Session WC12 - Dynamics of Thin Films.
ORAL session, Thursday afternoon, March 25
Room 268W, GWCC

[WC12.01] A Detailed Study of the Glass Transition in Freely-Standing Polystyrene Films

We have found that ellipsometry can be used to provide an accurate measure of the glass transition temperature T_g of thin freely-standing polystyrene films. We have used this technique to perform a detailed study of T_g as a function of the film thickness h, the molecular weight M_w of the polymer and the thermal cycling of the films. Using ellipsometry, we have verified quantitatively the large h-dependent and M_w-dependent reductions in T_g measured previously for freely-standing polystyrene films using Brillouin light scattering [1]. Also, we have extended the T_g measurements to different M_w values.

[1] J.A. Forrest, K. Dalnoki-Veress and J.R. Dutcher, Phys. Rev. E 56, 5705 (1997).

[WC12.02] Measurement of In-Plane CTE and Modulus of Polymer Thin-Films

Characterization of polymer thin-films for electronic applications is important for the advancement of the semiconductor industry. We have applied bending-plate technique to study the residual stresses of polymer thin-film materials. After coating polymer thin-film on a wafer at high temperature, the curvature of the wafer changed due to the difference in the thermal expansions. We measured the curvatures of the wafer before and after the coating of polymer. The stress of the polymer thin-film can be computed from the measured curvatures using Stoney's equation. The stress of the thin-film was measured as a function of temperature. From stress-temperature curves, the in-plane coefficient of thermal expansion and mechanical modulus can be deduced. This measurement technique does not require removal of thin-film from the substrate. This non-destructive data is interest and unique, because it involves polymer-substrate interface. Experimental results for several polymers are discussed.

[WC12.03] Onset of Hole Formation in Freely-Standing Polystyrene Films

We have obtained a sensitive measure of the onset of the formation of holes in thin freely-standing polystyrene films as the films are heated from room temperature. This measurement was performed using a membrane deflection experiment in which a small pressure difference across the film is measured and controlled to within 0.1 Pa. The experiment provides an objective measure of the initial stages of hole formation over the entire film surface. The dependence of the onset temperature for hole formation on the film thickness and the pressure difference across the film will be discussed.

[WC12.04] Mechanism of Hole Growth in Freely-Standing Polystyrene Films

Recently, we have measured the growth of holes in thin, freely-standing polystyrene films in the melt state. For all films, we observed exponential growth of the hole radius and uniform thickening, with reductions in the characteristic growth time as the film thickness was reduced. These results are interpreted in terms of the dependence of the viscosity on the shear strain rate associated with the hole growth process. The hole growth measurement provides a novel probe of non-linear viscoelastic effects in confined polymers.

[WC12.05] Viscoelastic properties of the free standing PS films

We have studied the viscoelastic properties of free standing PS films by measuring the rate of the growth of holes as a function of the molecular weight, thickness and temperature. Samples were imaged by optical microscopy and atomic force microscopy. No rim around the growing holes was observed, indicating "viscous bursting". The radius grows exponentially with time for thick films, but constant velocity is found for thin films. The results show the viscous effect dominates the bursting of the hole of thick free standing films where the bulk 3.4 power law for viscosity is found. however, for thin films, a weaker dependence of viscosity on molecular weight is observed. SIMS results show no observable diffusion perpendicular to the free surface on the time scale of the hole-opening. X-Ray reflectivity measurements show that thin free standing films contract upon annealing with the same behaviour as the supported film. These results will be interpreted in terms of surface tension and elasitc driving forces. Acknowledgement:We appreciated the suported of the National Science Foundation Polymer Program(DMR9316157),the Department of Energy(DE-FG02-93ER45481), and the MRSEC program of the national Science Foundation(DMR9632525)

[WC12.06] X-ray Scattering Measurement of Free-standing Polystyrene films

We used X-ray scattering to investigate surface capillary waves of thin freestanding polymer films. Our previous results showed that strong surface interactions can modify the capillary wave spectrum and introduce unusual cut-off in propagation frequencies(PRL,2731,1998). The study of freestanding film, the polymer film standing without substrate, can be a very good model. Different molecular weight polystyrene were spun on the glass and floated on aluminum plate having a 3mm-diameter hole in the center and pre-annealed in vacuum at 120 ^oC. The specular reflectivity, diffuse and longitudinal diffuse scattering are measured as a function of film thickness and temperature in vacuum. A distinct beating in the specula reflectivity profile was observed in freestanding films more than 1200Åthick. We postulate that the beating may be due to differences in film thickness when a resonant standing capillary wave mode is established. The slope of diffuse scattering profiles increased with film thickness.

