Rate-independent hysteretic energy dissipation in collagen fibrils.

Nanoindentation cycles measured with an atomic force microscope on hydrated collagen fibrils exhibit a rate-independent hysteresis with return point memory. This previously unknown energy dissipation mechanism describes in unified form elastoplastic indentation, capillary adhesion, and surface leveling at indentation velocities smaller than 1 µm s-1, where viscous friction is negligible. A generic hysteresis model, based on force-distance data measured during one large approach-retract cycle, predicts the force (output) and the dissipated energy for arbitrary indentation trajectories (input). While both quantities are rate independent, they do depend nonlinearly on indentation history and on indentation amplitude.

Nanomechanical 3D Depth Profiling of Collagen Fibrils in Native Tendon.

Connective tissue displays a large compositional and structural complexity that involves multiple length scales. In particular, on the molecular and the nanometer level, the elementary processes that determine the biomechanics of collagen fibrils in connective tissues are still poorly understood. Here, we use atomic force microscopy (AFM) to determine the three-dimensional (3D) depth profiles of the local nanomechanical properties of collagen fibrils and their embedding interfibrillar matrix in native (unfixed), hydrated Achilles tendon of sheep and chickens. AFM imaging in air with controlled humidity preserves the tissue's water content, allowing the assembly of collagen fibrils to be imaged in high resolution beneath an approximately 5-10 nm thick layer of the fluid components of the interfibrillar matrix. We collect pointwise force-distance (FD) data and amplitude-phase-distance (APD) data, from which we construct 3D depth profiles of the local tip-sample interaction forces. The 3D images reveal the nanomechanical morphology of unfixed, hydrated collagen fibrils in native tendon with a 0.1 nm depth resolution and a 10 nm lateral resolution. We observe a diversity in the nanomechanical properties among individual collagen fibrils in their adhesive and in their repulsive, viscoelastic mechanical response as well as among the contact points between adjacent collagen fibrils. This sheds new light on the role of interfibrillar bonds and the mechanical properties of the interfibrillar matrix in the biomechanics of tendon.

Conformational Selection in Anion Recognition: cGMP-Selective Binding by a Naphthalimide-Functionalized Amido-Amine Macrocycle.

Amido-amine macrocycles with two and four naphthalimide dyes were designed to bind nucleoside monophosphates and oligonucleotides in an aqueous buffered solution. Anion-templated synthesis was used to direct the macrocyclization reaction to the [2+2] product, while high dilution conditions favored the formation of the [4+4] macrocycle with an unprecedented geometry, as revealed from the X-ray analysis. The [2+2] product was found to exhibit a remarkable binding strength and fluorescence response for cyclic guanosine monophosphate (cGMP) in an aqueous solution. To our knowledge, this is the first synthetic receptor for cGMP, which also demonstrates a high preference to bind guanine-rich sequences accomplished by a strong fluorescence quenching. The receptor conformation is very sensitive to the guest structure in an aqueous solution, thus modeling the adaptive behavior of proteins. The study of synthetic systems with a detectable conformational equilibrium represents a great potential for understanding highly specific and tightly regulated interactions in biological systems.

Liquid-Crystalline Order and Film Thickness Determine the Semicrystalline Morphology in Diketopyrrolopyrrole-Based Copolymers.

Lyotropic liquid crystalline (LC) phases offer a means of controlling molecular order and orientation in thin films of conjugated polymers. Surface energy, surface-induced ordering, and film thickness are additional factors determining the molecular order in thin films. Through solvent vapor annealing and in situ atomic force microscopy in the swollen state, we show that in ultrathin films of a poly(dithiazolyldiketopyrrolopyrrole-tetrafluorobenzene) (PTzDPPTzF4) alternating copolymer stacks of monomolecular-thick layers with a 2.1 nm step height form, which resemble a lyotropic smectic LC phase. Within the smectic layers, the polymer backbones are aligned parallel to the film plane, with edge-on oriented diketopyrrolopyrrole (DPP) cores. Thicker films resemble a semicrystalline morphology with lamellae consisting of blocks. Such lamellae are typical for polymers crystallizing via Strobl's block-forming model. Our findings indicate that molecular order, molecular orientation, and the morphology of PTzDPPTzF4 copolymer films are tunable by LC order and by varying the film thickness according to the desired application of the particular organic electronic devices.

3D depth profiling of the interaction between an AFM tip and fluid polymer solutions.

