Theory of main chain nematic polymers with spacers of varying degree of flexibility

Main chain liquid crystal polymers are modelled as either worms or jointed rods. In reality they are composed of mesogenic units (rods) linked by spacers with varying degrees of flexibility. We present a molecular model to describe non-homogeneous nematic polymers. The model takes account of molecular parameters, such as the lengths of the mesogenic group and the spacer units, and the interactions between them. The spacers are found to have an order differing from the mesogenic units. If the spacer is not very long and thus in effect is inflexible, one end of the spacer can retain to some extent the orientation of the other end, allowing orientational correlation between spacers mediated by the intermediate mesogenic unit. This is important in giving the chain a global rod-like behaviour as the nematic field becomes strong or the temperature low. The nematic order of the two components (mesogens and spacers), the nematic-isotropic transition as well as the latent entropy are examined. Furthermore, the anisotropic conformations of the polymers are investigated, which show either rod-like or random walk behaviour. Comparison of our results with experiment is found to be satisfactory.     

"全反射"与课堂教学素质化

素质教育的主阵地是课堂教学.如何切实做到物理教学素质化呢?下面结合“全反射”的教学,谈几点看法和做法.1 课堂教学目标构建的多元性 课堂教学强调目标构建的多元性,以完成教学多方位的艰巨任务,这是素质教育的重要教学特征之一.因而在制定具体的物理教学目标时,既要有知识、技能方面的要求,     

Effect of track stiffness on vibration levels in railway tunnels

The paper discusses the difficulties in comparing and drawing conclusions from vibration measurements made in different railway tunnels. A number of new measurements are presented, which have been made and analyzed on a consistent basis. The track stiffness has been determined for each of the measurements. Graphs are presented which indicate the level of rail and tunnel floor vibration likely to occur for different track stiffnesses. The variance in the measurement results also indicates the potential error in using the data to predict vibration levels in new situations.     

带作用的群系的临界结构

在本文中，我们讨论了带作用群的群系的临界结构，给出了处理允许某些作用群的群的问题的一般方法，作为应用，我们特别给出了关于幂零群及ｐ－幂零群的一些结果。     

Kinetics of phenyl radical reactions with propane,n‐butane,n‐hexane,and n‐octane: Reactivity of C6H5 toward the secondary C?H bond of alkanes

The kinetics of C₆H₅ reactions with n‐C_nH_2n+2 (n = 3, 4, 6, 8) have been studied by the pulsed laser photolysis/mass spectrometric method using C₆H₅COCH₃ as the phenyl precursor at temperatures between 494 and 1051 K. The rate constants were determined by kinetic modeling of the absolute yields of C₆H₆ at each temperature. Another major product C₆H₅CH₃ formed by the recombination of C₆H₅ and CH₃ could also be quantitatively modeled using the known rate constant for the reaction. A weighted least‐squares analysis of the four sets of data gave k (C₃H₈) = (1.96 ± 0.15) × 10¹¹ exp[?(1938 ± 56)/T], and k (n‐C₄H₁₀) = (2.65 ± 0.23) × 10¹¹ exp[?(1950 ± 55)/T] k (n‐C₆H₁₄) = (4.56 ± 0.21) × 10¹¹ exp[?(1735 ± 55)/T], and k (n?C₈H₁₈) = (4.31 ± 0.39) × 10¹¹ exp[?(1415 ± 65)T] cm³ mol^?1 s^?1 for the temperature range studied. For the butane and hexane reactions, we have also applied the CRDS technique to extend our temperature range down to 297 K; the results obtained by the decay of C₆H₅ with CRDS agree fully with those determined by absolute product yield measurements with PLP/MS. Weighted least‐squares analyses of these two sets of data gave rise to k (n?C₄H₁₀) = (2.70 ± 0.15) × 10¹¹ exp[?(1880 ± 127)/T] and k (n?C₆H₁₄) = (4.81 ± 0.30) × 10¹¹ exp[?(1780 ± 133)/T] cm³ mol^?1 s^?1 for the temperature range 297‐‐1046 K. From the absolute rate constants for the two larger molecular reactions (C₆H₅ + n‐C₆H₁₄ and n‐C₈H₁₈), we derived the rate constant for H‐abstraction from a secondary C? H bond, k_s?CH = (4.19 ± 0.24) × 10¹⁰ exp[?(1770 ± 48)/T] cm³ mol^?1 s^?1. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 49–56, 2004     

Influence of Turbid Medium Parameters on Back Scattered Intensity and Pulsewidth of Picosecond Pulse

In this paper, we simulate a practical in vivo technique in which is produced influence of turbid medium parameters on backscattered intensity and pulsewidth of picosecond for turbid tissue surface of a semiinfinite medium by a small narrow linewidth laser beams. It is shown that the interaction of the ultra short pulse and the turbid tissue is very used as researching the optical parameters of the turbid medium.     

