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An augmented Bayesian optimization approach is presented for materials discovery with noisy and unreliable measurements.

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 resistivity upon exposure to 50-5500 ppm ammonia at ambient (50 ± 10% RH) and high (80% RH) humidity.

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.

© 2020 The Society of Rheology. 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 cross-linked ends, wherein the cross-links represent attachment to growing crystallites, and a modulus shift factor that increases with the degree of crystallinity. In contrast to our previous application of the slip-link model to isotactic polypropylene, in which the introduction of only bridging segments with cross-links at both ends was sufficient to describe the available data, for these LLDPEs, we find it necessary to introduce dangling segments, with cross-links 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 the two LLDPE grades.

The effects of interphase topology, entanglements, and chain dynamics on the mechanical response of semicrystalline polyethylene have been examined using atomistic simulations. In particular, the prevalence of the cavitation and melting/recrystallization mechanisms for yield and plastic flow were found to depend on both topological and dynamical properties of the molecular segments in the semicrystalline interphase. First, two different protocols were used during preparation of the interphase ensemble to modulate the distribution of (i) loops, bridges, and tails and (ii) entanglements within the noncrystalline domain. A protocol denoted “step-wise cooling” produced structures having a large fraction of long, entangled segments that yielded by the melting/recrystallization mechanism about 50% of the time. By contrast, the protocol denoted “instantaneous quench” produced structures that yielded by melting/recrystallization about 73% of the time. Second, two different united atom force fields, PYS and TraPPE-UA, that exhibit nearly identical topological characteristics of the noncrystalline domain but different mobilities were used to study the effect of chain dynamics on yield mechanisms. At the slower strain rate used in this work, yield and plastic flow proceeded exclusively via cavitation for the model using the TraPPE-UA force field, whereas both cavitation and melting/recrystallization were observed for the model using the PYS force field. The greater prevalence of melting/recrystallization in the latter case is attributed to faster chain-sliding dynamics in the crystalline domain. The dependences of the yield mechanism on topology and dynamics are found to be related. ISSN:1520-5835 ISSN:0024-9297

Many industrial polymer melts are entangled and undergo crystallization during processing. Several recent studies have reported experimental data for the rheology of crystallizing entangled polymers. Meanwhile, over the past few years, slip-link models have been demonstrated to describe the rheology of entangled melts under a variety of nonlinear deformation conditions. In this work, we present a modification of the slip-link model to describe the rheology of an entangled melt undergoing crystallization. The partially crystallized melt is represented by a blend of linear chains with free ends and cross-linked, bridgelike chains with fixed ends that resemble the tie molecules between developing crystallites. Two new parameters are introduced: the fraction of cross-linked chains and a modulus shift factor, both of which are functions of the degree of crystallinity. The model captures the evolution of viscosity and elasticity simultaneously over the whole range of available frequencies in the linear regime. The model is validated using experimental datasets for isotactic polypropylene.Many industrial polymer melts are entangled and undergo crystallization during processing. Several recent studies have reported experimental data for the rheology of crystallizing entangled polymers. Meanwhile, over the past few years, slip-link models have been demonstrated to describe the rheology of entangled melts under a variety of nonlinear deformation conditions. In this work, we present a modification of the slip-link model to describe the rheology of an entangled melt undergoing crystallization. The partially crystallized melt is represented by a blend of linear chains with free ends and cross-linked, bridgelike chains with fixed ends that resemble the tie molecules between developing crystallites. Two new parameters are introduced: the fraction of cross-linked chains and a modulus shift factor, both of which are functions of the degree of crystallinity. The model captures the evolution of viscosity and elasticity simultaneously over the whole range of available frequencies in the linear regime. ...

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.

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.

