自由电子激光对芳香聚酰胺纤维热处理的研究

用红外自由电子激光方法处理芳香聚酰胺纤维.  实验结果表明, 芳香聚酰胺纤维在自由电子激光辐照时在外力拉伸下可出现“缩颈”现象. 红外光谱研究结果表明, 经自由电子激光处理的芳香聚酰胺纤维样品未出现热裂解. 这种方法有可能发展成一种热处理芳香聚酰胺纤维的新方法.     

Modelling hydrological data with and without long memory

The paper focuses on the problem of long-range dependence when analysing time series of hydrological data. Three time series are analysed: the monthly rainfall in the town of Florence, Italy; the daily minimum temperatures in the same town; and, finally, the daily water inflow to Lake Maggiore, Italy. Heuristic methods and maximum likelihood estimation of a parametric model are used to investigate the Hurst phenomena and to detect whether long-range dependence is present in any of the time series. We found that long-range dependence is not present in the first series but it is present in the last two. The daily water inflow to Lake Maggiore was modelled by a fractionally differenced arima model (farima) which contains a long-range dependence component. It is shown that the fit is much better than the one provided by more traditional arima models that do not have such a component.

---

Sommario Lo studio considera il problema dell'identificazione dei fenomeni di dipendenza a lungo termine (long range dependence) nelle serie temporali di dati idrologici. Allo scopo sono state analizzate tre serie temporali, rispettivamente quella dei totali mensili di precipitazione rilevati alla stazione dell'Osservatorio Ximeniano de Firenze, quella delle temperature minime giornaliere per la stessa stazione e quella degli afflussi giornalieri al Lago Maggiore. Per identificare la presenza di fenomeni di dipendenza long range, attraverso la valutazione della consistenza del fenomeno di Hurst, sono stati utilizzati sia metodi euristici, sia la stima a massima verosimiglianza di un modello parametrico. Due delle tre serie analizzate sono risultate caratterizzate da tale dipendenza. Per la serie degli afflussi al Lago Maggiore, si è inoltre proceduto alla simulazione attraverso un modello arima a differenziazione frazionaria (farima), la cui struttura contiene una componente long-range. I risultati ottenuti, mostrano che tale modello fornisce risultati significativamente migliori dei tradizionali modelli arima, privi di tale componente.

     

The resolution of the Gibbs phenomenon for spherical harmonics

Spherical harmonics have been important tools for solving geophysical and astrophysical problems. Methods have been developed to effectively implement spherical harmonic expansion approximations. However, the Gibbs phenomenon was already observed by Weyl for spherical harmonic expansion approximations to functions with discontinuities, causing undesirable oscillations over the entire sphere.

Recently, methods for removing the Gibbs phenomenon for one-dimensional discontinuous functions have been successfully developed by Gottlieb and Shu. They proved that the knowledge of the first expansion coefficients (either Fourier or Gegenbauer) of a piecewise analytic function is enough to recover an exponentially convergent approximation to the point values of in any subinterval in which the function is analytic.

Here we take a similar approach, proving that knowledge of the first spherical harmonic coefficients yield an exponentially convergent approximation to a spherical piecewise smooth function in any subinterval , where the function is analytic. Thus we entirely overcome the Gibbs phenomenon.

     

Dynamical symmetry analysis of ionization and harmonic generation of atoms in bichromatic laser pulses

Recently the effect of the relative phase ? in a high‐intensity (～10¹⁴ W/cm²) two‐color (bichromatic) CW laser with frequencies ω and 2ω on the high‐order harmonic generation (HHG) was studied within the framework of the non‐Hermitian quantum mechanics (NHQM) [Phys Rev A 2004, 69, 043404/1]. Here we emphasize the study of symmetries in bichromatic HHG spectra within the framework of the conventional Hermitian QM, and in particular by taking the duration of the laser pulse into consideration (an effect that has not been included in the non‐Hermitian studies due to the time asymmetry problem in NHQM). The phase dependence of HHG and intense‐field ionization probability in a 1D Xe atom with symmetric field‐free potential and symmetric initial wave function were studied numerically and analytically. From simulations based on a single‐particle response it can be seen that the HHG spectra is symmetric with respect to inversion in the relative phase between the two colors ? only if ionization is forbidden in the system and the laser pulse is an adiabatic one. The HHG spectra is symmetric with respect to a π‐shift in ? whenever the laser pulse is an adiabatic one, either for bound or open (ionized) systems. The ionization probability is symmetric both to inversion or π‐shift in ?; the component probabilities (right‐ and left‐ionization probabilities) have the same ?‐dependence, up to a shift of π. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Heat transfer modelling in a polymer sample located in a microwave field

This work presents the modelling of heat transfer in a polymer sample submitted to a microwave field in the quartz column of a dilatometer surrounded by vacuum to avoid convection. The temperature rise is studied in transient state by the finite element method. It is assumed that a uniform and constant heat production is maintained in the entire volume of the sample. It is shown that it is possible to design the set composed of the sample and of the column to achieve a nearly uniform temperature in the whole volume of the sample while its temperature is raised from 20 to 250°C at a heating rate of 5 deg·min^?1.     

