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Topic: Glass transition temperature

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  Glass transition temperature - Wikipedia, the free encyclopedia
This means that glass transition is not merely a kinetic effect, i.e.
In contrast to the melting points of crystalline materials the glass transition temperature is therefore somewhat dependent on the time-scale of the imposed change.
is cooling-rate (or frequency) dependent as the glass is formed, the glass transition is not considered a true thermodynamic phase transition by many in the field.
en.wikipedia.org /wiki/Glass_transition_temperature   (1276 words)

 Glass transition temperature - Biocrawler   (Site not responding. Last check: 2007-11-03)
In polymers, Tg is often expressed as the temperature at which the Gibbs free energy is such that the activation energy for the cooperative movement of 50 or so elements of the polymer is exceeded.
In glasses (including amorphous metals and gels), Tg is related to the energy required to break and re-form covalent bonds in a somewhat less than perfect (may be regarded as an understatement) 3D lattice of covalent bonds.
The Tg is therefore influenced by the chemistry of the glass.
www.biocrawler.com /encyclopedia/Tg   (467 words)

A glass forms when a typical liquid, with a disordered molecular structure, is cooled to a temperature generally about 100dC (for many pure liquids) bellow its equilibrium crystalline melting temperature (Tm) or freezing point, at a rate sufficiently high to avoid crystallisation of the liquid.
The glass transition temperature is dependent on the water content of the food material, similarly to the influence of plasticisers on synthetic polymers.
The glass transition temperature denoted by Tg' is the glass transition temperature of the unfrozen portion of a maximally freeze concentrated unfrozen matrix.
www.rpi.edu /dept/chem-eng/Biotech-Environ/Porto/CRISTINA/TG.HTM   (1383 words)

 Polymer engineering FAQ - Eng-Tips
The glass transition occurs when there is enough vibrational (thermal) energy in the system to create sufficient free-volume to permit sequences of 6-10 main-chain carbons to move together as a unit.
The temperature at the midpoint of the transition from glassy to rubbery, the glass transition region, is defined as the glass transition temperature, Tg.
This is considered the “meltingtemperature of the polymer, or the first-order transition temperature.
www.eng-tips.com /faqs.cfm?fid=957   (747 words)

For the water/cryoprotectant mixture in a cryonics patient thermal conductivity varies inversely with temperature -- meaning that conductivity is necessarily the slowest at the beginning of a cryonics cooldown -- the very temperatures at which ischemic damage and cryoprotectant toxicity is the worst.
Glasses have lower thermal conductivity than crystalline materials, which makes the problem serious for fully vitrified cryonics patients than for partially frozen cryonics patients, despite the lower thermal expansion of glasses.
Temperature probes (thermocouples) are used to monitor the temperature of the gas in the cooling box, in the chest of the patient, under the skin, in a burr hole, in the throat (lower throat in neck) or deep in the nasopharynx (ie, deep down the nose into the upper throat).
www.benbest.com /cryonics/cooling.html   (5625 words)

 Glass Transition   (Site not responding. Last check: 2007-11-03)
Above the glass transition temperature there is an increase to the slope of the specific volume vs. temperature function.
The phenomenon called "the glass transition" is somewhat of a misnomer since the term 'transition' implies an equilibrium phenomenon that is invariant to the speed of the heating or cooling.
The crosslinking reaction of a thermoset polymer resin raises the glass transition temperature.
web.umr.edu /~wlf/Bulk/glasstrans.html   (610 words)

 Is glass liquid or solid? (via CobWeb/3.1 planetlab2.netlab.uky.edu)   (Site not responding. Last check: 2007-11-03)
It is sometimes said that glass in very old churches is thicker at the bottom than at the top because glass is a liquid, and so over several centuries it has flowed towards the bottom.
Glass can be made from pure silica, but fused silica has a high glass transition point at around 1200° C which makes it difficult to mould into panes or bottles.
Glass such as Pyrex (used for test-tubes and ovenware) is usually based on boro-silicates or alumino-silicates, which withstand heating better and typically have a higher glass transition temperature.
math.ucr.edu.cob-web.org:8888 /home/baez/physics/General/Glass/glass.html   (2574 words)

 DoITPoMS TLP - The Glass Transition in Polymers
The second stage of the phase transition is the growth of the nucleated seeds.
It is possible to calculate a value for the glass transition temperature by measuring the elastic modulus of the polymer as a function of the temperature, for example by using a torsion pendulum.
As the temperature is reduced, the viscosity of the polymer increases, so more of the elastic strain energy is dissipated, and the ball does not bounce as high.
www.doitpoms.ac.uk /tlplib/glass-transition/printall.php   (3317 words)

