Advanced 1H Solid‐State NMR Spectroscopy on Hydrogels, 1

Summary: The nature of the pH dependent collapse of poly(methacrylic acid) (PMAA) hydrogels is investigated using recent ¹H solid‐state NMR methods. In aqueous solution, PMAA changes from an expanded conformation at high pHs to a compact contracted form at low pHs, where hydrogen bonds play a central role. In solid‐state ¹H NMR spectra, recorded under fast magic angle spinning (MAS), dried PMAA samples previously collapsed at low pHs show characteristic signals in the spectral region of the carboxylic acid protons. With the aid of 2D ¹H‐¹H double‐quantum (DQ) MAS NMR spectra, three signals can be distinguished at 8, 10.5 and 12.5 ppm, which are attributed to free carboxylic groups and two different types of hydrogen bonded forms, respectively. The 12.5 ppm signal arises from the hydrogen bond with the shortest H? H distance, corresponding to the form that is most stable with respect to increasing temperature and pH. The weaker hydrogen‐bonded form (with a signal at 10.5 ppm) requires a slightly lower pH, while the free acid signal (at 8 ppm) emerges under the most acidic medium. Moreover, the stabilities of the hydrogen‐bonded carboxylic acid dimers can be inferred from the proton‐proton distances within the dimers, i.e. (275 ± 5) pm and (295 ± 15) pm for the protons at 12.5 and 10.5 ppm, respectively, which are determined by means of DQ MAS sideband patterns. Both the stability of the hydrogen bonds and the acidity of the protons may be related to the stereochemistry and the conformation of the PMAA chains.

     

Advanced 1H Solid‐State NMR Spectroscopy on Hydrogels, 2

Summary: Advanced ¹H solid‐state NMR methods are applied for studying the hydrogen bond formation occurring in polymer hydrogels based on N‐isopropylacrylamide (NIPAAm) and methacrylic acid (MAA). For P(NIPAAm‐co‐MAA) copolymers collapsed at low pH, two populations of water can be distinguished in ¹H magic‐angle spinning (MAS) NMR spectra, one of which is probably situated near stable hydrogen‐bonded regions, while the other behaves similarly to free water. The pH‐induced polymer collapse can be followed in detail using 2D ¹H‐¹H double quantum (DQ) MAS NMR experiments on dried samples. For collapsed copolymers as well as interpenetrating polymer networks, the appearance of characteristic signals shows that hydrogen bonding takes place between NIPAAm and MMA monomers. The temperature dependence of the DQ spectra indicates that acid‐amide hydrogen bonds formed between both comonomers are more stable than the acid‐acid hydrogen bonds formed among MAA moieties alone. Correspondingly, by analyzing ¹H‐¹H DQ sideband patterns, a relatively short distance of 270 pm is found for the NIPAAm‐MMA hydrogen bond. Finally, the pH dependence of the DQ spectra demonstrates that hydrogen bonding phenomena are directly related to the polymer collapse.

     

Study of Uniaxially Stretched Isotactic Poly(propylene) by 1H Solid‐State NMR and IR Spectroscopy

Changes in the amount of the rigid, semi‐rigid and soft fractions, molecular mobility and domain thickness of uniaxially stretched iPP were investigated as a function of temperature, draw ratio, drawing temperature and drawing rate. Correlations are established between the thicknesses of rigid domains and the amount of the rigid fractions of uniaxially stretched iPP. Also established are correlations between the thickness of rigid domains and the molecular mobility of the rigid fraction of uniaxially stretched iPP. The drawing temperature has an important effect on the strain‐induced transformation of the spherulitic morphology of iPP to a fibrillar one.

     

Nanocomposites Based on Layered Silicates and Epoxy Resins: Measurement of Clay Dispersion and Exfoliation using TEM,Solid‐State NMR,and X‐ray Diffraction Methods

In an epoxy/layered silicate nanocomposite containing different organically modified layered silicats, the effect of various organic modifications and the layered silicate content on the layer distances are investigated by transmission electron microscopy (TEM), X‐ray diffraction (XRD) measurements, and ¹H solid‐state NMR. TEM micrographs were evaluated using a semi‐quantitative method. The effect of paramagnetic impurities present in the clay is used to obtain overall sample information related to the degree of exfoliation by ¹H solid‐state NMR. A new model for determination of the degree of exfoliation by proton longitudinal relaxation rate measurements is developed.     

