Moisture absorption and absorption kinetics in polyelectrolyte films: influence of film thickness

Interplay between the Glass Transition Temperature, Analyte Diffusion, and Fluorescence Quenching for Detection of Nitro-Group Containing Explosives Using Organic Semiconducting Films

Efficient detection of chemical analytes using fluorescence-based sensors necessitates an in-depth understanding of the physical interaction between the analyte molecules and the sensor films. This study explores the interplay between the thermal properties of a series of triphenylamine-centered fluorescent dendrimers with different glass transition temperatures (Tg) for detecting nitroaromatic explosives. When exposed to 4-nitrotoluene (pNT) vapors, biphasic diffusion kinetics were observed for all the dendrimers, corresponding to Super Case II kinetics, suggesting rapid film swelling during initial analyte uptake. The diffusion kinetics were further analyzed using a diffusion-relaxation model, where a strong Tg dependence was observed for both the initial concentration-driven diffusion phase and the slower film relaxation phase. Additionally, a difference in kinetics between analyte uptake and release was observed. The photoluminescence (PL) kinetics also showed a Tg dependence, with more efficient PL recovery observed for films composed of dendrimers that had a lower Tg. Rapid quenching of over 40% with little PL recovery was seen in the dendrimer with the highest Tg (107 °C), while a smaller quench with efficient PL recovery was observed in the dendrimer that had a Tg close to room temperature. The results highlight the critical role of the thermal properties of sensor films in achieving rapid and sensitive detection.

Biodegradable films from the lignocellulosic fibers of wheat straw biomass and the effect of calcium ions

Plastics are hazardous to human health, and plastic waste results in environmental pollution and ecological catastrophe. Biobased polymers from renewable sources have recently become promising for developing biodegradable packaging films. Among them, lignocellulosic residue from agricultural biomass is inexpensive, renewable, and biodegradable. This study aims to develop biodegradable films using lignocellulosic residue from wheat straw biomass. The methodology is a green process that solubilizes lignocellulosic chains using Zn2+ ions and crosslinks with Ca2+ ions of different concentrations (200-800 mM). The results reveal that the increase of Ca2+ ions significantly decreases moisture content, water solubility, water vapor permeability, transparency, and elongation of films. The tensile strength is recorded as 6.61 ± 0.07 MPa with the addition of 800 mM of CaCl2, which is approximately 2.5 times higher than commercial polyethylene films. Around 90 % of films biodegrade within a month in soil containing 20 % moisture content. Overall, lignocellulosic residue from wheat straw biomass could be an excellent replacement for synthetic polymer to fabricate strong, transparent, and biodegradable plastic films.

Morphology and Mechanics of Star Copolymer Ultrathin Films Probed by Atomic Force Microscopy in the Air and in Liquid

Star copolymer films were produced by using spin-coating, drop-casting, and casting deposition techniques, thus obtaining ultrathin and thick films, respectively. The morphology is generally flat, but it becomes substrate-dependent for ultrathin films where the planarization effect of films is not efficient. The indentation hardness of films was investigated by Force Volume Maps in both the air and liquid. In the air, ultrathin films are in the substrate-dominated zone and, thus, the elastic modulus E is overestimated, while E reaches its bulk value for drop-casted ultrathin and thick films. In liquid (water), E follows an exponential decay for all films with a minimum soaked time t0 of 0.37 and 2.65 h for ultrathin and drop-casted ultrathin and thick films, respectively. After this time, E saturates to a value on average 92% smaller than that measured in the air due to film swelling. Such results support the role of film morphology in the antimicrobial activity envisaged in the literature, suggesting also an additional role of film hardness.

