Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S54: Polymer Nanocomposites V: Thermodynamics and DynamicsFocus
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Sponsoring Units: DPOLY Chair: Robert Hickey, Pennsylvania State University Room: BCEC 254A |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S54.00001: Effects of Matrix Chain Length on Miscibility of Nanoparticles Clement Koh, Amish Patel, Sanat Kumar We use molecular dynamics simulations to show, for the first time, that brush collapse is observable at the single particle level, and the small magnitude of the collapse points to the subtlety of the dispersion problem for a single nanoparticle. Brush collapse or collapse of the graft chains on a polymer grafted nanoparticle (PGNP), is observed experimentally by decreasing the graft/matrix chain ratio (N/M) for PGNPs in a homopolymer matrix of free chains. This brush collapse from varying N/M has not been observed in previous simulations of a single PGNP in homopolymer matrix. Here, the free chain length was varied systematically in a series of simulations, while NP radius, grafting density, and grafted chain length were held constant. While composition profiles of the graft/matrix chains do not reveal any signs of brush collapse, brush heights calculated using a second moment of the segment density highlights a subtle indication of brush collapse. Further, by measuring free chain fluctuations and free energies via indirect umbrella sampling, we are able to suggest that interactions at the surface are significant in studying changes in miscibility of the PGNP and might be driving the morphology of the entire brush. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S54.00002: Dynamics of Adsorbed Polymer Chains in Polymer Nanocomposite Melts Eric Bailey, Russell John Composto, Karen Winey The dynamics of polymer chains and chain segments near an enthalpically attractive interface can be significantly perturbed from the bulk state and can dictate properties in polymer nanocomposites (PNCs). In this work, we directly observe and quantify polymer chain-scale desorption from nanoparticles in the melt using elastic recoil detection and Rutherford backscattering spectrometry on model PNCs comprised of poly(2-vinyl pyridine) and 25-nm diameter silica nanoparticles. We observe polymer desorption ~103 times slower than bulk chain diffusion and desorption that is slowest for longer chains and at lower temperatures. In this highly attractive system, we observe bound polymer that persists after more than 106 reptation times, even at Tg+100°C. By correlating the measured bound polymer to the nanoparticle concentration, we measure the adsorbed chain areal density as ~0.05 chains per nm2 and extract a bound layer thickness that extends ~Rg from the nanoparticle surface, two parameters that have been difficult to experimentally probe in the melt state. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S54.00003: Rational Design of Polymer Nanocomposites to Advance Their Thermomechanical Performance via Predictive Multiscale Modeling Wenjie Xia Understanding and predicting the thermomechanical responses of polymer nanocomposites are challenging as they are greatly influenced by many factors, such as interfacial energy and filler volume fraction, giving rise to the presence of nanoscale interfaces. To better design of polymer nanocomposites, we have recently established an atomistically informed coarse grained (CG) modeling approach to investigate how the nanoscale interfaces and molecular characteristics influence the mechanical and glass transition properties of polymer nanocomposites. Taking cellulose reinforced polymer nanocomposite as a relevant model system, we present a multiscale materials by design framework using CG modeling combining with advanced computational algorithms for prediction of thermomechanical properties of nanocomposites. Our established framework is validated by recent experiments and breaks new ground in predicting key structure and property relationships for optimum and tailored design of polymer nanocomposite materials. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S54.00004: Phase Behavior of Polymer Nanocomposite Systems with Attractive Particle-Polymer Interactions Ben Lindsay, Francisco Buitrago, Peter A Gordon, Karen Winey, Robert Riggleman Polymer nanocomposites (PNCs) are a unique class of materials that can have improved mechanical, thermal, electrical, or optical properties compared to neat polymers. The property improvements are highly dependent on the dispersion state of the filler nanoparticles. PNC systems with attractive particle-polymer interactions have been explored as a way to improve particle dispersion, but the phase space of these systems has not been thoroughly explored with a method that accounts for thermal fluctuations. We have developed Polymer Field Theoretic methods to efficiently generate phase diagrams without ignoring thermal fluctuations. In this presentation, I will describe this method and demonstrate its ability to predict particle dispersion for real PNC materials. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S54.00005: Exchange of the Bound Polymer Layer on Silica Nanoparticles Sanat Kumar, Andrew Jimenez, Jacques Jestin It is now commonly accepted that a bound polymer layer (BL) naturally forms when a polymer melt is mixed with nanoparticles in the limit of favorable interactions. What is unclear is the temporal persistence of this BL – its very name implies that this layer is expected to be irreversibly adsorbed. We use contrast variation methods in conjunction with small angle neutron scattering to probe this issue in the canonical case of poly(2-vinylpyridine) mixed with 14 nm diameter silica nanoparticles. We find that there is essentially no long-term reorganization of the bound layer at 150 °C, but apparently a rapid reduction of the BL thickness at 175 °C. We believe that the dramatic temperature dependence arises from the polyvalency of the binding of a P2VP chain to a NP – that is the fact that each P2VP chain is adsorbed to the NP through multiple monomers. Thus, while the adsorption-desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a sharp process that occurs over a relatively narrow temperature range. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S54.00006: The complexity of linear viscoelastic properties of polymer nanocomposites: polymer dynamics and nanoparticle rearrangement Shiwang Cheng, Jie Yang, Wei Yang Polymer nanocomposites (PNCs) are widely used as structural and functional materials, whose lifetime are typically evaluated from the time-temperature superposition. Rheological measurements of PNCs usually show >30 decades variation in the horizontal shift factor, aT, over a finite temperature range [1], indicating an unrealistic fast polymer dynamics at high temperatures. In this work, we decoupled the dynamic spectra of PNCs through a combination of rheology and dielectric measurements. We found that polymer dynamics in PNCs at different time and length scales follow identical temperature dependence with the neat polymer, although the rheological measurements exhibit 30 decades variation in aT. Further analyses suggest the local rearrangements of nanoparticles, especially at high temperatures, lead to the breakdown of the time-temperature superposition principle in PNCs. These results clearly show the shift factors by the time-temperature superposition principles cannot be directly translated to evaluate the dynamics features of PNCs at different temperatures. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S54.00007: The Effect of Nanofillers on the Viscoelastic Creep Behavior of Thermoplastics Francisco Buitrago, Anita S Yang, Peter A Gordon, Robert Riggleman, Karen Winey The use of polymer nanocomposites (PNCs) in infrastructure applications requires a comprehensive understanding of the mechanism of nanoparticle reinforcement during viscoelastic creep. Some of the most relevant parameters that impact the mechanical reinforcement of nanoparticles to a thermoplastic matrix are the nanoparticle size and concentration, and the interaction between the nanoparticle surface and the polymer matrix. In this study, the long-term creep behavior of a model nanocomposite system is examined by applying time-temperature superposition to dynamic mechanical analysis (DMA) of PNC films at temperatures between Tg-60 °C and Tg+60 °C. The PNC system is composed of monodisperse 10-nm, 15-nm, and 28-nm silica nanoparticles dispersed in an amorphous polymer matrix of approximately 200,000 g/mol weight-average molecular weight. The interaction between nanoparticle surface and polymer matrix is adjusted by using bare silica featuring hydroxyl surface groups capable of hydrogen bonding versus nanoparticle surfaces treated with a phenyl-capping agent. The effect of these parameters on PNC morphology is quantified by small-angle X-ray scattering (SAXS) and by transmission electron microscopy (TEM), and correlated to the long-term viscoelastic creep behavior observed by DMA. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S54.00008: Thickness Effects on Morphology and Gas Permeability of Polystyrene-Grafted-Silica in a Polystyrene Matrix Sophia Chan, Connor Bilchak, Mayank Jhalaria, Andrew Jimenez, Sebastian Russell, Julia Pribyl, Brian C Benicewicz, Sanat Kumar Polymer-grafted-nanoparticles in a polymer matrix can self-assemble into anisotropic morphologies that can be controlled by varying system parameters, such as grafting density and film thickness. The relationships between film thickness, morphology, and gas permeability of these systems, however, are not well-understood. We present the film thickness effects on gas transport properties and the surface and bulk morphologies of polystyrene-grafted-silica (PS-g-SiO2) in a polystyrene (PS) matrix. Our thinnest films (2.5 μm) of the polymer nanocomposite exhibited a 90% reduction in CO2 permeability relative to that of PS, while those of bulk films (100 μm) increased by 200%. Morphology characterization suggests lightly grafted PS-g-SiO2 components are conducive to diffusing to the surface; this effect is more pronounced in thinner films, where distance to the surface is much shorter than bulk films. Nanoparticles at the surface of thinner films would then rearrange into structures that disallow gas transport. Conversely, the enhanced relative permeability in bulk films is likely due to the decreased interfacial density between the nanoparticle and the polymer chains, allowing gas molecules to more easily flow through the film. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S54.00009: Highly-Mobile Nanoparticles that Strongly Interact with Well-Entangled Polymer Melts Diffuse via the Vehicular Mechanism Karen Winey, Eric Bailey, Philip J Griffin, Russell John Composto When particles are large relative to the entanglement mesh in well-entangled polymer melts, the Stokes-Einstein (SE) relation predicts that particle diffusion scales as M-3.4. Using Rutherford backscattering spectrometry, we measure the diffusion coefficient of very small (radius ≈ 0.9 nm) octaaminophenyl silsesquioxane nanoparticles (NPs) in well-entangled poly(2-vinylpyridine) (P2VP) melts of varying molecular weight (1 – 26 entanglements/chain). We demonstrate that these small NPs diffuse between 10–10,000X faster in P2VP melts than predicted by SE, with the diffusion coefficients scaling weakly with molecular weight M–0.7±0.1. Furthermore, we characterize the local segmental relaxation process and chain-scale center-of-mass diffusion and find reductions relative to bulk of ~80% and ~60%, respectively, at a NP concentration of up to 25 vol%. Through the combined study of NP and polymer dynamics in this attractive nanocomposite system, we demonstrate experimentally that small and highly-mobile nanoparticles in well-entangled polymer melts diffuse via the vehicular mechanism, i.e. successive NP adsorption/desorption events that occur on Rouse length and time scales. |
Thursday, March 7, 2019 1:03PM - 1:39PM |
S54.00010: Using advanced field-based approaches to predict macroscale polymer nanocomposite phase behavior Invited Speaker: Jason Koski Predictive phase diagrams significantly improve the refinement and optimization of materials for advanced applications. Unfortunately, theoretical phase diagrams of polymer nanocomposites (PNCs) are lacking, which is a result of limitations with conventional modeling approaches such as computational expense or critical approximations. Here we describe the development of field-based approaches in modeling PNCs and demonstrate their efficacy in modeling macroscale phase behavior. The exciting advances of these field-based approaches have led to the development of phase diagrams for an array of polymer nanocomposite systems. Examples of these systems include grafted nanoparticles in a polymer melt and the assembly of mixed brush nanoparticles in solution. We find the introduction of thermal fluctuations have a significant impact on the overall phase behavior of these systems. Specifically, thermal fluctuations are necessary to properly capture depletion interactions in grafted nanoparticle systems and to describe solution-based assembly where it is understood that thermal fluctuations are significant. Excitingly, we further show the versatility of these methods to other PNC systems and provide comparisons with analogous experiments. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S54.00011: Local structure and phase behavior of dense polymer-particle mixtures: improved theory and comparison with simulation Yuxing Zhou, Kenneth Schweizer The polymer reference interaction site model (PRISM) has been extensively applied to study the equilibrium behavior of polymer nanocomposites (PNC). The theory predicts three typical phases or states of aggregation: depletion clustering, steric stabilization via discrete adsorbed layers, and polymer-mediated bridging or networking, depending on polymer-particle attractive interaction, size ratio, composition, chain length and total packing fraction. While the existence of such microstructures appears to qualitatively agree with simulations and experiments, the accuracy of the predicted pair correlation functions and phase behavior has not been thoroughly examined. By performing systematic simulations, we find the PRISM pair correlation functions, selected thermodynamic properties and phase behaviors based on the commonly used Percus-Yevick and Hypernetted Chain closures sometimes incur significant errors, especially near bridging and depletion spinodal boundaries. A variety of new closure approximations are explored and we find that over a wide range of parameter space the modified Verlet approximation provides a major improvement in accuracy of structural and thermodynamic behavior for both PNCs and the corresponding atomic or colloidal mixtures. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S54.00012: Deforming Interfacial Layers of Bare and Grafted Particle Nanocomposites in Large Amplitude Oscillatory Shear Siyang Yang, Pinar Akcora
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Thursday, March 7, 2019 2:03PM - 2:15PM |
S54.00013: Dissipative Particle Dynamics (DPD) Simulations of Polymer-Filler Blends: Understanding Dispersion and Hierarchical Structure in Polymer Nanocomposites. Ashish Gogia, Kabir Rishi, Alex McGlasson, Michael Chauby, Greg Beaucage, Vikram K Kuppa Nanoscale fillers are widely employed as cheap and effective additions for enhanced properties and functionality in polymeric systems. Such nanocomposites may contain fillers of varying miscibility, such as carbon black, silica, metal oxides, pigments, and/or various combinations thereof. In such systems, a complex partitioning of the components often results from the rich thermodynamics and kinetic history. Hence, the state of dispersion of the polymers and fillers is crucial to the behavior of polymer nanocomposites. In this research, we perform Dissipative Particle Dynamics (DPD) simulation of these blends, varying polymer-polymer, filler-filler and polymer-filler interaction energy, to understand the hierarchical structure and dispersion over multiple length and time-scales. The simulation results are validated against small angle x-ray scattering data to bridge a significant gap in our understanding of how complex hierarchical structure (across several decades in length) develops in these multicomponent systems. Additionally, the influence of parameters such as polymer chain stiffness and chain size on the formation of aggregates and agglomerates are explored. |
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