Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B33: Polymer Nanocomposites: InterfacesFocus
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Sponsoring Units: DPOLY DSOFT GSNP Chair: Shiwang Cheng, Michigan State Univ Room: 505 |
Monday, March 2, 2020 11:15AM - 11:51AM |
B33.00001: Designing Polymer Nanocomposites: Critical Role of the Interfacial Layer Invited Speaker: Alexei Sokolov It is now well recognized that interfacial layer controls macroscopic properties in polymer nanocomposites (PNCs). In this talk we overview recent studies on structure and dynamics of the interfacial layer in various PNCs. We employ broad array of experimental techniques and MD-simulations that provide detailed characterization of the interfacial layer. These studies revealed a gradient in the interfacial layer dynamics, but no “glassy” or “dead“ layer. The thickness of the interfacial layer increases upon cooling to Tg, and depends strongly on polymer rigidity, increasing from ~2nm in flexible polymers to ~5 nm in more rigid ones. We discuss a possible connection of the interfacial layer thickness to the dynamic heterogeneity length scale. We emphasize usually overlooked dynamic property of the interfacial layer – strong suppression of the amplitude of structural relaxation on time scale of segmental dynamics. At the end, we present a general picture how microscopic parameters control the interfacial layer, and how by tuning the interfacial layer we can tune macroscopic properties of polymer nanocomposites. |
Monday, March 2, 2020 11:51AM - 12:03PM |
B33.00002: Dynamics in polymer and polymer-grafted nanocomposites: it's the interfacial zone after all Emmanuel Mapesa, Dayton P. Street, S. Michael Kilbey II, Joshua Sangoro Polymers exhibit deviations from their bulk physical properties in the vicinity of solid substrates due to changes in various properties arising in the interfacial regions. Broadband dielectric spectroscopy and differential scanning calorimetry are used to study molecular dynamics in poly(methyl methacrylate)/silica nanoparticle composites. By systematically examining nanocomposites based on non-functionalized Si NPs dispersed in PMMA matrices and on PMMA-grafted Si NPs in PMMA matrices, we investigate the effects of interfacial interactions and confinement in each of these cases on Tg and the time-scales as well as breadth of the corresponding dielectric relaxations. We show that in addition to slower mobility, which is commonly reported in literature and assigned to interfacial relaxations, faster modes also arise due to confinement effects, and that these faster modes are more pronounced in nanocomposites with polymer-grafted nanoparticles. These faster relaxation modes are attributed to the increasing importance of chain wetting and packing in the interfacial zones around nanofillers. |
Monday, March 2, 2020 12:03PM - 12:15PM |
B33.00003: Theory of coupled activated relaxation in dense polymer-particle mixtures: effects of size ratio, particle loading and interfacial attraction Yuxing Zhou, Kenneth Schweizer Dense polymer-particle mixtures represent a wide range of systems including polymer nanocomposites, polymerized ionic liquids, and bio-related materials. Understanding and predicting the dynamical and mechanical properties of such hybrid systems is of practical importance and high theoretical interest. Here we study the dynamics of polymer nanocomposites using the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory with structural correlations obtained from Polymer Reference Interaction Site Model (PRISM) approach. The latter captures the depletion, steric stabilization, and tight bridging states of structural organization. We focus on the effects of particle-segment size ratio, interfacial (cross) attraction strength, and particle loading on the segmental relaxation time, glass or gel like mechanical properties, and kinetic arrest. Cooperative motions of segments and particles is treated in a dynamically self-consistent manner, and compared to the limiting case of pinned particles. We find a rich dynamical behavior with both plasticization and anti-plasticization regimes, and a nanocomposite shear modulus which can be either softened or reinforced. The key physics relates to the interplay between geometric packing and physical bonding effects. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B33.00004: The interfacial zone around nanoparticles in polymer nanocomposites and in thin polymer films Wengang Zhang, Hamed Emamy, Beatriz Pazmino Betancourt, Fernando Vargas-Lara, Francis Starr, Jack Douglas We perform coarse-grained simulations of polymer materials to quantify the range over which interfaces alter the structure and dynamics near the interface. We study the interfacial zone around nanoparticles (NPs) in polymer-NP composites with variable NP diameter, as well as the interfacial zone at the solid substrate and free surface of thin supported polymer films. These interfaces alter both the segmental packing and mobility in an interfacial zone. Variable NP size allows us to gain insight into the effect of boundary curvature, where the film is the limit of zero curvature. We find that the scale for perturbations of the density is relatively small and decreases on cooling for all cases. In other words, the interfaces become more sharply defined on cooling. In contrast, the interfacial mobility scale ξ for both NPs and supported films increases on cooling and is on the order of a few nanometers, regardless of the polymer-interfacial interaction strength. Additionally, the dynamical interfacial scale of the film substrate is consistent with a limiting value for polymer-NP composites as the NP size grows. These findings are based on a simple quantitative model to describe the distance dependence of relaxation that should be applicable to many interfacial polymer materials. |
