Microscopic Origins of the Nonlinear Behavior of Particle-Filled Rubber Probed with Dynamic Strain XPCS.

The underlying microscopic response of filler networks in reinforced rubber to dynamic strain is not well understood due to the experimental difficulty of directly measuring filler network behavior in samples undergoing dynamic strain. This difficulty can be overcome with in situ X-ray photon correlation spectroscopy (XPCS) measurements. The contrast between the silica filler and the rubber matrix for X-ray scattering allows us to isolate the filler network behavior from the overall response of the rubber. This in situ XPCS technique probes the microscopic breakdown and reforming of the filler network structure, which are responsible for the nonlinear dependence of modulus on strain, known in the rubber science community as the Payne effect. These microscopic changes in the filler network structure have consequences for the macroscopic material performance, especially for the fuel efficiency of tire tread compounds. Here, we elucidate the behavior with in situ dynamic strain XPCS experiments on industrially relevant, vulcanized rubbers filled (13 vol %) with novel air-milled silica of ultrahigh-surface area (UHSA) (250 m2/g). The addition of a silane coupling agent to rubber containing this silica causes an unexpected and counterintuitive increase in the Payne effect and decrease in energy dissipation. For this rubber, we observe a nearly two-fold enhancement of the storage modulus and virtually equivalent loss tangent compared to a rubber containing a coupling agent and conventional silica. Interpretation of our in situ XPCS results simultaneously with interpretation of traditional dynamic mechanical analysis (DMA) strain sweep experiments reveals that the debonding or yielding of bridged bound rubber layers is key to understanding the behavior of rubber formulations containing the silane coupling agent and high-surface area silica. These results demonstrate that the combination of XPCS and DMA is a powerful method for unraveling the microscale filler response to strain which dictates the dynamic mechanical properties of reinforced soft matter composites. With this combination of techniques, we have elucidated the great promise of UHSA silica when used in concert with a silane coupling agent in filled rubber. Such composites simultaneously exhibit large moduli and low hysteresis under dynamic strain.

Design of Interfacial Crowding for Elastomeric Reinforcement with Nanocrystals.

Design of the crowding of chains tethered at the faces of ß-sheet nanocrystals self-assembled from ß-alanine trimers grafted on polyisobutylene (PIB) rubber tailors nanocrystal size and thus the elastic matrix morphology, thereby altering the material's macroscopic elastic properties. Results from transmission electron microscopy, small-angle X-ray scattering, and small-angle neutron scattering characterizations of the morphology demonstrate that increasing the density of chain tethering at the crystalline nanodomain/matrix interface can sharply limit the nanodomain growth in the direction of hydrogen bonding in the crystals. The nanocrystal size, in turn, impacts the gradient in chain stretching away from the crystal surface and the macroscopic volume fraction of unperturbed chains. Nanocomposite mechanical and dynamic mechanical properties at low degrees of deformation are related to the structural hierarchy resulting from the control of interfacial tethering density.

Influence of Silane Coupling Agents on Filler Network Structure and Stress-Induced Particle Rearrangement in Elastomer Nanocomposites.

Filled rubber materials are key in many technologies having a broad impact on the economy and sustainability, the most obvious being tire technology. Adding filler dramatically improves the strength of rubber by reinforcement and tailoring the type of filler, and the chemistry of the interface between the filler and rubber matrix is important for optimizing performance metrics such as fuel efficiency. In a highly loaded, silica-filled, cross-linked model rubber closely mimicking commercial materials, both the filler network structure and the dynamics of the silica filler particles change when the silica surface is modified with silane coupling agents. Reduction in size scales characteristic of the structure is quantified using ultra-small-angle X-ray scattering (USAXS) measurements and the particle dynamics probed with X-ray photon correlation spectroscopy (XPCS). While the structure averaged over the scattering volume changes little with aging after step strain, the dynamics slow appreciably in a manner that varies with the treatment of the silica filler. The evolution of filler particle dynamics depends on the chemical functionality at the silica surface, and observing these differences suggests a way of thinking about the origins of hysteresis in nanoparticle-reinforced rubbers. These microscopic filler dynamics are correlated with the macroscopic stress relaxation of the filled materials. The combination of static and dynamic X-ray scattering techniques with rheological measurements is a powerful approach for elucidating the microscopic mechanisms of rubber reinforcement.

