Glass transition is per definitionem a transition ascribed to amorphous (glassy) materials without long range order. It is not a first order phase transition like melting (where you have a jump in enthalpy, volume etc.), but a relaxation transition determined (partly) by kinetic effect. (Although some believe that there is a second order transition somewhere close to the experimentally observed glass transition, proved by jumps in the first derivative of the basic quantities, such as heat capacity, volume expansion coefficient etc). If looked at from the phenomenological viewpoint the glass transition temperature is a temperature where the rigid glass becomes soft and flowable. This is not an abrupt transition, it has some breadth (depending on the rate of temperature change), nevertheless it is well defined and can be ovserved by mechanical, dielectric, magnetic realxation methods, by dilatometry or DSC (and several other effects). The glass formed is not in equilbirum and it shows volume and enthalyp relaxation. If glasses are stored long at temepratures below, but close to the glass transition are re-heated, an endothermal peak is observed, but it is not melting, but an enthalpy relaxation peak, which depends on the annearling conditions. There are several good review articles on these topics.
Glass transition is per definitionem a transition ascribed to amorphous (glassy) materials without long range order. It is not a first order phase transition like melting (where you have a jump in enthalpy, volume etc.), but a relaxation transition determined (partly) by kinetic effect. (Although some believe that there is a second order transition somewhere close to the experimentally observed glass transition, proved by jumps in the first derivative of the basic quantities, such as heat capacity, volume expansion coefficient etc). If looked at from the phenomenological viewpoint the glass transition temperature is a temperature where the rigid glass becomes soft and flowable. This is not an abrupt transition, it has some breadth (depending on the rate of temperature change), nevertheless it is well defined and can be ovserved by mechanical, dielectric, magnetic realxation methods, by dilatometry or DSC (and several other effects). The glass formed is not in equilbirum and it shows volume and enthalyp relaxation. If glasses are stored long at temepratures below, but close to the glass transition are re-heated, an endothermal peak is observed, but it is not melting, but an enthalpy relaxation peak, which depends on the annearling conditions. There are several good review articles on these topics.
Dr Banhegyi has provided a detailed answer. Only amorphous materials show glass transition. It is not an equilibrium structure and show a relaxation transition which is marked by induction of softness and flowability. In allloys, it is followed by a crystallization event/s on heating where some crystalline phase is formed. Crystallization does not happen in all polymers after glass transition. Similarly crystallization does not happen after glass transition on heating ordinary window glass (ceramic) to higher temperature.
Yes, it was an important note. Being a polymer scientist I described the phenomena occurring in polymers and in silicate glasses. Glassy metals (mostly alloys) can usually be formed only by extremely fast cooling. The glass transition of some chalcogenide, phosphate and fluoirde glasses may also be accompanied by crystallization. Even in polymer glasses secondary crystallization may occur after passing the glass transition if the polymer glass is formed by quenching in semi.crystalline polymers where rapid cooling prevents cyrstallization (which should occur BEFORE the vitrification during cooling).
I agree with everything that was said here by Drs Banhegyi and Abbas, but I would like to add that glass is actually an unsolidified liquid, but its viscosity is so high that it is considered a solid. Glass flows actually, but the flow is very hard to see. However, if you look at a large window installed many years ago you may see some thickening in the middle and some sagging under the frame. In order to have crystalline glass you need very well controlled cooling, but normally glass will remain glassy. Other materials have been discussed here. The glass transition temperature can be phenomenologically likened to the melting point, but we are talking about softening of a non-crystallized material. Metals have a very low crystallization energy, so it's very hard NOT to get them crystallized.
