Study of the self‐diffusion of poly(ethylene glycol)s in poly(vinyl alcohol) aqueous systems

We have measured the self‐diffusion coefficients of a series of oligo‐ and poly(ethylene glycol)s with molecular weights ranging from 150 to 10,000, in aqueous solutions and gels of poly(vinyl alcohol) (PVA), using the pulsed‐gradient spin‐echo NMR techniques. The PVA concentrations varied from 0 to 0.38 g/mL which ranged from dilute solutions to polymer gels. Effects of the diffusant size and polymer concentration on the self‐diffusion coefficients have been investigated. The temperature dependence of the self‐diffusion coefficients has also been studied for poly(ethylene glycol)s with molecular weights of 600 and 2,000. Several theoretical models based on different physical concepts are used to fit the experimental data. The suitability of these models in the interpretation of the self‐diffusion data is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2396–2403, 1999     

The self‐diffusion of macromolecules in binary blends of poly(ethylene glycol)

The concentration and molecular mass dependencies of the self‐diffusion coefficients were obtained for higher molecular mass component in binary blends of the homopolymer poly(ethylene glycol) (PEG) by a nuclear magnetic resonance method with pulsed magnetic field gradient. The shape of the diffusion decay and its dependence on the diffusion observation time in binary PEG blends have been investigated. The experimental results were explained by hypothesizing the existence of cluster formation in polymer melts and polymer blends and the possibility of molecular exchange between clusters. The entanglement time in such systems was evaluated. Copyright © 1999 John Wiley & Sons, Ltd.     

Self‐diffusion of styrene, 2,2′‐azobisisobutyronitrile,and polystyrene in bulk polymerization by pulsed‐gradient spin‐echo nuclear magnetic resonance spectroscopy

The self‐diffusion of styrene, polystyrene, and 2,2′‐azobisisobutyronitrile has been determined in the bulk polymerization of styrene with pulsed‐gradient spin‐echo nuclear magnetic resonance at 25 °C. Data on small molecules are discussed with respect to recent diffusion models. They can fit self‐diffusion coefficient data of small molecules in dilute or semidilute polymer solutions; in concentrated solutions, however, there is a breakdown. A semiempirical model based on scaling laws is used to describe the self‐diffusion of styrene and 2,2′‐azobisisobutyronitrile over the whole range of concentrations studied. The dependence of the polystyrene self‐diffusion coefficient on the polymer concentration is described with a stretched exponential function, D = D₀ exp(?αc^ν), where α depends on the molecular weight of the polymer and ν depends on the kind of solvent. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1605–1614, 2003     

NMR Spectroscopic Study of the Self‐Aggregation of 3‐Hexen‐1,5‐diyne Derivatives

The self‐assembly of polycatenar molecules derived from 1,6‐diphenyl‐3,4‐dipropyl‐3‐hexen‐1,5‐diyne has been studied in detail by solution NMR spectroscopy. The analysis of the concentration‐ and temperature‐dependent evolution of the chemical shifts and the diffusion coefficients in [D₁₂]cyclohexane agrees well with an isodesmic model of association in this solvent. The association constants for the stacking and entropy and enthalpy of the process have been obtained. The driving force for the aggregation process is provided by a negative enthalpy (ΔH), which is partially compensated by a negative entropy (ΔS). A structural study of the self‐assembly in solution has been carried out with the help of NOESY NMR spectroscopic experiments.     

Using Diffusion NMR To Characterize Guanosine Self‐Association: Insights into Structure and Mechanism

