Thick Polystyrene Films Research Articles

Nanobubbles give evidence of incomplete wetting at a hydrophobic interface

The appearance of a hydrophobic surface, namely a crystalline (111) Si wafer coated with a thick soft polystyrene film, and the morphological changes along this interface depending on the polarity of an adjoining liquid phase were studied with magnetic tapping mode atomic force microscopy. Interfacially associated nanobubbles of decreasing size and number are observed as the hydrophobicity of the subphase increases. The disturbance of the water structure in the contact region induces the formation of nanobubbles. The topology of the interface is visualized, starting with the dry polymer under normal atmosphere conditions and observing the changes as air is replaced by a series of liquids. With water, the surface coverage of the substrate with bubbles is almost a close-packed configuration. The bubble shape is well approximated by spherical caps of a rather low aspect ratio. The Gaussian size distributions of bubble shape parameters are discussed. The contact angle of the nanobubbles is substantially smaller than the corresponding number measured for a macroscopic droplet. This apparent discrepancy might be resolved if the nanobubbles were assumed to exist along the interface as a connecting sublayer between a depleted water film at the hydrophobic polymer surface and an adsorbed macrodroplet.

CO2-Enhanced Transport of Small Molecules in Thin Films: A Fluorescence Study

Steady-state fluorescence measurements have been used to measure the rate of transport of a fluorescent probe (pyrene) out of ca. 250 nm thick films of polystyrene in contact with CO2(sc) at pressures in the range 35−110 bar (estimated CO2 content in the film up to 0.11 weight fraction) and several temperatures (35, 40, 50, and 60 °C). At constant temperature, the estimated pyrene diffusion coefficient increases by approximately 4 orders of magnitude from the lowest to the highest CO2 content (e.g., from ca. 10-14 cm2/s for ca. 0.04 CO2 weight fraction to ca. 10-10 cm2/s for ca. 0.11 CO2 weight fraction at 60 °C). We estimate the glass transition pressure at a given temperature (Pg(T)) in two ways: (1) from the inflection point of the log D vs P(CO2) plot, (2) matching log D at a given temperature and P(CO2) to the value obtained for an unswollen film at Tg (D is measured by the rate of loss of the probe into a vacuum).

Spinodal-like dewetting of thermodynamically-stable thin polymer films.

Energetic considerations indicate that long-range Van der Waals forces stabilize thin polystyrene (PS) films against height fluctuations on silicon substrates. Nevertheless, we report here on the amplification of capillary waves of specific wavelengths for 15 nm thick PS films on silicon, ultimately leading to dewetting in a "spinodal-like" process. However, the temporal dependence of the wavelength of the growing instability does not agree with the "classical" spinodal dewetting mechanism. Therefore, this phenomenon is ascribed to the existence of "structural" forces resulting either from the restructuring of the films or from density variations within the films during annealing, in accordance with recent theoretical treatments. The process is shown not to be limited to polystyrene films, which indicates the generality of our findings.

Polymer film dynamics using X-ray photon correlation spectroscopy

A new method of X-ray photon correlation spectroscopy (XPCS) is applied for probing the dynamics of surface height fluctuations as a function of lateral length scale in supported polymer films. The short wavelength and slow time scales characteristic of XPCS extend the phase space accessible to scattering studies beyond some restrictions by light and neutron. Measurements were carried out on polystyrene films of thicknesses ranging from 84 to 333 nm at temperatures above the PS glass transition temperature. We present the experimental verification of the theoretical predictions for the thickness, wave vector and temperature dependence of the capillary wave relaxation times for supported polymeric films above the glass transition temperature.

Pulsed slow-positron beam for polymer films

A pulsed slow-positron beam using 22 Na as the positron source has been constructed for characterizing thin polymer films, coating polymer materials and their interfaces. The construction is very simple and the time resolution achieved is about 500 ps, which is sufficient for measuring long-lived positron lifetime components in thin polymer films. The positron lifetime in a 45 nm thick epoxy resin thin film held on a single-crystal silicon substrate has been measured using this pulsed slow-positron beam. Formation of o-positronium ( ortho-Ps) is not observed when the incident positron energy is more than about 1 keV. The surface effect of the glass-transition temperature of a 0.3 mm thick polystyrene (PS) film has been also studied. It has been found that the glass-transition temperature of the sub-surface region of PS is lower than that of the PS bulk.

