Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T43: Focus Session: Dynamics of Glassy Polymers Under Confinement II |
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Sponsoring Units: DPOLY Chair: Rodney Priestley, Princeton University Room: 214C |
Thursday, March 5, 2015 11:15AM - 11:51AM |
T43.00001: Cooperative Motion as a Unifying Principle to Understand Confinement Effects on Glass Formation Invited Speaker: Francis Starr We examine how confinement scale and interfacial interactions affect polymer glass formation, studied via molecular dynamics simulations. We consider both thin supported polymer films and polymer-nanoparticle composites. By varying the film thickness, nanoparticle loading fraction, or polymer-interfacial interactions, we can significantly alter both $T_g$ and the fragility of glass formation, leading to a seemingly intractable degree of complexity. However, we find that all our observations can be described in unified way by using the scale of collective motion as a measure of ``cooperatively rearranging regions'' in the Adam-Gibbs (AG) description of glass formation. For thin films, we show how the scale of cooperative motion relates to the scale of enhanced interfacial dynamics at the free surface, offering a promising route to experimentally determine the scale of cooperative motion. These string-like motions can further be described as a living polymerization. Combining polymerization theory with the AG approach, we theoretically predict the relaxation time at much lower $T$, which suggests a return to Arrhenius behavior that avoids a Kauzmann ``entropy crisis.'' Finally, we consider the applications of these ideas to ultra-stable polymer films formed by vapor deposition. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T43.00002: Effects of Substrate Interaction on Slow Dynamics and Vitrification in Confined Thin Films Stephen Mirigian, Kenneth Schweizer The nanoscopic confinement of a glass-forming liquid can have major effects on its slow dynamics which depend on the nature of the confining surface in a nonuniversal manner. We generalize our force-level theory for free-standing films [1] to incorporate the effects of solid surfaces in supported and capped films. A solid surface is treated as locally modifying the liquid density as a consequence of packing-induced layering and/or physical adsorption. For films supported on a neutral surface where the former densification mechanism is dominant, the theory predicts that the physical behavior is akin to a free-standing film but with one interface behaving in a nearly unperturbed bulk manner, in agreement with experiment. With increasing interfacial attraction, the relaxation near the solid surface slows down dramatically and a very large local mobility gradient emerges. The competition between the dynamical effects of the adsorbed layer and the mobile layer near the vapor interface results in a rich behavior of an apparent vitrification temperature. Representative calculations of the full spatial gradient of the relaxation time as a function of temperature, film thickness, and interfacial densification will be presented.\\[4pt] [1] J.Chem.Phys.-Comm., 141, 161103 (2014). [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T43.00003: Slow Relaxation, Vitrification, and Mobility Gradients in Free Standing Thin Films Kenneth Schweizer, Stephen Mirigian Glass forming molecular and polymeric liquids confined by free surfaces experience major changes of their slow dynamics beginning at relatively large film thickness. We have constructed a predictive, quantitative, force-level theory of relaxation in free-standing films that addresses the nature of the spatial mobility gradient [1]. The theory predicts a generic speed up of relaxation near a free surface due to two coupled effects: a local, direct surface reduction of caging forces near the vapor interface, and a weakening of the spatially long ranged component of the activation barrier associated with collective elasticity. Effective vitrification temperatures, dynamic length scales and mobile layer thicknesses naturally follow. At low temperatures in the vicinity of the thickness-dependent Tg, highly mobile and immobile regions are predicted to coexist near the surface and in the film interior, respectively. The latter can result in mechanical stiffening which can commence at a large film thickness. Our results provide a theoretical basis for reconciling a variety of experimental results (e.g., probe mobility, dielectric relaxation, particle embedding, ellipsometry, creep) within a single framework.