Robert Sinko

The role of mechanics in biological and bio-inspired systems

Natural systems frequently exploit intricate multiscale and multiphasic structures to achieve functionalities beyond those of man-made systems. Although understanding the chemical make-up of these systems is essential, the passive and active mechanics within biological systems are crucial when considering the many natural systems that achieve advanced properties, such as high strength-to-weight ratios and stimuli-responsive adaptability. Discovering how and why biological systems attain these desirable mechanical functionalities often reveals principles that inform new synthetic designs based on biological systems. Such approaches have traditionally found success in medical applications, and are now informing breakthroughs in diverse frontiers of science and engineering.

Tuning Glass Transition in Polymer Nanocomposites with Functionalized Cellulose Nanocrystals through Nanoconfinement

Cellulose nanocrystals (CNCs) exhibit impressive interfacial and mechanical properties that make them promising candidates to be used as fillers within nanocomposites. While glass-transition temperature (Tg) is a common metric for describing thermomechanical properties, its prediction is extremely difficult as it depends on filler surface chemistry, volume fraction, and size. Here, taking CNC-reinforced poly(methyl-methacrylate) (PMMA) nanocomposites as a relevant model system, we present a multiscale analysis that combines atomistic molecular dynamics (MD) surface energy calculations with coarse-grained (CG) simulations of relaxation dynamics near filler-polymer interfaces to predict composite properties. We discover that increasing the volume fraction of CNCs results in nanoconfinement effects that lead to an appreciation of the composite Tg provided that strong interfacial interactions are achieved, as in the case of TEMPO-mediated surface modifications that promote hydrogen bonding. The upper and lower bounds of shifts in Tg are predicted by fully accounting for nanoconfinement and interfacial properties, providing new insight into tuning these aspects in nanocomposite design. Our multiscale, materials-by-design framework is validated by recent experiments and breaks new ground in predicting, without any empirical parameters, key structure-property relationships for nanocomposites.

Design and test of an adaptive vibration absorber based on magnetorheological elastomers and a hybrid electromagnet

The focuses of this study are to design an adaptive-tuned vibration absorber based on a smart materials known as magnetorheological elastomers and to test its dynamic performance. A primary system replicating a miniature cryogenic cooler (i.e. mass and shape) was designed and fabricated in order to test the effectiveness of the vibration absorber. A hybrid magnetic system (electromagnet and permanent magnets) was also designed and fabricated in order to actuate the magnetorheological elastomers as an adaptive stiffness element in the vibration absorber. The vibration testing was conducted on both the primary system and vibration absorber individually in order to characterize the behavior and verify the design constraints. Further testing was performed on a 2-degrees-of-freedom system to measure and assess the feasibility of the magnetorheological elastomer material for use in an application requiring an adaptive vibration absorber. The results show that by using a hybrid design for the electromagnet within the vibration absorber, the stiffness of the magnetorheological elastomer material can be increased and decreased above its nominal value, therefore demonstrating the feasibility of this design as an alternative adaptive vibration absorber.

Effect of moisture on the traction-separation behavior of cellulose nanocrystal interfaces

Interfaces and stress transfer between cellulose nanocrystals (CNCs) dictate the mechanical properties of hierarchical cellulose materials such as neat films and nanocomposites. An interesting question that remains is how the behavior of these interfaces changes due to environmental stimuli, most notably moisture. We present analyses on the traction-separation behavior between Iβ CNC elementary fibrils, providing insight into how the presence of a single atomic layer of water at these interfaces can drastically change the mechanical behavior. We find that molecular water at the interface between hydrophilic CNC surfaces has a negligible effect on the tensile separation adhesion energy. However, when water cannot hydrogen bond easily to the surface (i.e., hydrophobic surface), it tends to maintain hydrogen bonds with other water molecules across the interface and form a capillary bridge that serves to increase the energy required to separate the crystals. Under shear loading, water lowers the energy barriers to sliding by reducing the atomic friction and consequently the interlayer shear modulus between crystals. Our simulations indicate that these nanoscale interfaces and physical phenomena such as interfacial adhesion, interlayer shear properties, and stick-slip friction behavior can be drastically altered by the presence of water.

Transient effects of drying creep in nanoporous solids: understanding the effects of nanoscale energy barriers.

The Pickett effect is the phenomenon of creep enhancement during transient drying. It has been observed for many nanoporous solids, including concrete, wood and Kevlar. While the existing micromechanical models can partially explain this effect, they have yet to consider nanoscale dynamic effects of water in nanopores, which are believed to be of paramount importance. Here, we examine how creep deformations in a slit pore are accelerated by the motion of water due to drying forces using coarse-grained molecular dynamics simulations. We find that the drying that drives water flow in the nanopores lowers both the activation energy of pore walls sliding past one another and the apparent viscosity of confined water molecules. This lowering can be captured with an analytical Arrhenius relationship accounting for the role of water flow in overcoming the energy barriers. Notably, we use this model and simulation results to demonstrate that the drying creep strain is not linearly dependent on the applied creep stress at the nanopore level. Our findings establish the scaling relationships that explain how the creep driving force, drying force and fluid properties are related. Thus, we establish the nanoscale origins of the Pickett effect and provide strategies for minimizing the additional displacements arising from this effect.