Flow-enhanced nucleation of the crystal phase under shear and uniaxial extension for a monodisperse melt of n-pentacontahectane (C150H302 or C150) chains was studied by nonequilibrium molecular dynamics simulation. The resulting acceleration in the crystal nucleation rate was correlated with macroscopically measurable properties of the flow field and with microscopic conformational statistics. Based on the fidelity of the observed correlations, several empirical models reported in the literature were evaluated for their abilities to account for the observed enhancement of the nucleation rate due to flow, and new models are proposed for data that do not comport with existing models. In agreement with prior reports, the nucleation rate was found to correlate well with first-normal stress difference, the second invariant of the deviatoric conformation tensor, and the stretch ratio, albeit with some differences from the existing models. New models based on conformational invariants for Kuhn segments are proposed and shown to describe the simulation data more accurately than those based on conformational behavior of entire chains. Within the applicability of the stress-optical rule, related models are proposed based on invariants of the extra stress tensor.Flow-enhanced nucleation of the crystal phase under shear and uniaxial extension for a monodisperse melt of n-pentacontahectane (C150H302 or C150) chains was studied by nonequilibrium molecular dynamics simulation. The resulting acceleration in the crystal nucleation rate was correlated with macroscopically measurable properties of the flow field and with microscopic conformational statistics. Based on the fidelity of the observed correlations, several empirical models reported in the literature were evaluated for their abilities to account for the observed enhancement of the nucleation rate due to flow, and new models are proposed for data that do not comport with existing models. In agreement with prior reports, the nucleation rate was found to correlate well with first-normal stress difference, the second invariant of the deviatoric conformation tensor, and the stretch ratio, albeit with some differences from the existing models. New models based on conformational invariants for Kuhn segments are propo...

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.

Electrospun mats with targeted wettability are a way forward to diversify their mainstream applications and can facilitate in creating a platform for a remarkable set of properties. Limited control over the fiber morphology and surface chemistry, along with the complex architecture of electrospun mats, pose some of the key challenges for maneuvering the wettability. Herein, we present the design principles that underpin the key determinants responsible for a metastable state of a water droplet on electrospun mats possessing wrinkled surface topography. A ‘first-of-its-kind’ analytical model of apparent contact angle based on the modified Cassie-Baxter state has been proposed that accounts for the dual-scale roughness originating from fiber wrinkles and protuberances created by fiber–fiber contacts. Three-dimensional (3D) fiber and structural parameters obtained via X-ray microcomputed tomography (microCT) analysis served as key inputs for predictive modeling. The analytical model of the apparent advancing and receding contact angles has been validated with electrospun mats having well-defined orientation characteristics. In general, the theory and experiments are in reasonable agreement. Although similar magnitudes of static and advancing contact angles of water on surfaces of randomly and aligned electrospun mats have been experimentally observed, the receding contact angles differed significantly.

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.

The compositions of oil-in-water emulsions encountered in industrial processes or in the environment vary widely in the nature of surfactants that stabilize them. This variety creates challenges for applications of membrane separation. Electrospun fiber membranes have shown high permeability and improved robustness against fouling in emulsion separation, but the interaction between emulsions and the membrane, and the fouling mechanism, remains unclear. In this paper, electrospun polyamide membranes were challenged by model emulsions of dodecane stabilized by anionic, cationic, non-ionic and zwitterionic surfactants in both dead-end and cross-flow filtration configurations under constant pressure of 2 psi. The membrane was shown to have high oil rejection (92.7 ± 1.5%), sufficient to meet EPA's regulatory limit, when separating emulsions stabilized by anionic surfactant in cross-flow filtration, while maintaining a steady flux of 44.3 ± 2.6 LMH. Analysis of permeate flux and oil rejection revealed that the types of surfactants influenced the membrane fouling in both dead-end and cross-flow systems, but in different ways. Fouling in dead-end filtration was found to be a function of the electrostatic interactions between the oil droplets and the membrane, while fouling in cross-flow filtration was mainly determined by the hydrophilic/hydrophobic interactions due to the adsorption of surfactants at the interfaces. Blocking filtration models are used to corroborate these findings and illustrate the transition between modes of fouling in dead-end filtration. A de-fouling process was found when separating emulsions stabilized by cationic surfactant in dead-end filtration, which was attributed to coalescence of oil droplets at the membrane surface, based on a kinetic model and direct observation. These results indicate that not only membrane-foulant but foulant-foulant interactions can influence the membrane fouling.

