Probing polymers with single fluorescent molecules

The use of single molecules to study local, nanoscale polymer dynamics is presented. Fluorescence lifetime fluctuations were used to extract the number of polymer segments (N_s) taking part in the rearranging volume around the probe molecule below the glass transition temperature. N_s was dependent on the temperature and it decreased with increasing temperature. Above the glass transition, rotational motion of single molecules was followed in time and typical time-scales of the rotational diffusion were extracted. These two approaches allowed us to obtain non-averaged information about the heterogeneous dynamics present in polymer systems, on the nanoscale, above and below glass transition temperatures.     

Evaluation of translational friction coefficients of micro-sized spherical probes in nematic liquid crystals

We study the translational friction coefficients of a spherical micrometric probe moving in nematic liquid crystalline fluids, by solving numerically the constitutive hydrodynamic equations of uncompressible isothermal nematic fluids (Leslie–Ericksen equations). The nematic medium is described by a vector field, which specifies the director orientation at each point and by the velocity vector field. Simulations of director dynamics surrounding the moving probe are presented, and the dependence of translational diffusion upon liquid crystal viscoelastic parameters is discussed. The time evolution of director field is studied in the presence of an orienting magnetic field in two characteristic situations, i.e. direction of motion parallel and perpendicular to field. In particular, a detailed analysis is given for the case of a spherical probe in rectilinear motion in nematic MBBA (4-methoxibenzylidene-4′-n-butylaniline), together with a comparison with other nematogens.     

Thin‐Layer Spectroelectrochemistry on an Aqueous Microdrop

Here we report a technique to perform thin layer spectroelectrochemistry using an aqueous microdrop. The chemical systems used to demonstrate the aqueous microdrop technique were an absorption based ionic probe [Fe(CN)₆]^3?/4? and an emission based ionic probe [Ru(bpy)₃]^3+/2+. The ability of the technique to perform semi‐infinite linear diffusion spectroelectrochemistry on an aqueous microdrop has been previously demonstrated; in this work we were able to demonstrate spectroelectrochemical behavior consistent with the restricted diffusion in a thin layer cell by reducing the analyte volume and the optical path length. The thin‐layer diffusion behavior was illustrated by substantial reduction in peak‐to‐peak separations of the cyclic voltammograms and the significant decrease in electrolysis time compared to the semi‐infinite linear diffusion behavior.     

Mobility of Spherical Probe Objects in Polymer Liquids

We use scaling theory to derive the time dependence of the mean-square displacement ?Δr(2)? of a spherical probe particle of size d experiencing thermal motion in polymer solutions and melts. Particles with size smaller than solution correlation length ξ undergo ordinary diffusion (?Δr(2) (t)? ~ t) with diffusion coefficient similar to that in pure solvent. The motion of particles of intermediate size (ξ < d < a), where a is the tube diameter for entangled polymer liquids, is sub-diffusive (?Δr(2) (t)? ~ t(1/2)) at short time scales since their motion is affected by sub-sections of polymer chains. At long time scales the motion of these particles is diffusive and their diffusion coefficient is determined by the effective viscosity of a polymer liquid with chains of size comparable to the particle diameter d. The motion of particles larger than the tube diameter a at time scales shorter than the relaxation time τ(e) of an entanglement strand is similar to the motion of particles of intermediate size. At longer time scales (t > τ(e)) large particles (d > a) are trapped by entanglement mesh and to move further they have to wait for the surrounding polymer chains to relax at the reptation time scale τ(rep). At longer times t > τ(rep), the motion of such large particles (d > a) is diffusive with diffusion coefficient determined by the bulk viscosity of the entangled polymer liquids. Our predictions are in agreement with the results of experiments and computer simulations.     

