Maya K. Endoh

Shear small-angle light scattering studies of shear-induced concentration fluctuations and steady state viscoelastic properties

We aimed at elucidating the influence of shear-induced structures (shear-enhanced concentration fluctuations and/or shear-induced phase separation), as observed by rheo-optical methods with small-angle light scattering under shear flow (shear-SALS) and shear-microscopy, on viscoelastic properties in semidilute polystyrene (PS) solutions of 6.0 wt % concentration using dioctyl phthalate (DOP) as a Theta solvent and tricresyl phosphate (TCP) as a good solvent. In order to quantify the effects of the shear-induced structures, we conducted a numerical analysis of rheological properties in a homogeneous solution based on the constitutive equation developed by Kaye-Bernstein, Kearsley, and Zapas (K-BKZ). In the low-to-intermediate shear rate gamma region between tau(w) (-1) and tau(e) (-1), where tau(w) and tau(e) are, respectively, terminal relaxation time and the relaxation time for chain stretching, the steady state rheological properties, such as shear stress sigma and the first normal stress difference N(1), for the PS/DOP and PS/TCP solutions are found to be almost same and also well predicted by the K-BKZ equation, in spite of the fact that there is a significant difference in the shear-induced structures as observed by shear-SALS and shear-microscopy. This implies that the contribution of the concentration fluctuations built up by shear flow to the rheological properties seems very small in this gamma region. On the other hand, once gamma exceeds tau(e) (-1), sigma and N(1) for both PS/DOP and PS/TCP start to deviate from the predicted values. Moreover, when gamma further increases and becomes higher than gamma(a,DOP) (sufficiently higher than tau(e) (-1)), above which rheological and scattering anomalies are observed for PS/DOP, sigma and N(1) for PS/DOP and PS/TCP are significantly larger than those predicted by K-BKZ. Particularly, a steep increase of sigma and N(1) for PS/DOP above gamma(a,DOP) is attributed to an excess free energy stored in the system via the deformation of interface of well-defined domains, which are aligned into the stringlike structure developed parallel to the flow axis, and stretching of the chains connecting the domains in the stringlike structures. Thus, we advocate that the effect of shear-induced structures should be well considered on the behavior of sigma and N(1) at the high gamma region above tau(e) (-1) in semidilute polymer solutions.

Introduction of molecular scale porosity into semicrystalline polymer thin films using supercritical carbon dioxide

We report supercritical carbon dioxide (scCO2) technology used for forming a large degree of molecular scale porosity in semicrystalline polymer thin films. The following three steps were integrated: (i) pre-exposure to an organic solvent which melted crystalline structures but did not cause a decrease in thickness, (ii) scCO2 exposure under the unique conditions where the anomalous absorption of CO2 occurred, and (iii) subsequent quick evaporation of CO2 to preserve the swollen structures. This unified process resulted in homogenous low-density polyphenylene vinylene films (a 15% reduction in density) with the sustained structure for at least 6 months at room temperature.

Directed self-assembly of nanoparticles at the polymer surface by highly compressible supercritical carbon dioxide

We report a versatile route for self-assembly of polymer-soluble nanoparticles at the polymer surface using highly compressible supercritical carbon dioxide (scCO2). Polystyrene and poly(methyl methacrylate)-based nanocomposite thin films with functionalized polyhedral oligomeric silsesquioxane and phenyl C61 butyric acid methyl ester nanoparticles were prepared on Si substrates and exposed to scCO2 at different pressures under the isothermal condition of 36 °C. The resultant structures could be then preserved by the vitrification process of the glassy polymersvia quick pressure quench to atmospheric pressure and subsequently characterized by using various surface sensitive experimental techniques in air. We found that the surface segregation of these nanoparticles is induced in the close vicinity of P = 8.2 MPa where the excess absorption of the fluid into the polymers maximizes. However, when the film thickness becomes less than about 4Rg thick (where Rg is the radius of polymer gyration), the uniform dispersion of the nanoparticles is favorable instead even at the same CO2 conditions. We clarify that the phase transition is correlated with the emergence of a concentration gradient of the fluid at the polymer/CO2 interface and is a general phenomenon for different polymer–nanoparticle interactions.