Nucleating agents are materials that enhance crystallization through their role in heterogeneous nucleation. They are frequently used to control kinetics and morphology in polymer crystallization. However, selection and design of nucleating agents is hindered by a lack of detailed, atomistic level information about the relationship between physicochemical properties and heterogeneous nucleation activity. We examined heterogeneous nucleation of n-pentacontane – a model surrogate for polyethylene – induced by graphene, and the family of 2D hexagonally coordinated nanoplatelet materials to which it belongs, using molecular dynamics simulation. High throughput computational screening was performed by systematic variation of the parameters that define the 2D nanoplatelet. Nucleation activity is characterized by induction time. These computations confirm the utility of graphene as a nucleating agent for n-pentacontane and, by extension, polyethylene. We identify a set of heuristics for the design of efficient nucleating agents. These include: epitaxial matching, material compatibility as measured by adhesive interactions, and optimal rigidity. We also observed a three-fold degeneracy in the orientation of epitaxial matches between the nucleating agent and the crystallizing organic, which increased the induction time. This study advances the prospect of high-throughput computation to build a “materials genome” of nucleating agents for polymers.

Thermoplastic polyurethanes (TPUs) are useful materials for numerous applications due in part to their outstanding resilience and ability to dissipate energy under large mechanical deformation. However, the mechanistic understanding of the origins of these mechanical properties at the molecular level remains elusive, largely due to the complex, heterogeneous structure of these materials, which arises from the segregation of chemically distinct segments into hard and soft domains. In this work, molecular simulations are used to identify the mechanism of mechanical response under large tensile deformation of a common thermoplastic polyurethane comprising 4,4′-diphenylmethane diisocyanate and n-butanediol (hard segment) and poly(tetramethylene oxide) (soft segment), with atomic resolution. The simulation employs a lamellar stack model constructed using the Interphase Monte Carlo method established previously for semicrystalline polymers, which models the interfacial zone between hard and soft domains with th...

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.

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.

The separation of emulsified oils from water represents a significant challenge for water reclamation and clean-up, because such droplets remain suspended for long periods of time. Membrane technology offers a solution to such separations, but its major drawback is the requirement for frequent cleaning to remove foulants, which reduces membrane lifetime and incurs additional economic and environmental costs. To address this shortcoming, we examine the separation performance of fibrous membranes of P84 polyimide, a copolymer known for its thermal stability and chemical resistance, fabricated by electrospinning technology. The P84 membranes were challenged by surfactant-stabilized emulsions of dodecane or motor oil in a constant pressure dead-end configuration. The P84 membranes exhibited moderate oil rejections between 85 and 90%, while subsequent foulant removal by heat treatment restored permeate fluxes to essentially 100% of their original values. A comparison of P84 membranes to other common membrane polymers with respect to robustness against fouling shows that the P84 membranes have some advantages, which we attribute to the oleophobic nature of the P84 membrane, as well as the ease of membrane regeneration. These results indicate that the electrospun P84 membrane is a promising candidate for microfiltration of water contaminated by emulsified oil.

Stiff, strong and tough ultrafine polyethylene fibers that rival the best high performance fibers, but with diameters less than one micron, are fabricated for the first time by “gel-electrospinning.” In this process, solution concentration and process temperatures are chosen to induce the formation of gel filaments “in flight,” which are subsequently drawn at high rates as a consequence of the whipping instability. The resulting submicron-diameter fibers exhibited Young’s moduli of 73 ± 13 GPa, yield strengths of 3.5 ± 0.6 GPa, and toughnesses of 1.8 ± 0.3 GPa, on average. Among the smallest fibers examined, one with a diameter of 490 ± 50 nm showed a Young’s modulus of 110 ± 16 GPa, ultimate tensile strength of 6.3 ± 0.9 GPa, and toughness of 2.1 ± 0.3 GPa, a combination of mechanical properties that is unparalleled among polymer fibers to date. The correlation of stiffness, strength and toughness with fiber diameter is attributed to high crystallinity and crystallite orientation, combined with fewer defects and enhanced chain slip associated with small diameter and high specific surface area. Gel-electrospinning improves the prospects for production of such fibers at scale., United States. Army. Natick Soldier Research, Development and Engineering Center

This work addresses the large strain behaviors of thermoplastic polyurethanes (TPUs) spanning a range of fractions of hard and soft contents in both experiment and theoretical modeling. The key mechanical features involve a combination of elasticity and inelasticity, and are quantified experimentally under a broad variety of loading scenarios. A finite deformation constitutive model is then presented to capture the main features of the stress-strain data, which are strongly dependent on fractions of hard and soft contents. The stress-strain behavior of these TPUs is characterized by highly nonlinear rate-dependent hyperelastic-viscoplasticity, in which substantial energy dissipation is accompanied by shape recovery as well as softening. Agreement between the model and the experimental data for the representative TPUs provides physical insight into the underlying deformation mechanisms in this important class of soft materials that exhibit both elastomeric and plastomeric characteristics.

