High-frequency acousto-optic effects in Bragg reflectors.

Picosecond acoustic interferometry was used to study the acousto-optic properties of a distributed Bragg reflector (DBR) manufactured from two immiscible polymers (cellulose acetate and polyvinylcarbyzole). Picosecond strain pulses were injected into the structure and changes in its reflectance were monitored as a function of time. The reflectance exhibited single-frequency harmonic oscillations as the strain pulse traversed the DBR. A transfer matrix method was used to model the reflectance of the DBR in response to interface modulation and photo-elastic effects. This work shows that photo-elastic effects can account for the acousto-optic response of DBRs with acoustically matched layers.

Origin of contact line forces during the retraction of dilute polymer solution drops.

The forced dewetting of water and dilute poly(ethylene oxide) solution (PEO) drops is investigated for syringe-driven flow. Comparisons are made with the free dewetting observed during drop impact. We provide strong evidence that during droplet retraction, polymer deposited on the substrate results in a velocity-dependent force at the contact line. These findings are in stark contrast to previous studies which attributed dissipation to bulk viscoelastic effects or normal stress effects at the contact line.

Improving secondary ion mass spectrometry C60(n+) sputter depth profiling of challenging polymers with nitric oxide gas dosing.

Organic depth profiling using secondary ion mass spectrometry (SIMS) provides valuable information about the three-dimensional distribution of organic molecules. However, for a range of materials, commonly used cluster ion beams such as C60(n+) do not yield useful depth profiles. A promising solution to this problem is offered by the use of nitric oxide (NO) gas dosing during sputtering to reduce molecular cross-linking. In this study a C60(2+) ion beam is used to depth profile a polystyrene film. By systematically varying NO pressure and sample temperature, we evaluate their combined effect on organic depth profiling. Profiles are also acquired from a multilayered polystyrene and polyvinylpyrrolidone film and from a polystyrene/polymethylmethacrylate bilayer, in the former case by using an optimized set of conditions for C60(2+) and, for comparison, an Ar2000(+) ion beam. Our results show a dramatic improvement for depth profiling with C60(2+) using NO at pressures above 10(-6) mbar and sample temperatures below -75 °C. For the multilayered polymer film, the depth profile acquired using C60(2+) exhibits high signal stability with the exception of an initial signal loss transient and thus allows for successful chemical identification of each of the six layers. The results demonstrate that NO dosing can significantly improve SIMS depth profiling analysis for certain organic materials that are difficult to analyze with C60(n+) sputtering using conventional approaches/conditions. While the analytical capability is not as good as large gas cluster ion beams, NO dosing comprises a useful low-cost alternative for instruments equipped with C60(n+) sputtering.

Effects of substrate constraint on crack pattern formation in thin films of colloidal polystyrene particles.

Crack formation and the evolution of stress in drying films of colloidal particles were studied using optical microscopy and a modified cantilever deflection technique, respectively. Drying experiments were performed using polystyrene particles with diameters of 47 ± 10 nm, 100 ± 16 nm, and 274 ± 44 nm that were suspended in water. As the films dried, cracks with a well-defined spacing were observed to form. The crack spacing was found to be independent of the particle size used, but to increase with the film thickness. The characteristic crack spacing was found to vary between 20 and 300 µm for films with thickness values in the range 3-70 µm. Cantilever deflection measurements revealed that the stresses that develop in the film increase with decreasing film thickness (increasing surface-to-volume ratio). The latter observation was interpreted in terms of the effects of a substrate constraint which causes the build up of stresses in the films. This interpretation was confirmed by crack formation experiments that were performed on liquid mercury surfaces in which removal of the substrate constraint prevented crack formation. Experiments were also performed on compliant elastomer surfaces in which the level of constraint was varied by changing the substrate modulus. The cracking length scale was found to increase with decreasing substrate modulus. A simple theory was also developed to describe the substrate modulus dependence of the cracking length scale. These combined experiments and theory provide convincing evidence that substrate constraints are an important factor in driving crack formation in thin colloidal films.

Thin film polymer photonics: Spin cast distributed Bragg reflectors and chirped polymer structures.

