Protein diffusion in charged polyacrylamide gels. Visualization and analysis

Protein diffusion in anionic, cross-linked polyacrylamide-based gels supported in fused-silica capillaries was characterized by a direct visualization method. Microphotography was used to obtain transient protein concentration profiles in these gels using cytochrome c as a probe molecule. Gels based on acrylamido-methylpropane sulfonic acid with 2.5-10% N,N'-methylene-bisacrylamide as a cross-linker and with a total polymer concentration of 0.21 g/cm3 yielded diffuse protein concentration profiles which were quantitatively consistent with a Fickian diffusion model. An analytical method was developed to calculate the diffusivity as a function of protein concentration in the gel from the experimental profiles. The diffusivity was found to assume values in the range 2.5-5.5x10(-8) cm2/s and varied somewhat with the protein concentration in the gel. The effects of some of the polymer properties, such as cross-link density, polymer concentration and charge, were also investigated for a limited range of conditions to derive qualitative trends. Results showed that the transport rates increased with a decrease in the cross-link density, were extremely reduced when the polymer concentration was doubled, and were slightly increased when the charge density was decreased by half by polymerizing a 1:1 mixture of acrylamide and acrylamido-methylpropane sulfonic acid monomers.     

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 effect of riboflavin-photoinduced degradation of alginate matrices on the diffusion of poly(oxyethylene) probes in the polymer network

The effect of irradiation, in the wavelength range of 310-800 nm, on the tracer diffusion of poly(oxyethylene) (POE) of different molecular weights embedded in various alginate matrices with the photosensitizer riboflavin (RF) present, has been investigated with the aid of pulsed field gradient spin-echo NMR (PGSE-NMR). Both alginate solutions of different polymer concentration were studied as well as corresponding acid gels of alginate produced by introduction of different amount of glucono-δ-lactone (GDL). In 2 wt.% alginate solutions, the values of the tracer diffusion coefficient suggest a strong obstruction effect as the probe molecular weight increases. Faster probe diffusion was observed for the irradiated samples, which indicates a photochemical scission of the polymer chains and the formation of a fragmented polymer network that facilitates the migration of the tracer chains. A semidilute alginate/RF solution was transformed into a gel by adding sufficient amount of GDL. GDL lowers the pH of the solution under the pK_a of alginate, favouring intermolecular associations and the evolution of a less homogeneous network with more open structure. Therefore, the POE chains were shown to diffuse faster in the acid gel matrix than in the corresponding more homogeneous alginate solutions. The photochemically induced cleavage again promoted faster migration of the probe chains in the irradiated samples. The probe diffusion of the eight-arm star-shaped POE sample revealed an augmented obstruction effect with increasing alginate concentration and higher values of the diffusion coefficient were found in gels. The evolution of a tighter network inhibits the diffusion of the probe molecules. At lower alginate concentrations the values of the tracer diffusion coefficients are higher for the irradiated samples than for the non-exposed systems.     

Fluorescence correlation spectroscopy in colloid and interface science

In the last two decades fluorescence correlation spectroscopy (FCS) has been increasingly applied to analyze systems and processes relevant to colloid and interface science. The method has become a routine tool to measure the hydrodynamic radii of small fluorescent molecules, macromolecules and nanoparticles, characterize their interactions and follow a possible aggregation. It was also used to study the diffusion of such species in inhomogeneous media like polymer melts, solutions, gels or porous structures. The formation kinetics and size of micelles of surfactants or block copolymers has been quantified. FCS has also been applied to characterize diffusion of tracers at fluid–liquid and solid–liquid interfaces and study the hydrodynamic boundary condition. The review is intended to summarize these applications and highlight perspectives but also limits of FCS in colloid and interface science.     

Microrheology of chemically crosslinked polymer gels by diffusing-wave spectroscopy

The Brownian motion of probe particles in aqueous solutions of poly(vinyl alcohol)(PVA) and in chemically crosslinked PVA gels has been studied by diffusing-wave spectroscopy (DWS). At long time scales the measurements allow us to determine the effect of the crosslinking ratio on the macroscopic viscosity of sols and the shear modulus of gels. The local shear modulus of gels as obtained from the characteristic length of the Brownian cage was found to agree with that measured by classical rheometry and dynamic light scattering (DLS). These microrheological techniques were applied to two polymer gel systems. Substrate induced gradient structure of hydrogels was studied from a microrheological point of view using DLS. It is clearly seen that hydrophobic substrate induces weakly crosslinked network formation at the interface region up to a few millimeters as expected from other experimental facts. Magnetic particle motion in gels under external magnetic field was investigated by DWS. The translational motion of the magnetic particles in gels due to the alternating magnetic force can be detected and found to be superimposed on the relaxation due to the thermal motion.     

