Studies on the radical polymerization of vinyl acetate in benzene,chlorobenzene and ethyl acetate by 1H NMR spectroscopy

Summary Polymerizations of vinyl acetate were carried out with AIBN in benzene, chlorobenzene and ethyl acetate, and the resultant polymers were analyzed for terminal group by using ¹H NMR technique. The results revealed that a part of the polymer molecules prepared in aromatic solvent contained one solvent fragment at the chain end, indicating the incorporation of aromatic molecule through the chain transfer reaction and not by the copolymerization. It was found that there existed at least 0.7 branching per a molecule in the polymer prepared in ethyl acetate, whereas almost no branching in the polymer prepared in aromatic solvents.     

Studies on the radiolytic degradation of cellulose acetate membranes

The effect of γ-irradiation on the performance of wet cellulose acetate membranes in the dose range of 2.5–10 Mrads was investigated using a ⁶⁰Co source. Changes in transport properties and inherent viscosity of the membranes suggested continued degradation as a result of irradiation. Solubility and specific gravity changes accompanying irradiation indicated some sort of structural aggregation occuring at higher doses. Consumption of dissolved oxygen during irradiation and the extent of deacetylation of the membrane polymer were determined to study the kinetics of the degradative process. Analysis of the end products of irradiation was attempted by UV spectroscopy. ESR spectra of membrane polymer after irradiation were analyzed to identify the free radicals generated. A tentative mechanism of radiolytic degradation causing the observed performance failure is proposed.     

Studies on absorbed water in cellulose by broad-line NMR

Nuclear magnetic resonance absorption spectra of linter cellulose containing various amounts of water were studied to clarify the relation between the amount of absorbed water in cellulose fiber and the molecular motion in such a cellulose–water system. An amorphous cellulose sample was used for the sake of comparison. The changes in line width and second moment of the spectra were measured at various temperatures. From the variation with temperature of the first-derivative curves and the second moments, it has been inferred that the proton movement of absorbed water changes markedly over the approximate range 180°–200°K and that the absorbed water lowers the glass transition temperature of cellulose to room temperature.     

Studies of water adsorbed on cellulosic materials by a high resolution NMR spectrometer

A study of water contained in cellulose samples softwoods dissolving pulp (DP) and hardwoods semichemical pulp (SCP) by a high resolution NMR spectrometer indicated the presence of two regions, one where the width at half value decreased rapidly with increasing water content and the other where the width at half value decreased slowly. As a result of a treatment for increasing the hydrophilicity of cellulose fibers such as beating, swelling by sodium hydroxide, and hydrolysis with hydrochloric acid, the differential energy of water binding increased in the low water content region, whereas it either did not change or decreased in the high water content region in spite of an increase in the amount of water, which is subject to the influence of the cellulosic materials. A comparison of different cellulosic materials showed that the differential energy of water binding of SCP was larger than that of DP. Moreover, an investigation of the relationship between the width at half value and the temperature indicated that different temperature ranges exist regarding binding of water, and such ranges are affected greatly by the degree of beating and the water content of the sample.     

The states of water in cellulose acetate membranes

Differential scanning calorimetric melting endotherms of wet and half-dried cellulose acetate membranes and salt distribution coefficients were studied to clarify the states of water in membranes. We have suggested that (a) there are four states of water in cellulose acetate membranes; (b) these states of water are those of completely free water, free water very weakly interacting with polymer, bound water which can contain salts, and bound water which rejects salts; (c) the semipermeability of membranes depends on the ratio of four states of water in membranes.     

