Cooperative Tridentate Hydrogen-Bonding Interactions Enable Strong Underwater Adhesion.

Multidentate hydrogen-bonding interactions are a promising strategy to improve underwater adhesion. Molecular and macroscale experiments have revealed an increase in underwater adhesion by incorporating multidentate H-bonding groups, but quantitatively relating the macroscale adhesive strength to cooperative hydrogen-bonding interactions remains challenging. Here, we investigate whether tridentate alcohol moieties incorporated in a model epoxy act cooperatively to enhance adhesion. We first demonstrate that incorporation of tridentate alcohol moieties leads to comparable adhesive strength with mica and aluminum in air and in water. We then show that the presence of tridentate groups leads to energy release rates that increase with an increase in crack velocity in air and in water, while materials lacking these groups do not display rate-dependent adhesion. We model the rate-dependent adhesion to estimate the activation energy of the interfacial bonds. Based on our data, we estimate the lifetime of these bonds to be between 2 ms and 6 s, corresponding to an equilibrium activation energy between 23kBT and 31kBT. These values are consistent with tridentate hydrogen bonding, suggesting that the three alcohol groups in the Tris moiety bond cooperatively form a robust adhesive interaction underwater.

Influence of Interfacial Bonding on the Mechanical and Impact Properties Ring-Opening Metathesis Polymer (ROMP) Silica Composites.

Polymers formed by ring-opening metathesis polymerization (ROMP) such as poly(dicyclopentadiene) (pDCPD) exhibit a technologically desirable combination of high toughness, high glass transition temperature, and outstanding low-temperature performance. However, because of their nonpolar molecular structure, they tend to suffer from relatively low elastic moduli and poor adhesion to common fillers, fibers, and substrates, limiting their utility as adhesives and composite binders without specialized bonding agents. Here, we investigate the mechanical properties of a pDCPD-based copolymer filled with well-defined spherical microparticles having four distinct surface chemistries capable of strong, moderate, or weak bonding to the matrix with surfaces ranging from polar to nonpolar. Measurements in uniaxial tension, quasi-static fracture, and high-velocity impact are complemented by digital image correlation (DIC), scanning electron microscopy (SEM) fractography, and X-ray computed tomography (X-µCT) of subcritically loaded crack tips, yielding insight into the complex roles played by interfacial bonding in strength, stiffness, and toughening mechanisms of an already tough polymer. Analysis using quantitative fracture and impact mechanism models provided valuable guidelines for designing heterogeneous systems that combine structural and tough performance.

Molecular Weight Control via Cross Metathesis in Photo-Redox Mediated Ring-Opening Metathesis Polymerization.

Photo-redox mediated ring-opening metathesis polymerization (photo-ROMP) is an emerging ROMP technique that uses an organic redox mediator and a vinyl ether initiator, in contrast to metal-based initiators traditionally used in ROMP. The reversibility of the redox-mediated initiation and propagation steps enable spatiotemporal control over the polymerization. Herein, we explore a simple, inexpensive means of controlling molecular weight, using alpha olefins as chain transfer agents. This method enables access to low molecular weight oligomers, and molecular weights between 1 and 30 kDa can be targeted simply by altering the stoichiometry of the reaction. This method of molecular weight control was then used to synthesize a functionalized norbornene copolymer in a range of molecular weights for specific materials applications.

Electrochemical Surface Treatment of Discontinuous Carbon Fibers.

We developed an operationally simple electrolytic design for the surface treatment of short carbon fibers. Using X-ray photoelectron spectroscopy (XPS), we demonstrated that the electrochemical surface treatment of discontinuous fibers is highly reproducible, uniform, and tunable. Specifically, total amounts of surface oxygen and nitrogen contents (0 to 17 atomic %) as well as surface oxygen-to-nitrogen ratio (1:0 to 1:2) vary significantly over the ranges of each processing parameter: applied voltage (1.5-21 V), location of carbon fiber (i.e., anode, cathode, or mixed mode), initial temperature (3-70.5 °C), and ammonium bicarbonate concentration (0.005-0.75 M). Optimized processing conditions afforded carbon fibers that have similar surface compositions (86.3 ± 1.1 at. % C, 8.9 ± 0.8 at. % O, 4.7 ± 0.6 at. % N) as those of commercially available continuous fibers. In addition, these fibers retain their mechanical properties (tensile strength and tensile modulus) and exhibit no detectable surface damage based on single fiber tensile tests and scanning electron microscopy (SEM). We also performed a number of control experiments to develop a proposed mechanism for the surface functionalization of the carbon fiber. These mechanistic studies demonstrated that water splitting contributes significantly to the oxidation of carbon fibers and that other species in the chemical equilibria of ammonium bicarbonate (and not just its individual ions) play a significant role in functionalizing carbon fiber surfaces.

