Dense Polymer Films Research Articles

Boltorn-modified polyimide gas separation membranes

This paper describes the preparation, characterization and permeation properties of polyimide BTDA-AAPTMI (Matrimid 5218) and co-polyimide BTDA-TDI/MDI (P84) dense polymer films containing aliphatic hyperbranched polyesters, Boltorn (H40). The H40 are dispersed in the polymers at various concentrations. For Matrimid–H40 1.0 wt% membrane the nitrogen permeability increases but with significant loss in selectivity, while at higher H40 concentrations (5.0 and 10.0 wt%) the permeability becomes lower than of the pure polymer and the selectivity generally stays constant. The dispersion of various concentrations of H40 (1.0, 5.0 and 10.0 wt%) in P84 membranes decreases gas permeability in comparison to pure P84, while the selectivity generally stays constant.

Boltorn-Modified Poly(2,6-dimethyl-1,4-phenylene oxide) Gas Separation Membranes

This paper describes the preparation, characterization and the permeation properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) dense polymer films containing aliphatic hyperbranched polyesters, Boltorn (H20, H30, and H40). The Boltorn are dispersed in PPO at various concentrations. The gas permeability results were very different between low and high concentration of Boltorn. The gas permeability of PPO with 1.0 wt % of Boltorn is 2−3 times higher than the pure polymer, while at higher concentration (9.1 wt %) of Boltorn the permeability becomes almost 50% of the pure polymer. The gas pair selectivity, however, stays constant. The increase in permeability at low concentration of Boltorn is due to the increase of the free volume, probably due to hydrogen bonds between Boltorn and the oxygen of PPO backbone. The decreased permeability of PPO containing higher concentration of Boltorn (9.1 wt %) is due to two reasons: decrease in free volume as determined by PALS as well as phase separation. The hyperbranced polyesters form aggregates that migrate to the top surface of the membrane.

Fullerene-Modified Poly(2,6-dimethyl-1,4-phenylene oxide) Gas Separation Membranes: Why Binding Is Better than Dispersing

This paper describes the preparation, characterization, and the permeation properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) dense polymer films containing fullerenes (C60). The C60 are either dispersed or covalently bonded to PPO at various concentrations. The gas permeability results are very different between covalently bonded and dispersed PPO−C60. The gas permeability of PPO−C60 bonded increases up to 80% with increasing fullerene concentration. The gas pair selectivity, however, stays constant. This behavior is probably due to stiffening of the polymer structure and increase of free volume. The gas permeability through the PPO−C60 dispersed decreases in comparison to pure PPO. This reduction is due to C60 clustering in the polymer. The clusters seem to induce polymer crystallinity, and they are probably aligned along the film plane, creating an extra barrier for gas permeation.

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.

Computer Simulations of Polymers Close to Solid Interfaces: Some Selected Topics

This paper presents a topical overview of molecular-dynamics and Monte Carlo simulations for polymer systems close to solid interfaces. The simulations utilize simplified coarse-grained models: The polymers are represented by bead-spring chains, and the walls by a crystalline layer of Lennard-Jones particles or by a smooth impenetrable barrier. This approach has two advantages. First, it reduces the complexity of the simulation. Often, it is only then possible that the interesting length and time scales can be studied at all. Second, the approach concentrates on generic features that are believed to determine the physics of the problem under consideration. The results of the simulation can thus help to single out those features which should be incorporated in an analytical treatment. In this paper, we want to illustrate the versatility of these models by applying them to a broad spectrum of different problems. The situations considered range from the adsorption of a polymer from dilute solution onto a wall, over the importance of sub-monolayer monomeric or polymeric lubricants for kinetic friction, to the crystallization or glass transition of dense polymer films.

Synthesis and characterization of porous polymeric low dielectric constant films

This paper reports the synthesis and dielectric properties of a porous poly(arylether) material with an ultra-low dielectric constant for interlayer dielectric applications in microelectronics. The porous polymer films were successfully fabricated by a method of organic phase separation and evaporation. A dielectric constant k of 1.8 was achieved for a porous film with an estimated porosity of 40% and average pore size of 3 nm. Electrical and mechanical properties as well as coefficient of thermal expansion for both dense and porous polymer films were measured.

