Statistics of Gaussian polymer chains in harmonic applied fields.

The model of an ideal polymer chain in a harmonic applied field has broad applicability in situations involving polymer confinement and deformation due to applied stress. In this work we (1) formulate a general analytical model for a continuous Gaussian chain under a harmonic applied potential and (2) evaluate the statistical mechanics of this model given the potential, obtaining partition functions and moment generating functions (MGFs) that describe the chain configurations. Closed-form expressions for the squared radius of gyration, potential energy, partition function, and MGF for the center of mass are obtained for a general and multidimensional harmonic field. The expressions are compared with results of Monte Carlo simulations of a discrete Gaussian chain as well as results for related systems obtained from the literature. The theory derived here is used to test the applicability of the current model assumptions to relations from the literature describing polymer confinement and deformation in experiment, theory, and simulations.

Molecular simulation of flow-enhanced nucleation of polyethylene crystallites in biaxial flows.

Flow-enhanced nucleation (FEN) of n-pentacontahectane (C150) under biaxial extensional flows of varying strain rate ratios is studied using nonequilibrium molecular dynamics simulation. The nucleation rates thus calculated are used to test previously published FEN models based on invariants of the conformation tensor of Kuhn segments and the extra stress tensor. Models based on the conformation tensor provide a more accurate description of FEN observed in biaxial flow simulations than those based on the extra stress tensor. In addition, the formation of nematic domains previously reported to be stabilized by shear or extensional flow is absent in equibiaxial flows. However, such domains do form in non-equibiaxial flows, and nucleation occurs in these domains preferentially. The shape and orientation of nuclei formed under biaxial flows of various strengths and strain rate ratios are also reported.

Mechanisms of Shock Dissipation in Semicrystalline Polyethylene.

Semicrystalline polymers are lightweight, multiphase materials that exhibit attractive shock dissipation characteristics and have potential applications as protective armor for people and equipment. For shocks of 10 GPa or less, we analyzed various mechanisms for the storage and dissipation of shock wave energy in a realistic, united atom (UA) model of semicrystalline polyethylene. Systems characterized by different levels of crystallinity were simulated using equilibrium molecular dynamics with a Hugoniostat to ensure that the resulting states conform to the Rankine-Hugoniot conditions. To determine the role of structural rearrangements, order parameters and configuration time series were collected during the course of the shock simulations. We conclude that the major mechanisms responsible for the storage and dissipation of shock energy in semicrystalline polyethylene are those associated with plastic deformation and melting of the crystalline domain. For this UA model, plastic deformation occurs primarily through fine crystallographic slip and the formation of kink bands, whose long period decreases with increasing shock pressure.

Three-Dimensional Imaging of Emulsion Separation through Liquid-Infused Membranes Using Confocal Laser Scanning Microscopy.

The removal of emulsified oils from water has always been a challenge due to the kinetic stability resulting from the small droplet size and the presence of stabilizing agents. Membrane technology can treat such mixtures, but fouling of the membrane leads to dramatic reductions in the process capacity. Liquid-infused membranes (LIMs) can potentially resolve the issue of fouling. However, their low permeate flux compared with conventional hydrophilic membranes remains a limitation. To gain insight into the mechanism of transport, we use 3D images acquired by confocal laser scanning microscopy (CLSM) to reconstruct the sequence of events occurring during startup and operation of the LIM for removal of dispersed oil from oil-in-water emulsions. We find evidence for coalescence of oil droplets on the surface of and formation of oil channels within the LIM. Using image analysis, we find that the rate at which oil channels are formed within the membrane and the number of channels ultimately govern the permeate flux of oil through the LIMs. Oil concentration in the feed affects the rate of coalescence of oil on the surface of the LIM, which, in turn, affects the channel formation dynamics. The channel formation dynamics also depend on the viscosity of the infused liquid and the operating pressure. A higher affinity to the pore wall for infused liquid than permeating liquid is essential to antifouling behavior. Overall, this work offers insight into the selective permeation of a dispersed liquid phase through a LIM.

