Dielectric normal mode relaxation

Dielectric Chain Dynamics of Side-Chain Liquid Crystalline Polymer

Both chain dynamics and statistics of side-chain liquid crystalline polymer are experimentally explored. Dielectric measurements over a wide range of frequencies and temperature windows demonstrate that the chain dynamics in the liquid crystalline phase (SmA) is retarded and has higher apparent activation energy compared to that in isotropic melt. The molecular weight dependence of the normal mode relaxation time in the isotropic melt conforms to the Zimm model with excluded volume effect, while it shows Rouse behavior in the liquid crystalline phase. The mean squared end-to-end distance of polymer chain in the liquid crystalline phase decreases compared to that in the isotropic melt. The main chain takes a self-avoiding walk with ⟨R_e²⟩ ∼ N^2ν, ν = 0.54-0.6 in the isotropic melt, whereas a random walk between smectic layers with ν ≅ 0.5, consistent with the results from chain dynamics.

Crowded, confined, and frustrated: dynamics of molecules tethered to nanoparticles

Above a critical chemistry-dependent molecular weight, all polymer molecules entangle and, as a result, exhibit slow dynamics, enhanced viscosity, and elasticity. Herein we report on the dynamics of low molecular weight polymers tethered to nanoparticles and find that even conventionally unentangled chains manifest dynamical features similar to entangled, long-chain molecules. Our findings are shown to imply that crowding and confinement of polymers on particles produce topological constraints analogous to those in entangled systems.

Polymer segmental cross correlations from dielectric relaxation spectra of block copolymers

Dielectric relaxation spectra of solutions of block polymers containing sequential type-A dipoles are considered from a theoretical correlation-function standpoint. Spectra of a specific set of block copolymers can be combined to isolate the dynamic cross-correlation between the motions of two distinct parts of the same polymer chain. Unlike past treatments of this problem, no model is assumed for the underlying polymer dynamics.

Reversible tuning of a block-copolymer nanostructure via electric fields

Block copolymers consisting of incompatible components self-assemble into microphase-separated domains yielding highly regular structures with characteristic length scales of the order of several tens of nanometres. Therefore, in the past decades, block copolymers have gained considerable potential for nanotechnological applications, such as in nanostructured networks and membranes, nanoparticle templates and high-density data storage media. However, the characteristic size of the resulting structures is usually determined by molecular parameters of the constituent polymer molecules and cannot easily be adjusted on demand. Here, we show that electric d.c. fields can be used to tune the characteristic spacing of a block-copolymer nanostructure with high accuracy by as much as 6% in a fully reversible way on a timescale in the range of several milliseconds. We discuss the influence of various physical parameters on the tuning process and study the time response of the nanostructure to the applied field. A tentative explanation of the observed effect is given on the basis of anisotropic polarizabilities and permanent dipole moments of the monomeric constituents. This electric-field-induced effect further enhances the high technological potential of block-copolymer-based soft-lithography applications.

Molecular dynamics study of the thermal and the density effects on the local and the large-scale motion of polymer melts: scaling properties and dielectric relaxation

Results from a molecular dynamics simulation of a melt of unentangled polymers are presented. The translational motion, the large-scale and the local reorientation processes of the chains, as well as their relations with the so-called "normal" and "segmental" dielectric relaxation modes are thoroughly investigated in wide temperature and pressure ranges. The thermodynamic states are well fitted by the phenomenological Tait equation of state. A global time-temperature-pressure superposition principle of both the translational and the rotational dynamics is evidenced. The scaling is more robust than the usual Rouse model. The latter provides insight but accurate comparison with the simulation calls for modifications to account for both the local chain stiffness and the nonexponential relaxation. The study addresses the issue whether the temperature or the density is a dominant control parameter of the dynamics or the two quantities give rise to comparable effects. By examining the ratio /alphatau//alphaP between the isochronic and isobaric expansivities, one finds that the temperature is dominant when the dynamics is fast. If the relaxation slows down, the fluctuations of the free volume increase their role and become comparable to those of the thermal energy. Detectable cross-correlation between the "normal-mode" and the "segmental" dielectric relaxations is found and contrasted with the usual assumption of independent modes.

Heterogeneous dynamics of poly(vinyl acetate) far above Tg: A combined study by dielectric spectroscopy and quasielastic neutron scattering

We have investigated the dynamics of poly(vinly acetate) using broadband dielectric spectroscopy (DS) covering over 14 decades in frequency up to 20 GHz and high-resolution quasielastic neutron-scattering (QENS) technique. The dielectric results have been interpreted in terms of the phenomenological Kohlrausch-Williams-Watts [G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Alvarez, A. Alegria, and J. Colmenero, Phys. Rev. B 47, 125 (1993)] description. Because of the wide frequency range covered by DS, it provides a precise determination of dynamics over a wide temperature range and it revealed a crossover in polymer dynamics at 387 K through different dielectric parameters, viz., characteristic times, asymmetric shape parameter, and dielectric strength. Moreover, shape parameter is found to be higher in comparison to other typical polymeric systems. The characteristic relaxation times observed by QENS displayed an anomalous dependence of momentum transfer, indicating the possible existence of heterogeneities in the system even at the high temperatures. In addition, spin-lattice relaxation times, T(1), were found to be decoupled from dielectric characteristic times. Based on these results, a model was proposed to account for heterogeneities where we consider coexistence of different regions with standard polymeric behavior but with different characteristic times, leading to a distribution of relaxation times. The model is found to account for the anomalous behavior and an inherent shape parameter is found to account for the shape of alpha relaxation. This model is also found to predict the T variation of T(1) characteristic time scales at all temperatures. The origin of the heterogeneous domains is believed to lie in the microstructure of polymer chains.

Relaxation dynamics of a polymer in a 2D confinement

The molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (MW=1000, 2000, and 4000 g/mol) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy. The alpha-relaxation and the normal mode relaxation processes were studied for all samples in bulk and confinement. The most prominent experimental observation was that for the normal mode process: the relaxation rate in the clay is drastically shifted to lower frequencies compared to that of the bulk material. This slowing down is probably caused by the strongly reduced number of accessible chain conformations in two dimensions. Also the temperature dependence of the relaxation time for the normal mode process is strongly affected by the confinement. In contrast, for the alpha-relaxation of the confined polymers we observed only a slight increase of the relaxation rate at high temperatures compared to the corresponding bulk samples, and a decrease of its relaxation strength relative to the beta relaxation. Thus, the glass transition is unaffected by the 2D confinement, suggesting that the underlying phenomena responsible for the glass transition is the same as in bulk. Moreover, in the clay the intensity of the normal mode is stronger than that of the alpha-process, in contrast to the bulk samples where the opposite behavior is observed.

Dielectric relaxation studies of poly(propylene glycol) confined in vermiculite clay

The molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (M(w)=1200, 2000 and 4000 g/mol) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy. In addition to the alpha-relaxation, the normal mode relaxation process was studied for all samples both in bulk and confinement. For the normal mode process the relaxation rate in the clay is drastically shifted to lower frequencies compared to that of the bulk material in contrast to the alpha-process whose relaxation time is only slightly affected by the confinement. Also the temperature dependence of the relaxation time for the normal mode process is strongly affected by the confinement. Moreover, in the clay the intensity of the normal mode is stronger than that of the alpha-process, in contrast to the bulk samples where the opposite is observed.