Polymer Absorption Coefficient Research Articles

Optical radiative properties of ablating polymers exposed to high-power arc plasmas

The radiative properties of polymers exposed to high-intensity radiation are of importance for the numerical simulation of arc-induced ablation. The paper investigates the optical properties of polymethylmethacrylate PMMA and polyamide PA6 films exposed to high-power arc plasmas, which can cause ablation of the material. A four-flux radiative approximation is first used to estimate absorption and scattering coefficients of the tested materials in the ultraviolet (UV) and in the visible (VIS) ranges from spectrophotometric measurements. The temperature-induced variation of the collimated transmissivity of the polymers is also measured from room temperature to the glass temperature of PMMA and the melting temperature of PA6. Furthermore, band-averaged absorption and scattering coefficients of non-ablating and ablating polymers are estimated from the UV to the short-wavelength infrared (SWIR), covering the range of interest for the simulation of arc-induced ablation. These estimates are obtained from collimated transmissivities measured with an additional in situ photometric system that uses a high-power, transient arc plasma to both illuminate the samples and to induce ablation. It is shown that the increase in the bulk temperature of PA6 leads to a strong reversible increase in collimated transmissivity, significantly reducing the absorption and scattering coefficients of the material. A weaker but opposite effect of temperature on the optical properties is found in PMMA. As a consequence, it is suggested that the absorption coefficient of polymers used for arc-induced ablation estimates should not be taken directly from direct collimated transmissivity measurements at room temperature. The band-averaged radiation measurements also show that the layer of products released by ablation of PMMA produces scattering radiation losses mainly in the VIS–SWIR ranges, which are only a small fraction of the total incident arc radiation. In a similar manner, the ablation layer of PA6 leads to weak absorption radiation losses, although mainly in the UV range.

Multi-scale modelling of heat transfer in polyurethane foams

The influence of morphology and cell gas composition on heat insulation properties of polyurethane (PU) foams was investigated using a multi-scale mathematical model. The polymer absorption coefficient was determined from quantum chemical computations. Reverse non-equilibrium molecular dynamics was used to calculate the thermal conductivity of polymer and gas mixtures relevant to PU foams. The equivalent foam conductivity was calculated using homogeneous phase approach. The individual models were coupled together using suitable surrogate models within MoDeNa framework. To validate the proposed model 9 foam samples were prepared using different recipes, their morphology was characterized and their thermal conductivity was measured. The difference between experimental and predicted values was comparable to experimental error. Developed multi-scale model was used to identify the most suitable relation for the calculation of thermal conductivity of gas mixtures in PU foams and to quantify the influence of foam density, cell size, and strut content on heat insulation properties of PU foams.

Model of Laser-Induced Nano-Cavitation in the Surface of Gelatin Confronted with the Fast Rise Time of the Optical Attenuation Measurements

In the laser-polymer interaction a particular mechanism is evidenced when the polymer absorption coefficient is small. In this case the laser ablation is most of the time, but not necessarily, accompanied by intense bubbling, which results from the interplay between ablation gas, polymer melt and pressure wave. In this work, the inception of this phenomenon called cavitation, is monitored for the gelatin case (as a model polymer) by the fast extinction of a probe cwHeNe laser for several fluences of the KrF laser pulses (1.13, 0.65, 0.470 J/cm²) used to trigger ablation whose threshold is 0.5 J/cm². A new model combining the time dependence of temperature, pressure and viscosity is developed for the simulation of this extinction due to the cavitation phenomenon and used to fit the experimental data. The model concludes that the cavity precursors with initial radius R0 = 3 nm and concentration n0 = 5×1013 cm-3 are suddenly expanded by the tension wave launched immediately after the absorption of the laser pulse. The probe laser radiation is attenuated by Rayleigh or Mie scattering with a rise time of the order of 110 ns, depending on the ablation fluence, and corresponding to the transit of the tension wave. The dynamics of the radius increase is given by the Rayleigh-Plesset equation and the thermodynamic kinetics of the growth is given by the classical theory of the homogeneous nucleation. The model shows that complete darkening of the sample surface is achieved only for a fluence larger than 1.1 J/cm² and that the increase of the radius is limited to the range 3–100 nm, in this case. The approximations are discussed in the paper. In the experiments, the increase of the opacity of the sample surface occurs also at lower fluence (0.65 and 0.470 J/cm²) although with a slower dynamics. This reveals that another mechanism may be invoked to account for the slower opacity rise observed at lower fluences in the experimental data. The model excludes melting in these cases and indicates rather a solid phase transition due to the gas evolution.

Absorption and reflection of infrared radiation by polymers in fire‐like environments

SUMMARYIn large‐scale fires, the input of energy to burning materials occurs predominantly by radiative transfer. The in‐depth (rather than just surface) absorption of radiant energy by a polymer influences its ignition time and burning rate. The present investigation examines two methods for obtaining the absorption coefficient of polymers for infrared radiation from high‐temperature sources: a broadband method and a spectral method. Data on the total average broadband transmittance for 11 thermoplastics are presented (as are reflectance data), and the absorption coefficient is found to vary with thickness. Implications for modeling of mass loss experiments are discussed. Copyright © 2011 John Wiley & Sons, Ltd.

