Volume 44 Issue 8, August 2012

Studies on NMR methodology of characterizing the structure of polymers and biopolymers based on understanding of NMR chemical shift/structure correlation by solution-state and solid-state NMR experiments, development of NMR chemical shift theory and their combination have been reviewed.

Proton spin diffusion (SD) experiments performed on low-field NMR spectrometers are a valuable means for the estimation of domain sizes in phase-separated polymers. We here investigate the interplay of the SD effect and fast T₁ relaxation by means of fits of data taken for two different polystyrene-polybutadiene di- and multiblock copolymers to numerical simulations. We demonstrate the necessity to explicitly include an interphase of intermediate mobility, and further show that a previously published, popular calibration of the SD coefficient for mobile polymer phases based on the transverse relaxation time T₂ should be revised.

The crystalline structure and molecular mobility of PVDF blended with PMMA at various fractions were investigated by solid-state ¹⁹F MAS NMR spectroscopy. The origin of the characteristic peaks observed for typical polymorphs of PVDF (α-, β- and γ-forms) are explained based on DFT calculations. The dominant crystalline form in the blends of [PVDF/PMMA]=[80/20], [70/20] and [50/50] is α-form containing a small portion of β-form, whereas that of [65/35], [60/40] and [55/45] consists of β-form and γ-form. In contrast, no crystalline structure is found in [45/55], [40/60] and [20/80], indicating that PVDF and PMMA chains are miscible at the molecular level.

Structure and molecular mobility of nascent iPP powder were investigated by differential scanning calorimetry, transmission electron microscopy and solid-state nuclear magnetic resonance (NMR). Combination of NMR with other analytical method provides the deep insight and concluded that the crystalline size of nascent powder is extremely small and the crystalline phase is easily affected by the surrounded noncrystalline phase.

The quaternary carbons linking to sulfur as cross-linking junction was assigned to the molecular singularity, which had an important role in the outstanding mechanical properties of natural rubber.

The natural rubber (NR) without vulcanization is easily deformed and strained during magic-angle spinning (MAS). The elongated NR during MAS forms molecular orientation. The static ¹³C NMR spectra of elongated NR became anisotropic, although the static ¹³C NMR spectra of NR before MAS show isotropic. Angle-dependent ¹³C NMR spectra of strips cut from the restricted elongation NR were different from those from the maximum elongated NR. The disagreement indicated that ¹³C chemical shift depends on not only the molecular orientation but also the magnetic susceptibility resulting from the alignment of NR chains.

Changes in the crystalline phase structures and molecular mobility of a semi-crystalline P(VDF₇₅/TrFE₂₅) have been analyzed using solid-state ¹⁹F MAS and ¹H→¹⁹F CP/MAS NMR spectroscopy. The conformational exchange motion at the head-to-tail linkage has a key role for the ferroelectric→paraelectric phase transition. The anomalous decrease in the signal intensity of the amorphous peaks and its T_1ρ^F value similar to the crystalline domain at 122 °C indicate the existence of cooperative motion occurring in both phases just above the transition temperature (T_c).

The relaxation behaviors for several commercial ultra-high molecular weight polyethylene reactor powders were compared. On the basis of the morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. The results of differential scanning calorimetry analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. In contrast, molecular motions detected by solid-state proton nuclear magnetic resonance techniques characterize the structural differences between these powders.

In this study, we investigated the molecular structure of the composite material of silk fibroin and polyurethane as coating agent of vascular grafts by using solid-state nuclear magnetic resonance (NMR) techniques. To elucidate the structure and dynamics, ¹³C cross-polarization (CP)/magic angle spinning (MAS) NMR studies including NMR relaxation experiments were performed. These observations showed that there is a partial interacted portion between soft segment of PU and SF, in particular, the domain size of PU:SU=1:1 composite was smaller than that of the other composites. This study gave basic information on development of new silk-like materials for vascular grafts.

Multivariate analysis was applied to the ¹³C nuclear magnetic resonance spectra of homopolymers, homopolymer blends and copolymers of methyl methacrylate (MMA) and tert-butyl methacrylate (TBMA). The first (PC1) and second (PC2) principal components reflected chemical composition of the copolymers and fraction of the MMA-TBMA dyad sequence (f_MT), respectively. Dyad and triad sequence distributions of copolymers that were obtained at higher conversions were successfully determined by partial least squares regression with those of copolymers obtained at low conversions as a training set.

