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