Session T19: Focus Session: Polymer-Nanoparticle Interactions I

Thermally Stable Gold Nanoparticles with a Crosslinked Diblock Copolymer Shell

The use of polymer-coated Au nanoparticles prepared using oligomeric- or polymeric-ligands tethered by Au-S bonds for incorporation into block copolymer templates under thermal processing has been limited due to dissociation of the Au-S bond at T $>$ 100$^{\circ}$C where compromises their colloidal stability. We report a simple route to prepare sub-5nm gold nanoparticles with a thermally stable polymeric shell. An end-functional thiol ligand consisting of poly(styrene-b-1,2{\&}3,4-isoprene-SH) is synthesized by anionic polymerization. After a standard thiol ligand synthesis of Au nanoparticles, the inner PI block is cross-linked through reaction with 1,1,3,3-tetramethyldisiloxane. Gold nanoparticles with the cross-linked shell are stable in organic solvents at 160$^{\circ}$C as well as in block copolymer films of PS-b-P2VP annealed in vacuum at 170$^{\circ}$C for several days. These nanoparticles can be designed to strongly segregate to the PS-P2VP interface resulting in very large Au nanoparticle volume fractions $\phi _{p}$ without macrophase separation as well as transitions between lamellar and bicontinuous morphologies as $\phi _{p}$ increases.   

Control on the Dispersion of Colloidal Quantum Dots with Unconventional Block Copolymers in Nanocomposite Films

Dispersion of colloidal quantum dots (QDs) with unconventional block copolymers (BCP) in thin films was investigated. Unconventional BCPs consisting of either thiol (SH) anchoring groups or fluorinated (F) phenyl groups in the minor block (with 11 repeat units) and either poly(triphenylamide) (PTPA) or polystyrene (PS) in the major block (with $\sim $ 60 repeat units) were synthesized by the Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization. QDs encapsulated with alkyl hydrocarbons were mixed with both types of BCPs and deposited by solvent casting on substrates to yield hybrid thin films with thicknesses less than 100 nm. With combined characterization with TEM, AFM, and Kelvin Probe Microscopy (KPM), we confirmed that QDs were uniformly distributed within the BCP matrix film, when thiol (SH) anchor blocks were used, due to the favorable enthalpic interaction between QD and BCP. In contrast, QDs were segregated from the BCP matrix film either at the top or at the bottom when fluorinated phenyl blocks were employed. Based on the uniformly distributed QDs in the BCP matrix film, we were able to realize QD-based light-emitting diodes containing organic-inorganic hybrid active layers with high quantum yield.   

Additive-Driven Assembly of Block Copolymers: A Strategy for Well Ordered Hybrid Nanocomposites

Ordered block copolymer - nanoparticle composite systems are an interesting class of materials in which the block copolymer guides the spatial distribution of nanoparticles. Important considerations for designing these systems include tuning the functionality of the ligands to promote compatibility with the desired block and tuning the size of the particles relative to the size of domains. Often, at high loadings, either the nanoparticles phase separate or the block copolymer morphology is disrupted. Here we discuss how strong interactions between the ligands and the polymer chains can lead to incorporation of high loadings of large molecular additives and nanoparticles to form well-ordered block copolymer morphologies. Block copolymers chosen for this purpose were disordered and selective hydrogen bonding interactions led to disorder to order transitions upon particle addition. Results for molecular additives as well as nanoparticles indicate a variety of routes to functionalization of block copolymer templates.   

Effects of Nanoparticle Size Polydispersity On the Tethered Nanoparticle Phase Diagram

Recent simulations predict that aggregating nanospheres functionalized with polymer ``tethers'' can self-assemble to form a cylinder, perforated lamallae, lamallae, and even the double gyroid phase also seen in block copolymer and surfactant systems [1]. We study the impact of nanoparticle size polydispersity on the properties of the phase diagram[2],[3]. We show that in the portions of the phase diagram characterized by an icosahedral packing motif, a low amount of polydispersity lowers the energy and a large amount of polydispersity raises the potential energy of the system by disrupting the icosahedral packing. In the portions of the phase diagram characterized by crystalline packing, polydispersity raises the energy of the system and induces a phase transition from crystalline to liquid within the nanosphere packing of the microphase.\\[4pt] [1] Iacovella, et al., PRE, 2007\\[0pt] [2] Phillips, et al., ``Stability of the double gyroid phase to nanoparticle polydispersity in polymer tethered nanosphere systems, preprint.\\[0pt] [3] Phillips, et al., ``Effects of Nanoparticle Size Polydispersity On the Tethered Nanoparticle Phase Diagram,'' preprint.   