Freezing and nonfreezing water in cellulose acetate membranes

The relative amounts of freezing and nonfreezing water in various cellulose acetate (CA) membranes were determined by differential scanning calorimetry. It was found that: (1) A significant fraction (17–40%) of the water (1.0–3.1 g H₂O per gram dry CA) in any membrane does not freeze at temperatures as low as ?60°C. (2) The amount of nonfreezing bound water (0.4–0.7 g nonfreezing water per gram dry CA) depends upon the nature of the membrane and is significantly higher than the total amount of water (all of which is nonfreezing) absorbed from liquid water by a dense film of the same polymer (～0.18 g water per gram dry CA). The structures of the membranes were studied by scanning electron microscopy. The results suggest that the amounts of nonfreezing water in cellulose acetate membranes decrease with the increase in the packing density (compactness) of the polymer within the membrane. In dense films, the extent of polymer–polymer interactions within the polymeric matrix is high, and therefore the macromolecular chains are less accessible to bind water.     

Characterization of water structure in cellulose acetate membranes by calorimetric measurements

The heat capacities of homogeneous and asymmetric cellulose acetate membranes have been measured at different water contents within the temperature range of ?40 to +20°C. The experimental heat capacity–temperature curves were verified by DTA measurements within a temperature range of ?120 to +20°C. The results for the partial heat capacity of water within the membranes as well as for the heat of fusion were interpreted by assuming two different states of water—unfreezing bound water due to a sorption process and unbound water due to capillary phenomena, which freezes with a freezing point depression and a reduced heat of fusion.     

The thermodynamic state of water in cellulose acetate membranes

Using experimental sorption data and corresponding experimental results from calorimetric investigations, the state of water in cellulose acetate (CA) membranes is discussed by applying a theoretical treatment of sorption reported previously (1–3). The sorption of water can be attributed to a gain in surface energy at the polymer/vapor interface. Using differential thermodynamic potentials of sorbed water together with experimentally determined heat capacities of sorbed water, the thermodynamic potentials G, H, and S of sorbed water are estimated for the temperature interval ?40 to + 40°C. At constant temperature, each thermodynamic potential depends on the water content. The resulting distribution function of G indicates that the sorbed water exists in different states. Comparing the Gibbs free energy of sorbed water with that of ice or liquid water at the same temperature leads to the conclusion that none of the sorbed water freezes to ice within the temperature interval used. Based on the Gibbs free energy of water in electrolyte solutions and the distribution function of G for sorbed water, partition coefficients of salts within CA membranes may be estimated. The results are in good agreement with experimentally determined partition coefficients which are available from the literature. As the partition coefficient of a salt is directly related to the salt rejection of the membrane, this provides a method of estimating the desalination performance of a membrane from its water sorption isotherm.     

Determination of diffusion coefficients of water in cellulose acetate membranes

The determination of the diffusion coefficient for water in various porosity cellulose acetate membranes by a gravimetric method, using a humidified carrier gas, is described. It was found to be impossible to obtain meaningful results for very porous membranes, although dense membranes gave limiting values of diffusion coefficient at high carrier gas velocities. This phenomenon is explained in terms of the dissipation of the heat of sorption by the forced convection provided by the carrier gas. The variation of diffusion coefficient with concentration of water in dense cellulose acetate is explained in terms of clustering of water molecules in the polymer at high concentration.     

Ion movements in cellulose acetate membranes

The behavior of ions in a cellulose acetate membrane was discussed from the standpoint of mobilities obtained from the membrane potentials. The mobilities of univalent ions larger in radius or divalent ions relative to Na⁺ ion mobility in the membrane are much different from those in aqueous solutions. The order of the mobilities of ions in the membrane is shown quantitatively. The mobilities of the ions with radii larger than a certain value in the membrane decrease with increase in their size. This could be explained by the physical friction between the ion and the membrane wall. This friction is of importance as the ionic radius comes close to the intermolecular gaps between polymer chains. The mobilities of Na⁺ anyd 1^? ions in the membrane are (2–3) × 10^?7 cm²/sec-V and are about three to four orders of magnitude less than those in aqueous solutions.     

