David L. VanderHart

Native Cellulose: A Composite of Two Distinct Crystalline Forms

Multiplicities in the resonances of chemically equivalent carbons, which appear in the solid-state carbon-13 nuclear magnetic resonance spectra of native celluloses, have been examined at high resolution. The patterns of variation are consistent with the existence of two distinct crystalline forms. One form is dominant in bacterial and algal celluloses, whereas the other is dominant in celluloses from higher plants.

Resolution in 13C NMR of organic solids using high-power proton decoupling and magic-angle sample spinning

Abstract The 13C NMR linewidths observed in organic solids by means of high-power proton decoupling and magic-angle sample spinning are roughly 10 to 100 times broader than resonances in the liquid phase. This paper investigates important 13C line-broadening mechanisms in organic solids and their dependences on experimental parameters, notably static and rf magnetic field strength. The discussion is limited to glassy (disordered, partially mobile) and crystalline (ordered, rigid) organics at natural isotopic abundance. Excluded are elastomers and systems with a third dipolar coupled nuclear species. Experimental data, primarily at 1.4 T, for glassy and semi-crystalline polymers as well as crystalline materials, illustrate and confirm the linebroadening mechanisms identified. For some specimens, 13C linewidths are compared at 1.4 and 4.7 T. It is found that a substantial linebroadening (0.5 to 6 ppm) corresponding to a dispersion of isotropic chemical shifts can arise from distributions of anisotropic sources of magnetic susceptibility, bond angles, or frozen molecular conformations; in other cases, the resonance lines may be split into many distinct lines by magnetic inequivalences present in the solid but not the liquid phase. For crystalline materials, methods for reducing the broadening from anisotropic bulk susceptibility are discussed. Other broadening mechanisms considered are: insufficient proton-decoupling fields, off-resonance decoupling, imperfections in magic-angle sample spinning, and motional modulation of both the carbon-proton dipolar coupling and the carbon chemical shift anisotropy. On consideration of these mechanisms, it is anticipated (and shown experimentally in limited cases) that no significant gain in resolution will be enjoyed at high magnetic fields, especially when variable-temperature operation is available. In some instances, degradation of resolution may occur at high field if large rf field strengths or high spinning rates cannot be achieved.

Measurement of 13C chemical shifts in solids

Abstract A pulse sequence and sample geometry which allows the measurement of 13 C chemical shifts of solid materials relative to liquid tetramethylsilane (TMS) are described. Using this technique, the chemical shifts of a series of common engineering plastics were measured and reported. A small number of candidate secondary shift reference materials were considered and their chemical shifts measured. Most of these materials proved to be unsuitable for general 13 C shift references for differing reasons. The most promising standard investigated was polydimethylsilane. The measurement of chemical shifts in solid materials is slightly complicated by anisotropic magnetic properties and sensitivity to magic-angle missetting when the material exhibits macroscopic orientation. These complications are discussed in detail and examples of misleading spectra are shown.

The role of solid state 13C NMR spectroscopy in studies of the nature of native celluloses

Published spectroscopic observations pertaining to the crystal structure of native celluloses are reviewed for the purpose of defining our current level of understanding about crystalline polymorphism in these materials. Emphasis is placed on observations from solid state 13C nuclear magnetic resonance (NMR), which first led to the postulate that most native, semicrystalline celluloses are composites of two crystalline allomorphs, labeled Ialpha and Ibeta. Historical background is presented, highlighting the structural controversies which mainly arose because different native celluloses were used, each one representing a different mixture of allomorphs. Input from Raman, infrared (IR) and electron diffraction data is included in the discussion of our current understanding of polymorphism in native celluloses. Also noted is the input from more recently studied celluloses (e.g., Halocynthia) as well as from newer processes that convert the Ialpha to the Ibeta form. On the basis of Raman and IR observations, it is argued that the Ialpha and Ibeta allomorphs differ in hydrogen bonding patterns only and that backbone conformations are nearly identical. Also, the point is made that the absence of correlation field splittings in the Raman spectra calls into question (although it does not disprove) whether the normal two-chain-per-unit-cell, monoclinic Ibeta allomorph really possesses two equivalent chains. Considerable discussion is devoted to the allomorphic composition of cellulose crystallites in higher plants. Published methods of NMR lineshape analysis for the higher plant celluloses are reviewed and critiqued, both from the point of view of lineshape theory and from the point of view of self-consistency of inferences that are based on lineshape analyses for different carbons (particularly C1 and C4). It is concluded that higher plant celluloses most likely possess a minor amount of the Ialpha allomorph where the Ialpha/Ibeta ratio is probably less than 0.25.

