We compare the effect of confinement on the glass transition temperature (Tg) of ultrathin supported films of polystyrene produced via emulsion polymerization (E-PS) and anionic polymerization ...(A-PS). Using spectroscopic ellipsometry and fluorescence to characterize Tg, we find that E-PS films supported on silica exhibit Tg-confinement effects that are suppressed and even eliminated when compared to A-PS films on silica. We prepared E-PS samples with varying dodecyl sulfate surfactant concentration made via sequential purification. Using Epton’s Method, we determined the amount of surfactant present in bulk unwashed E-PS sample and in several washed samples. Films made from E-PS containing as little as 0.023 wt% surfactant show elimination of the Tg-confinement effect within error down to a thickness of 14 nm. Films made from E-PS containing 0.0058 wt% surfactant show suppression of the Tg-confinement effect relative to that observed by both ellipsometry and fluorescence in A-PS films. Using X-ray photoelectron spectroscopy, we observe that the sulfur atom in the surfactant is localized at the top 2–4 nm of the film next to the polymer-air interface. The residual surfactant left from emulsion polymerization at or very near the free surface restricts the perturbation of Tg by the free surface, thereby suppressing and even eliminating the Tg-confinement effect in ultrathin E-PS films relative to that in A-PS films.
Display omitted
•We synthesized polystyrene by emulsion polymerization.•Dodecyl sulfate in polystyrene quantified by Epton's Method.•Prepared samples with residual surfactant ranging from 0.0031 to 3.5 wt%.•0.023–3.5 wt% surfactant can eliminate Tg-confinement effect in PS films.•XPS demonstrated that surfactant is located at free surface of supported film.
Stability of food is a great challenge that encompasses the interaction among the constituents, processing conditions and thermal history. The frozen storage of food sometimes incurs possible harmful ...effect due to the formation of large ice crystal and destruction of the cell structure. Glass transition state is a second-order transition of matter where a system reaches a thermodynamically non-equilibrium state due to the immobility of molecules, and it is a universal phenomenon observed when liquid goes to supercooled vitreous state because of extensive cooling or change in the composition. The cryostabilisation or storage at glassy conditions has been studied widely as it can prevent the quality degradation due to freezing.
The review provides an overview of the theories and assumptions related to the concept of the glass transition and the response of different food components such as moisture, carbohydrate, protein, and lipid at the glassy state. Influences of processing conditions including moisture removal, freezing rate, annealing time on the relaxation process or the glass transition are also elaborated. In addition, the effects of novel freezing techniques such as ultrasound assisted, high pressure assisted, electric and magnetic field assisted freezing are also discussed in the current review.
The glass transition is highly dependent on the presence of moisture and carbohydrate molecules for its great affinity to make hydrogen bond and increase viscosity. The fat and protein glass transitions take place at very low temperatures, at which commercial frozen storage is not considered feasible. Conventional freeze drying and dehydrofreezing require removal of water, which increases the glass transition temperature. Storage at or below the glass transition temperature is desired to increase stability and prevent any quality deterioration. Novel freezing processes such as high pressure, ultrasound, electric and magnetic assisted freezing incur changes in microstructure and metastable glassy states. The current review provides valuable information for designing products with optimized processing techniques and conditions.
•Glass transitions for various food components are discussed.•Effects of novel freezing technologies on glass transition are elaborated.•Glass transitions have high impacts on stability and other food quality parameters.•Glass transitions are highly dependent on constituents and freezing conditions.
