3-D printing is a revolutionary manufacturing technique which makes it possible to fabricate objects of any shape and size that are hard to reproduce by traditional methods. We develop a coarse-grained molecular dynamics simulation approach to model the continuous liquid interface production (CLIP) 3-D printing technique. This technique utilizes a continuous polymerization and cross-linking of the liquid monomeric precursor by the UV light within a thin layer while pulling the cross-linked polymeric object out of a pool of monomers. Simulations show that the quality of the shape of the 3-D printed objects is determined by a fine interplay between elastic, capillary, and friction forces. Using simulation results, we identify the source of the object shape deformations and develop a set of rules for calibration of the parameters to meet the accuracy requirements. Comparison between different continuous 3-D printing setups shows that proposed modifications of the printing process could improve quality and accuracy of the printed parts.

Combining high concentration of reversible hydrogen bonds with a loosely cross-linked chemical network in poly(N,N-dimethylacrylamide-co-methacrylic acid) hydrogels produces dual-network materials with high modulus and toughness on par with those observed for connective tissues. The dynamic nature of the H-bonded cross-links manifests itself in a strong temperature and strain rate dependence of hydrogel mechanical properties. We have identified several relaxation regimes of a hydrogel by monitoring a time evolution of the time-average Young’s modulus ⟨E(t)⟩ = σ(t)/ε̇t as a function of the strain rate, ε̇, and temperature. At low temperatures (e.g., 3 °C), ⟨E(t)⟩ first displays a Rouse-like relaxation regime (⟨E(t)⟩ ∼ t–0.5), which is followed by a temporary (physical) network regime (⟨E(t)⟩ ∼ t–0.14) at intermediate time scales and then by an associating liquid regime (⟨E(t)⟩ ∼ t–0.93) at the later times. With increasing temperature to 22 °C, the temporary network plateau displays lower modulus values, narrows, and shifts to shorter time scales. Finally, the plateau vanishes at 37 °C. It is shown that the energy dissipation in hydrogels due to strain-induced dissociation of the H-bonded cross-links increases hydrogel toughness. The density of dissipated energy at small deformations scales with strain rate as UT ∼ ε̇0.53. We develop a model describing dynamics of deformation of dual networks. The model predictions are in a good agreement with experimental data. Our analysis of the dual network’s dynamics provides general frameworks for characterization of such materials.

We use a combination of the coarse-grained molecular dynamics simulations and scaling analysis to study conformations of bottlebrush and comb-like polymers in a melt. Our analysis shows that a crossover between comb and bottlebrush regimes is controlled by the crowding parameter, Φ, describing overlap between neighboring macromolecules. In comb-like systems characterized by a sparse grafting of side chains (Φ < 1), the side chains and backbones belonging to neighboring macromolecules interpenetrate. However, in bottlebrushes with densely grafted side chains (Φ ≥ 1), the interpenetration between macromolecules is suppressed by steric repulsion between side chains. In this regime, bottlebrush macromolecules can be viewed as filaments with diameter proportional to size of the side chains. For flexible side chains, the crowding parameter is given by Φ ≈ [v/(lb)3/2][(nsc/ng + 1)/nsc1/2], which depends on both the architectural parameters (degree of polymerization of the side chains, nsc, and number of backbone bonds between side chains, ng) and chemical structure of monomers (bond length l, monomer excluded volume v, and Kuhn length, b). Molecular dynamics simulations corroborate this classification of graft polymers and show that the effective macromolecule Kuhn length, bK, and the mean-square end-to-end distance of the backbone, ⟨Re,bb2⟩, are universal functions of the crowding parameter Φ for all studied systems.

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. The brushes were synthesized 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. 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.

