Development of a transferable coarse-grained model of polydimethylsiloxane

Molecular Origin of Slippery Behavior in Tethered Liquid Layers

Slippery covalently attached liquid surfaces (SCALS) are a family of nanothin polymer layers with ultralow static droplet friction, characterized by a low contact angle hysteresis (CAH < 5°), which makes them ideally suited for self-cleaning, water harvesting, and antifouling applications. Recently, a Goldilocks zone of lowest CAH has been identified for polydimethylsiloxane (PDMS) SCALS of intermediate thickness (≈4 nm); yet, molecular-level insights are missing to reveal the underlying physical mechanism of this elusive, slippery optimum. In this work, the agreement between coarse-grained molecular dynamics simulations and atomic force microscopy data shows that nanoscale defects, as well as deformation for thicker layers, are key to explaining the existence of this "just right" regime. At low thickness values, insufficient substrate coverage gives rise to chemical patchiness; at large thickness values, two features appear: (1) a waviness due to a previously overlooked lateral microphase separation occurring in polydisperse brushes, and (2) layer deformation due to the contact line being larger than in thinner layers. The most pronounced slippery behavior occurs for smooth PDMS layers that do not exhibit nanoscale waviness. The converging insights from simulations, experiments, and a CAH theory provide design guidelines for tethered polymer layers with ultralow CAH.

Molecular Dynamics Simulation of Silicone Oil Polymerization from Combined QM/MM Modeling

We outline a molecular simulation protocol for elucidating the formation of silicone oil from trimethlyl- and dimethlysilanediole precursor mixtures. While the fundamental condensation reactions are effectively described by quantum mechanical calculations, this is combined with molecular mechanics models in order to assess the extended relaxation processes. Within a small series of different precursor mixtures used as starting points, we demonstrate the evolution of the curing degree and heat formation in the course of polymer chain growth. Despite the increasing complexity of the amorphous agglomerate of polymer chains, our approach shows an appealing performance for tackling both elastic and viscous relaxation. Indeed, the finally obtained polymer systems feature 99% curing and thus offer realistic insights into the growth mechanisms of coexisting/competing polymer strands.

Molecular Dynamics Study of Protein Aggregation at Moving Interfaces

Repeated compression and dilation of a protein film adsorbed to an interface lead to aggregation and entry of film fragments into the bulk. This is a major mechanism for protein aggregate formation in drug products upon mechanical stress, such as shaking or pumping. To gain a better understanding of these events, we developed a molecular dynamics (MD) setup, which would, in a later stage, allow for in silico formulation optimization. In contrast to previous approaches, the molecules of our model protein human growth hormone displayed realistic shapes, surfaces, and interactions with each other and the interface. This enabled quantitative assessment of protein cluster formation. Simulation outcomes aligned with experimental data on subvisible particles and turbidity, thereby validating the model. Computational and experimental results indicated that compression speed does not affect the aggregation behavior of preformed protein films but rather their regeneration. Protein clusters that formed during compression disassembled upon relaxation, suggesting that the particles originate from a partly compressed state. Desorption studies via steered MD revealed that proteins from compressed systems are more likely to detach as clusters, implying that compression effects at the interface translate into aggregates present in the bulk solution. With the possibility of studying the impact of different variables upon compression and dilation at the interface on a molecular level, our model contributes to the understanding of the mechanisms of protein aggregation at moving interfaces. It also enables further studies to change formulation parameters, interfaces, or proteins.

Mesoporous nanoperforators as membranolytic agents via nano- and molecular-scale multi-patterning

Plasma membrane lysis is an effective anticancer strategy, which mostly relying on soluble molecular membranolytic agents. However, nanomaterial-based membranolytic agents has been largely unexplored. Herein, we introduce a mesoporous membranolytic nanoperforators (MLNPs) via a nano- and molecular-scale multi-patterning strategy, featuring a spiky surface topography (nanoscale patterning) and molecular-level periodicity in the spikes with a benzene-bridged organosilica composition (molecular-scale patterning), which cooperatively endow an intrinsic membranolytic activity. Computational modelling reveals a nanospike-mediated multivalent perforation behaviour, i.e., multiple spikes induce nonlinearly enlarged membrane pores compared to a single spike, and that benzene groups aligned parallelly to a phospholipid molecule show considerably higher binding energy than other alignments, underpinning the importance of molecular ordering in phospholipid extraction for membranolysis. Finally, the antitumour activity of MLNPs is demonstrated in female Balb/c mouse models. This work demonstrates assembly of organosilica based bioactive nanostructures, enabling new understandings on nano-/molecular patterns co-governed nano-bio interaction.

Effect of Interfacial Modification on the Low-Temperature Fatigue Properties of Polymer/MXene Flexible Pressure Sensors

Maintaining an excellent force-electric response under cyclic bending at low temperatures is still challenging for resistive-type electrically conductive polymer composite-based pressure sensors. In this study, the effect of low temperature on the fatigue failure of flexible MXene/polymer pressure sensors was systematically investigated through the silane functionalization of MXene nanosheets embedded with different polymer matrixes. The results show that the MXene/polymer interfaces are the primary factors affecting the temperature-dependent bending fatigue of the Cu/MXene/polymer/Cu sensor. Using finite element analysis and theoretical calculations, we reveal that the MXene/polymer interfaces are affected by free volume changes and the molecular chain motion under different temperatures. At room temperature, the well-distributed free volume in the polydimethylsiloxane (PDMS) matrix permits local segmental mobility that promotes the affinity between the polymer and MXene. As the temperature decreases, the free volume in the matrix shrinks with less space left for molecular chains to slide relatively, weakening the polymer/MXene interfacial bonding strength. However, for PDMS/MXene sensors with the interface modified using the silane coupling agent KH550, the nanoconstrained structure formed by strong hydrogen bonds and covalent bonds at the PDMS/MXene interface can hinder the mobility of polymer chains, which greatly helps to dissipate the inter/intrachain friction. It thus alleviates the debonding energy dissipation during cyclic bending at subzero temperatures.

Energy renormalization for temperature transferable coarse-graining of silicone polymer

The bottom-up prediction of thermodynamic and mechanical behaviors of polymeric materials based on molecular dynamics (MD) simulation is of critical importance in polymer physics. Although the atomistically informed coarse-grained (CG) model can access greater spatiotemporal scales and retain essential chemical specificity, the temperature-transferable CG model is still a big challenge and hinders widespread application of this technique. Herein, we use a silicone polymer, i.e., polydimethylsiloxane (PDMS), having an incredibly low chain rigidity as a model system, combined with an energy-renormalization (ER) approach, to systematically develop a temperature-transferable CG model. Specifically, by introducing temperature-dependent ER factors to renormalize the effective distance and cohesive energy parameters, the developed CG model faithfully preserved the dynamics, mechanical and conformational behaviors compared with the target all-atomistic (AA) model from glassy to melt regimes, which was further validated by experimental data. With the developed CG model featuring tremendously improved computational efficiency, we systematically explored the influences of cohesive interaction strength and temperature on the dynamical heterogeneity and mechanical response of polymers, where we observed consistent trends with other linear polymers with varying chain rigidity and monomeric structures. This study serves as an extension of our proposed ER approach of developing temperature transferable CG models with diverse segmental structures, highlighting the critical role of cohesive interaction strength on CG modeling of polymer dynamics and thermomechanical behaviors.