Viscous Liquid State Research Articles

Glass formation and glass transition in supercooled liquids, with insights from study of related phenomena in crystals

We divide glass and viscous liquid sciences into two major research areas, the first dealing with how to avoid crystals and so access the viscous liquid state, and the second dealing with how liquids behave when no crystals form. We review some current efforts to elucidate each area, looking at strategies for vitrification of monatomic metals in the first, and the origin of the property ‘fragility’ in the second. Essential in the first is the ‘ideal glassformer’ concept. Essential in the second is the non-trivial behavior of the glassformer thermodynamics. We explore the findings on non-exponential relaxation and dynamic heterogeneities in viscous liquids, emphasizing the way in which direct excitation of the configurational modes has helped differentiate configurational from non-configurational contributions to the excess heat capacity. We then propose a scheme for understanding the relation between inorganic network and non-network glassformers which includes the anomalous case of water as an intermediate. In a final section we examine the additional insights to be gained by study of the glass-like, ergodicity-breaking transitions that occur in disordering crystals. Here we highlight systems in which the background thermodynamics is understood because the ergodic behavior is a lambda transition. Water, and the classical network glassformers, appear to be attenuated versions of the latter type of cooperative transition.

Discotic Liquid Crystals: Synthesis and Characterization of Radial Polyalkynylbenzene Derivatives

Abstract New radial polyalkynylbenzene derivatives, which consist of 4-phenylacetylene unit connected with a central phenyl or phenoxy or tolyl linking moiety, are presented. The mesomorphic properties of these compounds in their pure form and as donor–accepter (D–A) complexes with 2,4,7-trinitrofluoren-9-one (TNF) were investigated with polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) studies. The hexaalkynylbenzene derivatives in a pure form had a columnar mesophase texture, and pentaalkynylphenyl and pentaalkynyltoluene derivatives remained in a highly viscous liquid state even at room temperature, whereas D–A complexes showed a the columnar hexagonal (Colh) mesophase. The unexpected mesophase behavior of these radial polyalkynylbenzene derivatives comes from the molecular architecture arising from the branched aliphatic chain.

Peripherally alpha(α)-substituted novel phthalocyanines

Two novel, nickel phthalocyanines bearing peripherally alpha(α)-substituents, namely octa(1,3-bis(dodecyloxy)propan-2-ol) and octa(1,3-bis[2-(2-ethoxyethoxy)ethoxy]propan-2-ol) were synthesized by cyclotetramerisation of the corresponding nitriles. At room temperature, the peripherally α-substituted Ni(II) phthalocyanine complexes exhibited a relatively viscous liquid state; their spectroscopic properties and aggregation behaviour were investigated in different solvents and at different concentrations in chloroform. The two compounds showed Q-bands in the near-IR region of the electronic spectrum and did not aggregate in all solvents and under a wide range of concentrations in chloroform.

Amorphous Fe-based metal foam

A foam synthesis method that takes advantage of the viscous high-temperature liquid state of Fe-based bulk glass-forming alloys to produce amorphous steel foam is introduced. Zirconium hydride is utilized as a foaming agent taking advantage of the low hydrogen solubility of these glass-forming alloys. Amorphous foams with porosities up to 65% were produced having homogenous cellular morphologies that exhibit cell-size uniformity. Even though intracellular solid regions as thin as a few micrometers are detected, on a global scale the cellular structure is determined to be incapable of alleviating the foam from the brittle nature of the monolithic glass.

APPLICATION OF THE MR LIQUIDS IN SEMI-ACTIVE CONTROL DEVICES

Magneto-rheological liquids are controllable liquids that under the action of a magnetic field can reversibly pass from the linear viscous liquid state with free-flow to the semi-solid one with a controlled stress-state. They are composed of typically non-colloidal magnetic micronized particles and possess a load carrying capacity higher than other, more controllable, fluids, such as electro-rheological liquids; moreover they are less sensitive to impurities and contaminations that may possibly occur in manufacturing. in the paper, the most suitable models for simulation of such devices are investigated with emphasis on evaluation of their efficiency as structural control systems.

High frequency sound and the boson peak in amorphous silica

We present the results of extensive molecular dynamics computer simulations in which the high frequency dynamics of silica, nu>0.5 THz, is investigated in the viscous liquid state as well as in the glass state. We characterize the properties of high frequency sound modes by analyzing J_l(q,nu) and J_t(q,nu), the longitudinal and transverse current correlation function, respectively. For wave-vectors q>0.4 Angstrom^{-1} the spectra are sitting on top of a flat background which is due to multiphonon excitations. In the acoustic frequency band, i.e. for nu<20 THz, the intensity of J_l(q,nu) and J_t(q,nu) in the liquid and the glass approximately proportional to temperature, in agreement with the harmonic approximation. In contrast to this, strong deviations from a linear scaling are found for nu>20 THz. The dynamic structure factor S(q,nu) exhibits for q>0.23 Angstrom^{-1} a boson peak which is located nearly independent of q around 1.7 THz. We show that the low frequency part of the boson peak is mainly due to the elastic scattering of transverse acoustic modes with frequencies around 1 THz. The strength of this scattering depends on q and is largest around q=1.7 Angstrom^{-1}, the location of the first sharp diffraction peak in the static structure factor. By studying S(q,nu) for different system sizes we show that strong finite size effects are present in the low frequency part of the boson peak in that for small systems part of its intensity is missing. We discuss the consequences of these finite size effects for the structural relaxation.

