Showing papers on "Glass transition published in 2008"

Measuring the Surface Dynamics of Glassy Polymers

Abstract: The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk α relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk α relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.

Insights into phases of liquid water from study of its unusual glass-forming properties.

Abstract: The vitrification of pure water is compared with that of molecular solutions rich in water, and gross differences are noted. Thermodynamic reasoning and direct observations on noncrystallizing nanoconfined water indicate that the glass transition in ambient-pressure water is qualitatively distinct from that found in the usual molecular liquids. It belongs instead to the order-disorder class of transition seen in molecular and ionic crystalline materials. The distinctive "folding funnel" energy landscape for this type of system explains the extreme weakness of the glass transition of water as well as the consequent confusion that has characterized its scientific history; it also explains the very small excess entropy at the glass transition temperature. The relation of confined water behavior to that of bulk is discussed, and the "fragile-to-strong" transition for supercooled water is interpreted by adding a "critical point-free" scenario to the two competing scenarios for understanding supercooled bulk water.

The amount of immobilized polymer in PMMA SiO$_{2}$ nanocomposites determined from calorimetric data

Abstract: For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals aremelting first and simultaneously theRAFdevitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as theRAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do notmelt or undergo other phase transitions altering the polymer–nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO2 nanocomposites was shown on the basis of heat capacitymeasurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO2 nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transitionwas found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax. 2007 Elsevier Ltd. All rights reserved.

Ion exchange for glass strengthening

Abstract: This paper presents a short overview of silicate glass strengthening by exchange of alkali ions in a molten salt, below the glass transition temperature (chemical tempering). The physics of alkali inter-diffusion is briefly explained and the main parameters of the process, which control the glass reinforcement, are reviewed. Methods for characterizing the obtained residual stress state and the strengthening are described, along with the simplified modelling of the stress build-up. The fragmentation of chemically tempered glass is discussed. The concept of engineered stress profile glass is presented, and finally, the effect of glass and salt compositions is overviewed.

Universal scaling between structural relaxation and vibrational dynamics in glass-forming liquids and polymers

Abstract: If liquids, polymers, bio-materials, metals and molten salts can avoid crystallization during cooling or compression, they freeze into a microscopically disordered solid-like state, a glass1,2. On approaching the glass transition, particles become trapped in transient cages—in which they rattle on picosecond timescales—formed by their nearest neighbours; the particles spend increasing amounts of time in their cages as the average escape time, or structural relaxation time τα, increases from a few picoseconds to thousands of seconds through the transition. Owing to the huge difference between relaxation and vibrational timescales, theoretical3,4,5,6,7,8,9 studies addressing the underlying rattling process have challenged our understanding of the structural relaxation. Numerical10,11,12,13 and experimental studies on liquids14 and glasses8,15,16,17,18,19 support the theories, but not without controversies20 (for a review see ref. 21). Here we show computer simulations that, when compared with experiments, reveal the universal correlation of the structural relaxation time (as well as the viscosity η) and the rattling amplitude from glassy to low-viscosity states. According to the emerging picture the glass softens when the rattling amplitude exceeds a critical value, in agreement with the Lindemann criterion for the melting of crystalline solids22 and the free-volume model23.

Dielectric relaxations in PVDF/BaTiO3 nanocomposites

Abstract: The present work aims at the study of molecular relaxations in PVDF/BaTiO3 nanocomposites using broadband dielectric spectroscopy. The nanocomposites of PVDF with BaTiO3 (10–30% by wt%) are prepared using simple melt mixing method. In dielectric permittivity study, two relaxation processes are identified corresponding to the crystalline, glass transition in the PVDF/BaTiO3 nanocomposites. The peaks shift to higher frequencies as the temperature is increased. Electric modulus formalism is used to analyze the dielectric relaxations to overcome the conductivity effects at low frequencies. In M′′ spectra two peaks are observed only at high temperature and low frequency whereas a single relaxation peak appears at low temperatures. The single relaxation peak appearing at low temperatures is the αc relaxation attributed to crystalline chain relaxation in PVDF and the second relaxation peak which appears only at high temperatures and at a frequency lower than αc relaxation is identified as MWS relaxation. The temperature dependence of αc relaxation and MWS relaxation follows Arrhenius type behavior.