Measurement of mechanical properties for dense and porous polymer films having a low dielectric constant

We measured the mechanical properties of dense and porous polymeric films, the modified polyarylethers, which have a low dielectric constant varying from 2.7 to 1.8, by combining three different methods; membrane bulge test, nanoindentation, and single-substrate bending beam method. The elastic modulus and initial stress measured from these three methods are in good agreement. The substrate effect was observed in the measurements by nanoindentation. Data obtained by nanoindentation show a significant dependence on the film thickness and the displacement depth of the indenter. However, the hardness of the low dielectric constant thin film does not depend on thickness and only slightly depends on the indentation depth. A tentative analysis is proposed to explain the results.

Bicontinuous networks made of polyphosphates and of thermoplastic polymers

Composite films are obtained by casting aqueous dispersions obtained by admixture of PVAc or styrene-acrylic latexes with aluminium or iron polyphosphate powders. Film extraction with solution of aqueous EDTA (a solvent for polyphosphate) yields a dense polymer film, but using a polymer solvent (chloroform) a porous monolithic solid is obtained. Composite and solvent-extracted films were characterized by SEM coupled to energy dispersive X-ray analysis, and by Raman microspectrophotometry. The results from morphological, spectral and gravimetric experiments show that the cast films are bicontinuous, indicating a strong compatibility of the organic and inorganic phases. This is assigned to ionic bonds at the interfaces, in which aluminium ions act as bridging groups. © 1997 Elsevier Science Ltd.

Solid-state-concentration effects on the optical absorption and emission of poly(p-phenylene vinylene)-related materials.

We present measurements of the optical absorption and emission properties of poly(p-phenylene vinylene) (PPV) -related materials focusing on the differences between molecules isolated by dispersion in an inert host and concentrated molecular films. Optical absorption spectra, photoluminescence (PL) spectra, PL efficiency, and time-resolved PL spectra of dilute blends of PPV oligomers with 2--5 phenylene-phenyl rings are compared with those of dense oligomer and polymer films. In dilute oligomer-poly(methyl methacrylate) (PMMA) blends with high PL efficiency, the PL decay is exponential, independent of both temperature and oligomer length. This implies that the fundamental radiative lifetime of PPV oligomers is essentially independent of oligomer length. Concentrated spin-cast oligomer films and polymers have a faster and strongly temperature-dependent PL decay that approaches that of the dilute oligomer results at low temperature. The differences in PL decay correspond to changes in PL efficiency. The efficiency of the oligomer-PMMA blend is high and only weakly temperature dependent, whereas that of concentrated films is lower and strongly temperature dependent, decreasing by more than a factor of 3 from 10 to 350 K. The quenching of the PL efficiency in concentrated films is due to migration to extrinsic, impurity related centers as opposed to an intrinsic intermolecular recombination process. The PL spectrum of a dilute oligomer blend redshifts substantially, both as the excitation energy is decreased and as the emission time increases. This spectral redshift is due to disorder-induced site-to-site variation and not to diffusion to lower-energy sites. In contrast, no spectral shift with excitation energy or emission time was observed for dense oligomer films. \textcopyright{} 1996 The American Physical Society.

