Effect of syndiotacticity on the aqueous poly(vinyl alcohol) gel 2. Syneresis of gel

Summary The mechanism of syneresis of an aqueous gel of syndiotacticity-rich poly(vinyl alcohol) (PVA) with the rise of temperature was studied using the PVA having the content of syndiotacticity in diad from 58.4 to 66.2%. The extent of syneresis was estimated by gravimetry. Syneresis of gel with the rise of temperature depends upon the syndiotacticity and gelling temperature. The gel chilled at lower temperature of PVA having the content of syndiotacticity in diad above 60.9% showed remarkable syneresis with the rise of temperature. The maximum of the extent of syneresis appeared between the content of syndiotacticity in diad of 60.9% and that of 64.2%. The phenomena of syneresis was not thermally reversible. Syneresis occurs accompanying the breakdown of the unstable junction and further growth of the crystallite.

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Zusammenfassung Der Mechanismus der Synärese an wäßrigen Gelen von Polyvinylalkohol, reich an syndiotaktischem Anteil, wurde mit steigender Temperatur untersucht, wobei der Polyvinylalkohol in Bezug auf seinen Gehalt an syndiotaktischen Diaden zwischen 58,4 und 66,2% variiert. Die Synärese wurde gewichtsmäßig verfolgt. Die Synärese hängt von der Syndiotaktizität und der Gelierungstemperatur ab. Gele, erstarrt bei tieferen Temperaturen mit einer Syndiotaktizität von 60,9%, zeigten beträchtliche Synärese mit steigender Temperatur. Das Maximum an Synärese wurde zwischen 60,9 und 64,2% Syndiotaktizität beobachtet. Das Phänomen der Synärese ist nicht thermisch reversibel. Die Synärese läuft ab unter Zusammenbruch von unstabilen Bindungen und weiterem Wachstum der Kristallite.

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Part 1. cf. reference 4.     

Effect of syndiotacticity on the aqueous poly(vinyl alcohol) gel 3. Mechanical property of gel

Summary The mechanical property of aqueous gel was studied using the PVA having the content of syndiotacticity in diad from 58.4 and 66.2%. The elastic modulus of gels was 105–106 dyne/cm2 and the variation of the elastic modulus of gel with the rise of temperature was corresponded with the variation of the concentration of PVA in gel accompanying syneresis. The elastic modulus of gel having the same content of syndiotacticity and concentration of PVA increased in the order as follows, the first is the gel chilled at room temperature, the second is the gel chilled at 60 °C and last is the gel chilled at 0 °C. The variation of the elastic modulus of gel with the rise of temperature became smaller with increasing content of syndiotacticity. The increase of elastic modulus is considered to be attributed to the further growth of stronger junctions through the recrystallization followed by syneresis rather than the entropic effect of polymer networks with the rise of temperature.

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Zusammenfassung Es wurden die mechanischen Eigenschaften wßriger Gele aus Polyvinylalkohol untersucht, mit Gehalten an Syndioaktizität in Diaden zwischen 58,4 und 66,2%. Die elastischen Moduln der Gele lagen bei 105 bis 106 dyn/cm2 und die Variation der elastischen Moduln der Gele mit Steigerung der Temperatur entsprach der Konzentrationsänderung von Polyvinylalkohol im Gele gemäß der stattgefundenen Synärese. Der elastische Modul des Gels mit konstantem Gehalt an Syndiotaktizität und Konzentration des PVA steigen in folgender Weise: Zuerst das Gel abgekühlt bei Raumtemperatur, dann das Gel abgekühlt bei 60 °C und zuletzt das Gel erstarrt bei 0 °C. Die Variation der elastischen Moduln mit steigender Temperatur wird kleiner mit steigendem Gehalt an Syndiotaktizität. Das Anwachsen der Moduln sollte vermutlich dem weiteren Wachstum strengerer Kopp lungen durch Rekristallisation infolge der Synärese zuzuschreiben sein mehr als einem Entropieeffekt der polymeren Netzwerke mit steigender Temperatur.