High-sensitivity EUV Resists based on Tetrafluoroethylene contained Fluoropolymers

There is a growing interest in the fluorinization of resist materials in improving pattern formation efficiency for extreme ultraviolet (EUV) lithography. The increased polymer absorption coefficient obtained through this resist platform is expected to enhance acid production and in effect improve pattern formation efficiency. Our work over the past several years has shown that the main-chain fluorinated base resins realized by the co-polymerization of tetrafluoroethylene (TFE) and norbornene derivatives offer high dissolution rates. Based on this, a EUV resist which was prepared using the fluorinated polymers was investigated. Imaging evaluations, using the small field exposure tool (SFET by Canon / EUVA) with annular (σ outer 0.7 / σ inner 0.3) illumination conditions were performed. Relatively high sensitivity of 6.3mJ⋅cm-2 for half-pitch (hp) 45nm and satisfactory resolution limit of hp 40nm was achieved. At present, line width roughness (LWR) was measured at comparatively large values of more than 8.4nm at hp 45nm. This shows that further material and process optimizations may be necessary to improve its present lithographic capability. However, these initial results have shown the potential of fluorinated-polymer based platform as a possible solution for high sensitivity, high resolution and low LWR EUV resists. In this paper, we report recent results of high sensitivity of 5.1mJ?cm-2 for half-pitch (hp) 40nm, optimization of protecting groups and photo acid generators.

Fluorinated-Polymer Based High Sensitivity Extreme Ultraviolet Resists

There is a growing interest in the fluorinization of resist materials in improving pattern formation efficiency for extreme ultraviolet (EUV) lithography. The increased polymer absorption coefficient obtained through this resist platform is expected to enhance acid production and in effect improve pattern formation efficiency. Based on this, a EUV resist which was synthesized by co-polymerizing tetrafluoroethelyne (TFE) and functional norbornene derivative was investigated. Relatively high sensitivity of 6.3 mJ·cm-2 for half-pitch (hp) 45 nm and satisfactory resolution limit of hp 40 nm was achieved. However, at present, line width roughness (LWR) was measured at comparatively large values of more than 8.4 nm at hp 45 nm. Further material and process optimizations may be necessary to improve its present lithographic capability. However, these initial results have shown the potential of fluorinated-polymer based platform as a possible solution for high sensitivity, high resolution and low LWR EUV resists.

Transient Modeling of Thermal Degradation in Non-Charring Solids

For a more extensive investigation of polymers, a better understanding of its gasification process is extremely essential. Especially compared with the extensive studies in the gas phase, the solid phase received rather little attention. Thus, the purpose of this paper is in its comparison of several modeling approaches for thermal degradation and systematic demonstration of the effects of basic assumptions on the model results, while taking account of in-depth radiation. For this object, this study has examined in more detail the degradation for a horizontally positioned polymer which is exposed to external radiation. After a preliminary study, three different solid degradation models were chosen from the literature, and their corresponding mass and energy conservation equations and boundary conditions were assembled. The in-depth non-gray as well as gray radiation was taken into account by solving relevant radiative transfer equation. In order to concentrate on the fundamental mechanisms of polymer degradation, gas phase reactions and subsequent heat feedback from the gas were neglected. This corresponds to a nitrogen environment. Various other parameters such as the polymer refractive index, polymer absorption coefficient, convective heat loss, solid fuel thickness and external radiative heat flux were changed to discuss their effects on the quantitative as well as qualitative change in the mass loss rate. While the effect of the solid fuel thickness on the mass loss rate was negligible for a thick sample, each parameter among the polymer refractive index, polymer absorption coefficient, convective heat loss, and external radiative heat flux incurred a non-negligible change in the results. Furthermore, the convection term in the energy equation, which is usually neglected in the in-depth pyrolysis by many other works, was shown to account for 29% decrease in the mass loss rate. Finally, depending on the solid degradation model used, the addition of gray radiation to the energy equation was shown to augment or diminish the mass loss rate. It was further enhanced by taking account of the non-gray radiation. This results from the fact that the inner solid is more quickly heated due to the far-reaching effects of radiation from the surface.

Laser ablation of various polymer materials.

The laser ablative decomposition technique of various kinds of polymer materials is attracting attentions as one of the microfabrication techniques causing no thermal damages. We discuss here the etching process of polymer films such as polyimide, polystyrene, polymethylmethacrylate, polytetrafluoroethylene and etrafluoroethylene-hexafluoropropylene copolymer, by the ablative decomposition using excimer laser and YAG laser. The etched depth per laser energy density and the morphology of etched surface depend on the polymer's absorption coefficient at the given laser wavelength. Each polymer has a specific threshold energy density for decomposition. The energy provided in extremely thin polymer surface by irradiation of threshold laser energy density correlates appreciably with the energy needed for raising to the volatile temperature on each polymer. It can therefore be presumed that the behavior of laser ablative decomposition depends on the optical and thermal properties of polymer.