Sometimes the glass transition is described as a transition from "a hard, brittle, 'glassy' state" to a "molten or 'rubbery' state" (that one's from Wikipedia, but most sources say something similar).
The words "molten" and "rubbery" seem contradictory to me - in a rubbery material the elastic behavior should dominate over the viscous behavior, while in a molten material, the response should be primarily viscous. The classic examples seem to be similarly torn: balls bouncing at various heights depending on temperature, and silly putty flowing or not.
So then, is the glass transition a decrease in the Young's modulus of the elastic material, or a transition to a softer, plastically deforming phase, which therefore has no easily defined Young's modulus?
basically:
Modulus of a polymer depends on both cohesive energy and mobility
- below Tg, low molecular mobility --> high modulus (~ GPa)
- above Tg: high mobility --> low modulus (~ MPa)
In case of an amorphous thermoplastic polymer above Tg: sort of rubbery plateau where entanglement play the role of temporary crosslink nodes
In case of a semi-crystalline thermoplastic polymer, rubbery amorphous phase toughened by crystals (till to their melting point)
In case of a network, rubbery plateau with E = n.RT (n is related to the crosslinked density)
regards
Glass = lower possibility to move atoms in macromolecules and change the distance in the chain. If we talking about elastic/rubber material it has a lower E because E = Force / Elongation (but only in region where shape of material may back to virgin state). Using other words: where Hooke's law plays. In case of rubber the force to change the shape is not so high. After glass temperature (freeze macromolecules) the force in the same material, which have to be add to move frozen macromolecules is larger, but deformation (elongation) is lower, because atoms can;t move and rubber will fragile brake. So, I cant agree wit Your sentence: "So then, is the glass transition a decrease in the Young's modulus of the elastic material,".
basically:
Modulus of a polymer depends on both cohesive energy and mobility
- below Tg, low molecular mobility --> high modulus (~ GPa)
- above Tg: high mobility --> low modulus (~ MPa)
In case of an amorphous thermoplastic polymer above Tg: sort of rubbery plateau where entanglement play the role of temporary crosslink nodes
In case of a semi-crystalline thermoplastic polymer, rubbery amorphous phase toughened by crystals (till to their melting point)
In case of a network, rubbery plateau with E = n.RT (n is related to the crosslinked density)
regards
Hello Steven,
Stresses an develop in glass due to the thermal shock after firing and during melt quenching.... Glass transition is the temperature at which the glass starts to relieve the internal stresses within its atomic structure.. This is always found to be very useful in increasing not the durability of the material but also its strength...
In glass annealing, the glass is heated until the temperature reaches a stress-relief point. At this point the glass is hard enough to deform but soft enough for the stresses to relax.
Hope this helps
adel
So then even above the glass transition the material should deform elastically?
It depends on what you are doing with the material..
to my knowledge, the material is heat treated to the transition temperature to relieve stresses that were introduced into its structure during the rapid cooling... During glass annealing and after the material reaches the glass transition temperature for some time, the material is cooled slowly to room temperature.. that makes the material stronger and more ductile.. I guess this also increases its toughness..
It depends on the time frame used. I can't address the behavior of organic glasses, but for inorganic glasses, the "modulus" depends on the rate of application of the stress in the transformation region. If the measurement is much faster than the relaxation time, the material will behave elastically. If the rate is much slower the material will behave viscously. Hence the term "viscoelastic". As the temperature rise into the transformation region, the relaxation time will decrease and any elastic behavior will become more difficult to observe since the time frame will become so short.
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