The thermo-mechanical response of PP nanocomposites at high graphene loading

Abstract Authors have successfully fabricated polypropylene/graphene nanoplatelets (PP/GNPs), nanocomposites that are thermally conductive, processable, and flame resistant. Thermal conductivity measurements indicated that the thermal coefficient scaled linearly with GNP loading, where a value of 2.0 W m− 1 K− 1 was achieved at 40 wt-% loading. Tensile measurements indicated that the modulus increased linearly with GNP loading, while the Izod impact, after an initial decrease, remained constant for loadings up to 50 wt-%. Small angle X-ray scattering (SAXS) showed a large decrease in the amount of lamellar structure relative to the neat PP, while wide angle X-ray scattering (WAXS) showed a high degree of crystallinity. These results are consistent with formation of a new type of layered nanocomposite, composed of crystalline PP chains oriented onto layered GNPs.

Novel Effects of Compressed CO2 Molecules on Structural Ordering and Charge Transport in Conjugated Poly(3-hexylthiophene) Thin Films

We report the effects of compressed CO2 molecules as a novel plasticization agent for poly(3-hexylthiophene) (P3HT)-conjugated polymer thin films. In situ neutron reflectivity experiments demonstrated the excess sorption of CO2 molecules in the P3HT thin films (about 40 nm in thickness) at low pressure (P = 8.2 MPa) under the isothermal condition of T = 36 °C, which is far below the polymer bulk melting point. The results proved that these CO2 molecules accelerated the crystallization process of the polymer on the basis of ex situ grazing incidence X-ray diffraction measurements after drying the films via rapid depressurization to atmospheric pressure: both the out-of-plane lamellar ordering of the backbone chains and the intraplane π-π stacking of the side chains were significantly improved, when compared with those in the control P3HT films subjected to conventional thermal annealing (at T = 170 °C). Electrical measurements elucidated that the CO2-annealed P3HT thin films exhibited enhanced charge carrier mobility along with decreased background charge carrier concentration and trap density compared with those in the thermally annealed counterpart. This is attributed to the CO2-induced increase in polymer chain mobility that can drive the detrapping of molecular oxygen and healing of conformational defects in the polymer thin film. Given the universality of the excess sorption of CO2 regardless of the type of polymers, the present findings suggest that CO2 annealing near the critical point can be useful as a robust processing strategy for improving the structural and electrical characteristics of other semiconducting conjugated polymers and related systems such as polymer:fullerene bulk heterojunction films.

Structures and Dynamics of Adsorbed Polymer Nanolayers on Planar Solids

Solid-polymer interfaces play crucial roles in the multidisciplinary field of nanotechnology and are the confluence of physics, chemistry, biology, and engineering. There is now growing evidence that polymer chains irreversibly adsorb even onto weakly attractive solid surfaces, forming a nanometer-thick adsorbed polymer layer (“adsorbed polymer nanolayers”). In this Chapter, we review our recent experimental results on the structures and dynamics of the adsorbed polymer chains. Furthermore, we shed light on the mechanism giving rise to the extraordinary properties of the adsorbed nanolayers and on how these properties can propagate into the film interior over distances of several tens of nanometers, resulting in heterogeneities of local crystalline structures, viscosity, interdiffusive motions (in the melts and a solvent) of supported polymer thin films.

X-ray Photon Correlation Spectroscopy Study on Dynamics of the Free Surface in Entangled Polystyrene Melt Films

The dynamics of polymer chains near the surface of a melt and within thin films remains a subject of inquiry along with the nature of the glass transition in these systems. Recent studies show that the properties of the free surface region are crucial in determining the anomalous glass transition temperature (Tg) reduction of polymer thin films. In this study, by embedding dilute gold nanoparticles in polystyrene (PS) thin films as markers, we could successfully probe the diffusive Brownian motion which tracks the local viscosity both at the free surface and within the rest of the single PS thin film far above bulk Tg. The technique used was X-ray photon correlation spectroscopy with resonance-enhanced X-rays that allows us to independently measure the motion in the regions of interest at the nanometer scale. We found the presence of the surface reduced viscosity layer in entangled PS thin films at T>>Tg.

Surface Segregation of Nanoparticles Driven by Supercritical Carbon Dioxide

Surface segregation (i.e., preferential segregation of one component to the surface in multicomponent systems) is common to all material classes and is typically driven by a reduction in surface energy which compensates for the entropy loss and/or energy gain associated with the demixing of the components. However, the conventional surface segregation for polymeric systems requires high temperatures, typically close to 200°C, and long annealing time, in order to ensure enough polymer mobility. Here we show a low-temperature method to preferentially migrate organoclay nanoparticles to the polymer surface using supercritical carbon dioxide.