Critical Role of Boundary Conditions in Sorption Kinetics Measurements

In order to characterize the hygroscopic properties of cellulose-based materials, which can absorb large amounts of water from vapor in ambient air, or the adsorption capacity of pollutants or molecules in various porous materials, it is common to rely on sorption-desorption dynamic tests. This consists of observing the mass variation over time when the sample is placed in contact with a fluid containing the elements to be absorbed or adsorbed. Here, we focus on the case of a hygroscopic material in contact with air at a relative humidity (RH) differing from that at which it has been prepared. We show that the vapor mass flux going out of the sample follows from the solution of a vapor convection-diffusion problem along the surface and is proportional to the difference between the RH of the air flux and that along the surface with a multiplicative factor (δ) depending only on the characteristics of the air flux and the geometry of the system, including the surface roughness. This factor may be determined from independent measurements in which the RH along the surface is known while keeping all other variables constant. Then we show that the apparent sorption or desorption kinetics critically depend on the competition between boundary conditions and transport through the material. For sufficiently low air flux intensities or small sample thicknesses, the moisture distribution in the sample remains uniform and evolves toward the equilibrium with a kinetics depending on the value of δ and the material thickness. For sufficiently high air flux intensities or large sample thicknesses, the moisture distribution is highly inhomogeneous, and the kinetics reflect the ability of water transport by diffusion through the material. We illustrate and validate this theoretical description on the basis of magnetic resonance imaging experiments on drying cellulose fiber stacks.

Aptamer Conformational Dynamics Modulate Neurotransmitter Sensing in Nanopores

Aptamers that undergo conformational changes upon small-molecule recognition have been shown to gate the ionic flux through nanopores by rearranging the charge density within the aptamer-occluded orifice. However, mechanistic insight into such systems where biomolecular interactions are confined in nanoscale spaces is limited. To understand the fundamental mechanisms that facilitate the detection of small-molecule analytes inside structure-switching aptamer-modified nanopores, we correlated experimental observations to theoretical models. We developed a dopamine aptamer-functionalized nanopore sensor with femtomolar detection limits and compared the sensing behavior with that of a serotonin sensor fabricated with the same methodology. When these two neurotransmitters with comparable mass and equal charge were detected, the sensors showed an opposite electronic behavior. This distinctive phenomenon was extensively studied using complementary experimental techniques such as quartz crystal microbalance with dissipation monitoring, in combination with theoretical assessment by the finite element method and molecular dynamic simulations. Taken together, our studies demonstrate that the sensing behavior of aptamer-modified nanopores in detecting specific small-molecule analytes correlates with the structure-switching mechanisms of individual aptamers. We believe that such investigations not only improve our understanding of the complex interactions occurring in confined nanoscale environments but will also drive further innovations in biomimetic nanopore technologies.

Conformational entropy of hyaluronic acid contributes to taste enhancement

Natural taste/flavor enhancers are essential ingredients that could potentially address condiments overconsumption. For the first time, we report that hyaluronic acid (HA) could modulate taste perception, governed by the dynamic interactions among taste compounds, mucin, and HA. Various conformations of HA impact taste perception. The high molecular weight (Mw) of 1090 kDa HA inhibits the sense of taste due to its increased viscosity, which hinders the penetration of Na⁺ into the mucin layer. HA with low and medium Mw (100 kDa, 400 kDa) could enhance taste perception. Isothermal titration calorimetry analysis confirms the stronger binding between mucin and HA. The intensity of their interaction increases as the Mw of HA increases from 8 kDa to 400 kDa. Quartz crystal microbalance with dissipation characterization further indicates that the rigid conformation of 100 kDa HA facilitates the binding of Na⁺ with taste receptors, thereby enhancing taste perception. The flexible conformation of 400 kDa HA may conceal the taste receptor cells, reducing taste enhancement. Our work advances the understanding of conformational entropy of natural mucoadhesion and mucopenetration polymers, which lays the foundation for their potential use as taste enhancers.

Discrepancy of Effective Water Diffusivities Determined from Dynamic Vapor Sorption Measurements with Different Relative Humidity Step Sizes: Observations from Cereal Materials

Water diffusivity, a critical parameter for cereal processing design and quality optimization, is usually concentration-dependent. dynamic vapor sorption (DVS) system provides an approach to establishing the relationship between water concentration and diffusivity. However, the usual relative humidity (RH) jump during practical sorption processes is usually greater than that adopted in DVS measurements. Water vapor sorption kinetics of glutinous rice grains, glutinous rice flour and wheat flour dough films were measured using the DVS system to verify if varying RH step sizes can obtain identical diffusivities within the same range. The effective diffusivities were determined according to Fick’s second law. The results revealed that increasing RH step size led to a higher estimated diffusivity, regardless of whether the water concentration gradient or potential chemical gradient was considered a driving force for water diffusion. This finding was further confirmed by a linear RH scanning DVS measurement. The water concentration-dependent diffusivity obtained from a multi-step DVS measurement, according to Fick’s second law, will overestimate the required time for practical cereal drying or adsorption. Thus, this paradoxical discrepancy needs a new mass transfer mechanism to be explained.