Experiments and Modeling of Flow-Enhanced Nucleation in LLDPE.

A computational and experimental framework for quantifying flow-enhanced nucleation (FEN) in polymers is presented and demonstrated for an industrial-grade linear low-density polyethylene (LLDPE). Experimentally, kinetic measurements of isothermal crystallization were performed by using fast-scanning calorimetry (FSC) for melts that were presheared at various strain rates. The effect of shear on the average conformation tensor of the melt was modeled with the discrete slip-link model (DSM). The conformation tensor was then related to the acceleration in nucleation kinetics by using an expression previously validated with nonequilibrium molecular dynamics (NEMD). The expression is based on the nematic order tensor of Kuhn segments, which can be obtained from the conformation tensor of entanglement strands. The single adjustable parameter of the model was determined by fitting to the experimental FSC data. This expression accurately describes FEN for the LLDPE, representing a significant advancement toward the development of a fully integrated processing model for crystallizable polymers.

Electrospun Liquid-Infused Membranes for Emulsified Oil/Water Separation.

From an environmental perspective, microfiltration membranes are attractive for the separation of emulsified oils from contaminated water. However, fouling of the membrane is a major drawback of the technology. "Liquid-infused membranes" (LIMs) have the potential to eliminate membrane fouling. Here, we demonstrate the practical application of LIMs for the separation of oil from a stable oil-in-water emulsion and characterize their resistance to fouling. The base membrane is an electrospun nonwoven fibrous layer of the fluorinated copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP). The surface energy of the PVDF-co-HFP fibers was lowered by the covalent attachment of a fluorinated silane (PFOCTS), and then, the membrane was infused with a perfluoropolyether. The membrane was then challenged with model emulsions of dodecane in water in a cross-flow configuration. This PFOCTS-modified LIM showed better infused liquid stability, permeation selectivity, higher permeate flux than the unmodified LIM, and better anti-fouling properties than the bare membrane without infused liquid. We also examine the mechanism for transport of the dispersed oil phase through the liquid-infused membrane. We find a linear relationship between the dodecane flux and dodecane concentration in the feed and a higher dodecane flux through the PFOCTS-modified membrane than the unmodified one, which suggests that the capture of dodecane droplets from the feed plays an important role in determining the overall rate of permeation. Other factors such as lower viscosity of the infused liquid, larger pore size, and higher operating pressure also improved the permeate flux through the LIMs. Overall, this work provides some guidelines on the design of composite membranes comprising infused liquids and the choice of operating conditions for the filtration process.

Shape-Stable Composites of Electrospun Nonwoven Mats and Shear-Thickening Fluids.

To improve the flexibility of the fabric stacks used in protective clothing, shear-thickening fluids (STFs) have previously been incorporated into woven microfiber fabrics to enhance their impact resistance. However, the microfiber-STF composites can exhibit loss of the STF from the composite over time due to the large interstitial spaces between fibers, resulting in limited long-term shape stability. In this study, nonwoven mats of electrospun ultrafine fibers (UFFs) were used in place of woven microfiber fabrics to improve the STF retention within the fiber-STF composites by taking advantage of high specific surface area, small pore size, and large capillary force. The UFF-STF composite, comprising an electrospun polyamide (PA 6,6) UFF mat and a fumed silica (FS) STF, exhibited excellent shape stability with high breakthrough pressure and improved STF retention compared to composites based on conventional microfiber fabrics. The mechanical response of the composite is shown to depend on the rate of deformation. At strain rates lower than the shear-thickening threshold of the STF, the introduction of STF resulted in no stiffening or strengthening of fiber mats, allowing the composite to remain flexible. At high deformation rates above the onset of shear thickening, the incorporation of STF improved both the elasticity and the viscosity of the material. In addition, the shape stability and the mechanical properties of the composite were influenced by the STF viscosity and the UFF morphology. STF with high particle loading and UFF with small fiber diameter resulted in a more pronounced enhancement to membrane performance.

