聚卤乙烯单体性质及构型对链柔性和极性的影响

基于旋转异构态模型和生成矩阵统计方法,推导了极性取代基高分子链均方回转半径和均方偶极矩改进的计算公式,应用于研究聚卤乙烯链构象和构型依赖的性质,包括均方回转半径和偶极矩特征比与构象能、链规整程度和温度的关系.发现构型规整的聚卤乙烯链特征比、温度系数和构象能依赖性质均呈现较大的差异,尤其是间同和全同构型链.如聚卤乙烯间同链偶极矩特征比随主要相互作用Eη变化比例在-0.62～0.05(J/mol)-1之间.相反,构型不规整的聚卤乙烯链的无扰尺寸则比较接近.聚氟乙烯、聚氯乙烯和聚溴乙烯无规链均方偶极矩特征比分别为0.80、0.69和0.59,均方偶极矩温度系数为-0.07×10-3～-0.53×10-3 K-1,与实验结果符合.研究结果表明聚卤乙烯单体性质和构型对其链柔性和链极性的影响是显著的.     

Pb1-xTbxTi1-xMnxO3(O≤x≤0.10)固溶体的合成、结构与表征

用固相反应合成了Pb1-xTbxTi1-xMnxO3(0≤x≤0.10)固溶体,并用X射线粉末衍射进行了表征,室温下其空间群为P4mm.热分析仪测试结果显示,随着Tb和Mn掺杂量的增加,该固溶体的相变温度Tc降低.介电常数在Tc附近出现峰值,表明对应的相变是铁电相变.磁性测量显示,当x=0.08和x=0.10时,Pb1-xTbxTi1-xMnxO3分别在25和29 K附近有顺磁性向反铁磁性的转变.     

Pb_(1-x)Tb_xTi_(1-x)Mn_xO_3(0≤x≤0.10)固溶体的合成、结构与表征(英文)

用固相反应合成了Pb1-xTbxTi1-xMnxO3(0≤x≤0.10)固溶体,并用X射线粉末衍射进行了表征,室温下其空间群为P4mm.热分析仪测试结果显示,随着Tb和Mn掺杂量的增加,该固溶体的相变温度Tc降低.介电常数在Tc附近出现峰值,表明对应的相变是铁电相变.磁性测量显示,当x=0.08和x=0.10时,Pb1-xTbxTi1-xMnxO3分别在25和29 K附近有顺磁性向反铁磁性的转变.     

天然聚多糖的溶解机理及溶液性质

21世纪"绿色"化学已成为各国科学界和工业界研究与开发的方向.天然高分子中纤维素、甲壳素等聚多糖来源丰富,是重要的可再生资源,也是未来主要的化工原料之一.由于大量的分子内和分子间氢键,它们的溶解长期以来都是非常棘手的问题.另一方面,聚多糖的结构非常复杂,只有弄清楚其链构象和溶液性质,才可能进行合理的分子设计,实现其更好的应用.本文简要介绍了本课题组用碱/尿素水体系低温溶解纤维素和甲壳素等天然高分子的机理和溶液中分子链构象,以及活性多糖在水溶液中的链构象和溶液性质的研究进展.     

一种反铁电液晶薄膜相变过程的变温红外光谱研究

通过变温红外光谱对反铁电液晶MHOCPOOB薄膜相变过程中的分子构象、排布及相互作用的变化进行了研究.结果表明,室温时,薄膜中的分子烷基链同时含有zigzag和gauche两种构象.随着温度的升高,有序的zigzag构象转化为无序的gauche构象,链的扭曲程度增加.但S^*IA到S^*CA的转变并不引起烷基链构象和取向发生明显变化.刚性核中的羰基与相邻的苯环形成共轭体系,苯环之间相互倾斜排列,在相变过程中羰基与苯环的共平面作用逐渐被打破,且在相变点苯环间的二面角明显增大.     

