Persistence length of α-helical poly-L-lysine.

The α-helix has a significant role in protein function and structure because of its rigidity. In this study, we investigate the persistence length, lp, of α-helical poly-L-lysine, PLL, for two molecular weights. PLL experiences a random coil-helix transition as the pH is raised from 7 to 12. Using light scattering experiments to determine the radius of gyration (Rg), hydrodynamic radius, (Rh), the shape factor (Rg/Rh), and second virial coefficient (A2), and circular dichroism to determine the helical content, we find the structure and lp of PLL as a function of pH (7.4-11.4) and ionic strength (100-166 mM). With increasing pH, we find an increase in lp from 2 nm to 15-21 nm because of α-helix formation. We performed dissipative particle dynamics (DPD) simulations and found a similar increase in lp. While this lp is less than that predicted by molecular dynamics simulations, it is consistent with other experimental results, which quantify the mechanics of α-helices. By determining the mechanics of helical polypeptides like PLL, we can further understand their implications to protein function.

Combining dynamic Monte Carlo with machine learning to study nanoparticle translocation.

Resistive pulse sensing (RPS) measurements of nanoparticle translocation have the ability to provide information on single-particle level characteristics, such as diameter or mobility, as well as ensemble averages. However, interpreting these measurements is complex and requires an understanding of nanoparticle dynamics in confined spaces as well as the ways in which nanoparticles disrupt ion transport while inside a nanopore. Here, we combine Dynamic Monte Carlo (DMC) simulations with Machine Learning (ML) and Poisson-Nernst-Planck calculations to simultaneously simulate nanoparticle dynamics and ion transport during hundreds of independent particle translocations as a function of nanoparticle size, electrophoretic mobility, and nanopore length. The use of DMC simulations allowed us to explicitly investigate the effects of Brownian motion and nanoparticle/nanopore characteristics on the amplitude and duration of translocation signals. Simulation results were verified with experimental RPS measurements and found to be in quantitative agreement.

Simulation of the Coronal Dynamics of Polymer-Grafted Nanoparticles.

Polymer-grafted nanoparticles (PGNPs) are an important component of many advanced materials. The interplay between the nanoparticle surface curvature and spatial confinement by neighboring chains produces a complex set of structural and dynamical behaviors in the polymer corona surrounding the nanoparticle. For example, experiments have shown that the inner portion of the corona is more stretched and relaxes more slowly than the outer region. Here, we perform systematic core-modified dissipative particle dynamics (CM-DPD) simulations and analyze the relaxation dynamics using proper orthogonal decomposition (POD) of the monomer coordinates. We find that grafted chains relax more slowly than free chains and that the relaxation time of the grafted chains scales inversely with the confinement strength. For PGNPs in a polymer melt, the relaxation processes are always Rouse-like. However, we observe either Zimm-like or Rouse-like dynamics for PGNPs in solution depending on the confinement strength.

Bottlebrush polymers with flexible enantiomeric side chains display differential biological properties.

Chirality and molecular conformation are central components of life: biological systems rely on stereospecific interactions between discrete (macro)molecular conformers, and the impacts of stereochemistry and rigidity on the properties of small molecules and biomacromolecules have been intensively studied. Nevertheless, how these features affect the properties of synthetic macromolecules has received comparably little attention. Here we leverage iterative exponential growth and ring-opening metathesis polymerization to produce water-soluble, chiral bottlebrush polymers (CBPs) from two enantiomeric pairs of macromonomers of differing rigidity. Remarkably, CBPs with conformationally flexible, mirror image side chains show several-fold differences in cytotoxicity, cell uptake, blood pharmacokinetics and liver clearance; CBPs with comparably rigid, mirror image side chains show no differences. These observations are rationalized with a simple model that correlates greater conformational freedom with enhanced chiral recognition. Altogether, this work provides routes to the synthesis of chiral nanostructured polymers and suggests key roles for stereochemistry and conformational rigidity in the design of future biomaterials.