Anomalous Surface Relaxation Process in Polystyrene Ultrathin Films

Monodisperse polystyrene (PS) films with various thicknesses were spun-coated on silicon wafers with native oxide layer. Surface relaxation behavior in the PS films was studied as a function of thickness by lateral force microscopy (LFM). In the case of a thick PS film, a clear lateral force peak corresponding to surface αa-relaxation process of segmental motion was observed at a temperature much lower than the bulk glass transition temperature, Tg. As the film became thinner than 3−4 times the radius of gyration of an unperturbed consistent chain, the peak on lateral force vs temperature curve started to broaden out with decreasing thickness and eventually split into two peaks. The appearance temperature of the surface αa-relaxation peak was invariant with respect to the film thickness even in such an ultrathin state, meaning that surface Tg was insensitive to the thickness. On the other hand, the ultrathinning-induced relaxation process, called surface β-relaxation, was strongly dependent on the thickne...

Surface dynamics of polymer films.

The dynamics of supported polymer films were studied by probing the surface height fluctuations as a function of lateral length scale using x-ray photon correlation spectroscopy. Measurements were performed on polystyrene (PS) films of thicknesses varying from 84 to 333 nm at temperatures above the PS glass transition temperature. Within a range of wave vectors spanning 10(-3) to 10(-2) nm(-1), good agreement is found between the measured surface dynamics and the theory of overdamped thermal capillary waves on thin films. Quantitatively, the data can be accounted for using the viscosity of bulk PS.

Fabrication of Nanostructured Polymer Surfaces Using Colloidal Lithography and Spin-Coating

We present an original method for fabricating nanostructured polymer surfaces exhibiting homogeneous chemistry using colloidal lithography combined with polymer spin-coating. The first step of the strategy consists of creating gold surfaces with hemisphere-like protrusions, by adhesion of negatively charged polystyrene colloidal particles onto smooth positively charged surfaces, followed by coating with a thin layer of gold. In the second step, gold substrata are spin-coated with ultrathin polystyrene films. Using X-ray photoelectron spectroscopy and atomic force microscopy, we show that the nanotopography of the obtained polymer surfaces can be modulated by changing three parameters, i.e., the particle diameter, the particle concentration, and the polystyrene film thickness, while keeping the surface chemistry unmodified.

Dynamics of the leveling process of nanoindentation induced defects on thin polystyrene films

Using Atomic Force Microscopy (AFM) we study the effect of nanoindentation induced defects on 50 and 120nm thick unentangled polystyrene (PS) films, spin cast on silicon (Si) substrates. Indents with residual depths of penetration less than the film thickness level upon heating above the glass transition temperature (Tg) of bulk PS. The resulting leveling process is discussed in terms of a diffusion process driven by the curvature gradient. Calculated diffusivity values are close to the self-diffusivity of bulk PS.

Surface-Initiated Free Radical Polymerization of Polystyrene Micropatterns on a Self-Assembled Monolayer on Gold

We describe the in situ synthesis of nanometer thick films of polystyrene (PS) on a self-assembled monolayer (SAM) on gold by surface-initiated free radical polymerization and further demonstrate that three-dimensional polymer structures with micrometer lateral resolution and nanometer vertical resolution can be fabricated by combining microcontact printing (μCP) with surface-initiated polymerization (SIP). We implemented SIP onto a COOH-terminated SAM on gold using a sequential approach to couple an amine-terminated free radical initiator to the terminal COOH groups presented by the SAM, followed by free radical polymerization of styrene. Each step of SIP was characterized by X-ray photoelectron spectroscopy and ellipsometry, and the PS film and micropattern were also characterized by atomic force microscopy. We typically obtained PS films with a thickness of 10−20 nm for a polymerization time of 12−24 h and with a root-mean-square roughness of ≤0.5 nm. PS micropatterns were also successfully fabricated ...

Early Dewetting Stages of Thin Polymer Films Initiated by Nanoindentation

We study the effect of nanoindentation induced defects on thin (h = 50 nm) and thick (h = 100 nm) polystyrene (PS) films, spin cast on nonwettable silicon (Si) substrates. Indents with depths of penetration higher than the film thicknesses were imposed. Upon heating above the bulk glass transition temperature (Tg) of PS, some of the indents healed, resulting in a flat polymer surface, and some grew laterally and dewetted the substrate. Our goal is to establish a criterion that can predict the final result (healing or growing) of the indent evolution. We find that the applied force during the nanoindentation and the substrate area exposed to the air after the imposition of an indent can only be used as crude rules, since they do not provide a “cutoff” border, that distinguishes the growing from the healing indents. On the contrary the excess surface energy (ΔFγ) of the system, that is added to the initially flat coating with the formation of an indent, provides a clear critical value, ΔFγ,crit, which separ...

