Session A34: Focus Session: Dynamics of Glassy Polymers Under Nanoscale Confinment: Glass Transition

Calorimetry of Polymer Nanoparticles

Significant understanding regarding the dynamics of glassy polymers geometrically confined to the nanoscale has been obtained by investigating thin films. While thin films are an attractive model system to investigate the influence of confinement on material properties, measurements on other geometries is important from both scientific and technological viewpoints. Investigating glassy dynamics of polymer nanoparticles is useful for exploring the influence of geometry on the behavior of confined polymer, and thus, to gain insight into the generality of size-effects on material properties irrespective of the confining shape. Here, we use calorimetry to measure the glassy dynamics (e.g., glass transition temperature, fragility and structural relaxation) of polymers confined to the nanosphere geometry. We illustrate how nanoscale confinement can significantly alter the glassy dynamics of polymer nanoparticles. Our results suggest that interfaces are a key factor in modifying the glassy dynamics of confined polymer, irrespective of geometry.   

Viscoelasticity of Ultra Thin Films Probed via Temperature-Controlled Quartz Crystal Microbalance With Dissipation

Temperature-controlled quartz crystal microbalance with dissipation (QCM-D) is a powerful technique for probing glass transitions in ultra thin films via changes in viscoelasticity. QCM-D has the added benefit of monitoring such changes as a function of overtone, which allows for one to probe transitions at different locations vertically throughout the film. Here, we present a general approach towards discerning glass transitions in layer-by-layer (LbL) assemblies, which are formed via the alternate adsorption of oppositely charged polyelectrolytes. LbL assemblies consisting of strong polyelectrolytes or of weak polyelectrolytes are presented. A glass transition was only observed in the presence of water, which extensively plasticized the film and facilitates the breaking and reformation of ion pairs. Early results of glass transitions in homopolymers such as polystyrene are also presented. A strong dependence of glass transition temperature on overtone number was observed, suggesting a range of changes in viscoelasticity with respect to temperature and distance from the substrate.   

Making the Tg-Confinement Effect Disappear in Thin Polystyrene Films: Good Physics vs. Inappropriate Analysis

The Tg-confinement effect in polymers was first characterized in supported polystyrene (PS) films by Keddie et al. in 1994. Since then, many researchers have shown that (pseudo-)thermodynamic Tg measurements of supported PS films taken on cooling consistently yield the same qualitative results, with a decrease from bulk Tg beginning at 40-60 nm thickness and becoming very strong below 20 nm thickness. Some quantitative differences have been noted between studies, which may be ascribed to measurement method or the analysis employed. In 2004, we showed that the Tg-confinement effect in PS may be suppressed by adding several wt{\%} of small-molecule diluents such as dioctyl phthalate. Recently, Kremer and co-workers (Macromolecules 2010, 43, 9937) reported that there was no Tg-confinement in supported PS films based on an analysis of the second derivative of ellipsometry data and use of a ninth order polynomial fit. Here, we demonstrate a new method for suppressing the Tg-confinement effect. In particular, PS made by emulsion polymerization yields no Tg-confinement effect as measured by ellipsometry or fluorescence, while PS made by anionic or conventional free radical polymerization yield strong Tg-confinement effects. The difference is hypothesized to result from surfactant in the emulsion polymerized PS. We also show that the absence of the Tg-confinement effect reported by Kremer is due to inappropriate analysis of ellipsometry data and that correct analysis yields Tg-confinement effects.   

Reduced Calorimetric Tg in Confined Thin Polymer Films with Controlled Interface

When most prior studies on thin polymer film have shown that glass transition temperature (Tg) decreases under nano-confinement, differential alternating current (AC) chip calorimetric method shows little dependence of Tg on thickness for supported film. To reveal this contradiction, we have manipulated a free-interface by spin-coating polystyrene with an immiscible surfactant [tetraoctylammonium bromide (TOAB)], which had a melting point lower than Tg of polystyrene. When the sample was heated during AC chip measurement, TOAB molecules assembled on the interface and became a mobile layer. As a result, Tg was reduced for ultra thin polymer film. Moreover, stacked free-standing polymer films also show Tg dependence on thickness. The releasing of interface stresses caused by spin-coating is the major reason for reduction of calorimetric Tg. These data unambiguously show that thickness dependence of Tg is an intrinsic property of thin polymer film confined by geometry and dimensions.   