Neutron Reflectivity Study on the Suppression of Interfacial Water Accumulation between a Polypropylene Thin Film and Si Substrate Using a Silane-Coupling Agent

We measured the neutron reflectivity (NR) of isotactic polypropylene (PP) thin films deposited on Si substrates modified by hexamethyldisilazane (HMDS) at the saturated vapor pressure of deuterated water at 25 °C and 60 °C/85% RH to investigate the effect of HMDS on the interfacial water accumulation in PP-based polymer/inorganic filler nanocomposites and metal/resin bonding materials. We found that the amount of water accumulated at the PP/Si interface decreased with increasing immersion time of the Si substrate in a solution of HMDS in hexane prior to PP film deposition. During the immersion of the Si substrate, the HMDS molecules were deposited on the Si substrate as a monolayer without aggregation. Furthermore, the coverage of the HMDS monolayer on the Si substrate increased with increasing immersion time. At 60 ° C and 85% RH, only a slight amount of interfacial water was detected after HMDS treatment for 1200 min. As a result, the maximum concentration of interfacial water was reduced to 0.1 from 0.3, where the latter corresponds to the PP film deposited on the untreated substrate.

Vapor Sorption-Desorption Phenomena of HD and GB Simulants from Polyurethane Thin Films on Aluminum Oxide via a Quartz Crystal Microbalance

Protection and decontamination of surfaces after exposure to chemical warfare agents (CWAs) are of considerable interest to the homeland defense and battlespace operation communities. In this work, polyurethane was spin-coated onto aluminum oxide quartz crystal microbalance (QCM) sensors. Polyurethane film thickness was varied by altering the concentration of the polymer/chloroform solution used for spin-coating. Atomic force microscopy confirmed the formation of smooth, homogeneous films on the QCM sensor surface. Aluminum oxide QCM sensors coated with polyurethane were exposed to saturated vapors of dichloropentane (DCP), a mustard gas (HD) simulant, and dimethyl methylphosphonate (DMMP), a sarin gas (GB) simulant, and the mass uptake, diffusion coefficient, volume fraction, and partition coefficient of the simulant in the film were determined from QCM data. Results showed that both DCP and DMMP readily sorbed into the films although the mass uptake of DCP was greater than that of DMMP owing to DCP's higher vapor pressure. Additionally, the CWA simulant uptake increased with polyurethane film thickness. Sorption diffusion coefficients were 1 × 10^-13 cm²/s and 1 × 10^-12 cm²/s for DCP and DMMP vapor, respectively. Simulant desorption was also measured and showed that some DMMP remained in the film/substrate system, while DCP sorption was fully reversible. Reversible desorption for both CWA simulants was relatively quick and independent over the range of film thicknesses studied, with average desorption diffusion coefficients of 2 × 10^-9 cm²/s and 1 × 10^-11 cm²/s for DCP and DMMP, respectively. Collectively, this study is expected to inform protection and decontamination strategies of equipment and structures upon exposure to CWAs.

Metal organic framework/polyelectrolyte composites for water vapor sorption applications

Metal-organic framework (MOF) core particles of MIL-101(Cr), aluminum fumarate (Basolite® A520), MIL-53-TDC, zirconium fumarate, and UiO-66 were modified by adsorption of thin polyelectrolyte (PE)-based shells without deterioration of their crystal structure. By applying different PEs and depositing a single layer (MOF/PE) or one to three layer-by-layer assembled bilayers (MOF/LbL), the mass percent of shell material in the composite was varied from 0.6-2.5% to 50%. Under a constant relative pressure of water vapor, the moisture uptake by a MOF/PE and a MOF/LbL is rather comparable with its S-shaped curvature to that of pristine MOFs. The relevant differences, such as a shift of the ascending adsorption part to lower/higher relative pressure or an increase/decrease in water uptake in selected regions, are associated with the core-shell structure and related to the morphological changes of the MOF powders. The hydrophilic surface promotes the formation of liquid menisci at the points of contact between particles and accelerates the moisture uptake and loss. A decrease in water sorption under an atmosphere with high humidity by some composites can be associated with the inhibition of liquid water condensation by the more hydrophobic shells.

