Reaction-diffusion model for the preparation of polymer gratings by patterned ultraviolet illumination

A model is developed to describe the migration mechanism of monomers during the lithographic preparation of polymer gratings by ultraviolet polymerization. The model is based on the Flory–Huggins theory: a thermodynamic theory that deals with monomer/polymer solutions. During the photoinduced polymerization process, monomer migration is assumed to be driven by a gradient in the chemical potential rather than the concentration. If the chemical potential is used as the driving force, monomer migration is not only driven by a difference in concentration, or volume fraction, but also by other entropic effects such as monomer size and the degree of crosslinking of the polymer network, which is related to the ability of a polymer to swell. Interaction of the monomers with each other or the polymer is an additional energetic term in the chemical potential. The theoretical background of the model is explained and results of simulations are compared with those of nuclear microprobe measurements. A nuclear microprobe is used to determine the spatial monomer distribution in the polymer gratings. It is shown that two-way diffusion is expected if the monomers are both difunctional and have the same size. In some cases, if one monomer is considerably smaller than the other, it can eventually have a higher concentration in the illuminated regions, even when it has a lower reactivity. The model is used to simulate the grating formation process. This results in a calculated distribution of the monomer volume fractions as a function of position in polymer gratings. An excellent agreement with the nuclear microprobe measurements is obtained. ©2004 American Institute of Physics

Foration of Mesoscopic Polymer structures for Optical Devices : a nuclear Microprobe study

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Mass transport phenomena during lithographic polymerization of nematic monomers monitored with interferometry

Photopolymn. of liq.-cryst. diacrylates is a versatile tool to make optical films for liq.-crystal display (LCD) enhancement. The const. drive towards LCD's having an improved front-of-screen performance demands optical films with properties that can be adjusted on (sub) pixel level. Birefringent films made from liq.-cryst. diacrylates allow for the required adjustment of the optical property on (sub) pixel level. In this paper we report on the compn. of the acrylate mixt. that results in planarly aligned nematic films usable as optical retarder in transflective LCD's as well as the mass transport phenomena that take place during heating of a mask-exposed birefringent film of liq.-cryst. diacrylates. The mass transport phenomena are studied by interferometry as a function of temp. and time. Upon heating a pronounced surface corrugation arises from the latent image formed during the mask exposure. The surface profile largely depends on lateral feature sizes. For 1 ´ 1-mm2 areas the exposed areas rise compared to the nonexposed areas, whereas the opposite is obsd. for 100 ´ 100-mm2 areas. Finally, the direction of the mass transport depends on the mol. orientation of the liq.-cryst. diacrylate. The protrusion formed by lengthwise diffusion is 1.7 times higher than that formed by sidewise diffusion

Microbiome assembly in thawing permafrost and its feedbacks to climate

The physical and chemical changes that accompany permafrost thaw directly influence the microbial communities that mediate the decomposition of formerly frozen organic matter, leading to uncertainty in permafrost–climate feedbacks. Although changes to microbial metabolism and community structure are documented following thaw, the generality of post‐thaw assembly patterns across permafrost soils of the world remains uncertain, limiting our ability to predict biogeochemistry and microbial community responses to climate change. Based on our review of the Arctic microbiome, permafrost microbiology, and community ecology, we propose that Assembly Theory provides a framework to better understand thaw‐mediated microbiome changes and the implications for community function and climate feedbacks. This framework posits that the prevalence of deterministic or stochastic processes indicates whether the community is well‐suited to thrive in changing environmental conditions. We predict that on a short timescale and following high‐disturbance thaw (e.g., thermokarst), stochasticity dominates post‐thaw microbiome assembly, suggesting that functional predictions will be aided by detailed information about the microbiome. At a longer timescale and lower‐intensity disturbance (e.g., active layer deepening), deterministic processes likely dominate, making environmental parameters sufficient for predicting function. We propose that the contribution of stochastic and deterministic processes to post‐thaw microbiome assembly depends on the characteristics of the thaw disturbance, as well as characteristics of the microbial community, such as the ecological and phylogenetic breadth of functional guilds, their functional redundancy, and biotic interactions. These propagate across space and time, potentially providing a means for predicting the microbial forcing of greenhouse gas feedbacks to global climate change