Joonmyung Choi

Photo-isomerization effect of the azobenzene chain on the opto-mechanical behavior of nematic polymer: A molecular dynamics study

The opto-mechanical properties of a photo-responsive nematic polymer network (PRPN) are investigated using molecular dynamics simulation. For the implementation of the trans-to-cis isomerization of azo compounds, a switchable potential formalism for the N = N bond is applied to the crosslinked PRPN unit cell model. During the light switch-on and heating-up simulations at a wide range of temperatures, the scalar orientational order parameter for the mesogenic side group molecules, the effective photo-induced strain of the bulk polymer network, and the opto-mechanical properties are characterized. The correlation between the microstate which belongs to the molecular location and the macroscopically observed photostrain is identified according to the isomerization ratio of the diazene groups.

Nonlinear photomechanics of nematic networks: upscaling microscopic behaviour to macroscopic deformation

A liquid crystal network whose chromophores are functionalized by photochromic dye exhibits light-induced mechanical behaviour. As a result, the micro-scaled thermotropic traits of the network and the macroscopic phase behaviour are both influenced as light alternates the shape of the dyes. In this paper, we present an analysis of this photomechanical behaviour based on the proposed multiscale framework, which incorporates the molecular details of microstate evolution into a continuum-based understanding. The effects of trans-to-cis photoisomerization driven by actinic light irradiation are first examined using molecular dynamics simulations, and are compared against the predictions of the classical dilution model; this reveals certain characteristics of mesogenic interaction upon isomerization, followed by changes in the polymeric structure. We then upscale the thermotropic phase-related information with the aid of a nonlinear finite element analysis; macroscopic deflection with respect to the wide ranges of temperature and actinic light intensity are thereby examined, which reveals that the classical model underestimates the true deformation. This work therefore provides measures for analysing photomechanics in general by bridging the gap between the micro- and macro-scales.

Molecular Dynamics Study on the Photothermal Actuation of a Glassy Photoresponsive Polymer Reinforced with Gold Nanoparticles with Size Effect.

We investigated the optical and thermal actuation behavior of densely cross-linked photoresponsive polymer (PRP) and polymer nanocomposites containing gold nanoparticles (PRP/Au) using all-atom molecular dynamics (MD) simulations. The modeled molecular structures contain a large number of photoreactive mesogens with linear orientation. Flexible side chains are interconnected through covalent bonds under periodic boundary conditions. A switchable dihedral potential was applied on a diazene moiety to describe the photochemical trans-to-cis isomerization. To quantify the photoinduced molecular reorientation and its effect on the macroscopic actuation of the neat PRP and PRP/Au materials, we characterized the photostrain and other material properties including elastic stiffness and thermal stability according to the photoisomerization ratio of the reactive groups. We particularly examined the effect of nanoparticle size on the photothermal actuation by varying the diameter of the nanofiller (10-20 Å) under the same volume fraction of 1.62%. The results indicated that the insertion of the gold nanoparticles enlarges the photostrain of the material while enhancing its mechanical stiffness and thermal stability. When the diameter of the nanoparticle reaches a size similar to or smaller than the length of the mesogen, the interfacial energy between the nanofiller and the surrounding polymer matrix does not significantly affect the alignment of the mesogens, but rather the adsorption energy at the interface generates a stable interphase layer. Hence, these improvements were more effective as the size of the gold nanoparticle decreased. The present findings suggest a wider analysis of the nanofiller-reinforced PRP composites and could be a guide for the mechanical design of the PRP actuator system.

Liquid crystalline elastomers with gold nanoparticle cross-linkers

Embedding nanoparticles in a responsive polymer matrix is a formidable way to fabricate hybrid materials with predesigned properties and prospective applications in actuators, mechanically tunable optical elements, and electroclinic films. However, achieving chemical compatibility between nanoparticles and organic matter is not trivial and often results in disordered structures. Herein, it is shown that using nanoparticles as exclusive cross-linkers in the preparation of liquid-crystalline polymers can yield long-range-ordered liquid-crystalline elastomers with high loadings of well-dispersed nanoparticles, as confirmed by small-angle XRD measurements. Moreover, the strategy of incorporating NPs as cross-linking units does not result in disruption of mechanical properties of the polymer, and this phenomenon was explained by the means of all-atom molecular dynamics simulations. Such materials can exhibit switchable behavior under thermal stimulus with stability spanning over multiple heating/cooling cycles. The presented strategy has proven to be a promising approach for the preparation of new types of hybrid liquid-crystalline elastomers that can be of value for future photonic applications.

Theoretical modeling of PEB procedure on EUV resist using FDM formulation

Semiconductor manufacturing industry has reduced the size of wafer for enhanced productivity and performance, and Extreme Ultraviolet (EUV) light source is considered as a promising solution for downsizing. A series of EUV lithography procedures contain complex photo-chemical reaction on photoresist, and it causes technical difficulties on constructing theoretical framework which facilitates rigorous investigation of underlying mechanism. Thus, we formulated finite difference method (FDM) model of post exposure bake (PEB) process on positive chemically amplified resist (CAR), and it involved acid diffusion coupled-deprotection reaction. The model is based on Fick’s second law and first-order chemical reaction rate law for diffusion and deprotection, respectively. Two kinetic parameters, diffusion coefficient of acid and rate constant of deprotection, which were obtained by experiment and atomic scale simulation were applied to the model. As a result, we obtained time evolutional protecting ratio of each functional group in resist monomer which can be used to predict resulting polymer morphology after overall chemical reactions. This achievement will be the cornerstone of multiscale modeling which provides fundamental understanding on important factors for EUV performance and rational design of the next-generation photoresist.