The Calorimetric Glass Transition of Polystyrene Ultrathin Films

The glass transition temperature (Tg) for nanoconfined materials have been widely studied since the early 1990s. For supported polystyrene ultrathin films, Tg differs from bulk value. Recent work has attributed nanoconstrained Tg effects to artifact. In this study, we attempted to resolve this controversy and measure Tg for single polystyrene ultrathin films using Flash DSC. Films have been prepared in two ways: spincast films placed on a layer of inert oil or grease and films directly spincast on the back of the calorimetric chip. For the films on oil or on grease, the 160 nm thick films show no Tg depression. On the other hand, thinner films on oil and on grease show a Tg depression which decreases with increasing cooling rate. The depression reverts to the bulk values over the course of a day at 160 $^{\circ}$C due to dewetting and thickening. For directly spincast films, no Tg depression is observed, consistent with results from other nanocalorimetry work. Our results are consistent with literature results that Tg decreases with decreasing substrate surface energy, and they also demonstrate that the Tg depression observed is not due to degradation or to plasticization effects.   

Local Variation of Fragility and Glass Transition Temperature of Ultra-thin Supported Polymer Films

Extensive studies have shown that the properties of ultra-thin polymer films can differ significantly from the bulk. The effect of the film thickness $h$ on the glass transition temperature has been widely examined, but this does not account for the fragility of glass-formation, which quantifies how rapidly relaxation time varies with temperature $T$. Accordingly, we simulate polymer films of a bead-spring model on a smooth or rough surface and determine both $T_g$ and fragility, both as function of $h$ and film depth. We find that the commonly invoked free-volume layer model does not describe our results. In addition, as opposed to the bulk, we find that $T_g$ and fragility do not generally vary proportionally. Therefore, determination of fragility is essential for the characterization of dynamic changes in film. Finally, we relate these changes of fragility to changes in the cooperative monomer dynamics.   

Fragility of an Isochorically Confined Polymer Glass

When polymers are confined to the nanometer length-scale, the glass transition temperature ($T_{g})$ and its associated dynamics can deviate substantially from the bulk. As confined polymers continue to play an important role as enabling materials in technologies ranging from drug delivery to plastic electronics, a greater understanding of size effects on glass transition dynamics is warranted. Here, we present the effect of isochoric confinement on the dynamic fragility of a polymeric glass-former, $i.e.$, polystyrene (PS). Utilizing silica-capped PS nanospheres as a model system, the fictive temperature ($T_{f})$ and the isochoric heat capacity ($C_{v})$ are measured as a function of diameter \textit{via} differential scanning calorimetry (DSC). By examining $T_{f}$ as a function of cooling rate for each sample, the isochoric fragility ($m_{v})$ is obtained, which decreases significantly as the diameter of the nanospheres is reduced from 462 nm to 129 nm. Hence, the contribution of thermal effects on structural relaxation is reduced with isochoric confinement for PS geometrically restricted by silica. Furthermore, we explore the impact of chemical structure, \textit{via} PS derivatives, on the observed confinement effect on the dynamic fragility.   

Effects of aging on glass-forming polymers

Despite nearly twenty years of active research, the effects of nanoscale confinement on the properties of glass forming polymers remain poorly understood. Furthermore, molecular simulations have so far only played a limited role in our understanding of these confinement effects, which are important for applications in both membrane separation and semiconductor manufacturing. We have used molecular dynamics simulations to investigate the effects of aging on bulk and free-standing thin-film glass-forming polymers. We demonstrate that in the vicinity of the bulk glass transition temperature, there are regimes where the free surface is in equilibrium while the center of the film exhibits bulk-like aging. We compare our results with those published from recent experiments, and we provide a microscopic picture on the differences in physical aging in bulk and free-standing polymer films.   

Physical Aging of Thin Polystyrene Films Quenched and Measured Free-Standing

High molecular weight (MW) free-standing polymer films exhibit unusual and yet unexplained nanoconfinement effects. We have recently demonstrated that such ultrathin, high MW free-standing polystyrene (PS) films show two reduced glass transition temperatures (Tgs) which can be separated by more than 60 K, indicating that two separate mechanisms act simultaneously to propagate enhanced mobility into the film from the free surface. These studies indicate that the majority of the film transitions to a glass at the upper Tg leaving only a small fraction of the material mobile to much lower temperatures. In an effort to gain insight into the properties of these films between the two reduced Tgs, we aim to measure the physical aging characteristics at temperatures both above and below the lower transition temperature. To this end, we have developed a method using ellipsometry to measure the physical aging rate of thin free-standing PS films that remain free-standing after being thermally quenched. Measurements on thicker free-standing films, greater than 500 nm, supported by rigid, circular sample holders show no thickness dependence to the aging rate, consistent with the thickness independent stress applied to these films by the thermal expansion mismatch between film and holder. Measurements on thinner films will also be presented.   

