Why many polymers are so fragile

Many polymers exhibit extremely high fragility and deviate from most of the trends known for non-polymeric glass-forming systems. The analysis of literature data presented here demonstrates that chain rigidity might be the main reason for this peculiar behaviour. Based on this analysis a simple scenario is proposed: (i) oligomers (short polymeric chains) follow trends characteristic for non-polymeric systems; (ii) increase in molecular weight (MW) does not affect structural relaxation significantly in the case of flexible chains; (iii) however, it slows down structural relaxation in rigid chains tremendously. As a result, a strong increase of Tg with MW is observed for rigid polymers. This rise in Tg leads to extremely high 'apparent' fragility and failure of usual correlations between the fragility and other parameters in rigid polymers.