Controlled Stepwise Growth of Siloxane Chains Using Bivalent Building Units With Different Functionalities
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1272-LL11-08
Controlled Stepwise Growth of Siloxane Chains Using Bivalent Building Units With Different Functionalities
Nils Salingue, Dominic Lingenfelser, Pavel Prunici, and Peter Hess Institute of Physical Chemistry, University of Heidelberg Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany
ABSTRACT Organic/inorganic hybrids of silicon and their subsequent chemical modification are of interest for tailoring and structuring surfaces on the nanoscale. The formation of monolayers on hydroxylated silicon surfaces was employed to synthesize molecular dimethylsiloxane chains by wet-chemical condensation reactions, using dimethylmonochlorosilane as the precursor. The SiH group of the resulting dimethylsilyl termination could be selectively oxidized to the SiOH group, which opened the possibility of bonding another species. By repeating the condensation and oxidation cycle the stepwise growth of one-dimensional dimethylsiloxane chains was achieved. The ongoing chain growth was characterized by attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and determination of the surface energy by contact-angle experiments.
INTRODUCTION Poly(dimethylsiloxane) (PDMS) is a polymeric material with growing applications due to its excellent properties such as biocompatibility, nontoxicity, optical transparency, low surface energy, hydrophobicity, robust backbone, high flexibility, and excellent thermal properties [1]. Therefore, besides many established practical applications, PDMS is also of increasing interest in current research, for example, in bio-microelectromechanical system (bio-MEMS) devices for microfluidics to control the nanoscale properties of the surface (labs-on-a-chip) [2]. PDMS is also of interest in microcontact printing (µCP), offering functionalization and thus extension to the use of polar molecules as inks by surface treatment and low cost surface patterning [3]. To achieve these goals it is necessary to carefully control and tailor the physical and chemical surface properties of thin layers of PDMS. A more detailed discussion of the field of surfacefunctionalized silicone elastomer networks can be found in [4]. It is important to note that several of the reported properties are superior to those of self-assembled monolayers (SAMs) of longchain alkyl silanes,which may also be used in microfluidics, cell growth, and soft lithography. Despite the needs to understand and control the surface chemistry on the molecular level, results on the controlled formation of siloxane monolayers and the elucidation of their properties have been reported only in very few publications. This includes the chemisorption of monolayers of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) on flat substrates of TiO2 [5]. In this work sequential cycles of TMCTS chemisorption and the photoinduced oxidation to uniform SiOx
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