Diffusive desorption of small solute molecules from amorphous polymers: poly(methyl methacrylate), poly (vinyl acetate) and poly(n-alkyl 2-cyanoacrylates)

Diffusion coefficients of some aromatic substances in poly(vinyl acetate) and poly(methyl methacrylate) have been measured from desorption rates into aqueous buffers from solid solutions of diffusant in cast polymer films. With ordinary spectrophotometric instrumentation, extension of the measurable range of the diffusion coefficient at infinite dilution to below 10^?21 m² sec^?1 is sometimes possible. Measures values below 10^?12 m² sec^?1 are generally much more quickly obtained, and are apparently at least as reproducible as those obtained by time-lag methods, permitting differential effects of diffusant molecular weight and shape to be studied, with solutes of higher molecular weight than those to which previous published studies were confined. Diffusion coefficients of the same solutes in a number of poly(n-alkyl 2-cyanoacrylates) were measured by desorption from polymer particles and films. The rank order and general dependence of diffusivity upon molecular weight and shape is similar in all three polymers. There is a tendency for the ionic and basic solutes to become bound, probably by covalent linkage, to poly(n-alkyl 2-cyanoacrylate) molecular chain ends.     

极性和非极性溶剂在聚乙烯中的无限稀释扩散系数的测定

 气相法研究小分子溶剂与高分子聚合物材料之间的相互作用是一个快速、准确、方便的方法 ,可以测量多种小分子溶剂在聚合物中的溶解、扩散参数。通过气相法测定了 5种小分子溶剂 (正己烷、正庚烷、正癸烷、乙醇和水 )在固定液聚乙烯中的保留时间和半峰宽 ,运用vanDeemter模型进行数据处理 ,得到上述 5种小分子在聚乙烯中的无限稀释扩散系数 ,获得了十分有意义的结果。     

Application of size exclusion chromatography with surfactant eluent to the study of polymer–surfactant interactions: oligomeric and micellar chromatographic effects

Complexation of sodium dodecyl sulphate (SDS) with a wide range of molecular weights of poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO) has been studied by size exclusion chromatography using aqueous SDS eluent. A multi-angle laser light scattering detector and a differential refractometer were applied to give direct measurement of the molecular weight of complexes without reference to elution volume, since the latter is not a reliable indicator of the complex size. Background light scattering from micellar eluents hampered quantitative size measurements, but was minimal in sub-micellar eluent, where saturated binding was observed for polymers larger than 1000 g mol⁻¹. Multiple peaks and voids were observed in the elution profiles of low molecular weight polymers (up to a mass of 600 g mol⁻¹) in eluent at micellar concentrations. Several sources contribute to this behavior, including micellar chromatographic separation of the PEG oligomers due to their different distribution coefficients between the micellar and water phases. Preliminary results are reported for distribution coefficients of individual oligomers in a 600 g mol⁻¹ PEG sample. Three distinct binding behaviors are observed with increasing degree of polymerization of PEG: no interaction for small glycols, equilibrium partitioning of intermediate oligomers in and out of micelles, and binding of micelles to the larger polymers.     

Measurement of the infinite dilution diffusion coefficients of small molecule solvents in silicone rubber by inverse gas chromatography

The infinite dilution diffusion coefficients of n-hexane, n-heptane and n-decane in crosslinked silicone rubber with different crosslinking agent concentrations were measured in the temperature range of 348.15 K-368.15 K by inverse gas chromatography. The crosslinked silicone rubber was obtained by dissolving PDMS prepolymer, crosslinking agent and catalyst in n-heptane solvent and characterized by FTIR spectra. The Van Deemter equation was used to determine diffusion coefficients from the variation in chromatographic peak width with carrier gas flow rate. The good linear relation indicated the Van Deemter equation used in this work was reliable. The influences of small molecule solvent, crosslinking agent concentration and temperature on the infinite dilution diffusion coefficient were investigated. The results showed that the infinite dilution diffusion coefficient decreased with an increasing number of CH₂ group in the alkane series. The increase in crosslinking agent concentration resulted in decrease of the infinite dilution diffusion coefficient. The infinite dilution diffusion coefficient increased with the rising of temperature. The interdependence on the infinite dilution diffusion coefficient and temperature accorded with Arrhenius equation well. Diffusion constant and activation energy obtained from the Arrhenius equation provided straight lines with the specific critical volume and crosslinking agent concentration.     