Influence of molecular packing on solid-state 13C chemical shifts: The n-alkanes

Abstract The question of the influence of molecular packing on isotropic chemical shifts (ICS) in solids is probed experimentally. The n -alkanes are found in four crystallographic forms. Since the isolated chain geometry is considered to be the same in all of these forms and since these solids lack specific interactions (e.g., hydrogen bonds), observed ICS variations for corresponding carbons should be related to packing effects. It is found that the ICS of the interior methylene carbons is very constant in three forms plus orthorhombic polyethylene. The exception is the triclinic form represented by C-20, for which this resonance is shifted 1.3 + 0.4 ppm downfield. Beyond the uniqueness of the triclinic subcell, the origin of this shift is not obvious. Magnetic susceptibility effects are considered and dismissed as inadequate. It is suggested that until solid-state chemical shifts are better understood, care should be taken in attributing observed shifts (less than 2 ppm) for a given carbon to changes in conformation or specific interactions.

Some perspectives on the interpretation of proton NMR spin diffusion data in terms of polymer morphologies.

Proton spin diffusion data yield morphological information over dimensions covering approximately the 2-50 nm range. In this article, the interpretation of such data for polymers is emphasized, recognizing that the mathematical framework for much of this interpretation already exists in the literature. Practical issues are considered, for example, a useful scaling of plotted data is suggested, key attributes of the data are identified and ambiguities in the mapping of data into morphological models are spelled out. Discussion is limited to two-phase systems, where it is assumed that, by employing multiple-pulse methods polarization gradients can be generated, whose spunal sharpness is limited solety by the morphological definition of the interfaces. Interpretation of data in terms of morphology and stoichiometry is emphasized, where stoichiometric issues pertain only to chemically heterogeneous systems. Extraction of stoichiometric information from spin diffusion data is not commonly attempted; the discussion included herein allows for the possibility that the composition of phases may be chemically mixed. Methods for generating gradients are discussed only briefly. A standardized spin diffusion plot is proposed and the initial slope of this plot is tocussed on for providing information about morphology and stoichiometry. Ambiguities of interpretation considered include the dimensionality of the deduced morphology and, for systems with chemical heterogeneity the uniqueness of the compositional characterization of each phase. In addition, funite difference methods are used to simulate entire spin diffusion curves for idealized lamellar and hexagonal rod/matrix morphologies. Comparisons of these curves show that distinguishing 1-D and 2-D morphologies on the basis of experimental data is unlikely to be successful over the range of stoichiometrics where such morphologies are expected. Several examples of spin diffusion data are presented. Brief treatments of the following topics are included: finite interface width, estimation of spin diffusion constants, and incorporation of longitudinal relaxation effects. Finally, a short experimental discussion on the preparation of polarization gradients is given including those preparations which make use of differences in the multiple-pulse relaxation time, T1xz. It is noted that T1xz decays may be strongly perturbed in the presence of magic angle spinning, therefore, strategies are also outlined for minimizing these effects.

Quantitative determination of the monoclinic crystalline phase content in polyethylene by 13C n.m.r.

Abstract Solid-state 13 C n.m.r. spectroscopy involving the techniques of cross-polarization (CP), magic angle spinning (MAS), and high power proton decoupling, has been used to determine quantitatively the ratio of monoclinic to orthorhombic crystalline phases in compression moalded ultra-high molecular weight polyethylene (UHMWPE) sheet which had been stretched uniaxially. Criteria for expecting quantitative relative intensities in 13 C CP-MAS spectra are discussed. Attenuation of the non-crystalline (NC) signals relative to crystalline signals was observed. Experiments were therefore carried out to ascertain whether measurable relative intensity distortions exist between the monoclinic crystalline phase (MCP) and the orthorhombic crystalline phase (OCP) resonances due to possible differences in proton ‘spin diffusion’ between the NC and the two crystalline phases during cross-polarization. No relative intensity distortions were detected. This result, coupled with experiments in which spin diffusion was monitored at times longer than those used for cross-polarization, suggests that the average distance from the protons in a given crystalline phase to the nearest protons in the NC regions is the same for the MCP and the OCP. Finally, non-spinning 13 C spectra of the deformed polyethylene were recorded to determine the orientation of the chains in the crystalline and NC regions. The Hermans orientation function, F c , was determined independently for the crystalline (combined OCP and MCP) and NC regions, and found to be 0.66 + 0.06 and 0.23 + 0.04 respectively. The occurrence of orientation in the NC regions may be evidence for internal stresses, which, it is suggested, also stabilize the metastable MCP in the stretched sample.