二维HP格点模型中的紧密高分子链构象及其热力学性质的研究

采用二维HP模型用精确计数法和MonteCarlo方法研究了链长为N(≤ 2 2 )的紧密高分子链的构象和热力学性质 .发现不同HP序列的紧密高分子链的平均自由能和平均配分函数与链长N存在关系 :〈F〉=aN+b ,　ln〈Z〉=a′N +b′ .同时发现对于可折叠成基态且简并度为 1的紧密高分子链 ,其平均自由能和平均配分函数与链长N也存在相似的关系 .在HP模型中对于链长为N的紧密高分子链 ,存在着 2 N + 1 个不同的HP序列 .我们发现可以折叠成基态且简并度为 1的蛋白质分子的HP序列数目NS 为NS =a× 2 N+ 1 　 (a =0 0 2 5 ) ,对应的HP序列中 ,疏水基团 (H)数目的含量为 4 0 %～ 6 0 %的序列出现的几率最大 .同时在这些紧密高分子链中有些具有相同的结构 ,发现结构的‘简并度’为 3 3～ 4 0 (10≤N≤ 16 ) .在紧密高分子链折叠过程中 ,折叠的初期能量下降比较快 ,折叠的中期能量下降比较缓慢 ,折叠的后期能量下降也是比较快     

拉力诱导下的紧密高分子链相变行为

采用相互作用自回避行走(interacting self-avoiding walks,ISAWS)模型研究了一端固定的紧密高分子链在拉伸过程中的低温相变行为,观察到在拉伸过程中当温度T<0.1时平均拉力会出现一个震荡,随着温度的升高这种震荡现象又渐渐消失,这是由于紧密高分子链在低温时类似于β折叠的"冻结构象"被拉开而引起的.比较吸附条件下和无吸附作用下平均拉力、自由能以及相变行为的差别,发现在吸附条件下在拉伸的初始阶段为了克服表面吸附的相互作用,拉力会出现一个峰.吸附作用也使得外界作用到高分子链上的实际有效拉力减小,造成崩塌相态(collapsed phase)区域面积减少.另外发现在吸附条件下平均拉力还受温度变化的影响.在拉伸的初期由于单体间存在体积排除效应,平均拉力是随着温度的升高而降低,随着拉伸的深入当末端距到达一定长度时平均拉力是随着温度的升高而增加.并同Kumar等人在不考虑吸附作用下拉伸紧密高分子链得到的结果进行了比较.这些研究对于进一步研究外力诱导下吸附紧密高分子的相变有一定的参考价值.     

(Nd1-xYx)3Fe27.31Ti1.69化合物的结构与磁性研究

制备了(Nd1-xYx)3Fe27.31Ti1.69(0≤x≤0.6)系列化合物,通过X射线衍射和磁性测量等手段研究了它们的结构和磁性.这些化合物均为Nd3(Fe,Ti)29型结构,A2/m空间群.化合物晶胞体积随Y含量增加而单调减少,居里温度Tc随Y含量增加略有降低,说明化合物的居里温度主要由Fe-Fe之间的交换相互作用所决定.温度为5K时,饱和磁化强度Ms随Y含量的增加而单调降低,与一个简单的稀释模型预期结果一致.所得化合物在低温下均发生自旋重取向,自旋重取向温度Tsr随Y含量增加而单调降低,从x=0时的232K减小到x=0.6时的121K.基于磁晶各向异性的研究结果确定了所得化合物的自旋相图.     

两性高分子的溶液性质

两性高分子是分子链上同时含有正负电荷基团的一类高聚物、具有独特的溶液性质。本文就两性高分子等电点的构象状态、影响等电点的因素、两性高分子的溶解性以及高分子结构和组成、ｐＨ、溶液离子强度、混合溶剂、温度对流体力学行为的影响进行了综述。     

聚N-异丙基丙烯酰胺接枝聚苯乙烯高分子刷的构象

当长链高分子高密度接枝到一个表面上时,由于分子链间的相互作用使得接枝的高分子链扩张而形成伸直链的构象,这种形态被称为高分子刷.     