Time Dependent Structure and Property Evolution in Fibres during Continuous Carbon Fibre Manufacturing.

Here we report on how residence time influences the evolution of the structure and properties through each stage of the carbon fibre manufacturing process. The chemical structural transformations and density variations in stabilized fibres were monitored by Fourier Transform Infrared Spectroscopy and density column studies. The microstructural evolution and property variation in subsequent carbon fibres were studied by X-ray diffraction and monofilament tensile testing methods, which indicated that the fibres thermally stabilized at longer residence times showed higher degrees of structural conversion and attained higher densities. Overall, the density of stabilized fibres was maintained in the optimal range of 1.33 to 1.37 g/cm³. Interestingly, carbon fibres manufactured from higher density stabilized fibres possessed lower apparent crystallite size (1.599 nm). Moreover, the tensile strength of carbon fibres obtained from stabilized fibres at the high end of the observed range (density: 1.37 g/cm³) was at least 20% higher than the carbon fibres manufactured from low density (1.33 g/cm³) stabilized fibres. Conversely, the tensile modulus of carbon fibres produced from low density stabilized fibres was at least 17 GPa higher than those from high density stabilized fibres. Finally, it was shown that there is potential to customize the required properties of resultant carbon fibres suiting specific applications via careful control of residence time during the stabilization stage.

Influence of molecular weight between crosslinks on the mechanical properties of polymers formed via ring-opening metathesis.

The apparent molecular weight between crosslinks (Mc,a) in a polymer network plays a fundamental role in the network mechanical response. We systematically varied Mc,a independent of strong noncovalent bonding by using ring-opening metathesis polymerization (ROMP) to co-polymerize dicyclopentadiene (DCPD) with a chain extender that increases Mc,a or a di-functional crosslinker that decreases Mc,a. We compared the ROMP series quasi-static modulus (E), tensile yield stress (σy), and fracture toughness (KIC and GIC) in the glassy regime with literature data for more polar thermosets. ROMP resins showed high KIC (>1.5 MPa m0.5), high GIC (>1000 J m-2), and 4-5 times higher high rate impact resistance than typical polar thermosets with similar Tg values (100 °C to 178 °C). The overall E values were lower for ROMP systems. The σy dependence on Mc,a and T-Tg for ROMP resins was qualitatively similar to more polar thermosets, but the overall σy values were lower. In contrast to more polar thermosets, the KIC and GIC values of the ROMP resins showed strong Mc,a and T-Tg dependence. High rate impact (∼104-105 s-1) trends were similar to the KIC and GIC behavior, but were also correlated to σy. Overall, a ductile failure mode was observed for quasi-static and high rate results for a linear ROMP polymer (Mc,a = 1506 g mol-1 due to chain entanglement), and this gradually transitioned to a fully brittle failure mode for highly crosslinked ROMP polymers (Mc,a ≤ 270 g mol-1). Molecular dynamics (MD) simulations showed that low Mc,a ROMP resins were more likely to form molecular scale nanovoids. The higher chain stiffness in low Mc,a ROMP resins inhibited stress relaxation in the vicinity of these nanovoids, which correlated with brittle mechanical responses. Overall, these differences in mechanical properties were attributed to the weak non-covalent interactions in ROMP resins.

Polydopamine and Polydopamine-Silane Hybrid Surface Treatments in Structural Adhesive Applications.

Numerous studies have focused on the remarkable adhesive properties of polydopamine, which can bind to substrates with a wide range of surface energies, even under aqueous conditions. This behavior suggests that polydopamine may be an attractive option as a surface treatment in structural bonding applications, where good bond durability is required. Here, we assessed polydopamine as a surface treatment for bonding aluminum plates with an epoxy resin. A model epoxy adhesive consisting of diglycidyl ether of bisphenol A (DGEBA) and Jeffamine D230 polyetheramine was employed, and lap shear measurements (ASTM D1002 10) were made (i) under dry conditions to examine initial bond strength and (ii) after exposure to hot/wet (63 °C in water for 14 days) conditions to assess bond durability. Surprisingly, our results showed that polydopamine alone as a surface treatment provided no benefit beyond that obtained by exposing the substrates to an alkaline solution of tris buffer used for the deposition of polydopamine. This implies that polydopamine has a potential Achilles' heel, namely, the formation of a weak boundary layer that was identified using X-ray photoelectron spectroscopy (XPS) of the fractured surfaces. In fact, for longer deposition times (2.5 and 18 h), the tris buffer-treated surface outperformed the polydopamine surface treatments, suggesting that tris buffer plays a unique role in improving adhesive performance even in the absence of polydopamine. We further showed that the use of polydopamine-3-aminopropyltriethoxysilane (APTES) hybrid surface treatments provided significant improvements in bond durability at extended deposition times relative to both polydopamine and an untreated control.

