Session Q17: Focus Session : Glass Transition in Thin Films II

Local and Average Glass Transitions in Polymer Thin Films

No quantitative, predictive model has yet been developed to account for the dramatic suppression of the glass transition (Tg) of a polymeric film in the neighborhood of a free surface. Experimental results provide evidence not only for the correlation between Tg and total film thickness, but also partially illuminate how more local slices of the film behave. Our model predicts the local Tg relative to distance from a free surface. In using these results we are lead to consider the kind of average nature evidently performs in revealing the film-averaged response measured experimentally. In this talk I will summarize our progress to date, compare with some existing data, and suggest directions for future experimental work.   

Dye reorientation in freestanding polystyrene films exhibits fast and slow subpopulations

The segmental dynamics of freestanding polystyrene films as thin as 14 nm were studied by measuring the reorientation of dilute dye molecules. Both temperature ramping and isothermal anisotropy measurements reveal the existence of two subsets of dye molecules with differing dynamics. The dynamics of the slow subset matches the dynamics observed in thick films. The reorientation time scale of the fast subset is a few hundred seconds at Tg-25 K and has weak temperature dependence. At Tg- 5 K, the average relaxation time of these two subsets differs by 4 orders of magnitude. The fraction of the sample with fast dynamics increases with temperature and does not depend on the molecular weight of the polymer. We interpret the fast subset as a mobile surface layer and estimate a thickness of 6 nm at the bulk Tg.   

Effect of Free Surface on Physical Aging of Polystyrene Films and Its Connection to Glass Transition Temperature Reductions: Evidence for a Gradient in Dynamics

The glass transition and physical aging in nanoconfined polymer films have been heavily studied. Recently there have been an increasing number of studies pointing towards a gradient in mobility emanating from the free surface in these films. Using a new streamlined ellipsometry technique, we have measured the temperature dependence of the physical aging rate Beta for both bulk (2430 nm) and thin (29 nm) polystyrene (PS) films supported on silicon. We find that the thinner films have reduced physical aging rates at all temperatures that are inconsistent with a simple shift in the temperature dependence of Beta corresponding to the shift in Tg observed in these films. The reduced Beta values measured at all physical aging temperatures are consistent with a gradient in dynamics originating from the free surface of the film. Our data is well fit by a simple two-layer model that has been previously employed to explain the Tg reductions in PS thin films, suggesting that the enhanced dynamics present at the free surface are responsible for both effects. The surface layer thickness of this two-layer model, which increases with decreasing temperature, characterizes the depth to which the enhanced mobility at the free surface propagates into the film.   

Molecular Dynamics Simulations of Glass-forming Polymer Films

We employ molecular dynamics simulations to explore structure and dynamics of freely standing and supported polymer films as they are cooled toward the glass transition. Our simulations are concerned with the features of a coarse-grained bead-spring model in a temperature regime above the critical temperature $T_{\mathrm{c}}$ of mode-coupling theory. We find that the film dynamics is spatially heterogeneous. Monomers at the free surface relax much faster than they would in the bulk at the same temperature $T$. The fast relaxation of the surface layer continuously turns into bulk-like relaxation with increasing distance $y$ from the surface. This crossover remains smooth for all $T$, but its range grows on cooling. We show that it is possible to associate a gradient in critical temperatures $T_{\mathrm{c}}\c(y)$ with the gradient in the relaxation dynamics. This finding is in qualitative agreement with experimental results on supported polystyrene (PS) films. Furthermore we show that the $y$-dependence of $T_{\mathrm{c}}(y)$ can be expressed in terms of the depression of $T_{\mathrm{c}}(h)$---the global $T_{\mathrm{c}}$ for a film of thickness $h$---if we assume that $T_{\mathrm{c}}(h)$ is the arithmetic mean of $T_{\mathrm{c}}(y)$ and parametrize the depression of $T_{\mathrm{c}}(h)$ by $T_{\mathrm{c}}(h)=T_{\mathrm{c}}/(1+h_0/h)$, a formula suggested in the literature for the reduction of the glass transition temperature in supported PS films. We demonstrate the validity of this formula by comparing our simulation results to results from other simulations and experiments.   