Solvent response of mixed polymer brushes

We have performed classical density functional theory calculations to study the behavior of mixed polymer brushes tethered to a planar surface. We assume no lateral segregation of the polymer at the grafting density studied and consider an implicit solvent. For a binary mixture of short and long athermal polymer chains, the short chain is compressed while the long chain is stretched compared with corresponding pure polymer chains at the same grafting density, which is consistent with simulation. This results from configurational entropy effects. Furthermore, we add a mean-field interaction for each polymer brush to simulate their different response towards a solvent. The long chain is forced to dislike the solvent more than the short chain. Through the interplay between the solvent effects and configurational entropy effects, a switch of the polymer brush surface (or outer) layer is found with increasing chain length of the long chain. The transition chain length (long chain) increases with increasing the solvent selectivity, and decreases with increasing the grafting density of the long chain. These results can provide guidance for the design of smart materials based on mixed polymer brushes.     

The glass transition temperature of polymer melts

We develop an analytic theory to estimate the glass transition temperature T(g) of polymer melts as a function of the relative rigidities of the chain backbone and side groups, the monomer structure, pressure, and polymer mass. Our computations are based on an extension of the semiempirical Lindemann criterion of melting to locate T(g) and on the use of the advanced mean field lattice cluster theory (LCT) for treating the thermodynamics of systems containing structured monomer, semiflexible polymer chains. The Lindemann criterion is translated into a condition for T(g) by expressing this relation in terms of the specific volume, and this free volume condition is used to calculate T(g) from our thermodynamic theory. The mass dependence of T(g) is compared to that of other characteristic temperatures of glass-formation. These additional characteristic temperatures are determined from the temperature variation of the LCT configurational entropy, in conjunction with the Adam-Gibbs model for long wavelength structural relaxation. Our theory explains generally observed trends in the variation of T(g) with polymer microstructure, and we find that T(g) can be tuned either upward or downward by increasing the length of the side chains, depending on the relative rigidities of the side groups and the chain backbone. The elucidation of the molecular origins of T(g) in polymer liquids should be useful in designing and processing new synthetic materials and for understanding the dynamics and controlling the preservation of biological substances.     

Polymer adsorption on a fractal substrate: numerical study

We study the adsorption of flexible polymer macromolecules on a percolation cluster, formed by a regular two-dimensional disordered lattice at critical concentration p(c) of attractive sites. The percolation cluster is characterized by a fractal dimension d(s) (p(c))=91/49. The conformational properties of polymer chains grafted to such a fractal substrate are studied by means of the pruned-enriched Rosenbluth method. We find estimates for the surface crossover exponent governing the scaling of the adsorption energy in the vicinity of transition point, φ(s)(p(c))=0.425±0.009, and for adsorption transition temperature, T(A)(p(c))=2.64±0.02. As expected, the adsorption is diminished when the fractal dimension of the substrate is smaller than that of a plain Euclidean surface. The universal size and shape characteristics of a typical spatial conformation which attains a polymer chain in the adsorbed state are analyzed as well.     

Interplay of the main chain, chiral side chains, and solvent in conformational transitions: poly([(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene).

Light scattering, sedimentation equilibrium, viscosity, circular dichroism (CD), and UV absorption (UV) measurements were made on dilute solutions of poly([(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene)(PRS) as functions of molecular weight. From light scattering and viscosity data, PRS is found to be a very stiff polymer of persistence length q as large as 103 nm at 25 degrees C, essentially a 7(3) helix found in the solid state; q increases only gradually with lowering temperature between -15 and 25 degrees C. The CD data show that PRS undergoes a conformational transition around 3 degrees C in isooctane (transition temperature T(c)). The CD signal is largely positive at low temperatures, passes through zero at T(c), and becomes largely negative at higher temperatures; T(c) is independent of sample's chain length N. This is a highly cooperative helix (M)-to-helix (P) transition depending remarkably on N, as PRS is substantially rodlike. The CD data are converted to the fraction f(P) of P helix as a function of N and analyzed successfully by a statistical mechanical theory based on a helix reversal model, where a polymer chain consists of M and P helices intervened by helix reversals, with the result that the free energy difference DeltaG(h) between P and M shows a temperature dependence similar to that of 2f(P) - 1, whereas the helix reversal energy is substantially constant at 1.2 x 10(4) J mol(-1); the latter value means that the helix reversal occurs only once in 100 Si units or less. This DeltaG(h) change and solvent dependence of T(c) are explained by a double-well potential for the rotation about Si-Si bonds, which incorporates into DeltaG(h) the solvent interactions with the helical grooves of side chains surrounding the main chain. Detailed features of UV absorption spectra at different temperature and molecular weights are also presented.     

