Tagit Oya, Tomczak Nikodem, Vancso G Julius
Materials Science and Technology of Polymers, Faculty of Science and Technology, and MESA Institute for Nanotechnology, University of Twente, PO Box 217, 7500AE Enschede, The Netherlands.
Small. 2008 Jan;4(1):119-26. doi: 10.1002/smll.200700260.
Submicrometer-sized particles of poly(N-isopropylacrylamide) (PNIPAM) are synthesized by surfactant-free radical polymerization. The morphology and nanomechanical properties of individual, isolated PNIPAM microgel particles at the silicon/air and silicon/water interfaces, below and above the PNIPAM volume-phase-transition temperature (VPTT), are probed by atomic force microscopy. In air, and in water below the VPTT, the PNIPAM spheres are flattened and adopt a pancakelike shape. Interestingly, above the VPTT the microgels adopt a more spherical form with increased height and decreased width, which is attributed to reduced interactions of the particles with the substrate. The elastic modulus calculated from force-indentation curves obtained for individual microgel spheres reveals that the stiffness of the particle's surface decreases by two orders of magnitude upon swelling in water. Additionally, the modulus of the PNIPAM spheres in water increases by one order of magnitude when crossing the VPTT from the swollen to the collapsed states, indicating a more compact chain packing at the particle surface.
通过无表面活性剂自由基聚合合成了聚（N-异丙基丙烯酰胺）（PNIPAM）的亚微米级颗粒。利用原子力显微镜研究了在PNIPAM体积相变温度（VPTT）以下和以上，硅/空气和硅/水界面处单个、孤立的PNIPAM微凝胶颗粒的形态和纳米力学性能。在空气中以及VPTT以下的水中，PNIPAM球体变平并呈薄饼状。有趣的是，在VPTT以上，微凝胶呈现出更球形的形态，高度增加而宽度减小，这归因于颗粒与基底之间相互作用的减弱。从单个微凝胶球体获得的力-压痕曲线计算出的弹性模量表明，颗粒在水中溶胀时，其表面刚度降低了两个数量级。此外，当PNIPAM球体从溶胀态转变为塌陷态越过VPTT时，其在水中的模量增加了一个数量级，这表明颗粒表面的链堆积更加紧密。