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With 6 figures and 2 tables

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Part 2. cf. reference 2.     

Ion-specific swelling behavior of poly(vinyl alcohol) gel in aqueous solutions containing some divalent ions

Swelling behaviors of poly(vinyl alcohol) (PVA) gel were investigated in solutions of salts containing certain divalent ions, namely, alkaline-earth-metal halides and sulfates of alkali-metals, alkaline-earth-metals, and transition metals. The gel deswelled in the solutions of the alkaline-earth-metal chlorides but swelled in those of the bromides. However, the cation specificities of the swelling for both of the halides were roughly the same: the degrees of swelling are Sr²⁺2+2+. The gel deswelled for all sulfate solutions with specificities for alkali-metal and alkaline-earth-metal cations: the swelling degrees are K⁺;Na⁺2++. There was no order for transition metals.     

Preparation of poly(vinyl pivalate) microspheres by dispersion polymerization in an ionic liquid and saponification for the preparation of poly(vinyl alcohol) with high syndiotacticity

We report here a successful free-radical dispersion polymerization of vinyl pivalate (VPi) in an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][TFSI]) using poly(vinyl pyrrolidone) (PVP) as a stabilizer. Morphological analysis by FE-SEM revealed that poly(vinyl pivalate) (PVPi) obtained from dispersion polymerizations were in the form of spherical particles. Micron-sized, PVPi particles with a number-average molecular weight (M_n) of 166,400 g/mol could be obtained using 5% stabilizer (w/w to monomer) at 65 °C for 20 h. The effects of varying concentration of stabilizer, initiator and monomer upon polymer yield, molecular weight, and morphology of PVPi were also investigated. Analogous polymerizations in dimethyl sulfoxide (DMSO) and bulk served as references. In addition, the preparation of poly(vinyl alcohol) (PVA) by saponification of the resultant PVPi was described.     

Long branching in poly(vinyl acetate) and poly(vinyl alcohol). II. Polymerization of vinyl acetate in the presence of crosslinked poly(vinyl alcohol)

It is a common view that poly(vinyl acetate) has many branches at the acetyl side group, but that the corresponding poly(vinyl alcohol) has little branching. In order to study the branching in poly(vinyl acetate) and poly(vinyl alcohol) which is formed by chain transfer to polymer, the polymerization of ¹⁴C-labeled vinyl acetate in the presence of crosslinked poly(vinyl acetate), which was able to be decrosslinked to give soluble polymers, was investigated at 60°C and 0°C. This system made it possible to separate as well as to distinguish the graft polymer from the newly polymerized homopolymer. Furthermore, the degree of grafting onto the acetoxymethyl group and onto the main chain were estimated. It became clear that, in the polymerization of vinyl acetate, chain transfer to the polymer main chain takes place about 2.4 times as frequently at 60°C as that to the acetoxy group and about 4.8 times as frequently at 0°C.     

X-ray diffraction and infrared studies on annealed highly syndiotactic poly(vinyl chloride)

X-ray diffraction scans and infrared absorption spectra for the C-CI stretching region were obtained for a highly syndiotactic poly(vinyl chloride) (PVC) sample made by the urea clathrate method. When the polymer was annealed at a series of increasing temperatures up to 180°C, x-ray diffraction measurements showed that the crystallinity increases steadily with annealing temperature. Even at 50°C an increase above the original value of 63% was detectable and by 180°C it had reached 70% with a further increase to 78% after cooling to ambient temperature. There is a concurrent significant increase in the lateral crystallite dimensions. However, the infrared spectrum did not change, in agreement with the recent prediction of Moore and Krimm that there is no observable band splitting from interchain interaction in crystalline regions, thus indicating that these C-CI bands cannot be used as a measure of crystallinity. The results from computer curve fitting of the spectra suggest that at least 85% of the polymer consists of long planar syndiotactic sequences and there is therefore substantial order along the chain direction. A mechanism for the increase in crystallinity on annealing, involving the lateral ordering of these regular chains, is discussed. Furthermore, as the temperature is raised some amorphous material is converted to a nematic phase, and this may crystallize during the subsequent cooling.     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.     

