Mark Geoghegan

Wetting at polymer surfaces and interfaces

Abstract Experimental research on wetting in polymer films is a subject that is reaching maturity. We review progress from the past few years in research into the influence of a boundary in polymer blends, concentrating largely on the wetting transition, and the growth of wetting layers, where we pay particular attention to blends in which hydrodynamic flow plays a dominant role. A summary of work over the same period concerning the dewetting of polymer films is also included, along with a discussion of the role of pattern formation caused by dewetting and topographically and chemically patterned substrates. We conclude by summarising some experiments that we believe may inspire future research.

Organic field effect transistors from ambient solution processed low molar mass semiconductor–insulator blends

The morphology and organic field effect transistor (OFET) properties of two component blends of semi-crystalline 6,13-bis(triisopropylsilylethinyl)pentacene (TIPS-pentacene) with selected amorphous and semi-crystalline side chain aromatic low permittivity insulating binders deposited at room temperature under vacuum from a good solvent are reported. When blended with an amorphous binder there is evidence from X-ray photoelectron spectroscopy (XPS) of a strong interaction between TIPS-pentacene and the binder in the solidified film giving rise to twisted TIPS-pentacene crystals containing dislocations. Due to this strong interaction we see no evidence of segregation of TIPS-pentacene towards the active interface and hence we observe a rapid fall off in saturated hole mobility at an active concentration less than 50 wt%. When blended with a crystalline binder there is no evidence from XPS of any interaction between TIPS-pentacene and the binder in the solidified film. We propose that when a semi-crystalline binder is used, which crystallizes more slowly from solution than TIPS-pentacene, we observe stratification of the active material to both interfaces and as a result retention of saturated hole mobility even down to 10 wt%. The potential applications of the approach are in the formulation of low-cost organic semiconductors whose solution and solid state properties can be fine-tuned by careful binder selection.

Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H‐atom production in the dissociative recombination of some protonated ions

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H‐atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H‐atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showe...

Surface segregation and self-stratification in blends of spin-cast polyfluorene derivatives

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,-dioctylfluorene) (F8) and poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ?nm at an F8 volume fraction = 0.5, dropping down to ?2?nm for = 0.2 and?0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.

Quantitative evaluation of evaporation rate during spin-coating of polymer blend films: Control of film structure through defined-atmosphere solvent-casting

Abstract.Thin films of polymer mixtures made by spin-coating can phase separate in two ways: by forming lateral domains, or by separating into distinct layers. The latter situation (self-stratification or vertical phase separation) could be advantageous in a number of practical applications, such as polymer optoelectronics. We demonstrate that, by controlling the evaporation rate during the spin-coating process, we can obtain either self-stratification or lateral phase separation in the same system, and we relate this to a previously hypothesised mechanism for phase separation during spin-coating in thin films, according to which a transient wetting layer breaks up due to a Marangoni-type instability driven by a concentration gradient of solvent within the drying film. Our results show that rapid evaporation leads to a laterally phase-separated structure, while reducing the evaporation rate suppresses the interfacial instability and leads to a self-stratified final film.

Effect of brush thickness and solvent composition on the friction force response of poly(2-(methacryloyloxy)ethylphosphorylcholine) brushes

The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.

Optimization of the Bulk Heterojunction Composition for Enhanced Photovoltaic Properties: Correlation between the Molecular Weight of the Semiconducting Polymer and Device Performance

Herein we propose an approach toward the optimization of the photovoltaic performance of bulk heterojunctions by tuning the composition of the active layer with respect to the molecular weight of the semiconducting polymer. We used a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend as a typical system and varied the molecular weight of the P3HT semiconducting polymer in order to determine its influence on the bulk heterojunction morphology as well as on the optoelectronic characteristics of the device. We have systematically mapped out the phase diagram for different molecular weight P3HTs blended with PCBM and observed the presence of a eutectic composition, which shifts to higher content of P3HT for lower molecular weight P3HTs. This shift inherent to the P3HT molecular weight is also apparent in the photovoltaic performance as the eutectic composition corresponds to the best of the photovoltaic properties. The P3HT molecular weight dependence of the eutectic composition is due to the molecular weight dependence of the P3HT crystallization behavior, which leads to dramatic morphological changes of the bulk heterojunction.