Glass transition temperature and its distribution in nanoconfined free-standing polymer films

The confinement effect on the glass transition temperature (Tg) of free-standing polystyrene (PS) films is determined using the temperature dependence of a pyrene-label fluorescence intensity ratio. A strong confinement effect on Tg is evident at thicknesses $<$ 80-90 nm; a 41-nm-thick PS film (Mn = 701 kg/mol) exhibits a 47 K reduction in Tg relative to bulk PS. A strong molecular weight dependence of the Tg-confinement effect is observed, in reasonable agreement with the study by Dutcher group (PRE 2001). Employing multilayer films, measurements of Tg at specific locations within PS free-standing films are possible, e.g., a 14-nm-thick pyrene-labeled PS layer atop a 500-nm-thick PS free-standing film exhibits a 32 K reduction in Tg relative to bulk PS, the same level of free-surface effect on Tg as supported PS films. This study is the first to experimentally address the distribution of Tgs across free-standing films and provide a route to test the fundamental basis for the large Tg-confinement effect in free-standing films and in particular the theory proposed by de Gennes (EPJE 2000).   

Molecular Weight Changes and Crosslinking Kinetics in Glassy and Elastomeric Thin Films

The quantitative and qualitative kinetics of molecular bridging through hydrogen extraction from the tertiary carbon in Polymer backbones are explored through HPLC with MALLS in 300nm films of Polystyrene, Poly(n-butyl acrylate), and other polymers above and below the glass transition temperature. Changes in molecular weight distribution and the appearance of peaks at double and triple the original molecular weight allow the study of the initial stages of network formation. The relative merits of multiple bridging molecules are explored, as well as their effects on kinetics and distribution. When our compounds are mixed into a polymer and exposed to UV radiation, they abstract hydrogen atoms from any chains in proximity, thereby initiating a cascade of free radical reactions that include several mechanisms that can lead to covalent polymer crosslinking.   

Influence of Molecular Size and Shape to Viscoelastic Properties of Polyethylene Glycol (PEG) Solution Boundary Layers

Viscosity and shear modulus of polyethylene glycol (PEG) solution boundary layers with different molecular weights and shapes were studied using a quartz crystal (coated with Au) resonator technique. Based on the resonant frequency shift and the dissipation broadening of the quartz crystal resonator, the viscosity and shear modulus of the solution boundary layers near the solution-Au interface as a function of the concentration and molecular weight of PEG molecules were determined. The results show that near the semidilute concentration, the viscosity of the boundary layer increases rapidly following a power law with an exponent that depends on the molecular weight of PEG molecules. For solutions with small PEGs, the viscosity increases of the boundary layers are similar to that in bulk solutions; the shear modulus of the boundary layer remains negligible up to the highest concentration measured. However, for solutions with large PEGs, the increases in viscosity deviate from that of the corresponding bulk solutions, and the boundary layers display noticeable nonzero shear modulus at the higher concentrations. The implication of these results will be discussed.   

Unified description of the slip phenomena in sheared polymer films: A molecular dynamics study

The dynamic behavior of the slip length in shear flow of polymer melts past atomically smooth surfaces is investigated using MD simulations. The polymer melt was modeled as a collection of FENE-LJ bead-spring chains. We consider shear flow conditions at low pressures and weak wall-fluid interaction energy so that fluid velocity profiles are linear throughout the channel at all shear rates examined. In agreement with earlier studies we confirm that for shear- thinning fluids the slip length passes through a local minimum at low shear rates and then increases rapidly at higher shear rates. We found that the rate dependence of the slip length depends on the lattice orientation at high shear rates. The MD results show that the ratio of slip length to viscosity follows a master curve when plotted as a function of a single variable that depends on the structure factor, contact density and temperature of the first fluid layer near the solid wall. The universal dependence of the slip length holds for a number of parameters of the interface: fluid density and structure (chain length), wall-fluid interaction energy, wall density, lattice orientation, thermal or solid walls.   

