Viscoplastic fracture transition of a biopolymer gel.

Physical gels are swollen polymer networks consisting of transient crosslink junctions associated with hydrogen or ionic bonds. Unlike covalently crosslinked gels, these physical crosslinks are reversible thus enabling these materials to display highly tunable and dynamic mechanical properties. In this work, we study the polymer composition effects on the fracture behavior of a gelatin gel, which is a thermoreversible biopolymer gel consisting of denatured collagen chains bridging physical network junctions formed from triple helices. Below the critical volume fraction for chain entanglement, which we confirm via neutron scattering measurements, we find that the fracture behavior is consistent with a viscoplastic type process characterized by hydrodynamic friction of individual polymer chains through the polymer mesh to show that the enhancement in fracture scales inversely with the squared of the mesh size of the gelatin gel network. Above this critical volume fraction, the fracture process can be described by the Lake-Thomas theory that considers fracture as a chain scission process due to chain entanglements.

Free Surface Relaxations of Star-Shaped Polymer Films.

The surface relaxation dynamics of supported star-shaped polymer thin films are shown to be slower than the bulk, persisting up to temperatures at least 50 K above the bulk glass transition temperature T_{g}^{bulk}. This behavior, exhibited by star-shaped polystyrenes with functionality f=8 arms and molecular weights per arm M_{arm}

Structural relaxations of thin polymer films.

Time-dependent changes of thermodynamic properties due to structural relaxations and physical aging occur in all glasses. We show that the physical aging of thin supported films of star-shaped macromolecules, with f arms of length N(arm), exhibits average aging dynamics that are sensitive to f and N(arm). Regions of the films in proximity to interfaces age at substantially different rates than the interior of the film; this is also true of linear chain systems. This behavior may be reconciled in terms of a universal picture that accounts only for changes in the local T(g) of the films.

Wetting of a multiarm star-shaped molecule.

The equilibrium contact angles and line tensions of macroscopic droplets, composed of star-shaped polystyrene (PS) macromolecules, on silicon oxide substrates, are shown to be smaller than their linear analogs, by up to approximately 1 and 2 orders of magnitude, respectively, depending on the size and functionality of the star-shaped molecule. A precursor layer, of lateral dimensions and of thicknesses on the order of nanometers, surrounds each droplet of low molecular weight linear PS chains. Droplets composed of star-shaped molecules possessing a sufficient number of arms, reside on a layer adsorbed to the substrate.

Role of molecular architecture on the vitrification of polymer thin films.

We show that thin film star-shaped macromolecules exhibit significant differences in their average vitrification behavior, in both magnitude and thickness dependence, from their linear analogs. This behavior is dictated by a combination of their functionality and arm length. Additionally, the glass transition temperature at the free surface of a star-shaped molecule film may be higher than that of the interior, in contrast to their linear analogs where the opposite is true. These findings have implications for other properties, due largely to the origins, entropic, of this behavior.

Mechanical Response of Thermally Annealed Nafion Thin Films.

Perfluorinated ionomers, in particular, Nafion, are a critical component in hydrogen fuel cells as the ion conducting binder within the catalyst layer in which it can be confined to thicknesses on the order of 10 nm or less. It is well reported that many physical properties, such as the Young's modulus, are thickness dependent when the film thickness is less than 100 nm. Here we utilize a cantilever bending methodology to quantify the swelling-induced stresses and relevant mechanical properties of Nafion films as a function of film thickness exposed to cyclic humidity. We observe a factor of 5 increase in the Young's modulus in films thinner than 50 nm and show how this increased stiffness translates to reduced swelling or hydration. The swelling stress was found to increase by a factor of 2 for films approximately 40 nm thick. We demonstrate that thermal annealing enhances the modulus at all film thicknesses and correlate these mechanical changes to chemical changes in the infrared absorption spectra.

Design considerations for electrode buffer layer materials in polymer solar cells.

Electrode buffer layers in polymer-based photovoltaic devices enable highly efficient devices. In the absence of buffer layers, we show that diode rectification is lost in ITO/P3HT:PCBM/Ag (ITO = indium tin oxide; P3HT = poly(3-hexylthiophene); PCBM = phenyl C61-butyric acid methyl ester) devices due to nonselective charge injection through the percolated phase pathways of a bulk heterojunction active layer. Charge-selective injection, and thus rectification and device function, can be regained by placing thin, polymeric buffer layers that break the direct electrode-active layer contact. Additionally, we show that strong active layer-buffer layer interactions lead to unwanted vertical phase separation and a kinked current-voltage curve. Device function is regained, increasing power conversion efficiency from 3.6% to 7.2%, by placing a noninteracting layer between the buffer and active layer. These results guide the design and selection of future polymeric electrode buffer layers for efficient polymer solar cell devices.

Surface dynamics of miscible polymer blend nanocomposites.

Diverse processes that include energy conversion, wettability, lubrication, adhesion, and surface-directed phase separation in mixtures fundamentally depend on the structure and dynamics of materials' surfaces and interfaces. We report an unusual phenomenon wherein the surface viscosity of polymer nanocomposites of polystyrene (PS), polyvinyl methyl ether (PVME), and PS-coated gold nanoparticles (PS/PVME/PS-Au) is over an order of magnitude smaller than that of the neat miscible PS/PVME blend. Our X-ray photon correlation spectroscopy studies of the surface dynamics also reveal that the polymer chains manifest dynamics associated with two separate average compositional environments: a PVME-rich region, significantly in excess of its bulk concentration, and a separate PS-rich environment, where the dynamics are approximately 2 orders of magnitude slower. The unusually rapid surface dynamics in the PS/PVME/PS-Au nanocomposite are due largely to the excess PVME chains and the polymer/brush-coated nanoparticle interactions at the free surface.

Surface Layer Dynamics in Miscible Polymer Blends.

In thin film A/B polymer/polymer mixtures, the formation of a layer at the free surface, with average composition that differs from the bulk, due to the preferential segregation of the lower cohesive energy density component, is well understood. While much is also understood about this surface layer formation and growth to date, virtually nothing is known about the surface dynamics of the chains in such mixtures. Questions about the surface chain dynamics in relation to the bulk have remained unanswered. With the use of X-ray photon correlation spectroscopy (XPCS) we show that the dynamics of poly(vinyl methyl ether) (PVME) chains at the free surface of polystyrene (PS)/PVME thin film mixtures can be orders of magnitude larger than the PVME chains in the bulk. These dynamics manifest from differences between the local compositions of the blend at the free surface and the bulk, as well as film thickness constraints.

Physical Aging of Star-Shaped Macromolecules.

Time-dependent structural relaxations, physical aging, of films with thicknesses in the range 0.4 µm < H < 2 µm of star-shaped polystyrene (SPS) macromolecules were investigated. Our studies reveal that the aging rates of star-shaped PS macromolecules are appreciably slower than their linear chain analogs. The magnitude of the difference between the aging rates of the linear and star-shaped macromolecules increases with increasing functionality, f, and decreasing molecular weight per arm, Mnarm, of the stars. Our results are consistent with the notion that constraints imposed due to the architecture of the macromolecule suppress relaxations associated with and accommodate the reduction of the free volume of the system.

Effect of thickness-dependent microstructure on the out-of-plane hole mobility in poly(3-hexylthiophene) films.

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, µ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100-200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10(-4) cm(2)/V·s, for a 80 nm thick film, to a value of 10(-3) cm(2)/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.