This paper presents results from a series of pulsed field gradient (PFG) NMR studies on lipophilic guanosine nucleosides that undergo cation‐templated assembly in organic solvents. The use of PFG‐NMR to measure diffusion coefficients for the different aggregates allowed us to observe the influences of cation, solvent and anion on the self‐assembly process. Three case studies are presented. In the first study, diffusion NMR confirmed formation of a hexadecameric G‐quadruplex [G 1 ]₁₆ ? 4 K⁺ ? 4 pic^? in CD₃CN. Furthermore, hexadecamer formation from 5′‐TBDMS‐2′,3′‐isopropylidene G 1 and K⁺ picrate was shown to be a cooperative process in CD₃CN. In the second study, diffusion NMR studies on 5′‐(3,5‐bis(methoxy)benzoyl)‐2′,3′‐isopropylidene G 4 showed that hierarchical self‐association of G₈‐octamers is controlled by the K⁺ cation. Evidence for formation of both discrete G₈‐octamers and G₁₆‐hexadecamers in CD₂Cl₂ was obtained. The position of this octamer–hexadecamer equilibrium was shown to depend on the K⁺ concentration. In the third case, diffusion NMR was used to determine the size of a guanosine self‐assembly where NMR signal integration was ambiguous. Thus, both diffusion NMR and ESI‐MS show that 5′‐O‐acetyl‐2′,3′‐O‐isopropylidene G 7 and Na⁺ picrate form a doubly charged octamer [G 7 ]₈ ? 2 Na⁺ ? 2 pic^? 9 in CD₂Cl₂. The anion's role in stabilizing this particular complex is discussed. In all three cases the information gained from the diffusion NMR technique enabled us to better understand the self‐assembly processes, especially regarding the roles of cation, anion and solvent.     

PGSE‐WATERGATE,a new tool for NMR diffusion‐based studies of ligand–macromolecule binding

A new pulsed gradient spin‐echo NMR diffusion sequence, PGSE‐WATERGATE, which is based on the extremely efficient WATERGATE solvent suppression sequence, is presented. The sequence is simple to set up and particularly suited to measuring the diffusion coefficients of small ligands in aqueous solution such as is commonly required in pharmaceutical and combinatorial applications. It also affords the possibility of measuring the diffusion of exchangeable resonances, which is often impossible in conjunction with other suppression schemes. Further, a trivial modification of the sequence affords the possibility of multiple solvent suppression, thereby increasing its suitability to LC–NMR applications. The utility of the sequence is demonstrated on the salicylate–bovine serum albumin system. The dissociation constant, K_d, and the number of binding sites were found to be 0.030 M and 33, respectively. Importantly, the extremely high degree of suppression provided by the new sequence allowed the salicylate diffusion coefficients to be measured over a very wide concentration range sufficient to show that the salicylate–bovine serum albumin system is not well described by a simple two‐site model. Previous studies in the literature have been based on data from a smaller concentration range, for which this model gives an apparently good fit. Copyright © 2002 John Wiley & Sons, Ltd.     

Drug–Polymer Interactions in Hydrogel‐based Drug‐Delivery Systems: An Experimental and Theoretical Study

In drug‐delivery systems, drug transport is a key step, but the interpretation of the transport mechanism is still controversial. Here, we investigated a promising hydrogel library loaded with the anticonvulsant drug ethosuximide (ESM). The self‐diffusion coefficient of ESM was measured using two methods: a direct and advanced measurement with a pulsed field gradient spin‐echo (PFGSE) method, using an NMR spectrometer equipped with high‐resolution magic angle spinning (HR‐MAS) probe, and an indirect one based on fitting in vitro drug‐delivery data. Starting from the experimental data a mathematical model without fitted parameters was developed and all the phenomena involved, that is, adsorption and diffusion, were considered. At low drug concentrations, adsorption prevails and consequently the diffusivity in the gels is lower than that in water. At high drug concentrations, where all adsorption sites are saturated, the diffusion in the gels is similar to that in a water solution. This study may pave the way for better device design.     

An NMR investigation on the phase structure and molecular mobility of the novel exfoliated polyethylene/palygorskite nanocomposites

Novel exfoliated polyethylene (PE)/palygorskite nanocomposites prepared by in situ polymerization are characterized by solid‐state nuclear magnetic resonance (NMR). The phase structure and molecular mobility are investigated by a combination of proton and carbon NMR. The results showed that incorporation of small amounts of palygorskite had great influence on the phase structure and molecular mobility. The incorporated palygorskite hindered the crystallization process and introduced motion‐hindered chains in the NMR crystalline and amorphous phase. ¹³C cross‐polarization and magic‐angle spinning NMR revealed two orthorhombic crystalline phase with different line‐width. The chain mobility of orthorhombic crystalline phase with broad resonance line is obviously hindered compared with the phase with narrow resonance line when the filler is introduced. Additionally, the results of pulsed field gradient NMR technique show those the tortuosities in the nanocomposites are much higher than that in the bulk PE. The self‐diffusion process of probe molecules is also influenced by the palygorksite load. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1363–1371, 2010     

