Christopher J. Durning

A mechanistic picture of the effects of rubbing on polyimide surfaces and liquid crystal pretilt angles

A mechanistic picture of the effects of rubbing on polyimide alignment layer (AL) surfaces and nematic liquid crystal (LC) pretilt angles (ΘP), leading to establishing a mechanism of determination of ΘP’s, was proposed on the basis of the study of the change in surface polarity of a polyimide AL with the rubbing force and density, and the subsequent changes in the ΘP and the azimuthal anchoring energy. In the regime of relatively weak rubbing where inhomogeneous, patchy surface modification (the microscopic reorientation of polymer functional groups and the orientation of polymer backbones) occurs, the surface polarity, the anchoring energy, and the ΘP all increase monotonically with rubbing strength. These increases correlate to an increase in the area fraction of the reoriented/oriented AL surface which generates in-plane orientation of the first LC monolayer at the surface and subsequently bulk alignment. Beyond this region in which the AL surface gets modified sufficiently to induce fully developed in...

Penetrant transport in semicrystalline poly(ethylene terephthalate)

Abstract This paper examines the unsteady transport of liquid methanol, acetone, and dimethylformamide (DMF) in thin films of unoriented, semicrystalline poly(ethylene terephthalate) (PET). Three different semicrystalline fine structures were prepared; one by solvent induced crystallization (SINC) and two by thermal annealing procedures. Penetrant mass uptake and sample dimensional changes were followed during integral sorption experiments at temperatures in the range 25–75°C. Optical cross-sections of partially swollen films and penetrant desorption from equilibrated films were also studied. For films prepared by thermal annealing, the methanol transport is classical, while in SINC films, the process shows some non-classical (non-Fickian) characteristics. For both acetone and DMF, the transport is non-Fickian; the non-Fickian effects are consistent with current phenomenological models. The influence of semicrystalline fine structure on the transport is subtle for methanol, but very strong for acetone and DMF. The profound effect on the non-Fickian transport results from the significant influence of fine structure on the polymers mechanical properties. The appearance of non-Fickian behaviour is successfully predicted by an appropriate Deborah number correlation.

Effects of polymer molecular weight and temperature on case II transport

We investigated the effects of polymer molecular weight and temperature on Case II transport in the poly(methyl methacrylate)/methanol (PMMA/MeOH) system by a laser interferometric technique, using monodisperse polymer samples. Both the induction process and the steady-state front propagation were investigated. The data gave the volume fraction of MeOH in the swollen layer behind the moving front, ϕ, the steady state front speed, υ, and the characteristic induction time, tind. Values of ϕ separated into two groups, independent of molecular weight within each group. Significantly lower values of ϕ were found for polymers with molecular weight above the critical threshold for entanglement which can be explained by unrelaxed entanglements in the swollen layer. The Case II front velocity was independent of molecular weight for molecular weights at, or above, the critical weight for entanglement, suggesting that anelastic deformation processes other than simple viscous flow control the front propagation. Analysis of induction time data shows that the film surface properties differ from those of the bulk.

Nonlinear viscoelastic diffusion in concentrated polystyrene/ethylbenzene solutions

Nonlinear gradient-driven diffusion was studied in concentrated polystyrene (PS)/ethylbenzene (EB) solutions using vapor sorptions with a finite driving force. The nonlinear sorptions were carried out on thin films (≅2.05, 3.50 μ thick) at conditions where non-Fickian, “viscoelastic” effects appear. These data were modeled with the nonlinear diffusion equation studied by Tang. Four dimensionless material parameters in the model were determined from a limited amount of linear-response, differential sorption data on PS/EB mixtures measured in the same range of experimental conditions as for the nonlinear sorptions. The nonlinear model successfully predicts the observed nonlinear response either above or below the glass transition (Tg). In order to simultaneously capture the nonlinear response both above and below Tg, the abrupt change in the concentration dependence of physical properties at Tg must be accounted for.

Effect of molecular weight on two-stage sorption in concentrated polystyrene-ethylbenzene solutions

We examine the effect of polymer molecular weight on two-stage weight uptake kinetics in concentrated polymer solutions just below the glass transition. New results are reported for ethylbenzene sorption in two monodisperse polystyrenes (M = 100 000 and 350 000). The two-stage sorption data were fitted with a linear model coupling diffusion and viscoelasticity. A data-fitting scheme is described, efficient enough for routine analysis of sorption data with small bench-top computers. Two dimensionless parameters, α and θ, and a collective diffusion coefficient, D, are determined from each two-stage sorption curve. α, proportional to the ratio of the shear and osmotic moduli of the mixture, is nearly constant over the entire concentration range examined, and is insensitive to molecular weight. The diffusion Deborah number, θ, decreases monotonically with ethylbenzene content, while D increases with ethylbenzene content, but no systematic changes in θ or D were found with molecular weight. These results support the view that two-stage sorption results from a coupling of diffusion to high-frequency viscoelastic modes, which are insensitive to polymer molecular weight.

