Engineering topics

Dobrynin, Andrey V.

Publications and source records attributed to Dobrynin, Andrey V..

6 records

DOE OSTI2019

Brush-Like Polymers and Entanglements: From Linear Chains to Filaments

Dynamics of melts and solutions of high molecular weight polymers and biopolymers is controlled by topological constraints (entanglements) imposing a sliding chain motion along an effective confining tube. For linear chains, the tube size is determined by universal packing number P e , the number of polymer strands within a confining tube that is required for chains to entangle. In this work, we show that in melts of brush-like (graft) polymers, consisting of linear chain backbones with grafted side chains, P e is not a universal number and depends on the molecular architecture. Specifically, we use coarse-grained molecular dynamics simulations to demonstrate that the packing number is a nonmonotonic function of the ratio R nsc /R ng of the size of the side chains R nsc to that of the backbone spacer between neighboring grafting points R ng . This parameter characterizes the degree of mutual interpenetration between side chains of the same macromolecule. We demonstrate that P e of brush-like polymers first decreases with increasing side chain grafting density in the dilute side chain regime (R nsc < R ng ), then begins to increase in the regime of overlapping side chains (R nsc > R ng ), approaching the value for linear chains in the limit of densely grafted side chains. This dependence of the packing number reflects a crossover from chain-like entanglements in systems with loosely grafted side chains (comb-like polymers) to entanglements between flexible filaments (bottlebrush-like polymers). Our simulation results are in agreement with the experimental data for the dependence of a plateau modulus on the molecular architecture of graft poly(n-butyl acrylates) and poly(norbornene)-graft-poly(lactide) melts.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Programming temporal shapeshifting

Shapeshifting enables a wide range of engineering and biomedical applications, but until now transformations have required external triggers. This prerequisite limits viability in closed or inert systems and puts forward the challenge of developing materials with intrinsically encoded shape evolution. Herein we demonstrate programmable shape-memory materials that perform a sequence of encoded actuations under constant environment conditions without using an external trigger. We employ dual network hydrogels: in the first network, covalent crosslinks are introduced for elastic energy storage, and in the second one, temporary hydrogen-bonds regulate the energy release rate. Through strain-induced and time-dependent reorganization of the reversible hydrogen-bonds, this dual network allows for encoding both the rate and pathway of shape transformations on timescales from seconds to hours. In conclusion, this generic mechanism for programming trigger-free shapeshifting opens new ways to design autonomous actuators, drug-release systems and active implants.

42 ENGINEERING↗

DOE OSTI2016

Distribution of Chains in Polymer Brushes Produced by a “Grafting From” Mechanism

The molecular weight and polydispersity of the chains in a polymer brush are critical parameters determining the brush properties. However, the characterization of polymer brushes is hindered by the vanishingly small mass of polymer present in brush layers. In this study, in order to obtain sufficient quantities of polymer for analysis, polymer brushes were grown from high surface area fibrous nylon membranes by ATRP. We synthesized the brushes with varying surface initiator densities, polymerization times, and amounts of sacrificial initiator, then cleaved from the substrate, and analyzed by GPC and NMR. Characterization showed that the surface-grown polymer chains were more polydisperse and had lower average molecular weight compared to solution-grown polymers synthesized concurrently. Furthermore, the molecular weight distribution of the polymer brushes was observed to be bimodal, with a low molecular weight population of chains representing a significant mass fraction of the polymer chains at high surface initiator densities. Moreover, the origin of this low MW polymer fraction is proposed to be the termination of growing chains by recombination during the early stages of polymerization, a mechanism confirmed by molecular dynamics simulations of brush polymerization.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Computer Simulations of Bottle Brushes: From Melts to Soft Networks

We use a combination of Molecular dynamics simulations and analytical calculations, and study dens bottle-brush systems in a melt and network State. Analysis of our simulation results shows that bottle-brush macromolecules in melt behave as ideal chains with effective Kuhn length b K . Simulations show that the bottle-brush-induced bending rigidity is due to an entropy decrease caused by redistribution of the side chains upon backbone bending. The Kuhn length of the bottle:brushes increases with increasing the side-chain degree of polymerization n sc as b K proportional to n sc 0.46 . Moreover, this model of bottle brush macromolecules is extended to describe mechanical properties of bottle brush networks in linear and nonlinear deformation regimes. In the linear deformation regime, the network shear modulus scales with the degree of polymerization of the side chains as G 0 proportional to (n sc + 1) -1 as long as the ratio of the Kuhn length, b K , to the size of the fully extended bottle-brush backbone between cross-links, R-max, is smaller than unity, b K /R max << 1. Bottle-brush networks With b K /R max proportional to 1 demonstrate behavior similar to that of networks Of semiflexible chains with G 0 proportional to n sc -0.5 . Finally, in the nonlinear network deformation regime, the deformation-dependent shear modulus is a universal function of the first strain invariant I 1 and bottle-brush backbone deformation ratio beta describing stretching ability of the bottle-brush backbone between cross-links.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2015

Polymer/Pristine graphene based composites: from emulsions to strong, electrically conducting foams

The unique electrical, thermal, and mechanical properties of graphene make it a perfect candidate for applications in graphene/graphite based polymer composites, yet challenges due to the lack of solubility of pristine graphene/graphite in water and common organic solvents have limited its practical utilization. In this paper, we report a scalable and environmentally friendly technique to form water-in-oil type emulsions stabilized by overlapping pristine graphene sheets, enabling the synthesis of open cell foams containing a continuous graphitic network. Our approach utilizes the insolubility of graphene/graphite in both water and organic solvents and so does not require oxidation, reduction, surfactants, high boiling solvents, chemical functionalization, or the input of large amounts of mechanical energy or heat. At the heart of our technique is the strong attraction of graphene to high-energy oil and water interfaces. This allows for the creation of stable water-in-oil emulsions with controlled droplet size and overlapping graphene sheets playing the role of surfactant by covering the droplet surface and stabilizing the interfaces with a thin graphitic skin. Finally, these emulsions are used as templates for the synthesis of open cell foams with densities below 0.35 g/cm 3 that exhibit remarkable mechanical and electrical properties including compressive moduli up to ~100 MPa, compressive strengths of over 8.3 MPa (1200 psi), and bulk conductivities approaching 7 S/m.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Adhesion and Wetting of Soft Nanoparticles on Textured Surfaces: Transition between Wenzel and Cassie-Baxter States

We use a combination of the molecular dynamics simulations and scaling analysis to study interactions between gel-like nanoparticles and substrates covered with rectangular shape posts. Our simulations have shown that nanoparticle in contact with substrate undergo first order transition between Wenzel and Cassie-Baxter state which location depends on nanoparticle shear modulus, the strength of nanoparticle-substrate interactions, height of the substrate posts and nanoparticle size, R p . There is a range of system parameters where these two states coexist such that the average indentation δ produced by substrate posts changes monotonically with nanoparticle shear modulus, G p . We have developed a scaling model that describes deformation of nanoparticle in contact with patterned substrate. In the framework of this model the effect of the patterned substrate can be taken into account by introducing an effective work of adhesion, W eff , which describes the first order transition between Wenzel and Cassie-Baxter states. There are two different shape deformation regimes for nanoparticles with shear modulus G p and surface tension γ p . Shape of small nanoparticles with size R p < γ p 3/2 G p -1 W eff -1/2 is controlled by capillary forces while deformation of large nanoparticles, R p > γ p 3/2 G p -1 W eff -1/2</sup, is determined by nanoparticle elastic and contact free energies. The model predictions are in a good agreement with simulation results.

36 MATERIALS SCIENCE↗