Determan Michael D, Guo Liang, Thiyagarajan P, Mallapragada Surya K
Department of Chemical and Biological Engineering, Iowa State University and Ames Laboratory, 144 Spedding Hall, Ames, Iowa 50011, USA.
Langmuir. 2006 Feb 14;22(4):1469-73. doi: 10.1021/la0527691.
A unique pH-dependent phase behavior from a copolymer micellar solution to a collapsed hydrogel with micelles ordered in a hexagonal phase was observed. Small-angle neutron scattering (SANS) was used to follow the pH-dependent structural evolution of micelles formed in a solution of a pentablock copolymer consisting of poly((diethylaminoethyl methacrylate)-b-(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)-b-(diethylaminoethyl methacrylate)) (PDEAEM25-b-PEO100-b-PPO65-b-PEO100-b-PDEAEM25). Between pH 3.0 and pH 7.4, we observed the presence of charged spherical micelles. Increasing the pH of the micelle solution above pH 7.4 resulted in increasing the size of the micelles due to the increasing hydrophobicity of the PDEAEM blocks above their pKa of 7.6. The increase in size of the spherical micelles resulted in a transition to a cylindrical micelle morphology in the pH range 8.1-10.5, and at pH >11, the copolymer solution undergoes macroscopic phase separation. Indeed, the phase separated copolymer sediments and coalesces into a hydrogel structure that consists of 25-35 wt % water. Small-angle X-ray scattering (SAXS) clearly indicated that the hydrogel has a hexagonal ordered phase. Interestingly, the process is reversible, as lowering of the pH below 7.0 leads to rapid dissolution of the solid into homogeneous solution. We believe that the hexagonal structure in the hydrogel is a result of the organization of the cylindrical micelles due to the increased hydrophobic interactions between the micelles at 70 degrees C and pH 11. Thus we have developed a pH-/temperature-dependent, reversible hierarchically self-assembling block copolymer system with structures spanning nano- to microscale dimensions.
观察到一种独特的pH依赖性相行为，即从共聚物胶束溶液转变为具有六方相排列胶束的塌陷水凝胶。小角中子散射（SANS）用于跟踪由聚（（甲基丙烯酸二乙氨基乙酯）-b-（环氧乙烷）-b-（环氧丙烷）-b-（环氧乙烷）-b-（甲基丙烯酸二乙氨基乙酯））（PDEAEM25-b-PEO100-b-PPO65-b-PEO100-b-PDEAEM25）组成的五嵌段共聚物溶液中形成的胶束的pH依赖性结构演变。在pH 3.0至pH 7.4之间，我们观察到带电球形胶束的存在。将胶束溶液的pH提高到pH 7.4以上会导致胶束尺寸增大，这是由于PDEAEM嵌段在其pKa 7.6以上时疏水性增加。球形胶束尺寸的增加导致在pH范围8.1 - 10.5内转变为圆柱形胶束形态，并且在pH > 11时，共聚物溶液发生宏观相分离。实际上，相分离的共聚物沉淀并聚结成一种水凝胶结构，该结构含有25 - 35 wt%的水。小角X射线散射（SAXS）清楚地表明该水凝胶具有六方有序相。有趣的是，这个过程是可逆的，因为将pH降低到7.0以下会导致固体迅速溶解成均匀溶液。我们认为水凝胶中的六方结构是由于在70℃和pH 11时胶束之间疏水相互作用增加，使得圆柱形胶束有序排列的结果。因此，我们开发了一种具有pH/温度依赖性、可逆的分级自组装嵌段共聚物体系，其结构跨越纳米到微米尺度。