水、甲醇和乙醇溶剂在PVA膜材料中无限稀释扩散系数测定

用气相色谱法研究小分子溶剂与聚合物材料之间的相互作用是一种快速、准确和方便的方法,该方法可以测量多种小分子溶剂在聚合物中的溶解和扩散行为.采用该方法测定了水、甲醇、乙醇在固定液PVA中的保留时间和半峰宽,运用van Deemter模型进行数据处理,得到了上述几种小分子在PVA膜材料中的无限稀释扩散系数,获得了有意义的结果.     

Poly(ethylene glycol)--oligolactates with monodisperse hydrophobic blocks: preparation, characterization, and behavior in water

Methoxypoly(ethylene glycol)-b-oligo-L-lactate (mPEG-b-OLA) diblock oligomers with monodisperse OLA blocks were obtained by fractionation of polydisperse block oligomers using preparative HPLC. The fractionated oligomers were composed of an mPEG block with a molecular weight of 350, 550, or 750 and an OLA block with a degree of polymerization of 4, 6, 8, or 10. The diblock oligomers with a low PEG content were fully amorphous, with glass transition temperatures ranging from -60 to -20 degrees C, indicating that the blocks were miscible. Upon heating aqueous dispersions of the block oligomers, cloud points, depending on the PEG/OLA ratio of the block oligomer, were observed at temperatures above 40 degrees C. The monodispersity of the hydrophobic block enabled the amphiphilic molecules to form nanoparticles in water with a hydrodynamic radius of 130-300 nm, at concentrations above the critical aggregation concentration (0.4-1 mg/mL), whereas polydisperse mPEG-b-OLAs gave formation of large aggregates. Static light scattering measurements showed that the nanoparticles have a low density (0.6-25 mg/mL), indicating that the particles are highly hydrated. In agreement herewith, the (1)H NMR spectra of nanoparticles in D2O closely resembled spectra in a good solvent for both blocks (CDCl3). It is therefore suggested that the nanoparticles contain a hydrated core of mPEG-b-OLA block oligomers, stabilized by a thin outer PEG layer. The particles were stable for two weeks, except for the mPEG350 series and mPEG750-b-OLA4, indicating that both the PEG block size and the PEG weight fraction of the oligomers determine their stability. The evident self-emulsifying properties of mPEG-b-oligo-l-lactates with monodisperse hydrophobic blocks as demonstrated in this study, together with their expected biocompatibility and biodegradability, make these systems well suitable for pharmaceutical applications.     

非极性烷烃溶剂在聚乙烯膜中无限稀释扩散系数的测定

气相色谱法研究小分子溶剂与聚合物材料之间的相互作用是一种快速、准确、方便的方法,该方法可以用来研究多种小分子溶剂在聚合物中的溶解和扩散行为.通过气相色谱法测定了3种小分子烷烃溶剂(正庚烷、正壬烷和正癸烷)在固定液聚乙烯中的保留时间和半峰宽,用vanDeemter模型进行数据处理,得到3种小分子在聚乙烯膜中的无限稀释扩散系数.     

Water mass transfer in W/O emulsions

Water transportation through the oil phase in W/O emulsions and in W1/O/W2 systems (W/O emulsion in contact with water) was examined. Substance diffusion through interfaces led to interface instability and spontaneous emulsification which caused nanodispersion formation. The photomicrographs of Pt/C replicas of emulsions showed the presence in the continuous oil phase a lot of nanodispersion droplets with a diameter in the range 17-25 nm. Diffusion coefficient (D) of water calculated on the base of Lifshiz-Slezov-Wagner (LSW) equation was about 15 times lower than the coefficients of molecular diffusion. Since such emulsions were extremely unstable toward coalescence, the growth of water droplets took place through as Ostwald ripening as coalescence. In three-phase W1/O/W2 systems diffusion of water, Rhodamine C, and ethanol was studied. D calculated on the base of the equation of nonstationary diffusion were approximately 1000 times lower than molecular ones. It was assumed, that nanodispersion droplets were more likely water carriers in investigated W/O emulsions stabilized by sorbitan monooleate.     