\\[4pt] [1] J.Chem.Phys.-Comm., 141, 161103 (2014). [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T43.00004: Fragility Nanoconfinement Effect in Thin Polymer Films: Novel Characterization by Ellipsometry Tian Lan, John Torkelson A novel ellipsometry-based method was introduced to determine kinetic fragility in polymer films and to investigate the effect of nanoscale confinement on polymer fragility. Three systems were studied: polystyrene (PS), polycarbonate (PC), and PS doped with small molecule diluents of 1,10-bis-(1-pyrene)decane (BPD). In bulk-like films, fragility index measured by ellipsometry agreed very well with that by differential scanning calorimetry. With confinement, a dramatic decrease in fragility was observed in highly fragile PS and PC. The fragility decreased by 58{\%} from 166 to 69 in PS and by 65{\%} from 214 to 75 in PC as film thickness decreased from bulk to 27-28 nm; a substantially muted response was observed in the strongest of the three: PS $+$ 2 wt{\%} BPD, where the fragility decreased only 21{\%} from 134 to 106 from a bulk film to a 27-nm-thick film. The larger fragility-confinement effect in more fragile polymers strongly correlates with a previous discovery of the $T_{\mathrm{g}}$-confinement effect: the strength of the $T_{\mathrm{g}}$-confinement effect increases with increasing fragility of bulk polymers. It indicates that bulk fragility is associated with the susceptibility of polymers to effects of nanoscale confinement. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T43.00005: Vitrification of thin polymer films: from linear chain to soft-colloid like behavior Emmanouil Glynos, Bradley Frieberg, Georgios Sakellariou, Alexandros Chremos, Peter Green The glass transition temperature $T_{\mathrm{g}}$ of sufficiently thin, supported, polymer films is dependent on the film thickness. Based on the nature of the polymer substrate interactions $T_{g}$ may increase, $\Delta T_{\mathrm{g}}$ \textgreater 0, or decrease, $\Delta T_{\mathrm{g}}$ \textless 0, in relation to the bulk. We show that for star-shaped macromolecules the value of $\Delta T_{\mathrm{g}}$ depends on the functionality $f$ of the molecule, for polymer films supported by the same substrate. Specifically in the case of polystyrene (PS) macromolecules, with arms of molecular weight M$_{\mathrm{arm}}$\textless 10 kg./mol., supported by silicon oxide substrates, $\Delta T_{\mathrm{g}} $\textless 0, when f\textless 4. For much higher functionalities, f $\ge $ 32, where the polymer exhibits soft-colloid like behavior $\Delta T_{\mathrm{g}}$ $\sim$ 0. For values of 4\textless f\textless 32, $\Delta T_{\mathrm{g}} $\textgreater 0. The transition from the linear-chain to the soft-colloid behavior is gradual and occurs with increasing $f$ and/or decreasing M$_{\mathrm{arm}}$. With the help of molecular dynamics simulations we rationalize this behavior in terms of competing entropic effects, associated with changes in $f$ and $M_{\mathrm{arm}}$, which drives the ability of these molecules to efficiently pack at interfaces. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T43.00006: Dynamics of Hyperbranched Polymers under Confinement Krystallenia Androulaki, Kiriaki Chrissopoulou, Spiros H. Anastasiadis, Daniele Prevosto, Massimiliano Labardi The effect of severe confinement on the dynamics of three different generations of hyperbranched polyesters (Boltorns) is investigated by Dielectric Spectroscopy. The polymers are intercalated within the galleries of natural Na$^{+}$-MMT, thus, forming ~1nm polymer films confined between solid walls. The $T_{g}$'s of the polymers determined by DSC show a clear dependence on the generation whereas the transition is completely suppressed when all the polymer chains are intercalated. The dynamic investigation of the bulk polymers reveals two sub-$T_{g}$ processes, with similar behavior for the three polymers with the segmental relaxation observed above the $T_{g}$ of each. For the nanocomposites, where all polymers are severely confined, the dynamics show significant differences compared to that of the bulk polymers. The sub-$T_{g}$ processes are similar for the three generations but significantly faster and with weaker temperature dependence than those in the bulk. The segmental process appears at temperatures below the bulk polymer $T_{g}$, it exhibits an Arrhenius temperature dependence and shows differences for the three generations. A slow process that appears at higher temperatures is due to interfacial polarization. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T43.00007: Enhanced Tg-Confinement Effect in Crosslinked Polystyrene Characterized by Ellipsometry Kailong Jin, John Torkelson The effects of nanoscale confinement on the glass transition temperature, Tg, and related behavior are studied in crosslinked polystyrene (PS). Crosslinked PS films are achieved by thermally annealing the spin-cast linear precursor (polystyrene-vinylbenzocyclobutene) films with varying thicknesses at 250 $^{\mathrm{o}}$C. Tg reductions are observed with ellipsometry measurements of both supported linear and crosslinked PS films, with confinement effects being enhanced in crosslinked polystyrene compared to the linear precursor. The greater magnitude of Tg reduction observed in confined crosslinked PS films can be rationalized by the increased bulk fragility induced by crosslinking. Effects of confinement on fragility and physical aging in the crosslinked PS will also be discussed. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T43.00008: Reaction Rate Acceleration and Tg Depression of Polycyanurate Under Nanopore Confinement Evelyn Lopez, Sindee L. Simon Material properties such as Tg and the reaction kinetics are known to deviate from the bulk when subjected to nano-sized confinement. Previous work from our laboratory on the trimerization of cyanate esters found that the reaction kinetics were faster for a monofunctional reactant compared to a difunctional monomer, whereas the Tg depression was greater for the crosslinked product of the latter compared to the low molecular weight trimer of the former. The origin of the changes in nanoconfined reaction rates differs from those that govern changes in the Tg. The research objective is to further explore the effect that confinement has on reaction kinetics and Tg using a mixture consisting of mono- and di- cyanate ester monomers. The product is an uncrosslinked polycyanurate with Mn $=$ 5240 g/mol and PDI $=$ 1.78. The confinement mediums are controlled pore glasses with diameters ranging from 8.1 to 111.1 nm. The nanopore-confined material was synthesized in-situ and the reaction kinetics are followed by DSC; after the reaction, the Tg values of the nanoconfined polymer where also measured by DSC. An acceleration factor of 13 and a Tg depression of 38 $^{\circ}$C are observed for the material confined in the smallest 8.1 nm-diameter pores. The Tg depression is between those of the trimer and network previously studied, while the acceleration of the reaction rate is lower. Our results are consistent with the reaction acceleration arising from packing effects at the pore wall and the Tg depression arising from intrinsic size effects. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T43.00009: The Effect of Nanoconfinement on Free Radical Equilibrium Polymerization Haoyu Zhao, Sindee Simon Free radical polymerization under nanoconfinement results in changes in reaction kinetics, reaction thermodynamics, and polymer properties. In this work, hydrophilic and hydrophobic nanoporous media (d \textless 13 nm) are employed as the nanoconfined matrix to perform polymerization of acrylate monomer. Differential scanning calorimetry (DSC) is used to study the reaction kinetics and thermodynamics, whereas gel permeation chromatography (GPC) is used to measure the molecular weight of the polymer produced. Although the polymerization is thermodynamically feasible at low temperature, as reaction temperatures increase, the depropagation rates in acrylate polymerization become appreciable resulting in equilibrium polymerization at high temperature. For polymer synthesized in nanoconfined environment, the change in entropy upon propagation becomes a larger negative number resulting in a decrease in equilibrium conversion and a shift of the ceiling temperature to lower temperatures. The results are analyzed in the context of the scaling of the change in confinement entropy of chains on chain length. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T43.00010: Effect of film processes on the chain conformations of adsorbed polymer nanolayers Mani Sen, Maya K. Endoh, Tadanori Koga, Daisuke Kawaguchi, Keiji Tanaka Polymer chains adsorb even onto weakly attractive solid surfaces, resulting in the formation of adsorbed polymer nanolayers (``PNs''). We report how film processing affects the chain conformations composed of PNs. 50 nm thick polystyrene (PS, Mw$=$290 kDa) thin films were prepared onto hydrogen-passivated silicon substrates by using two different processes (i.e., spin coating and dip coating). The PNs were then formed by high temperature thermal annealing and subsequent rinsing with a good solvent. We characterized the PNs using x-ray reflectivity (XR), atomic force microscopy (AFM), and sum-frequency generation spectroscopy. The XR and AFM data reveal that the homogenous PNs are composed of the two different architectures regardless of the film processing: flattened polymer chains that constitute the inner higher density region of PNs and loosely adsorbed polymer chains that form the outer bulk-like density region although adsorbed chain conformations from the two processes are different. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T43.00011: Investigation of the Temperature-Dependent Specific Volume of Supported Polystyrene Films Upon Confinement Xinru Huang, Connie Roth The experimentally observed large changes in the glass transition temperature Tg of ultrathin supported and free-standing polymer films with decreasing thickness h have puzzled the field for more than two decades. An open question at present is what material property changes correspond to the large shifts in film dynamics upon confinement. Thermodynamic theories have predicted that the observed Tg(h) decrease in ultrathin polymer films may be tied to small shifts in the specific volume of the liquid-line above Tg. Here we use ellipsometry to investigate the temperature-dependent specific volume for supported polystyrene (PS) films of different thicknesses. Using the Lorentz-Lorenz parameter as a measure of the relative change in film density, we calculate the specific volume from temperature-dependent measurements of the index of refraction. While the slope of the liquid-line (thermal expansion coefficient) remains constant upon confinement, the Tg(h) decrease is accompanied with a broadening of the transition and a small increase in the glassy-line thermal expansion, consistent with a larger fraction of the sample remaining liquid to lower temperatures. We find that both the liquid and glass specific volume lines shift together with decreasing thickness indicative of small 0.5-1{\%} changes in overall film density with decreasing thickness. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T43.00012: Quantitative Relations Between Cooperative Motion and Emergent Elasticity in Model Glass-Forming Polymer Materials Beatriz A Pazmino Betancourt, Paul Hanakata, Francis W Starr, Jack F. Douglas There are many semi-empirical models that allow us to understand the dynamics of glass-forming liquids. Some of them emphasize the importance of a progressively growing cooperative motion which grows while the configuration entropy of the liquid drops. Others from a solid-like nature of glass perspective look at the emergent elasticity. However, there has been limited success in finding a unify framework of understanding such different perspectives. In this work, we find quantitative relations between emergent elasticity in terms of the average local volume accessible for particle motion, and the growth of the collective motion in super cooled liquids. Surprisingly, we find that each of these models of glass-formation can equally well describe the relaxation data for a large range of fragility variations of glass-forming liquids, such as polymer nanocomposites, and thin films, as well as in a BLJ liquid that we have simulated. [Preview Abstract] |
(Author Not Attending)
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T43.00013: Molecular-weight Dependent $T_{\mathrm{g}}$ Depression of Silica-supported Poly($\alpha $-methyl styrene) Films Kun Geng, Fei Chen, Ophelia K. C. Tsui The glass transition temperature ($T_{\mathrm{g}})$ of poly($\alpha $-methyl styrene) (P$\alpha $MS) films supported by silica is studied as a function of film thicknesses from 17 to 168 nm at three molecular weights of 1.3, 20 and 420 kg/mol. For the 20 and 420 kg/mol films, the glass transition temperature decreases with decreasing film thickness, consistent with previous results. But for the 1.3 kg/mol films, it becomes independent of the film thickness. We tentatively suggest the $T_{\mathrm{g}}$ depression to be caused by free volume excess at the polymer-air interface and that its influence diminishes at low enough molecular weights because of a chain stiffness effect. [Preview Abstract] |
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