结合理论课堂的知识点设计高分子材料合成实验课程

尝试在"高分子材料合成创新实验"课程中将理论课的知识点巧妙地融入到实验教学中，有目的地设置实验环节，通过对实验现象的分析，帮助学生巩固理论课知识点。为了充分利用实验的等待时间，在有目的地组织学生对实验进行预习的基础上，还尝试"激励学生参与"实验课的教学，鼓励学生以实验小组的形式讲解与实验相关联的理论知识，进一步巩固相关知识点，并扩充知识面。2年的实验教学反馈表明，这些尝试既有效巩固了理论课堂的知识点，做到了活学活用，又激发了学生的学习兴趣，提高了分析问题的能力。     

Lee-Yang zeros and stokes phenomenon in a model with a wetting transition

We consider the statistical mechanics of a fluctuating string (1D solid-on-solid model) ofN columns with a contact energy term displaying a critical wetting transition. For this model we derive a contour integral representation for the finite-size partition function. From this representation we derive a polynomial representation and obtain the Lee-Yang zeros forN , 100. Through the asymptotic evaluation of the contour integral we evaluate the zeros for higherN. This asymptotic evaluation displays a Stokes phenomenon providing a different viewpoint of the mechanism by which a phase transition can arise, supplementing the picture of Lee and Yang. We also reproduce and extend somewhat the results of Smith for the finite-size scaling limit of the partition function.     

Phase transition in swollen gels 31. Swelling and mechanical behaviour of interpenetrating networks composed of poly(1-vinyl-2-pyrrolidone) and polyacrylamide in water/acetone mixtures

Swelling and mechanical behaviour of interpenetrating positively charged polymer networks (IPNs), composed of poly(1-vinyl-2-pyrrolidone) (PVP) networks and polyacrylamide (PAAm) networks, was investigated in water/acetone mixtures. The first PVP networks were prepared by radiation polymerization at room temperature; after that the PVP networks were swollen in PAAm aqueous solutions and the networks were prepared by thermal copolymerization at 65 °C. The IPNs were prepared with various amounts of the two charged comonomers (quaternary ammonium salts) in the presence of crosslinkers. Two transition regions, detected in the dependence of swelling ratio X on acetone concentration a, suggest that a two-phase structure was formed. The first transition, located between 44 and 60 vol% of acetone, corresponds to PAAm networks, while the second transition, located at 75 vol% of acetone, corresponds to PVP networks. Depending on the amount of positive charges bound to chains, both transitions exhibit continuous or discontinuous character; this fact indicates that intermolecular interactions between the two components occur with the formation of IPNs (e.g., more polar, charged PVP component increases the extent of hydrogen bonding and makes acetone less effective solvent for IPNs at the PAAm transition). The dependences of log G on log X are roughly the same regardless of charge concentrations; this means that the mechanical behaviour is predominantly determined by the degree of swelling for all gels.     

Structural and dynamical aspects of PEG/LiClO4 in solvent mixtures via NMR spectroscopy

Motivated by the potential usefulness of polyethylene glycol (PEG)/Li⁺ salt mixtures in several industrial applications, we investigated the structure and dynamics of PEG/LiClO₄ mixtures in D₂O and its mixtures with CD₃CN and DMSO-d₆, in a series of PEG-based polymers with a wide variation in their molecular weights. ¹H NMR chemical shifts, T₁/T₂ relaxation rates, pulsed-field gradient NMR diffusion experiments, and 2D HOESY NMR studies have been performed to understand the structural and dynamical aspects of these mixtures. Increasing the temperature of the medium results in a significant perturbation in the H-bonded structure of PEG in its PEG/LiClO₄/D₂O mixtures as observed from the increase in chemical shifts. On the other hand, the addition of molecular cosolvents has a negligible effect. The hydrodynamic structure of PEG shows a pronounced variation at low temperature with increasing molecular weight, which, however, disappears at higher temperatures. Increasing the temperature leads to a decrease in the hydrodynamic structure of PEG, which can be explained on the basis of solvation–desolvation phenomena. The 2D HOESY NMR spectra reveal a new finding of Li⁺-water binding in the PEG/LiClO₄/D₂O mixtures with the addition of molecular solvents, suggesting that the Li⁺ cation diffuses freely in the D₂O mixtures of polymers as compared with the polymer mixtures with DMSO or CD₃CN.     

A measurement method for distinguishing the real contact area of rough surfaces of transparent solids using improved Otsu technique

An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented,intending to support the investigation of friction characteristics,heat conduction,and energy dissipation at the contact interface.A laser sheet illuminates the contact interface,and the transmitted laser sheet is projected onto a screen.Then the contact information is acquired from the screen by a camera.An improved Otsu method is proposed to process the data of experimental images.It can compute the threshold of the overall image and filter out all the pixels one by one.Through analyzing the experimental results,we describe the relationship between the real contact area and the positive pressure during a continuous loading process,at different loading rates,with the polymethyl methacrylate(PMMA)material.A hysteresis phenomenon in the relationship between the real contact area and the positive pressure is found and explained.