Stereochemical Control Yields Mucin Mimetic Polymers.

All animals except sponges produce mucus. Across the animal kingdom, this hydrogel mediates surface wetting, viscosity, and protection against microbes. The primary components of mucus hydrogels are mucins-high molecular weight O-glycoproteins that adopt extended linear structures. Glycosylation is integral to mucin function, but other characteristics that give rise to their advantageous biological activities are unknown. We postulated that the extended conformation of mucins is critical for their ability to block microbial virulence phenotypes. To test this hypothesis, we developed synthetic mucin mimics that recapitulate the dense display of glycans and morphology of mucin. We varied the catalyst in a ring-opening metathesis polymerization (ROMP) to generate substituted norbornene-derived glycopolymers containing either cis- or trans-alkenes. Conformational analysis of the polymers based on allylic strain suggested that cis- rather than trans-poly(norbornene) glycopolymers would adopt linear structures that mimic mucins. High-resolution atomic force micrographs of our polymers and natively purified Muc2, Muc5AC, and Muc5B mucins revealed that cis-polymers adopt extended, mucin-like structures. The cis-polymers retained this structure in solution and were more water-soluble than their trans-analogs. Consistent with mucin's linear morphology, cis-glycopolymers were more potent binders of a bacterial virulence factor, cholera toxin. Our findings highlight the importance of the polymer backbone in mucin surrogate design and underscore the significance of the extended mucin backbone for inhibiting virulence.

Dynamic Interfacial Trapping of Janus Nanorod Aggregates.

Taking advantage of both shape and chemical anisotropy on the same nanoparticle offers rich self-assembly possibilities for nanotechnology. Through dissipative particle dynamics calculations, in the present work, the directed assembly of Janus nanorod aggregates and their capability to assemble into metastable novel structures at an interfacial level have been assessed. Symmetric Janus rods become kinetically trapped and exhibit either parallel or antiparallel alignment with respect to their long axis (different compositions). This depends on several factors that have been mapped herein and that can be precisely tuned: Flory-Huggins interaction parameter χ between polymer phases; concentration; shear rate; and even aggregate shape. Ultimately, two different aggregate structures result from rod tumbling that are not observed under quiescent conditions: monolayer-like aggregates exhibiting trapped rods with antiparallel configuration; and stacked nanorod arrays similar to superlattice sheets. These different structures can be controlled by the likelihood with which tumbling Janus rods encounter other aggregate portions showing parallel alignment. Hence, the present study offers fundamental insight into relevant parameters that govern the directed assembly of Janus nanoparticles at an interfacial level. Novel applications may potentially derive from the resulting aggregate structures, such as peculiar displays and sensors.

Translocation of soft phytoglycogen nanoparticles through solid-state nanochannels.

Phytoglycogen nanoparticles are soft, naturally-derived nanomaterials with a highly uniform size near 35 nm. Their interior is composed of a highly-branched polysaccharide core that contains more than 200% of its dry mass in water. In this work, we measure the translocation of phytoglycogen particles by observing blockade events they create when occluding solid-state nanochannels with diameters between 60 and 100 nm. The translocation signals are interpreted using Poisson-Nernst-Planck calculations with a "hardness parameter" that describes the extent to which solvent can penetrate through the interior of the particles. Theory and experiment were found to be in quantitative agreement, allowing us to extract physical characteristics of the particles on a per particle basis.

Persistent Multiexcitons from Polymers with Pendent Pentacenes.

Singlet fission has emerged as a key mechanism of exciton multiplication in organic chromophores, generating two triplet excitons from a single photon. Singlet fission is typically studied in crystalline films or in isolated dimers. Here, we investigate an intermediate regime where through-space interactions mediate singlet fission and triplet pair recombination within isolated polymer chains. Specifically, we investigate how appending pentacenes to a polynorbornene backbone can lead to macromolecules that take advantage of through-space π-π interactions for fast singlet fission and rapid triplet pair dissociation. Singlet fission in these systems is affected by molecular dynamics, and triplet-triplet recombination is a geminate process where the rate of recombination scales with molecular-weight. We find that these pendent pentacene polymers yield free triplets with lifetimes that surpass those of crystalline chromophores in both solution as isolated polymers and in thin films.

