Study of the influence of physical aging on macroradical decay in poly(methyl methacrylate)

The macroradical decay in poly(methyl methacrylate) samples with different thermal histories was investigated in the temperature interval 20–100 °C using ESR spectroscopy and the second order kinetic model. The rate constants exhibit two different regimes with the transitions atT _tr=68±1°C which are independent of thermal treatment. ForT<T _tr andT>T _tr the rate constants as well as the corresponding activation parameters are sensitive to history because of different physical microstructures. The compensation law, i.e., the linear relation between lnk _{o, eff} andE _eff, was analyzed in terms of the so-called compensation quantitiesk _c andT _c and a proximity betweenT _c=T _tr andT _o=53±3 °C — Vogel temperature for -segmental dynamics was found. A comparison of kinetic and dynamic data suggests that the decay of terminal macroradicals in the low-temperature region is controlled by secondary relaxations and that the -mobility contributes to a more rapid decay at higher temperatures belowT _g.     

Phase uniqueness and correlation length in diluted-field Ising models

The diluted-field Ising model, a random nonnegative field ferromagnetic model, is shown to have a unique Gibbs measure with probability I when the field mean is positive. Our methods involve comparisons with ordinary uniform field Ising models. They yield as a corollary a way of obtaining spontaneous magnetization through the application of a vanishing random magnetic field. The correlation lengths of this model defined as (lim _n-(1/n) log ₀; _n)^-1, wheren is the site on the first coordinate axis at distancen from the origin and ₀; _n is the origin ton two-point truncated correlation function, is non-random. We derive an upper bound for it in terms of the correlation length of an ordinary nonrandom model with uniform field related to the field distribution of the diluted model.     

Cluster description of water-in-oil microemulsions near the critical and percolation points

The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behavior in the vicinity of room temperature. The phase diagram in the temperature-volume fraction (of the dispersed phase) plane exhibits a lower consolute critical point at about 40 degrees centigrades and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to high volume fraction side at progressively lower temperatures. In this paper we review the evidence that allows to interpret the phase behavior of our system in terms of interacting spherical droplets. We also investigate the dynamics of droplets, below and approaching the critical point by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation is also reviewed and interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

The O(N) vector model in the large-N limit revisited: multicritical points and double scaling limit

The multicritical points of the O(N)-invariant N vector model in the large-N limit are re-examined. Of particular interest are the subtleties involved in the stability of the phase structure at critical dimensions. In the limit N → ∞ while the coupling g → g_c in a correlated manner (the double scaling limit) a massless bound state O(N) singlet is formed and powers of 1/N are compensated by IR singularities. The persistence of the N → ∞ results beyond the leading order is then studied with particular interest in the possible existence of a phase with propagating small mass vector fields and a massless singlet bound state. We point out that under certain conditions the double scaled theory of the singlet field is non-interacting in critical dimensions.     

Thermodynamics of mixtures of hexane and heptane isomers with normal and branched hexadecane

Molar excess mixing enthalpies h ^E , Gibbs free energies g ^E and hence entropies s ^E have been obtained using calorimetry and the vapor sorption method at 25°C for hexane isomers+2,2,4,4,6,8,8-heptamethylnonane, a highly branched C ₁₆ . The h ^E and g ^E are negative while Ts ^E are positive, but small. The values are explained by the Prigogine-Flory theory through negative free volume contributions to h ^E and Ts ^E , counterbalanced in the case of Ts ^E by the positive combinatiorial Ts ^E for mixing molecules of different size. No contribution is seen from the interaction between methyl and methylene groups. The excess quantities are also obtained for hexane and heptane isomers mixed with n-hexadecane. Values of h ^E and Ts ^E are now strongly positive, while those of g ^E are only slightly less negative. The interpretation requires two recently advanced contributions in addition to those of the Prigogine-Flory theory: 1) a decrease of order when correlations of orientations between n-C ₁₆ molecules in the pure liquid are replaced in the solution by weaker correlations whose strengths depend on the shapes of the lower alkane isomers. For lower alkane isomers of the same shape, but highly sterically hindered, h ^E and Ts ^E are small, manifesting, 2) a negative contribution, ascribed to a rotational ordering of n-C ₁₆ segments on the sterically-hindered molecule. Enthalpy-entropy compensation is observed for these new contributions, arising from their rapid fall-off with increase of temperature.     