Thin Plasma-Polymerized Coatings as a Primer with Polyurethane Topcoat for Improved Corrosion Resistance.

Use of a plasma-polymerized (pp) layer under a polyurethane (PU) coating on aluminum dramatically improves the corrosion resistance. Compared to conventional polymer coatings, pp coatings are highly cross-linked, have better adhesion to substrates, and result in lower emission of volatile organic contents. Although past research has focused on the properties of comparatively thick pp films and on the use of pp films alone to protect metals, we consider here very thin pp coatings as a primer layer to improve corrosion resistance. Electrochemical impedance spectroscopy combined with salt spray lab tests show that the corrosion resistance of a PU coating on top of a pp coating from hexamethyldisiloxane (HMDSO) is much better than that of a PU coating directly on Al 3003. The relatively poor pull-off adhesion between PU and pp-HMDSO is readily addressed using a gradient coating by depositing a pp maleic anhydride layer over the pp-HMDSO coating or by modifying the surface composition of the pp-HMDSO coating with N2 plasma. X-ray photon spectroscopy analysis of the failure interface from pull-off tests makes clear that failure does not occur at the interface between the pp coating and the metal substrate. Field tests show the performance of the coating system with PU on a gradient coating on Al 3003 to be superior to that of a coating system of PU on chromate-treated Al 3003.

Following the Morphological Disruption by an Electrolyte of a Buried Interface.

A challenge of broad interest in both materials science and biology is the study of interfaces that are buried within a structure, particularly multilayer structures. Despite the enormous costs of corrosion and many decades of corrosion research, details of the mechanisms of various sorts of corrosion are still not clear, in part due to the difficulty in interrogating the interface between the corroding metal and an organic coating, which is typically used to mitigate corrosion. Generally, the performance of such coatings is evaluated by visual inspection after exposure or by modeling impedance data, which is a process not straightforwardly connected to physical interface structures. "Rocking-curve" X-ray scattering measurements provide a means of probing such interfaces due to the ability of X-rays to penetrate materials. Here, variations in the morphology of an interface between a protective coating and a metal substrate due to exposure to an electrolyte are derived from analysis of rocking-curve data in conjunction with atomic force microscopy imaging of the outer coating surface. The interfaces of cross-linked epoxy coatings with aluminum are irreversibly changed after 12 h of contact between the electrolyte solution and the face of the coating. The character of this change varies with the molecule used to cross-link the coating. Since X-ray off-specular scattering is sensitive to changes on the nanometer scale, it is also able to register interface degradation on time scales shorter than those probed by many other techniques, potentially expediting the evaluation of coatings for protection against degradation of the interface.

Surface Layer Matrix-Assisted Laser Desorption Ionization Mass Spectrometry Imaging: A Surface Imaging Technique for the Molecular-Level Analysis of Synthetic Material Surfaces.

Surface layer matrix-assisted laser desorption ionization mass spectrometry imaging (SL-MALDI-MSI) is a powerful new surface sensitive imaging technique to establish surface component localization of multicomponent polymer materials. This study demonstrates the ability of SL-MALDI-MSI to image defects from foreign materials, material absence, mechanical scribing, and solvent perturbation at the surface of low-molecular-weight poly(methyl methacrylate) and polystyrene thin films. The surface specificity of the SL-MALDI-MSI technique is validated by imaging polystyrene on poly(methyl methacrylate) bilayer films; only polystyrene ions are detected from the surface of the unperturbed polystyrene layer. A key process enabling SL-MALDI-MSI is the solvent-free sublimation of matrix and salt uniformly on the sample's surface.

