S. Arrese-Igor

Effect of hydration on the dielectric properties of C-S-H gel

The behavior of water dynamics confined in hydrated calcium silicate hydrate (C-S-H) gel has been investigated using broadband dielectric spectroscopy (BDS; 10(-2)-10(6) Hz) in the low-temperature range (110-250 K). Different water contents in C-S-H gel were explored (from 6 to 15 wt%) where water remains amorphous for all the studied temperatures. Three relaxation processes were found by BDS (labeled 1 to 3 from the fastest to the slowest), two of them reported here for the first time. We show that a strong change in the dielectric relaxation of C-S-H gel occurs with increasing hydration, especially at a hydration level in which a monolayer of water around the basic units of cement materials is predicted by different structural models. Below this hydration level both processes 2 and 3 have an Arrhenius temperature dependence. However, at higher hydration level, a non-Arrhenius behavior temperature dependence for process 3 over the whole accessible temperature range and, a crossover from low-temperature Arrhenius to high-temperature non-Arrhenius behavior for process 2 are observed. Characteristics of these processes will be discussed in this work.

Phenylene ring dynamics in bisphenol-A-polysulfone by neutron scattering

We have investigated the dynamics of phenylene rings in a glassy polysulfone (bisphenol-A-polysulfone) by means of quasielastic neutron scattering. Nowadays it is well known that these molecular motions are directly connected with the mechanical properties of engineering thermoplastics in general. The particular system investigated by us has the advantage that by selective deuteration of the methyl groups, the neutron scattering measured is dominated by the incoherent contribution from the protons in the phenylene rings. In this way, the dynamics of such molecular groups can be experimentally isolated. Two different types of neutron spectrometers: time of flight and backscattering, were used in order to cover a wide dynamic range, which extends from microscopic (10(-13) s) to mesoscopic (10(-9) s) times. Moreover, neutron diffraction experiments with polarization analysis were also carried out in order to characterize the structural features of the sample investigated. Fast oscillations of increasing amplitude with temperature and pi-flips are identified for phenylene rings motions. Due to the structural disorder characteristic of the amorphous state, both molecular motions display a broad distribution of relaxation times, which spreads over several orders of magnitude. Based on the results obtained, we propose a model for phenylene rings dynamics, which combines the two kinds of molecular motions identified. This model nicely describes the neutron scattering results in the whole dynamic range investigated.

Dielectric relaxation of 2-ethyl-1-hexanol around the glass transition by thermally stimulated depolarization currents

We explore new routes for characterizing the Debye-like and α relaxation in 2-ethyl-1-hexanol (2E1H) monoalcohol by using low frequency dielectric techniques including thermally stimulated depolarization current (TSDC) techniques and isothermal depolarization current methods. In this way, we have improved the resolution of the overlapped processes making it possible the analysis of the data in terms of a mode composition as expected for a chain-like response. Furthermore the explored ultralow frequencies enabled to study dynamics at relatively low temperatures close to the glass transition (Tg). Results show, on the one hand, that Debye-like and α relaxation timescales dramatically approach to each other upon decreasing temperature to Tg. On the other hand, the analysis of partial polarization TSDC data confirms the single exponential character of the Debye-like relaxation in 2E1H and rules out the presence of Rouse type modes in the scenario of a chain-like response. Finally, on crossing the glass transition, the Debye-like relaxation shows non-equilibrium effects which are further emphasized by aging treatment and would presumably emerge as a result of the arrest of the structural relaxation below Tg.

Anomalous molecular weight dependence of chain dynamics in unentangled polymer blends with strong dynamic asymmetry

We address the general question of how the molecular weight dependence of chain dynamics in unentangled polymers is modified by blending. By dielectric spectroscopy we measure the normal mode relaxation of polyisoprene in blends with a slower component of poly(ter-butylstyrene). Unentangled polyisoprene in the blend exhibits strong deviations from Rouse scaling, approaching ‘entangled-like’ behavior at low temperatures in concomitance with the increase of the dynamic asymmetry in the blend. The obtained results are discussed in the framework of the generalized Langevin equation formalism. On this basis, a non trivial relationship between the molecular weight dependence of the longest chain relaxation time and the nonexponentiality of the corresponding Rouse correlator is found. This result is confirmed by molecular dynamics simulations.

On the non-exponentiality of the dielectric Debye-like relaxation of monoalcohols

We have investigated the Debye-like relaxation in a series of monoalcohols (MAs) by broadband dielectric spectroscopy and thermally stimulated depolarization current techniques in order to get further insight on the time dispersion of this intriguing relaxation. Results indicate that the Debye-like relaxation of MAs is not always of exponential type and conforms well to a dispersion of Cole-Davidson type. Apart from the already reported non-exponentiality of the Debye-like relaxation in 2-hexyl-1-decanol and 2-butyl-1-octanol, a detailed analysis of the dielectric permittivity of 5-methyl-3-heptanol shows that this MA also presents some extent of dispersion on its Debye-like relaxation which strongly depends on the temperature. Results suggest that the non-exponential character of the Debye-like relaxation might be a general characteristic in the case of not so intense Debye-like relaxations relative to the α relaxation. Finally, we briefly discuss on the T-dependence and possible origin for the observed dispersion.

