A multiscale model for the rupture of linear polymers in strong flows

Abstract Polymer-containing solutions used across research and industry are commonly exposed to mechanically harsh fluid processes, for example shear and extensional forces during flow through porous media or rapid micro-dispensing of biopharmaceutical molecules. These forces are strong enough to break the covalent bonds in the polymer backbone. As this scission phenomenon can change the functional and fluid-flow properties as well as introduce reactive radicals into the solution, it must be understood and controlled. Experiments and models to-date have only provided partial or qualitative insights into this behaviour. Here we build a link between the molecular-scale degradation models and the macro-scale laminar flow of dilute solutions in any given geometry. A free-draining bead-rod model is used to investigate rupture events at the molecular scale. It is shown by uniaxial extension simulations of an ensemble of chains that scission can be conveniently described at the macroscopic scale as a first order reaction whose rate is a function of the conformation tensor of the macromolecules and the velocity gradient of the flow. This approach is implemented in the finite volume code OpenFOAM by elaborating an appropriate constitutive equation for the conformation tensor. The macroscopic model is run and analysed for ultra-dilute solutions of poly(methyl methacrylate) in ethyl acetate and polyethylene oxide in water, using the geometry of an abrupt contraction flow and neglecting any viscoelastic effect. This multi-scale approach bridges the gap between phenomenological observations of mechanically-induced chemical degradation in large scale applications and the rich field of molecular-scale models of macromolecules under flow.King Abdulaziz City for Science and Technology (KACST

Laminar flow-induced scission kinetics of polymers in dilute solutions

AbstractKing Abdulaziz City for Science and Technology (KACST) EPSRC (Grant No. EP/S009000/1

Dewetting of the residual layer of annealed nanoimprinted polystyrene films

The presence of a residual layer at the end of a classical NanoImprint Lithography (NIL) process is at the root of many issues when NIL is used to manufacture resist patterns used later as an etching mask. In this paper, we investigate how the spontaneous break-up of annealed polystyrene films, a phenomenon called dewetting, can produce residual-layer-free polymer patterns. Although the dewetting of flat polymer films has been characterized for decades, little interest has been focused on patterned films. We present exploratory experiments conducted on polystyrene 30 kg/mol ultra-thin films. The samples are embossed using thermal NIL with a specific spatially modulated space/line patterned mold, and then annealed at 50 °C above the glass transition temperature. The resulting patterns are then interpreted as a competition between surface tension and van der Waals forces. Finally, a quantitative analysis is made in the framework of lubrication theory, with the help of a finite volume simulation tool

Viscoelastic leveling of annealed thin polystyrene films

Theoretical and experimental work on nanoscale viscoelastic flows of polystyrene melts is presented. The reflow above the glass transition temperature (Tg) of a continuous patterned film is characterized. Attention is paid to the topographical consequences of the flow rather than to the temporal description of the leveling of the film. In the framework of capillary wave theory, it is shown that only the shortest spatial wavelengths of the topography exhibit an elastic behavior, while long waves follow a viscous decay. The threshold wavelength depends on the surface tension, on the elastic plateau modulus, and, for ultrathin films, on the film thickness. Besides, for polystyrene, this threshold is a nanoscale parameter and weakly depends on the temperature of annealing. Experiments are conducted on polystyrene 130 kg/mol submicrometer films. The samples are embossed using thermal nanoimprint technology and then annealed at different temperatures between Tg + 10 °C and Tg + 50 °C. The smoothed topographies of the films are measured by atomic force microscopy and compared to a single-mode Maxwell leveling model and a more elaborated model based on reptation theory. © 2014 American Chemical Society

AFM characterization of annealed nanoimprinted patterns applied to rheological properties measurement of thin polymer films with shear rate control

In several applications where flows at the nanoscale play an important role, such as nanoimprint lithography or nanopatterns reflow, in situ rheological properties measurements are required. We developed a fast and cost-effective method to measure the rheological properties of a thin polymer film from the reflow of purposely designed nanoimprinted patterns. The novelty of our approach is based on the accurate spatial determination of the film surface with use of atomic force microscopy (AFM), rather than on its temporal evolution. Here, a particular attention is given to the verification of the linear rheological response of the material. Indeed, if the shear rate in the flowing film reaches a critical value, non-linear behaviors such as shear thinning can be expected. Using the framework of capillary wave theory, we show that it is possible to analytically assess the shear rate at any point within the film, only knowing the position of the free interface (measured by AFM) and the zero-shear viscosity. We show that in our experimental setup, the shear rate never reaches critical values and we confirm the linear rheology hypothesis. © 2013 Elsevier B.V. All rights reserved

Computation of eddy currents in highly conductive particles dispersed in a moderately conductive matrix

