Li Destri Giovanni, Miano Fausto, Marletta Giovanni
Laboratory for Molecular Surfaces and Nanotechnology (LAMSUN), Department of Chemistry, University of Catania and CSGI , V.le A Doria 6, 95125 Catania, Italy.
Langmuir. 2014 Apr 1;30(12):3345-53. doi: 10.1021/la4043777. Epub 2014 Mar 21.
The linear viscoelastic behavior in the low-frequency regime at the water/air interface of three different polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) copolymer monolayers, with block length ratio varying from 66-33 to 50-50 and 25-75 in molecular units, was studied and related to the interfacial behavior, characterized by means of Langmuir isotherms, and their structure, characterized by means of the atomic force microscopy technique. The two monolayers with the highest PMMA amount showed a single phase transition at about 12 mN/m, the viscoelastic behavior changing from a predominantly elastic to a viscoelastic one. This change in the viscoelastic properties was ascribed to the beginning of entanglement among the PMMA coronas of the predominantly circular quasi-2D micelles formed by the two copolymer systems. Conversely, the polymer with the lowest PMMA amount, despite having the same PMMA block length of the PS-PMMA 50-50 block copolymer, was found to behave as a viscoelastic system at any surface pressure value. This characteristic behavior cannot therefore be simply related to the molecular weight difference, but it has been put in connection to the irregular micelle structure observed in this case, consisting of a mixture of spherical and wormlike micelles, and to the different conformation adopted by the PMMA block. By blending this copolymer with an immiscible elastic homopolymer, namely poly(2-vinylpyridine), it was possible to tune the micelle nanostructure, obtaining regular circular quasi-2D micelles, with viscoelastic properties as expected for the PMMA-rich copolymer monolayers. To the best of our knowledge, this study shows for the first time the explicit dependence upon the relative block length and, in turn, upon the nanostructure of the quasi-2D micelles, of the viscoelastic properties of Langmuir monolayers and suggests that molecular weight and intermolecular interactions are not the only parameters governing the polymer conformation and, in turn, the polymer rheology and dynamics in quasi-2D confined systems.
研究了三种不同的聚苯乙烯 - b - 聚（甲基丙烯酸甲酯）（PS - b - PMMA）共聚物单分子层在水/空气界面低频区域的线性粘弹性行为，并将其与通过朗缪尔等温线表征的界面行为及其通过原子力显微镜技术表征的结构相关联。具有最高PMMA含量的两个单分子层在约12 mN/m处显示出单一的相变，粘弹性行为从主要为弹性转变为粘弹性。粘弹性性质的这种变化归因于由两种共聚物体系形成的主要为圆形的准二维胶束的PMMA冠之间开始缠结。相反，具有最低PMMA含量的聚合物，尽管其PMMA嵌段长度与PS - PMMA 50 - 50嵌段共聚物相同，但发现在任何表面压力值下都表现为粘弹性体系。因此，这种特征行为不能简单地与分子量差异相关，而是与在这种情况下观察到的不规则胶束结构有关，该结构由球形和蠕虫状胶束的混合物组成，并且与PMMA嵌段采用的不同构象有关。通过将这种共聚物与不混溶的弹性均聚物即聚（2 - 乙烯基吡啶）共混，可以调节胶束纳米结构，获得规则的圆形准二维胶束，其具有富含PMMA的共聚物单分子层所预期的粘弹性性质。据我们所知，这项研究首次表明朗缪尔单分子层的粘弹性性质明确依赖于相对嵌段长度，进而依赖于准二维胶束的纳米结构，并表明分子量和分子间相互作用不是控制聚合物构象以及进而控制准二维受限体系中聚合物流变学和动力学的唯一参数。