Design and Test of an Adaptive Tuned Vibration Absorber for Reducing Cryogenic Cooler Vibrations

The focuses of this study are to design an adaptive tuned vibration absorber based on a smart material known as magnetorheological elastomers (MRE) and to test its dynamic performance. A primary system replicating a cryogenic cooler was designed and fabricated in order to test the effectiveness of the vibration absorber. A hybrid magnetic system (electromagnet and permanent magnets) was also designed and fabricated in order to actuate the MRE as an adaptive stiffness element in the vibration absorber. Vibration testing was conducted on both the primary system and vibration absorber individually in order to characterize the behavior and verify the design constraints. Further testing was performed on the two degree of freedom system to measure and assess the feasibility of the MRE material for use in an application requiring an adaptive vibration absorber. The results show that by using a hybrid design for the electromagnet within the vibration absorber, the stiffness of the MRE material can be both increased and decreased above its nominal value; therefore demonstrating the feasibility of this design as an alternative adaptive vibration absorber.Copyright

A nanoscale perspective on the effects of transverse microprestress on drying creep of nanoporous solids

The Pickett effect describes the excess non-additive strain developed during drying of a nanoporous solid material under creep. One explanation for its origins, developed using micromechanical models, is the progressive relaxation of internally developed microprestress. However, these models have not explicitly considered the effects of this microprestress on nanoscale energy barriers that govern the relative motion and displacement between nanopore walls during deformation. Here, we evaluate the nanoscale effects of transverse microprestresses on the drying creep behaviour of a nanoscale slit pore using coarse-grained molecular dynamics. We find that the underlying energy barrier depends exponentially on the transverse microprestress, which is attributed to changes in the effective viscosity and degree of nanoconfinement of molecules in the water interlayer. Specifically, as the transverse microprestress is relaxed (i.e. its magnitude decreases), the activation energy barrier is reduced, thereby leading to an acceleration of the creep behaviour and a stronger Pickett effect. Based on our simulation results, we introduce a new microprestress-dependent energy term into our existing Arrhenius model, which describes the relative displacement of pore walls as a function of the underlying activation energy barriers. Our findings further verify the existing micromechanical theories for the origin of the Pickett effect and establish a quantitative relationship between the transverse microprestress and the intensity of the Pickett effect.

Materials by Design for Stiff and Tough Hairy Nanoparticle Assemblies

Matrix-free polymer-grafted nanocrystals, called assembled hairy nanoparticles (aHNPs), can significantly enhance the thermomechanical performance of nanocomposites by overcoming nanoparticle dispersion challenges and achieving stronger interfacial interactions through grafted polymer chains. However, effective strategies to improve both the mechanical stiffness and toughness of aHNPs are lacking given the general conflicting nature of these two properties and the large number of molecular parameters involved in the design of aHNPs. Here, we propose a computational framework that combines multiresponse Gaussian process metamodeling and coarse-grained molecular dynamics simulations to establish design strategies for achieving optimal mechanical properties of aHNPs within a parametric space. Taking poly(methyl methacrylate) grafted to high-aspect-ratio cellulose nanocrystals as a model nanocomposite, our multiobjective design optimization framework reveals that the polymer chain length and grafting density are the main influencing factors governing the mechanical properties of aHNPs, in comparison to the nanoparticle size and the polymer-nanoparticle interfacial interactions. In particular, the Pareto frontier, that marks the upper bound of mechanical properties within the design parameter space, can be achieved when the weight percentage of nanoparticles is above around 60% and the grafted chains exceed the critical length scale governing transition into the semidilute brush regime. We show that theoretical scaling relationships derived from the Daoud-Cotton model capture the dependence of the critical length scale on graft density and nanoparticle size. Our established modeling framework provides valuable insights into the mechanical behavior of these hairy nanoparticle assemblies at the molecular level and allows us to establish guidelines for nanocomposite design.

Effect of Surface Modification on Water Adsorption and Interfacial Mechanics of Cellulose Nanocrystals

With increasing environmental concerns about petrochemical-based materials, the development of high-performance polymer nanocomposites with sustainable filler phases has attracted significant attention. Cellulose nanocrystals (CNCs) are promising nanocomposite reinforcing agents due to their exceptional mechanical properties, low weight, and bioavailability. However, there are still numerous obstacles that prevent these materials from achieving optimal performance, including high water adsorption, poor nanoparticle dispersion, and filler properties that vary in response to moisture. Surface modification is an effective method to mitigate these shortcomings. We use computational approaches to obtain direct insight into the water adsorption and interfacial mechanics of modified CNC surfaces. Atomistic grand-canonical Monte Carlo simulations demonstrate how surface modification of sulfated Na-CNCs impacts water adsorption. We find that methyl(triphenyl)phosphonium (MePh3P+)-exchanged CNCs have lower water uptake than Na-CNCs, supporting experimental dynamic vapor sorption measurements. The adsorbed water molecules show orientational ordering when distributed around the cations. Steered molecular dynamics simulations quantify traction-separation behavior of CNC-CNC interfaces. We find that exchanging sodium for MePh3P+ effectively changes the surface hydrophilicity, which in turn directly impacts interfacial adhesion and traction-separation behavior. Our analysis provides guidelines for controlling moisture effects in cellulose nanocomposites and nanocellulose films through surface modifications.

Understanding emergent functions in self-assembled fibrous networks.

Understanding self-assembly processes of nanoscale building blocks and characterizing their properties are both imperative for designing new hierarchical, network materials for a wide range of structural, optoelectrical, and transport applications. Although the characterization and choices of these material building blocks have been well studied, our understanding of how to precisely program a specific morphology through self-assembly still must be significantly advanced. In the recent study by Xie et al (2015 Nanotechnology 26 205602), the self-assembly of end-functionalized nanofibres is investigated using a coarse-grained molecular model and offers fundamental insight into how to control the structural morphology of nanofibrous networks. Varying nanoscale networks are observed when the molecular interaction strength is changed and the findings suggest that self-assembly through the tuning of molecular interactions is a key strategy for designing nanostructured networks with specific topologies.