A New Tool in Peptide Engineering: A Photoswitchable Stilbene‐type β‐Hairpin Mimetic

Peptide secondary structure mimetics are important tools in medicinal chemistry, as they provide analogues of endogenous peptides with new physicochemical and pharmacological properties. The development, synthesis, photochemical investigation, and conformational analysis of a stilbene‐type β‐hairpin mimetic capable of light‐triggered conformational changes have been achieved. In addition to standard spectroscopic techniques (nuclear Overhauser effects, amide temperature coefficients, circular dichroism spectroscopy), the applicability of self‐diffusion measurements (longitudinal eddy current delay pulsed‐field gradient spin echo (LED‐PGSE) NMR technique) in conformational studies of oligopeptides is demonstrated. The title compound shows photoisomerization of the stilbene chromophore, resulting in a change in solution conformation between an unfolded structure and a folded β‐hairpin.     

Construction of Three‐Dimensional DNA Hydrogels from Linear Building Blocks

A three‐dimensional DNA hydrogel was generated by self‐assembly of short linear double‐stranded DNA (dsDNA) building blocks equipped with sticky ends. The resulting DNA hydrogel is thermoresponsive and the length of the supramolecular dsDNA structures varies with temperature. The average diffusion coefficients of the supramolecular dsDNA structures formed by self‐assembly were determined by diffusion‐ordered NMR spectroscopy (DOSY NMR) for temperatures higher than 60 °C. Temperature‐dependent rheological measurements revealed a gel point of 42±1 °C. Below this temperature, the resulting material behaved as a true gel of high viscosity with values for the storage modulus G′ being significantly larger than that for the loss modulus G′′. Frequency‐dependent rheological measurements at 20 °C revealed a mesh size (ξ) of 15 nm. AFM analysis of the diluted hydrogel in the dry state showed densely packed structures of entangled chains, which are also expected to contain multiple interlocked rings and catenanes.     

Effect of polymer architecture on self‐diffusion of LC polymers

Two LC side‐group poly(methacrylates) were synthesized, and their melt dynamics were compared with each other and a third, main‐chain side‐group combined LC polymer. A new route was developed for the synthesis of the poly(methacrylate) polymers which readily converts relatively inexpensive perdeuteromethyl methacrylate to other methacrylate monomers. Self‐diffusion data was obtained through the use of forward recoil spectrometry, while modulus and viscosity data were measured using rotational rheometers in oscillatory shear. Diffusion coefficients and complex viscosity were compared to previous experiments on liquid crystal polymers of similar architecture to determine the effect of side‐group interdigitation and chain packing on center of mass movement. The decyl terminated LC side‐group polymer possessed an interdigitated smectic phase and a sharp discontinuity in the self‐diffusion behavior at the clearing transition. In contrast, the self‐diffusion behavior of the methyl terminated LC side‐group polymer, which possessed head‐to‐head side‐group packing, was seemingly unaffected by the smectic–nematic and nematic–isotropic phase transitions. The self‐diffusion coefficients of both polymers were relatively insensitive to the apparent glass transition. The presence of moderately fast sub‐T_g chain motion was supported by rheological measurements that provided further evidence of considerable molecular motion below T_g. The complex phase behavior of the combined main‐chain side‐group polymer heavily influenced both the self‐diffusion and rheological behavior. Differences between the self‐diffusion and viscosity data of the main‐chain side‐group polymer could be interpreted in terms of the defect structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 405–414, 1999     

