Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S56: Polymer Nanocomposites III: FundamentalsFocus
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Sponsoring Units: DPOLY Chair: Guoliang Liu, Virginia Tech Room: LACC 515B |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S56.00001: Concentration Dependence of Polymer Adsorption on Clay Surfaces by Small Angle Neutron Scattering Alan Nakatani, Carol Mohler, Michael Poindexter, Stephanie Hughes Understanding the basis of polymer adsorption behavior on inorganic surfaces is critical in a variety of industrial applications, including water treatment, pigmented architectural coating formulations, cement formulations, and pulp and paper manufacture. To further the understanding of the adsorption process, the adsorption behavior of select high molecular weight polymers onto a model clay surface in aqueous formulations is studied as a function of polymer concentration by small angle neutron scattering (SANS). The model clay used in the study is Laponite, which has a well-defined disk-like morphology and narrow distribution of platelet sizes. The SANS results were fit using a core-shell cylinder model allowing extraction of the adsorbed polymer layer dimensions on both the radial and face of the clay particles. The polymer layer thickness and density were found to depend on the polymer concentration, progressively adsorbing onto the radial and then facial locations of the clay particles. The results are compared with other studies of polymer adsorption onto surfaces and put in context with existing theories for polymer adsorption. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S56.00002: Structure, Entanglements, and Mechanical Properties of Adsorbed Polymer-Grafted Nanoparticles from Molecular Dynamics Simulations Jeffrey Ethier, Lisa Hall Inorganic nanoparticles with polymer chains grafted to their surface, or polymer-grafted nanoparticles (PGNs), have potential as a means to create functional materials with a controllable nanoscale structure. For instance, a thin film of PGNs deposited on a surface can self-assemble into a hexagonally packed structure with mechanically robust and precise spacing. We perform molecular dynamics (MD) simulations of neat PGNs adsorbed on a flat surface with various graft densities, graft lengths, and surface adsorption strengths, to show how these synthetically controllable parameters affect the structure and entanglements. Specifically, we model a PGN as a spherical nanoparticle to which graft chains are tethered at random points on the surface, and use a simple bead-spring model with finitely extensible bonds to allow us to capture polymer entanglements. We first consider two nearby PGNs adsorbed on a surface (isolated from other PGNs) and analyze their interparticle spacing and entanglements. We then simulate adsorbed hexagonally packed PGN monolayers and relate their stress-strain behavior to nanoparticle arrangement and interparticle entanglements. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S56.00003: Modeling the Morphology and Phase Behavior of One-Component Polymer-Grafted Nanoparticle Systems Valeriy Ginzburg Polymer-grafted, or “hairy” nanoparticles (HNP) represent an important and relatively new class of materials. One-component (no matrix) HNP materials, in particular, are not prone to macroscopic phase separation and have a variety of interesting microphase-separated, anisotropic morphologies, similar to surfactants or block copolymers. Here, we develop a new self-consistent field theory describing the behavior of one-component HNP systems, and apply it to predict the morphology as function of the ligand molecular weight and grafting density. As in the case of block copolymers, we observe lamellar, cylindrical, and spherical morphologies, and elucidate phase boundaries as function of the core (nanoparticle) volume fraction and the ratio of the particle radius to the ligand radius of gyration. We also observe the formation of a novel phase, labeled as “sheets”, where the lamellar-like ordering of particle-rich and ligand-rich layers is characterized by the hexagonal ordering of the particles within the particle-rich layer. The predictions agree qualitatively with experimental results. Our theoretical approach can be easily extended to HNPs with mixed ligands and block-copolymer ligands. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S56.00004: Coarse-Grained Simulations of Gas Penetrants within Polymer Nanocomposites James Barnett, Sanat Kumar Recent experimental studies have shown that gas diffusion in polymer nanocomposites is enhanced compared to diffusion in corresponding neat systems for a range of chain lengths. Additionally coarse-grained simulations have been shown that in order to simulate a gas separation membrane, one must meet several criteria and operate in the correct parameter space. Here we use that knowledge to explore coarse-grained simulations of gas particles within both nanocomposite and neat systems with the puprose of uncovering the underlying mechanisms of diffusion enchancment exhibited in such nanocomposite systems. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S56.00005: Controlling the Stability of Imprinted Nanostructures Using Grafted Nanoparticles Sonal Bhadauriya, Xiaoteng Wang, Praveen Pitliya, Dharmaraj Raghavan, Christopher Stafford, Jack Douglas, Alamgir Karim Structural decay in imprinted polymer-nanocomposite thin films presents an important problem from both fundamental and application standpoint as they find use in various applications such as nanoimprint and E-beam lithography. This work investigates the kinetics of structural decay in imprinted composite thin films of Polymethyl methacrylate (PMMA) grafted TiO2 nanoparticles(NP) incorporated within PMMA matrix of a lower molecular weight compared to the grafted chains. Addition of grafted NP significantly improved the thermal stability above Tg and acted as stabilizing agents for the imprinted nanostructures. Utilizing ex-situ atomic force microscopy measurements, decay kinetics of the imprinted structures provide insights into stress relaxation of the nanocomposite film. We demonstrate that as the NP loading is increased, the imprinted nanostructure exhibited a higher relaxation time, indicative of slower decay kinetics. Furthermore, extrapolation of the relaxation behavior for this composite reveals a transition from antiplastication to plastication regime at 97°C below which NP decrease the relaxation time of the composite.These results provide a simple strategy to control high temperature nano-structural integrity in imprinted polymer-nanocomposite films by using grafted NPs. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S56.00006: Polymer dynamics and nanoparticle diffusion in nanocomposites Nigel Clarke, Argyrios Karatrantos, Karen Winey, Russell Composto Nanoparticle and polymer dynamics in nanocomposites containing spherical and rod-like nanoparticles were investigated by means of molecular dynamics simulations. We show that the polymer diffusivity decreases with spherical nanoparticle concentration due to an increase of the interfacial area created by nanoparticles, and that small sized nanoparticles can diffuse much faster than that predicted from the Stokes-Einstein relation in the dilute regime1. We show that the spherical nanoparticle diffusivity decreases at higher nanoparticle concntration due to nanoparticle–polymer interface. We also show how nanorod diffusion from our MD simulations compares with theoretical predictions. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S56.00007: Interplay of the structure and properties of hierarchical functional nanocomposites Eileen Buenning, Christopher Durning, Dimitris Vlassopoulos, Sanat Kumar Our recent work shows that membranes comprised solely of polymer grafted nanoparticles exhibit remarkable, enhanced, molecular transport compared to that of the corresponding bulk polymer or “traditional” ungrafted, mixed-matrix composites. At a fixed grafting density, these enhancements show a non-monotonic dependence on the grafted chain length. Extensive structural characterization in the dry (non-solvated) state indicate only monotonic changes to the interparticle spacing as a function of molecular weight. Additionally, in the swollen state, we observe homogeneous solvent distribution within the brush. Interestingly, the linear mechanical response of these materials show a liquid-to-solid transition, whose location coincides with the peak in permeability. These results demonstrate that rational design of hierarchical materials with desired functionalities relies heavily on control of structure-property relationships over multiple length scales. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S56.00008: Pattern and Phase Miscibility Directed-Assembly of Polymer-Grafted Nanoparticles within Polymer Matrix Xiaoteng Wang, Michael Bockstaller, Alamgir Karim The modification of nanoparticles with polymer brushes has emerged as an effective way of controlling the interactions and organization of nanoparticles in polymer nanocomposites. Herein we investigate the phase behavior of polystyrene-grafted silica nanoparticles in poly (vinyl methyl ether) (PS-g-SiO2/PVME) homopolymer blend films, where the graft and matrix polymers exhibit attractive/repulsive enthalpic interactions at low/high temperatures. In ultra thin films (~100nm), phase separation into polymer-grafted nanoparticles (PGNPs) rich and matrix polymer rich phases occurs at a temperature lower than the LCST of classic binary linear polymer blends. Thermal cycling of the blend thin film through the critical temperature enables the reversible formation and dissolution of nanoparticle domain structures. We further illustrate that the PGNPs could be driven into spatially organized sub-micron domains under a symmetry-breaking soft elastomer topographic pattern. Selective segregation of nanoparticle domains is observed in both miscible and immiscible states. Such guided assembly of PGNPs in polymer nanocomposites open pathways to create novel hybrid materials for many technological applications. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S56.00009: Nanoparticle Diffusion in Polymer Nanocomposites Koteswararao Medidhi, Venkat Padmanabhan Studies on Polymer Nanocomposites (PNCs) have increased prominently over the past few decades because of their enhanced properties compared to their near counterpart. Using molecular dynamics simulations we investigated the diffusivity of nanoparticles in polymer nanocomposites containing blends of polymer grafted and bare nanoparticles. Our results indicate that presence of bare particles in the melt significantly affects the dynamics of polymer-grafted nanoparticles and the change is sensitive not only to grafting density and polymer backbone rigidity but also on the mutual interactions between the two types of nanoparticles. The enhancement in the diffusivity of polymer-grafted nanoparticles is in part due to the coverage of the particle core by smaller ‘bare’ particles. |
Thursday, March 8, 2018 1:03PM - 1:39PM |
