Novel thin film polymer foaming technique for low and ultra low-k dielectrics

The results presented show a novel route for the preparation of thin ultra-low-k polymer films based on commercial and "non-exotic" (non-expensive) polyimide by a foaming technique. Dependent on the glass transition temperature of the polyimide mechanically and thermally stable (> 300 °C) films having porosities of ca. 40 % and k-values below 2.0 are formed. A further reduction into the ultra low k region may be accomplished by tailoring the shape of the pores from spherical into disc-like void

1-Butanol adsorption in poly(styrene-divinylbenzene) ion exchange resins for catalysis

The swelling behaviour of poly(styrene-co-divinylbenzene), P(S-DVB), ion exchange resins in 1-butanol (BuOH) has been studied by means of atomistic classical molecular dynamics simulations (MD). The topological characteristics reported for the resin in the dry state, which exhibited complex internal loops (macropores), were considered for the starting models used to examine the swelling induced by BuOH contents ranging from 10% to 50% w/w. Experimental measurements using a laser diffraction particle size analyzer indicate that swelling causes a volume variation with respect to the dry resin of 21%. According to MD simulations, such a volume increment corresponds to a BuOH absorption of 31-32% w/w, which is in excellent agreement with the indirect experimental estimation (i.e. 31% w/w). Simulations reveal that, independently of the content of BuOH, the density of the swelled resin is higher than that of the dry resin, evidencing that the alcohol provokes important structural changes in the polymeric matrix. Thus, BuOH molecules cause a collapse of the resin macropores when the content of alcohol is ≤20% w/w. In contrast, when the concentration of BuOH is close to the experimental value (∼30% w/w), P(S-DVB) chains remain separated by pores faciliting the access of the reactants to the reaction centers. On the other hand, evaluation of both bonding and non-bonding interactions indicates that the mixing energy is the most important contribution to the absorption of BuOH into the P(S-DVB) resin. Overall, the results displayed in this work represent a starting point for the theoretical study of the catalytic conversion of BuOH into di-n-butyl ether in P(S-DVB) ion exchange resins using sophisticated electronic methods

Surface effects in nucleation and growth of smectic B crystals in thin samples

We present an experimental study of the surface effects (interactions with the container walls) during the nucleation and growth of smectic B crystals from the nematic in free growth and directional solidification of a mesogenic molecule ($C_4H_9-(C_6H_{10})_2CN$) called CCH4 in thin (of thickness in the 10 $\mu$m range) samples. We follow the dynamics of the system in real time with a polarizing microscope. The inner surfaces of the glass-plate samples are coated with polymeric films, either rubbed polyimid (PI) films or monooriented poly(tetrafluoroethylene) (PTFE) films deposited by friction at high temperature. The orientation of the nematic and the smectic B is planar. In PI-coated samples, the orientation effect of SmB crystals is mediated by the nematic, whereas, in PTFE-coated samples, it results from a homoepitaxy phenomenon occurring for two degenerate orientations. A recrystallization phenomenon partly destroys the initial distribution of crystal orientations. In directional solidification of polycrystals in PTFE-coated samples, a particular dynamics of faceted grain boundary grooves is at the origin of a dynamical mechanism of grain selection. Surface effects also are responsible for the nucleation of misoriented terraces on facets and the generation of lattice defects in the solid.Comment: 15 pages, 24 figures, submitted to PR

A molecular dynamics simulation study of solvation thermodynamical quantities of gases in polymeric solvents

In this paper, we discuss the sorption thermodynamics of gases in polymeric solvents with particular emphasis on the contributions of solute binding and solvent reorganization to the partial heat of gas sorption. Molecular interpretation of these contributions provides useful insights in molecular mechanisms of gas sorption and explanations of endo- and exo-thermic heat effects, which may both occur depending on the nature of the gas molecule and the (polymeric) solvent. Molecular dynamics simulations have been performed to calculate several solvation quantities for a series of gas penetrants in liquid dodecane and liquid (rubbery) poly(ethylene), as well as for glassy poly(vinyl chloride) and poly(vinyl alcohol). The differences in solvation behavior of the liquid solvents are discussed in detail and it is shown that, at the glass transition temperature of the glassy polymeric solvents, the Van’t Hoff representation of temperature dependent sorption data may pass through a minimum

Efficient sampling of solvent free energies in polymers

Solvent excess chemical potentials in amorphous polymers were calculated by means of molecular dynamics simulations using two different methods, the thermodynamic integration method and an extended ensemble method, during which the coupling of a test particle with the rest of the system is continuously changing. The latter method was found to probe the simulation box very efficiently whereas, in the thermodynamic integration, only limited regions of the box were sampled. Excess free energies of chloroform in swollen polydimethylsiloxane were found to decrease with increasing solvent swelling in qualitative agreement with experiment