Cyclic methacrylic acid oligomer as a novel pH‐sensitive carrier material

The pH effect of inclusion of methylene blue (MB) in cyclic methacrylic acid oligomer (CMAA) with degree of polymerization 14, which was a novel cationic host material with pK_a 4.83, was investigated in methanol. The self‐diffusion coefficients of MB in methanol at pH 7.0 and 4.0 with and without CMAA were measured by ¹H‐NMR spectrometry with pulsed‐field‐gradient stimulated‐echo pulse (PGSE) analysis. At both pH values, the addition of CMAA to MB solution resulted in the complex formation between CMAA and MB. MB was included in CMAA in methanol at a wide range of pH. The molar fraction of the slow diffusion component corresponding to the complex increased by varying the pH from 7.0 to 4.0. Then, SiO–CMAA was synthesized by immobilization of CMAA on silica gel beads. The time dependence of adsorption of MB to SiO–CMAA was investigated by UV–Vis spectrometry. At pH 7.0, the absorbance of MB–methanol solution gradually decreased by the addition of SiO–CMAA. The absorbance drastically decreased on varying the pH from 7.0 to 4.0, and the absorbance quickly increased on varying the pH from 4.0 to 7.0. The inclusion of MB in CMAA was fast and reversible on changing the pH of the solution. CMAA was the novel pH sensitive host material. Inclusion of MB in CMAA enhanced at pH 4.0 rather than at pH 7.0. Copyright © 2008 John Wiley & Sons, Ltd.     

Co‐Micellization Investigated by Pulsed Field Gradient‐NMR Spectroscopy

Summary: Pulse field gradient‐NMR (PFG‐NMR) spectroscopy is determined to be a more suitable method for the investigation of self‐association processes in multi‐component (co)polymer systems than light scattering methods. Here the co‐micellization of mixtures of the diblock copolymer polystyrene‐block‐(hydrogenated polyisoprene) (PS‐HPI) and the triblock copolymer polystyrene‐block‐(hydrogenated polybutadiene)‐block‐polystyrene (PS‐HPB‐PS) in decane is investigated by PFG‐NMR spectroscopy and the results compared to those experimentally determined by static (SLS) and dynamic (DLS) light scattering. As expected, diffusion coefficients determined by PFG‐NMR spectroscopy are systematically lower than those from DLS. The PFG‐NMR measurements provided higher values of cequation/tex2gif-stack-1.gif(X)/c_tot than the model calculations, illustrating that the basic assumption used in the calculations, i.e., that the number concentration of co‐micelles in mixed solutions follows the dilution with a triblock copolymer solution, 1 − X, is not fully valid at high X (weight fraction of PS‐HPB) values.

Comparison of PFG‐NMR spectroscopy and SLS (cequation/tex2gif-stack-2.gif/c_tot = equilibrium concentration of free PS‐HPB‐PS over the total concentration of copolymers in solution, X = weight fraction of PS‐HPB).     

The structure‐dependent self‐association of five phenolic acids in aqueous solution

Weak self‐interaction plays an important role in interpreting the biomechanisms and modes of drug action. The structure‐dependent self‐association of five phenolic acids with various bioactivities, including danshensu (DSS), caffeic acid (CA), rosmarinic acid (RA), lithospermic acid (LA), and salvianolic acid B (SA), was investigated by ¹H NMR. These phenolic acids have similar condensed structures, with a CA moiety and varying numbers of DSS moieties. The strengths of the self‐association constants are in the order DSS < CA < RA < LA < SA, which corresponds to the increasing molecular size of these phenolic acids and roughly corresponds to the increasing number of DSS moieties. The binding site for the self‐aggregation of these phenolic acids has been identified to be on the CA moiety, rather than on the DSS moiety, as a result of CA's stronger aromatic π–π interactions, which cause larger chemical shift variations. The thermodynamic parameters for the self‐association of these phenolic acids show that the self‐association is spontaneous and enthalpically favorable at room temperature in all cases. It was inferred that π–π interactions and intermolecular hydrogen bonding stabilize the stacking structures of the phenolic acids. Knowledge of self‐association processes will enable us to quantitatively assess the possible effects of self‐aggregation on the interaction between drug and protein. Copyright © 2014 John Wiley & Sons, Ltd.     

柔性金属—有机骨架材料中已烷的扩散研究

本工作将MM3力场进行了扩展,使其可用于描述其它IRMOF材料的柔韧性;在此基础上,我们采用分子动力学模拟研究了柔性IRMOF-1和-16材料中已烷的扩散。 本文重点研究了温度和分子数对己烷自扩散系数、扩散机理,以及骨架柔性的影响。结果表明,分子数是影响扩散路径的重要因素。其次,IRMOF-16的柔性强于IRMOF-1。 工作的结论有助于进一步研究链状分子在柔性MOF材料中的扩散。     