Rheological investigation of aqueous solutions of poly(vinyl alcohol) during ageing

The changes in viscosity and normal stress difference during the ageing of concentrated aqueous solutions of poly(vinyl alcohol) prepared at 80 °C and the effect of the procedure used in the preparation of solutions on the course of these changes are described. The results are interpreted by means of shear complianceJ _e and relaxation time ₀.     

Characterization of poly(vinyl alcohol) during the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as emulsifier

During the emulsion polymerization of vinyl acetate (VAc) using poly(vinyl alcohol) (PVA) as stabilizer and potassium persulfate as initiator, the VAc reacts with PVA forming PVA-graft-PVAc. When the grafted polymer reaches a critical size it becomes water-insoluble and precipitates from the aqueous phase contributing to the formation of polymer particles. Since particle formation and therefore the properties of the final latex will depend on the degree of grafting, it is important to quantify and to characterize the grafted PVA. In this work, the quantitative separation and characterization of the grafted water-insoluble PVA was carried out by a two-step selective solubilization of the PVAc latex, first with acetonitrile to separate PVAc homopolymer, followed by water to separate the water-soluble PVA from the remaining acetonitrile-insoluble material. After the separation, the water-soluble and water-insoluble PVA were characterized by Fourier Transform Infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance (NMR) analyses, from which the details of the PVA-graft-PVAc structure were obtained. © 1996 John Wiley & Sons, Inc.     

Stabilization of electrospun poly(vinyl alcohol) nanofibrous mats in aqueous solutions

<正>The stability ofpoly(vinyl alcohol)(PVA) nanofibrous mats in water media was improved by post-electrospinning treatments.Bifunctional glutaraldehyde(GA) in methanol was used as a crosslinking agent to stabilize PVA nanofiber,but fiber twinning was observed frequently,and the highly porous structure of PVA nanofibrous mats was destroyed when the crosslinked fiber was soaked in water.To overcome this shortcoming,chitosan(CS) was introduced into the PVA spinning solution to prepare PVA/CS composite nanofibers.Their treatment in GA/methanol solution could retain the fiber morphology of PVA/CS nanofibers and porous structure of PVA/CS nanofibrous mats even if they were soaked in aqueous solutions for 1 month.Scanning electron microscopy(SEM),X-ray diffraction(XRD),thermal gravimetric analysis(TGA) and differential scanning calorimetry(DSC) were applied to characterize the physicochemical structure and thermal properties of PVA nanofibers.It was found that the water resistance of PVA nanofibrous mats was enhanced because of the improvement of the degree of crosslinking and crystallinity in the electrospun PVA fibers after soaking in GA/methanol solution.     

High molecular weight syndiotacticity-rich poly(vinyl alcohol) gel in aging process

The properties of the aged gels of high molecular weight syndiotacticity-rich poly(vinyl alcohol)s (HMW S-PVAs) with different syndiotactic diad (s-diad) contents were investigated. HMW S-PVA gels with s-diad content of 61.5% and 58.2% showed the rapid increases of the syneresis and the turbidity from the early stage of aging time, which is ascribable to the phase separation, while that with s-diad content of 55.7% did not. From the morphological study, it was confirmed that the phase separation in HMW S-PVA gel with s-diad content of 61.5% occurred without the liquid-liquid phase separation in sol state, whereas both the liquid-liquid phase separation in sol state and the subsequent phase separation in gel state occurred in the case of HMW S-PVA gel with s-diad content of 58.2%. On the other hand, HMW S-PVA gel with s-diad content of 55.7% showed neither the liquid-liquid phase separation in sol state nor the phase separation in gel state in the long period of time. It was also confirmed from wide angle X-ray diffractogram that the crystallization was accompanied by the phase separation in gel state in the aging process of PVA gel. However, the crystallization was hindered by the fast network formation at the initial stage of time. Later the syndiotacticity promoted the crystallization. The tensile modulus of HMW S-PVA gel with higher syndiotacticity increased more significantly with time. Received: 2 December 1999/Accepted: 12 July 2000     

