平行壁面间平衡聚合物吸附行为的自洽场分析

采用自洽场理论研究了平衡聚合物在平行壁面间的吸附规律.结果表明,平衡聚合物的吸附行为可根据壁面吸附强度划分为弱吸附和强吸附两个区间.在强吸附区间,平衡聚合物可以形成明确的吸附层,增大吸附强度能够引起平均分子链长的陡增,但不会改变链长分布的指数形式.平衡聚合物的长链分子在强吸附条件下较短链分子更靠近吸附壁面,在弱吸附条件下,则更接近体系中心.通过计算壁面压强发现:在弱吸附作用下,吸附壁面始终受到分子链的推斥作用;在强吸附作用下,分子链对壁面的作用随壁面间距的加大由推斥转变为吸引.平衡聚合物的溶解性和排空效应以及壁面吸附作用之间存在竞争关系,加大排除体积作用参数会引起分子链密度分布的均一化和壁面承受压强的降低.     

两亲性聚合物刷的合成和溶液自组装

结合可控自由基聚合和铜催化的叠氮-炔环加成(Cu AAC)反应,合成了一系列Ab BA型两亲性的聚合物刷.其中A段为亲水性的聚(N,N'-二甲基丙烯酰胺)(PAm);B段为高密度接枝的聚合物刷,其侧链为疏水性的聚苯乙烯(PS).通过核磁氢谱(1H-NMR),凝胶渗透色谱(GPC)表征了聚合物刷的组成及结构,其侧链接枝密度约为100%.研究了亲水链段含量不同的聚合物刷在多种条件下的溶液自组装行为,通过透射电子显微镜(TEM)表征了组装体的形貌.在相同的组装条件下,两亲性聚合物刷中亲水链段含量较高时(5%),组装体形貌为球形,而随着亲水链段含量降低,形貌向片层状胶束和囊泡转变.对聚合物刷PAm195-b-(PA-g-PS67)153-b-PAm195的自组装研究表明,提高初始浓度促使组装体形貌向片层和囊泡转变;采用与PS的溶度参数相近的良溶剂,可提高胶束核的活动性,组装体更容易形成囊泡(热力学稳定态);而采用与PS的溶度参数相差大的选择性溶剂时,组装体倾向于聚集形成大尺寸的囊泡.     

疏水作用对光化学和光物理过程的影响 Ⅱ.不良溶剂中长链水杨酸烷基酯的簇集和荧光光谱的变化

前文报道了长链β-萘甲酸烷基酯在不良溶剂中形成激基缔合物,为长链分子在不良溶剂中发生簇集提供了新的证据.本文报道证明长链分子在不良溶剂中发生簇集的另一个实验事实.长链水杨酸烷基酯在二甲基亚砜-水(DMSO-H_2O)和乙二醇-水(EG-H_2O)混合溶剂中的荧光光谱分布以及荧光量子产率与短链水杨酸烷基酯的显著不同,这些差别可以用长链分子发生簇集来加以解释.     

疏水作用对光化学和光物理过程的影响II: 不良溶剂中长链水杨酸烷基酯的簇集和荧光光谱的变化

本文报道证明长链分子在不良溶剂中发生簇集的实验事实.长链水杨酸烷基酯在二甲基亚砜-水(DMSO-H2O)和乙二醇-水(EG-H2O)混合溶剂中的荧光光谱分布以及荧光量子产率与短链水杨酸烷基酯的显著不同,这些差别可以用长链分子发生簇集来加以解释.     

环形侧链聚合物刷的合成与表征

通过grafting-onto方法合成高侧链接枝密度的环形侧链聚合物刷并进行结构表征.先通过开环易位聚合(ROMP)制备带有炔基侧基的线形聚降冰片烯衍生物主链(l-PND),通过原子转移自由基聚合(ATRP)和铜催化的叠氮-炔环加成(Cu AAC)反应制备带有一个叠氮基的环形聚丙烯酸叔丁酯侧链(c-Pt BA),再利用Cu AAC反应将环形侧链高密度地接入主链,得到环形侧链聚合物刷.通过核磁氢谱(1H-NMR)、凝胶渗透色谱(GPC)和红外光谱(IR)对聚合物主/侧链及聚合物刷的结构和组成进行了表征.该聚合物刷主链聚合度为400,高接枝密度的环形侧链使主链构象伸展.聚合物刷在原子力显微镜(AFM)下呈单分子蠕虫形貌(柱状形貌),分子长度为200 nm,直径为30 nm.     

