Session A33: Polymer Nanocomposites: Dynamics

Nanoparticle Structure and Dynamics in Polymer Nanocomposites

The physical properties of polymers can be significantly altered both by embedding nanoparticles within them and by grafting them to nanoparticle surfaces. The ability to predict and measure the structure, dynamics, and thermodynamics of grafted polymers is central to purposefully creating new nanocomposite materials. In particular, one powerful technique for studying these aspects of nanocomposites is neutron scattering. This talk will review recent work we have performed to study the behavior of nanoparticles in nanocomposite materials, with a focus on nanorods and nanospheres. Small-angle neutron scattering measurements of poly(ethylene oxide)-grafted Au nanorods, poly(methyl acrylate)-grafted SiO₂ nanospheres, and poly(methyl methacrylate)-grafted Fe₃O₄ nanosheres will be discussed, along with approaches to interpret the scattering results. The conformation of the grafted polymers will be compared between solution and nanocomposite states. Neutron spin echo measurements of the relaxation dynamics of grafted chain will be discussed, along with opportunities for future research and measurements.   

Hybrid nanoparticles with continuously tunable scattering length density for the analysis of phase separation in mixed colloidal systems

The modification of nanoparticle surfaces with polymeric chains has emerged as effective tool to control the interactions and assembly behavior of colloidal systems. Recent results have shown that interactions between chemically distinct polymeric ligands can drive the phase separation of mixed particle systems. This provides opportunities for the fabrication of microstructured hybrid materials that derive functionality from the organization of nanoparticle constituents into microscopic domains.
We will present recent results on the phase separation behavior of polymer tethered particles in thin films. In the thin film state, phase separation resembles the corresponding linear polymer blends provided the length of polymer ligands exceeds a threshold value. However, domain growth kinetics differs from the prediction for diffusion-controlled systems. To elucidate the origin of this deviatory behavior, small angle neutron scattering is performed in the bulk state. A method enabling the continuous variation of the scattering length density of nanoparticles will be presented and its application to the SANS analysis of phase separation in mixed brush particle systems will be demonstrated.   

Fast solvent induced switchable phase-states of binary polymer-grafted nanoparticle blends

Polymer-grafted nanoparticles (PGNPs) have potential applications in nanoelectronics, photonic devices, and as tough materials with tunable enhanced mechanical properties. However, these applications require the particles to form well-controlled structures. We hypothesize that these can be achieved through liquid-enabled phase-separation as a facile approach. To this end, we developed a method to switch the state of phase-separated structures in a binary blend film of PGNPs, poly (methyl methacrylate) silica (PMMA-SiO₂) and poly(styrene) silica (PS-SiO₂), by using a direct solvent immersion annealing (DIA) method. Our results show that by varying the solvents in the DIA solution, interchangeable phase-separated and homogeneous morphologies are formed in the PMMA-SiO₂/PS-SiO₂ blends within 1 minute. Such homopolymer matrix free PGNP only blend systems are novel, and these switchable transitions are not readily obtainable by thermal annealing due to the large masses and the athermal property of PGNPs involved.   

PEO / SiO2 nanocomposites: Correlating Polymer Morphology to Rheological Properties

Polymer nanocomposites, comprised of a polymer matrix and inorganic additives, possess improved and often innovative physicochemical properties compared to conventionally filled systems. In this work we report on the rheological behavior of a series of poly(ethylene oxide) / silica, PEO/SiO₂, nanocomposites through oscillatory shear rheology measurements. The nanohybrids were synthesized by dispersing spherical SiO₂ nanoparticles of two different radii within high molecular weight (Mw) PEO at different compositions in order to investigate the effect of the additive on the material rheological properties. PEO crystallinity was found to depend on the degree of spatial confinement that the nanoparticles impose as well as their adsorption capacity. Dynamic time and strain sweep tests verify the material thermal stability and linear viscoelastic behavior whereas dynamic frequency sweeps probe its dynamic response. The effect of nanoparticle size and concentration on the behavior is examined to correlate the morphological changes to the rheological response of the materials in an attempt to better understand the structure-properties relationship. Acknowledgements: This research has been co-financed by EU and Greek national funds (Action RESEARCH – CREATE - INNOVATE, MIS: 5030174).   