Conductive, Acid-Doped Polyaniline Electrospun Nanofiber Gas Sensing Substrates Made Using a Facile Dissolution Method.

A novel dissolution method that allows for the total solvation of high-concentration, high-molecular-weight polyaniline (PANi) doped with (+)-camphor-10-sulfonic acid (CSA) is reported. Preparation of 12-16 wt % 65,000 Da PANi solutions in N,N-dimethylformamide is achievable using a simple one-pot method. Doped polyaniline solutions in common organic solvents were processed into nanofibers using a convenient single-nozzle electrospinning technique. The electrospinning of PANi-CSA into nanofibrous membranes generated substrates that were subsequently employed in colorimetric gas sensing. These substrates demonstrated linearity of response upon exposure to 50-5500 ppm ammonia at ambient (50 ± 10% RH) and high (80% RH) humidity.

Competitive Wetting: A New Approach to Prevent Liquid Penetration through Porous Materials with Superior Synergistic Effect.

Blocking liquid penetration in porous materials is a key function for several applications including chemical protective clothing (CPC), wound healing, and hygiene products. Enormous efforts are made to prevent liquid penetration through porous media by the modification of materials. CPC is used as an example to demonstrate the effect of the synergistic effect on liquid penetration. A common strategy to achieve liquid protection is the use of liquid-repellent surfaces with the aid of a liquid absorption liner layer. However, this strategy demonstrates limited success for low surface energy liquids. Herein, a novel approach is reported to prevent the permeation of liquid across porous materials by a synergistic effect. Both fabrics are individually susceptible to be wetted by low surface tension liquids. However, when they are assembled, they can prevent low surface tension liquids from penetrating because of the wettability gap between the two fabrics. The fabric assembly demonstrates an increase in the liquid prevention capacity by 70-1000 times compared with a commercial CPC material. This novel synergistic effect may offer a breakthrough in the development of various applications including protective clothing baby nappies, hygiene products, food preparation, soil water retention, and sporting/camping/ski equipment and clothing.

Examination of Nanoparticle Filtration by Filtering Facepiece Respirators During the COVID-19 Pandemic.

The onset of the COVID-19 pandemic in spring 2020 resulted in a spike in the demand for face masks and respirators. Due to their effectiveness at filtering aerosols that could potentially contain viruses, the N95-type filtering facepiece respirators (FFRs) are frequently used by healthcare workers and first responders. However, due to a shortage of domestic N95 FFRs in the US at the beginning of the pandemic, internationally produced respirators were imported and deployed under an Emergency Use Authorization by the Food and Drug Administration. Due to concerns raised at the time, there was an urgent need to verify their effectiveness and usability. In this study, we summarize our characterization of the nanoparticulate filtration performances of 136 such respirators, measured between April 1 and June 30, 2020. Our results indicate that about 42% of the respirators showed filtration efficiencies better than 90% (≤10% penetration), but only 17% performed better than 95% (≤5% penetration). On the other hand, about 35% showed filtration efficiencies below 80% (≥20% penetration). A representative subset of devices was analyzed for the origin of such variations in filtration performance. We found that filtration efficiency increased with the level of electrostatic charge on the FFRs and that the poor performance of the internationally sourced FFRs could be traced to a lack of electrostatic filtration mechanisms. Furthermore, electrostatics shifted the particle size at which aerosol penetration through the FFR was maximal from around 200 nm to less than 100 nm for the highest-performing FFRs, a size range that largely goes undetected in standardized tests.

Rheology of Crystallizing LLDPE.

