Rheological Behavior of Polydimethylsiloxane Langmuir Layers at the Air-Water Interface

On the Preparation of Thin Films of Stearyl Methacrylate Directly Photo-polymerized at the Air-Water Interface

Characterization of bidimensional polymeric films at the air-water interface in the Langmuir trough, despite being a recurrent topic, usually refers to films of already formed polymeric materials, with very scarce reports on direct polymerization at the air-water interface. In the present work, we studied the photo-polymerization of stearyl methacrylate directly at the air-water interface under a nitrogen atmosphere, with the radical initiator solubilized in the aqueous phase. Two-dimensional (2D) polymerization was monitored by measuring the pressure-area isotherm at different irradiation times. The polymerization leads to a film with an isotherm different from that observed for the monomer, where the surface pressure is directly related to the irradiation time. The shape of this isotherm confirms the presence of a compressed liquid phase, where a higher order can be attained as a consequence of stronger packing forces involving polymer chains. The presence of inter-chain interactions allows rearrangements on the surface of the subphase, and even before the collapse a dense 2D ordering (with a solid phase-like behavior) can be observed. We present a new one-step, solvent-free procedure to obtain a photo-polymeric film directly at the air-water interface, which can be transferred to a solid surface by the Langmuir-Blodgett method, allowing film preparation of controlled thickness. Films were characterized by measuring properties such as thickness, roughness, and hydrophobicity and comparing them with films obtained from a conventional polymer. We report the differences between the interfacial behavior of amphiphilic molecules and nanomaterials such as films obtained by photo-polymerization, PSMA, directly on the air-water interface.

Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties

New non-crystallizable low-dispersity star-shaped polydimethylsiloxanes (PDMS) containing stereoregular cis-tetra(organo)(dimethylsiloxy)cyclotetrasiloxanes containing methyl-, tolyl- and phenyl-substituents at silicon atoms and the mixture of four stereoisomers of tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane as the cores were synthesized. Their thermal and viscous properties were studied. All synthesized compounds were characterized by a complex of physicochemical analysis methods: nuclear magnetic resonance (NMR), FT-IR spectroscopy, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), viscometry in solution, rheometry, and Langmuir trough study.

Silicone Surface Fundamentals

Many of the applications of the most familiar silicone polymer, polydimethylsiloxane (PDMS), are a consequence of its hydrophobic nature. The key quantities underlying this behavior are the water contact angle with water droplets, the surface tension of the polymer, and its interfacial tension with water. These quantities are reviewed for PDMS and the fluorsilicone polymethyltrifluoropropylsiloxane (PMTFPS) as well as some other less common, more highly fluorinated, fluorosilicones. As aliphatic fluorocarbons are usually introduced into polymers to lower surface tension, it is unexpected that the surface tension of PMTFPS is higher than PDMS. However, this observation is consistent with Zisman's early extensive studies. It is also somewhat surprising that there are no definitive values accepted for the water contact angle with PDMS and the interfacial tension at the PDMS/water interface. Some reasons for this are explored and relevant limitations considered. The variety of ways in which a PDMS surface can be presented must have a major effect on the range of water contact angles reported.