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

Abstract

Small-molecular-induced intermolecular hydrogen bonding (inter-HB) interactions were reported to increase the glass transition temperature (<i>T</i>_g) while decrease the dynamic fragility (<i>m</i>) of polymers. Herein, enthalpy relaxation parameters heat capacity jump (Δ<i>C</i>_p) at <i>T</i>_g and enthalpy hysteresis (Δ<i>H</i>_R) were investigated to help clarify the effect of macromolecular-induced inter-HB on <i>T</i>_g and <i>m</i> 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 <i>T</i>_g and <i>m</i> increased after copolymerization due to the increased chain rigidity. Nevertheless, increasing steric hindrance leads to an increased <i>T</i>_g while anomalously reduced <i>m</i>. Further results found that <i>m</i> can be well correlated to <i>T</i>_g·Δ<i>C</i>_p/Δ<i>H</i>_R. Δ<i>C</i>_p increases more significantly than Δ<i>H</i>_R in co-polyamides, and thus the entropy change dominates the activation free energy of cooperative rearrangement. By contrast, Δ<i>H</i>_R increases more significantly than Δ<i>C</i>_p with increasing steric hindrance, and thus it is reasonable that <i>T</i>_g increases while <i>m</i> decreases. Most importantly, Δ<i>C</i>_p and Δ<i>H</i>_R decrease with increasing inter-HB strength regardless of the variation of <i>T</i>_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.