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Recently, slow molecular dynamics of poly(l‐lactic acid) (PLLA) by using 1D and 2D exchange NMR are investigated. In this work, slow molecular dynamics of PLLA chains in the α′, a stereocomplex (SC) with poly(d‐lactic acid), and glassy states are investigated in terms of centerband‐only detection of exchange (CODEX) NMR. The mixing‐time dependence of the CODEX data demonstrates that the molecular dynamics of stems become slower in the order of α′, α, and SC. The temperature dependence of the correlation time 〈τ_c〉 of the helical jump motions in the α and SC phases simply exhibits Arrhenius behaviors, with activation energy,E_a, values of 91 ± 1 and 97 ± 1 kJ mol⁻¹, respectively. In contrast, the temperature dependence of 〈τ_c〉 in the α′ sample exhibits two Arrhenius lines with substantially differentE_avalues of 273 ± 12 and 16 ± 14 kJ mol⁻¹at temperatures below and above 84 °C. The obtained kinetics of molecular dynamics not only establish the relationship between packing structure and dynamics in PLLA polymorphs and in the SC, but also allow for an understanding of the coupled dynamics between the crystalline and amorphous regions at approximatelyT_g.

 

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 (Φc) (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 (Tm), while in the crystalline regions of i- and a-hPNB, small amplitude motions occur in a slow dynamic regime of 10–2–102 s. The molecular dynamics follows Arrhenius behavior in a-hPNB up to the crystal–crystal transition temperature (Tcc), 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. 

In response to the stringent requirements for future DC-link capacitors in electric vehicles (EVs), it is desirable to develop dielectric polymer films with high-temperature tolerance (at least 105 °C) and low loss (dissipation factor, tan δ < 0.003). Although the biaxially oriented poly(ethylene terephthalate) (BOPET) film has an alleged temperature rating of 120 °C, its dielectric performance in terms of breakdown strength and lifetime cannot satisfy the stringent requirements for power electronics in EVs. In this work, we carried out a structure–electrical insulation property relationship study to understand the working mechanism for various PET films, including a commercial BOPET film, an amorphous PET (AmPET) film, and two annealed PET films (AnPET, i.e., cold-crystallized from AmPET). Structural analyses revealed a uniform edge-on crystalline orientation in BOPET with the a* axis in the film normal direction. Meanwhile, a high content of the rigid amorphous fraction (RAF) was identified for BOPET, which resulted from biaxial stretching during processing. On the contrary, AnPET films had a random crystal orientation with lower RAF contents. From dielectric breakdown and lifetime studies, the high-crystallinity AnPET film exhibited better electrical insulation than BOPET, and AmPET had the worst electrical insulation. Electrical conductivity results revealed that the high RAF content in BOPET led to reasonably high breakdown strength and long lifetime only at low temperatures (<100 °C). Meanwhile, PET crystals were more insulating than the amorphous phase, whether mobile, rigid, or glassy. In particular, the flat-on lamellae in the AnPET film were more effective than the edge-on lamellae in BOPET in blocking the conduction of charge carriers (electrons and impurity ions). This understanding will help us design high-temperature semicrystalline polymer films for DC-link capacitors in EVs. 

Chain-level structure of semicrystalline polymers in melt- and solution-grown crystals has been debated over the past half century. Recently, 13C–13C double quantum (DQ) Nuclear Magnetic Resonance (NMR) spectroscopy has been successfully applied to investigate chain-folding (CF) structure and packing structure of 13C enriched polymers after solution and melt crystallization. We review recent NMR studies for (i) packing structure, (ii) chain trajectory, (iii) conformation of the folded chains, (iv) nucleation mechanisms, (v) deformation mechanism, and (vi) molecular dynamics of semicrystalline polymers.