Decoupling the polymer dynamics and the nanoparticle network dynamics of polymer nanocomposites through dielectric spectroscopy and rheology

The dynamics of polymer nanocomposites (PNCs) are dictated by two intertwining components, the polymer matrix and the nanoparticle network, whose characteristics have not been clearly elucidated. Here, we unravel the salient features of the polymer matrix dynamics and the nanoparticle network dynamics through dielectric spectroscopy and rheology. Dielectric measurements show that the dynamics of the polymer matrix of PNCs are almost identical to the neat polymer. In contrast, rheological measurements exhibit a strong deviation in the dynamics of PNCs from that of the neat polymer. Detailed analyses show that the rheology captures both the contributions of the polymer matrix and the nanoparticle network while dielectric measurements are only sensitive to polymer matrix dynamics. Moreover, the dynamics of the polymer matrix and the nanoparticle network have very different temperature dependences, leading to a dynamic decoupling phenomenon and the breakdown of the time-temperature superposition principle in PNCs.   

Segmental dynamics in matrix-free polymer grafted nanoparticles

Polymer grafted nanoparticle based composite materials display a rich spectrum of complex chain and nanoparticle dynamics. Specifically, composite materials constructed using only polymer grafted nanoparticles display several dynamic anomalies–spanning a large range of time and length scales. We focus on the segmental and local dynamics of polymer chains using spatial and temporally sensitive probes with the view of identifying the primary dynamic driving force for penetrant transport in polymers. The composite materials used in the study exhibit enhanced light gas diffusivities highly dependent on the graft chain length and the grafting density, providing a chemically homogenous platform for identification of the relevant dynamics. We find that the segmental motions are highly accelerated in the composites; these speeded up dynamics appear to correlate with the diffusivities observed for these materials, in contrast to localized side group dynamics which are less affected by grafting and do not seem to affect transport. In fact, side group motion is invariant in the presence of high pressures of CO₂ further supporting this assertion.

  

Modeling the Entanglement Distribution in Polymer-grafted Nanoparticle Systems

Polymer nanocomposites have become increasingly useful materials due in part to their ability to provide improved strength and mechanical reinforcement over pure polymer systems. Difficulty controlling nanoparticle dispersion in these nanocomposites has lead researchers to use polymer grafted nanoparticles embedded in matrices of free polymer chains. We believe the nature of the improved mechanical properties in grafted systems is the result of graft chains on different filler particles forming an entanglement network. A computational modeling technique was developed that utilizes slipsprings to model the mobility constraints that physical entanglements impose on individual polymer chains. Focusing on the distribution of entanglements, we found that graft chains appear to be more highly entangled than free chains, irrespective of filler particle loading. The amount of inter-particle graft interactions is also always higher than we would expect if the entanglement pairs were randomly distributed based solely on their probability of occurrence.   

Disentangling the role of chain conformation on the mechanics of polymer grafted nanoparticle materials

The linear elastic properties of isotropic materials of polymer tethered nanoparticles (NPs) are evaluated using noncontact Brillouin light spectroscopy. While the mechanical properties of dense brush materials follow predicted trends with NP composition, a surprising increase in elastic moduli is observed in the case of sparsely grafted particle systems at approximately equal NP filling ratio. Complementary molecular dynamics simulations reveal that the stiffening is caused by the coil-like conformations of the grafted chains, which lead to stronger polymer–polymer interactions compared to densely grafted NPs with short chains. Our results point to novel opportunities to enhance the physical properties of composite materials by the strategic design of the “molecular architecture” of constituents to benefit from synergistic effects relating to the organization of the polymer component.   

Suppression of Creep in Model Polymer Nanocomposites

While the elastic properties of polymer nanocomposites (PNCs) have been widely studied, the ability of nanoparticles (NPs) to suppress creep in a polymer matrix has received comparatively less attention, and creep suppression is essential for the use of composites in structural applications. It is believed that the primary mechanism of reinforcement in PNCs is the presence of a layer near the NPs' surfaces where the polymer monomers have modified mobility. Thus, understanding how the dynamics in this layer changes as a function of stress, NP size, and polymer-nanoparticle interaction is critical. In this work, we use molecular dynamics simulation to investigate the PNC's creep response with different NP sizes and polymer-nanoparticle interactions. Our results indicate that the small NPs with strong polymer-nanoparticle interaction can best suppress materials' creep response and stiffen the material. A recently developed, machine-learning field called softness is applied to describe the local structures in our composites. We find that the softness is modified near the NP surfaces and largely reduced for attractive polymer-particle interactions, though the range over which softness is modified differs from the range over which the relaxation time is different from the bulk.   