Polymers on nanoparticles: structure & dynamics.

Grafting polymers to nanoparticle surfaces influences properties from the conformation of the polymer chains to the dispersion and assembly of nanoparticles within a polymeric material. Recently, a small body of work has begun to address the question of how grafting polymers to a nanoparticle surface impacts chain dynamics, and the resulting physical properties of a material. This Review discusses recent work that characterizes the structure and dynamics of polymers that are grafted to nanoparticles and opportunities for future research. Starting from the case of a single polymer chain attached to a nanoparticle core, this Review follows the structure of the chains as grafting density increases, and how this structure slows relaxation of polymer chains and affects macroscopic material properties.

Local Structure and Relaxation Dynamics in the Brush of Polymer-Grafted Silica Nanoparticles.

When grafted to spherical nanoparticles at high grafting densities, polymers adopt a variety of conformations. Because of strong confinement by neighboring chains, portions of the polymer near the nanoparticle core are highly stretched in the concentrated polymer brush region (CPB) of the polymer layer. Farther away from the core, where the polymer is less confined, the conformation becomes more ideal in the semidilute polymer brush (SDPB) region. Using a combination of small-angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy, we directly characterized both the structure and dynamics of the CPB and SDPB on poly(methyl acrylate) (PMA) grafted SiO2 nanoparticles (NPs). Analysis of SANS measurements using a new core-chain-chain (CCC) model confirmed that the portion of the chain in the CPB region is highly stretched, and transitions to a more random conformation. Dynamics in the CPB region were found to be much slower than the SDPB region.

Poly[n]catenanes: Synthesis of molecular interlocked chains.

As the macromolecular version of mechanically interlocked molecules, mechanically interlocked polymers are promising candidates for the creation of sophisticated molecular machines and smart soft materials. Poly[n]catenanes, where the molecular chains consist solely of interlocked macrocycles, contain one of the highest concentrations of topological bonds. We report, herein, a synthetic approach toward this distinctive polymer architecture in high yield (~75%) via efficient ring closing of rationally designed metallosupramolecular polymers. Light-scattering, mass spectrometric, and nuclear magnetic resonance characterization of fractionated samples support assignment of the high-molar mass product (number-average molar mass ~21.4 kilograms per mole) to a mixture of linear poly[7-26]catenanes, branched poly[13-130]catenanes, and cyclic poly[4-7]catenanes. Increased hydrodynamic radius (in solution) and glass transition temperature (in bulk materials) were observed upon metallation with Zn2.

Biomimetic Reversible Heat-Stiffening Polymer Nanocomposites.

Inspired by the ability of the sea cucumber to (reversibly) increase the stiffness of its dermis upon exposure to a stimulus, we herein report a stimuli-responsive nanocomposite that can reversibly increase its stiffness upon exposure to warm water. Nanocomposites composed of cellulose nanocrystals (CNCs) that are grafted with a lower critical solution temperature (LCST) polymer embedded within a poly(vinyl acetate) (PVAc) matrix show a dramatic increase in modulus, for example, from 1 to 350 MPa upon exposure to warm water, the hypothesis being that grafting the polymers from the CNCs disrupts the interactions between the nanofibers and minimizes the mechanical reinforcement of the film. However, exposure to water above the LCST leads to the collapse of the polymer chains and subsequent stiffening of the nanocomposite as a result of the enhanced CNC interactions. Backing up this hypothesis are energy conserving dissipative particle dynamics (EDPD) simulations which show that the attractive interactions between CNCs are switched on upon the temperature-induced collapse of the grafted polymer chains, resulting in the formation of a percolating reinforcing network.

