectric spectroscopy and can be well described by the common Vogel-Fulcher-Tammann law, which is typical for cooperative processes. A secondary process is shown by dielectric spectroscopy to be Arrhenius activated, indicating a local process which leads to a broadening of the Mössbauer resonance line. The corresponding times can be determined via line-shape analysis. A third process, the so-called cage process, is responsible for an anomalous decrease of the Debye-Waller factor. Its characteristic times are assumed to be in the picosecond region.
Dynamics of Amorphous and Semicrystalline 1,4- trans -Poly(isoprene) by Dielectric Spectroscopy
Macromolecules, 2008
We have studied the dynamics of amorphous 1,4-trans-poly(isoprene) (trans-PI) with different molecular weights using broadband dielectric spectroscopy over a wide range of temperature ∼T g -70 K to T g + 130 K, and frequencies (10 -1 to 10 6 Hz). Avoiding crystallization by quenching, three dielectric processes (global chain, segmental and local relaxations) were observed in order of decreasing temperature. Both the segmental and local relaxations of the here studied trans-PI, were found to be markedly different compared with the corresponding processes in cis-PI. In contrast, the temperature dependence of the relaxation time related to the global chain mode was found to be independent of the chain configuration. However, its relaxation strength was 1 order of magnitude lower in trans-than in cis configuration indicating a small parallel component of the dipole moment in trans configuration. In addition we also studied the dielectric spectra of trans-PI upon crystallization. The kinetics during crystallization as observed by dielectric spectroscopy results independent of molecular weight. During isothermal crystallization the dielectric spectra can be analyzed by a combination of three processes: the R m -relaxation related with the modified amorphous phase, the so-called R CAP -relaxation induced by crystallization and the -relaxation, the two later only varying in relaxation strength. At the end of isothermal crystallization a percentage of crystallinity about 30% was estimated.