Three-body Interactions in Polymer Nanocomposites

We use the modified iSAFT density functional theory (DFT) to calculate interactions among nanoparticles immersed in a polymer melt. Because a polymer can simultaneously interact with more than two nanoparticles, three-body interactions are important in this system. We treat the nanoparticles as spherical surfaces, and solve for the polymer densities around the nanoparticles in three dimensions. The polymer is modeled as a freely-jointed chain of spherical sites, and all interactions are repulsive. The potential of mean force (PMF) between two nanoparticles displays a minimum at contact due to the depletion effect. The PMF calculated from the DFT agrees nearly quantitatively with that calculated from self-consistent PRISM theory. From the DFT we find that the three-body free energy is significantly different in magnitude than the effective three-body free energy derived from the two-particle PMF. [This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000).]   

Conformational Study of Rigid Polymers Grafted on Silica Nanoparticle

Atomistic molecular dynamics simulations have been used to study the structure and conformation of dialkyl poly \textit{para} phenyleneethynylenes (PPEs), an electro-optically active polymer, confined to silica nanoparticles (NP), with the goal to define the factors that control the assembly of polymer-nanoparticle complexes. The conformation of PPEs determines the conjugation length and their assembly mode which in turn affects the electro-optical properties of the NP-polymer complexes. The current work investigates the structure of diethylhexyl PPE confined to silica nanoparticles as a function of solvent quality. In comparison with grafted flexible hydrocarbon chains, the PPE backbones remain stretched out away from the surface of the NP. The nature of solvents affects the distribution of the chains around the NP. In good solvent, the radial distribution of the polymer is isotropic whereas in a poor solvent distinct clustering is observed. Further studies are on their way to investigate the effects of molecular parameters including the length of the polymer and nature of the side chains coupled with molecular interactions, on the structure and the conformation of the confined PPE and extends the single particle study to an ensemble of NPs.   

Phase Behavior of Thin Film Polystyrene(PS)-Coated Nanoparticles/PS Mixtures

We show that the phase behavior of supported thin film mixtures of polystyrene (PS) brush-coated spherical nanoparticle and PS homopolymers is characterized by three regimes, depending on P, the degree of polymerization of the PS host, and N, the degree of polymerization of the grafted chains. Phase separation between the nanoparticles and the host chains occurs in samples for which N $<$ N$^{\ast }$ and P$>>$N. Specifically, the nanoparticles segregate exclusively at the substrate and free surface in these samples, forming a trilayered structure. When P$>>$N and N$>$ N*, preferential segregation of the grafted nanoparticles to the interfaces is accompanied by a structural instability (surface roughening). We identify this as Regime I and the former as Regime II. The system is miscible in Regime III (P $<$ N and N $>$ N*); the nanoparticles are dispersed throughout the film. The characteristics of Regime I are reminiscent of phase separation in polymer/polymer thin film mixtures, whereas Regime II is reminiscent of the interfacial segregation of hard spheres in an athermal melt of polymer chains.   

Competing Phase Separation and Dewetting in Nanofilled Polymer Blend Films

The simultaneous phase separation and dewetting of polymer blend films provides for an interesting interplay between the two thermodynamic transitions. Previously we have shown that polystyrene (PS) and polybutadiene (PB) polymer film dewetting can be suppressed by the addition of nanoparticles that segregate to the polymer substrate interface. In the current work, we report the effects of fullerene (C$_{60})$ nanoparticles on PS/PB blend thin film morphology in both the single phase and two phase regions of phase separation. The incorporation of the nanoparticles leads to distinct effects in the different regimes of phase stability. In the blend one phase region we see film dewetting and contact line pinning of the growing holes in a fashion similar to former homopolymer observations; whereas, in the phase separated regime we see complex patterns that apparently reflect the competitive segregation of the nanoparticles to the polymer-polymer and polymer-substrate interfaces.   

