Elucidation Of Chain Folding Structure And Crystallization Mechanism Of Semicrystalline Polymer 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.

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.

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.

Molecular Characterization of Polymers presents a range of advanced and cutting-edge methods for the characterization of polymers at the molecular level, guiding the reader through theory, fundamentals, instrumentation, and applications, and supporting the end goal of efficient material selection and improved material performance. Each chapter focuses on a specific technique or family of techniques, including the different areas of chromatography, field flow fractionation, long chain branching, static and dynamic light scattering, mass spectrometry, NMR, X-Ray and neutron scattering, polymer dilute solution viscometry, microscopy, and vibrational spectroscopy. In each case, in-depth coverage explains how to successfully implement and utilize the technique. This practical resource is highly valuable to researchers and advanced students in polymer science, materials science, and engineering, and to those from other disciplines and industries who are unfamiliar with polymer characterization techniques. Introduces a range of advanced characterization methods, covering aspects such as molecular weight, polydispersity, branching, composition, and tacticity Enables the reader to understand and to compare the available technique, and implement the selected technique(s), with a view to improving properties of the polymeric material Establishes a strong link between basic principles, characterization techniques, and real-life applications

At alleviated temperatures, some semicrystralline polymers can be stretched to very large deformation ratios. Such deformations of semicrystalline polymers have been extensively studied since 1960s. Based on experimental observations and theoretical investigations, solid-state transformation (three stage model) proposed in 1971 and local melting and recrystallization in 1978 have been considered two major mechanisms to explain the deformations of polymer crystals. With the elucidation of molecular dynamics in the last two decades, it was proposed in 1999 that helical jump motion plays an important role in crystal deformation. On the other hand, the new structures induced by deformation also influence the molecular motions and resultant properties of deformed polymers. Such processing-structure-property relationship is very important to understand the polymer behaviors as well as to inform the polymer industry. However, to conclude the deformation mechanism of semicrystalline polymer is still challenging, because there is no appropriate tools to trace structural evolution of polymer chains inside the polymer bulk. And detailed understanding of the relationships between hierarchical structures and specific motions and properties need to be achieved. In this dissertation, using the advanced tool of solid-state NMR (ss-NMR), we achieve three goals: Firstly, we investigate the hierarchical crystalline structural changes of isotactic polypropylene (iPP) upon high temperature stretching to understand the deformation process. Secondly, we evaluate the roles of local packing structure and crystal thickness in determining the stem motions and thermal properties of deformed a-form iPP. Thirdly, we utilize 13C-labeled isotactic polypropylene (iPP) to trace the change of chain folding number as a function of e to conclude molecular-level deformation mechanism. To realize the first and second goals, the chain packing, crystal thickness, molecular dynamics, and melting temperature (Tm) of a-form iPP drawn uniaxially at high temperatures of 100 - 150 °C were investigated using solid-state (SS) NMR and DSC. Two types of iPP samples with disordered (a1) and relatively ordered (a2-rich) packing structures were prepared via different thermal treatments and drawn up to an engineering strain (e) of approximately 20. High-resolution 13C NMR detected continuous a2-to-a1 transformations in the original a2-rich samples over the entire deformation range at all drawing temperatures (Tds). A sudden a1-to-a2 transformation was found to occur in the original a1 sample in the small e range of approximately 3 - 8 at Td = 140 °C. Then, in the late stage, the newly grown a2 structure reversely transformed into a1 structure with further increase in e, as observed in the original a2-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of crystallographic constraints, the continuous a2-to-a1 transformation over the entire deformation range is attributed to molecular-level melting and recrystallization facilitated by chain diffusion. The steep a1-to-a2 transformation in the smaller e range is assigned to isotropic melting and recrystallization induced by stress. After the large deformations (e ̃ 20) of the original a2-rich and a1 samples at Td = 150 and 140 °C, respectively, 1H spin diffusion verified increases in the crystal thickness in both the former (14.1 nm at e = 0 and 20.1 nm at e = 20) and the latter (9.2 to 17.0 nm). Centerband-Only Detection of EXchange (CODEX) NMR at 120 °C demonstrated that the correlation time (tc) of the helical jump for the former drastically decreased from tc = 52.4 ± 5.2 at e = 0 to 9.3 ± 1.8 ms at e = 20 but slightly increased from 4.2 ± 1.3 to 7.1 ± 0.9 ms for the latter. Additionally, DSC indicated that the melting temperature (Tm) for the former decreased considerably from 173 °C at e = 0 to 165 °C at e = 20, whereas the melting temperature (Tm) remained nearly invariant at 163 °C for the latter. Based on these findings, we conclude that the local packing structure plays a crucial role in determining the molecular dynamics of the stems and Tm of largely deformed iPP materials. The established relations among the structures, the dynamics, and the thermal properties provide a useful guide to achieving improved properties of iPP materials under processing. To realize the third goal, 13C-13C Double Quantum (DQ) NMR was applied to trace the structure evolution of 13C-labeled iPP chains inside the crystallites under stretching at 100 oC. DQ NMR based on spatial proximity of 13C labeled nuclei proved conformational changes from the folded chains to the extended ones of the iPP chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays critical role to achieve the large deformability of iPP.

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 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.

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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.