选择性溶剂中的嵌段共聚物胶束

嵇培军　蔡良元　徐坚　马军　叶美玲　施良和

摘要:综述了近20年来在选择性溶剂中嵌段聚合物缔合行为的研究进展，探讨了胶束结构、表征方法及胶束形成的热力学、动力学和流体力学。
关键词:嵌段聚合物；胶束；热力学；动力学；流体力学；综述
中图分类号:TQ314.251 文献标识码:A
文章编号:1000－1255(2000)02－0118－05

Block copolymer micelles in selective solvents

Ji Peijun and Cai Liangyuan
（ Beijing Institute of Aeronautical Materials）
Xu Jian,Ma Jun,Ye Meiling and Shi Lianghe
(Polymer Physics Laboratory, Institute of Chemistry, The Chinese Academy of Sciences)

Abstract: Micellization of block copolymers in selective solvents was reviewed with 69 references. The structure, kinetics, dynamics and hydronamics behavior of the formation of block copolymer micelles were discussed.
Keywords: block copolymer; micelle; kinetics; dynamics; hydronamics; review

　　在选择性溶剂（对一嵌段为良溶剂，同时对另一嵌段为劣溶剂）中嵌段聚合物能够缔合形成胶束，由溶解性差的嵌段形成胶束的核，溶解性好的嵌段形成胶束的壳。大多数情况下嵌段共聚物遵循封闭缔合模型形成胶束，在胶束和溶解的大分子之间存在缔合平衡。嵌段共聚物的化学结构、溶剂选择性和温度影响胶束的性质，如缔合数、胶束相对分子质量、胶束的相对密度、临界胶束温度和临界胶束浓度^[1,2]。研究嵌段共聚物在选择性溶剂中胶束的形成机理和结构，对于开发嵌段共聚物作表面活性剂^[3]、增溶剂^[3,4]、药物载体^[3,5～11]和纳米材料^[12,13]，以及了解嵌段共聚物结构和性能的关系有重要的理论和实际意义。

1　胶束的结构

　　由电镜^[14～18]观察得到的胶束结构为球形。小角X射线^[19～21]和中子散射法^[22～28]证明胶束的核为均匀球形。不同于低分子表面活性剂胶束的是，嵌段共聚物胶束的核通常是溶胀的。
　　当溶剂对中间嵌段有选择性时，三嵌段共聚物胶束有其特殊性，例如庚烷是聚（苯乙烯－氢化异戊二烯－苯乙烯）嵌段共聚物中间嵌段的选择性溶剂。Tuzar^[29]发现，30个大分子形成1个胶束分子，三嵌段共聚物的两端嵌段形成核，中间嵌段在壳中形成环，只有少数大分子选择构象是一端在核中，另一端伸展在溶液中形成触角，即形成了章鱼模型。章鱼模型解释了这种类型胶束的特殊性质，如高粘度、低浓度下胶束的立体有序和复杂弛豫时间光谱等。最近章鱼模型得到了进一步验证^[30,31]。

2　胶束的表征方法

　　研究嵌段共聚物胶束的报道较多，典型的嵌段共聚物和选择性溶剂如表1所示。表征嵌段共聚物胶束的方法主要有光散射法^{[15,16,32～58]}、凝胶色谱法^{[38,45,48～58]}、超速离心沉降法^[36,40,59]、电镜法^{[14～18,48,58]}、粘度法^{[35～37,49,50]}、荧光法^{[39,43,60,61]}、核磁共振波谱法^[21,22,62]、中子散射法和小角X光散射法^[18～28,63]。
　　实验^[3]证明嵌段共聚物胶束有如下规律：(1)对于给定的嵌段共聚物，随着其相对分子质量的

hPI:hydrogenatedpolyisoprene;hPB:hydrogenatedpolybutadiene.

Table1　Some typical block copolymer micelle systems

3　胶束的形成机理

3.1　胶束形成的热力学
　　嵌段共聚物胶束的形成和解缔合可以用带有光散射检测器的停流技术来研究[64]。将沉淀剂和溶解在良溶剂中的嵌段共聚物快速共混，嵌段共聚物将发生缔合；当嵌段共聚物胶束溶液与良溶剂快速共混时，胶束发生解缔合。在形成胶束或胶束发生解缔合时，及时测量光散射强度。
　　对于两嵌段或三嵌段的共聚物，形成胶束的弛豫时间需要几十毫秒，而对于两嵌段共聚物，胶束解缔合弛豫时间小于仪器检测极限（1ms），三嵌段共聚物胶束解缔合弛豫时间超过100ms。在解缔合过程中嵌段共聚物的两端嵌段必须离开较为粘稠的核区域，所以三嵌段共聚物解缔合的弛豫时间较长。
　　Quintana等^[65～67]利用光散射、凝胶色谱和粘度法分别研究了聚苯乙烯－聚（乙烯/丁二烯）－聚苯乙烯（SEBS）在正庚烷和4－甲基－2－戊酮溶剂中胶束形成的热力学（见表2）。

Sample

4　胶束的流体力学

　　通常情况下，嵌段共聚物胶束的形成不仅伴随着热力学变化，而且由于胶束结构的收缩导致其粘度下降。这个特性非常实用，例如降低温度可使聚苯乙烯－氢化聚异戊二烯共聚物在矿物油中形成胶束，溶液粘度降低5倍，因此可将这种共聚物开发成为粘度改进剂^[3]。
　　线性聚合物在稀溶液中的粘度和扩散系数随浓度变化较小，而嵌段共聚物胶束的变化则可达10％。浓度继续增加时，由于形成了微晶格，导致零剪切粘度剧增。
　　在亚浓溶液区域中，中间嵌段为核的三嵌段共聚物胶束具有显著不同的流体力学特性。由于胶束的触角相互吸引，稀溶液和亚浓溶液之间的交叉浓度明显小于封闭缔合模型的交叉浓度，胶束仍然保持核壳结构和大小，因此这种胶束可以用来改善溶液的粘度。

5　结束语

　　嵌段共聚物在选择性溶剂中的缔合行为不同于低分子表面活性剂，有其特殊性质。对于二嵌段共聚物而言，计算机模拟在胶束的结构和性质研究方面已经取得很大进展，但目前还不能准确预测多嵌段共聚物胶束的一系列特征参数，这有待于进一步研究。嵌段共聚物胶束将在医药、农业和生态等行业得到广泛应用。

作者简介:嵇培军，博士。从事聚合物复合材料性能的研究工作。已发表论文14篇。
嵇培军（北京航空材料研究院，100095）
蔡良元（北京航空材料研究院，100095）
徐坚（中国科学院化学研究所高分子物理开放实验室，北京）
马军（中国科学院化学研究所高分子物理开放实验室，北京）
叶美玲（中国科学院化学研究所高分子物理开放实验室，北京）
施良和（中国科学院化学研究所高分子物理开放实验室，北京）

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收稿日期:1999－04－08；修订日期:2000－02－22。