Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W22: Focus Session: Dynamics of Polymers Under Nanoscale Confinement III |
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Sponsoring Units: DPOLY Chair: Robert Riggleman, University of Pennsylvania Room: 407 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W22.00001: Physical Aging within Hairy NanoParticle Assemblies H. Koerner, M. Bockstaller, A. Dang, C. Mahoney, K. Matyjaszewski, C.-M. Hui, R. Vaia Polymer grafted nanoparticles provide solutions to overcome dispersion challenges in conventional polymer-inorganic nanocomposites (NCs). While most research has focused on blends of these hairy nanoparticles (HNPs) into polymer matrices, recent work has demonstrated substantial promise for solvent- or matrix-free assemblies of HNPs (aHNPs). Significant progress has been made in understanding the relationship between the structure of the polymer corona at intermediate and high graft densities and the morphology, mechanical properties and melts dynamics of the assembly. However, very little is known about the behavior of aHNPs with low graft densities ($\sigma $\textless 0.05 nm$^{-2})$ of high molecular weight chains that are above entanglement (\textgreater 60kDa). Such aHNPs contain more than 30 vol{\%} inorganic, with maximum separation between particle surfaces less than 10 nanometers. For such materials, we discuss the physical aging characteristics from enthalpy relaxation experiments of these highly confined poly(styrene) and poly(methylmethacrylate) grafts. Physical aging is substantially suppressed in the low $\sigma $ ($\sigma $\textless 0.05) regime, as compared to conventional NCs at similar nanoparticle loadings. Furthermore, relaxation rate, distribution and fragility indicate that aHNPs with high $\sigma $ exhibit behavior deep within the glass similar to conventional NCs and their neat polymers, however deviate substantially from Arrhenius behavior as Tg-T approaches 0. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W22.00002: Glassy structural relaxation of star-shaped polymers Bradley Frieberg, Emmanouil Glynos, Georgios Sakellariou, Peter Green Time-dependent changes of thermodynamic properties due to structural relaxations, physical aging, occur in all glasses. In the case of linear polymers, the aging rate is independent of the degree of polymerization at a given aging temperature, relative to the average glass transition temperature. In contrast, we demonstrate that star-shaped macromolecules exhibit average structural relaxations that are dependent on both the number of arms, f, and the degree of polymerization of each arm, Narm. In particular, while increasing f, and/or decreasing Narm, the average segmental relaxation rate decreases, and can be up to a factor of two times lower for star-shaped molecules compared to their linear analogs. We reconcile these differences in terms of the free volume diffusion and its relation to the segmental motions in the glass state. We propose that this ideal class of polymeric materials, star-shaped molecules, can be used in order to tailor the physical properties on a molecular level, by simply changing the polymer architecture. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W22.00003: Glass transition of star-shaped thin polymer films Emmanouil Glynos, Bradley Frieberg, Georgios Sakellariou, Peter Green We show that the glass transition temperatures, $T_{\mathrm{g}}$, of thin polystyrene (PS) films, supported by oxidized silicon substrates (SiOx), depends on functionality, $f$, and the degree of polymerization of the arm, N$_{\mathrm{arm}}$, of the macromolecule. The $T_{\mathrm{g}}$s of star-shaped PS films with thickness H$=$30nm, $T_{\mathrm{g}}$(30nm) with $f$ as high as 64 were investigated. The $T_{\mathrm{g}}$(30nm) of linear PS thin films, is less than the average bulk $T_{\mathrm{g}}$, $T_{\mathrm{g}}$(bulk). For molecules of N$_{\mathrm{arm}}$ about100, the $T_{\mathrm{g}}$(30nm) of a macromolecule with $f=$3 was equal to that of a linear PS. However, the $T_{\mathrm{g}}$(30nm) increased with increasing $f$, reaching a maximum of at $f=$8 where $T_{\mathrm{g}}$(30nm) was higher than $T_{\mathrm{g}}$(bulk). For larger values of $f$, T$_{\mathrm{g}}$ (30nm) decreased monotonically with increasing $f$ and for $f=$64the$ T_{\mathrm{g}}$(30nm) became comparable to $T_{\mathrm{g}}$(bulk). The magnitude of this effect is weaker for much larger values of N$_{\mathrm{arm}}$. We rationalized these observations in term of two competing entropic: the increasing entropic attraction of the macromolecules to interfaces, with increasing $f$, and an increasing intermolecular entropic repulsion of these macromolecules with increasing f, and/or decreasing N$_{\mathrm{arm}}$. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W22.00004: Dynamics of Hyperbranched Polymers in the Bulk and under Confinement: Effect of Dendritic Generation Kiriaki Chrissopoulou, Krystalenia Androulaki, Spiros H. Anastasiadis, Daniele Prevosto, Massimiliano Labardi The structure and dynamics of three generations of a hyperbranched polyester polyol (Boltorn) and their nanocomposites with natural montmorillonite (Na$^{+}$-MMT) are investigated to offer a detailed picture of the behavior in bulk and under confinement. The structure was studied with X-ray diffraction (XRD) and differential scanning calorimetry (DSC), while the dynamics using dielectric spectroscopy (DS). XRD reveals that the polymer chains reside within the galleries of the Na$^{+}$-MMT producing an intercalated nanocomposite. The glass transition temperature, Tg, of the bulk polymers shows a dependence on the generation whereas the transition is completely suppressed when all chains are intercalated. The dynamics of the polymers and nanocomposites with $\sim$50wt\% polymer, where all chains are confined, were investigated for temperatures both below and above the polymer Tg. A sub-Tg process was found, showing similar features for the three polymers whereas the segmental relaxation was observed around Tg. For the nanocomposites, the dynamics that are observed show similarities and differences with the respective of the pure polymers depending on the specific process. Partially sponsored by EU (COST Action MP0902) and by the Greek GSRT (Research Funding Program: THALES (MIS 377278)) [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W22.00005: Confinement Effects on Molten Thin Cyclic Polystyrene Films Qiming He, Suresh Narayanan, David Wu, Mark Foster The surface fluctuations of melt film of 6k cyclic polystyrene (CPS) and its linear analog were measured using X-ray photon correlation spectroscopy (XPCS) for films of various thicknesses. The surface fluctuations of the 6k linear PS melt films 17 nm and thicker and the 6k cyclic melt films 28 nm and thicker can be described using a hydrodynamic continuum theory (HCT) that assumes the film is characterized only by the bulk viscosity. When a film of CPS is 24 nm or thinner, the behavior can no longer be captured using the HCT with bulk viscosity. The surface fluctuations behave as though the film has an effective viscosity higher than the bulk value. There is no evidence of an effective modulus in the very thin films of cyclic chains. The thickness at which confinement effects are seen for the 6k CPS chains is larger than that for the linear analogs. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W22.00006: Chain configurations, glass transition and polymer dynamics in polymer nanoparticles under 3D-confinement Aurora Nogales, Daniel E. Martinez-Tong, Michelina Soccio, Alejandro Sanz, Tiberio A. Ezquerra Polymer nanospheres with different size distributions of several polymer systems are prepared by a variety of methods, including miniemulsion and flash precipitation among others. The physical properties of the obtained nanoparticles have been studied. The calorimetric trace of these spheres shows an increase of the glass transition temperature that has been evaluated by means of an entropy model. This 3D-confinement, imposed by the nanospheres, leads to a limiting number of repeating polymer units in the sphere and thus to a reduction of the possible configuration states of the polymer chains, which is ultimately related to variations in the bulk value of the glass transition temperature. Our model is evaluated against our calorimetric measurements as well as with the data available in the literature. Good agreement between data and model is found for many cases, proving that confinement is related to reductions in entropy for these systems. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W22.00007: Structural Relaxations in Bulk and Thin Film Polymers: Role of Macromolecular Architecture Invited Speaker: Peter Green Structural relaxations that occur in polymers quenched below the glass transition temperature, T$_{\mathrm{g}}$, are responsible for time-dependent changes in physical properties that include optical, specific volume and the enthalpy. This phenomenon, physical aging, has been of interest in bulk polymers for decades and much is understood. The aging rate, $R$, is known to increase as the temperature at which the glass ages, $T',$ decreases below $T_{g}$, due to the increasing departure from structural equilibrium; it then decreases with further decreases in T. We show that the aging rates of