Helical Jump Motions in Isotactic Poly(4-methyl-1-pentene) Crystallites Revealed by 1D MAS Exchange NMR Spectroscopy

Chain Trajectory, Chain Packing, and Molecular Dynamics of Semicrystalline Polymers as Studied by Solid-State NMR

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.

Molecular Dynamics in the Crystalline Regions of Poly(ethylene oxide) Containing a Well-Defined Point Defect in the Middle of the Polymer Chain

The chain mobility in crystals of a homopolymer of poly(ethylene oxide) (PEO) with 22 monomer units (PEO₂₂) is compared with that of a PEO having the identical number of monomer units but additionally a 1,4-disubstituted 1,2,3-triazole (TR) point defect in the middle of the chain (PEO₁₁-TR-PEO₁₁). In crystals of PEO₂₂, the characteristic α_c-relaxation (helix jumps) is detected and the activation energy of this process is calculated from the pure crystalline ¹H FIDs to 67 kJ/mol. PEO₁₁-TR-PEO₁₁ exhibits a more complex behavior, i.e. a transition into the high temperature phase HTPh is noticed during heating in the temperature range between -5 and 10 °C which is attributed to the incorporation of the TR ring into the crystalline lamellae. The crystal mobility of the low temperature phase LTPh of PEO₁₁-TR-PEO₁₁ is in good agreement with PEO₂₂ since helical jump motions could also be detected by analysis of the ¹H FIDs and the corresponding values of their second moments M₂. In contrast, the high temperature phase of PEO₁₁-TR-PEO₁₁ shows a completely different behavior of the crystal mobility. The crystalline PEO chains are rigid in this HTPh on the time scale of both, the ¹H time-domain technique and in ¹³C MAS CODEX NMR spectroscopy, i.e. the α_c-mobility of PEO in the HTPh of PEO₁₁-TR-PEO₁₁ is completely suppressed and the PEO₁₁ chains are converted into a crystal-fixed polymer due to the incorporation of the TR rings into the crystal structure. However, the TR defect of PEO₁₁-TR-PEO₁₁ shows in the HTPh characteristic π-flip motions with an Arrhenius type activation energy of 223 kJ/mol measured by dielectric relaxation spectroscopy. This motion cannot be observed by corresponding ¹³C MAS CODEX NMR measurements due to an interfering spin-dynamic effect.

Investigation on the artificial exchange signals induced by the RIDER effect in CODEX experiments

The CODEX (center-band only detection of exchange) NMR experiment is widely used for the detection of slow motions in organic solids, especially polymers. However, the RIDER (relaxation-induced dipolar exchange with recoupling) effect may result in artificial exchange signals in the CODEX pure exchange spectrum, which greatly limits the application of CODEX method. Herein, we investigate the distance range that the RIDER effect can reach by performing CODEX experiments on two typical organic solids, hexadecyltrimethylammonium bromide (CTAB) and semi-crystalline polyamide-6 (PA6) where there are no slow molecular motions at room temperature. Our experimental results demonstrate that generally two-bond distance is far enough to ignore the RIDER effect resulted from the dipolar interactions between (13)C and the fast relaxing heteronucleus (14)N. From the built-up curve of RIDER signals as a function of recoupling time and mixing time, it is clearly revealed that the RIDER effect can greatly affect the signal from (13)C directly bonded with (14)N. However, this RIDER effect accounts less than 3% of the reference intensity for signals from (13)C not directly bonded with (14)N if typical recoupling (~0.5 ms) and mixing times (~0.5 s) are used for the investigation of slow motions. When longer recoupling and mixing time are used, there are small RIDER signals even for the (13)C far away from the (14)N. These signals, to a large degree, result from the spin diffusion effect and/or the special microscopic molecule arrangement. However, they are so small compared to the reference signal (~5%) that they can be ignored. Finally, according to the simulation results, it is worth noting that the RIDER signal is still generally negligible compared to the signals due to slow motions if the chemical shift anisotropy reorientation during the mixing time is not too small(larger than 20°) under the condition of 4t(r) recoupling time at the magic-angle-spinning speed of 6.5 kHz.

Hydration dependence of backbone and side chain polylysine dynamics: A13C solid-state NMR and IR spectroscopy study

The molecular dynamics of solid poly-L-lysine has been studied by the following natural abundance (13)C-NMR relaxation methods: measurements of the relaxation times T(1) at two resonance frequencies, off-resonance T(1rho) at two spin-lock frequencies, and proton-decoupled T(1rho). Experiments were performed at different temperatures and hydration levels (up to 17% H(2)O by weight). The natural abundance (13)C-CPMAS spectrum of polylysine provides spectral resolution of all types of backbone and side chain carbons and thus, dynamic parameters could be determined separately for each of them. At the same time, the conformational properties of polylysine were investigated by Fourier transform infrared spectroscopy. The data obtained from the different NMR experiments were simultaneously analyzed using the correlation function formalism and model-free approach. The results indicate that in dry polylysine both backbone and side chains take part in two low amplitude motions with correlation times of the order of 10(-4) s and 10(-9) s. Upon hydration, the dynamic parameters of the backbone remain almost constant except for the amplitude of the slower process that increases moderately. The side chain dynamics reveals a much stronger hydration response: the amplitudes of both slow and fast motions increase significantly and the correlation time of the slow motion shortens by about five orders of magnitude, and at hydration levels of more than 10% H(2)O fast and slow side chain motions are experimentally indistinguishable. These changes in the molecular dynamics cannot be ascribed to any hydration-dependent conformational transitions of polylysine because IR spectra reveal almost no hydration dependence in either backbone or side chain absorption domains. The physical nature of the fast and slow motions, their correlation time distributions, and hydration dependence of microdynamic parameters are discussed.