Chen Chunyan, Clarke Matthew L, Wang Jie, Chen Zhan
Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.
Phys Chem Chem Phys. 2005 Jun 7;7(11):2357-63. doi: 10.1039/b501910a.
Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate and compare the chemical structures of poly(ethyl methacrylate) (PEMA) and poly(ethyl acrylate) (PEA) in air, in water, and in a non-polar solvent, FC-75. SFG spectra from both polymer surfaces in air are dominated by vibrational modes from the ester ethyl side groups. The average orientation of these ester ethyl groups on the two polymer surfaces is slightly different. In water, the two polymers show markedly different restructuring behavior. The ester ethyl side chains on the PEMA surface in water reorient to tilt more toward the surface, yet remain ordered. Such a restructuring of the PEMA surface in water is reversible. However, no SFG signal was detected from the PEA/water interface, showing that the surface of PEA becomes disordered upon contacting water, and this process is irreversible. SFG results collected from the C=O range indicate that hydrogen bonding is observed for both polymer/water interfaces, but the order of C=O at the PEA/water interface is much lower than that at the PEMA/water interface. Supplemental experiments support our hypothesis that the PEA surface becomes rough and loses order gradually as it interacts with water. We have demonstrated, for the first time, that the loss of surface structural order is due to the interaction between soft PEA chains with water molecules followed by reorganization of the polymer backbone. This causes the polymer surface to become rough and disordered. However, the surface structures of PEMA and PEA in FC-75 are similar and are also similar to those in air. This indicates that not only T(g), but also the contacting medium plays an important role in determining the surface restructuring behavior of polymer materials.
和频振动光谱已被用于研究和比较聚甲基丙烯酸乙酯（PEMA）和聚丙烯酸乙酯（PEA）在空气、水和非极性溶剂FC - 75中的化学结构。空气中两种聚合物表面的和频光谱主要由酯基乙基侧链的振动模式主导。这两种聚合物表面上这些酯基乙基的平均取向略有不同。在水中，这两种聚合物表现出明显不同的重构行为。水中PEMA表面的酯基乙基侧链重新取向，更倾向于向表面倾斜，但仍保持有序。PEMA表面在水中的这种重构是可逆的。然而，在PEA/水界面未检测到和频信号，这表明PEA表面在与水接触时变得无序，且此过程不可逆。从C = O范围收集的和频结果表明，在两种聚合物/水界面都观察到了氢键，但PEA/水界面处C = O的有序度远低于PEMA/水界面。补充实验支持了我们的假设，即PEA表面在与水相互作用时会逐渐变得粗糙并失去有序性。我们首次证明，表面结构有序性的丧失是由于柔软的PEA链与水分子之间的相互作用，随后聚合物主链发生重组。这导致聚合物表面变得粗糙且无序。然而，PEMA和PEA在FC - 75中的表面结构相似，且与在空气中的也相似。这表明不仅玻璃化转变温度（T(g)），而且接触介质在决定聚合物材料的表面重构行为中也起着重要作用。