Thermoplastic polyurethanes constitute a versatile family of materials with a broad variety of engineering applications. However, connection between their chemical structure and mechanical properties remains elusive, in large part due to their heterogeneous nature, arising from segregation of chemically distinct segments into separate domains, with resulting complex morphologies. Using atomistic simulations, we examine the structure and mechanical properties of a common family of thermoplastic polyurethanes (TPU) comprising 4,4′-diphenylmethane diisocyanate and n-butanediol (hard segment) and poly(tetramethylene oxide) (soft segment). A lamellar stack model previously developed for the study of semicrystalline polymers is applied here for the first time to a phase-segregated copolymer. Equilibrium structure and properties were evaluated for TPUs with different ratios of hard and soft components, using a combination of Monte Carlo and molecular dynamics simulations. Stress–strain behaviors were then evalua...

The vibrational spectra of semicrystalline polyethylene and its distinct domains were investigated using molecular dynamics (MD) simulations. A method for the vibrational analysis of the domains within the lamellar stack model of semicrystalline polymers has been developed and demonstrated on semicrystalline polyethylene using force fields having either united atom (UA) or explicit atom (EA) detail. In the UA description, the calculated vibrational spectra were found to differ from the observed skeletal vibrations of polyethylene with the force field used in this work. Therefore, a modified UA force field with different stretching and bending force constants is proposed, which was found to reproduce the observed frequencies of the skeletal vibrations. In the EA description, the vibrational spectra of semicrystalline polyethylene were in satisfactory agreement with typical infrared and Raman signatures of polyethylene melts and crystals. Experimental interpretations regarding the assignment of peaks in the...

As Macromolecules celebrates its 50th anniversary, we reflect on the impact of polymer chemistry and engineering on the advancement of synthetic polymer fibers. In this Perspective, we focus on two exemplary cases: (i) high performance fibers and (ii) ultrafine electrospun fibers. High performance in this context refers to fibers like Kevlar and Spectra, which emerged as a consequence of novel chemistry and processing innovations to convert synthetic polymers into fibers with exceptional specific stiffness and strength. More recently, the development of “ultrafine” (i.e., submicrometer diameter) fibers by technologies such as electrospinning has advanced dramatically, resulting in interesting, emergent structures and properties, such as surface and internal morphologies, electrical and mechanical properties, and growth applications like tissue engineering and sensors, which are subjects of current research. In both cases, challenges and opportunities exist in the development of polymer processes to advanc...

We performed molecular dynamics simulations to investigate the effects of layer thicknesses of both crystalline and noncrystalline domains and chain tilt within the crystalline lamellae on tensile deformation mechanisms of the lamellar stack model of semicrystalline polyethylene. For equal thicknesses of crystalline and noncrystalline regions, similar stress–strain profiles were obtained with two different initial orientations of the crystal stem relative to the tensile direction. Repeated melting/recrystallization transitions were observed, at the slower strain rate of 5 × 106 s–1, characterized by oscillating stress–strain profiles. With increasing thickness of the crystalline regions, these oscillations occurred less frequently. For systems with initially tilted chain stems in the crystalline domain, decreasing the thickness of the noncrystalline region increased the number of short bridge segments in the noncrystalline region connecting the two crystalline regions and induced significant shear stresse...