Topological Insight into Superabsorbent Hydrogel Network Structures: a 1H Double‐Quantum NMR Study

Superabsorbent polymer (SAP) hydrogels have pronounced water‐absorbing and water‐storing capacities, which are essential for numerous potential applications. It remains a challenge to better understand the network topology because of their amorphous and anisotropic structures. Synthesis parameters such as monomer concentration, degree of neutralization and crosslinking, and surface crosslinking are varied to correlate structural changes in the network with low‐field proton double‐quantum (¹H DQ) NMR results. ¹H DQ‐NMR data are processed by a reliable, user‐independent analysis approach to determine the fractions of network defects, of mobile sol components, and of network chains as well as the residual dipolar coupling distribution in SAPs. In addition, results obtained by applying different distributions to describe the DQ buildup curves are quantified and compared. The correlation between topological and synthesis parameters as well as the impact of temperature, swelling, and solvent of SAP on DQ signals is investigated and discussed.     

Chain Dynamics of Poly(oxyethylene) in Nanotubes of α‐Cyclodextrin by Solid‐State 2H NMR

Summary: Solid‐state ²H NMR spectroscopy was used to examine the chain dynamics of perdeuterated poly(oxyethylene) (d‐POE) inside α‐cyclodextrin (α‐CD) nanotubes. The nanotubes were prepared by aqueous self‐assembly of α‐CD onto either low‐molecular‐weight (1.5 kg/mol) or high‐molecular‐weight (25.8 kg/mol) monodisperse d‐POE. At a given temperature, POE chain segments exhibit faster dynamics when included inside the CD nanotubes as compared to the bulk. Even at 150 K, when no large‐angle dynamics are detected in bulk POE, evidence for chain motions in the nanotube‐confined POE is observed. The ²H line shapes representing this motion were modeled by a discrete 3‐site jump using a ln‐Gaussian distribution of correlation times. An activation energy of 15 ± 1 kJ/mol was determined and the motion envisioned as gauche defects propagating along the primarily trans chains included within CD nanotubes. As the nanotube length decreased, the jump angle became much less defined and more isotropic motions were observed for POE segments at elevated temperatures (>270 K).

     

Structure and Segmental Motions in a Substituted Polythiophene: A Solid‐State NMR Study

Thermochromic poly[3‐(2‐methyl‐1‐butoxy)‐4‐methylthiophene] has been investigated in the solid state. Some important changes have been observed by UV‐Vis absorption and XRD measurements. Solid‐state NMR spectroscopy, which has the ability to provide information about the structure and dynamics of polymers over a wide range of length scales and time scales, has been utilized to better understand the motion of polythiophene chains during this thermochromic transition. Different solid‐state NMR analyses led to the determination of the relaxation rates for the main chain and the side chain as well as to the characterization of a twisting of the conjugated backbone chain during the thermochromic transition.

     

Solid‐State NMR Characterization of the Multiphase Structure of Polypropylene In‐reactor Alloy

A variety of solid‐state NMR techniques were used to characterize the chain dynamics, miscibility and the micro‐phase structure of a polypropylene (PP) in‐reactor alloy system. The alloy was physically separated into three fractions, and the molecular dynamics and relaxation behavior of the pure fractions was then compared with the components in the alloy to achieve comprehensive understanding of the phase structure of the PP in‐reactor alloy. The miscibility among different components of the alloy was studied by the rotational frame spin‐lattice relaxation time. Proton spin‐diffusion methods were used to quantify the domain thicknesses of different regions in the alloy. The results show that the alloy is composed of three phases, namely, a homo‐polyethylene (HPE) matrix, a homo‐polypropylene (HPP) dispersed phase, and a linear low‐density polyethylene (LLDPE) interphase. The thickness of the LLDPE interphase is estimated to be 7.7 nm at room temperature, and changes dramatically with temperature. Finally, based on all the solid‐state NMR results, a model for the micro‐phase‐structure of the PP in‐reactor alloy is proposed, and a correlation between the micro‐phase structure and the excellent mechanical property is established.