The polymer physics and chemistry of microbial cell attachment and adhesion

The attachment of microbial cells to solid substrata is a primary ecological strategy for the survival of species and the development of specific activity and function within communities. An hypothesis arising from a biological sciences perspective may be stated as follows: The attachment of microbes to interfaces is controlled by the macromolecular structure of the cell wall and the functional genes that are induced for its biological synthesis. Following logically from this is the view that diverse attached cell behaviour is mediated by the physical and chemical interactions of these macromolecules in the interfacial region and with other cells. This aspect can be reduced to its simplest form by treating physico-chemical interactions as colloidal forces acting between an isolated cell and a solid or pseudo solid substratum. These forces can be analysed by established methods rooted in DLVO (Derjaguin, Landau, Verwey and Overbeek) theory. Such a methodology provides little insight into what governs changes in the behaviour of the cell wall attached to surfaces, or indeed other cells. Nor does it shed any light on the expulsion of macromolecules that modify the interface such as formation of slime layers. These physical and chemical problems must be treated at the more fundamental level of the structure and behaviour of the individual components of the cell wall, for example biosurfactants and extracellular polysaccharides. This allows us to restate the above hypothesis in physical sciences terms: Cell attachment and related cell growth behaviour is mediated by macromolecular physics and chemistry in the interfacial environment. Ecological success depends on the genetic potential to favourably influence the interface through adaptation of the macromolecular structure, We present research that merges these two perspectives. This is achieved by quantifying attached cell growth for genetically diverse model organisms, building chemical models that capture the variations in interfacial structure and quantifying the resulting physical interactions. Experimental observations combine aqueous chemistry techniques with surface spectroscopy in order to elucidate the cell wall structure. Atomic force microscopy methods quantify the physical interactions between the solid substrata and key components of the cell wall such as macromolecular biosurfactants. Our current approach focuses on considering individually mycolic acids or longer chain polymers harvested from cells, as well as characterised whole cells. This approach allows us to use a multifactorial approach to address the relative impact of the individual components of the cell wall in contact with model surfaces. We then combine these components to increase complexity step-wise, while comparing with the behaviour of entire cells. Eventually, such an approach should allow us to estimate and understand the primary factors governing microbial cell adhesion. Although the work addresses the cell-mineral interface at a fundamental level, the research is driven by a range of technology needs. The initial rationale was improved prediction of contaminant degradation in natural environments (soils, sediments, aquifers) for environmental cleanup. However, this area of research addresses a wide range of biotechnology areas including improved understanding of pathogen survival (e.g., in surgical environments), better process intensification in biomanufacturing (biofilm technologies) and new product development.

Determination of the electron-ion dissociative recombination coefficients for several molecular ions at 300 K

The electron-ion dissociative recombination rate coefficients, alpha e, have been determined for ten molecular ions, seven of which have not previously been measured. The data were obtained under truly thermal conditions at 300 K using a flowing afterglow/Langmuir probe (FALP) apparatus. Recombination coefficients were obtained for the following ions: CO2+ ( alpha e=3.1*10-7 cm3 s-1), N2+ (2.0), CO+ (1.6), KrH+ (<0.2), XeH+ (<0.4), HSO2+ (2.7), CH3CNH+ (3.3), CH3CHOH+ (3.9), C2H5CNH+ (4.7) and ((CH3)2CO)2H+ (14). The dissociative recombination of many of the ions studied may be important in planetary, cometary and terrestrial atmospheres and be involved in the synthesis of interstellar molecules.

Lamellar structure in a thin polymer blend film

Abstract We have fully characterized the three-dimensional morphology of thin films of mixtures of polystyrene and polybutadiene cast from a toluene solution, using nuclear reaction analysis, neutron reflectometry and transmission electron microscopy. Polystyrene-rich phases wet both the air and substrate interfaces and are separated by a polybutadiene-rich phase; these layers are very well defined and the interfaces between them are sharp (down to A ). Within the polybutadiene-rich central layer, lateral phase separation is also evident, with polystyrene-rich domains of oblate spheroidal shape. Under certain circumstances thin polystyrene-rich layers exist within the polybutadiene-rich phase. We discuss possible mechanisms for this unusual morphology in terms of surface effects on the mechanism of phase separation in the ternary polymer-solvent system from which the films are cast.