Dynamic Geometry and Kinetics of Polymer Confined in Self-Assembly via Cooperative Hydrogen Bonding: A Solid-State NMR Study under Paramagnetic Doping

Abstract

Well-defined arrangements of the secondary intermolecular interactions play important roles for the design of new materials, which leads to unique self-assemblies consisting of multicomponent molecules with different sizes and shapes. Much work has gone into characterizing the static structures of such selfassembled systems. 1 Cooperative secondary interactions influence the molecular dynamics of the components and the bulk properties. Therefore, understanding relationship between microscopic dynamics and secondary interactions in self-assemblies is an important subject in this field. 2 Poly(ethylene oxide) (PEO) is a hydrogen acceptor and can form supramolecular crystals with a small molecule of UREA, which is a hydrogen donor, in methanol solutions. XRD revealed that two-thirds of the UREA molecules form nanochannels with a diameter of ∼1 nm, inside which isolated PEO chains and the remaining UREA molecules are included ( Figure 1a). 3 Several groups have investigated PEO dynamics in this unique morphology by conventional NMR relaxation measurements and have suggested that the dynamics of PEO is restricted by cooperative hydrogen bonding. 4,5 However, the details of the geometry and kinetics of the molecular dynamics of PEO in this supramolecular system are not well understood.Solid-state (SS) NMR techniques using magnetically anisotropic interactions can provide not only the kinetics and but also the geometry of the molecular dynamics. 6 In the past decade, various sophisticated SS-NMR techniques have been developed to investigate molecular dynamics in a wide frequency range in natural abundance. 7-11 Among them, center bands only detection of exchange (CODEX) 7,8 NMR can provide both the geometry and kinetics of molecular dynamics in a slow range and has been successfully applied to investigate the molecular dynamics of small molecules, 7 polymers, 12-14 liquid crystals, 15 and polymer blends. 16 However, this robust technique suffers from low NMR sensitivity, which has limited its application in chemically complex systems and/or systems with long T 1H values. Recently, Wickramasinghe et al. proposed a simple approach using paramagnetic doping for sensitivity enhancement. 17 Adding a small amount of a paramagnetic compound into a protein crystal could effectively shorten the T 1H value without disturbing the structures. Similar strategies may be applicable to self-assemblied systems. In this Communication, CODEX NMR and paramagnetic doping have been used to elucidate the dynamic nature of PEO in a PEO-UREA supramolecular system in natural abundance. Figure 1a shows the 13 C CPMAS NMR spectrum for PEO-UREA supramolecules at 213 K and their structures. PEO shows a very sharp 13 C signal at 70 ppm. The single resonance supports the result the PEO chains adopt a uniform 4 1 helical conformation which was obtained by XRD. 3 The observed single resonance is in stark contrast to a bulk PEO signal, which has multiple splitting on its 13 C signal (Figure 2b). Using 13 C-13 C double quantum NMR combined with relaxation...

Roles of Conformational Flexibility in the Crystallization of Stereoirregular Polymers

“… For a -hPNB, the CODEX decaying curves get deeper and deeper with increasing temperature. Similar observations were reported in several mobile polymer crystals such as PEO, i -PP, , i -P4M1P, PLLA, and so forth. For i -hPNB, the CODEX t mix decaying curve for the C5,6 carbon of i -hPNB initially follows a similar trend with a -hPNB.…”

“…This implies that the cyclopentyl ring is not limited to two conformations but is flexible and adopts multiple conformations. The characteristic reorientation angles of 5–30° in a -hPNB are quite unique compared to those of other mobile crystals such as PE, i -PP, , PEO, i -P4M1P, i -PB1 form III, PLLA, and so forth, which achieve high reorientation angles (75–200°) depending on their conformations. Register and co-workers investigated crystal unit-cell parameters of a -hPNB below T cc and reported the 0.5% contraction of the c -axis dimension between 23 and 95 °C .…”

Characterization of the Slow Molecular Dynamics of Poly(l‐Lactic Acid) in α and α′ Phases, in a Glassy State, and in a Complex with Poly(d‐Lactic Acid) by Solid‐State NMR

“…In this work, molecular dynamics of the PLA chains in the α′, SC, and glassy samples in a slow dynamic regime for correlation time, 〈τ c 〉, of 10 −3 to 10 2 s are investigated by centerband‐only detection of exchange (CODEX) NMR . The evolution‐time dependence of the CODEX decay curve revealed that PLLA chains in the SC and α′ in addition to the α form exhibit large‐amplitude helical jump motions, similar to those previously observed for several polyolefins and other polymers . Moreover, the temperature dependence of the 〈τ c 〉 values, as determined by mixing‐time dependence CODEX experiments, indicated the kinetics of the molecular motions in three crystalline phases largely different from each other.…”

“…[35] The evolution-time dependence of the CODEX decay curve revealed that PLLA chains in the SC and α′ in addition to the α form [34] exhibit large-amplitude helical jump motions, similar to those previously observed for several polyolefins [4][5][6][7]9,14] and other polymers. [36,37] Moreover, the temperature dependence of the 〈τ c 〉 values, as determined by mixing-time dependence CODEX experiments, indicated the kinetics of the molecular motions in three crystalline phases largely different from each other. Comparisons of the reported crystalline structures and dynamic data obtained during the current study provide detailed information about the relationship between structure and dynamics.…”

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

“…Dipolar-based ssNMR techniques have successfully revealed the three dimensional structure of peptides, as well as biomacromolecules, in the past decade [28,29,30]. In polymers, molecular motions including time scale and amplitude of re-orientations in the glassy [27], crystalline [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47], and melt [48,49] states, and local packing [50] and conformation [51] in the glassy state have been elucidated by various NMR techniques.…”