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 .