Stability Of Thin Polymer Films Research Articles

Nonlinear activity and long-term stability of thin polymer films based on poly(3,5,7,3′,4′-pentahydroxyflavone-8-sulfonic acid) sodium salt

Cross-linked polymers with chromophore fragments built-in main chains can, depending on their structure and synthesis conditions, keep during a long time the orientation of chromophores that is necessary to create new organic polymer materials demonstrating nonlinear optical properties. The present work is devoted to cross-linked polymers on the basis of di-, tri-, and tetraglycidyl ethers of quercetin-8-sulfonic acid sodium salt (3,5,7,3′,4′-penthahydroxyflavon-8-sulfonic acid sodium salt) which were synthesized, and then, poled in electrical field of corona discharge. Study of thermal, optical, and nonlinear optical parameters of the polymer films was carried out. It was found that the polymers obtained from di- and triglycidyl ethers of quercetin-8-sulfonic acid sodium salt are characterized by the higher values of macroscopic quadratic susceptibilities comparing with such a value for polymer obtained from tetraglycidyl ether of quercetin-8-sulfonic acid sodium salt. However, the latter polymer demonstrates a minimal intensity of relaxation processes that evidence its highest long-term stability, and thus, could be used as a nonlinear optical material. It is supposed that the difference of the nonlinear optical properties is due to peculiarities of intermolecular interactions in physical network of the polymers, namely, to the difference between numbers of hydrogen bonds formed by hydroxyl groups of chromophore fragments and number of hydrogen bonds linked of inter fragmental parts of the polymeric chains.

Dewetting of a thin polymer film under shear

The objective of this work is to give new insight into the stability of thin polymer films under shear, in order to pave the way to a better control of the nanolayer coextrusion process. To do so, a finite-difference numerical scheme for the resolution of the thin film equation was set up taking into account capillary and van der Waals (vdW) forces. This method was validated by comparing the dynamics obtained from an initial harmonic perturbation to established theoretical predictions. With the addition of shear, three regimes have then been evidenced as a function of the shear rate. In the case of low shear rates the rupture is delayed when compared to the no-shear problem, while at higher shear rates it is even suppressed: the perturbed interface goes back to its unperturbed state over time. In between these two limiting regimes, a transient one in which shear and vdW forces balance each other, leading to a non-monotonic temporal evolution of the perturbed interface, has been identified. While a linear analysis is sufficient to describe the rupture time in the absence of shear, the nonlinearities appear to be essential otherwise.

Effect of Interfacial Adsorption on the Stability of Thin Polymer Films in a Solvent-induced Process

The stability of ultrathin polymer films plays a crucial role in their technological applications. Here, we systematically investigated the influence of interfacial adsorption in physical aging and the stability of thin polymer films in the solvent-induced process. We further identify the stability mechanism from the theory of thin film stability. Our results show that the aging temperature and film thickness can strongly influence the stability of thin PS films in acetone vapor. Physical aging can greatly improve the stability of thin polymer films when the aging temperature Taging1 > Tg. A thinner PS film more quickly reaches a stable state via physical aging. At short aging time, the formation of the adsorbed layer can reduce the polar interaction; however, it slightly influences the stability of thin polymer films in the solvent-induced process. At later aging stage, the conformational rearrangement of the polymer chains induced by the interfacial effect at the aging temperature Taging1 plays an important role in stabilizing the thin polymer films. However, at Taging2 < Tg, the process of physical aging slightly influences the stability of the thin polymer films. The formation of the adsorbed layer at Taging2 can reduce the short-range polar interaction of the thin film system and cannot suppress the instability of thin polymer films in the solvent-induced process. These results provide further insight into the stable mechanism of thin polymer films in the solvent-induced process.

Effect of ion irradiation on the thermal stability of thin polymer films

We report on the stability of poly(methyl methacrylate) thin films in vacuum after exposure to moderate doses of 300keV H+, 2MeV H+, and 18MeV Au7+ ions combined to thermal treatments. A small but steady increase in roughness of the films with increasing fluence was observed for bombardments at room temperature at a rate that varied strongly with dE/dx. For irradiations at 100°C, the roughness also increased sharply at very low fluences, but it was followed by smoothing and stabilization of the surfaces at larger doses. In situ post-irradiation annealing of samples after a fixed irradiation dose caused evolution on the surface topography that was markedly different for the H+ and Au7+ beams. For samples bombarded with H+ and annealed at 100°C, the radiation effect was to slow down the roughness increase, stabilizing its value at levels below those of the films not exposed to the beam. Irradiation with 18MeV Au ions, on the contrary, destabilized the films, causing strong changes in surface morphology and roughness. Such differences in behavior are attributed to the type of damage introduced by each beam and to the synergistic effects of radiation-induced bond breaking and heating.

