Effect of the polar group content on the glass transition temperature of ROMP copolymers

Polar groups have long been recognized to greatly influence the glass transition temperature (T_g) of polymers, but understanding the underlying physical mechanism remains a challenge. Here, we study the glass formation of ring-opening metathesis polymerization (ROMP) copolymers containing polar groups by employing all-atom molecular dynamics simulations. We show that although the number of hydrogen bonds (N_HB) and the cohesive energy density increase linearly as the content of polar groups (f_pol) increases, the T_g of ROMP copolymers increases with the increase of f_pol in a nonlinear fashion, and tends to plateau for sufficiently high f_pol. Importantly, we find that the increase rate of Gibbs free energy for HB breaking gradually slows down with the increase of f_pol, indicating that the HB is gradually stabilized. Therefore, T_g is jointly determined by N_HB and the strength of HBs in the system, while the latter dominates. Although N_HB increases linearly with increasing f_pol, the HB strength increases slowly with increasing f_pol, which leads to a decreasing rate of increase in T_g.