Electric Field Induced Self Assembly and Template Patterning of Polymer Microstructures

ABSTRACTWe have developed a method for fabricating polymer microstructures based on electric field induced self assembly and pattern formation. A dielectric fluid placed in between to conductive plates experiences a force in an applied electric field gradient across the plates, which can induce a diffusive surface instability and self construction of the fluid surface. This process is exploited for the fabrication of self assembled polymer microstructures as well as replicated patterns through the use of pre-patterned plates or electrodes. We have used silicon wafers and transparent ITO (Indium-Tin Oxide) coated quartz substrates to fabricate the capacitor structures. The bottom silicon plate is spin coated with a 100-200 nm thick polystyrene film. The ITO substrate was placed over the polymer surface at a distance to leave a thin air gap using spacers. For directed pattern transfer, patterned ITO substrates were used. The capacitor setup was heated above the glass transition temperature of the polymer and a voltage was applied across the plates (50-150 Volts), which induces electric fields on the order of 107–108 V/m. The capacitor structure was quenched to observe the structures using optical microscopy and atomic force microscopy. The method described can be used to fabricate a variety of structures in the micron and nanometer scales including bio-fluidic MEMS, polymer optoelectronic devices and patterned templates for nanolithography.

Scanning near-field infrared microscopy and spectroscopy with a broadband laser source

A scanning near-field microscope that allows the fast acquisition of midinfrared absorption spectra is described. The microscope couples the nanoscale spatial resolution of a scanning probe microscope with the chemical specificity of vibrational spectroscopy. Key design elements of the microscope include a tunable broadband infrared light source; an infrared focal plane array-based spectrometer which allows parallel detection of the entire pulse bandwidth (200 cm−1); and a single mode, fluoride glass, near-field probe fabricated with a chemical etching protocol. Infrared transmission images of a micropatterned thin gold film are presented that demonstrate spatial resolution conservatively estimated to be λ/7.5 at 3.4 μm, in the absence of optical artifacts due to topography. Constant height mode images of a polymer nanocomposite demonstrate instrumental sensitivity to fractional transmission changes of 1×10−3. Near-field absorption spectra (λ=3.4 μm) of a 2 μm thick polystyrene film are presented which demonstrate the instrumental sensitivity required for high spatial resolution, near-field absorption imaging. These spectra are acquired in 2 s and indicate a film thickness detection limit for polystyrene of 200 nm. Measurements exploring the coupling between the infrared absorption magnitude and changes in tip–sample separation suggest that near-field absorption imaging is relatively insensitive to topographic artifacts.

Nanostructure formation in spin-cast polystyrene films

The dependence of spin-cast polystyrene film thickness on silicon substrates is examined as a function of concentration (in toluene) and molecular weight. For uniform films ranging in thickness from 100 A to 1 µm, thickness follows a power law dependence on concentration, with the power varying from 1.25 at low molecular weights, to greater than 2 at the highest molecular weights examined. For very dilute concentrations, atomic force microscopy reveals that the films break up into a uniform distribution of voids or islands with submicron dimensions, as has been observed previously. The apparent film thickness (at molecular weight 690 000), is determined by secondary ion mass spectrometry, and is found to vary linearly with concentration for film thicknesses below ∼30 A, the thickness at which the film begins to break up. At this molecular weight, X-ray reflectivity results indicate that the limiting thickness of the remnant polymer regions is ∼30 A. Effects of annealing are also investigated. © 2000 Society of Chemical Industry

Evanescent (surface) wave holographic recording in thin polymeric film

Holographic grating recording with surface evanescent waves is reported. Owing to the small penetration depth the interference pattern is sorted in a 200 nm thick polystyrene film, sensitized with ON-stilbene. The absorption distribution in the recording medium is analysed in the small attenuation index approximation. The diffraction efficiency dependence on time and temperature is also investigated.

Direct Measurement of Retarded van der Waals Attraction

Total internal reflection microscopy is used to measure the total potential energy of interaction between a 6 μm polystyrene (PS) latex bead and either a bare glass microscope slide or a glass slide spin-coated with a 1 μm thick PS film, when the two interacting bodies are separated by 10−300 nm of aqueous solution having an ionic strength between 0.5 and 3 mM. In particular, these are the first measurements of van der Waals interaction between microscopic bodies of PS across water, for which the dielectric spectra are well-known. Under these conditions the bead is levitated above the slide by double-layer repulsion. After the gravitational contribution is subtracted, the potential energy profile displays a minimum of 0.5−2.3kT formed by long-range van der Waals attraction and shorter-range double-layer repulsion. The attraction was detectable at distances up to 200 nm. At separation distances greater than 100 nm (energy < 0.5kT), the measurements agree well with predictions using Lifshitz theory to predict the interaction of two PS half spaces, coupled with Derjaguin's approximation to account for the curvature of the sphere. At all separations, both retardation and screening are very important to the van der Waals interaction. As the separation becomes smaller than 100 nm, the measured interaction becomes weaker than predicted. Using atomic force microscopy (AFM), we observed asperities with heights up to 10 nm on the spin-coated PS film and up to 30 nm on the latex bead. If our experimental “zero” separation corresponds to contact of the largest asperities, the separation distance used in the theory should be larger than that measured. Shifting the theoretical curve by the sum of the asperity heights causes the theory to shift from overpredicting the van der Waals attraction to underpredicting it. We suggest a new theory in which roughness is treated as a diffuse film whose composition varies from pure PS at the inner surface to pure water at the outer surface. The composition profile can be determined independently from the histogram of elevations measured with AFM.

