Distribution Of Glass Transition Temperature Research Articles

Laponite-graphene oxide hybrid particulate filler enhances mechanical properties of cross-linked epoxy

A new hybrid of Laponite and graphene oxide (LGO), prepared in aqueous media by ultrasonication followed by solvent evaporation was used to reinforce epoxy matrix. The hybrid system was dispersed in liquid epoxy using a two-step solvent-assisted process. The suspensions showed negligible enhancements in processing barrier as revealed by rheology. A combinatorial analysis of small-angle x-ray scattering (SAXS) and microscopy suggested uniform dispersion of nanofillers in the matrix. The fillers showed fractal dimensions in polymer matrix as inferred from SAXS studies. Below 0.5 wt% LGO, the structure showed surface fractal and above 0.5 wt% the composites showed mass fractal, indicating a transformation from well-dispersed to agglomerated composites as the filler content increases. The composites exhibited substantial improvements in various mechanical properties. Notably, the flexural strength and modulus increased by ~23% and ~29%, respectively, with only 0.5 wt% LGO and the fracture toughness showed an increment of ~23% with 0.3 wt% LGO in epoxy matrix. A bimodal distribution of glass transition temperature (T g ) with improved T g was obtained for the composites. The simultaneous strengthening and toughening effects of nanofillers are explained by means of fractography.

The Distribution of Glass Transition Temperatures in Ultrathin Polymer Films Controlled by Segment Density or Interfacial Interaction

Nowadays, the microscopic mechanism controlling the distribution of local glass transition temperatures (Tgs) across thin polymer films is still unclear and thus large‐scale applications of polymer films are restricted. Dynamic Monte Carlo simulations are performed to investigate the key factors dominating the distribution of layer Tgs in two kinds of capped ultrathin films with and without attractive polymer–substrate interactions, respectively. For the film without polymer–substrate interaction, the interfacial layer Tg is lower than the middle layer Tg. Additionally, the layer Tgs and the layer segment densities below Tg are linearly correlated, indicating that polymer density determines the distribution of layer Tgs. However, for the films with polymer–substrate interactions, the interfacial layer Tg increases dramatically with the raise of interfacial interactions, while the middle layer Tg decreases slightly. The interfacial layer Tg is proportional to the strength of interfacial interaction, while the middle layer Tg is linearly correlated with the segment density of the middle layer below Tg. Namely, interfacial interaction is the main factor dominating the interfacial layer Tg, while segment density controls the middle layer Tg. image

Structure-reinforcement correlation and chain dynamics in graphene oxide and Laponite-filled epoxy nanocomposites

The present work attempts at a relative deduction of correlations between structure-reinforcement and chain dynamics in Laponite- and graphene oxide (GO)-dispersed epoxy nanocomposites. The fillers were reasonably well dispersed in the epoxy matrix as revealed by wide-angle X-ray diffraction, transmission electron microscopy and small-angle X-ray scattering studies. The scattering from the nanocomposites exhibited power-law behaviour at low q region with fractal dimensions, implying presence of platelets and tactoids of varying thicknesses. A comprehensive study on the thermomechanical properties of the nanocomposites was made in terms of tensile, dynamic mechanical analysis and flexural and fracture toughness measurements. The studies revealed simultaneous reinforcement as well as toughening effects in the nanocomposites; ~42 and ~34 % increases in flexural strength and mode I fracture toughness (K IC), respectively, with 0.1 wt% GO; and ~25 and ~20 % enhancements in flexural modulus and K IC with 0.1 and 0.3 wt% Laponite, respectively. A unique phenomenon of bimodal distribution of glass transition temperatures was observed as two overlapped peaks in terms of Gaussian contributions in the tan δ versus temperature profiles of the nanocomposites (from dynamic mechanical analysis) and derivative of reversible heat capacity with respect to temperature, dC p,rev/dT versus temperature profiles (from modulated differential scanning calorimetric measurements); as against a single symmetric profile for the unfilled matrix. We attempt to understand the nanofiller-induced alteration in the primary relaxation mechanisms as well as notable reinforcement and toughening effects by invoking the filler/polymer interactions, filler dispersion and fractographic investigations.

Distribution of glass transition temperature in multilayered poly(methyl methacrylate) thin film supported on a Si substrate as studied by neutron reflectivity

We studied the distribution of glass transition temperature (Tg) through neutron reflectivity in a poly(methyl methacrylate) (PMMA) thin film supported on a silicon substrate with a five-layered PMMA thin film consisting of deuterated-PMMA and hydrogenated-PMMA. The depth distribution of Tg was successfully observed in the PMMA thin film. Compared to the previously reported distribution of Tg in a polystyrene thin film, the presence of a long-range interfacial effect, supposedly caused by an interaction between PMMA and the substrate, is considered to be responsible for the differences in both the distribution of Tg and the thickness dependence of Tg in both polymers. Therefore, it is expected that the thickness dependence of Tg reported for single-layered polymer thin films can, in principle, be understood from the viewpoint of the difference in the depth distribution of Tg.

Glass Transition Distribution in Miscible Polymer Blends: From Calorimetry to Rheology

At their glass transition, simple liquids and polymers exhibit a broad spectrum of relaxation—with a width of more than 4 decades in time. Blends of miscible polymers exhibit an even broader relaxation spectrum. Here, we assume that a blend can be considered as an ensemble of domains of various local glass transition temperatures. Using calorimetric data, with or without aging, we probe the distribution of glass transition temperature (Tg) of miscible polymer blends of polybutadiene (PB) and styrene butadiene rubber (SBR). Using the self-consistent averaging method inspired by the Olroyd–Palierne model, we predict quantitatively, with no adjustable parameter, the viscoelastic spectrum of our blends from the Tg distribution obtained by calorimetry. This quantitative prediction confirms thus the assumption that mechanically a blend can be considered as an ensemble of domains, each of which have a different glass transition temperature.

Distribution of Glass Transition Temperature as Studied by Neutron Reflectivity

Distribution of Glass Transition Temperature as Studied by Neutron Reflectivity

Distribution of Glass Transition Temperatures in Free-Standing, Nanoconfined Polystyrene Films: A Test of de Gennes’ Sliding Motion Mechanism

Effects of nanoscale confinement on the distribution of glass transition temperatures (Tgs) in free-standing polystyrene (PS) films are determined via a multilayer/self-referencing fluorescence method employing a pyrene dye label. Average film Tgs yield a Tg-confinement effect in agreement with the molecular weight (MW) dependence reported by Forrest, Dalnoki-Veress, and Dutcher. Multilayer films, with one pyrene-labeled layer, reveal that a 14 nm thick free-surface layer in sufficiently thick films (≥∼56 nm) exhibits Tg = Tg,bulk – ∼34 K, independent of film thickness and indicative of a strong Tg gradient near a surface. In sufficiently thin films (≤∼56 nm), a 14 nm thick free-surface layer reports Tg that decreases with decreasing film thickness and is equal to the Tg of a 14 nm thick middle layer and the average film Tg. Thus, the strongly perturbed Tg at the two surfaces affects Tg several tens of nanometers into and across the film, resulting in greater Tg reductions than observed in supported films...

Distribution of Segmental Mobility in Ultrathin Polymer Films

We investigated by dielectric relaxation spectroscopy the distribution of glass transition temperatures and dielectric relaxation strength inside ultrathin polymer films capped between metallic layers. Measurements of the local dielectric properties were achieved by selectively placing layers of dye-labeled polystyrene at different depth inside films of neat polystyrene of different thickness. We show experimental evidence for an interfacial nature of the deviations from bulk behavior; in particular, the value of the dielectric strength and the glass transition temperature strongly depend on the distance from the solid interface. These peculiar profiles of static and dynamic dielectric properties are discussed in terms of a physical picture based on competition between chain adsorption and packing frustration at different annealing conditions. Such a picture was able to rationalize common features observed in properties of ultrathin films like reduction of the relaxation strength, broadening of the dynami...

Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at Tg, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (Tg) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10K as compared to the pure matrix).