     

A New Class of Organosoluble Rigid‐Rod,Fully Aromatic Poly(1,3,4‐oxadiazole)s and Their Solid‐State Properties, 2. Solid‐State Properties

New symmetrical and unsymmetrical dialkoxy‐substituted aromatic poly(1,3,4‐oxadiazole)s are investigated by cyclovoltammetric measurements. The electrochemical redox behaviour of these materials is compared with the chemical structure. The reduction behaviour allows the estimation of the electron affinity, and shows that these polymers are suitable candidates for electron‐transport materials in electronic devices. Surprisingly it is found from UV‐vis and fluorescence spectroscopic measurements that the polyoxadiazole with linear alkoxy side chains (C16) shows a reversible thermochromism in solution as well as in the solid state. The changes in this temperature range are ascribed to a phase transition between aggregated chains, in solution as well as in the film, at room temperature and molecularly dissolved polymer chains at higher temperature. The other poly(1,3,4‐oxadiazole)s with branched alkoxy side chains exhibit an improved solubility and they are completely soluble in o‐dichlorobenzene at room temperature. The absorption and emission behaviour does not change in the temperature range from 25°C to 100°C. The fluorescence excitation studies of these poly(1,3,4‐oxadiazole)s show a blue emission in solution and in the film. Due to these results the poly(1,3,4‐oxadiazole)s are used as electron transport and blue emitting layers in electronic devices like diodes or polymer light emitting diodes (PLED).     

Multilayer Thin Films by Layer‐by‐Layer Assembly of Hole‐ and Electron‐Transport Polyelectrolytes: Optical and Electrochemical Properties

Summary: In this paper, we present the synthesis of a series of p‐type and n‐type semiconducting polyelectrolytes with triarylamine, oxadiazole, thiadiazole and triazine moieties. The synthesized polymeric hole and electron transport materials were examined optically and electrochemically using UV/Vis spectroscopy, PL spectroscopy and CV. Based on the optical and electrochemical data, each of the energy levels were calculated and all values suggested that they were promising hole‐ (p‐type) or electron‐transport (n‐type) materials for devices. Moreover, the synthesized ionic polymers were suitable for LBL thin film deposition from dilute polymer solutions and the multilayers were fully characterized by UV/Vis, PL spectroscopy and CV.

     

Online Coupling of Size‐Exclusion Chromatography and Low‐Field 1H NMR Spectroscopy

Current results and limitations of a novel SEC–MR–NMR (size‐exclusion chromatography–medium resolution nuclear magnetic resonance) system are presented, delivering a direct correlation of chemical structure and molecular weight. A 20 MHz ¹H NMR spectrometer based on permanent magnets was combined with a SEC setup. The NMR sensitivity was improved by digital filters, shims, and a self‐built flow probe when compared with time‐domain instruments. For optimum NMR signal‐to‐noise ratio (S/N) of polymer fractions and concomitantly reduced S/N of solvent signals, deuterated solvents or protonated solvents, with solvent suppression, were applied. Results are presented after optimization of both NMR and chromatographic components such as column dimension and volume flow.     

Radical Copolymerization of Vinylidene Fluoride with 8‐Bromo‐1H,1H,2H‐perfluorooct‐1‐ene: Microstructure,Crosslinking and Thermal Properties

An original synthesis of 8‐bromo‐1H,1H,2H‐perfluorooct‐1‐ene (BDFO) and its radical copolymerization with vinylidene fluoride (VDF), initiated by 2,5‐bis(tert‐butylperoxy)‐2,5‐dimethylhexane at 134 °C, are presented. The fluorinated bromoalkene was obtained by dehydrobromination of 1,8‐dibromo‐1H,1H,2H,2H‐perfluorooctane in a satisfactory yield. Although BDFO did not homopolymerize under radical initiation, it did copolymerize with VDF. The compositions of the resulting random type copolymers were calculated by means of ¹⁹F NMR spectroscopy and allowed the quantification of the respective amounts of both comonomers in the copolymers, showing good incorporation of the brominated monomer. Nevertheless, obtaining PVDF copolymers containing a high molar percentage of BDFO in good yields was difficult to achieve from initial molar ratios of BDFO higher than 9.2 mol‐%. Radical terpolymerization of VDF, BDFO and hexafluoropropene (HFP) was also successfully achieved. BDFO contents in these co‐ or terpolymers ranged from 3.6 to 12.2 mol‐%. The bromoalkene acted as a cure site monomer and the resulting poly(VDF‐co‐BDFO) copolymers were crosslinked via the bromine atom in the presence of a triallyl isocyanurate/peroxide system. The materials obtained led to more thermally stable copolymers than the uncured ones and their thermostabilities were compared to those of commercially available poly(VDF‐co‐HFP) copolymers crosslinked using diamines.

