Glass Transition Temperature(Tg):
The temperature below which a polymer is hard, brittle, glassy and above which is soft, flexible or rubbery is called the glass transition temperature (Tg).
The hard, brittle state is known as the glassy state and soft, flexible state as rubbery state. If the polymer becomes highly viscous liquid & starts flowing, this state is visco-fluid state.
Importance of Glass Transition Temperature (Tg):
The glass transition temperature is an important parameter of a polymeric materials.
1/ It is used as a measure for evaluating the flexibility of a polymer molecule & the type of response the polymeric material would exhibit to mechanical stress.
2/ The Tg value of polymer, decides whether a polymer at the use temperature will behave like a rubber or plastic.
3/ It would suffice to know that polymer above their Tg will be soft &flexible & exhibit a delayed elastic response while those below their Tg will.be hard & brittle & will posses dimensional stability.
4/ The Tg value along with the Tm value gives an indication of the temperature region at which a polymeric material transform from a rigid solid to a soft viscous state.
5/ Tg helps in choosing the right processing temperature. i.e. the temperature region at which the material can be converted into finished products through a different processing techniques such as moulding, calendering & extrusion.
Glass Transition of co-polymer:
The polymer is a mixture of two homo-polymer. The Tg value of random co-polymer depends on the Tg value of the individual homo-polymer & on the ratio of two monomeric compounds. Equationally expressed of Tg of co-polymer-
1/Tg(ab) = W(a)/Tg(a) + W(b)/Tg(b)
Where, Tg(a), Tg(b), Tg(ab) are the glass transition temperature of the homo-polymer of a & b and copolymer ab respectively and W(a), W(b) is the weight ftaction of respective monomer.
Factors affecting the Glass Transition Temperature (Tg):
1. Highly crystalline polymers possessing a regular chain geometry show a high Tg. That's why the Tg of linear polyethylene is greater than the Tg of branched polyethylene.
Here compactness is more because it is oriented & Tg is also more.
2. Bulky side groups come in the way of segmental motion & increase the Tg of the polymer. The Tg of the poly-vinyl-carbazole is 150°c as it contains bulky side groups.
3. But in the presence of long pendant group decrease the Tg of polymer.
4. Presence of aromatic rings in the backbone chain increase Tg. Example: Nylon - 6 ( Tg = 50°c)
5. The amount of plasticizers even in small quantities reduce the Tg of the polymer.