In Situ Nanoscale Characterization of Water Penetration through Plasma Polymerized Coatings.

The search continues for means of making quick determinations of the efficacy of a coating for protecting a metal surface against corrosion. One means of reducing the time scale needed to differentiate the performance of different coatings is to draw from nanoscale measurements inferences about macroscopic behavior. Here we connect observations of the penetration of water into plasma polymerized (PP) protective coatings and the character of the interface between the coating and an oxide-coated aluminum substrate or model oxide-coated silicon substrate to the macroscopically observable corrosion for those systems. A plasma polymerized film from hexamethyldisiloxane (HMDSO) monomer is taken as illustrative of a hydrophobic coating, while a PP film from maleic anhydride (MA) is used as a characteristically hydrophilic coating. The neutron reflectivity (NR) of films on silicon oxide coated substrates shows that water moves more readily through the hydrophilic PP-MA film. Off-specular X-ray scattering indicates the PP-MA film on aluminum is less conformal with the substrate than is the PP-HMDSO film. Measurements with infrared-visible sum frequency generation spectroscopy (SFG), which probes the chemical nature of the interface, make clear that the chemical interactions between coating and aluminum oxide are disrupted by interfacial water. With this water penetration and interface disruption, macroscopic corrosion can occur much more rapidly. An Al panel coated with PP-MA corrodes after 1 day in salt spray, while a similarly thin (∼30 nm) PP-HMDSO coating protects an Al panel for a period on the order of one month.

Detection of Surface Enrichment Driven by Molecular Weight Disparity in Virtually Monodisperse Polymers.

The preference for a shorter chain component at a polymer blend surface impacts surface properties key to application-specific performance. While such segregation is known for blends containing low molecular weight additives or systems with large polydispersity, it has not been reported for anionically polymerized polymers that are viewed, in practice, as monodisperse. Observations with surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS), which distinguishes surface species without labeling and provides the entire molecular weight distribution, demonstrate that entropically driven surface enrichment of shorter chains occurs even in low polydispersity materials. For 6 kDa polystyrene the number-average molecular weight (Mn) at the surface is ca. 300 Da (5%) lower than that in the bulk, and for 7 kDa poly(methyl methacryalate) the shift is ca. 500 Da. These observations are in qualitative agreement with results from a mean-field theory that considers a homopolymer melt with a molecular-weight distribution matched to the experiments.

Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films.

The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10Rg to 107Rg. For 15k four-arm star polystyrene (SPS), confinement appears at a thickness between 112 nm (40Rg) and 72 nm (26Rg), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7Rg) and 6k and 14k cyclic (24 and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6Rg) thick highly viscous layer at the substrate, which is significantly thicker than the 1Rg thick "irreversibly adsorbed" layer. For a 29 nm (10Rg) thick film, more striking confinement occurs due to the combined influence of both interfaces. These results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.

Subtle End Group Functionalization of Polymer Chains Drives Surface Depletion of Entire Polymer Chains.

The surface of a blend of 6 kDa polystyrene and 6 kDa polystyrene functionalized with hydroxymethyl ends not only is depleted of the higher energy end groups but also is depleted of any segments belonging to the functionalized chains. This is demonstrated using the emerging technique of surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS), which detects entire chains that have any repeat unit at the outer surface, and requires no labeling. Detecting entire chains provides information about the relationship of chain functionalization to surface segregation behavior of entire chains. That the surface is depleted of interior segments of functionalized chains as well as of the ends is remarkable, since the functionality at the single chain end involves less than 0.5 wt % of the functionalized polymer chain.

Tunable, Liquid Resistant Tip Enhanced Raman Spectroscopy Probes: Toward Label-Free Nano-Resolved Imaging of Biological Systems.