Natural-abundance 133C13C spin exchange in rigid crystalline organic solids

Abstract Natural-abundance 13C13C spin exchange, in the presence of proton dipolar couplings, has been investigated for the crystalline regions of two semicrystalline polymers, linear polyethylene (LPE), and cellulose. Because of very long longitudinal relaxation times, T1C, in these materials, spin exchange could be followed to times exceeding 100 s. This paper focuses on the behavior of spin exchange at the longer times. The incentive for investigating LPE was that other published work on this system suggested that spin exchange proceeded more rapidly than theory would predict. Our results on an oriented sample were similar to those reported. In addition, however, the spin-exchange behavior had an unexpected temperature dependence which, in turn, was attributed to temperature-dependent chain transport through the crystalline lattice. In the low-temperature limit, spin exchange was consistent with theory. In cellulose whose monomer contains six different carbons and whose crystal structure imposes magnetic inequivalence on certain carbons, there also exist questions about crystalline polymorphy. Spin exchange was studied following a selective population perturbation of only one multiplet component within the C1 resonance pattern. Results reenforced the hypothesis of polymorphy. Mathematical modeling of spin exchange in LPE and cellulose was carried out using an isolated-pair approximation. This model was shown to be a good description for only a few seconds. A correspondence between spin-exchange time and the distance of magnetization transport was estimated both by analyzing the isolated-pair decay and by considering the probability profile for isolated n-spin clusters as a function time. This correspondence between time and distance shows that classical diffusion behavior is not followed. Influence of spin exchange on T1C measurements is also discussed.

High Throughput Methods for Polymer Nanocomposites Research: Extrusion, NMR Characterization and Flammability Property Screening

A large number of parameters influence polymer-nanocomposite performance and developing a detailed understanding of these materials involves investigation of a large volume of the associated multi-dimensional property space. This multi-dimensional parameter space for polymer-nanocomposites consists of the obvious list of different material types under consideration, such as “polymer” and “nano-additive,” but also includes interphase surface chemistry, and processing conditions. This article presents combinatorial library design and high-throughput screening methods for polymer nanocomposites intended as flame-resistant materials. Here, we present the results of using a twin-screwn extruder to create composition-gradient library strips of polymer nanocomposites that are screened with a solid-state NMR method to rapidly evaluate the optimal processing conditions for achieving nanocomposite dispersion. In addition, we present a comparison of a new rapid Cone calorimetry method to conventional Cone calorimetry and to the gradient heat-flux flame spread method.

Solid state NMR spectroscopy of specifically 13C-enriched lignin in wheat straw from coniferin

Abstract Three coniferins, specifically 13 C-enriched at side chain α, β and γ carbons, and natural abundance (unenriched) coniferin were administered to internode cavities of lignifying culms of dwarf wheat. Difference 13 C CP/MAS spectra were obtained between the spectra of the coniferin-fed and the unfed wheat straws, or between the spectra of straws fed with enriched coniferin and unenriched coniferin. The difference spectra indicated that, although the feeding of coniferin increased the lignin content slightly, the normal lignification process was not affected seriously by feeding of the precursor. The lignin derived from the coniferin in the wheat stalk was specifically 13 C-enriched at the positions corresponding to the fed precursors. It is estimated that of the total lignin associated with the bottom and top sections of the wheat straws, respectively, 15.4±2.0 and 9.5±2.5% of the lignin originated from the labelled coniferin. The percentages of the major dimeric substructures of enriched lignin in the top and bottom of internodes, respectively, are: β - O -4 including β - O -4/ α - O - R (R = carbohydrates and lignols), 74±1.5 and 65±1.5%; combined β-5, β-β and β-1 structures, 18±1.5 and 28±1.5%; and combined coniferyl alcohol and aldehyde end groups, 8±1.5 and 6±1.5%.