Effect of Au Nanoparticle Spatial Distribution on the Stability of Thin Polymer Films

The stability of thin poly(methyl-methacrylate) (PMMA) films of low molecular weight on a solid substrate is controlled by the areal coverage of gold nanoparticles (NPs) present at the air-polymer interface. As the polymer becomes liquid the Au NPs are free to diffuse, coalesce, and aggregate while the polymer film can change its morphology through viscous flow. These processes lead at the same time to the formation of a fractal network of Au NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed, while for thicker films two wavelengths compete. With continued heating the aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies, while the polymer films flatten again. The disordering occurs first on the smallest scales and coincides (in thicker films) with the disappearance of the smaller wavelength. The subsequent disordering on larger scales causes the films to flatten.

The stability of thin polymer films as controlled by changes in uniformly sputtered gold

The stability of polystyrene thin films of low molecular weight on a solid substrate is shown to be controlled by the presence of uniformly distributed gold sputtered at the air–polymer interface. Continuous gold coverage causes the formation of wrinkles. High coverage and Au nanoparticle (NP) density leads to the development of a spinodal instability while low coverage and NP density retards the nucleation dewetting mechanism that beads up the thin polymer film into drops when no coverage is present. Heating at temperature larger than the polymer glass transition temperature for extended periods allows the gold NPs to coalesce and rearrange. The area of polymer surface covered by NPs decreases as a result and this drives the films from unstable to metastable states. When the gold NPs are interconnected by polymer chains a theoretically predicted spinodal instability that patterns the film surface is experimentally observed. Suppression of the instability and a return to a flat film occurs when the polymer interconnections between particles are broken. While the polymer films maintain their physical continuity changes in their chemical surface composition and thickness are observed. The observed film metastability is nevertheless in agreement with theoretical prediction that includes these surface changes.

Stability of thin polymer films: influence of solvents.

The interface and surface properties and the wetting behavior of polymer-solvent mixtures are investigated using Monte Carlo simulations and self-consistent field calculations. We carry out Monte Carlo simulations in the framework of a coarse-grained bead-spring model using short chains (oligomers) of N(P)=5 beads and a monomeric solvent, N(S)=1. The self-consistent field calculations are based on a simple phenomenological equation of state for compressible binary mixtures and we employ Gaussian chain model. The bulk behavior of the polymer-solvent mixture belongs to type III in the classification of van Konynenburg and Scott [Phil. Trans. R. Soc. London, Ser. A 298, 495 (1980)]. It is characterized by a triple line on which the polymer-liquid coexists with solvent-vapor and a solvent-rich liquid. The solvent is not homogeneously distributed across the dense polymer film but tends to accumulate at the surface and the polymer-vapor interface. This solvent enrichment at the interface and surface becomes more pronounced upon increasing the vapor pressure and alters the surface and interface tensions. This effect gives rise to a nonmonotonic dependence of the contact angle on the vapor pressure and one might observe reentrant wetting. The results of the Monte Carlo simulations and the self-consistent field calculations qualitatively agree. The profiles of drops are investigated by Monte Carlo simulations and a pronounced solvent enrichment is observed at the wedge formed by the substrate and the liquid-vapor interface at the three-phase contact line.

Effect of Mechanical Confinement on an Immiscible Polymer-Polymer Interface

The interfacial width formed between two immiscible polymers when they are subject to mechanical confinement has been studied using the neutron reflection technique. Deuterated polystyrene (dPS) and high-density polyethylene (HDPE) of high molecular weight were used in this study. The measurements were performed in situ at 175°C using a melt cell where the thickness of the dPS layer, coating the silicon substrate, was varied and was held onto a thick HDPE plate. The interfacial roughness extracted from the reflectivity curves was around 1.5 nm for thicker dPS films and decreased to around 1.1 nm for thinner film. In contrast to the prediction that long range van der Waals forces would destabilise the system, particularly for the thinner dPS films, it is stable within the time scale of the measurements. This could indicate a possible mechanical confinement effect on the stability of thin polymer films.

Stability of thin polymer films on a corrugated substrate

We study the wetting behaviour of thin polystyrene (PS) films on regularly corrugated silicon substrates. Below a critical film thickness the PS films are unstable and dewet the substrates. The dewetting process leads to the formation of nanoscopic PS channels filling the grooves of the corrugated substrates. Films thicker than the critical thickness appear stable and follow the underlying corrugation pattern. The critical thickness is found to scale with the radius of gyration of the unperturbed polymer chains.

Low dielectric constant Parylene-F-like films for intermetal dielectric applications

We report on the dielectric properties and thermal stability of thin polymer films that are suitable candidates for replacing silicon dioxide as the intermetal dielectric material in integrated circuits. Parylene-F-like films, (–CF2–C6H4–CF2–)n, were produced by plasma deposition from a mixture of Ar and 1,4-bis(trifluoromethyl)benzene (CF3–C6H4–CF3) discharges and characterized using infrared absorption spectroscopy, spectroscopic ellipsometry, and capacitance measurements. The dielectric constant and the magnitude of the electronic and ionic contributions to the dielectric constant were determined through capacitance measurements and Kramers–Kronig analysis of the infrared absorption data. The film’s dielectric constant ranges between 2 and 2.6 depending on the deposition conditions and the largest contribution to the dielectric constant is electronic. The films deposited at 300 °C are stable above 400 °C and further optimization could push this limit to as high as 500 °C.