柔性树枝形大分子溶液的自洽场理论计算

树枝形大分子是一种新型高分子材料,其浓度分布、末端官能团分布和分子尺寸决定了材料的性质.在柔性支化臂条件下,推导了均聚物树枝形大分子在溶剂中的自洽场理论,计算了不同代数G的树枝形大分子的链节浓度分布.计算结果表明：（1）在良溶剂（或绝热溶剂）中,柔性支化臂的树枝形大分子符合"dense-core"模型,链节浓度沿径向缓慢下降;（2）由于柔性链的折回构象,末端链没有向球形分子的外缘球壳层聚集的倾向;（3）不同代数的支化臂所受的拉伸强度不同,近中心处的第一代支化臂所受的拉伸程度最大;（4）自洽场计算与Flory平均场理论标度律估算结果一致,得到树枝形大分子在良溶剂中的分子尺寸具有标度律R~（GP）^1/5N^2/5,其中P是单个支化臂的聚合度,N是分子的总的链段数;（5）在G固定的情况下,该标度律简化为R~P^3/5.     

无定形1,2-聚丁二烯的分子链结构与分子链的相互作用

本工作用聚合物玻璃化转变温度,内聚能密度和分子链内旋转势垒及异构化能表征分子链的相互作用;提出了在混合溶剂中用特性粘数法测定内聚能密度,和从聚合物玻璃化转变时的自由体积及稀溶液性质计算分子链内旋转参数等方法。     

聚合物分子刷的高效合成与应用

当聚合物链通过稳定的共价或非共价键高密度地接枝到另一聚合物链、二维平面结构或球形或柱形的三维结构上时,这类聚合物体系分别称为一维、二维和三维聚合物分子刷.与具有相似化学组成的线性聚合物相比,一维聚合物分子刷具有一些独特的性质,其中包括蠕虫状构象、紧凑的分子尺寸和链端效应.将聚合物链引入平面、球形或圆柱形基体的表面不仅能显著地改变基体的表面性质,还可赋予所得杂化聚合物分子刷新的功能.因此,聚合物分子刷在催化、纳米光刻、生物矿化、药物递送、医学诊断和光电子材料等领域具有十分广泛的应用.虽然活性/可控聚合的出现为各种一维、二维和三维聚合物分子刷的制备提供了更高效的合成手段,但是具有精确组成、结构和功能的聚合物分子刷的高效可控制备仍然是该领域的关键挑战之一.本文总结和评述了课题组近些年在发展一维、二维和三维聚合物分子刷的高效可控制备策略中取得的成果,并且通过相关实例展现了聚合物分子刷基材料在药物输送、防污涂料、催化和锂离子电池等领域的广阔应用前景.     

两亲性嵌段共聚物在选择性溶液中吸附行为的理论研究

通过自洽场理论方法研究了在圆形孔道以及水平基板受限情况下两亲性嵌段共聚物在选择性稀溶液中的自组装形态,考察受限管壁、管壁极性以及基板间距对聚合物吸附行为的影响.中性圆形孔道中,随着溶剂对疏水链的排斥作用增强和对亲水段的亲水作用增大,聚合物以蘑菇形状吸附于管壁上.疏水链、亲水链与溶剂的相互作用差异性越大,越容易产生吸附现象.在亲水性孔道中,吸附现象有所缓解,随着管壁对亲水段吸引作用的增强,聚合物在管壁附近形成规则排列的球状胶束.当疏水链、亲水链与溶剂的相互作用相差非常大时,即使管壁对亲水嵌段的吸附作用增强,也不能消除聚合物吸附现象.在中性水平基板中,随着基板间距的增加,两亲性嵌段共聚物在基板附近依次出现球状胶束-蘑菇状胶束-单层球状胶束-高分子刷-双层球状胶束-对称高分子刷的自组装形态.     

聚合物分子刷的合成与应用

本文综述了近年来在线形聚合物、树枝状大分子及树枝化聚合物等不同聚合物的每个重复单元上,高密度地、通过不同联接方式接枝上新的聚合物侧链而形成的刷形接枝共聚物,即聚合物分子刷的研究进展.主要介绍聚合物分子刷的三种合成途径,即“大单体聚合法“,“从主链接枝法“及“接枝到主链法“,并对不同路线的特点进行分析,同时对分子刷在制备纳米杂化材料、纳米导线及智能材料等方面的应用进行了综述.     

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.     

Spectral collocation methods for polymer brushes

We provide an in-depth study of pseudo-spectral numerical methods associated with modeling the self-assembly of molten mixed polymer brushes in the framework of self-consistent field theory (SCFT). SCFT of molten polymer brushes has proved numerically challenging in the past because of sharp features that arise in the self-consistent pressure field at the grafting surface due to the chain end tethering constraint. We show that this pressure anomaly can be reduced by smearing the grafting points over a narrow zone normal to the surface in an incompressible model, and/or by switching to a compressible model for the molten brush. In both cases, we use results obtained from a source (delta function) distribution of grafting points as a reference. At the grafting surface, we consider both Neumann and Dirichlet conditions, where the latter is paired with a masking method to mimic a confining surface. When only the density profiles and relative free energies of two comparison phases are of interest, either source or smeared distributions of grafting points can be used, but a smeared distribution of grafting points exhibits faster convergence with respect to the number of chain contour steps. Absolute free energies converge only within the smeared model. In addition, when a sine basis is used with the masking method and a smeared distribution, fewer iterations are necessary to converge the SCFT fields for the compressible model. The numerical methods described here and investigated in one-dimension will provide an enabling platform for computationally more demanding three-dimensional SCFT studies of a broad range of mixed polymer brush systems.     

