Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E9: Glass Formation and Dynamics in Nanostructured Polymers and Glasses IFocus
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Sponsoring Units: DPOLY GSOFT GSNP Chair: Connie Roth, Emory University Room: 268 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E9.00001: EMPTY SLOT |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E9.00002: EMPTY SLOT |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E9.00003: EMPTY SLOT |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E9.00004: Long-range correlated dynamics in amorphous selenium Tianyi Liu, Richard Stephens, Zahra Fakhraai Recent works in supported polymer and organic molecular films have shown that the thin and bulk films exhibit very different average relaxation time due to the influence of the free surface in enhancing the dynamics in layers close to the free surface. When increasing the film thickness to up to 100nm, there exists a transition from liquid-like to glassy behavior. The mid point of the transition for both polymer and organic molecular glasses are found to be \textasciitilde 30nm. We use cooling rate-dependent Tg measurements to demonstrate in this work that an inorganic network glass, amorphous selenium, exhibits the similar enhanced dynamics in thin films. This observation suggests a long-range correlated dynamics in amorphous selenium. The result can help elucidate the origin of enhanced dynamics in thin glasses, and also broaden the existing literatures on correlated dynamics in amorphous thin films. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E9.00005: The Nanoconfined Free Radical Polymerization: Reaction Kinetics and Thermodynamics Haoyu Zhao, Sindee Simon The reaction kinetics and thermodynamics of nanoconfined free radical polymerizations are investigated for methyl methacrylate (MMA) and ethyl methacrylate (EMA) monomers using differential scanning calorimetry. Controlled pore glass is used as the confinement medium with pore diameters as small as 7.5 nm; the influence of both hydrophobic (silanized such that trimethylsilyl groups cover the surface) and hydrophilic (native silanol) surfaces is investigated. Propagation rates increase when monomers are reacted in the hydrophilic pores presumably due to the specific interactions between the carbonyl and silanol groups; however, the more flexible EMA monomer shows weaker effects. On the other hand, initial rates of polymerization in hydrophobic pores are unchanged from the bulk. In both pores, the onset of autoacceleration occurs earlier due to the reduced diffusivity of confined chains, which may be compensated at high temperatures. In addition to changes in kinetics, the reaction thermodynamics can be affected under nanoconfinement. Specifically, the ceiling temperature (T$_{\mathrm{c}})$ is shifted to lower temperatures in nanopores, with pore surface chemistry showing no significant effects; the equilibrium conversion is also reduced at high temperatures below T$_{\mathrm{c}}$. These observations are attributed to a larger negative change in entropy on propagation for the confined system, with the MMA system again showing greater effects. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E9.00006: Ultras-stable Physical Vapor Deposited Amorphous Teflon Films with Extreme Fictive Temperature Reduction Gregory McKenna, Heedong Yoon, Yung Koh, Sindee Simon In the present work, we have produced highly stable amorphous fluoropolymer (Teflon AF$^{\mathrm{\mbox{\textregistered }}}$ 1600) films to study the calorimetric and relaxation behavior in the deep in the glassy regime. Physical vapor deposition (PVD) was used to produce 110 to 700 nm PVD films with substrate temperature ranging from 0.70 $T_{g}$ to 0.90 $T_{g}$. Fictive temperature ($T_{f})$ was measured using Flash DSC with 600 K/s heating and cooling rates. Consistent with prior observations for small molecular weight glasses [J. Chem. Phys. 2007, 127, 154702], large enthalpy overshoots were observed in the stable amorphous Teflon films. The $T_{f}$ reduction for the stable Teflon films deposited in the vicinity of 0.85 $T_{g}$ was approximately 70 K compared to the $T_{g} $of the rejuvenated system. The relaxation behavior of stable Teflon films was measured using the TTU bubble inflation technique and following Struik's protocol in the temperature range from $T_{f}$ to $T_{g}$. The results show that the relaxation time decreases with increasing aging time implying that devitrification is occurring in this regime. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:48AM |
E9.00007: Measuring glassy correlation lengths in ultra-thin polymer films Invited Speaker: Zahra Fakhraai We have recently demonstrated that ultra-thin films of organic glass-formers have significantly enhanced dynamics compared to bulk dynamics at temperatures well below the glass transition temperature, Tg. We showed that a sharp glass to liquid transition occurs when the thickness of the layer is decreased below 30 nm. The sharp decrease in fragility in these systems resembles those measured polymeric systems. Here, I discuss how these results can help resolve a long-standing ambiguity in the origin of enhanced dynamics and Tg reduction in thin polymer films. In particular, we show how one can separate ``glassy effects'' from those induced by the ``polymer chain''. We show that the free surface and substrate effects present as perturbations in the dynamics that can propagate up to 30 nm into the bulk of the film. These data can explain the extent of Tg reduction and dramatic change in polymer fragility in ultra-thin films in various polymer systems, including homopolymers and miscible blends. For example, we show that in the presence of strong substrate interactions, such as poly (2-vinylpyridine) on silicon, strongly correlated dynamics can result in the emergent of two distinct Tgs, one increasing and the other one increasing upon decreasing film thickness. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E9.00008: Disconnecting structure and dynamics in glassy thin films Daniel Sussman, Samuel Schoenholz, Ekin Cubuk, Andrea Liu The dynamics near the surface of a glassy film are markedly different from those in the bulk. We investigate whether the differences between surface and bulk dynamics can be explained by differences in local microscopic structure. We show that machine learning methods that successfully identify strong correlations between local structure and particle rearrangement dynamics in the bulk completely fail to detect key aspects of thin-film glassy dynamics. Furthermore, we show that no combination of local structural features drawn from a very general set of two- and multi-point functions is able to distinguish between particles at the center of film and those in near the surface where the dynamics are strongly perturbed. We note that the behavior of glassy films has often been interpreted via a model with a glassy, immobile layer near the center of the film and liquid-like mobile layers near free interfaces. Our results suggest that the two populations are indistinguishable in structure, necessitating a purely dynamical theory for additional relaxation processes in the film. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E9.00009: Glass formation in a continuum of nanostructured polymers from surfactant-laden nanolayers to block copolymers Weston Merling, David Simmons Evidence suggests that dynamics and glass formation behavior of polymers and other glass forming liquids can be altered in the nanoscale vicinity of an interface. While much of the early literature in this field focused on polymer thin films, recent efforts have elucidated similar effects in polymers with internal nanostructure. However, the precise physical nature of these alterations and their dependence on details of the polymer and interface remain poorly understood. One open question is to what extent trans-interface covalent bonds, such as those in block copolymers, alter interface effects on dynamics. To answer this question, here we report molecular dynamics simulations of a continuum of nanostructured polymers ranging from pure nanolayers to surfactant-modified nanolayers to block copolymers. Results suggest that the chi parameter of the polymers constituting the two domains can play a larger role in determining glass formation behavior than does the density of interfacial covalent grafting. These findings suggest an equivalence between glass formation behavior in block copolymers and nanolayered polymers, while pointing to the potential to decouple interfacial energy from near-interface dynamics via surfactants. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E9.00010: The Influence of Additives on Polymer Matrix Mobility and Tg Jane Lipson, Jeffrey DeFelice In this talk, we model the effects of additives on a polymer matrix and demonstrate how they can shift the physical properties of the matrix relative to those of its pure bulk state. Experimental measurements indicate that different additive species, such as: small organic molecules, gases, ionomers, and nanoparticles, can have a range of effects on the segmental relaxation time and glass transition temperature, Tg, of the polymer matrix. For example, additives can cause Tg suppression (plasticization) or Tg enhancement (antiplasticization) relative to the pure bulk Tg. We have applied a simple kinetic lattice model, the Limited Mobility (LM) model, to probe the connection between Tg and the influence of additives on the local mobility of a polymer matrix. Using our LM model characterizations of additives and their effects on the matrix, we analyze trends in our results and make comparisons with experimental data for a variety of real polymer/additive mixtures. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E9.00011: Effective Viscosity of Ultraviolet Ozone Modified Polymer Nanometer Films\textbf{.} Xuanji Yu, Fei Chen, Ophelia KC Tsui Reductions in the glass transition temperature and effective viscosity have been reported for polystyrene (PS) films supported by silica upon thickness reduction. The most common attribute is enhanced mobility at the free surface. Here, we chemically modified the topmost region of the films by ultraviolet ozone (UVO) and measured their effective viscosity. We found that even with a brief UVO exposure of 1 second under typical exposure conditions, the reduction in effective viscosity was eliminated and for film thickness below about 15 nm effective viscosity exhibited a rapid increase with thickness reduction. A layer model, consisting of a viscous oxygenated surface layer topped with some median mobile species, atop a bulk-like layer, was able to fit the data well. Our result demonstrated the potential of UVO treatment to adjust the dynamic properties of polymer thin films. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E9.00012: Polymer Diffusion in Confined Thin Films via Fluorescence Recovery after Photobleaching Laura A. G. Gray, Clifford P. Brangwynne, Rodney D. Priestley Over the past twenty years many studies have shown a reduction in the glass transition temperature (Tg) of thin polymer films confined to the nanoscale when supported on non-attractive substrates or in the free-standing film geometry. The depth dependence of Tg has been measured using thin layers of fluorescently tagged polymer to localize the dye within a larger polymer film stack, revealing a decrease in local Tg tens of nanometers into the film. These results have been explained by the propagation of enhanced mobility from the free-surface into the polymer film. Fewer direct measurements of molecular mobility have been made in confined polymer systems. Here, we present the results of fluorescence recovery after photobleaching (FRAP) experiments investigating the mobility of fluorescently labeled methacrylate-based polymers confined in thin film geometries. By quantifying the percentage of mobile fluorophores, FRAP allows us to probe entanglement density as a function of confinement. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E9.00013: Unusual Stiffening and Elastic Response of Polyisobutylene Nanometric Thin Films Heedong Yoon, Caleb Wigham, Gregory McKenna The TTU bubble inflation technique was used to study the elastic response and unusual stiffening behavior of nanometirc polyisobutylene (PIB) films. Mechanical properties and surface tension of PIB films were measured through the strain-stress response for film thicknesses ranging from 13 nm to 126 nm. The tests were performed at room temperature, far above the glass transition temperature of PIB. It is found that the stiffening increases with decreasing film thickness, while the surface tension is independent of the film thickness. Similar to the prior bubble inflation measurements in polymeric thin films, the thickness dependence of the stiffening followed a power law behavior in this case of D$_{\mathrm{s}}\propto $ h$^{\mathrm{1.5}}$. These results are consistent with the Ngai et al [J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 214] proposition that rubbery stiffening is related to the separation of the $\alpha $ relaxation and Rouse modes. In addition, we compare stiffening index (S) with fragility ($m)$ based on our prior observation that the S follows a linear behavior with dynamic $m$. Unlike other polymeric materials seen in prior bubble inflation measurements [Macromolecule 2015, 48, 6329], the S of PIB does not follow the linear behavior with $m$. [Preview Abstract] |
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