Hydrogels in aqueous phases of polyvinylalcohol (PVA), surfactants and clay minerals

Aqueous solutions of synthetic clay minerals have been studied in the presence of surfactants and water-soluble polyvinylalcohol (PVA). The PVAs (PVA 1, PVA 2) had a molecular weight of about 10⁵ Dalton and a degree of hydrolysis of 82%. The PVA-samples were surface active and lowered the surface tension to 43 mN/m. As a consequence of their amphiphilic nature the PVA molecules bind strongly to clay mineral particles. On saturation the clay mineral particles adsorb the fivefold weight of PVA of their own weight. It is concluded that the thickness of the adsorbed layers on both sides of the clay mineral is in the range of the hydrodynamic diameter of the PVA-coils in the bulk phase.When the clay mineral particles are not saturated with PVA, they act as cross-linking agents for the PVA. The whole systems are physically cross-linked and assume gel-like properties. Rheological measurements show that samples behave like soft matter with a yield stress value. All of them have a frequency independent storage modulus which is an order of magnitude larger than the loss modulus. The hydrogels become stronger as PVA concentration increases.Small amounts of cationic surfactants bind on the clay mineral. The interface of the clay mineral becomes more hydrophobic and the binding of the PVA on the clay mineral is strengthened. With rising concentration of the surfactant the surfactant molecules bind on PVA and the PVA becomes hydrophilic. As a consequence the PVA can no longer bind on the clay mineral and the gels transform to viscous and turbid solutions. Small amounts of cationic surfactants therefore stiffen the hydrogels while larger amounts cause phase separation and a solution with low viscosity. Anionic surfactants like SDS do not bind on the clay mineral, but strongly on the PVA. With increasing SDS concentration, the hydrogels become stiffer at first but thereafter they break and transform to viscous fluids.In PVA-solutions without the clay minerals both cationic and anionic surfactants bind to the PVAs in the aqueous solution. With increasing concentration of surfactant, the viscosities of the solutions pass over a maximum. In this respect the PVAs behave like hydrophobically modified water soluble polymers. The surfactants bind to the hydrophobic microdomain and thereby crosslink the polymer molecules. On saturation the polyvinyl alcohol with anionic surfactant become hydrophilic and the network character disappears to a certain extent.     

Emulsion polymerization of hydrophobia monomers

Emulsion polymerization is the most important industrial polymerization process for manufacturing water based polymers. The heterogeneous nature of the process requires the diffusion of monomers from the emulsified droplets, through the aqueous medium, into the polymer particles where the polymerization takes place. Adequate solubility of the monomer is necessary for the diffusion process to occur effectively. Consequently, very hydrophobic monomers cannot be readily incorporated by emulsion polymerization. The use of a catalytic level of cyclodextrin allows the use of very hydrophobic monomers in emulsion polymerization.^[1] The mechanism of the process is believed to involve a catalytic cycle in which cyclodextrin acts as a “Phase Transport Catalyst”, continuously complexing and solubilizing the hydrophobic monomers and releasing them to the polymer particles. The kinetics and thermodynamics are favorable for the reaction to proceed.     

低分子物质在聚乙烯醇水凝胶中的扩散

水凝胶是指一种网络结构中含大量水,而不溶于水的高分子材料。近年来,发现聚乙烯醇(PVA)水溶液在低温冷冻一段时间,可形成强度较高的水凝胶弹性体,用这种水凝胶制得的固定化增殖细胞凝胶,在生物工程材料开发中取得了很好的效果。本文旨在研究低分子物质在PVA水凝胶中的扩散性能。     

Studies on bound water in poly(vinyl alcohol). Hydrogel by DSC and FT-NMR

The phase transition of water bound in poly(vinyl alcohol)(PVA) hydrogel was measured by differential scanning calorimetry. The cross-link density calculated from the swelling ratio of PVA hydrogel ranged from 3 × 10^?5 to 1 × 10^?3 according to the total dose of ⁶⁰Co γ-ray irradiation. From the enthalpy of melting, it was found that water in PVA gel can be classified into at least three groups; ca. 1–1.5 mol non-freezing water, 5–6 mol freezing bound water, and the rest free water, corresponding to each hydroxyl group of PVA. The characteristic properties of polymer hydrogel were attributed to the large number of molecules of freezing bound water in the network. The amount of free water depended markedly upon cross-link density; however, it was not apparent in PVA gel with cross-link density >2.0 × 10^?4. The decreases in the nuclear magnetic resonance relaxation times of water and PVA were related to the cross-link density and were found to level off at a cross-link density 2.0 × 10^?4.     