Rapid Large-Scale Assembly and Pattern Transfer of One-Dimensional Gold Nanorod Superstructures.

The utility of gold nanorods for plasmonic applications largely depends on the relative orientation and proximity of the nanorods. Though side-by-side or chainlike nanorod morphologies have been previously demonstrated, a simple reliable method to obtain high-yield oriented gold nanorod assemblies remains a significant challenge. We present a facile, scalable approach which exploits meniscus drag, evaporative self-assembly, and van der Waals interactions to precisely position and orient gold nanorods over macroscopic areas of 1D nanostructured substrates. By adjusting the ratio of the nanorod diameter to the width of the nanochannels, we demonstrate the formation of two highly desired translationally ordered nanorod patterns. We further demonstrate a method to transfer the aligned nanorods into a polymer matrix which exhibits anisotropic optical properties, allowing for rapid fabrication and deployment of flexible optical and electronic materials in future nanoscale devices.

Grafted polymer chains suppress nanoparticle diffusion in athermal polymer melts.

We measure the center-of-mass diffusion of poly(methyl methacrylate) (PMMA)-grafted nanoparticles (NPs) in unentangled to slightly entangled PMMA melts using Rutherford backscattering spectrometry. These grafted NPs diffuse ∼100 times slower than predicted by the Stokes-Einstein relation assuming a viscosity equal to bulk PMMA and a hydrodynamic NP size equal to the NP core diameter, 2Rcore = 4.3 nm. This slow NP diffusion is consistent with an increased effective NP size, 2Reff ≈ 20 nm, nominally independent of the range of grafting density and matrix molecular weights explored in this study. Comparing these experimental results to a modified Daoud-Cotton scaling estimate for the brush thickness as well as dynamic mean field simulations of polymer-grafted NPs in athermal polymer melts, we find that 2Reff is in quantitative agreement with the size of the NP core plus the extended grafted chains. Our results suggest that grafted polymer chains of moderate molecular weight and grafting density may alter the NP diffusion mechanism in polymer melts, primarily by increasing the NP effective size.

Semibatch monomer addition as a general method to tune and enhance the mechanics of polymer networks via loop-defect control.

Controlling the molecular structure of amorphous cross-linked polymeric materials is a longstanding challenge. Herein, we disclose a general strategy for precise tuning of loop defects in covalent polymer gel networks. This "loop control" is achieved through a simple semibatch monomer addition protocol that can be applied to a broad range of network-forming reactions. By controlling loop defects, we demonstrate that with the same set of material precursors it is possible to tune and in several cases substantially improve network connectivity and mechanical properties (e.g., ∼600% increase in shear storage modulus). We believe that the concept of loop control via continuous reagent addition could find broad application in the synthesis of academically and industrially important cross-linked polymeric materials, such as resins and gels.

Polymer Structure and Conformation Alter the Antigenicity of Virus-like Particle-Polymer Conjugates.

Covalent conjugation of water-soluble polymers to proteins is critical for evading immune surveillance in the field of biopharmaceuticals. The most common and long-standing polymer modification is the attachment of methoxypoly(ethylene glycol) (mPEG), termed PEGylation, which has led to several clinically approved pharmaceuticals. Recent data indicate that brush-type polymers significantly enhance in vitro and in vivo properties. Herein, the polymer conformation of poly(ethylene glycol) is detailed and compared with those of water-soluble polyacrylate and polynorbornene (PNB) when attached to icosahedral virus-like particles. Small-angle neutron scattering reveals vastly different polymer conformations of the multivalent conjugates. Immune recognition of conjugated particles was evaluated versus PEGylated particles, and PNB conjugation demonstrated the most effective shielding from antibody recognition.

Miscanthus Giganteus: A commercially viable sustainable source of cellulose nanocrystals.

