Chain Dynamics In The Crystalline Region Of Polyethylene Oxide Peo As Investigated By Solid State Nmr

Several semi-crystalline polymers show molecular dynamics of chains in the crystalline regions in the mechanical ac relaxation temperature range and are categorized as ac mobile crystals. Recently, several groups focused on understanding morphological effects on the molecular dynamics of ac mobile crystals. Until now, it was suggested that local ordering of interfacial segments, lamellae thickness, and entanglements in the amorphous regions, affect crystalline stem dynamics of semi-crystalline polymers. However, several structural effects on dynamics are still controversial in literatures. To clarify structural effects on molecular dynamics of the crystalline chains, well controlled polymer systems are demanded. In this work, we investigated how morphology, lamellae thickness, chain-folding number, and artificial defects made at the surface affect crystalline chain dynamics of poly(ethylene oxide) (PEO) with Mn=5,000 and PDI = 1.05. We prepared single crystals with different chain-folding numbers of 0-2, different thickness of 10- 32 nm, and melt-grown crystals. Besides, we synthesized PEO5k-isobutyl-substituted polyhedral silsesquioxane (BPOSS). BPOSS nanoparticle (giant defects) appeared at the surface of single crystals. One dimensional exchange NMR demonstrated that fully extended chain conformation and giant defect lead to restricted and similar dynamics of PEO stems while presence of chain folding in the single crystals and melt-grown crystals show relatively fast dynamics.

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In this dissertation, we have focused on the study of the interplay of structure and molecular dynamics of soft materials including supramolecules and semicrystalline polymers at the molecular scale through various state-of-the-art solid state NMR techniques. The dissertation is consisting of three parts. In Chapter IV, we focus on the atomic scale dynamics for a Janus bisamide supramolecule. Relationship between unique structure and dynamics will be demonstrated. On the basis of the determined conformations and packing structures of the alkyl chains in ordered and disordered crystalline phases, along with the geometry and kinetic parameters of the structural elements' dynamics, the self-assembly, the phase-transition mechanisms, and the relationship between the structure and dynamics of the asymmetric Janus bisamide supramolecules were addressed.In Chapter V, we investigate molecular dynamics of semicrystalline polymers including poly-lactic acid-PLA, polyethylene oxide PEO, and polyoxymethylene POM in well controlled morphologies. Based on dynamic frequency and geometry of molecular motions, we've discussed possible structural factors that influence chain dynamics in the crystalline regions. In Chapter VI, we investigate the chain-trajectory of PLA stereocomplex by 13C Double-Quantum -DQ NMR in combination with spin-dynamics simulation. Poly-L-lactide PLLA and poly-D-lactide -PDLA alternatively pack with each other and form stereocomplex crystals (SCs). The habits of SCs formed in the dilute solution depend highly on the molecular weight Mw. It was demonstrated that the ensemble average of the successive adjacent re-entry number n for the l-PLLA chains drastically changes depending on Mws of the counter PDLA chains in the SCs. It was concluded that the limited space for two kinds of PLA chains at the fold surface significantly influence the chain-folding patterns inside the SCs and as a result led to the unique Mw dependence of the crystal morphology.

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In this work, we mainly studied the molecular dynamics of polyoxymethylene in fully extended single crystals (whiskers) and folded lamellae by using the solid-state NMR (SSNMR). It was found that spin lattice relaxation time in the laboratory frame (T1[subscript ,H]) in the latter is 1.5 s at 298 K while the former shows 80.3 s. The reason for the ultra-long relaxation time was attributed to the perfect crystals with the fully extended chains. And the length of the POM whisker chains was determined to be around 20 [micrometers] by Transmission Electron Microscopy. The copper (II) acetylacetonate (CuAA) was used to shorten the T1[subscript ,H] values down to 5.9 s. Crystallinity of POM whisker crystals ([Chi]c = ~100 %) and POM lamellar crystals ([Chi]c = 63.25 %) was determined by 13C direct polarization and magic angle spinning (DP/MAS) spectrum. Centerband-only Detection of Exchange (CODEX) experiment was conducted to study the slow molecular dynamics of POM whisker crystals and folded lamellae at various temperatures. The correlation time in the former was much longer than the latter. The activation energy E[subscript a] (97 [plus or minus] 4kJ/mol) of POM lamellar crystal was slightly larger than that of the POM whisker crystal (60 [plus or minus] 5kJ/mol). The lower E[subscript a] value suggested that the whisker crystals have better thermal stability than that for the folded crystals.