Manipulating the degradation rate of PVA nanoparticles by a novel chemical‐free method

The high water solubility of poly (vinyl alcohol) (PVA) is one of the challenging problems in its application. In order to rectify this problem, PVA needs to be crosslinked. Freeze‐thawing in solid state as a novel physical crosslinking method was employed for enhancement the stability of PVA nanoparticles in aqueous solutions during this study. PVA nanoparticles were successfully prepared by electrospraying and electrospray conditions were optimized in the view points of polymer concentration and solvent system. The morphology of nanoparticles was tailored from collapsed particles and mixture of particles/fibers to spherical particle by manipulating of polymer solution concentration and solvent system. After preparation of PVA nanoparticles in optimum condition, they were frozen at ?20°C and subsequently thawed at 25°C for different cycles of 1, 2, and 3. Field‐emission scanning electron microscope (FE‐SEM), Fourier‐transform infrared (FTIR), X‐ray diffraction (XRD), differential scanning calorimeter (DSC), and biodegradation were used to evaluate the effect of freeze‐thawing on properties of PVA nanoparticles. FE‐SEM showed the spherical morphology of the PVA nanoparticles with sizes ranging from 200 to 300 nm. The FTIR spectroscopy indicated that the crystallinity of PVA nanoparticles increases after freeze‐thawing process. Moreover, by increasing the number of cycles, degree of crystallinity of nanoparticles increases. The XRD and DSC analysis of PVA nanoparticles again demonstrated the increasing of crystallinity of nanoparticles after freeze‐thawing process. The biodegradation behavior of PVA nanoparticles after freeze‐thawing exhibited the decreasing of degradation rate by increasing the number of cycles. Our overall results present a solvent‐less and safe method for crosslinking of PVA nanoparticles in solid state, which make it suitable for biomedical applications.     

Effect of soluble polymer binder on particle distribution in a drying particulate coating

Soluble polymer is frequently added to inorganic particle suspensions to provide mechanical strength and adhesiveness to particulate coatings. To engineer coating microstructure, it is essential to understand how drying conditions and dispersion composition influence particle and polymer distribution in a drying coating. Here, a 1D model revealing the transient concentration profiles of particles and soluble polymer in a drying suspension is proposed. Sedimentation, evaporation and diffusion govern particle movement with the presence of soluble polymer influencing the evaporation rate and solution viscosity. Results are summarized in drying regime maps that predict particle accumulation at the free surface or near the substrate as conditions vary. Calculations and experiments based on a model system of poly(vinyl alcohol) (PVA), silica particles and water reveal that the addition of PVA slows the sedimentation and diffusion of the particles during drying such that accumulation of particles at the free surface is more likely.     

Relation between the gel structure and the mobility of tracers in globular protein gels

Diffusion of fluorescent-labeled dextran with different molecular weights was investigated in β-lactoglobulin (β-lg) solutions and gels over a wide range of salt and protein concentrations at pH 7 by combining confocal laser scanning microscope (CLSM) with fluorescence recovery after photobleaching (FRAP). Effects of the protein concentration, the salt concentration and the tracer size were investigated in detail. Diffusion in turbid heterogeneous gels formed at 0.2 M NaCl depended weakly on the probe size and the protein concentration and remained close to that in unheated solutions. A strong decrease of the diffusion coefficient with increasing tracer size and protein concentration was observed in more homogeneous gels formed at lower salt concentrations. Larger dextran chains were trapped in transparent gels formed at NaCl concentration below 0.1 M. The present investigation complements an earlier study of tracer diffusion of larger spherical probes in β-lg gels using multi-particle tracking.     

