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Self-healing polymers such as poly(ethylene-co-methacrylic acid) ionomers (PEMAA) can heal themselves immediately after a projectile puncture which in turn lowers environmental pollution from replacement. In this study, the thermal-mechanical properties and self-healing response of a library of 15 PEMAA copolymers were studied to understand the effects of the ionic content (Li, Na, Zn, Mg) and neutralization percentage (13 to 78%) on the results. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and tensile testing were used to study the thermo-mechanical properties of PEMAA copolymers while the self-healing response was studied using the projectile test. Puncture sites were observed using scanning electron microscopy (SEM) and the healing efficiency was quantitatively measured using the water leakage test. Five different self-healing responses were observed and correlated to ionic content and neutralization. At high neutralization, divalent neutralizing ions (Zn and Mg) that have stronger ionic interactions exhibited brittle responses during projectile testing. PEMAA samples neutralized with Mg and Li at low concentrations had a higher healing efficiency than PEMAA samples neutralized with Zn and Na at low neutralization. The PEMAA copolymers with higher tensile stress and two distinct peaks in the graph of loss factor versus temperature that indicate the presence of sufficient ionic aggregate clusters had improved healing efficiency. By increasing the neutralization percentage from 20% to 70%, the tensile strength and modulus of the samples increased and their self-healability generally increased. Among the investigated samples, the copolymer with ~50% neutralization by Li salt showed the highest healing efficiency (100%). Overall, the strength and elastic response required for successful self-healing responses in PEMAA copolymers are shown to be governed by the choice of ion and the amount of neutralization.

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The search for materials with better performance, longer service life, lower environmental impact, and lower overall cost is at the forefront of polymer science and material engineering. This has led to the development of self-healing polymers with a range of healing mechanisms including capsular-based, vascular, and intrinsic self-healing polymers. The development of self-healable systems has been inspired by the healing of biological systems such as skin wound healing and broken bone reconstruction. The goal of using self-healing polymers in various applications is to extend the service life of polymers without the need for replacement or human intervention especially in restricted access areas such as underwater/underground piping where inspection, intervention, and maintenance are very difficult. Through an industrial and scholarly lens, this paper provides: a) an overview of self-healing polymers; b) classification of different self-healing polymers and polymer-based composites; c) mechanical, thermal, and electrical analysis characterization; d) applications in coating, composites, and electronics; e) modeling and simulation; and f) recent development in the past 20 years. This review highlights the importance of healable polymers for an economically and environmentally sustainable future, the most recent advances in the field, and current limitations in fabrication, manufacturing, and performance.

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HYPOTHESIS: Confinement causes a change in the amount of surfactant adsorbed and in the adsorption morphology. EXPERIMENTS: Two cationic surfactants, tetradecyltrimethylammonium bromide (TTAB) and cetylpyridinium chloride (CPC) were adsorbed at the silica-water interface. Atomic force microscopy (AFM) force curves were measured on 50 nm and 80 nm wide trenches. Force curves were also measured on silica pillars, and the results were quantified based on distance from the edge. FINDINGS: Trenches: Adsorbed surfactants films in 50 nm and 80 nm trenches showed the same break-through values. However, compared to unconfined values, TTAB in trenches had decreased break-through and adhesion forces while CPC in trenches had increased break-through and adhesion forces, indicating that surfactant identity varies the confinement effect. Pillars: Near the edge, few surfactants adsorb, and those that do extend in the direction normal to the surface. While the experimental data agree qualitatively with previous coarse-grained molecular dynamic simulations, the length scales at which the phenomena are detected differ by ~ half-order of magnitude. Specifically, experimental data show measurable effects on adsorbed surfactant morphology at a distance from the edge 10-20 times the length of a surfactant molecule after accounting for the ~8 nm size of the probe.