Dynamic Protein−Water Relationships during β-Sheet Formation

Abstract: We investigated the polymer−water interaction in a model fibrous protein. Bombyx mori silk fibroin film, a typical model of biodegradable material, was cast from aqueous solution and analyzed in this study. Differential scanning calorimetry (DSC), its temperature-modulated variant (TMDSC), and the time-resolved technique of Fourier transform infrared spectroscopy (FTIR) were used for the first time to monitor the detailed structural changes of silk fibroin during heating and during isothermal crystallization above the glass transition temperature, Tg ∼ 451 K (178 °C). Results show that intermolecular bound water molecules, acting as a plasticizer, will strongly affect the secondary structure of silk fibroin. DSC study shows that silk fibroin initially displays a water-induced glass transition around 80 °C during heating, resulting from a temporary bound water-silk structure. Quantitative thermal analysis of the heat capacity changes of this system during heating revealed that no β-sheet crystals were form...

Influence of Particle Size and Polymer−Filler Coupling on Viscoelastic Glass Transition of Particle-Reinforced Polymers

Abstract: The viscoelastic glass-to-rubber softening transition is analyzed for various cross-linked polymers reinforced with filler particles. We find that the loss modulus peak corresponding to the segmental relaxation process (glass transition) is not significantly affected by the particle surface area in carbon black-filled polybutadiene or by silane chemical coupling of poly(styrene-co-butadiene) to silica. Large differences in shape and magnitude of the peak in the loss tangent (tan δ) vs temperature are noted for these materials; however, this is due to variations in the storage modulus at small strains in the rubbery state, which is influenced by the nature of the jammed filler network. The use of a simple relaxation model demonstrates this feature of the viscoelastic glass transition in filled rubber. It is not necessary to invoke concepts involving a mobility-restricted polymer layer near the filler surfaces to explain the viscoelastic results. Atomic force microscopy conducted with an ultrasharp tungsten tip indicates that there may be some stiffening of the elastomer in the proximity of filler particles, but this does not translate into an appreciable effect on the segmental dynamics in these materials.

Glass transition temperature and thermal decomposition of cellulose powder

Abstract: Cellulose powder and cellulose pellets obtained by pressing the microcrystalline powder were studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermal gravimetry (TG). The TG method enabled the assessment of water content in the investigated samples. The glass phase transition in cellulose was studied using the DSC method, both in heating and cooling runs, in a wide temperature range from −100 to 180 °C. It is shown that the DSC cooling runs are more suitable for the glass phase transition visualisation than the heating runs. The discrepancy between glass phase transition temperature T g found using DSC and predictions by Kaelbe’s approach are observed for “dry” (7 and 5.3% water content) cellulose. This could be explained by strong interactions between cellulose chains appearing when the water concentration decreases. The T g measurements vs. moisture content may be used for cellulose crystallinity index determination.

Effect of Bi2O3 on physical, optical and structural studies of ZnO–Bi2O3–B2O3 glasses

Abstract: Zinc bismuth borate glasses with composition 10ZnO–xBi2O3–(90 − x)B2O3 (where x is in mol%, ranging from 25 to 50 in steps of 5) have been prepared using conventional melt quenching technique. Differential scanning calorimetry (DSC) studies showed that the glass transition temperature (Tg) decreases from 473 °C to 449 °C as the content of Bi2O3 increases. The optical absorption studies revealed that the cutoff wavelength increases while optical band gap energy (Eopt) and Urbach energy (ΔE) decreases with increase of Bi2O3 content. The Eopt values of these glasses are found to be in the range 3.464–3.169 eV where as the values of ΔE lies in the range 0.503–0.178 eV. The DSC and optical absorption studies revealed that the glass network becomes less tightly packed and degree of disorder increases with increase in Bi2O3 concentration in the present glass system. The IR studies indicate that these glasses are made up of [BiO6], [BO3] and [BO4] basic structural units. The values of optical basicity evaluated using oxide ion polarizability α b 2 − ( E opt ) obtained from optical band gap are in agreement with those values calculated from optical basicity values of the constituent oxides.