Regular microstructures in gel–surfactant complexes: Influence of water content and comparison with the surfactant structure in water

Interaction of slightly crosslinked hydrogels of poly(diallyldimethylammonium chloride) (PDADMACI) and of copolymer DADMACI/acrylamide (AAm) with sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfate (SDBS) results in significant shrinking of the gels due to the formation of polymer-surfactant complexes. Jump-wise transitions in the collapsed state were observed for the networks with the content of cationic groups 100 and 75 mol %. The structure of complexes was studied by means of X-ray scattering method. The scattering curves for collapsed gels, where most chloride anions were replaced by anions of SDS, show a set of well-pronounced narrow diffraction maxima. Fully charged “wet” complexes studied at the equilibrium swelling conditions exhibit high degree of ordering, which diminishes upon drying with the simultaneous transition from hexagonal to lamellar type of ordering. In contrast to this, for DADMACl/AAm copolymer gels (75 mol % of DADMACl monomers in the initial polymerization mixture) the ordering is less pronounced in the “wet” state and becomes more perfect upon drying. The SDS aqueous solutions of the same concentration in the absence of gel do not show such high degree of ordering, while the system of SDS/neutral AAm gel exhibits lamellar ordering typical for low-temperature phases of SDS solutions. © 1996 John Wiley & Sons, Inc.     

A Study of cryostructuring of polymer systems. 41. Complex and composite poly(vinyl alcohol) cryogels containing soluble and insoluble forms of chitosan,respectively

Complex macroporous poly(vinyl alcohol) (PVA) cryogels have been obtained by cryogenic treatment (freezing at–20°C for 12 h followed by defrosting at a rate of 0.03°C/min) of PVA–chitosan hydrochloride mixed solutions. The subsequent alkaline treatment of the cryogels has resulted in the transformation of the water-soluble salt form of chitosan into its insoluble basic form, which coagulates inside the bulk of the continuous phase of PVA cryogel into small particles with sizes of 2–5 µm. In the resulting composite cryogels, these particles play the role of an “active” filler, which increases the rigidity and heat endurance of the gel material. It has been shown that the sorption capacity of such chitosan particles entrapped into the bulk of composite cryogels with respect of bivalent copper ions is noticeably higher than the sorption capacity of ground chitosan particles incorporated as a discrete filler into the continuous phase PVA cryogels. The study of the properties of PVA–chitosan hydrochloride mixed solutions revealed that these polymers are, to a large extent, compatible with one another in a common solvent at a low ionic strength. Therefore, liquidliquid phase separation of these systems due to the thermodynamic incompatibility of macromolecules of different natures is observed only upon increasing the ionic strength by adding a low-molecular-mass salt (NaCl, 0.15 mol/L) to the solution.     

Polycondensation reaction of dimethyl tartrate with hexamethylenediamine in the presence of various matrices

The polycondensation reaction of dimethyl tartrate (DMT) with hexamethylenediamine (HMD) was carried out in dimethyl sulfoxide (DMSO) at 60°C in the presence of various polymer matrices, which were expected to interact with DMT or the resulting polyamide which had pendant hydroxyl groups due to hydrogen bonding. It was found that the rate of polycondensation was enhanced by polymer matrices such as poly(vinyl pyrrolidone) (PVP), Pullulan (polysaccharide) (PF), and poly(vinyl alcohol) (PVA). The rate enhancement became more pronounced with increasing molecular weight of the polymer matrix. When polycondensation in the presence of PVA was carried out in DMSO, a polymer complex was produced. The formation of the polymer complex between the resulting polyamide and PVA during polycondensation was dependent on the concentration of monomers and also on PVA; a gelation of the solution was observed at a concentration of PVA.     

荧光关联光谱在高分子单链研究中的应用

荧光关联光谱(fluorescence correlation spectroscopy,FCS)是一项用于研究体系动力学性质的统计光谱技术,随着它被引入材料与化学研究领域,近年来取得了大量全新的研究成果.该技术在高分子科学研究中也逐渐发挥出越来越大的作用,特别是在聚合物结构和动力学方面,这表明它在高分子领域的巨大潜力.本文将从FCS的基本原理、实验技巧以及在一些具有挑战性体系中的应用等方面展开,着重介绍它在高分子溶液,如聚电解质溶液、高分子混致不溶现象,以及不同的表界面体系中取得的新成果,展示FCS区别于其他传统技术的特点和优势.     

