Growney David J, Mykhaylyk Oleksandr O, Armes Steven P
Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
Langmuir. 2014 Jun 3;30(21):6047-56. doi: 10.1021/la501084a. Epub 2014 May 22.
The micellar self-assembly behavior of a near-monodisperse polystyrene-hydrogenated polyisoprene (PS-PEP) diblock copolymer is examined in non-polar media (either n-heptane or n-dodecane). Direct dissolution of this diblock copolymer leads to the formation of relatively large polydisperse colloidal aggregates that are kinetically frozen artifacts of the solid-state copolymer morphology. Dynamic light scattering (DLS) and transmission electron microscopy studies indicate that heating such copolymer dispersions up to 90-110 °C leads to a structural rearrangement, with the generation of relatively small, well-defined spherical micelles that persist on cooling to 20 °C. Variable temperature (1)H NMR studies using deuterated n-alkanes confirm that partial solvation (plasticization) of the polystyrene micelle cores occurs on heating. This increased mobility of the core-forming polystyrene chains is consistent with the evolution from a kinetically-trapped to a thermodynamically-favored copolymer morphology via exchange of individual copolymer chains, which are observed by DLS. These micellar self-assembly observations are also consistent with small-angle X-ray scattering (SAXS) studies, which indicate the formation of star-like micelles in n-heptane, with a mean polystyrene core diameter of about 20 nm and an overall diameter (core plus corona) of about 80 nm. Micelle dissociation occurs on addition of chloroform, which is a good solvent for both blocks. Finally, physical adsorption of this PS-PEP diblock copolymer onto a model colloidal substrate (carbon black) has been confirmed using X-ray photoelectron spectroscopy. A Langmuir-type adsorption isotherm has been constructed using a supernatant depletion assay based on UV spectroscopy analysis of the aromatic chromophore in the polystyrene block. Comparable results were obtained using thermogravimetric analysis to directly determine the amount of adsorbed copolymer. Based on the maximum adsorbed amounts observed at 20 °C, these studies strongly suggest that individual PS-PEP copolymer chains adsorb onto carbon black from chloroform, whereas micellar adsorption occurs from n-alkanes.
在非极性介质（正庚烷或正十二烷）中研究了近单分散聚苯乙烯-氢化聚异戊二烯（PS-PEP）二嵌段共聚物的胶束自组装行为。这种二嵌段共聚物直接溶解会导致形成相对较大的多分散胶体聚集体，这些聚集体是固态共聚物形态的动力学冻结产物。动态光散射（DLS）和透射电子显微镜研究表明，将这种共聚物分散体加热到90-110°C会导致结构重排，生成相对较小、定义明确的球形胶束，冷却至20°C时这些胶束仍能保持。使用氘代正构烷烃进行的变温（1）H NMR研究证实，加热时聚苯乙烯胶束核会发生部分溶剂化（增塑）。形成核的聚苯乙烯链的这种流动性增加与通过DLS观察到的单个共聚物链的交换，从动力学捕获的共聚物形态向热力学有利的共聚物形态的演变一致。这些胶束自组装观察结果也与小角X射线散射（SAXS）研究一致，SAXS研究表明在正庚烷中形成了星状胶束，聚苯乙烯核的平均直径约为20 nm，总直径（核加冠层）约为80 nm。加入氯仿后胶束会解离，氯仿是两种嵌段的良溶剂。最后，使用X射线光电子能谱证实了这种PS-PEP二嵌段共聚物在模型胶体基质（炭黑）上的物理吸附。基于对聚苯乙烯嵌段中芳香发色团的紫外光谱分析，使用上清液耗尽分析法构建了朗缪尔型吸附等温线。使用热重分析直接测定吸附的共聚物量也得到了类似的结果。基于在20°C下观察到的最大吸附量，这些研究强烈表明，单个PS-PEP共聚物链从氯仿中吸附到炭黑上，而胶束吸附则发生在正构烷烃中。
