Physics of Interfaces and Nanomaterials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands.
J Phys Chem B. 2012 Aug 2;116(30):9261-8. doi: 10.1021/jp304364m. Epub 2012 Jul 24.
We demonstrate the potential of in situ spectroscopic ellipsometry for the investigation of the chain segment density profile and layer thickness during the temperature-induced, reversible collapse-expansion transition of poly(N-isopropylacrylamide) (PNIPAM) grafted layers. Here, we study PNIPAM films with variable grafting densities in aqueous systems, which were produced by atom-transfer radical polymerization (ATRP). In our attempt to obtain a realistic quantitative description of the thickness of our swollen PNIPAM layers, various models were implemented to fit the ellipsometric data. As expected, we found that the swelling ratio is strongly dependent on the grafting density. From the ellipsometry results, the density and thickness variation accompanying the collapse transition across the lower critical solution temperature (LCST) was characterized. The collapse can be adequately explained by considering the PNIPAM film to consist of two layers: (i) a dense layer near the surface and (ii) a more diluted layer on the side of the film exposed to the solvent. Analysis of the optical response reveals a gradient density profile within these layers.
我们展示了原位光谱椭圆偏振术在研究聚（N-异丙基丙烯酰胺）（PNIPAM）接枝层在温度诱导的可逆坍塌-扩张转变过程中链段密度分布和层厚的潜在应用。在这里，我们研究了在水相体系中通过原子转移自由基聚合（ATRP）制备的具有不同接枝密度的 PNIPAM 薄膜。在尝试对我们的溶胀 PNIPAM 层的真实定量描述中，我们实施了各种模型来拟合椭圆偏振数据。正如预期的那样，我们发现溶胀比强烈依赖于接枝密度。从椭圆偏振结果中，我们可以确定在较低临界溶液温度（LCST）下坍塌转变时的密度和厚度变化。通过考虑 PNIPAM 膜由两层组成，可以很好地解释坍塌：（i）靠近表面的致密层和（ii）暴露在溶剂侧的更稀释层。对光学响应的分析揭示了这些层内的梯度密度分布。
