Session T44: Focus Session: Interparticle Interactions in Polymer Nanocomposites - Grafted Chains

Colloidal assembly on soft substrates

We studied -- by Monte Carlo computer simulations - ordering of hard sphere colloidal particles subject to gravity and soft interactions induced by polymer-coated substrates. The polymers are randomly anchored to a flat surface with an average density ranging from very dilute values to a situation where a dense polymer brush with the average height h forms. Studying a single colloid subject to such substrate and the gravity we observed a transition from a regime where it is confined to the hard surface at z=0, via a bimodal regime where it is equally likely to find it at the bottom or at the top, to the regime where it lies on top of the brush at z=h. In a system of many colloids the polymers induce an effective colloid-colloid interaction of entropic origin. We have performed Grand canonical Monte Carlo simulations and explored the structure formation as a function of the anchoring density and the effective gravity of the colloids. Colloids are initially attracted and grow into elongated assemblies with a well-defined lateral width. At low grafting densities such assemblies form percolated networks, while at high enough grafting densities finite clusters are observed. We discuss the relevance of our results to applications like particle sorting, reactions catalysis and passive diffusion.   

Molecular Dynamics Simulations on the Thermal and Mechanical Properties of Blend of Polymer and Polymer Grafted Nanoparticles

Grafting polymers onto the surface of NPs has become one of the most effective approaches to integrate NPs into polymer melts. It then becomes crucial to be able to understand how the presence of grafted chains affects the effective interactions between NPs as well as the mechanical properties of the resulting composites. Using molecular dynamics simulations we first measure the potential of mean force between grafted NPs from two-particle simulations. Simulations of systems containing many grafted NPs are then performed to determine the phase behavior and structure of grafted NPs in explicit polymer matrices. Finally, we cool the nanocomposites to temperatures below their glass transition and stress the systems to investigate how the presence of grafted NPs changes their mechanical properties.   

Self Assembly of Tethered Nanoparticle Telechelics

Simulations, theory, and experiement predict that aggregating nanoparticles functionalized with polymer tethers can self-assemble to form phases seen in block copolymer and surfactant systems, but with additional nanoparticle ordering and mesophase complexity. Here we consider a novel class of ``telechelic'' tethered nanoparticle building blocks, where two nanoparticles are connected together by a polymer tether. The architecture is similar to a triblock copolymer, but with additional geometric constraints imposed by the rigid particle end groups. Using Brownian dynamics simulations, we explore the phase diagrams of several examples of this class of nano-building-block, and present predictions of novel phases and their dependence on particle size, tether length, and thermodynamic parameters. We compare our results with recent simulations of di-tethered nanospheres [1, 2] and mono-tethered nanospheres [2, 3]. \begin{enumerate} \item Iacovella, C. R.; Glotzer, S. C.; \textit{Soft Matter} \textbf{2009}, 5, 4492-4498. \item Iacovella, C. R.; Keys, A.S..; Glotzer, S. C. \textit{PNAS}, in press. arXiv:1102.5589. \item Phillips, C. L.; Iacovella, C. R.; Glotzer, S. C.; \textit{Soft Matter}\textbf{ 2010}, 6, 1693-1703. \end{enumerate}   

Controlling the Phase Behavior of Gold Nanoparticles within Polymer Matrix by Varying Composition and Length of Ligands

Nanocomposites have been investigated for many years due to their facsinating features. In spite of the captivative properties, compatibility problem has been an obstacle for manufacturing the nanocomposites. To achieve the compatibility between NPs and polymer matrix, thiol-terminated polymeric ligands have been used for tuning the surface property of the NPs. However, in case of Au NPs, Au-thiol bond is unstable above 60 $^{\circ}$C. In our recent work, we designed thermally stable Au NPs by using thiol terminated photo crosslinkable block copolymer, PS-b-PSN3-SH. With the thermally stable Au NPs, we demonstrated that Au NPs are stable at high temperature and can serve as compatibilizers for PS/PMMA blends. Herein, we prepared Au NPs using photo crosslinkable polymeric ligands which have various ligand composition and lengths. As the ligands characteristics were changed, the phase behaviors of Au NPs in both homopolymer and block copolymer bulk sample were significantly different. For example, the NPs become more dispersed within the polymer matrix with longer polymer ligands, while they are aggregates when shorter ligands were used. In this work, morphologies of Au NPs within polymer matrix were systematically investigated using the cross-sectional transmission electron microscopy.   

