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ABSTRACT Biological 'catch' bonds display the distinctive attribute that the bond lifetime can increase under an applied force. Insertion of biomimetic catch bonds into hybrid materials could lead to composites that exhibit remarkable mechanical properties. We model the tensile behavior of polymer-grafted nanoparticle (PGN) networks interconnected by a mixture of catch bonds and conventional 'slip' bonds, whose lifetimes decrease with force. We formulate a kinetic master equation that provides the complete probabilistic description of a system of two PGNs. The master equation is used to analyze the parameter space that determines the rupture behavior of the catch bonds in the two-particle system. We then utilize two exemplary sets of catch bond parameters in three-dimensional computer simulations of larger PGN networks under strain-controlled tensile deformation. We demonstrate that the strain at break and toughness of the networks can be altered by 'tuning' the attributes of the catch bonds and varying the fraction of catch bonds in the network. We show that networks encompassing the catch bonds could exhibit a several-fold increase in the strain-at-break and toughness relative to those interconnected solely by the slip bonds. Our studies can provide valuable guidelines for tailoring the mechanical properties of novel, bio-inspired nanocomposites. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 105-118 [ABSTRACT FROM AUTHOR]

The ability to precisely tune the mechanical properties of polymeric composites is vital for harnessing these materials in a range of diverse applications. Polymer-grafted nanoparticles (PGNs) that are cross-linked into a network offer distinct opportunities for tailoring the strength and toughness of the material. Within these materials, the free ends of the grafted chains form bonds with the neighboring chains, and tailoring the nature of these bonds could provide a route to tailoring the macroscopic behavior of the composite. Using computational modeling, we simulate the behavior of three-dimensional PGN networks that encompass both high-strength "permanent" bonds and weaker, more reactive labile bonds. The labile connections are formed from slip bonds and biomimetic "catch" bonds. Unlike conventional slip bonds, the lifetime of the catch bonds can increase with an applied force, and hence, these bonds become stronger under deformation. With our 3D model, we examined the mechanical response of the composites to a tensile deformation, focusing on samples that encompass different numbers of permanent bonds, different bond energies between the labile bonds, and varying numbers of catch bonds. We found that at the higher energy of the labile bonds (Ul = 39kBT), the mechanical properties of the material could be tailored by varying both the number of permanent bonds and catch bonds. Notably, as much as a 2-fold increase in toughness could be achieved by increasing the number of permanent bonds or catch bonds in the sample (while the keeping other parameters fixed). In contrast, at the lower energy of the labile bonds considered here (Ul = 33kBT), the permanent bonds played the dominant role in regulating the mechanical behavior of the PGN network. The findings from the simulations provide valuable guidelines for optimizing the macroscopic behavior of the PGN networks and highlight the utility of introducing catch bonds to tune the mechanical properties of the system. [ABSTRACT FROM AUTHOR]

Nematic liquid-crystalline elastomers (LCEs) are weakly cross-linked polymeric networks that exhibit rubber elasticity and liquid-crystalline orientational order due to the presence of mesogenic groups. Three end-on side-chain nematic LCEs were investigated using real-time synchrotron wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and thermogravimetry (TG) to correlate the thermal behaviour with structural and chemical differences among them. The elastomers differed in cross-linking density and mesogen composition. Thermally reversible glass transition temperature,Tg, and nematic-to-isotropic transition temperature,Tni, were observed upon heating and cooling. By varying the heating rate,Tg0andTni0were determined at zero heating rate. The temperature dependence of the orientational order parameter was determined from the anisotropic azimuthal angular distribution of equatorial reflections seen during real-time WAXS. Results show that the choice of cross-linking unit, its shape, density, and structure of co-monomers, all influence the temperature range over which the thermal transitions take place. Including multi-ring aromatic groups as cross-linkers increased the effective stiffness of the cross-linking, resulting in a higher glass transition temperature. The nematic-to-isotropic transition temperature increased in the presence of multi-ring aromatic structures, as either cross-linkers or mesogens, particularly when the multi-ring structures were larger than the low-molar-mass mesogen common to all three samples. [ABSTRACT FROM PUBLISHER]

