Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T21: Focus Session: Polymer Nanocomposites II - Dynamics |
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Sponsoring Units: DPOLY GSNP Chair: Jason Bochinski, North Carolina State University Room: 406 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T21.00001: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T21.00002: Direct Neutron Scattering Measurements of Grafted Polymer Chain Conformations from Functionalized Nanoparticles Michael J.A. Hore, Boualem Hammouda The conformations of grafted polymers play an important role in determining the physical properties of polymer nanocomposites. Small-angle neutron scattering (SANS) is performed to quantify the conformation of poly(methyl methacrylate)($M_{w} > $ 27,000 g/mol) and polystyrene chains ($M_{w} >$ 57,000 g/mol) which are attached to iron oxide nanoparticles ($R_{np} = 2.5$ nm, $\sigma = 0.73$ chains/$\mathrm{nm^{2}}$) and small fractal aggregates ($R \approx 11$ nm, $\sigma = 0.2$ chains/$\mathrm{nm^{2}}$), respectively. Unlike light scattering or microscopy, SANS can directly measure the grafted polymer chain conformations. In a homopolymer melt, we find the grafted chains adopt stretched conformations near the nanoparticle surface, and transition to ideal, random coils past a cutoff distance $r_{c}$, in agreement with scaling arguments in the literature. We find the conformation of the polymer chains is largely unaffected by the ratio of the degree of polymerization of the matrix ($P$) to that of the brush ($N$). Finally, we extend this work to measure grafted polymer conformation in solution as a function of solvent quality, and find the grafted chains behave as swollen coils with an excluded volume parameter $\nu$ that decreases as the solvent cools to the $\Theta$ temperature. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T21.00003: Many Body Effects on Particle Diffusion in Polymer Nanocomposites Zachary E. Dell, Kenneth S. Schweizer Recent statistical mechanical theories of nanoparticle motion in polymer melts and networks have focused on the dilute particle limit. By combining PRISM theory predictions for microscopic structural correlations, and a new formulation of self-consistent dynamical mode coupling theory, we extend dilute theories to finite filler loading. As a minimalist model, the polymer dynamics are first assumed to be unperturbed by the presence of the nanoparticles. The long time particle diffusivity in unentangled and entangled melts is determined as a function of polymer tube diameter and radius of gyration, nanoparticle diameter, and polymer-filler attraction strength under both constant volume and constant pressure situations. The influence of nanocomposite statistical structure (depletion, steric stabilization, bridging) on dynamics is also investigated. Using recent theoretical developments for predicting tube diameters in nanocomposites, the consequences of filler-induced tube dilation on nanoparticle motion is established. In entangled melts, increasing filler loading first modestly speeds up diffusion, and then dramatically when the inter-filler separation becomes smaller than the tube diameter. At very high loadings, a filler glass transition is generically predicted. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T21.00004: Enhanced Nanorod Diffusion in Polymer Melts Russell J. Composto, Nigel Clarke, Karen I. Winey, Jihoon Choi Using Rutherford backscattering spectroscopy (RBS), the translational diffusion of titanium oxide (TiO$_{2})$ nanorods ($l=$43.1 nm and $d=$4.6 nm) is measured in entangled and unentangled polymer melts, polystyrene (PS; $M_{n}=$9-2000 kg/mol). Nanorods in entangled systems ($M_{n}=$160, 650, and 2000 kg/mol) are found to diffuse up to two orders of magnitude faster than predicted by classical theory. However, diffusion of nanorods in unentangled systems ($M_{n}=$9 and 65 kg/mol) is captured by this continuum theory. Below or near the entanglement limitation, $M_{n} \quad \le \quad M_{e}$ ($M_{e}$: entanglement molecular weight), unentangled polymer melts described by Rouse dynamics can be modeled as a continuum matrix against nanoscale inclusions. However, in highly entangled systems ($M_{n}$ \textgreater \textgreater $M_{e})$ the standard continuum models are no longer valid and lead to local non-hydrodynamic friction at the length scale of the tube diameter (i.e., $d_{t}=$8 nm for PS) [1]. Thus, enhanced diffusion of nanorods parallel to the tubes may be responsible for the faster than expected translational diffusion in entangled polymer melts. These experiments provide new insight into the relevant parameters that govern the diffusion of anisotropic nanoparticles in complex fluids. [1] Yamamoto et. al., J. Chem. Phys., 135, 224902 (2011). [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T21.00005: The Dynamics of Nanoparticles in Polymer Solutions and Melts Ashis Mukhopadhyay, Sharmine Alam, Indermeet Kohli Polymer nanocomposites (PNCs) has received a lot of attention in the recent years because of their potential applications in fabricating materials with novel mechanical, electrical, and photonic properties. The mobility of nanoparticles (NPs) play crucial role in determining various properties of PNC systems. Computer simulations and recent experiments have suggested that properties such as the toughness of a composite depend upon particle mobility. Even nanocomposites with ``self-healing'' properties that can restore strength in damaged regions have been proposed and some early work of their feasibility has been demonstrated. In this talk I will present some of our experimental work on the diffusion of nano-sized gold particles in polymer solutions and melt. Unusually fast diffusion of NPs when their size is smaller than the tube diameter in an entangled polymer was observed. Comparison with current theories and simulations will be shown. If time permits, our recent results on gold nanorod diffusion in polymer solution using polarized fluorescence correlation spectroscopy will be presented. