Effect of intermolecular hydrogen bonding strength on the dynamic fragility of amorphous polyamides.

Small-molecular-induced intermolecular hydrogen bonding (inter-HB) interactions were reported to increase the glass transition temperature () while decrease the dynamic fragility () of polymers. Herein, enthalpy relaxation parameters heat capacity jump (Δ) at and enthalpy hysteresis (Δ) were investigated to help clarify the effect of macromolecular-induced inter-HB on and using amorphous polyamides as model polymers. The inter-HB strength was weakened by random copolymerization with varied chain rigidity, but was enhanced by decreasing steric hindrance. It was found that and increased after copolymerization due to the increased chain rigidity. Nevertheless, increasing steric hindrance leads to an increased while anomalously reduced . Further results found that can be well correlated to ·Δ/Δ. Δ increases more significantly than Δ in co-polyamides, and thus the entropy change dominates the activation free energy of cooperative rearrangement. By contrast, Δ increases more significantly than Δ with increasing steric hindrance, and thus it is reasonable that increases while decreases. Most importantly, Δ and Δ decrease with increasing inter-HB strength regardless of the variation of . These results indicate that the inter-HB strength may be very strong and insensitive to temperature in polyamides, thus behaving like physical cross-linking.