On the flexibility of cyclochain polymers

Conformational parameters for a wide variety of aromatic cyclochain polymers have been computed assuming free rotation about virtual bonds. The flexibility with free rotation is shown to be determined by the geometry of the repeat unit and can vary over a wide range (15–1500 Å). Experimental values of the Kuhn segment length A are calculated from literature data on the hydrodynamic behavior of polymeric macromolecules of this class in solution. For most polymers A_fr for free rotation is equal to the experimental A_exp, and hindered rotation is mainly due to bulky substituents. © 1993 John Wiley & Sons, Inc.     

Associativity and Operator Hamiltonian Quantization of Gauge Theories

We show that the associative algebra structure can be incorporated in the BRST quantization formalism for gauge theories such that extension from the corresponding Lie algebra to the associative algebra is achieved using operator quantization of reducible gauge theories. The BRST differential that encodes the associativity of the algebra multiplication is constructed as a quadratic second-order differential operator on the bar resolution.     

Hydrodynamic behavior of anionically prepared linear polyisoprenes and polystyrenes in carbon tetrachloride

Intrinsic viscosity, [η], weight-average molecular weight, M_w, relationships are reported for narrow molecular weight distribution linear polyisoprene and polystyrene samples in CCl₄ at 25°C. Molecular weight values cover a range nearly two orders in magnitude, extending as low as 3 × 10³. In the case of polystyrene there exists a molecular weight range (around M_w = 16,700) corresponding to a change in the Mark-Houwink-Sakurada (MHS) exponent from α = 0.71 to α = 0.54. Comparisons between the viscometric and hydrodynamic radii, from literature data, are made. For polyisoprene the MHS relationship is reported in CCl₄, for the first time. For this case α = 0.713 for the whole range of molecular weights studied. Values for the second virial coefficient from low-angle light-scattering measurements support the conclusions drawn from viscometry that CCl₄ is a good solvent for both polymers studied. The different behavior of the MHS exponent may be attributed to the difference in chain flexibility. © 1995 John Wiley & Sons, Inc.     

Logistic方程的紊动现象

对于单参数二维映射ｆ（ｘ，ｙ）＝（ｙ，μｙ（１－ｘ）），本文给出使ｆ存在Ｓｍａｌｅ马蹄的两个参数区间，从而对Ｌｏｇｉｓｔｉｃ方程的一些数值计算结果作出理论上的解释。     

A hybrid approach for addressing ring flexibility in 3D database searching

A hybrid approach for flexible 3D database searching is presented that addresses the problemof ring flexibility. It combines the explicit storage of up to 25 multiple conformations ofrings, with up to eight atoms, generated by the 3D structure generator CORINA with thepower of a torsional fitting technique implemented in the 3D database system UNITY. Acomparison with the original UNITY approach, using a database with about 130,000 entriesand five different pharmacophore queries, was performed. The hybrid approach scored, on anaverage, 10–20% more hits than the reference run. Moreover, specific problems withunrealistic hit geometries produced by the original approach can be excluded. In addition, theinfluence of the maximum number of ring conformations per molecule was investigated. Anoptimal number of 10 conformations per molecule is recommended.     

Conformational study of a guaiacyl beta-O-4 lignin model compound by NMR. Examination of intramolecular hydrogen bonding interactions and conformational flexibility in solution

Intramolecular H-bonding interactions were investigated in solution for the threo and erythro diastereomeric forms of a guaiacyl beta-O-4 lignin model compound by using the NMR data obtained from hydroxyl protons. Temperature coefficients of the chemical shifts (ddelta/dT) and coupling constants (3J(HCOH)) were measured in aprotic and protic solutions: DMSO-d6, acetone-d6 and acetone-d6-water. The NMR parameters do not support the existence of strong and persistent intramolecular H-bonds that could participate in the stabilization of the guaiacyl beta-O-4 structure in solution, but instead indicate that intermolecular H-bonds to solvent predominate. 1D NOE experiments nevertheless revealed the presence of a direct chemical exchange between the hydroxyl protons, suggesting the possible existence of weak and transient intramolecular H-bonding interactions. The conformational flexibility of the threo structure was also investigated in acetone solution from the measurement of long-range 1H, 1H and 1H, 13C coupling constants and from NOESY experiments. The NMR data are not consistent with any single conformation, indicating that different conformers co-exist in solution. The experimental results support the conformational flexibility predicted by molecular dynamics simulations performed in a previous study. Finally, both experimental and theoretical approaches indicate that weak intramolecular H-bonds can exist transiently in solution, breaking and reforming as the beta-O-4 molecule undergoes conformational interconversion, but cannot be invoked as possible means of conferring rigidity to the beta-O-4 structure.     

基于颗粒离散元法的连接键应变软化模型及其应用

基于颗粒间的有限接触假设,提出了可表述颗粒间力、力矩传递的连接键模型. 为了表征连接键的塑性、损伤及断裂过程,在连接键中引入了考虑应变软化效应的Mohr-Coulomb 准则及最大拉应力准则. 单一连接键的单向拉伸测试及直剪测试表明了上述连接键应变软化模型的计算精度. 研究了颗粒体系的宏观应变能与颗粒平均配位数的对应关系. 通过计算发现,对于二维颗粒体系,当平均配位数为5 时,颗粒体系的宏观应变能与相同参数下连续介质方法(如有限元等) 计算获得的应变能基本一致. 利用上述连接键应变软化模型对岩石的单轴压缩过程进行了模拟,计算结果表明:岩石单轴压缩的应力应变曲线经历了线性上升段、非线性上升段、非线性下降段及缓变段等4 个阶段,并给出了上述4 个阶段与岩石内部损伤破裂状态的内在联系. 计算结果还表明,随着断裂应变的增大,岩石的破裂模式逐渐由拉剪复合型破裂向单一压剪型破裂转化;随着断裂应变的增大,峰值应力及达到峰值应力时的应变均逐渐增大,但峰值时的破裂度及终态时的破裂度将逐渐减小.      

大理岩动态劈裂拉伸的SHPB实验研究

利用直径100 mm的Hopkinson压杆和薄圆形铝片作为波形整形器,用不同弹速径向冲击平台巴西圆盘试样以研究大理岩的动态拉伸强度。分析了试样的应变率、破坏时间、破坏模式,以及破坏过程中的载荷-应变关系,得到了关于大理岩在高应变率下拉伸强度及弹性模量的一些结论。考虑了试样的尺寸大小及两个平台附近应力的时间不均匀性与空间不均匀性对实验结果的影响。同时,利用有限元法对平台巴西圆盘试样的动态应力分布进行了数值模拟,验证了动态劈裂拉伸实验的有效性。     

基于有限单元柔度法的材料与几何双重非线性空间梁柱单元

从有限单元柔度法的基本思想出发,基于完全拉格朗日格式(TL格式),建立了能考虑材料与几何双重非线性的一般化空间梁柱单元,适用于满足Euler-Bernoulli梁柱二阶分析理论假定的空间杆系结构非线性分析。另外,在梁柱单元截面分析中引入纤维模型,使其适用于解决钢筋混凝土这类复合材料结构的非线性分析问题。对钢筋混凝土双向偏心受压柱试验结果的模拟分析表明,本文所提方法是正确、可靠的,能有效地分析钢筋混凝土框架柱的材料与几何双重非线性问题。