Effect of the morphology of hydrophilic polymeric matrices on the diffusion and release of water soluble drugs

Drug release mechanisms from, and diffusion processes in, hydrophilic crosslinked polymeric systems were investigated in two macromolecular states: in the glassy and rubbery states during the early part of countercurrent water diffusion, and in the rubbery state after thermodynamic equilibrium between the network and the surrounding dissolution medium (water) was attained. Dilute, aqueous poly(vinyl alcohol) (PVA) solutions containing theophylline were crosslinked with glutaraldehyde. The crosslinking ratio, X, varied between 0.01 and 0.20 moles glutaraldehyde per mole of PVA repeating unit. Theophylline release from these rubbery matrices was followed as a function of time. It was determined that, within the range of crosslinking ratios studied, the crosslinked macromolecular structure affected the solute diffusion process. Theophylline release from crosslinked PVA slabs, which were originally dehydrated at 30°C, was also measured. The drug release process was significantly impeded in these systems, especially for samples with crosslinking ratio X ≥ 0.10. This behavior was explained in terms of relaxation of the macromolecular chains and possible existence of ordered chain structures. Glass-to-rubber transitions, a result of the countercurrent diffusion of water into the originally dried (glassy) polymer, shifted the fractional release of theophylline from a f(t^1/2) to a f(tⁿ) time dependence, with n taking values between 0.50 and 0.76. This type of release behavior indicates anomalous diffusion mechanisms. These results may be helpful in the development of swelling controlled drug delivery systems.     

Relaxational mode structure for optical probe diffusion in high molecular weight hydroxypropylcellulose

We studied translational diffusion of dilute monodisperse spheres (diameters 14 < d < 455 nm) in aqueous 1 MDa hydroxypropylcellulose (0 ≤ c ≤ 7 g/L) at 25°C using quasielastic light scattering. Spectra are highly bimodal. The two spectral modes (“slow,” “fast”) have different physical properties. Probe behavior differs between small (d < R_h) and large (d ≥ R_g) probes; R_h and R_g are the matrix polymer hydrodynamic radius and the radius of gyration, respectively. We examined the dependences of spectral lineshape parameters on d, c, scattering vector q, and viscosity η for all four probe-size and mode-type combinations. We find three time scale-separated modes: (1) a large-probe slow mode has properties characteristic of particle motion in a viscous medium; (2) a large-probe fast mode and small-probe slow modes share the same time scale, and have properties characteristic of probe motion coupled to internal chain dynamics; and (3) a small-probe fast mode has properties that can be attributed to the probe sampling local chain relaxations. In the analysis, we also attempted to apply the coupling/scaling (CS) model of Ngai and Phillies [Ngai, K. L., Phillies, G. D. J. J. Chem. Phys., 105 , 8385 (1996)] to analyze our data. We find that the second mode is described by the coupling/scaling model for probe diffusion; the first and third modes do not follow the predictions of this model. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3087–3100, 1998     

Fluorescence Correlation Spectroscopy Study of Probe Diffusion in Poly(vinyl alcohol) Solutions and Gels

In this study, we demonstrate how the diffusion of probe particles in aqueous poly(vinyl alcohol) (PVA) solutions and gels is affected by: (i) the presence of cross-links, (ii) the cross-link density, (iii) the polymer concentration. We apply fluorescence correlation spectroscopy (FCS) to measure the diffusion time of a rhodamine-based fluorescent particle (TAMRA) and TAMRA-labeled dextran in PVA solutions and gels prepared at various polymer concentrations (1% to 8.6% w/v) and cross-link densities (1/400 to 1/50 cross-link monomers per PVA monomers). The measurements indicate that the probe particles are slowed down with increasing polymer concentration and with increasing cross-link density. Also, FCS can detect differences in the diffusion times measured in “fresh” and “aged” PVA solutions. We find that FCS provides a quantitative measure of network inhomogeneities.     

