高分子研究中的一个知识问题——柔性链“从线团到小球”的构象变化

一条大分子链由许多小分子通过共价键连接而成.正是这一"连接"导致了大分子一些独特的物理性质及相关问题.本文希望阐明的就是这样一个小分子物理中没有的知识问题:小分子在溶剂中仅有溶或不溶2个状态;而自60年代起,理论学家们就预言一条柔性大分子链在溶解的状态下,其构象随着溶剂性质变差可以从无规线团蜷缩成一个单链小球.为了证明这一构象变化,实验学家们从70年代末起进行了大量的研究,直至90年代初期仍未观察到稳定的单链蜷缩小球.实验上这一长期悬而未解的问题困惑着众多研究者.甚至有理论学家在1993年报道,当今的样品制备和实验手段无法观察到一个热力学稳定的单链蜷缩小球.中国钱人元先生和一些其他研究者自80年代末期也开始关注与单链有关的问题.我们实验室从1993年开始另辟蹊径,通过制备和采用窄分布的热敏性水溶性高分子超长链,终于在1995年利用激光光散射首次观察到理论上预测的"线团到小球"的构象变化.随后,又揭示了变化过程中存在着一个全新的"融化球"构象以及在单链蜷缩小球中并无理论上预计的额外链互穿和打结.从得到的稳定单链蜷缩小球出发,我们又首次在实验上研究了"小球到线团"的过程,意外地观察到其在准理想状态附近滞后于"线团到小球"的构象变化,并证明该滞后可归于链蜷缩过程中形成的额外链内氢键.最后,借用红外纳秒脉冲激光加热的方法研究了"线团到小球"的蜷缩动力学,并发现其包含了在单个高分子链上"成核"和"粗化"先后2个过程.其中,"成核"过程与链长无关.经过近20年的努力,我们终于基本解决了这一近代高分子物理研究中与知识有关的重要问题,揭示了与其相关的一些大分子特有的物理性质.

One Intellectual Problem in Polymer Research-the Coil-to-Globule Conformation Transition of Flexible Chains

A macromolecule consists of many repeating units connected via covalent bonds.Such a chain connectivity results in some unique physical properties and related problems of macromolecules.In this review,we will clarify one intellectual problem that is absent in small molecular systems;namely,small molecules only have soluble and insoluble two states;in 1960's,theorists predicted that even in its soluble state a flexible linear polymer chain can change its conformation from a swollen random coil to a collapsed globule as the solvent quality varies from good to poor.Since the end of 1970's,experimentalists tried hard to confirm such a coil-to-globule transition.However,a stable single-chain collapsed globule had not been experimentally observed up to the middle of 1990's.This unsolved problem has long puzzled many researchers.One theorist even claimed in 1993 that a thermodynamically stable single-chain collapsed globule cannot be observed with our modern instruments and current sample preparation technique.In China,the late Professor Renyuan Qian and some researchers started to study some single-chain problems in the end of 1980's.In 1993,our laboratory used a novel approach to attack such a problem by successfully preparing and using some narrowly distributed high-molar mass linear thermally sensitive water-soluble homopolymers.In 1995,we observed this long-predicted coil-to-globule transition by using laser light scattering.Further,we discovered a novel "molten globule" state during the coil-to-globule transition and revealed that there is no additional knotting and entanglement inside individual single-chain globules.We also studied,for the first time,the opposite globule-to-coil transition and found an unexpected hysteresis in comparison with the coil-to-globule transition,which is related to the formation of additional intrachain hydrogen bonds inside the collapsed globular state.Finally,we investigated the chain folding kinetics by using the infrared pulsed laser-induced temperature jump and unearthed that the coil-to-globule transition has two distinct kinetic stages:the nucleation (formation of small "pearls" made of a limited number of collapsed chain segments along the chain),in dependent of the chain length;and the coarsening (merging of "pearls").After persisting for nearly twenty years,we have essentially concluded the study of this important intellectual problem in modern polymer physics and revealed some of its related unique physical properties.