Studies of convective transport in evaporative casting of dense polymer films

Although several models have been developed to describe the evaporative casting of dense polymer films, none of them has included the convective transport terms which arise owing to the densification which occurs. In this paper we first describe a new finite element solution to the binary nonisothermal evaporative casting process which is used to confirm the predictions of the finite difference solution recently developed by Shojaie et al. (J. Mater. Process. Manu. Sci., 1 (1992) 181). This comparison then establishes that systematic deviations between experimental measurements and the model predictions of Shojaie et al. do not arise because of any inaccuracies in the numerical solution methodology. We then explore whether these deviations arise owing to omission of the convective transport terms by Shojaie et al. and all other modeling efforts. We employ a scaling analysis to demonstrate that convective transport effects can be significant in the evaporative casting process. The convective transport terms then are incorporated into the nonisothermal evaporative casting model of Shojaie et al. for the cellulose acetate/acetone system. The model predictions both with and without convective transport are compared to real-time gravimetric and surface-temperature data. This study indicates that ignoring convective transport can result in differences as large as 40% in the model predictions. The model predictions incorporating convective transport are in quantitative agreement with real-time data at short evaporation times, but progressively deviate at longer times. This deviation is thought to be due to the use of an equilibrium equation-of-state for the solution density as a function of concentration.

Dense polymer film and membrane formation via the dry-cast process part II. Model validation and morphological studies

In order to validate the dry-cast model developed by us, a multifaceted experimental approach was undertaken whereby three process variables can be followed independently in real time. Since it is extremely difficult to determine experimentally the concentration and temperature profiles within the cast polymer solution, experiments were designed so as to provide information on the coupled mass- and heat-transfer processes by measuring other process variables. The experimental data-acquisition technique combined gravimetric, inframetric, and light-reflection analyses which provided information on the overall mass change, surface temperature, and the onset and duration of phase separation, respectively. Structural studies were conducted using scanning electron microscopy. These studies revealed that macrovoids or “fingers” can be fomred in dry-cast membranes. A hypothesis for the formation of fingers based on the model predictions and experimental observations is proposed.

Dense polymer film and membrane formation via the dry-cast process part I. Model development

The dry-cast membrane-formation process is a major phase-inversion technique by which asymmetric membranes are manufactured. In this paper a fully predictive model which incorporates coupled heat and mass transfer is developed to describe the evaporation of both solvent and nonsolvent from an initially homogeneous polymer/solvent/nonsolvent system. This unsteady-state, one -dimensional, coupled heat- and mass-transport model allows from local film shrinkage owing to excess volume of mixing effects as well as evaporative solvent and nonsolvent loss. The model can predict composition paths into the ternary phase diagram which determine the onset of phase transition. The ternary phase diagram is predicted using the Flory-Huggins theory allowing for composition-dependent interaction parameters. The model is applied to the well-characterized cellulose acetate/acetone/water-system for which sufficient experimental data are available to permit determination of the friction coefficients in the ternary mass-transport model. The model is solved numerically using a software package based on the method of lines which is capable of handling moving boundary problems. The modeling studies indicate that for a given polymer/solvent/nonsolvent/support system, the most influential parameters are the gas-phase mass transport, initial cast film thickness, and initial composition. Of particular importance, the model can predict the general morphological characteristics associated with the formation of dense polymer films and symmetric as well as asymmetric membranes.

Permeation of carbon dioxide and water vapour in plasticized poly(vinylchloride)-starch blends: anomalous behaviour

A method is presented for studying anomalies in the diffusion of gases and vapours in dense polymer films in the transient regime. It was applied to the diffusion of CO 2 and water vapour in plasticized-poly (vinylchloride)(PVC)-potato-starch blends. The apparent diffusion coefficient of CO 2 in plasticized PVC remains constant in the transient regime, while that of water vapour first increases, passes through a maximum and then finally decreases to a constant value. The anomalous process observed for water vapour permeation in the transient regime can be explained by the interaction between the diisooctylphthalate (DIOP) plasticizer and the deformation of the polymer network due to stresses generated by the starch particles swelling. For water vapour in the stationary regime, the diffusion coefficient decreases and sorption coefficient increases when the starch content increases. There is a strong increase in both sorption and diffusion coefficients when pure PVC is replaced by DIOP-plasticized PVC.

Carbon-13 Nuclear Magnetic Resonance Study of the Separation of Styrene from Ethylbenzene by Dense Polymer Films

Carbon-13 Nuclear Magnetic Resonance Study of the Separation of Styrene from Ethylbenzene by Dense Polymer Films