含铅聚合物微凝胶与硫化氢反应前后在良溶剂中的分子扩散行为

用溶液聚合法制备出轻度交联的含铅微凝胶,用光子相关光谱技术测定其在良溶剂中与H2S反应前、后的扩散行为,由外推法得到在浓度无限稀时的分子扩散系数,给出微凝胶的流体力学半径。结果表明:相同量的含铅微凝胶在不同的初始浓度下与H2S反应,生成含PbS纳米微粒的凝胶；但其体积增大不同,这说明H2S与含铅微凝胶的反应既可以在分子内也可以在分子间进行,分子间的反应使含硫化铅微凝胶扩散系数随浓度的变化曲线的线性范围变小。     

含铅聚合物微凝胶与硫化氢反应前后在良溶剂中的分子扩散行为

用溶液聚合法制备出轻度交联的含铅微凝胶,用光子相关光谱技术测定其在良溶剂中与H₂S反应前、后的扩散行为,由外推法得到在浓度无限稀时的分子扩散系数,给出微凝胶的流体力学半径。结果表明:相同量的含铅微凝胶在不同的初始浓度下与H₂S反应,生成含PbS纳米微粒的凝胶;但其体积增大不同,这说明H₂S与含铅微凝胶的反应既可以在分子内也可以在分子间进行,分子间的反应使含硫化铅微凝胶扩散系数随浓度的变化曲线的线性范围变小。     

四臂星形聚苯乙烯在良溶剂和0溶剂中分子扩散的光子相关光谱的研究

用光子相关光谱研究了四臂星形聚苯乙烯在良溶剂(四氢呋喃THF)和θ溶剂(环己烷CH)中扩散系数与溶剂的浓度和温度的依赖关系。用累积量方法分析光子相关数据给出了多分散样品的Z均扩散系数。在θ溶剂中,高于或低于θ温度时,聚合物在溶液中的分子扩散分别表现出具有在良溶剂与不良溶剂中的行为。外推浓度至零,得到无限稀时不同温度的分子扩散系数,借助Stokes-Einstein方程,给出了聚合物的流体力学半径。通过InD对I/T作图,得到了星形聚苯乙烯在THF与CH中的扩散活化能。     

Pulsed field gradient NMR study of anomalous diffusion in a lecithin-based microemulsion

Self-diffusion measurements in microemulsion systems composed of a naturally occurring soybean lecithin mixture, an aqueous phase, either water or a 1% aqueous PDADMAC solution, and isooctane were accomplished by pulsed field gradient (PFG) 1H NMR spectroscopy at oil dilution lines of low and intermediate water/lecithin ratios. The concentration-dependent diffusion data reveal water-in-oil (W/O) reverse micellar aggregates with dimensions on the nanometer scale being slightly smaller at low water content. With increasing micellar volume fractions, both hydrodynamic as well as direct interactions between particles significantly slow aggregate diffusion. The surfactant mean square displacements (msd's) in dilute and concentrated polymer-free systems studied as a function of diffusion time (20-1000 ms) are characterized by a crossover from Gaussian diffusion, due to slow aggregate motion, to anomalously enhanced diffusion, due to fast surface-bulk surfactant exchange at intermediate times revealing weak, barrier-controlled adsorption behavior. Upon addition of the polycation PDADMAC, the diffusion characteristics change to exclusively superdiffusive behavior with surfactant msd scaling with time as t(3/2) over the entire time range studied. This is caused by surfactant molecules performing Levy walks along the surface of reverse micelles mediated by the dilute bulk. The bulk-mediated surface diffusion is a consequence of the diffusion-controlled micelle-bulk exchange dynamics induced by interactions of PDADMAC with surfactant headgroups.     

