Nigel Clarke

The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics

The domain size, domain purity and interfacial width between domains for a bulk heterojunction are controllably altered through use of Cahn–Hilliard modeling and their relative effect on OPV performance is predicted using Monte Carlo modeling. It is found that locally sharp, well-connected domains of only 4 nm extent out perform morphologies with broadened interfaces and/or impure domains even when domain sizes were at the ‘optimum’ size of ∼10 nm. More generally, these data provide information on the most effective method to optimize the as-cast bulk heterojunction morphology depending upon initial domain purity and the nature of interfaces between domains. Further, it indicates why morphology optimization is more effective for some blends than others. It is shown that the quench depth of the blend can be used as a general technique to control the interfacial structure of the morphology and realize substantial increases in short circuit photocurrent.

Anion tuning of chiral bis(urea) low molecular weight gels

A series of chiral bis(urea) compounds based on oligomethylene spacers with S-phenylethyl end groups have been investigated as low molecular weight gelators. The series shows gelation of a variety of liquids and their structural, morphological and rheological properties are reported. The bis(urea) compounds also act as supramolecular hosts for anions such as chloride and acetate and the weakening of the gels by competitive anion complexation on this series of archetypical, low molecular weight gels is reported. Modification of gel properties using gel mixtures is also described and the gelation process studied using a fluorescent napthyl analogue.

In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating

Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. Applications for such layers include photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase separation morphology of the spun film. Furthermore, at micrometer scales, phase separation provides a route to produce self-organized structures for templating applications. Understanding the factors that determine the final phase-separated morphology in these systems is consequently an important goal. However, it has to date proved problematic to fully test theoretical models for phase separation during spin coating, due to the high spin speeds, which has limited the spatial resolution of experimental data obtained during the coating process. Without this fundamental understanding, production of optimized micro- and nanoscale structures is hampered. Here, we have employed synchronized stroboscopic illumination together with the high light gathering sensitivity of an electron-multiplying charge-coupled device camera to optically observe structure evolution in such blends during spin coating. Furthermore the use of monochromatic illumination has allowed interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and phase separates with nanometer precision. We have used this new method to directly observe the phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing new insights into the spin-coating process and opening up a route to understand and control phase separation structures.

Modeling of Polymer Structure and Conformations in Polymer Nanocomposites from Atomistic to Mesoscale: A Review

ABSTRACT Over the past two decades polymer nanocomposites have received tremendous interest from industry and academia due to their advanced properties comparative to polymer blends. Many computational studies have revealed that the macroscopic properties of polymer nanocomposites depend strongly on the microscopic polymer structure and conformations. In this article we review computer simulation studies of the fundamental problem of homopolymers structure and dimensions in nanocomposites containing bare or grafted spherical or rod nanoparticles. Experimentally, there is controversy over whether the addition of nanoparticles in a polymer matrix can perturb the polymer chains.

Polymer diffusion in a polymer nanocomposite: effect of nanoparticle size and polydispersity

The tracer diffusion of deuterated polystyrene (dPS) is measured in a polystyrene nanocomposite containing silica nanoparticles (NPs), with number average diameters dn of 28.8 nm and 12.8 nm, using elastic recoil detection. The volume fractions of the large and small NPs (ϕNP) range from 0 to 0.5, and 0 to 0.1, respectively. At the same volume fraction of NPs, the tracer diffusion of dPS is reduced as NP size decreases because the interparticle distance between NPs (ID) decreases. The reduced diffusion coefficient, defined as the tracer diffusion coefficient in the nanocomposite relative to pure PS (D/D0), plotted against the confinement parameter, namely ID(dn) relative to tracer size, ID(dn)/2Rg, nearly collapses onto a master curve, although D/D0 is slightly greater for the more polydisperse, smaller NPs. Using a log normal distribution of NP size from SAXS, the average ID of the smaller NPs is shown to increase by 25% at ϕNP = 0.1 as polydispersity (σ) increases from 1 to 1.39. By accounting for polydispersity, the confinement parameter better represents the effect of NP spacing on polymer diffusion. These experiments demonstrate that polymer tracer diffusion in polymer nanocomposites is empirically captured by the confinement parameter and that an increase in the average ID due to NP polydispersity has a secondary effect on model NP systems with a narrow distribution of sizes. However, for commercial systems, where polydispersity can be quite large, the effect of size distribution can significantly increase ID which in turn will influence polymer dynamics.