“Grafting through” polymerization involving surface-bound monomers

“Grafting through” polymerization represents copolymerization of free monomers in solution and polymerizable units bound to a substrate. Free polymer chains are formed initially in solution and can incorporate the surface-bound monomers, and thereby, get covalently bonded to the surface during the polymerization process. As more growing chains attach to the surface-bound monomers, an immobilized polymer layer is formed on the surface. We use a combination of computer simulation and experiments to comprehend this process for monomers bound to a flat impenetrable substrate. We concentrate specifically on addressing the effect of spatial density of the surface-bound monomers on the formation of the surface-attached polymers. We employ a lattice-based Monte Carlo model utilizing the bond fluctuation model scheme to provide molecular-level insight into the grafting process. For experimental validation, we create gradients of density of bound methacrylate units on flat silicon wafers using organosilane chemistry and carry out “grafting through” free radical polymerization initiated in bulk. We report that the proximity of the surface-bound polymerizable units promotes the “grafting through” process but prevents more free growing chains to “graft through'' the polymerizable units. The “grafting through” process is self-limiting in nature and does not affect the overall density of the surface-bound polymer layer, except in case of the highest theoretical packing density of surface-bound monomers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 263–274     

Pulsed NMR study of the structure of poly(vinyl alcohol)-poly(sodium acrylate) composite hydrogel

The heterogeneous structure of poly(vinyl alcohol)-poly(sodium acrylate) composite hydrogels (PVA-PAA gels), prepared by repeatedly freezing and thawing mixed aqueous solutions of PVA and PAA, has been studied using pulsed NMR. The free induction decay (FID) signals of the transverse magnetization for the PVA-PAA gels were measured by both Carr-Purcell-Meiboom-Gill and solid-echo pulse sequences. The FID signal was decomposed into two or three decaying signals with various spin-spin relaxation times, based upon differences in the molecular mobility that reflected structural heterogeneity. The structural heterogeneity of the PVA-PAA gel is quite different from that of PVA gel free of PAA. We have made observations on “crosslinked” domains in the PVA-PAA gels. They are formed by PVA solid chains, probably phase-separated in the freezing-thawing process. The formation of crosslinked domains is influenced by the number of freezing-thawing cycles. © 1994 John Wiley & Sons, Inc.     

Fluorescence correlation spectroscopy studies of diffusion of a weak polyelectrolyte in aqueous solutions

We apply fluorescent correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The technique brings single molecule sensitivity and molecular specificity to dynamic measurements of polyelectrolyte solutions. Translational diffusion of Alexa-labeled poly(methacrylic acid), PMAA*, chains was studied in very dilute, 10(-4) mg/ml, solutions as a function of solution pH and ionic strength. The observed changes in diffusion coefficients were consistent with about twofold expansion of PMAA* coils when pH was changed from 5 to 8, and with chain contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The dependence of the hydrodynamic size of PMAA* chains on the counterion type followed the sequence: Li(+)>Na(+) approximately equal to Cs(+)>K(+). The dependence of translational diffusion on polyacid concentration was weak at the low concentration limit, but chain motions were significantly slower at higher polymer concentrations when PMAA chains overlapped. Finally, measurements of dynamics of PMAA* chains in "salt-free" solutions showed that self-diffusion of PMAA* chains significantly slowed down when PMAA concentration was increased, probably reflecting the sensitivity of PMAA* translational motions to the onset of interchain domain formation. These results illustrate the utility of the FCS technique for studying hydrodynamic sizes of polyelectrolyte coils in response to variation in solution pH or concentration of salt and polyelectrolytes. They also suggest that FCS will be a promising technique for selective observation of the dynamics of polyelectrolyte components in complex polymer mixtures.     

Comparison of the different emulsion polymerization routes between acrylic and vinyl acetate monomers using poly(vinyl alcohol) as the sole stabilizer

The emulsion polymerization of acrylic and vinyl acetate monomers using poly(vinyl alcohol) (PVA) as protective colloid has been studied for a long time, whereas rare reports ranged over the lateral comparison of the two systems and the reason for the unstability of the PVA/MMA polymerization system was still indistinct. Here in this paper, a collection of experiments of methyl methacrylate (MMA) and vinyl acetate (VAc) were performed respectively with varied amount of PVA as the sole stabilizer. The grafting extent of the polymerisate was characterized through the fraction and FTIR and it was found that the grafted amount of PVA was even larger in the PVA/MMA copolymers than PVA/VAc copolymers, so the grafted mode of PVA was considered. Based on the kinetics, the slower initiation rate of sulfate radical towards MMA was found to be responsible for the unstability of corresponding colloids accompanied with the relatively quicker hydrogen abstraction of radical to PVA, which resulted in “layer-by-layer” grafting structure inside and the particle surface-grafting density was lowered thereof. This was proved with TEM and static contact angle measurements and a pseudo-homopolymer model was employed to describe the relationship of colloidal stability and the PVA density on surfaces.