Application of molecular dynamics simulations in molecular property prediction II: diffusion coefficient

In this work, we have evaluated how well the general assisted model building with energy refinement (AMBER) force field performs in studying the dynamic properties of liquids. Diffusion coefficients (D) have been predicted for 17 solvents, five organic compounds in aqueous solutions, four proteins in aqueous solutions, and nine organic compounds in nonaqueous solutions. An efficient sampling strategy has been proposed and tested in the calculation of the diffusion coefficients of solutes in solutions. There are two major findings of this study. First of all, the diffusion coefficients of organic solutes in aqueous solution can be well predicted: the average unsigned errors and the root mean square errors are 0.137 and 0.171 × 10(-5) cm(-2) s(-1), respectively. Second, although the absolute values of D cannot be predicted, good correlations have been achieved for eight organic solvents with experimental data (R(2) = 0.784), four proteins in aqueous solutions (R(2) = 0.996), and nine organic compounds in nonaqueous solutions (R(2) = 0.834). The temperature dependent behaviors of three solvents, namely, TIP3P water, dimethyl sulfoxide, and cyclohexane have been studied. The major molecular dynamics (MD) settings, such as the sizes of simulation boxes and with/without wrapping the coordinates of MD snapshots into the primary simulation boxes have been explored. We have concluded that our sampling strategy that averaging the mean square displacement collected in multiple short-MD simulations is efficient in predicting diffusion coefficients of solutes at infinite dilution.     

三种芳烃溶剂在低密度聚乙烯膜材料中无限稀释扩散系数的测定

用气相色谱法测定了苯、甲苯和乙苯在固定液低密度聚乙烯中的保留时间和半峰宽,运用vanDeemter模型进行数据处理,得到3种芳烃小分子在低密度聚乙烯膜材料中的无限稀释扩散系数.     

Determination of preferential interaction parameters by multicomponent diffusion. applications to poly(ethylene glycol)-salt-water ternary mixtures

Poly(ethylene glycol) (PEG) is a hydrophilic nonionic polymer used in many biochemical and pharmaceutical applications. We report the four diffusion coefficients for the PEG-KCl-water ternary system at 25 degrees C using precision Rayleigh interferometry. Here, the molecular weight of PEG is 20 kg mol(-1), which is comparable to that of proteins. The four diffusion coefficients are examined and used to determine thermodynamic preferential interaction coefficients. We find that the PEG preferential hydration in the presence of KCl is 1 order of magnitude larger than that previously obtained under the same conditions for lysozyme, a protein of similar molecular weight. In correspondence, the coupled diffusion in the PEG case was greater than that observed in the lysozyme case. We attribute this difference to the greater exposure of polymer coils to the surrounding fluid compared to that of globular compact proteins. Moreover, we observe that the PEG preferential hydration significantly decreases as salt concentration increases and attribute this behavior to the polymer collapse. Finally, we have also employed the equilibrium isopiestic method to validate the accuracy of the preferential interaction coefficients extracted from the diffusion coefficients. This experimental comparison represents an important contribution to the relation between diffusion and equilibrium thermodynamics.     

Montmorillonite alignment induced by magnetic field: evidence based on the diffusion anisotropy of water molecules

Diffusion coefficients of water in Na-montmorillonite (Na-mon) suspensions have been determined by pulsed-field gradient spin-echo (PGSE) NMR spectroscopy for three directions (x, y, and z), where x and y mean the directions perpendicular to the static magnetic field, and z the direction parallel to it. Diffusion anisotropy was observed in the suspensions with Na-mon weight fractions of 0.63, 1.82, and 3.32%; i.e., the diffusivity of water in the z direction is faster than that in the x or y direction. The largest diffusion anisotropy of water was observed at the Na-mon fraction of 3.32%. However, diffusion anisotropy disappeared in the suspensions with Na-mon fraction more than 5.02%. The fast diffusivity in the z direction was slightly enhanced in a stronger static magnetic field (14.1 T). These results indicate that the platelike Na-mon particles are aligned with their platelike faces parallel to the static magnetic field of NMR. We also measured diffusion coefficients of water for the z direction in the temperature range from 24 to 85 degrees C. The plot of diffusion coefficients of water against reciprocal temperature showed a refraction point at 65 degrees C. This phenomenon explicitly means that the alignment is gradually relaxed at higher temperatures.     