Hiking down the Energy Landscape: Progress Toward the Kauzmann Temperature via Vapor Deposition

Abstract: Physical vapor deposition was employed to prepare amorphous samples of indomethacin and 1,3,5-(tris)naphthylbenzene. By depositing onto substrates held somewhat below the glass transition temperature and varying the deposition rate from 15 to 0.2 nm/s, glasses with low enthalpies and exceptional kinetic stability were prepared. Glasses with fictive temperatures that are as much as 40 K lower than those prepared by cooling the liquid can be made by vapor deposition. As compared to an ordinary glass, the most stable vapor-deposited samples moved about 40% toward the bottom of the potential energy landscape for amorphous materials. These results support the hypothesis that enhanced surface mobility allows stable glass formation by vapor deposition. A comparison of the enthalpy content of vapor-deposited glasses with aged glasses was used to evaluate the difference between bulk and surface dynamics for indomethacin; the dynamics in the top few nanometers of the glass are about 7 orders of magnitude faster tha...

Electrical conductivity of thermoresponsive shape-memory polymer with embedded micron sized Ni powder chains

Abstract: The electrical resistivity of a thermoresponsive polyurethane shape-memory polymer (SMP) filled with micron sized Ni powders is investigated in this letter. We show that, by forming conductive Ni chains under a weak static magnetic field (0.03T), the electrical conductivity of the SMP composite in the chain direction can be improved significantly, which makes it more suitable for Joule heat induced shape recovery. In addition, Ni chains reinforce the SMP significantly but their influence on the glass transition temperature is about the same as that of the randomly distributed Ni powders.

Effect of recycling on mechanical behaviour of biocompostable flax/poly(L-lactide) composites

Abstract: The biocompostability of natural fibre-reinforced biopolymers, also known as biocomposites, makes them attractive alternative to glass fibre-reinforced petrochemical polymers. The aim of this work is to study the capacity of flax/PLLA (poly(l-lactide)) biocomposite (20% and 30% fibres by weight) to be recycled. Mechanical properties were evaluated initially, and shown to be similar to those of glass/PP and superior to hemp/PP and sisal/PP composites. Then after repeated injection cycles tensile properties were shown to be conserved until the third cycle. Matrix degradation and fibre aspect ratio were followed using molecular weight measurements, thermal and rheological analyses, image analysis of sections and SEM fractography. These techniques revealed a lower molecular weight, lowering of glass transition temperature, reduction of fibre length, and separation of fibre bundles with injection cycles. Nevertheless, the property retention after three cycles under extreme recycling conditions (100% recycling with no added virgin polymer) indicate the promising recyclability of these materials.

Effect of pulsed vacuum and ultrasound osmopretreatments on glass transition temperature, texture, microstructure and calcium penetration of dried apples (Fuji)

Abstract: The effect of pulsed vacuum and ultrasound pretreatments on glass transition, texture, rehydration, microstructure and other selected properties of air- and freeze-dried apples (Fuji) were investigated. Apple cylinders (15 mm height × 15 mm diameter) were first osmoconcentrated in a 60 g/100 g high-fructose corn syrup solution containing 7.5 g/100 g Gluconal Cal® combined with agitation, pulsed vacuum (PV), or ultrasound for 3 h, then hot-air or freeze dried. Changes in glass transition temperature, hardness, crispness, and rehydration rate were measured, microstructure was observed using scanning electronic microscopy (SEM), and calcium ions distributions were analyzed by a laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Under the same drying method, ultrasound led to a higher glass transition temperature, lower water activity, moisture content and rehydration rate, severer structure collapse, less cavities and calcium uptake than PV did. Different osmoconcentration pretreatment had no significant (P

Solvent and concentration effects on the properties of electrospun poly(ethylene terephthalate) nanofiber mats

Abstract: This study describes the preparation and characterization of nanofibrous mats obtained by electrospinning poly(ethylene terephthalate) (PET) solutions in trifluoroacetic acid/dichloromethane (TFA/DCM). Special attention was paid to the effect of polymer concentration and solvent properties on the morphology, structure, and mechanical and thermal properties of the electrospun nonwovens. The results show that the spinnable concentration of PET solution in TFA/DCM solvents is above 10 wt %. Mats have nanofibrous morphology with fibers having an average diameter in the range of 200–700 nm (depending on polymer concentration and solvent composition) and an interconnected pore structure. Higher solution concentration favors the formation of uniform fibers without beads and with higher diameter. Morphology and fiber assembly changed with the solvent properties. Solvent mixtures rich in TFA, i.e., those with higher dielectric constant and lower surface tension, originated fibers with small diameter. However, due to the lower volatility, those solvent mixtures also produced more branched and crosslinking fibers, with less morphologic uniformity. Mechanical properties (Young's modulus, ultimate strength, and elongation at break) and thermal properties (glass transition, crystallization, and melting) have been studied for the PET electrospun nanomats and compared with those of the original polymer. Solvent effect on fiber crystallinity was not significant, but a complex effect was observed on the mechanical properties of the electrospun mats, as a consequence of the different structural organization of the fibers within the mat network. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 460–471, 2008