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Rheological properties of the solution of an extended surfactant, sodium alkoxy sulfate (C8-(PO)4-(EO)1-SO4Na), are investigated as a function of the presence of various paraffinic oils over a range of salt conditions in the Winsor III microemulsion region at oil fractions where the microemulsion is "oil-starved". The addition of as small as 3 vol % alkane to 2 wt % surfactant solutions at salt concentrations where the oil-water interfacial tension is minimized induces a sudden shift in the rheological behavior. The solution viscosity increases by 5 orders of magnitude, with solid-like behaviors (G' > G″) being observed in the entire frequency region investigated (0.01-100 rad/s). Commonly, in the cases where wormlike micelles are present in the solution, alkanes are believed to be solubilized in the core of micelles, leading to a radial growth of the cylindrical part of the wormlike micelle, resulting in a drop of end-cap energy (EC) and micelle length and a reduction in viscosity. In this study, however, the addition of oil causes the formation of wormlike micelles. The viscosity of solubilized-oil samples does, however, decrease with an increase in incorporated oil volume. We hypothesize that this "abnormal oleo-responsive" viscoelastic behavior is related to a spacer of intermediate hydrophilicity, that is, polypropylene oxide (PO) segment of the alkoxy sulfate, being inserted between the hydrophobic tail and hydrophilic head (the ethoxylated sulfate segment) of the extended surfactant. The addition of a small amount of oil likely extends the PO moiety and increases the tail length of the surfactant in the aggregates as well as reducing the headgroup size, driving the formation of wormlike micelles from a solution that initially had a viscosity consistent with the absence of such structures.

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Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.

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Mixtures of fumed fractal metal oxide nanoparticles (np's) dispersed in water, at a solution pH where one species is positively charged and the other is negatively charged, form yield stress gels at volume fractions as low as 1.5%, due to electrostatic heteroaggregation into networks as confirmed by small-angle neutron scattering. These gels exhibit a measurable yield stress and an apparent viscosity that follows a power law relationship with shear rate. Rotational and oscillatory shear rheology is presented for binary mixtures of fumed silica, fumed alumina, and fumed titania in aqueous dispersions. Gels were characterized at various particle concentrations, solution pHs, mixture ratios, and salt concentrations. The strength of the gel network, as evaluated by the storage modulus and yield stress, is maximized when the mixture contains a mixture of particles with an approximately equal, but opposite, number of charged groups.

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Using coarse-grained dissipative particle dynamics (DPD) simulations, we systematically study the effect of surface heterogeneity on surfactant adsorption. Here we investigate the adsorption and aggregation of surfactants on hydrophobic stripes crossing each other perpendicularly (i.e., crossing stripes) and on hydrophobic steps. The results are compared with those obtained for isolated stripes. We find that on crossing stripes of moderate stripe widths (e.g., L = 0.61LS, 1.22LS and 1.83LS, where LS is the length of one surfactant molecule) the crossing region hinders the formation of defect-free adsorbed surfactant structures. By increasing the stripe width and/or by increasing the length of one of the two perpendicularly crossing stripes (i.e., lowering the surface density of defects/intersections), the crossing region is found to have a weaker effect on the features of the adsorbed structures. Regarding surfactant adsorption on steps, our simulation results show that the self-assembled aggregates can be stretched along the step corner, and the resultant elastic deformation can hinder adsorption. This qualitative observation can facilitate a description of surfactant adsorption that takes into consideration also the deformation of the self-assembled film. As suggested by such a general model, increasing the convex angle of the step, increasing the size of the surfactant head groups, and changing other physical parameters can reduce the elastic energy penalty, and yield larger amounts of surfactants adsorbed. The results presented could assist in understanding and sometimes predicting surfactant adsorption on heterogeneous surfaces, suggest methods to formulate surfactant mixtures to control surface coverage on heterogeneous surfaces, and perhaps facilitate new methods for the fabrication of nano-structured surfaces.

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Self-assembly is widely seen as the method of choice for the bottom-up manufacture of supra-colloidal aggregates. Surfactants have been used extensively to appreciate qualitatively and quantify driving forces and methodologies for controlling self-assembling processes and the resultant self-assembled aggregates. However, not much is known regarding self-assembled surfactant aggregates formed on heterogeneous surfaces. If heterogeneous surface features affect the morphology of surfactant aggregates, it is possible that new templating methodologies could be designed by engineering surfaces. Here we report equilibrium dissipative particle dynamics simulation results for surfactants adsorbed on model heterogeneous surfaces. Our simulation results reveal that, depending on the morphological and chemical properties of the solid substrate, a number of not-before-reported structures can be obtained for the self-assembled aggregates. The results presented could be useful for the manufacture of new coatings and materials, e.g., via the admicellar polymerization procedure, as well as for interpreting experimental data for surfactant adsorption on heterogeneous surfaces.