star-shaped polymers are slower than their linear analogs of the same degree of polymerization, N. Whereas the temperature dependence of $R$ is independent of N for linear chains, it depends on the functionality, $f,$ and on the degree of polymerization per arm, N', of the star-shaped macromolecules. $R$ decreases with increasing $f, $for N' less than a threshold degree of polymerization, which increases with increasing $f$. The aging of very thin films (h $\sim$ 100 nm) is dependent on the distance, z, from an external interface. With the use positron annihilation spectroscopy (PALS) we determined the z-dependence of the T$_{\mathrm{g}}$ in thin films and showed that the aging rate is largely determined by the difference between the local glass transition temperature of the films and T'. Finally we show that the aging rates of linear and star-shaped macromolecules are consistent with experimental findings of glassy dynamics in both systems. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W22.00008: Effect of Confinement on Glass Transition Behavior in Polymeric Nanotubes and Nanorods: Comparison of DSC and Fluorescence Measurements Anthony Tan, John Torkelson The effects of nanoscale confinement on the glass transition temperature, Tg, and related behavior are studied in polystyrene nanotubes and nanorods made using anodized aluminum oxide templates. Tube thickness as small as 19 nm has been achieved by melt infiltration methods. Substantial Tg reductions are observed with both DSC and fluorescence measurements of nanotubes supported by the templates, with confinement effects being comparable in magnitude to those obtained via ellipsometry and fluorescence for supported polymer films. Free-standing nanotubes can also be characterized by DSC, yielding much larger Tg reductions than observed in supported nanotubes. Effects of confinement on fragility and physical aging in the supported polystyrene nanotubes and nanorods will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W22.00009: Glass Transitions in Polymer Nanocomposites Dong Meng, Sanat Kumar For polymers are under geometric confinement, it is generally believed that the glass transition temperature (T$_{\mathrm{g}})$ increases with favorable interfacial interactions. Experiments [1] and simulations [2] have reported that T$_{\mathrm{g}}$ increases almost proportionally to the attractive polymer-surface interactions. However, recent studies [3,4] have reported the contradictory finding that the T$_{\mathrm{g}}$ shift is rather modest and insensitive to the strength of interfacial attractions. In this study, we investigate the glass transition in polymer nanocomposites using molecular dynamics simulations. With attractive polymer-nanoparticle (NP) interactions, we find that T$_{\mathrm{g}}$ is increased by $\sim$ 3{\%} at moderate loadings and that the shift stays almost unchanged when the polymer-NP attractions are further increased by one order of magnitude. Both are in agreement with the recent experiments at comparable NP loadings [4]. We show that this is because the strongly adsorbed polymer segments do not participate in the glass transition. In other words, strong polymer-NP attractions create immobile polymer ``coatings'' around NPs that shield them from direct contact with the mobile polymers. \\[4pt] [1] Tate, R. S.; de Pablo, J. J.; Nealey, P. F. Journal of Chem. Phys. 2001, 115 (21), 9982-9990. \\[0pt] [2] Torres, J. A.; Nealey, P. F.; de Pablo, J. J. Phys. Rev. Lett. 2000, 85 (15), 3221$-$3224. \\[0pt] [3] Lu, H. Y.; Chen, W.; Russell, T. P. Macromolecules 2009, 42 (22), 9111$-$9117. \\[0pt] [4] Moll, J.; Kumar, S.K. Macromolecules 2012, 45 (32), 1131$-$1135. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W22.00010: Different Effects of Confinement on the Glass Transition Behavior of Supported Polymer Films and Model Polymer Nanocomposites Made with Carbon Based vs. Silica Based Substrates Lawrence Chen, John Torkelson While the effect of confinement on the glass transition temperature, Tg, of polymeric materials has been studied for two decades, only limited work has focused in a systematic way on the effect of different substrates, in the case of polymer films, or different nanofillers, in the case of nanocomposites. We employ both silica based and carbon based substrates on which films have been spin coated to study how the potential for pi-pi bonding interactions between polymer and substrate can modify the Tg-confinement effect in thin polystyrene (PS) films. Characterization is done in single-layer supported films by fluorescence spectroscopy and ellipsometry and in multilayer films by fluorescence. Model nanocomposite studies are also done by layering films supported on substrates, yielding a film sandwiched between substrates. Major differences in the Tg-confinement effect are observed in sufficiently thin PS films, with silica supported films possessing a free surface exhibiting major Tg reductions while the carbon supported analogs exhibit little Tg reduction. Bilayer film studies demonstrate an enhancement in Tg in a sufficiently thin PS layer in contact with a carbon substrate, which is not observed with a silica substrate. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W22.00011: Creating monodisperse polyacrylamide free-radically via thermal frontal polymerization in confined geometries Preeta Datta, Kirill Efimenko, Jan Genzer Bulk free radical polymerization reactions lead to highly polydisperse polymers (polydispersity index, PDI $\gg$ 1.5). In the past, researchers have shown that polymerization in porous microreactors can lower polydispersity (PDI $\sim$1.5-1.7) by promoting gelation. We employ free-radical thermal frontal polymerization reaction of acrylamide (AAm) in DMSO in highly confined reactors (height \textless 1mm) to produce high molecular weight ($\sim$300 kDa) PAAm of relatively low PDI ($\sim$1.2). In frontal polymerization systems, a localized reaction zone propagates in space along the direction of heat transfer, sustained by the interplay of heat diffusion and Arrhenius reaction kinetics. The directional heat transfer assists in maintaining the uniformity of the front temperature. While convection improves thermal transport, it causes inhomogeneity in the propagating front in horizontal reactors. In highly confined systems, convection is heavily suppressed, as manifested by the ``flattening'' of the reaction front and the absence of ``fingering''. Gelation lowers termination rate and increases the life time of the active reaction centers. Elimination of convection in confined geometries coupled with directional heat transfer and gelation results in polymers with high molecular weights and low PDIs. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W22.00012: The Effect of Nanoconfinemnt on Methyl Methacrylate Polymerization: Reactivity and Resulting Properties Haoyu Zhao, Ziniu Yu, Ronald Hedden, Sindee Simon The effect of nanoconfinement is well known to affect the properties of polymers. In this work, free radical polymerization of methyl methacrylate (MMA) is performed in hydrophilic or hydrophobic 13 nm diameter controlled pore glass (CPG). Changes in polymerization kinetics and the properties of the synthesized polymer are quantified. Reaction kinetics and glass transition temperatures are followed by differential scanning calorimetry (DSC). After polymerization, the changes in the molecular weights and tacticity are measured using gel permeation chromatography (GPC) and 1H nuclear magnetic resonance (1H NMR). Nanoconfinement is found to result in earlier onset of autoacceleration leading to the increase in both number-average and weight-average molecular weights, whereas the polydispersity index at full conversion decreases relative to the bulk value. Moreover, for both pore surfaces, the glass transition temperature increases compared with the bulk, but the increase in hydrophilic pores is more pronounced at 20 $^{\circ}$C. In addition to the changes in molecular weight and Tg, the tacticity changes from syndiotactic-rich triads for the bulk PMMA to a higher percentage of isotacticity under nanoconfinement. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W22.00013: Finite size effects on irreversible chain adsorption: a new probe of dynamics under nanoscale confinement Simone Napolitano, Caroline Housmans, Michele Sferrazza We investigated the role of finite size effects on the dynamics of thin polymer films, analyzing the thickness dependence of the kinetics of irreversible chain adsorption of polystyrene onto silicon oxide. We identified two growth regimes - linear at short times and logarithmic at long times - separated by a molecular weight independent crossover time, and by a crossover thickness scaling as predicted by the reflected random walk. Film thickness did not affect the dynamics at short time scales, while in the logarithmic growth regime we observed slower adsorption for melts confined in slabs thinner than 4-6 R$_{g}$'s. Given the correlation between the dynamics of the whole film and the structure of the adsorbed layer [1, 2], our findings suggest that the time necessary to equilibrate a polymer melt increases upon confinement. [1] Napolitano et al. Nature Comm., 2, 260 (2011) [2] Napolitano et al. EPJE 36, 61 (2013) [Preview Abstract] |
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