Non-destructive probing of mechanical anisotropy in polyimide films at nanoscale

The present work demonstrates the first application of Brillouin light scattering (BLS) to probe elastic excitations in highly anisotropic polymer thin films. Both the in-plane and out-of-plane longitudinal and shear moduli of poly(biphenyl dianhydride--$p$--phenylenediamine) (BPDA-PDA) polyimide were accessed by utilizing the transmission and reflection scattering geometries [Macromolecules 2007, 40, 7283/ 2009, 42, 7164]. A series of thin BPDA-PDA polyimide coatings on transparent substrates with thickness in the range of 0.1-20 $\mu $m were explored, and no noticeable trend in elastic properties and mechanical anisotropy with thickness has been found, in conformity with earlier results. The earlier studies were restricted to thick films ($>$1.5$\mu $m) and to the in-plane moduli.   

Self-wrinkling of a thin polymer film on a soft elastic substrate

We discuss a new method to induce wrinkling in a thin glassy film. In these experiments, a thin polystyrene film is floated onto water and attached to polydimethylsiloxane substrate~as it is either pushed into or pulled out of a water bath at a chosen speed and angle.~ As attachment proceeds, well-aligned wrinkles form parallel to the three point contact line. Several methods (osmotic swelling, thermal expansion, mechanical strain) have been used extensively to induce wrinkling of thin glassy films, with the mechanics of these systems understood through elastic instability theory. The process presented here offers important advantages compared to these methods. The wavelength is controlled by the mechanical properties of the bilayer, consistent with classical wrinkling theory, while the amplitude (or applied strain) is controlled mainly by the speed of the attachment process. Furthermore, wrinkles do not form for speeds exceeding a critical value, theoretically ascribed to a liquid entrainment limit. ~By controlling the curvature and velocity of the three point contact line, we demonstrate that this method provides a powerful tool for creating localized patterns across laterally extensive lengths.   

Molecular Weight and nanoparticle size dependence of Nanoparticle embedding into Glassy Polymer films

We present a study of nanoparticle embedding into polystyrene films over a range of temperatures both above and below the glass transition temperature . In this study we vary the nanoparticle size to examine the effect of probe size scale on the apparent viscoelastic properties. Utilizing polymers of different M$_{w}$ reveals a surprising dependence of the embedding lifetimes on polymer size. These studies suggest new evidence for a reduced entanglement density near the polymer free surface. Collectively the results indicate nanoparticle embedding may provide a new probe of entanglements in thin polymer films.   

Capillary force induced assembly of high aspect-ratio polymeric micropillar arrays

We present experimental evidence and theoretical calculations to show that when a liquid is evaporated off the surface of 2D arrays of micropillars, the micropillars cluster together due to lateral capillary meniscus interaction force rather than due to often reported Laplace pressure difference resulting from isolated capillary bridges. Experiments show that the pillars assemble while still completely surrounded by wetting liquid except at the tips and calculations reveal that the lateral capillary meniscus interaction forces are much smaller than those estimated from Laplace pressure approach. The cluster sizes of the collapsed micropillars as a function of their elastic modulus and the critical modulus for stability as estimated from capillary meniscus interaction approach agree well with the experimental observation. We also discuss the utility of such clustered micropillars as ultrathin whitening layers.   

Varying Properties of Polymers in Thin Films: The Influence of Solvent Quality on Dewetting

The transition from a solution to a dry melt or glass is shown to have an impact on the behavior of polymeric samples. This transition may result in residual stresses arising from out-of-equilibrium chain conformations due to rapid solvent loss. These non-equilibrium conformations and resultant residual stresses, in turn, can give rise to changes in the viscoelastic properties and ageing behavior of the samples. We investigate the effect of sample preparation on the dewetting behavior of spincast thin polymer films from unfavorable substrates, paying particular attention to the quality of the solvent used to dissolve the polymer. By tuning the solvent quality, we can affect the macromolecular configurations and the extent that the polymer chains are out of equilibrium. We observed differences in ageing and relaxational behavior dependent upon solvent quality, thus sample preparation has a crucial influence on the properties of polymers in thin films.