THE ROLE OF DOUBLE-HELIX FORMATION IN WATER DIFFUSION AND AGING OF INJECTION-MOLDED STARCH*

Native potato starch has been processed without the use of any additive. Moldings of initial high strength and ductility have been achieved. However, mechanical properties change with time. Thermogravimetric experiments at different temperatures show that aging takes place in three different steps. The first is characterized by a strong formation of double helices (as revealed by wide-angle X-ray scattering [WAXS]) in parallel with strong water loss. Hardness, strength, and elongation at break increase in this time range: Double helices act as reinforcing elements. When the yield elongation is surpassed, double helices are unraveled, providing a higher elongation at break. With further aging (drying), due to a shrinking process, a temporary reduction of double helices, causing a resoftening, seems to take place. In a third step, the starch moldings become quite brittle due to the lack of molecular mobility. At room conditions, it takes approximately 1 year (beginning of range 3) to lose the good mechanical properties. It is proposed that the mechanical behavior of starch moldings is controlled by a molecular network that consists of singlehelix molecules bound by double helices within an amorphous structure.

     

Исследование механизма свободнорадикальных реакций в растворе с помощью дейтерия

Die Diffusionsmethode eignet sich sehr gut zur Herstellung von radioaktiven Kryptonaten. In dieser arbeit ist die Versuchseinrichtung der Diffuysionskryptonierung für das Temperaturgebiert 20…300°C und bis zu einem Druck von etwa 8 ata beschrieben. Abschlieβend sind einige Beispiele für die Krysptonierung anorganischer Verbindungen angegeben einschlieβlich der Kryptonierungs-bedingungen und der erreichten Werte der relativen spezifischen Aktivitäten.     

Separation of Nitrogen Isotopes by Thermal Diffusion

The prototype of a thermal diffusion column for the use as basic unit in a N¹⁴N¹⁵ cascade is described. The hot wire separation column consists of a brass tube of effective length of 250 cm and inner diameter of 10 mm. Four columns are connected in series on a square cascade, the separation factors of which are compared with the values obtained by other investigators.     

随机双指数记忆耗散系统的非马尔可夫扩散

针对具有双指数耗散记忆核函数的两自由度耦合系统, 本文利用Laplace变换导出了热宽带噪声激励下该系统响应二阶矩的解析表达式. 并观察到位移二阶矩不同于单自由度情形下的反常扩散:<x²(t)> ∝ t^α (0<α<2, α≠1), 而是随时间及噪声等参数变化呈现普遍的振荡扩散现象.分析可得, 阻尼耦合因子B使粒子远离简谐势场的束缚, <x²(t)>随B的增大扩散加剧而摩擦系数增大却使其趋于平稳状态.进一步, 若两热噪声互关联时, 较小的互关联时间对二阶矩的影响较大, 反之作用较小. 伴随互关联强度递增, 位移二阶矩的扩散加剧, 位移间的相关性加强, 与物理直观相符. 关键词： 热噪声 非马尔可夫扩散 广义朗之万方程 关联性     

Computer‐Aided Optimization of Conditions for Fast and Controlled ICAR ATRP of n‐Butyl Acrylate

The potential of initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) for the synthesis of well‐defined poly(n‐butyl acrylate) is analyzed by means of simulations. The kinetic model accounts for reactivity differences between secondary and tertiary macrospecies and considers the possible influence of diffusional limitations. CuBr₂ is used as transition metal salt and the commercially available N,N,N′,N″,N″‐pentamethyldiethylenetriamine as ligand. For targeted chain lengths (TCLs) up to 1000, the ICAR ATRP can be performed relatively quickly, and with ppm levels of ATRP catalyst. For moderate TCLs, slightly higher ppm levels are required if excellent control over chain length is also desired. In all cases, limited loss of end‐group functionality (EGF) results.

     

Time-varying human mobility patterns with metapopulation epidemic dynamics

In this paper, explicitly considering the influences of an epidemic outbreak on human travel, a time-varying human mobility pattern is introduced to model the time variation of global human travel. The impacts of the pattern on epidemic dynamics in heterogeneous metapopulation networks, wherein each node represents a subpopulation with any number of individuals, are investigated by using a mean-field approach. The results show that the pattern does not alter the epidemic threshold, but can slightly lower the final average density of infected individuals as a whole. More importantly, we also find that the pattern produces different impacts on nodes with different degree, and that there exists a critical degree _kc$k_c$. For nodes with degree smaller than _kc$k_c$, the pattern produces a positive impact on epidemic mitigation; conversely, for nodes with degree larger than _kc$k_c$, the pattern produces a negative impact on epidemic mitigation.     

