Mechanochemically responsive polymer enables shockwave visualization.

Understanding the physical and chemical response of materials to impulsive deformation is crucial for applications ranging from soft robotic locomotion to space exploration to seismology. However, investigating material properties at extreme strain rates remains challenging due to temporal and spatial resolution limitations. Combining high-strain-rate testing with mechanochemistry encodes the molecular-level deformation within the material itself, thus enabling the direct quantification of the material response.

The Projectile Perforation Resistance of Materials: Scaling the Impact Resistance of Thin Films to Macroscale Materials.

From drug delivery to ballistic impact, the ability to control or mitigate the puncture of a fast-moving projectile through a material is critical. While puncture is a common occurrence, which can span many orders of magnitude in the size, speed, and energy of the projectile, there remains a need to connect our understanding of the perforation resistance of materials at the nano- and microscale to the actual behavior at the macroscale that is relevant for engineering applications. In this article, we address this challenge by combining a new dimensional analysis scheme with experimental data from micro- and macroscale impact tests to develop a relationship that connects the size-scale effects and materials properties during high-speed puncture events.

Self-Assembled Asperities for Pressure-Tunable Adhesion.

Control of adhesion is important in a host of applications including soft robotics, pick-and-place manufacturing, wearable devices, and transfer printing. While there are adhesive systems with discrete switchability between states of high and low adhesion, achieving continuously variable adhesion strength remains a challenge. In this work, a pressure-tunable adhesive (PTA) that is based on the self-assembly of stiff microscale asperities on an elastomeric substrate is presented.

Tuning the mechanical impedance of disordered networks for impact mitigation.

Disordered-Network Mechanical Materials (DNMM), comprised of random arrangements of bonds and nodes, have emerged as mechanical metamaterials with the potential for achieving fine control over their mechanical properties. Recent computational studies have demonstrated this control whereby an extremely high degree of mechanical tunability can be achieved in disordered networks a selective bond removal process called pruning. In this study, we experimentally demonstrate how pruning of a disordered network alters its macroscopic dynamic mechanical response and its capacity to mitigate impact.

Controlling toughness of polymer-grafted nanoparticle composites for impact mitigation.

Toughness in an entangled polymer network is typically controlled by the number of load-bearing topological constraints per unit volume. In this work, we demonstrate a new paradigm for controlling toughness at high deformation rates in a polymer-grafted nanoparticle composite system where the entanglement density increases with the molecular mass of the graft. An unexpected peak in the toughness is observed right before the system reaches full entanglement that cannot be described through the entanglement concept alone.

Supersonic Impact Response of Polymer Thin Films via Large-Scale Atomistic Simulations.

Recent nanoscale ballistic tests have shown the applicability of nanomaterials for ballistic protection but have raised questions regarding the nanoscale structure-property relationships that contribute to the ballistic response. Herein, we report on multimillion-atom reactive molecular dynamics simulations of the supersonic impact, penetration, and failure of polyethylene (PE) and polystyrene (PS) ultrathin films. The simulated specific penetration energy (*) versus impact velocity predicts to within 15% the experimentally determined * for PS.

Cutting to measure the elasticity and fracture of soft gels.

The fracture properties of very soft and/or brittle materials are challenging to measure directly due to the limitations of existing fracture testing methods. To address this issue, we introduce a razorblade-initiated fracture test (RIFT) to measure the mechanical properties related to fracture for soft polymeric gels. We use RIFT to quantify the elasticity, crack initiation energy, and the fracture energy of gellan hydrogels as a function of gellan concentration.

Using microprojectiles to study the ballistic limit of polymer thin films.

The dynamic impact between a particle and a planar material is important in many high impact events, and there is a growing need to characterize the mechanical properties of light-weight polymeric materials at dynamic loading conditions. Here, a laser-induced projectile impact test (LIPIT) is employed to investigate the ballistic limit (V0) and materials properties at impact velocities ranging from 40 m s-1 to 70 m s-1. An analytical expression describing the various energy dissipation mechanisms is established to estimate the yield stress and elasticity for polycarbonate thin films.