Polymer crystallization occurs in many plastic manufacturing processes, from injection molding to film blowing. Linear low-density polyethylene (LLDPE) is one of the most commonly processed polymers, wherein the type and extent of short-chain branching (SCB) may be varied to influence crystallization. In this work, we report simultaneous measurements of the rheology and Raman spectra, using a Rheo-Raman microscope, for two industrial-grade LLDPEs undergoing crystallization. These polymers are characterized by broad polydispersity, SCB and the presence of polymer chain entanglements. The rheological behavior of these entangled LLDPE melts is modeled as a function of crystallinity using a slip-link model. The partially crystallized melt is represented by a blend of linear chains with either free or crosslinked ends, wherein the crosslinks represent attachment to growing crystallites, and a modulus shift factor that increases with degree of crystallinity. In contrast to our previous application of the slip-link model to isotactic polypropylene (iPP), in which the introduction of only bridging segments with crosslinks at both ends was sufficient to describe the available data, for these LLDPEs we find it necessary to introduce dangling segments, with crosslinks at only one end. The model captures quantitatively the evolution of viscosity and elasticity with crystallization over the whole range of frequencies in the linear regime for two LLDPE grades.

Spectroscopic analysis in molecular simulations with discretized Wiener-Khinchin theorem for Fourier-Laplace transformation.

The Wiener-Khinchin theorem for the Fourier-Laplace transformation (WKT-FLT) provides a robust method to obtain the single-side Fourier transforms of arbitrary time-domain relaxation functions (or autocorrelation functions). Moreover, by combining an on-the-fly algorithm with the WKT-FLT, the numerical calculations of various complex spectroscopic data in a wide frequency range become significantly more efficient. However, the discretized WKT-FLT equation, obtained simply by replacing the integrations with the discrete summations, always produces two artifacts in the frequency-domain relaxation function. In addition, the artifacts become more apparent in the frequency-domain response function converted from the relaxation function. We find the sources of these artifacts that are associated with the discretization of the WKT-FLT equation. Taking these sources into account, we derive discretized WKT-FLT equations designated for both the frequency-domain relaxation and response functions with the artifacts removed. The use of the discretized WKT-FLT equations with the on-the-fly algorithm is illustrated by a flow chart. We also give application examples for the wave-vector-dependent dynamic susceptibility in an isotropic amorphous polyethylene and the frequency-domain response functions of the orientation vectors in an n-alkane crystal.

Direct Three-Dimensional Visualization of Membrane Fouling by Confocal Laser Scanning Microscopy.

Membrane-based separation is an important technique for removing emulsified oil from water. However, the mechanisms of fouling are complex because of the deformability and potential for coalescence and break-up of the oil droplets. Here, we report for the first time direct, three-dimensional (3D) visualization of oil droplets on electrospun fiber microfiltration membranes after a period of membrane-based separation of oil-in-water emulsions. High-resolution 3D images were acquired by a dual-channel confocal laser scanning microscopy (CLSM) technique in which both the fibers and the oil (dodecane) were fluorescently labeled. The morphology of dodecane as the foulant was observed for two different types of fibers, an oleophobic nylon (PA6(3)T), and oleophilic polyvinylidene fluoride (PVDF). Through direct visualization, the rejected oil was found to form droplets of clam-shell shape on the PA6(3)T fibers, whereas the oil tended to wet the PVDF fibers and spread across the membrane. The morphology was also analyzed as a function of separation time (i.e., "4D" imaging), as the oil accumulated within and upon the membranes. The observations are qualitatively consistent with a transition from blocking of individual pores in the membrane to coalescence of oil droplets into coherent liquid films with increasing filtration time. Analysis of permeate flux using blocking filtration models corroborate the transition of fouling modes indicated by the 3D images. This direct, 3D visualization CLSM technique is a powerful tool for characterizing the mechanisms of fouling in membranes used for liquid emulsion separations.

Empirical potential for molecular simulation of graphene nanoplatelets.

A new empirical potential for layered graphitic materials is reported. Interatomic interactions within a single graphene sheet are modeled using a Stillinger-Weber potential. Interatomic interactions between atoms in different sheets of graphene in the nanoplatelet are modeled using a Lennard-Jones interaction potential. The potential is validated by comparing molecular dynamics simulations of tensile deformation with the reported elastic constants for graphite. The graphite is found to fracture into graphene nanoplatelets when subjected to ∼15% tensile strain normal to the basal surface of the graphene stack, with an ultimate stress of 2.0 GPa and toughness of 0.33 GPa. This force field is useful to model molecular interactions in an important class of composite systems comprising 2D materials like graphene and multi-layer graphene nanoplatelets.

