Chad Daley

Comparing surface and bulk flow of a molecular glass former

In this work we measure the response of the molecular glass former 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB Tg = 347 K) to the presence of 20 nm gold nanoparticles placed on the material surface. At times ranging from a few minutes to many hundreds of minutes at temperatures below Tg − 2 K the surface evolves with no change in the apparent height of the nanoparticle. At temperatures Tg − 9 K < T < Tg, and after sufficiently long times, the nanospheres are observed to embed into the material. We employ a simple model for embedding in order to estimate a bulk material viscosity (the material properties ∼10–20 nm into the film) and obtain good agreement with previously reported values over the temperature range 338–345 K. The surface evolution that is observed prior to nanoparticle embedding has a much weaker temperature dependence than the embedding process. The surface evolution is modelled as a thin film with uniformly enhanced mobility, and alternately as surface diffusion. In the context of a decreased viscosity in the entire film, the measured time scales correspond to a viscosity value of 107–1010 Pa·s. Restricting the surface flow to a smaller layer results in correspondingly decreased viscosity values. In the context of a surface diffusion model, the timescale for surface evolution corresponds to a range of surface diffusion coefficients of Ds from 10−14 (at 318 K) to 10−11 m2/s (at 345 K). By measuring both surface and bulk dynamics we provide a quantitative measure for the enhancement of surface dynamics relative to the bulk.

Highly Enhanced Raman Scattering of Graphene using Plasmonic Nano-Structure

Highly enhanced Raman scattering of graphene on a plasmonic nano-structure platform is demonstrated. The plasmonic platform consists of silver nano-structures in a periodic array on top of a gold mirror. The gold mirror is used to move the hot spot to the top surface of the silver nano-structures, where the graphene is located. Two different nano-structures, ring and crescent, are studied. The actual Raman intensity is enhanced by a factor of 890 for the G-peak of graphene on crescents as compared to graphene on a silicon dioxide surface. The highest enhancement is observed for the G-peak as compared to the 2D-peak. The results are quantitatively well-matched with a theoretical model using an overlap integral of incident electric field intensities with the corresponding intensities of Raman signals at the G- and 2D-peaks. The interaction of light with nano-structures is simulated using finite element method (FEM).

Molecular weight dependence of near surface dynamical mechanical properties of polymers

We have made a detailed characterization of the molecular weight dependence of isothermal gold nanoparticle embedding into the surface of atactic polystyrene films. Polymers in the range 3 × 103 < Mw < 1.2 × 106 g mol−1 were used within a temperature range that encompassed the bulk glass transition temperature, around which both Mw dependence and heterogeneous dynamics were measured. The range in Mw also allows for comparison to recent studies on molecular glass surfaces – essentially covering the entire range of possible polymer sizes. The results show that there is a different manifestation of enhanced surface mobility for large and small polymer molecules. The transition between the mechanisms provides some bound on the size of a surface region with enhanced surface dynamics on glassy polystyrene surfaces.

A Brief Survey of β-Detected NMR of Implanted 8Li+ in Organic Polymers

Unlike the positive muon, we expect the chemistry of the implanted 8Li+β-NMR probe in organic polymers to be simply that of the monovalent ion, but almost nothing is known about the NMR of isolated Li+ in this context. Here, we present a brief survey of 8Li+β-NMR in a variety of insulating polymers at high magnetic field, including polyimide, PET, polycarbonate, polystyrene and polyethylene oxide. In all cases, we find a large-amplitude, broad Lorentzian resonance near the Larmor frequency, consistent with the expected diamagnetic charge state. We also find remarkably fast spin-lattice relaxation rates 1/T1. There is very little dependence of either linewidth or 1/T1 on the proton density, the main source of nuclear dipolar magnetic fields, leading us to conclude the main contribution to both broadening and spin relaxation at room temperature is quadrupolar in origin. This behaviour is very different from crystalline insulators such as MgO and Al2O3, and suggests that 8Li+β-NMR will be an important probe of polymer dynamics. Additionally, we note dramatically different behaviour of one sample above its glass transition, motivating the construction of a high temperature spectrometer to enable further exploration at elevated temperature.

Crystallization of low molecular weight atactic polystyrene

We observe and characterize the crystallization of atactic polystyrenes (PS) of nearly oligomeric Mw using atomic force microscopy. We find that the low Mw polystyrene exhibits observable crystals on the surface. The crystals appear to be a few nm thick and nm to microns wide. These crystals grow at all temperatures less than ∼290 K. Melting of crystals was probed over an extended temperature range, and some fraction of the crystals start to melt at 302 K, but some fraction persist to higher temperatures and do not exhibit complete melting until 343 K. The tacticity of the molecules is tested with NMR spectroscopy and found to be atactic. We suggest that the crystals form due simply to the distribution of isomerism along the molecule which necessarily leaves some fraction of the molecules with uniform stereoregularity. This natural crystallinity may be related to previously observed and not definitively explained gel formation in atactic PS (a-PS), as well as cluster formation. The measurements are compared with the theory by Semenov (Macromolecules, 2009, 42, 6761) and together suggest that such crystallinity is possible over a wide range of polymerization index (N), and is limited only by the vanishingly small volume fractions and sluggish growth.