Neutron scattering investigation of a diluted blend of poly(ethylene oxide) in polyethersulfone

By using quasielastic neutron scattering (QENS) with isotopic labeling we have investigated the component dynamics in a miscible blend of polyethersulfone (PES) and poly(ethylene oxide) (PEO) with 75% content in weight of PES. Due to the large difference in the glass-transition temperatures, T(g)s, of the two polymers (T(g) (PEO) approximately equal to 220 K, T(g) (PES) approximately equal to 382 K) the dynamic asymmetry in the system dramatically increases when approaching the average T(g) of the blend, . For the fast (PEO) component, this leads to a behavior which hints a crossover from typical glass-forming liquidlike dynamics at high temperatures to confined dynamics close to induced by the freezing of the segmental motions of the slow PES. The features of the confined PEO motion observed by QENS are similar to those of the secondary gamma-relaxation detected for pure (semicrystalline) PEO. A neutron diffraction study of the short-range order of the homopolymers and the blend suggests that this coincidence could be due to similarities in the intermolecular packing of PEO and PES polymers.

Short-time dynamics of phenylene-rings in bisphenol based engineering thermoplastics

Abstract We have recently performed one of the first approaches by means of quasielastic neutron scattering (QENS) to the problem of identifying the molecular motions giving rise to the secondary relaxations of engineering thermoplastics. Preliminary results point to phenylene ring π-flips as the main motion causing the observed quasielastic broadening in the ∼ 10 −10 –10 −9 s time scale below the glass transition temperature T g . Continuing our study of sub- T g dynamics in these systems by QENS, measurements on polycarbonate (PC) and polysulfone (PSF) with deuterated methyl groups (d6) in the ∼10 −13 –10 −11 s time scale have been performed. The intermediate scattering function shows a smooth second decay in addition to that of vibrations and “fast dynamics” at T ≳ 200 K. The extrapolation of phenylene π-flip motion to faster times does not explain the decay observed. However, a non-negligible contribution of π-flips at T ⩾350 K in PCd6 is noticeable, whereas for PSFd6 some effect can be inferred above ∼450 K. In the temperature region where the π-flips do not contribute to the spectra we have characterised the signal by assuming a temperature dependent distribution of small angle oscillations of phenylene rings leading to an activation energy of 0.18 eV.

Dielectric spectroscopy of ionic microgel suspensions

The determination of the net charge and size of microgel particles as a function of their concentration, as well as the degree of association of ions to the microgel backbone, has been pursued in earlier studies mainly by scattering and rheology. These methods suffer from contributions due to inter-particle interactions that interfere with the characterization of single-particle properties. Here we introduce dielectric spectroscopy as an alternative experimental method to characterize microgel systems. The advantage of dielectric spectroscopy over other experimental methods is that the polarization due to mobile charges within a microgel particle is only weakly affected by inter-particle interactions. Apart from electrode polarization effects, experimental spectra on PNIPAM-co-AA [poly(N-isopropylacrylamide-co-acrylic acid)] ionic microgel particles suspended in de-ionized water exhibit three well-separated relaxation modes, which are due to the polarization of the mobile charges within the microgel particles, the diffuse double layer around the particles, and the polymer backbone. Expressions for the full frequency dependence of the electrode-polarization contribution to the measured dielectric response are derived, and a theory is proposed for the polarization resulting from the mobile charges within the microgel. Relaxation of the diffuse double layer is modeled within the realm of a cell model. The net charge and the size of the microgel particles are found to be strongly varying with concentration. A very small value of the diffusion coefficient of ions within the microgel is found, due to a large degree of chemical association of protons to the polymer backbone.

A multi-scale approach for percolation transition and its application to cement setting

Shortly after mixing cement grains with water, a cementitious fluid paste is formed that immediately transforms into a solid form by a phenomenon known as setting. Setting actually corresponds to the percolation of emergent network structures consisting of dissolving cement grains glued together by nanoscale hydration products, mainly calcium-silicate-hydrates. As happens in many percolation phenomena problems, the theoretical identification of the percolation threshold (i.e. the cement setting) is still challenging, since the length scale where percolation becomes apparent (typically the length of the cement grains, microns) is many times larger than the nanoscale hydrates forming the growing spanning network. Up to now, the long-lasting gap of knowledge on the establishment of a seamless handshake between both scales has been an unsurmountable obstacle for the development of a predictive theory of setting. Herein we present a true multi-scale model which concurrently provides information at the scale of cement grains (microns) and at the scale of the nano-hydrates that emerge during cement hydration. A key feature of the model is the recognition of cement setting as an off-lattice bond percolation process between cement grains. Inasmuch as this is so, the macroscopic probability of forming bonds between cement grains can be statistically analysed in smaller local observation windows containing fewer cement grains, where the nucleation and growth of the nano-hydrates can be explicitly described using a kinetic Monte Carlo Nucleation and Growth model. The most striking result of the model is the finding that only a few links (~12%) between cement grains are needed to reach setting. This directly unveils the importance of explicitly including nano-texture on the description of setting and explains why so low amount of nano-hydrates is needed for forming a spanning network. From the simulations, it becomes evident that this low amount is least affected by processing variables like the water-to-cement ratio and the presence of large quantities of nonreactive fillers. These counter-intuitive predictions were verified by ex-professo experiments that we have carried out to check the validity of our model.