In the atomic force microscopy (AFM) investigation of soft polymers and liquids, the tip-sample interaction is dominated by long-range van der Waals forces, capillary forces and adhesion. Furthermore, the tip can indent several tens of nanometres into the surface, and it can pull off a polymer filament from the surface. Therefore, measuring the unperturbed shape of a polymeric fluid can be challenging. Here, we study the tip-sample interaction with polystyrene droplets swollen in chloroform vapour, where we can utilize the solvent vapour concentration to adjust the specimen's mechanical properties from a stiff solid to a fluid film. With the same AFM tip, we use two different AFM force spectroscopy methods to measure three-dimensional (3D) depth profiles of the tip-sample interaction: force-distance (FD) curves and amplitude-phase-distance (APD) curves. The 3D depth profiles reconstructed from FD and APD measurements provide detailed insight into the tip-sample interaction mechanism for a fluid polymer solution. The fluid's intrinsic relaxation time, which we measure with an AFM-based step-strain experiment, is essential for understanding the tip-sample interaction mechanism. Furthermore, measuring 3D depth profiles and using APD data to reconstruct the unperturbed surface comprise a versatile methodology for obtaining accurate dimensional measurements of fluid and gel-like objects on the nanometre scale.

Scaling properties of ageing orientation fluctuations in stripe phases.

We investigate the non-equilibrium dynamics of an ordered stripe-forming system free of topological defects. In particular, we study the ageing and the coarsening of orientation fluctuations parallel and perpendicular to the stripes via computer simulations based on a minimal phase-field model (model B with Coulomb interactions). Under the influence of noise, the stripe orientation field develops fluctuations parallel to the stripes, with the dominant modulation length λ*∥ increasing with time t as λ*∥ ∼ t1/4 and the correlation length perpendicular to the stripes ξ⊥θ increasing as ξ⊥θ ∼ t1/2. We explain these anisotropic coarsening dynamics with an analytic theory based on the linear elastic model for stripe displacements first introduced by Landau and Peierls. We thus obtain the scaling forms and the scaling exponents characterizing the correlation functions and the structure factor of the stripe orientation field. Our results reveal how the coarsening of orientation fluctuations prevents a periodically modulated phase free of topological defects from reaching equilibrium.

Unraveling capillary interaction and viscoelastic response in atomic force microscopy of hydrated collagen fibrils.

The mechanical properties of collagen fibrils depend on the amount and the distribution of water molecules within the fibrils. Here, we use atomic force microscopy (AFM) to study the effect of hydration on the viscoelastic properties of reconstituted type I collagen fibrils in air with controlled relative humidity. With the same AFM tip, we investigate the same area of a collagen fibril with two different force spectroscopy methods: force-distance (FD) and amplitude-phase-distance (APD) measurements. This allows us to separate the contributions of the fibril's viscoelastic response and the capillary force to the tip-sample interaction. A water bridge forms between the tip apex and the surface, causing an attractive capillary force, which is the main contribution to the energy dissipated from the tip to the specimen in dynamic AFM. The force hysteresis in the FD measurements and the tip indentation of only 2 nm in the APD measurements show that the hydrated collagen fibril is a viscoelastic solid. The mechanical properties of the gap regions and the overlap regions in the fibril's D-band pattern differ only in the top 2 nm but not in the fibril's bulk. We attribute this to the reduced number of intermolecular crosslinks in the reconstituted collagen fibril. The presented methodology allows the mechanical surface properties of hydrated collagenous tissues and biomaterials to be studied with unprecedented detail on the nanometer scale.

Pathways to equilibrium orientation fluctuations in finite stripe-forming systems.

Small-angle orientation fluctuations in ordered stripe-forming systems free of topological defects can exhibit aging and anisotropic growth of two length scales. In infinitely extended systems, the stripe orientation field develops a dominant modulation length λ_{∥}^{*}(t) in the direction parallel to the stripes, which increases with time t as λ_{∥}^{*}(t)∼t^{1/4}. Simultaneously, the orientation correlation length ξ_{⊥}(t) in the direction perpendicular to the stripes increases as ξ_{⊥}(t)∼t^{1/2} [Riesch et al., Interface Focus 7, 20160146 (2017)2042-889810.1098/rsfs.2016.0146]. Here we show that finite systems of size L_{⊥}×L_{∥} with periodic boundary conditions reach equilibrium when the dominant modulation length λ_{∥}^{*}(t) reaches the system size L_{∥} in the stripe direction. The equilibration time τ_{eq}^{∥} is solely determined by L_{∥}, with τ_{eq}^{∥}∼L_{∥}^{4}. In systems with L_{⊥}

Monitoring Nanoscale Deformations in a Drawn Polymer Melt with Single-Molecule Fluorescence Polarization Microscopy.

