Instituto de Ciencia y Tecnología de Polímeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain.
Instituto de Física de Buenos Aires (IFIBA-CONICET) and Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, C1428EHA Buenos Aires, Argentina.
Soft Matter. 2017 Mar 29;13(13):2453-2464. doi: 10.1039/c7sm00206h.
The surface properties of soft nanostructured hydrogels are crucial in the design of responsive materials that can be used as platforms to create adaptive devices. The lower critical solution temperature (LCST) of thermo-responsive hydrogels such as poly(N-isopropylacrylamide) (PNIPAm) can be modified by introducing a hydrophilic monomer to create a wide range of thermo-responsive micro-/nano-structures in a large temperature range. Using surface initiation atom-transfer radical polymerization in synthesized anodized aluminum oxide templates, we designed, fabricated, and characterized thermo-responsive nanopillars based on PNIPAm hydrogels with tunable mechanical properties by incorporating acrylamide monomers (AAm). In addition to their LCST, the incorporation of a hydrophilic entity in the nanopillars based on PNIPAm has abruptly changed the topological and mechanical properties of our system. To gain an insight into the mechanical properties of the nanostructure, its hydrophilic/hydrophobic behavior and topological characteristics, atomic force microscopy, molecular dynamics simulations and water contact angle studies were combined. When changing the nanopillar composition, a significant and opposite variation was observed in their mechanical properties. As temperature increased above the LCST, the stiffness of PNIPAm nanopillars, as expected, did so too, in contrast to the stiffness of PNIPAm-AAm nanopillars that decreased significantly. The molecular dynamics simulations proposed a local molecular rearrangement in our nanosystems at the LCST. The local aggregation of NIPAm segments near the center of the nanopillars displaced the hydrophilic AAm units towards the surface of the structure leading to contact with the aqueous environment. This behavior was confirmed via contact angle measurements below and above the LCST.
软纳米结构水凝胶的表面性质在设计响应性材料中至关重要，这些材料可作为平台来创建自适应器件。例如聚（N-异丙基丙烯酰胺）（PNIPAm）等温敏水凝胶的低临界溶液温度（LCST）可以通过引入亲水性单体来修改，从而在较大温度范围内形成广泛的温敏微/纳米结构。我们使用在合成的阳极氧化铝模板中进行的表面引发原子转移自由基聚合，设计、制造和表征了基于 PNIPAm 水凝胶的温敏纳米柱，通过掺入丙烯酰胺单体（AAm）可调节机械性能。除了 LCST 之外，亲水实体的掺入使我们系统的拓扑和机械性能发生了突然变化。为了深入了解纳米结构的机械性能、其亲水/疏水性行为和拓扑特征，我们结合使用原子力显微镜、分子动力学模拟和水接触角研究。当改变纳米柱的组成时，其机械性能会发生明显的相反变化。当温度升高超过 LCST 时，PNIPAm 纳米柱的硬度会如预期那样升高，而 PNIPAm-AAm 纳米柱的硬度则会显著降低。分子动力学模拟在 LCST 时提出了我们纳米系统中的局部分子重排。纳米柱中心附近的 NIPAm 段的局部聚集将亲水性 AAm 单元推向结构表面，导致与水相环境接触。这种行为通过在 LCST 以下和以上进行接触角测量得到了证实。