 Glass Transition Temperature — Tg   (Site not responding. Last check: 2007-11-03)
A rubbery is above the temperature at which molecular motions take place on the time scale of the experiment.
The glass transition temperature (Tg) separates glassy from rubbery behavior.
At slower the cooling process Tg tends to occur at lower temperatures and the resulting polymer is denser.
www-ee.technion.ac.il /people/nir/Glass_Transition_Temperature.htm   (281 words)

 eFunda: Glossary of Polymers (Plastics)
Therefore, when metal inserts are placed in a molded part, it is important to verify that with temperature excursions the maximum tensile strength of the plastic is not exceeded.
Heat Deflection Temperature The overall definition of heat deflection temperature is a temperature at which a polymer sample deflects a certain amount under heat and load.
The heat deflection temperature is a function of the temperature, strain rate, and stress.
www.efunda.com /materials/polymers/definitions/polymer_definitions.cfm   (651 words)

 Glass Transition
The temperature at which the transition in the amorphous regions between the glassy and rubbery state occurs is called the glass transition temperature.
At body temperature the gum is soft and pliable, which is characteristic of an amorphous solid in the rubbery state.
The glass transition temperature of the gum is somewhere between 0
www.lasalle.edu /academ/chem/ms/polymersRus/Resources/GlassTrans.htm   (530 words)

 Glass Transition Temperature, Tg   (Site not responding. Last check: 2007-11-03)
Glass Transition Temperature, Tg Slide 28 of 33
Transition is called “Glass Transition” because the properties below it are similar to ordinary glass.
Transition range is not one temperature but a range over a relatively narrow range (10 degrees).
www.csuchico.edu /~jpgreene/itec041/m41_ch03/sld028.htm   (95 words)

 Glass Transition Temperature of Polymers (rubber band and polystyrene)   (Site not responding. Last check: 2007-11-03)
The glass transition temperature is a physical property of polymers and can be measured.
They are going to find that the glass transition temperature is a range of temperatures instead of one temperature as they would find in a melting point or freezing point experiment.  Although the substances are homogenous the polymer chains that make them up are of different lengths which accounts for the range of temperatures.
Extensions of this investigation can be to make "shrinky dinks" from the polystyrene or to find the glass transition of "happy/sad" balls.  The students can also make and use their own latex bands.  These lessons are available on the Polymer Ambassadors on the IPEC website;www.uwsp.edu/chemistry/ipec/pa/.
agpa.uakron.edu /k12/lesson_plans/glass_transition_temperature.html   (334 words)

 Glass transition
The glass transition is a transition which happens to amorphous polymers; that is, polymers whose chains are not arranged in ordered crystals, but are just strewn around in any old fashion, even though they are in the solid state.
To be sure, by the time we get down to the glass transition temperature, it is already too cold for the polymer molecules, tangled up in each other as they are, to move any distance in one direction.
The only change we see at the glass transition temperature is an increase in the slope, which means, of course, that we have an increase in heat capacity.
www.pslc.ws /welcome/tour/macrog/tg.htm   (2865 words)

 Materials Science   (Site not responding. Last check: 2007-11-03)
We have shown that the key parameter is the glass transition temperature of the polymer.
If the operating frequency and temperature are in the vicinity of the glass transition, the absorption will be high.
Since we have a considerable background in controlling the glass transition temperature through changes in chemical composition, we are able to design polymers with the desired level of absorption.
www.dt.navy.mil /code60/code645/polymers/accomplish.htm   (640 words)

 Polymer engineering - What is the Glass Transition Temperature, Tg?
This second-order, or ?-transition is the temperature at which the mechanical behavior of the polymer changes from that of a brittle glass to more of a tough, leathery behavior.
Polycarbonate, for instance, has a third-order or ?-transition at about -90°C, whereas, the glass transition temperature is reported to be around 150°C. With a high glass transition temperature, polycarbonate should behave as a brittle glass at room temperature, similar to other amorphous polymers like PS or PMMA.
The second-order, or glass transition temperature, is also referred to as the “alpha”-transition, while a third-order transition is referred to as a “beta”-transition.
www.eng-tips.com /viewthread.cfm?qid=94856   (1499 words)

 Glass transition temperature : Tg
Glass transition temperature : Tg Glass transition temperature : Tg A simplistic view of a material's glass transition temperature (Tg) is the temperature below which molecules have very little mobility.
On a larger scale, polymers are rigid and brittle below their glass transition temperature and elastic above it.
A fuller discussion of the Tg requires an understanding of mechanical loss mechanisms (vibrational and resonance modes) of specific (and usually common in a given material) functional groups and molecular arrangements.
www.factspider.com /tg/tg.html   (485 words)

 Glass transition temperature   (Site not responding. Last check: 2007-11-03)
In polymers Tg is often expressed as temperature at which the gibbs free energy such that the activation energy for the movement of 50 or so elements of polymer is exceeded.
In glasses (including amorphous metals and gels) Tg is related to the energy required break and re-form covalent bonds in a less than perfect (may be regarded as understatement) 3D lattice of covalent bonds.
The Space Shuttle Challenger disaster was caused by a rubber that was below its glass transition temperature thus could not flex adequately to form proper seal around one of the two rocket boosters.
www.freeglossary.com /Glass_transition_temperature   (404 words)

 Low-Cost, High Glass-Transition Temperature, Thermosetting Polyimide   (Site not responding. Last check: 2007-11-03)
PMR-15 polyimide, developed in the mid-1970’s at the NASA Lewis Research Center, is recognized as a state-of-the-art high-temperature resin for composite applications in the temperature range of 500 to 550 °F (260 to 288 °C).
Current efforts at Lewis are focused on raising the use temperature of polyimide composites by increasing the glass-transition temperature of the matrix resins.
Achieving this dramatic increase in the upper use temperature without sacrificing polymer and composite processability is a major technical challenge.
www.grc.nasa.gov /WWW/RT1998/5000/5150chuang.html   (311 words)

 glass transition
The glass transition is common in quite different and important classes of materials (for example, polymers, oxides, chalcogenides, metals, and molecular solids), and continues to attract much attention from a fundamental and a practical point of view.
Experimental results for the apparent activation energy that is measured when approaching the glass transition from below, using both TSDC and dielectric permittivity, record a very high activation energy (enthalpy) at temperatures slightly below the glass transition (a transition in polymers).
Near the glass transition the change of configurational energy with temperature is very large and causes the observation of an anomalously high apparent activation energy.
www.elp.uji.es /juan_home/research/glass_transition.htm   (341 words)

 What is the difference between Glass Transition Temperature
The entire apparatus is submerged in an oil bath, with the temperature of the oil raised at a set rate.
When the cast bar deflects 0.010-inches under the load, the oil temperature is recorded as the HDT.
Glass Transition Temperature (Tg) is a computerized method of measuring the molecular change.
www.ptm-w.com /index.asp?pgid=54   (321 words)

 Tg - Glass transition temperature - BioForum
Glass transition temperature is the temperature at which a material's characteristics change from that of a glass to that of rubber.
The temperature region around Tg is often called the leathery region.
The glass transition is not a sharp transition but a gradual transition.
www.protocol-online.org /forums/index.php?showtopic=6624   (219 words)

 [No title]
The glass transition is exhibited as an endothermic peak or a shift in the base line in Differential Scanning Calorimetry (DSC) due to change in specific heat.
However, in chalcogenide glasses, such an endothermic peak can also be observed due to a fast change in enthalpy when the glassy system relaxes quickly due to a decrease in viscosity at the transition temperature.
Therefore, a study of the kinetics of glass transition is one of the most important problems in the area of chalcogenide glasses The glass transition temperature Tg represents the strength or rigidity of the glassy structure of the alloys.
www.chalcogen.infim.ro /Mehta.doc   (1600 words)

 Polymer Chemistry Glossary
The temperature at which the pressure exerted by molecules leaving a liquid equals the pressure exerted by the molecules in the air above it.
The temperature where the molecules of a polymeric solid can begin to move relative to one another, giving a substance that behaves like a rubber, rather than a brittle glass.
Alternatively, you can think of it as the temperature where the molecules of a polymeric solid can no longer move relative to one another, giving a substance that behaves like glass, rather than a rubber that can be stretched without breaking.
www.kcpc.usyd.edu.au /discovery/glossary-all.html   (6465 words)

 MSE 310 Experiment 10   (Site not responding. Last check: 2007-11-03)
In Fig 8-3-1, the modulus is plotted versus absolute temperature, and the time dependent nature of strain in each region is identified.
This general behavior is characteristic of amorphous thermoplastic polymers; all such polymers show almost identical behavior as a function of temperature, referenced to their respective glass transition temperatures.
At low temperature, most of the energy is stored and the amount lost to internal friction is very small.
www.tu-darmstadt.de /fb/ms/student/fs/german/lab/w8/mse8_3.htm   (867 words)

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