Predicting reaction behavior of tethered polymers in shear flow.

Kinetics of force-mediated chemical reactions of end-tethered polymers with varying chain length N in varying shear rate flow γ̇ are explored via coarse-grained Brownian dynamics simulations. At fixed γ̇, force F along a polymer increases linearly with N as previously predicted; however, contrary to existing theory, the F(N) slope increases for N above a transition length that exhibits minimal dependence on γ̇. Force profiles are used in a stochastic model of a force-mediated reaction to compute the time for x percent of a polymer population to experience a reaction, tx. Observations are insensitive to the selected value of x in that tx data for varying N and γ̇ can be consistently collapsed onto a single curve via appropriate scaling, with one master curve for systems below the transition N (small N) and another for those above (large N). Different force scaling for small and large N results in orders of magnitude difference in force-mediated reaction kinetics as represented by the population response time. Data presented illustrate the possibility of designing mechano-reactive polymer populations with highly controlled response to flow across a range in γ̇.

Replica-averaging: An algorithm to study mechano-reactive processes for polymers under flow conditions.

A new method based on quasi-independent parallel simulation approach, replica-averaging, has been developed to study the influence of flow on mechanical force-mediated polymer processes, such as denaturation and breaking of bonds. This method considerably mitigates the unphysical prediction of force-mediated events inherent in Brownian dynamics (BD) polymer chain simulations that employ instantaneous force profile-based criteria to identify the occurrence of such events. This inaccuracy in predicting force-mediated event kinetics is due to high fluctuations of the instantaneous force profile around the average force. Replica-averaging reduces such high fluctuation effects by computing a force profile that faithfully represents the average force profile of the polymer chain conformation, which is then used to predict reactive events. For transient conformation conditions, the replica-averaged method more accurately predicts the mechano-reactive kinetics than the time-averaged method typically employed to reduce the unphysical prediction of force-mediated events in BD simulations. Furthermore, the influence of the proposed replica-averaging method parameters on the accuracy of predicting the true average force profile along the polymer is discussed.

Atomic-Layer-Deposited Aluminum Oxide Thin Films Probed with X-ray Scattering and Compared to Molecular Dynamics and Density Functional Theory Models.

A better understanding of amorphous aluminum oxide's structure and electronic properties is obtained through combined experimental and computational approaches. Grazing incidence X-ray scattering measurements were carried out on aluminum oxide thin films grown using thermal atomic layer deposition. The corresponding pair distribution functions (PDFs) showed structures similar to previously reported PDFs of solid-state amorphous alumina and molten alumina. Structural models based on crystalline alumina polymorphs (PDFgui) and amorphous alumina (molecular dynamics, MD) were examined for structural comparisons to the experimental PDF data. Smaller MD models were optimized and verified against larger models to allow for quantum chemical electronic structure calculations. The electronic structure of the amorphous alumina models yields additional insight into the band structure and electronic defects present in amorphous alumina that are not present in crystalline samples.