Self-folding Structural Design Using Multiscale Analysis on the Light-absorption Folding Behaviour of Polystyrene Sheet

Concentrated light-absorption on specific areas of polystyrene (PS) sheet induces self-folding behaviour. Such localized light-absorption control is easily realized by black-coloured line pattern printing. As the temperature in the line patterns of PS sheet increases differently due to the transparencies in each line pattern, localized thermal contraction generates folding deformation of the PS sheet. The light-activated folding technique is caused by the shape memory effect of PS sheet. The shape memory creation procedure (SMCP) is described by using molecular dynamic (MD) simulation, and the constitutive model of PS sheet is identified. This study employs the shell/cohesive line element for the folding deformation of PS sheet, and utilizes the constitutive model obtained from the MD simulation. Based on the continuum-model analysis of the PS sheet folding deformation activated by light, various self-folding structures are designed and manufactured.

Computational approach on PEB process in EUV resist: multi-scale simulation

For decades, downsizing has been a key issue for high performance and low cost of semiconductor, and extreme ultraviolet lithography is one of the promising candidates to achieve the goal. As a predominant process in extreme ultraviolet lithography on determining resolution and sensitivity, post exposure bake has been mainly studied by experimental groups, but development of its photoresist is at the breaking point because of the lack of unveiled mechanism during the process. Herein, we provide theoretical approach to investigate underlying mechanism on the post exposure bake process in chemically amplified resist, and it covers three important reactions during the process: acid generation by photo-acid generator dissociation, acid diffusion, and deprotection. Density functional theory calculation (quantum mechanical simulation) was conducted to quantitatively predict activation energy and probability of the chemical reactions, and they were applied to molecular dynamics simulation for constructing reliable computational model. Then, overall chemical reactions were simulated in the molecular dynamics unit cell, and final configuration of the photoresist was used to predict the line edge roughness. The presented multiscale model unifies the phenomena of both quantum and atomic scales during the post exposure bake process, and it will be helpful to understand critical factors affecting the performance of the resulting photoresist and design the next-generation material.

An interphase design strategy for multifunctional polymer nanocomposites using multiscale method

In this study, a multiscale model which integrates molecular dynamics (MD) simulation and finite element (FE) analysis has been developed to design multifunctional polymer nanocomposites and their effective interphase. Both the global stiffness of the polymer nanocomposite model and the internal stress distribution on the nanofiller surface during mechanical loadings were quantitatively characterized. Through MD simulations, crosslinked epoxy resin (crosslinking ratio: 0.45) and nano-sized filler (spherical SiC and zigzag single walled carbon nanotube) embedded epoxy nanocomposite models were prepared with full atomistic detail. For each model, uniaxial tensile tests were carried out to obtain the elastic behavior of the nanocomposites and the strain energy distribution in the vicinity of a nanofiller surface. Meanwhile, a three-dimensional FE model of a three-phase was prepared, consisting of a nanofiller, polymer networks adsorbed on the nanofiller surface (interphase), and polymer networks non-adsorbed on the nanofiller surface (bulk matrix). The unknown mechanical response and thickness of the interphase were numerically characterized through homogenization and deformation energy matching to that of the full atomic molecular model, respectively. The present multiscale method, therefore, yields an effective region of the interphase as well as its mechanical properties. The suggested multiscale model accurately predicts virial local stresses at both the interphase and bulk matrix regions of the full-atomic model and explains the reinforcing mechanism at the interphase region.

Influence of crosslink ratio on the thermomechanical properties of polymer nanocomposites and interphase: a molecular dynamics simulation

The effect of different crosslink ratios on the thermal and mechanical properties of thermoset epoxy-based nanocomposites are investigated by molecular dynamics (MD) simulations and a sequential scale bridging method. For establishing molecular models crosslinked epoxy structures composed of epoxy resin EPON 862 and curing agent TETA with a wide range of crosslink ratios are considered. Silica (SiO2) nanoparticles having different radii are introduced as a filler material in order to analyze the characteristics of interphase regions regarding the particle size effect. The elastic modulus and thermal expansion coefficient of various unit cells with different crosslink ratios and particle sizes are investigated using MD simulations. The results illustrate that the degree of bulk property changes of nanocomposites with respect to crosslink ratios is lower than the cases of pure epoxy structures. For the quantification of the properties of interphase regions, the interaction energy densities are investigated and a sequential scale bridging method is applied. The interaction between the particle and matrix is weakened as the crosslink ratio increases. Moreover, the elastic modulus of interphase is reduced and the size effects on thermal expansion of interphase are hindered with the presence of more crosslinked networks. The results indicate that the interphase behaviors are significantly diminished as crosslink ratio increases.

Exceptional interfacial properties of structurally defected CNT/polymer nanocomposites: a molecular dynamics study

The interfacial properties of structurally defective single-walled carbon nanotube (CNT) embedded polymeric nanocomposites are quantitatively characterized through molecular dynamic simulations. Three different kinds of point defects including single vacancy, single adatom, and Stone-Wales are generated to pristine CNT respectively and nanotube pulling out simulation in CNT/PP nanocomposites unit cells are carried out using molecular mechanics calculation. The CNT-polymer interaction energy, CNT pulling out energy, interfacial bonding energy, and interfacial shear strength of modeled nanocomposites are derived and compared with each other according to the types and number of defect. Vacancy defect decreases the interfacial shear strength linearly as the number of defect increases whereas adatom and Stone-Wales defects increase the interfacial shear strength due to the strong adsorption characteristics of the heptagonal carbon ring in these two defect types with the polymer chains close to the nanotube. Those exceptional dependencies on the types of structural defect also can be observed from the other interfacial properties and local deformation of the matrix phase induced by the pulled CNT.