Tip enhanced Raman spectroscopy (TERS) has been established as a powerful, noninvasive technique for chemical identification at the nanoscale. However, difficulties, including the degradation of probes, limit its use in liquid systems. Here TERS probes for studies in aqueous environments have been demonstrated using titanium nitride coatings with an alumina protective layer. The probes show enhancement in signal intensity as high as 380% in liquid measurements, and the probe resonance can be tuned by varying deposition conditions to optimize performance for different laser sources and types of samples. This development of inexpensively produced probes suited for studies in aqueous environments enables its wider use for fields such as biology and biomedicine in which aqueous environments are the norm.

Evidence and Limits of Universal Topological Surface Segregation of Cyclic Polymers.

If you mix lines and circles, what happens at the edge of the mixture? The problem is simply stated, but the answer is not obvious. Twenty years ago it was proposed that a universal topological driving force would drive cyclic chains to enrich the surface of blends of linear and cyclic chains. Here such behavior is demonstrated experimentally for sufficiently long chains and the limit in molecular weight where packing effects dominate over the topological driving force is identified.

Modifying Surface Fluctuations of Polymer Melt Films with Substrate Modification.

Deposition of a plasma polymerized film on a silicon substrate substantially changes the fluctuations on the surface of a sufficiently thin melt polystyrene (PS) film atop the substrate. Surface fluctuation relaxation times measured with X-ray photon correlation spectroscopy (XPCS) for ca. 4Rg thick melt films of 131 kg/mol linear PS on hydrogen-passivated silicon (H-Si) and on a plasma polymer modified silicon wafer can both be described using a hydrodynamic continuum theory (HCT) that assumes the film is characterized throughout its depth by the bulk viscosity. However, when the film thickness is reduced to ∼3Rg, confinement effects are evident. The surface fluctuations are slower than predicted using the HCT, and the confinement effect for the PS on H-Si is larger than that for the PS on the plasma polymerized film. This deviation is due to a difference in the thicknesses of the strongly adsorbed layers at the substrate which are impacted by the substrate surface energy.

Effect of tethering on the surface dynamics of a thin polymer melt layer.

The surface height fluctuations of a layer of low molecular weight (2.2k) untethered perdeuterated polystyrene (dPS) chains adjacent to a densely grafted polystyrene brush are slowed dramatically. Due to the interpenetration of the brush with the layer of "untethered chains" a hydrodynamic continuum theory can only describe the fluctuations when the effective thickness of the film is taken to be that which remains above the swollen brush. The portion of the film of initially untethered chains that interpenetrates with the brush becomes so viscous as to effectively play the role of a rigid substrate. Since these hybrid samples containing a covalently tethered layer at the bottom do not readily dewet, and are more robust than thin layers of untethered short chains on rigid substrates, they provide a route for tailoring polymer layer surface properties such as wetting, adhesion and friction.

Scaling Behavior and Segment Concentration Profile of Densely Grafted Polymer Brushes Swollen in Vapor.

The scaling of the thickness, hs, of a densely grafted polymer brush of chain length N and grafting density σ swollen in vapor agrees quantitatively with the scaling reported by Kuhl et al. for densely grafted brushes swollen in liquid. Deep in the brush, next to the substrate, the shape of the segment concentration profile is the same whether the brush is swollen by liquid or by vapor. Differences in the segment concentration profile are manifested primarily in the swollen brush interface with the surrounding fluid. The interface of the polymer brush swollen in vapor is much more abrupt than that of the same brush swollen in liquid. This has implications for the compressibility of the swollen brush surface and for fluctuations at that surface.

Confinement Effects with Molten Thin Cyclic Polystyrene Films.