The critical slowing down of the dynamics of atoms and molecules with decreasing temperature results in “kinetic-arrest” of first order liquid-crystal phase transition making the behavior of supercooled liquid nonergodic. The kinetic arrest basically ceases the homogeneous nucleation of the supercooled liquid into crystalline solid and finally terminating into a liquid to “Glass-transition”[1]. Although the phenomenon of glass-transition is known since long the understanding of its physics is still the most important among the major unsolved scientific problems [2, 3]. The term “glass” is often loosely taken as synonyms of the “structural disorder” but the most widely accepted definition is the liquid in which the molecular motion has under gone “kinetic-arrest”. The term “kinetic-arrest” has mostly been mentioned in relation to disorder-disorder (liquid to glass) transition, but now it has been realized in order-order transitions too. Mukhopadhta et al. [4] first showed that the pseudobinary alloy Ce(Fe0.96Ru0.04) under goes kinetic-arrest of the first-order ferromagnetic (FM) to antiferromagnetic (AFM) transition when the applied field is above a certain critical value at low-temperatures. The occurrence of kinetic-arrest (or dynamic freezing) in order-order type transitions has been rather more frequently reported in perovskite (ABO3) related manganites and multiferroics. Using magnetic force microscopic imaging Weida et al. [5] have reported La5/8−yPryCa3/8MnO3 (y = 0.4) going to a magnetic-glass state at low-temperatures due to cooperative, dynamic freezing of the first-order AFM(charge ordered) to FM transition and not to a spin-glass state. Banerjee et al [6] and Chaddah et al [7] have reported similar magnetic-glass state in half-doped manganites Pr0.5 Ca0.5Mn0.975Al0.025O3 are La0.5 Ca0.5MnO3 respectively. They have demonstrated the occurrence of field-induced dearrest of stable FM phase out of kinetically arrested metastable AFM phase 6 and kinetic-arrest of metastable FM phase out of stable AFM phase [7] at low-temperatures. Kinetic-arrest (dearest) has also been observed in multiferroic perovskites too [8]. It has been argued that the magnetic-glass states arise perhaps due to the deep association between the kinetics of the phase-transition and the accommodation strain[5].
The occurrence of kinetic-arrest in an order-order transition appears to extend its fundamental role, and offer advantage to explore and generalize the concept of “Glass-transition”, to other first-order phase transitions too. In conventional glassy materials the high-temperature equilibrium phase (liquid) with order parameter zero, under goes a glass-transition, either by rapid quenching or by kinetic arrest, to a metastable solid state phase at lower temperatures with order parameter still zero. The entropy of the high-temperature liquid phase is preserved in the form of disorder of the atomic arrangement in the glassy phase. Thus the study of kinetic-arrest phenomenon in order-order transition becomes important from point of view that in what form the entropy of high-temperature is preserved in the arrested phase at low-temperature. It was far more intriguing of observation of such a kinetic arrest in an order-order structural phase transition, which has been reported by our group [9].
1. P. G. Debenedetti and F. H. Stillinger, Nature London 410, 259 (2001).
2. J. Kurchan, Nature London 433, 222 2005.
3. P. W. Anderson, Science 267, 1615 (1995)
4. M. K. Chattopadhyay, S. B. Roy, and P. Chaddah Phys. Rev. B 72, 180401(R) (2005).
5. Weida, W.U, Casey I., Namjung H., Soonyong P., Sang-Wook C. and Alex De L. Nature materials 5, 881 (2006).
6. A. Banerjee, K Mukherjee, Kranti Kumar, and P. Chaddah, Phys. Rev. B 74, 224445 (2006).
7. P. Chaddah, Kranti Kumar, and A. Banerjee, Phys. Rev. B 77, 100402(R) (2008).
8. Y. J. Choi, C. L. Zhang, N. Lee, and S-W. Cheong, Phys. Rev. Lett. 105, 097201 (2010).
9. Aga Shahee, Dhirendra Kumar, Chandra Shekhar, and N. P. Lalla, "Kinetic arrest of first-order R-3c to Pbnm phase-transition in supercooled LaxMnO3+δ
(X= 1 & 0.9) "J. Phys.: Condens. Matter 24 (2012) 225405.
Beside all aforementioned, you should consider that most of polymers are semi-crystalline materials which means that they have crystals and amorphous parts in their structure. In those materials, glass transition temperature (also known as Tg) is attributed to the amorphous part of the polymer chains, as the crystalline part are interlocked in crystals and cannot relax. Therefore, semi-crystalline materials can also show Tg.
@R Manory: The idea that window glass flows over time is just a myth.
To Dr Weimer: I have not measured this myself, but this was passed on to me many years ago when I was taking a subject in ceramics. To check that it is a myth I would have to check the dimensions old windows and I don't have any occasion to do it. Thanks for filling one of the gaps in my knowledge.
Glassy state: If vetrification of state turns out to be a relaxation process and obeys kinetic rather that thermodynamic laws or in other ways at vitrification the structure of the substance freezes in under conditions where molecular rearrangements are so slow that a change of structure can’t keep up with changes in external parameters. Then the state which show kinetic dependent relaxation is called as glassy state.
Mostly this state occurring during the blockage of disorder to order (i.e. liquid to crystal) phase transition and the phase which got freezed is a liquid ( where atoms/ ions/ molecules are in disorder form) and there after we start to feel that disorder solid system is called a glassy state. Such a situation can occur even during order to order (i.e. crystal to crystal or ferromagnetic to anti-ferromagnetic or vice verse) phase transition (references for which are give in my first comment ) and in all that cases crystalline materials will show the glass transition and glass transition temperature(Tg) can be observed their too .
As your question is why only amorphous materials alone has Tg. Imagine a class where students sitting in ten rows and ten columns ( total 100) and perfectly arranged (crystalline materials) and the same class with less students around 60 randomly arranged with some gaps in seats (amorphous materials)... now heat is increased in room upto a certain temperature all students will sit in a same place ( assume everybody as a same character as comparing with atoms or molecules) and reaching a limit they suddenly gets up and moves (MELTING Tm)... please note that every student wants to move as heat is increasing continuosly but for them there is no place to move here and there as seats are completly occupied and after limit exceeds everybody moved from their place at same time (like bursting , sudden struggle by people against king due to pressure of him over a long time). so sharp movement or say melting at exact temperature BUT in case of amorphous materials as heat increases the 60 students present here in random also wants to move from there i.e vibrating, rotating, moving. the difference here is the students near by him if know one seated he is having a space to move or show reaction for heat. this is Tg. transition . i.e many other students who seated continuously in some areas of class cannot move due to no space. but some other students in thier same class at same temperature can able to move due to place availability. so amorphous materials have tg prior to tm. and at time of still increase heat upto tm everybody moves even arranged persons as limit reached. so crystalline has sharp tm while amorphous has first tg and tm.
here is a nice analogy with Snakes on the Tg (The Snake Pit).
http://pslc.ws/macrog/tg.htm
Most of the polymers are not perfectly crystalline. So amorphous regions in a semi-crystalline polymer will display Tg while ordered domains will display melting point.
If the polymer is completely amorphous it will only have Tg & there will be no melting point. Tg does not occur jumpwise & it has a range so what is usually reported is the average of this range.
Some comments:
1- About flowing of glass at room temperature, see attachment below.
2- Sometimes the words glassy and amorphous are used for distinguish
Glassy: disordered materials that exhibit Tg (silicate glasses for example)
Amorphous: disordered materials that not exhibit Tg (some metalic amorphous materials). It doesnt mean that there is not Tg, but it can be overlapped with the crystallization peak (in a DTA or DSC measurement)
3- Tg is a kinetics phenomena, in a temperature interval, that goes from liquid in equilibrium to liquid in a not equilibrium state (glass) with mechanical properties of a solid . On cooling the molecular movements are limited and the material cannot get the equilibrium structure of the corresponding liquid.
@M Prado: Thanks for the interesting article on "flowing" glass.
I would say that amorphous and glassy are two different characteristics of local, short-range, and long-range order parameters. Amorphous materials have no order correlation at any length scale. Glassy materials have some degree of order correlation at local length scales. For reference: Liquids are a glassy fluids, while gases are amorphous fluids.
Solidification is a thermodynamic process. The notion that kinetics may control it arise from experimental constraints. When we could solidify something while maintaining perfect reversibility, we would have no need to discuss the kinetics of the fundamental components of nucleation, growth, or diffusion.
The doubt still remains as to whether amorphous and glass are same or different? if different how to distinguish? There may be instances where, both glass and amorphous shows glass transition. will a heat-cool-heat DSC help?
Each glass is considered as amorphous material, but not each material in amorphous state is glass, and possesses Tg. Some materials are nanocrystalline and appear as amorphous on XRD diffractograms, i.e. they possess the so called X-ray amorphous plateau (their particles are so small, so the XRD cannot show "orientation" pattern). Then one has to prove the glassy (amorphous) nature by other experiments. According to some scientists the bulks are called glasses, and the powders are called amorphous. It is tricky to distinguish them, but anyways the amorphous/glassy condition is defined by lack of defined crystalline long order.
can i say glasses are quench-cooled solids whereas non-glassy amorphous are not quench cooled solids but obtained maybe by spray-drying?
No. :-) Not any solid, obtained by quenching is glassy. Each composition for outlining of a region of glass formation is commonly quench-cooled, but not each of them is glassy. This is the point of the glass formation - you need to have materials that tend to from disordered matrix. The quenching is used to "freeze" the liquid state, but the nature does not like chaos and many of the materials crystallise during cooling or even in solid state. A glass is considered as such if it lasts in its disordered state at least (roughly) one week.
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