White matter maturation in the brains of Long Evans shaker myelin mutant rats by ex-vivo QSI and DTI

The brains of Long Evans shaker (les) rats, a model of dysmyelination, and their age- matched controls were studied by ex-vivo q-space diffusion imaging (QSI) and diffusion tensor imaging (DTI). The QSI and DTI indices were computed from the same acquisition. The les and the control brains were studied at different stages of maturation and disease progression. The mean displacement, the probability for zero displacement and kurtosis were computed from QSI data while the fractional anisotropy (FA) and the eigenvalues were computed from DTI. It was found that all QSI indices detect the les pathology, at all stages of maturation, while only some of the DTI indices could detect the les pathology. The QSI mean displacement was larger in the les group as compared with their age-matched controls while the probability for zero displacement and the kurtosis were both lower all indicating higher degree of restriction in the control brains. Since all the DTI eigenvalues were higher in the les brains as compared to controls, the less efficient DTI measure for discerning the les pathology was found to be the FA. Clearly, the most sensitive DTI parameter to the les pathology is λ₃, i.e. the minimal diffusivity. Since the QSI and DTI data were obtained from the same acquisition, despite the somewhat higher SNR of the QSI data compared to the DTI data, it seems that the higher diagnostic capacity of the QSI data in this experimental model of dysmyelination, originates mainly from the higher diffusing weighting of the QSI data.     

Diffusion tensor parameters and principal eigenvector coherence: Relation to b-value intervals and field strength

Diffusion-weighted MRI images acquired at b-value greater than 1000 s mm^− 2 measure the diffusion of a restricted pool of water molecules. High b-value images are accompanied by a reduction in signal-to-noise ratio (SNR) due to the application of large diffusion gradients. By fitting the diffusion tensor model to data acquired at incremental b-value intervals, we determined the effect of SNR on tensor parameters in normal human brains, in vivo. In addition, we also investigated the impact of field strength on the diffusion tensor model. Data were acquired at 1.5 and 3 T, at b-values 0, 1000, 2000 and 3000 s mm^− 2 in twenty diffusion-sensitised directions. Fractional anisotropy (FA), mean diffusivity (MD) and principal eigenvector coherence (κ) were calculated from diffusion tensors fitted between datasets with b-values 0–1000, 0–2000, 0–3000, 1000–2000 and 2000–3000 s mm^− 2. Field strength and b-value effects on diffusion parameters were analysed in white and grey matter regions of interest. Decreases in FA, κ and MD were found with increasing b-value in white matter. Univariate analysis showed a significant increase in FA with increasing field strength in highly organised white matter. These results suggest there are significant differences in diffusion parameters at 1.5 and 3 T and that the optimal results, in terms of the highest values of FA in white matter, are obtained at 3 T with a maximum b = 1000 s mm^− 2.     

Experimental Investigation of Saturated Flow Boiling Heat Transfer to TiO2/R141b Nanorefrigerant

Heat transfer in two-phase flow boiling of a dilute mixture of TiO₂ nanoparticles in R141b base fluid in a smooth tube is investigated experimentally. Examining the obtained results reveals that enhancement of the convective heat transfer coefficient for the particle volume fractions of 0.01% and 0.03% in comparison with pure R141b is more pronounced for a higher volume fraction. The measured data also show that at higher vapor qualities, the improvement in heat transfer coefficient is greater. Moreover, the heat transfer coefficient decreases substantially with mass flux while an increase in saturation temperature leads to an improvement in this coefficient.     

Preferential etching of ion bombarded GaAs

Abrupt changes in the near-band-edge luminescence of n-type undoped GaAs after implantation with 400 keV Zn ions and vacuum annealing at 580°C are reported. The good agreement of the spectral position, half-width and temperature dependence of the emission obtained after implantation and annealing with that of melt-doped GaAs: Zn indicates that implanted Zn ions have been incorporated at Ga lattice sites. The larger number of Zn substitutions obtained when bombardment is made on the Ga face than for an equivalent fluence on the As face demonstrates the existence of a polar implant effect.