Chain Folding And Crystallization Mechanism Of Polyl Lactide As Investigated By Solid State Nmr

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.

Semi-crystalline polymers undergo drastic structural changes from random coils in the melt states to folded chains in the crystalline lamellae during crystallization. One fundamental question in this process is the long-standing controversy about the chain-folding structures in semi-crystalline polymers. The isotactic polypropylene (iPP) is one of the most important polymer materials in commercial application as well as in academic field. Also, the iPP has different crystalline forms, in which the chain packing structures, such as fraction of different forms and spatial heterogeneity, were not fully understood previously. When cooled from melt, iPP crystallize into a form, which can be further divided into ordered a2 and disordered a1 depending on the methyl group orientations. Using modern high-resolution solid-state NMR (SS-NMR), the chain-packing structure of a1 and a2 forms can be quantitatively separated by the dramatic lineshape differences. The effects of chemical structures of iPP on the chain-packing structure in the crystalline region have been studied. The higher stereo regularity sample forms more a2 form at Tc > 135 oC. Moreover, 1H spin diffusion experiments indicated that the a1 and a2 form will form domain structures at average side of ca. 40 nm when iPP crystallized at 150 oC Furthermore, high-resolution NMR spectra demonstrated that the stereo regularity defects are excluded in the crystalline region at Tc = 150 oC, whereas 4% of the defect remains in the crystalline region quench annealed a1 form. Finally, the chain-folding structures and ensemble average of successive chain-folding number are revealed by 13C-13C double quantum (DQ) NMR. The average chain-folding number is determined to be 5 to 7 for both a1 and a2 forms. However, different chain-folding structures are concluded based on the chain-packing structures and conformational constrains. Through obtained packing and folding structures, crystallization mechanism was discussed at molecular levels.

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.

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.

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.

Polymers with side-chain groups capable of crystallization independent of the backbone have been of interest for many years. Recent reviews summarize the syntheses and characterization of a large number of different families of comb polymers. The easiest to synthesize and evaluate with respect to property trends in a homologous series are those containing linear alkyl side-chains. Derivatives with greater than 8-12 carbons form side-chain crystals with hexagonal packing of the alkyl moieties. In this type of packing, individual alkyl groups adopt an all-trans conformation but with rotational disorder between side-chains. In the process of characterizing the various DHA polymers synthesized, we became interested in the use of solid state C NMR to evaluate several thermal transitions that we have tentatively ascribed to separate side-chain and backbone molecular relaxation processes. In the course of this project, a closely related paper appeared on the use of variable temperature CP/MAS to study side-chain and backbone molecular relaxation processes. In the course of this project, a closely related paper appeared on the use of variable temperature CP/MAS to study side-chain and backbone motion of poly(eta-n-octadecyl L-glutamate) (POG). We describe here the variable temperature C CP/MAS NMR behavior of the C18 DHA polymer along with several commercially available comb polymers for comparison. Keywords: Molecule molecule interactions, Molecular structure, Nuclear magnetic resonance. (kt).

The series Topics in Current Chemistry presents critical reviews of the present and future trends in modern chemical research. The scope of coverage is all areas of chemical science including the interfaces with related disciplines such as biology, medicine and materials science. The goal of each thematic volume is to give the non-specialist reader, whether in academia or industry, a comprehensive insight into an area where new research is emerging which is of interest to a larger scientific audience. Each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole. The most significant developments of the last 5 to 10 years are presented using selected examples to illustrate the principles discussed. The coverage is not intended to be an exhaustive summary of the field or include large quantities of data, but should rather be conceptual, concentrating on the methodological thinking that will allow the non-specialist reader to understand the information presented. Contributions also offer an outlook on potential future developments in the field. Review articles for the individual volumes are invited by the volume editors. Readership: research chemists at universities or in industry, graduate students.

This book provides the whole spectrum of polysaccharides from basic concepts to commercial market applications. Chapters cover various types of sources, classification, properties, characterization, processing, rheology and fabrication of polysaccharide-based materials and their composites and gels. The applications of polysaccharides include in cosmetics, food science, drug delivery, biomedicine, biofuel production, marine, packaging, chromatography and environmental remediation. It also reviews the fabrication of inorganic and carbon nanomaterials from polysaccharides. The book incorporates industrial applications and will fill the gap between the exploration works in the laboratory and viable applications in related ventures.

In the context of polymer crystallization there are several still open and often controversially debated questions. The present volume addresses issues such as novel general views and concepts. It presents new ideas in a connected and accessible way. The intention is thus not only to provide a summary of the present state-of-the-art to all active works but to provide an entry point to newcomer and graduate students entering the field.