Phenol transport in cellulose acetate membranes

Past detailed studies of solute transport through reverse-osmosis membranes have been conducted only with simple salts. The present work with phenol was undertaken largely because of the practical observation that the transport of low molecular weight organics is much more rapid than that of the salts. Studies of phenol sorption from dilute aqueous solution indicate that the diffusion coefficient for phenol in water-saturated 39.8 wt.-% acetyl cellulose acetate is 9.6 × 10^-10 cm.²/sec., and the equilibrium distribution coefficient between the acetate phase and water is 42. Thus, the diffusion coefficient is quite close to that measured for sodium chloride, and the higher permeability of the membranes to phenol can be attributed entirely to their greater sorption of this solute. In direct osmosis experiments performed with significant water flow a measurable interaction or positive coupling between water and phenol flows has been observed. Further evidence of flow coupling is derived from reverse osmosis experiments in which significant negative solute rejection is observed; i.e., the permeate is enriched in phenol by as much as 20%. It is shown that a solution-diffusion transport model is not adequate to rationalize the results, and a more complex transport model is apparently required.     

Immobilization of polymers on cellulose acetate membranes

Using models of dead-end filtration theory, the kinetics of forming dynamic layers of sulfate lignin (SL) and sodium carboxy-methylcellulose (Na-CMC) onto CA membranes during cross-flow filtration of dilute solutions of polymers was studied. It was found for both polymers (compact SL and linear Na-CMC), that the polymer layer with the least hydraulic resistance, which yields a small reduction in membrane water permeability (10–20%), but a significant increase in salt rejection, is formed, if the process kinetics corresponds to J-V linear dependence predicted by the model of 'blocking a pore by a single particle's. The results obtained may be used to define the optimum conditions for immobilizing the available catalytic active polymers on regular semipermeable membranes during the membrane filtration process.     

Membrane potential measurements on cellulose acetate membranes

Membrane potentials and apparent transport numbers of the cation are reported for cured cellulose acetate membranes bounded by HCl, NaCl, KCl and MgCl₂ solutions, using Ag/AgCl electrodes and a flow-cell method. Membranes cured at 70°, 80° and 90° are used. Bounding solution concentrations vary from 0.005 to 0.05 M at the high concentration side (bounding the dense side of the membrane), and are kept constant at 0.002 M for the low concentration solution. In the KCl 90° membrane case the low concentration solution is varied as well, from 0.0001 to 0.002 M. Results show that cured cellulose acetate membranes are permselective towards univalent cations. This is interpreted as resulting from a low cation-exchange capacity of the dense layer of the cured membrane. The permselectivity increases with increased curing temperature. Addition of a non-electrolyte to the low concentration side reverses the osmotic flow and leads to higher apparent transport numbers of the cation. It is concluded that diffusion in small pores contributes significantly to the transport of ionic solutes through uncompacted membranes.     

The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content

Highly productive cellulose acetate membranes were cast under conditions of very short air exposure periods from cellulose acetate–acetone–formamide casting solutions having a high cellulose acetate (CA) content and lying close to the phase boundary. Air exposure periods as short as 0.05 sec were used with CA content up to 32 wt-%. Membranes from a casting solution containing 30 wt-% cellulose acetate (E-398-3), 45 wt-% acetone, and 25 wt-% formamide perform as well as membranes from other compositions at all salt rejection levels for a 0.5 wt-% NaCl feed at 600 psig. Partial replacement of acetone by dioxane in the casting solution substantially increases the water flux from membranes cast with short air exposure periods at any given salt rejection level below 96% salt rejection. Addition of small amounts of ZnCl₂ to nondioxane casting solutions with 32 wt-% CA improves membrane performances remarkably for lower salt rejection levels, while the improvement in performance of membranes from 30 wt-% CA casting solutions with dioxane due to ZnCl₂ addition is marginal. Variation in air exposure from 0.05 to 2 sec results in minor performance variations in the membranes having any of these compositions. With air exposure periods beyond 2–3 sec, membrane fluxes drop drastically. The concept of a thinner skin satisfactorily explains the improvement in mixed solvent systems, whereas ZnCl₂ acts as a swelling salt. A Kimura-Sourirajan-type membrane performance plot indicates that for a 0.5 wt-% NaCl feed at 600 psig, membranes of the present work perform as well as the best performing membranes reported in the literature for conversion of brackish water.     