     

Investigation of relative metabolic changes in the organs and plasma of rats exposed to X‐ray radiation using HR‐MAS 1H NMR and solution 1H NMR

Excess exposure to ionizing radiation generates reactive oxygen species and increases the cellular inflammatory response by modifying various metabolic pathways. However, an investigation of metabolic perturbations and organ‐specific responses based on the amount of radiation during the acute phase has not been conducted. In this study, high‐resolution magic‐angle‐spinning (HR‐MAS) NMR and solution NMR‐based metabolic profiling were used to investigate dose‐dependent metabolic changes in multiple organs and tissues – including the jejunum, spleen, liver, and plasma – of rats exposed to X‐ray radiation. The organs, tissues, and blood samples were obtained 24, 48, and 72 h after exposure to low‐dose (2 Gy) and high‐dose (6 Gy) X‐ray radiation and subjected to metabolite profiling and multivariate analyses. The results showed the time course of the metabolic responses, and many significant changes were detected in the high‐dose compared with the low‐dose group. Metabolites with antioxidant properties showed acute responses in the jejunum and spleen after radiation exposure. The levels of metabolites related to lipid and protein metabolism were decreased in the jejunum. In addition, amino acid levels increased consistently at all post‐irradiation time points as a consequence of activated protein breakdown. Consistent with these changes, plasma levels of tricarboxylic acid cycle intermediate metabolites decreased. The liver did not appear to undergo remarkable metabolic changes after radiation exposure. These results may provide insight into the major metabolic perturbations and mechanisms of the biological systems in response to pathophysiological damage caused by X‐ray radiation. Copyright © 2016 John Wiley & Sons, Ltd.     

Molecular Structure Modulated Organization and Properties of Azobenzene‐Substituted Polydiacetylene Assemblies Films: Photopolymerization,Morphology, and Thermal Stability

Three accessible azobenzene mesogen‐substituted diacetylene monomers (DA1, DA2, and DA3) were synthesized and the effect of the molecular structure on the photopolymerization behavior and self‐assembled structures of the resulting azobenzene‐substituted polydiacetylene (PDA) films had been investigated in detail. The films derived from DA1 were substantially polymerized into blue phase. However, the films derived from DA2 and DA3 were polymerized into red phase directly. The morphology, packing structure, and thermal stability of the resulting azobenzene‐substituted PDA films were characterized in detail by SEM, XRD as well as POM measurements. PDA1 formed micrometer‐sized lamellar crystal structures, while PDA2 and PDA3 both formed spherulitic crystal structures. The strong π–π stacking interaction among side chains was essential for maintaining high stability upon thermal stimulus.

     

Investigation of Chain Dynamics in Poly(n‐alkyl methacrylate)s by Solid‐State NMR: Comparison with Poly(n‐alkyl acrylate)s

PnAMAs exhibit a local nanophase separation associated with intriguing chain dynamics (Macromol. Chem. Phys. 2005 , 206, 142). PnAMAs of high molar mass, as determined by SEC and MHKS parameters, were investigated in the melt with a recently‐developed solid‐state NMR method (NOE with dipolar filter; Solid State Nucl. Magn. Res. 2005 , 28, 160). The correlation times are assigned to the relaxation of the alkyl nanodomains, as coupled motions of the main chain and hindered local modes in the side chain. Comparison with poly(n‐alkyl acrylates) shows a higher anisotropy of the main chain motions and a better organized local nanophase separation in PnAMAs.

     

Morphology and Motional Heterogeneity in PS/PMMA Diblock Copolymers Studied by 1H and 13C Solid‐State NMR Spectroscopy

The domain thicknesses and the heterogeneity of the chain dynamics of PS‐b‐PMMA diblock copolymers with different molar masses was studied by ¹H and ¹³C solid‐state NMR. The chain dynamics heterogeneity of different components and inside the interfacial region was investigated by different NMR methods, including ¹H residual second van Vleck moments of PS and high‐resolution ¹³C longitudinal magnetization relaxation. The effect of changes in the morphology on chain dynamics heterogeneity is demonstrated. The interplay between microscopic and mesoscopic properties of diblock copolymer is helpful for a better assessment of their role in the mechanical properties of these systems.

     

Investigation of Soft Component Mobility in Thermoplastic Elastomers using Homo‐ and Heteronuclear Dipolar Filtered 1H Double Quantum NMR Experiments

Summary: Information about segmental mobility in thermoplastic elastomers was obtained using static ¹H double quantum (DQ) NMR experiments in combination with homo‐ and heteronuclear dipolar filters, e.g. ¹³C editing of ¹H DQ buildup curves. Block copolymers of poly(butylene terephthalate) (PBT) as hard blocks and poly(tetramethylene oxide) (PTMO) as soft blocks (PBT‐block‐PTMO) were investigated by varying composition and block length. By simulation of the DQ buildup curves, residual dipolar couplings and with this the average order parameter were deduced for the mobile PTMO blocks which are sensitive to the segmental mobility responsible for the viscoelastic properties of thermoplastic elastomers. A strong correlation exists between residual dipolar coupling and composition. Furthermore, the average order parameter correlates linearly with the amount of PTMO in a PTMO‐rich phase as determined in previous studies. Additionally, ¹H transverse magnetization relaxation measurements revealed a direct correlation between the effective T₂ relaxation time of the soft domain and the composition of the thermoplastic elastomers.