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T21.00006: Dynamics at the Polymer/Nanoparticle Interface in Poly(2-vinylpyridine) Nanocomposites Adam Holt, Vera Bocharova, Philip Griffin, Alexander Agaprov, Adam Imel, Mark Dadmun, Joshua Sangoro, Alexei Sokolov The intriguing thermodynamic properties of polymer nanocomposites (PNCs) have often been attributed to the formation of an interfacial polymer region at the nanoparticle surface and a better understanding of how the interfacial region affects the PNC dynamics is desired. The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and small angle x-ray scattering (SAXS). The SAXS data revealed a core-shell structure formed in interfacial region and BDS data detected the slower relaxation process associated with the interfacial polymer layer. Both static and dynamic measurements estimated the layer thickness to be 4-6 nm. We also demonstrated that the presence of interfacial polymer layer has negligible influence on the glass transition temperature and segmental dynamics of the remaining polymer. These results potentially offer an explanation to recent controversies in studies of polymer nanocomposites due to different experimental techniques. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 1:03PM |
T21.00007: Nanoscale Organic Hybrid Materials (NOHMs) -- Structure and Dynamics Invited Speaker: Lynden Archer Polymer-particle composites are used today in virtually every field of technology. When the particles approach nanometer dimensions, large interfacial regions are created in their polymer hosts, which present opportunities and challenges for research, as well as for applications. This talk will focus on a novel class of polymer-particle composite fluids created by densely grafting short organic polymer chains or ionic liquid molecules to inorganic nanostructures. By manipulating the nanoparticle size, polymer molecular weight and surface chemistry, we show that it is possible to create self-suspended suspensions of nanoparticles in which each particle in suspension carries around a discrete share of the suspending medium. The talk will explore consequences of the self-suspended state on fluid structure, rheology, and tethered polymer {\&} particle dynamics in these so-called \textit{nanoscale organic hybrid materials} (NOHMs). The talk will also discuss particle and tethered polymer dynamics in single-component NOHMs and phase stability, structure, and rheology of NOHMs/polymer blends. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T21.00008: Dynamics of entangled polymers in the presence of obstacles Nigel Clarke, Karen Winey, Russell Composto We have observed that, for a wide range of spherical nanoparticles, the polymer diffusion coefficient relative to the pure melt value as a function of the interparticle distance relative to the chain radius of gyration collapses onto a master curve. In order to gain insight into the molecular basis for this behaviour, we use the Evans-Edwards Monte Carlo model for reptation dynamics in which the chains are coarse-grained such that each bead within the simulation represents one entanglement segment. We investigate the long time diffusion behaviour when the chains are constrained by a lattice structure with regularly spaced holes each the size of an entanglement spacing. We find that as the dimensions of the lattice decreases, the power law for the scaling of the diffusion coefficient with molecular weight changes from the well known result for melt diffusion of entangled chains of approximately -2 to approximately -3. We present a simple physical model that captures this result. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T21.00009: The Effect of Nanoparticle Radius of Gyration on the Diffusion of Polystyrene in a Nanocomposite Adam Imel, Brad Miller, Wade Holly, Durairaj Baskaran, J.W. Mays, Mark D. Dadmun Controlling the dispersion of nanoparticles throughout a polymer matrix is difficult. We have found that nanoparticle dispersion can be achieved by incorporating soft, organic nanoparticles with complementary chemical moieties, thus achieving favorable enthalpic interactions. The rational design of soft nanoparticles can create an interface that allows interpenetration of the polymer chains and particles reducing the depletion of entropy that is the main contributing force to the flocculation of nanoparticles. The nanoparticles are produced by intra-molecularly crosslinking a single polystyrene chain via a nano-emulsion technique with divinyl benzene. This synthetic approach allows the effects from structure, size and softness of the nanoparticle to be examined as they contribute to the dynamics of the polymer matrix by varying the crosslink density. This report focuses on the effect that these nanoparticles have on the diffusion coefficient of polystyrene. Neutron reflectivity was used to monitor the interdiffusion of deuterated polystyrene and protonated polystyrene with and without the soft nanoparticles in the respective layers. It has been proposed that the ratio of the radius of gyration (Rg) of the polymer chain to the nanoparticle controls the dynamics, thus the molecular weights of the matrix in this study have been varied from 535, 173, to 68 kg/mol. Initial results suggest when the Rg of the polymer is larger than that of the nanoparticle Rg the dynamics are impacted the most. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T21.00010: XPCS Studies of Nanoparticle Motion within Glassy Polymer Melts Hongyu Guo, S.K. Ghosh, S.K. Sinha, M. Cui, T.P. Russell, W. Cha, J. Carnis, H. Kim, Z. Jiang, S. Narayanan We report x-ray photon correlation spectroscopy (XPCS) experiments to investigate the motion of nanoscale gold particles within polystyrene (PS) melts of molecular weight between 30K and 900K g/mol. The particles, with diameter span from 5 nm to 22 nm, are dispersed in a highly dilute concentration (volume fraction 0.005) and are functionalized with PS chains to stabilize them against aggregation. We already know that for low molecular weight PS melts there are dynamics crossovers from diffusive motion to hyper-diffusive motion when quenching to lower temperature. When polymer chains are longer than the entanglement length, things are more complicated. At low temperature, similar hyper-diffusive motion are observed. At high temperature, i.e. 70 K higher than Tg, the dynamics changed from overdamped behavior to underdamped oscillatory behavior, indicating that entanglement strongly affects the particle motion. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T21.00011: Microscopic Theory for Entangled Polymer Dynamics in Rod-Sphere Nanocomposites Umi Yamamoto, Kenneth Schweizer We have developed a self-consistent microscopic theory for the long-time dynamics of needles in an array of static spherical fillers. The approach exactly enforces the dynamical two-body rod topological uncrossability and sphere impenetrability constraints, leading to a generalized concept of entanglements that includes the filler excluded volume effect. How the diffusion anisotropy (transverse versus longitudinal motion) depends on the filler-needle aspect ratio, polymer concentration, and filler volume fraction is established. Due to the steric blocking of the longitudinal reptative motion by obstacles, a literal localization transition is predicted that is generically controlled by the ratio of filler diameter to the pure polymer tube diameter or needle length. For a window of filler sizes and loadings, the needle is predicted to diffuse via a ``renormalized'' reptation dynamics where the tube is compressed and the longitudinal motion is retarded in a manner that depends on all system variables. At high filler volume fractions the needle diffusivity is strongly suppressed, and localization ultimately occurs in the unentangled needle regime. Generalization of the approach to treat mobile fillers, flexible chains, and nonrandom microstructure is also possible. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T21.00012: Polymer Diffusion in Nanocomposites with Nanorods: Bridging the Gap between Nanosphere and Nanotube fillers Jihoon Choi, Nigel Clarke, Karen I. Winey, Russell J. Composto The tracer diffusion of deuterated polystyrene (dPS; 168-3200 kg/mol) is measured in polystyrene (650 kg/mol) nanocomposites containing phenyl-capped nanorods with a similar aspect ratio (AR $=$ 9) but different sizes, NR-short (TiO$_{2}$; $l=$43.1 nm and $d=$4.6 nm) and NR-long (SiO$_{2}$-[Ni(N$_{2}$H$_{4})_{3}$]Cl$_{2}$; $l=$371 nm and $d=$43 nm). For NR-long where $l$ \textgreater 2$R_{g}$, the diffusion coefficient initially decreases as nanorod volume fraction increases but then begins to increase for near the percolation threshold. In this system, $R$ \textless $R_{g}$ and the diffusion behavior is consistent with previous studies of carbon nanotubes (i.e., $l $\textgreater \textgreater 2$R_{g})$. However, for NR-short (i.e., $l$ \textless 2$R_{g})$, diffusion shows a monotonic slowing down as the volume fraction increases despite the small values of $R$/$R_{g}$. This behavior is similar to the slowing down observed for isotropic nanoparticles. These experiments demonstrate that not only radius but also length of the nanoparticle plays a key role in diffusion. Moreover, these results indicate that a comprehensive model for polymer dynamics should include the geometry of the nanoparticle relative to $R_{g}$. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T21.00013: The structure and dynamics of polymer nanocomposites containing anisotropic nanoparticles Chia-Chun Lin, Kohji Ohno, Nigel Clarke, Karen Winey, Russell Composto, Michael Hore The tracer diffusion of deuterated polystyrene (dPS; 49-532 kg/mol) is measured in polystyrene (PS: 270 kg/mol) nanocomposites containing PS-grafted (132 kg/mol) anisotropic nanoparticles (NP). The NP's are small aggregates containing iron oxide spheres (5nm). These NP's uniformly disperse in PS up to 100{\%} loading. The structure of the polymer nanocomposites is probed using (ultra)small angle x-ray scattering (USAXS,SAXS). Peaks shift to high Q region with increasing NP loadings, indicating a decrease in spacing between particles. The interparticle distance for the pure NP case is 30nm, consistent with TEM, and a brush thickness of 15nm. The brush profile is also measured using SANS. The reduced tracer diffusion coefficient initially decreases as NP loadings increase and then reaches a minimum (35{\%} reduction) near 0.25 vol{\%} (core) for all dPS. With a further increase in NP loading, diffusion recovers to 90{\%} of the unfilled case. Penetration of the tracer (i.e., wetting) into the brush will affect the effective interparticle distance. Diffusion of dPS (1866 kg/mol) will be examined to determine if the dry brush case influences the recovery at high loading. These experiments demonstrate that polymer brushes grafted to anisotropic nano particles can affect the tracer diffusion pathway and indicate that diffusion models should incorporate the interfacial structure between brush and matrix. [Preview Abstract] |
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