Synthesis and Characterization of Aminopropyltriethoxysilane-Polydopamine Coatings.

Polydopamine coatings are of interest due to the fact that they can promote adhesion to a broad range of materials and can enable a variety of applications. However, the polydopamine-substrate interaction is often noncovalent. To broaden the potential applications of polydopamine, we show the incorporation of 3-aminopropyltriethoxysilane (APTES), a traditional coupling agent capable of covalent bonding to a broad range of organic and inorganic surfaces, into polydopamine coatings. High energy X-ray photoelectron spectroscopy (HE-XPS), conventional XPS, near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), and ellipsometry measurements were used to investigate changes in coating chemistry and thickness, which suggest covalent incorporation of APTES into polydopamine. These coatings can be deposited either in Tris buffer or by using an aqueous APTES solution as a buffer without Tris. APTES-dopamine hydrochloride deposition from solutions with molar ratios between 0:1 and 10:1 allowed us to control the coating composition across a broad range.

Nanovoid formation and mechanics: a comparison of poly(dicyclopentadiene) and epoxy networks from molecular dynamics simulations.

Protective equipment in civilian and military applications requires the use of polymer materials that are both stiff and tough over a wide range of strain rates. However, typical structural materials, like tightly cross-linked epoxies, are very brittle. Recent experiments demonstrated that cross-linked poly(dicyclopentadiene) (pDCPD) networks can circumvent this trade-off by providing structural properties such as a high glass transition temperature and glassy modulus, while simultaneously exhibiting excellent toughness and high-rate impact resistance. The greater performance of pDCPD was attributed to more facile plastic deformation and nano-scale void formation, but the chemical and structural mechanisms underlying this response were not clear. Here, we use atomistic molecular dynamics to compare the molecular- and chain-level properties of pDCPD and epoxy networks undergoing high strain rate deformation. We quantify the tensile modulus and yield strength of the networks as well as the prevalence and characteristics of nanovoids that form during deformation. Networks of similar molecular weight between cross-links are compared. Two key molecular-level properties are identified - monomer flexibility and polar chemistry - that influence the behavior of the networks. Increasing monomer flexibility reduces the modulus and yield strength, while strong non-covalent interactions (e.g., hydrogen bonds) that accompany polar moieties provide higher modulus and yield strength. The lack of strong non-covalent interactions in pDCPD was found to account for its lower modulus and yield strength compared to the epoxies. We examine the molecular-level properties of nanovoids, such as shape, alignment, and local stress distribution, as well as the local chemical environment, finding that nanovoid formation and growth are increased by the monomer rigidity but decreased by polar chemistry. As a result, the pDCPD network, which has a stiff chain backbone with nonpolar alkane chemistry, exhibits more and larger nanovoids that grow more readily during deformation, which could account for the higher toughness and more ductile behavior observed in pDCPD.

Expanded Functionality of Polymers Prepared Using Metal-Free Ring-Opening Metathesis Polymerization.

Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is an alternative to traditional metal-mediated ROMP that avoids the use of transition metal initiators while also enabling temporal control over the polymerization. Herein, we explore the effect of various additives on the success of the polymerization in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP. The use of protected alcohol monomers allows for homo- and copolymers to be prepared that contain functionality beyond simple alkyl groups. Several other functional groups are also tolerated to varying degrees and offer insight into future directions for expansion of monomer scope.

Nanoscale phase analysis of molecular cooperativity and thermal transitions in dendritic nonlinear optical glasses.