Nanoparticles with controllable dispersion and localization in immiscible polymer blends

Polymer blending is a versatile route to the development of new polymeric materials with enhanced properties and hence applications. However, the unfavorable interaction between most polymer pairs leads to phase-separated systems with unstable morphologies, weak interfaces and poor properties. The use of nanoparticles (NPs) as interfacial stabilizers has gained momentum recently due to their high surface area and strong adsorption at interfaces, but controlling their dispersion and localization in immiscible polymer blends is a major challenge. Recent work has demonstrated that polymer-grafted NPs, which exhibit surfactant-like properties, can self-assemble into a variety of superstructures that depend on the polymer grafting density (σ) and the graft chain length (N). Here, we will show that the location of polystyrene-grafted NPs within an immiscible blend of polymethyl methacrylate (PMMA)/polystyrene (PS) can be determined using σN^0.5, the brush crowding parameter, and, 1/α, which describes the entropic effects associated with a mismatch in brush/matrix polymer chain lengths. Using these parameters, the spatial control over NP segregation in immiscible polymer blends is probed and understood.   

Polymer/Star-Polymer composites: structure and dynamics of bulk and confined materials

Polymer-grafted nanoparticles (NP) are versatile building blocks to create tunable particle superstructures and polymer nanocomposites with customizable properties. Polymer grafted NP are topologically very similar to star polymers. While nanocomposites with polymer-grafted NP have been explored recently, comparatively little is known about composites consisting of star polymers and chain polymers, or star polymers in polymer thin films. We investigate how both the number of arms and molecular weight affect the morphology of self-assembled structures in both a bulk polymer material and an ultra-thin polymer film. Additionally, we study how star polymers affect the glass transition of these composites and films. In doing so, we also examine how star polymers affect the nature of cooperative molecular motions in the matrix and film, and how this potentially relates to changes in the mechanical and rheological properties of the material.   

Tuning structure and dynamics of segmented ionenes with added spherical nanoparticles

Segmented ionenes can be synthesized such that short ion-containing segments alternate with uncharged segments. By changing the ratio of relatively soft and hard uncharged segment types, material properties can be easily tuned. Incorporating nanoparticles may allow for further manipulation of structure and mechanical behavior. However, the multiple length/time scales involved, the large parameter space, and the nanoparticles’ tendency to aggregate complicates material design. Using efficient coarse-grained molecular dynamics simulations, we aim to establish architecture-structure-property relationships to guide synthetic efforts.
Inspired by experimental ammonium ionenes with short polyethylene segments and long, soft segments (such as polyethylene oxide), we study segmented ionenes and spherical nanoparticles using a simple bead-spring model. Soft segment length, the ratio of the two polymer types, and nanoparticle surface chemistry is tuned and structure and dynamics analyzed. We find weight fraction dependent local microphase segregation of soft and hard polymer segments. We also determine the nanoparticle's impact on the local structure as a function of weight fraction and chemistry.   

Molecular Dynamics Study of Structural and Flow properties of Polyelectrolyte-Grafted Nanoparticles in Solution

Polyelectrolytes have found applications in fuel cells, solar cells, membranes, etc. Here, we investigate the structural and ow properties of polyelectrolyte grafted nanoparticles (PENP) in a solution using coarse-grained molecular dynamics simulations. The degree of ionization (pH) and the concentration of PENP in solution are systematically varied. For low pH, the ow properties are dominated by grafted chain entanglements, while at high pH, the strong electrostatic repulsions between the ionized groups are the major factor. At intermediate pH, the hydrogen bonding between the ionized and non-ionized groups along with the concentration of PENP plays a significant role in dictating the solution viscosity. At low PENP concentrations, intra-particle hydrogen bonds are formed that lowers the viscosity, while at higher concentrations, inter-particle hydrogen bonds are formed enhancing the viscosity of the solution.