We use molecular simulations with a united atom force field to examine the effect of short chain branching (SCB) on the noncrystalline, interlamellar structure typical of linear low density polyethylene (LLDPE). The model is predicated on a metastable thermodynamic equilibrium within the interlamellar space of the crystal stack and accounts explicitly for the various chain topologies (loops, tails, and bridges) therein. We examine three branched systems containing methyl, ethyl, and butyl side branches and compare our results to high density polyethylene (HDPE), without branches. We also compare results for two united atom force fields, PYS and TraPPE-UA, within the context of these simulations. In contrast to conventional wisdom, our simulations indicate that the thicknesses of the interfacial regions in systems with SCB are smaller than those observed for a linear polyethylene without branches and that branches are uniformly distributed throughout the interlamellar region. We find a prevalence of gauche...

Superhydrophobic fabrics have diverse applications in both textile and non-textile fields. Most of the waterborne materials for superhydrophobic treatment of fabrics use fluoro-containing substances, which have potential issues with bio-accumulation in both the human body and animals. Fluorine-free waterborne coatings are highly desirable for superhydrophobic treatment. In this study, we have prepared a fully waterborne coating solution through dispersion of an alkyl silane (hexadecyl trimethoxysilane) in an aqueous dopamine solution. After applying to fabrics through a wet-chemical process, the coating made the fabrics have a superhydrophobic surface with a water contact angle and a sliding angle of 163° and 8.6°, respectively. The treated fabrics are durable enough to withstand multiple washing. The coating is self-healable against acid/base etching and plasma damage. Such a fluorine-free, durable coating may be useful for the development of various superhydrophobic fabric products.

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.

Segmented polyurethanes exhibit versatile mechanical properties, where their outstanding resilience and toughness enable material designs with high energy-absorption capability. However, there is a lack of fundamental understanding regarding the underlying molecular pathways toward rapid dissipation of high-pressure fields. Here, we design a set of 4,4′-methylenediphenyldiisocyanate (MDI)−butanediol (BDO)−poly(tetramethylene oxide) (PTMO)-based polyurethanes and elucidate the influence of composition on thermal transition characteristics, crystallinity, segmental dynamics of PTMO, as well as high-strain-rate impact response on the microscale. Furthermore, simulations of shock compression, performed using an isotropic, constant-stress Hugoniostat method, comparing a MDI-BDO-PTMO-based polyurethane with polyethylene models of varying crystallinity suggest that the high-rate mechanical response is dominated by a soft domain response, which in turn can be sensitive to specific interactions present in the PTMO component that are not present in PE.

We report molecular simulation, at the atomistic level, of crystalline 4,4′-diphenylmethane diisocyanate (MDI) with n-butanediol (BDO) as chain extender, henceforth denoted as MDI/BDO, which is one of the most important components of thermoplastic polyurethanes. This work studies the structure and properties of crystalline MDI/BDO at equilibrium and under deformation. An atomistic molecular model of the MDI/BDO unit cell was constructed from fractional coordinates available for related model compounds and space group symmetry, and bulk properties of the subsequently equilibrated crystal were estimated by molecular dynamics. Overall stress-strain behavior of the crystal to small strains was simulated. The full stiffness matrix of crystalline MDI/BDO was extracted, allowing for the complete characterization of the linear elastic behavior of the crystal. Keywords: Atomistic simulation; Crystal elasticity; Polyurethane

Enhancement of the heterogeneous electron transfer (HET) activities of graphene materials toward redox-active molecules assumes a crucial role in numerous graphene-based electrochemical technologies. Here we discover that carbon-nanotube-supported reduced graphene oxide (rGO/CNT) exhibits unusually higher HET activities (including electrocatalytic performance toward dopamine, electron transfer kinetics with Ru(NH3)63+/2+ and Fe(CN)63–/4–, and direct electron transfer efficiencies with cytochrome c and horseradish peroxidase) than does CNT-free rGO with an identical electrochemical surface area and surface chemistry. Through examination of the electronic structure combined with Gerischer–Marcus calculations, the critical factors responsible for this anomalous enhancement of the HET activities in rGO/CNT are identified to be a high density of π electronic states, up-shifting of the Fermi level, and appearance of a pronounced quantum-capacitance-dominating character. These results indicate a general strategy...