Investigation of Interfacial Water Accumulation between Polypropylene Thin Film and Si Substrate by Neutron Reflectivity

We performed neutron reflectivity (NR) measurements of isotactic polypropylene (PP) thin films deposited on a Si substrate at the saturated vapor pressure of deuterated water to investigate interfacial water accumulation between the PP and metal surfaces in PP-based polymer/inorganic filler nanocomposites and metal/resin bonding materials. The PP thin films prepared on a Si substrate by a spin-coating technique were adequate as a model system for the PP/metal interface in these materials. A water-rich layer with a maximum water concentration of 0.5, which was considerably higher than those reported in previous studies of organic/inorganic interfaces, was observed within a width of approximately 3 nm at the interface under saturated vapor conditions. This could be attributed to the weak interaction between the PP thin film and the Si substrate. The pathway of moisture transport to the interfacial region was along the interface rather than through the PP film because the hydrophobic PP thin film does not entirely swell with water vapor.

Layered Structure in the Crystalline Adsorption Layer and the Leaching Process of Poly(vinyl alcohol) Revealed by Neutron Reflectivity

We investigated the structure of the crystalline adsorption layer of poly(vinyl alcohol) (PVA) in hot water by neutron reflectivity in two cases: when the adsorption layer is exposed on the substrate by leaching the upper bulk layer and when it is deeply embedded between a relatively thick PVA film and substrate. In both cases, the PVA adsorption layer consists of three layers on the Si substrate. The bottom layer, consisting of amorphous chains that are strongly constrained on the substrate, is not swollen even in hot water at 90 °C. The middle layer, consisting of amorphous chains that are much more mobile compared with those in the bottom layer, has no freedom to assume a crystalline form. Only the molecular chains in the top layer are crystallizable in the adsorption layer, leading to a heterogeneous layered structure in the film thickness direction. This layered structure is attributed to the crystallizable chains of PVA during the formation of the adsorption layer driven by hydrogen bonding. However, the structure and dynamics in the adsorption layer may differ in both cases because the molecular chains in the vicinity of the surface seem to be affected by surface effects even in the adsorption layer.

Gas-flow humidity control system for neutron reflectivity measurements

We developed a novel humidity control system for neutron reflectivity measurements based on the two-way gas-flow method that can generate up to 85% relative humidity (RH) within a temperature range of 5 °C-85 °C. The system consists of a gas-flow-type humidity generator and a thermostatic sample chamber, each of which can independently control the temperature. The key features include rapid humidity response and long stable operation time. The humidity reaches equilibrium within 2 to 5 min during the humidity change, and the system exhibited acceptable stability over a three-day, nonstop experimental measurement duration, with a precision of ±1% RH at 85 °C and 85% RH. The sample chamber is capable of measuring substrate samples with dimensions of up to 2-in. in diameter and 5-mm in thickness. We demonstrate the reflectivity data measured at a pulsed neutron facility, MLF BL17, in the Japan Proton Accelerator Research Complex. The combined use of this system with neutrons permits in situ, time-resolved studies of the swelling process of polyvinyl alcohol and adhesive materials.

Thickness Dependence of the Diffusivity and Solubility of Cyclohexane in Nanoscale Bitumen Films

Diffusivity and solubility of cyclohexane in nanoscale bitumen films coated on hydrophilic substrates at ambient conditions were studied using a gravimetric analyzer. Three substrates were used, and they are as follows: sample A, monodisperse spherical glass beads; sample B, polydisperse spherical glass beads mixed with polydisperse irregular-shape kaolin clay particles; and sample C, irregular-shape residual solids generated from a solvent extraction process of an oil sand ore. All of the above samples had a mean diameter of 150 μm. Diffusion coefficients were determined based upon the initial rates of cyclohexane absorption when bitumen-coated samples at various amounts (thicknesses) were exposed to a carrier gas with cyclohexane vapors at two levels of relative saturations (RSs), and they were found to be in the range of 10^-18 to 10^-16 m²/s. A double-first-order kinetics model fits well to the absorption data, suggesting that there exists a concentration gradient of polar (or nonpolar) bitumen molecules in the nanoscale films. This is because the hydrophilic substrates attract the relatively polar fraction of bitumen molecules to the region close to the substrates and the nonpolar fraction resides in the region near the free surface. As a result, the measured diffusion coefficients exhibited positive thickness dependence when the thickness of the bitumen films was at the nanoscale. The molecules near the substrates tended to diffuse slower than those in the free surface region. However, diffusivity was insensitive to the cyclohexane RS. On the other hand, the measured solubility of cyclohexane in the nanoscale bitumen films exhibited no thickness dependence but strong cyclohexane RS dependence. These results suggest that solubility is not affected by the inhomogeneous distribution of bitumen molecules in the nanoscale films and that it follows more or less Henry's law.

