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

Abstract

Polar groups have long been recognized to greatly influence the glass transition temperature (<i>T</i>_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 (<i>N</i>_HB) and the cohesive energy density increase linearly as the content of polar groups (<i>f</i>_pol) increases, the <i>T</i>_g of ROMP copolymers increases with the increase of <i>f</i>_pol in a nonlinear fashion, and tends to plateau for sufficiently high <i>f</i>_pol. Importantly, we find that the increase rate of Gibbs free energy for HB breaking gradually slows down with the increase of <i>f</i>_pol, indicating that the HB is gradually stabilized. Therefore, <i>T</i>_g is jointly determined by <i>N</i>_HB and the strength of HBs in the system, while the latter dominates. Although <i>N</i>_HB increases linearly with increasing <i>f</i>_pol, the HB strength increases slowly with increasing <i>f</i>_pol, which leads to a decreasing rate of increase in <i>T</i>_g.