What do you mean by glass transition temperature of polymers ?

Question

In case of polmers it happened that there is another temperature than its melting point which play an important role . So I want to know it

Answer 1

The glass transition temperature is a temperature below which an amorphous material (one that lacks crystalline structure) behaves comparably to a crystalline solid. Above this temperature, the material may remain solid but is "rubbery" in character, able to withstand large deformations without fracturing.

Answer 2

Glass Transition Temperature Definition

Answer 3

Glass Transition Temperature:
The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a crystalline phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). A material’s glass transition temperature, Tg, is the temperature below which molecules have little relative mobility. Tg is usually applicable to wholly or partially amorphous phases such as glasses and plastics. For inorganic or mineral glasses, such as common silicon dioxide (SiO2) glass, it is the mid-point of a temperature range in which they gradually become more viscous and change from being liquid to solid. Thermoplastic (non-crosslinked) polymers are more complex because, in addition to a melting temperature, Tm, above which all their crystalline structure disappears, such plastics have a second, lower Tg below which they become rigid and brittle, and can crack and shatter under stress. Small molecular weight pure substances such as water have just one such condensed-phase temperature, below which they are solid crystals (or amorphous ice if cooled below Tg fast enough) and above which they are liquids.

Above Tg, the secondary, non-covalent bonds between the polymer chains become weak in comparison to thermal motion, and the polymer becomes rubbery and capable of elastic or plastic deformation without fracture. This behavior is one of the things which make most plastics useful. But such behavior is not exhibited by crosslinked thermosetting plastics which, once cured, are set for life and will shatter rather than deform, never becoming plastic again when heated, nor melting.

Glass transition temperature of some materials
Polymer Tg (°C)
Polyethylene (LDPE) −125 or −30 also cited
Polypropylene (atactic) −20
Poly(vinyl acetate) (PVAc) 28
Polyethylene terephthalate (PET) 79
Poly(vinyl alcohol) (PVA) 85
Poly(vinyl chloride) (PVC) 81
Polystyrene 95
Polypropylene (isotactic) 0
Poly(3-hydroxybutyrate) (PHB) 0
Poly(methylmethacrylate) (atactic) 105
Chalcogenide AsGeSeTe 245
ZBLAN 265
Tellurite 279
Fluoroaluminate 400
Tyre Rubber 160
Silica 1175

These are only mean values, as the glass transition temperature depends on the cooling-ratio, molecular weight distribution and could be influenced by additives.

Note also that for a semi-crystalline material such as Polyethylene that is 60-80% crystalline at room temperature the quoted glass transition refers to what happens to the amorphous part of the material as the temperature is dropped

Answer 4

The other scraped answers are correct. Polymers only, the Tg is whereby the polymer goes from a more glass like state to a rubber state. On a DSC graph (nonreversing heat flow), this transition can be gradual, or it can be sudden. It all depends on the polymer. In more general terms, it is like a freezing point. the liquid nitrogen rubber ball demo is a great example.

In thermalset resin systems, there really is not a melt temperature. there would only be a decomposition temp at high temp. For thermalplastics (like polyethylene), there is a melt point in addition to the glass transition. I suppose one could consider the Tg to be the first softening point (in thermalplastics only).

Much debate in the materials science field about what goes on in the polymer to cause the Tg.

Answer 5

the temperature below which the polymer becomes brittle and shatters like glass on hammering.

Answer 6

its the temperature below which an amorphous (powdery) substance behaves like a crystalline (glass like) substance. this is because the molecules movement is decreased