International audienceIn this article, we report 3D numerical simulations of highly conductive non-magnetic particles dispersed in a moderately conductive matrix, subject to an AC magnetic field in a range of several hundred kHz. We address the issue of the scaling of current loops and heating power with respect to the volume fraction of the dispersed phase. Simulations are performed in two steps. First, a static electric potential gradient is imposed between two opposite faces of the simulation domain and an effective conductivity is computed in good agreement with percolation models. Second, the particles are constrained in a spherical sub-region and an AC magnetic field is imposed at the boundary of the domain. For small volume fractions, the induced Joule power is in good agreement with an analytical model of dilute dispersions. As the volume fraction increases, wider current loops form, until the percolation threshold is reached. Then the induced power in the spherical aggregate is well described by the power induced in an equivalent sphere with a volume-fraction-dependent conductivit

Viscosity measurements of thin polymer films from reflow of spatially modulated nanoimprinted patterns

We present a method to measure the viscosity of polymer thin films. The material is spin coated onto a silicon substrate and specially designed nanopatterns are imprinted on the film using thermal nanoimprint. A brief reflow is performed during which patterns flow under surface tension. Spectral densities of the topology before and after annealing are compared and the rheologic properties, such as viscosity, are extracted as fitting parameters of an evolution model. Contrary to previous similar approaches, emphasis was put on the spatial description rather than the temporal decay of the patterns. We used this method to measure the viscosity of polystyrene for two molecular weights at various temperatures and successfully recovered results of previous authors. © 2011 American Physical Society

Viscoelastic Leveling of Annealed Thin Polystyrene Films.

International audienceTheoretical and experimental work on nanoscale viscoelastic flows of polystyrene melts is presented. The reflow above the glass transition temperature (T-g) of a continuous patterned film is characterized. Attention is paid to the topographical consequences of the flow rather than to the temporal description of the leveling of the film. In the framework of capillary wave theory, it is shown that only the shortest spatial wavelengths of the topography exhibit an elastic behavior, while long waves follow a viscous decay. The threshold wavelength depends on the surface tension, on the elastic plateau modulus, and, for ultrathin films, on the film thickness. Besides, for polystyrene, this threshold is a nanoscale parameter and weakly depends on the temperature of annealing. Experiments are conducted on polystyrene 130 kg/mol submicrometer films. The samples are embossed using thermal nanoimprint technology and then annealed at different temperatures between T-g + 10 degrees C and T-g + 50 degrees C. The smoothed topographies of the films are measured by atomic force microscopy and compared to a single-mode Maxwell leveling model and a more elaborated model based on reptation theory

Viscoelastic properties measurements of thin polymer films from reflow of nanoimprinted patterns

The authors describe in this paper a fast and cost-effective method to measure the viscoelastic properties of a thin polymer film from the reflow of nanoimprinted patterns. The material is spin-coated onto a silicon substrate and specially designed nanopatterns are imprinted on the film using thermal nanoimprint. A first measurement of the imprinted profile is done by atomic force microscopy (AFM). The film is then heated at a definite temperature above the glass transition temperature during a definite time. The film is rapidly cooled down and the reflowed profile is again measured by AFM. Spectral densities of the profiles are computed using standard Fourier transform algorithms, and the viscoelastic properties are computed as fitting parameters of an evolution model for the spectral density of the topology. The originality of our method is based on the accurate spatial description of the imprint rather than on its temporal decay. Using our approach, we measured the viscoelastic properties of a 205 nm-thick polystyrene (molecular weight 130 kg/mol) film, assuming a single relaxation time Maxwell model. © 2012 American Vacuum Society

A multiscale model for the rupture of linear polymers in strong flows

Abstract Polymer-containing solutions used across research and industry are commonly exposed to mechanically harsh fluid processes, for example shear and extensional forces during flow through porous media or rapid micro-dispensing of biopharmaceutical molecules. These forces are strong enough to break the covalent bonds in the polymer backbone. As this scission phenomenon can change the functional and fluid-flow properties as well as introduce reactive radicals into the solution, it must be understood and controlled. Experiments and models to-date have only provided partial or qualitative insights into this behaviour. Here we build a link between the molecular-scale degradation models and the macro-scale laminar flow of dilute solutions in any given geometry. A free-draining bead-rod model is used to investigate rupture events at the molecular scale. It is shown by uniaxial extension simulations of an ensemble of chains that scission can be conveniently described at the macroscopic scale as a first order reaction whose rate is a function of the conformation tensor of the macromolecules and the velocity gradient of the flow. This approach is implemented in the finite volume code OpenFOAM by elaborating an appropriate constitutive equation for the conformation tensor. The macroscopic model is run and analysed for ultra-dilute solutions of poly(methyl methacrylate) in ethyl acetate and polyethylene oxide in water, using the geometry of an abrupt contraction flow and neglecting any viscoelastic effect. This multi-scale approach bridges the gap between phenomenological observations of mechanically-induced chemical degradation in large scale applications and the rich field of molecular-scale models of macromolecules under flow