Roles of Conformational Flexibility in the Crystallization of Stereoirregular Polymers

Stereoregularity significantly influences the crystallization, mechanical, and thermal properties of polymers. In this work, we investigate crystallization behaviors and molecular dynamics of atactic (a)-, isotactic (i)-, and syndiotactic (s)hydrogenated poly(norbornene) (hPNB)s by using small-angle X-ray scattering and solid-state (ss) NMR. a-hPNB exhibits a much higher crystallinity (Phi(c)) (82%) and long period (L) (80 nm) than i- and s-hPNB (50-55% and 17-21 nm). Moreover, in the s-hPNB crystalline region, chain dynamics is not thermally activated up to the melting temperature (T-m), while in the crystalline regions of i- and a-hPNB, small amplitude motions occur in a slow dynamic regime of 10(-2)- 10(2) s. The molecular dynamics follows Arrhenius behavior in a-hPNB up to the crystal-crystal transition temperature (T-cc), while these dynamics are surprisingly saturated in i-hPNB under these conditions. Temperature dependence of the molecular dynamics leads to different crystal-crystal transitions between i- and a-hPNBs: i-hPNB changes the trans conformation to the gauche one due to the localized bond rotations where chain dynamics is restricted, whereas a-hPNB keeps a nearly trans conformation and conducts fast chain dynamics due to the amplified C-C bond rotations in the high-temperature phase. Such fast chain dynamics leads to unique crystallization behaviors of hPNB, specifically in the atactic configuration due to configurational disorder coupled with conformational flexibility.

Automated summary

Stereoregularity significantly affects the crystallization, mechanical, and thermal properties of polymers. Research shows that atactic hPNB exhibits higher crystallinity and longer period compared to isotactic and syndiotactic hPNB. Furthermore, differences in molecular dynamics among the different stereochemistries lead to unique crystal-crystal transitions in hPNB.