We use molecular dynamics simulations to study mechanical properties of polymeric nanocomposites of liquid inclusions in polymeric network matrix. The shear modulus of nanocomposite is shown to be a universal function of the elastocapillary number γNL/(GNR0), where γNL is the surface tension of the liquid/network interface, GN is the shear modulus of the network and R0 is the initial size of liquid inclusions. First, in the range of elastocapillary numbers, γNL/(GNR0) < 1, the composite shear modulus increases with increasing elastocapillary number. In this interval of elastocapillary numbers, liquid inclusions soften the network such that the composite modulus GC is smaller than GN. However, for elastocapillary numbers γNL/(GNR0) ≈ 2, the liquid inclusions begin to reinforce the network resulting in GC > GN. In such composites, the surface energy of the deformed liquid inclusions stiffens the composite. When the elastocapillary number increases further, γNL/(GNR0) ≫ 1, the interfacial energy of network/liquid interface dominates the mechanical response of the composite. Elongation ratio of the liquid inclusions monotonically decreases with increasing elastocapillary number γNL/(GNR0).

The ability of nanoparticles to remain adsorbed at an interface between two soft materials makes it possible to use them as efficient adhesives. Using a combination of the molecular dynamics simulations and theoretical calculations, we establish conditions for different regimes of interfacial confinement of nanoparticles between two polymeric gels. Depending on the relative strength of the capillary and elastic forces acting on a nanoparticle in contact with substrates, a nanoparticle could be in the bridging, the Pickering, or the submerged state. The work of adhesion for a nanoparticle reinforced interface was derived analytically and obtained in simulations from the potential of mean force for separation of two gels. Simulations show that the work required for separation of two gels with nanoparticles confined at interface could be up to 10 times larger than the work of adhesion between two neat gels without the nanoparticle reinforcement. These results provide a valuable insight in understanding the mechanism of gluing soft materials, including gels and biological tissues, by nano- and microsize particles.

The deformation dynamics of bottlebrush networks in a melt state is studied using a combination of theoretical, computational, and experimental techniques. Three main molecular relaxation processes are identified in these systems: (i) relaxation of the side chains, (ii) relaxation of the bottlebrush backbones on length scales shorter than the bottlebrush Kuhn length (bK), and (iii) relaxation of the bottlebrush network strands between cross-links. The relaxation of side chains having a degree of polymerization (DP), nsc, dominates the network dynamics on the time scales τ0 < t ≤ τsc, where τ0 and τsc ≈ τ0(nsc + 1)2 are the characteristic relaxation times of monomeric units and side chains, respectively. In this time interval, the shear modulus at small deformations decays with time as G0BB(t) ∼ t–1/2. On time scales t > τsc, bottlebrush elastomers behave as networks of filaments with a shear modulus G0BB(t) ∼ (nsc + 1)−1/4t–1/2. Finally, the response of the bottlebrush networks becomes time independent at times scales longer than the Rouse time of the bottlebrush network strands, τBB ≈ τ0N2(nsc + 1)3/2, where N is DP of the bottlebrush backbone between cross-links. In this time interval, the network shear modulus depends on the network molecular parameters as G0BB(t) ∼ (nsc + 1)−1N–1. Analysis of the simulation data shows that the stress evolution in the bottlebrush networks during constant strain-rate deformation can be described by a universal function. The developed scaling model is consistent with the dynamic response of a series of poly(dimethylsiloxane) bottlebrush networks (nsc = 14 and N = 50, 70, 100, 200) measured experimentally.

The propensity of boron nitride sheets to stack creates obstacles for their application as multifunctional materials despite their unique thermal, mechanical, and electrical properties. To address this challenge, we use a combination of molecular dynamics simulations and experimental techniques to demonstrate surfactant-like properties of BN sheets at the interface between immiscible solvents. The spreading of two-dimensional BN sheets at a high-energy oil/water interface lowers the free energy of the system, creating films of overlapping BN sheets that are more thermodynamically favorable than stacked sheets. Coating such films onto polymers results in composite materials with exceptional barrier and dielectric properties.Keywords: boron nitride; coating; composites; dielectrics; exfoliation; interface trapping; molecular dynamics simulations; polymeric films