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 T_g with MW is observed for rigid polymers. This rise in T_g leads to extremely high 'apparent' fragility and failure of usual correlations between the fragility and other parameters in rigid polymers.