高分子凝胶体积相变中的氢键作用

我们把关联模型(association models)推广应用到高分子凝胶体系,研究高分子与溶剂分子间的氢键和溶剂分子与溶剂分子间的氢键在高分子凝胶体积相变中的作用.首先通过分析凝胶体积分数与温度的关系发现,由于两种氢键作用,随着温度变化高分子凝胶出现连续、不连续体积相变,结果表明在体积相变过程中两种氢键都起着重要作用.其次,对不同氢键分数条件下的旋节线的研究发现,对于高分子凝胶体积相变中出现的UCST和LCST(上临界共溶温度和下临界共溶温度)现象也是由于高分子与溶剂分子间氢键和溶剂分子与溶剂分子间氢键共同作用的结果.     

中性/聚电解质高分子混合刷对蛋白质的吸附/解吸附

本文应用分子理论,研究中性(A)/聚电解质(B)高分子混合刷对蛋白质的吸附/解吸附特性.理论模型考虑蛋白质与中性高分子A的排斥、以及与聚电解质高分子B的静电吸引.研究发现,在pH=4～6、中性高分子A处于弱水合状态时,混合刷中A高分子链塌缩,B聚电解质链溶胀.由于蛋白质和B聚电解质链间的静电吸引,导致高分子混合刷对蛋白质的吸附.当A高分子水合性增强时,A高分子链溶胀,B聚电解质链塌缩.由于蛋白质与A高分子链间的排斥作用增强,与B聚电解质链间的静电吸引减弱,混合刷对蛋白质解吸附.     

中性/聚电解质高分子混合刷对蛋白质的吸附/解吸附

本文应用分子理论,研究中性(A)/聚电解质(B)高分子混合刷对蛋白质的吸附/解吸附特性.理论模型考虑蛋白质与中性高分子A的排斥、以及与聚电解质高分子B的静电吸引.研究发现,在pH=4～6、中性高分子A处于弱水合状态时,混合刷中A高分子链塌缩,B聚电解质链溶胀.由于蛋白质和B聚电解质链间的静电吸引,导致高分子混合刷对蛋白质的吸附.当A高分子水合性增强时,A高分子链溶胀,B聚电解质链塌缩.由于蛋白质与A高分子链间的排斥作用增强,与B聚电解质链间的静电吸引减弱,混合刷对蛋白质解吸附.     

水合作用与 pH 调控两性离子聚合物刷的相变行为

本文基于Flory-Huggins理论,建立理论模型研究水合作用与pH调控两性离子聚合物(ZP)刷的相变行为.理论模型考虑ZP的水合作用,以及ZP刷体系中的静电作用.研究发现,在不同pH条件下,ZP刷的体积分数随着水合作的减弱而的增加.随着pH的变化,ZP刷构象随着水合性转变行为明显改变,这是由于pH调控ZP链单体带有过多的净电荷(正电荷或负电荷),致使ZP链内出现静电排斥导致ZP刷溶胀.另外,当ZP链单体呈现过多的净电荷,会在很大程度上决定ZP刷体系静电势,影响ZP刷的相变行为.通过考察体系自由能,我们还发现,ZP刷体系自由能呈现了极大值,随着pH值的变化,自由能呈现的极大值随之改变,由此表明了体系的不稳定性,进而导致ZP刷体系发生相变.     

亲水性两性离子聚合物刷的构象与结构

本文应用分子场理论,研究暴露于水蒸气中的亲水性两性离子聚合物(HP)刷的构象与结构.理论模型考虑HP-水(P-W)氢键和水-水(W-W)氢键效应,以及HP单体之间的偶极-偶极相互作用.研究发现,P-W与W-W氢键决定着HP的水合性,P-W氢键形成,会诱导HP刷溶胀.我们通过考察HP单体间的偶极-偶极相互作用发现,随着偶极-偶极相互作用增强,HP链在垂直培基表面沿着链方向,形成了结节状结构.这是由于HP单体之间的偶极-偶极静电吸引作用导致单体间汇聚结节,这种结节在刷内产生了较强的排斥体积作用,因此,这种HP刷具有抗污性能.在较高的接枝密度环境下,由于HP链间单体之间的偶极-偶极静电吸引作用,会形成链间单体-单体的结节,在刷内形成结节网络状凝胶结构,这种结构的出现,会使得HP刷呈现极强的抗污性.另外,当体系中水蒸气浓度增加、水合相互作用增强时,增加的P-W氢键将平衡HP单体之间的偶极-偶极相互作用,使得结节解开,聚合物链伸展.我们的理论结果符合实验观测,由此表明,P-W氢键效应,以及HP单体之间的偶极-偶极相互作用决定着HP刷的构象转变和结构特性,刷内出现的两性离子聚合物链内单体间的结节和链间单体结节状凝胶结构,是两性离子聚合物刷呈现较强抗污性的本质特性.     