A novel kind of dimmer (excimer)-induced-AIE compound 2-phenylisothiazolo[5,4-b]pyridin-3(2H)-one as high selective bisulfite anion probe

An interesting dimmer (excimer)-induced-AIE characteristic of 2-phenylisothiazolo[5,4-b]pyridin-3(2H)-one was observed. By using a ring-opening reaction, we developed a novel fluorescent probe based on sub-micron particles of 2-phenylisothiazolo[5,4-b]pyridin-3(2H)-one in water.     

Diffusion of sodium alginate in aqueous solutions at T = 298.15 K

Taylor dispersion technique was used for measuring mutual diffusion coefficients of sodium alginate aqueous solutions at T = 298.15 K, by using as carrier stream solution both pure water and solutions of this polyelectrolyte at a slightly different concentration. The limiting values found at infinitesimal ionic strength, D⁰, were determined by extrapolating to c → 0. These studies were complemented by molecular mechanics calculations. From the experimental data, it was possible to estimate both the limiting conductivity and the tracer diffusion coefficient values for the alginate anion, and the hydrodynamic radius of the sodium alginate (NaC₆H₇O₆), as well as to discuss the influence of the kinetic, thermodynamic and viscosity factors on the diffusion of sodium alginate in aqueous solutions at finite concentrations. Thus, the aim of our innovative research is to contribute to a better understanding of the structure and the thermodynamic behavior of these polymeric systems in solution and supplying the scientific and technological communities with data on these important parameters in solution transport processes.     

Simulation Study of Passive Rod Diffusion in Active Bath: Nonmonotonic Length Dependence and Abnormal Translation-Rotation Coupling

Diffusion of tracer particles in active bath has attracted extensive attention in recent years. So far, most studies have considered isotropic spherical tracer particles, while the diffusion of anisotropic particles has rarely been involved. Here we investigate the diffusion dynamics of a rigid rod tracer in a bath of active particles by using Langevin dynamics simulations in a two-dimensional space. Particular attention is paid to how the translation (rotation) diffusion coefficient \begin{document}$ D_{ \rm{T}} $\end{document} (\begin{document}$ D_{ \rm{R}} $\end{document}) change with the length of rod \begin{document}$ L $\end{document} and active strength \begin{document}$ F_{ \rm{a}} $\end{document}. In all cases, we find that rod exhibits superdiffusion behavior in a short time scale and returns to normal diffusion in the long time limit. Both \begin{document}$ D_{ \rm{T}} $\end{document} and \begin{document}$ D_{ \rm{R}} $\end{document} increase with \begin{document}$ F_{ \rm{a}} $\end{document}, but interestingly, a nonmonotonic dependence of \begin{document}$ D_{ \rm{T}} $\end{document} (\begin{document}$ D_{ \rm{R}} $\end{document}) on the rod length has been observed. We have also studied the translation-rotation coupling of rod, and interestingly, a negative translation-rotation coupling is observed, indicating that rod diffuses more slowly in the parallel direction compared to that in the perpendicular direction, a counterintuitive phenomenon that would not exist in an equilibrium counterpart system. Moreover, this anomalous (diffusion) behavior is reentrant with the increase of \begin{document}$ F_{ \rm{a}} $\end{document}, suggesting two competitive roles played by the active feature of bath particles.     

Spatial heterogeneity in a lyotropic liquid crystal with hexagonal phase

Non-ionic surfactant hexaethylene glycol, C(12)E(6), in water self-assembles into various kinds of mesophases by varying the surfactant concentration. A spatial heterogeneity was discussed on the basis of the diffusion of probe particles dispersed in the C(12)E(6)-water solution. Interestingly, at 50 wt% C(12)E(6) where the hexagonal structure was formed, two kinds of motion of probe particles were observed: some particles normally diffused while others were restricted, indicating the existence of a heterogeneity in the physical properties. Such heterogeneity can be explained in terms of heterogeneous structures composed of hexagonal domains with isotropic-like regions.     