With a goal of identifying a new scalable source for cellulose nanocrystals (CNCs), we successfully isolated CNCs from a sustainable, non-invasive grass, Miscanthus x. Giganteus (MxG). Subjecting MxG stalks to base hydrolysis, bleaching and acid hydrolysis allowed access to cellulose nanocrystals (MxG-CNC) in high yields. X-ray diffraction studies showed the crystallinity of the MxG-CNCs increased with subsequent treatment conditions (>90% after HCl hydrolysis). Transmission electron microscopy showed that the MxG-CNC exhibit relatively high aspect ratios (60-70), and small angle neutron scattering showed the crystals were ribbon-like with a width and thickness of 8.5 and 2.8nm respectively. As expected, thermomechanical analysis of nanocomposites fabricated with carboxylic acid functionalized MxG-CNC (MxG-CNC-COOH) and PVAc showed an increase in modulus (above Tg) as filler content was increased. Comparing the properties to PVAc nanocomposites containing CNCs from wood showed at least as good, if not slightly better, reinforcement at the same loading level.

Highly branched and loop-rich gels via formation of metal-organic cages linked by polymers.

Gels formed via metal-ligand coordination typically have very low branch functionality, f, as they consist of ∼2-3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal-organic cages (MOCs) as junctions. The resulting 'polyMOC' gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

Polymer-mediated nanorod self-assembly predicted by dissipative particle dynamics simulations.

Self-assembly of nanoparticles in polymer matrices is an interesting and growing subject in the field of nanoscience and technology. We report herein on modelling studies of the self-assembly and phase behavior of nanorods in a homopolymer matrix, with the specific goal of evaluating the role of deterministic entropic and enthalpic factors that control the aggregation/dispersion in such systems. Grafting polymer brushes from the nanorods is one approach to control/impact their self-assembly capabilities within a polymer matrix. From an energetic point of view, miscible interactions between the brush and the matrix are required for achieving a better dispersibility; however, grafting density and brush length are the two important parameters in dictating the morphology. Unlike in previous computational studies, the present Dissipative Particle Dynamics (DPD) simulation framework is able to both predict dispersion or aggregation of nanorods and determine the self-assembled structure, allowing for the determination of a phase diagram, which takes all of these factors into account. Three types of morphologies are predicted: dispersion, aggregation and partial aggregation. Moreover, favorable enthalpic interactions between the brush and the matrix are found to be essential for expanding the window for achieving a well-dispersed morphology. A three-dimensional phase diagram is mapped on which all the afore-mentioned parameters are taken into account. Additionally, in the case of immiscibility between brushes and the matrix, simulations predict the formation of some new and tunable structures.

Probing the Structure, Composition, and Spatial Distribution of Ligands on Gold Nanorods.

The structure and size of ligands attached to the surfaces of gold nanorods, such as adsorbed surfactants or grafted polymers, are important considerations that facilitate the use of such nanoparticles in the human body, in advanced materials for energy harvesting, or in devices for single molecule detection. Here, we report small-angle neutron scattering (SANS) measurements from surfactant or poly(ethylene glycol) (PEG) coated gold nanorods in solution, which quantitatively determine the location, structure, and composition of these surface layers. In addition, by synthesizing gold nanorods using seed crystals which are coated with deuterated cetyltrimethylammonium bromide (dCTAB), we are able to exploit the isotopic sensitivity of SANS to study, for the first time, the retention of surfactant from the seed crystals to the final gold nanorod product, finding that very little exchange of the deuterated with hydrogenated surfactant occurs. Finally, we demonstrate that, when Au NRs are PEGylated using standard techniques, the surfactant bilayer remains intact, and while mass spectrometry detects the presence of both surfactant and PEG, the composition as measured from SANS is predominantly that of the surfactant. These measurements not only provide new insight into the synthesis and functionalization of gold nanorods but provide a quantitative picture of the structure of grafted polymer and surfactant layers on gold nanorod surfaces which has implications for the fabrication of plasmonic and biomedical materials.