Matsuoka Hideki, Yamakawa Yuta, Ghosh Arjun, Saruwatari Yoshiyuki
†Department of Polymer Chemistry, Kyoto University, Katsura, Nishikyo, Kyoto 615-8610, Japan.
‡Osaka Organic Chemical Industries Ltd., 7-20 Azuchi-Machi, 1-Chome, Chuo-ku, Osaka 541-0052, Japan.
Langmuir. 2015 May 5;31(17):4827-36. doi: 10.1021/acs.langmuir.5b00637. Epub 2015 Apr 20.
Zwitterionic amphiphilic diblock copolymer, poly(ethylhexyl acrylate)-b-poly(carboxybetaine) (PEHA-b-PGLBT), was synthesized by the reversible addition-fragmentation chain transfer (RAFT) method with precise control of block length and polydispersity. The polymers thus obtained were spread onto the water surface to form a polymer monolayer. The fundamental property and nanostructure of the block copolymer monolayer were systematically studied by the surface pressure-molecular area (π-A) isotherm, Brewster angle microscopy (BAM), and X-ray reflectivity (XR) techniques. The π values of the monolayer increased by compression in relatively larger A regions. After showing a large plateau region by compression, the π value sharply increased at very small A regions, suggesting the formation of poly(GLBT) brush formation just beneath the water surface. The domain structure of μm size was observed by BAM in the plateau region. XR profiles for the monolayer at higher surface pressure regions clearly showed the PGLBT brush formation in addition to PGLBT carpet layer formation under the hydrophobic PEHA layer on the water surface, as was observed for both anionic and cationic brush layer in the water surface monolayer studied previously. The critical brush density, where the PGLBT brush is formed, was estimated to be about 0.30 chains/nm(2) for the (EHA)45-b-(GLBT)60 monolayer, which is relatively large compared to other ionic brushes. This observation is consistent with the fact that the origin of brush formation is mainly steric hindrance between brush chains. The brush thickness increased by compression and also by salt addition, unlike the normal ionic brush (anionic and cationic), whose thickness tended to decrease, i.e., shrink, by salt addition. This might be a character unique to the zwitterionic brush, and its origin is thought to be transition to an ionic nature from the almost nonionic inner salt caused by salt addition since both the cation and anion of the GLBT chain obtained counterions by the addition of salt. This stretching nature of the PGLBT brush depends on the ion species of the salt added, and it followed the Hofmeister series, i.e., more stretching in the order of Li(+) > Na(+) > K(+). However, it was rather insensitive to the anion species (Cl(-), Br(-), SCN(-)), which suggests that the carboxylic anion has a more dominant effect than the quaternized cation in GLBT although the former is a weak acid and the latter is believed to be a strong base.
通过可逆加成-断裂链转移（RAFT）方法合成了两性离子两亲性二嵌段共聚物聚（丙烯酸乙基己酯）-b-聚（羧酸甜菜碱）（PEHA-b-PGLBT），实现了对嵌段长度和多分散性的精确控制。将由此得到的聚合物铺展在水面上形成聚合物单分子层。通过表面压力-分子面积（π-A）等温线、布鲁斯特角显微镜（BAM）和X射线反射率（XR）技术系统地研究了该嵌段共聚物单分子层的基本性质和纳米结构。在相对较大的A区域，单分子层的π值通过压缩而增加。在压缩后出现一个大的平台区域后，在非常小的A区域π值急剧增加，这表明在水面下方形成了聚（GLBT）刷。在平台区域通过BAM观察到了微米尺寸的畴结构。在较高表面压力区域，单分子层的XR图谱清楚地表明，除了在水面上疏水的PEHA层下方形成PGLBT地毯层外，还形成了PGLBT刷，这与之前研究的水面单分子层中的阴离子和阳离子刷层情况相同。对于（EHA）45-b-（GLBT）60单分子层，形成PGLBT刷的临界刷密度估计约为0.30链/nm²，与其他离子刷相比相对较大。这一观察结果与刷形成的起源主要是刷链之间的空间位阻这一事实一致。与正常的离子刷（阴离子和阳离子刷）不同，正常离子刷的厚度往往会因加盐而减小，即收缩，而PGLBT刷的厚度通过压缩以及加盐都会增加。这可能是两性离子刷独有的特性，其起源被认为是由于加盐导致几乎呈非离子状态的内盐转变为离子性质，因为加入盐后GLBT链的阳离子和阴离子都获得了抗衡离子。PGLBT刷的这种伸展性质取决于所加盐的离子种类，并且遵循霍夫迈斯特序列，即按Li⁺>Na⁺>K⁺的顺序伸展程度更大。然而，它对阴离子种类（Cl⁻、Br⁻、SCN⁻）相当不敏感，这表明在GLBT中羧酸根阴离子比季铵化阳离子具有更主导的作用，尽管前者是弱酸而后者被认为是强碱。