Dynamics of poly(p‐phenylene) ladder polymers in solution

Photon correlation spectroscopy in both polarized and depolarized geometry was employed to investigate the dynamics of a ribbon‐type polymer exhibiting good solubility. In dilute solution, the translational diffusion for all examined molecular weights has confirmed the picture of wormlike chains with rather short (∼ 7 nm) persistence length (Macromolecules 1997, 30, 273). In the semidilute regime, the total concentration fluctuations display, besides the fast dominant cooperative diffusion, a second slower diffusive process that exhibits weak concentration dependence and is not related to the self‐diffusion measured by pulse‐field‐gradient NMR. The concentration dependence of the cooperative and the self‐diffusion coefficient as well as of the zero‐shear viscosity cannot be consistently described by neither flexible nor stiff chain models. Presence of aggregates was revealed at high concentrations. Owing to the short persistence length, the rotational diffusion is too fast to be adequately investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2211–2220, 1999     

Gradient polymer networks formed by photopolymerization with self‐floating polysiloxane‐containing nanogel

Polysiloxane‐containing nanogels can be used as a fast, convenient and environmentally friendly method to control gradient photopolymerization and to obtain gradient polymer network because of its self‐floating feature. The chain length of polysiloxane is a key factor that influences the self‐floating capability of the polysiloxane‐containing nanogel. This paper reports a series of nanogels compositions synthesized with methacrylate‐modified polysiloxanes with different chain lengths, urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) at a molar ratio of 10:20:70 in the presence of a thiol chain transfer agent. The effect of polysiloxane chain length on self‐floating capability of the nanogel and gradient polymer network was researched. The results show that polysiloxane chain length is the main driving force for the self‐floating capability of the nanogels. The nanogel with long polysiloxane chain length exhibits good self‐floating capability in the monomer–polymer matrix because of the lower surface tension of polysiloxane. Furthermore, the gradient polymer network containing the nanogel with long polysiloxane chain length presents lower dispersion surface energy and greater hardness and thermostability. Copyright © 2016 John Wiley & Sons, Ltd.     

A fast method for the measurement of diffusion coefficients: one‐dimensional DOSY

A new experiment for the single‐scan measurement of diffusion coefficients is presented. The principle is to introduce a spatial variation in the parameters of a conventional pulse sequence, so that all of the scans required to determine some physical parameter can be recorded simultaneously from different parts of the sample. The spectrum is acquired in the presence of a weak read gradient so that the resulting lineshapes contain the information required. The pulse sequence is described in detail and demonstrated on a sample containing three components; its advantages and limitations are discussed in relation to those of existing techniques. For uncrowded spectra with high signal‐to‐noise ratio, this experiment provides an order of magnitude reduction in experiment time compared with conventional methods and is likely to be of most benefit where samples are changing rapidly with time or where a long period of polarization, which may be difficult to reproduce accurately, prohibits the use of multiple‐scan techniques. Copyright © 2003 John Wiley & Sons, Ltd.     

Self‐diffusion coefficients obtained from proton‐decoupled carbon‐13 spectra for analyzing a mixture of terpenes

In the limit of sufficient sensitivity, natural abundance ¹³C offers a much better spectral resolution than proton NMR. This is due to an important chemical shift range and to proton‐decoupling conditions that yield one peak per carbon with practically no overlap. However, pulsed gradient spin echo experiments, which lead to the diffusion coefficient associated with each peak, have scarcely been employed. In this article, we present and compare different ways to access this quantity and we have effectively verified that, without any precaution, diffusion coefficients cannot be properly determined from standard procedures. The cause of such a failure is decoupling during the gradient pulses. We have used a very simple remedy that proved to be very successful on a model mixture of three monoterpenes and that appears as being of general applicability. Copyright © 2009 John Wiley & Sons, Ltd.     

Catalysis‐Driven Self‐Thermophoresis of Janus Plasmonic Nanomotors

It is highly demanding to design active nanomotors that can move in response to specific signals with controllable rate and direction. A catalysis‐driven nanomotor was constructed by designing catalytically and plasmonically active Janus gold nanoparticles (Au NPs), which generate an asymmetric temperature gradient of local solvent surrounding NPs in catalytic reactions. The self‐thermophoresis behavior of the Janus nanomotor is monitored from its inherent plasmonic response. The diffusion coefficient of the self‐thermophoresis motion is linearly dependent on chemical reaction rate, as described by a stochastic model.