Polymer based photonic structures were produced by spin coating up to 50 alternating layers of polystyrene (PS) and poly(vinylpyrrolidone) (PVP) from mutually exclusive (orthogonal) solvents. The resulting thin film multi-layer structures were studied using a simple optical reflectivity apparatus and were shown to have narrow (10-20nm wide) reflectance bands in the visible region. The position of the reflectance bands was controlled by varying the spin speed used during production of the multi-layers and peak reflectance values of 55% were obtained for samples containing 50 layers. The results were shown to be in agreement with modified optical transfer matrix method calculations which include the effects of diffuse polymer interfaces. This modelling approach revealed that the width of the polymer/polymer interfaces formed by spin coating was in the range 15-20nm. Data and calculations were also obtained for chirped polymer photonic structures. These results were also shown to be in good agreement. These experiments demonstrate that simple processing methods such as spin coating can be used to produce organic photonic structures with tailored optical properties.

Insulin fibril nucleation: the role of prefibrillar aggregates.

Dynamic light scattering and Fourier transform infrared spectroscopy were used to study the formation of prefibrillar aggregates and fibrils of bovine pancreatic insulin at 60 degrees C and at pH 1. The kinetics of disintegration of the prefibrillar aggregates were also studied using these techniques after a quench to 25 degrees C. These experiments reveal that formation of prefibrillar aggregates is reversible under the solution conditions studied and show that it is possible to significantly reduce the nucleation (lag) times associated with the onset of fibril growth in bovine pancreatic insulin solutions by increasing the concentration of prefibrillar aggregates in solution. These results provide convincing evidence that less structured prefibrillar aggregates can act as fibril-forming intermediates.

Mechanically driven wrinkling instability in thin film polymer bilayers.

Optical microscopy and atomic force microscopy were used to study a mechanically induced wrinkling instability in thin film poly(caprolactone)/polystyrene and poly(ethylene oxide)/poly(methyl methacrylate) bilayers. The instability in these samples was shown to be driven by changes in the interfacial area between a semicrystalline polymer underlayer and a glassy polymer capping layer that occurred when the underlayers were melted. The wrinkling instability resulted in the formation of one-dimensional corrugations at the surface of the bilayer samples that had a well-defined wavelength on the micrometer length scale. A linear stability analysis was used to derive a simple model of the wrinkling process in these samples. This model considered the flow and deformation of material in the molten underlayer as well as the balance of stresses in the glassy polymer capping layers. Rheological data were also obtained from polymers similar to those used to form the bilayers. These data were used to show that the model is capable of quantitatively predicting the capping layer and underlayer thickness dependencies of the characteristic wrinkling wavelengths, if the mechanical properties of the two layers and the strain in the capping layers can be determined.

Watch your step! A frustrated total internal reflection approach to forensic footwear imaging.

Forensic image retrieval and processing are vital tools in the fight against crime e.g. during fingerprint capture. However, despite recent advances in machine vision technology and image processing techniques (and contrary to the claims of popular fiction) forensic image retrieval is still widely being performed using outdated practices involving inkpads and paper. Ongoing changes in government policy, increasing crime rates and the reduction of forensic service budgets increasingly require that evidence be gathered and processed more rapidly and efficiently. A consequence of this is that new, low-cost imaging technologies are required to simultaneously increase the quality and throughput of the processing of evidence. This is particularly true in the burgeoning field of forensic footwear analysis, where images of shoe prints are being used to link individuals to crime scenes. Here we describe one such approach based upon frustrated total internal reflection imaging that can be used to acquire images of regions where shoes contact rigid surfaces.

Thickness dependence of the dynamics in thin films of isotactic poly (methylmethacrylate).

The film thickness dependence of both the glass transition temperature (T(g)) and the 1 kHz alpha relaxation were studied for thin films of isotactic Poly (methylmethacrylate) (i-PMMA) supported on aluminium substrates. Films in the thickness range 7-200 nm were studied. The ellipsometrically determined T(g) was found to show reductions for films thinner than 60 nm, with the largest observed reduction being 12 K for a 7 nm thick film. Measurements of the T(g) were also performed on i-PMMA films supported on silicon substrates. Dielectric studies of the temperature dependent 1 kHz alpha relaxation peak, showed that the position (T(alpha)) and shape of the peak have no film thickness dependence. This was shown to hold for films with one free surface and films with a 30 nm thermally evaporated capping layer. Capping the films was shown to have no effect on the thickness dependence of either T(g) or T(alpha). The implications of these results are discussed further and the different film thickness dependencies of T(g) and T(alpha) are discussed. This is done within the framework of the Vogel-Fulcher-Tamann (VFT) theory of glass forming materials and also in the context of the existence of a dynamic correlation length xi.