This paper reports multivariate analyses such as principal component analysis and partial least-squares regression of NMR spectra of poly(N-isopropylacrylamide)s [poly(NIPAAm)s]. Principal component analysis successfully interpreted the assignments of ¹H and ¹³C NMR spectra of methylene protons and methine carbons, respectively, of poly(NIPAAm)s in terms of stereostructures. Furthermore, triad tacticity was successfully predicted by partial least-squares regression of ¹H NMR spectra of the methine and methylene groups. Consequently, it is assumed that chemometric analysis of NMR spectra is a promising method for the characterization of synthetic polymers.

¹²⁹Xe NMR spectra of ¹²⁹Xe in polyphenyleneoxide, PPO were measured at various temperatures below T_g. From the analysis of ¹²⁹Xe NMR chemical shifts, the mean volume of individual microvoids in PPO, v could be determined. The temperature when value of v becomes same as a Xe atom, was very close to T_g of PPO. Temperature dependence of individual microvoid’s volume was very similar with that of C_H′, which is obtained by dual-mode sorption model from Xe sorption isotherms and is corresponding to the unrelaxed volume of PPO in the glassy state.

Xe sorption properties of tetramethyl bisphenol A polycarbonate (TMPC)/polystyrene (PS) blends indicated the reduction of microvoids by blending. From the analysis of ¹²⁹Xe NMR chemical shifts of the ¹²⁹Xe in these blends, the mean volume of individual microvoids (v) was calculated. The v values for these blends were smaller than those of predicted by a simple additive rule, as well as the cases of variation of density and Xe sorption properties. This fact indicates that the volume contraction induced by blending for TMPC/PS blends is mainly attributed to contraction volume of individual microvoids.

The hydrogen dissolved in acrylonitrile butadiene rubber due to high-pressure hydrogen exposure was analyzed by solid-state ¹H NMR. On the basis of pressure dependency of chemical shift of free hydrogen and relaxation time of dissolved hydrogen, the hydrogen with two different characters at 4.3 and 4.8 p.p.m. were ensured to be with different mobility. Conversion of the ratio of the two states of hydrogen suggested the transition of hydrogen from the state constrained to rubber chemical structure to free volume during hydrogen elimination process.

Isotactic and syndiotactic poly(lactic acid) dimer model compounds were synthesized, and studied by ¹H and ¹³C NMR in three solvents, CDCl₃/CCl₄ (20/80 v/v), CDCl₃ and DMSO-d₆. All the peaks in the ¹H and ¹³C NMR spectra were assigned with the help of two-dimensional NMR. The tacticity splitting of the dimers showed no significant difference among the solvents. The chemical shifts were calculated and compared with the experimental shifts to understand the origin of the tacticity splitting in PLA. The observed tacticity splitting of the chemical shifts between isotactic and syndiotactic ¹H and ¹³C NMR peaks were reproduced particularly well using the combination of Becke’s three parameter hybrid method for conformational energy calculation and Hartree-Fock for chemical shift calculation.

A sulfonated aromatic block copolymer (SABC), consisting of hydrophobic and hydrophilic blocks, was analyzed by 2D NMR techniques of HSQC, HMBC and HSQC–TOCSY. Because of its complicated chemical structure with five different phenylene rings, 12 types of ¹H signals and 24 types of ¹³C signals were observed in a narrow chemical shift range (7.0–8.0 p.p.m. for ¹H and 118–162 p.p.m. for ¹³C). As a result of these studies, complete ¹H and ¹³C NMR signal assignment of SABC was performed.

The proton and carbon NMR resonances have been assigned for 3A-amino-3A-deoxy(2AS,3AS)-α-, β- and γ-CyDs (3α, 3β and 3γ). In these CyDs, a glucose residue has been replaced by an altro-pyranose unit with an axial hydroxyl group. Assignments were made with one-dimensional NMR and COSY, TOCSY, ROESY and CHSHF (heteronuclear shift correlation) spectra. Titration by NMR shift changes gave amino-group pK_a values of 7.73 and 8.84 for 3β (left) and 6A-amino-6A-deoxy-β-CyD (6β, right), respectively.

Two topics concerning the utilization of internal and external magnetic field gradients are discussed. The first topic concerns the transport of conformons in the quasi-ordered phase of a regioregular, π-conjugated polymer, poly(4-methylthiazole-2,5-diyl), using a combination of longitudinal and transverse relaxation dispersion measurements. The effects of conformon diffusion on the relaxation exponent were successfully estimated via a numerical approach that used the Fourier spectrum solution of the one-dimensional Bloch-Torrey equation. The second topic demonstrates a low-cost magnetic resonance imaging method using a magnetic field gradient from a neodymium ferromagnet. Spin echo experiments are shown for three-layered thin films of PDMS/PTFE/PDMS under a large magnetic field.

