Glass Transition Temperature of Polymer
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.
Glass transition temperature (Tg) is defined as the temperature at which the mechanical properties of a plastic/adhesive radically changed due to the internal movement of the polymer chains that form the plastic/adhesive.
In the first part of the definition, the Tg identified as the temperature at which the mechanical properties of the adhesive change drastically, so the glass transition temperature indirectly define the temperature range in which it can be exposed or work the adhesive / plastic, so the Tg is also known as the working temperature and the knowledge of its value is of vital importance in the design phase.
In the second part of the definition, we identify the origin of the Tg for ease or restrict movements of the polymer chains that occurs inside the plastic/adhesive. When the adhesive or plastic is exposed to temperatures below its Tg, the movement of the polymer chains decreases doing the material acquires a hard and brittle behavior, at temperatures above the Tg polymer chains motion increases doing the material acquires an elastic behavior.
Factors Influencing Glass Transition Temperature
In general the glass transition temperature depends on five other factors which are:
- Free volume of the polymer vf, which is the volume of the polymer mass not actually occupied by the molecules themselves. The higher vf is the more room the molecules have to move around and the lower Tg is. For all polymers the ratio of the free volume vs the total volume (Vf/V) is about 0.025 at Tg.
- The attractive forces between the molecules. The more strongly the molecules are bound together , the more thermal energy must be applied to produce motion.
- The internal mobility of the chains, or their freedom to rotate about the bonds.
- The stiffness of the chains. Stiff chains cannot easily coil and fold, causing Tg to be higher for polymers with stiff chains. Polymers with parallel bonds in the backbone, like polyimides, and polymers with highly aromatic backbones have extremely stiff chains and thus high Tg's.
- The chain length. The glass transition temperature varies according to the relation:
Tg = Tinf - C/x where,
- C is a polymer specific constant,
- Tinf is the asymptotic value of the glass transition temperature for a chain of length infinity and
- x is the length of the chain.
This relationship shows that shorter chains can move easier than longer chains. For most commercial polymers Tg ~ Tinf, since x is quite long.