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Competitive surfactant adsorption of anionic surfactant AOT and nonionic surfactant Tween 20 on gold was investigated by using a quartz crystal microbalance with dissipation (QCM-D) at 25 °C. The adsorption isotherm of pure AOT did not reach a plateau at the CMC, but rather adsorption continued to increase gradually at concentrations higher than the CMC before reaching a plateau. This behavior is evidence of competitive adsorption between AOT and impurities. The adsorbed layer of AOT on gold became more viscoelastic as the concentration of AOT increased. Tween 20 reached the plateau adsorption on gold before its concentration reached the CMC, suggesting that the attraction between Tween 20 and gold is very strong. The Tween 20 adsorbed layer was rigid when compared to the AOT adsorbed layer, as indicated by low dissipation. The addition of Tween 20 to a surface covered by AOT resulted in an increase in adsorbed mass, suggestive of the insertion of Tween 20 into the AOT adsorbed layer as expected because Tween 20 is able to separate the repulsive headgroups of AOT. When AOT was added to a preformed Tween 20 layer, a drop in the adsorbed amount was found between 0 and 0.1 CMC, and then no change was observed until the CMC of AOT was reached; the adsorbed amount then increased, reaching a final adsorption greater than that of pure AOT. All data support the formation of mixed surfactant layers on the surface. Although a two-step model fit both AOT and Tween 20 adsorption kinetic data well, AOT was found to adsorb much more slowly than Tween 20.

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The adsorption and self-assembly of surfactants are ubiquitous processes in several technological applications, including the manufacture of nano-structured materials using bottom-up strategies. Although much is known about the adsorption of surfactants on homogeneous flat surfaces from experiments, theory, and simulations, limited information is available, in quantifiable terms, regarding the adsorption of surfactants on surfaces with chemical and/or morphological heterogeneity. In an effort to fill this knowledge gap, we report here results obtained using equilibrium dissipative particle dynamics (DPD) simulations for the adsorption of model surfactants onto patterned flat surfaces (i.e., flat surfaces with chemical heterogeneity). The patterns consist of one or two stripes of variable width on which the surfactants could adsorb. The adsorbing stripes are surrounded by a surface that effectively repels the surfactants. This repelling surface, perhaps not realistic, allows us to quantify the effect of lateral confinement on the morphology of surfactant aggregates. When the stripe width is large (effectively providing a homogeneous flat surface), the surfactants yield a flat monolayer. Our simulations suggest that the flat monolayers become hemi-cylinders, hemi-spheres, and individual surfactants as the stripe width decreases, a consequence of lateral confinement. In some cases our simulations show evidence of cooperative effects when two adsorbing stripes are present on the surface. If the distance between the stripes and the widths of the stripes are both less than about one surfactant length, hemi-cylindrical shells and irregular structures are observed because of cooperativity; otherwise the results match those found for a single isolated stripe. Our predictions could be useful for the design of new nano-structured materials and coatings, for applications ranging from nano-fluidic devices to nano-reactors.

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HYPOTHESIS: Magnetically susceptible PLGA nanoparticles will effectively target the round window membrane (RWM) for delivery of dexamethasone-acetate (Dex-Ac) to the scala tympani. BACKGROUND: Targeted delivery of therapeutics to specific tissues can be accomplished using different targeting mechanisms. One technology includes iron oxide nanoparticles, susceptible to external magnetic fields. If a nanocomposite composed of biocompatible polymer (PLGA), magnetite, and Dex-Ac can be pulled into and across the mammalian RWM, drug delivery can be enhanced. METHOD: In vitro targeting and release kinetics of PLGA-magnetite-Dex-Ac nanoparticles first were measured using a RWM model. Next, these optimized nanocomposites were targeted to the RWM by filling the niche in anesthetized guinea pigs. A permanent magnet was placed opposite the RWM for 1 hour. Cochlear soft tissues, perilymph, and RWM were harvested after euthanasia and steroid levels were measured using HPLC. RESULTS: Membrane transport, in vitro, proved optimal targeting using a lower particle magnetite concentration (1 versus 5 or 10 mg/ml). In vivo targeted PLGA-magnetite-Dex-Ac particles had an average size of 482.8 ± 158 nm (DLS) and an average zeta potential -19.9 ± 3.3 mV. In 1 hour, there was significantly increased cochlear targeted delivery of Dex or Dex-Ac, compared with diffusion alone. CONCLUSION: Superparamagnetic PLGA-magnetite-Dex-Ac nanoparticles under an external magnetic field (0.26 mT) for 1 hour significantly increased Dex-Ac delivery to the inner ear. The RWM was not completely permeated and also became loaded with nanocomposites, indicating that delivery to the cochlea would continue for weeks by PLGA degradation and passive diffusion.