单个纳米粒子对聚合物结晶行为的影响

采用粗粒化模型,应用分子动力学方法研究了单个纳米粒子对聚合物结晶行为的影响.通过改变纳米粒子与聚合物单体之间作用方式(吸引作用或排斥作用)、纳米粒子与聚合物单体之间作用强度和聚合物分子链的长度,计算整个系统和局部区域的有序参数,研究了三个不同因素下纳米粒子对聚合物结晶行为的不同影响.研究表明,在聚合物基体中添加单个纳米粒子,纳米粒子对整个系统的结晶影响不明显,但是纳米粒子对其周围聚合物单体的结晶存在局部强化作用.当纳米粒子与聚合物单体之间为吸引作用且作用强度较大时,纳米粒子对聚合物结晶表现出明显的局部强化作用,聚合物分子链长度也有着一定的影响,在较大吸引作用强度下,长链样本比短链样本有着更为显著的局部强化作用.     

水合作用与pH调控聚电解质刷的构象转变

我们基于Flory-Huggins理论,建立理论模型研究水合作用与pH调控聚电解质刷的构象转变.理论模型考虑聚电解质链与水分子间的作用(聚电解质链的水合作用)、体系中的静电作用.研究发现,随着水合作用的改变,聚电解质刷出现由溶胀到塌缩的构象转变.由此表明了水合作用可在很大程度调节聚电解质刷的相变.通过分析pH的调控效应我们还发现,在碱性环境中(pH=8),聚电解质链单体的解离度增大,静电排斥会使得聚电解质刷溶胀.由此表明,聚电解质刷内水合作用与静电效应的耦合,将会共同决定聚电解质刷的构象转变特性.理论结果深刻揭示了水合作用的改变,会使得聚电解质刷体系发生相变,pH可在很大程度上改变其相变特性.     

多元醇固—固相变机理的研究

多元醇在一定温度下发生固—固相变,大量文献都报道了它们的固—固相变焓、相变温度以及相图,并认为相变焓与氢键有关.本文以IR 光谱测试结果以及量热实验结果为基础,定量探讨了NPG(neopentylglycol),PG(pentaglycerino),PE(pentaerythritol)的固—固相变焓与氢键的关系,进一步解释了相变的机理:NPG,PG,PE分子在发生固—固相变前,—OH形成分子间氢键,分子具有层状结构.相变后,NPG,PG,PE分子沿着层面移动,移动过程中部分氢键断裂,分子发生相变时所吸收 关键词： 固—固相变 相变焓 氢键 红外光谱     

紧密高分子链从无规线团到球形相变的蒙特卡洛模拟：链单体的相互作用

本文采用蒙特卡洛模拟研究紧密高分子单链的无规线团到球形转变过程. 首先,研究某一固定链单体的相互作用下的有效尺寸效应,并发现短链表现出一次的坍塌相变,而长链则出现两次,这从热熔曲线上可以明显看出. 随着链单体相互作用的减小,热熔曲线上峰值对应的转变温度朝着低温转移. 从体系的能量、均方回转半径和形状因子可以进一步看出,高分子相变发生在更低的温度区间. 这些结果有助于加深理解高分子坍塌相变过程中链单体相互作用的影响.     

退火温度对高分子薄膜中掺杂小分子的结晶和光谱学响应特性的影响

研究了小分子材料苝(EPPTC)在高分子材料聚芴衍生物(F8BT)薄膜中的结晶特性随温度的变化规律,以及由此引起的两种材料构成的异质结中激发复合体荧光发射特性的变化。实验结果表明,退火温度的升高会加强小分子与高分子材料在固体薄膜中的相分离。而小分子相在析出过程中,会在分子间作用力诱导下发生π—π团聚而结晶。晶体的尺度在达到小分子材料相变温度之前基本上随温度升高而增大。这一过程将破坏异质结结构,减小小分子与高分子间的接触面积,从而降低激发复合体的形成及其荧光发射强度。同时,由高分子向小分子发生的能量转移过程被显著减弱,而小分子晶相的荧光发射成分提高。这对于调控有机半导体异质结结构,进而改善光伏器件性能具有重要意义。     