The paper presents a study about the glass transition of commercially available epoxy resins used for structural strengthening of concrete members for instance by means of Carbon Fiber Reinforced ...Polymer (CFRP) strips. Prior to an experimental investigation with a dynamic mechanical analysis (DMA), an overview on differences between definitions for the glass transition temperature Tg is given. Several testing recommendations are listed in this respect. Subsequently, DMA tests on three commercially available products are presented. A first focus is put on the different evaluation methods for one specific test result. It is visible that considerable differences in the finally adapted glass transition temperature might arise if one or the other procedure is followed. Additional parameters, such as curing procedure, specimen age, temperature history, and ultimate temperature during heating are considered, too. In all the above mentioned cases, differences in the glass transition can be found. Higher specimen age, higher ultimate temperature during testing, accelerated curing, as well as a lower heating rate implicate higher glass transition temperatures, showing that the glass transition temperature is not a fixed material characteristic. In a final step, the relevance for Tg for civil engineering applications is described. The various design code provisions for defining the service temperature in structures related to Tg are presented. The overall aim of the investigation is to show that structural engineers and end users have to be aware of the different influential parameters on the final results regarding the glass transition temperature, which also highlights the need of a potential deeper product investigation in case technical data sheets lack detailed information.
Molecular dynamics and molecular mechanics simulations are used to establish well-equilibrated, validated molecular models of the EPON 862-DETDA epoxy system with a range of crosslink densities using ...a united atom force field. Molecular dynamics simulations are subsequently used to predict the glass transition temperature, thermal expansion coefficients, and elastic properties of each of the crosslinked systems. The results indicate that glass transition temperature and elastic properties increase with increasing levels of crosslink density and the thermal expansion coefficient decreases with crosslink density, both above and below the glass transition temperature. The results demonstrate reasonable agreement with thermomechanical properties in the literature. The results also indicate that there may be a range of crosslink densities in epoxy systems beyond which there are limited changes in thermomechanical properties.
Display omitted
We use machine-learning methods on local structure to identify flow defects-or particles susceptible to rearrangement-in jammed and glassy systems. We apply this method successfully to two very ...different systems: a two-dimensional experimental realization of a granular pillar under compression and a Lennard-Jones glass in both two and three dimensions above and below its glass transition temperature. We also identify characteristics of flow defects that differentiate them from the rest of the sample. Our results show it is possible to discern subtle structural features responsible for heterogeneous dynamics observed across a broad range of disordered materials.
Display omitted
•Phase separation of ESBA disrupts the original dispersion of SBS in the asphalt.•The size of SBS domains in ESBAs increases in the SBS loading.•ESBA’s viscosity increases in the SBS ...loading.•The thermal stability of ESBAs enhances with the increase of SBS loading.•The neat EA’s elongation at break increases by 56% with the addition of 3 wt% SBS.
Both styrene–butadiene-styrene copolymer (SBS) and epoxy resin have been widely applied in the asphalt modification. The influence of SBS concentration on the morphology, viscosity, thermal stability, glass transition temperature (Tg), damping performance and mechanical behaviors of the neat EA binder was studied. Double phase separation occurred in the epoxy SBS modified asphalt (ESBA): main phase separation between epoxy and the SBS modified asphalt (SBA) and secondary phase separation between asphalt and SBS. The occurrence of phase separation in the ESBA disrupted the original dispersion of SBS particles in SBA and resulted in the redistribution of SBS in the form of smaller spherical particles. The inclusion of SBS increased the viscosity of the neat EA. Furthermore, the viscosity of ESBAs increased with the SBS content. ESBAs had as long as 150-min construction time for the mixture pavement. The presence of SBS improved the thermostability of the neat EA. In terms of ESBAs, the thermostability increased with the SBS content. The addition of SBS lowered the Tg of the neat EA when the SBS content was lower than 4 wt%. The Tg of ESBAs increased with the SBS content. The incorporation of SBS significantly enhanced the damping behaviors of the neat EA. The tensile strength of the neat EA was improved with the addition of 2 wt% SBS. The inclusion of SBS improved the elongation at break and the toughness of the neat EA when the SBS loading was greater than 1 wt%.
Scanning calorimetric determination of the glass transition temperatures (Tg) of sodium disilicate doped individually with 25 wt.% of the oxides of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, ...and Lu reveals the presence of the so‐called “lanthanide tetrad effect” in their glass transition temperatures. This is to the best of our knowledge the first observation of a tetrad effect in the macroscopic properties of a high‐temperature silicate phase.
