Glass Transition Temperature (Tg)

Glass Transition Temperature (Tg): The temperature below, which a polymer is hard and above which it is soft, is called the glass transition temperature (Tg). The hard, brittle state is known as the glassy state and the soft flexible state as rubbery or viscous state.

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The molecular mobility is just starts at that temperature called Glass Transition Temperature.

The molecular mobility is just starts above that temperature or below which mobility arrested called Tg. Tg is unique to amorphous thermoplastics. It occurs at a specific temperature that depends on pressure and specific volume and is lower than melting point. The glass transition temperature is a measure of evaluating the flexibility of polymer molecules and the type of response the polymeric material would exhibit to mechanical stress. The glass transition temperature depends on the structure and polarity of a polymer, which affects both chain flexibility and intermolecular interaction. Glass is an amorphous (non-crystalline) solid material. Glasses are typically brittle and optically transparent.

Tg is unique to amorphous thermoplastics. It occurs at a specific temperature that depends on pressure and specific volume and is lower than melting point.

The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica (SiO2) plus Na2O, CaO, and several minor additives. Often, the term glass is used in a restricted sense to refer to this specific use. In science, however, the term glass is usually defined in a much wider sense, including every solid that possesses a non-crystalline (i.e., amorphous) structure and that exhibits a glass transition when heated towards the liquid state. In this wider sense, glasses can be made of quite different classes of materials: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers.

For many applications (bottles, eyewear) polymer glasses (acrylic glass, polycarbonate, and polyethylene terephthalate) are a lighter alternative to traditional silica glasses.

1. The glass transition temperature is a measure of evaluating the flexibility of polymer molecules and the type of response the polymeric material would exhibit to mechanical stress.

2. The glass transition temperature depends on the structure and polarity of a polymer, which affects both chain flexibility and intermolecular interaction energy.

3. Since  non-polar  polymers  have  lower  potential energy barrier for rotation, their chain remains flexible and they have low Tg. Chains based on aliphatic C-C bond and C-O bound are quite flexible, on the other hand introduction of ring structure into main chain has a marked stiffening effect which results in higher Tg of such polymer.

4. Rotation about a single C-C bond is also impended by the substitutions of attached hydrogen atoms by methyl or other attached hydrogen atoms by methyl or other attached to the main chain carbon atom can influence the Tg.

5. Inclusion of double bonds will stiffen the chain at the point of inclusion but at the same time decrease the flexibility of adjacent bond. The net effect may therefore be to reduce the glass transition temperature.

6. The presence of polar groups or atoms will result in high glass transition temperature. Hydrogen bonding has a similar effect to that of a polar group.