Elucidation Of Molecular Dynamics Of Polymer Chains In Fully Extended Single Crystals By Solid State Nmr Polyoxymethylene

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.

Since Keller found single crystal of polyethylene (PE) in 1957, he first proposed the long polymer chains are more or less regularly folded in thin lamellae and the chain stems between successive folds oriented preferentially normal to the plane of the lamellae. The discovery has triggered the study of how long polymer molecules are embedded in the thin lamellae of semicrystalline polymers. Subsequently, several different crystallization mechanisms had been proposed such as Lauritzen-Hoffman kinetic theory, multistage model, aggregation model, and bundle model, etc. In order to prove these crystallization models, chain trajectory of semicrystalline polymers have been investigated prominently by neutron scattering (NS) and infrared (IR) spectroscopy combined with 1H/2H polymers because the chain-level structure would reflect the process during the crystallization. Later on, other techniques such as atomic force microscopy (AFM) and decoration method on the surface of PE crystals have been developed. Irrespective of the tremendous efforts over the last half century, the detailed chain trajectory of semicrystalline polymers still remains missing due to insufficient resolution of available techniques and intrinsic polymer structures that consist of repeating monomer units. Therefore, various crystallization theories could not be verified until now and hence a new approach is required to clarify the molecular level structure. In this dissertation, we have developed a novel strategy to investigate chain trajectory of semicrystalline polymers as a function of concentration and crystallization temperature. We have used solid-state nuclear magnetic resonance (SS-NMR) spectroscopy combined with selectively 13C isotopic labeling approach. Since the SS-NMR approach based on 13C-13C magnetically dipolar interactions has atomic level resolutions, the approach was able to investigate the chain trajectory of isotactic poly(1-butene) (iPB1). 13C-13C double quantum (DQ) NMR and spin-dynamics simulations determined adjacent re-entry parameters of the re-entrance site, chain-folding fraction (F), average successive chain-folding number n, and molecular dimension of folded chains of iPB1 with a relatively low Mw of 37 K g/mol in melt- and solution-grown crystals in a wide range of crystallization temperature (T[subscript c]). The determined chain trajectory of form I iPB1, which is one of the type of iPB1 crystal form, turned out that the re-entrance site of iPB1 is independence of the concentration and crystallization temperatures while the lower concentration induces long-range order and higher fraction of adjacent re-entry chain-folding. The n and F values were nearly invariant of T[subscript c] in each the solution- and melt-grown systems. In addition, we studied the effects of T[subscript c] on the lamellar thickness (l[subscript c]), crystallinity ([Chi][subscript c]), and morphology of iPB1 crystallized in both states. The combined data obtained at different length scales demonstrated that kinetics plays different roles for the structural formations from molecular to morphological levels. Lastly, another iPB1 form III displayed three dimensional clusters of folded chains instead of the two dimensional one expected by classical surface nucleation model of crystallization. Through the molecular level structures, [Chi][subscript c], l[subscript c], morphology of single crystal, and the dimension of folded chains as well as the molecular dynamics information reported in the literature, we discussed the crystallization mechanisms of semicrystalline polymer from a molecular level of view.

Since the discovery of single crystals of polyethylene by Keller using transmission electron microscopy (TEM)1, various methods have been developed to unravel detailed chain-level structures (chain-folding) of semi-crystalline polymers in both bulk and single crystals. Nevertheless, understanding of molecular structure basis, is still a controversial issue due to experiment limitations. Such experimental situations are largely different from theory or simulations. Recently, our group developed a unique approach, which can investigate the chain trajectory of the synthetic polymer in bulk crystals, using 13C-13C double quantum (DQ) NMR combined with 13C labeling samples2. In this thesis, we propose systematic research on chain-level structure of a semi-crystalline polymer prepared under different conditions. We investigated chain-trajectory of 13C CH3-labeled isotactic poly(3-methyl-butene-1) (iP3MB1) in melt-grown crystals and solution-grown single crystals blended with non-labeled iP3MB1 using solid-state NMR. Comparisons of 13C-13C double quantum (DQ) NMR results with spin dynamics simulation revealed individual chains in melting grown crystals fold in three different directions and chains in single crystals prefer direction parallel to the long side (crystallographic a axis) of the rectangle-size single crystal is determined.

"Research on the dynamic properties and related molecular reorientation processes in polymer chains has attracted special attention in recent years. The solid-state nuclear magnetic resonance (SSNMR) is one of the most important complementary techniques in the scientific testing of macromolecules. When polymer molecules make reorientation motions, a fluctuating local magnetic field is generated. Thus, the shape of the resonance line emerges as a function of temperature and reveals the activation energy of the polymer chain"--

In polymer science and technology, the advanced development of various new polymer materials with excellent properties and functions is desirable. For this purpose it is necesary to determine the exact relationship between physical properties and molecular structure-dynamics with powerful techniques. One such technique is solid state NMR. Recently, high resolution NMR studies of solids have been realized by using advanced pulse and mechanical techniques, which has resulted in a variety of structural and dynamical information on polymer systems. Solid state NMR has provided characteristic information which cannot be obtained by other spectroscopic methods. This book is divided into two parts. The first part covers the principles of NMR, important NMR parameters such as chemical shifts, relaxation times, dipolar interactions, quadrupolar interactions, pulse techniques and new NMR methods. In the second part, applications of NMR to a variety of polymer systems in the solid state are described. Features of this book: • Contains an up-to-date and comprehensive account of solid state NMR of polymers by leading researchers in the field • Provides a compilation of solid state NMR of polymers, which makes it an ideal reference book for both NMR researchers and general polymer scientists. This book will be of interest to the NMR community, and will be invaluable for both the beginner and the expert.

NMR spectroscopy has emerged as one of the most important methods for the solid-state characterisation of polymers. This report gives an overview of the methods and applications of NMR to relevant polymer problems with an emphasis on how NMR can be used for materials characterisation and to understand structure-property relationships in polymers. An additional indexed section containing several hundred abstracts from the Rapra Polymer Library database gives useful references for further reading.

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.

In NMR, it is well-known that the chemical shift conveys structural informa tion, e. g. a carbonyl carbon will have a resonance frequency appreciably dif ferent from a methyl carbon, etc. The relation between structure and chemical shift is mostly established by empirical rules on the basis of prior experience. It is only quite recently that the advent of both comparatively cheap comput ing power and novel quantum chemistry approaches have provided feasible routes to calculate the chemical shift at the ab initio level for molecules of reasonable size. This raises the question whether application of these novel theoretical concepts offers a means of obtaining new structural information for the complex chain molecules one deals with in polymer science. Solid state 13C-NMR spectra of glassy amorphous polymers display broad, partially structured resonance regions that reflect the underlying disorder of the polymer chains. The chemical shift responds to the variation of the ge ometry of the chain, and the broad resonance regions can be explained by an inhomogeneous superposition of various chain geometries (and thus chem ical shifts). In this review, we present a novel approach to combine polymer chain statistical models, quantum chemistry and solid state NMR to pro vide quantitative information about the local chain geometry in amorphous polymers. The statistical model yields the relative occurrence of the various geometries, and quantum chemistry (together with a force field geometry op timization) establishes the link between geometry and chemical shift.