Surface diffusion and relaxation of partially adsorbed polymers

We studied by lattice simulation the surface diffusion and relaxation of isolated, self-avoiding polymers partially adsorbed onto a flat surface. The key parameters describing the system are the number of segments in the chain, N, the adsorption energy of a segment, expressed as a dimensionless surface temperature T s , and the segmental friction factor on the surface relative to that in the bulk, ζ s /ζ b . The simulation data indicate Rouse scaling of the surface diffusion coefficient, D∥, and in-plane relaxation time, T, versus N for all values of T s and ζ s /ζ b studied. A simple application of the Rouse model to a partially adsorbed chain, which ignores fluctuations in adsorbed trains, yields a formula for D | with the correct N-scaling. It can account for the effects of T s when ζ s /ζ b is finite (≤10), but it fails when ζ s /ζ b diverges, predicting no surface diffusion at all, whereas simulations indicate finite surface mobilities facilitated by a caterpillar-like motion.

Effect of the environment on the measured values of the adhesion of polyimide to silicon

We have studied the effects of liquid environments on the measured adhesion of PMDA-ODA polyimide to a silicon substrate by the blister and peel tests. The debonding pattern and adhesion strength were very sensitive to the liquid. For example, introducing pure methanol at the crack tip caused stick-slip debonding, with a high debonding angle, and severe plastic deformation of the polyimide film in blister tests, and greatly increased the adhesion strength in the peel test. The effects were progressively enhanced for water/methanol mixtures as the content of methanol increased, and for pure alcohols as the molecular size decreased. Evidently, as the wettability between the environment fluid and the surfaces involved improves, transport of the fluid to the crack tip region increases, allowing more diffusion of the fluid into the film, which causes a considerable decrease in its yield stress. Under these conditions, local plastic deformation and consequent blunting of the crack tip are facilitated, resulting...

Annealing and Microstructural Characterization of Tin-Oxide Based Thick Film Resistors

The sheet resistance of tin oxide based thick-film resistors exhibits two regions of temperature-dependence, described by hopping (23°C–200°C) and diffusion mechanisms (200°C–350°C), respectively. Annealing these samples causes the sheet resistance to increase in both regions. In the post-annealed samples, the hopping conduction range is extended by 50°C (23°C–250°C) while the hopping parameter, T0, is decreased by more than 50%. The activation energy of diffusion (0.60 eV) is the same for both pre- and post annealed samples, but the magnitude of resistance in the diffusion controlled region is increased significantly as a result of annealing. These changes are explained in terms of a net decrease in the concentration of tin ions in the glass matrix. From a careful microstructural study it was found that a conduction path composed of tin-oxide grains or their clusters in contact with each other does not exist in the present system. HREM micrographs showed the presence of nanocrystalline tin-oxide particles in the glass phase separating the tin-oxide grain clusters. Estimated average separation between the nanocrystals in 4 nm, consistent with a variable-range hopping conduction via the dissolved tin ions in the glass matrix.

Polymer adsorption at a planar interface

Abstract The Hookean dumbbell model interacting with a Lennard-Jones (LJ) interface is studied. The number density profiles for dumbbells near the interface show three features: (a) the dumbbells are excluded from a region immediately adjacent to the solid boundary; (b) the number density reaches a local maximum at the distance from the solid boundary where the LJ potential reaches its local minimum; (c) beyond the well, but less than a distance equal to the equilibrium extension of the dumbbell, the number density may fall below the bulk value. Within a certain region of the LJ parameter space, the surface adsorption, Γ, and the equivalent homogeneous film thickness, δ, are double valued, which may cause ambiguity in the interpretation of adsorption data. A large parameter limit for the flexible polymers accounting for multiple contacts between real chains and a surface is developed. In this limit the inverse functions for Γ(ϵ, σ) and δ(ϵ, σ) exist and are presented. The model also predicts that the adsorption of dumbbells at the interface will remain unchanged for simple shear flow and decrease by a constant factor for pressure-driven flow past the interface, relative to the adsorption at equilibrium. The observable adsorbed film thickness will remain unchanged in flow for any interface potential based on van der Waals forces in agreement with the results for adsorbed polymers in weak flows.

Adsorption of free-draining, Gaussian, bead-spring macromolecules at a plane interface

Abstract Equilibrium measurements of the ellipsometric adsorbance and film thickness are used to estimate the number of beads, within a free-draining, Gaussian, bead-spring model macromolecule, and at the attraction well depth of a wall potential, with which the macromolecules interact, to predict the adsorption behavior in a flow situation. The model predicts a film thickness unchanged from its equilibrium size in flow. The predicted adsorption of polymer in flow can be either more or less than that at equilibrium depending upon the values of the parameters used to fit the model to the equilibrium situation. The analysis demonstrates the need for modeling the interaction of polymers with a wall using multibead chains. The value of the density and its gradient at the wall, and the density boundary layer thickness, predicted by multibead chains, can differ substantially from those predicted by dumbbell models.