Showing papers by "Sylwester J. Rzoska published in 2020"

Composition and pressure effects on the structure, elastic properties and hardness of aluminoborosilicate glass

Abstract: A series of calcium-aluminoborosilicate glasses with different ratios of B2O3/SiO2 have been isostatically compressed at 1 and 2 GPa at temperatures above their glass transition temperatures. Boron and aluminum coordination numbers were quantified for recovered samples by 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) and 27Al MAS NMR spectroscopy at ambient pressure and temperature. The average coordination numbers increase as the glass was compressed to higher pressure and the glass with higher concentration of boron shows higher recoverable densification. The atomic structure changes can be represented by oxygen packing density, Young's modulus and Vickers hardness number show good correlations with oxygen packing density as well. In general Poisson's ratio decreases as the average coordination number of network formers increases, however the interpretation of the changes in Poisson's ratio with pressure for each glass composition presents a more complicated case and no straightforward trend with compaction was found.

Impact of Pressure on Low-Molecular Weight Near-Critical Mixtures of Limited Miscibility.

Abstract: Near-critical mixtures of limited miscibility are significant for chemical physics, soft matter physics, and a variety of challenging applications. Their basic properties can be tuned by compressin...

Dynamics and Pretransitional Effects in C60 Fullerene Nanoparticles and Liquid Crystalline Dodecylcyanobiphenyl (12CB) Hybrid System

Abstract: The report shows the strong impact of fullerene C60 nanoparticles on phase transitions and complex dynamics of rod-like liquid crystal dodecylcyanobiphenyl (12CB), within the limit of small concentrations. Studies were carried out using broadband dielectric spectroscopy (BDS) via the analysis of temperature dependences of the dielectric constant, the maximum of the primary loss curve, and relaxation times. They revealed a strong impact of nanoparticles, leading to a ~20% change of dielectric constant even at x = 0.05% of C60 fullerene. The application of the derivative-based and distortion-sensitive analysis showed that pretransitional effects dominate in the isotropic liquid phase up to 65 K above the clearing temperature and in the whole Smectic A mesophase. The impact of nanoparticles on the pretransitional anomaly appearance is notable for the smectic–solid phase transition. The fragility-based analysis of relaxation times revealed the universal pattern of its temperature changes, associated with scaling via the “mixed” (“activated” and “critical”) relation. Phase behavior and dynamics of tested systems are discussed within the extended Landau–de Gennes–Ginzburg mesoscopic approach.

Polyvinylidene difluoride-based composite: glassy dynamics and pretransitional behaviour

Abstract: This paper presents results of broadband dielectric spectroscopy studies in the composite system for which particularly strong interactions between polyvinylidene difluoride (PVDF: ferroelectric polymer, TC = 453−473 K) matrix and barium strontium titanate (BST) ferroelectric micro-particles can be expected. For PVDF the super-Arrhenius (SA) dynamics, associated with segmental motions freezing at the glass temperature Tg = 235 K, is evidenced. The addition of BST particles qualitatively changes dynamics, converting the SA-type behaviour in PVDF to the clear Arrhenius one in BST/PVDF composite. The latter crossovers to the relaxor-type SA dynamics on cooling, exactly at the glass temperature of PVDF. The preliminary model explaining such unique behaviour is proposed. For the consistent portraying of the SA evolution of primary relaxation times in PVDF and BST/PVDF, the activation energy index analysis was carried out and the new equation, entropy and symmetry controlled, is introduced. Studies are accomplished by the analysis of the ferroelectric-paraelectric transition in PVDF and for the composite system. They led to the discovery of the strong pretransitional anomaly of de∕dT, extending even to the vicinity of the room temperature, The semi-discontinuous nature of melting in PVDF and its composites, with the discontinuity metric △T ≈ 20 K is suggested.

High-pressure behavior of dielectric constant in a binary critical mixture.

Abstract: An alternative design of the measurement capacitor for high-pressure studies of complex liquids or soft matter systems is presented. Subsequently, results for the precritical anomaly of dielectric constant in 1-nitropropane-octane critical mixture are reported. First, the pressure dependence of the critical consolute temperature T_{C} up to P=0.55GPa was determined and portrayed using the derivative-based analysis. Second, temperature and pressure evolutions of dielectric constant on approaching the critical consolute point at (T_{C}=304.1K and P_{C}=403MPa) were studied. They revealed that the pretransitional anomaly ɛ(P→P_{C}) is notably more pronounced than for ɛ(T→T_{C}). For both paths, the static domain extends even to as low frequency as f=100Hz, whereas for tests under atmospheric pressure, they require at least f=100kHz. The discussion of the impact of correction-to-scaling terms, including the unique case of the pressure paths, is also presented.