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The influence of surface roughness on surfactant adsorption was studied using a quartz crystal microbalance with dissipation (QCM-D). The sensors employed had root-mean-square (R) roughness values of 2.3, 3.1, and 5.8 nm, corresponding to fractal-calculated surface area ratios (actual/nominal) of 1.13, 1.73, and 2.53, respectively. Adsorption isotherms measured at 25 °C showed that adsorbed mass of cetyltrimethylammonium bromide per unit of actual surface area below 0.8 cmc, or above 1.2 cmc, decreases as the surface roughness increases. At the cmc, both the measured adsorbed amount and the measured dissipation increased dramatically on the rougher surfaces. These results are consistent with the presence of impurities, suggesting that roughness exacerbates well-known phenomena reported in the literature of peak impurity-related adsorption at the cmc. The magnitude of the increase, especially in dissipation, suggests that changes in adsorbed amount may not be the only reason for the observed results, as aggregates at the cmc on rougher surfaces are more flexible and likely contain larger amounts of solvent. Differences in adsorption kinetics were also found as a function of surface roughness, with data showing a second, slower adsorption rate after rapid initial adsorption. A two-rate Langmuir model was used to further examine this effect. Although adsorption completes faster on the smoother surfaces, initial adsorption at zero surface coverage is faster on the rougher surfaces, suggesting the presence of more high-energy sites on the rougher surfaces.

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Competitive counterion binding of sodium and calcium to micelles, and mixed micellization have been investigated in the systems sodium dodecylsulfate (NaDS)/sodium decylsulfate (NaDeS) and NaDS/sodium 4-octylbenzenesulfonate (NaOBS) in order to accurately model the activity of the relevant species in solution. The critical micelle concentration (CMC) and equilibrium micelle compositions of mixtures of these anionic surfactants, which is necessary for determining fractional counterion binding measurements, is thermodynamically modeled by regular solution theory. The mixed micelle is ideal (the regular solution parameter ß(M)=0) for the NaDS/NaOBS system, while the mixed micelle for NaDS/NaDeS has ß(M)=-1.05 indicating a slight synergistic interaction. Counterion binding of sodium to the micelle is influenced by the calcium ion concentration, and vice versa. However, the total degree of counterion binding is essentially constant at approximately 0.65 charge negation at the micelle's surface. The counterion binding coefficients can be quantitatively modeled using a simple equilibrium model relating concentrations of bound and unbound counterions.

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The structure of aqueous sodium dodecyl sulfate (SDS) surfactant aggregates formed on small graphene sheets and graphene nanoribbons has been studied using all-atom molecular dynamics simulations. Because the edges of the carbonaceous supports confine laterally the surfactant aggregates, by changing the size of the support (diameter of graphene sheets and width of graphene nanoribbons) it is possible to investigate lateral confinement effects on the aggregate morphology. The results are compared to those available on graphite, with no lateral confinement. Aqueous SDS aggregates were studied on 2.0 nm, 5.0 nm, and 10.0 nm circular graphene sheets and on 2.0 and 5.0 nm wide graphene nanoribbons. For the first time our results show that, because of lateral confinement provided by the graphene edges, SDS yields multiple layers, hemispheres, hemicylinders or multiple hemispheres depending on the graphene size and shape. Results are quantified in terms of morphology of the surfactant aggregates, order parameter of the adsorbed surfactant aggregates, and number of water molecules at contact with the carbonaceous support.

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Negatively charged colloidal poly(methyl methacrylate-co-butyl acrylate) (P(MMA-BA)) particles and positively charged dissolved poly(ethyleneimine) (PEI) were adsorbed onto a cement block using a layer-by-layer (LBL) assembly technique. The block was fashioned so as to have a cylindrical hole running from one face to another along the long axis of the rectangular block, and a fluid containing either of the two charged materials was pumped through the block. The result was a film tens of micrometers thick, and the pressure required to crack the cement block was measured after one end of the hole was sealed. Latex particles with a T(g) near the use temperature showed the maximum improvement in the cracking stress of the blocks. In a multilayer coating with identically sized particles, the cracking stress of the blocks increased to an improvement of 25% and then dropped off with increasing number of layers, even though the relationship between film thickness and the number of layers was linear. An improvement of about 30% in the cracking stress of the coated blocks was obtained when using multiple layers with different particle sizes. The effects of the number of layers and particle size on the cracking stress suggest that both the morphology and the thickness of the film play a role in performance. Tests done under confinement, e.g., with an external stress applied to the outside of the blocks, suggest that not only does a film-forming mechanism contribute to performance but that filling of microcracks in the rock may also play a role.

