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 (Tg) while decrease the dynamic fragility (m) of polymers. Herein, enthalpy relaxation parameters heat capacity jump (Delta Cp) at Tg and enthalpy hysteresis (Delta HR) were investigated to help clarify the effect of macromolecular-induced inter-HB on Tg and m 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 Tg and m increased after copolymerization due to the increased chain rigidity. Nevertheless, increasing steric hindrance leads to an increased Tg while anomalously reduced m. Further results found that m can be well correlated to Tg center dot Delta Cp/Delta HR. Delta Cp increases more significantly than Delta HR in co-polyamides, and thus the entropy change dominates the activation free energy of cooperative rearrangement. By contrast, Delta HR increases more significantly than Delta Cp with increasing steric hindrance, and thus it is reasonable that Tg increases while m decreases. Most importantly, Delta Cp and Delta HR decrease with increasing inter-HB strength regardless of the variation of Tg. These results indicate that the inter-HB strength may be very strong and insensitive to temperature in polyamides, thus behaving like physical cross-linking. Increasing inter-HB strength leads to a decreased fragility m, heat capacity Delta Cp, and enthalpy hysteresis Delta HR although the decrease of Tg, which indicates that the inter-HB interactions behave like physical cross-linking.