Two nematic shells brought in contact coalesce in order to reduce their combined interfacial tension, and, following this topological transition, relax to an equilibrium state. In this work, we study the defect textures as the combined shell shape evolves. By varying the sizes of the shells, we perform a quasistatic investigation of the director field and the defect valence on the doublet. Regimes are found where positive and negative defects exist due to the large negative Gaussian curvature at the neck. Using large-scale computer simulations, we determine how annihilating defect pairs on coalescing shells are selected and the stage of coalescence at which annihilation occurs. [ABSTRACT FROM AUTHOR]

We study, numerically and theoretically, defects in an anisotropic liquid that couple to the extrinsic geometry of a surface. Though the intrinsic geometry tends to confine topological defects to regions of large Gaussian curvature, extrinsic couplings tend to orient the order along the local direction of maximum or minimum bending. This additional frustration is generically unavoidable, and leads to complex ground-state thermodynamics. Using the catenoid as a prototype, we show, in contradistinction to the well-known effects of intrinsic geometry, that extrinsic curvature expels disclinations from the region of maximum curvature above a critical coupling threshold. On catenoids lacking an "inside-outside" symmetry, defects are expelled altogether above a critical neck size. [ABSTRACT FROM AUTHOR]

Various experimental and theoretical studies demonstrate that complex stimulus-responsive out-of-plane distortions such as twist of different chirality, emergence of cones, simple and anticlastic bending can be engineered and pre-programmed in a liquid crystalline rubbery material given a well-controlled director microstructure. Via 3-d finite element simulation studies, we demonstrate director-encoded chiral shape actuation in thin-film nematic polymer networks under external stimulus. Furthermore, we design two complex director fields with twisted nematic domains and nematic disclinations that encode a pattern of folds for an auto-origami box. This actuator will be flat at a reference nematic state and form four well-controlled bend distortions as orientational order changes. Device fabrication is applicable via current experimental techniques. These results are in qualitative agreement with theoretical predictions, provide insight into experimental observations, and demonstrate the value of finite element methods at the continuum level for designing and engineering liquid crystal polymeric devices. [ABSTRACT FROM AUTHOR]

Using both experiments and finite element simulations, we explore the shape evolution of off-axis twist nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90° left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample midplane. Both experiments and theoretical studies show that when the director at midplane is parallel to either the ribbon's long or short axes, ribbons form either helicoids or spirals depending on aspect ratio and temperature. Simulation studies show that if the director at midplane is off-axis, ribbons never form helicoids, instead evolving to distorted spiral shapes. Experimental studies for two samples with off-axis geometry show agreement with this prediction. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulations predict off-axis samples also change their macroscopic chirality at fixed temperature, depending on the angular offset. These results provide insight into the mechanisms driving shape evolution and macroscopic chirality, enabling engineering design of these materials for future applications. [ABSTRACT FROM AUTHOR]

Mechanical deformation can potentially provide an effective means of controlling the nanoscale morphology in hybrid materials. The challenge, however, is establishing optimal couplings of the deformation and mechano-responsive components in the material to achieve nanoscopic structural reorganization without causing catastrophic damage. Through computational modeling, we investigate how torsion can be utilized to induce controllable structural changes in networks formed from binary mixtures (A and B) of polymer-grafted nanoparticles (PGNs). The nanoparticles' rigid cores are decorated with a corona of grafted polymers, which contain reactive functional groups at the chain ends. With the overlap of the neighboring coronas, these reactive groups form labile bonds, which can reform after breakage. The labile bond energy between similar PGNs (U AA , U BB ) is different than the energy between dissimilar species (U AB ). By tailoring the relative values of these bond energies and the boundary conditions acting on the system, the application of a torsional deformation can result in a controllable reconfiguration of the network, leading to intertwining helical structures, or homogeneously mixed nanocomposites. In effect, our mechano-mutable system resembles a "Rubik's cube" material, whose nanostructure, and hence global properties, can be tailored by mechanically twisting the sample.

Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. Using a three-dimensional finite element elastodynamics simulation, we investigate one such instability, the onset of stripe formation in a monodomain film stretched along an axis perpendicular to the nematic director. In our simulation, we observe the formation of striped domains with alternating director rotation. This model allows us to explore the fundamental physics governing dynamic mechanical response of nematic elastomers and also provides a potentially useful computational tool for engineering device applications.