Dielectric and ellipsometric studies of the dynamics in thin films of isotactic poly(methylmethacrylate) with one free surface.

We have performed dielectric loss measurements at 1 kHz on thin films of isotactic poly(methyl methacrylate). A key distinction of our studies is that the samples measured were supported films with one free surface rather than films that have metallic electrodes covering both surfaces. This unique sample geometry allows us to eliminate any effects due to evaporation of metal onto the top film surface and provides a unique opportunity to make direct comparisons between dielectric loss and glass transition measurements. Film thicknesses in the range from 6 microm to 7 nm were prepared on Al coated substrates. The dielectric loss peak and ellipsometric glass transition temperature of all films were measured. The dielectric loss was found to exhibit no discernible film thickness dependence in either the temperature of the maximum loss value or the shape of the loss curve. In contrast, the measured T(g) values were found to decrease with decreasing film thickness with a maximum shift of 10 K for a 7-nm film. Dielectric measurements were also made on Al coated films and these samples also showed no shift in the temperature of the loss peak. Finally, the T(g) measurements were also made on Si substrates. These values exhibited an increasing T(g) value with film thickness with a maximum increase of approximately 15 K being measured for a 7-nm film.

Swelling-induced morphology in ultrathin supported films of poly(d,l-lactide).

In this study we describe a surface morphology that arises when ultrathin supported films of poly(d,l-lactide) are immersed in water. The films are initially flat with a rms roughness of approximately 2 nm. After immersion the surfaces of the films are covered with craters. The craters have a narrow distribution of sizes and are typically micrometers in diameter. They have depths in the 10-100 nm range. In situ atomic force microscopy shows that the craters occur as a result of a blistering process, which occurs when the films delaminate from the silicon substrate. The films buckle away from the substrate to give a nonzero initial diameter and then the blisters proceed to grow until they reach a maximum size. At any point during the growth process, the blisters can be made to collapse by removing the films from water. This phenomenon is explained in terms of a laterally confined swelling film, which has a buckling instability and releases excess strain energy by wrinkling. An expression for the initial buckling wavelength is extracted using the expressions for a buckling plate. Information about the mechanical properties of the films and the surface interaction between the film and substrate can also be obtained by considering the kinetics of blister growth.

Nucleation and growth of insulin fibrils in bulk solution and at hydrophobic polystyrene surfaces.

A technique was developed for studying the nucleation and growth of fibrillar protein aggregates. Fourier transform infrared and attenuated total reflection spectroscopy were used to measure changes in the intermolecular beta-sheet content of bovine pancreatic insulin in bulk solution and on model polystyrene (PS) surfaces at pH 1. The kinetics of beta-sheet formation were shown to evolve in two stages. Combined Fourier transform infrared, dynamic light scattering, atomic force microscopy, and thioflavin-T fluorescence measurements confirmed that the first stage in the kinetics was related to the formation of nonfibrillar aggregates that have a radius of 13 +/- 1 nm. The second stage was found to be associated with the growth of insulin fibrils. The beta-sheet kinetics in this second stage were used to determine the nucleation and growth rates of fibrils over a range of temperatures between 60 degrees C and 80 degrees C. The nucleation and growth rates were shown to display Arrhenius kinetics, and the associated energy barriers were extracted for fibrils formed in bulk solution and at PS surfaces. These experiments showed that fibrils are nucleated more quickly in the presence of hydrophobic PS surfaces but that the corresponding fibril growth rates decrease. These observations are interpreted in terms of the differences in the attempt frequencies and energy barriers associated with the nucleation and growth of fibrils. They are also discussed in the context of differences in protein concentration, mobility, and conformational and colloidal stability that exist between insulin molecules in bulk solution and those that are localized at hydrophobic PS interfaces.

Spinodal wrinkling in thin-film poly(ethylene oxide)/polystyrene bilayers.