Surface properties of ω-perfluorooctyl-alkyl polyacrylates: odd–even effect

A series of ω-perfluorooctyl-alkyl polyacrylates has been prepared and analysed. The odd–even effect, already observed in the case of liquid crystalline polymers has been exhibited for perfluorinated ones. Values of the contact angles to advanced (θ_a), with withdrawal (θ_r), as well as the contact angle hysteresis (Δθ) of various ω-perfluorooctyl-alkyl polyacrylates in water at 20 °C are described. Contrary to the advancing contact angle which is almost independent of spacer length, the receding one varies strongly with it.     

New breathable and waterproof coatings for textiles: effect of an aliphatic polyurethane on the formation of PEEK-WC porous membranes

PEEK-WC is an amorphous polyetheretherketone with high chemical stability, excellent thermal resistance and significant solubility in various solvents. It has been used to prepare flat membranes by the phase inversion technique. The water vapour permeability through a porous PEEK-WC film was 1350 g/m² day at 26 °C and its liquid entry pressure (LEP) of water equivalent to a column of 2.0 m. These values were remarkably improved by addition of an aliphatic ether polyurethane (PU) into the PEEK-WC/DMF dopes: the water vapour flux was increased up to 2000 g/m² day and the LEP was equivalent to 12.5 m. This improvement is correlated to the different structure of the membranes: a spongy, porous and almost symmetric structure for the PEEK-WC/PU membranes, and an asymmetric structure with fingers for PEEK-WC membrane. The presence of PU influences also the mechanical properties of the blend membranes. The role of the PU on the resulting membrane morphology is rationalised on the basis of the mechanism of phase separation.     

A role of non-hydrogen framework vibrational modes in formation of ferroelectric properties of H-bonded K3H(SO4)2-like materials

An effect of molecular vibrational modes within the dimers [O₃AO–(H/D)OAO₃]³⁻ on behavior of H-bond protons or deuterons is studied theoretically for H-bonded antiferroelectrics and quantum paraelectrics of the M₃(H/D)(AO₄)₂ family (M=alkali metal, A=S, Se). The ‘heavy’ reorganization modes in D-species of this family and both reorganization and promoting modes in corresponding H-analogues are examined. It is shown that the analysis with due regard for the reorganization modes results in insignificant decreasing of the effective tunneling frequency of light nuclei. Hence the reasonable estimations of the critical temperature of the low-temperature phase transition in M₃D(AO₄)₂ materials are available only with additional corrections related to potential profiles of a deuteron. It is noted that in the limits of the proposed model the promoting mode in the case of H-species can favor, in principle, the increasing as well as the decreasing of effective proton level splitting.     

超声波作用下的糖精相转移催化N—苄基化反应的研究

超声波作用下的糖精相转移催化Ｎ－苄基化反应的研究嵇学林刘华黄昆（江苏镇江医学院有机化学教研室镇江２１２００１）关键词超声波相转换催化糖精苄基化有机声化学近年来，由于超声波（Ｕｌｔｒａｓｏｕｎ，缩写Ｕ．Ｓ）在有机合成中应用的不断发展，逐渐形成了一门新兴...     

纳米氧化锆的物相与尺寸效应

用溶胶凝胶法制得ＺｒＯ２粉体。通过控制晶体尺寸得到了室温下稳定的立方相、四方相和单斜相。用Ｘ射线衍射（ＸＲＤ）、透射电镜（ＴＥＭ）、激光拉曼谱（ＬＲＳ）、电子顺磁共振（ＥＳＲ）等技术研究了晶体结构和晶粒尺寸的相互关系。试验表明：单斜相、四方相、立方相ＺｒＯ２的比表面能依次递减。因而，当晶粒尺寸减小至纳米级时，四方相和立方相都可变为室温下的稳态或亚稳态。