We report the use of atomistic simulation to study semicrystalline poly(tetramethylene oxide) (PTMO), which is one of the major components of thermoplastic polyurethanes. This work reports the first application of an Interphase Monte Carlo model previously developed for polyethylene to a more complex chemistry involving heteroatoms, about which much less is known experimentally. The interface between the crystalline and amorphous domains of PTMO has been modeled in detail, complete with the equilibrium distributions of tails, loops and bridges. In doing so, a criterion has been established for selecting the relevant interface between domains, and a methodology developed that identifies the energetically most favorable interface in a heterogeneous material. A representative sample of configurations was then simulated by molecular dynamics, and analysis of deformation to small strains at different strain rates is described. Estimation of the full stiffness matrix of semicrystalline PTMO is reported for the ...

A kinetic model is proposed to describe the structure and rate of advancement of the growth front during crystallization. Solidification occurs through the mechanisms of surface nucleation and lateral spreading of the solid phase within layers in the vicinity of the growth front. The transformation from liquid to solid within each layer is described by an equation similar to the two-dimensional variant of the Johnson–Mehl–Avrami (JMA) equation, but in which the finite size and shape of the critical nucleus and the dynamic evolution of the solid fraction of the underlying layers are taken into account. Connection to the regime theory of Hoffman and co-workers, for surface nucleation and spreading in one or two dimensions, is also made. Given only molecular level information regarding surface nucleation rates, lateral spreading rates, and critical surface nucleus geometry, the resulting set of coupled nonlinear equations for solidification in each layer is numerically integrated in time to obtain the struct...

Metal oxide nanofibers are promising candidates for gas sensing applications due to their high surface area, pseudo one-dimensional structure and semi-conducting characteristics. The sensitivity of metal oxide nanofibers to different gases can be further maximized by controlling their morphology and microstructure. In this context, nickel oxide nanofibers were synthesized with controlled morphology and microstructure through the electrospinning technique using a solution composed of nickel acetate salt and poly(vinyl alcohol) polymer followed by calcination and annealing. These nanofibers were then employed as hydrogen and ammonia sensors. The diameter and crystallinity of the nanofibers increased, whereas the surface area decreased, with increasing ratio of salt to polymer in the solution. Nanofibers with intermediate diameter, crystallinity and surface area were found to undergo the largest resistance changes when exposed to hydrogen and ammonia at elevated temperatures. Furthermore, we found that appropriate annealing of these nickel oxide nanofibers led to full crystallinity, with a drastic increase in crystallite size and several fold decrease in surface area. These fully crystalline nanofibers possessed much lower sensitivity to hydrogen and ammonia but faster recovery as compared to their semicrystalline counterparts. In addition to surface area variations, different dominant sensing mechanisms are proposed to explain the observed differences in sensing behavior.

Crystal growth from the melt of n-pentacontane (C50) was studied by molecular dynamics simulation. Quenching below the melting temperature gives rise to propagation of the crystal growth front into the C50 melt from a crystalline polyethylene surface. By tracking the location of the crystal–melt interface, crystal growth rates between 0.02 and 0.05 m/s were observed, for quench depths of 10–70 K below the melting point. These growth rates compare favorably with those from a previous study by Waheed et al. [Polymer 2005, 46, 8689−8702]. Next, surface nucleation was identified with the formation of two-dimensional clusters of crystalline sites within layers parallel to the propagating growth front. Critical nucleus sizes, waiting times, and rates for surface nucleation were estimated by a mean first passage time analysis. A surface nucleation rate of ∼0.05 nm–2 ns–1 was observed, and it was nearly temperature-independent. Postcritical “spreading” of the surface nuclei to form a completely crystallized layer...

The production of nanofibers from an aqueous solution of poly(vinyl alcohol) by the method of free surface electrospinning from a wire electrode has been examined. The results are interpreted in terms of a previously reported model that was based on the production of fibers from polymeric solutions in ethanol by the same method. Differences in behavior were observed between the processing of aqueous and ethanolic solutions, arising from the more viscoelastic nature of the aqueous solution, the higher surface tension, the fewer number of droplets jetting simultaneously from the wire, and the different electrical current profile observed for jetting from a single droplet. These differences necessitated changes in equipment design, operation of the process, and modeling of productivity. The result is a more robust model for the productivity of fibers by free surface electrospinning from a wire electrode.