Elongating a polymer melt causes polymer segments to align and polymer coils to deform along the drawing direction. Despite the importance of this molecular response for understanding the viscoelastic properties and relaxation behavior of polymeric materials, studies on the single-molecule level are rare and were not performed in real time. Here we use single-molecule fluorescence polarization microscopy for monitoring the position and orientation of single fluorescent perylene diimide molecules embedded in a free-standing thin film of a polymethyl acrylate (PMA) melt with a time resolution of 500 ms during the film drawing and the subsequent stress relaxation period. The orientation distribution of the perylene diimide molecules is quantitatively described with a model of rod-like objects embedded in a uniaxially elongated matrix. The orientation of the fluorescent probe molecules is directly coupled to the local deformation of the PMA melt, which we derive from the distances between individual dye molecules. In turn, the fluorescence polarization monitors the shape deformation of the polymer coils on a length scale of 5 nm. During stress relaxation, the coil shape relaxes four times more slowly than the mechanical stress. This shows that stress relaxation involves processes on length scales smaller than a polymer coil. Our work demonstrates how optical spectroscopy and microscopy can be used to study the coupling of individual fluorescent probe molecules to their embedding polymeric matrix and to an external mechanical stimulus on the single-molecule level.

Monitoring Demineralization and Subsequent Remineralization of Human Teeth at the Dentin-Enamel Junction with Atomic Force Microscopy.

Using atomic force microscopy, we monitored the nanoscale surface morphology of human teeth at the dentin-enamel junction after performing successive demineralization steps with an acidic soft drink. Subsequently, we studied the remineralization process with a paste containing calcium and phosphate ions. Repeated atomic force microscopy imaging of the same sample areas on the sample allowed us to draw detailed conclusions regarding the specific mechanism of the demineralization process and the subsequent remineralization process. The about 1-µm-deep grooves that are caused by the demineralization process were preferentially filled with deposited nanoparticles, leading to smoother enamel and dentine surfaces after 90 min exposure to the remineralizing agent. The deposited material is found to homogeneously cover the enamel and dentine surfaces in the same manner. The temporal evolution of the surface roughness indicates that the remineralization caused by the repair paste proceeds in two distinct successive phases.

Nanoscale Swelling Heterogeneities in Type I Collagen Fibrils.

The distribution of water within the supramolecular structure of collagen fibrils is important for understanding their mechanical properties as well as the biomineralization processes in collagen-based tissues. We study the influence of water on the shape and the mechanical properties of reconstituted fibrils of type I collagen on the nanometer scale. Fibrils adsorbed on a silicon substrate were imaged with multiset point intermittent contact (MUSIC)-mode atomic force microscopy (AFM) in air at 28% relative humidity (RH) and in a hydrated state at 78% RH. Our data reveal the differences in the water uptake between the gap and overlap regions during swelling. This provides direct evidence for different amounts of bound and free water within the gap and overlap regions. In the dry state, the characteristic D-band pattern visible in AFM images is due to height corrugations along a fibril's axis. In the hydrated state, the fibril's surface is smooth and the D-band pattern reflects the different mechanical properties of the gap and overlap regions.

The effect of diiodooctane on the charge carrier generation in organic solar cells based on the copolymer PBDTTT-C.

Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices. We analyze the impact of a systematically increased amount of the solvent additive diiodooctane (DIO) on the morphology of PBDTTT-C:PC71BM blends and related changes in free carrier formation and recombination by combining surface imaging, photophysical and charge extraction techniques. We identify agglomerates visible in AFM images of the 0% DIO blend as PC71BM domains embedded in an intermixed matrix phase. With the addition of DIO, a decrease in the size of fullerene domains along with a demixing of the matrix phase appears for 0.6% and 1% DIO. Surprisingly, transient absorption spectroscopy reveals an efficient photogeneration already for the smallest amount of DIO, although the largest efficiency is found for 3% DIO. It is ascribed to a fine-tuning of the blend morphology in terms of the formation of interpenetrating donor and acceptor phases minimizing geminate and nongeminate recombination as indicated by charge extraction experiments. An increase in the DIO content to 10% adversely affects the photovoltaic performance, most probably due to an inefficient free carrier formation and trapping in a less interconnected donor-acceptor network.