Monday, March 2, 2020 12:27PM - 1:03PM |
B33.00005: Polymer NanoComposites, Interfaces and Data Invited Speaker: Catherine Brinson For polymer composites, nanocomposites and polymer thin film systems, the local properties of polymers can be altered by the chemical and physical interactions with substrates and embedded particles over a substantial length scale. In order to better understand and design nanocomposites, polymer coatings and electronic components, it is essential to develop better understanding and robust design strategies. Two key missing links are understanding of altered polymer properties near surfaces/particles and ability to quantitatively leverage prior data for these systems, predictively in a robust manner. Therefore there is great interest in utilizing scanning probe methods to quantify the local property changes in the polymer region near surfaces. Additionally, there is great need to harvest, record and be able to learn from the vast amount of data archived in journal articles. In this work, both experimental characterization and development of a data framework and infrastructure is presented. The ability of Atomic Force Microscopy (AFM) to characterize the local mechanical properties (elastic and viscoelastic) of the interphase region in model composites is presented and combined with numerical simulations to refine the analysis. A new platform for data, analysis tools and simulation portals for polymer nanocomposites will be presented: NanoMine. NanoMine utilizes a robust schema to hold the data in a software infrastructure with query, visualization and microstructure analysis tools. Case studies are demonstrated which connect the property-structure-property domains through a combination of machine learning and physics-based modeling, demonstrating the ability to identify the most critical features influence properties. |
Monday, March 2, 2020 1:03PM - 1:15PM |
B33.00006: Chemical Heterogeneities and Architectures of Interfacial Layers in Polymer Nanocomposites Di Wu, Siyang Yang, Pinar Akcora Polymer coated nanoparticles have been widely used to achieve good dispersion in polymer matrices. Dynamics and glass-transition temperature of bound interfacial layers on nanoparticles of different sizes have been recently investigated to understand the effect of interfacial relaxations as they govern reinforcement in composites. We investigate the effect of chemical heterogeneities and varying chain architectures around nanoparticles on tuning the dynamics of interfacial polymers, hence the mechanical properties such as thermal-stiffening. Fe3O4 nanoparticles adsorbed or grafted with PMMA chains and dispersed in PMA matrices are prepared and their rheological behavior is characterized. Our findings show that short adsorbed chains lead to thermal-stiffening, whereas long adsorbed chains yield softening with increasing temperature. Conformations of adsorbed and grafted chains on the same particle sizes will be discussed together to reveal this unusual behavior which relies on the interfacial chemical heterogeneities in polymer nanocomposites. Further, other types of adsorbed chains of PA, P2VP, PMMA, with decreasing rigidity, are compared to understand the role of rigid chains on interfacial dynamics and relaxations. |
Monday, March 2, 2020 1:15PM - 1:27PM |
B33.00007: Interfacial mechanics and viscoelastic properties of patchy graphene oxide reinforced nanocomposites Zhaoxu Meng Graphene oxide (GO) is a promising building block for nanocomposites due to its excellent mechanical properties and tunable interfacial interactions with polymers. While experiments have shown that GO sheets consist of graphitic regions clustering into patches and oxidized regions constituting the remaining areas, the role that these heterogeneous patches play on interfacial and mechanical properties of GO reinforced nanocomposites have not yet been investigated. To address this issue at spatiotemporal scales beyond atomistic simulations, we employ recently developed coarse-grained models of GO sheet and polybutadiene to model patchy GO sheets and a representative GO/polybutadiene nanocomposite with GO sheets serving as fillers. We quantify how interfacial adhesion energy and polymer conformations depend on the size of patches and corroborate these findings with the viscoelastic behaviors of the nanocomposite. We find that heterogeneous patchy structures on GO sheets are responsible for variations in interfacial and viscoelastic properties of GO-based nanocomposites. Our study provides fundamental insights into the interfacial mechanisms of GO-polymer nanocomposites and the influence of heterogeneous functionalized surfaces on the mechanical properties of polymer nanocomposites. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B33.00008: Theory and Simulation of Polymer Brushes - Interaction and Structure Sabin Adhikari, Sanat Kumar Addition of polymer-grafted nanoparticles (NPs) to a polymer melt can result in hybrid materials with improved properties. Understanding of the structure and the interactions of polymer-grafted NPs in a melt is essential to improve their practical applicability. In this study, we focus on the fundamental aspects of the interactions and structure of polymer brushes in either a poor solvent (i.e., incorporating compressibility effects in the brush) or in a (compressible) polymer-matrix. First, we study two planar brushes in a poor solvent and then in a matrix-polymer melt by using theory and simulations. We study the dependence of monomer density distribution, brush height, and the interaction force between two brushes as a function of graft density and polymer chain length. Then we discuss the generalization of our results to a spherical geometry of relevance to the practically important testbed of polymer grafted nanoparticles. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B33.00009: Aggregation of Grafted Nanoparticles in a Polymeric Matrix Clement Koh, Gary Grest, Sanat Kumar A common phenomenon for polymer-grafted nanoparticle (PGNP) systems is that NP aggregation spontaneously occurs at a specific ratio of the chain length Ng of the grafted chains to the matrix chain length N. This aggregation is generally considered to be entropic in origin and can be attributed to the autophobic dewetting of the grafted chains. Here, we incisively probe the microscopic driving force governing PGNP aggregation through a series of large-scale multi-particle simulation studies. The matrix chain length, graft chain length, and volume fraction were varied. Analysis of our simulations indicate that aggregation take place only at unexpectedly low ratios of Ng/N. Further, simulations tuning the interaction mismatch between nanoparticles and polymer chains indicate that some form of enthalpic interplay between the polymer chains and NPs is significant in controlling the PGNP aggregation that is observed experimentally. These results suggest that entropy alone is probably not the only factor driving nanoparticle aggregation in nanocomposites. |
Monday, March 2, 2020 1:51PM - 2:03PM |
B33.00010: Surface Segregation and Wetting of Nanoparticles in Polymer Nanocomposites Shawn Maguire, John Derek Demaree, Connor Bilchak, Nadia Krook, Michael J. Boyle, Andreea-Maria Pana, Patrice Rannou, Manuel Maréchal, Kohji Ohno, Russell Composto The segregation and wetting of polymer grafted nanoparticles (NP) to the free surface of a polymer nanocomposite film is driven by both surface energy and thermodynamic forces. Here, we probe these two contributions in a model system of PMMA grafted silica NPs in a poly(styrene-ran-acrylonitrile) (SAN) matrix using Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM) as a function of thermal annealing temperature and time. Studies are performed above and below the critical point of this LCST system to decouple the thermodynamic and interfacial energy contributions. With increasing time, a monotonic increase in surface excess of PMMA grafted NPs is observed in the miscible regime, which is attributed to the difference in surface energies between the PMMA brush and SAN matrix. Upon annealing above the critical point, a much stronger increase in PMMA NP surface coverage is observed because of the thermodynamic driving force, namely the Flory-Huggins interaction parameter between the PMMA grafts and SAN matrix. Using the measured surface excess values of PMMA NPs at multiple annealing times and temperatures, we then extract the apparent diffusion coefficients of the particles and compare them to the systems homopolymer analogue. |
Monday, March 2, 2020 2:03PM - 2:15PM |
B33.00011: Initial Solvent-Driven Nonequilibrium Effect on the Adsorption Layer of Polymer Nanocomposites Sol Mi Oh, Mozhdeh Abbasi, Tae Joo Shin, Kay Saalwaechter, So Youn Y Kim There have been extensive efforts to characterize interfacial layers in polymer nanocomposites (PNCs), which determines the final structures and properties of PNCs. While tremendous studies have focused on intrinsic parameters of components such as size, shape, chemistry, an understanding for the variation of the structure and dynamics under different processing conditions is relatively lacking. In this work, we report that the initial dispersing solvent, which is not present in final PNCs, induces nonequilibrium effects on polymer chain dynamics at interfaces. By employing 1H NMR free induction decay, we probe that the quantity rather than the mobility of interfacial polymers can be changed depending on the initial solvent, which leads to a difference in thickness of interfacial layers. Accordingly, the particle microstructures and rheological properties are greatly influenced observed by small-angle X-ray scattering and oscillatory rheometry experiments. In addition, we reveal that the outcome of the nonequilibrium effect driven by the initial solvent becomes more prominent at effective range of particle volume fraction related to the polymer chain dimension. |
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