The environmental and social concerns regarding environmental‐friendly materials lead to alternatives in replacing synthetic fibers for natural ones on polymeric composites. This study focused on ...modeling dynamic mechanical curves of kenaf/polyester composites using response surface methodology (RSM). Composites with three different reinforcement contents (13.5, 22.33, and 36.27 vol%) were produced and subjected to the dynamic mechanical analysis (DMTA). From the experimental DMTA curves, a 3D surface plot using RSM was done. The results showed that the fiber dynamic mechanical behavior and fiber/matrix interface had a low influence on the glass transition temperature but significantly changed the tan δ peak height. On the other hand, the kenaf fibers presented an enormous difference in the elastomeric region. The constrained region (calculated using the tan delta height) increased ~4 times for the composite reinforced with 36.27 vol% when compared to the composite reinforced with 13.5 vol%. The RSM enabled the viscoelastic modeling using different fiber volumes with high reliability and low error (R2 > 0.99). The RSM approach proved to be an intelligent and reliable technique to access a higher range of results, reducing experimental time and cost and keeping statistical significance. Also, the present methodology can be extended to model other properties and/or optimize parameters.
We combine VI photometry from OGLE-III with VISTA Variables in The Via Lactea survey and Two Micron All Sky Survey measurements of E(J -Ks) to resolve the longstanding problem of the non-standard ...optical extinction toward the Galactic bulge. We show that the extinction is well fit by the relation AI = 0.7465xE(V - I) + 1.3700xE(J - KS), or, equivalently, AI = 1.217 x E(V - I)(1 + 1.126x(E(J - KS)/E(V -I) - 0.3433)). The optical and near-IR reddening law toward the inner Galaxy approximately follows an RV approximate 2.5 extinction curve with a dispersion sigmaRV approximate 0.2, consistent with extragalactic investigations of the hosts of Type Ia SNe. Differential reddening is shown to be significant on scales as small as our mean field size of 6'. The intrinsic luminosity parameters of the Galactic bulge red clump (RC) are derived to be (MI,RC, sigmaI,RC,0, (V - I) sub(RC,0), sigma sub(()V-I)RC, (J - KS) sub(RC,0 )) = (-0.12, 0.09, 1.06, 0.121, 0.66). Our measurements of the RC brightness, brightness dispersion, and number counts allow us to estimate several Galactic bulge structural parameters. We estimate a distance to the Galactic center of 8.20 kpc. We measure an upper bound on the tilt alpha approximate 40degrees between the bulge's major axis and the Sun-Galactic center line of sight, though our brightness peaks are consistent with predictions of an N-body model oriented at alpha approximate 25degrees. The number of RC stars suggests a total stellar mass for the Galactic bulge of ~2.3 x 10 super(10) M sub(middot in circle) if one assumes a canonical Salpeter initial mass function (IMF), or ~1.6 x 10 super(10) M sub(middot in circle) if one assumes a bottom-light Zoccali IMF.
We use molecular dynamics (MD) to perform an extensive characterization of the thermo-mechanical response of a thermoset polymer composed of epoxy EPON862 and curing agent DETDA. Our simulations, ...with no adjustable parameters, show that atomistic simulation can capture non-trivial behavior of amorphous thermosets including the role of polymerization degree, thermal history, strain rate and temperature on the glass transition temperature (
T
g
) and mechanical response (including ultimate properties) and lead to predictions in quantitative agreement with experiments. We find a significant increase in
T
g
, Young’s modulus and yield stress with degree of polymerization while yield strain is significantly less sensitive to it. For structures cured beyond the gel point (percolation of a 3D network) conversion degree and temperature affect yield stress in a similar way with yield stress linearly dependent on
T−
T
g
; however, we find non-linear and non-universal relationship below the gel point. Our results show that a relative small variation in polymerization degree (∼5%) can explain the spread in experimental measurements of
T
g
and elastic constants available in the literature.
Display omitted
PDF
