A direct quantitative measure of surface mobility in a glassy polymer.

Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.

When Does a Glass Transition Temperature Not Signify a Glass Transition?

We present a simple model that links enhanced mobility at the free surface to the dilatometric glass transition temperature, Tg in thin films. The model shows that what is typically measured as a dilatometric Tg, characterized by the hallmark "kink" in the plot of film thickness versus temperature, only represents the dynamics of an infinitesimally thin layer of the sample. In other words, the measured dilatometric Tg value in thin films is no longer a good reporter of the dynamics. Calculations based on the model are found to agree with a vast body of thin film Tg measurements. While mathematically simple, the model contains all the necessary physics of a near surface layer with enhanced dynamics and a length scale over which the surface dynamics monotonically varies from surface enhanced to bulk-like. The model demonstrates that the typical dilatometric measurement of the glass transition is not necessarily a real glass transition.

Reduced glass transition temperatures in thin polymer films: surface effect or artifact?

We have examined the direct effect of manipulating the number of free surfaces on the measured glass transition temperature T(g) of thin polystyrene films. Thin films in the range 35 nm < h < 114 nm with molecular weights of 592 kg/mol and 1144 kg/mol were studied. Ellipsometry was used to determine the temperature dependence of the thickness and refractive index of freestanding films. By noting the change in slope in each of these quantities, a T(g) value can be assigned in quantitative agreement with previously reported results. For thin freestanding films this value is reduced from that of the bulk. The exact same films are then transferred to a Si substrate and the T(g) of the resulting supported film was determined. The T(g) values of the now supported films are the same as the bulk value and the same as previous reports of similar supported films. These experiments unambiguously show that free interfaces are the dominant cause of the T(g) reductions for the film thicknesses studied.

Dynamics of interacting edge defects in copolymer lamellae.

It is known that terraces at the air-polymer interface of lamella-forming diblock copolymers do not make discontinuous jumps in height. Despite the underlying discretized structure, the height profiles are smoothly varying. The width of a transition region of a terrace edge in isolation is typically several hundreds of nanometres, resulting from a balance between surface tension, chain stretching penalties, and the enthalpy of mixing. What is less well known in these systems is what happens when two transition regions interact with one another. In this study, we investigate the dynamics of the interactions between copolymer lamellar edges. We find that the data can be well described by a model that assumes a repulsion between adjacent edges. While the model is simplistic, and does not include molecular level details, its agreement with the data suggests that some of the the underlying assumptions provide insight into the complex interplay between defects.

Reversible sphere-to-lamellar wetting transition at the interface of a diblock copolymer system.

We use ellipsometry to investigate a transition in the morphology of a sphere-forming diblock copolymer thin-film system. At an interface the diblock morphology may differ from the bulk when the interfacial tension favours wetting of the minority domain, thereby inducing a sphere-to-lamella transition. In a small, favourable window in energetics, one may observe this transition simply by adjusting the temperature. Ellipsometry is ideally suited to the study of the transition because the additional interface created by the wetting layer affects the polarisation of light reflected from the sample. Here we study thin films of poly(butadiene-ethylene oxide) (PB-PEO), which order to form PEO minority spheres in a PB matrix. As temperature is varied, the reversible transition from a partially wetting layer of PEO spheres to a full wetting layer at the substrate is investigated.

Surface nucleation in the crystallisation of polyethylene droplets.

The division of semi-crystalline polymeric material into small domains is an effective tool for studying crystal nucleation. The scaling behavior of the nucleation rate as a function of domain size can reveal important information about the mechanism responsible for the birth of a crystal nucleus. We have investigated the process of crystal nucleation in a system of dewetted polyethylene droplets. Through the use of a correlation sample analysis, we are able to differentiate between heterogeneous and homogeneous nucleation mechanisms in a droplet sample. An analysis of the dependence of the nucleation rate on droplet size reveals that the nucleation probability scales with the surface area of the droplet.

Adhesion and membrane tension of single vesicles and living cells using a micropipette-based technique.

The fundamental study of the adhesion of cells to each other or to a substrate is a key research topic in cellular biophysics because cell adhesion is important to many biological processes. We report on the adhesion of a model cell, a liposome, and a living HeLa cell to a substrate measured with a novel experimental technique. The cells are held at the end of a micropipette mounted on a micromanipulator and brought into contact with a surface. The adhesion energy and membrane tension are measured directly using the deflection of the micropipette when binding or unbinding the cell from the substrate. Since the force applied on the cells is known throughout the experiment, the technique presented enables the measurement of dynamics such as changes in the adhesion, elasticity, and membrane tension with time.

Spreading of diblock copolymer droplets: a probe of polymer micro-rheology.

We present an experimental study of the spreading dynamics of symmetric diblock copolymer droplets above and below the order-disorder transition. Disordered diblock droplets are found to spread as a homopolymer and follow Tanner's law (the radius grows as R approximately t(m), where t is time and m = 1/10). However, droplets that are in the ordered phase are found to be frustrated by the imposed lamellar microstructure. This frustration is likely at the root of the observed deviation from Tanner's law: droplet spreading has a much slower power law (m approximately 0.05+/-0.01). We show that the different spreading dynamics can be reconciled with conventional theory if a strain-rate-dependent viscosity is taken into account.

Kinetics of layer hopping in a diblock copolymer lamellar phase.