Effect of poly(ethylene oxide) on the structure and properties of poly(vinyl alcohol)

To improve the drawability of poly(vinyl alcohol) (PVA) thermal products, poly(ethylene oxide) (PEO), a special resin with good flexibility, excellent lubricity, and compatibility with many resins, was applied, and the Fourier transform infrared spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WXRD) were adopted to study the hydrogen bonds, water states, thermal properties, crystal structure, and nonisothermal crystallization of modified PVA. It was found that PEO formed strong hydrogen bonds with water and PVA, thus weakened the intra‐ and inter‐hydrogen bonds of PVA, changed the aggregation states of PVA chains, and decreased its melting point and crystallinity. Moreover, the interactions among PVA, water, and PEO retarded the water evaporation and made more water remain in the system to plasticize PVA. The existence of PEO also slowed down the melt crystallization process of PVA, however, increased the nucleation points of system, thus made more and smaller spherulites formed. The weakened crystallization capability of PVA and the lubrication of PEO made PVA chains to have more mobility under the outside force and obtain high mechanical properties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1946–1954, 2010     

Fractionation of poly(vinyl alcohol) derived from poly(vinyl acetate) from its aqueous solution induced by shearing force-IV

Summary In order to clarify the mechanism of crystallization under molecular orientation, the fractionation for poly(vinyl alcohol) (VAc-PVA) derived from vinyl acetate from its aqueous solution was carried out under shearing force. The fibrillar crystals grown by shearing force from solutions were produced on the stirrer and in solution simultaneously. The fibrillar crystals had a diameter of approximately 1 mm and a length of 1 cm in the case of the initial polymer concentration of 2.0%, and approximately 1 mm and 5 cm in the case of 0.5%. The initial rate of precipitation (Rs) for the precipitates produced in solution was higher than that (Rr) on the stirrer. The difference betweenRs andRr increased with an increase in initial polymer concentration. The difference in the percentages of syndiotactic diads of precipitates grown on the stirrer and those in solution was not found, and the percentages of syndiotactic diads of each fraction decreased in the order of fractionation. Furthermore, the difference in the molecular weight of precipitates grown on the stirrer and those in solution was not found in the case of the higher polymer concentration, whereas it was found clearly in the case of lower one.

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Zusammenfassung Um den Mechanismus der Kristallisation unter molekularer Orientierung zu klären, wurde die Fraktionierung von Polyvinylalkohol (VAc-PVA), der aus Vinylacetat hergestellt war, in wäßriger Lösung unter Scherkräften durchgeführt. Die von Lösung unter Scherung gewachsenen flbrillären Kristalle erscheinen gleichzeitig in Lösung und auf dem Rührer. Die aus Lösung gewonnenen Faserkristalle hatten einen Durchmesser von etwa 1 mm und eine Länge von etwa 1 cm bei einer anfänglichen Konzentration des Polymeren von 2.0%, etwa 1 mm und 5 cm resp. für 0.5%. Die anfängliche Fällungsgeschwindigkeit (Rs) der in Lösung wachsenden Niederschläge war höher als diejenige (Rr) auf dem Rührer. Ein Unterschied im Gehalt an syndiotaktischen Diaden zwischen den vom Rührer und aus der Lösung entnommenen Faserkristallen ließ sich nicht feststellen, und die syndiotaktischen Diaden nahmen in beiden Proben mit der Ordnung der Fraktionierung ab. Darüber hinaus ließ sich ein Unterschied im Molekulargewicht zwischen den auf dem Rührer und in der Lösung gewachsenen faserigen Kristallen bei der höheren Anfangskonzentration des Polymeren nicht feststellen, aber bei der niedrigeren Anfangskonzentration des Polymeren war die Auswirkung des Molekulargewichtes deutlich.

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Mechanical Denaturation of High Polymers in Solutions, XXV.     

