Subsurface Mapping of Amorphous Surface Layers on Poly(3-hexylthiophene)

Atomic force microscopy nanoindentation kinetics and subsurface visualization of soft inhomogeneous polymer

Modern techniques of nanoindentation by atomic force microscopy (AFM) produce maps of topography and physical-mechanical properties of the material. Analysis of the interaction rate of the AFM tip with the soft surface reveals the surface and subsurface structure and expands standard analysis of the material behavior. Phase-separated polymer (polyurethane, elastic modulus-6 MPa) is studied. Reversible inelastic changes of the surface at different stages of indentation were established in dependence on peculiarities of velocity and position of the AFM-tip in the material: uniform soft nanofilm covering the outer surface gradually passes into fibrillar heterogeneous structure of the polymer. The point of stable mechanical contact is defined, and the elastic moduli of soft and hard blocks of the polymer are estimated using certain intervals of the indentation. The presented methods of surface analysis are useful in the study of a wide class of soft heterogeneous materials.

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.

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.

Molecular and nanoscale compositional contrast of soft matter in liquid: interplay between elastic and dissipative interactions

We demonstrate that the phase contrast observed with an amplitude modulation atomic force microscope depends on two factors, the generation of higher harmonics components and the energy dissipated on the sample surface. Those factors are ultimately related to the chemical composition and structure of the surface. Our findings are general, but they specifically describe the results obtained while imaging soft materials in liquid. Molecular resolution experiments performed on a protein membrane surface in liquid confirm the theory.