The “food polymer science” approach to flour functionality and ingredient technology in biscuit baking

An overview is made on the food polymer science approach developed by the authors. The quality and performance of flours for the production of cookies and crackers have been shown to depend upon the major functional polymeric components of flour: gluten protein, damaged starch, and pentosans. Of these, damaged starch and soluble pentosans in soft-wheat flours are detrimental to the commercial production of low-moisture cookies and crackers. The detrimental effects of soluble pentosans in flours can be eliminated through the use of pentosanase enzyme in cookie and cracker doughs. Three commercialized applications of this industrial enzyme technology have been patented by Nabisco. The food polymer science approach to baking technology has also been used to study finished-product attributes such as texture, in the context of the thermomechanical properties (e.g. modulus) of glassy solid and rubbery liquid matrices. Results of various studies have clearly demonstrated that products in a glassy solid physical state (at T < T g ) are hard and crisp in texture, but upon increasing plasticization by water (such that T g is depressed below the observation T), are transformed to a rubbery or viscous liquid state, wherein textural hardness (and mechanical modulus) and crispness are dramatically reduced.

Methanofullerenes with mesogenic groups: Bulk properties and Langmuir films

New C 60 derivatives functionalized with mesogenic groups have been synthesized. Their behaviour at the nitrogen-water interface was investigated and appeared to depend on the nature of their functional group: if it contains a fluorinated chain, a stable monomolecular film is obtained; if not, then a multilayer organization is observed. In the bulk state, the structural organization of the methanofullerenes has been studied by polaried optical microscopy, DSC and X-ray diffraction experiments. At room temperature, a non-crystalline periodic structure is shown, and a phase transition leading to a viscous liquid state occurs when temperature increases.

Dielectric relaxation in the fragile viscous liquid state of toluene

Dielectric measurements were carried out on viscous toluene covering a frequency range from 0.1 Hz to 1 MHz. In order to suppress the pronounced crystallization tendency of this supercooled liquid it was contained in thin walled capillaries with outer diameters of 300 μm. From the temperature dependence of the characteristic dielectric relaxation times it was found that toluene is one of the most fragile low molecular weight glass-forming liquids, with a fragility index m=105. By comparison with time constants available from other experimental techniques it appears that near the glass transition the dielectric relaxation mode is not the slowest one.

Electrically conducting polymers from phenylacetylene substituted Schiff's base momomers

Electrically conducting polymers have been synthesized from novel mono- or difunctional phenylacetylene-substituted Schiff's base monomers. These monomers melt to a viscous liquid state, and on continued heating above about 300°C, thermally polymerize to form an electrically insulating thermoset polymers. On further postcure heat treatment, the polymers become electroconductive showing a bulk conductivity of approximately 10 −2 S/cm. Because the monomers remain in a liquid stage for 1–2 h, depending on the cure conditions, moldings, castings, and pre-pregs can be fabricated using conventional processing techniques. In addition, since reactive dopants are not used, the resulting polymers and conductivity are stable in ambient as well as more aggressive environments, allowing them to function at 300°C and above. The monomers are well characterized, but the structure of the cured and postcured polymers are not yet well defined because of their high stability and intractability in the cured state.

Fluorescence study of the motional states of core and surface lipids in native and reconstituted low density lipoproteins.

Low density lipoproteins (LDL) consist of an apolar core of cholesterol esters and triglycerides surrounded by a monolayer of phospholipid, cholesterol, and a single molecule of apolipoprotein B (apoB-100). To determine the influence of core and surface constituents on the surface of LDL, we have measured core and surface order parameters for native LDL, and reconstituted LDLs (rLDL) whose apolar core lipids were extracted and replaced with either cholesterol oleate (CO) or triolein (TO). Order parameters were measured by fluorescence depolarization of diphenylhexatriene (DPH), which is located primarily in the core, and of trimethylammoniumdiphenylhexatriene (TMA-DPH), which is anchored at the water-phospholipid interface. DPH order parameters for LDL reconstituted with TO (r-[TO]LDL) were much lower than those for LDL reconstituted with CO (r-[CO]LDL), consistent with the physical properties of TO, a nonviscous liquid at all temperatures studied, and CO, which exists in a liquid crystalline or viscous liquid state at the temperatures studied. Although core cholesterol esters in r[CO]LDL and native LDL undergo distinct order-disorder transitions, these transitions were not detected by DPH. This is most likely due to the difference between the time scale for end-over-end tumbling of cholesterol esters and the fluorescence lifetime of DPH. Despite the fact that the core lipids of r-[CO]LDL were much more ordered than those of r-[TO]LDL, surface order parameters for both lipoproteins were similar. We conclude that the motional states of the core and surface lipids are relatively independent. Surface order parameters for native LDL were higher than those for reconstituted LDLs.(ABSTRACT TRUNCATED AT 250 WORDS)