Correlation of the average order parameter vs. the fraction of PTMO in the PTMO‐rich phase.     

Order parameters of the orientation distribution of collagen fibers in Achilles tendon by 1H NMR of multipolar spin states

The angular distribution function of collagen fibrils in a sheep Achilles tendon was investigated by (1)H NMR of multipolar spin states represented by dipolar-encoded longitudinal magnetization and double-quantum filtered signals. For the first time the angular distribution function based on the Legendre moment expansion is used. Order parameters were obtained from the anisotropy of (1)H residual dipolar couplings of bond water, which were determined model-free from the excitation efficiency of the multipolar spin states and from double-quantum filtered line splitting. The orientation distribution function of collagen fibrils in Achilles tendon measured from the anisotropy of the residual dipolar couplings is characterized by the average values of beta0 = 1.8+/-0.2 degrees and order parameters [P2] = 0.93+/-0.04, [P4] = 0.78+/-0.04 and [P6] = 0.58+/-0.04. The order of many biological tissues in the presence of ageing, injuries or regeneration can be quantified by the order parameters of the angular distribution function.     

A New Class of Organosoluble Rigid‐Rod Fully Aromatic Poly(1,3,4‐oxadiazole)s and Their Solid‐State Properties, 1. Synthesis

Since it is known, that poly(p‐phenylene‐1,3,4‐oxadiazole) exhibits reversible reduction behaviour in cyclovoltammetric measurements, it may be used as an electron transport material in electronic devices such as light emitting diodes (LEDs). However, the polymers are difficult to process, for example as thin films. By introducing linear or branched alkoxy side groups into the backbone it is possible to obtain rigid rod, fully aromatic poly(1,3,4‐oxadiazole)s which are soluble in organic solvents. Various polycondensation routes were investigated to prepare symmetrical and unsymmetrical dialkoxy‐substituted aromatic poly(1,3,4‐oxadiazole)s. Initially the interfacial polycondensation and the low‐temperature solution polycondensation between aromatic dihydrazides and diacid chlorides in N‐methylpyrrolidone were used. These methods were not successful for the synthesis of the polyoxadiazole with branched side chains. Thus, a new synthetic method for the preparation of such polymers was developed. The polycondensations are carried out in hot 1,2‐dichlorobenzene in the presence of a basic acceptor like pyridine. These conditions are unusual for such types of reaction.     

1H NMR spectroscopy of glioblastoma stem‐like cells identifies alpha‐aminoadipate as a marker of tumor aggressiveness

Patients suffering from glioblastoma multiforme (GBM) face a poor prognosis with median survival of about 14 months. High recurrence rate and failure of conventional treatments are attributed to the presence of GBM cells with stem‐like properties (GSCs). Metabolite profiles of 42 GSC lines established from the tumor tissue of adult GBM patients were screened with ¹H NMR spectroscopy and compared with human neural progenitor cells from human adult olfactory bulb (OB‐NPCs) and from the developing human brain (HNPCs). A first subset (n = 12) of GSCs exhibited a dramatic accumulation of the metabolite α‐aminoadipate (αAAD), product of the oxidation of α‐aminoadipic semialdehyde catalyzed by the ALDH7A1 aldehyde dehydrogenase (ALDH) family in lysine catabolism. αAAD was low/not detectable in a second GSC subset (n = 13) with the same neural metabolic profile as well as in a third GSC subset (n = 17) characterized by intense lipid signals. Likewise, αAAD was not detected in the spectra of OB‐NPCs or HNPCs. Inhibition of mitochondrial ATP synthase by oligomycin treatment revealed that the lysine degradative pathway leading to αAAD formation proceeds through saccharopine, as usually observed in developing brain. Survival curves indicated that high αAAD levels in GSCs significantly correlated with poor patient survival, similarly to prostate and non‐small‐cell‐lung cancers, where activity of ALDH7A1 correlates with tumor aggressiveness. Copyright © 2015 John Wiley & Sons, Ltd.