Magnet-responsive, superhydrophobic fabrics from waterborne, fluoride-free coatings.

In this study, durable superhydrophobic fabrics with magnet responsive properties were prepared by a two-step coating technique using polydopamine (PDA), Fe3O4 nanoparticles, and hexadecyltrimethoxysilane as coating materials. The coated fabrics exhibit fast magnetic responsivity and a water contact angle of 156°. The coating is durable enough to withstand at least 50 cycles of home laundering and 500 cycles of Martindale abrasions without losing its superhydrophobicity and magnetic properties. The PDA pre-coating plays a significant role in improving the adhesion of hydrophobic Fe3O4 nanoparticles on fabric surface. The coated fabric is highly oleophilic (oil contact angle = 0°). When used for absorbing oil, the coated fabric floats naturally on the surface of oily water, and it can be moved to approach oil drops under magnetic actuation. The fabric is reusable for at least 10 cycles. This may offer an environmentally friendly way to prepare "smart" oil-recovery materials.

Heterogeneous Nucleation of an n-Alkane on Tetrahedrally Coordinated Crystals.

Heterogeneous nucleation refers to the propensity for phase transformations to initiate preferentially on foreign surfaces, such as vessel walls, dust particles, or formulation additives. In crystallization, the form of the initial nucleus has ramifications for the crystallographic form, morphology, and properties of the resulting solid. Nevertheless, the discovery and design of nucleating agents remains a matter of trial and error because of the very small spatiotemporal scales over which the critical nucleus is formed and the extreme difficulty of examining such events empirically. Using molecular dynamics simulations, we demonstrate a method for the rapid screening of entire families of materials for activity as nucleating agents and for characterizing their mechanism of action. The method is applied to the crystallization of n-pentacontane, a model surrogate for polyethylene, on the family of tetrahedrally coordinated crystals, including diamond and silicon. A systematic variation of parameters in the interaction potential permits a comprehensive, physically based screening of nucleating agents in this class of materials, including both real and hypothetical candidates. The induction time for heterogeneous nucleation is shown to depend strongly on crystallographic registry between the nucleating agent and the critical nucleus, indicative of an epitaxial mechanism in this class of materials. Importantly, the severity of this registry requirement weakens with decreasing rigidity of the substrate and increasing strength of attraction to the surface of the nucleating agent. Employing this method, a high-throughput computational screening of nucleating agents becomes possible, facilitating the discovery of novel nucleating agents within a broad "materials genome" of possible additives.

Analysis of nucleation using mean first-passage time data from molecular dynamics simulation.

We introduce a method for the analysis of nucleation using mean first-passage time (MFPT) statistics obtained by molecular dynamics simulation. The method is based on the Becker-Döring model for the dynamics of a nucleation-mediated phase change and rigorously accounts for the system size dependence of first-passage statistics. It is thus suitable for the analysis of systems in which the separation between time scales for nucleation and growth is small, due to either a small free energy barrier or a large system size. The method is made computationally practical by an approximation of the first-passage time distribution based on its cumulant expansion. Using this approximation, the MFPT of the model can be fit to data from molecular dynamics simulation in order to estimate valuable kinetic parameters, including the free energy barrier, critical nucleus size, and monomer attachment pre-factor, as well as the steady-state rates of nucleation and growth. The method is demonstrated using a case study on nucleation of n-eicosane crystals from the melt. For this system, we found that the observed distribution of first-passage times do not follow an exponential distribution at short times, rendering it incompatible with the assumptions made by some other methods. Using our method, the observed distribution of first-passage times was accurately described, and reasonable estimates for the kinetic parameters and steady-state rates of nucleation and growth were obtained.

WO3 Nanofiber-Based Biomarker Detectors Enabled by Protein-Encapsulated Catalyst Self-Assembled on Polystyrene Colloid Templates.