Small angle neutron scattering study of vanadium pentoxide gels swelling

Concentration dependent changes in the structure of Y 2O 5, nH 2O gels has been followed by small angle neutron scattering (SANS) in the composition range 10 ⩽ n ⩽ 800. Up to n = 50, a one-dimensional mode, where the interlamellar distance increases up to a value comparable with the width of the V 2O 5-ribbons, is first observed. Then a two-dimensional swelling corresponding to a viscous liquid state is obtained between 150 and 800 H 2 O V 2O 5 . The mean interparticle distance increases up to about 700 Å. A transition region appears between two swelling regimes corresponding to the thixotropic state of the gel.

Blends of normal high density and ultra-high molecular weight polyethylene, ? irradiated at a low dose

The kinetics of a nonisothermal crystallization and melting ofγ irradiated with dose of 6 Mrad blends of an ultra-high molecular-weight polyethylene (UHMWPE) and a high-density polyethylene with normal molecular weight (NMWPE) is investigated by means of DSC. The blends have been prepared at temperature below the flow temperature of UHMWPE: The enthalpies of melting of the polyethylenes increase, while those of their blends decrease after irradiation. The enthalpies of crystallization of the pure polyethylenes are higher, while those of their blends almost do not change or are a bit higher after irradiation. The rates of a nonisothermal crystallization and melting of the polyethylenes increase, while those of the polyethylenes in the blends decrease after irradiation. Thermomechanical measurements under constant load in wide-temperature interval of irradiated polyethylenes and their blends have been made. A high-elastic plateau in viscous-liquid state is established on the thermomechanical curves of UHMWPE, and the blends with high content of UHMWPE. On the basis of results obtained assumptions have been made about the processes taking place in the blends under the action of irradiation, as well as about the character of the mutual influence between the components in the process of irradiation.

Pecularities of the thermomechanical behaviour of ultra-high molecular weight linear polyethylene and its blends with linear polyethylene of normal molecular weight

Ultra-high molecular weight polyethylene UHMWPE (Mw=4 · 106,Is=O g/ 10 min), high density polyethylene of normal molecular weight NMWPE (Is= 4.8 g/10 min) and their blends have been investigated by means of thermomechanical loading in constant and impulse regime. It has been established that after melting, NMWPE passes to a viscous-liquid state. After melting at 138 °C UHMWPE passes to a high-elastic state. The transition of UHMWPE to a viscous-liquid state takes place at temperatures higher than 180 °C and is accompanied by a high-elastic reversible deformation. The blends of UHMWPE with 10 and 20 mass % of NMWPE show a plateau on the thermomechanical curves, corresponding to a high-elastic state, in a shorter temperature range where the deformation is greater. The blends containing the higher percent of NMWPE show thermomechanical curves lacking such a plateau. All blends are characterized by a singular thermomechanically defined temperature of melting, which increases with increase of UHMWPE content. The existence of the high-elastic state in the curves of UHMWPE and its blends containing NMWPE less than 30 mass % above their melting temperatures is explained by the high degree of physical crosslinking of UHMWPE.

Field desorption of sucrose studied by combined optical microscopy and mass spectrometry

Field ion emission from single sample droplets deposited from an aqueous solution of sucrose on bare 10μm W wires has been studied by combined optical microscopy and mass spectrometry. The experiments reveal molecular ion emission from an amorphous nearly solid or glassy state of the sample, having a loose structure with many cavities. This state is formed via the growth of field-enhancing protuberances from a viscous liquid state and the evaporation of solvent molecules from the surface of the protrusions. The temperature range of molecular ion emission is significantly below the melting point of sucrose. Observed changes in the ion energy distribution and intensity fluctuations are consistent with ion emission from transient glassy protuberances of low ionic conductivity. The desolvation of ions is explained by a cooperative mechanism of ion extraction from a molecularly rough surface. In this mechanism, successive ruptures of intermolecular bonds are accomplished by rearrangements of molecules in the surface upon the field stress. The principal features of the desolvation mechanism are suggested to be the same for most polar biochemicals forming a glassy state after evaporation of the solvent.