The surface fluctuations of a melt film of a low molecular weight cyclic polystyrene (CPS) manifest confinement effects for a film thickness (14Rg) much larger than that for which a melt film of the linear chain analog manifests confinement. This is true both in terms of absolute thickness and thickness relative to chain size, Rg. In fact, the linear analog polymer does not manifest confinement effects even at a thickness of 7Rg. Both types of films have a strongly adsorbed layer at the substrate that plays a role in slowing the surface fluctuations for the thinnest films. This layer is 70% thicker for the cyclic chains than for the linear chains. At the interface with the substrate the packing of the cyclic chains is perturbed much more strongly than is the packing of the linear chains.

Surface segregation driven by molecular architecture asymmetry in polymer blends.

The contributions of chain ends and branch points to surface segregation of long-branched chains in blends with linear chains have been studied using neutron reflectometry and surface-enhanced Raman spectroscopy for a series of novel, well-defined polystyrenes. A linear response theory accounting for the number and type of branch points and chain ends is consistent with surface excesses and composition profile decay lengths, and allows the first determination of branch point potentials. Surface excess is determined primarily by chain ends with branch points playing a secondary role.

Anomalous surface relaxations of branched-polymer melts.

The dynamics of thermally stimulated surface fluctuations of 100 nm thick films of long-branched polymers are measured for the first time. In contrast to comparable films of linear or cyclic chains that show no change in viscosity upon confinement, films of 6-pom, 6-star, and 6-end end-branched stars show viscosities, inferred from x-ray photon correlation spectroscopy, as much as 100 times higher than in the bulk. This difference varies in magnitude with chain architecture. Branching has a profound effect on confinement, even for these unentangled chains.

Manipulation of polymer/polymer interface width from nonequilibrium deposition.

We demonstrate, using neutron reflectivity, that the width of a nonequilibrium interface between an organo-soluble aromatic polyimide film and triacetate cellulose (TAC) support film created by spin-coating or solution-casting can be broadened in a controllable way using a "swelling agent" in the deposition process. In a favorable case, the adhesion, as measured by T-peel tests, can be increased by a factor of 7 by adjustment of the solvent composition. The morphologies of the TAC fractured surfaces after peeling tests measured by AFM reveal that broadening of the interfacial width causes an interconnected network in the interface, leading to a sharp increase in the interfacial adhesion. Differences in the chemistry (solubility) of the materials being deposited do make a difference in the effectiveness of this strategy of using a "swelling agent". For one polyimide, a 3-fold increase in adhesion can be obtained by optimizing the deposition temperature, but this approach for improving adhesion is less effective than that of adding "swelling agent". The formation of robust interfaces of this type is important because of the critical roles that multilayer films containing polymers with special properties and tailored structures play in applications as diverse as computer displays, photovoltaic devices, and polymeric electronics. The "swelling agent" strategy makes it possible to produce polymer multilayer structures in a cost-effective way with roll-to-roll mass production using direct solution coating.

Differentiation of linear and cyclic polymer architectures by MALDI tandem mass spectrometry (MALDI-MS2).

[M + Ag](+) ions from cyclic and linear polystyrenes and polybutadienes, formed by matrix-assisted laser desorption ionization (MALDI), give rise to significantly different fragmentation patterns in tandem mass spectrometry (MS(2)) experiments. In both cases, fragmentation starts with homolytic cleavage at the weakest bond, usually a C-C bond, to generate two radicals. From linear structures, the separated radicals depolymerize extensively by monomer losses and backbiting rearrangements, leading to low-mass radical ions and much less abundant medium- and high-mass closed-shell fragments that contain one of the original end groups, along with internal fragments. With cyclic structures, depolymerization is less efficient, as it can readily be terminated by intramolecular H-atom transfer between the still interconnected radical sites (disproportionation). These differences in fragmentation reactivity result in substantially different fragment ion distributions in the MS(2) spectra. Simple inspection of the relative intensities of low- versus high-mass fragments permits conclusive determination of the macromolecular architecture, while full spectral interpretation reveals the individual end groups of linear polymers or the identity of the linker used to form the cyclic polymer.