High‐resolution solid‐state NMR study of the domain structure and molecular motion in drawn films of a vinylidene fluoride and trifluoroethylene copolymer with 73 mol % vinylidene fluoride

The heterogeneous higher order structure and molecular motion in a single crystalline film of a vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer with 73 mol % VDF was investigated with the ¹H–¹³C cross‐polarization/magic‐angle spinning NMR technique. A transient oscillation was observed in plots of the ¹³C peak intensity versus the contact time for the CH₂, CHF, and CF₂ groups. On the basis of the extended cross‐relaxation theory of spin diffusion, we determined that the oscillation behavior was caused by the TrFE‐rich segments in the chain and that the crystal consisted of VDF‐rich and TrFE‐rich domains. The former had TrFE‐rich segments in VDF and TrFE fractions of 0.24 and 0.27, respectively, and the latter had VDF‐rich segments in a VDF fraction of 0.49. The spin–lattice relaxation time T_1ρH in the rotating frame for each group was minimal in the three temperature regions of β, α_b, and α_c (↑) on heating and in the two temperature regions of α_1D and α_c (↓) on cooling. The α_c (↑) and α_c (↓) processes depended on the first‐order ferroelectric phase‐transition regions on heating and cooling, respectively. The motional modes for the other processes were confirmed by the T_1ρH minimum behavior of the VDF and TrFE groups in the TrFE‐rich domain and the VDF‐rich segments in the VDF‐rich domain. The β and α_b processes were attributed to the flip–flop motion of the TrFE‐rich segments and the competitive motion of the TrFE‐ and VDF‐rich segments in the ferroelectric phase, respectively. The α_1D process was due to the one‐dimensional diffusion motion of the conformational defects along the chain in the paraelectric phase, accompanied by the trans and gauche transformation of the VDF conformers of ttg⁺tg^? and g⁺tg^?tt. The effect of the competitive motion of the TrFE‐rich segment on the thermal stability of the VDF‐rich segment in the chain near the Curie temperature was examined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1026–1037, 2002     

Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

The spectral diffusion of singlet and triplet excitons in 9,9-dioctylfluorene-based conjugated copolymers is investigated using photoluminescence spectroscopy at both low (5 K) and room temperature (300 K). Inclusion of a N,N-diphenyl-4-(pyridin-2-yl)aniline moiety into the polymer backbone allows subsequent cyclometalation with platinum acetylacetonate to increase the spin-orbit coupling and yield radiative decay from the triplet state. For suitably low fractions (≤5%) of bulky ligand inclusion, cyclometalated or not, the resulting longer sequences of fluorene units are able to adopt the chain-extended β-phase conformation. Comparison between the phosphorescence spectral diffusion in glassy- and β-phase Pt-copolymer samples provide insight into the triplet exciton transfer in more- or less-disordered conjugated polymer films. It is found in the glassy-phase samples with shorter conjugation lengths that the triplet exciton relaxation becomes frustrated at low temperature due to a freezing out of the thermally activated hops required to move from one conjugated segment to another. In contrast, for films containing β-phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through intra-segment motion. This work demonstrates controlled use of changes in molecular conformation to optimize triplet diffusion properties in a member of the widely deployed fluorene-based conjugated copolymers.     

十二烷基硫酸钠水溶液的动态表面吸附性质

利用MPTC型气泡压力张仪研究了十二烷基硫酸钠(SDS)溶液在不同NaCl 浓度下的动态表面吸附性质, 分析了离子型表面活性剂在表面吸附层和胶束中形成双电层结构产生表面电荷对动态表面扩散过程和胶束性质的影响. 结果表明, SDS在表面吸附过程中, 表面电荷的存在会产生5.5 kJ·mol^-1的吸附势垒(E_a), 显著降低十二烷基硫酸根离子(DS^-)的有效扩散系数(D_eff). 十二烷基硫酸根离子的有效扩散系数与自扩散系数(D)的比值(D_eff/D)仅为0.013, 这表明SDS与非离子型表面活性剂不同, 在吸附初期为混合动力控制吸附机制. 加入NaCl可以降低吸附势垒. 当加入不小于80 mmol·L^-1 NaCl后, E_a小于0.3 kJ·mol^-1, D_eff/D在0.8-1.2之间, 表现出与非离子型表面活性剂相同的扩散控制吸附机制. 同时, 通过分析SDS胶束溶液的动态表面张力获得了表征胶束解体速度的常数(k₂). 发现随着NaCl 浓度的增大, k₂减小, 表明SDS胶束表面电荷的存在会增加十二烷基硫酸根离子间的排斥力, 促进胶束解体.     