Effect of Adjacent Hydrophilic Polymer Thin Films on Physical Aging and Residual Stress in Thin Films of Poly(butylnorbornene-ran-hydroxyhexafluoroisopropyl norbornene)

The properties of thin supported polymer films can be dramatically impacted by the substrate upon which it resides. A simple way to alter the properties of the substrate (chemistry, rigidity, dynamics) is by coating it with an immiscible polymer. Here we describe how ultrathin (ca. 2 nm) hydrophilic polymer layers of poly(acrylic acid), PAA, and poly(styrenesulfonate), PSS, impact the aging behavior and the residual stress in thin films of poly(butylnorbornene-ran-hydroxyhexafluoroisopropyl norbornene), BuNB-r-HFANB. The aging rate decreases as the film thickness (h) is decreased, but the extent of this change depends on the adjacent layer. Even for the thickest films (h>500 nm), there is a decrease in the aging rate at 100 °C when BuNB-r-HFANB is in contact with PSS. In an effort to understand the origins of these differences in the aging behavior, the elastic modulus and residual stress (σR) in the films were determined by wrinkling as a function of aging time. The change in the elastic modulus during aging does not appear to be directly correlated with the densification or expansion of the films, but the aging rates appear to roughly scale as hσR 1/3. These results illustrate that the physical aging of thin polymer films can be altered by adjacent polymers.

Control of Probe Density at DNA Biosensor Surfaces Using Poly(l-lysine) with Appended Reactive Groups

Biosensors and materials for biomedical applications generally require chemical functionalization to bestow their surfaces with desired properties, such as specific molecular recognition and antifouling properties. The use of modified poly(l-lysine) (PLL) polymers with appended oligo(ethylene glycol) (OEG) and thiol-reactive maleimide (Mal) moieties (PLL-OEG-Mal) offers control over the presentation of functional groups. These reactive groups can readily be conjugated to, for example, probes for DNA detection. Here we demonstrate the reliable conjugation of thiol-functionalized peptide nucleic acid (PNA) probes onto predeposited layers of PLL-OEG-Mal and the control over their surface density in the preceding synthetic step of the PLL modification with Mal groups. By monitoring the quartz crystal microbalance (QCM) frequency shifts of the binding of complementary DNA versus the density of Mal moieties grafted to the PLL, a linear relationship between probe density and PLL grafting density was found. Cyclic voltammetry experiments using Methylene Blue-functionalized DNA were performed to establish the absolute probe density values at the biosensor surfaces. These data provided a density of 1.2 × 10¹² probes per cm² per % of grafted Mal, thus confirming the validity of the density control in the synthetic PLL modification step without the need of further surface characterization.

Surfactants as mesogenic agents in layer-by-layer assembled polyelectrolyte/surfactant multilayers: nanoarchitectured "soft" thin films displaying a tailored mesostructure

Interfacial supramolecular architectures displaying mesoscale organized components are of fundamental importance for developing materials with novel or optimized properties. Nevertheless, engineering the multilayer assembly of different building blocks onto a surface and exerting control over the internal mesostructure of the resulting film is still a challenging task in materials science. In the present work we demonstrate that the integration of surfactants (as mesogenic agents) into layer-by-layer (LbL) assembled polyelectrolyte multilayers offers a straightforward approach to control the internal film organization at the mesoscale level. The mesostructure of films constituted of hexadecyltrimethylammonium bromide, CTAB, and polyacrylic acid, PAA (of different molecular weights), was characterized as a function of the number of assembled layers. Structural characterization of the multilayered films by grazing-incidence small-angle X-ray scattering (GISAXS), showed the formation of mesostructured composite polyelectrolyte assemblies. Interestingly, the (PAA/CTA)n assemblies prepared with low PAA molecular weight presented different mesostructural regimes which were dependent on the number of assembled layers: a lamellar mesophase for the first bilayers, and a hexagonal circular mesophase for n ≥ 7. This interesting observation was explained in terms of the strong interaction between the substrate and the first layers leading to a particular mesophase. As the film increases its thickness, the prevalence of this strong interaction decreases and the supramolecular architecture exhibits a "bulk" mesophase. Finally, we demonstrated that the molecular weight of the polyelectrolyte has a considerable impact on the meso-organization for the (PAA/CTA)n assemblies. We consider that these studies open a path to new rational methodologies to construct "nanoarchitectured" polyelectrolyte multilayers.