In the ordered state, symmetric diblock copolymers self-assemble into an anisotropic lamellar morphology. The equilibrium thickness of the lamellae is the result of a delicate balance between enthalpic and entropic energies, which can be tuned by controlling the temperature. Here we devise a simple yet powerful method of detecting tiny changes in the lamellar thickness using optical microscopy. From such measurements we characterize the enthalpic interaction as well as the kinetics of molecules as they hop from one layer to the next in order to adjust the lamellar thickness in response to a temperature jump. The resolution of the measurements facilitate a direct comparison to predictions from self-consistent field theory.

Effect of atmosphere on reductions in the glass transition of thin polystyrene films.

We have used nulling ellipsometry to measure the glass transition temperature, T (g) , of thin films of polystyrene in ambient, dry nitrogen, and vacuum environments. For all environments, the measured T (g) values decrease with decreasing film thickness in a way that is quantitatively similar to previously reported studies in ambient conditions. These results provide strong reinforcement of previous conclusions that such reduced T (g) values are an intrinsic property of the confined material. Furthermore, the results are in contrast to recent reports which suggest that the T (g) reductions measured by many researchers are the results of artifacts (i.e. degradation of the polymer due to annealing in ambient conditions, or moisture content).

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.

Measurement of adhesion energies and Young's modulus in thin polymer films using a novel axi-symmetric peel test geometry.

We present a novel method of probing adhesion energies of solids, particularly polymers. This method uses the axi-symmetric deformation of a thin spincast polymer membrane brought into contact with a flat substrate to probe the work of adhesion. The use of a thin membrane minimizes uncertainty in the radius of contact, while the use of spincast films provides very smooth surfaces by means of a very simple method. The experimental profile of the deformed membrane shows good agreement with the expected logarithmic profile. The experimental setup enables the measurement of Young's modulus and the solid-solid work of adhesion for thin films. The value obtained for Young's modulus of polystyrene (PS) was found to be in agreement with other conventional measurement techniques. In addition, measurement of the work of adhesion at the PS/silicon oxide interface was possible. The apparatus is well suited to studying the dependence of Young's modulus, work of adhesion and fracture energy on membrane thickness, temperature, pulling rate, and ageing of the interface, and can readily be modified to study biologically relevant samples.

Direct visualisation of homogeneous and heterogeneous crystallisation in an ensemble of confined domains of poly(ethylene oxide).

We present a study of homogeneous and heterogeneous nucleation in polymer crystallisation. In bulk samples the crystallization is typically dominated by nucleation from defects (heterogeneous nucleation), and consequently studies must rely on sample preparation to minimize this effect. We present a study of nucleation within discrete droplets of poly(ethylene oxide) that are formed by the dewetting of a thin film on an unfavourable substrate. The samples provide an ensemble of impurity-free droplets, with length scales that can easily be measured. We show that the data for heterogeneous and homogeneous nucleation is qualitatively different, and that the data mirrors the fundamental differences in the underlying mechanisms for the two nucleation processes. The experiments presented here provide a simple method that can be used to study heterogeneous and homogeneous nucleation in great detail.

First inelastic neutron scattering studies on thin free standing polymer films.

Glass transition studies in free standing polymer films have revealed values of the transition temperature, T(g), which were substantially reduced below the bulk for sufficiently thin films. Here we report on the preparation of two stacks of free standing polystyrene films: 70 films with a thickness of h approximately 107 nm and 140 films with h approximately 55 nm with equivalent total sample thicknesses of approximately 7.5 microm. We have performed the first measurements on such samples using inelastic neutron scattering, and demonstrate that inelastic neutron scattering experiments, performed on the time-of-flight spectrometer IN6 and the backscattering spectrometer IN16 at the Institut Laue-Langevin, are feasible.

Crystallization kinetics and crystal morphology in thin poly(ethylene oxide) films.

We present a detailed study of the kinetics of crystallization for thin films of poly(ethylene oxide) (PEO). Measurements of the growth rate have been carried out using optical-microscopy techniques on films of monodisperse PEO. Films with thicknesses from 13 nm to approximately 2 microm were crystallized isothermally at temperatures approximately 20 degrees C below the melting point. A remarkable non-monotonic slowing-down of the crystal growth is observed for films with thickness less than approximately 400 nm. The changes in the growth rate from bulk-like values is significant and corresponds to a factor of 40 decrease for the thinnest films studied. The morphologies of isothermally crystallized samples are studied using atomic-force microscopy. We find that a morphology, similar to diffusion-controlled growth (dendritic growth and densely branched growth), is observed for films with h<150 nm. In addition, changes in the morphology occur for thicknesses consistent with changes in the growth rate as a function of film thickness.

Molecular weight dependence of reductions in the glass transition temperature of thin, freely standing polymer films.

We have used transmission ellipsometry to perform a comprehensive study of the glass transition temperature T(g) of freely standing polystyrene films. Six molecular weights M(w), ranging from 575 x 10(3) to 9100 x 10(3), were used in the study. For each M(w) value, large reductions in T(g) (as much as 80 degrees C below the bulk value) were observed as the film thickness h was decreased. We have studied in detail the dependence of the T(g) reductions on M(w) in a regime dominated by chain confinement effects. The empirical analysis presented is highly suggestive of the existence of a mechanism of mobility in thin freely standing films that is inhibited in the bulk and distinct from the usual cooperative motion associated with the glass transition.