DNA diffusion in aqueous solution in presence of suspended particles

Although nanoparticles, which are comparable in size to polymer chains, are widely used as fillers to polymer matrices for developing functional and high performance materials, the dynamics of polymers constrained between solid particles has not been well elucidated. In this study, dynamics of individual polymer under such condition was investigated with fluorescent microscopy using DNA solutions as model systems. For individual T4 and λ DNA molecules in aqueous suspensions of spherical polystyrene particles with diameter of 1 μm, it was found that (i) the radius of gyration of DNA is independent of the particle volume fraction, ?_p, (ii) DNA diffusion is not sensitive to ?_p up to a certain critical ?_p where the average distance between particle surfaces is close to DNA size, and (iii) the DNA diffusion becomes slower at higher ?_p. The diffusion coefficient of DNA was larger, by a factor of 2, in the suspensions at intermediate ?_p than in the corresponding confined geometry (channel/slit between fixed walls), whereas this difference asymptotically vanished with increasing ?_p. This result suggested that the DNA diffusion in the suspensions with intermediate ?_p is accelerated by the particle motion. In fact, the diffusion coefficient measured for DNA in the suspensions was semiquantitatively described by the Rouse constraint‐release model considering the matrix effect on the probe chain diffusion. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1103–1111, 2009     

Diffusion kinetics investigations of Nano Ag-doped holographic polymer dispersed liquid crystal gratings

ABSTRACT

In this paper, we use multicomponent mutual diffusion method to derive a one-dimensional non-local diffusion dynamic model to describe the diffusion kinetics of a dynamic holographic polymer dispersed liquid crystal grating (H-PDLC) doped with nano-silver. The physical mechanism of diffusion between monomer and liquid crystal, monomer and nano-silver particles is analysed using this model. Using coupled-wave theory, the H-PDLC’s diffraction efficiency curve with the expose time are simulated due to the vivid changing of effective refractive index modulation caused by the movement of concentration of each component with the expose time. Correspondingly, in the experiment, the diffraction efficiency of the grating is measured in real time, which shows the improvement for the holographic properties because of nano-silver doped H-PDLC. The simulation results have a good agreement with experimental data by fitting the corresponding parameters of the model. In addition, through comparing with simulation and experimental results with doping different concentrations of nano-silver particles, the recipe and diffraction characteristics of H-PDLC grating can be improved. Thus, the diffusion Kinetics model can be used to optimise the phase separation of the PDLC grating, and finally to improve the opto-electrical properties of H-PDLC gratings.     

Effect of hydrophobic chain length of the chiral surfactant on enantiomeric separations by electrokinetic chromatography: Comparison between micellar and vesicular pseudo-stationary phases

Two novel amino acid based surfactants sodium N-(4-n-decyloxybenzoyl)-l-valinate (SDeBV) and sodium N-(4-n-octyloxybenzoyl)-l-valinate (SOBV) have been synthesized and used as chiral selectors for enantiomeric separations by micellar electrokinetic chromatography (MEKC). The aggregation behavior of the surfactants was studied in buffered aqueous solution using surface tension and fluorescence probe techniques. The microenvironment of the aggregates was studied using pyrene, and 1,6-diphenyl-1,3,5-hexatriene (DPH) as probe molecules. Results of these studies indicate that these two surfactants form micelles in buffered aqueous solution. Successful enentioseparation has been achieved for 1,1′-bi-2-naphthol (BOH), 1,1′-binaphthyl-2,2′-diylhydrogenphosphate (BNP), 2,8-dimethyl-6H-5,11-methanodibenzo[b,f] [1,5]diazocine (Tröger's base, TB), and benzoin (BZN) using the two chiral selectors SDeBV and SOBV. The separations were optimized with respect to surfactant concentration, pH, and buffer concentration. The results are discussed in light of the aggregation behavior of the surfactants. A comparison of the results of this study has been made with the data from literature to investigate the effect of self-assembly morphology on enantiomeric separations.     