Steric stabilization of spherical colloidal particles: Implicit and explicit solvent

We present the results of Monte Carlo simulations and density functional theory treatment of interactions between spherical colloidal brushes both in implicit (good) solvent and in an explicit polymeric solution. Overall, theory is seen to be in good agreement with simulations. We find that interactions between hard-sphere particles grafted with hard-sphere chains are always repulsive in implicit solvent. The range and steepness of the repulsive interaction is sensitive to the grafting density and the length of the grafted chains. When the brushes are immersed in an explicit solvent of hard-sphere chains, a weak mid-range attraction arises, provided the length of the free chains exceeds that of the grafted chains.     

双分布聚乙二醇接枝纳米二氧化硅的高效可控制备

在混合溶剂中通过"grafting to"的方法将2种分子量不同的聚乙二醇单甲醚(MPEG M_w=750,4000)接枝到氨基修饰的St?ber法二氧化硅(SiO_2-NH_2)表面,制备双分布纳米接枝复合物.采用二步法,先将带环氧端基的低分子量聚乙二醇单甲醚(MPEG-EO)与SiO_2-NH_2在甲苯溶剂中充分反应后,与高分子量的MPEG-EO在甲苯和正癸烷的混合溶剂中使用相同的反应条件和后处理方法,能便捷制备出具有双分布接枝的纳米复合物.在接枝反应体系中,分子链的链段尺寸和接枝密度之间存在着密切关系.一定的范围内,接枝密度随链段尺寸减小而增大.通过改变混合溶剂比例来调控接枝链段的尺寸,可以很好控制聚合物的接枝密度.在双分布接枝的纳米复合物中,低分子量的接枝密度为0.85 chains/nm~2,高分子量的接枝密度能达到0.40 chains/nm~2,体现出了简单、高效、可控的特点,与聚环氧乙烷(PEO)共混后分散良好,对于制备出均匀分散的纳米复合材料起到了一定的指导作用.     

Simple model for grafted polymer brushes

The first theories of grafted polymer brushes assumed a step profile for the monomer density. Later, the real density profile was obtained from Monte Carlo or molecular dynamics simulations and calculated numerically using a self-consistent field theory. The analytical approximations of the solutions of the self-consistent field equations provided a parabolic dependence of the self-consistent field, which in turn led to a parabolic distribution for the monomer density in neutral brushes. As shown by numerical simulations, this model is not accurate for dense polymer brushes, with highly stretched polymers. In addition, the scaling laws obtained from the analytical approximations of the self-consistent field theory are identical to those derived from the earlier step-profile-approximation and predict a vanishing thickness of the brush at low graft densities, and a thickness exceeding the length of the polymer chains at high graft densities. Here a simple model is suggested to calculate the monomer density and the interaction between surfaces with grafted polymer brushes, based on an approximate calculation of the partition function of the polymer chains. The present model can be employed for both good and poor solvents, is compatible with a parabolic-like profile at moderate graft densities, and leads to an almost steplike density for highly stretched brushes. While the thickness of the brush depends strongly on solvent quality, it is a continuous function in the vicinity of the temperature. In good and moderately poor solvents, the interactions between surfaces with grafted polymer brushes are always repulsive, whereas in poor solvents the interactions are repulsive at small separations and become attractive at intermediate separation distances, in agreement with experiment. At large separations, a very weak repulsion is predicted.     

A combinatorial approach to study solvent-induced self-assembly of mixed poly(methyl methacrylate)/polystyrene brushes on planar silica substrates: effect of relative grafting density

This article reports the study of the effect of relative grafting densities of two polymer chains on solvent-induced self-assembly of mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes through a combinatorial approach. Gradient-mixed PMMA/PS brushes were synthesized from a gradient-mixed initiator-terminated monolayer by combining atom transfer radical polymerization (ATRP) and nitroxide mediated radical polymerization (NMRP) in a two-step process. The gradient-mixed initiator-terminated monolayer was fabricated by first formation of a gradient in density of an ATRP initiator through vapor diffusion followed by backfilling of an NMRP-initiator-terminated trichlorosilane. After treatment of a gradient-mixed brush whose PS Mn was slightly lower than that of PMMA with glacial acetic acid, a selective solvent for PMMA, relatively ordered nanodomains were observed in the region where the ratio of PS to PMMA grafting density (number of polymer chains/nm2) was in the range from 0.67 to 2.17 and the overall grafting density was approximately 0.85 polymer chains/nm2. Contact angle hysteresis were high (> or =40 degrees ) in this region and XPS studies confirmed that the PMMA chains were enriched at the outermost layer. The nanodomains are speculated to be of a micellar structure with PS chains forming the core shielded by PMMA chains.     

