Effect of Chain Length

The chain length indirectly controls the molecular weight of a polymer. It is noted that:

• As molecular weight increases, Tg also increases
• As molecular weight decreases, Tg also decreases

The possible reasons could be -

– More end groups
– Easier movement of molecules
– More inherent free volume in the polymer

The Tg of a polymer may be determined as follows:

T_g = Tg∞ - C/M_n

Where, T_g∞ = Glass transition temperature at infinite chain length C – a polymeric constant
M_n = ΣMi Ni/ ΣMi