Shear-Induced Fracture at the Interface of PDMS and a Rigid Slab Modified with Polyelectrolyte Layers

Characterization of Buried Interfaces of Silicone Materials in Situ to Understand Their Fouling-Release, Antifouling, and Adhesion Properties

Poly(dimethylsiloxane) (PDMS) has numerous excellent properties and is extensively used as the main component of many silicone products in a variety of research fields and practical applications such as biomedical materials, aviation, construction, electronic devices, and automobiles. Interfacial structures of PDMS and other components in silicone systems are important for such research and applications. It is difficult to probe interfacial molecular structures of buried solid-liquid and solid-solid interfaces of silicone materials due to the lack of appropriate analytical tools. In this feature article, we presented our research on elucidating the molecular structures of PDMS as well as other additives in silicone samples at buried interfaces in situ at the molecular level using a nonlinear optical spectroscopic technique, sum frequency generation (SFG) vibrational spectroscopy. SFG was applied to study various PDMS surfaces in liquid environments to understand their fouling-release and antifouling activities. SFG has also been used to study buried solid-solid interfaces between silicone adhesives and polymers, elucidating the molecular adhesion mechanisms. Our SFG studies provide important knowledge on interfacial structure-function relationships of silicone materials, helping the design and development of silicone materials with improved properties through optimization of silicone interfacial structures.

Icephobic Durability of Branched PDMS Slippage Coatings Co-Cross-Linked by Functionalized POSS

Ice accretion poses a severe impact on diverse aspects of human life. Although great efforts have been dedicated to prevent or alleviate ice adhesion to the surface of substrates by developing various icephobic coatings, it is still needed to improve the integrated performance. Herein, we present a novel strategy to prepare poly(dimethylsiloxane) (PDMS) slippage coatings by combining a soft architecture-driven branched PDMS with partial short PDMS-functionalized polyhedral oligomeric silsesquioxane (POSS) as a co-cross-linker, in which silicone oil with certain viscosity was added as a lubricant. The chemical structure, surface morphology, and icephobic durability of the prepared coatings were investigated with concerns for the potential anti-icing uses. The PDMS slippage coating shed light on extraordinary icephobic durability with the ice shear strength at approximately 11.2 kPa and maintained low values below 14 kPa even after 50 icing/deicing cycles. Due to the elaborate control of the cross-link density, the side chains of the branched PDMS provided a rich storage space for entrapped silicone oil for the formation of the interfacial slippage. Moreover, the introduction of the functionalized POSS brought about significantly improved mechanical resistance in abrasion and elastic modulus. It is suggested that the branched PDMS slippage coating is a promising candidate in practical anti-icing applications.

Soft matter dynamics: Accelerated fluid squeeze-out during slip

Using a Leonardo da Vinci experimental setup (constant driving force), we study the dependency of lubricated rubber friction on the time of stationary contact and on the sliding distance. We slide rectangular rubber blocks on smooth polymer surfaces lubricated by glycerol or by a grease. We observe a remarkable effect: during stationary contact the lubricant is only very slowly removed from the rubber-polymer interface, while during slip it is very rapidly removed resulting (for the grease lubricated surface) in complete stop of motion after a short time period, corresponding to a slip distance typically of order only a few times the length of the rubber block in the sliding direction. For an elastically stiff material, poly(methyl methacrylate), we observe the opposite effect: the sliding speed increases with time (acceleration), and the lubricant film thickness appears to increase. We propose an explanation for the observed effect based on transient elastohydrodynamics, which may be relevant also for other soft contacts.

Adhesion-induced instabilities and pattern formation in thin films of elastomers and gels

A hydrostatically stressed soft elastic film circumvents the imposed constraint by undergoing a morphological instability, the wavelength of which is dictated by the minimization of the surface and the elastic strain energies of the film. While for a single film, the wavelength is entirely dependent on its thickness, a co-operative energy minimization dictates that the wavelength depends on both the elastic moduli and thicknesses of two contacting films. The wavelength can also depend on the material properties of a film if its surface tension has a pronounced effect in comparison to its elasticity. When such a confined film is subjected to a continually increasing normal displacement, the morphological patterns evolve into cracks, which, in turn, govern the adhesive fracture behavior of the interface. While, in general, the thickness provides the relevant length scale underlying the well-known Griffith-Kendall criterion of debonding of a rigid disc from a confined film, it is modified non-trivially by the elasto-capillary number for an ultra-soft film. Depending upon the degree of confinement and the spatial distribution of external stress, various analogs of the canonical instability patterns in liquid systems can also be reproduced with thin confined elastic films.

Quantitative molecular level understanding of ethoxysilane at poly(dimethylsiloxane)/polymer interfaces

Because of the wide applications of silicone adhesives, it is important to study adhesion mechanisms of silicone elastomers to polymers. Adhesion properties are believed to be directly related to the molecular structures at the adhesive/substrate interfaces. To improve adhesion, adhesion promoters such as silanes are commonly used to modify the interfacial structures. It is difficult to study buried interfacial molecular structures between two materials in situ using conventional analytical techniques. In this study, sum frequency generation (SFG) vibrational spectroscopy was used to investigate molecular structures at buried silicone/poly(ethylene terephthalate) (PET) interfaces. Environmental-friendly epoxysilanes including (3-glycidoxypropyl)triethoxysilane (γ-GPES), (3-glycidoxypropyl)methyldiethoxysilane (γ-GPDES), and (3-glycidoxypropyl)dimethylethoxysilane (γ-GPDMES) and their mixtures with methylvinylsiloxanol (MVS) were used as adhesion promoters to modify silicone adhesion properties to PET. Various PET/silane, PET/uncured silicone, and PET/cured silicone interfaces were examined. The interfacial structures deduced from SFG spectra were correlated to adhesion testing results. It was found that silane headgroup order at the polymer interfaces is an important factor for improving adhesion. The decrease of silane headgroup order due to chemical reaction and disordering of such groups at the polymer interfaces can be associated with improved adhesion. The molecular level understanding on polymer/adhesive interfacial structures helps to design and develop adhesion promoters and polymer adhesives with improved performance.