SLO: Elastomeri iz tekočih kristalov (LCE) --- polimerne mreže z vgrajenimi enotami iz tekočih kristalov --- so funkcionalni materiali, za katere je značilno izrazita sklopitev med elastično napetostjo in tekoče kristalno orientacijsko ureditvijo. Ko so pripravljeni s polimerizacijo in zamreženjem v izotropni fazi in nato ohlajeni, nastali polodominski materiali v "izotropni genezi" kažejo izjemno mehko elastično obnašanje pri enosmernem vlečenju, s platojem podobno območje nizkega elastičnega modela v krivulji napetost-deformacija, preden se spremeni v monodomenski LCE, kjer se povrne standardni elastični upor [1,2].
Ker se zdi, da je mikroskopski mehanizem tega vedenja zelo zanimiv in ni popolnoma razumljen, v tem delu [3] raziskujemo njegov izvor z izvedbo obsežnih molekularnih izo-stresnih Monte Carlo simulacij otekle polidomene LCE glavne verige. Naše simulacije temeljijo na potencialu interakcije Gay-Berne z mehkim jedrom [4] in reproducirajo eksperiment napetosti-obremenitve izkazujoč platoju podobno vedenje. Globlji vpogled v molekularno organizacijo simuliranih vzorcev razkriva, da sta osnovna mehanizma mehke elastičnosti rotacija in rast lokalne domene, izključujoč orientacijski red dekonstrukciji-rekonstrukcije. To prav tako predlaga, da bi tem mehanizmom lahko pomagalo odvajanje proste prožne energije, shranjene v topoloških napakah, nastalih med postopkom izotropne geneze, kar je združljivo nereverzibilnostjo obremenjevanja in napetosti, opaženo v nekaterih LCE glavne verige.
Skupaj s fiziko obravnavanega sistema LCE bodo obravnavani tudi različni računski in metodološki vidiki študije, kot je paralelizacija kode in vizualizacija molekularnih posnetkov ter metode napovedovanja izbranih eksperimentalnih opazovanj, kot so spektri magnetne resonance devterija in razpršeni rentgenski vzorci.
[1] K. Urayama, E. Kohmon, M. Kojima in T. Takigawa, Macromolecules 42, 4084 (2009).
[2] J. S. Biggins, M. Warner in K. Bhattacharya, Phys. Rev. Lett. 103, 037802 (2009).
[3] G. Skacej in C. Zannoni, Macromolecules 47, 8824 (2014).
[4] R. Berardi, C. Zannoni, J. S. Lintuvuori in M. R. Wilson, J. Chem. Fiz. 131, 174107 (2009).
ENG: Liquid crystal elastomers (LCE) --- polymer networks with embedded liquid crystal units --- are functional materials characterized by a pronounced coupling between elastic strain and liquid crystalline orientational ordering. When prepared by polymerization and cross-linking in the isotropic phase, and then cooled, the resulting ``isotropic-genesis'' polydomain materials exhibit an extraordinary soft elastic behavior under unidirectional pulling, with a plateau-like low elastic modulus region in the stress-strain curve, before turning into a monodomain LCE where a standard elastic resistance is recovered [1,2].
As the microscopic mechanism of this behavior appears to be of great interest and is not fully understood, in this work [3] we investigate its origin by performing large-scale molecular iso-stress Monte Carlo simulations of swollen polydomain main-chain LCE. Our simulations are based on the soft-core Gay-Berne interaction potential [4] and reproduce the stress-strain experiment featuring the plateau-like behavior. Deeper insight into the molecular organization of our simulated samples reveals that the basic mechanisms behind soft elasticity are local domain rotation and growth, excluding orientational order destruction-reconstruction. It also
suggests that these mechanisms may be assisted by a dissipation of elastic free energy stored in topological defects created during the isotropic genesis process, which is compatible with the stress-strain irreversibility observed in some main-chain LCE.
Along with the physics of the LCE system considered, various computational and methodological aspects of the study will also be discussed, such as code parallelization and the visualization of molecular snapshots, as well as the methods of predicting selected experimental observables like deuterium magnetic resonance spectra and scattered X-ray patterns.
[1] K. Urayama, E. Kohmon, M. Kojima, and T. Takigawa, Macromolecules 42, 4084 (2009).
[2] J. S. Biggins, M. Warner, and K. Bhattacharya, Phys. Rev. Lett. 103, 037802 (2009).
[3] G. Skačej and C. Zannoni, Macromolecules 47, 8824 (2014).
[4] R. Berardi, C. Zannoni, J. S. Lintuvuori, and M. R. Wilson, J. Chem. Phys. 131, 174107 (2009).
Udeleženci / Target audience
SLO: Splošna javnost, ki jo zanima fizika tekočih kristalov in računski vidiki Monte-Carlo simulacij.
ENG: General public interested in physics of liquid cristals and computational aspects of Monte-Carlo simulations.
O avtorju / About the author
SLO: Gregor Skačej je docent na Fakulteti za matematiko in fiziko in se v glavnem ukvarja z molekularno dinamiko tekočih kristalov z uporabo Monte Carlo simulacij.
ENG: Gregor Skačej is an assistant professor at the Faculty of Mathematics and Physics and primarily deals with the molecular dynamics of liquid crystals using Monte Carlo simulations.