Friction and wear behaviour of carbon ion-implanted PS against steel

A 25 μm thick polystyrene (PS) film was implanted with 120 keV ions of carbon to doses of 1×10 14, 1×10 15, 3×10 15, 5×10 15 and 1×10 16 ions cm −2, respectively. The structure of the implanted PS was characterized by X-ray photoelectron spectroscopy (XPS), whereas the friction and wear behaviour of both implanted and unimplanted PS were investigated using an one-way reciprocating friction tester against a steel ball. Friction results indicate that ion implantation at a dose of 1×10 14 ions cm −2 increases friction coefficient, whereas the ion implantation at a dose of 1×10 15 ions cm −2 or higher decreases friction coefficient, and all the tests show that ion implantation lowers wear rate. XPS analyses reveal that ion implantation at dose of 1×10 14 ions cm −2 results in partial break of benzene ring, and at a dose of 1×10 16 ions cm −2 results in complete break of benzene ring.

The response time of optical sensors based on luminescence quenching

The response time of optical sensors based on dynamic quenching usually shows an asymmetry. The response is faster when the signal decreases than when it increases. This has been adequately explained by Opitz and Lübbers for a sensing film (SF) obeying strictly the Stern–Volmer relationship and for a homogeneous quencher concentration. In the first part of this paper, we extend this treatment to any response function and to a non-homogeneous quencher concentration in the SF. For downward curved or linear Stern–Volmer (SV) plots, the response time is always shorter when the signal decreases than when it increases; whereas for upward curved SV plots the asymmetry of the response depends on the definition of the response time, i.e. the fraction of the final response that must be reached (usually 90 or 99%). In the second part, various computer simulations were performed with a view to test the effect of gas mixing during transport from the cylinders to the measuring cell, the relative importance of the rate constants for crossing the gas/polymer phase interface and the rate constant for gas diffusion inside the SF. The calculated data are compared with those obtained on two polystyrene films of different thicknesses. It is found that gas mixing in the inlet tubing has a marginal effect on the response time, and that a quencher gradient in the film affects the response time. A by-product of the computations is an approximate value of the O 2 diffusion coefficient in polystyrene which is found to be in reasonable agreement with that recently measured by Ogilby: D O 2 (computed)=7.7×10 −7 cm 2 s −1; D O 2 (experimental)=2.3×10 −7 cm 2 s −1 at 25°C.

Brillouin light scattering studies of the mechanical properties of polystyrene/polyisoprene multilayered thin films

We have used Brillouin light scattering (BLS) to investigate the mechanical properties of thin polystyrene (PS) and polyisoprene (PI) films incorporated in a multilayer geometry consisting of alternating layers of the two polymers. All samples had a total thickness h ∼1600 A, and consisted of between 2 and 10 individual polymer films ; individual layer thicknesses were as small as 160 A. Ellipsometry was used to determine the thickness of the PS and PI layers. The velocities of several high-frequency film-guided mechanical waves were measured using BLS and the results are compared with values calculated using an effective medium approach. The effective elastic constants of the multilayered films were obtained from those determined for thick films of PS and PI. Remarkable agreement was obtained between the measured and calculated velocities even for samples in which the individual layer thicknesses were much less than the unperturbed size of the polymer molecules. These results suggest that the mechanical properties of polymers change very little even when the molecules are forced into highly confined geometries.

Porphyrins as probe molecules in the detection of gaseous pollutants I: Diffusion of pyridine in polystyrene films containing zinc-tetraphenylporphyrin

Zinc-porphyrins can be coordinated by amines, resulting in a red shift in the absorption spectrum. We have taken advantage of this property to study the diffusion of gaseous pyridine in polystyrene films. We have studied the absorption properties of polystyrene films of different thicknesses and calculated the extinction coefficients of Zn-porphyrin in these films. It has been demonstrated that there is no aggregation of porphyrins in the film at the concentrations used in this study. We have demonstrated that diffusion of pyridine is dependent on the concentration of porphyrins in the film but independent of the film thickness. A fast response (≈ 2 min) and a detection limit of 10 μmol m −3 of pyridine in atmosphere are obtained. The film can be regenerated by heating at 383 K under vacuum (1 mmHg). Since the films are found to be sensitive to other amines, a possible application is the realization of cheap dosimeters for amines.