Topological entanglement length in polymer melts and nanocomposites by a DPD polymer model

We investigate the topological constraints (entanglements) in polymer–nanorod nanocomposites in comparison to polymer melts using dissipative particle dynamics (DPD) polymer model simulations. The nanorods have a radius smaller than the polymer radius of gyration and an aspect ratio of 7.5. We observe an increase in the number of entanglements (50% decrease of Ne with 11% volume fraction of nanorods dispersed in the polymer matrix) in the nanocomposites as evidenced by larger contour lengths of the primitive paths. The end-to-end distance is essentially unchanged with the nanorod volume fraction (0–11%). Interaction between polymers and nanorods affects the dispersion of nanorods in the nanocomposites.

The Interplay between Cell Wall Mechanical Properties and the Cell Cycle in Staphylococcus aureus

The nanoscale mechanical properties of live Staphylococcus aureus cells during different phases of growth were studied by atomic force microscopy. Indentation to different depths provided access to both local cell wall mechanical properties and whole-cell properties, including a component related to cell turgor pressure. Local cell wall properties were found to change in a characteristic manner throughout the division cycle. Splitting of the cell into two daughter cells followed a local softening of the cell wall along the division circumference, with the cell wall on either side of the division circumference becoming stiffer. Once exposed, the newly formed septum was found to be stiffer than the surrounding, older cell wall. Deeper indentations, which were affected by cell turgor pressure, did not show a change in stiffness throughout the division cycle, implying that enzymatic cell wall remodeling and local variations in wall properties are responsible for the evolution of cell shape through division.

Polymer and spherical nanoparticle diffusion in nanocomposites

Nanoparticle and polymer dynamics in nanocomposites containing spherical nanoparticles were investigated by means of molecular dynamics simulations. We show that the polymer diffusivity decreases with nanoparticle loading due to an increase of the interfacial area created by nanoparticles, in the polymer matrix. We show that small sized nanoparticles can diffuse much faster than that predicted from the Stokes-Einstein relation in the dilute regime. We show that the nanoparticle diffusivity decreases at higher nanoparticle loading due to nanoparticle-polymer interface. Increase of the nanoparticle radius slows the nanoparticle diffusion.

Primitive path network, structure and dynamics of SWCNT / polymer nanocomposites

We investigate the entanglements, structure and dynamics of monodisperse polymer melts in the presence of a single wall carbon nanotube (SWCNT) in comparison to inclusion-free polymer melts by molecular dynamics simulations. The SWCNT has an infinite aspect ratio and radius smaller than the polymer radius of gyration. In the presence of SWCNT with or without attractive interactions, the contour length of the primitive path increases indicating more entanglements. We also find that the overall configuration, as characterized by the radius of gyration, is not perturbed by either the interaction energy between the polymer and the SWCNT or by variations in the SWCNT radius. We also find that there is a large heterogeneity in the polymer dynamics of the polymer melts with a SWCNT due to the polymers in contact with the SWCNT.

Dynamics of roughening and growth kinetics of CdS–polyaniline thin films synthesized by the Langmuir–Blodgett technique

Thin films of cadmium sulphide (CdS) nanoparticle induced polyaniline (PANI) nanocomposites have worked as a better system for application in photovoltaics due to the efficient charge separation and charge transfer. In this communication, we have chosen such a system of varying thickness deposited by the Langmuir–Blodgett (LB) technique in order to study the growth and roughness phenomena by dynamic scaling theory. Different techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray reflectivity (XRR) are used for characterization of these films. In the study, the growth exponent, β, is found to have a correlation with the invariant logarithmic scaling law. A very large value of β = 1.18 ± 0.23 as calculated is a result of the rapid roughening in the multilayer film growth process. We have investigated the dynamic scaling behavior of the multilayer system which shows a difference in the value of the coarsening factor and this confirms the breakdown of self-affinity as a consequence of some nonlocal growth effects. The normal grain formation with deposition at the initial stage and the increase in grain abnormality with increase in thickness is also verified in support of the dynamic scaling ansatz. In the present study the layer-by layer deposition is confirmed by XRR fitting for a 3 layer film whereas the multilayer shows diffusing behavior due to H-bonding or electrostatic interactions.