A broad nanoscopic study of a wide-range of dendritic organic nonlinear optical (NLO) self-assembly molecular glasses reveals an intermediate thermal phase regime responsible for both enhanced electric field poling properties and strong phase stabilization after poling. In this paper, the focus is on dendritic NLO molecular glasses involving quadrupolar, liquid crystal, and hydrogen bonding self-assembly mechanisms that, along with chromophore dipole-dipole interactions, dictate phase stability. Specifically, dendritic face-to-face interactions involving arene-perfluoroarene are contrasted to coumarin-containing liquid crystal mesogen and cinnamic ester hydrogen interactions. Both the strength of dendritic interactions and the impact of dipole fields on the relaxation behavior have been analyzed by nanoscale energetic probing and local thermal transition analysis. The presence of dendritic groups was found to fundamentally alter transition temperatures and the molecular relaxation behavior. Thermal transition analysis revealed that molecules with dendritic groups possess an incipient transition (T(1)) preceding the glass transition temperature (T(2)) that provides increased stability and a well-defined electric field poling regime (T(1) < T < T(2)), in contrast to molecular groups lacking dendrons that exhibit only single transitions. On the basis of enthalpic and entropic energetic analyses, thermally active modes below T(1) were found to be intimately connected to the dendron structure. Their corresponding activation energies, which are related to thermal stability, increased moving from cinnamic ester groups to coumarin moieties to arene-perfluoroarene interacting groups. While dendritic NLO materials were found to possess only enthalpic stabilization energies at temperatures relevant for device operation (T < T(1)), the apparent molecular binding energies above T(1) contain a substantial amount (up to ~80%) of cooperative entropic energy. The multiple interactions (from dipole-dipole interactions to local noncovalent dendritic interactions) are discussed and summarized in a model that describes the thermal transitions and phases.

Manipulation of interfacial amine density in epoxy-amine systems as studied by near-edge X-ray absorption fine structure (NEXAFS).

In this work, we investigate the ability to tune the quantity of surface amine functional groups in the interfacial region of epoxy-diamine composites using NEXAFS, a technique that is extremely sensitive to surface composition. Thereby, we employ a model surface (silicon wafer with the native oxide present) and, after deposition of an epoxy functionalized silane, we immersed the wafers in various diamines, followed by reaction with a diepoxy acting as a molecular probe. These results show that the number of available surface amines depends on the diamine chosen, wherein smaller molecular weight diamines provide more reaction sites. Subsequent experiments with mixtures of diamines undergoing competitive adsorption show that the amine quantity can be tailored by choice of the diamine mixture. Further experiments of diamine treated 3-(glycidoxypropyl) trimethoxysilane layers in a reacting epoxy/diamine showed that the surface reaction site density differences observed for adsorption experiments persisted in the reacting epoxy, implying that the surface reaction rate (and by extension, the surface amine concentration) dictate interfacial cross-link density up to the point of gelation.

Nano-engineering lattice dimensionality for a soft matter organic functional material.

A high performing electro-optic (EO) chromophore with covalently attached coumarin-based pendant groups exhibits intermolecular correlation of coumarin units through molecular dynamics (MD) simulations. Unique, orthogonal molecular orientations of the chromophore and coumarin units are also evident when investigated optically. Such molecular orientation translates to reduced lattice dimensionality of the bulk C1 soft matter material system, leading to increased acentric order and EO activity. Results are corroborated by nanorheological experimental methods.

Molecular friction dissipation and mode coupling in organic monolayers and polymer films.

The impact of thermally active molecular rotational and translational relaxation modes on the friction dissipation process involving smooth nano-asperity contacts has been studied by atomic force microscopy, using the widely known Eyring analysis and a recently introduced method, dubbed intrinsic friction analysis. Two distinctly different model systems, i.e., monolayers of octadecyl-phosphonic acid (ODPA) and thin films of poly(tert-butyl acrylate) (PtBA) were investigated regarding shear-rate critical dissipation phenomena originating from diverging mode coupling behaviors between the external shear perturbation and the internal molecular modes of relaxation. Rapidly (ODPA) versus slowly (PtBA) relaxing systems, in comparison to the sliding rate, revealed monotonous logarithmic and nonmonotonous spectral shear rate dependences, respectively. Shear coupled, enthalpic activation energies of 46 kJ∕mol for ODPA and of 35 and ∼65 kJ∕mol for PtBA (below and above the glass transition) were found that could be attributed to intrinsic modes of relaxations. Also, entropic energies involved in the cooperative backbone mobility of PtBA could be quantified, dwarfing the activation energy by more than a factor of five. This study provides (i) a material specific understanding of the molecular scale dissipation process in shear compliant substances, (ii) analyses of material intrinsic shear-rate mode coupling, shear coordination and energetics, (iii) a verification of Eyring's model applied to tribological systems toward material intrinsic specificity, and (iv) a valuable extension of the Eyring analysis for complex macromolecular systems that are slowly relaxing, and thus, exhibit shear-rate mode coupling.

Molecular mobility in self-assembled dendritic chromophore glasses.