Removal of sodium acetate in poly(vinyl alcohol) and its quantification by 1H NMR spectroscopy

A major impurity in poly (vinyl alcohol) (PVA) is sodium acetate which remains after its preparation by a base catalyzed hydrolysis of poly(vinyl acetate), and the amount of sodium acetate in commercial PVA samples may reach several percentages. To establish an optimal condition for the removal of sodium acetate, several washing parameters such as washing period, solvent polarity, and temperature were investigated in this study. Nuclear magnetic resonance (NMR) spectroscopy was successfully applied to determine the residual amounts of sodium acetate in the purified poly(vinyl alcohol). The relative integral value for the methyl peak of sodium acetate in PVA was converted to a relative mass value and finally to the sodium acetate content contained in PVA. The results showed that over 95% of sodium acetate in PVA was removed by a washing of PVA with distilled water within 2 h. When methanol was used as a washing solvent, a higher temperature than room temperature was required for an effective removal of sodium acetate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Hydroxyl group determination in prepolymers by 1H NMR spectroscopy

A fast and specific method has been developed for the determination of hydroxyl content in prepolymers that contain hydroxymethylene functional groups. This method involves reaction of the sample with either trifluoroacetic anhydride or hexafluoroacetone to separate the terminal oxymethylene signals in the ¹H NMR spectrum from those of the repeating unit and integration against an internal trinitrobenzene standard. This procedure generally requires less than 50 mg of polymer and has been employed in the analysis of prepolymers with number average molecular weights ranging up to 10000. The analysis has been tested using several different prepolymers. Precision of analysis was determined to be within 2.7 percent using a Varian XL-200 spectrometer.     

A study on reproducibility of cellulose acetate membranes

The variation in flux and rejection of replicate cellulose acetate membranes is measured within a membrane sheet as well as between membranes. Although the casting conditions were carefully controlled and various influences were studied. the source of variation in replicates was not identified. It is shown that the cause of variation is not due to hand or machine casting technique, casting solution composition, heat treatment, dust, overall film thickness or pressure history.     

Electrokinetic transport of water and methanol in Nafion membranes as observed by NMR spectroscopy

Electrophoretic NMR (eNMR) and pulsed-field-gradient NMR (PFG-NMR) methods were used to study transport processes in situ and in a chemically resolved manner in the electrolyte of an experimental direct methanol fuel cell (DMFC) setup, constituted of several layers of Nafion 117. The measurements were conducted at room temperature for membranes fully swollen by methanol-water mixtures over a wide concentration interval. The experimental setup and the experimental protocol for the eNMR experiments are discussed in detail. The magnitude of the water and methanol self-diffusion coefficients show a good agreement with previously published data while the ratio of the two self-diffusion coefficients may indicate an imperfect mixing of the two solvent molecules. On the molecular level, the drag of water and methanol molecules by protons is roughly of the same magnitude, with the drag of methanol molecules increasing with increasing methanol content. The electro-osmotic drag defined on mass-flow basis increased for methanol from a low level with increasing methanol concentration while that of water remained roughly constant.     

Movements of univalent ions in cellulose acetate membranes

Membrane potentials across the asymmetric membranes of cellulose acetate with various salt rejection properties have been measured for univalent ions. The behavior of ions in the membranes is discussed from the viewpoint of relative ionic mobilities calculated from the membrane potentials. The relationship between ionic mobilities and ionic radii in the membranes having salt rejections lower than 80% is almost the same as that in aqueous solutions. This implies that the ions in these membranes behave as if they exist in bulk water. However, the ionic mobilities in the membranes having salt rejections higher than 86% differ significantly depending on the ionic radii. It seems probable that the bound water influences the ionic mobilities in these membranes.