ABSTRACT
We investigated the depth, temperature, and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted 8Li+ and ß-detected nuclear magnetic resonance. Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average 8Li spin-lattice relaxation time (T1 avg) was measured near the free surface and in the bulk. Spin-lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence T1 avg. There is no clear correlation of the average entropy of activation (ΔSÌ) and enthalpy of activation (ΔHÌ) with MW, but there is a clear correlation between ΔSÌ and ΔHÌ, i.e., entropy-enthalpy compensation. This results in the average Gibbs energy of activation, ΔG, being approximately independent of MW. Measurements of the temperature dependence of T1 avg as a function of depth below the free surface indicate the inherent entropic barrier, i.e., the entropy of activation corresponding to ΔHÌ = 0, has an exponential dependence on the distance from the free surface before reaching the bulk value. This results in ΔG near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a "double-exponential" depth dependence. This model can explain the depth dependence of 1/T1 avg in polystyrene films. The characteristic length of enhanced dynamics is â¼6 nm and approximately independent of MW near room temperature.
ABSTRACT
We have used ellipsometry to characterize the anisotropy in stable polymer glasses prepared by physical vapor deposition. These measurements reveal birefringence values (as measured by the magnitude of in-plane vs out-of-plane refractive index) less than 0.002 in vapor-deposited polystyrenes with N from 6 to 12 and with fictive temperatures between 10 K and 35 K below the Tg values. We have measured the thermal expansivity of these stable glasses and compared to ordinary rejuvenated glass. The thermal expansivity of the stable glasses is less than that of ordinary glass with a difference that increases as the fictive temperature Tf decreases.
ABSTRACT
Stable glasses prepared by vapour deposition are an analogue of glassy materials aged for geological timescales. The ability to prepare such materials allows the study of near-ideal glassy systems. We report the preparation and characterization of stable glasses of polymers prepared by physical vapour deposition. By controlling the substrate temperature, deposition rate and polydispersity, we prepared and characterized a variety of stable polymer glasses. These materials display the kinetic stability, low fictive temperatures and high-density characteristic of stable glasses. Extrapolation of the measured transformation times between the stable and normal glass provides estimates of the relaxation times of the equilibrium supercooled liquid at temperatures as much as 30 K below the glass transition temperature. These results demonstrate that polymer stable glasses are an exciting and powerful tool in the study of ultrastable glass and disordered materials in general.
ABSTRACT
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 â¼290 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.
ABSTRACT
To date, detailed studies of the thickness of coatings using surface plasmon resonance have been limited to samples that are very uniform in thickness, and this technique has not been applied quantitatively to samples that are inherently rough or undergo instabilities with time. Our manuscript describes a significant improvement to surface plasmon resonance imaging (SPRi) that allows this sensitive technique to be used for quantitative tracking of the thickness and roughness of surface coatings that are rough on the scale of tens of nanometers. We tested this approach by studying samples with an idealized, one-dimensional roughness: patterned channels in a thin polymer film. We find that a novel analysis of the SPRi data collected with the plane of incidence parallel to the patterned channels allows the determination of the thickness profile of the channels in the polymer film, which is in agreement with that measured using atomic force microscopy. We have further validated our approach by performing SPRi measurements perpendicular to the patterned channels, for which the measured SPR curve agrees well with the single SPR curve calculated using the average thickness determined from the thickness profile as determined using AFM. We applied this analysis technique to track the average thickness and RMS roughness of cellulose microfibrils upon exposure to cellulolytic enzymes, providing quantitative determinations of the times of action of the enzymes that are of direct interest to the cellulosic ethanol industry.