Analysis of transient wave scattering from an inclusion with an unilateral smoothly contact interface by BEM

Summary A 2D time-domain Boundary Element Method (BEM) is applied to solve the problem of transient scattering of plane waves by an inclusion with a unilateral smooth contact interface. The incident wave is assumed strong enough so that localized separations take place along the interface. The present problem is indeed a nonlinear boundary value problem since the mixed boundary conditions involve unknown intervals (separation and contact regions). In order to determine the unknown intervals, an iterative technique is developed. As an example, we consider the scattering of plane waves by the cross section of a circular cylinder embedded in an infinite solid. Numerical results for the near field solutions are presented. The distortion of the response waves and the variation of the interface states are discussed. The financial support by the China National Natural Science Foundation under Grant No. 19872001 and No. 59878004 is gratefully acknowledged. The second author is also grateful to the support of the National Science Fund for Distinguished Young Scholars under Grant No. 10025211.     

Homothetic self-similar solutions of three-dimensional Brans-Dicke gravity

All homothetic self-similar solutions of the Brans-Dicke scalar field in three-dimensional spacetime with circular symmetry are found in closed form.     

The influence of hard‐segments on two‐phase structure and shape memory properties of PCL‐based segmented polyurethanes

Poly(ε‐caprolactone)‐based segmented polyurethanes (PCLUs) were prepared from poly(ε‐caprolactone) diol, diisocyanates (DI), and 1,4‐butanediol. The DIs used were 4,4′‐diphenylmethane diisocyanate (MDI), 2,4‐toluenediisocyanate (TDI), isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI). Differential scanning calorimetry, small‐angle X‐ray scattering, and dynamic mechanical analysis were employed to characterize the two‐phase structures of all PCLUs. It was found that HDI‐ and MDI‐based PCLUs had higher degree of microphase separation than did IPDI‐ and TDI‐based PCLUs, which was primarily due to the crystallization of HDI‐ and MDI‐based hard‐segments. As a result, the HDI‐based PCLU exhibited the highest recovery force up to 6 MPa and slowest stress relaxation with increasing temperature. Besides, it was found that the partial damage in hard‐segment domains during the sample deformation was responsible for the incomplete shape‐recovery of PCLUs after the first deformation, but the damage did not develop during the subsequent deformation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 557–570, 2007     

Synthesis of graft copolymers with “V‐shaped” and “Y‐shaped” side chains via controlled radical and anionic polymerizations

A combination of nitroxide‐mediated radical polymerization and living anionic polymerization was used to synthesize a series of well‐defined graft (co)polymers with “V‐shaped” and “Y‐shaped” branches. The polymer main chain is a copolymer of styrene and p‐chloromethylstyrene (PS‐co‐PCMS) prepared via nitroxide‐mediated radical polymerization. The V‐shaped branches were prepared through coupling reaction of polystyrene macromonomer, carrying 1,1‐diphenylethylene terminus, with polystyryllithium or polyisoprenyllithium. The Y‐shaped branches were prepared throughfurther polymerization initiated by the V‐shaped anions. The obtained branches, carrying a living anion at the middle (V‐shaped) or at the end of the third segment (Y‐shaped), were coupled in situ with pendent benzyl chloride of PS‐co‐PCMS to form the target graft (co)polymers. The purified graft (co)polymers were analyzed by size exclusion chromatography equipped with a multiangle light scattering detector and a viscometer. The result shows that the viscosities and radii of gyration of the branched polymers are remarkably smaller than those of linear polystyrene. In addition, V‐shaped product adopts a more compact conformation in dilute solution than the Y‐shaped analogy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4013–4025, 2007