Sangah Gam

Gold nanorods dispersed in homopolymer films: optical properties controlled by self-assembly and percolation of nanorods.

In this paper, polymer nanocomposite films containing gold nanorods (AuNRs) and poly(2-vinyl pyridine) (P2VP) have been investigated for their structure-optical property relationship. Using transmission electron microscopy (TEM), the assembly of AuNRs (7.9 nm × 28.4 nm) grafted with a P2VP brush in P2VP films is examined as a function of the AuNR volume fraction Ø(AuNRs) and film thickness h. For h ∼ 40 nm, AuNRs are confined to align parallel to the film and uniformly dispersed at low Ø(AuNRs). Upon increasing Ø(AuNRs), nanorods form discrete aggregates containing mainly side-by-side arrays due to depletion-attraction forces. For Ø(AuNRs) = 2.7%, AuNRs assemble into a 2D network where the discrete aggregates are connected by end-to-end linked nanorods. As Ø(AuNRs) further increases, the polymer-rich regions of the network fill in with nanorods and rod overlap is observed. Monte Carlo simulations capture the experimentally observed morphologies. The effect of film thickness is investigated at Ø(AuNRs) = 2.7%, where thicker films (40 and 70 nm) show a dense array of percolated nanorods and thinner films (20 nm) exhibit mainly isolated nanorods. Using Rutherford backscattering spectrometry (RBS), the AuNRs are observed to segregate near the substrate during spin-casting. Optically, the longitudinal surface plasmon resonance (LSPR) peaks are correlated with the local orientation of the AuNRs, where side-by-side and end-to-end alignments induce blue and red shifts, respectively. The LSPR undergoes a red shift up to 51 nm as Ø(AuNRs) increases from 1.6 to 2.7%. These studies indicate that the optical properties of polymer nanocomposite films containing gold nanorods can be fine-tuned by changing Ø(AuNRs) and h. These results are broadly applicable and provide guidelines for dispersing other functional nanoparticles, such as quantum dots and carbon nanotubes.

Polymer diffusion in a polymer nanocomposite: effect of nanoparticle size and polydispersity

The tracer diffusion of deuterated polystyrene (dPS) is measured in a polystyrene nanocomposite containing silica nanoparticles (NPs), with number average diameters dn of 28.8 nm and 12.8 nm, using elastic recoil detection. The volume fractions of the large and small NPs (ϕNP) range from 0 to 0.5, and 0 to 0.1, respectively. At the same volume fraction of NPs, the tracer diffusion of dPS is reduced as NP size decreases because the interparticle distance between NPs (ID) decreases. The reduced diffusion coefficient, defined as the tracer diffusion coefficient in the nanocomposite relative to pure PS (D/D0), plotted against the confinement parameter, namely ID(dn) relative to tracer size, ID(dn)/2Rg, nearly collapses onto a master curve, although D/D0 is slightly greater for the more polydisperse, smaller NPs. Using a log normal distribution of NP size from SAXS, the average ID of the smaller NPs is shown to increase by 25% at ϕNP = 0.1 as polydispersity (σ) increases from 1 to 1.39. By accounting for polydispersity, the confinement parameter better represents the effect of NP spacing on polymer diffusion. These experiments demonstrate that polymer tracer diffusion in polymer nanocomposites is empirically captured by the confinement parameter and that an increase in the average ID due to NP polydispersity has a secondary effect on model NP systems with a narrow distribution of sizes. However, for commercial systems, where polydispersity can be quite large, the effect of size distribution can significantly increase ID which in turn will influence polymer dynamics.

A jamming morphology map of polymer blend nanocomposite films

The addition of nanoparticles (NPs) to polymer blends is an attractive route for controlling their morphology. Here, we investigate the phase separation of poly(methyl methacrylate) (PMMA) : poly(styrene-ran-acrylonitrile) (SAN) films with thicknesses from 140 nm to 2500 nm and silica NP concentrations from 1 to 10 wt%. Atomic force microscopy (AFM) and focused ion beam (FIB) etching combined with scanning electron microscopy (SEM) are used to identify the morphology as discrete or bicontinuous. FIB/SEM is introduced as a facile method to locate NPs at the PMMA/SAN interface and construct 3D images of the morphology of thick films. With increasing film thickness, the concentration of NPs required to stabilize a bicontinuous morphology decreases from 10 wt% to 2 wt%. A jamming map of the discrete and bicontinuous morphologies is constructed to examine the interplay between NP concentration and film thickness. The delineation between these jammed morphologies agrees with a simple geometric model based on arranging spherical NPs at the PMMA/SAN interface. The bicontinuous morphology is an especially attractive structure for applications requiring high interfacial area such as organic solar cells, membranes, catalysis, and fuel cells.

Excluded Volume Model for the Reduction of Polymer Diffusion into Nanocomposites

An analytic model for the slowing down of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chains diffuse. The reduction of polymer diffusion in each cylinder is equated with the reduction of diffusion for a sphere through a cylinder. The distribution of cylinder diameters within the ensemble is determined from statistical mechanical theories based on the packing of spherical particles. For low loadings of spherical particles in nanocomposites, this model results in a master curve for the reduced diffusion coefficient. With no adjustable parameters, the model agrees with recent data for tracer diffusion measurements in polymer nanocomposites at low loading.