NMR Studies of Polymer-Nanoparticle Interfaces in Biological and Synthetic Nanocomposites

Nuclear magnetic resonance (NMR) provides unique capabilities for studying buried interfaces in organic-inorganic (specifically phosphate-based) nanocomposites, in terms of local composition as well as distances between, and mobility of, structural units near the interface. The organic-inorganic interface is crucial for the mechanical coupling between the polymer and the inorganic nanoparticles. We have studied the organic-inorganic nanocomposite in bone and characterized the interface between the organic matrix (the triple-helical fibrous polypeptide collagen) and the inorganic, reinforcing bioapatite (a calcium phosphate) that accounts for 45 vol{\%} of the material and is present as $\sim $3-nm thick nanocrystals. By solid-state $^{13}$C{\{}$^{31}$P{\}} NMR, we can obtain selective spectra of the collagen residues at the interface; ionic and hydroxyproline C-OH groups of significant mobility are dominant. $^{1}$H-$^{31}$P and $^{1}$H-$^{13}$C NMR prove that water with isotropic mobility, which accounts for about 7{\%} of the total volume, forms a monomolecular interfacial layer between apatite and collagen. Its rotational correlation time is about five orders of magnitude longer than that of liquid water. We propose that this water layer can be considered as ``viscous glue'' that holds the components of the nanocomposite together. It would avoid stress concentration and, by virtue of its flexible H-bonding, reduce the requirement of matched lock-and-key binding sites for collagen sidegroups on the apatite surface. In nanocomposites of phosphate glass with polyamides, $^{1}$H-$^{13}$C NMR reveals a reduced crystallinity of the polyamide near the inorganic particles.\\[4pt] Coauthors: Yan-Yan Hu, Aditya Rawal (Ames Laboratory), Joshua Otaigbe (University of Southern Mississippi)   

Dynamics of the Polyisoprene Matrix with the Addition of Polystyrene-Grafted Gold Nanoparticles

The relaxation dynamics of polyisoprene (PI) chains in mixtures of PI, of varying molecular weights, with small quantities (\textit{$\varphi $} $\le $ 0.5wt{\%}) of polystyrene (PS)-grafted gold nanoparticles is studied using Dielectric Spectroscopy. The normal and segmental dynamics of unentangled PI chains decreased with the addition of the nanoparticles, whereas the dynamics of entangled PI chains remained virtually unaffected. Moreover, we show that with the addition of the NPs, a new relaxation peak, not reported before, appears near the frequency range of the $\beta $-relaxation, below 200 K; the $\beta $-relaxation remains invariant. The intensity of this new peak, which increases with increasing NP concentration, exhibits a very weak dependence on the temperature. Additionally, the peak shifts approximately one order of magnitude, increasing dynamics, as the molecular weight of PI increases from 10k to 138k. This phenomenon, together with the reported behavior of $\beta $-relaxation, reflects the influence of the nano-scale structure of the dynamics of the system.   

Polymer diffusion in silica nanoparticle / polymer nanocomposites

Nanoparticles (NPs) added to polymers can enhance mechanical, electrical and thermal properties. Moreover, the polymer processing conditions (e.g., viscosity) can be changed by the addition of NPs because molecular relaxation is perturbed by the filler. Using elastic recoil detection, the tracer diffusion coefficient D* of deuterated polystyrene (dPS) in polystyrene (PS): silica NP nanocomposites was measured for NP loadings up to 50 vol{\%}. TEM studies show that the phenyl-capped silica NPs are well dispersed in the PS matrix. For low molecular weight dPS ($\sim $200k), D* decreases very slightly as NP loading increases. However, for high molecular weight dPS ($\sim $2M), D* decreases more strongly as NP concentration increases up to 30 vol{\%} and then slightly increases at higher concentrations. The tracer diffusion studies are compared with DMA results as well as tracer diffusion in carbon nanotube nanocomposites that show a minimum in D* near the percolation threshold followed by an increase in D*.   

Effective diffusion rate through a random polymer network in tension

Random networks of elastic material, such as polymer gels and cytoskeletal structures, are frequently found in synthetic and biological materials. Diffusion of nanoparticles through these networks and their mechanical properties has been extensively investigated over the past years. However, little attention has been paid to the diffusion rate through networks subject to mechanical deformation. Here, using two mesoscopic simulation methods, we examine the effective diffusion of nanoscopic particles through a random network of elastic material in tension. The mesoscopic methods used are as follows: 1) Bond-bending lattice spring model (LSM) that consists of lattice sites connected by one-dimensional harmonic spring and captures the deformation of a random elastic network. 2) Dissipative particle dynamics (DPD) that explicitly resolves the hydrodynamic interactions between the network and particles.