S56.00010: Polymer grafted nanoparticles for designed interfaces and controlled assembly in polymer nanocomposites Invited Speaker: Brian Benicewicz Over the past 15 years, the application of controlled radical polymerization techniques to grafted polymer chains has enhanced our ability to design the critical interface between inorganic particles and polymer matrices. These interfaces can be prepared with control over many molecular variables such as chain density, chain length, polydispersity, chain architecture, end group chemistry, etc. In particular, we have used the RAFT polymerization method with polymer grafting techniques as an approach to modify the surfaces of nanoparticles with a variety of functional polymers. In our work, we developed multiple approaches for attaching RAFT agents to the surface of nanoparticles with variable graft densities. Using these surface-immobilized RAFT agents, styrenic, acrylate, methacrylate, and diene monomers can be polymerized on the nanoparticle surfaces via surface-initiated RAFT polymerization in a controlled manner. However, this diversity of design strategies is not limited to a single population of chains. Another set of RAFT agents can be attached to the remaining free surface and a second (or third) population of polymer chains can be polymerized from the surfaces of the same particles. This subsequent set of chains can possess a completely independent set of molecular variables (chain density, molecular weight, chemistry, architecture, etc.) from the initial population of grafted chains. Thus, a almost limitless design space is available to create highly specified interfaces on nanoparticles which affect assembly. This presentation will provide a timely update of the chemistry used to prepare multimodal polymer-grafted nanoparticles with precise control over multiple polymer chain variables, and examples in select applications. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S56.00011: Decoupling nanoparticle motion from elastic modulus in polymer nanocomposites in confined regime Erkan Senses, Suresh Narayanan, Yimin Mao, Antonio Faraone We investigated slow motion of nanoparticles individually dispersed in entangled polymer melts at particle volume fractions up to 42 %. At such high loadings, the nanoparticles serve as both fillers for the resulting polymer nanocomposites and probes for the network dynamics therein. The results from x-ray photon correlation spectroscopy revealed that the particle relaxation closely follows the mechanical reinforcement in the nanocomposites only at the intermediate concentrations below the critical value for the chain confinement. Interestingly, the particles do not further slowdown at higher volume fractions- when all chains are practically on nanoparticle interface- and their motion decouple from the elastic modulus of the nanocomposites that further increases orders of magnitude. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S56.00012: Molecular Dynamics Simulations of Small and Sticky Nanoparticles in Polymer Nanocomposites Jan-Michael Carrillo, Umi Yamamoto, Vera Bocharova, Bobby Sumpter, Kenneth Schweizer, Alexei Sokolov Large-scale molecular dynamics simulations indicate that the diffusion of small nanoparticles, where the diameter of the nanoparticle is of the same size as the Kuhn segment length and smaller than the entanglement tube diameter, is influenced by its affinity to the segments of the polymer matrix. In the simulations, this affinity is characterized by the desorption time, τdes, which is defined as the characteristic time for a nanoparticle to desorb from a polymer segment. The sticky nanoparticle diffusion in a polymer melt can be represented as a nanoparticle with an effective radius, due to an adsorbed polymer layer, if its size is sufficiently large compared to matrix chains. While for longer matrix chains, there is decoupling of the diffusivity of the nanoparticle with the polymer matrix. We observe that the diffusion coefficient of small and sticky nanoparticles decreases as a function of polymer matrix degree-of-polymerization, N, and then the value saturate and becomes N independent. The crossover to N independence occurs at a higher N as τdes increases. This behavior is akin to non-adsorbing nanoparticle of increasing size, where matrix entanglements begin to impede its diffusion as its size increases. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S56.00013: Molecular Dynamics Simulation on Static and Rheological Properties of Polymer-Nanoparticle composite Systems with Chemically Identical Brush and Matrix chains Wei Peng, Pawel Keblinski, Rahmi Ozisik Polymer grafted nanofillers are widely used to reinforce the polymer matrix. Recently, Giovino et al. (Giovino M, Pribyl J, Benicewicz B, Kumar S, Schadler L, Polymer 2017, doi: 10.1016/j.polymer.2017.10.016.) studied the rheology of the polystyrene(PS)-grafted silica NPs mixed with the PS matrices of different chain lengths in experiment. They found the relative viscosity increase caused by the NPs in the system with shorter PS chains was much greater than that caused by the NPs in the system with longer PS chains, while the dispersion states of NPs and glass transition temperatures were very similar. To expose the mechanism responsible for the behavior, we used molecular dynamics to simulate nanocomposite systems with chemically identical brush and matrix chains. The viscosity increases observed in our simulations exhibit similar trends as those in the experiment. However, we find that the brush chain conformation undergoes a transition from self-avoiding walk to ideal Gaussian when the matrix chain length is of the order of square root of the grafted chain length, indicating that in the experimental studies the brush chains were Gaussian. To further expose the specifics of reinforcement mechanism, we evaluate microscopic effects of grafted NPs on the dynamics of matrix chains. |
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