Janne T. Hirvi

Molecular dynamics simulations of water droplets on polymer surfaces

Molecular dynamics simulations were used to study the wetting of polymer surfaces with water. Contact angles of water droplets on crystalline and two amorphous polyethylene (PE) and poly(vinyl chloride) (PVC) surfaces were extracted from atomistic simulations. Crystalline surfaces were produced by duplicating the unit cell of an experimental crystal structure, and amorphous surfaces by pressing the bulk polymer step by step at elevated temperature between two repulsive grid surfaces to a target density. Different-sized water droplets on the crystalline PE surface revealed a slightly positive line tension on the order of 10(-12)-10(-11) N, whereas droplets on crystalline PVC did not yield a definite line tension. Microscopic contact angles produced by the simple point charge (SPC) water model were mostly a few degrees smaller than those produced by the extended SPC model, which, as the model with lowest bulk energy, presents an upper boundary for contact angles. The macroscopic contact angle for the SPC model was 94 degrees on crystalline PVC and 113 degrees on crystalline PE. Amorphicity of the surface increased the water contact angle on PE but decreased it on PVC, for both water models. If the simulated contact angles on crystalline and amorphous surfaces are combined in proportion to the crystallinity of the polymer in question, simulated values in relatively good agreement with measured values are obtained.

Micro-micro hierarchy replacing micro-nano hierarchy: a precisely controlled way to produce wear-resistant superhydrophobic polymer surfaces.

Superhydrophobic polymer surfaces are typically fabricated by combining hierarchical micro-nanostructures. The surfaces have a great technological potential because of their special water-repellent and self-cleaning properties. However, the poor mechanical robustness of such surfaces has severely limited their use in practical applications. This study presents a simple and swift mass production method for manufacturing hierarchically structured polymer surfaces at micrometer scale. Polypropylene surface structuring was done using injection molding, where the microstructured molds were made with a microworking robot. The effect of the micro-microstructuring on the polymer surface wettability and mechanical robustness was studied and compared to the corresponding properties of micro-nanostructured surfaces. The static contact angles of the micro-microstructured surfaces were greater than 150° and the contact angle hysteresis was low, showing that the effect of hierarchy on the surface wetting properties works equally well at micrometer scale. Hierarchically micro-microstructured polymer surfaces exhibited the same superhydrophobic wetting properties as did the hierarchically micro-nanostructured surfaces. Micro-microstructures had superior mechanical robustness in wear tests as compared to the micro-nanostructured surfaces. The new microstructuring technique offers a precisely controlled way to produce superhydrophobic wetting properties to injection moldable polymers with sufficiently high intrinsic hydrophobicity.

CO oxidation on PdO surfaces

Density functional calculations were performed in order to investigate CO oxidation on two of the most stable bulk PdO surfaces. The most stable PdO(100) surface, with oxygen excess, is inert against CO adsorption, whereas strong adsorption on the stoichiometric PdO(101) surface leads to favorable oxidation via the Langmuir-Hinshelwood mechanism. The reaction with a surface oxygen atom has an activation energy of 0.66 eV, which is comparable to the lowest activation energies observed on metallic surfaces. However, the reaction rate may be limited by the coverage of molecular oxygen. Actually, the reaction with the site blocking molecular oxygen is slightly more favorable, enabling also possible formation of carbonate surface species at low temperatures. The extreme activity of strongly bonded surface oxygen atoms is more greatly emphasized on the PdO(100)-O surface. The direct reaction without adsorption, following the Eley-Rideal mechanism and taking advantage of the reaction tunnel provided by the adjacent palladium atom, has an activation energy of only 0.24 eV. The reaction mechanism and activation energy for the palladium activated CO oxidation on the most stable PdO(100)-O surface are in good agreement with experimental observations.

Hydrosilylation of cinchonidine and 9-O-TMS-cinchonidine with triethoxysilane: application of 11-(triethoxysilyl)-10,11-dihydrocinchonidine as a chiral modifier in the enantioselective hydrogenation of 1-phenylpropane-1,2-dione

The detailed synthesis and characterization of (−)-11-(triethoxysilyl)-10,11-dihydrocinchonidine (4), a starting material for the immobilization of (−)-cinchonidine on silica based supports, is described. Compound 4 together with its precursors 9-O-(trimethylsilyl)cinchonidine (2) and 9-O-(trimethylsilyl)-11-(triethoxysilyl)-10,11-dihydrocinchonidine (3) were employed as chiral modifiers in the hydrogenation of a prochiral diketone, 1-phenylpropane-1,2-dione, over a heterogeneous Pt/Al2O3 catalyst using cinchonidine (1) as a reference modifier. The unexpected enhancement of ee induced by 4, demonstrating the positive effect of distal modifier substitution, is discussed in the light of molecular modeling and NMR studies.

Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single‐Site α‐Olefin Polymerization Catalysis

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n (AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis-acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2(+) cation. Thermodynamics suggest that activation through AlMe2(+) transfer is the dominant mechanism because sites that are able to release AlMe2(+) are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.

Stability of dioctahedral 2:1 phyllosilicate edge structures based on pyrophyllite models

Phyllosilicates and related clay minerals are of interest due to a variety of technological applications and impact on natural soils. The important properties of these layered minerals arise from their surface chemistry, and therefore understanding the characteristics of their surfaces is desirable. The common focus has been on the basal surfaces, whereas the edge surfaces are little studied. One of the underlying reasons is that the edge surfaces exhibit various possible geometries making it difficult to assume a certain structure with a confidence. The present paper is dedicated to the stability of the edge structures and introduces the largest quantum chemical study on the subject to date. Pyrophyllite was used as a model species. Edge stabilities were determined as cleavage energies, including edge termination by dissociative sorption of water with variable proton configurations. The results show similar stabilities for various edge structures parallel to the (010), (130), (110) and (100) lattice planes, but the edges cleaved by cutting the fewest bonds are suggested to be the most stable on the basis of free energy estimation. The dominant edge is predicted to appear on the (110) crystal face.

Interaction of octahedral Mg(II) and tetrahedral Al(III) substitutions in aluminium-rich dioctahedral smectites

Smectites are clay minerals of interest due to their natural availability, extraordinary surface properties, and high swelling capacity in the presence of water. The behaviour of smectites and other layered phyllosilicates is largely affected by the appearance of isomorphous substitutions with a lower nuclear charge in their layer structures. These substitutions are the origin of negative net layer charge, and their distribution in the mineral layer impacts on the electrostatic properties of the mineral surfaces. The present density functional theory study shows that octahedral Mg(II) and tetrahedral Al(III) substitutions in aluminium-rich dioctahedral smectites tend to avoid each other and disperse in the layer structure. This behaviour is explained and demonstrated to originate from geometry deformations around the substitutions which are larger than the unsubstituted species.

Promoter effect of BaO on CO oxidation on PdO surfaces.

The effect of bulk BaO promoter on CO oxidation activity of palladium oxide phase was studied by density functional calculations. A series of BaO(100) supported Pd(x)O(y) thin layer models were constructed, and energy profiles for CO oxidation on the films were calculated and compared with corresponding profiles for the most stable PdO bulk surfaces PdO(100) and PdO(101). The most stable of the thin films typically exhibit the same PdO(100) and PdO(101) surface planes; the PdO(100) dominates already with double layer thickness. The supporting promoter improves the CO oxidation activity of the Pd(x)O(y) phase via a direct electronic effect and introduced structural strain and corrugation. Changes in CO adsorption strength are reflected in oxidation energy barriers, and the promoting effect of even 0.3 eV can be seen locally. Easier oxygen vacancy formation may partially facilitate the reaction.

Computational approach to study the influence of Mn, Fe, and Ni as additives toward rubber–brass adhesion

The effect of different transition metals (manganese, iron, and nickel) as alternative for cobalt additives toward the performance of rubber adhesion to brass has been investigated by employing modeling approach at density functional theory level. Out of the three different dopants, manganese shows positive results on both sulfide surfaces with notable improvement in adhesion on copper sulfide via carbon–carbon double bond. However, it exhibits lower promotional effect on zinc sulfide than cobalt dopant. Iron, on the other hand, only enhances the adhesion on copper sulfide, while inclusion of nickel displays the lowest promotional effect.

Effect of cobalt additives and mixed metal sulfides at rubber–brass interface on rubber adhesion: a computational study

Interaction of rubber models with dopant atoms on ZnS(110) and Cu2S(111) surfaces, present in rubber–brass adhesive interlayer, has been studied via computational approach using density functional theory. Carbon–carbon double bonds and thiol groups in rubber are generally responsible of the interaction with metal atoms of ZnS(110) and Cu2S(111) surfaces. Inclusion of a copper atom into a zinc sulfide environment weakens the interaction with rubber. The incorporation of cobalt atoms as additives on ZnS(110) and Cu2S(111) surfaces enhances the adsorption strength with rubber. The cobalt dopant increases the adsorption strength of all functional groups of rubber with zinc sulfide, while in doped copper sulfide only the carbon–carbon double bond interaction with rubber is enhanced. It is noteworthy that also the adsorption of saturated hydrocarbons on zinc sulfide is improved by cobalt doping indicating enhanced carbon–hydrogen bond interaction with the sulfide surface.