Probing the thermal collapse of PNIPAM grafts by quantitative in situ ellipsometry
- PDF / 28,335,428 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 81 Downloads / 148 Views
Probing the thermal collapse of PNIPAM grafts by quantitative in situ ellipsometry E.S. Kooij1, X.F. Sui2, M.A. Hempenius2, H.J.W. Zandvliet1, G.J. Vancso2 1
Physics of Interfaces and Nanomaterials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands 2
Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands ABSTRACT We demonstrate the potential of 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 with variable grafting densities in aqueous systems. To obtain a quantitative description of the thickness of our swollen PNIPAM layers, various models were implemented to fit the ellipsometric data. 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 with a gradient density profile on the side of the film exposed to the solvent. INTRODUCTION Stimuli-responsive polymer layers consisting of surface-tethered macromolecules have been widely applied in many technological areas, including fabrication of sensors, regulating cell cultures, and in controlling wetting and adhesion properties. Owing to their fascinating conformational changes in response to variation of a number of environmental parameters, this class of materials has also attracted the scientific community focused on renewable and sustainable energy applications. For example, the anti-fouling properties of polymer brush layers have been investigated, while recently poly(N-isopropylacrylamide) (PNIPAM) has been suggested as rooftop coating to control water adhesion, and therewith heat transfer into and out of buildings. PNIPAM is one of the most frequently studied responsive polymers [1-3]. The thermally induced collapse transition of PNIPAM in aqueous liquids as well as in mixed solvents has been investigated extensively. At temperatures below the lower critical solution temperature (LCST), the polymer is soluble in water due to cooperative hydration [4], caused by a positive correlation between adjacent bound water molecules. Due to steric interactions, consecutive sequences of bound water appear along the PNIPAM molecule. Upon heating the system to temperatures above the LCST, which is around 32°C in aqueous media, each sequence is considered to collectively dehydrate, resulting in a collapse of the grafted molecules. Surprisingly, despite the numerous studies in which spectroscopic ellipsometry is employed to characterize polymer brushes in general and also PNIPAM [5-14], actual spectra have not been presented. Moreover, the quantitative discussion of t