Maximizing Contact of Supersoft Bottlebrush Networks with Rough Surfaces To Promote Particulate Removal.

Efficient removal of particulates from a rough surface with a soft material through a "press and peel" method (i.e., an adhesion and release approach) depends on good conformal contact at the interface; a material should be sufficiently soft to maximize contact with a particle while also conforming to rough surface features to clean the entire substrate surface.

Characterizing salt permeability in polyamide desalination membranes using electrochemical impedance spectroscopy.

Improving the performance of desalination membranes requires better measurements of salt permeability in the polyamide separating layer to elucidate the thermodynamic and kinetic components of membrane permselectivity. In this work, electrochemical impedance spectroscopy (EIS) is introduced as a technique to measure the salt permeability and estimate the salt partition coefficient in thin polyamide films created using molecular layer-by-layer deposition. The impedance of supported polyamide films ranging in thickness from 3.

Entanglement Density-Dependent Energy Absorption of Polycarbonate Films via Supersonic Fracture.

The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as entanglement density as well as extrinsic parameters such as strain rate and test temperature. Here we use laser-induced projectile impact testing (LIPIT) to study the extreme strain rate (≈10 s) puncture behavior of free-standing polycarbonate (PC) thin films. We demonstrate that changes to the PC molecular mass and the degree of plasticization can lead to substantial changes in the specific puncture energy.

Creating Aligned Nanopores by Magnetic Field Processing of Block Copolymer/Homopolymer Blends.

We describe the phase behavior of a cylinder-forming block copolymer (BCP)/homopolymer blend and the generation of aligned nanopores by a combination of magnetic field alignment and selective removal of the minority-block-miscible homopolymer. Alignment is achieved by cooling through the order-disorder transition temperature () in a 6 T field. The system is a blend of poly(styrene--4-vinylpyridine) (PS--P4VP) and poly(ethylene glycol) (PEG).

Size effects in plasma-enhanced nano-transfer adhesion.

Plasma bonding and layer-by-layer transfer molding have co-existed for decades, and here we offer a combination of the two that drives both techniques to the nanoscale. Using fluorinated elastomeric stamps, lines of plasma-treated poly(dimethylsiloxane) (PDMS) were stacked into multi-layer woodpile structures via transfer molding, and we observe a pronounced size effect wherein nanoscale lines (≤280 nm period) require ultra-low plasma dose (<20 J) and fail to bond at the much higher range of plasma dose (600 J to 900 J) recommended in the PDMS plasma bonding literature. The size effect appears to be related to the thickness of the oxide film that develops on the PDMS surface during treatment, and we employ an empirical relationship, , to estimate the thickness of this film in the low plasma dose (<100 J) regime.

Functional group quantification of polymer nanomembranes with soft x-rays.

Polyamide nanomembranes are at the heart of water desalination, a process which plays a critical role in clean water production. Improving their efficiency requires a better understanding of the relationship between chemistry, network structure, and performance but few techniques afford compositional information in ultrathin films (<100 nm). Here, we leverage resonant soft x-ray reflectivity, a measurement that is sensitive to the specific chemical bonds in organic materials, to quantify the functional group concentration in these polyamides.

Viscoplastic fracture transition of a biopolymer gel.

Physical gels are swollen polymer networks consisting of transient crosslink junctions associated with hydrogen or ionic bonds. Unlike covalently crosslinked gels, these physical crosslinks are reversible thus enabling these materials to display highly tunable and dynamic mechanical properties. In this work, we study the polymer composition effects on the fracture behavior of a gelatin gel, which is a thermoreversible biopolymer gel consisting of denatured collagen chains bridging physical network junctions formed from triple helices.

Studying water and solute transport through desalination membranes via neutron radiography.