Qualitatively and quantitatively characterizing water adsorption of a cellulose nanofiber film using micro-FTIR spectroscopy

The efficient utilization of cellulose is very important for environmental protection and sustainability of resources.

Sorption of Water and Initial Stages of Swelling of Thin PNIPAM Films Using in Situ GISAXS Microfluidics

The sorption of low-molecular penetrants by thin polymer films, as well as structural changes provoked therein, is of high relevance for many fields of application. Complex permeation, diffusion, swelling, and dissolution processes are often induced within films by solvents or gases. Here, we use a novel in situ microfluidics-grazing incidence small-angle X-ray scattering (GISAXS) setup to examine changes in film thickness and in the surface structure of a thin polymer film that sorbs a good solvent. Thus, this technique is highly complementary to the established techniques on the field of diffusion in polymers. The initial stages of water uptake and swelling are resolved for a 50 nm thin, hydrophilic poly(N-isopropylacrylamide) (PNIPAM) film, before its dissolution sets in. The initial stages of swelling are tentatively described by anomalous swelling induced by a time- and space-dependent diffusion coefficient.

Fruit calcium accumulation coupled and uncoupled from its transpiration in kiwifruit

Accumulation of Ca in several fleshy fruit is often supposed to depend, among others, by climatic variables driving fruit transpiration. This study tests the whole causal chain hypothesis: VPD → fruit transpiration → Ca accumulation. Also there are evidences that relationship between fruit transpiration and Ca content is not always clear, hence the hypothesis that low VPD reduces the fraction of xylemic water destined to transpiration was tested by examining the water budget of fruit. Attached fruits of Actinidia deliciosa were subjected to Low (L) and High (H) VPD. Their transpiration was measured from early after fruit-set to day 157 after full bloom (DAFB). Fruits were picked at 70, 130 and 157 DAFB for Ca and K determinations and for water budget analysis. Cumulative transpired water was ∼ 70 g and ∼ 16 g H2O f(-1) in HVPD and LVPD, respectively. Calcium accumulated linearly (R(2) = 0.71) with cumulative transpiration when VPD was high, while correlation was weaker (R(2) = 0.24) under LVPD. Under low VPD the fraction of xylem stream destined to transpiration declined to 40-50%. Results suggest that Ca accumulation is coupled to cumulative transpiration under high VPD because under that condition cumulative transpiration equals xylem stream (which carry the nutrient). At LVPD, Ca gain by fruit is uncoupled from transpiration because ∼ 60% of the xylemic water is needed to sustain fruit growth. Results will apply to most fruits (apples, tomatoes, capsicum, grapes etc.) since most suffer Ca deficiency disorders and grow in changing environments with variable VPD, also they could be supportive for the implementation of fruit quality models accounting also for mineral compositions and for a reinterpretation of certain field practices aimed at naturally improve fruit Ca content.

Mechanisms of criticality in environmental adhesion loss

Moisture attack on adhesive joints is a long-standing scientific and engineering problem. A particularly interesting observation is that when the moisture level in certain systems exceeds a critical concentration, the bonded joint shows a dramatic loss of strength. The joint interface plays a dominant role in this phenomenon; however, why a critical concentration of moisture exists and what role is played by the properties of the bulk adhesive have not been adequately addressed. Moreover if the interface is crucial, the local water content near the interface will help elucidate the mechanisms of criticality more than the more commonly examined bulk water concentration in the adhesive. To gain a detailed picture of this criticality, we have combined a fracture mechanics approach to determine joint strength with neutron reflectivity, which provides the moisture distribution near the interface. A well-defined model system, silica glass substrates bonded to a series of polymers based on poly(n-alkyl methacrylate), was utilized to probe the role of the adhesive in a systematic manner. By altering the alkyl chain length, the molecular structure of the polymer can be systematically changed to vary the chemical and physical properties of the adhesive over a relatively wide range. Our findings suggest that the loss of adhesion is dependent on a combination of the build-up of the local water concentration near the interface, interfacial swelling stresses resulting from water absorption, and water-induced weakening of the interfacial bonds. This complexity explains the source of criticality in environmental adhesion failure and could enable design of adhesives to minimize environmental failure.