[WC12.07] New Developments in the Glass Transition of Freely Standing Polymer Films

We have used Brillouin Light Scattering (BLS) to characterize the molecular weight dependence of the glass transition temperature, T_g, for thin, freely standing films of polystyrene. The freely standing films, with thicknesses as small as 200 Åwere prepared from monodisperse polymer with M_w values ranging from 115k to 2100k. For values of M_w (> 400k) we observe a T_g value which depends on both the film thickness and the M_w value; in agreement with previous BLS studies. For M_w < \sim 350k, the measured T_g exhibited no discernible variation with M_w value, suggesting a much weaker M_w dependence. The behavior for these samples is characterised by large decreases in the T_g value for films with thickness h \leq 400 ÅThe contrast between the thermal expansivities of the melt and glassy states was studied and found not to be a strong function of the film thickness. This is in contrast to the case for supported polymer films where the T_g value becomes progressively harder to measure for samples with smaller film thickness. The results are discussed in terms of models used to describe anomalous T_g behavior in confined systems.

[WC12.08] Temperature Effect on Dielectric Loss Response of Confined Ploymer

Frequency dependence of the dielectric loss of thin cis-polyisoprene films was measured at temperatures from 25oC to -60oC. To avoid the undesirable effects of surface roughness, freshly cleaved atomically smooth mica was placed between silver electrodes. The polymer samples of different thickness, confined between mica to a definite thickness, were spin-coated onto the mica and the thickness was accurately measured by multiple beam interferometry. Though the normal mode loss peak of the confined polymer did not shift so much as the terminal zone peak of viscoelastic relaxation, it was observed that not only the normal mode but also segmental mode relaxation peaks were shifted to lower frequency with decreasing sample thickness. Preliminary measurements show that normal mode and segmental mode relaxation display a different dependence on temperature in thin film than in the bulk.

[WC12.09] Molecular Mobility in Polymer Thin Films

Fluorescence recovery after photobleaching (FRAP) was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene (PS) supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, Tg, and in the range of film thicknesses from 5-100 times the radius of gyration Rg of individual polymer molecules. As the film thickness decreases towards Rg the value of D increase above the bulk values, with significant effects first appearing in films about 20 times Rg. In the thinnest films studied, about 4 Rg, the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both Tg and the thermal expansion coefficient for liquid state decrease as the film thickness decreases. The possible effects of surface segregation of the dye-probe are discussed.

[WC12.10] Ultrathin Polymer Films: Probe Diffusion and Physical Aging Studies

There are conflicting results in the research literature regarding the range of influence of polymer-substrate affinity on the dynamics of ultrathin polymer films. We address this issue with fluorescence methods for in situ monitoring of small molecule probe diffusion (coupled to cooperative segmental mobility) and physical aging in ultrathin polymer films. Studies to date near Tg in supported films (on fused silica) of polystyrene, poly(isobutyl methacrylate), and poly(2-vinyl pyridine) show little effect of film thickness or polymer-substrate affinity on polymer relaxation dynamics; only polystyrene shows probe diffusion yielding any significant dependence on film thickness for thicknesses < 150 nm. Physical aging of polystyrene supported on (hydrophilic) fused silica and on a hydrophobically modifed silica show little effect of film thickness or substrate polarity in films as thin as 10 nm. These studies indicate that specific polymer-substrate interactions do not typically have a range influence on ultrathin polymer film mobility of more than a few tens of nanometers or less. Studies are underway on systems that may show greater effects through major modification of Tg with film thickness.

[WC12.11] Fluorescence Probe Determination of the Glass Transition Temperature (Tg) for Supported and Free-Standing Ultra-Thin Polymer Films.

Recently, several different experimental results employing a variety of techniques have suggested that Tg for ultra-thin flexible polymer films becomes a function of film thickness at approximately 100nm. These experiments can be divided into two categories: experiments on supported films and experiments on free-standing films, with the free standing films showing the largest affect. There have been no systematic studies on the effect of the degree of support on the deviation of Tg. In this paper, results from a fluorescence probe determination of the glass transition temperature on a series of supported and partially supported ultra-thin flexible polymer films will be discussed in addition to the experimental details.