Temperature dependence of the electrical conductivity of imidazolium ionic liquids

Abstract: The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10(7) Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion.

Effect of Silica Nanoparticles on the Local Segmental Dynamics in Poly(vinyl acetate)

Abstract: The effect of nanosized silica particles on the properties of poly(vinyl acetate) (PVAc) was investigated for a range of silica concentrations encompassing the percolation threshold. The quantity of polymer adsorbed to the particles (“bound rubber”) increased systematically with silica content and was roughly equal to the quantity shielded from shear stresses (“occluded rubber”). This bound and occluded polymer attained a level of ∼12% at a silica volume content of 28%; nevertheless, the glass transition properties of the PVAc, including the glass transition temperature, local segmental relaxation function and relaxation times, and the changes in thermal expansion coefficient and heat capacity at Tg, were unaffected by the interfacial material. That is, there is no indication that the local segmental dynamics of the chains adjacent to silica particles differ from the motions of the bulk chains. Interestingly, the volume sensitivity of the segmental dynamics, as determined from the scaling exponent γ in th...

Correlation of Phase Behavior and Charge Transport in Conjugated Polymer/Fullerene Blends

Abstract: The phase diagrams for three conjugated polymer/fullerene blends of interest for polymer solar cells, namely semicrystalline poly(3-hexylthiophene) (P3HT):methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene) (MDMO-PPV):PCBM, and poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV):PCBM, have been constructed based on X-ray scattering data and differential scanning calorimetery (DSC). Both melting point depression and glass transition temperature elevation were observed in the P3HT:PCBM blends as a function of increasing PCBM wt %. The PCBM solubility limit, i.e., the phase-separation point, was determined to be 30, 40, and 50 wt % PCBM for P3HT:PCBM, MDMO-PPV:PCBM, and MEH-PPV:PCBM mixtures, respectively. The phase behavior of the blends is directly correlated with electrical transport behavior determined by measuring field effect conduction in a transistor testbed. Specifically, below the solubility limit for PCBM...

Thermal and dynamic mechanical behavior of bionanocomposites: Fumed silica nanoparticles dispersed in poly(vinyl pyrrolidone), chitosan, and poly(vinyl alcohol)

Abstract: Various bionanocomposites were prepared by dispersing fumed silica (SiO2) nanoparticles in bio- compatible polymers like poly(vinyl pyrrolidone) (PVP), chitosan (Chi), or poly(vinyl alcohol) (PVA). For the biona- nocomposites preparation, a solvent evaporation method was followed. SEM micrographs verified fine dispersion of silica nanoparticles in all used polymer matrices of compo- sites with low silica content. Sufficient interactions between the functional groups of the polymers and the surface hydroxyl groups of SiO2 were revealed by FTIR measurements. These interactions favored fine dispersion of silica. Mechanical properties such as tensile strength and Young's modulus substantially increased with increas- ing the silica content in the bionanocomposites. Thermog- ravimetric analysis (TGA) showed that the polymer matrices were stabilized against thermal decomposition with the addition of fumed silica due to shielding effect, because for all bionanocomposites the temperature, corre- sponding to the maximum decomposition rate, progres- sively shifted to higher values with increasing the silica content. Finally, dynamic thermomechanical analysis (DMA) tests showed that for Chi/SiO2 and PVA/SiO2 nanocomposites the temperature of b-relaxation observed in tand curves, corresponding to the glass transition tem- perature Tg, shifted to higher values with increasing the SiO2 content. This fact indicates that because of the reported interactions, a nanoparticle/matrix interphase was formed in the surroundings of the filler, where the macromolecules showed limited segmental mobility. V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 1739-1749, 2008