Optical microscopy and atomic force microscopy were used to study a novel roughness-induced wrinkling instability in thin-film bilayers of poly(ethylene oxide) (PEO) and polystyrene (PS). The observed wrinkling morphology is manifested as a periodic undulation at the surface of the samples and occurs when the bilayers are heated above the melting temperature of the semi crystalline PEO (T(m) = 63 Celsius) layer. During the wrinkling of the glassy PS capping layers the system selects a characteristic wavelength that has the largest amplitude growth rate. This initial wavelength is shown to increase monotonically with increasing thickness of the PEO layer. We also show that for a given PEO film thickness, the wavelength can be varied independently by changing the thickness of the PS capping layers. A model based upon a simple linear stability analysis was developed to analyse the data collected for the PS and PEO film thickness dependences of the fastest growing wavelength in the system. The predictions of this theory are that the strain induced in the PS layer caused by changes in the area of the PEO/PS interface during the melting of the PEO are sufficient to drive the wrinkling instability. A consideration of the mechanical response of the PEO and PS layers to the deformations caused by wrinkling then allows us to use this simple theory to predict the fastest growing wavelength in the system.

Free surfaces cause reductions in the glass transition temperature of thin polystyrene films.

The effect of free surfaces on the glass transition temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm

The properties of free polymer surfaces and their influence on the glass transition temperature of thin polystyrene films.

We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (T(g)) of thin polystyrene (PS) films. Direct measurements of the near-surface properties of PS films are made by monitoring the embedding of 10 and 20 nm diameter gold spheres into the surface of spin-cast PS films. At a temperature T = 378 K( > T(g)), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below T(g) (363 K < T < 370 K), both sets of spheres embed 3-4 nm into the PS films and stop. These studies suggest that a liquid-like surface layer exists in glassy PS films and also provide an estimate for the lower bound of the thickness of this layer of 3-4 nm. This qualitative idea is supported by a series of calculations based upon a previously developed theoretical model for the indentation of nanoscale spheres into linear viscoelastic materials. Comparing data with simulations shows that this surface layer has properties similar to those of a bulk sample of PS having a temperature of 374 K. Ellipsometric measurements of the T(g) are also performed on thin spin-cast PS films with thicknesses in the range 8 nm < h < 290 nm. Measurements are performed on thin PS films that have been capped by thermally evaporating 5 nm thick metal (Au and Al) capping layers on top of the polymer. The measured T(g) values (as well as polymer metal interface structure) in such samples depend on the metal used as the capping layer, and cast doubt on the general validity of using evaporative deposition to cover the free surface. We also prepared films that were capped by a new non-evaporative procedure. These films were shown to have a T(g) that is the same as that of bulk PS (370+/-1 K) for all film thicknesses measured (> 7 nm). The subsequent removal of the metal layer from these films was shown to restore a thickness-dependent T(g) in these samples that was essentially the same as that observed for uncapped PS films. An estimate of the thickness of the liquid-like surface layer was also extracted from the ellipsometry measurements and was found to be 5+/-1 nm. The combined ellipsometry and embedding studies provide strong evidence for the existence of a liquid-like surface layer in thin glassy PS films. They show that the presence of the free surface is an important parameter in determining the existence of T(g) reductions in thin PS films.

Surface denaturation and amyloid fibril formation of insulin at model lipid-water interfaces.

We consider the effects that different lipid surfaces have upon the denaturation and subsequent formation of amyloid fibrils of bovine insulin. The adsorption and unfolding kinetics of insulin being adsorbed onto the different lipid surfaces under denaturing conditions are studied using FTIR ATR spectroscopy and are compared to the bulk solution behavior of the protein. Atomic force microscopy studies are also performed to compare the fibrils growing on the different surfaces. This study shows that both the adsorption and unfolding kinetics of insulin can be described by a sum of exponential processes and that different surfaces behave differently, with respect both to one another and to the bulk protein solution. The proteins adsorbed onto the surfaces are observed to have faster unfolding kinetics than those in the bulk, and the fibril-like structures formed at the surfaces are shown to be different in a number of ways from those found in bulk solution. The beta-sheet content and growth kinetics of the adsorbed proteins also differ from those of the bulk system. An attempt is made to describe the observed behavior in terms of simple physical arguments involving adsorption, unfolding, and aggregation of the proteins.

The timescale of spinodal dewetting at a polymer/polymer interface.

We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate) (PMMA) thin films on polystyrene (PS) "substrates" is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface.