Glass transition temperature:
the glass transition temperature is the temperature at which an amorphous solid becomes soft upon heating or brittle upon cooling. The glass transition temperature is lower than the melting point of its crystalline form, if it has one.
Melting temperature:
The temperature response of linear polymers may be viewed as divided into three distinctly separate segments:
1. Above Tm :
In this segment, the polymer remains as a melt or liquid whose viscosity would depend on molecular weight and on the temperature of observation.
2. Between Tm and Tg :
This domain may range between near 100% crystalline and near 100% amorphous chain molecular clusters depending on the polymer structural regularity and on experimental conditions. The amorphous part behaves much like super cooled liquid in this segment. The overall physical behaviour of the polymer in this intermediate segment is much like a rubber.
3. Below Tg :
The polymer material viewed as a glass is hard and rigid, showing a specified coefficient of thermal expansion. The glass is closer to a crystalline solid than to a liquid in behavioural pattern in terms of mechanical property parameters. In respect of molecular order, however, the glass more closely resembles the liquid. There is little difference between linear and cross linked polymer below Tg.
Tm and TgValues of Several Polymers:
Polymer | Repeat Unit | Tm, 0C | Tg, 0C |
Polyethylene | – CH2 – CH2 – | 137 | -115,-60 |
Polyoxymethylene | – CH2 – O – | 181 | -85,-50 |
Polypropylene (isotactic) | – CH2 – CH (CH3) – | 176 | - 20 |
Polyisobutylene | – CH2 – C (CH3)2 – | 44 | - 73 |
Polybutadine (1, 4 cis) | – CH2 – CH = CH – CH2 – | 2 | - 108 |
Polyisoprene (1, 4 cis), (NR) | – CH2 – C(CH3) = CH – CH2 – | 14 | - 73 |
Poly (dimethyl siloxane) | – OSi (CH3)2 – | - 85 | - 123 |
Poly (vinyl acetate) | – CH2 – CH (OCOCH3) – | --- | 28 |
Poly (vinyl chloride) | – CH2 – CH Cl – | 212 | 81 |
Polystyrene | – CH2 – CH (C6H5) – | 240 | 95 |
Poly (methyl methacrylate) | – CH2 – C(CH3)( COOCH3) – | 200 | 105 |
Poly tetrafluoroethylene | – CF2 – CF2 – | 327 | 126 |
Poly caprolactam (Nylon 6) | – (CH2)5 CONH – | 215 | 50 |
Poly(hexamethylene adipamide) (Nylon 66) | –HN(CH2)6-NHCO–(CH2)4CO – | 264 | 53 |
Poly (ethylene terephthalate) | – O(CH2)2 – OCO – (C6H4) CO – | 254 | 69 |
Poly (ethylene adipate) | – O(CH2)2 – OCO – (CH2)4 CO – | 50 | -70 |
Cellulose triacetate | 306 | 49-478 | |
Cellulose trinitrate | 700 | 53 | |
Ethyl cellulose | 43 | ||
Natural rubber | 36 | -73 | |
Polybutyl acrylate | 47 | -54 | |
Polyvinyl carbazole | >320 | 150 | |
Polyvinylidene chloride | 190 | -18 |