Neutron radiography, a non-destructive imaging technique, is applied to study water and solute transport through desalination membranes. Specifically, we use neutron radiography to quantify lithium chloride draw solute concentrations across a thin-film composite membrane during forward osmosis permeation. This measurement provides direct visual confirmation of incomplete support layer wetting and reveals significant dilutive external concentration polarization of the draw solution outside of the membrane support layer.

Compressing and Swelling To Study the Structure of Extremely Soft Bottlebrush Networks Prepared by ROMP.

To fully explore bottlebrush polymer networks as potential model materials, a robust and versatile synthetic platform is required. Ring-opening metathesis polymerization is a highly controlled, rapid, and functional group tolerant polymerization technique that has been used extensively for bottlebrush polymer generation but to this point has not been used to synthesize bottlebrush polymer networks. We polymerized a mononorbornene macromonomer and dinorbornene cross-linker (both poly(-butyl acrylate)) with Grubbs' third-generation catalyst to achieve bottlebrush networks and in turn demonstrated control over network properties as the ratio of macromonomer and cross-linker was varied.

Quantifying the sensitivity of the network structure and properties from simultaneous measurements during photopolymerization.

We present a method that combines experimental and computational approaches to assess a comprehensive set of structural and functional evolution during a network formation process via photopolymerization. Our work uses the simultaneous measurement of the degree of conversion, polymerization stress, the change in reaction temperature, and shrinkage strain in situ. These measurements are combined with the theory of viscoelastic materials to deduce the relaxation time and frequency-dependent modulus of the polymerizing network.

Nanoscale Pillar-Enhanced Tribological Surfaces as Antifouling Membranes.

We present a nonconventional membrane surface modification approach that utilizes surface topography to manipulate the tribology of foulant accumulation on water desalination membranes via imprinting of submicron titanium dioxide (TiO) pillar patterns onto the molecularly structured, flat membrane surface. This versatile approach overcomes the constraint of the conventional approach relying on interfacial polymerization that inevitably leads to the formation of ill-defined surface topography. Compared to the nonpatterned membranes, the patterned membranes showed significantly improved fouling resistance for both organic protein and bacterial foulants.

Using Indentation to Quantify Transport Properties of Nanophase-Segregated Polymer Thin Films.

Indentation of hydrated Nafion thin films reveals that both the in-plane diffusivity of water and the intrinsic permeability of the phase-segregated network decrease dramatically with decreasing film thickness. Using pore-network theory, this decrease in diffusivity is attributed to both an increase in ionic-domain heterogeneity and a reduction in ionic-domain connectivity upon confinement.

Bilayer mass transport model for determining swelling and diffusion in coated, ultrathin membranes.

Water transport and swelling properties of an ultrathin, selective polyamide layer with a hydrophilic polymer coating, i.e., a polymer bilayer, are studied using quartz crystal microbalance with dissipation (QCM-D).

Deswelling of ultrathin molecular layer-by-layer polyamide water desalination membranes.

The selective layer of pressure-induced water desalination membranes is an ultrathin and highly crosslinked aromatic polyamide (PA) film that separates salt from water based on differences in permeability, which is a product of diffusivity and solubility. Characterizing the transport properties of the selective layer is necessary in understanding its permselective performance. However, measuring transport of ultrathin films in general is nontrivial.

Tailoring the permselectivity of water desalination membranes via nanoparticle assembly.

Thin film composite membranes can selectively separate mono- and divalent ions from water via solution-diffusion of each species through a dense but ultrathin, highly cross-linked polymer "skin" layer; water is transported across the membrane faster than associated salts. Changing the selectivity of the "skin" layer typically requires adjusting the monomer chemistries that make up the polymer "skin" layer, but doing so also impacts a host of other membrane properties. Here, we employ electrostatic layer-by-layer deposition of inorganic nanoparticles to enhance the permselectivity of an existing commercial nanofiltration membrane.