Molecular Weight Distribution Effects on the Structure of Strongly Adsorbed Polymers by Monte Carlo Simulation

Monte Carlo simulations are used to investigate the adsorption of polymers from solution onto strongly attractive, perfectly smooth substrates. Using a coarse-grained united atom model for freely rotating polymer chains, three systems with different polydispersities are studied. The structure of the adsorbed layers, exemplified by density profiles, bond orientation order parameters, radii of gyration, and distribution of the adsorbed chain fractions, is shown to be highly dependent on the molecular weight distribution of the polymer phase. The results for the more monodisperse polymer systems are qualitatively similar to experimental and theoretical investigations, but devolve from very different chain conformations and statistics. For the first time ever, equilibrium polymer adsorption on highly attractive surface is studied, with all molecules in the adsorbed layers demonstrated to be indistinguishable from each other. The ergodicity of states explored by the polymer chains is in contrast to the kinetically constrained viewpoint of irreversible adsorption, and the observed behavior is explained in the context of the competition between polymers to make contact with the surface.   

Role of Stretchable Arms, Grafting Density and Parallel Reformable Bonds in the Self-Healing of Cross-Linked Star Nanogel Particles

We investigate the role of stretchable and reformable bonds in the self-healing of a network formed by star-like nanogel particles. The individual particles of the network are composed of a cross-linked gel core and a corona of grafted polymeric arms with sticky end groups. The sticky groups in the coronas of adjacent particles interact to form multiple labile bonds (up to N) that lead to the formation of the nanogel network. Interaction between soft colloids with polymeric arms is combined with the Bell model for rupture and formation of bonds to model the interaction of array of particles. While the stretch of the bonds is captured through the bond spring constant (k) and cutoff radius for bond breaking (rc), the equilibrium distance (req) at which the labile bonds reform is obtained from the corona thickness. We show that the presence of stretchable arms allows for rearrangements leading to either increase or decrease of the strength and ductility of the nanogel network depending on the grafting density. We also show that while the force required to rupture the nanogel network is proportional to the number of parallel bonds (N), the ductility is a more complex function of N.   

Poly(ethylene-oxide)/clay/silica nanocomposites: Morphology and thermomechanical properties

Poly(ethylene oxide) PEO/clay/silica nanocomposites were prepared via solution intercalation by exploiting phase separation based on the bridging of particles by polymer chains. The intercalated morphology of nanocomposites was confirmed by XRD. Vibrational modes of the ether oxygen of PEO in the hybrids are shifted due to the coordination of the ether oxygen with the sodium cations of clay and the H-bonding interactions of the ether oxygen with the surface silanols of hydrophilic fumed silica. Based on SEM, the overall density of nanoparticle aggregates in the interspherulitic region was observed to be higher compared to that inside spherulites. PEO/clay/silica hybrids show significant property improvements compared to PEO/clay hybrids and pure PEO. The system containing 10 wt.{\%} clay and 5 wt.{\%} silica has substantially higher modulus and much lower crystallinity compared to the 15 wt.{\%} clay system. The physics behind the reinforcement effect and the reduction of crystallinity as a function of fumed silica loading is discussed based on the morphological characterization of the hybrids. Lastly, PEO/clay/silica hybrids display good thermal stability and are much stiffer compared to pure PEO and PEO/clay nanocomposites.   