Phase transitions in lyotropic nematic gels

In this paper, we discuss the equilibrium phases and collapse transitions of a lyotropic nematic gel immersed in an isotropic solvent. A nematic gel consists of a cross-linked polymer network with rod-like molecules embedded in it. Upon decreasing the quality of the solvent, we find that a lyotropic nematic gel undergoes a discontinuous volume change accompanied by an isotropic-nematic transition. We also present phase diagrams that these systems may exhibit. In particular, we show that coexistence of two isotropic phases, of two nematic phases, or of an isotropic and a nematic phase can occur. Received 15 February 2002 and Received in final form 14 June 2002     

Phase transition in polymer gels due to local heating by illumination of light

The volume phase transition in gels induced by visible light and its related properties are presented, the mechanism of which is based on local heating of a polymer network by illumination of light. The gels consist of a covalently cross-linked copolymer network of thermosensitive N-isopropylacrylamide and a chromophore. Without light illumination, the gel volume changes sharply, but continuously at approximately 34°C when the temperature is varied. At a fixed temperature of an appropriate value, a discontinuous volume transition is observed when the light intensity is gradually changed. The phase transitions can be understood in terms of the temperature increment at the immediate vicinity of polymer chains due to the local heating via light absorption and subsequent thermal dissipation of light by the chromophore. The results can be qualitatively described by the Flory-Huggins mean-field equation of state of gels. In order to make clear the mechanism of the light-induced phase transition in the present system, we measured the light transmitting properties and the swelling as well as shrinking kinetics. These preliminary results are described semi-qualitatively by making use of a simple phenomenological model.     

Orientational Ordering of a Liquid-Crystal Suspension of Carbon Nanotubes in a Magnetic Field

A statistical theory is proposed to describe a suspension of carbon nanotubes in a nematic liquid crystal. The mean-field approach is used, and dispersion attraction, the excluded volume effects, the diamagnetism of liquid crystal molecules, and the strong diamagnetism of nanotubes are taken into account. The influence of the volume fraction of impurity, temperature, and magnetic field on the orientational ordering of a liquid crystal matrix and carbon nanotubes is studied. The concentration and temperature phase transitions in the suspension are investigated for various magnetic fields. The concentration and field shifts of the point of the phase transition between nematic and isotropic or paranematic phases are studied.     

Freezing and microphase separation in random copolymers

Summary A two-letter random copolymer with attraction between similar monomers and repulsion between different ones is investigated using the replica method. This type of interactions favors microphase separation (MPS) in a compact state of a polymer or in a melt. Frustrations between interactions and polymeric bonds may lead to freezing transition in a phase where only a few conformations dominate and replica symmetry is broken. Our analysis reveals that stiff polymers have a frozen phase and do not undergo transition to a phase with microdomain structure. In flexible polymers the microphase transition may occur before freezing. A complete phase diagram showing the interplay between the two phase transitions is constructed for the two-letter random copolymer. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

DNA Intercalation in Neutral Multilamellar Membranes: Experiments and Theory

We report both an experimental and a theoretical work on the effect of confinement on a biological polymer (DNA) incorporated to multilamellar phase. We can see that even when no electrostatic interaction are involved this semi-flexible polymer can be incorporated in between the membranes. X-ray provides experimental evidence for the existence of a mixed DNA/Phospholipid structure. The nature of the DNA ordering is briefly described. A theoretical model is developed to describe the phase diagram. We emphasize the role of the isotropic/nematic transition of the DNA molecules to explain the ability to confine DNA in membranes.     

Reversible Gelation and Phase Transition of Aqueous Solution of Hydrophobically Modified Poly(N‐Isopropylacrylamide)

Abstract

We investigated physical gelation and phase transition of aqueous solutions of hydrophobically modified poly(N‐isopropylacrylamide) (HM‐PNIPAAm) with stearyl acrylate (SA). Aqueous solutions of HM‐PNIPAAm form a reversible gel cross‐linked with hydrophobic aggregations of stearyl groups. The physically cross‐linked HM‐PNIPAAm gel with relatively low SA content shows a two‐step volume transition with increasing temperature. Moreover, overlapping of the reversible gel–sol transition and phase separation results in the phase diagrams of the aqueous solutions of HM‐PNIPAAm, having five distinct regions: dilute sol, gel, gel–sol, sol–sol, and condensed sol regions. The overlapping of the two different phase transitions causes the two‐step volume transition.     

Effect of phase transitions on the kinetic parameters of polytetrafluoroethylene deformation

The effect of solid-state phase transitions on the kinetic parameters of creep deformation in polytetrafluoroethylene has been studied by differential scanning calorimetry and a laser-interferometric creep rate meter. It has been shown that the deformation activation volumes and the elementary phase transition volume differ from each other by more than an order of magnitude. The crystalline component of this polymer becomes a liquid-crystal phase coexisting with an amorphous phase.