Impact of Xylose on Dynamics of Water Diffusion in Mesoporous Zeolites Measured by NMR

Zeolites are known to be effective catalysts in biomass converting processes. Understanding the mesoporous structure and dynamics within it during such reactions is important in effectively utilizing them. Nuclear magnetic resonance (NMR) T₂ relaxation and diffusion measurements, using a high-power radio frequency probe, are shown to characterize the dynamics of water in mesoporous commercially made 5A zeolite beads before and after the introduction of xylose. Xylose is the starting point in the dehydration into furfural. The results indicate xylose slightly enhances rotational mobility while it decreases translational motion through altering the permeability, K, throughout the porous structure. The measurements show xylose inhibits pure water from relocating into larger pores within the zeolite beads where it eventually is expelled from the bead itself.     

Dynamic light scattering from spherical particles

The dynamic light scattering behavior of poly(butylmethacrylate) PBMA microgels and of kappa-casein micelles is compared with that from hard sphere latex particles. The latex particles and the kappa-casein micelles exhibited a single exponential decay of the time correlation function (TCF). For the microgels, progessively stronger deviations from a single exponential were observed as the scattering angle was made larger. These deviations are interpreted as being the result of internal modes of motion. From measurement of the first cumulant of the TCF, extrapolated towards zero angle, the translational diffusion coefficients D were determined, and the hydrodynamically effective radii were calculated via the Stokes-Einstein relationship. The ratio of the radius of gyration to the hydrodynamic radius was found to be?=0.775+0.012 for the latex particles, in good agreement with theory. The microgels, however, exhibit much lower?-parameters of 0.49 to 0.58, while the kappa-casein micelles showed the opposite behavior with values between 1.1 and 2.5. The results are interpreted on the basis of the DebyeBueche and Deutsch-Felderhof theory for porous spheres.     

Effect of dry density on 125I diffusion in GMZ bentonite

Gaomiaozi(GMZ) bentonite is regarded as the favorable candidate backfilling material for a potential repository in China.It is important to understand the diffusion behavior of 125 I in GMZ bentonite and compare the diffusion behavior in GMZ and other types of bentonite like MX-80,Avonlea,etc.Therefore,through-and out-diffusion experiments were conducted to obtain the effective diffusion coefficient(D e) and distribution coefficient(K d).A computer code named Fitting for diffusion coefficient(FDP) was used for the experimental data processing and theoretical modeling.At the dry density of GMZ bentonite from 1600-2000 kg/m 3,the D e values of 125 I were(2.4-20.4) × 10-12 m 2 /s and K d values were constants.At dry density above 1800 kg/m 3,the diffusion behaviors were almost the same,indicating that the anion exclusion was ineffective.Out-diffusion results showed that the species of 125 I may be changed during the diffusion processing.It was probably caused by some organic matters or reducing substances in GMZ bentonite.Since the main composition of bentonite is montmorillonite,similar diffusion parameters were obtained in GMZ and other types of bentonite.The relationship of D e and accessible porosity(acc) could be described by Archie’s law with exponent n = 1.2-2.8 for 125 I diffusion in bentonite,whereas n = 2.0 in GMZ bentonite.Furthermore,bentonite with the dry density of 1800 kg/m 3 was proposed as the backfilling materials used in the construction of high level radioactivity waste repository.