Continuous self-avoiding walk with application to the description of polymer chains

We develop the continuous self-avoiding walk (CSAW) methodology for investigating temperature dependent thermodynamic properties of finite polymer chains without imposing a lattice. This leads to a new concept: the free energy theta temperature, T(theta)(F), at which the free energy is proportional to chain length. Above T(theta)(F), the polymer chain-solvent mixture leads to a single phase, whereas below T(theta)(F) the polymer solvent system has a positive surface tension with a tendency to phase separation to form a globular phase. For finite chains this coil-globule transition lies above the geometric theta temperature at which the distribution describes a Gaussian coil. CSAW provides the basis for a new approach to predict globular properties of real polymers.     

在电场和磁场下手性侧链丙烯酸酯液晶聚合物的行为

在电场和磁场下手性侧链丙烯酸酯液晶聚合物的行为江雪平金顺子漆宗能张树范（中国科学院化学研究所北京１０００８０）关键词手性侧链丙烯酸酯液晶聚合物，红外二向色性，液晶分子排列含有手性液晶基团的聚合物，分子中含有不对称碳原子，在一定条件下使液晶聚合物...     

Molten globule state in homopolymers:A Monte Carlo study

Simple models of polymer chains were based on a simple cubic lattice. The model chains were star‐branched with f = 3 and f = 6 branches. The attractive potential between polymer segments was introduced to study the properties of polymer chains in the different temperature regimes. The computer simulations were carried out by means of the dynamic Monte Carlo method. It was found that contrary to recent real experiments, the ratio of the radius of gyration to the hydrodynamic radius did not exhibit a maximum near the coil‐globule transition but decreased monotonically with the temperature. The distribution of polymer‐polymer contacts and their lifetimes were also studied. It appeared that in homopolymer chains the lifetimes of these contacts were very short. At low temperatures contacts were distributed over the entire chain and at high temperatures only contacts that were close to the chain survived longer times.     

Fragility of glass-forming polymer liquids

The fragility of polymeric glass-forming liquids is calculated as a function of molecular structural parameters from a generalized entropy theory of polymer glass-formation that combines the Adam-Gibbs (AG) model for the rate of structural relaxation with the lattice cluster theory (LCT) for polymer melt thermodynamics. Our generalized entropy theory predicts the existence of distinct high and low temperature regimes of glass-formation that are separated by a thermodynamically well-defined crossover temperature T(I) at which the product of the configurational entropy and the temperature has an inflection point. Since the predicted temperature dependence of the configurational entropy and structural relaxation time are quite different in these temperature regimes, we introduce separate definitions of fragility for each regime. Experimentally established trends in the fragility of polymer melts with respect to variations in polymer microstructure and pressure are interpreted within our theory in terms of the accompanying changes in the chain packing efficiency.     

Finite-size scaling analysis on the phase transition of a ferromagnetic polymer chain model

The finite-size scaling analysis method is applied to study the phase transition of a self-avoiding walking polymer chain with spatial nearest-neighbor ferromagnetic Ising interaction on the simple cubic lattice. Assuming the scaling M2(T,n) = n(-2beta/nu)[phi0 + phi1n(1/nu)(T-T(c)) + O(n(2/nu)(T-T(c))2)] with the square magnetization M2 as the order parameter and the chain length n as the size, we estimate the second-order phase-transition temperature T(c) = 1.784 J/k(B) and critical exponents 2beta/nu approximately 0.668 and nu approximately 1.0. The self-diffusion constant and the chain dimensions (R2) and (S2) do not obey such a scaling law.