Increasing complexity in bottom-up molecular designs of amorphous structures with multiple relaxation modes demands an integrated and cognitive design approach, where chemical synthesis is guided by both analytical tools and theoretical simulations. In particular, this is apparent for novel organic second-order nonlinear optical materials of self-assembling molecular glasses involving dendritic arene stabilization moieties (phenyl, naphthyl, and anthryl) with electro-optical activities above 300 pm/V. In this study, nanoscale thermo-mechanical analyses yield direct insight into the molecular enthalpic and entropic relaxation modes. Arene-perfluoroarene interactions for coarse self-assembly are found to impose three phase relaxation regimes, with intermediate regimes of 8-15 degrees C in width and apparent activation energies between 40 and 60 kcal/mol to be the most effective for poling. Energetic analyses based on intrinsic friction microscopy (IFA) identify increasing temporal stability with increasing arene size for the low-temperature regime. Electric field poling efficiency is found to be inversely proportional to entropic cooperative contributions that can make up 80% of the overall apparent relaxation energy for the high-temperature regime. The origin for the activation energies below the incipient glass transition temperature, based on complementary molecular dynamic simulations, is tied primarily to noncovalent interactions between chromophore (dipole), dendritic (quadrupole) moieties, and combinations thereof.

Intrinsic friction analysis--novel nanoscopic access to molecular mobility in constrained organic systems.

Condensed organic materials designed for nanotechnological applications are impacted significantly by internal and external constraints. Internal constraints are inherent to the molecular architecture, and generally result from direct bonding, electrostatic interactions, or steric effects. Internal constraints can be incorporated a priori into molecular designs, as a prescription for desired material properties. Today's challenge lies in obtaining information about the molecular mobility, in particular, in thin organic structured or unstructured films. A method that has been recently introduced, and is well suited for free surface analysis of complex organic thin film materials is intrinsic friction analysis (IFA), a spectroscopic analysis method based on scanning force microscopy (SFM). Here, we present a critical assessment of the method with practical illustrations towards the study of intramolecular and molecular mobilities in amorphous organic non-linear optical (NLO) materials, specifically self-assembling molecular organic glasses and complex organic polymer side-chain systems.

Cooperative and submolecular dissipation mechanisms of sliding friction in complex organic systems.

Energy dissipation in single asperity sliding friction was directly linked to submolecular modes of mobility by intrinsic friction analysis, involving time-temperature superposition along with thermodynamic stress and reaction rate models. Thereby, polystyrene served as a representative tribological sample for organic and amorphous complex systems. This study reveals the significance of surface and subsurface (alpha-, beta-, and gamma-) relaxational modes, which couple under appropriate external conditions (load, temperature, and rate) with shear induced disturbances, and thus gives rise to material specific frictional dissipation. At low pressures and temperatures below the glass transition point, the phenyl pendant side groups of polystyrene, known for their preferential orientation at the free surface, were noticed to be the primary channel for dissipation of kinetic sliding-energy. While this process was found to be truly enthalpic (activation energy of 8 kcalmol), energy dissipation was shown to possess both enthalpic and cooperative entropic contributions above the loading capacity of the surface phenyl groups (9.9 kcalmol) or above the glass transition. Apparent Arrhenius activation energies of frictional dissipation of 22 and 90 kcalmol, respectively, and cooperative contributions up to 80% were found. As such, this study highlights issues critical to organic lubricant design, i.e., the intrinsic enthalpic activation barriers of mobile linker groups, the evaluation of cooperative mobility phenomena, and critical tribological parameters to access or avoid coupling between shear disturbances and molecular actuators.

Mesoscale dynamics and cooperativity of networking dendronized nonlinear optical molecular glasses.

First, molecular scale insight into the mobility of a novel class of organic materials for photonic applications with electro-optical activities larger than 300 pm/V is presented. A representative second order nonlinear optical (NLO) material of this class of self-assembling molecular glasses involving quadrupolar phenyl-perfluorophenyl (Ph-PhF) interactions is analyzed based on its molecular relaxation phenomena and phase behavior. Thereby, a new and straightforward nanoscale methodology, involving shear modulation force microscopy and intrinsic friction analysis is introduced. It provides both the submolecular enthalpic and entropic dynamics in nanoconstrained systems (e.g., ultrathin films), and thus, insight into local motion of single molecules due to dissociation of Ph-PhF pairs as well as the cooperative dynamics of the assembled network. This nanoscale model-independent thermomechanical methodology is shown to be very effective in fundamentally evaluating appropriate poling conditions of organic NLO materials. It promises to be a straightforward analysis tool to guide organic material synthesis from a molecular mobility perspective, particularly for applications that impose nanoscale constraints on the system.