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.

Automated summary

Polar groups have a significant impact on the glass transition temperature of polymers, and this study reveals that the increase in the content of polar groups leads to a nonlinear increase in T-g. The strength of hydrogen bonds plays a dominant role in determining T-g, and the rate of increase in T-g decreases with the increase of polar group content.