Glass transition temperatures in nanoscale equilibrated polystyrene droplets

Measurements of thin film glass transition temperature (Tg) in thin polymer films are only made possible through the metastability of the film with respect to dewetting. Even in the melt state, such samples are not in thermal equilibrium, and resulting Tg values may not be conclusive. In this talk we discuss recent measurements of Tg for equilibrium polystyrene droplets on silicon substrates as measured through their thermal expansion with true non-contact atomic force microscopy. These measurements show promise to not only definitively address the continuing controversy surrounding thin film Tg measurements, but are also readily applied to study non-polymeric glass formers.   

Confinement effects on the glass transition of nanolayered polymers

Despite numerous studies of glass transition confinement effects in liquids confined in freestanding films, on rigid substrates, and in pores, many outstanding questions remain regarding the origin, nature, and magnitude of these effects. In recent years, studies have demonstrated that these effects are also present in materials under soft confinement, including in internally nanostructured polymers such as nanolayered polymers and block copolymers. This latter class of materials offers a new platform for exploration of confinement effects in the absence of issues surrounding substrate selection and preparation. In this talk, we describe the results of coarse-grained molecular dynamics simulations probing the glass-formation behavior of nanolayered polymers, with a focus on the role of `cooperatively rearranging regions' in nanoconfinement effects in these systems. Furthermore, we discuss the role of miscibility and bulk T$_{g}$ of the nanolayered polymers in determining the magnitude and direction of changes to T$_{g}$ and mobility of polymers under this form of `soft' nanoconfinement.   

Dynamic Cluster Size Effects on the Glass Transition of Thin Films

During cooling from the melt of amorphous materials, it has been shown experimentally that dynamic rigid clusters form in equilibrium with the liquid and their relaxation behavior determines the kinetic nature of T$_{\mathrm{g}}$ [Stanzione et al, J. Non Cryst Solids 357(2): 311-319 2011]. The fractal clusters of size R $\sim$ 5-60 nm (polystyrene) have relaxation times $\tau $ $\sim$ R$^{1.8}$ (solid-to-liquid). They are analogous to sub critical size embryos during crystallization as the amorphous material tries to crystallize due to the strong intermolecular forces at T \textless\ T$_{\mathrm{m}}$ ; they are not related to density fluctuations or surface capillary waves. In free-standing thin films of thickness h, several important events occur: (a) The large clusters with R \textgreater\ h are excluded and the thin films have an average faster relaxation time compared to the bulk; consequently T$_{\mathrm{g}}$ decreases as h decreases. (b) The segmental dynamics at the 1 nm scale are largely not affected by nanoconfinement since T$_{\mathrm{g}}$ is determined only by the cluster dynamics with R $\gg$ 1 nm. (c) The mobile layer on the surface of free standing films is due to the presence of smaller clusters on the surface which will disappear with increasing rate of testing. (d) With adhesion to a solid substrate, the surface mobile layer disappears as the surface clusters size grow and the change in T$_{\mathrm{g}}$ is suppressed. (e) Physical aging is controlled by the relaxation of the rigid fractal clusters and in thin films, physical aging will occur more rapidly compared to the bulk. (f) The large effect of molecular weight M on T$_{\mathrm{g}}$ appears to be related to the effect on the cluster size distribution giving smaller clusters and faster relation times with increasing M. These results are in accord with the Twinkling Fractal theory of the glass transition.   

How does Tg reduction affect the chain mobility in confined PS films?

It is well established that the glass transition temperature (Tg) of supported polystyrene (PS) thin films decrease with decreasing film thickness. This Tg reduction due to the free surface effect is associated with enhanced mobility. However, the correlation between the enhanced mobility and Tg reduction has not been studied yet. To understand the effect of Tg reduction on the vertical mobility of PS chains across the interfaces we have investigated the interdiffusion between PS and deuterated PS (dPS) films in bilayer and trilayer geometries using neutron reflectivity (NR). Bilayer films of 42 nm thick dPS bottom layer and 20 nm thick PS top layer are created in such a way to mimic the films where large Tg reductions has been demonstrated by recent fluorescence measurements. Trilayer films were created using the same bottom layer but floating a 10 nm thick PS middle layer and 10 nm thick dPS top layer to compare the mobilities at the interfaces between the top/middle and middle/bottom layers. NR results showed that there is almost no mixing between the layers up to 90-95 C for both bilayer and trilayer films which is not consistent with large Tg reductions observed in the literature. Our results also indicate no difference in the mobility of PS chains at the top/middle and middle/bottom interfaces in the trilayer film which argues against the enhanced mobility reported in the literature for the top 10 nm of PS thin films. Diffusion of PS chains across the interface gets faster as the MW decreases.