The mechanism for the redox activity of electroactive polymers (EAPs) with unconjugated backbones (e.g., polyvinylferrocene, PVFc) is fundamentally different from that of intrinsically conducting polymers. Here we investigate, for the first time, the redox transformation efficiency (RTE) of unconjugated EAPs with discrete electron donor/acceptors. We find that the RTE of PVFc can be modulated systematically by incorporation of carbon nanotubes (CNTs) via a facile and controllable solution process. Furthermore, we show that the improvement in the RTE of PVFc translated to enhanced performance of PVFc-based applications, such as energy storage, electrochemically responsive heterogeneous catalysis, and enzymatic biosensing. Our study provides valuable insights into the charge transport mechanism of unconjugated EAP/nanocarbon hybrids and the accelerated electron transfer kinetics by incorporation of materials with a high density of electronic states near the Fermi level. Moreover, this report suggests a gene...

Macroporous WO₃ nanotubes (NTs) functionalized with nanoscale catalysts were fabricated using coaxial electrospinning combined with sacrificial templating and protein-encapsulated catalysts. The macroporous thin-walled nanotubular structures were obtained by introducing colloidal polystyrene (PS) particles to a shell solution of W precursor and poly(vinylpyrrolidone). After coaxial electrospinning with a core liquid of mineral oil and subsequent calcination, open pores with an average diameter of 173 nm were formed on the surface of WO₃ NTs due to decomposition of the PS colloids. In addition, catalytic Pd nanoparticles (NPs) were synthesized using bio-inspired protein cages, i.e., apoferritin, and uniformly dispersed within the shell solution and subsequently on the WO₃ NTs. The resulting Pd functionalized macroporous WO₃ NTs were demonstrated to be high performance hydrogen (H₂) sensors. In particular, Pd-functionalized macroporous WO₃ NTs exhibited a very high H₂ response (R[subscript air]/R[subscript gas]) of 17.6 at 500 ppm with a short response time. Furthermore, the NTs were shown to be highly selective for H₂ compared to other gases such as carbon monoxide (CO), ammonia (NH₃), and methane (CH₄). The results demonstrate a new synthetic method to prepare highly porous nanotubular structures with well-dispersed nanoscale catalysts, which can provide improved microstructures for chemical sensing., Intel Corporation

Flow-enhanced crystal nucleation in a lightly entangled melt of n-pentacontahectane (C150H302, or C150) is examined by nonequilibrium molecular dynamics simulation under steady shear and uniaxial extension. Under sufficiently strong flow fields above the melting temperature, strong fluctuations give rise to domains with high local alignment on the Kuhn length-scale. Upon quenching, the nucleation of small crystallites was found to occur preferentially within these domains. This behavior is at odds with classical nucleation theory, wherein it is typically assumed that nucleation occurs uniformly throughout the melt with equal rate. Comparisons are drawn between the simulation results and theories involving the formation of precursors that precede the formation of crystals.

Binary liquid aerosols were filtered using electrospun fiber membranes for the purpose of demonstrating the feasibility of changing aerosol composition through size selection. The impacts of filtration on the particle size distribution and the overall composition of the aerosols were studied and correlated to the filter properties. The binary liquid aerosols comprised di(2-ethylhexyl) sebacate and di(2-ethylhexyl) phthalate, with median mobility diameters between 200 nm and 300 nm. Droplet size-resolved measurements of aerosol composition, performed using a combination of cascade impactor and gas chromatography-mass spectrometry (GC–MS), revealed size-dependent compositional variation. Electrospun membranes of polyacrylonitrile with four different fiber diameters, ranging from 300 nm to 1 μm, were tested. The membranes with small fiber diameters were more likely to have significant impact on both the size and composition of the aerosols. Electrospun membranes consisting of 300 nm diameter fibers altered the median particle diameter by as much as 4%, and the overall composition of the aerosols by as much as 30%. Membranes with large fiber diameters, on the other hand, exhibited negligible effects on the aerosol systems with regard to both aerosol size and composition. Electrospun cellulose acetate membranes of different thicknesses were also tested on a binary aerosol system with the same components. Thicker filters resulted in greater decrease in mean particle size and greater change of aerosol composition, up to 16%. The changes in aerosol size distribution and composition upon filtration were correlated through the droplet size-dependence of the aerosol composition.