Use of solute fixed coordinate system and method of lines for prediction of drying kinetics and surface stickiness of single droplet during convective drying

The distribution of moisture within fructose and lactose droplets, subjected to convective drying conditions, was predicted using a convective-diffusion equation based on a solute fixed coordinate system. The partial differential equation was solved using the method of lines. Neumann and Robbins type boundary conditions were imposed at the centre and at the droplet–air interface, respectively. The temperature history was predicted by coupling the heat balance equation with the rate of change in average moisture. The distribution of glass transition temperature ( T g) within the droplet and at the surface layer was predicted. The concept of sticky point temperature ( T sticky) was introduced and used as an indicator of the development of stickiness at the droplet surface. Single droplets were dried in a controlled air stream and their moisture and temperature history were measured experimentally. These data were used to compare with the model predictions. A custom built in situ surface stickiness testing instrument was used to experimentally determine the development of surface stickiness at the droplet surface. These data were used to validate the model on surface stickiness. The model predicted the experimental moisture and temperature histories of fructose and lactose droplets within ±5–7% absolute error and ±1–2 °C accuracy. It also predicted the development of surface stickiness of fructose and lactose droplets reasonably well. In test conditions, lactose surface attained non-sticky state through crystallization or transformation into glassy state. Fructose droplets remained sticky even in a bone dry state due to their low T g.

The distribution of glass-transition temperatures in nanoscopically confined glass formers.

Despite the decade-long study of the effect of nanoconfinement on the glass-transition temperature (T(g)) of amorphous materials, the quest to probe the distribution of T(g)s in nanoconfined glass formers has remained unfulfilled. Here the distribution of T(g)s across polystyrene films has been obtained by a fluorescence/multilayer method, revealing that the enhancement of dynamics at a surface affects T(g) several tens of nanometres into the film. The extent to which dynamics smoothly transition from enhanced to bulk states depends strongly on nanoconfinement. When polymer films are sufficiently thin that a reduction in thickness leads to a reduction in overall T(g), the surface-layer T(g) actually increases with a reduction in overall thickness, whereas the substrate-layer T(g) decreases. These results indicate that the gradient in T(g) dynamics is not abrupt, and that the size of a cooperatively rearranging region is much smaller than the distance over which interfacial effects propagate.

Mobile silver ions and glass formation in solid electrolytes.

Solid electrolytes are a class of materials in which the cationic or anionic constituents are not confined to specific lattice sites, but are essentially free to move throughout the structure. The solid electrolytes AgI and Ag2Se (refs 1, 2, 3, 4, 5, 6, 7) are of interest for their use as additives in network glasses, such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential applications for batteries, sensors and displays. Here we show that these composite glasses can exhibit two distinct types of molecular structures-an intrinsic phase-separation that results in a bimodal distribution of glass transition temperatures, and a microscopically homogeneous network displaying a single glass transition temperature. For the first case, the two transition temperatures correspond to the solid-electrolyte glass phase and the main glass phase (the 'base glass'), enabling us to show that the glass transition temperatures for the AgI and Ag2Se phases are respectively 75 and 230 degrees C. Furthermore, we show that the magnitude of the bimodal glass transition temperatures can be quantitatively understood in terms of network connectivity, provided that the Ag+ cations undergo fast-ion motion in the glasses. These results allow us to unambiguously distinguish base glasses in which these additives are homogeneously alloyed from those in which an intrinsic phase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.

Modelling of the glass transition temperature of free radical copolymers: An approach for control purposes

A method that allows one to determine the relationship between the evolution of copolymer composition and the distribution of glass transition temperatures is presented in this work. The experimental validation of the model is based on histograms of T g distributions which were computed from DSC thermograms by using a new model-based optimisation technique. This technique allows us to obtain both the average T g of the overall copolymer and the cumulative distribution of T g as a function of the degree of advancement of the copolymerisation reaction, even in the case of reaction with composition drift. The modelling strategy was used successfully to analyse the evolution of the T g with conversion of several free radical copolymers of styrene and butyl acrylate in emulsion. In particular, it has been shown that the value of the glass transition temperature of an ideally alternating copolymer, T g 12 used in the Johnston (1973 Macromolecule 6, 453–456) equation, is in fact a function of the copolymer composition, and may vary by as much as 50°C. The model of T g presented here can be used to control profiles of glass transition temperature distributions (not just averages) during semi-continuous polymerisation processes.