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Glass Transition Region (glass + transition_region)

Distribution by Scientific Domains
Polymers and Materials Science100%

Selected Abstracts

Relaxation and Glass Transition in an Isostatically Compressed Diopside Glass

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2007
Lothar Wondraczek
Relaxation of isostatically compressed CaMgSi2O6 (diopside) glass is explored by ex situ differential scanning calorimetric (DSC) experiments. A diopside melt was compressed above its glass transition temperature under isostatic conditions at a pressure of 500 MPa. DSC analysis was performed at ambient pressure after slowly cooling the compressed melt under pressure (i.e., after freezing-in the densified state). Compression-induced enhancement of the overshoot in heat capacity was observed in the glass transition region. This indicates that a densified, quenched glass possesses a lower apparent fictive temperature, TfA, than a glass that was cooled under ambient pressure at the same cooling rate. However, a thermodynamic analysis indicates that the fictive temperature produced under pressure, Tf0, is actually much higher than that determined from DSC experiments at ambient pressure (Tf0>TfA). [source]

Energy Release in Isothermally Stretched Silicate Glass fibers

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2006
Lasse Hornbøll
Three types of silicate glass fibers are annealed, simultaneously stretched in the glass transition region for certain time lengths, then slowly cooled to room temperature under load, and subsequently scanned by differential scanning calorimetry (DSC). During the DSC scanning, a broad exothermic peak (representing energy release) occurs in the stretched fibers well below the glass transition temperature, while it does not occur in the non-stretched fibers. The peak indicates that mechanical stretching can result in an energy enhancement in the fibers. It also confirms that the energy released during reheating of the fibers formed using an industrial continuous fiber drawing process originates not only from thermal quenching but also from mechanical stretching. However, the mechanical stretching-induced energy is much lower than the thermal hyperquenching-induced energy in glass fibers. The effect of annealing temperature and time on the energy release behavior is discussed in terms of viscoelasticity. [source]

Enthalpy relaxation in bisphenol-A polycarbonate/poly(methyl methacrylate) blends

POLYMER ENGINEERING & SCIENCE, Issue 3 2007
Maurizio Penco
Bisphenol-A polycarbonate (PC)/poly(methyl methacrylate) (PMMA) blends (PC/PMMA) were prepared by melt mixing with and without a trans-esterification catalyst (tetrabutylammoniun tetraphenylborate), which is able to promote inter-exchange reactions between PC and PMMA. Evidences of the ester,ester exchange reaction taking place were pointed out via Fourier Transform Infra-Red and size exclusion chromatography analyses. A series of enthalpy relaxation measurements were carried out for the pure polymers and for blends thereof. The data were analyzed on the basis of the Tool,Narayanaswamy,Moynihan/Kohlraush,Williams,Watts model. This model characterizes the structural relaxation in the glass transition region by means of four parameters: the apparent activation energy (,h*), the nonlinearity (x), the nonexponentiality (,), and the pre-exponential factor (,0). The apparent activation energy can be determined from the cooling rate dependence of the fictive temperature (Tf) measured using DSC. ,h* values of homogeneous blend and of the starting polymers were determined in this way, while an estimation of x, , and ,0 was proposed. Comparisons were made between a single glass transition temperature PC/PMMA blend and homopolymers data. The experimental values of ,h* suggest that the degree of cooperativity decreases on going from the starting polymers to the blend. POLYM. ENG. SCI., 47:218,224, 2007. © 2007 Society of Plastics Engineers. [source]

A thermally stimulated depolarization current study of polymers in the glass transition region

POLYMER ENGINEERING & SCIENCE, Issue 1 2001
Joseph S. Sedita
The low-frequency dielectric properties of a number of polymers, composites and blends have been studied using a thermally stimulated depolarization current (TSDC) apparatus that was designed and constructed in-house. The TSDC technique can be used to determine the glass transition of a polymer sample. This TSDC glass transition temperature has been shown to be very similar to that obtained from differential scanning calorimetry (DSC). The actual difference between these two values depends on the heating rates used with each technique, however. TSDC data can also be combined with AC dielectric data to produce a data set, which possesses a very wide frequency range. Finally, individual TSDC relaxation peaks can be fit with the Williams-Walts distribution function to obtain an estimate of their distributions. This is especially useful when studying polymer blends, but could also be utilized in the study of other systems. [source]

Mechanical and viscoelastic properties of semi-interpenetrating polymer networks of poly(vinyl chloride)/thermosetting resin blends

POLYMER ENGINEERING & SCIENCE, Issue 6 2000
Charles U. Pittman Jr.
Semi-interpenetrating polymer networks (SIPNs) of PVC/thermoset were prepared by premixing porous, 150 ,m diameter particles of PVC and a small quantity (from 5 to 15% by weight) of a single thermosetting liquid preresin from one of five types (e.g. methylene bis-phenyl diisocyanate (MDI), oligomeric MDI isocyanates (PAPI), toluene diisocyanate (TDI) prepolymer, epoxy, and vinyl ester resins, respectively). Two roll milling of these mixtures was followed by hot-press curing. Mechanical testing indicated that most of these blends exhibited increased tensile, impact, and flexural strengths. The strength increments were greater when going from 0 to 5% thermoset content than when going from 5 to 10% or 10 to 15% thermoset. In many cases, increasing thermoset content from 10 to 15% gave slightly decreased or unchanged tensile, impact, and flexural strengths. This behavior is in accord with a "thermoset dilution effect" in PVC. Most of these SIPN blends exhibited a tan , peak temperature lower than that for pure PVC in the glass transition region. The tan , peak temperatures were progressively lowered as the amounts of thermoset increased. Also, a single distinct peak existed in the E, curves for most of the blends. Only the PVC/epoxy (90/10) blend showed two peak maxima in E, vs. temperature curves. All blends exhibited peak E, values at a lower temperature than those of PVC which had been exposed to the same processing temperatures. These observations seem to rule out the presence of large domains of PVC, which are phase-separated from PVC/thermoset SIPN, and pure thermoset domains. A substantial amount of the added thermoset appears to exist in SIPN type phases in these five blend types. [source]