Aging of orientation fluctuations in stripe phases.

Stripe patterns, observed in a large variety of physical systems, often exhibit a slow nonequilibrium dynamics because ordering is impeded by the presence of topological defects. Using computer simulations based on a well-established model for stripe formation, we show that a slow dynamics and aging occur also in stripe patterns free of topological defects. For a wide range of noise strengths, the two-time orientation correlation function follows a scaling form that is typical for systems exhibiting a growing length scale. In our case, the underlying mechanism is the coarsening of orientation fluctuations, ultimately leading to power-law spatial correlations perpendicular to the stripes. Our results show that even for the smallest amount of noise, stripe phases without topological defects do not reach equilibrium. This constitutes an important aspect of the dynamics of modulated phases.

Multi-Set Point Intermittent Contact (MUSIC) Mode Atomic Force Microscopy of Oligothiophene Fibrils.

We developed MUSIC-mode atomic force microscopy (AFM) to emulate intermittent contact mode AFM without a feedback loop and in the absence of lateral forces. This single-pass approach is based on maps of amplitude-phase-distance curves and allows the height and phase images to be simultaneously obtained for almost any amplitude set point. This is advantageous for determining the shape and nanomechanical properties of very soft and fragile samples. As an example, we studied supramolecular aggregates of oligothiophenes, which form ≈15 nm wide fibrils with a rigid core and a soft shell.

Locally auxetic behavior of elastomeric polypropylene on the 100 nm length scale.

We observe unexpected locally auxetic behavior in elastomeric polypropylene, a semicrystalline polymer with a natural microstructure and a low degree of crystallinity. Our series of scanning force microscopy images show the nanomechanical deformation processes that occur upon stretching a thin film of elastomeric polypropylene. Upon uniaxial stretching, the angle between epitaxially grown lamella branches remains constant and the lamellae elongate, resulting in locally auxetic behavior (negative Poisson's ratio) on the 100-nanometer scale. This mechanism causing auxetic behavior, which was previously proposed on the basis of geometric arguments, appears to be an intrinsic property of certain semicrystalline polymers.

Subsurface imaging of soft polymeric materials with nanoscale resolution.

Nondestructive depth-resolved imaging of ∼20-nm-thick surface layers of soft polymeric materials is demonstrated using amplitude modulation atomic force microscopy (AM-AFM). From a map of amplitude-phase-distance curves, the tip indentation into the specimen is determined. This serves as a depth coordinate for reconstructing cross sections and volume images of the specimen's mechanical properties. Our method reveals subsurface structures which are not discernible using conventional AM-AFM. Results for surfaces of a block copolymer and a semicrystalline polymer are presented.

Nanoscale compositional mapping with gentle forces.

Microscopists have always pursued the development of an instrument that combines topography and materials properties analyses at the highest resolution. The measurement of the tiny amount of energy dissipated by a vibrating tip in the proximity of the sample surface has provided atomic force microscopes with a robust and versatile method to determine the morphology and the compositional variations of surfaces in their natural environment. Applications in biology, polymer science and microelectronics illustrate the potential of phase-imaging force microscopy for nanoscale analysis.

Defect evolution in block copolymer thin films via temporal phase transitions.

We study the details of the defect dynamics in thin films of a cylinder-forming polystyrene-block-polybutadiene (SB) diblock copolymer melt. The high temporal resolution of in-situ scanning force microscopy (SFM) uncovers elementary dynamic processes of structural rearrangements on time scales not accessible so far. Short-term interfacial undulations and the formation of transient phases (spheres, perforated lamellae, and lamellae) are observed. We demonstrate that the well-known structural defects are annihilated by short-term phase transitions into what may be considered excited states. These temporary phase transitions are reproduced in simulations based on dynamic self-consistent field theory. We discuss the role of the observed structural evolution in the context of the equilibrium phase behavior in SB thin films.

Rapid transitions between defect configurations in a block copolymer melt.

With in situ scanning force microscopy, we image the ordering of cylindrical microdomains in a thin film of a diblock copolymer melt. Tracking the evolution of individual defects reveals elementary steps of defect motion via interfacial undulations and repetitive transitions between distinct defect configurations on a time scale of tens of seconds. The velocity of these transitions suggests a cooperative movement of clusters of chains. The activation energy for the opening/closing of a connection between two cylinders is estimated.