Copolymer mediated interactions in dense nanocomposites: role of chemical heterogeneity and sequence

Microscopic PRISM integral equation theory is applied to study the structure and dispersion of nanospheres in AB copolymer melts as a function of architecture and chemical heterogeneity of the interfacial attractions spanning the depletion, stabilization and bridging regimes. For a random copolymer (RCP) with coexisting weak and strong monomer-particle attractions, the nanoparticle potential-of-mean-force (PMF) at intermediate copolymer compositions varies non-monotonically between the two homopolymer limits. Non-adsorbing monomers are generically found to be crucial for achieving good filler dispersion in RCP melts. For 50/50 multiblock melts the role of block length, R, has been studied over a wide range (R=1,2,5,10,25,50). Filler miscibility dramatically increases when one of the monomers is weakly adsorbing (depletion). Dispersion is also improved for some block lengths when one of the monomers strongly adsorbs (bridging) due to the emergence of a long-ranged repulsive barrier in the PMF just beyond a local non-contact minimum, resulting in a positive second virial coefficient. Strikingly, better stability is predicted as the particle-to-monomer diameter ratio increases from 2 to 10, contrary to the behavior in homopolymer or RCP melts.   

Effect of bidispersity in grafted chain length on potential of mean force between polymer grafted nanoparticles in a homopolymer matrix

In polymer nanocomposites consisting of polymer grafted nanoparticles in a polymer matrix the molecular weight of the grafted polymers plays a key role in dictating the effective inter-particle interactions. Despite the importance of graft molecular weight on effective inter-particle interactions in monodisperse polymer grafted nanoparticles, and evidence of non-trivial polydispersity effects in systems containing polymers grafted on flat surfaces, not much work has been done to explore polydipsersity effects in polymer grafted nanoparticles. In this talk we will present self-consistent PRISM theory-Monte Carlo simulation studies showing how bidispersity in grafted chain lengths affects the grafted chain conformations and inter-particle interactions in a dense homopolymer polymer matrix. The value of the potential of mean force (PMF) between bidisperse grafted particles at contact is governed by the short grafts and values at large inter-particle distances are governed by the longer grafts. Our results suggest that by introducing bidispersity in grafted chains one can change the rules seen in monodisperse polymer grafted particles for wetting/dewetting of grafted polymers by matrix polymer.   

Concentration, Interaction and Temperature Dependent Chain Dynamics in Polybutadiene / Clay Nanocomposites Characterized by Solid State 1H Double Quantum NMR Spectroscopy

A concentration, interaction and temperature dependent multimode segmental chain dynamics of carboxyly terminated polybutadiene (CTPB) in CTPB/organo-clay (C18-clay) nanocomposite was investigated by solid state 1H double quantum (DQ) NMR spectroscopy. Here, the measurements were performed on a series of samples of CTPB physically attached to the surface of C18-clay. NMR results showed that the nanocomposite exhibit discrete dynamic component with stepwise increase in their motional freedom with decreasing clay content. A remarkable change of CTPB chain dynamics at organo-clay concentration of 40 wt{\%} was found, indicating a saturation effect of the polymer adsorbed on the clay surface. The transverse magnetization relaxation experiment help us quantitatively analysis the rigid, intermediate, and mobile components. Removal of either the end-group of CTPB or of the modifier on the clay changed the polymer-clay interaction profoundly, and thus changed polymer chain dynamics, especially for chains in proximity to the clay surface. The measurement confirmed our earlier hypothesis that the strong polymer-clay interaction was the key to the synergy effect in polymer/clay nanocomposites.   

Structural Transitions of Polymer Grafted Magnetic Nanoparticles

We decorate iron oxide nanoparticles with polymers to investigate the various interactions between particles and particle-polymer on the formation of different morphologies. We show that very low to intermediate grafting densities (2-20 chains/particle) can be achieved by controlling the concentration of free polymer chains in solution by grafting-to method. Nanostructural transitions of poly(styrene) grafted Fe3O4 nanoparticles are investigated upon varying the grafting densities and brush lengths. The roles of these parameters as well as dipolar force in the formation of various aggregates, such as star-like shape and chains, are discussed. This morphological transition is found to be sensitive to small grafting density change in very low to intermediate regime (0.01-0.04 chains/nm2). Interestingly, the matrix shows reverse effects on nanostructures with increment of brush lengths.