Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A55: Broadband Dielectric Spectroscopy of Polymers and Soft MatterFocus
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Sponsoring Units: DPOLY GSOFT DCOMP Chair: Simone Napolitano, Universite libre de Bruxelles Room: BCEC 254B |
Monday, March 4, 2019 8:00AM - 8:36AM |
A55.00001: Broadband dielectric spectroscopy on miscible polymer blends in the bulk and in nanometer thick films - Comparison of the different confinement situations Invited Speaker: Andreas Schoenhals Broadband dielectric spectroscopy in the frequency range from 10-1 to 109 Hz is employed to revisit the segmental dynamics of the miscible blend system of poly(vinyl methyl ether)/polystyrene (PVME/PS) in dependence on the composition first in the bulk state. Here, the case of high polystyrene concentrations is considered especially. It is important to note that the dipole moment of polystyrene is negligible, thus only the molecular dynamics of PVME segments, as affected by PS, is observed. Three relaxation processes are found, which are due to fluctuations of differently constrained or confined PVME segments. The degree of the confinement of PVME segments due to PS is discussed in dependence on the composition. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A55.00002: Thin supported polymer films: Modified mobility at the interfaces Emmanuel Mapesa, Nobahar Shahidi, Emmanouil Doxastakis, Joshua Sangoro Using a nanostructured electrode arrangement that effectively offers two different interfaces to the thin polymer layers under study, Broadband Dielectric Spectroscopy (BDS) is employed to characterize interfacial dynamics in thin supported poly(vinyl acetate) (PVAc) films. By analyzing the dielectric loss peak due to the dynamic glass transition, it is demonstrated that although the mean relaxation times for thin films remain bulk-like, there are indeed layers at the polymer/substrate and polymer/air interfaces that exhibit modified mobility in dependence on both overall film thickness and temperature. Preliminary molecular dynamics simulations corroborate these findings. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A55.00003: The Cooperative Free Volume Rate Model: Applications to Pressure Dependent Dynamics and Dynamics under Confinement Ronald White, Jane E Lipson In this talk we discuss our recent work in modeling and predicting alpha relaxation times (τ) using the cooperative free volume rate model (CFV), an approach where the system's free volume determines the molecular cooperativity, and thus the activation energy. The term "free volume" carries some historical baggage, having been ill-defined and, at times, over-enthusiastically applied. However, our definition of free volume is grounded in analysis of experimental volumetric information (PVT data) using our locally correlated lattice (LCL) model equation of state. We have shown, for both polymers and small molecules, that the LCL predicted free volume is very effective in representing a system's volume-based contribution to dynamics. Another essential feature (ignored in historical models) is that the CFV model incorporates an independent temperature-based contribution. We have shown this contribution acts in a product form together with an inverse Vfree dependence, a result that follows mechanistically from the rate model. This presentation will summarize some of our applications of the CFV τ(T,Vfree) form to experimental systems, with a particular focus on pressure dependent dynamics and dynamics under confinement. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A55.00004: Dielectric Phenomena in Polymers and Multilayered Dielectric Films Lei Zhu High dielectric constant and low dielectric loss are desirable electrical properties for next-generation polymer dielectrics that show promise for applications in pulsed power, power electronics, and printable electronics. Unfortunately, the dielectric constant of polymers is often limited to 2-5, much lower than that of inorganic dielectrics, because of the nature of hydrocarbon covalent bonds for electronic and atomic polarizations. It is essential to understand the fundamental physics of different types of polarization and the associated loss mechanisms in polymers. In this presentation, we discuss the characteristics of each polarization and explain how to enhance the polarization using rational molecular designs without causing significant dielectric losses. Among various approaches for high dielectric constant and low loss polymers, the multilayer film technology is of particular interest because a multilayer film is a unique one-dimensional system with tailored material choices, layer thicknesses, and interfaces. By minimizing the disadvantageous polarizations and enhancing the advantageous polarizations, multilayer films hold promise as advanced dielectrics for future polymer film capacitors. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A55.00005: Deciphering Relaxation Spectra of Amorphous Polymers through Dielectric Spectroscopy of an Epoxy Daniel Wilcox, Grigori Medvedev, Yelin Ni, Akash Patil, Brett Savoie, Bryan Boudouris, James M Caruthers In contrast to the crystalline solid and gaseous phases, the mechanism behind the behavior of liquids and amorphous solids remains a significant challenge, where current descriptions of relaxation behavior in these materials are highly empirical with tenuous physical significance. Recently, we have found that the dynamic mechanical relaxation behavior of an epoxy resin is better described using a small, finite number of discrete relaxation processes instead of the continuous spectrum of relaxation times that are typically employed. In this work, the broadband dielectric spectroscopy of the same epoxy material is investigated, where isotherms were measured from liquid nitrogen temperatures to well above the glass transition temperature. By comparing the loss spectra between the two techniques, we observe that the same discrete set of relaxation times unifies the description of both mechanical and dielectric data. Each of the discrete relaxation processes exhibits its own temperature dependence that is the same for mechanical and dielectric experiments, although the strength of the individual spectral components is different. The implications of the existence of discrete relaxation spectrum for developing theory of amorphous materials will be discussed. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A55.00006: Rigid Amorphous Phase in Polymer Nanocomposites as Revealed by Dielectric Relaxation Spectroscopy and Fast Scanning Calorimetry Paulina Szymoniak, Andreas Schoenhals For inorganic/polymer nanocomposites a so-called Rigid Amorphous Phase (RAF) is formed in the interfacial region by adsorption of polymer segments onto the nanoparticles. The segmental dynamics of RAF is expected to be altered, as compared to the pure matrix, which might percolate into the entire system, affecting the overall nanocomposite properties. A combination of two relaxation spectroscopy techniques (Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS)), as well as Fast Scanning Calorimetry (FSC) was employed to investigate the structure and molecular mobility of nanocomposites based on Epoxy and Layered Doubled Hydroxides. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A55.00007: Network dynamics in hydrogen-bonding telechelic polymers: associate lifetime, structural relaxation and phase separation Martin Tress, Kunyue Xing, Peng-Fei Cao, Shiwang Cheng, Tomonori Saito, Vladimir N Novikov, Alexei P Sokolov Supra-molecular networks formed by reversible bonds between polymer chains exhibit extraordinary properties, e.g. extreme toughness and elongation at break, self-healing. We study short telechelic polymers with different H-bonding end-groups and backbone flexibility [1]. The H-bonds increase the glass transition temperature (Tg), though in flexible polydimethyl siloxanes (PDMS) it does not vary with H-bond strength whereas in much stiffer telechelic polypropylene glycol (PPG) Tg varies significantly. In contrast, viscosity strongly depends on the H-bond strength in PDMS while it remains similar in PPG. Complementary measurements of shear modulus and dielectric relaxation indicate that this can be explained by competing lifetimes of the supra-molecular associations and the chain relaxation. Only if the former exceeds the latter, viscosity increases. Furthermore, the viscoelastic properties can be enhanced tremendously by phase separating ends, controlled primarily by Tg of the aggregations. Our analysis reveals that the concept of bond lifetime renormalization [2] describes the results qualitatively but fails on a quantitative level. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A55.00008: WITHDRAWN ABSTRACT
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Monday, March 4, 2019 10:00AM - 10:12AM |
A55.00009: Hard versus soft confinement effects on molecular dynamics of 4-hexyl-4′-isothiocyanatobiphenyl liquid crystals Malgorzata Jasiurkowska-Delaporte, Tomasz Rozwadowski, Anna Baranowska-Korczyc, Ewa Ewa Juszynska-Galazka, Maria Massalska-Arodz The purpose of this work is to exam the dynamic properties of 4-hexyl-4′-isothiocyanatobiphenyl (6BT) experiencing different forms of confinement. Hard confinement was achieved by the infiltration of LCs into nanoporous aluminium oxide (AAO) templates with non-intersecting, cylindrical, channels. Soft confinement derived from the interactions between polymer and guest liquid crystalline molecules was investigated on an example of electrospun polymer/liquid crystal composite fibres. We prepared composite fibres for three different mass ratios of polycaprolactone (PCL) and 6BT. The formation of liquid crystalline domains in fibres was confirmed by polarizing microscope observations and their distribution was illustrated by Raman. The intra-molecular interactions in composite fibres were revealed by analysis of the temperature dependencies of specific infrared (IR) absorption bands. By a combination of broadband dielectric spectroscopy with the other spectroscopic methods, the microscopic picture of role of soft and hard confinement on molecular dynamics and a crystallization process in liquid crystals is obtained. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A55.00010: On the Fragility of Hydroxypropyl Methylcellulose as Measured via Broadband Dielectric Spectroscopy William Woodward, Michael Lesniak, Tirtha Chatterjee, Kevin O'Donnell, Robert Sammler, Travis McIntire, Yongfu Li, Mark Rickard, Dave Meunier A special class of hydroxypropyl methylcellulose polymer (HPMC) has been developed for the production of amorphous solid dispersions of active pharmaceutical ingredients by hot-melt extrusion. The new HPMC materials are novel in that they have low glass transition temperatures (Tg ≈ 100 °C) that enable them to be melt processed without plasticizers. This has produced the opportunity to acquire frequency-dependent Tg measurements of HPMC via broadband dielectric spectroscopy (BDS). Our study revealed that the fragility (m) of this polymer is very low (m = 16.9 ± 0.4). Although the measurements herein are limited to this type of HPMC, it is hypothesized that this low fragility is not unique to this particular grade of HPMC. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A55.00011: Role of α and β relaxations in Collapsing Dynamics of a Polymer Chain in Supercooled Glass-forming Liquid Mrinmoy Mukherjee, Jagannath Mondal, Smarajit Karmakar Understanding the effect of glassy dynamics on the stability of bio-macromolecules and investigating the underlying relaxation processes governing degradation processes of these macromolecules are of immense importance in the context of bio-preservation. In this work we have studied the stability of a model polymer chain in a supercooled glass-forming liquid at different amount of supercooling in order to understand how the dynamics of supercooled liquids influence the collapse behavior of the polymer. Our systematic computer simulation studies find that apart from long time relaxation processes (α relaxation), short time dynamics of the supercooled liquid, known as β relaxation plays very important role in controlling the stability of the model polymer. These observations are in stark contrast with the common belief that only long time relaxation processes are the sole players (vitrification hypothesis). We also show that anti-plasticizing effect found in this context can be rationalized using the β-relaxation process. We believe that our results will lead to understand the primary factors in protein stabilization in the context of bio-preservation. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A55.00012: Insights into the role of dynamic heterogeneity in reorientational and translational dynamic measurements from simulations in the isoconfigurational ensemble Daniel Mauricio Diaz Vela, David Simmons A major question in the study of the glass transition over the last 50 years has been the origin of stretched exponential relaxation and its connection to the observations of non-Gaussian dynamics and decoupling phenomena such as Stokes-Einstein breakdown. The proposition that these phenomena all emerge from averaging over spatial dynamic heterogeneity has emerged as a major view of the common origin of these phenomena. However, significant questions remain, such as the frequent observation that stretching exponents obtained from dielectric spectroscopy commonly do not exhibit the pronounced temperature dependence expected to accompany growing dynamic heterogeneity on cooling. Here we employ molecular dynamic simulations of glass-forming dimers and bead-spring polymers in the isoconfigurational ensemble to quantify the extent to which each of these phenomena emerge from spatial averaging vs locally anomalous dynamics. Results point to important differences between the role of dynamic heterogeneity in reorientational and translational dynamic quantities such as those measured by dielectric spectroscopy and neutron scattering respectively. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A55.00013: Configurational Enthalpy Model for Describing Temperature and Pressure Dependence of the Relaxation Time of Amorphous Polymers above Tg Grigori Medvedev, James M Caruthers Recently we reported [1] that the super-Arrhenian temperature dependence of the relaxation time for 21 molecular glass formers is quantitatively described by a single-parameter model based on the excess enthalpy, where the excess entropy based model of the Adam-Gibbs form fails to describe the data. For these molecular glass formers the liquid/crystalline enthalpy/entropy were obtained by straightforward integration of the heat capacity data, where excess enthalpy/entropy was obtained by subtracting the crystalline value from the liquid value and the independently measured heat of fusion fixes the value of integration constant. For polymeric glass formers the crystalline values are unavailable; consequently, the glassy heat capacity was used in place of the crystalline heat capacity to obtain the configurational (vs. excess) enthalpy/entropy, and the glassy PVT surface is used to determine these quantities at elevated pressures. We show that the resulting configurational enthalpy model describes the mobility data for 12 amorphous polymers, including both the temperature and pressure dependence of the relaxation time above Tg. The configurational entropy model in the Adam-Gibbs form fails to describe the mobility data. |
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