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Hyaluronic acid-poly(de-co-glycolide) nanoparticles (HA-PLGA NPs) were synthesized to stabilize the porous structure of porcine small intestinal submucosa (SIS), to improve surface biocompatibility and to enhance performance in tissue regeneration. HA-PLGA NPs were characterized for size, zeta potential, surface morphology, and HA loading. Human microvascular endothelial cells responded to HA-PLGA NPs and HA-PLGA modified SIS (HA-PLGA-SIS) with elevated cell proliferation. HA-PLGA-SIS significantly enhanced neo-vascularization in an in ovo chorioallantoic membrane angiogenesis model. The angiogenic capability of the newly fabricated HA-PLGA-SIS was tested in a canine bladder augmentation model. Urinary bladder augmentation was performed in beagle dogs following hemi-cystectomy using HA-PLGA-SIS. The regenerated bladder was harvested at 10 weeks post augmentation and vascularization was evaluated using CD31 immunohistochemical staining. Bladder regenerated with HA-PLGA-SIS had significantly higher vascular ingrowth compared to unmodified SIS. This study shows that HA-PLGA NPs may represent a new approach for modifying naturally derived SIS biomaterials in regenerative medicine.

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The adsorption of sodium dodecyl sulfate and a polyethoxylated nonylphenol, and well defined mixtures thereof, was measured on gamma-alumina. A pseudo-phase separation model to describe mixed anionic/nonionic admicelle (adsorbed surfactant aggregate) formation was developed, analogous to the pseudo-phase separation model frequently used to describe mixed micelle formation. In this model, regular solution theory was used to describe the anionic/nonionic surfactant interactions in the mixed admicelle and a patch-wise adsorption model was used to describe surfactant adsorption on a heterogeneous solid surface. The formation of mixed anionic/nonionic admicelles in the absence of micelles was accurately described by regular solution theory; mixed admicelle formation exhibited stronger negative deviations from ideality than mixed micelle formation. An adequate description of mixed anionic/nonionic admicelle formation in the presence of mixed micelles was obtained through a simultaneous solution of the pseudo-phase separation models for mixed admicelle and mixed micelle formation, and the appropriate mass balance equations. Anionic/nonionic mixed adsorption in the presence of mixed micelles was shown to correspond to an admicelle composition of approximately a 1:1 anionic/nonionic mole ratio throughout Regions II and III of the adsorption isotherm.

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The ability to control the dispersion of carbon nanotubes in polymers is key to most applications of nanotube-polymer composites. This feature article describes recent advances in methods used to disperse carbon nanotubes and considers how these methods affect dispersion on different length scales. It is becoming increasing clear that perfect dispersion is not desired for many applications, in particular for electrical conductivity, and controlling the dispersion is key for proper function of the composite in its intended application.

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The ability to pattern a surface with proteins on both the nanometer and the micrometer scale has attracted considerable interest due to its applications in the fields of biomaterials, biosensors, and cell adhesion. Here, we describe a simple particle lithography technique to fabricate substrates with hexagonally patterned dots of protein surrounded by a protein-repellent layer of poly(ethylene glycol). Using this bottom-up approach, dot arrays of three different proteins (fibrinogen, P-selectin, and human serum albumin) were fabricated. The size of the protein dots (450 nm to 1.1 microm) was independent of the protein immobilized but could be varied by changing the size of the latex spheres (diameter=2-10 microm) utilized in assembling the lithographic bead monolayer. These results suggest that this technique can be extended to other biomolecules and will be useful in applications where arrays of protein dots are desired.

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In this work, we evaluate the effects of the low molecular weight compounds toluene, phenol, and 1-hexanol on the adsorption of two surfactants on one solid surface. The surfactants are cetyltrimethylammonium bromide (CTAB, cationic) and hexaethylene glycol monododecyl ether (C12E6, nonionic). The surface is gold, although X-ray photoelectron spectroscopic analysis reveals the presence of a large number of oxygenated sites that render the surface hydrophilic (contact angle 10 degrees). Adsorption isotherms are measured using a quartz crystal microbalance with dissipation monitoring (QCM-D). Although our measurements do not allow the determination of the morphology of the aggregates directly, we rationalize our results by referring to AFM images from the literature. On the basis of primarily the dissipative signal and on AFM studies done by others, our results are consistent with CTAB forming a patchy cylindrical structure and C12E6 likely yielding a monolayer structure. The presence of cosolutes almost doubles the mass of surface aggregates and increases the rigidness of the aggregates for CTAB, consistent with a morphological change from cylinders to flat bilayers. Part of the increase in adsorbed mass is likely due to increased surface area covered by admicelles. For C12E6, cosolutes cause small changes in the mass adsorption and essentially no change in the flexibility of surface aggregates.

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