Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T22: Focus Session: Dynamics of Glassy Polymers under Nanoscale Confinement II |
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Sponsoring Units: DPOLY Chair: Daniel Sussman, University of Pennsylvania Room: 407 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T22.00001: Extended Tg Gradient Profile Across a Glassy-Rubbery Polymer-Polymer Interface with an 80 K Tg Difference Roman Baglay, Connie Roth For decades Tg in confined systems has been studied with the hopes of uncovering the length scales that impact the glass transition. However, understanding length scales of Tg gradients near a free surface have been hampered by limitations of how to treat the enhanced mobility at the free surface theoretically. Here, we use a glassy-rubbery polymer-polymer interface to establish an 80 K Tg gradient from one well-defined Tg value to another. Multilayer films of high molecular weight polystyrene (PS) and poly(n-butyl methacrylate), a weakly immiscible system with a 7 nm interfacial width, are constructed. Ultrathin (10-15 nm) pyrene-labeled layers are inserted into the multilayer structure at different positions (z) from the glassy-rubbery interface. Temperature-dependent fluorescence intensity is collected to determine the local Tg(z) at a given position z from the interface. Using a series of different samples, we are able to map the Tg(z) profile across this glassy-rubbery interface. Our work reveals an asymmetric local mobility gradient propagating hundreds of nanometers away from the interface into the glassy PS side before bulk PS Tg is recovered. These results demonstrate that cooperative segmental Tg dynamics can be coupled across long length scales spanning multiple cooperatively rearranging regions (CRRs). [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T22.00002: Soft spots in amorphous thin films: a structural signature of free surfaces Daniel Sussman, Andrea Liu, Sidney Nagel While it is known that the dynamics in thin films strongly depend strongly on the distance from a free surface, standard measures of the static structure in these systems (e.g., the density, the radial distribution function, or the distribution of under-coordinated particles) typically find at most a monolayer of particles at the surface that differ from those in the bulk. We investigate energy-minimized, thin-film configurations of Lennard-Jones particles and find that the presence of a free surface leads to low-energy vibrational surface modes with properties very different from those in the bulk. By analyzing the structure of these modes, we find that the density of ``soft spots,'' local regions of high mode amplitude, is higher near the surface. These soft spots have well-defined length scales characterizing both how far they penetrate into the bulk and how extended along the surface each one is. Furthermore, these soft spots have a high correlation to particle rearrangements or enhanced mobility. We discuss the implications of surface soft spots for existing results on glassy thin films. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T22.00003: Free surface facilitation of the dynamics of entangled polymer films Ethan Glor, Zahra Fakhraai Recent work in polymer physics shows that the structural relaxation time near a free surface of a thin polystyrene film is significantly different from that of the bulk polymer. This can have a large influence on their properties. For instance, studies have shown that polystyrene thin films exhibit a decreased glass transition temperature as the thickness decreases below 60 nm. A puzzling aspect of this phenomenon is that most studies indicate that there is no molecular weight dependence on T$_{g}$ reduction in supported films, while the same phenomenon in free-standing polystyrene films shows a strong molecular weight dependence. In this study, we use cooling-rate dependent T$_{g}$ measurements to indirectly probe the relaxation dynamics of thin polystyrene films, and show they are directly influenced by the dynamics of the free surface. Furthermore, we show that the relaxation dynamics of supported polystyrene films slow down slightly as the molecular weight of polystyrene is increased. Finally, this study elucidates the importance of the time scale of the measurement on the magnitude of the observed T$_{g}$ reduction, and discusses the nature of the apparent onset of observable interfacial effects. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:27PM |
T22.00004: Molecular simulation of the dynamics in thin polymer films Invited Speaker: Robert Riggleman After more than 15 years of study since the original article by Keddie et al. demonstrating the effects of confinement on the glass transition temperature (Tg) in nanoscopic polymer films supported on a silicon substrate, there is not yet a consensus on the origins of the Tg shift. Understanding and controlling the effects of confinement on glass-forming polymers is essential to further development of photolithography and semiconductor manufacturing, as well as several emerging technologies that will depend on the properties of confined glasses, such as stable glasses, flexible displays, and responsive materials. A growing body of experimental literature exists that suggests that the dynamics near a free surface are not only enhanced but are fundamentally different in their nature compared to a bulk glass-forming material. However, despite the fact that the experimentally-relevant length scales can be easily captured by molecular simulation, there are comparatively few simulation studies examining the dynamics of glass-forming polymers in confined geometries relative to the extensive experimental work. In this talk, I will describe some of our recent efforts to understand how the dynamics of glass-forming polymers change under nanoscale confinement. First, I will describe our results on the changes in the entanglement network of an entangled polymer under both planar and cylindrical confinement, where we find that the density of entanglements is strongly decreased. Second, I will describe our work examining the nature of the dynamics near the free surface at temperatures close to the simulated glass transition and below. Finally, I will describe some of our work trying to understand how the view of confinement effects found in model systems studied with molecular dynamics compares with recent experiments. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T22.00005: A simple view of $T_g$ measurements in thin polymer films James Forrest, Kari Dalnoki-Veress In the past two decades, there have been numerous measurements of the glass transition temperature, $T_g$, in thin polymer films. These results have been the subject of significant controversy. While it does appear that the surface of glassy polymer films exhibits an anomalously high mobility, how this results in measured values of $T_g$ less than that of the bulk is not yet clear. Here we present a simple model that shows how an enhanced surface mobility that penetrates into the material with a characteristic length scale can lead to what appears as a reduced dilatometric $T_g$. We will show that despite the strong similarities to a $T_g$ measurement, the signature observed in experiments does not necessarily correspond to a glass transition in the thin polymer film. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T22.00006: Instrumentation origin of the glass transition temperature depression in thin films measured by ellipsometry Mikhail Efremov, Paul Nealey Ellipsometry is one of the standard methods for observation of glass transition in thin films. However, sensitivity of the method to surface morphology can complicate the manifestation of the transition in a few nm thick samples. In particular, an onset of the free surface roughness in the glass transition temperature range affects the experimental data in a way that leads to biased glass transition temperature assignment. Two possible mechanisms of surface roughening in the vicinity of glass transition are discussed: the roughness due to lateral heterogeneity and roughness associated with thermally activated capillary waves. Effective medium approximation models are used to introduce the surface roughness into optical calculations. The results of optical modeling for a 5 nm thick polystyrene film on silicon are presented. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T22.00007: Dependence of Tg on interfacial energy and ``hardness'' of confinement in multi-nanolayered polymers David Simmons, Ryan Lang, Weston Merling Numerous studies have demonstrated that polymers and other glass-forming materials confined to dimensions under 100 nm can exhibit large deviations from bulk glass formation, mechanical, and transport behavior. The magnitude and direction of these alterations appears to depend on both the interfacial energy and the ``softness'' of confinement, among other variables, with implications both for the practical design of nanoscale materials and for the mechanistic understanding of nanoconfinement effects. Here we describe molecular dynamics simulations of multinanolayered polymers in which we systematically vary the interfacial energy and the relative glass transition temperatures of the domains. Results suggest a simple functional form that describes the combined dependence of nanoconfined $T_{g}$ on interfacial energy and the relative Debye-Waller factors of the two domains. We suggest that this functional form may describe the $T_{g}$ of nanoconfined materials more broadly, with implications for the design and understanding of nanostructured materials. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T22.00008: Methacrylate-Based Polymer Films Exhibit Different Tg-Confinement Effects at High and Low Molecular Weight Tian Lan, John Torkelson The effects of confinement on the properties of polymer films are important in applications related to photoresists. To optimize resolution, methacrylate-based polymers in photoresists are often low molecular weight (MW). Here, we have used ellipsometry and fluorescence to determine how the glass transition temperature, Tg, is affected by confinement in silica-supported films of low and high MW poly(1-ethylcyclopentyl methacrylate) (PECPMA) and poly(methyl methacrylate) (PMMA). With decreasing nanoscale thickness, Tg is nearly invariant at high PECPMA MW but decreases dramatically at low MW, with Tg- Tg(bulk) $=$ -15 K in a 17-nm-thick film. Fluorescence studies of a single 20-nm-thick dye-labeled layer in multilayer PECPMA films reveal a much greater perturbation to Tg in the free-surface layer for low MW polymer. The effect of MW in PMMA films is even more striking; Tg increases with decreasing thickness for high MW but decreases for low MW. The strong influence of MW on the confinement effect in PECPMA and PMMA is in strong contrast to the previously reported invariance of the effect with MW in supported polystyrene films, reconfirmed in our study. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T22.00009: A Thermodynamic Model for Glass Transition Shifts in Freestanding and Supported Films Chris Price, Ronald White, Jane Lipson The thickness dependence of the glass transition temperature in polymer thin films is investigated via an analytical approach relying only on bulk material data. Previously, this method had been used to successfully model freestanding polystyrene (PS) films. In this discussion, new model results are shown for freestanding poly (methyl methacrylate) (PMMA) that capture the difference in its thickness dependent glass transition shift relative to polystyrene. Furthermore, the simple model is generalized to study supported polymer films. We show that the inclusion of a polymer-substrate interaction can cause a flip in the glass transition shift from depression to enhancement. We estimate the strength of this interaction for the case of PMMA on a silicon oxide substrate using a simple physical model. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T22.00010: Physical Aging of Polymer Glasses Vitrified under Stress Laura A.G. Gray, Connie B. Roth How stress and mechanical deformation impart mobility to polymer glasses has been studied primarily for materials where the glassy state was formed stress free. Here, we investigate the stability of polymer glasses after a constant stress is applied during the formation of the glassy state (thermal quench). We have constructed a unique jig to apply a known stress to free-standing films during the thermal quench. Ellipsometry is used to measure the physical aging rate of polystyrene films transferred onto silicon wafers by quantifying the time-dependent decrease in film thickness that results from an increase in average film density during aging. Stress values above a threshold result in less stable polymer glasses with faster physical aging rates. Initial measurements of the rubbery plateau creep compliance indicate these thin films are stiffer than bulk by two orders of magnitude, consistent with other studies in the literature. However, our results appear to be independent of film thickness over the range studied (150-700 nm). Current efforts are now focused on computer-controlled application of stress and strain during the quench to investigate these unusual material properties in thin films. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T22.00011: Structural Recovery of Single Polystyrene Ultrathin Films Yung P. Koh, Sindee L. Simon Glasses are not at equilibrium and, thus, structure evolves towards equilibrium in a process termed structural recovery. In this work, nanocalorimetry is used to investigate structural recovery for single polystyrene ultrathin films. In addition to being able to study single films with this technique, we can also investigate the response at aging times as short as 0.01 s, as well as aging at temperatures as high as T$_{\mathrm{g}} \quad +$ 15 K for high fictive-temperature glasses obtained at high cooling rates. The results indicate that structural recovery progresses as expected when the aging temperature is low compared to the initial fictive temperature. In this case, the fictive temperature evolves towards the aging temperature at a rate that depends on the aging temperature and initial fictive temperature (i.e., on the cooling rate prior to aging). At equilibrium, the fictive temperature T$_{\mathrm{f}} \quad =$ T$_{\mathrm{a}}$. For cases where the aging temperature is higher than the fictive temperature, the results of the calorimetric experiment can be explained by the relaxation that occurs both during isothermal aging and cooling. The influence of film thickness on the structural recovery response will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T22.00012: Understanding the Physical Aging Behavior of Glassy Polystyrene Layers in Close Contact with Rubbery Domains Connie Roth, Phil Rauscher, Justin Pye, Roman Baglay Recent advances in synthesis strategies and processing methods have led to new nanostructured polymer blend and block-copolymer materials containing domain sizes less than 100 nm with glassy and rubbery domains in close proximity. Given the outsized role interfacial perturbations have played in causing large changes in the glass transition temperature Tg and physical aging of ultrathin single-layer films, we are interested in studying how the presence of glassy-rubbery interfaces between neighboring polymer domains may alter the local stability and physical aging of confined glassy layers. Using a polystyrene (PS) / poly(n-butyl methacrylate) (PnBMA) weakly immiscible system with 7 nm interfacial width, we demonstrate how ellipsometry can be used to isolate the physical aging rate of thin PS layers atop rubbery PnBMA layers. Despite a 25-30 K reduction in the average Tg of 84 nm thick PS layers atop PnBMA as measured by fluorescence, we observe no change in the PS aging rate relative to bulk. These results are in contrast with previous works on single-layer polymer films that have found the local aging rate to often be correlated with local Tg changes. This appears not to be the case for glassy PS layers atop rubbery PnBMA suggesting some additional factor is affecting the structural relaxation occurring near the glassy-rubbery interface. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T22.00013: Limitations in interpretation of Quartz Crystal Microbalance (QCM) beyond the rigid (Sauerbrey) to viscoelastic (lossy) transition Clinton Wiener, Robert Weiss, Christopher White, Bryan Vogt Since Sauerbrey's 1959 discovery of the mass-frequency relationship in quartz, the QCM has been utilized to probe deposited mass layers. The mass to frequency (imaginary component of the impedance) relationship breaks down when the added mass is not rigidly coupled to the sensor surface and viscous dissipation of the quartz occurs. This dissipation is important in the deposition of soft materials such as polymers or biological molecules. By using a viscoelastic model for frequency and dissipation; the mass, viscosity, and shear modulus can be accurately determined. Here, we demonstrate an additional breakdown in the coupling of the imaginary component of the impedance to the mass by simultaneous QCM-D and spectroscopic ellipsometry (SE) measurements by examination of the swelling behavior of thin physically crosslinked poly-n-isopropylacrylamide films. A film swollen beyond 3 times its dry thickness shows a frequency increase (mass loss) and dissipation increases (increasing lossy film character) on cooling, but SE results show increased swelling of the film. This behavior was found to be thickness invariant for dry thicknesses of 32 nm and greater. Modeling of this QCM-D data shows non-physical results. Scaling concepts associated with this high loss limit will be discussed. [Preview Abstract] |
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