Chromatography of inorganic anions on poly(vinyl alcohol) gel columns with acidic eluents

Summary Poly(vinyl alcohol) (PVA) gel shows ionic retention properties for common inorganic anions when an acidic eluent is used. The ionic property of the PVA gel is due to the proton-acceptable nitrogen atoms of the cross-linking agent and the carboxylic residues being comprised in the gel matrix. The extent of the net charge on the gel surface depends on the pH of the eluent. At a pH ranging from 2.3 to 5.3, the PVA gel behaves as a weak anion exchanger with very low ion-exchange capacity. At these conditions four UV-absorbing inorganic anions (bromate, bromide, nitrate, and nitrite) are separated by eluting with aqueous sulfuric acid. Alkyl groups introduced on the gel surface hinder the ionized solute molecules from accessing to the positively charged functional groups on the gel surface. A neutral solute (HNO₂) is retained with non-ionic interactions.     

Effect of solvent for vinyl acetate polymerization on microstructure of poly(vinyl alcohol)

The solution polymerization of vinyl acetate was carried out in several solvents at 0 to 100°C, using 2,2′-azobisisobutyronitrile as initiator. For the resulting poly(vinyl alcohol) (PVA), iodinecoloration, 1,2-glycol structure and tacticity were observed. The pentad tacticity of PVA was estimated from its methine carbon spectra by means of ¹³C-FTNMR spectrometer. Iodine-coloration ability of PVA varied markedly with the type of polymerization solvent and decreased in the following order: phenol > aq. phenol > methyl alcohol > ethyl acetate > DMSO, ethylene carbonate. The syndiotactic fraction in PVA also decreased with polymerization solvent in the same order as that of iodine coloration, while 1,2-glycol content of PVA was not almost affected by polymerization solvent except for phenol and aq. phenol. In solution polymerization performed, effect of polymerization temperature on tacticity was less than that of solvent.     

Degrees of order from X-ray diffraction in highly crystalline poly(vinyl chloride)

Convenient x-ray diffraction instrumentation for assessing PVC order is described. A reflectance diffractometer, copper radiation, a monochromator, and a proportional detector are used to make measurements on low molecular weight, highly crystalline PVC prepared using chain-transfer agents. Interpretation includes a correction for air scattering, employment of a trimodal noncrystalline pattern, and the calculation of crystallinity from the ratio of the crystalline to the total integrated intensity. The results are reasonably insensitive to the assumptions made. The noncrystalline pattern used is taken to indicate a single, partially ordered mesomorphous phase. A whole polymer prepared in the presence of butyraldehyde, a fraction therefrom prepared by precipitation, and a similar fraction terminated by butyl mercaptan have similar molecular weights (M?_n ca. 1600) and crystallinities of 30, 44, and 44%, respectively. Fractionation apparently effects a separation with respect to syndiotactic content, and the resultant crystallinity appears to be the highest valid x-ray value reported to date for a PVC polymer, irrespective of polymerization variables, fractionation procedure, or molecular weight. The order in these polymers is due in part to their greater syndiotacticity (about 64% for the fractions by ¹³C NMR spectroscopy), although a more favorable tactic placement distribution may be involved in addition to a possible effect of molecular weight itself.     

Radiation effects on the thermal degradation of poly(vinyl chloride) and poly(vinyl alcohol)

Radiation effects on the formation of conjugated double bonds in the thermal degradation of poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) were investigated. Thin films of PVC and PVA were either irradiated with γ-rays at ambient temperature (pre-irradiation) and then subjected to thermal treatment, or irradiated at elevated temperatures (in situ irradiation). An extensive enhancement of the thermal degradation was observed for the pre-irradiation of the PVC films, which was more effective than the effect of the in situ irradiation at the same absorption dose. For the PVA degradation, however, the effect of the in situ irradiation was larger than that of the pre-irradiation. The results were explained and related mechanisms were discussed based on radiation-induced chemical reactions and their individual contributions to the thermal degradation behaviors of the two polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3089–3095, 1998