Mesoscale simulations of the behavior of charged polymer brushes under normal compression and lateral shear forces

Dissipative particle dynamics (DPD) was used to investigate the behavior of two opposing end-grafted charged polymer brushes in aqueous media under normal compression and lateral shear. The effect of polymer molecular weight, degree of ionization, grafting density, ionic strength, and compression on the polymer conformation and the resulting shear force between the opposing polymer layers were investigated. The simulations were carried out for the poly(tert-butyl methacrylate)-block-poly(sodium sulfonate glycidyl methacrylate) copolymer, referred as PtBMA-b-PGMAS, end-attached to a hydrophobic surface for comparison with previous experimental data. Mutual interpenetration of the opposing end-grafted chains upon compression is negligible for highly charged polymer brushes for compression ratios ranging from 2.5 to 0.25. Under electrostatic screening effects or for weakly charged polymer brushes, a significant mutual interpenetration was measured. The variation of interpenetration thickness with separation distance, grafting density, and polymer size follows the same scaling law as the one observed for two opposing grafted neutral brushes in good solvent. However, compression between two opposing charged brushes results in less interpenetration relative to neutral brushes when considering equivalent grafting density and molecular weight. The friction coefficient between two opposing polymer-coated surfaces sliding past each other is shown to be directly correlated with the interpenetration thickness and more specifically to the number of polymer segments within the interpenetration layer.     

Evaluation of an atomic force microscopy pull-off method for measuring molecular weight and polydispersity of polymer brushes: effect of grafting density

The accuracy of the molecular weights Mn and polydispersities of polymer brushes, determined by stretching the grafted chains using atomic force microscopy (AFM) and measuring the contour length distribution, was evaluated as a function of grafting density sigma. Poly(N,N-dimethylacrylamide) brushes were prepared by surface initiated atom transfer radical polymerization on latex particles with sigma ranging between 0.17 and 0.0059 chains/nm2 and constant Mn. The polymer, which could be cleaved from the grafting surface by hydrolysis and characterized by gel permeation chromatography (GPC), had a Mn of 30,600 and polydispersity (PDI) of 1.35. The Mn determined by the AFM technique for the higher density brushes agreed quite well with the GPC results but was significantly underestimated for the lower sigma. At high grafting density in good solvent, the extended structure of the brush increases the probability of forming segment-tip contacts located at the chain end. When the distance between chains approached twice the radius of gyration of the polymer, the transition from brush to mushroom structure presumably enabled the formation of a larger number of segment-tip contacts having separations smaller than the contour length, which explains the discrepancy between the two methods at low sigma. The PDI was typically higher than that obtained by GPC, suggesting that sampling of chains with above average contour length occurs at a frequency that is greater than their spatial distribution.     

Stochastic dynamics simulation of grafted polymer brushes under shear deformation

We present results of computer simulations of polymer brushes (layers of polymer chains attached at one end onto an impermeable planar surface) under shear deformation at constant shear rate. As the first stage of calculations the behavior of a single brush was studied. The monomer density profile, the distribution of the chain ends, the positions and orientations of different monomers along the chain were calculated. Dimensions of the polymer chains as functions of the shear rate were obtained for different grafting densities. An increase in the brush thickness over the grafting plane with an increase in the shear rate as predicted by the theory of Barrat was observed. However, the magnitude of the effect appears to be small. We explain this by finite extensibility of the grafted chains.     

Solvent size effect on the depletion layer of a polymer solution near an interface

By varying polymer concentration phi0p and Flory-Huggins parameter chi, the effect of solvent size on the depletion interaction between polymer coils and a hard wall was investigated by the real-space version of self-consistent field theory (SCFT). The depletion profiles and depletion thickness indicated that the depletion effect is strong in less good solvent with large molecular volume. Through the analysis of the respective free energies of polymer coils and solvent molecules, we found that the increment in the translation entropy of the solvent is the key to strengthening the depletion interaction. On the basis of the SCFT results, we define a solvent with volume about one to six times that of the polymer segment as a "middle-sized solvent". The density oscillations previously studied by Van der Gucht et al. and Maassen et al. were also observed in our simulation, and the addition of middle-sized solvent will magnify the amplitude of the oscillations. The solvent-size-dependent depletion interaction may be an explanation for the reduced entanglement and promoted crystallization behavior of polymer coils prepared from the solution with middle-sized solvent.