Surfactant-triggered disassembly of electrostatic complexes probed at optical and quartz crystal microbalance length scales

A critical advantage of electrostatic assemblies over covalent and crystalline bound materials is that associated structures can be disassembled into their original constituents. Nanoscale devices designed for the controlled release of functional molecules already exploit this property. To bring some insight into the mechanisms of disassembly and release, we study the disruption of molecular electrostatics-based interactions via competitive binding with ionic surfactants. To this aim, free-standing micrometer-size wires were synthesized using oppositely charged poly(diallyldimethylammonium chloride) and poly(acrylic acid) coated iron oxide nanoparticles. The disassembly is induced by the addition of sodium dodecyl sulfates that complex preferentially the positive polymers. The process is investigated at two different length scales: the length scale of the particles (10 nm) through the quartz crystal microbalance technique and that of the wires (>1 μm) via optical microscopy. Upon surfactant addition, the disassembly is initiated at the surface of the wires by the release of nanoparticles and by the swelling of the structure. In a second step, erosion involving larger pieces takes over and culminates in the complete dissolution of the wires, confirming the hypothesis of a surface-type swelling and erosion process.

Chitosan adsorption on hydroxyapatite and its role in preventing acid erosion

Polymer adsorption onto an artificial saliva (AS) layer is investigated using quartz-crystal microbalance with dissipation (QCM-D) and chitosan as the model polymer. QCM-D is utilized in an innovative manner to monitor in situ adsorption of chitosan (CH) onto a hydroxyapatite (HA) coated crystal and to examine the ability of the adsorbed layer to "protect" the HA upon sequential exposure to acidic solutions. After deposition of a thin AS layer (16 nm), the total thickness on the HA substrate increases to 37 nm upon exposure to CH at pH 5.5 for 10 min. Correspondingly, the surface charge changes from negative (i.e., AS) to positive, consistent with the adsorption the polycationic CH onto or into the AS layer. Upon exposure to an oxidizing agent, the chitosan cross-links and collapses as noted by a decrease in thickness to 10 nm and an increase in the shear modulus by an order of magnitude. Atomic force microscopy (AFM) is used to determine the surface morphology and RMS roughness of the coated and HA surfaces after citric acid challenges. Both physisorbed and cross-linked chitosan are demonstrated to limit and prevent the erosion of HA, respectively.

New poly(N,N-dimethylaminoethyl methacrylate)/polyvinyl alcohol copolymer coated QCM sensor for interaction with CWA simulants

A series of copolymers, namely, poly (N, N-dimethylaminoethyl methacrylate)/polyvinyl alcohol (PDMAEMA/PVA), have been obtained through emulsion polymerization. Then, a synthetic copolymer (PDMAEMA/PVA)-coated quartz crystal microbalance sensor (PC-QCM) has been developed to apply in evaluating the interactions between the copolymers and three chemical warfare agent (CWA) simulants, such as diethyl chlorophosphate (DCP), 2,2'-dichlorodiethyl ether (DCE), and dimethyl methylphosphonate (DMMP). The results from the QCM measurements indicate that the PDMAEMA/PVA copolymers not only directly degradate sarin simulants (DCP and DMMP), but also show obvious absorption in mustard gas simulant (DCE). In addition, the mass of degraded DCP by the copolymer is calculated on the basis of resonance frequency shifts (Δf) of quartz crystal in the QCM sensor, which may be applied to quantitatively evaluate the reactivity of PDMAEMA/PVA copolymers. Furthermore, the fluorescence of PDMAEMA/PVA copolymers in DMSO solution gradually quenched with the addition of DCP solution, which can be explained that the reactions take place between DCP and reactive groups of PDMAEMA/PVA copolymers. Thus, this PDMAEMA/PVA membrane may be a good candidate for the application in chemical warfare protective materials, and this new PC-QCM technique shows potential application in the rapid, real-time, and sensitive detection of the chemical warfare agents and hazardous chemicals.