Hydrodynamic behavior of high molar mass linear polyglycidol in dilute aqueous solution

Six high molar mass polyglycidol samples were obtained by fractionation of polyglycidol synthesized by means of cationic polymerization of ethoxyethyl glycidyl ether followed by cleavage of the protective groups. The fractions covering the molar mass range from 0.1 to 2.4 x 10(6) were studied by dynamic and static light scattering. The weight-average molar masses (Mw), second virial coefficients (A2), radii of gyration (Rg), diffusion coefficients (D0), hydrodynamic radii (Rh), and dynamic virial coefficients (kDphi) were determined for the single coil in dilute aqueous solution at 25 degrees C, and scaling equations were established. It was found that polyglycidol in water does not exhibit the expected asymptotic good solvent behavior. The scaling exponents for A2, D0, and Rh are even closer to those for polymer coils in marginal solvents than to the expected ones in the excluded-volume region. The values of the interpenetration parameter, psi, and kDphi are far from reaching limiting values even for the fractions of the highest molar masses. The scaling exponent for Rg as well as the Rg/Rh ratio, which was found to increase with increasing molar mass, imply elongated coil conformation in the high molar mass region.     

Sizing of Reverse Micelles in Microemulsions using NMR Measurements of Diffusion

This paper reports the size of reverse micelles (RMs) in AOT/octane/H(2)O and CTAB/hexanol/H(2)O microemulsions using magnetic resonance (MR) pulsed field gradient (PFG) measurements of diffusion. Diffusion data were measured using the pulsed gradient stimulated echo (PGSTE) experiment for surfactant molecules residing in the RM interface. Inverse Laplace transformation of these data generated diffusion coefficients for the RMs, which were converted into hydrodynamic radii using the Stokes-Einstein relation. This technique is complementary to those previously used to size RMs, such as dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), but also offers several advantages, which are discussed. RM sizes, determined using the PGSTE method, in the AOT (sodium bis(2-ethylhexyl) sulfosuccinate) and CTAB (cetyltrimethylammonium bromide) microemulsions were compared with previous DLS and SAXS data, showing good agreement. Methods for determining number distributions from the PGSTE data, through the use of scaling factors, were investigated.     

Capillary electrophoresis of small solutes in linear polymer solutions: relation between ionic mobility, diffusion coefficient and viscosity

Electrophoretic mobilities, mu, and diffusion coefficients, D, of a small ion (molecular weight 579) were determined in dependence on the viscosity, eta, of aqueous buffer solutions containing ethylene glycol, or polyethylene glycol (PEG) with average molecular weights of 400, 20000, 100000 or 2000000, respectively, as additives. The values for mu and D are inversely proportional to the viscosity for the solutions with small-sized additives (ethylene glycol and PEG400), in accordance to Walden's rule. In contrast, for the longest polymers the mobilities and the diffusion coefficients approximate the values observed for pure water, and are nearly independent of the viscosity. This result agrees with the model of fractional free volume and the obstruction theory. For solutions with equal viscosity, three ranges can be differentiated for mu and D in relation to the size of the additive: for small additives, on the one hand, and the long-chained polymers, on the other hand, the values for mu and D are nearly independent of the size of the additive. In contrast, a pronounced increase of mu and D is found with increasing polymer size in the molecular weight range between 20000 and 100000. The ratio mu/D, occurring in a number of expressions for the plate height contributions, exhibits a remarkably small change over the entire polymer size and viscosity range (between 1 and 7 cP) under consideration. Consequently, the separation efficiency, expressed by the plate number, is found to be nearly constant, and is independent of viscosity.