[WC12.12] Glass Transition Behavior of Polymer Thin Films

The glass transition behavior of polymer thin films has recently received a great amount of attention. A thin film has a large surface to volume ratio and polymer/substrate interaction which may lead to shift in the glass transition temperature. The glass transition behavior might be different with bulk and show thickness dependence. In this work, we propose a novel method for the characterization of surface glass transition behavior for the polymer thin films. The method to measure the glass transition temperature, Tg involves pressing a sharp cantilever tip against a sample with a certain load. The drive signal in the lateral direction was modulated sinusoidally and applied to the scanner of an atomic force microscopy (AFM). The depth of penetration can be evaluated by measuring the response amplitude of the cantilever in lateral direction. The response amplitude is monitored as a function of the temperature of the sample. Tg is determined from the temperature-amplitude curve. Spin coated poly(n-butyl methacrylate) (PBMA) and polystyrene (PS) thin films of thickness ranging from 15~250 nm were studied. It was found that for the PBMA films thinner than 50 nm or approx. 6Rg of this polymer Tg increases gradually as the film thickness decreases. These results will be directly compared with x-ray reflectivity measurements of changes in the expansion coefficients on the same films.

[WC12.13] The Character of the Glass Transition of Thin Supported Polymer Films

We have used ellipsometry to study the thermal expansivity of thin polystyrene films on silicon substrates with thicknesses of 10-200nm. Using a cooling scan at 2K/min from the liquid state that was employed to avoid any effect of the thermal history we find well defined glass transitions; films with thickness > 60 nm show bulk like behaviour while for films with thickness < 60 nm the glass transition is shifted to lower temperatures, in agreement with many other studies. However more detailed analysis of the expansivities shows that for thinner films the transition width is broadened, while the strength of the transition, defined by the difference between the expansivities in the liquid and glassy state, is reduced, with the expansivity in the glassy state higher than in the bulk. We have also studied ageing effects by conducting a heating scan after isothermal annealing 20K below the bulk glass transition temperature. These show a relaxation peak in the expansivity for thicker films, with the strength of the peak reducing in magnitude for thinner films until for a 10 nm film no peak is visible. Both of these phenomena are consistent with the idea that a layer of roughly constant thickness, of order 10 nm, near the surface of the film has liquid like thermal properties at all temperatures.

[WC12.14] Dielectric Response of Polymer Films Confined between Mica Surfaces

Dielectric response of organic thin films when confined between opposed atomically-flat mica surfaces has been investigated. The main idea, to prevent deleterious effects of surface roughness, was to buffer the metal electrode by an intervening dielectric layer of cleaved mica. Normal-mode relaxation of cis-polyisoprene (Mn = 6,000 and 14,500 g-mole-1) within a surface forces apparatus and also sandwiched between parallel sheets of mica were measured as a function of film thickness. Although the peak of the dielectric loss shifted to lower frequency by factor of only 2, the response at lower frequencies grew more slowly than directly proportional to frequency, indicating a distribution of relaxation times, and at a given frequency a huge reduction of dielectric loss was observed as the film thickness decreased. In a comparison with frequency dependent shear dynamics, the peak of dielectric response fell at significantly higher frequency than the peak of mechanical response, showing that single-molecule motions were not retarded so much as viscoelastic shear relaxations.

[WC12.15] Calorimetric Measurements of the Glass Transition in Nano-meter Pores

Controlled Pore Glasses of pore diameter ranging from 10 to 50 nm have been used as matrices to study the glass transition (Tg) of solutions of polystyrene in ortho-terphenyl in constrained geometries. Differential scanning calorimetry (DSC) techniques have been used to examine the change in the glass transition as a function of both pore size and concentration. Concentrations were chosen to vary from c* to 4 c*, where c* is the overlap concentration for the polymer chains. These results show the first calorimetric evidence that constraint and geometry may exhibit opposite effects on the glass transition. The DSC thermograms exhibit two glass transitions. That of a "core" liquid is depressed from the bulk (unconstrained) Tg. In addition, there is evidence that there is a surface layer that is constrained and has an increased Tg relative to the bulk value. Interestingly, this two-layer model depends on the observation that the total heat capacity change over the two glass transitions is the same as that for the bulk material.

Part W of program listing