A novel catalyst functionalization method, based on protein-encapsulated metallic nanoparticles (NPs) and their self-assembly on polystyrene (PS) colloid templates, is used to form catalyst-loaded porous WO3 nanofibers (NFs). The metallic NPs, composed of Au, Pd, or Pt, are encapsulated within a protein cage, i.e., apoferritin, to form unagglomerated monodispersed particles with diameters of less than 5 nm. The catalytic NPs maintain their nanoscale size, even following high-temperature heat-treatment during synthesis, which is attributed to the discrete self-assembly of NPs on PS colloid templates. In addition, the PS templates generate open pores on the electrospun WO3 NFs, facilitating gas molecule transport into the sensing layers and promoting active surface reactions. As a result, the Au and Pd NP-loaded porous WO3 NFs show superior sensitivity toward hydrogen sulfide, as evidenced by responses (R(air)/R(gas)) of 11.1 and 43.5 at 350 °C, respectively. These responses represent 1.8- and 7.1-fold improvements compared to that of dense WO3 NFs (R(air)/R(gas) = 6.1). Moreover, Pt NP-loaded porous WO3 NFs exhibit high acetone sensitivity with response of 28.9. These results demonstrate a novel catalyst loading method, in which small NPs are well-dispersed within the pores of WO3 NFs, that is applicable to high sensitivity breath sensors.

Molecular simulation of flow-enhanced nucleation in n-eicosane melts under steady shear and uniaxial extension.

Non-equilibrium molecular dynamics is used to study crystal nucleation of n-eicosane under planar shear and, for the first time, uniaxial extension. A method of analysis based on the mean first-passage time is applied to the simulation results in order to determine the effect of the applied flow field type and strain rate on the steady-state nucleation rate and a characteristic growth rate, as well as the effects on kinetic parameters associated with nucleation: the free energy barrier, critical nucleus size, and monomer attachment pre-factor. The onset of flow-enhanced nucleation (FEN) occurs at a smaller critical strain rate in extension as compared to shear. For strain rates larger than the critical rate, a rapid increase in the nucleation rate is accompanied by decreases in the free energy barrier and critical nucleus size, as well as an increase in chain extension. These observations accord with a mechanism in which FEN is caused by an increase in the driving force for crystallization due to flow-induced entropy reduction. At high applied strain rates, the free energy barrier, critical nucleus size, and degree of stretching saturate, while the monomer attachment pre-factor and degree of orientational order increase steadily. This trend is indicative of a significant diffusive contribution to the nucleation rate under intense flows that is correlated with the degree of global orientational order in a nucleating system. Both flow fields give similar results for all kinetic quantities with respect to the reduced strain rate, which we define as the ratio of the applied strain rate to the critical rate. The characteristic growth rate increases with increasing strain rate, and shows a correspondence with the nucleation rate that does not depend on the type of flow field applied. Additionally, a structural analysis of the crystalline clusters indicates that the flow field suppresses the compaction and crystalline ordering of clusters, leading to the formation of large articulated clusters under strong flow fields, and compact well-ordered clusters under weak flow fields.

Advances in electrospun carbon fiber-based electrochemical sensing platforms for bioanalytical applications.

Electrochemical sensing is an efficient and inexpensive method for detection of a range of chemicals of biological, clinical, and environmental interest. Carbon materials-based electrodes are commonly employed for the development of electrochemical sensors because of their low cost, biocompatibility, and facile electron transfer kinetics. Electrospun carbon fibers (ECFs), prepared by electrospinning of a polymeric precursor and subsequent thermal treatment, have emerged as promising carbon systems for biosensing applications since the electrochemical properties of these carbon fibers can be easily modified by processing conditions and post-treatment. This review addresses recent progress in the use of ECFs for sensor fabrication and analyte detection. We focus on the modification strategies of ECFs and identification of the key components that impart the bioelectroanalytical activities, and point out the future challenges that must be addressed in order to advance the fundamental understanding of the ECF electrochemistry and to realize the practical applications of ECF-based sensing devices.