Supercooled nano-droplets of water confined in hydrophobic rubber.

Hydrophobic elastomers are capable of absorbing a small amount of water that forms droplets around hydrophilic sites. These systems allow the study of confinement effects by a hydrophobic environment on the dynamics and thermodynamic behaviour of water molecules. The freezing-melting properties and the dynamics of water inside nano-droplets in butyl rubber are affected, as revealed by differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H-NMR). Upon cooling down, all water crystalizes with a bimodal droplet population (da = 3.4 nm and db = 4.4 nm) in a temperature range associated with the droplet size distribution. However, the melting temperature is not shifted according to the Gibbs-Thomson equation. The relative decrease of the 2H-NMR longitudinal magnetization is not a single exponential and, by inverse Laplace transformation, it was deduced to be bimodal in agreement with the DSC measurements (T1,a ∼ 10 ms and T1,b ∼ 200 ms). The deduced correlation time of molecular reorientation is longer than that of bulk water and the behaviour with temperature follows the Vogel-Fulcher-Tammann (VFT) equations with a changing fragility as the droplet size is reduced when reducing hydration.

Bulk supercooled water versus adsorbed films on silica surfaces: specific heat by Monte Carlo simulation.

Between 150 and 230.6 K, bulk supercooled water freezes upon cooling, and amorphous ice crystallizes upon heating: bulk water thus exists only in its stable ice form. To circumvent this problem, experiments are generally performed on water adsorbed in SiO2 based porous systems. In this work, we take advantage of Monte Carlo simulations to explore this metastable supercooled region inaccessible to experiments. Using three rigid, non-polarizable water models, namely SPC, TIP4P and TIP4P/2005, we investigate the isobaric specific heat capacity (Cp), between 100 and 300 K, of bulk water and water films of few monolayers adsorbed on different SiO2 surfaces: a smooth surface, a non-hydroxylated (0001) surface of quartz, and a fully hydroxylated (001) surface of cristobalite. As Cp is directly related to the entropy fluctuations and we focus on low temperatures, the convergence of the Monte Carlo simulations is a critical point of this work. Also, due to the small mass of the hydrogen atoms, quantum corrections are taken into account, and lead to an excellent agreement of the simulated and experimental Cp values at low temperature (100 K region). Altogether, we conclude that, in bulk, Cp is shown to exhibit a broad peak around 225 K for the SPC and TIP4P models, and around 250 K for the TIP4P/2005 model, in qualitative agreement with the experimentally observed features in Cp measurements. For interfacial water, in all cases, the broad Cp peak disappears. This result, at odds with experimental observations, suggests that disorder and hydrogen bonding at the interface (not yet taken into account) have a fundamental role in confined water transitions.

Freezing, melting and dynamics of supercooled water confined in porous glass.

The freezing, melting and dynamics of supercooled water at different hydration of controlled porous glass with mean pore sizes 10 nm, 30 nm, 50 nm and 70 nm are studied using differential scanning calorimetry and deuteruim nuclear magnetic resonance (2H-NMR). For saturated samples, the melting temperature follows the Gibbs-Thomson relation despite a clear linear decrease of the melting enthalpy when the transition is shifted due to confinement. For partially filled porous glasses the crystallization and melting temperatures as well as enthalpies are lower than for the saturated samples. 2H-NMR confirms the existence of a non-crystallizable part of water adsorbed on the surface of pores. At room temperature, spin-lattice relaxation rate (1/T 1) is proportional to the inverse of the mean pore size indicating that the relaxation is governed by a surface limited process. At low temperature relaxation rate follows the Vogel-Fulcher-Tammann (VFT) relation.

Water dynamics in silanized hydroxypropyl methylcellulose based hydrogels designed for tissue engineering.

Silanized hydroxypropyl methylcellulose based hydrogels were developed for cartilage and intervertebral disc tissue engineering. Herein, study of dynamics of confined water showed two different populations, identified as hydration and bulk-like water. The diffusion coefficient showed that bulk-like water diffuses over distances ∼10 µm without being affected by the hydrogel matrix. Addition of silica nanofibers leads to improved mechanical properties and enhanced diffusion coefficient. Good diffusion within hydrogels is essential for the application.

Enhanced ionic liquid mobility induced by confinement in 1D CNT membranes.

Water confined within carbon nanotubes (CNT) exhibits tremendous enhanced transport properties. Here, we extend this result to ionic liquids (IL) confined in vertically aligned CNT membranes. Under confinement, the IL self-diffusion coefficient is increased by a factor 3 compared to its bulk reference. This could lead to high power battery separators.

Chain orientation in natural rubber, Part II: 2H-NMR study.

Stress-induced crystallisation (SIC) and stress-induced melting (SIM) in natural rubbers (NR), unfilled and filled with carbon black (CB) have been studied by (2)H-NMR measurements. Various materials have been swollen with small amount (< 2%) of deuterated alkane chains. The orientation of the amorphous chains, then the local deformation of the amorphous chains during deformation cycles and during stress relaxation, permits to clarify the SIC and SIM processes during hardening and recovery. By mechanical, WAXS and NMR measurements one determines the same critical draw ratio for appearance lambda(A) and disappearance lambda(E) of the crystallites. It is demonstrated that the hysteresis observed by the different techniques (stress sigma, crystallinity chi, NMR splitting Deltanu) are due to the supercooling effect ( lambda(A) > lambda(E), at constant temperature). During hardening at constant strain rate it is found that the local draw ratio remains constant and equal to lambda(A), whereas the crystallinity increases linearly with the macroscopic draw ratio lambda. The hardening sigma approximately (lambda - lambda(A))(2) is then interpreted as a reinforcement effect due to the crystallites, which act as new crosslinks. This confirms the prediction of Flory. In filled rubber the same effects are observed, and the stress amplification factor is determined as a function of the CB content. It is found that the fillers act as nucleation centres for the NR crystallites. The reinforcement of such materials is due principally to this nucleation effect and to the presence of a super network formed by both the NR crystallites and the CB fillers.

Synthesis and properties of two new liquid crystals: an analytical and thermodynamic study.

Synthesis, analytical performances, thermodynamic and surface properties of two new liquid crystals substituted with poly(ethylene oxide) chains are described. The first of them is N,N'-diphenyl-[4-[2,3,4-tri[2-(2-metoxyethoxy)ethoxy]benzylidene]i mine]piperidine (LC1) and the second is 2-hydroxy-3-methyl-4-[4-[2-(2-butoxyethoxy)ethoxy]] 4'-[4-[2-(2-butoxyethoxy)ethoxy]styryl]azobenzene (LC2). The nematic ranges of the two liquid crystals, determined by differential scanning calorimetry do not interfere. The analytical and thermodynamic studies of LC1 and LC2 in the solid, nematic and liquid state were done using a series of appropriate solutes. Comparison of the analytical performances shows a better efficiency in the nematic state.