Bacteriorhodopsin (bR) has a retinal with all-trans and 13-cis, 15-syn configurations whose isomeric ratio is close to 1 in the dark and the population of 13-cis, 15-syn configuration can be increased under the pressurized condition. Increase of pressure by fast magic angle spinning (MAS) induced dynamics change at around Ala81 and Ala84 in bR together with retinal isomerization from all-trans to 13-cis configurations. We demonstrated that fast MAS NMR provides a pressure sauce for investigating the structure and dynamics change of biomacromolecules.

The images of ¹H spin density, ¹H T₂ and diffusion coefficient (D) of water molecules in the layer of silk-based vascular grafts (VG) coated by different materials that are silk fibroin or polyurethane were non-destructively observed in water using ¹H MR imaging. The inner and outer coating materials affect significantly the amount and mobility of water molecules in the VG.

An isotope-labeled hornet (Vespa) cocoon in which the Ala carbons of silk proteins were substituted with ¹³C was successfully obtained by feeding a mixture of larval saliva with [¹³C₃]Ala to mature larvae. The Ala fraction in the α-helix conformation of hornet silk was estimated by obtaining high-resolution ¹³C solid-state NMR spectra of the labeled cocoon, which also helped in understanding the molecular structure of hornet silk.

It was observed that the peptides resulted in β-sheet structure when the peptide solutions were mixed in combination of the side-chain charges being neutralized by each other. Additionally, formation speeds of the β-sheet structure were different between systems, AD12 and AK12 or AE12 and AK12. These phenomena were also observed under the higher concentration condition by ¹³C CP/MAS NMR. These results indicate that the mixing of the two kinds of peptide solutions will be a trigger for the peptide self-assembly and will be able to control the formation speed by the combination.

A chemical profiling methodology is described for seaweed biomass based on comprehensive characterization of the water-soluble, water-insoluble and solid-state components. For nuclear magnetic resonance (NMR) measurements of seaweed water-insoluble components, an extraction method was developed by modification and optimization of physical pretreatments and extraction processes. Compositional variations were evaluated by NMR and attenuated total reflectance Fourier transform infrared spectroscopy, and subsequently indicated red, brown and green algae-specific profiles derived from multivariate statistical analysis.

A multivariate analysis was used to extract components in solid-state nuclear magnetic resonance (NMR) spectra from bacterial cellulose (BC). Polymers such as cellulose have several domain structures, and their structure and dynamics are reflected in the variety of solid-state spectra derived from different parameters. Multivariate analysis, such as principle component analysis (PCA), is suggested as an option to improve analyses of complex NMR spectra from relaxation measurements. In this study we demonstrate the extraction of peak components using PCA from cross polarization experiment data with variable contact time spectra of BC.

The hydrodynamic screening length ξ in κ-carrageenan solution was estimated by observing the restriction on the diffusion of pullulan probes with different molecular weights. Signal intensity of κ-carrageenan chains without gradient I_kappa(0) decreased and ξ increased below the sol-to-gel transition temperature T_sg. It is considered that κ-carrageenan chains formed helical aggregates, and enlarged the networks of solute κ-carrageenan chains and the aggregates.

A fluorinated heme was incorporated into Mb, and the field dependence of the line width of ¹⁹F NMR signals of the protein with S values ranging from 0 to 5/2 was analyzed in order to gain a quantitative insight into the ¹⁹F transverse relaxation mechanism in the protein. In the cases of S=2 and S=5/2 forms, the significant contribution of Curie spin relaxation to the relaxation was demonstrated and analysis of the Curie spin relaxation was useful to estimate the overall correlation time of the protein.

The local structure of the acidic-treated model peptides,(E)₈GGLGGQGAG(A)₆GGAGQGGYGG, derived from the consensus sequence of Nephila clavipes fibroin major ampullate spidroin 1 was determined using two-dimensional proton-driven spin-diffusion solid-state NMR under off-magic-angle spinning coupled with ¹³C isotope double labeling of specific residues. We observed a positional dependence on the torsion angles of Ala residues in the β sheet. Torsion angle of the Ala residue at the center of poly-Ala domain concentrates around at (ϕ, ψ) = (−150°, 150°), while that of its periphery is distorted and more distributed.

The August special issue of the Polymer Journal, titled the NMR of Polymers - recent advances and innovations, presents the usefulness and importance of NMR in recent polymer studies, emphasizing the significance of NMR in a wide-variety of research areas within polymer science. Further background information about this technique in polymer studies is available through the accompanying web focus which links to related articles from across Nature Publishing Group